THE GEOLOGY OE TÎI5 HASAMT AREA^ ^

™VALLEY, SOUTH TURKAI'IA,

KE2TÏA AMO r t f B H f^ m y c F r if à ^ PL\ccE?Ht= vcicftroic a c e k :- 3

Tliesis presented for the Degree of Doctor of Philosophy in the University of London

\ \

Stephen Donald Weaver ProQuest Number: 10098242

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ABSTRACT

The geology of a previously unmapped 170 square miles

( 440 sq. kms. ) area of South Turkana, lying within the

rift, is described. A contoured ard coloured geological map is presented.

The stratigraphical succession established which totals

about 6000 feet ( 18 OO m. ) in thickness and consists largely of volcanic rocks, ranges in age from about 25 to 4 million years b.p.

New potassium/argon age determinations on some of the lavas are presented and reinforce the established stratigraphy • Particular

attention is paid to the Pliocene succession which consists of a thick, widespread basalt formation overlain by a number of

trachyte volcanoes. The morphology of the multicentred trach^rte volcanoes suggests that they are a type not described before,.

The petrography of the lavas of the Nasaken area is described in detail.

Chemical analyses of rocks and minerals are presented and the petrogenesis of the Pliocene lavas is discussed with Ill

particular reference to the relationship between the basalts

and the voluminous trachytes and the origin of the chemical variation within the trachytes. The trace-element geochemistry

of the Pliocene lavas is compared with the geochemistry of

lavas from other areas within the East African rift system • IV

CONTENTS

page

CHAPTER 1 : INTRODUCTION

a) Scope of work 2

b) Situation and. access 2

c ) Physiography 3

d.) Drainage and water resources 5

e) Climate 6

f ) Population 9

g) Flora and fauna 9

h) Conduct of field work 10

i) Previous geological work 10

j) Geological setting 11

k) Note on time-scale employed 14

l) Note on systems of measurement 15

CHAPTER 2 : STRATIGRAPHY

1) Basement System 17

2) a Turkana basalt formation 21

b Correlations and age 26

3 ) a Kowun volcano 27

b Correlations and age 33

4 ) a Kasorogol melanephelinite 35

b Nakasuw and Kanitiriam trachyphonolites 35

0 Kanitiriam microfoyaite 36

d Correlations and age 37

e History of the area west of the watershed 38

5) Tirioko basalt formation

a Introduction 39

b Kagumnyikal basalts 41 page

5 ) 0 Napeitom mugearites 44

d Napeitom basalts 45

e Lotokongolea basalts 46

f Napeitom limestone 48

g Correlations and age 50

6) Pliocene trachyte volcanoes

a Introduction 51

b Trachyte intrusions of the Kasamanang valley 53

c Kafkandal volcano

i Introduction 56

ii Ngapawoi unit 58

iii %ong unit 59

iv Moru Angitak unit 60

d Nasaken trachyte complex

i Introduction 61

ii Nasaken area 63

iii Balakaicha area 6?

iv Nakwamoroi centre 70

V Tirioko basalt inliers 71

vi Emus and the cyclic nature of activity 73

vii Edilteniro area 76

viii Karimorega centre 77

ix Katatarope valley 79

X Morphology of trachyte flows 81

e Kanatim volcano 82

f South Turkana group of volcanoes 82

g Geochronology of the South Turkana group 84

7 ) Recent deposits 86 VI

page

CHAPTER 3 : STRUCTURE

1 ) Regional structure 89

2) Structure of the Nasaken area 90

3) Form and structure of the major volcanic units 94

CHAPTER 4 : PETROGRAPHY

1 ) Classification 99

2 ) Mineralogical components

a Plagioclase feldspars 102

h Alkali feldspars 102

c Olivines 105

d pyroxenes 109

e Amphiboles 111

f Aenigmatite 113

g Micas 113

h Magnetite 114

i Quartz 114

j Feldspathoids and analcite 115

k Apatite 115

1 Secondary minerals 115

3) Textures

a Basalts and intermediate lavas 117

b Trachytic lavas 118

4) Petrography of the lavas

a Introduction 120

b Turkana basalt formation 120

c Kowun trachytes 121

d Kasorogol melanephelinite 123

e Kanitiriam microfoyaite 123

f Kanitiriam and Nakasuw trachyphonolites 124 vil

page

4 ) g Tirioko basait formation 124

h Nasaken volcanics - basic and intermediate lavas 128

i Kafkandal and Nasaken volcanics - trachytic lavas 129

j Pulaskite syenites 132

k Pétrographie features of welded tuffs 134

CH/iPTER 5 : GEOCHEMISTRY and PETROLOGY

1 ) Introduction 163

2 ) Conpositions of the Miocene lavas 166

3) Geochemistry of the Pliocene lavas

a Introduction 171

b Mineral chemistry 172

c Chemistry of the lavas 177

4 ) Petrogenesis of the Pliocene volcanic rocks

a Relationship between basalt and trachyte 197

b Origin of the syenites 201

c Differentiation within the salic lavas 202

d Chemical variation within a single trachyte flow 2O4

e Rare-earth element distributions in feldspars 2O5

f Residual trace-element ratios and magma-types 206

CONCLUSIONS 208

REFERENCES 211

APPENDICES

1) Analytical methods 226

2 ) C.I.P.W. norm 228

3 ) Specimen localities 229 viii

LIST OP FIGURES

after page

1. Location of area 8 3

2 . Relief map 3

3. Physiographic divisions 4

4. Basement drainage pattern 5

5. V/ater supplies 6

6 . E.A.G.R.U. areas 11

7. East African rift system 11

8 . Basement outcrop in South Turkana 17

9. Basement map for area 8 20

10. Successions within the Turkana basalt formation 21

11. Turkana basalt lava succession 23

12. Extent of Kowun volcano 28

13. Cross-section through tholoid and basalt cone 31

14. Kowun fault-strip succession 31

15. Geological evolution of area west of the watershed 38 l6. Serial E-W sections along Kasamanang valley 40

17. Members of the Tirioko basalt formation - 40

18 . N-S longitudinal section, Cheror to Napeitom 42

19. Radiating basalt cooling columns 48

20. Napeitom gorge succession 48

21. Structural contour map for base of trachyte volcanoes 52

22. Form of trachyte intrusion along fault plane 55

23. Map of Kafkandal volcano 56

24. Section through Epong 59

25. Section through Moru Angitak 59

26. Moru Angitak from Plate 24 60

27. Nasaken volcano, source and flank zones 62

28 . Rose diagram of dyke orientations 64 IX

after page

29. Succession at Nasaken 66

30. Sketch of hill at 683678 69

31. Sketch of hill at 705772 69

32. Succession at Nakv/amoroi 71

33. Succession at Emus 73

34. Formation of ash and ash-flov; eruptions after Rittmann 74

35. V/elded tuff specimen 8/ll4a 76

36. Successions in Edilteniro river 76

37 . Flow directions in Katirr basalt 77

3 8 . Succession in Karimorega area 78

39. Section through a trachyte flow 81

40. Kanatim succession in Edilte river 82

41. South Turkana group of volcanoes 82

42. Geochronology of South Turkana group 86

43. Regional structure 89

44. Cross-section from the Kerio to the Suguta 89

45. Modal classification of volcanic rocks 99

46. Normative classification of alkali rhyolites 99

47 . Graph of v femics for and Nasaken trachytes 100

48 . 2V determinations of alkali feldspars IO3

49. Alteration of olivine 108

50. Graph of modal quartz v dark minerals, Nasaken trachytes 129

51. Crystallization sequences and rection series 134

52. Alkali / silica plot, rift volcanics 163

53. Compositions of olivines and pyroxenes 172

54. Compositions of feldspars 175

55. Normative feldspars 176

56» Alkali / silica plot. Pliocene rocks of Nasaken area I86

57 . A-F-M diagram, Pliocene lavas I86

58 . Major-oxides plotted against Solidification Index 187 X

after page

59» Alkali / silica plot, Nasaken and Ribkwo areas 189

60. Graph of 2r against Solidification Index 194

61. Si02 3 CaO against Zr 194

62. AI2O3 , MgO against Zr 194

63. Total iron-oxide, total alkalis against Zr 194

64. Peralkalinity against Zr 200

65. Si02 - AI2O2 - total alkalis diagram 201

66. R.E.E. patterns in feldspars 201

67 . R.E.E. patterns in lavas 202

68. Residua system 2O3

69. Alumina - alkali ratio diagram 2O3

70 . Chemical variation through a single flow 204

71* Zr/Nb against Zr for northern rift volcanics 207

72 . Zr/Nb against Zr for East African volcanics 207 XI

LIST OP PLATES after page

1. Photomicrograph, garnet-plagioclase symplectite 19

2. Photomicrograph, layered plagioclase-amphiholite 19

3. Nakasuw trachyphonolite hills 21

4. Agglomerate, Turkana basalts 23

5. Botryoidal oalcite, Turkana basalts 23

6, Agglomerate ridge, Kowun 28

7. Agglomerate centre, Kowun 32

8. Panorama west to north from Kowun 36

9. Kanitiriam hill 36

10. Kov/un trachyte dipping beneath Lotokongolea basalt 41

11. Kagumnyikal hill 41

12. Pahoehoe basalt 47

13. Basalt dyke cutting Kowun trachytes 47

14. Radiating basalt cooling columns 48

15. Basalts in Napeitom gorge 48

16, Earth pillars in Napeitom gorge 49

17. Napeitem limestone 49

18 . Lodukai dyke 54

19. Lodukai dyke 55

20. Cone sheets near Cheror 55

21. Trachyte dyke, Kasamanang valley 56

22. Panorama, Ngapawoi lavas 58

23. Centre in Ngapawoi unit, Kafkandal 59

24. Moru Angitak centre 60

25. ditto 60

26. ditto 61

27. Explosion breccia, Nasaken 64

28 . ditto 64 Xll

after page

29. Exfoliation weathering in basalts at Nasaken 65

30. Bedded pumice tuffs, Nasaken 65

31. Bills west of Nasaken 66

32. ditto 66

33. Balakaicha area 67

34. Trachyte plugs 68

35. Trachyte outliers 70

36. Polongus 70

37. Tuffs resting on basalt inlier within Nasaken volcano 71

38 . Nasaken ash-flow tuff field 80

39. Nasaken flank deposits and Oliyamur 80

40. Zones within trachyte flows 81

41. ditto 81

42. ditto 81

43. Kanatim-Nasaken unconformity 84

44. River gravels, Nasaken 84

45. Mud volcano in Nasaken river 86

46. ditto 86

47. ditto 87

48. Kagumnyikal hill 91

49. Fault scarps in Kowun trachyte 91

50. Small fault in Nasaken trachytes 92

51. Photomicrograph, zoned augite phenocryst 110

52. ditto , hedehbergite phenocrysts 110

53. ditto , ditto 116

54. ditto , ditto 116

55. ditto , ankaramite, Tirioko basalts 117

56. ditto , soda rhyolite, Nasaken 117

57. ditto , soda trachyte, Nasaken 119

58 . ditto , ditto 119 xiii

after page

59. Phot omic rograph, anorthoclase trachyte, Kowun 119

60, ditto , olivine-hasalt, Tirioko basalts 125

61. ditto , feldsparphyric basalt, Tirioko basalts 125

62. ditto , resorbed augite phenocryst 126

63. ditto , ditto 126

64. ditto 9 sieve texture in augite phenocryst 126

65. ditto , res orbed plagioclase phenocryst 126

66# ditto , quartz-trachyte, Nasaken 130

67. ditto , soda rhyolite , Nasaken 131

68 . ditto , pantelleritic obsidian, Nasaken 131

69. ditto , soda trachyte, Nasaken 131

70 . ditto , welded tuff, Nasaken 135

71. ditto , welded tuff, Kowun 135

72. ditto , welded tuff, Nasaken 136

73 . ditto , welded tuff, Kafkandal 136 XXV

LIST OP TABLES

page

1. Ana lys ici of Nasaken water 7

2 . Temperature and rainfall data 8

3. Analyses of magnesian limestones 25

4. Compositions of feldspar phenocrysts 104

5. Compositions of feldspars from syenites 106

6 . Petrography : Turkana basalt formation 138

7. ditto : ditto 139

8 . ditto : Kowun trachytes 140

9. ditto : ditto 141

10. ditto : ditto 142

11. ditto ; ditto 143

12. ditto : Kasorogol melanephelinite 144

13. ditto : Kanitiriam microfoyaite 144

14. ditto I Kanitiriam and Nakasuw trachyphonolites 145

15. ditto : Tirioko basalt formation 146

16. ditto : ditto 147

17. ditto : ditto 148

18 . ditto : ditto 149

19. ditto : ditto 150

20. ditto : ditto 151

21. ditto ! Nasaken basic and intermediate lavas 152

22. ditto : ditto 153

23. ditto ; Kafkandal trachytes 154

24. ditto ; ditto 155

25. ditto : Nasaken - soda trachytes 156

26. ditto : ditto 157

27. ditto : ditto 158 XV

page

28 . Petrography ; Nasaken - quartz-trachytes 159

29. ditto : Nasaken - soda rhyolites 160

30. ditto : ditto 161

31. Chemical analyses : averages for different types of lavas 164

32. ditto : Miocene basic rocks 167

33. ditto : Miocene rocks of the Nasaken area 168

34. ditto : Kowun feldspars 170

35. ditto : olivines and pyroxenes 173

36 . ditto : feldspars, Pliocene lavas 175

37. ditto : feldspars. Pliocene rocks 176

38 . ditto : Tirioko basalt formation 178

39. ditto I ditto 179

40. ditto : Tirioko and Nasaken lavas 180

41. ditto : Nasaken lavas 181

42. ditto : ditto 182

43. ditto : ditto 183

44. ditto ; ditto 184

43. ditto : Nasaken, Ribkwo and Menengai lavas 185

46. ditto : trace-elements, Tirioko basalts 190

47. ditto : trace-elements, Nasaken lavas 191

48 . ditto : trace-elements, Nasaken lavas 192

49. ditto ; trace-elements, syenites 193

30 . ditto : trace-elements, feldspars 194

51 . Residual trace--element ratios for 7 rift volcanoes 196

52. Trace-element analyses of TJ.S.G.S. standard rocks 226 XVI

Acknowledgments

I would like to acknowledge with gratitude the receipt

of a Natural Environment Research Council Studentship and a N.EoRoC,

research assistantship during the course of this work.

I am indebted to Professor B.C. King who supervised the

research for his valuable and patient criticism. In particular I

must thank Dr. Ian Gibson for long and fruitful discussions on the

geochemistry and petrogenesis of the lavas but mainly for his infectious

enthusiasm for the subject.

Dr. N. Snelling and Mrs. Maureen Brook carried out the

K/Ar age determinations at the Institute of Geological Sciences.

Mr, N, Sinclair-Jones performed the tedious job of contouring the

aerial photographs. He also made an excellent final draught of the map. Fh?, H, Lloyd painstakingly carried out the wet chemical analysis.

Thanks are also due to my colleagues at Bedford College for

much discussion, in particular Dr. J.N, Carney, Dr. G.R. Chapman,

Dr, S.J, Lippard, Ftc. J.S.C, Sceal and Mr, P.H. Truckle.

Finally I should like to thank Dr. and Mrs. L.A.J. Williams

for their organization and help with the fieldwork in Kenya. CHAPTER 1

INTRODUCTION a) Scope of work

This work Is a contribution to the studies of the East

African Geological Research Unit and has been carried out in the field in Kenya and at Bedford College, University of London, under the direct­ ion of Professor B,C# King, The East African Geological Research Unit

(E.A.G.R.U.) was formed in 1965 to map in detail large parts of the northern section of the vdiich had previously received little or no attention from geologists. The author*s original intention was to map an area of about 4OO sq, miles north of 1° 30' N and west of the

Kapeddo-Lokori road. The tribal boundary between the Pokot and Turkana tribes bisects this area and it was the escalation of hostilities between these tribes and towards members of E.A.G.R.U. which caused the author to vacate the area in the first month of the second field season, in October

1969 . Subsequently, the Kenya government withdrew its permission for geologists to work in the closed tribal district of Turkana. This ban on further geological mapping persisted for about 18 months. During this time the author abandoned his intention to return for a second field season and decided to undertake petrological and geochemical work based on the volcanics mapped in the first field season, a period of 7 months between September I968 and April I969. Since the collecting done in this period was never intended as a basis for detailed geochemical work, it was not as exhaustive as the author would have wished.

This thesis consists, therefore, of a description of the geology of an area known as Nasaken and a detailed geochemical and petrol­ ogical account of the volcanic rocks from Nasaken and adjacent areas with particular emphasis on the Pliocene trachytic volcanoes which characterize this part of the Rift valley,

b) Situation and access

The area mapped is situated in the South Turkana district in the northern part of the Gregory Rift valley, Kenya# The Nasaken area,

designated niunber 8 of the E.A.G.R.U# research areas, is 170 sq. miles

(440 sq, kms.) in extent. It is bounded in the east by the Lokori-

Kapeddo road and in the south and north by the 63 and 90 grid lines taken

from the available D.O.S. maps of the area. The western boundary extends

3 kilometres west of longitude 36° 00* E except north of grid line 80

where a further 50 sq. kms. west of this line have been mapped (Fig. l).

That part of the area north of 1° 50* ^ is covered by the

D.O.S. 1 : 100,000 Amaler (Y633/63) and Lokori (Y633/64) sheets which have

aerial photographs marked relative to a kilometre grid. South of 1° 30* N

is covered by the 1 : 50,000 Tiati (76/2) and Lomelo (77/l) sheets. None

of these maps is contoured and in many places the river patterns marked

on them are inaccurate. The area forms part of the Geological Survey’s

quarter degree sheets 26 (N.W, and S.E.) and 27 (N.E, and S.V.).

Access is by the D6OI/3 road which is motorable by Land Rover.

This road extends northwards from through Nginyang and Kapeddo

to Lokori township which lies about 20 miles north of the Nasaken area and

consists of a few native shops and a missionary hospital. Mapping more

than 4 miles from the road was accomplished using donkey safaris. A main

camp was established where the road crosses the Nasaken river at the locat­

ion of the only permanent water supply within the area.

c) Physiography

The area is one of relatively h i ^ ground between the Kerio

and Suguta rivers, the watershed passing through its western half. The relief interval is about 3sP00 feet; ground near the road at Napeitom and

Nasaken stands at about 2,000 feet whereas parts of Kafkandal are above

5>000 feet (Fig. 2).

The area can be divided into four main physiographic zones in which erosion, volcanicity and tectonism have contributed to produce a 3 6 ° \0 0 'e PO

Lokori

o

80

o>

3 0

a m a l e r LOKORi /■J0 0 , 0 0 0 '■.100,000 shsQt Y6 33 /6 3 skaot Y 633/64

r 30 N

Kapoddo 63 !lfi

Fi g. ! Area 8 : Location, boundan'Qs and_ mop coverage

sca/e il 200,000 1$

1 : 125.000

E

THE NASAKEN

AREA

j Fig. 2 Relief map

> 4 5 0 0 feet 4 0 0 0 - 4 5 0 0

3 5 0 0 - 4 0 0 0 3 0 0 0 - 3 5 0 0

2 5 0 0 - 3 0 0 0 2 0 0 0 - 2 5 0 0

< 2 0 0 0

interva 1 on map variety of land-forms (Plg.)),

I s A flat-lying Basement surface extends into the north-west of the area. Isolated, conioal monadnocks protrude from this surface which stands at about 2,700 feet and dips gently to the north. It is thou^t Pulfrey to represent the * sub-eocene peneplain* (Bixey, 1948*/l960) and is strewn with angular feldspar and sub-angular quartz pebbles which occur in sheets.

II : The h i ^ ground occupied by the Kowun volcanics and bounded in the west by a marked erosion scarp and in the east by the Napeitom faults, forms the watershed between the Kerio and Suguta rivers. It has an essentially smooth topography unlike the highly dissected younger volcanic formations and slopes from about 4,000 feet in the south to 3,000 feet in the north.

III s The valley of the Kasamanang river and its tributaries bisects the area along a KNB-SSW line. This wide valley is bounded by the step­ like Napeitom fault scarps in the west and the high ground of the Kafkandal and Nasaken volcanics in the east. It has the appearance of a grab en structure, however, the westward facing boundary scarp is an erosional feature which has developed along the line of a large fault which down­ throws several hundred feet to the east. The thicker trachytic volcanics on the downthrow side of this fault have proved more resistant than those on the upthrow side, where erosion has penetrated deep into the basaltic formation beneath.

The Kasamanang river descends from a height of about 3,200 feet in the south of the area to about 2,000 feet at Napeitom, an average gradient of about 60 feet per mile,

IV s The Pliocene trachytic volcanics occupy over half the area mapped. They consist of lavas and tuffs from several volcanoes and are highly dissected and faulted. The rugged and deeply eroded central zone of the Nasaken volcano occupies lower ground to the north of the older

Kafkandal massif. To the south-east the uniformly stratified flank de­ posits of Nasaken and Kanatim have givOTi rise to a much more regular mesa- - \

Fig. 3 Physiographic divisions of area 8

j [ Boscmant plain

i i Ken/O’- Suguta ^atarsPad \

/// /kasamanang yallay i \ .

' ■ S' iVa Nasakan '"cantrai zona d/ssQcied Pl/ocane ' ' D Kafkandal TTjassIf / trachyte volcanoes c Nasakan and Kanatim flanks

scale I: 2 0 0 J 0 0 0 like topography with broad U-shaped valleys.

d) Drainage and water resources

The area as a whole forms part of the Lake Rudolph basin into which both the Kerio and Suguta rivers drain. Most of their tributaries are sand-rivers which carry water only immediately after rainfall. In view of the low annual rainfall, the large number of dry sand-rivers can only be ascribed to localized flash floods when relatively small areas have to cope with large amounts of water in short periods of time.

The courses of the two main rivers, the Suguta and Kerio, are controlled by the overall northward slope of the rift floor which falls steadily from 6,000 feet at to 1,200 feet at Lake Rudolph.

Joubert (1966 ) has suggested that in the Loperot area in previous times, the discharge of water was mostly to the north but that eastward tilting during the late-Pleistocene has resulted in a change in the main drainage direction. The fact that the rivers in the area are actively re-excavating their beds suggests either uplift of the area or subsidence of base-level.

Since downcutting in South Turkana is more pronounced towards Lake

Rudolph, the latter seems more probable and it is likely that the re­ juvenation of the rivers in the area is due to the lowering of the level of Lake Rudolph.

The dissection of the Basement surface in the north-west of the area has produced a characteristic dendritic drainage pattern

(Pig,4). In some parts, where the strike of the rock has controlled many of the subsequent streams, a trellised drainage pattern has resulted whereas the isolated residual Basement hills have radial patterns of drainage. In volcanic terrain, the rivers are more linear and far less complicated. The larger streams have cut deep into the lavas to produce canyons with nearly vertical sides. Valleys cut into tuffs tend to be more V—shaped due to a lower angle of rest. The spectacular valley out I Q Qj I <0 <8 II i?U« ! -tu O Q.

I o O k I O Q O O 2^ cn "by the Ngapawoi river into thick puiiiioe-tiiffs just east of Mora Angitak is 1,200 feet deep.

The hot spring in the river at Nasaken is one of the few permanent water supplies within the area. Although the author found the water quite palatable, the flourine content has been measured as being above the level recommended as safe for human consumption (S, Ehemtulla pers. comm.). However, except after periods of exceptional drought most of the major tributaries such as the Kasamanang, Hasaken and Katatarope contain water at depth, trapped in the sand behind natural rock barriers.

A map has been prepared showing the pattern, of drainage within the area and indicates the localities of the water supplies (Fig, $). Table 1 gives an analysis of the water (cold) collected from the water-hole at the Nasaken camp (M. Gwynne, pers. comm.).

e) Climate

South Turkana is true semi-desert with rainfall always less than 12 inches per year. The region falls within the only strip of the arid Ecological Zone VI to be found in East Africa (Pratt et al., 1966).

The months November to February are very dry, the rain coming mainly between March and May but both the periodicity and quantity of rainfall are hi^ily variable and it is concentrated on or around hills. There are no meteorological stations within the area but that at Lodwar (l,600 feet) has recorded an average of 5.99 inches of rain per year, over a period of 34 years. Rainfall and temperature were recorded at Lokori during the months the author was in the field by members of the Royal Geo­ graphical Society's South Turkana Expedition. The figures for the year from September I968 to August I969 are reproduced in Table 2 and have been taken from Morgan (l97l)* Of the 7.39 inches recorded for the year, 65% of the rain fell in the three months between March and May. In the low lying areas the temperatures are h i ^ from 8.00 a.m. onwards until 5-OOpjn. 1 ; 12 5,000

THE NASAKEN

AREA

Fig. 5 m Wafer supplies and thermal sources

0 water hole - permanent Ô " ” temporary

A spring — fresh or paiatobty saline A " - saline h

jik fiot spring -f- steam jet T a b l e 1 : /uialysi.s of v/ater from Nasaken water-hole.

( N.D,Grrr'nne, pers. comm. )

parts per million

nil Free and saline ammonia as N

nil Albuminoid ammonia as N

0,3 Og absorbed (4 hrs. at 27°C)

nil Nitrates as N '

nil Nitrites as N

420 Total dissolved solids dried at 180°C

18 Chlorides

10 Sulphates

52 Carbonate alkalinity as CaCO^

206 Bicarbonate alkalinity as CaCO^

52 Carbonate hardness as CaCO^

nil Non-carbonate hardness as CaCO^

52 Total hardness as CaCOj

218 Excess alkalinity as NagCO^

7 Flourides

0.1 Iron

50 Silica

104 Sodium

8 Potassium

19 Calcium

1 Magnesium

pH 8.1

Comparisons may be made with water analyses given in

McCall (1967), Table III, p .76 . S' ro to Vx vx vn CO P CO CD .s to O I 8 c+ % 1 3. O 4 (O Vx O P P f" f-o 05CD h) eC+ o\ et fo Vx c: 8 K c+ H* c+ Vx a to to I to O!Z^ I VJI I r 03 VO < ÎS S' o VJ1 10 to vx to V71 S’ 5^ (D o ?o Vt o e+ vx Vji to vn p f- o\ a f f- r § I VjJ ch VaJ VD 3 Ov w I VO I CD to vxVJI S bd vx 4P VO o\ a\ o VO IÎ3* a* LN a\ to f- I to vx cr\ P VD o +- «

VxU1 r Po\ 09 B" 05

—j et p>b • o a VMVO ctP aÇ3 H H and the nights warm, and with little oloud cover, rates of evaporation

are high.

f) Population

The area is inhabited by the Nilo-hamitio Turkana people who

are nomadic pastoralists owning cattle, camels, sheep, goats and donkeys

from which they obtain milk, blood, meat and skins. A considerable pro­

portion of their diet is composed of berries and fruits.

The movements of the Turkana are influenced by the availa­

bility of water and the state of grazing. They live in huts constructed

from thorn branches or palm leaves and realistically make no attempt to

cultivate crops. Some interesting details on the Turkanas' use of land

in the Kerio valley near Lokori are contained in Morgan (1971 ). The

Turkana language is particularly difficult for Europeans and fortunately

many speak Swahili.

g) Flora and Fauna

The area is covered by Acacia thorn bush which is virtually

impenetratable on parts of the h i ^ ground of the Kerio-Suguta watershed.

Various species of taller Acacia trees line the larger river courses.

Permanent and semi-permanent water-holes and the hot springs at Nasaken

are marked by clusters of Hyphaene palms# Above about 4*000 feet on the

slopes of Kafkandal, Euphorbia candelabrum is common. Morgan (l97l) has made many specific identifications among the flora of the Kerio valley.

A recent survey of wild and domestic animals within the whole of South Turkana has been made by Watson (1969 )# The fauna of the Nasaken area is very restricted but the numbers of different species to be seen , increases considerably with proximity to the . Grant's gazelle are common on the lower ground; dik-dik are frequently seen on rooky slopes and a herd of about twenty greater kudu confines itself to the 10

hipest parts of Kafkandal. Baboons are ubiquitous and other animals

seen include leopard, Thomson*s gazelle, ostrich, silver-backed jackal, bat-eared fox, striped hyaena and genet.

The original fauna has been much reduced by poaching and

over-grazing. Many Turkana place-names make reference to wild animals,

some of which are no longer found in the area, such as lion, elephant, rhinoceros, giraffe and oryx. This lends support to the supposition that

the present low wild-animal intensities are a relatively recent phenomenon.

h) Conduct of field work

Mapping was carried out by marking geological information on

to aerial photograph overlays which are on a scale of approximately

1? 40» 000» The area was topographically surveyed by the author using both telescopic alidade and indian clinometer. Since no trigonometrical points are situated within the area, hei^t control was achieved by long

traverses to trig, points north and south of the area. Contours were drawn for the author at 100 foot intervals by photogrametric techniques and geological information was transferred to the contour map which has been prepared at a scale of 1 §50,000«

Localities are referred to in the text by six figure map ref­ erences where place names are insufficient. Specimen numbers (which correspond to locality numbers) are given with the prefix *8* referring to area 8 of the E.A.G.R.ÏÏ. project. Rock specimens, powders, thin- sections, field overlays and notebooks are housed in the Geology Depart­ ment, Bedford College, London University.

i) Previous geological work

The earliest geological work in South Turkana was carried out in 1888 by members of Count Teleki's expedition which discovered Lake

Rudolph. Gregory (1921 ) devoted a chapter to the northern section of the 11 rift valley which was later to assume his name, althou^ he did not visit

Turkana. Murray-Hughes (l933) produced a small-scale map of the western half of Kenya and named flat-lying sediments which rest on the Basement

System, the Turkana Grits. Champion (1935» 1937a, 1957b) published sev­ eral accounts of the geology and physiography of the country south-west of Lake Rudolph and his geological specimens were petrographically des­ cribed by Campbell Smith (l938). Fuchs (1939) was concerned mainly with the geological history of the Lake Rudolph basin, Bixey (1948) has dis­ cussed the geology and erosion surfaces of Turkana,

The only previous work on the area mapped by the author is by

Mason and Gibson (1957) who made a reconnaissance survey of the ground west of 36° 00* N. Joubert (1966 ) mapped the Loperot district to the north of the present area and reference will frequently be made to his report. In recent years members of the Geological Survey of Kenya have mapped large parts of central and northern Turkana. Rhemtulla (1970) reconnoitred much of South Turkana as geologist attached to the R.G.S.

South Tbrkana Expedition and the author was pleased to provide details of the geology of the Nasaken area for inclusion in Mr. Rhemtulla*s report. The area to the south has been mapped by Dr. P.K. Webb as part of the E,A,GcR,U. project (Webb, 1971)• The position of the other re­ search areas is indicated in Fig, 6 .

j) Geological Setting

The Eastern, Kenya or Gregory Rift is part of the East African

Rift System which extends from Mozambique to the Mediterranean (Fig. 7)*

Unlike many sections of the rift system as a whole, the Gregory rift is characterised by an abundance of volcanic products. In Kenya it is essentially a tectonic trough averaging about 40 miles (65 km.) in width.

North of 1° 50* N the rift zone widens as the Uganda escarpment trends away to the north-east until at a latitude of 2® 00* N the rift zone is 37" 35“E 36" CZ%- L.Rudolf

IM

"o

Erv-

Scale 1=2,000,000

1 J.E.Mortyn 2 G.R.Chopmon T Tiati Hills 3 M.P.MGCIenoghon K Kamasia Hills 4 J.S.CSceol S Silall B L.Baringo 5 P.K.Webb ; Area 6 S.Rhemtulla H L. Hannington of 7 G.J.H.MCC0 II 8 ’ S.D.Weaver 9 S.J.LIppard 10 J.N.Carney 11 ' P.Truckle ; 12 M.Golden ! 13 P.S.Grlffllhs

FIG.6 E.A.G.R.U. Research areas. N

z o 03

f

N

% N V ! a N ! 13

South of the Hibkwo volcano the representatives of Group III are the Kaparaina Basalts (McClenaghan, 1971)» to the north are the

Tirioko Basalts (Webh, 19?l)« Althou^ these two formations are almost

certainly equivalent they can not be proved to be continuous under the

cover of the Hibkwo voloanics and both names are in current use.

Group IVg This comprises the Plio-Pleistocene trachytic shield volcan­ oes which are so impressively developed in South Turkana, The Kapkut volcano north of ELdama Ravine (Lippard, 1972) and the Tirr-Tirr Series

(Baker, 1963) may also be included in this group. Some volcanoes consist

of undersaturated lavas, phonolitic trachytes and trachyphonolites, whereas others are highly oversaturated and contain alkali rhyolites.

Group Vs a) The initial Quaternary volcanic rocks are trachytic or

trachyphonolitio flood lavas and have followed a second major faulting

episode. They include the Lake Hannington 'phonolites* (McCall, 1967) o

b) The central volcanoes of the axial grab en are also placed

in group V, , Silali and Enuruangogolak are trachyte/basalt volcan­ oes whereas the more southerly volcanoes Menangai, Eburru, Longonot and

Suswa contain little basalt.

Here follows a short introduction to the stratigraphy estab­ lished in the author's area.

The oldest rocks are .those of the metamorphic Basement and are described briefly. Although the Basement contains a wide variety of rock-;types and has a complex structural history, it is not the subject of this thesis.

Within the area mapped the Turkana Grits do not occur, the metamorphio rocks being directly overlain by the Turkana Basalts, A few miles to the west, in the Kerio valley, are over 1,500 feet of arkosic sandstones which represent the pre-volcanic erosion of an early fault- 14

scarp that extended along the line of the Kula faults.

The Turkana Basalts are an extrusive, wholly-basaltic form­

ation outcropping in the north-western part of the area where they dip beneath the Kowiin trachytes. The junction between these two formations

is marked by a latoritic horizon. The Kowun lavas are an isolated example

of trachytic volcanism of Miocene age, since within the rift as a whole,

trachytes are uncommon in the dominantly basaltic group I or phonolitic

group II, This latter group is poorly represented in the Nasaken area

by the Kanitiriam microfoyaite and the Nakasuw and Kanitiriam phonolites.

The Tirioko Basalts are a thick, widespread and varied Pliocene

sequence of basalts, liawaiites and mugearites. Prior to the building of

the huge multicentred trachyte complexes of Kafkandal and Nasaken, the

Tirioko Basalts were subjected to a monoclinal flexure along a NNE - SSW

axis which tilted the eroded surface of the basalts gently towards the

centre of the rift.

No Quaternary volcanic rocks occur within the area, the

youngest lavas encountered being the Kanatim trachytes, thought to be late-

Pliocene in age,

k) Note on time scale employed

There is as yet no general agreement on the radiometric time

scale to be used to date the Miocene, Pliocene and Pleistocene epochs.

It is proposed here to continue using 25, 12 and 2 million years (after

Holmes, I960 ) for the beginnings of these respective periods. This

time-scale is that most generally used in East Africa (Baker et al,,

1971 ), Recent scales proposed include those by Punnell (1964 )» 26, 7

and 1,5-3*5 million years and Berggren (1969 ), 22,5» 5*5 a^id 1,0 million

years. 15 l) Note on the use of English and metrio systems of measurement.

The 1:100,000 D.0.8. maps have been prepared with a one kilometre-square grid which is used to locate the aerial photograph centres. However, contours have been drawn at one hundred feet inter­ vals to conform with those established in areas to the south. Thus heights, vertical distances and thicknesses are all quoted in feet.

Horizontal distances are readily reckoned by reference to the number of grid squares traversed on the map and are therefore quoted in kilometres.

It is recognized that the use of both systems of units is not ideal but this has been done in order to avoid confusion. The compromise has therefore arisen as a matter of convenience and is the product of the long period of time it takes for topographical and geological maps (and those who make them) to be converted to the metric system. CHAPTER 2

STRATIGRAPHY 17

1, The Basement System

The metamorphio rooks of the area are referred to the Kenya

Basement System which forms part of the Mozambique erogenic belt (Holmes,

1951 ). The Basement System rocks consist of a thick sequence of géo­ synclinal sediments of calcareous, psammitic, semi-pelitic and politic types which, together with basic magmatic rocks have suffered regional metamorphism in the almandine-amphibolite facies. Ur/Pb radiometric determinations on samarskite from three pegmatites from Turkana (l° 59* N,

55° 04* E) have yielded an average age of 655 + 25 m.y, (Bamley et al.,

1961 ). A K/Ar date of 6I5 + 15 m.y. has been obtained on muscovite from a similar pegmatite (Cahen and Snelling, I966 ). Since these pegmatites cut folded plagioclase amphibolites and marbles, they set a younger limit of about 625 m.y. for the age of the metamorphism.

Brief notes only on the metamorphio rock types encountered by the author are given here. Details on the metamorphio rocks are con­ tained in numerous reports of the Geological Survey of Kenya, in partic­ ular, excellent descriptions of the Basement System in the region of the author*s area may be found in Dodson (I965 )» McCall (1964 ) » Joubert

(1966 ) and Fairbum and Matheson (1970).

The outcrop of the Basement System in the north-west of the area mapped is part of a much larger area lying within the Rift, east of the Kula fault system. The Tèrtiary lavas in this section of the Rift are essentially confined to the east of this fault-line by the h i ^ ground formed by the Basement hills of Kailongol, Laiteruk and Masol, and provide only a thin, partial cover to the metamorphio rocks. (Pig. 8).

Much of the metamorphio terrain consists of foliated biotite and hornblende gneisses in which garnet porphyroblasts have developed.

Specimen 8/455 is a cream coloured gneiss with a marked foliation and anhedral garnet porphyroblasts. It is composed of an allotriomorphic mosaic of micro dine, quartz and plagioclase with myrmekitic intergrowths. z o o o R o *

k. 0 C isu

00 ___ / 1 \\ // O o \ X o "y k ILl o X 8 1 ^ ç m 0 I 1 § e i ! -JOA/i^ /'•N— _ _ ^ 4 ^7 ^"-^'^=&zr °//ay "— \

1 i V \ OÔ '' X 1 1 LU ' o •5 ^ m \n i? m 18

Small laths of red-brown biotite define the foliation and zircon and sphene are present as accessory minerals, A similar biotite-gamet gneiss (B/437) has anhedral porphyroblasts of pink garnet and consists of allotriomorphic granular quartz, oligoclase and orthoclase. Often small, well-rounded grains of quartz are enclosed in felspar. Brown biotite and a little muscovite define a crude foliation. Specimen 8/l57a is a well- foliated hornblende gneiss consisting of allotriomorphic plagioclase, quartz and orthoclase with large anhedral crystals of green hornblende and a few irregular magnetite grains. Similar gneisses in which hornblende has grown at the expense of biotite also occur. The biotite, hornblende and biotite—hornblende gneisses are considered to be metamorphosed semi- pelitic sediments.

A number of psammitic bands, which are more resistant to erosion than the other rock types, form conspicuous ridges which may be followed long distances over what is usually poorly exposed ground due to the extensive blanket of Basement-derived gravels. The Nakasuw river crosses two such psammites and in the Basement inlier due north of Kowun two psammite bands are very prominent, the Kowun trachytes resting directly upon the metamorphio rocks. A specimen, 8 /II5 , from one of these bands is a granitoid leucogneiss consisting of allotriomorphic microcline quartz and a little sodic plagioclase showing development of myrmekite. Sparse biotite and a little muscovite are idiomorphic but show no preferred orientation. Quartz occurs as large anhedral crystals but also as much smaller rounded grains reminiscent of sedimentary particles. A few hun­ dred yards south-east of this psammite band in poorly exposed low ground, a thin marble band dips beneath Turkana basalts. Specimen B/I58 from this marble consists of large anhedral interlocking plates of calcite with a few flakes of muscovite, rounded grains of apatite and rare scapolite. It is clearly the product of the metamorphism of a very pure limestone and has a creamy white^ coarsely crystalline appearance in the field. 19

Amphibolites and plagioclase amphibolites which axe thought to represent metamorphosed basic igneous rocks are common in Turkana (Dodson,

1963 ? McCall; 1964 ? etc.), A thick band of plagieclase-gamet amphi- bolite can be seen in the western bank of the Kasarogol river near the northern margin of the author's area. The well foliated amphibolite has sharp contacts with the enclosing biotite gneisses and is characterized by large (2 cms-across) euhedral porphyroblasts of pink gamet. It is composed of green hornblende and andesine plus a little anhedral magne­ tite. Relics of pale green diopside enclosed in hornblende are common.

The amphibolitization consists of the conversion of pyroxenes to horn­ blende and the growth of sodic plagioclase at the expense of plagioclase of a more calcic composition. A plagioclase amphibolite (8/l57c) from the Basement inlier north of Kowun consists essentially of mafic and felsic bands a few rams, across. In the mafic bands, pale green diopside has largely been converted to green hornblende. Small amounts of epidote and plagioclase also occur. The felsic bands consist almost entirely of plagioclase of bytownite-anorthite composition displaying remarkable symplectic in ter growths of gamet (probably grossular) (Plate l). The small beads of weakly biréfringent gamet have grown randomly in the host plagioclase which in some areas has been extensively altered to muscovite.

The development of gamet is confined to the symplectic intergrowths in the plagioclase bands, the minor amounts of plagioclase in the mafic bands being free from gamet. Discontinuous thin bands of sphene and magnetite octahedra occur between certain of the plagioclase-gamet bands and the diopside-homblende bands (Plate 2). This fine banding is replaced by the homogeneous assemblage hornblende, plagioclase, diopside and magnetite with apatite and a little quartz as accessory minerals. The banding is thou^t to represent relict igneous layering of what must have been essentially monomineralic layers of pyroxene and plagioclase. Subraman- iam (1956 ) has described an amphibolite facies metamorphosed layered igneous complex containing anorthositic rocks, in which grossular has re— #

Plate 1: Specimen 8/l9?c; a layered plagioclase-

ampMbolite; p.p.l., x 100. The symplectite

occurring in the feldspar bands ccnsists of

small beads of garnet which have grown randomly

in the anorthite host. t

H r

@ @ m # w V f !

Plate 2: Specimen 8/l97c; a layered plagioclase-

amphibolite, p.p.l., x 2$. The conspicuous

layering consists essentially of plagioclase-garnet,

symnlectite bands and hornblende-diopside bands.

Discontinuous thin bands of magnetite plus sphene

also occurs. 20

placed plagioclase and given rise to a characteristic sieve texture.

The banded amphibolite described here is poorly exposed in low gravel-

strewn ground but the parallelism of the banding, the foliation in the

amphibolite and the foliation in the adjacent gneiss and marble, suggest

a concordant sill-like intrusion in which some form of igneous layering

was produced.

Numerous concordant and discordant intrusive pegmatites occur

in the area. They consist of large allotriomorphic crystals of quartz

and micro dine, the latter often showing exsolution stringers of albite,

and may contain books of biotite up to a few inches across.

Throughout the area, distinct bands of psammitic gneiss and

marble follow the trend of the foliation indicating that the foliation is

closely related to the original sedimentary bedding. Within the main

outcrop of the Basement System mapped, the strike of the foliation

swings in a gentle manner about a regional trend estimated to be about

015° (Fig. 9 ), Along the northern margin of the area, the strike changes

gradually from 035° ia the west to 340^ in the east. Throu^out, the dip

of the foliation is between 40-50° and always to the east. No major fold

closures could be mapped in this small area so that the structure within

the Basement is unknown. Joubert (1966 ) has mapped in detail the meta-

morphic rocks to the north and has shown that large, isoclinal and tight

folds are present. The only lineations measured by the author indicate

fold axes plunging at about 15® in a direction of 015°. These are small

folds folding the main foliation and have been seen only in one exposure

in the western bank of the Kasorogol river at 299896* They appear to

represent a local area of minor complexity, no minor structures having

been seen at any other locality. The foliation trend within the inlier north of Kowun is markedly different from that of the nearest part of the main outcrop. In the former, the rocks are striking at about 060° and

dipping at about 20® to the south. It is possible that this change in 0

§ 1 I 0\

6) iC

'y

U1

5 O §>

•S % Q o 0 -<0 -Q CJ §) §, ^ 1 c CJ s s o i o c 21 strike indicates the proximity of a major fold closure hidden beneath the volcanic cover.

2(a) The Turkana Basalts

The Turkana Basalts are a widespread formation of Miocene age which is represented in the area mapped by a succession at least 1,100 feet in thickness. This consists of a lower unit composed mainly of pumice and welded tuffs, an agglomeratic lens and an upper unit consisting of basalt lavas (Fig* 10). The dominantly basaltic formation rests directly on metamorphio rocks of the Basement System. The junction be­ tween the volcanics and the Basement is a surface of essentially low relief (Plate 3)» the so-called **sub-Miocene peneplain" (.Pulfroy,i960) which was approaching maturation just prior to the eruption of the Miocene volcanic rocks.

The Turkana Basalts cover about 45 sq. kms. in the north-west part of the area. They outcrop along the upper reaches of the Nakasuw and Kasarogol rivers and are preserved beneath the erosion scarps of the

Kowun trachyte and Nakasuw phonolitic trachyte. In the extreme west of the area (245890) a 250 feet thick succession of basaltic, pumice tuffs and welded tuffs rests on Basement and is capped by thick flows of Nakasuw phonolitic trachyte. The tuffs are purple in colour and contain numerous lithic fragments, mainly of vesicular basalt and brown pumice. Within the pumice tuffs is a welded tuff member about 80 feet thick which forms a prominent feature across the Nakasuw river. The welded tuffs are purple or buff coloured rocks, very hard, with a oonchoidal fracture and contain numerous streaked-out and flattened pumiceous fiamme. They contain broken anorthoclase crystals and lithic fragments of trachyte. Secondary oxidation and alteration has been intense and the opaque red-brown meso- stases are heavily charged with haematite and calcium carbonate. The welded tuffs probably represent trachytic compositions. Above them occur CoI c:

H co o

C>^ ^ r> 1 ï . 4 ? l rv o $ 1 :K"'

(f) I 8 M. . % o I % O* Z; co

5- >s Q S- (/) 10

IQ (n O Q S' ? Cb § Q (n § O r~K Q -g 5 : ^ s % Q o ‘ (D

•H x: eg I •H -j in CD •HP> 1-4CD CO P u cu s CD ■p O c -P B CO (U •H % Xj m CD X: Ui § p> p> t OÛ C/J .5 +3 CÙ PS CO p> 0 C-' -p(D B 5 •H (D r-^ 13 f § § >:■ rC 0 X 0 O X: Cti o cti % P> -p 44 O -P CO § C C ^ CO 0 VÛ cd & 0 CD rH cti O po U) bl) U 0 o •S x: I—I -p •H 4' CC;

1: o 'D 'S rr\ S cti rP4 - l 22 about 120 feet of purple lithic and pumice tuffs containing numerous basalt bombs and lava fragments ranging in size up to a few inches across.

The top two feet of the tuffs immediately below the phonolitic trachyte are intensely welded and reddened. The succession of tuffs described dips at an angle of about 5° to the south-west or west-south-west.

The basaltic tuffs occur as outliers on the metamorphio rocks and continue to the east and south across the Nakasuw river. At 277&6Q the welded tuff member has increased to about 200 feet in thickness. It consists of several cooling units and contains numerous fragments of basement gneiss. This member can be traced along the eastern side of

Kanitiriam hill where it is cut by the Kanitiriam microfoyaite, the lithic and welded tuffs forming the envelope to the intrusion. On the south side of the hill, just above the river (276831 ) an exposed thickness of $0 feet of a lower welded tuff dips beneath coarse basaltic agglomerates, The tuffs and agglomerates have been disrupted by the intrusion of the micro- foyaite so that they dip away from the plug at angles in excess of 50^»

From the Kasorogol river to the scarp formed by the Kowun trachyte at 275820, a vertical distance of 250 feet, is a succession of flat-lying tuffs which are mainly basaltic but include a prominent welded horizon near the base. To the east, the tuff succession passes under­ neath scoriaceous basalt lavas.

Bedded basaltic agglomerates overlie welded tuffs in the banks of the Kasorogol river. At 2878$5, 20 feet of weathered and reddened welded tuff outcrop below 10 feet of purple-grey, bedded, basaltic agglomerates which are capped by 15-20 feet of grey, aphyric, phonolitic trachyte. The agglomerates thicken rapidly downstream becoming chaotic and containing pumice and basalt fragments. Blocks of aphyric, purple basalt, some rounded, may be in excess of one foot in diameter. The

Kasorogol passes between cliffs, ^0 feet high, composed of agglomerate

(Plate 4 ) and at 292855 an aphyric basalt dyke, 20 feet in diameter, can 23 be seen in the bed of the river, cutting the agglomerate, The agglomer­ ates thin out and are absent north of 880. They have not been traced west of Kanitiriam or to the south where the tuff-welded tuff succession continues up to the scarp formed by the Kowun trachyte. East of the

Kasarogol river, at 292838 and 292869, the bedded agglomerates dip gently eastwards under feldsparphyric basalt lavas. The Kasorogol has thus cut throu^ an agglomeratic lens which separates the tuff-welded tuff sequence from the succeeding lavas. This lens is greatly elongated north-south and appears to represent an important fissure-like source within the

Turkana Basalts.

From 29 O84 O eastwards, feldsparphyric basalts can be traced up the steepening slope to the scarp formed by the Kowun trachyte. In this vertical distance of 450 feet, seven or eight flows occur with an average thickness of about 60 feet. The basalt flows have vesicular upper sur­ faces, are grey or purple in colour and frequently contain zeolite or calcite-filled amygdales. Lath-like, creamy-white phenocrysts of plagio- clase up to 3cms. in length are common but some flows are microporphyritic containing microphenocryst plagioclase crystals up to in length.

The lavas assume a general dip of about 10° to the east.

Tô the north, along the scarp (314850), trachyte rests upon about 20 feet of reddened basaltic pumice tuffs which are underlain by feldsparphyric basalts. Farther north, the trachyte cap of Lotow hill

(33O887 ) rests on a thin (lO feet), black, aphyric basalt beneath which a sequence of 7 feldsparphyric basalt flows can be traced. In the low ground around the Kasorogol river on the northern edge of the area a purple, aphyric analcite-mugeaxite (8 /484 ), occurring below the felds­ parphyric basalt flows, rests upon an aphyric, silvery-grey, somewhat fissile flow which has proved to be trachymugearite (Specimen 8 /426)

(Fig. 11). Here the basaltic flows dip at about 5° to the north-east.

The Turkana Basalt lavas do not occur east of the Kowun trachyte Lotow Kowun scarp

hill (a/ong northing 840)

[7; /(ocoun traclyte \

apkjr/c 6aSait i éASACé'/C \ ^ / K / ^ / ^ ifd s 'p iw y U jrù écLfa.Cà ~ - \ X. / \ Lcts^ft (^//K fk«‘V.t>c'fifsLs L*'. ôf jf^dspa.'Tj fjn'onene c oùvlne /êk/io/xi^ (dspoiypk^wfc ) \ / /f ?dspa/^ ~7kucn>vk (fr^c - ' I ■fe ^d ^pa Y- 'tu ^crzrpk^y /'c 7 rr -epocos b u o itt > ■^e^ds'pAypiiÿYfC SaSaCt \ / I I èaSA^ô \

fe^drp«/^pk,(pic a'na.£c}te,‘''^u

'SoLje'nit'yit

'ne/sS A A A hAS^tt Ajj£oM€TA.àe A A A A , A

/cm.=z sort, = 16m.'

K/g. // : Turkana Basalts , lava successions Plate h: Basaltic agglomerate oi the Turkana basalt

formation in the Kasorogol river is affected by

minor faulting.

I

Plate 9* Botryoidal calcite growing on the surfaces of

Turkana basalt flows. 24

erosion scarp except in a small inlier at 665855 where 40 feet of deeply-

weathered basalt is exposed below Kowun trachytes, and is cut by trachyte

dykeso To the east, the Napeitom step-faults have downthrown the Turkana

Basalts and the Kowun Trachytes deep beneath the covering of Pliocene

volcanics.

The feldsparphyric basalts are particularly prone to weathering

breakdown, the exposed surfaces often being crumbly, friable and assoc­

iated with numerous different forms of secondary calcite growth. The

calcite occurs as clusters of dog-tooth crystals, groups of radiating

acicular crystals and in various superficial botryoidal or ooraloidal

forms (Plate 5). Most of the growths are complex and delicate and it is

inconceivable that they were once buried by later lavas. The calcite

is probably forming on the surfaces of weathered basalt under the present

conditions of ground and surface water movement. Growths tend to occur

on flat basalt surfaces where drainage is impeded. It seems likely that

the calcium carbonate is produced from the breakdown of plagioclase;

certainly most calcite growths occur on the surfaces of feldsparphyric

basalts which due to their strongly porphyritic nature are particularly

Ga-rich.

A small area of magnesian limestone occurs at 294847 as a

thin, horizontal bed which describes a very restricted sedimentary basin

set into weathered basalt. The limestone does not exceed one foot in

thickness and is a porcellanous, cream-coloured rock with an orange-brown

weathered skin. A chemical analysis (Table 3) shows that it (Specimen

8 /5O8 ) is a siliceous magnesian limestone containing about 4 wt. per cent

iron-oxide, mainly in the ferrous state. Since the outcrop is on a

flattish basalt surface, it is not clear whether the limestone is inter- volcanic or of a more recent origin. Joubert (1966 ) and Dodson (1971 )

have recently described magnesian limestones which are interbedded with

Miocene basalts in the Loperot and Lodwar areas, Fuchs (1939) believed

these limestones represent precipitates from thermal waters and gave two 25

Tàble 5» Analyses of magnesian limestones from South Turkana

8 /5O8 8/5 6a 8/414b

SiOg 7.14 7.85 10.04

AlgiO^ 0.39 1.94 2.67 0.82 2.02

TiOg 0.02 0.16 0.19

^®2^3 0.59 2.00* 1.64 1.95* 2.74*

FeO 3.60 - 0.25

NgO 14.59 18.59 18.46 18.70 19.04

CaO 31.28 25.90 24.87 27.92 31.71

Na20 - 0.37 0.47

KgO - 0.41 0.51

MnO 0 .19 0.35 0.20

P2O5 - 0.02 0.03

H2O+ 0.04 0.78 1.05

H^O— 0.35 1.65 1.38

COg 41.31 40.30 37.33

TOTAL 99.50 100.32 99.09

* includes FeO

8 /5O8 s intervolcanic limestone from Turkana basalts at 294847

8 /56a s limestone from Napeitom

8/414b s calcified basalt from Napeitom

1 8 limestone from near Lodwar, Fuchs (1939)

2 8 limestone from near Lodwar, Fuchs (l939) 26

partial chemical analyses which are reproduced for comparison in Table 3.

The general disposition of the bedded pyroclastics and lava

flows relative to the agglomerate lens suggests that here was the source

area for much of the basaltic volcanism. Members within the pyroclastic

sequence thicken towards the agglomerates of the Kasarogol river. Simi­

larly the lava flows appear to thin away from the agglomerates, at least

towards the north. The lower, pyroclastic unit is absent in the north of

the area where basalt lavas rest directly on Basement rocks. Farther

south (around 279873), welded tuff and basalt outliers rest on Basement

within close proximity to one another and at the same level. This

suggests that the topography of the pyroclastic unit was one of apprec­

iable relief prior to the eruption of the lavas. This may be consistent

with there being a volcanic source nearby and need not imply any great

time interval between the cessation of pyroclastic activity and the

eruption of the basalts, since erosion can be particularly rapid and

effective in volcanic source areas where soft tuffaceous material has

accumulated.

The junction between the Turkana Basalts and the Kowun

trachyte approximates to a planar surface which dips gently to the north.

This surface is discordant to the underlying basalt flows and is every­

where marked by a zone of deeply weathered and reddened rock. At 274822,

under the lowest trachyte flow, a lateritically weathered zone about 60

feet thick is imposed on basaltic welded tuffs. The nature of the pre-

Kowun surface implies an important break during which the basalts suffered

planation, although K/At dating suggests that the time interval was only

of the order 0,5 m.y,

2(b) Correlations and age

The Turkana Basalts extend south of the area mapped by the author where they are continuous with the Kapchererat Formation of Webb 27

(l9?l) and McClenaghan (l97l). Webb has described a succession up to

1,800 feet thick consisting of basaltic lavas overlying tuffs. The

lavas are varied; feldsparphyric basalts, olivine and pyroxene phyric

basalts predominate and mugearites and trachytes are subordinate.

In the Loperot area, to the north, a thick basalt series,

resting on Basement or Turkana Grits, has been described by Joubert

(1966 ), The basalts of the Loperot area are the southern extensions of

those of the Lake Rudolph basin which overlie sediments yielding a Miocene

fauna (Arambourg, 19435 Bixey, 1943). Joubert correlated these lavas

with the Samburu Series (Shackleton, 1945) which occurs on the eastern

side of the rift. This correlation has been broadly confirmed by iso­

topic age determinations but because of the lack of physical continuity

across the floor of the rift valley, the basalts on the western side were

distinguished by Baker et al. (1970 ) by naming them the Turkana Basalts,

This name is adopted here.

A specimen of analcite-mugearite (8 /484 ) from the lava success­

ion west of Lotow hill has yielded a K/Ar age of 15,7. +0,6 m,y. (N.J.

Snelling, pers, comm,), A trachyte at the base of the Kapchererat Form­

ation has been dated at 16,6+0.5 m.y, (Webb, 1971). Three specimens of

Turkana Basalts from the Loperot area have yielded dates of 17.5 + 0,9

m.y.; 16,8 + 0,5 m.y. and 16.7 + 0.8 m.y, (Baker et al, 1970), The

preceding dates suggest that the Turkana Basalts of South Turkana are be­

tween 18 and 16 million years old, A much wider age range is obtained when basalts from northern Turkana are included, ages ranging from 32 to

14 m.y. For the Samburu Basalts, ages ranging from 23 to 18,5 m.y, have been determined (Baker et al., 1970),

3(a) The Kowun volcano

The trachytic volcano Kowun is a Miocene shield-like volcano forming much of the watershed between the Kerio and Suguta rivers. It 28

once covered a much wider area but west of the watershed the Kowun rocks

have been removed by erosion whereas to the east, north and south they

have been buried after partial erosion beneath younger formations. The

probable present extent is about I50 sq, kms. of which 85 sq, kms, fall

into the area mapped (Fig, 12), The maximum thickness of lavas attrib­

utable to the Kowun volcano is about 900-1,000 feet occurring under Kowun

hill which rises to 3,850 feet.

One main centre of extrusive activity has been recognized

together with numerous intrusions. The volcano has a broad shield-like

profile in a north-south direction culminating at the agglomeratic in­

trusions of Kowun hill which are thou^t to represent a small caldera

structure. The lavas are wholly trachytic and occur in massive, thick

flows. Locally, welded tuffs, pumice and crystal tuffs are prominent.

Because of their age and the fact that large areas of the

flank deposits have been exhumed from beneath younger lavas, the Kowun

rocks are commonly highly oxidized and secondarily altered thus making

fresh specimens difficult to obtain.

On the northern margin of the area, Kowun trachyte overlaps

flows of the Turkana basalt formation to rest directly on psammites of

the Basement System, With this single exception, the Kowun trachytes

rest on a sub—planar surface of Turkana basalts, a marked red horizon

being developed at the junction.

On Kowun hill, two arcuate ridges of trachytic agglomerate

define a discontinuous crescent (Plate 6), The agglomerate rises 200

feet above gently dipping trachyte lavas and appears to have been in­

truded into them. An arcuate fault can be traced between the two agglom­

erate ridges marked by a narrow line of agglomerate or breccia; this also

extends to the south-east. The continuation of this feature has not been traced on the east. The agglomerate crescent encloses a topographical depression, the walls of which consist of flat-lying trachyte flows. on/es

1 vr\ older on/cs vole a EXTENT

KOWUN VOLCANO

mapped \ in fe rre d from F/g. 12 Jierial_ photogrjiph^ \o 0) -p 29

This ill-defined structure probably represents a small caldera (l km. in diameter) which was a major source for much of the Kowun trachyte. A leucotrachyte dyke about 20 feet across and of limited lateral extent cuts the trachyte lavas within the caldera structure. There is a general qua- quaversal dip of flows about the centre which is located on the thickest part of the Kowun volcanic pile. hips are generally a little hi^er to the east implying a subsequent tilting in that direction of a few degrees.

About 8 or 9 flows of trachyte can be traced above the Turkana basalts west of the Kowun centre. To the north and on Moru Ngararai many of the original flow surfaces are preserved and flow fronts form distinctive features. The individual flows are typically 60 to 100 feet thick. To the east and south of Kowun hill the dissection of the lava pile has re­ sulted in the formation of a less even topography.

From Kowun hill to the north, the trachytes thin rapidly. On the outlier of Lotow they are about 200 feet thick and thin to nothing

10 kms. north of the 90 grid line in the area currently being mapped by

P. Truckle.

Typically, the trachytes dipping away from Kowun hill are grey to purple, massive, non-fissile rocks, in which alkali feldspar pheno­ crysts up to 1 cm, in length may make up 15 per cent. Some have a purple/creamy-white ^mottled appearance due to the development of a

'mossy' texture (see Ch.4s part 5b).

In a river gorge at 51^745 massive trachyte lava can be traced from the river bed, just west of the fault, to the top of the gorge, a thickness of 350 feet with no visible flow boundaries. The trachyte is traversed by numerous chert veins up to 5 cms. across and has suffered considerable secondary silicification. Faint flow-banding of darker - more.mafic,and lighter,more felsic,bands can sometimes be seen. In general the bottom parts of what appears to be a single flow are more mafic and appear to contain a hi^er proportion of feldspar phenocrysts. 30

Crude columnar jointing has developed in the top 50 to 60 feet of the flow.

The eastern flanks of Kowun have been cut by a N.N.E./S.S.Vo trending system of step faults which have downthrown the trachytes to the east. In the northern part of the area, Kowun rocks are preserved in a number of tilted fault-blocks. The Tirioko basalts banked up against the Kowun volcano and much of the present outcrop of the trachytes on the northern and eastern sides has been exhumed by subsequent erosion.

In the Kowun volcano pyroclastic rocks are only locally abund­ ant. Part of a fault-strip west of Lodukai and Kagumnyikal is dominated by thin trachytic welded tuffs, crystal tuffs and pumice tuffs with sub­ ordinate trachyte lavas. To the north and south the pyroclastic rocks thin ra.pidly and the area has been arbitrarily delineated by the local dominance of these rocks. This lens of pyroclastics is also exposed in the Kasamanang valley on the downthrow wide of the main fault. The welded tuffs show eutaxitic texture with conspicuous grey-green fiamme in a brick-red matrix. The matrix colour is due to the presence of much haematite and in general these rocks have been intensely oxidised.

Secondary, blue, opaline silica often occurs in small vugs.

Many of the pumice tuffs are brick-red, porcellanous rocks, rich in crystals and trachyte lapilli. The alteration is almost certainly due to baking by the Tirioko basalts. Fresh, unaltered, pumice tuffs are typically yellow and friable. Some contain such an abundance of stubby anorthoclase crystals (up to 1 cm. in length) that they may be called crystal tuffs.

Cutting throu^ the pyroclastic rocks are numerous intrusive bodies of a medium-grained 1 euootrachyte (or microsyenite). Two kilo­ metres west of Kagumnyikal (338735) is a small hill composed of leuco- trachyte which consists of alkali feldspar phenocrysts (up to 1 cm. in length) set in a trachytic textured groundmass containing only 10-15 per 31 cent mafic minerals (Specimen 8/186). The rocks have a distinctive sheen due to the h i ^ reflectivity from the faces of the parallel feldspar laths. The leucotrachyte intrusion has a doraal form centred on a thick, sinuous dyke composed of the same material. In the fault strip to the north-west, numerous similar intrusions of leuco trachyte, all showing the highly characteristic sheen, have been mapped. Dyke and sill-like apophyses emanate from some of these. Although intrusive contacts can be demonstrated in most cases, a large mass of leucotrachyte at 328767 appears to have flowed out onto a surface composed of welded tuff. This flow is of limited extent, varies rapidly in thickness, probably concealing its own source and may be called a tholoid (Fig. I3). The upwelling of what must have been highly viscous trachyte to form the intrusions and tholoid-like masses described here appears to represent the culmination of volcanic activity in this source area.

A cylindrical intrusive plug of leuco trachyte, similar to those described, occurs at 335867. The plug is itself cut by a thick dyke of the same rock type. On its northern margin the plug can be seen to cut trachyte lavas. Elsewhere the Kowun lavas have been removed owing to preferential erosion around the resistant plug, so that the contacts of the latter are almost entirely with lavas of the Turkana basalt formation.

One kilometre to the south-east a leuco trachyte dyke cuts lavas of a small inlier of Turkana basalt,

A succession of about 120 feet of tuffs and lavas has been traced in a fault strip 4 kms. east of the Kowun caldera (Fig. I4 ).

The volcanics dip at about 7° to the west and contain a welded tuff about

40 feet thick which forms a prominent feature. The welded tuff is a green, eutaxitic rock containing dark green fiamme and numerous lithic fragments of a grey—green porphyritic trachyte. The welded tuff caps about 25 feet of yellow pumice tuffs which show air-fall graded bedding.

Above the welded tuff and below the pumice tuffs are trachyte lavas. r/C/. /J

3 1 0 7 7 0 670770

horizon t a! zeala / ; 25 j 0 0 0 W t vertical exaggeration X

670020 694020 \!/

Fia. i 4

## /)c//y?/cc kxe/c/cc/ /7r/o/^o

/

A' 0 32

The lower one is a green, massive trachyte containing- fresh, glassy, alkali feldspar phenocrysts. The trachyte above the welded tuff is a grey-brown lava permeated by secondary white chalcedony veinlets. The rock contains about 20 per cent of alkali feldspar phenocrysts which are pinky-red owing to the intense oxidation and alteration caused by burial beneath the Pliocene basalts which are preserved a few hundred metres to the north and south. This four-member succession can be traced in another fault-bounded inlier of Kowun volcanics 2 kms. west of Napeitom,

The welded tuff is here only 10 feet thick and the pumice tuffs beneath contain light, friable, pure white tuffs which are locally diatomaceous.

This succession dips at about 15^ to the north-west.

At 343705 s Kowun lavas have been intruded by a li^t grey medium-grained rock notable for its large and abundant anorthoclase pheno­ crysts. The phenocrysts which occur in glomerophyric aggregates are creamy-white in colour and may measure up to 2 cms. in length. They make up about 30 per cent of the rock and are randomly arranged within the granular matrix, A pétrographie description of this rock (8 /94 ) which may be classed as a solvsbergite is given in chapter 4s part yo. The contact, which is obscured in the field, must be near vertical so that its intrusive nature is not in doubt.

Three hundred metres west of the Kasamanang river at Napeitom, thin flows of rubbly scoriaceous trachyte dip away from a small centre at angles of about 20-24° (Plate 7). The lavas are deeply reddened owing to the alteration of the mafic constituents to ferric-iron oxides. The central area consists of chaotic trachyte agglomerate and 'agglutinate* as small lens or dyke—like bodies forming a sub—circular area 60 metres in diameter. The breccias contain fragments of reddened trachyte lava and brick-red porcellancus pumice tuff presumably owing its appearance to baking or fumarolic activity. The area, which represents a minor Kowun so-urce, was probably the site of intense hydrothermal activity judging by ' î • 1: ., it-raï: " (D -P c d

0 m - 3 Ê I afl

44 û eu S (Xt I

ih â i t m •S-XS

CQ ^ ■ m 44 O 4 4 \0 co 3 ON O T > l>- CbC P co cti g K r - < P 4 4 C 0 ü

Cd o ?4 -P M

g

0 "ë r H

KM» m m â L : i m 33 the pitted and honeycombed nature of some of the lava surfaces. A small spring is located at this site, the water of which is however cool.

3(b) Correlations and Age

North of the author's area, Kowun trachytes thin out and are absent to the north of the 99 grid line. They occur as erosional rem­ nants in unmapped ground to the west (Fig, 12) but are absent from the

Ribkwo area to the south (Webb, 1971 ) •

The following K/Ar whole rock age dates have been obtained on lavas from Kowun (Dr. N. Snelling, pers. comm.) :

8/147 (686876 ) ...... 15«0 + 3*4 m.y. 8/504 (317822 ) ...... 15.2 + 0.5 m.y. 8/507 (315853 ) ...... 18.1 + 0.6 m.y.

The specimens 8 /5O4 and 8 /507 are stratigraphically equivalent whereas

8/147 is younger. In view of the age of 15*7 + 0.6 m.y. obtained for a

Turkana basalt lava (8 /484 ) and those obtained for the Loperot area, the date given by 8/507 may be rejected with some confidence as being too old.

The dates of I5 .O and 15.2 m.y. obtained for Kowun lavas appear to be acceptable. Thus the planation of the Turkana basalts and the production of the lateritized surface onto which the Kowun lavas were erupted seems to have taken place between 15«7 + 0.6 and 15.2 + 0.5 million years.

The Kowun volcano, consisting of trachytes and quartz-trachytes, is the only example known of a 'saturated* or * oversaturated * volcano occurring at this stratigraphical position within the Gregory rift valley.

Elsewhere the Turkana and Samburu basalts are succeeded by the 'plateau* phonolites (Baker et al., 1911$ King and Chapman, 1972). In South

Turkana the representatives of the latter group post-date the Kowun trach­ ytes.

The 'rhyolites* and 'andésites* of north-west Turkana are thought to be Pliocene in age, because of the extent of their dissection

(Walsh and Dodson, 1969» Fairbum and Matheson, 1970). These rocks 34

which include pantellerites, trachytes and mugearites (Williams, 1970)

could thus be broadly equated with those of the large trachyte/quartz-

trachyte/pantellerite volcanoes of South Turkana (Nasaken, Kafkandal etc.)

which have been dated radiometrically as Pliocene. With the discovery of

a Miocene trachyte volcano (Kowun) the age of the north-west Turkana

'rhyolites* posed an additional problem. Two specimens from the Survey

collection provided by Dr. J. Walsh yielded whole rock K/Ar age dates as

follows (s. Lippard, pers. comm.)§

l/l (Walsh and Dodson, 19&9; p.21) ....38.4 + 1.0 m.y. I8/552 (Pairbum and Matheson, 1970; p.30) .... 24.3 ± 0.7 m.y.

The Muruasigar rhyolite (18/552) is a dark red-brown rock in which the

mafic minerals have been altered to haematite. It also contains numerous

zeolite-filled vesicles. The Puch Prasir rock (l/l) is a fresh crystall­

ine pantellerite. In view of the differences in freshness of the two

specimens, greater emphasis may be placed on the 38 m.y. date. Since

the effect of oxidation and weathering is to cause the preferential loss

of radiogenic argon (Pitch, 1972) a discrepantly low age may be suspected

for 18 /552. Of interest here are the dates of 32.2 + 0.5 and 31*5 + 0.7

obtained on separate runs on a basalt from north-west Turkana, thought to underlie the Turkana 'rhyolites* (Reilly et al., I966 ) although the poss­

ibility of interdigitation with the 'rhyolites* cannot be ruled out.

This 32 m.y. date has been rejected by Baker ct al. (1971 ) a,s being of doubtful reliability since it falls outside the range of Turkana-Samburu basalt ages which they currently accept. Whilst it is true that basalts often yield discrepant ages, the dates obtained from the 'rhyolites* and the basalt from north-west Turkana suggest that the volcanic stratigraphy there may not correlate with that established for the main section of the rift. It seems possible that volcanic activity in north-west Thrkana commenced in the Oligocene and that further work may produce correlations with the Tcap Series of Ethiopia (Mohr, I96 I). 35

4(a) Kasorogol melanephelinite

On the east hank of the Kasorogol river at 284888 outcrops a

massive black rock in which micro phenocrysts of olivine occur in a fine,

granular matrix* This rock (Specimen 8/428), mistaken in the field for

a basalt, proves in thin section to be a melanephelinite close to ankara-

trite (see ch. 4 » pt. 4&)« A chemical analysis of 8/428 is given in

Table 33 « The melanephelinite cuts the Turkana basalt lavas and forms

a dyke-like intrusion some 100 metres by 6OO metres. The contacts where

visible are vertical and brecciation of the feldsparphyric basalt is seen

at the northern margin.

Specimen 8/428 has given a K/Ar whole rock age date of 20.6 +

3.0 m.y. (n, Snelling, pers. comm.). In view of the dates obtained for

the Turkana basalts both from the Loperot area and the author's (see

ch. 2, pt. 2b) this date appears to be too old,

Nephelinite dykes occur in the Loperot area to the north

(Joubert, I966) and are common in the area south of Lodwar (Dodson, 1971)

where they cut the Türkana basalts and are associated with microfoyaite

intrusions similar to that forming Kanitiriam hill.

4(b) Kanitiriam and Nakasuw trachyphonolites

The capping rocks of the Nakasuw scarp are porphyritic trachy- phonolite lavas dipping gently to the south-west. TWo thick flows form

the scarp above the pyroclastic member of the Türkana basalts. Farther west, the eastward-facing flow fronts form a series of soarp features.

Specimens of Nakasuw trachyphonolite are described in section ch*4

The lower flow consists of green-brown trachytoid rock (8/440) containing about IO-I5 per cent anorthoclase phenocrysts orientated parallel to a fissility. In hand specimen it resembles certain Kowun trachytes but analcite has been identified in the groundmass and the flow is classed as a trachyphonolite. The capping flow is a sli^tly fissile, dark, grey- 36

green, fine-grained lava (8 /44I). There is a small scale mottling and

the rock contains sparse alkali feldspar phenocrysts. Thin section

examination suggests that this rook is a phonolitic trachyte containing

a lower proportion of analcite than 8 /44O.

On the east side of Kanitiriam hill, dipping away at about

15-20° is a single flow of analcite-trachyphonolite. The rock (Specimen

8 /487 ) is aphyric and silvery-grey in colour. It is non-fissile and has

a li^t brown weathering skin. The petrography of specimen 8/487 is

described in ch. 4 » pt. 4^ and a chemical analysis is given in Table 33

This flow of 'Kanitiriam trachyphonolite* forms a sharp feature above the

Kasorogol river at 278833 where it overlies Turkana basalt agglomerates.

It is of limited extent having been traced only in the west bank of the

Kasorogol. At 292865 the trachyphonolite rests on feldsparphyric

basalts and appears to be about 120 feet thick. Farther north and to the

east it has been removed by erosion. Nowhere in the field can the strati­

graphical relationship of the Nakasuw and Kanitiriam trachyphonolites to

the Kowun trachytes be demonstrated but age determinations confirm the

Kowun trachytes as being considerably older.

4 (0) Kanitiriam microfoyaite

Kanitiriam hill, which is in fact two peaks separated by a

col (Plate 8), consists of an intrusion of nepheline-microsyenite, The

surface of the hill reveals large blocks composed wholly of microsyenite

(Plate 9 ) and nowhere can the microsyenite be found 'in situ*. It is a

medium-grained, granular rock, light grey in colour. Phenocrysts of

alkali feldspar may make up to about 15 per cent by volume and individual

crystals are up to 3 cms. in length. Li^t brown * squares * of altered nepheline can be readily seen in hand specimen. Little variation in the relative abundance of feldspar and nepheline phenocrysts could be dis­

cerned in the field. Specimen 8/489a is described in section 4e, oh. 4 Plate 6s Panorama looking from west to nurta from Kowun. The ^rnitiriaTû

microfoyaite intrusion is in the middle distance beyond the kasorgol

The Kowun trachyte erosion scan:) stretches away to the north (right;. O a r C -P I o .H rj & s (44 O c P 0) O Ü •H 0 •H cd 3 S r C bn

I g A o -P •H Td § w On I rH 37

and since the feldspars are perthitic the rook is more accurately termed a microfoyaite. A chemical analysis of this rock is given in Table -13

The margins of the intrusions can be delineated with some accuracy by the sharp change in a slope corresponding to the passage from the tuff/welded tuff envelope to the bouldery microsyenite, althou^ no actual contact was found. The intrusion of the microfoyaite appears to have disturbed the country rocks such that the tuffs dip away from

Kanitiriam hill at h i ^ angles on all sides.

4(d) Correlations and Age

The trachyphonolites and phonolitic trachytes of the present

area may be the representatives of the extensive * plateau* phonolite

group II of King and Chapman (1972). However no true phonolites have

been found and it seems likely that the Nakasuw and Kanitiriam lavas are

merely local expressions of the phonolitic volcanism which characterized

the rift between 11.0 and 13«5 million years (Baker et al., 1971)*

Joubert (1966 ) has described phonolitic trachytes from the

Loperot area which are similar to the Nakasuw and Kanitiriam lavas. At

Kakhapit hill, phonolitic trachyte surrounds a central core of micro­

foyaite , a situation analagous to that of the Kanitiriam trachyphonolite

and microfoyaite. Joubert states that the phonolitic trachytes precede

the emplacement of the microfoyaite and this sequence is suggested from

the dating of the Kanitiriam rocks. The trachyphonolite (8 /487 ) has

yielded a date of 11.8 + O.4 m.y. and the microfoyaite (8/489a) has been

dated at 9.9 + 0.5 m.y. (N. Snelling, pers. comm.),

Microfoyaite plugs and dykes occur extensively in Turkana

(Joubert, 19665 Dodson, 1971) and the 9*9 m.y. date is the only one

currently available ; moreover the Kanitiriam intrusion is the southern­ most yet recorded. At Kamutile, south of Lodwar, Dodson (l97l) Las described microfoyaite plugs and dykes associated with nephelinites, 38

teschenites and lamprophyres. Farther north, the Muruangapoi hills are

made up of phonolite and nephelinite lavas and microfoyaite intrusions»

This complex is regarded as a dissected central volcano of Miocene age

(williams, 1970) and is comparable with the younger volcanic complexes of

Karamoja, E» Uganda, in particular Moreto and Yelele. Age dates of

12,5 and 14*5 m*y. are available for Moroto (Bishop et al,, 1969) the

lavas of which include nephelinites, basalts and phonolites. The central

area contains nepheline-syenite intiusions as does that of Yelele the

lavas of which are mainly phonolites and nephelinites (King et al,,

1972)0 Thus in eastern Uganda and Turkana the extrusion of phonolitic

lavas during the Upper Miocene occurred from a number of centres with

which nephelinites and nepheline-syenites (including microf oyait es) were

associated. It therefore seems likely that the microfoyaite plugs of

Turkana represent centres of phonolite extrusion from which the lavas

have been almost completely removed. It is significant that the chemical

composition of the Kanitiriam microfoyaite falls within the rather re­

stricted range determined for 'plateau* phonolites (Table 31 ), Compared

with the main section of the rift which is characterized by a thick pile

of phonolite lavas, a deeper erosional level may be inferred for Turkana,

Lippard (1972) has suggested that the *plateau* phonolites emanated from

a number of very large centres rather than fissures along the crest of the

Kenya dome (Williams, 1970)* Whereas nephelinites associated with the

*plateau' phonolites are rare, a number of small nepheline-syenite intrus­

ions have been found in the dissected Sigatgat and Tiati phonolite com­ plexes (McClenaghan, 1971, Webb, 1971)*

4 (e) History of the area west of the watershed

The geological history of the area west of the watershed is summarised in Fig, I5 . It is thought that the cycle of erosion which produced the sub—Kowun surface was continuous into post—Kowun times.

Away from the Kowun volcano the planation of the Turkana basalts continue' G(?o/o^/cc7/ . (?(/o/u^/on o/"

a % 9 C o c X

^^^C4gx4X4vw/^r ^/^\$2bL7 /o

b

^7M#6x6g 6?. ;<:4\6L^Y:/6*( (7^ /(^)6J644< 6%-

/4u2ZV6L4< dC4/L6^

^iduOdL^^^' /C%%>4y2&cj? /4<. 0 4t%. u . L/

d

J/%4 f?L6b9/Ÿn4 6%^/4dU4(f(:Y4A<4yw>f 2. //f?/»M:. 6/. ' ;fZj%S/Kf (%n?dr/dt

C/' 39

so that the phonolitic lavas were erupted onto a surface of very low relief. The phonolite lavas west of the present area are flat-lying

(Mason and Gibson, 1957) occurring in mesa-like form above a now dis­

sected pre-phonolite surface. The upstanding Kowun shield volcano

suffered considerable dissection and erosion of its flanks and it is likely that the phonolitic lavas *lapped* around the residual Kowun mas­

sif. In the area of the Nakasuw scarp, the Kowun trachytes were removed before the eruption of the phonolites since the former extent of the trachytes was almost certainly as far to the west as the area of the

Nakasuw scarp. The intrusion of the Kanitiriam microfoyaite marked the termination of igneous activity in this area. Volcanism shifted to the east of the watershed where the widespread Tirioko basalts were erupted between about 8 and 6 million years.

5. Tirioko basalt formation a) Introduction

The Tirioko basalts are a widespread formation of lower Plio­ cene age named after a locality in Webb's area, to the south, where they predominate. The formation is composed largely of basaltic lavas, porphyritic and non-porphyritic basalts and hawaiites being most abundant.

Other types include^basanites, gWmramites and mugearites. Salic rocks are uncommon but trachymugearites and a trachytic welded tuff occur in a restricted area between Kagumnyikal and Napeitom. The basaltic lavas abutted against the high ground formed by the upstanding Miocene volcanics,

Much of the eastern flanks of Kowun and the Tiati complex in the south

(Webb, 1971 ) were buried beneath the products of subsequent Tirioko basalt volcanism. No representatives of the Tirioko basalts have been mapped

0ccurring on the western side of the Kerio—Suguta watershed formed by the upstanding Miocene lavas. The regional picture is one of migration of volcanism and accompanying flexuring and faulting eastwards towards what is now the central graben of the rift. The Tirioko basalts were in 40

their turn eroded, downwarped and downfaulted towards the centre of the rift with the subsequent build up of trachyte complexes east of this active

zone.

The outcrop of the Tirioko basalt formation within the author's area is confined to a narrow strip averaging 5 kms, in width and extending some 30 kmso across the area in a north-north-east/south-south-west trend.

The western boundary is either the base of the formation where it rests upon Kowun trachytes or a fault contact between the two. The eastern boundary is either the top surface of the formation, dipping beneath the trachytes of Kafkandal and Nasaken or a fault contact between the Tirioko basalts and the younger trachytes (Fig. 16).

Attention has been drawn to the reddened and baked nature of the Kowun rocks where the Tiroko basalts have been stripped-off. However, judging by the extent of the reddening, the basalts appear to have been confined to the east of what is now the watershed. The unconformity be­ tween Kowun lavas and the basalts may be quite marked, Plate 10 shows deeply reddened Kowun pumice tuffs dipping at about 20 degrees beneath basaltic lavas at 683898 . The unconformity may be traced in the fault strips west of Napeitom where the Kowun rocks occur as inliers exhumed from beneath the thin cover of basalts. In the south around 305^90 the basalts climb up and onto the older rocks where the Kowun lavas assume a southerly dip.

The Tirioko basalt formation can be divided into a number of distinct units on the criteria of rock type, aerial extent, source and direction of prevailing dip. These members constitute separate basaltic volcanoes or volcanic sources which were however closely related in space and time. The formation as a whole represents the physical coalescence of the volcanic products from numerous sources (Pig. I?)*

Because of its extent throughout the northern section of the

Gregory rift, the Tirioko basalt formation (and its equivalent, the Kapar- ^ M In \o N

-N'aportom^ m u g e a rR o s 2ftom basa _ KagumnyfRdI ~ brrsalts otokongofQo bosoms

/ 7 Z)/c 7_yr(7/77(7f/c o/" /7?e/776ers o/" 7"//-/o/ro ùcscy/f /or/7?(7f/o/7. 41

aina formation of I^Iartyn (1969 )) has been thought of as a ’plateau' form­

ation being the product of fissure eruptions (Williams, 1970? Baker et

al., 1971 )' The classification of a volcanic formation or group as of

'plateau* type is often due to the age of the formation. Erosion and

planation have tended to obliterate the sources of lavas of Miocene and

Pliocene age. Recently, Lippard (l972) has placed the 'plateau* phono­

lites into perspective by the identification of large shield volcanoes

within what was previously thought to be a fissure-erupted pile. Simi­

larly the Pliocene basaltic formation can be shown to be more 'central'

in origin. Inevitably the more accurate picture is more complex and

different, often adjacent, areas emphasise differing aspects of the same

volcanism. Thus in the author's area the Tirioko basalts appear to have

been fed from central volcanoes with dykes being scarce, whereas the area

to the north and east, recently mapped by Truckle, contains an abundance

of dykes with central, volcanoes and cones scarce, A notable dyke-swarm

occurs in the Kaparaina basalts west of Lake Baringo (Martyn, I969 ).

The Pliocene basalts therefore have been fed both by dykes and from vol­

canoes and cones. The dykes tend to occur as swarms in restricted

areas. Elsewhere dykes may be almost completely absent and central-type

basalt volcanoes identifiable. The description is that of fissure vol­

canism similar to that occurring in Iceland (Walker, 19^5) where volcanicfn

is largely controlled by tectonic lineations. Such volcanism does not

however preclude the building of quite large central-type volcanoes which

may be situated upon the controlling lineations.

Four members have been recognised within the Tirioko basalt

formation. These are the Kagumnyikal basalts, the Napeitom mugearites,

\ / the Napeitom basalts and the Lotokongolea basalts (Fig, 17),

5b) Kagimnyikal basalts

The Kagumnyikal basalts originated from an agglomerate-fill

centre just outside the author's area in the region of 280652, Their Plate 10; Very reddened Kowun trachyte dipping beneath

Lotokongolea basalt at 69I469S.

Plate 11: Kagumnyikal hill 42 maximum present thickness is at least 1,800 feet. The Kagumnyikal lavas occur beneath Cheror and to the east where they dip beneath the Kafkandal trachytes. They thin systematically northwards to Kagumnyikal hill

(Plate 11 ) and disappear beneath the Napeitom basalts just south of

Napeitom (Fig, 18). The Kagumnyikal basalts can be divided into four units on a lithological basis, each unit consisting of numerous lava flows.

The upper, aphyric unit consists mainly of non-porphyritic basalts and hawaiites with minor olivine phyric flows. It is about 450 feet thick beneath Cheror and also occurs further south where it is faulted against the lower, aphyric unit. The lavas are black, massive basalts and are usually fresh. Some flows are purple-grey in colour, often amygdaleidal with white zeolitic material filling the vesicles.

The upper aphyric unit is not represented on Kagumnyikal hill and possibly did not extend that far north.

Beneath the upper aphyric unit is one composed dominantly of basalts containing abundant phenocrysts of pyroxene and/or olivine. This mafic, porphyritic unit is about 4OO feet thick around Cheror and thins rapidly to the south, east and north. On Kagumnyikal, 200 feet of mafic porphyritic unit occur and south of Napeitom the single remaining flow terminates at about northing 855° The lavas are black and massive, in­ cluding picritic basalts and ankaramites. The clinopyroxenes appear as lustrous, black, euhedral crystals up to 2 cms. across. On the dark brown weathering skins the shiny black pyroxenes stand out. Resinous yellow^green olivines can sometimes be seen on fresh broken surfaces but weather preferentially giving a pitted appearance to the exposed surfaces.

The olivine phenocrysts can be seen to be frequently altered to a red- brown material (iddingsite)•

Just above the river at 702814, in the top part of a flow con­ taining large augite phenocrysts, large vesicles and gas pipes occur. Kagumnyikal

Lod u kai

/S'ÿ. /8 Secf/o/) f/ie T/r/o/ro /^o/-/77af/o/) c/o/ogr fAe /c7//ey horizontal scale I:S O ,000 vertical exaggeration K J /i

Key as fig- 16 and j'. ' aphyric Lotokongolea basalt

(_J| Napeitom limestone 43

The vesicles axe subspherical, filled with calcite and chalcedony and many are over one centimetre in diameter. The gas pipes may he np to

6 cmso in diameter and penetrate at least 30 cms. vertically down from the top of the flow.

Beneath the mafic ; porphyritic unit occurs the felsic porphy*- ritic unit which consists predominantly of feldsparphyric basalts and hawaiites. These rocks tend to be grey or dark grey in colour and are packed with creamy-white or glassy crystals of plagioclase often 3-4 cms. in length. Some flows are estimated to contain over 70 per cent feldspar phenocrysts which, because of their platy form, are fluxionally arranged, often going into complex swirls mimicking the flow of the lava. Because of their high content of phenocrysts, these rocks are friable and weather easily. *Pahoehoe* toes are common in the low ground in the Kasamanang valley on the southern margin of the area. The felsic, porphyritic unit is at least 600 feet thick beneath the Kafkandal scarp. Thinning rapidly to the north, 300 feet are present beneath Kagumnyikal. This unit has a minimum thickness of 60 feet where it is covered by the Napeitom basalts south of Napeitom. East of the author*s area, this unit has been traced farther downstream in the Kasamanang where four flows give a maximum thickness of only 85 feet.

Around the base of Kagumnyikal hill can be found an aphyric, black basalt. At a lower level, in the stream courses, a purple-grey, vesicular, aphyric basalt occurs. Tliis lower aphyric unit occupies much of the central section of the Kasamanang valley and can be seen to rest directly on Kowun rocks in a number, of places. Vest of Kengolereng, its thickness is about 4OO feet thinning northwards. Just north of Cheror the thickness of this unit is unknown but it is likely to considerably exceed 4OO feet. On the upthrow side of the major fault west of Cheror, the ground is wholly occupied by lava flows, the vast majority of which are aphyric. No trace of the felsic or mafic porphyritic units could be 44

found in traversing from the junction with the Kowun trachytes at 312693

to the top of the prominent ridge at 517650® This represents a thickness

of about 1,300 feet of what is inferred to be lower, aphyric unit* This

implies a minimum throw of about 1,400 feet for the fault west of Cheror.

Althou^ the present maximum thickness of the Tirioko basalts is about

1,800 feet under Cheror, extrapolation of the observed thicknesses of the

felsic, mafic and upper, aphyric units westwards towards the Kagumnyikal

centre gives a minimum figure of about 2,700 feet. Thus a considerable

thickness of Kagumnyikal basalts has been removed by erosion.

The crest of the ridge running from 307653 to 3O8643 consists

of a linear vent of agglomerate containing a high proportion of spindle

bomhs. The agglomerate has intruded into the basalts of the lower

aphyric basalt only.

3 c) Napeitom mugearites

Above the pyroxene-phyric basalts at Kagumnyikal hill are two

flows of aphyric trachymugearite, each about 50 feet thick. The rocks

are fissile and red-brown in colour being rather oxidised. Similar flows

inferred to be the lateral equivalents of those at Kagumnyikal are found

on the high ground east of the Kasamanang river at 702815 and 705805»

Below these flows are three mugearite lavas with a thickness of about 85

feet above the mafic porphyritic unit. The mugearites are purple-grey,

slightly fissile and with sparse feldspar phenocrysts. The tops of the

flows are vesicular containing abundant white zeolites. A thickness of

up to three feet of red bole is developed on the porphyritic basalt below

its junction with the lowest mugearite.

Above the trachymugearite flows is a welded tuff about 20 feet

thick. The welded tuff contains lapilli of trachyte and has a eutaxitic

texture due to the inclusion of numerous flattened, pumiceous fiamme in a

red—brown porcelanous matrix. This welded tuff can be traced around the 43

outlier of Napeitom basalt and to pass under the main outcrop of the

latter.

The succession of mugearites - trachymugearites and welded

tuff constitute the * Napeitom mugearites'. This member appears to be a

local development of more salic rocks within the Tirioko basalt formation.

Its maximum thickness is about 250 feet occurring approximately 1.5 kms.

north of the lodukai dyke in an area where numerous mugearite dykes cut

the lavas. It is not known whether the Napeitom mugearites were fed from

these dykes or emanated from the Lodukai centre.

3d) Napeitom basalts

The Napeitom basalts, the youngest member of the Tirioko

basalt formation within the area mapped, occupy the high ground east of

the Kasamanang between northings 800 and 850 . The Napeitom basalts rest

with sli^t angular unconformity upon the Napeitom mugearites. Two

kilometres south of Napeitom they overlap onto the felsic porphyritic

unit of the Kagumnyikal basalts.

The Napeitom basalts are at least 550 feet thick around 7 IO8 IO

and dip consistently at about 5° iu north-easterly or east-north-easterly

directions, thinning downdip. The basalts are characterized by an

abundance of black augite and light green olivine phenocrysts set in fresh

black matrices. Certain flows may be classed as ankaramites or picrite

basalts. Some contain plagioclase phenocrysts in addition to pyroxene

and olivine although in number feldspar phenocrysts are always subordinate

to the mafic ones.

Centred on 722842 is a round, flat-topped hill composed of

grey, aphyric basalt lying above the main mafic unit of the Napeitom

basalts.

It is likely that the Lodukai centre was the source for the

Napeitom basalts ; no other possible source has been found and the lava 46

flows dip away from the Lodukai structure. Unfortunately, no match can

be made between lithologies within the caldera and those of the Napeitom

basalts. The Lodukai dyke is an annular, inward—dipping intrusion of

coarse trachyte which has come up the bounding ring fracture of a small

(1,5 kms. in diameter) caldera structure within the Torioko basalts. On

the south side, where the trachyte dyke is absent, feldsparphyric basalts

outside the caldera have a fault contact with bedded basaltic agglomerates

and lapilli tuffs inside the caldera. On the north side, aphyric black

basalt appears to have lapped up against the trachyte dyke and to have

breached the dyke in one place flowing out onto Kagumnyikal basalt. This

flow of 'Lodukai basalt* therefore post-dates the formation of the caldera

and the trachyte intrusion.

5 e) Lotokongolea basalts

The Lotokongolea basalts are exposed in tilted fault strips

around Napeitom and to the west and north. They are probably strati-

graphically equivalent to the Kagumnyikal basalts but have originated from

sources locally and to the north and east of the area. Three units have

been recognizeds a lower aphyric unit, a felsic porphyritic unit and an

upper mafic porphyritic unit. Thus the lithostratigraphy correlates with

the lower three units of the Kagumnyikal basalts. The Lotokongolea

basalts are nowhere in contact with the Kagumnyikal basalts although 4:he

lower aphyric unit of the latter is exposed less than one kilometre away

from the mafic porphyritic unit of the Lotokongolea basalts, in a fault

strip between 680822 and 678829 .

Just south of Napeitom the Napeitom basalts are downfaulted

into contact with the aphyric Lotokongolea basalts and Kowun trachytes.

The aphyric unit of the Lotokongolea basalts is exposed in the

Napeitom gorge beneath the Napeitom limestones and along the road north

of Napeitom. The aphyric basalts provide a thin cover to the hummocky 47

topography of the Ko\>run trachytes, filling in the low ground and lapping

around Kowun inliers. The basalts are massive, black, aphyric lavas con­

taining occasional spherical, opal-filled amygdales. They are granular

and coarser than other aphyric basalts encountered and this more doleritic

aspect is due to the development of ophitic texture in the groundmass.

The ropy top surface of a basalt flow is well exposed at 715866 (Plate 12).

The succession in Napeitom gorge is described in section 5f •

Basalts in which plagioclase phenocrysts are common occur in the north-east comer of the area and continue northwards to the hill

called Lotokongolea. Between 683900 and 695894 the plagioclase phyric basalts can be traced unconformably on top of reddened Kowun trachytes.

A prominent dyke trending at 040° cuts both the plagioclase phyric basalts and Kowun trachytes. The dyke is composed of basalt containing about

IO-I5 per cent black augite crystals and is about 5 metres wide (Plate I3).

Along a line between 692887 and 703881 the felsic porphyritic unit dips under mafic porphyritic lavas which occupy the higher ground to

the south. The felsic porphyritic lavas of the Lotokongolea member rarely

consist of more than 20 per cent phenocrysts and the plagioclase crystals are smaller than those in the feldsparphyric basalts of the similar

Kagamnyikal unit.

The lavas of the mafic porphyritic unit are basalts and basanites. All contain abundant black augite phenocrysts but only rarely

can olivine be discerned in hand specimen. These lavas occur in two

fault strips west of Napeitom where they rest unconf ormably upon Kowun

rooks. The basalts only thinly cover the Miocene rocks and the fault

blocks have been tilted by as much as 20° to the west. Since considerable

erosion followed the tilting, the basalts outcrop to the west of the un­

conformity with the Kov/un lavas which occur as fault bounded inliers.

The mafic porphyritic basalts of this area were fed from a prominent cone centred on 692854* Two small conical hills at 328770 and Plate 12; 'Pahoehoe’ surface on basalt at 713866.

##

Plate 13; Basalt dyke cutting reddened surface of Kowun

trachytes looking south-south-west ironi 6908p6,

Kowun is in the far distance. 48

333770 are composed of fresh basalt containing abundant augite pheno—

crysts. Lava seems to have flowed from these two small isolated cones

and defines an outlier on pyroclastic rocks and leucotrachyte intrusions

of Kowun age. Although geographically distant from the main outcrop, this

occurrence may be included with the mafic porphyritic unit of the Lotokon­

golea basalts since it bears no relation to the similar lavas of the

Kagumnyikal member.

Tie form of the cone at 692854 is of interest in that it con­

sists of radiating basalt columns (Plate I4 ). The columns occur on a

conical hill about 80—90 feet high which is situated on the flattish top

of a larger hill. The individual columns are regular hexagons in cross-

section and may be over 50 feet in length tapering uniformly to the top

of the hill. Joints occur at h i ^ angles to the lengths of the columns,

every foot or so. These cross-joints split the columns up into blocks

which have convex upper surfaces and concave lower surfaces when in situ.

The columns radiate from the top of the hill throu^ 560° and their in­

clination from the horizontal is between 40° and 50°» At the very top of

the hill they steepen to about 70 ° loosing their ' regularity and terminate

in a complexly jointed mass of basalt. The columns are thought to have

foimed due to contraction whilst basalt cooled within the vent of a vol­

canic cone. The shapes of the inferred cooling surfaces therefore suggest

that the superstructure of the cone has been stripped away by erosion in

the manner indicated in Fig. 19»

5 f) Napeitom limestone

The Napeitom limestone outcrops at Napeitom over an area a little

less than one square kilometre. The dolomitic limestone defines what was

a small, shallow lake basin cut into Lotokongolea basalts and Kowun

trachytes. The hard, blocky limestone has given rise to a flat pavement

on which Napeitom settlement was founded. A small outcrop of pyroxene F/g. 19 Basalt cone at 692Q54 show/ng inf a r red cooling surfaces.

u

^ d o; o o lO io !^^’/ cczb X

^Xü(;yr^:z6z

7%^ ^ ^ /ÿvd^

rY'^^LZ y <3yZ);^V^ J/z ^<:

^ /r & 9 " ^ 'd

: /O feet 7 i/V

Fig. 20 Napeitom gorge succession at 7UQ5Q. Plate Ibî Radiating basalt coding columns on the hill at

69285Ü.

Plate 1.^: Aphyric Lotokongolea basalt in Napeitom gorge is

criss-crossed by dolomitic limestone. Lacustrine

dolomitic limestone caps the basalt. 49 phyric basalt at 714857 is topographically higher than the limestone which may thus be inter-volcanic with respect to the Tirioko basalt form­ ation although the contacts of the basalt and the limestone are obscured by recent gravels. The succession in Napeitom gorge is shown in Fig. 20.

Above the river bed are two flows of aphyric basalt. Next occurs a 2-3 feet thick bed of what appears to be completely calcified basalt. The lava has been so honeycombed by veins and stringers of carbonate and on such a fine scale that only small, discrete cores of rotten basalt remain. This calcification has developed on top of the underlying basalt,

A gradation exists through less calcified and less rotten basalt into fresh rock. Above the calcified layer is a flow of granular aphyric basalt. This is criss-crossed by a close network of carbonate veins mainly horizontal and vertical and presumably joint controlled (Plate 13). In places the basalt is so rotten that it forms 'earth pillars' beneath the more resistant layers of carbonate (Plate 16),

On top occurs a horizontally-bedded, nodular limestone which varies in thickness from 0 to 15 feet. The limestone is very hard, compact, creamy-white or pink in colour (Plate 17) and contains numerous concretionary, tubular and nodular structures some of which have been identified as being algal in origin (hr, W.V, Bishop, pers. comm,). In the top few inches small 'viviparus-like' gastropods have been found.

These are of a type which are indicative of comparatively fresh water conditions.

Chemical analyses show that the limestones are dolomitic

(Table 3). Moreover, the calcified basalt (specimen 8/414i>) gives a similar analysis to that given by the compact limestone (specimen 8/56a),

Other, similar analyses have been obtained for the inter-volcanic lime­

stone (specimen 8 /5O8 ) from the Turkana basalts and for limestones from near Lodwar, The occurrence of chemically similar dolomitic limestones

in formations of Miocene and Pliocene age throughout Turkana suggests that ripte 16: Earth pillars in Napeitom gorge. Pillais of

thoroughly rotten basalt are protected by

cappings of limestone.

pi r te 17 ; Mass il'e dolomitic limestone bed containing algal

structures. 50

they may have a common mode of origin. It may be significant that in each

case the limestones have developed upon basalts 5 rocks which are of course

rich in CaO and MgO, It is thus pertinent that Hall (l938) has shown that

the weathering of diabase releases large quantities of lime and magnesia

which are available for reprecipitation.

The presence of algal structures and gastropods at Napeitom

is indicative of a lacustrine environment, the final conditions of pre­

cipitation being less alkaline than the first. Alkalinity is regarded as

being the principal influence on solubility of CaGo^ (Degens, I965),

However organic processes may play a role throu^ the reduction of CO2

in lake waters. Any process tending to lower COg content or raise pH will

also tend to precipitate CaCo^. Algae and certain lacustrine plants are

of importance here due to their ability to reduce the content of carbon-

dioxide (Coudie, 197 O ).

It is thou^t that the anastomosing system of carbonate veins

in the basalt beneath the Napeitom limestones is due to the penetration of

carbonate-charged water into small cracks and pores by capillary action.

The high force of crystallization attributed to calcite (Hothrock, 1925)

would then facilitate penetration deeper into the underlying basalts and

account for their thoroughly rotten nature. The prominent 'bed* of

calcified basalt has been formed along the junction between two lava flows

and crystallization and permeation of carbonates may have been aided by

the top part of the flow being vesicular.

3s) Correlations and age

The Tirioko basalt formation is the representative in this area

of King and Chapman’s (1972 ) lithostratigraphical group III. It can be

traced south and disappears beneath the lavas of Ribkwo volcano where they

overlap onto the rocks of the Tiati complex (Webb, 1971)» South of Ribkwo,

the formation in the same s trat igraphi cal position is the Kaparaina basalt.

Although these two formations are very probably stratigraphically equivalent 51 they can not be proved to continue beneath the cover of the Ribkwo vol­ cano.

Radiometric dates obtained from lavas of these two formations suggest that group III lavas between latitudes 1° and 2° N may be dia­ chronous, Chapman (l9?l) gives two K/Ar dates of 5.3 + 0,2 m.y. and

5,4 + 0.2 m.y, for trachyte lavas intercalated with Kaparaina basalts near the top of the formation. From dates obtained for rocks from the

Nasaken area, the Tirioko basalts can not be younger than 6 million years in age. The trachyte dyke which forms the Lodukai intrusion has yielded a date of 6.1 + 0.3 m.y. whereas a specimen (8 /II9) from a flow of felds­ parphyric basalt which is cut by the dyke gives an age of 6.6 + 0,7 m.y.

Two lavas from the Kafkandal volcano which overlies the Kagumnyikal basalts have yielded ages of 5*9 + 0.3 m.y. and 5*7 + O.3 m.y. A second specimen of feldsparphyric basalt (8/122) from Kagumnyikal hill has given a date of 5*3 + 0.7 m.y. which in view of the ages obtained for the trachytes from Lodukai and Kafkandal appears to be too young, A specimen of mugearite (8/82) from the Napeitom mugearites has yielded an age of

6.6 + 0 .3 m.y. Thus the Tirioko basalt formation can be inferred to be between about 7 and 6 million years old in this section of the rift. The

Kaparaina basalts have yielded ages between 8.2 and 5*3 m.y. (Chapman,

1971)0 Other correlations between Pliocene basaltic formations are given in Baker et al. (l97l)o Notably the Kirikiti basalts west of Magadi

(Baker, 1958) have yielded ages of about 5 m.y. The precise northward extension of Pliocene basalts is at present uncertain but an isotopic date of 8,3 m.y, has been obtained from a basalt at Lothagam, South Turkana,

(Patterson et al., 1970).

6, Pliocene multicentred, trachyte volcanoes a) Introduction

The Tirioko basalts are overlain by a number of large trachyte. 52

multicentred volcanoes. These include Kafkandal, Nasaken and Kanatim

in the present area and Rlhkwo to the south. The volcanoes are low-angle

shields composed of trachyte lavas and subordinate pyroclastic rocks in­

cluding a high proportion of welded tuffs. Following the cessation of basalt volcanism was a period of between one and a half million years as

judged by radiometric dating during which erosion was extensive. The

Tirioko basalts were tilted gently to the east and prior to the initiation of trachyte volcanism, the basalts were downfaulted towards the centre of the rift. The trachyte lavas of Kafkandal and Nasaken thus rest uncon- formably upon the basalts and in several places banked up against fault scarps. Since the fault scarps were not degraded it may be that this period of strong faulting actually triggered the trachyte volcanism.

The sub-trachyte surface was one of low relief generally sloping to the east and north-east and cut by a number of upright, east­ ward facing fault scarps (see Webb, 1971» page 47)° A structural-contour map for the base of the trachyte volcanoes is reproduced as Fig, 21,

The trachyte/basalt junction is at 4» 000 feet on Cheror and about 3,400 feet on Polongus, Kengolereng and Kagumnyikal, The surface has an average slope of about 3° between Cheror and the Kafkandal scarp and 2° between Cheror and Kagumnyikal,

Judging by the displacement of the basalt/trachyte surface, the

Balakaicha fault has a maximum throw of about 900 feet opposite Kagumn­ yikal, It is possible that movement along this fault preceded the extru­

sion of Nasaken lavas but there is no way of knowing what thickness of

trachyte has been removed from the upthrow side by erosion. However,

the almost complete removal of trachyte from the upthrow side leaving only

the outliers Kagumnyikal, Kengolereng and Polongus may indicate a much

thinner cover owing to the presences of an early Balakaicha scarp.

The Napeitom basalts were faulted before the extrusion of Nasaken trachytes such that the latter banked up against a scarp running 8/

///

2/

structura! — contour map

for base of trachyte

vo/canoes

JO 75

I /

I W X ^ s cale / 5 0 ,0 0 0 53 from 734837 to 720801.

The existence of a number of fault-bounded inliers of Tirioko basalts within the Nasaken volcano allows the tracing of the junction between these two formations. This surface always maintains an eastward component of dip but downwarping due to the Balakaicha fault has modified this to produce a north-eastward dipping surface,

A phase of trachyte intrusion was the harbinger of the spectacular trachyte volcanism which resulted in the building of several shield-like volcanoes in South Turkana and the establishment of a pattern of activity which lasted throu^out the Pliocene,

6(b) Pliocene trachyte intrusions in the Kasamanang valley

There are in the Kasamanang valley numerous intrusive bodies of trachyte which cut the lavas of the Tirioko basalt formation. Large bodies include the Lodukai dyke, a large dyke-like mass of trachyte two kilometres north of Cheror and a prominent sill cutting feldsparphyric basalts beneath the Kafkandal erosion scarp, 1,5 kilometres south of

Polongus, There are also many smaller dykes and steeply dipping sheets often having a sinuous, irregular form. These intrusions are related to the trachyte volcanoes Kafkandal and Nasaken and were in some cases feeders for trachyte lavas, A high density of dykes, sheets and sills occurs west of Kafkandal, The dip of several of the sheets and their centres of curvature suggest that there may have been a large body of trachyte magma below Kafkandal from which they originated in the form of cone-sheets.

It is likely that trachyte volcanism was initiated by an intrusive phase prior to the focusing of activity at individual centres and the build-up of volcanoes. Thus most of the intrusive bodies appear to be geographic­ ally and/or structurally related to Kafkandal which is the oldest trachyte complex. The notable exception is the Lodukai dyke which, in spite of its proximity to Nasaken source areas, on the evidence of isotopic dating, 54

predates much of the Kafkandal complex.

It is clear that the intrusions as seen are at a very high level? the maximum cover at the time of intrusion was between 1,000 and

1,500 feet of basalt lavas. Some intrusions have taken advantage of pre­ existing structures such as the Lodukai dyke and the intrusion along the fault north of Cheror. Many cut across the regional structural trend and their irregularity and relationship to trachyte centres are again com­ paratively near surface expressions.

The Lodukai dyke is an annular intrusion of coarse porphyritic trachyte or microsyenite (Plate 18), It has come up the bounding ring^ fault of a small caldera structure within the Tirioko basalts. The dyke stands-up, over 200 feet in places, above the surrounding basalts (Plate

19 ). The intrusion is foliated, dipping at about 50° at the inner contact and steepening to near vertical at the outer, intrusive contact with the plagioclase phyric basalt. The trachyte at the outer contact is chilled to a dark green, fine-grained facies and fragments of basalt breccia have been included. The Lodukai basalt inside the dyke rests upon the trachyte with no disturbance and in one place has breached the dyke. On the south­ ern margin where the dyke is not continuous, basaltic tuffs within the caldera are faulted against feldsparphyric basalts. The fault is marked by coarsely brecciated basalt and a line along which pneumatolytic action has produced white "trap".

The trachyte is a grey—green, mesocratio rock containing alkali feldspar phenocrysts up to 2 centimetres in length. Small pheno— crysts of green pyroxene are also visible and the common occurrence of interstitial alkali amphiboles produces a mottled appearance in hand specimen. Small veins of trachyte, a few centimetres across, criss-cross the main dyke.

The intrusion north of Cheror has a dyke or funnel-shaped form

55 opening upwards (Pigo22), This intrusion appears to have taken advantage of a fault plane but rejuvenation of this structure has caused the in­ trusion to be faulted itself. The trachyte is a very fissile, light, grey-green rook containing about 15 per cent euhedral, alkali feldspar phenocrysts. The fissility is vertical at the centre of the intrusion and dips inwards at either margin. In the central part of the intrusion is a dyke of identical lithology which has intruded the main mass of trachyte. The fissility in this second intrusion is oblique to that present in the first. On the western side, a thick flow of trachyte at

315702 rests upon Kowun rocks. Its lithological similarity to the in­ trusive 2?ock suggests that it may have been fed by the latter. The in­ trusion has disturbed both the Tirioko basalts and the Kowun rocks so that near the contacts the flows dip at high angles.

It is to be noted that the Kowun rocks here are mostly trachytes but are purple-grey or purple-brown in colour and contrast sharply with the fresh grey-green or green of the Pliocene rocks.

The sill south of Polongus is at its maximum about 70 feet thick and is composed of fresh, grey-green, porphyritic trachyte. It cuts the feldsparphyric basalts and both a welded tuff and trachyte flow from

Kafkandal. At its lower contact with the basalt, are about six feet of fine-grained, green trachyte with microphenocrysts of alkali feldspar.

The basalt has been brecciated and apophyses of trachyte penetrate into it. The upper contact of the basalts in the stream at 6T^666 also shows a chilled facies. Here the sill is slightly discordant to the basalt flows such that reaching a higher level it cuts Kafkandal rocks. Where it cuts the promontary of Kafkandal rocks the sill is inclined to the south—south­ east at about 20^. Tracing the intrusion southwards, the inclination steepens so that the sill becomes a dyke inclined at an angle of about 70 to the east and 40 feet thick. In several places the inclination of the trachyte sheet changes sharply from about 70 00° to about 20 , always Qost/ng J 2 0

northing 70 6

I

V77. ////

I

700

I

6 9 3

ij intrusive trachyte

I I Tirioko basait

Kowun trachyte

Tg._22 SQrial E-W sections through a trachyte intrusion north of Cheror Plate 19; The Lodukai. dyke standing up against kagumnyikal

basalts.

Plate 20: Cone sheets east of Cheror. 56 inclining to the east. These rapid changes in the degree of discordance give rise to the lobes along the length of the intrusion as marked on the map9 corresponding to differing widths of outcrop.

East of Cheror are a number of cone-sheets dipping towards the

Moru Angitak centre of Kafkandal. They have arcuate outcrops and dip at angles of between 40 and 60° (Plate 20)« All are composed of fissile trachyte with a fine-grained facies at the margins. Rarely they have glass selvages, a few centimetres in width. Numerous other dykes and inclined sheets occur, always composed of fissile trachyte (Plate 2l),

The fissility is due to the strong alignment of groundmass feldspar laths whilst flowing during emplacement. The intense parallelism of the feld­ spar laths produces a sheen and many of these rocks could be mistaken for mica-schists at first glance, although they of course contain no mica.

6(c) The Kafkandal volcano i) Introduction

Kafkandal is the oldest of the Pliocene, multicentred trachyte volcanoes. It has a roughly circular outline, 10 kilometres in diameter and its present area is about 95 sq. kilometres of which 40 sq, kilo­ metres fall into the author * s area, the remainder having been mapped by

Webb (1971)9 The original extent of the Kafkandal volcanics must have been far greater. Most of the flank deposits have been removed by erosion. In the west and south, Kafkandal lavas rest on Tirioko basalts whereas in the north and east they are hidden beneath the younger rocks of the Nasaken volcano .> The maximum thickness of Kafkandal volcanics is about 1,800 feet. The highest peak in the area is Moru Angitak (4,915 feet) although Nasrewa, 2 kilometres to the south reaches ^^002 feet.

The volcanic products of Kafkandal are attributable to three

Diain centres, Ngapawoi, %)ong and Moru Angitak (Pig. 23)# Centres or source areas are generally characterized by quaquaversal dips of trachyte Fig.23 Kafkandal volcano

North

W eaver

Webb

scale /; i 0 0 lOOO \ older volcanics

younger n

O pyrodostic rocks Plate 21' Trachyte dyke cutting basalt in the river bed at

658335. A thin selvage of chilled trachyte remains

in contact with brecciated basalt. Cemented river

gravels occur behind. 57

flows away from them and it is mainly the structural attitude of the flows

which has facilitated the division of Kafkandal into three units. Indiv­

idual flows thicken systematically towards the source area which is marked

by a great thickness of pumice tuffs, often unstratified, together with

numerous trachyte plugs and dykes.

The lavas of Kafkandal are, without exception, trachytes, no

more basic types having been found. The trachytes are saturated with

respect to silica or may contain up to 10 per cent modal quartz. They are

grey to green in colour, massive and individual thicknesses of between 60

and 100 feet are common. Normally a trachyte lava has a thick basal breccia produced by the fragmentation of the lowest chilled part due to flowage on the still viscous interior.

The trachytes are usually porphyritic containing abundant anorthoclase phenocrysts. Rarely mafic phenocrysts or microphenocrysts are present, green pyroxene or alkali amphibole.

The pumice tuffs are white or yellow, friable rocks, exten­ sively altered to clay minerals. They often show air-fall stratification and graded bedding. They contain lapilli of trachyte and rarely syenite, fragments of volcanic glass and abundant broken feldspar crystals. Pumice tuffs tend to be laterally impers is tent showing great variations in thick­ ness. Frequently they blanket an earlier topography and primary dips of up to 30° have been inferred. Welded tuffs are not common in Kafkandal but at least three occur in the Epong succession. They are eutaxitic flows, welded from top to bottom and form convenient marker horizons.

The whole of the Kafkandal complex has been tilted towards the east by between 5 and 10°. Thus lava flows on the eastern sides of Epong and Ngapawoi are more steeply dipping than their counterparts on the west.

Most of the flows forming the Kafkandal escarpment on the western edge of the volcano are now horizontal. 58 ii) Ngapawoi

The lavas and tuffs of the Ngapawoi unit form the main shield of Kafkandal. They occur in the north-west, west and south of the complex and are buried beneath the younger Epong and Moru Angitak units in the other areas. Initial eruptions were from dispersed small centres which are exposed at the base of the complex. They consist of short, thick, massive trachyte flows, together with numerous small, intrusive plugs and dykes.

Eventually activity was focused and a low-angle shield was formed.

Younger flows are more regular, widespread and have a general quaquaversal disposition.

Under the Kafkandal scarp, the base of the Ngapawoi unit is marked by a thick, pumice-lapilli tuff, up to 80 feet thick, resting upon two feet of red boll formed on feldsparphyric (Tirioko) basalt. At

670649 pumice tuffs are cross-bedded and appear to have been water depos­ ited. Rounded fragments of basalt are contained in these tuffs. In the

Ngapawoi valley, trachytic pumice tuffs again rest on reddened feldspar­ phyric basalts which occur in a fault-bounded inlier. A traverse up the steep eastern slope of the V-shaped valley covers a vertical distance of

1,200 feet. The Ngapawoi succession here contains two prominent welded tuffs, a pumice tuff horizon and 10 trachyte flows. The average thickness of these flows is about 90 feet.

Farther north other fault-bounded inliers of basalt occur beneath Ngapawoi volcanics. At 699678 trachytic pumice tuffs rest upon a purple vesicular, aphyric basalt about 20 feet thick which is underlain by feldsparphyric basalt. Here a fissile trachyte dyke forms a resistant rib across the stream course and cuts both the basalt and the tuffs.

East of here NasaJken lavas dipping to the south-east at about

5° bank up against and bury the Ngapawoi trachytes which dip almost due north at angles of between 5 and 13° (Plate 22). The unconformity with the Nasaken lavas can be traced with some certainty around the north and -pw cd 0)

ÎHI ÜO ■S o H rn MDco S g mu C (D

-P Oj ’Il ' I T3 h W rj

•H O :s eu& i euhC -pCQ IS et ? rCm -p f tH 0 o CO 1

g o w § o pL, -p CNJ ! ü east of Kafkandal but its position immediately east of Polongus and across

the Ngapawoi river is uncertain. On the east side of Kafkandal a reddened,

trachyte surface dips beneath a purple-grey, aphyric, vesicular basalt

which has been traced northwards and proved to be part of the Nasaken

succession. This basalt, the 'Katirr basalt', flowed from the north-west

and was deflected around the high ground formed by the Ngapawoi shield.

The area around 703663 contains numerous spine-like intrusions

of trachyte. These have vertical, brecciated contacts with trachyte lava flows. Surfaces of the intrusive trachyte exhibit vertical slickensides.

The plugs intrude an area in which pumice-lapilli tuffs are preponderant

(Plate 23)0 Stratified tuffs may dip at high angles. Others show no stratification and their chaotic nature is taken to indicate proximity to the vent. These tuffs contain large, angular blocks of trachyte up to one metre across. Away from this area, lava flows maintain radial dips suggest­ ing that this was a major source area for the Ngapawoi volcanic rocks. iii) Epong The Epong unit rests unconformably upon Ngapawoi rocks and represents a migration of the focus of activity eastwards. Thus the Epong centre formed on the eastern flank of the Ngapawoi shield.

Pyroclastic rocks form a high percentage of the visible part of the unit and in the south, Webb (l97l) has shown them to be steeply inward dipping at angles of up to 40° defining a crater-like structure.

In the author's area dips are much flatter and eventually become reversed dipping to the north-west and north instead of the south. It is thought that the crater may be a collapse structure.

Trachyte lavas intercalate with the pyroclastic rocks of the crater area and several trachyte flows overlie the tuffs.

Centred on 733637 is a circular mass of trachyte about 800 metres in diameter (Pig. 24), Its vertical fissility contrasts sharply with the 750 63 0 7006s o rSOOO

- 4 0 0 0

LJOOO

Fig. 24 Section through the Epong unit of Kafkandal

horizontal sco / q i : SO ^0 0 0 vortical exaggeration %

■ O tra c h y te > Epong j Moru Angitak D tu ffs

H Ngapawoi trachytes

EH Tirioko has alts

horizontal scale l.'ESjOOO vertical exaggeration X lX% rSO OO

-4000

3 0 0 0

Fig. 25 Section through the Moru Angitak centre Plate 23: A small centre within the Wga.pawoi unit,

Kafkandal. A cylindrical trachyte plug

occurs in the foreground. Behind are a

succession of thin trachytes and tuffs,

the latter showing high original angles

of bedding. 60

low angled attitude of that in the surrounding trachyte lavas. This

apparently cylindrical trachyte intrusion occurred late in the history of

the Epong unit. The trachyte is a grey-green rock containing about 10 per

cent anorthoclase phenocrysts and does not differ in hand specimen from

that forming the lava flows.

Lavas and intercalated welded tuffs dip away from the central

crater and the above-mentioned plug. At 7 ÔI658 a welded tuff and thin pumice tuff overlie massive trachyte flows inferred to belong to the

Ngapawoi unit. The pumice tuff has included in it abundant trachyte frag- ments and in addition blocks of a coarse syenite. The syenite blocks are angular5, up to 10 centimetres across and consist of large, white crystals of alkali feldspar with interstitial, black mafic minerals. The latter prove to be intergrown aegirine and arfvedsonite. A little fayalite and magnetite are also present. The interstices between many of the feldspar crystals are empty giving a rather open texture suggestive of a cumulate origin.

Two hundred metres to the north the welded tuff and pumice tuff disappear beneath the Katirr basalt. The Epong volcanics can be traced around the north side of Kafkandal where they disappear beneath Nasaken trachytes, The unconformity between Epong and Ngapawoi can not be traced above northing 650. South of this line the opposing dips of the two units witness to their separate identities. iv) Moru Angitak

The Moru Angitak centre represents the final phase of activity of the Kafkandal volcano. It is situated on the western flank of the

Ngapawoi shield (Eig. 25). At 688639» the Ngapawoi river has cut through about 900 feet of chaotic, white, trachytic pumice tuffs. Small plugs and feeder dykes of trachyte penetrate this structureless mass (Plates 24 and

25, and Eig. 26). The summit of Moru Angitak is a steeply dipping mass of c -k, 5: s5

c: à C5 k 'b

C> Q

Zi o

VD CNj Ph

! - p o c o0 0 Æ0 •P I

•HÜ0 Plate 25: Moru Angitak showing pmice tuffs

cut by plugs and dykes. 61

trachyte about 150 feet thick. A feeder dyke can be seen to pass up into this trachyte flDW. Both to the south and west trachyte flows capping pumice tuffs dip away from the Moru Angitak centre (Plates 25 and 26).

About 400 metres south of the Moru Angitak, stratification in the pumice tuffs can be discerned.

Related to the Moru Angitak centre is a large trachyte dyke running approximately north-south and cutting Ngapawoi lavas from 680647 to 685663. The sill south of Polongus and the cone sheets east of Cheror may also be related to this centre.

Pumice tuffs cover a wide area around Moru Angitak and thicken rapidly towards it losing their stratification. The junction between the Moru Angitak tuffs and the Ngapawoi lavas in the Ngapawoi river must be at a high angle and it is likely that Moru Angitak represents a tuff- filled explosion crater. Tbwards the north, lava flows inferred to have originated from the Moru Angitak centre dip northwards and lie unconf orm­ ably upon Ngapawoi rocks. The Moru Angitak trachytes have been confined to a valley cut into Ngapawoi trachytes. A conical hill at 686677 has a succession of lava flows with dips consistent with derivation both from the south and the north (Pig. 30). It is thus probable that lavas from

Moru Angitak and Nasaken interdigitated,

^(d) The Nasaken trachyte complex i) Introduction

The volcanic rocks attributed to the Nasaken complex origin­

ated from numerous sources. These were however geographically restricted

and active over a short time-interval such that Nasaken, as a volcanic

3^d structural unit, can be fairly well distinguished from the older

Kafkandal and the younger Kanatim volcanics, in spite of the similarity

^ ^ook types.

Nasaken volcanics mostly dip at angles of between 5 and 15 io sp H Çh 1—1 £T3 !h ■P Ü (D U CD Xi P g O kl H CD b d 13 h Cf CD ü a •H CD T> X P CD P g X •H ü X c\3 o kl cc3 P O kl ■vs p . k! A cc5 CD CD bC PI X P •H 0 -X P TJ 0 X U cti kl p •H •H 0 ÜD X 1 P 0 ;0 W Jh q O i—i s (0 p po X P p 0 o 03 p .. vO CM (U P O) rH P 6Z

the east and south-east. In part these dips are due to a postulated east­

ward tilting of about 5°» Primarily the eastward component of dip is due

to the location of the source areas almost entirely on the western edge of

the complex. This edge is an axis of monoclinal flexuring and step-

faulting active prior and subsequent to Nasaken activity. The complex was

built up on the eastern downthrown side of this faulted flexure. Its

various sources are more or less aligned along this axis which follows the

main structural trend of this section of the rift. The irregular outcrop

pattern of the Nasaken complex (Pig, 2?) is due mainly to this strong

structural control. To the west, the Nasaken flows were restricted by the high ground and eastward-facing fault scarps in the Tirioko basalts and

Kowun trachytes, Tb the south, Nasaken volcanics bank against the dis­ sected slopes of Kafkandal and have flowed around the eastern side of this massif.

The present extent of Nasaken is about 220 sq, kms, (85 sq, miles) but before erosion an area of perhaps 550 sq. kms. (14O sq. miles) seems likely. About I50 sq, kms. fall onto the area mapped by the author.

To the north-east, Nasaken rocks disappear beneath the Lopirapira basalts which is an extensive flood basalt formation (S, Rhemtulla, pers, comm,).

To the east and south-east, Nasaken volcanics are buried beneath the trachytic rocks of the Kanatim and Olyamur volcanoes which were built up on its flanks as the foci of trachytic activity migrated towards the centre of the rift.

As in the building of the Ngapawoi shield of Kafkandal, the earliest activity was a period of widespread trachyte intrusion. The most striking Intrusions are the Lodukai dyke and the funnel-shaped mass north of Cheror (Pig. 22) although neither of these can be directly correlated

^ith Nasaken lavas. In several areas of the oldest Nasaken trachytes,

0Warms of closely spaced trachytic dykes have been found. In some cases

can be seen to have fed flows. Thus thick, irregular flows of trach- /O kms

north

T i rioko

K A F K AN D A L

K AC MI L A

Fig.27ThQ NosakQn complex 63

yte lava occurred throughout a wide area, pyroclastic rocks being compara­ tively rare.

Activity was then focused on one or two central areas. The

Lodukai centre may have been an important source but the main one was what is now a pumice tuff and basalt-filled crater near Nakwamoroi. Lavas and pyroelastics flowed away from these centres along easterly and south­ easterly directions, eventually building up stratiform flank deposits. In the Katatarope valley the dissection of evenly stratified, alternating, trachyte lavas and welded, ash-flow tuffs has resulted in a mesa-like topography.

Within the Nasaken volcano, rocks more basic in composition than trachyte probably occupy only about 5 per cent of the volume, Pyroclastic rocks probably constitute about one third of the volume. Pumice tuffs, pumice-lapilli tuffs, crystal tuffs, ash-flow tuffs (most of which are welded), ashes and agglomerates have all been recognized. In addition there are small pockets of intra-vo1canic epiclastic sediments often with thin diatomaceous horizons.

The trachyte lavas are nearly all peralkaline, soda trachytes or quartz-trachyteso More extreme, pantelleritic compositions occur rarely.

In contrast the trachyte lavas of the Ribkwo volcano are silica undersat- nrated and are phonolotic trachytes. The majority of trachytes from

Kafkandal contain neither modal quartz nor feldspathoids, ii) The Nasaken area

The oldest lavas of the complex outcrop around Nasaken itself.

Low ground to the west of the road is occupied by irregular, short, coalescing trachyte flows which have been fed largely from dykes, Through- oiit this area trachyte dykes are common but they also occur in dense swarms cutting the flows. The dykes are very strongly aligned along a north—north­ east/ south-south-west direction but do not persist for great distances along 64

the strike. Two such swarms have been located about one kilometre west of

Nasaken, The dyke rocks are marked by a penetrative, vertical fissility and occur as a series of ribs cutting horizontal trachyte lava. Their occurrence is accentuated by oxidation and weathering at their margins producing an orange-brown colour which contrasts sharply with the greens of the fresh rock. The dykes are so numerous that they can only be re- presented diagrammatically on the map. Estimates of intrusion density have been obtained for a swarm exposed on flat ground around 76276 O.

Walking at right angles to the general strike of the dykes, a distance of 450 paces (approximately 4OO metres), 37 dykes were measured. Their widths varied from 1 to 25 feet averaging about 11 feet. Thus the dilat­ ion here can be estimated to be between 30-40 per cent. The dykes have a pronounced preferred direction along 035° (Eig. 28) which is the regional structural trend shov/n by both the major faults and the strike of the

Basement, The occurrence of the dykes in swarms and their orientations within those swarms imply a deeper tectonic control than that of the

•trachyte intrusions in the Kasamanang valley. The latter are less dis­ cordant to the volcanic strata and can in some cases be related to ex­ trusive centres.

Vertical foliation within dykes has been observed rarely to fan out, diverging upwards or to swing into a near horizontal attitude.

The paucity of pyroclastic rocks and the irregularities among the trachyte flows within -this complex area also suggest that the lavas were mainly dyke fed.

Where the road crosses the Nasaken river is a small area of trachyte agglomerate or explosion breccia exposed mainly in the west bank.

The surrounding trachytes are purple and green horizontal lavas and there fs a passage from this undisturbed trachyte into coarsely brecciated rock

(Plate 27 ), The breccia contains large angular blocks up to 3 feet (l

®etre) across (Plate 28) set in a yellow-brown matrix composed of pumiceous Fig. 28 Dyke orientât ions within a swarm at 762 760 yNasakon trachytes

2 7 0 9 0

/80

samp 12 3 7 Plate 27: Explosion breccia at Nasaken.

Plate 28: Explosion breccia at Nasaken

containing large angular blocks of

trachyte. 65 material and powdered trachyte. The blocks are composed of oxidised, flow-banded trachyte, purple in colour. The banding in the trachyte is often contorted and it seems likely that a lava which may not have com­ pletely solidified was disrupted by explosive activity. The trachyte surrounding the breccia pipe is deeply honeycombed to a depth of several inches and this may have been caused by the corrosive waters of a geyser falling onto the trachyte surface. The area is still the site of a hot spring emanating from the brecciated rock.

The east bank of the river at Nasaken is about 50 feet high and composed of black, aphyric, hawaiite lava. The exfoliation weathering of this rock is very striking (Plate 29)» Upstream, the outcrop of this flow is terminated by a fault and a large dyke of the same rock-type occurs in the river bed. Downstream the hawaiite flow can be shown to rest between trachyte lavas. Farther north where the road crosses several small stream courses, hawaiite and basalt are exposed in the valleys. These appear to be thin, local, intercalations of mainly basic lava within the older Nasaken trachytes. Small pockets of tuffs occur within this area above the basic lavas (Plate 50)» The pyroclastic rocks are mainly friable, white pumice tuffs in fine, graded beds and coarser tuffs containing lithic fragments ranging from basalt to trachyte.

Channelling and cross-bedding suggest that the tuffs have been reworked in part by water and thin, pure white beds of very light mudstone may be diatomaceous. Various horizons within the tuffs are reddened to a depth of a few inches indicating a more prolonged exposure than usual during which oxidation of the surface took place. Leaf impressions have been found in bedded ashes exposed on either side of the road at 751804»

The tuffs are overlain by a trachymugearite flow which has a thick (5-10 feet) basal breccia where the lava has slumped into and chumed-up the underlying tuffs. This lava is highly vesicular throughout.

The vesicles are very elongated in the direction of flow and some contain Plate 29: Exfoliation weathering of basalt in

the river bank a t Nasaken.

Plate 30: A pocket of bedded pumice tufls

resting on basalt at 7^1823 within the

Nasaken volcano. On top remains the

basal breccia of a trachymugearite flow. 66

sulphur and pale pink heulandite.

Between Nasaken and the Lodukai river are a group of flat-

topped hills composed of a succession of trachyte lavas, welded tuffs and

pumice tuffs lying on top of the older, irregular, dyke-fed trachytes

(Plate 51). The succession is represented in Fig. 29 from three measured

sections. At 725794 and 726788 , the top trachyte is a 90 feet thick flow

of dark green, fissile rock containing about 15 per cent of alkali felds­

par phenocrysts and small ( ( . 5 cm.) orange prisms of hydrobiotite after

hedenbergite (see ch.4 , pt,2i)<> Beneath this flow at both these local­

ities is a thick, welded ash-flow. This is a light grey-green, porcell­

aneous rock but with no eutaxitic texture and contains abundant unsorted

angular fragments of trachyte, black volcanic glass, relatively unflatt­

ened pumice clasts and alkali feldspar crystals constituting in all

2O-4O per cent by volume. Crudely hexagonal, vertical cooling columns are

developed in the lower two-thirds of this flow (Plate 52), The welded

tuff rests on a coarse trachyte agglomerate and bedded, graded, pumice

tuffs. At 726788 two trachyte lavas have been mapped below the tuffs.

These flows are 115 and 175 feet thick respectively. The underlying ash-

flow tuff has a welded, eutaxitic lower part containing elongated, glassy

fiamme, passing into an unwelded, unbedded, tuffaceous, upper part con­

taining numerous unsorted lava fragments and pumice clasts. Since the degree of welding of ash-flow tuffs is a function of temperature and load pressure (Ross and Smith, I961) it seems likely that the weight of overlying material was only sufficient to cause welding towards the base of this flow.

Fig. 29 illustrates the difficulty of making detailed strati-

ëraphioal correlations even over quite short distances in such a complex volcanic situation. Such rapid variations in thicknesses and changes in

"the succession are indicative of proximity to a centre. At 708802 what

S'PPoars to be the top trachyte at 725794 and 726788 is overlain by light Fig.29 S tratigraphica! variations within the area west of Nasaken

70QQ02 725794 7267Q8

/k A A A A

A A Â *A' A

too ft

/ / trachytQ lava % z r z we/ded osh^f/ow tuff vTLTiy unwe'fded " " p um ice / /ith it t uff A A A oggiomee rot ro e a o\ Î CM w d 50

CD •H 'd(D O a 3 u •p mo CD C o d •H •H U) U) a CD Ü dÜ I w d 'W w CD •H • p !> S’ ItH Î - p •H c 8 "d o 0 d a d g i •H 1w •H % d •to p rd CD a a I £ 0 Ü 3 1 «H P> I S■ë I •H I I I 5 I IH

I r4 (D s g 5 ■P I -P 3 to w K q) % •HCO 1 I C U o> o s to 3 o rO %

a 67

greys eutaxitic welded tuff containing dark grey, feathery fiamme»

Beneath the trachyte is a thick succession of pyroclastic rocks. Bedded, lithic and pumice tuffs and a thin welded tuff rest upon a thick series of agglomerateso One horizon is made up almost entirely of extremely rounded blocks (up to 3 feet across) of coarse leucosyenite» The syenite is a mesh of alkali feldspar plates with interstitial black mafic minerals

(arfvedsonite and aegirine) and is described petrographically in ch.4» ptThe occurrence of thick agglomerates at 708802, thinning to the east, suggests derivation from a source to the west. This evidence sug­ gests that the Lodukai centre may have been a source for these Nasaken volcanics since there is no other known source in this direction.

The succession described can be traced, downfaulted, in a series of small hills east of the main outcrop. Farther south at 743760,

10 feet of trachyte lava cap a 20 feet thick welded tuff which is very similar in appearance to that capping the hill at 708802 in possessing a pronounced eutaxitic texture formed by dark grey fiamme in a grey-green matrix. Beneath this, a porphyritic trachyte flow rests upon thick pumice tuffs which have been mapped as part of the Nakwamoroi succession.

The thick welded tuff occurring at 725794 and 726788 is thus absent here.

Large variations in thickness are to be expected in ash-flow deposits since due to their fluidity and mobility during flowage they are charact­ eristically absent from steep slopes, accumulating only on flat ground or on depressions (Gibson, 1970, Sparks and Walker, 1973)»

iii) The Balakaicha area

Under this heading it is proposed to consider that part of the

Nasaken complex occurring within a 5 kilometre wide strip east of the

Lalakaicha fault (Plate 33)» The rocks of this area are similar in age and type to those of the Nasaken area. Dykes are common and feed irregu­ lar trachyte flows. Some flows are however very thick, covering great Î..>r7lâià^.-2â5a>fi5 •»L-

Plate 33: The area of irregular trachyte flows

east of the Balakaicha fault.

Kagumnyikal hill is prominent in the

middle distance. 68 areal extents, and these generally occur above the irregular, dyke-fed trachytes* Pockets of bedded, yellow and white pumice tuffs occur within the trachytes but ash-flow tuffs are absent.

About 70 feet of bedded, graded tuffs occur beneath trachyte lavas about one kilometre south of the Lodukai centre and these are cut by the Balakaicha fault. Elsewhere tuffs rest as thin cappings upon the trachytes and may be associated with local vents. At 683715 a thickness of up to 90 feet of pumice tuffs, lapilli tuffs and crystal tuffs blankets earlier trachyte flows. A single flow of porphyritic trachyte sits above the tuffs and has ploughed into them contorting the bedding. Nearby, two circular plugs of fissile trachyte cut through the lavas beneath the tuffs

(Plate 34).

Farther south at 685695 a hill is capped by a thin trachyte flow resting on thick (about 60 feet) pumice tuffs, Bovn the valley to the north of this locality occur several north-south trending trachyte dykes. These cut only the lowermost flows and again represent the early dyke-fed phase of activity which occurs throughout the complex as a whole.

In the river bed at 687702 occurs an inlier of Tirioko basalt age. An aphyric basalt lava about 40 feet thick rests upon a basalt agglomerate containing numerous fine examples of spindle bombs.

About 50 feet of feldsparphyric basalt is exposed on the west bank of the Ngapawoi at 688688. The basalt is identical to that of the felsic phyric unit of the Kagumnyikal member. This inlier is fault- bounded towards the east. The basalt is reddened to a depth of three

^’eet at its junction with the trachyte. A trachyte dyke some 8 feet in

width cuts both the basalt and the lowest trachyte flow. This dyke has

■'^bin selvages composed of black glass.

The succession of tuffs and lavas composing the hill at 685678

S-Ppears to indicate an interdigitation of material derived both from the 2 0)CÜD u o Tl 4-1 CD TD CD T) m m r C (D - P r O

P3 !>s M r O

. TD (D ;''-4 I> - p 4 M S É i M O (d o SB co •H Iti T) w g ;3 •H o H (D 4 4 n

-ii g M rj 'O rH •H H CD t. - § Æ:o E h cd p W -p CD -p 0 ü O cd

CD

ü 69

north and the south (Fig, 30)» It is this evidence which suggests that

the volcanics of Moru Angitak and those low down on the Nasaken succession

were contemporaneous.

Several occurrences of basalt lava have been found in the

Balakaicha area, either intercalated with the trachytic rocks or resting

on top of them, A small hill at 694736 is composed of basalt containing

about 10 per cent black, pyroxene phenocrysts, resting upon trachytic

ashes. The bedding in the fine ashes has been picked out on the weathered

surfaces. Thin layers occur within the ashes consisting wholly of deli­

cate, well sorted, pumice clasts. Just to the north the basalt lava rests

upon trachyte underlying the ashes and is thus unconformable upon the

local Nasaken succession, infilling erosional hollows. Numerous other

outcrops of this basalt have been found within this area. The hill at

694736 is however capped by a thin trachyte flow (about 15 feet thick).

Thus the extrusion of the basalt and what is inferred to be an extensive

period of erosion may be intra-Nasaken events. Farther north, aphyric

black basalt appears to have flowed into hollows within the trachyte/tuff

succession (Fig, 3l)« The Lodukai basalt outcropping a few hundred metres

to the west is an identical rock-type. This basalt is a single flow, much younger than the basaltic tuffs with which it shares the Lodukai caldera

since, in the north, it has breached the trachyte dyke and flowed out

onto Kagumnyikal basalt. The throw of the Balakaicha fault here is prob­ ably about 3-400 feet yet the Lodukai basalt and its presumed equivalent on the downthrow side are at the same height. Thus basaltic activity appears to have occurred at the Lodukai centre after the last major move­ ment of the Balakaicha fault. The Lodukai basalt is itself affected by

■this fault but the amount of post Lodukai basalt movement is very small.

Such inferences depend very much on the certainty with which one can

correlate similar lithologies and lava-types. In the case of the basalts m and around Lodukai, proximity and petrographical similarity make their F ig . 3 0 /g e /c/ s/refcA" o f /?///\

N S

^ Ù ’ra.ck^ÔEL

é ecCcCeûC/ce.

/ O O f t .

1 0 0 ft.

Fi g. 31 Field sketch : a t 7 05 7 7 2 see

E IV

i/es/ca ^ r fto p

t r a c h y t e _ aphyr/c basa/t 5 0 ft

T'LVtY hedf

i - 70

correlation reasonable.

It is appropriate here to mention the trachyte outliers,

Kagumnyikal, Kengolereng, Polongus (Plate 35) and Cheror, Each of these impressive hills is composed of a thick capping of trachyte lava resting upon a reddened surface of Tirioko basalts which dips gently ((10°) to the east or north-east, What appears to be a single flow of trachyte, at least 280 feet thick rests on oxidised trachymugearite (Napeitom mugear- ite) at Kagumnyikal, The trachyte is a grey, leucocratic, coarsely mottled rock containing about 10 per cent alkali feldspar phenocrysts up to 2 cms, in length, Polongus (Plate 36) and Kengolereng have cappings of similar trachyte about 200 and 24O feet thick respectively, the former consisting of two flows. The Napeitom mugearite is missing on these two hills, the trachyte resting upon pyroxene phyric Kagumnyikal basalt as does that on Cheror where the trachyte is about I50 feet thick.

The outliers represent the thinning of the Pliocene trachyte lava succession against the upstanding, eroded hills of Tirioko basalt and Kov/un trachyte, A large volume of Pliocene trachyte has been stripped off the upthrow side of the Balalcaicha fault to reveal the Tirioko basalt succession in the Kasamanang valley. The large individual thicknesses of the trachyte flows suggest that these lavas may have been ponded between high ground formed by the older volcanics in the west, south and north, and that formed by an active source area in the east,

iv) The Nakwamoroi centre

The Nakwamoroi centre is an ill-defined, tuff-filled crater Plate 39: The trachyte outliers, Polongos,

Kengolerengand Kagumnyikal, looking north

from Kafkandal. The Kasamanang valley is

to the west (left) of them.

Plate 36; Polongos looking south from just east

of Kengolereng. The Balakaicha fault

passes east (left of Polongos)and below

the dark trachyte feature at top left. 71 about 1.5 kilometres in diameter. Within its walls is a thick succession of air-fall, pyroclastic rocks. These are generally bedded and graded, consisting of pumice tuffs, lapilli tuffs, crystal tuffs and agglomerates.

Lava blocks in the coarser deposits are only of trachyte. Many angular blocks of leucosyenite similar to the type found at 708802 are included.

Outside the crater, the pyroclastic rocks thin away rapidly. The tuffs within the crater walls are overlain by trachyte flows but no trace of trachyte remains within the crater which has been deeply eroded. Basalt, containing phenocrysts of olivine and pyroxene, covers the floor of the crater and appears to have flowed from two small vents on its northern edge. Considerable erosion of the crater tuffs took place prior to the extrusion of the basalt which in the north has flowed out onto the surr­ ounding trachyte lavas. This basalt is petrographically similar to that occurring east of Kagumnyikal around 694736,

Tracing the pyroclastic rooks out of the crater towards the east, they pass under a thick trachyte flow at 743760 which is thought to be part of the succession at Nasaken (section ii and Fig, 29), Strati- graphical successiongat 731764 on the north side of the crater, 743760 on the east and 734747 on the south are given in Fig, 52. At this last locality, the tuffs and trachyte in the crater wall are outward dipping at about 50° to the east-south-east. Tuffs and trachyte which are thought to be part of the Emus succession sit unconformably on top of them,

v) Tirioko basalt inliers

Several inliers of Tirioko basalt occur within the Nasaken complex. At 710737 a thickness of about 15 feet of li^t grey basalt containing abundant plagioclase phenocrysts is exposed in the Nakasen river, A thin red soil has been developed upon the basalt above which are white pumice tuffs, about 50 feet thick (Plate 37)? these are capped by a thick porphyritic trachyte flow. The tuffs, traced northwards along the 731764 7 4 3 7 6 0 73474 7

\ a 6 4 a \ A A A A Ù

\

A A A A A • • • ' . \ •% ;

A.A Â A 'A

A A A 'A A A A A A A A A A A A

A A A A A

SO ft. Key os for F i g .

F/g. 32 Strotigraphica/ variotions in the Nakwamoroi area m

4 k

Plate 37; Bedded pumice tuffs of the Nasaken

volcano rest up on feldsparphyric basalt

which occurs as an inlier at 710739. 72

river valley, thicken towards the Nakwamoroi centre and appear to he the

equivalent of those exposed in the crater walls.

Following the basalt southwards, a thin flow of basalt contain­

ing pyroxene and olivine phenocrysts comes in above the feldsparphyric flow. Thus the trachytic tuffs follow with a low angular unconformity.

The inlier is fault-bounded in the west, the outcrop of the basalt term­ inating against a linear feature.

Another inlier of pyroxene phyric basalt occurs about one kilo­ metre east of that just described. Here the basalt is faulted rendering a greater thickness visible on the eastern, upthrow, side. The western contact with the trachytic rocks is probably an unconformity since no evidence for a fault contact could be found although exposures are poor in this low ground.

The inlier of feldsparphyric basalt exposed at the confluence of the Ngapawoi and Nasaken rivers has a fault-bounded western contact with the trachytes. The lithologies occurring in the basalt inliers and the succession of pyroxene phyric basalt above feldsparphyric basalt correlate with those of the Kagumnyikal basalt. The low relief, pre-

Nasaken surface of Tirioko basalt (Kagumnyikal member) occurs in this area beneath a thin cover of trachytic rocks, to be revealed here and there by subsequent antithetic faulting and erosion.

Around 722700 an area of juot over one square kilometre con­

sists of basalt containing abundant pyroxene .and olivine phenocrysts. The

basalt is in the form of a dome, no flow junctions being observed. At the

top of the dome the basalt is 200 feet above the level of its junction

with the overlying Nasaken rocks on the periphery. At the junction the

basalt is weathered and very reddened up to depths of about 20 feet. The

tuffs and trachytes overlying it, thin onto the basalt dome. It seems

likely that this inlier of Tirioko basalt age represents an endogenous 73 dome which was particularly resistant to erosion and remained as high ground above the general level of the pre-Nasaken surface.

vi) Emus and the cyclic nature of activity

Emus is a prominent ridge about 4 kilometres in length and trending north-north-west/south-south-east. It is entirely an erosional feature reaching up to a height of 3»270 feet.

From the basalt of the inlier at 720730 to the top of the ridge (at 735725) a 900 feet succession of Nasaken volcanics can be traced (Fig. 33)» The pumice tuffs overlying the basalt at 720730 are thought to be the lateral equivalents of those in the walls of the

Nakwamoroi crater. Although these tuffs represent the base of the Nasaken volcano under Emus, in the Nasaken area a thickness of at least 400 feet of trachytic rocks occurs beneath them. The volcanic strata on Emus dip gently (averaging 5°) to the east-south-east. They consist of regular alternations of laterally persistent flows of trachyte, thin welded ash- flow tuffs and bedded air-fall pyroclastic rocks. The flows have not been fed by dykes which are rare in this area. The Emus succession re­ presents the focusing of activity to a central source area or vent, either

Nakwamoroi or Lodukai and the termination of the early dyke-fed phase of activity. Stratiform flank deposits in which pyroclastic rocks constitute about one third of the volume were built up. The pyroclastic rocks on

Emus are mostly bedded, air-fall deposits, Ash-flow tuffs are thin and this predominance of air-fall over pyroclastic flow deposits is indicative of proximity to the central vent.

Towards the top of the Emus succession is a single flow about

70 feet thick of aphyric olivine basalt. The basalt has an intergranular texture and the olivine in the groundmass has been almost completely pseudomorphed by iddingsite. This flow (the Katirr basalt) forms a dis­ tinctive marker horizon within the trachytic rocks and has been traced as Fig. JJ Section from the top of Emus ridge [7JS72s) to the baso/t inh'cr at 720730.

dojŸkjg^ZM à^cick^te,

JONUM. , 'M&dtù.djpo/^ly'T^'c

impjs/tiù/ttYvi^ tAfL^axitlc l o ^ c ù d à u ^ A A A A A j^U/VHtCÛ ^7U^S (7-J^ ^CAAA.e7YVid^ A A AA‘*A

j jhof^U^TuJc E^ticU^'G.

/ tuJraociJbic c ^ c h c L

y X M M l O L y t c H o i c Om J[ ( y i f Y ' t ^

SO y i .

j^mvt^'\AA,fFûtù.d. tytccli^

miftyUr/c iMEdud

E ^ i X M A i a i

t T ^ c l u ^ d j L

fUU\M(U CfM-d àüM UùUk. (KJ^CAMWtG

h

(I'l/ffoko b o A O ^ ^ . 74

far south as Katirr where it passes into the area mapped by Webb (1971),

The volcanic rocks on Bnus are cyclic deposits, each tripartite

cycle beginning with pumice tuffs followed by an ash-flow tuff and ending with the extrusion of trachyte lava. The same cyclic pattern of activity has been recognized in the Pleistocene trachyte volcanoes, Karau and

Chepchuk, in the Baringo area (Carney, 1972). In Nasaken, pumice tuffs frequently overlie older rocks unconformably and start the cyclic sequence of a new phase of activity. It is not known whether pumice eruption took place in part at the same time as an ash-flow. If so, the air-fall mat­ erial would become indistinguishably incorporated into the ash-flow deposit.

The mechanism of generation of ash-flows is generally accepted to be due to an extreme vésiculation of gas-oversaturated magma In the vent and upper part of the conduit. The expansion of the gas inflates the magma to a froth and then explosively disrupts it to form angular, cuspate shards. These portions of the froth which escape disruption sur­ vive as pumice claSts, The emulsion of gas and semi-solid fragments then overflows the lip of the vent. The hot fluid mixture flows with a high velocity often confined to one or more eruptive pathways to accumulate on flat ground and in depressions. The fragments are lubricated by the interstitial gas and the residual heat results in the welding of the shards and flattening of the pumice clajts into fiamme as the flow deflates.

The difference between a normal ash eruption and an ash-flow eruption according to Rittmann (1962 ) is illustrated in Fig. 34» The level in the conduit at which intense vésiculation occurs must be an important factor but as Rittmann implies so may be the actual shape of the conduit.

The first parts of the cycles recognized in the Nasaken deposits represent the long sustained and repetitive emission of pumice and ash from an ex­ plosively vesiculating trachyte magma column. The nature of the * trigger’ which causes an ash-flow eruption followed by the eruption of voluminous, Overflowing glowing cloud

Ash • eruption

bicontinuous

pyromogmo osh suspension (glov;ing fog)

oversoturated X- hypomogmo .«o' pyromogmo

I T - p Zoversoturated hypomogmo hypomogmo

FIGURE J ^ Diagram illuslraling llic clifl'crcncc between a normal ash eruption and an ash-flow eruption. (From Rittmann, 1962a.) The hypo- inagma at depth (/one a) is saturated or undersaturatcd with gas. As it rises into /ones of lower pressure it becomes ovcrsaturaled (zone a') and bubbles start to form in it (pyromagma, zone b) but the expansion of the , liquid-and-gas mixture is restrained by the high viscosity of the liquid. In zone c the gas becomes so abundant that llierc are present two continuous phases, instead of a gas phase dispersed in a liquid. Eventually, with further rise, the explosion level (X - X ) is reached, and the gas mshes out, bearing in it a cloud of liquid and solid particles. In the asli cniption (right) the » explosion level is deep, and the expanding gas is directed upward as though by a gun barrel. In the ash-flow eruption (left) the shallow explosion level results in an overflow of the gas-and-ash mixture with ash flows and glow­ ing clouds spreading out from the vent. The higher the viscosity of the magma, the lower the external pressure that is reipured to initiate explosion, hence the shallower the explosion level, so that the more viscous siliceous magmas (e.g. rhyolite) are more apt to produce ash flows than arc the less viscous basic ones. ' * . 75

essentially degassed magma to produce a holecrystalline trachyte lava, is unknowno Perhaps an ash-flow follows a particularly violent eruption when the upper part of the conduit is destroyed producing an open9 coll­ apsed vent where the strongly vesiculating magma is very shallow* The lack of a pipe-like conduit to direct the expanding mixture thus may result in an overflowing of this material. What clearly must control the kind of eruption is the rate of vésiculation of the magma. This is linked directly to the rate of rising of the magma in an open conduit. The vent however may be blocked by congealed lava and lithic debris. The pressure due to the rising magma and the vapour pressure produced by exsolution and crystallisation in the magma column will eventually overcome the resist­ ance of this plug. The sudden, free escape of gases would result in an immediate lowering of pressure at the head of the column, explosive vés­ iculation and the vertical eruption of pumice • Slower, more con­ trolled release of pressure or release before the pressure reached too high a level would result in strong but not explosive vésiculation and an ash-flow might result, {An analogy with the opening of a bottle of beer seems to be particularly attractive!).

It is important to note that the fiamme in the ash-flow tuff deposits of Nasaken are more or less equidimensional in horizontal section and have been formed by flattening of equidimensional pumice clasts or blebs of lava after the flow had come to rest. Thus the fiamme have suffered little or no stretching in the direction of the flow. The degree of welding and the concomitant development of fiamme in ash-flow tuffs varies with several factors, the principal of which are the thickness of the flow, its temperature on coming to rest and the amount of gas it contained. That thickness alone is not the only or dominant factor is shown by the occurrence of thin flows which are welded throughout. The

Sulur tuff in eastern Iceland is strongly welded in places where it is only two feet thick (Walker, I962 ). The Fantale tuff is welded throu^out 76

where its thickness is about three feet (Gibson, 19?0)o Several welded ash-flow tuffs in Nasaken are less than three feet in thickness; one is in places only 5 cms. thick (2 inches) yet it is densely welded in its lower part (Fig. 35)» Such very thin flows showing more than incipient welding must have been emplaced at very high temperatures.

vii) The Edilteniro area

The succession on Emus can be traced south-eastwards, downdip and forms part of that occurring within the gorge of the Edilteniro river.

The river has cut down a vertical distance of over 700 feet, its bed being cut into the Katirr basalt. The lavas at the southern end of the

Emus ridge dip at about 5° to the south-east. Approaching the Edilteniro the dip increases sudden ly to between 15 and 20°. The strata east of the river maintain a dip of about 10° to the south-east. These changes in dip may be indicative of the burial by the Nasaken deposits of an east­ ward facing fault scarp. Upstream, Nasaken tuffs and trachytes are banked against an eastward facing, steeply rising surface of Epong trach­ ytes. The unconformity here trends north-south in line with the inflexion within the Nasaken rocks.

That the Nasaken flows actually thinned against Kafkandal is shown by the succession in the hill at 746684 (Fig. 36). Several of the units, particularly the ash-flow tuffs, in this hill are thinner than those less than 1 kilometre distant in the gorge succession at 755680

(Fig, 56). A purple, higlily vesicular trachyte occurring at 746684 has not however been traced east of the Edilteniro. This thin lava is dis­ tinctive, consisting of about 25 per cent large (up to 3 cms.) anortho- clase plates.

The purple-grey, aphyric, Katirr basalt in the bed of the

Edilteniro can be traced downstream for some considerable distance before its outcrop is obscured by gravels. Above it occur pumice and lapilli J5

/4 ajolt-^ouj ^ v w ^ c < f& ô ÿ v ^

C m i^ (m J^ S ( % W . fk/ck . Tka J C c ^ cc^caa TCckcÛA^ol ^

OX/dcMôL ^ T k s i k i< y tto w L ^cvKi

'j)CfyxÆ.Qj\A

çf/a/m m (K. (a epuc dùm^HA/cMa£ m kûH^cMÀraC 4^o6rovi . / % d ip ^ crj^ u j^ c ii^ dscA^ûMô / c m a .

C c^uiitcA^ àj^ uM i^^ded m ù d it^o ^: UA (& ^cU uM.J^AÂd:eM£cC

jiaaaaiol cÙ Ats Com. ife f^cajA i.iz£c{, Ab(AAe /kz a o k'^^o tj ( ih ^

(sxAMA^ adocuA A i ^f(9c44/wj ^ ^ h e c l A z A )

ü i ^ é d f j (paUn ûUôtiM.(dr tA^i ^m { /fe U M ( ^ d j 2 d

TTuâ skjztck S^^-€/CIAAA£AA, //A CK. "7 é k 7 k- lû

tiMAJi Scajtz , * Fig. J6 Sections tAroc/gi) Nos ok en ■fiank deposits across the Ediiteniro gorge

746684 755680

yy9f.A\ tr«icU^t^ 7 ^^orpk^^itic

h ff/ckrtdf ^6rce//

ox idt'Sfd y styoM^^^ porpkjyd/c jfu/rple-jrt^y^ tyaciit^t^ sf'roy^yîy forphy'rit'/c

pvjffle j V^/ctifa'T j s6yoMa7tj truckytt / / / poy pU^yiir/c trAeUyt^

y €M^AX

dej*k yr^evi. ^ pcrpUyy/t/'c y it ic trucl^tç (A^Çdtd. i7Uiff

pu'**xicç jY«eu j pcypUy'rdcc | hoicUtjti. — pyxio'KitAxtS , âf uyz^Jed ^ / f A x 6&rc d^eccia. j

'■ 1 bedded, j

Tracing the Katirr basalt upstream, it disappears beneath the tuffs in the bed of a tributary?’ of the Ediiteniro. The junction with the overlying tuffs rises in height from about 2,500 feet at 752692 to about

2,800 feet at 756664^ Thus along the river bed the basalt appears to have a dip (probably to the north-east) with an average northerly compon­ ent of 3° « From 756664 southwards the dip of the basalt is 10° to the east-south-east. The succeeding tuffs and trachytes do not show this subtle reversal in the direction of dip always dipping to the south-east or east-south-east. The flows within the Ediiteniro gorge do however thin considerably southwards against the Katirr basalt. The configuration of this basalt lava has almost certainly been controlled by the shape of the dissected surface of the Kafkandal rocks over which it flowed and the existence of the pre-Nasaken fault scarp along the line of the lower

Ediiteniro valley (Fig. 37)»

viii) The Karimorega centre

Karimorega is an area of hummocky ground just west of the road and south of the Katatarope river. It contrasts sharply with the smooth, flat topography of the ash-flow tuffs to the south. It is com­ posed of thick, short flows of trachyte dipping more or less quaqua- versally at angles up to 30° away from three small trachyte plugs.

Trachyte dykes radiate from this central area. They are composed of fissile, green, porphyritic trachyte and have darker, finer grained mar­ gins, Several cut the pumice tuffs between the Ediiteniro and Katatarope N 1 : 5 0 , 0 0 0 /N

Fig. J7 The present outcrop of the Katirr basait flow directions and be fine of the buried fault 78

rivers. Little apophyses and stringers of trachyte have penetrated into the tuffs hut in spite of the incompetancy of the latter, the main dykes maintain linearity. A thick dyke about 120 feet (40 metres) in width cuts trachytes in the Katatarope about 200 metres upstream from the river crossing. The Kanatim welded tuff is not cut by this dyke and sits un- conformably upon the Karimorega rocks. Also in the Katatarope, poorly exposed at 797722 occurs an agglomerate containing large (up to 2 feet across) angular blocks of massive, green trachyte and rare rounded blocks of syenite set in a yellow-brown tuffaceous matrix.

The tuffs between the Ediiteniro and the Katatarope are mainly pumice tuffs and crystal tuffs. In the coarser, lapilli tuff horizons, fragments of purple-grey basalt occur. These are reminiscent of the

Katirr basalt which occurs stratigraphically just below the tuffs. The tuffs in the Milteniro gorge are about 80 feet thick (Fig, 36) having systematically thinned from Emus, At 783706 these tuffs measure at least

150 feet and this increased thickness is due to contribution from the

Karimonega centre. Several of the units traced in the Ediiteniro area

(Fig, 36) thin eastwards towards Karimorega where they interdigitate with the volcanic products of the latter (Fig, 38), The thick welded ash-flow tuff which forms the river bed over much of the length of the Katatarope was not deposited on the high ground around Karimorega, It is 100 feet thick at 783696 thinning to about 5 feet at 796698 (Fig, 58 ) and dis­ appearing completely a little farther east.

The Karimorega trachytes are overlain unconformably by a thick, eutaxitic ash-flow tuff which is part of the main Fasaken flank success­ ion, At 798695 , this tuff has a sinuous contact with the underlying trachytes, filling in hollows. Outliers of ash-flow tuff rest upon the

Karimorega trachytes farther north. The tuff is a buff-green rock con­ taining numerous dark, elongate fiamme throughout. Much of the welding, especially towards the top of the flow, has been destroyed by devitrific- F ig.JSStrat igra ph i ca/ va ria i ions in the Karimoraga area.

SQction in tfye Kototorope

volley from 798 695

io 7 9 5 6 9 9

Section from 78J696

io ihe Kangomo

river at 76J706

key os for Fig.

5 0 ft. 25m. 79

ation and the tuff has a flaggy appearance tending to break parallel to the eutaxitic foliation.

The trachyte lavas from the Karimorega centre are green» porphyritic rocks and several contain the small, orange prisms of hydro- biotite (a pseudomorph after green pyroxene) that occur in the trachytes around the Nakwamoroi centre. They differ in no way in hand specimen from the lavas of the main Nasaken volcano, A single exception is a flow occurring beneath the ash-flow tuff at 308700, This flow is about 40 feet thick and possesses a hexagonal columnar structure toward the base. It is a pantelleyitic obsidian. In hand specimen the rock is a dull black with dark green streaks. Its non-glassy appearance is due to a weak crystall­ isation of the groundnass, the green streaks representing a stronger, coarser, crystallisation. Phenocrysts of alkali feldspar and fresh hedenbergite occur (Table 30,8/281),

Undulating ground 1,5 kilometres north of where the road crosses the Naregekamar river, is the site of a small extrusive centre which was probably active at the same time as that at Karimorega, The ash-flow tuffs of the main Nasaken succession rest unconformably against pumice tuffs and a trachyte lava flow. The lava is cut by two large trachyte dykes which may have acted as feeders. Another similar centre from which short, thick flows of trachyte emanated is situated about 4 kilometres south of the author*s area, along the road. These three centres contributed locally to the Nasaken flank deposits. They are approximately on a north-south line and were contemporaneously active.

They represent the eastward migration of trachytic volcanism during the final stages of the activity of the Nasaken complex,

ix) The Katatarope valley.

The area between the eastern slopes of Kafkandal and the road contains a succession of ash-flow tuffs and trachyte lavas which constitute 80

the outermost flank deposits of the Nasaken volcano. The flows are very

regular maintaining constant thicknesses or thinning systematically and

gradually to the south and east. The Katatarope river and its tributaries

are incised into these stratiform flanks and have given rise to broad

U-shaped valleys with flat, mesa-like interfluves. The strata dip to the

south-east at angles of about 5° (Plate 38) and disappear beneath the

younger Kanatim volcanics just east of the road and beneath Oliyamur

trachytes farther south (Plate 39)•

There are three thick ash-flow tuffs within this area. The

lower one occurs in the river bed of the Katatarope and its tributaries

and is about 160 feet thick. It consists of eutaxitic rock containing

dark green fiamme in a pale green ma,trix and shows little vertical var­

iation being welded throughout, At 783660, this tuff is overlain by three

green porphyritic trachyte flows. The succeeding ash-flow tuff is about

220 feet thick and is largely unwelded except for the bottom 80 or so

feet. Here the tuff consists of unbedded green-brown pumiceous material

containing trachyte fragments of a variety of sizes. The lack of both bedding and sorting, together with the gradational passage into eutaxitic

rock below are parameters which distinguish such unwelded pyroclastic

flows from air-fall deposits. This flow is welded throughout in the

Ediiteniro gorge thus demonstrating that the degree of welding decreases with increasing distance from the source.

Two, green, aphyric, trachyte flows occur above this tuff as

782660 whereas only one was recognized in the Ediiteniro gorge at 755680

(Pig, 36). Above the trachytes occurs a thickness of about I40 feet of pyroclastic rocks, stratigraphically the youngest known deposits of the

Nasaken complex. At 780638, 50 feet of orange-green, flaggy, eutaxitic, welded tuff are overlain by about 40 feet of bedded ptimice-lapilli tuffs.

The latter are capped by a thickness of about 5O feet of grey-green, porcellanous welded tuff which has a crude columnar jointing. r %) i g CO X> I fi

CQ Sm â •s I

co Is;n3 fi I Î3 (H 4 - t o "d ■H (D cd •H "H 4h E-h (H X) B "3 X) 44I I x: c3 : oorn (D L'5- -P fi A. Plate 39: The flank deposits of the Nasaken volcano

dip gently beneath the Oliyamur volcano in

the distance. 81

x) The morphology of trachyte flows

Most of the lava flows of Nasaken are composed of fissile

trachyte. The fissility is due to the marked parallelism of alkali

feldspar laths which is produced during flowage. The development of

fissility is also dependent upon the rate of cooling of the lava, chilled,

fine-grained trachytes being massive rather than fissile,

A vertical section through a typical trachyte flow (that ex­

posed in the west bank of the Nasaken at 730784) is illustrated in Pig,

39, Zone 1 is essentially a coarse, mechanical breccia formed by the

flow on the vesicular top of the preceding one (Plate 40) in response to

flowage in the still molten interior of the lava. Within the breccia fine­

grained, dark green fragments of chilled trachyte can be distinguished

from the vesicular, purplish ones derived from the underlying lava.

Zone 11 contains smaller clac ts which have been welded together by the rapid solidification of a trachyte matrix. Above is the chilled zone

(ill) which is free from trachyte clasts (Plates 4^ and 42), The fissile

zone (rv) constitutes between about 80-90% of the flow thickness.

There is a certain amount of pétrographie variation between

the zones established, Tiachyte from zone 11 and the base of zone 111 is very glassy and contains few feldspar phenocrysts. The fine-grained rocks of these two zones contain much aegirine as small interstitial prisms, often fringed with arfvedsonite. In the holocrystalline fissile zone, arfvedsonite is abundant and aegirine sparse becoming absent higher up.

It may be that the reaction --

aegirine ^ arfvedsonite goes to completion in the slowly cooled, middle and upper parts of the flow. It is in this zone that the mossy texture consisting of poikilitic arfvedsonite enclosing feldspar laths is developed on a coarse scale and phenocrysts may constitute about 10-15 per cent of the rock. Fig.J9VQrtico! sQct/on through a typical trachyte flow

\ T vulculoj^ : /ki oMfi S!^ ff vSic&ô. LLfiiocurrU . V^àic&o e i ^ g c U e d ^C4^) & M £ ^ / c M . 7%1 /ô~p Jûtté Iy p ^ Mû. Lo J'UApilok iM Cc^auA' oLfJi to ox/djtt/cM . olaaA ha. kuAO l^û iCuCceûdiuû^ à iô . TtiyoO (kô et OiM u kûtû^tK S /X e f /-h ^ fkû UMÀüJt^lMif t:ntLCl^^ÙL. b

jV fOytdU. tYGLclMjtk. : tbjL^ c^dû 'W^atj h & ' t h oaaA’ (XCYtriô. Th. tyAcU^ \ :

J/n. /O f t .

)> m ^ ( c k t t lt d ) 4 - t 1- cL^k (FytLclc^^ e <» _ — - -"o -a o FL^CI o .’3 0 CO, ^

1

Plate Ul: The basal breccia zone has been eroded out above the vesicular top of the preceeding flow. At the base of the massive trachyte can be seen the welded breccia. Plate U2: The zonal nature of a typical trachyte

flow: The basal breccia (zone 1) has largely

been eroded out. The welded breccia (zone 2)

can be seen below massive trachyte (zone 3)

at the hammer. Fissile trachyte (zone L)

can be seen behind left. 82

6(g ) Kanatim volcano

This Pliocene trachytic volcano is situated to the south­ east of Nasaken and has been mapped by S. Rhemtulla (pars, comm.).

Within the area mapped by the author» pyroclastic products of Kanatim outcrop over an area of about 7 square kilometres between the Edilte and Katatarope rivers.

Considerable erosion of the Nasaken complex took place prior to the eruption of Kanatim. The Kanatim tuffs rest in an erosional erabayment within the Nasaken rocks and accumulated on a gravel-strewn pediment.

At 779740 a Kanatim welded ash-flow tuff rests upon about

18 feet of reddened waterlaid gravels. Coarse, rounded boulders of trachyte at the base of this deposit rest on trachyte lava. The welded tuff can be traced into the succession at 788756 (Pig. 4o) where it occurs above a thin trachyte, tuffs and an aphyric hawaiite which represent the local Nasaken succession. The pumice tuff occurring above the welded tuff has a thin black soil developed upon it. These two units of the

Kanatim volcano are flat-lying between the Edilte and the Katatarope.

South of the latter the beds are inclined at about 10° to the north-east and rest unconformably on the trachyte lavas of the Karimorega centre.

East of the road the ash-flow tuffs of the Nasaken flank deposits dip beneath the Kanatim strata and this unconformable relationship is well illustrated in Plate 43»

6(f) The South Turkana group

The northern part of the Gregory rift is characterized by a group of trachyte, multicentred stratovolcanoes of which Kafkandal,

Nasaken and Ribkwo are examples (McClenaghan et al., 1971)« These form a continuous north-south trending belt between 1° N and 2° N on the western side of the rift (Pig. ^^) and are represented on the eastern side Fig. 40 Successions of Kanatim and Nasaken rocks along the Edilte river.

788 7S6

é'yûujM. pxHX/cc. K n F

779740

J a.jgcJ ,

WdcitA^ed ^ caaJhattaiM jtte M / j tc. jrs^vtCs (J/ 7L 4mAMdc4 z % a htdcUd ofik c i u d tuji' èot(.tdôri ( f Â’-’-ÂV-A-a -(L^^CcMtCŸtiài a d h ^ s c x b 6aS€. xpbjjHc , t-Ttychtc Cxvx Lx ù ü d u à t .

r 0

feef

-fO

l20 J ô ' o O E j ô '/s 'e 2 D O N

The strati graphical sequence is as follows

0 1 — O h y a m u r / Kp— Kaptagni r 4 5 ' N K n K a n a t i m K c Kachita f/s ^Nasaken R b — Ri b k w o K f — Kafkandal also K! — KaUime rlim

_ /'j o W K f

_/ /J/V

/0 kms.

r o o N

^ig.4/ The South Turkona group of Pliocene trachyte volcanoes 83

by the Tirr Tirr Series (Baker, I963 ? Dodson, I963 ) and volcanics yet to

be mapped east of Silali and Emuruangogolak (M, Golden, pers. comm.).

The trachyte volcanoes of South Turkana are morphologically

transitional between shields and plateaux. Some have the form of low-

angle cones (Ribkwo, Olyamur); others are essentially flat-lying without

quaquaversal dips (Nasaken). Nasaken may be thought of as the product of

a type intermediate between fissure and central volcanism in that the

grouping of the source areas is elongated along the main structural trend

and early lavas emanated from dykes and sheets, the intrusion of which

was facilitated by the faulted monoclinal structure. To the south where

the faulting of this flexure dies out, Ribkwo displays a more central

appearance and even possesses a small caldera.

These trachyte volcanoes appear to form a type not previously

described. They are in general very large low-angle, multicentred

shields or plateaux consisting almost wholly of trachyte lava and a high proportion of trachytic pyroclastic rocks, Vdien preserved, a stratiform

* flank zone ' made up of even and extensive lava and pyroclastic flows,

contrasts sharply with the 'central zone' in which the eruptive vents were concentrated. This source zone is characterized by short, irreg­ ular trachyte flows, dykes, plugs and thick pockets of pumice tuffs.

Ash-flow tuffs are rare tending only to accumulate on the flatter ground of the flanks or in depressions. The volcanoes typically do not have

calderas or parasitic cones and craters are not always clearly defined.

The 'flank zone* deposits originated from within the 'central zone* and dip away from the source areas. The steep dips, irregular topography and the occurrence of thick pockets of relatively soft pumice tuffs facilitate more rapid erosion of the source zones compared with the flanks. The deposits in the latter may therefore be considerably younger than those preserved near the sources.

The South Turkana volcanoes together with the other occurrences 84 of this type in the Kenya rift may well constitute the world’s largest accumulation of the products of trachyte volcanism, certainly of

composition.

6(g) Geo chronology of the South Turkana trachyte volcanoes

The trachytic activity which gave rise to the South Turkana group of volcanoes was more or less continuous from about 6 million years b.p. to about 2 million years b.p. The time span of individual volcanoes or complexes was of the order of 0,5-1 million years. The foci of act­ ivity moved eastwards with time, toward what is now the central graben of the rift. In the Suguta valley, trachyte lavas from Oliyamur are buried by those from the Quaternary volcano Emuruangogolak, Late Pliocene trachytes also occur east of Emuruangogolak, stratigraphically above

Pliocene basalts. Thus the disposition of the major volcanic formations is more or less symmetrical about the Suguta graben at this latitude

(l° 50' N). However the Pliocene trachytes east of Emuruangogolak are more deeply dissected than those of Oliyamur and may be older. No dates are yet available for the Pliocene trachytes on the eastern side of this section of the rift except for the following three determinations on a single specimen from the Tirr Tirr trachytes (Baker, 1965)? 5»9 + 0.4,

5,8 + 0,4 and 5,6 + 0,4 million years (Baker et al., 1971),

Three K/Ar whole-rock age dates are available for the Kaf­ kandal volcano, A date of 5«9 ± 0,5 m.y. has been obtained for a trachyte lava (specimen 8 /584 ) from 67565O within the Ngapawoi unit. A specimen

(8 /573) from the Epong unit (at 727653) yielded a date of 5*7 ± O.5 m.y.

Since Moru Angitak lavas probably interdigitated with Nasaken volcanics of about 5,4 m.y. in age, the span of the Kafkandal volcano may be in­ ferred to be of the order of 0,5 m.y. A specimen (8 /6OO) from a Moru

Angitak feeder dyke has given an age of 6.2 _+ 0,5 m,y. which, because it is inconsistent with the established stratigraphy, may be too old. How- Plate 43: The Kanatini-Nasaken unconformity.

Nasaken flows (foreground) dip beneath

Kanatim laves (background).

Plate 4L: Cemented boulders and river gravels

at Nasaken. 85

ever all three dates on Kafkandal rocks overlap within the limits of experimental error and it may be that Kafkandal was built over a shorter period tha^ 0,5 m.y, although reference to the dated durations of other trachyte volcanoes suggests that this may not be the case.

The Lodukai dyke has been dated at 6,1 + 0,5 m.y. (specimen

8/67a).

K/Ar age determinations on trachyte specimens from the Nasaken

are as follows :

8/7a 735778 5.5 ± 0.2 m.y.

8/89 744763 4.3 i 0 .5 m.y.

8/205 754690 5 .4 + 0.3 m.y.

8/218 745684 5.7 ± 0.2 m.y.

8/314 814665 5.7 + 0 .4 m.y.

In addition specimen 6/121 collected by Rhemtulla from a position 5 kms. north-east of Nasaken has given a date of 5-6 + 0.5 m.y. A discrepancy exists between the dates given by specimens 8/7a and 8/89 which are from the same flow and it seems likely that the date obtained from the latter specimen is too young. The closely grouped Nasaken dates suggest that the trachytes which make up the Nasaken complex were erupted continuously and rapidly over a period of time probably less than 0,5 m.y.

A welded tuff (specimen 8/244) from near the base of the

Kanatim succession (787753) has yielded an age of 4»8 + 0.2 m.y. whereas a trachyte from near the top of the succession has given 5«8 ± 0.5 m.y,

(specimen 6/15),

Trachytes from the Oliyamur volcano which overlies Kanatim have given ages of 2.7 + 0.2 (specimen 6/l8) and 2.4 + 0,1 (specimen

6/92 ). Age dates obtained from the Ribkwo volcano range from 5«2 m.y. to

4.4 m.y, (McClenaghan, I97 I; Webb, I971 ). Trachyte flows provisionally assigned to the Nasaken lavas by Webb and yielding dates of 2.6 and 2.9 m.y, 86

may be almost certainly attributed to the Oliyamur centre owing to their similarity in age and proximity to this volcano.

Age determinations on lavas from the Pliocene trachyte volcanoes are summarized in Pig. 42

7 . Recent deposits

The Recent deposits of the area are those formed by the active erosion of the volcanic hills. The main stream and river courses are marked by large rounded trachyte boulders and coarse gravels. In the river banks at Nasaken gravels have become cemented due to the precipit­ ation of salts from the hot spring water (Plate 44).

Flat areas and pediments are covered with gravel wash, many of the pebbles being coated with a glossy 'desert varnish*.

Thin black soils have been developed on the Kanatim pumice tuffs and on flat ground underlain by the Napeitom and Lotokongolea basalts.

The only other deposits of recent origin are mud volcanoes occurring in the Nasaken river at Nasaken. A number of old mud volcanoes can be-seen downstream from the road within a distance of about 500 metres. Over a period of three days in mid-October I968 , a mud volcano appeared and grew in the bed of the Nasaken about 40 metres upstream from the road crossing. The volcano consisted of a low cone about 8 feet across and 5*5 feet high. Its walls were composed of dried mud containing pebbles and boulders such as occur on the river bed (Plate 45 ) « The walls enclosed a pool of red-brown liquid mud which was perched at least 10 feet above the water-table within the sand river. The wet mud remained for a period of about 5 months during which time water lost by evaporation must have been replenished from below. The wet mud was always cold and ex­ tended to a depth in excess of 8 feet. Twice the liquid mud was able to Ut ç o <« (/) b co •S cr> O g H Q o Çj C>) "ü .S w o

X Qj ç L o X co C) «s >o *o M L_ _JL _J_ _ l_ _L_ 7 ^ H jnUJD/f/Q h— H b h C F - * I O A: L. tJUfgûUD)/^

I—#— I c -I . Z^î— -H I------———H o (/)

s<. I— '— I O I— •— I (U6l'99^M)oM)ig!y I "— 4 X. o( > o I *— ( Q h • I O I--- #--- 1 k . ÜO)jfOSOh/ C \ » 1 O I ------1 O I—m 1 CD

I----- •—I I # I /OpuO)fJD)l f\j 4—— »-- \ \h (//6/'9Ç3/^)’^oân/)/( l -4 dkfXp fokfnpoj^ H #-- 1 I “ I "T" é < ) *o M f\l Plate 49: Mud volcano in the bed of the Nasaken

river.

as

%/-f . ' , TSr' S_ - »•

Plate 46: Mud flows from the volcano. 87

break through the walls and gave rise to small flows (Plate 46),

Following the mud flows, the mud remaining within the walls hardened and collapse produced a caldera-like structure (Plate 47 ). Later fresh liquid mud was introduced from below but eventually the volcano dried up and became completely solid. The cause of the mud volcano is unknown.

It appeared about one week after a heavy rain shower and it may be that rain water found a new channel in the river bed to connect with the spring water which provided the necessary hydrostatic pressure head that was maintained for some time. The spring water issuing from the river bed about 50 metres downstream is however hot (c. 45° C) and no diminut­ ion of flow occurred during the period the mud volcano was active. Plate U7: Caldera-like structure formed within

the mud volcano after a mud flow. CHAPTER 3

STRUCTURE 89

1. Regional Structure

The Nasaken area is situated in the northern part of the Gregory

rift valley at a latitude where the rift is widening into a less

well-defined structure (Fig. 7). The eastern boundary has not yet been

geologically examined in detail but it appears at this latitude to be

a monocline affected by relatively minor faults. West of the Nasaken

area in the Kerio Valley, the Kula fault system is a continuation of

the main Elgeyo fault (Fig. 1a3). South of Tot, the latter has a throw

approaching 10,000 feet (3000 metres) and that of the Kula faults may

be several thousand feet. In the Kerio Valley north of Tot the

*Turkana grits* consist of over one thousand feet of conglomerates and

sandstones (Pig. Wi) largely composed of Basement-derived material.

Fragments of volcanic rocks occur near the top of the sequence. The

sediments are the result of the erosion of an eastward facing escarp­

ment formed by early (pre-volcanic) movements along the Kula faults.

Conglomerate *wedges* thickening westwards testify to repeated movements along these faults* Laiteruk (6L70 feet) and Kailongol

(6780 feet) are relics from the erosion of the Kula scarp. Movement

along the Elgeyo fault almost certainly took place at the same time but the present and its continuation towards Uganda

are essentially younger features.

The Kamasia fault (Chapman, 1971) (Fig. 7) has a total throw of

about 6000 feet (I8 OO metres) and its continuation northwards is the

Saimo fault and the Klto Pass faults. The Kamasia-Kito Pass line marks

the limit to which Basement rocks can be proved to occur within the

rift on the western side of the central axis. McClenaghan (1971) has suggested that faulting took place along the Kito Pass line before the onset of volcanism. Thus major escarpments were produced along 3 6 E 3 6 2 0 E 2 N

L A I T E RU

! 3 0 N

T / A T!

TOT R!BKWO

0 45 N

F/g .43 R Qgio^nal S tructur e

major faults only ara shown k/

Lai ta r uk

6 0 0 0 -

trachypbonoHtQ [Nakasuw) c. /2 m.y. trach yte àosa/tsj / gri^ c./Sm.y.

Section from the western A/asaken Kamuge valley . bosa/ts 8 ~ 6 /i trachytes 6-5m.y. t rachyte c. 5.3 rn.y.

line of section approximately east-west , A —B n o r th in g 900 B - C 2 4 6 9 0 0 to 33 2 8 4 7 , . - C~D northing 847 [latitude l'40 N)

Suguta valley ^ along latitude / 40 N E

Suguta river - 6 0 0 0 i- 5 ft, Recent Suguto — ^ Kamuge valley sediments, ba sa lt\ V-

D 90

lines of the Elgeyo-Kula and the Kamasia-Kito Pass faults in the

Lower Miocene (or earlier) and constitute the first recordable rift

movements•

The Kito Pass faults die out south-east of Tiati (7713 feet) and

it is largely to this non-continuation of the faults that Tiati owes its

elevation. East of Tiati the strata are downwarped towards the centre

of the rift in a gentle monocline (Webb, 1971)• Farther north this

pattern of faulting continues and the Napeitom faults of the

author*s area are evidently an extension of a major structural

lineament which runs from the southern Kamasia to Lake Rudolf*

On a regional scale the rift faults are parallel to Pre-

Cambrian features and there is much evidence that the fault pattern

has followed a variety of Basement grains (Dixey, 1956; King 1970).

2. Structure of the Nasaken area

Within the Nasaken area major and minor faults of all ages have

a dominant north-north-east/south-south-west trend. This Is the

average strike direction of the foliation within the Basement rocks

of the region. Structurally, the Nasaken area is not complex and is

dominated by the Napeitom step-faults (Fig. 16). These are normal,

synthetic faults with small hades. Basement rocks do not outcrop east of the faults and the total effect of the faulting is a down­ throw towards the centre of the rift of at least 2500 feet (750 metres).

On the upthrow side of the main fault vest of Cheror, basalts of the lower, aphyric Kagumnyikal unit occur at liliOO feet. On the downthrow side, the top surface of this unit is at 3000 feet. Since 91

the main fault dies out rapidly towards the south, the throw farther

north near Kagumnyikal is likely to be greater than the lUOO feet

(U20 metres) minimum throw estimated near Cheror.

Judging by the vertical displacement of the Nasaken trachyte-

Tirioko basalt junction, the Balakaicha fault opposite

Kagumnyikal has a minimum throw of 900 feet (270 metres) (Plate 1|8).

Whereas towards the south, the Napeitom faults die out and are

replaced by the Tiati monocline (Webb, 1971) towards the north the

major fault (that west of Kagumnyikal and Cheror) passes into a number

of synthetic splay faults. These are continued ’©n echelon’ by those

mapped by P. Truckle north of the area and west of the line of the

Napeitom faults (Fig, h3)*

The most recent movements along the Napeitom faults have

produced a series of near-vertical scarps (Plate 1:9) in Kowun

trachyte, often with thick fault breccias at the bases of the scarps.

Within the fault-blocks west of Napeitom, the Kowun and Tirioko lavas

are back-tilted and dip generally at angles of up to 10 degrees to the west. Lotokongolea basalt immediately on the downthrow side of the main (most westerly) fault has suffered fault drag such that the lavas dip 5-10 degrees to the east and the fault here is marked by a breccia zone composed of secondarily silicified, angular fragments of hydro thermally altered trachyte and basalt.

Repeated movements have taken place along the major faults.

Different types of Lotokongolea basalt resting upon Kowun trachyte in adjacent fault strips imply the existence of fault scarps before the basaltic eruptions and the restriction of the basalts by these scarps.

The junction between the Napeitom basalt and the Nasaken Plate L8: Kagumnyikal from east of the Balakaicha

fault at 697733. The top flow is of

trachyte. In the foreground are trachytes

and tuffs of the Nasaken complex. The

estimated throw of the fault here is about

900 feet.

Plate U9; Fault scarps in Kowun trachytes looking south­

west from near 700860. Kowun summit is in the

background; Lotokongolea basalt is in the

foreground. 92

trachyte from 731:837 to 723803 is veiy steep. Here the trachytes are

banked up against a near-vertical fault scarp within the basalts.

At its southern end the Nasaken lavas bury the fault scarp itself.

Along the length of this scarp trachyte infills small erosional

embayments and in a few places trachyte remnants are preserved

against the scarp of the basalt. The formation of this fault scarp

within the basalts must have immediately proceeded the eruption of

the Nasaken trachytes. It is also probable that the main movements

on all the Napeitom faults are the same age and pre-date the Nasaken

trachytes. Movement along the Balakaicha fault in part post-dates

Nasaken but the increased thickness of trachyte east of the fault

suggests that a scarp was present against which the trachytes flowed

and that much of the inferred minimum throw of 900 feet (270 metres)

could have been pre-Nasaken in age.

In summary, it seems likely that movement along some or all of

the Napeitom faults took place at least three times. The earliest

movements were post-Kowun trachyte and pre-Tirioko basalt in age and

formed scarps against which the Lotokongolea basalts were ponded.

The second, major phase of movement, immediately proceeded the

trachyte volcanism of Nasaken and Kafkandal. The faults appear to

have had little influence on the forms of the Kasamanang trachyte

intrusions. The form of the intrusion illustrated in Fig. 22 is the main exception.

The Nasaken, Kafkandal and Kanatim lavas were only affected by minor faulting without any systematic direction of throw. In spite of their small throws and limited extensions these faults are strongly aligned along the regional. Basement trend. Quite minor faults may have thick zones of brecciation (Plate 5o). Plate 50: Small fault within Nasaken trachytes at 720750,

The fault breccia zone is about 5 feet across. 93

The’upward’ limit of the faulting affecting the Turkana basalts

and Kowun trachytes west of the watershed is unknown and the volcanics

are gently arched about an approximate north-south axis.

Pyroclastic rocks of the Turkana basalt formation west of the

Nakasuw river dip gently to the west whereas lavas of the same

formation dip gently to the east beneath the Kowun trachytes east of

Kanitiriam. Erosion across the axis of this arch has exposed metamorphic Basement. The Kanitiriam intrusion is situated along the

axis of the arch but only locally disturbs the strata. West of the

arch, the strata are either horizontal or dip gently towards the

Kerio while eastwards the lavas are downwarped towards the centre of the rift (Fig. kh)->

The Turkana basalts dip more steeply to the east than the

overlying Kowun trachytes which have themselves been tilted in the same direction. This pattern of easterly dipping formations is interrupted by the westward tilting in the Napeitom fault blocks.

East of the Balakaicha fault the lavas again have a general eastward dip. The Tirioko basalts dip beneath horizontal Kafkandal trachytes at angles of about 10 degrees. On the eastern side of Kafkandal, lavas of the same Ngapawoi unit dip at 20-2^ degrees towards the centre of the rift. Thus an eastwards tilting of about 10 degrees of the whole Kafkandal volcano is implied (see Webb, 1971), East of Kafkandal the Nasaken flank suc«ession dips gently beneath

Kanatim lavas. Progressive gentle tilting towards the centre of the rift has taken place so that the dip of the older formations is generally greater than that of the younger (Fig. LL). 94

3. Form and structure of the major volcanic units

Detailed descriptions of the volcanic units of the Nasaken area have been given in Chapter 2* This summary outlines the main structural features of the different volcanic formations within the northern part of the Kenya rift. Basaltio ’flood* eruptions The basalt formations are very broad, almost flat domes with numerous, thin, overlapping flows. These plateau-like formations are formed of many flows from widely separated vents overlapping one another (e.g.: Kagumnyikal, Napeitom and Lotokongolea basalts).

Lavas of one shield-like volcanic centre rest with small angular discordance on those of another. The slopes of the individual shields are typically less than 5 degrees but are often difficult to recognise in the older, eroded formations, a situation analagous to that in Iceland (Rutten, 1964). The flows are of the pahoehoe type and ash and tuff are typically absent; characteristics that are generally true of basaltic flood eruptions (Macdonald, 1972). In some places parallel fissure source areas have been proved; an example is the dyke swarm in the Kaparainabasalts (Chapman, 1971).

The ’plateau' basalt formations of Kenya are structurally similar to the Snake River basalts and Columbia River basalts (for general descriptions, read Macdonald, 1972). The flood basalts of the north-western U.S. are more voluminous and generally angles of slope are less than 1 degree. Similarities also exist between the

Kenyan ’plateau’ basalts and the Icelandic shield volcanoes

(Thorarinsson, I960) which have slopes averaging 7 to 8 degrees.

’Plateau’ phonolites

The ’plateau’ or flood phonolites of Kenya are very extensive 95

flows which are locally very thick infilling the underlying topography. They are commonly thought to have been erupted from either rift-margin fissure zones (Wright, 1965) or fissures on the crest of the Kenya dome (Williams, 1970). Examination of these areas has failed to identify linear dyke swarms or fissure sources and the detailed investigation of the Kamasia area by Lippard (1972) suggests that the ’plateau' successions are the flanks of large, low-angle, coalescing shield volcanoes (Lippard, in press) with radii of 35-40 kilometres (22-2^ miles). The phonolites must have ' been erupted rapidly in a very fluid condition and in enormous volumes.

Multicentred trachyte volcanoes '

The multicentred trachyte volcanoes of the South Turkana group

(Ribkwo, Nasaken, Kafkandal, etc.) are of a type not previously recognised. They are low-angle, shield-like edifaces up to $0 kilometres (30 miles) in diameter and consist almost wholly of rocks of peralkaline trachytic composition, including both lavas and ash flows. Flows of basaltic and intermediate composition are uncommon. A stratiform flank zone built up of extensive lava and pyroclastic flows contrasts with a more or less centrally located area in which the eruptive vents were concentrated and containing a high proportion of air-fall pyroclastic material. The source zone is typified by the presence of short, thick, lava flows, plugs, tuff craters and dykes which often occur in intense swarms over very limited area. Generally, the volcanoes do not have a clearly defined single crater or centre and calderas are characteristically rare. A small caldera, 4 kilometres (2.5 miles) in diameter occurs on Ribkwo (Webb, 1971). 96

The northern Kenya trachyte volcanoes are comparable in size

to the strato-volcanoes Mont Dore and Cantal in the Auvergne

(Jung, 1946), These too have a large proportion of 'differentiated*

lavas but of alkaline and calc-alkaline affinitios #

Several individual trachyte flows in Ribkwo, Nasaken and

Oliyamur taper out in all directions away from the central source

area. Similar **lenticular discs" have been described by Vincent

(1963 , 1970 ) from the Tibesti province, Sahara but these are ash-flow

sheets not lavas. The "ignimbrite" shields of Tibesti are central

volcanoes fed from relatively small central vent areas but the

actual feeding conduits are dykes of rhyolitic and trachytic

composition.

The South Turkana volcanoes are composed of trachytic lavas which were erupted in a very fluid state and in large volumes, khyolite'

dacite magma has been erupted in large volumes in the south-western

U.S. but the fluidity was caused by the movement of magma as ash-flows

(Smith and Bailey, 1966) moreover these large ash-flow deposits are usually associated with calderas. Although the rocks of the ash-flow centres in south-west Nevada, are of the calc-alkaline type, the

Black Mountain and Silent Canyon centres are characterised by peralkaline rhyolites and quartz-trachytes (Noble, 1968)

Central volcanoes

The Quaternary central volcanoes of the Kenya rift (Paka,

Suswa, Emuruangogolak, Silali, Menengai, Teleki's and Longonot) are typical composite volcanoes. They consist of stratifications of lava and tephra and each one has a caldera. The central volcanoes have a conical or domal form with angles of slope typically 10-20 97

degrees. The salie lava-type is usually trachyte which ranges from phonolitic to pantelleritic affinity. Unlike the Pliocene trachyte centres, the Quaternary volcanoes may have high proportions of basaltic rocks (Paka - 25 per cent, Emuruangogolak -

35 per cent, Silali - 45 per cent). CHAPTER h

PETROGRAPHY 99

1, Classification.

The volcanic rocks of the area are members of the Alkaline

Olivine-Basalt Association, Their classification is by modal mineralogy

and is based on the nomenclature of Macdonald (1960), Muir and Tilley

(1961) and Tilley and Muir (1964)* There is gradational variation in rock

types between the members of the association but in general the modal

criteria of the classification can be applied. Difficulties do arise

because of fine grain-size^ the presence of all three components in the

feldspars together with zoning and in the determination of the precise

nature of the interstitial groundmass material. In such cases bulk chem­

istry and norms have used to assess the modal mineralogy. It is reassuring

that most rocks named here on modal criteria receive the same names using

the chemical classification of Irvine and Barager (l97l)* Note that

’trachymugearite' is preferred to 'benmoreite*,

The full modal classification used is shown diagrammatically

in Fig, 45* The nomenclature of the under saturated ()> 5 per cent modal

nepheline or feldspathoid) rocks which are not well represented in the

Nasaken Area, is that of Chapman (19713-9 b). Trachyte is defined as having

less than 5 per cent quartz or nepheline (+ feldspathoid). All the trach­

ytes described are alkaline and a qualification as to the dominant alkali

is made. Hence many of the Nasaken lavas are termed soda trachytes, like­

wise those containing 9-10 per cent modal quartz may be termed soda

quartz-trachytes (although 'quartz-trachyte’ is preferred for brevity).

Lavas with)> 10 per cent modal quartz are called alkali rhyolites, those

from the author's area being soda rhyolites.

The classification of the alkali rhyolites used in the recent

literature has a normative basis in view of the fine grained or glassy nature of these lavas which include pantellerites and comendites. An

arbitrary division at 12,5 per cent normative femics was suggested by

Lacroix (1927 ) between comendites (12,5)» NOMENCLATURE OF THE ALKALINE OLIVINE- BASALT

V O L C A N 1C A S S O C ! ATI ON ^ FQldspar bearing rocks^

M o do! -I- /S2/o(s^(7f/)o/(ys ObC7rfz

I S 1 0 S 5 10

A ikaii i I Trachyte o§ rhyoiite feldspar

N O R M A T i V E CLASSIFICA TION Nepheline — Trachym ugearite

trachy— Benmoreitej y 'A alkali mugeoHte feldspar

Ana/cite — 3 0 - {nepheiine^ Oligoclase Mugeatite mugeorite dominant 20 -Com endit e Pante lie rite An Anoicite — /O - 1 andesine {nepheiin^ Hawaiite Comenditic Panteiieritic hawaiite ^ dominant trachyte trachyte ■■ — 1— 0 iO 20 40 1 Basanite Aikaii oiivine - abradonte femics y ephnt e basait F ig 45 : dominant Fig 46 . /o 100

This definition is that currently used by most workers on peralkaline

silicic lavas (Noble 1968; Macdonald et al. 1970, Sutherland 1971) and is

applied here when chemical analyses are available. Since the division

between trachyte and rhyoiite is taken at the conventional 10 per cent

normative quartz, four fields may be defined on a plot of normative quartz

and femics (Fig. 46); those of pantellerites, comendites, pantelleritic

trachytes and comenditic trachytes. The modal and normative classific-

atory schemes for the silicic rocks are not completely compatible.

Because of the combining conventions of the C.I.P.W, norm, modal estimates

of quartz appear to be about 5 per cent higher than the calculated values

of normative quartz. Thus pantelleritic trachyte roughly includes quartz-

trachytes and alkali rhyolites with 10-15 per cent modal quartz. No

attempt has been made to define a modal equivalent of Lacroix's 12,5 per

cent femics division but in pétrographie tables, an estimate of the per­

centage of dark minerals in trachytic and rhyolitic rocks has been made,

A modification of Lacroix's division currently being proposed

(Macdonald and Bailey, in press) is, in the present case at least, unsat­

isfactory, An 'arbitrary' line on a plot of normative quartz v normative

femics, together with the 10 per cent normative quartz division defines the

same four fields as before (Fig, 47). This line has end points qz=40 per

cent, femics=9.2 per cent and qz=0 per cent and femics=22 per cent and has been defined by the distribution of over 100 analyses of comendites and pantellerites, yet only 9 analyses have been used to define the fields of

comenditic and pantelleritic trachytes. Lavas from Nasaken (and other peralkaline trachyte centres in Kenya) define a trend across and almost at

ïight angles to the line dividing comenditic and pantelleritic rocks.

Thus some lavas would be called comenditic trachytes, others pantelleritic

trachytes, and all these are from a single suite of closely related rocks

the end products of which are pantellerites. Clearly the associated trach­ ytes are of pantelleritic affinity and the use of the earlier definition I/O y E

CD > \0. _ o O E O

_ o

z4Jonb 9A14D01JOU 101,

of Lacroix demonstrates this.

Other rook names used are based on the following definitions

Piorite-basalt — olivine-basalt with> 30 per cent phyric olivine and

augite in roughly equal proportions.

Ankaramite — picrite-basalt with phyric augite in excess of phyric

olivine.

Feldsparphyric-basalt — basalt or olivine-basalt withi> 30 per cent

phyric plagioclase,

Nephelinite — highly undersaturated basaltic rock devoid of feldspar,

Ankaratrite — a biotite-olivine-melanephelinite,

Microfoyaite — medium-grained nepheline-syenite with perthitic or

antiperthitic feldspars.

Microsyenite — the medium-grained equivalent of trachyte,

Pulaskite-syenite — a sodipotassic leucosyenite (Read 1931) consisting

of antiperthitic feldspars and about 10 per cent or

less mafic constituents. It is saturated with respect

to silica containing neither modal quartz nor

nepheline (or feldspathoid).

It is important to note that the terms 'basic', 'intermediate'

and 'salic* are used by the author to describe the chemical and pétro­

graphie variation of the rock types encountered within the thesis area,

'Basic' is used to describe rocks with a silica content of less than 50 per

cent, *Salic' is used in the sense of Cross et al, (1903) to describe cer­

tain rocks and mnemonically recalls the siliceous and aluminous character

of the minerals within those rocks. Since trachyte is the normal salic product, the term 'intermediate' is used to describe lavas which are

intermediate in composition between trachyte and basalt. Thus mugearitic and trachymugearitic lavas with Si02 between 30-58 wt. per cent are desig­ nated 'intermediate'. The author is aware that his use of these descript­ ions as terras of convenience constitutes a mixing of terminology but it 102

facilitates a useful tripartite division of the chemical and pétrographie variation between basalt and trachyte.

2. Mineralogical Components. a) Plagioclase Feldspars The Anorthite contents of the phenocryst and groundmass plagioclase feldspars have been determined using recognised twin-1aw tech­ niques, although phenocrysts of the basic and especially the intermediate rocks are often strongly zoned. It is on the measurement of the An content that the definitions of hawaiite and mugearite are based. The anorthite content ranges from Acqq in ankaramites and olivine-basalts to about Ango in some mugearites. Plagioclase feldspar is absent from the trachytes of

Nasaken. Polysynthetic twinning according to the albite law is always present but becomes indistinct in the oligoclase range of composition,

Carlsbad twins are also common.

Phenocrysts are often tabular and tend to occur in glomero- phyric aggregates, sometimes showing partial resorption. Some basalts from the Tirioko formation (8/II9, 8/122) are highly feldsparphyric with pheno­ crysts up to 3 cms. in length but typically 0,3-1 cm. One flow was en­ countered with large plagioclase phenocrysts making up about 70 per cent by volume of the rock and showing a strong fluidal texture,

b) Alkali Feldspars Alkali feldspars are the most common minerals in trachytic rocks where they occur both as phenocrysts and in the groundmass. Inter­ stitial alkali feldspar is present in mugearites and probably in hawaiites.

From their composition and optical properties the volcanic alkali feld­ spars belong to the high albite-anorthoclase-sanidine series (Tuttle,

1952 ), in which the optic axial plane is perpendicular to

(010), The series is divided into anorthoclase (^ Or^y) and sanidine

(^Or^y) by the change in symmetry from triclinic at the Na rich end to 103

mono clinic at the K rich end. Compositions in the range 0^25 - Orgo on a very fine scale (< iyu.) which is detectable only by X-rays, thus giving anorthoclase and sanidine crypto-perthites.

The symmetry change is temperature dependent occurring at the composition Or^^Abg^ at room temperature. Above 1000 C, compositions nearly up to pure albite are monoclinic (Beer et al. I963), Anorthoclase usually shows cross-hatched twinning because it originally crystallised with monoclinic symmetry and inverted on slow cooling. However, alkali feldspars which do not show cross-hatched twinning have been shown to be of anorthoclase composition (S/89, 8/206b - Table 4) and presumably either cooled too rapidly for inversion to take place or the twinning is sub- microscopic, The absence of cross-hatched twinning,therefore, cannot serve to distinguish sanidine (>Or^y) from anorthoclase, but feldspars exhib­ iting this twinning must be anorthoclase (<(0r^^) in composition.

The X-ray diffraction method of Carmichael and Mackenzie (1963) using the 201 feldspar reflection and an internal quartz standard has been used to determine the Or content of alkali feldspar phenocrysts from

Nasaken trachytes. The range in composition measured is not large

(8^2.2 “ and in most cases unmixing has taken place. In some pheno­ crysts the unmixing is complete. The compositions determined all fall within the anorthoclase field so that all feldspar phenocrysts, whether showing cross-hatched twinning or not, are referred to as anorthoclases.

This adoption of the strict chemical definition is in contrast to the practice of using anorthoclase for cross-hatched feldspars and sanidine for feldspars in which cross-hatched twinning is absent (McClenaghan 1971»

Webb 1971)9 definitions which have no compositional control.

Table 4 gives X.R.B. determinations of orthoclase contents and average optic axial angles of Nasaken anorthoclase phenocrysts. The ranges in 2V measurements of these phenocrysts are plotted (Fig. 48) against comp­ osition on the graph of Tuttle (1952), from which it can be seen that the 3 0 2Vcx

4 0

5 0

(mean values are marked as bars j

to 20 3 0 4 0 5 0 60 0

rig . 4 8 Variatior) of the optic axial angle with

composition for alkali feldspar phenocrysts

from Nasaken trachytes, on graph of

Tuttle (^I9S.2\ Iü4

Table 4^ : X-Ray determinations and 2V measurements on phenoorj/st

alkali feldsoars from Nasaken and Ribkwo volcanoes.

Feldspar Composition Unmixinr 2Vç^avera,2;e

8/69,Nasaken Orjj.Orj^t partial Org,Org^ 44

8/129,Nasaken Orj7 % extreme Or^,Orgg 44

8/2 0 6 a,Nas aken 0p1 partial 0rg,0r^2 46 8/206b,Nasaken extreme Or^,Or^^ —

8/222,Nasaken °po partial Org,Or^^ 46 8/249,Nasaken partial 0r^^,0ré5 44

8/313,Nasaken partial Org,Or^^ 45

8/125,Nasaken homogeneous 54

8/123,Nasaken — 44

5/l6,Ribkv;o Or, 2 partial Or^,Or^^ —

5/225,Ribkwo partial Or^ ^,Oryg —

3/204b,Riblcwo — extreme Org,Or^i* 42

3/382|a,Ribkwo — extreme Org,Or^^ • —

— — 3/575,Ribkwo “P S * — 3/7 82, Rib Icwo ° P 8 *

-)- determined by XRF analysis X determined by wet chemical analysis

o groundmass feldspar

* observations by HcGlenaghan

2V determinations on U-stage 105

alkali feldspars lie close to the low temperature side of the sanidine-

anorthoclase series. The average 2Y for groundmass alkali feldspars in

8/125 is 44^ and their composition is about Or^^. Since the composition

of the phenocrysts (Table 4) is Or^g, ü may be inferred that the cryst­

allization trend in the alkali feldspars is one of orthoclase enrichment.

The anorthoclase cross-hatched twinning is a combination of the

pericline and albite types. Uninverted monoclinic feldspars show Carlsbad

and rarely Baveno twins and are flattened parallel to (OlO). It is this

type of feldspar that is most common as phenocrysts in the trachytes and

is invariably present in the groundmass. It is the parallel orientation

of the (010) faces during flow which produces the trachytic texture of the

groundmass and the fissility of the trachytes observable in hand specimen.

Certain phenocrysts with the appearance of monoclinic symmetry

show incipient albite or cross-hatched twinning. This is thought to be

indicative of partial inversion to the triclinic state. Many monoclinic

laths show characteristic transverse cracks ;• possibly due to deformation

during flow.

The Plutonic feldspars of the syenite bombs from Nasaken,

Kafkandal etc. are antiperthites, the unmixing being easily visible under

the microscope. Chemical analyses have shown them to be of similar comp­ osition to the volcanic phenocrysts. The unmixing in these feldspars is

extreme, the feldspar from a Kafkandal syenite (8 /9 OOO) consisting of almost pure albite (Ory) and pure orthoclase (Or^y), (Table 5)*

c) Olivines Phenocrysts of olivine are common in the basaltic rocks, less

so in rocks of intermediate composition and rare in trachytes. They have a considerable range in optical properties and composition, from 2V = 91°,

Pa 17 in an ankaramite (8 /8 I), to 2V = 150°, Pa 92 in the quartz-trachytes of the Nasaken complex. 106

Table S : X-Ray determinations on antiperthitio feldspars

from syenite bombs.

Feldspar Composition Unmixing

8/128,Nasaken extreme - Or^, Org^

8/128a,Nasaken extreme - Ori, Or95

8/9000,Kafkandal extreme - Or^, Or^y

5/4, Ribkwo “ '•38 extreme - Or^, 0 rg2

(Composition determined by v/et chemical analysis) 107

Olivines typical of the basalts are euhedral, colourless, with a variable development of (OlO) cleavage, A second generation of pheno­ crysts can sometimes be recognised, Phenocrysts reach 1 cm, in size and may show zoning. Olivine shows alteration ranging from that confined to fractures and margins to complete pseudomorphism. The alteration products of the olivines in the basaltic rocks are usually green-yellow serpentine- type minerals,

Hawaiites and mugearites contain olivine in the groundmass and as small phenocrysts. The olivine of these rocks seems particularly sub­ ject to alteration and is frequently completely pseudomorphed by red-brown

'iddingsite*,

Fayalitic olivines are present in trachyte lavas as small (up to 1 mm,) rounded phenocrysts. These are pale yellow and faintly pleo- chroic. They often show reaction rims of opaque ore (magnetite), Pheno­ crysts in 8/206b, a welded tuff, show a series of reaction rims from faya­ litic olivine-magnetite-soda pyroxene?-arfvedsonitic amphibole. Similar pétrographie features are shovm by fayalites from the amphibole-fayalite granites of Northern Nigeria, The fayalite crystals are surrounded by reaction rims of alkali amphibole and are associated with magnetite

(Jacobson et al,, 1958),

Alteration to ‘serpentine’ is confined to olivines in the more basic (Mg rich) rocks, ’Iddingsitic' alteration occurs more frequently in lavas of intermediate and salic composition and appears to be generally indicative of olivines with higher Pe/Mg ratios. However, the factors governing the crystallization of olivine and its alteration to ‘serpentine* and ‘iddingslte’ are more complex than mere compositional control, Pheno­ crysts and microphenocrysts of olivine from specimen 8/260, a highly por- phyritic Tirioko basalt, show a variety of alteration patterns involving both ‘iddingsite* and ‘serpentine’, Many phenocrysts show alteration to

'iddingsite* at the margins and along cracks in the typical manner. Each 1018

of certain large phenocrysts has a core of ’iddingsite' mantled by fresh olivine which has a thin zone of ’iddingsite’ at the margins. In these phenocrysts only rarely can the core ’iddingsite’ be seen to have contact with the outside of the crystal directly or via cracks. The observation that ’iddingsite’ rims or ’iddingsite’-olivine-’iddingsite’ rims follow the margins of strongly resorbed crystals having irregular outlines, suggests that there is no compositional control on the formation of iddingsite with­ in each crystal. Some olivines show marginal alteration to ’iddingsite’ and are surrounded by a zone of green-yellow serpentine-type mineral(s) containing relics of fresh olivine in optical continuity with that forming the core of each crystal. In some cases a second zone of ’iddingsitic’ alteration has developed outside the serpentine-like zone. The different patterns of olivine alteration are illustrated in Fig. 49*

The mantling of iddingsite by fresh olivine has been described by numerous authors (Edwards, 1958| Macdonald, 1940? Harkin, I960; Sheppard,

1962 ) and in each case the textural evidence was taken to suggest that the alteration of the olivine phenocrysts was just prior to and during extrus­ ion. Macdonald (1940 ) postulated that water-rich volatiles accumulated at the top of the vent and altered the olivine phenocrysts prior to extrusion.

After extrusion and on crystallization of the groundmass, overgrowths of olivine were added to the altered phenocrysts. These inferences seem very plausible with respect to the author's own observations on specimen 8 /26O.

It seems likely that the thin rims of ’iddingsite’ outside the fresh oli­ vine mantles in this specimen may be the product of deuteric oxidation or weathering, although the accompanying green-yellow serpentine-like alter­ ation is a further complication. However Baker and Haggerty (1967 ) argue that the hydrous smectite-bearing assemblage of ’iddingsite’ could not exist under conditions allowing the reprecipitation of olivine to form a i"im. They also point out that goethite (another component of ’iddingsite') is unstable at temperatures in excess of 140^0 and state that although the F ig .4 9 Altérât ion products of oiiv i no

a

G

f

I iddi ngsitQ* — d a r k rpQnt/ne - H k 2 mineroU — st/pp/ed 109

reasons for ’iddingsitization' to occur within olivine are not -understood,

alteration to ’iddingsite' is in all oases under oxidising conditions and

at low temperat-ureso They have also shown that the high temperature alt­

eration (oxidation) of olivine does not produce 'iddingsite' but the

assemblage haematite + forsterite (Haggerty and Baker, I967 ).

d) Pyroxenes The pyroxenes show considerable variation within the suite

from augites in the basaltic rocks to those rich in the acmite molecule

in the trachytic rocks. Because of the continuous chemical variation

between diopside, hedenbergite and aegirine, it is not always possible to

positively identify these minerals on the basis of their optical prop­

erties, This is particularly the case for the pyroxene phenocrysts in the

intermediate and trachytic rocks. Analogy is made with other areas having

lavas of similar composition, such as the Japanese alkali basalt-trachyte

series (Aoki 1964)9 the Otago mugearite series (Muir and Tilley I96 I),

the alkali basalt-pantellerite volcanics of Pantelleria (Carmichael 1962)

and the Go-ugh Island volcanics (Le Maitre I962 ), In each case, green,

strongly pleochroic pyroxenes with 2V^of about 60^ and angles of^^ c of

about 50°, similar to those fo-und in certain Nasaken trachytes, have been

shown to be hedenbergite-sodic ferrohedenbergite in composition. More­

over, Yagi (1966 ) has shown that under the PO^ conditions of volcanic

crystallisation, pyroxenes rich in the acmite molecule are -unlikely to

occur as phenocrysts, but that hedenbergite is stable under such condit­

ions.

A-ugite phenocrysts may be so ab-undant in -the alkali basalts

that some of the latter can be classed as ankaramites, Phenocrysts are up to 2 cmso across, euhedral, less commonly rounded and strongly zoned.

Zoning is simple or the hour-glass type. Twinning does not appear to be

common but when present is simple on (lOO), Measurements of 2V^for the

a-ugites in the basaltic lavas of the Tirioko Basalts, range from 92° to 110

56° with %/\ o from 39° to 44°• According to the data of Tomita (1934),

pyroxenes with = 56° and<)$/y c = 44° as measured on phenocrysts in

8/Ï06; a basanite from the Lotokongolea basalts (Table 16), are approach­

ing ferroaugite in composition. Colours range from colourless, pale

brown, pale purplish-brown to mauve. The purplish-brown to mauve colours

are associated with a high titanium content (Deer et al,, 1963)0 These

titanaugites are strongly pleochroic with mauve in the K vibration direct­

ion, Commonly, phenocrysts are colourless or pale with striking, strongly

coloured, mauve rims (Plate 5l)o This outermost zone of the phenocrysts

often forms after partial resorption and may be impregnated with iron-ore

granules. It usually corresponds to the crystallisation of the groundmass

pyroxene which commonly shows an ophitic relationship to the plagioclase

feldspar. Some phenocrysts display a marked sieve texture, enclosing

patches of groundmass. Here again the sharply defined thin outer zones

and zones adjacent to enclosed sieve patches are titaniferous,

Augites and titanaugites are usually fresh but alteration,

mainly to chlorite, does occur.

In the mugearites, pyroxenes are pale green or brown-green and

faintly pleochroic. They are not common as phenocrysts but occur mainly

in the groundmass. Mauve, ophitic titanaugites in 8 /II9 have thin pale

green rims. These pale green pyroxenes of the intermediate lavas may be

ferroaugite or approaching hedenbergite in composition.

Small (up to 2 mm,) hedenbergite phenocrysts are present in

some Nasaken trachytes (Plate 52), Those in 8/125 have 2V^'= 63° and

° “ 52°y data which compare well with those of Carmichael (1962 ) for

sodic ferrohedenbergites from the pantellerites of Pantelleria, These

data just plot on the standard curves for 2Vc

(1956 ) for the aegirine-augite series and correspond to a composition of less than 5 per cent acmite molecule. The trachyte phenocrysts are pleo­

chroic from pale to bright green and are usually zoned, the rims being Plate 91: Specimen 8/l9l; an olivine-basaIt from the

Lotokongolea basalts; p.p.l., x 90. An augite

phenocryst with strongly coloured titanaugite rims,

Plate 92: Specimen 8/89; a Nasaken quartz-trachyte; p.p.l.,

X 100. Euhedral hedenbergite phenocrysts (dark

areas) have been almost completely pseudomorphed

by hydrobiotite-vermiculite (grey). 111

deeper in colour; a feature which is associated with Na and/or Pe enrich­

ment. They are often associated with magnetite grains and frequently en­

close them. Peripheral alteration to arfvedsonitic amphibole may occur

but more commonly the hedenbergite phenocrysts are rimmed or pseudo­

morphed by an orange-brown pleochroic mineral, provisionally identified

as a hydrobiotite. In the amphibole granites of northern Nigeria, heden­

bergite is rarely seen without a reaction rim of alkali amphibole (Jacobson

et al.; 1958 )o

Aegirine-augite is often present in the groundmass of trachytes.

It is characteristically a bright green, strongly pleochroic with 2V#

between 70° and 90° and & c less than 20°, It is commonly intergrown

with soda-amphiboles, Hedenbergitic phenocrysts in 8/313) a Nasaken

trachyte, are rimmed by a bright green, highly pleochroic pyroxene with

c about 10° and probably of aegirine-augite composition. The transit­

ion between the two pyroxenes appears to be sharp, the crystallisation of

the aegirine-augite corresponding to that of the groundmass pyroxene.

Bright green, pleochroic pyroxene with 2V of about 60° and

optics is approaching aegirine in composition. True aegirine is present in

the syenite bombs found in the trachytic pyroclastics and is intergrown

with arfvedsonite,

e) Amphiboles

Alkali amphiboles, occurring in the groundmass, are the most

abundant mafic minerals present in the trachytes of Nasaken and Kafkandal,

They evidently crystallised late in the sequence of minerals and are either

interstitial in the feldspar-rich rocks or, more commonly, occur as micro-

poikilitic patches enclosing groundmass alkali feldspar laths and giving

rise to the characteristic 'mossy’ texture of the trachytes.

Owing to the fine-grained texture of the groundmass in the 112

trachytic rooks, detailed optical examination of the alkali amphiboles is

difficult. Moreover, classification of these amphiboles is not possible

on optical data alone. Data obtained on alkali amphiboles separated from

coarse-grained igneous rocks however, show that there is a broad correl­

ation between pleochroic schemes and chemical compositions (Jacobson et

al»; 1958s Borley, 1963? Deer et al,, I963? Vlasov et al,, I966), Using

this data, the alkali amphiboles in the trachytes can be divided on a

threefold basis on their pleochroic schemes, as follows §

OC O' Kataphorite yellow green-brown, brown red-brown, green- brown

Arfvedsonite blue-green blue-grey, light green, yellow-green brown

Riebeckite blue-black blue-grey, blue green, blue-green, dark blue brown-green

Kataphorite has characteristic yellow or yellow-brown absorp­ tion colours, Arfvedsonite usually shows a brown or violet-brown colour

not shown by riebeckite and often exhibits anomalous extinction, Riebeckite

is characterised by its deep blue-black colour and almost complete absorp­

tion parallel to the OC vibration direction. Miyashiro (1957) has shown

that the chemical variation of the alkali amphiboles is continuous from

kataphorite to arfvedsonite and from arfvedsonite to riebeckite. Many

amphiboles in the Kenyan trachytes show pleochroic schemes which are inter­ mediate between that regarded as diagnostic for kataphorite and arfvedson­

ite respectively. Similarly, there appear to be amphiboles intermediate between arfvedsonite and riebeckite, thus the terms 'arfvedsonite- riebeckite' and ’arfvedsonite-kataphorite* are used.

Specimen 8/125, from the trachyte outlier of Kagumnyikal, has small phenocrysts (up to ,5 mm.) of zoned alkali amphibole. The pleo-

chroism is pale brown to green-brown in the centres and blue-grey to blue- green at the margins of the crystals, the latter scheme corresponding to 113

that of the groundmass amphibole which is probably near arfvedsonite in

composition. Amphibole in 8/II6 and 8/89 has a zonation from that showing

the pleochroic scheme attributed to kataphorite in the cores, to that-

showing the pleochroism of arfvedsonite at the margins. Alkali amphiboles

contained in rocks from the alkali massif of Lovozero show a zonation from

kataphorite at the core to arfvedsonite at the margins (Vlasov et al.,

1966, po29l) and have similar optical properties to the zoned amphiboles

in specimens 8/II6 and 8/89«

Most trachytes contain two amphiboles as well as aenigmatite.

That possessing the pleochroic scheme attributed to arfvedsonite is by far

the most common. Alkali amphibole frequently rims earlier formed mafic

phases such as hedenbergite, aegirine-augite and magnetite.

The pulaskite syenites of the trachytic complexes possess

alkali amphiboles intergrown with aegirine, that from 5/4» a Ribkwo

syenite, having a chemical analysis close to arfvedsonite.

Small (up to .3 mm.); euhedral crystals of an amphibole with

(X - pale yellow, - yellow-brown and ^ - pale salmon-pink, are present in

the groundmass of an altered, carbonated hawaiite (8/394? Tirioko Form­

ation) .

f) Aenigmatite

Aenigmatite has a similar paragenesis to the alkali amphiboles.

It occurs abundantly in the groundmass of trachytes as ’mossy* poikilitic

patches but never as phenocrysts. In general, its absorption is so great

that the interference colours are completely masked, so that it appears

black or dark brown, Pleochroism, when visible, is as follows §

cx — deep red-brown, - dark brown and 3T — dark brown, black.

g) Micas

Micas are rare in the volcanics of the Gregory Rift other than 114

in phonolites which are not well represented in the author’s area. Two

occurrences of hiotite-bearing rocks are known.

Small crystals of pleochroic, pale pink - orange-brown biotite

are abundant in the groundmass of an ankaratrite (8/428), from the Turkana

Basalt Formation.

A biotite with strong pleochroism (dark brown - orange - red- brown) is found in multiple reaction rims around fayalite in 8/l28a, a

Nasaken pulaskite syenite. The fayalite has a thin veneer of magnetite

and is rimmed by arfvedsonitic amphibole. A partial rim of biotite is

developed on the arfvedsonite.

h) Magnetite

Magnetite (or titanomagnetite) is ubiquitous as an accessory mineral in the groundmass of many of the basic lavas. The typical occurr­

ence is shown by a feldsparphyric basalt (8 /161), from the Turkana Basalts.

This has octahedra of magnetite as phenocrysts which occur in glomero- phyric aggregates, and minute octahedra disseminated throughout the matrix. Magnetite granules often form at the margins of partially re­

sorbed augites, as in 8/393? an olivine-basalt from the Tirioko Formation.

Magnetite occurs in the groundmass or as sparse phenocrysts in mugearites and trachymugearites. In 8/Ï25, a Nasaken trachyte, pheno­

crysts of hedenbergitic pyroxene have crystallised on earlier formed magne­

tite grains. Magnetite is absent from the groundmass of trachytic lavas, occurring only as sparse phenocrysts or as rims around fayalitic olivines,

themselves rare.

i) Quartz

Quartz is common in the trachytic lavas of the Kafkandal and

Nasaken volcanoes and in some lavas from Kowun. It occurs in the ground­ mass as poikilitic patches enclosing feldspar laths. It appears to cryst­ 115

allise with, or later than, the alkali amphiboles and may approach 20 per cent in the mode.

Although quartz never occurs as phenocrysts in the lavas, single, large crystals are sometimes seen in welded and crystal tuffs associated with these trachyte lavas. It also occurs as a devitrification product in welded tuffs.

j) Feldspathoids and Analcite

Nepheline phenocrysts, up to 2 ram, in length, occur in the analcite-microfoyaite intrusion of Kanitiriam (8/489a, b, c). They show a cloudy alteration to cancrinite and fibrous zeolites, Nepheline, in the groundmass of the trachyphonolites (8 /487 , 8 /44I) which are related to the microfoyaites, is replaced by calcite. Small grains of nepheline are present in the groundmass of an ankaratrite (8 /428 ), from the Turkana

Basalts.

Analcite is common in the groundmass of many of the basic volcanics and it is the presence of this mineral which makes these lavas nepheline normative. Modal quantities only rarely exceed 10 per cent,

k) Apatite

Apatite occurs only rarely as small needles in the basic lavas.

1) Secondary minerals

The alteration products of olivine have been described under section c)• Various minerals are produced from the alteration of feld­ spars, including muscovite, zeolites and calcite but in general the feld­ spars are fresh,

Calcite is commonly found in the basic lavas as a deuteric or secondary mineral filling interstices and vesicles. A mugearite (8 /426^ from the Turkana Basalts, has beautifully spherical vesicles, about 2 mm. 116

in diameter, filled with a zeolite, possibly heulandite, around the margins and calcite in the centre.

Of particular interest is a distinctive orange mineral which will be described in some detail. Many of the hedenbergite phenocrysts in the trachytic lavas of the Pliocene volcanoes are rimmed or pseudo­ morphed by a pleochroic orange mineral identified as a hydrobiotite- vermiculite, In 8/89, 3l Nasaken trachyte, complete pseudomorphs after euhedral hedenbergite phenocrysts occur. Other grains show cores of green hedenbergite remaining. The mineral is bri^t yellow-orange in colour and pleochroic with a 2Vc

A qualitative microprobe analysis performed by Dr, J, Dixon

(at Leicester University), found the elements Pe, Ca, Al, Si and K,

An X-ray diffraction study of the orange mineral has produced patterns consistent with its being a clay mineral. The in ter layer spacing

(dgoi) was measured as 15.3A, but on heating the specimen for two days at

110°c, the basal spacing decreased to 14«6A. These data suggest that the mineral is a hydrobiotite-vermiculite.

The paragenesis of this alteration product is restricted to hedenbergitic pyroxenes in oversaturated rocks. Pyroxenes of aegirine- augite or aegirine composition remain fresh and hydrobiotite has not been found in undersaturated lavas.

It seems likely that the ’iddingsite-like* pseudomorphs after Piste : Specimen 8/209; ^ Nasaken quartz-trachyte p.p.l.,

X 100. Hedenbergite phenocrysts, enclosing

magnetite octahedra are rimmed and veined by

hydrobiotite-vermiculite. 1

Plate 9U: Specimen 8/29L; a Nasaken soda trachyte p.p.l.,

X 100. A hedenbergite phenocryst rimmed and

veined by hydrobiotite-ver-miculite. aegirine-augite’ described by Martyn (1969) in a comendite (I/890 ) from the Kabamet Trachytes, are in fact hydrobiotite-vermiculite after heden­ bergite phenocrysts, although this alteration is analagous to that of olivine to iddingsite,

Dodson (1963, p,3l) has described a trachyte from the Tirr

Tirr series containing biotite and abundant reddish-brown prism-like grains which appear to be the alteration product of a ferromagnesian min­ eral.

The orange mineral is also present in a syenite (8/l28a) as pseudomorphs after an early crystallising mineral (hedenbergite?). In the same slide, fresh biotite, itself clearly late stage, can be seen rimming arfvedsonite. This occurrence of hydrobiotite-vermiculite and fresh biotite in the same rock, together with the similar observation by

Dodson, suggest that the alteration from hedenbergite to vermiculite is direct, involving no reaction to form biotite as an intermediate stage,

3. Textures The textures of the basaltic lavas are the typical ones of holecrystalline basic volcanics and may be described by well known text­ ural terms. Those of the trachytic lavas are not so well known and the textural terminology is not obvious. Thus the various textures of the trachytes are described in some detail, and an attempt has been made to classify the trachytic lavas accordingly. a) Basalts and intermediate lavas

Basalts (and lavas of basaltic affinity) are mostly pcrphy- ritic, the phenocrysts often occurring in glomerophyric aggregates,

Phenocrysts include olivine, augite, plagioclase and magnetite. In some, such as 8/81, an ankaramite from the Napeitom basalts, pyroxene and oli­ vine are thought to be cumulate (Plate 55)* Others (8/119? 8/112, 8 /483 ) Plate 99: Specimen 8/8I; an ankaramite from the Napeitem

basalts; p.p.l., x 10. Abundant phenocrysts oi

augite and olivine, together with sparse

plagioclase feldspar, are set in a groundmass

having an intergranulai' texture.

Plate 96: Specimen 8/II6 ; a soda rhyolite from Nasaken; p.p.l.,

x 17. The groundmass has a distinctive ’messy’

texture. Micropoikilitic patches of aenigmatite (black)

arfvedsonite (grey) and quartz (white) are abundant. 118

are referred to as feldsparphyrio basalts and in these plagioclase may be cumulateo Here, the alignment of the plagioclase plates during flow has produced a marked fluidal texture in hand specimen. Aphyric basalts do occur, and in many of the intermediate lavas (mugearites) phenocrysts are sparse.

Some basalts show a typical ophitic texture with titanaugite moulded around and enveloping euhedral crystals of plagioclase. Others exhibit sub-ophitic or intergranular textures, the latter becoming pre­ dominant in the intermediate lavas. It is in these rocks that the feld­ spar microlites begin to show a rude parallel orientation along the flow direction and may then be said to have a sub-trachytic texture,

b) Trachytic lavas

Four textural types, together with transitional varieties, are recognised among the trachytes. These are:

1) ’mossy'

2) trachytic

3) orthophyric

4) ’macrophyri c'

The majority of trachytes are of types 1 and 2, and there is every grad­ ation between them. Types 3 and 4 are rare but of a distinctive nature,

1 ) Trachytes exhibiting the ’mossy* texture may be porphyritic or aphyric. They are characterised by an abundance of mafic minerals and quartz in the groundmass, Aenigmatite, alkali amphiboles and pyroxenes occur as discrete, poikilitic patches enclosing laths of alkali feldspar and giving the appearance of mossy growth against a light background

(Plate 56)0 In spite of its ragged appearance, each patch is not an agg­ regate but is in optical continuity throughout. The quartz, crystallis­ ing apparently somewhat later than the mafic minerals, occurs similarly in discrete poikilitic ’pools’. These may be seen against the background 119

of feldspar laths under crossed polars,

Anderson (1969) has described poikilitic patches of quartz, enclosing alkali feldspar in the matrix of a welded-tuff. This texture he termed 'snowflake’ texture and proposed that its presence is a general criterion by which to distinguish devitrification of the glassy matrices of welded tuffs from late-stage crystallisation in lava flows. Since poikilitic quartz is ubiquitous in Nasaken trachytes (it is also developed weakly as a devitrification texture in welded tuffs) Anderson's hypothesis would seem to be invalid,

'Mossy', poikilitic patches of aenigmatite, aegirine and alkali amphiboles have been described from some Kenyan phonolites and are char­ acteristic of the 'Kamasia type'. (Prior I903),

2) In trachytes which are rich in alkali feldspar, the mafic minerals occur int^rstitially to the feldspar microlites, A marked trachytic texture is developed with a strong alignment of the groundmass laths (Plate 57). If phenocrysts are present, the feldspars swirl around them in the direction of flow (Plate 58).

As the percentage of mafic minerals and quartz increase and the percentage of feldspar correspondingly decreases, gradation from the trachytic to the 'mossy' texture takes place,

3) The orthophyric texture is well developed in 8 /5II, a Kowun leucotrachyte. The alkali feldspar of the groundmass occurs in short, stumpy crystals which are randomly orientated, Groundmass mafic minerals occur interstitially,

4 ) A single example of the 'macrophyric ' texture is known.

8/94; a Kowun trachyte contains abundant large (up to 2 cms, ) anortho­ clase phenocrysts set in a groundmass rich in feldspar microlites showing a sub-trachytic texture (Plate 59)» The rock is highly leucocratic and may be intrusive although field relations are not clear. It may be Plate 97: Specimen 8/178; a soda trachyte from the

K asamanang intrusives; crossed polars^ x 20. The

rock has a well developed trachytic texture. Plate 98; Specimen 8/129; a soda trachyte from Nasaken;

crossed polars x 20. The slender feldspar laths

of the groundmass show a strong parallelism. A

large anorthoclase phenocryst with fine cross-

hatched twinning appears to have rotated from

this alignment. A

m

■S.'*t

Plate 59: Specimen 8/9U: an anorthoclase-trachyte from the

Kowm trachytes; p.p.l., x 10. The large

anorthoclase phenocrysts set in a leucociatic

groundmass give a distinctive 'macrophyric* texture. 120

classed as a solvsbergite,

4, Petrography of the volcanic rocks a) Introduction

Pétrographie descriptions of thin sections of representative rocks from the area are given in Tables 6 to 30 , The volcanics and other igneous rocks are described on a Pormational basis according to stratigraphie sequence. Percentages of phenocrysts, relative proportions of minerals etc., are rough estimates only and, in general, minerals are described as 'rare’, 'sparse', 'common' or 'abundant'. The abbreviat­ ions of mineral names used in the tables are self-explanatory. The letters

R, R, A etc, are used to denote the following:

R = Radiometric date

P = Photomicrograph

A = Analysis of major oxides and trace elements

Am = Analysis of major oxides only

At = Analysis of trace elements only

b) Turkana basalt formation (Tables 6 and 7 )

The Turkana basalts are composed entirely of basic and inter­ mediate lavas. These are dark grey to black in colour with dark to light brown weathering skins and are mainly basalts and hawaiites but mugearites and trachymugearites do occur. The majority of specimens are porphyritic with feldsparphyric varieties dominant.

The plagioclase phenocrysts (Ang^ to An^) occur as glomero­ phyric plates. They frequently show oscillatory zoning and partial re­ sorption, and may make up to 30 per cent of the volume of a rock.

Olivine and pyroxene are not common, the former being frequently altered and the latter pale in colour, Analcite is common in the groundmass of many basalts and hawaiites and also infills vesicles. Textures are mostly 121

intergraaular but 8 /48 I provides a particularly good example of an ophitic texture.

A 500ft. succession of Turkana basalts is exposed in the West facing erosion scarp, beneath Kouun trachytes, along the E.V, grid line

89 o It consists of 10-12 flows of feldsparphyric basalts and hawaiites with a purplish, aphyric, somewhat fissile lava at the base. This flow

(8 /484 ) is an analcite bearing mugearite with oligoclase in the groundmass which has a sub-trachytic texture. On the west side of the Kasorogol river, at the same latitude, a flow with a silvery-grey colour and feld­ spar phenocrysts occurs. This rock (8 /426) has strongly resorbed anortho- clase phenocrysts and pale green pyroxenes. The groundmass feldspar which appears to be anorthoclase or potash oligoclase has an acicular form and the rock has a well developed intersertal texture, the meso- stasis being a brown-glass. This specimen is almost identical to 8/447

(Grid Refs 287882) and may be best described as a trachymugearite,

The lavas described here are not representative of the Form­ ation as a whole; the feldsparphyric types seem to predominate locally.

Varieties containing abundant olivine and pyroxene phenocrysts, and those with kaersutitic amphibole, have not been encountered in this area but are plentiful further south (McClenaghan, 1971? Webb, 1971)»

The lavas of the Turkana basalt formation are petrographically similar to those of the Pliocene Tirioko and Kaparaina formations. In general, however, the abundance of analcite in the Miocene lavas bears witness to their more undersaturated nature (McClenaghan, 1971)« Also, the basaltic lavas of the younger formations do not contain kaersutite xenocrysts, with the exceptions of 8 /IO6 and 8/255 from the Tirioko basalt formation, c) The Kowun Trachytes (Tables 8-11)

These are typically alkali quartz—trachytes and are in many 122

respects petrographically similar to the much younger Pliocene trachytes.

The majority of flows are porphyritic and only rarely are phenocrysts

other than alkali feldspar present. The feldspar phenocrysts occur either

as rectangular plates of anorthoclase showing cross-hatch twinning or

’sanidine-like’ laths with simple Carlsbad twins. The former appears to

be the predominant type. The groundmass consists of slender alkali feld­

spar laths with alkali amphiboles and aenigmatite interstitially or as

sub—poikilitic patches. Pyroxene has been identified in the matrices of

only a few flows, Quartz is usually present as sub-poikilitic to poikil­

itic pools in the groundmass, in quantities estimated as up to 20 per

cent. However it is quite possible that some quartz has been secondarily

introduced. A section through Kowun trachytes was sampled at 320750«

The percentage of groundmass quartz appears to increase from about 5 per

cent near the top to about 20 per cent near the base of the visible

section. In places, small stringers and veins of quartz and chalcedony

were seen cutting the lavas.

Textures in the trachytes are usually sub-trachytic; mafic

minerals (amphiboles and aenigmatite) are scarce and rarely form poikil­

itic crystals, 8/94 has already been described as an example of the

'macrophyric' texture. The abundant anorthoclase phenocrysts appear as

white aggregates set in a medium grained light grey granular matrix. A pale-grey, massive leucocratic rock (8 /5II) shows an orthophyric texture.

The more characteristic flows are grey-green to purple-grey in hand

specimen. The Kowun lavas generally are red-brown or red in colour; this appears largely due to the visual alteration of the mafics of the grount^- mass to ferric iron oxides.

In general, the Kowun trachytes are more leucocratic (comen- ditic) than the trachytes of the Pliocene. The latter are characterised by the * mossy* texture which is only rarely or crudely developed in the

Kowun lavas. Also, the feldspar phenocrysts in the older formation apr"^-- 123

to be more frequently of the anorthoclase cross-hatched type. The Kowun. rocks differ from those of Nasaken and Kafkandal in that pyroxene is almost entirely lacking in the groundmass, although this may be due to alteration preventing identification. The amphiboles tend to be of the kataphorite type rather than the arfvedsonite-riebeckite that is ubiquit­ ous in the Nasaken lavas,

d) The Kasorogol melanephelinite (Table 12)

This (8 /428 ) is an isolated example of a nephelinite, of a type approaching ankaratrite. Olivine and pyroxene are abundant as phenocrysts and occur together with biotite, magnetite and nepheline in the ground­ mass. In hand specimen, the rock is black and small olivine phenocrysts can be discerned. Similar nephelinite intrusives have been described from

Turkana (Joubert, 1966; Walsh and Dodson, I969; Dodson, 1971)? in areas to the north of the author’s where this rock type is more common though never abundant,

e) The Kanitiriam microfoyaite intrusion (Table 13)

The hill Kanitiriam consists of an intrusive mass of medium- grained nepheline-syenite with perthitic feldspars and is hence termed microfoyaite. The rocks have a light-brown weathering skin with a pale- grey matrix set with glomerophyric aggregates of white feldspar and glass;- nepheline. Only slight pétrographie variation across the intrusion has been observed (compare 8 /489 % with 8/489b), but in every case the alkali feldspars are extremely cloudy. This turbidity is thou^t to be due to analcitization under late-stage hydro thermal conditions and has been commented upon by Joubert (1966) who has described a much wider pétro­ graphie range than is encountered in the Kanitiriam example, (See also

Walsh and Dodson, I969? Dodson, 1971)» 124

f) The Kanitiriam and Nakasuw trachyphonolites (Table I4)

The Kanitiriam and Nakasuw trachyphonolites which consist of

only 4 or 5 thin flows, are thought to be the representatives of the vast volumes of Miocene phonolites of the Kamasia succession (Chapman, I971).

The Kanatiriam lavas are dark-green, aphanitic, aphyric lavas with light brown weathering skins. They do not show the fissility associated with

trachytes, nor is their conchoidal fracture as distinctive as that in the

true ’flinty' phonolites of the Kamasia, The groundmass consists of aegirine-augite, kataphorite and aenigmatite, occurring interstitially between crudely orientated feldspar laths. About 10 per cent of analcite occurs though nepheline has not been identified. The specimens described

(8 /452, 8 /487 ) have IO-I5 per cent light brown glass.

The Nakasuw lavas (8/44O, 8/441) are similar to the above but are holocrystalline, with magnetite taking the place of aenigmatite and rare microphenocrysts of pale green pyroxene, 8/441 appears to contain c5 per cent analcite and on the modal classification adopted (Fig, 45) is designated a phonolitic trachyte.

Although Joubert (I966) has described phonolitic trachytes from

Kakhapit hill which bear a similar relationship to the Kaldiapit microfoy­ aite as the Kanitiriam lavas do to the Kanitiriam microfoyaite, the two groups of lavas differ in their detailed petrography. Suffice to say that the Kanatiriam and Kakhapit examples suggest a consanguinity between lavas of phonolitic affinity and the microfoyaite intrusives,

g) The Tirioko basalt formation (Tables 15-20)

The lavas of the Tirioko basalt formation are predominantly alkaline olivine-basalts, Hawaiites and mugearites are well represented but appear to be abundant only locally. No trachytes have been mapped in this formation by the writer but anorthoclase-trachytes occur intercalated between basaltic flows at several horizons farther south (Chapman, 1971? 125

Webb; 1971 ). The lavas of the formation as a whole are described since the component members are only distinct structurally, not petrographically.,

The basalts are frequently rich in phenocrysts (Plate 60); picritic, ankaramitic and feldsparphyric types having been recognised.

The phenocrysts minerals are typical of alkali basalts; namely forster- itic olivine, augite and a Ca-rich plagioclase, usually labradorite.

Black, euhedral; augite prisms stand out from the dark brown to red-brown weathering skins of the basalts, whereas plagioclase may weather prefer­ entially into hollows. Yellow-green olivine is best seen on freshly fractured surfaces. The most common phenocryst assemblages in the basalts are as follows s

olivine + augite augite + olivine augite + plagioclase plagioclase plagioclase + augite + olivine

— but virtually all combinations and proportions of augite, olivine and plagioclase phenocrysts + magnetite have been encountered.

Basalts may contain up to 10 per cent analcite in the ground­ mass as isotropic interstitial patches. Ophitic texture is rare, most matrices having an intergranular texture in which small prisms of augite, anhedra of olivine and octahedra of magnetite occur interstitially be­ tween plagioclase laths, Plagioclase phenocrysts frequently make up to

40 per cent of a feldsparphyric basalt. Such lavas have creamy-brown weathering skins and even in thin section the fluxion orientation of the tabular plagioclases may be observed (Plate 6I) although this is most marked in the field,

Hawaiites and mugearites do not contain abundant phenocrysts and are frequently a purple-brown colour in hand specimen. They tend to be fine grained containing abundant magnetite octahedra disseminated through the groundmass. Olivine is invariably pseudomorphed by idding- site and the clinopyroxene is pale—green in contrast to the neutral to Plate 60: Specimen 8/37; an olivine-basalt from the Tirioko

Basalt formation; p.p.l., x 12. Euhedral

phenocrysts of olivine show alteration to a

serpentine-like mineral. Phenocrysts of augite and

plagioclase also occur and are set in a groundmass

which has an intergranular texture and is rich in

minute magnetite octahedra. Plate 6l: Specimen 8/122; a feldsparphyric basalt from the

Kaguninyikal Basalts; partially crossed polars x 20.

Flow orientated laths of plagioclase are

abundant. The rock also contains olivine and augite

phenocrysts^ the latter have purple titanaugite

rims. 126

mauve colours of those in the basalts. Small euhedra of pleochroic

(yellow to pink) amphibole have grown preferentially in carbonate veinlets in 8/394» Kataphorite occurs in the groundmass of a mugearite (8/126) and a trachymugearite (8 /124), In these intermediate rocks the slender, groundmass, plagioclase laths may be orientated, approaching a trachytic texture and giving rise to a fissility.

Many phenocrysts in the basic lavas of the Tirioko basalt formation show pétrographie features which are consistent with their having been in disequilibrium with the liquid from which the host groundmass has crystallised, I^larginal resorption of euhedral crystals has produced corroded and lobate outlines on olivine, pyroxene (Plates 62 and 63) and plagioclase phenocrysts,

Augite phenocrysts may show a sieve texture in which a euhedral crystal contains numerous rounded pores filled with groundmass minerals.

Narrow zones adjacent to the enclosed sieve pores have the darker, mauve colour of the marginal zones of the phenocryst. Thus a zoning, inferred to relate to Ti enrichment, occurs both towards the crystal margins and the enclosures. It seems likely that the sieve texture has resulted from the trapping of liquid during the growth of the phenocrysts. Some sieve pores are filled with stilbite,'calcite, serpentine and magnetite (Plate 64).

This mineral assemblage probably formed by late-stage alterations perhaps involving reaction with the pyroxene host.

Sieve texture is shown rarely by plagioclase phenocrysts in feldsparphyric basalts. Usually the plagioclase plates enclose numerous individual grains of the groundmass minerals rather than the groundmass assemblage. The feldspar phenocrysts are often marginally resorbed pro­ ducing lobate outlines and they may show complex zoning suggestive of gcowth-resorption cycles (Plate 65),

Pyroxene ’phenocrysts' in a basanite (8 /IO6) are of particular mmmm «

Plate 62; Specimen 8/81; an anksramite from the Napeitom

basalts; p.p.l,, x 25. An augite phenocryst that has

suffered strong resorption.

Plate 63; Specimen 8/399; an olivine-basaIt from the

Kaguninyikal basalts; p.p.l., x 50. A partially

resorbed augite phenocryst. b ' i s W A ."‘•Ù g ##

#f'-./

’Hxm

# # g

»;

Plate 64: Specimen 8/399; an olivine-basalt from the

Fagumnyikal basalts, p.p.l., x 50* An augite

phenocryst showing sieve texture; the sieve pores

are filled with stilbite, calcite, serpentine

and magnetite. m

Plate 65: Specimen 8/122; a feldsparphyric basalt from the

Kagumnyikal basalts; crossed polars, x 50. A

partially resorbed plagioclase phenocryst showing

complex zoning. 127

interest, being of three types. The phenocrysts proper are euhedral augite prisms, neutral in colour with pale mauve rims, the rims resembling the groundmass pyroxene. This type of zoning is usual for pyroxenes in alkali basalts. Euhedral phenocrysts with pale mauve rims but with irreg­ ularly shaped cores of green or light—green pyroxene constitute a second typa# The pale green inner zones have 2Y^— 56^ and c = 44^ suggesting a composition near ferroaugite. The mauve rims have 2$/^ c = 36° and are probably more Mg and Ti rich than the cores (Tomita, 1934).

Pyroxene also occurs in 8 /IO6 replacing xenocrystic, kaersutitic amphibole. This basanite is unusual in that it is one of only two Pliocene

(or younger) lavas known from the research area that contains kaersutite, a mineral that has been commonly recorded from the Miocene phonolites and

Turkana and Samburu basalts. The other occurrence of kaersutite, as small

(1 mm.) prisms, is in a feldsparphyric basalt (8 /255) which is an inlier of the Tirioko basalts exposed beneath Nasaken trachytes. The kaersutite in 8/106 is pleochroic from pale-yellow to dark yellow-brown and is largely replaced by clinopyroxene which is neutral in colour. The pyroxene which resembles augite contains numerous exsolution lamellae of a pleochroic deep red-brown to dark green-brown mineral. Small laths of plagioclase and magnetite octahedra are contained in the augites which show strong reaction rims hi^ly charged with magnetite granules. Similar kaersutite xeno­ crysts showing all stages of replacement have been found in a basaltic dyke

(10/1029 ) from the Samburu Basalts (j, Carney, pers, comm,). Le Maitre

(1969 ) has described xenoliths from Tristan da Cunha in which kaersutite is breaking down to a granular mosaic of titanaugite, iron ore, plagioclase and a high relief, pleochroic red-brown to green-brown mineral. This min­ eral is referred to by Dunne (l94l) as rhonite (an alkali-poor variety of aenigmatite) and a similarly coloured mineral after kaersutite has been described by Uchimizu (1966 ) as an alkali amphibole. It is likely that the clinopyroxene replacing the kaersutite in the basanite 8 /IO6, has exsolved 128

excess Ti as lamellae of a titanium-rich mineral, probably aenigmatite.

There is considerable pétrographie evidence from lavas of the

Tirioko Basalt Formation of the concentration of early-formed crystals.

Phenocrysts often constitute 40 per cent by volume of specimens of ankar- amites and feldsparphyric basalts. Such great abundances of phenocrysts, and the chemical analyses of these rock-types strongly suggest an origin involving the accumulation of crystals. There is also the rare occurrence of xenoliths showing cumulate textures, A small xenolith in a feldspar­ phyric basalt (8 /257) is composed almost entirely of zoned labradorite laths which have a crude lamination parallel to 010, A single large plate of neutral coloured augite which poikilitically encloses labradorite appears to be an intercumulus phase. In many of the feldsparphyric bas­ alts, the plagioclase phenocrysts occur in glomerophyric aggregates with individual crystals sub-parellel to 010, Feldspar plates can frequently be seen in the act of breaking apart with groundmass occurring as thin wedges between the plates. These observations suggest that at least some of the phenocrysts in the feldsparphyric basalts may have been picked up by the basalt on disruption of plagioclase accumulations in which an igneous lamination was prominent,

h) Nasaken volcanics — Basic and Intermediate lavas (Tables 21 and 22)

Rare basalts and hawaiites occur within the superstructure of the Nasaken volcanic complex, interbedded with trachytes, Petrographically these lavas are indistinguishable from types occurring within the Tirioko

Basalt Formation. None of the six specimens described has a phenocryst content in excess of about 10 per cent, A marked ophitic texture is shown by 8 /526, a hawaiite, the remainder having intergranular textures, Anal­ cite has not been identified in any of the Nasaken basaltic lavas.

No basic lavas have been found associated with the Kafkandal volcano• 129

i) Kafkandal and Nasaken volcanoes — Trachytic rocks (Tables 23-30)

The trachytic lavas of the volcanoes Kafkandal and Nasaken are described together since they conform to the same pétrographie types. The lavas are classified as soda trachytes, quartz-trachytes and soda rhyolites

(Fig, 50)• These divisions cannot be recognised in hand-specimen, indeed the phonolitic trachytes of Ribkwo volcano cannot be distinguished in the field from the quartz-trachytes of Nasaken.

The trachytes are usually fissile with red-brown weathering skins. Fresh surfaces are light to dark green with a fine mottled or speckled appearance, the darker areas of which correspond to the mossy poikilitic patches of the alkali amphiboles and aenigmatite. Some speci­ mens show a diffuse streakiness with feldspar phenocryst plates lying flat in the planes of the flow. More leucocratic trachytes are grey-green or grey in colour and do not show mottling, Phenocrysts of alkali feldspar about 2-5 mms, in length are quite common and hydrobiotite pseudomorphs after hedenbergite can be recognised in hand specimen as orange-brown prisms a few mms. in length.

Trachytic lavas of Kafkandal and Nasaken usually contain 5-20 per cent by volume alkali feldspar phenocrysts which are of anorthoclase composition (Or^g - Or^^, Tabke 4)* They are clear 'sanidine-like' laths with Carlsbad twins and sometimes have patchy or indistinct extinction and only rarely show cross-hatched twinning of the anorthoclase type. It is inferred that this reflects rapid cooling through the inversion temperature from monoclinic to triclinic symmetry.

Small hedenbergite euhedra are often present as phenocrysts or microphenocrysts. They are frequently rimmed or pseudomorphed by orange hydrobiotite vermiculite. The hedenbergite has crystallised around magne­ tite octahedra which occur as microphenocrysts. Neutral to pale yellow euhedra of fayalite occur as sparse phenocrysts. Those in 8/329a, a

Nasaken quartz-trachyte show a good 100 cleavage and have 2V

x:

o - o .Q I

o o - o

oo oo

o o - o

c r

O 130 ..

corresponds to a composition of Fa.g (Henriques, 1958). Other dark miner­

als oocurring as phenocrysts are arfvedsonite as small anhedral grains in

a soda trachyte, 8/67> a single occurrence of euhedral prisms of aenig^

matite in a glassy soda rhyolite (8/253). Fayalite phenocrysts in 8/373

are rimmed and replaced by magnetite which is in turn rimmed by arfved­

sonite.

The matrices of the trachytic lavas consist largely of slender

alkali feldspar laths and alkali amphiboles plus aenigmatite in varying

proportions. Different textures are inferred to reflect variations in the

relative proportions of felsic and mafic minerals and their order of crys­

tallization. Pyroxene is usually present in small amounts in the ground­

mass but may be completely absent. Hedenbergite, aegirine-augite and

aegirine have been identified as groundmass pyroxenes in the trachytic

lavas on the criteria given in section 2d, The green pyroxenes occur as

individual euhedral prisms interstitially to the feldspar microlites, but

exceptionally the alkali amphiboles and aegigmatite also form euhedral

prisms in leucocratic trachytes when the percentage of dark minerals is

low (about 5-12 per cent). Such lavas show typical trachytic textures

with strong parallel alignment of the alkali feldspar microlites. Rarely

the groundmass feldspars are randomly orientated and the texture inter­

granular .

The typical occurrence of alkali amphiboles is in "mossy*

poikilitic patches (Plate 66), In many cases the amphiboles show a pleo­

chroic scheme which is intermediate between those regarded diagnostic of

arfvedsonite and kataphorite and are therefore termed arfvedsonite-

kataphorite (similarly arfvedsonite-riebeckite). Thus amphiboles in 8 /II6 and 8/89 show a zonation with kataphorite at the inner parts of each patch passing with optical continuity into amphibole having the pleochroic scheme of arfvedsonite.

Modal quartz may be as high as 20 per cent in some alkali rhyo— ..V

f

Plate 66; Specimen 8/89; a quartz-trachyte from Nasaken;

p.p.l., X 20. Euhedral alkali feldspar phenocrysts

show characteristic transverse cracks and are crudely

flow-orientated. The groundmass contains abundant

'mossy' patches of aenigmatite (black) and

arfvedsonite-kataphorite (grey). White areas represent groundmass feldspar & micropoikilitic quartz. 131

lites and occurs as well developed poikilitic 'pools', the * snowflake' texture. There is in general a tendency for the modal percentage of quartz to increase with that of dark minerals (Fig. ^0 ). Thus the major­ ity of lavas classed as alkali rhyolites have well developed ’mossy' textures while the soda trachytes tend to have trachytic textures. Lavas showing 'mossy' texture are usually finer grained than those showing good trachytic texture. Also the latter are holocrystalline but the 'mossy' lavas frequently possess a glassy mesostasis (Plate 67 ). A single example of an obsidian (Plate 68), which has partially devitrified is described

(8/281, Table 30).

In some lavas the 'mossy' amphiboles (plus aenigmatite) are elongated parallel to the alignment of the feldspar microlites and the inferred direction of flow (Plate 69). In others the 'mossy' patches are circular in cross-section and the quartz 'pools' always crystallise in this manner. It is likely that this more equant form indicates crystall­ isation after all flow had ceased and the lava was stationary. When an order of crystallisation can be established, quartz always follows the alkali amphiboles. Similarly in those rare trachytes which have a parallel orientation of the feldspar phenocrysts but a random orientation of feld­ spar microlites, it appears that the crystallisation of the groundmass feldspars was from a stationary liquid.

Many trachytic intrusives in the form of dykes, cone-sheets and irregular masses are associated with Kafkandal and Nasaken. Only one sill has been found, this intrusion being associated with the Moru Angitak centre on Kafkandal. A specimen from it (8 /6II) proves to be a leuco­ cratic trachyte with about 10 per cent of dark minerals. Euhedral prisms of aegirine-augj.te are common in the groundmass but alkali amphibole is rare. The Lodukai annular intrusion is composed of soda trachyte and a specimen (8 /67 ) is described in Table 25> • The masses of trachyte which intrude the Tirioko basalts and the Kowun volcanics on either side of the Piste 67: Specimen 8/233; a sods rhyolite from Nessken;

p.p.l., X 19. Micropoikilitic 'pools' of quartz

occur in a turbid, glassy mesostasis. The

distinctive texture may be termed -*■ snowflake ' *

Plate 68: Specimen 8/281; a pantelleritic obeidian from

Nasaken; p.p.l., x 10. The obsidian contains

abundant alkali feldspar phenocrysts & scarce, small prisms of hedenbergite. Discrete centres & diffuse

patches (dark) of devitrification consist mainly

of alkali amphiboles and aenigmatite. mam:##

m W #

Plate 69: Specimen 8/61*0; a soda trachyte from Nasaken;

p.p.l., X 10. Sparse alkali feldspar phenocrysts

set in a groundmass showing well developed 'mossy*

texture. Aenigmatite appears black and

arfredsonite, grey. 132

Kasamanang river are all very similar. They are grey, leucocratic rocks approaching microsyenite in grain-size. A specimen, 8/337, from one of the cone-sheets which cut the Tirioko basalts has a trachytic texture and contains .3 mm, prisms of aegirine which form microphenocrysts. This rock contains only a little arfvedsonite, as does a specimen from a soda trachyte dyke, 8/209b, which cuts Nasaken lavas near the Katatarope river.

In general, the intrusive rocks of Nasaken and Kafkandal are more leucocratic, containing a lower percentage of dark minerals than their extrusive associates. Most of the lavas contain alkali amphibole in abundance, with Na-pyroxene being sparse or absent. The reverse appears to be true for many of the intrusive rocks which usually have well developed trachytic textures. Rocks of the more extreme compositions

(i.e. alkali rhyolites) have not been found as intrusives.

j) Pulaskite Syenites

Rounded leucocratic syenite bombs have been found in pumice tuffs from each of the volcanoes Nasaken, Kafkandal and Ribkwo. Modally these syenites consist of between 85 and 95 per cent alkali feldspar which occurs as stout crystals about a centimetre or so in length. The feld­ spars are antiperthitic, although the proportion of sodium to potassium feldspar in many crystals is about 1:1. The Na-feldspar shows albite twinning and the K-feldspar is un twinned. Under plane polarized light an incipient cloudy alteration to these feldspars can be seen. The range of compositions of the feldspars from syenites of the three volcanoes is very limited; Or^^ - Or^g (Table 5). The degree of unmixing is extreme, the components of a Kafkandal antiperthite being Or^ and Or^^. The syenites have a cumulo-phyric texture defined by the feldspar plates.

Chemical evidence suggesting that these rocks are indeed crystal cumulates will be presented later.

A variety of dark minerals occur in the syenites, the most 133

common being aegirine which is pleochroic from bright green to yellow and is frequently intergrown with an arfvedsonite amphibole. The aegirine and arfvedsonite have crystallised in the interstices between the feldspar plates. Magnetite octahedra and iddingsite pseudomorphs are present in

8/128, a Nasalcan syenite. The feldspars in another syenite, 8/l28a, from the same locality enclose hydrobiotite and rounded fayalite crystals. By analogy with its occurrence in the lavas, the hydrobiotite which occurs in small eight sided prisms is inferred to pseudomorph euhedral hedenbergite pyroxene. The fayalites have multiple rims of magnetite, arfvedsonite and biotite. The hedenbergite pseudomorphs are mantled by arfvedsonite. In contrast the aegirine-arfvedsonite intergrowths are interstitial to the feldspar plates.

A small amount of interstitial red-brown (basaltic?) glass occurs in a Ribkwo syenite, 5/4(i)» and in a second Ribkwo specimen,

5/4(ii)s which has only about 5 per cent of dark minerals, aegirine has crystallised in stellate form in the open structure formed by the feldspar plates.

No quartz or nepheline occur in any of the syenites described,

A small syenite xenolith has been found in a soda trachyte dyke (8/658a).

It consists of interlocking plates of perthitic feldspar with fine grained assemblage of arfvedsonite, magnetite and alkali feldspar between the plates. Another soda trachyte (8 /384 ) contains a glomerophyric aggregate of magnetite and alkali feldspar.

Blocks end bombs of syenite, similar to those described, have been reported from the slopes of Kilombe and Menengai volcanoes (McCall,

1964 ; McCall, 1967 ). Leucocratic alkali syenites with antiperthitic feld­ spars and sodic pyroxene and amphibole have been called pulaskites. The

Kenyan examples are petrographically and chemically very similar to the pulaskites of Ben Loyal and Loch Ailsh, Assynt, referred to by Read (l93l). 134

Crystallisation sequences have been established from pétro­

graphie observations for the pulaskite syenites, (Fig.5^ ) and the trach-

ytio lavas (Fig.^^ ) from the Nasaken complex. The main differences are

the occurrence of aegirine in the plutonic environment and aenigmatite and

quartz in the volcanic one.

k) Pétrographie features of welded tuffs.

Excellent descriptions of welded pyroclastic rocks are given

in Webb (l9?l) together with a brief account on the development of con­

cepts concerning them. It is proposed merely to supplement this work with

a few descriptions of the welded tuffs from the Nasaken complex. Descript­

ions of non-welded pyroclastic rocks and epiclastic sediments have been

given in the appropriate sections in Chapter 2.

Welded ash-flow tuffs are common in the Nasaken volcano and

frequently occur as part of the eruptive cycle, pumice tuff - welded tuff - lava flow. By contrast welded tuffs are rare in Kafkandal, There is no doubt that welded tuffs are more common on the flanks of Nasaken than in

the source areas. It seems likely that welded tuff deposits are more per­ sistent and thin out less quickly tlian the lavas, due to a higher mobility.

The observations of various authors (Smith, I96 O5 Boss and Smith, I96 I;

Gibson, 1970) that welded tuffs may thicken distally, apparently having very low angles of rest on volcanic slopes, receives some support from the author’s observations on the flank deposits of Nasaken. The Nasaken welded tuffs have even upper surfaces maintaining low angles of dip (usually less than 10 degrees). Their evident ability to flow around obstacles has given rise to the mesa-like topography (See Plate Villa, Rhemtulla, 1970) cut by the rivers within the flank deposits of the Katatarope area which has a succession of welded tuffs and thin lava flows. The even top surfaces of the welded tuffs are in contrast to the blanketing of topography by air-fall F/g. 5/ Crystallizütion ■ sequencQ and raact/on series:

alkali feldspar

c. O r JS

fayalite -1 a e g / n n e

'^magnetite arfvedsonite ^ .

^ — : — hedenbergite biotite

aj Nasaken pulaskite syenites

phenocrysts g roundm ass A______r

alkali feldspar fayalite aegirine — augite I— — — I

\magnetite arfvedsonite

...4'— ------—' hedenbergite aenigmatite

quartz 1--- -4

Nasaken trachytes

reaction and order of crystallization 135

tuffsu Specimen 8/313b is a welded tuff with a grey-green matrix and dark grey fiamme which are very elongate, being several centimetres in length and less than half a centimetre in width. The fiamme have been flattened into one plane and have rounded or elliptical cross-sections parallel to this plane. The rock has a eutaxitic texture and the inclusion of broken feldspar crystals and angular fragments of trachyte witnesses its clastic nature. In thin section, the matrix is a turbid, green-brown, structure­ less material with a patchy crystallisation or devitrification. The fiamme are elongate areas of devitrification containing spherulites and acicular feldspars which are arranged perpendicularly to the margins of the fiamme. Many welded tuffs show similar pétrographie features and in the majority no flow lineation in the plane of flattening can be discerned.

Recognizable pumice fragments occur in specimen 8/252a (Plate

70 ), The pumice fragments are flattened with frayed ends and contain tube vesicles. The matrix is faintly banded and consists of yellow-brown, turbid material containing incipient feldspar microlites. Most of the lithic fragments are trachyte but one of mugearite has been observed. In a similar welded tuff, 8/312 (Plate 71)» a, 4 cms, diameter inclusion of feldsparphyric basalt occurs. Of the crystals included in the welded tuffs, only feldspar is abundant but fayalite and hedenbergite pseudomorphs also occur. Specimen 8/278 contains large intergrown crystals of aegirine- arfvedsonite similar to those described in the syenites, 'Phenocrysts' of alkali feldspar, hedenbergite and fayalite are contained within the fiamme of specimen 8/3O8 and are surrounded by spherulitic crystallisation of feldspar microlites. The fiamme in 8/3I2 consist of acicular feldspar set perpendicularly to the margins with spherulitic feldspar and quartz towards the centre of the fiamme. The devitrification structure of the fiamme described here is similar to axiolitic structure occurring in shards

(Ross and Smith, I96I) but has developed on a much coarser scale. It is the result of crystal growth beginning at the fiamme boundaries and meeting #

Piste 70: Specimen 8/2^2a; a welded tuff from Nasaken;

p.p.l,, X 10, The welded tuff consists of

lenticular turbid fiamme showing a high degree of

compaction around lithic fragments. Plate 71: Specimen 8/312; a welded tuff from Nasa.ken;

p.p.l., X 10. The welded tuff consists of elongate

devitrified fiamme and numerous broken

anorthoclase crystals set in a dense, tubid matrix. 13^

at a central line of discontinuity. Photomicrographs of devitrified pumice fragments in welded tuffs from the Valles Mountains, New Mexico (Ross and

Smith, 1961 — Figs, 57 and 58» p.6?) show identical textural features to the fiamme contained in specimens 8/312 and 8/315b. These authors also comment (on p.37 of the same paper) "Axiolitic structure has been observed only in ash-flow tuffs and it seems to provide criteria for distinguishing between ash-flow and ash-fall tuffs".

In general, fiamme may be present throughout the vertical extent of a welded tuff although, like the occurrence of lithic fragments, they may be concentrated at particular horizons. Welding in all cases occurs from top to bottom of the flows, Lithostatic loading was not therefore important in controlling the welding. Little variation in vertical section is seen in the welded tuffs; some had denser, more vitric bases, others pumiceous bases, -

It is extremely difficult to identify shards in the matrices of the welded pyroclastio rocks. Recognizable shards are ubiquitous in

•the oalo-allcaline welded tuffs of the south-western U.S.A. (Smith, I96 O) but in the Nasaken rocks, intense welding, deformation and devitrification have given rise in general to dense turbid matrices. Occasionally dis­ torted plate-like shards can be seen bent around feldspar fragments. Only one example containing abundant cuspate shards lias been found (Plate 72) and it is clear that the different forms represent the interstitial glass between several bubbles.

An intriguing specimen, 8/599» from Kafkandal is a banded glassy rock containing spectacular flow folds (Plate 73)« The banding is defined by lighter and darker turbid areas, some of which are completely colourless and consist of glass showing perlitic cracks. Lithic and crystal fragments are included with the flow banding wrapping round them. In hand specimens the rock contains several large gas cavities elongated parallel to the flow banding. In the field this rock appeared to pass rapidly up into a typically Plete 72: Specimen 8/l70a; a welded tuff from the Kowun

trachytes; p.p.l., x 100. Numerous undeformed

gless sherds showing faint axiolitic structure are

set in a dense, opaque mesostasis. \

K

»

Pl?te 73: Specimen 8/599; p.p.l., x 10 (negative print).

Flow folds in a banded obsidian fran Kafkandal< 137

eutaxitic welded tuff containing fiamme. In retrospect it is thought likely that this is a banded obsidian underlying the welded tuff and that the folding was caused by slumping into soft pumice tuffs beneath. i:$8

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H P P P PP P !=x •H •rl S x •rH ■ AC P in H rH m _ r-H a o P, m p O P O m P O o p a d ■Cd S x P a d b P d ' P r H o A d o A d H O f M m p l/lp W CDP

XO o P p "C P d • <

P tl p 1 p P 01 p <1 m O p r4 01 X V?. G •rl •rl •rl P ■ p p 0 0 0] UX A to to 4 4 rH p. iH u Xi p p iH UX •rl P (H to ü ad p G CM G GG p P G p p P IX iO o p 43 •H 10 A 1,1 M ü H II •H •rl P G 01 g ■« ° a 01 G P (0 S>P X G P 0 Ed'S 2 o •rl a rH p A4 P P P •rl G P G ai B p to B •Cd ad 43 e 0 W A

P P f) •H •rl P P •P P •rl m G H H U) P to •rl A ad •rl Al o G G •rl p G PO P A P O- r- P ad I P P N 7Î 8 51 p r-l t I t ü o cd ^ > P p CM C P O P • 3 •H 'Td p p, O

p p •rl O P T1 O P p w tH w • p m p o 'p iH p p ad •rl p % P -H Ad 'd p p . o > AC o p 4-1 •H p p +3 p O p A3 Jh *h »h cd p pv pi p cd *H s p j

,—1 P Xi p S •H P p 1 P •rl •rl c 1— 1 •H ai O P Â3 S M to p. P P O fcO H G G 01 01 P G •rl Cd P P PC P A B G 01 G A A3 p p o td fd p o 1 P PG p P p p ra 6 to A •rH P P A A k i ad p ^ B P G GG p. P G m îx 'H p o p p P o P P p. •H 01 A P p, A3 P P ü Ad M o o

^ C p 01 G iH •H cd A Sx ad c 1—i P A r j p _ Sx iH p p p H P G 4-, A3 p P td ad 44 G P 01 0 A3 Ad ad to td 0 10 P H Ad p p p •rl P A4 UX •rl SxP b ^ (8 rH P p p fH A3 rH C ) 0 G •rl 0 P p iH C) P p P P 0 0

UX AG P q) q) m P • ^ O 43 A h , p M p p p G •H p ad ad UX G •H A ü P rH ad p P A P 0 01 P p e > 0) GA G G SXrC G 4-1 P to A 01 43 P P P G P P PG 01 •H A P. 4-, P P ‘rl BP P p. 01

rH ^ P H I 'Cd p T- p g w o p *H _ p td ad B ad Ad P c ad p p w P C- o p p « ad Ad rl p M 'rl ad Ad 'H P S I p p t, P O A3 p G C C o Ad p Ci. C- S Ad Ad p p.4-1 o

-H" ad fc I G ^ p to p. m rH P P I G "P G P ra A 4-, •rl G w (H p M m rl' A S P p P p A H S 1 1 1 H P O ad h G ^B A4 p p G (H P ^ 4-1 iH p o o 1—! rH P p c5 rH O P‘ O ~ O A O O p a r H ü As a p ü

P G p Pi .1 Xi P O ad o p o P •rl »r4 C - P n « & p 'rl A M G p , O o o r - CO p AM CM cS* CHAPTER 5

GEOCnaiISTRY

and

PETROLOGY 163

Petrology

1. Introduction

Recent suimnaries of the petrology of the volcanic rocks of

the Kenya rift are those of King and Chapman (l9?2) and Williams (l9?2).

With reference to the northern part of the Kenya rift, emphasis has heen

placed on the differences in chemistry between Miocene and post-Miocene

lavas. The more undersaturated suite of groups I (Turkana and Samburu

basalts) and II (’plateau’ phonolites) may be distinguished on an alkalis/

silica diagram (Pig* 52) from the alkali basalt-mugearite^trachyte trend

of the younger groups III (Pliocene basalts) and IV (Pliocene trachytes).

Chapman (l9?l) showed tliat phonolitic lavas are of two petrographically

and chemically distinct types, Phonolites (sensu stricto) are entirely

restricted to the lower part of the succession (groups I and II) whereas

trachyphonolites occur throughout the succession from Mid-Miocene.

Average compositions for the two types are given in Table 51» In Figure

52; the ’plateau’ phonolites form a tightly clustered group of points

towards the top centre of the diagram and are distinct from the trachy­

phonolites which grade compositionally into trachytes.

Basaltic magma which has been available throughout the volcanic history of the northern section of the rift shows a decrease in

silica undersaturation in time (Fig, 52). Computed averages of Miocene,

Pliocene and Quaternary basalts, taken from McClenaghan (l97l)> are given

in Table 31° The basalts have been recalculated water-free and have passed the chemical screen of Manson (196?) which rejects altered, oxidised or cumulate compositions. The group V basalts of the Quaternary volcanoes are of ’transitional' compositions and a few have normative hypersthene.

The values of normative nepheline for the three averages are 6.6, 3.2 and

0.9 weight per cent. The change in chemistry with time is analagous but less extreme than that in the'Afro-Arabian dome. Compositions vary from the alkali basalts of the Trap Series through 'transitional' types to the o ?; o I. 0 HI 5 5 c» C» C» _ 1 — o ' c :5k -j- o g § + s ± 'K. k. C;

C) • • • X Q o . +

E (y

:S § k o Q % + Cl) ;S o°-Ap„ o o • o < \l o 0

CO f + \ 1

o o o • o• + - + \ o o \ t» Î? o \ o

o k o S.:§ ■ i'% (/) b

o • \ b :c : 0 ° ° .0 *\ ■5 b \ _ O o * b V) o o b £ (M b K <«» J L x _ L I I I l \ o Co O Co oiv + 0^^/v ? 1 6 4

Table 31

1 2 3 4 5 SiOg 46.05 46.28 47.51 58,22 55.00

TiOg 2.52 2.50 2,29 0,65 0.53 AlgO^ 15.01 16.23 17.01 16,58 19.94

Feg03 4.52 3.85 3.15 3.97 2,41

FeO 7.50 8.15 8.24 3.11 2,27

MnO 0.10 0.18 0,17 0.26 0,27

MgO 8.75 7.26 6.33 0.56 0.51

CaO 9.95 10.84 11.26 1.47 1.42

NagO 3.74 3.10 3.13 6.87 8.34

KgO 1.25 1.15 0,65 5.21 5.94

H2O+ 1.99 3.19

H2O— 1.45 0 0 66

P2O5 0.51 0.46 0.26 0.06 0,77

100,0 100.0 100.0 ];2C«J;0

q

or 7.4 6,8 3.8 30.4 38.4

ab 19.5 20.2 24.7 41*7 24.1

an 20.5 27.0 30.4

ne 6.6 3.2 0.9 7.5 25.5

ac 6.7 1.9 ns

di 20.4 19.2 19.3 5.9 4.4 hy

ol 13.1 12,1 11.2 1,2 1.9

il 4.8 4.7 4.3 1,2 0.9 mt 6.6 5.6 4.6 3.2 1.0

hm ap 1.2 1,1 0.6 0,1 0,1 CO

1 Average Miocene basalt (McClenaghan, 1971) tt 2 n Pliocene basalt 3 " Quaternary basalt " 4 " traohyphonolite (Lippard, in press) 5 ” phonolite (s,s,) " .165

tboleiitio associations of the Red Sea and Gulf of Aden (Gass, I970 ). No

lavas ofthoieiitlc composition have heen recorded from the Kenya rift.

A trace element study hy Camey (l972 - Fig, 67) suggests that

the Miocene basalts are more obviously parental to Miocene trachytes than

to the 'plateau' phonolites. Both Camey (1972) and Lippard (1972 and in press) suggest that possible parents for the phonolites are analcite- basanites and analcite-hawaiites of the Noroyan formation, Kamasia hills

(chapman, 1971) which occurs within the phonolite sequence.

One of the dominating petrogenetic problems of the Eastern rift volcanics is the presence of at least two distinct parallel, fract­ ionation trends, one strongly undersaturated and the other mildly under­ saturated. These chemical characteristics appear to have been inherited from parental magma types of different compositions (Harkin, 19595 King and Sutherland, I96 O5 Saggerson and Williams, 1964? King, 1965; King,

1970)0 Wright (1963) proposed the existence of two parental magmas of melanephelinitic and alkali olivine-basalt compositions. In southern

Kenya and northern Tanzania, Saggerson and Williams (1964) proposed co­ existing magma series, the strongly alkaline (more undersaturated) melanephelinite-nephelinite-phonolite series and the mildly alkaline

(less under saturated) alkali basalt-mugearite-trachyte series.

The origin of the abundant salic/silicic volcanics of the

Kenya rift constitutes a major petrogenetic problem, A recent assessment of the relative volumes of basic and salic/silicic lavas is given by

Williams (1972) as followss basic salic/silicic

Miocene 1.5 “ 1

Pliocene 0,7 ° 1

Qua ternary 2,1 : 1 Volume estimates of phonolitic lavas in Kenya are given by Lippard (in press). The huge volumes of trachyte, phonolite and rhyolite and the lack 156

of quanbitatively significant associations of related basic and/or inter­ mediate rocks have led many authors to reject crystal fractionation as an important petrogenetic process (Wright, 1965; CcCall, I966 ; Williams,

I97 O5 Wright; 1970 ). Bailey (1964 ) suggested the production of the salic of dll© *to lavas by partial melting/mantle material y relief of pressure and heat and volatile concentration at the base of the crust. Wright (1965 ) and Nash et al. (1969 ) have rejected the assimilation or fusion of gneissic rocks as a possible process for generating the compositions of rift volcanics.

The present geochemical study of the lavas from the Nasaken area is almost entirely confined to the Pliocene volcanic rocks which are so well represented within the area. Five analyses only of lavas from groups I and II are presented and their compositions are discussed in the context of earlier work on the lüocene rocks.

2, Composition of the Miocene lavas.

The Turkana and Samburu basalts ar-e associated with nephelin- ites, rare basanites, teschenites and lamprophyres (joubert, 19665

Dodson, 1971 ? Carney, 1972). These associations are indicative of the more undersaturated nature of this group compared with the basalts of groups III - 7,

No analyses of Turkana basalts from the Nasaken are are avail­ able but two analyses of porphyritic basalts from the area to the south are given in Table 32. These have normative nepheline values of 13.7 and

6,9 per cent. The intrusive ankaratritic melanephelinite of Kasorogol

(Table 12) has ne of greater than 17 per cent (Table 35) and its compos­ ition is similar to those of melanephelinites from Napak and Moroto

(Table 33).

Two analyses of Kowun rocks are presented (Table 33). Pétro­ graphie descriptions of these rocks are given in Table 8, Specimen 8 /IO7 is a slightly over saturated, peralkaline (ac = 9*4 per cent) trachyte 16 ?

Table 32

5/592 5/421 K4I 73 SiOg 45.94 45.90 41.51 42.1 TiOg 2,00 2.22 2.25 2,6 AlgOj 16.13 12,85 12,66 12,4 PegO^ 5.89 4.01 7.72 5 5 FeO 5.55 6.72 6.32 7.4 MnO 0,20 0,17 0,18 0,11 MgO 4.63 9.08 6,49 7.8 CaO 8.55 12,66 11,91 12.4 Na^O 5.59 5.51 4.65 4.0 KgO 2.39 0,76 1.65 2.0 HgO+ 2 .71* 2.57* 5.51 5.5 HgO— 0.41 1.5 P2O5 0.70 0.55 1.05 0.8 COg 0.25 0.10

100 oil 100.58 100,59 99.70 * total water

q or 14.12 4.49 9.4 11.82 ab 20.32 15.22 5.2 0,05 an 12.76 17.91 9.2 9.97 ne 15.70 6.95 18,5 18.31 ac ns di 18.60 55.94 54.1 57.15 hy ol 5.57 8,68 2,2 6.03 il 3.80 4.22 4.5 4 . 9 4 mt 8.54 5.81 11,1 4.78 hm ap 1.62 0,81 2.4 1.85 CO 0.57 0.2

5/592 - basalt with augite and olivine phenocrysts (Webb, 1971) 5/421 - basalt with augite and olivine phenocrysts " K4I - melanephelinite, Napak (King, 1965) 75 - melanephelinite, Moroto (7ame, 1968) 168

Table 33

8/107 8/94 8/428 8/489a 8/487 SiOg 64.58 65.75 42.59 55.24 57.25 TiOg 0.55 0.55 2.14 0,26 0.77 AlgO^ 15.40 16.55 15.41 19.40 17.47 ^®2^3 5.47 4.67 5.71 4.82 2.47 FeO 1.77 0.88 5.57 1.04 5.75 MnO 0.31 0.19 0,20 0.41 0.31 MgO 0.36 0.16 10,10 0.27 0,84 CaO 0.64 0.67 11.96 1,12 1.32 NagO 7.02 6084 4.25 9.10 6.19 KgO 5.48 4.70 1.59 5.03 5.97 H2O+ 0.72 0.85 2.25 5.45 1,65 HgO— 0.49 0.31 0.26 0.20 0,28 P2O5 0.05 0.11 0.42 0,04 0,12 CO2 0,16 1.85 100.64 100.21 100.19 100,38 100.22

a b q 2.63 5.03 2.95 or 52.58 27.77 8.21 29.72 55.28 ab 48.70 57.88 5.55 54.05 41,20 an 0.58 15.50 ne 17.47 20,44 ac 9.42 4.60 ns di 2.44 0.86 55.03 1.45 hy 2.38 6.20 ol 8.25 il 1.04 1.04 4.06 0.49 1.46 mt 0.31 1.86 8.28 5.95 5.58 hm 5.59 0.52 ap 0.12 0.26 0.97 0.09 0,28 cc 0.36 2.07

8/107 - trachyte, Kowun 8/94 - macrophyric trachyte, Kowun 8/428 - melanephelinite, Kasorogol 8/489a - microfoyaite, Kanitiriam 8/487 - phonolitic trachyte, Kanitiriam

a wollastonite I.4 b corundum 3.0? Nag00^ 2.3 169

similar in composition to many lavas of the Pliocene trachyte volcanoes.

Specimen 8/94 is a highly leucocratio rook containing less than 10 per cent

maficSo There is no visible modal quartz and the occurrence of 5 per cent

is mainly due to the sli^tly oxidised state of the rock (hm = 3*4 per

cent). Many of the Kowun lavas are oxidised and secondarily silicified

rendering them unsuitable for chemical analysis. Compositions of feldspars

from Kowun rocks are given in Table 34» The feldspars are of anorthoclase

composition and are very low in the anorthite molecule. The trend of feld­

spar crystallization in 8/94 is of increasing content of the orthoclase

molecule and decreasing albite and anorthite (Table 34s 1 and 2).

An analysis of the Kanitiriam microfoyaite (Table 13) is given

in Table 35* The microfoyaite has more than 20 per cent normative nepheline

and greater than I4 per cent alkalis. Its composition is strikingly similar

to that of the average 'plateau' phonolite (Table 31) thus confirming the

microfoyaites of South Turkana as the hypabyssal equivalents of the Miocene

phonolites. The high percentage of combined water in 8/489a, 8/428 and

other analyses of Miocene lavas may be related to the presence of signifi­

cant amounts of modal analcite. It is the presence of anal cite detected by X-ray diffraction methods which gives rise to the 'flinty' properties

of the 'plateau' phonolites and this may be due to the isotropic nature of

the mineral.

The Kanitiriam traohyphonolite, 8 /487 , has modal analcite

(Table I4) yet is slightly quartz normative (Table 35). This is due to the high content of carbon dioxide and the combining conventions of the

C.I.P.W. norm which make calcite and Na^CO^ first using soda and lime which would otherwise form feldspars and hence leaving an excess of alumina and silica. The Kanitiriam lava is stratigraphically associated with the micro­ foyaite althou^ the two rock-types are not chemically equivalent. Lavas of similar composition to the analysed Kanitiriam traohyphonolite occur in the Tiati phonolite succession and their chemical analyses are given in 1 7 0

Table 34* Pelds-nar analyses from Kowun

1 2 3 PegO^ 0.45 0.51 v± % CaO 0.43 0,20 0.13 NagO 8.15 6.99 7*92 KgO 4.10 5.16 5.28

Or 25.7 35.6 31.5 wt % Ab 71.4 65.3 67.9 An 2.9 1.1 0.6

1 Anorthoclase phenocrysts from trachyte 8/94» Kowun 2 Groundmass feldspar from 8/94 3 Anorthoclase crystals from Kowun crystal tuff 1 7 1

Webb (1971, Table 22).

3. Geochemistry of the Pliocene volcanic rocks,

a) Introduction

This section deals with the petrology and geochemistry of the

lavas of the Tirioko basalt formation and the Nasaken complex. The bas­

altic formation constitutes the substructure of the trachytic volcano and

according to K/At dating no great time elapsed between the two formations.

The change from dominantly basaltic to dominantly trachytic eruptions took

place between 6 and 6,5 million years b.p, and was preceded (or accompan­

ied) by an episode of faulting,

Lava-types within the Tirioko formation are mainly alkali bas­

alts and hawaiites with subordinate mugearites and trachymugearites (ben- moreites). Trachytes occur within the formation south of the Nasaken area

(Webb ; 1971 ). Basalts and mugearites are rare within the Nasaken volcano

but are petrographically similar to Tirioko formation lava-types. The

lavas of the Nasaken complex are dominantly soda trachytes and panteller-

itic trachytes.

Compositionally the lavas of the two formations appear to re­ present a typical alkali basalt-mugearite-trachyte suite comparable with

those of Hawaii, the Hebridean province and East Otago, New Zealand,

The geochemical study of the lavas is based on whole-rock major-element and trace-element analyses. The lavas are mostly porphyr­ itic. Basic and intermediate rocks contain phenocrysts of olivine,

clinopyroxene, plagioclase and ore. Trachytes commonly contain pheno—

crysts of alkali feldspar. Less common are hedenbergitic pyroxene rnd ore. Payalitic olivine, aenigmatite and arfvedsonite—riebeckite occur as rare phenocrysts. Mineralogical data in addition to the optical data given in Chapter 4 is presented overleaf. 1 7 2

3b) Mineral chemistry.

Olivine is an abundant phenocryst in the basalts and hawaiites

of the Tirioko basalt formation. The compositional range measured is Fal6

to Fa51o Groundmass olivine in mugearitic lavas is frequently altered to

iddingsite and phenocrysts are rare in these lavas. Fayalitic olivine

(Fa84 “ Fa92) occurs as small phenocrysts in some of the Nasaken trachytes.

Where both olivine and clinopyroxene are present, the olivine is richer in +2 Fe than the clinopyroxene, the relationship usually found (Muir and

Tilley, 19615 Aoki, I964 ). Compositional data on olivines are given in.

Fig. 53 and Table 35=

Clinopyroxene s Phenocrysts of clinopyroxene are present in most basic

lavas and are diopsidic augite or augite Those in the intermediate lavas appear to be ferroaugites. The trachytes contain phenocrysts of hedenbergitic pyroxene while aegirine-augite.and aegirine occur as groundmass minerals. Compositions derived from optical data are plotted in Fig. 55# Attempts to separate clinopyroxenes for chemical analysis were unsuccessful. However three analyses of pyroxene megacrysts from the Samburu basalts, Paka and volcanoes are presented in Table

55= These compositions plotted in Fig, 55 fall close to those determined optically for pyroxenes from the Tirioko basalts. It is important to note the high Al^O^ contents of these pyroxenes, that of the Paka sample being exceptionally high. Precipitation-, of aluminous pyroxene together with calcic plagioclase would tend rapidly towards the production of a per­ alkaline condition.

The predominant trend in the optically determined pyroxenes is 12 extensive replacement of Mg by Fe with an almost constant, high Ca content and is similar to those of pyroxenes from Japanese alkalic lavas (Aoki,

1964 ), the Nandewar volcano, N.S.W. (Abbott, 19^9) and Suswa volcano,

Kenya (Nash et al., I969 ). Such trends are typical of clinopyroxene fract­ ionation under low oxygen fugacities (Yagi, 1966), That clinopyroxene S

•kj c: %* o

t* Q.

o <. o

Q «o o

•o t 1 7 3

Table 55 « Olivine and pyroxene analyses

8/31+ El PI 81 SiOg 57.86 48.08 49.65 50.37 Ti02 0.05 0.90 0.84 0.96 AlgO^ 0.54 5.72 9.37 4.49 FegO^ 2.00 1.87 2.55 FeO 16.50 6.57 3.87 3.27 MnO 0.25 0,16 0.14 0.11 MgO 43.69 18.81 15.65 15.40 CaO 0.58 16.75 18.08 21.20 Ha^O 0.14 1.53 0.74 KgO 0.05 0.01 0.54 0.10 CrgO^ 0.41 _OaM 99.09 99.93 2 2 M

Si 0.98 Si 1.79 1.81 1.86 A1 0.01 Alz 0.21 0.19 0,14 Ti 0.00 Aly 0.04 0,21 0.05 Mg 1.68 Ti 0.05 0.02 0.05 0.06 0.05 0.07 Pe+2 0.54 Fe+3 Mn 0.01 Mg 1.04 0.74 0.84 Ca 0.02 Fe+^ 0.20 0.12 0.10 K 0.00 Mn 0.01 0.00 0.0c Ca 0.67 0.71 0.84 Atomic % Na 0.01 0.11 0.05 Fa 17 K 0.00 0.05 0.00 Fo 85 Cr 0.01 0.02

+ X.R.F, analysis Z 2.00 2.00 2.00 X + Y 2.06 2.01 2.02

Mg 52.5 45.7 45.4 Fe 15.6 10.5 9.2 Ca 33.8 43.8 45.4

8/Bl - Olivine phenocrysts, basalt, Tirioko formation KL - Pyroxene from gabbro nodxile, Korosi volcano PI - Pyroxene megacryst from tuffs, Paka volcano SI — Pyroxene megacryst from tuffs, Samburu basalts 1 7 4

salie phenocrysts occurring in peralkaline/lavas are always of hedenbergitic composition is indicated by the studies of Carmichael (l962) on pantell- erites, Abbott (I969 ) on trachytes and comendites, Nash et al. (1969 ) on trachytes and trachyphonolites and Nicholls and Carmichael (1969 ) on comendites and pantellerites.

Feldspars s Phenocrysts of feldspar are found in all lava—types. Many

Tirioko lavas are 'big feldspar* basalts and hawaiites possessing abund­ ant, large, white, plagioclase phenocrysts, Plagioclase phenocrysts also occur with clinopyroxene and olivine in basalts and intermediate rocks.

Optical determinations using recognized twin-law techniques suggest a compositional range from An80 to Anl5 'in basalts to trachymugearites.

Phenocrysts from two Tirioko lavas have been analysed (Table 36) and are plotted in Pig, 54* Pig* 54 also shows the compositions of alkali feld­ spars (Tables 56 and 3?) from the Nasaken volcano determined chemically or by the X.R.3.201 method. Most of the feldspar compositions,including those from the syenites ^cluster around the minimum point on the Ab-Or join although there is a range in Or content from Gr30 to Or37»4« All the analysed alkali feldspars fall into the anorthoclase field and are unmixed to varying degrees (Tables 4 and 5)« The alkali feldspar pheno­ crysts with one exception have An contents of less than 3 per cent. A point plotting near the albite section of the diagram represents the composition (Ab84, An5, Orll) of phenocrysts separated from 8 /125, a soda trachyte. The groundmass feldspars in this lava have a composition (Ab65

An?,Or33) similar to those of the phenocrysts of the more evolved quartz- trachytes and allcali rhyolites.

5^© normative feldspars have been plotted in Pig, 55»

The normative feldspar trend of the Pliocene lavas is inteirmediate be­ tween that for the East Otago basalt-trachyte series (Coombs and Wilkin­ son, 1969 ) and that for the Hawaiian alkalio suite (Macdonald and Eat sura,

1964 ). 1 7 5

Table 36 Peldspar analyses

8/393"*" 8 /119 + 8 /125+ 8/2066+ 8/129 8/89 Si02 49.07 52.57 66.18 67.03 TiOg 0.00 0.00 0.00 0,00 Al£0^ 32.22 29.25 19.53 18.39 PegO^ 0.84 0.76 1.20 1.31 1.44 0.75 PeO MnO 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 CaO 15.80 12,62 0.50 0.23 0.03 0.24 Na20 2.59 4.11 7.45 7.26 7.11 7.71 KgO 0.28 0.51 5.81 5.82 6.07 6.33 P2O5 0.19 0.07 0.04 0.01 100.98 99.90 100J2 100.05

Si 8.94 9.58 11.78 A1 6.92 6.29 4.10 Pe+3 0.11 0.10 0.16 Ca 3.08 2.47 0.10 Na 0.91 1.45 2.57 K 0.07 0.12 1.32 P 0.00 0.00 0.00

Z 15.97 15.97 16.04 X 4.06 4.04 3.99

¥t Ab 22.50 36.0 64.5 64.7 62.6 61.4 % An 75.90 60.9 • 2.4 1.2 0.1 1.2 Or 1.60 3.1 33.1 34.1 37.3 37.4

+ analysis by X. R. P, remainder are partial

8/393 - plagioclase phenociysts, basalt, Tirioko formation 8/119 - plagioclase phenocrysts, basalt, Tirioko formation 8/125 - groundmass feldspar, trachyte, Nasaken 8/2066 - anorthoclase phenocrysts, trachyte, Nasaken 8/129 - anorthoclase phenocrysts, trachyte, Nasaken 8/89 “ anorthoclase phenocrysts, trachyte, Nasaken ilC5 S

"b

IIS

X H-

l i t 176

Table 37 ï'eldspar analyses

8 /l25a 8/128 8/l28a+ 8/800* 5/4 SiOg 66.60 66.45 66,71 66.00 Ti02 0.00 0.00 0.00 0.05 AlgO^ 18.95 19.49 19.41 19.10 ^'©2^3 0.45 0.85 1.11 0.52 FeO 0.07 MnO 0.00 0.00 0.00 0.02 MgO 0.04 0.00 0.00 0.00 0.00 CaO 0.64 0.05 0.25 0.23 0.47 NagO 10.06 7.79 7.67 7.29 7.28 K2O 1.86 6.40 6.27 6.45 6.56 P2O5 0.01 0.02 0.01 0.02 100.32 100.99 1.1.20 100.02

Si 11.91 11.81 11.83 11.86 A1 3.99 4.08 4.06 4.00 Fe"*"^ 0.06 0.11 0.15 0.07 Ca 0.01 0.05 0.04 0,08 Na 2,70 2,65 2.51 2.50 K 1.46 1.42 1.46 1.46 P 0.00 0.00 0,00

Z 15.96 16.01 16.04 15.93 X 4.17 4.11 4.01 4.04

Ab 83.9 63.4 64.5 62.5 60.0 An 4.7 0.2 1.1 1.1 2.2 Or 11.4 56.4 54.6 36.4 37.8

+ analysis by X. R. F, remainder wet chemical analyses

8/l25a - anorthoclase phenocrysts, trachyte, Nasaken. 8/128 - antiperthite feldspar, syenite, Nasaken. 8/l28a - antiperthite feldspar, syenite, Nasaken. 8/800 - antiperthite feldspar, syenite, Kafkandal. 5/4 - antiperthite feldspar, syenite, Ribkwo,

1 7 7

The variation in feldspay compositions in the Nasaken area lavas is in accord with theoretical trends (Tuttle and Bowen, 19585 Carmichael,

I96 O5 19655 Nicholls and Carmichael, I969 ), and suggests that the sequence has developed by crystallization differentiation. As a liquid of Nasaken trachyte composition or any pantelleritic liquid is cooled, the precip­ itating feldspar becomes progressively more potassic whereas the residual liquid becomes steadily more sodic (and more peralkaline providing there is an An component to the feldspar). Such a trend may be modified by the precipitation of a sodic ferromagnesian mineral in sufficient amount or by the expulsion of soda as the liquid phase diminishes,

3c) Chemistry of the lavas»

In Tables 58-45 are presented 44 major-oxide analyses of lavas

(and one syenite) from the Tirioko basalt formation and the Nasaken volcano. Of these, 5I were by wet chemical methods and the remainder by

X-ray flourescence (see appendix l). The analyses have been arranged in order of increasing SiO^ contents. Also given are the C.I.P.W, normative minerals (see appendix 2), The I5 X.E.P. analyses were made to supplement the wet chemical data and are partial analyses in that ferrous iron, water and carbon dioxide have not been determined. (Values obtained for oxides determined by the two methods are quite comparable since 10 wet chemically analysed,Nasaken area lavas were used together with other standards to erect the X-ray calibrations), In order to calculate the normative miner­ alogy, estimates of PeO in the X-ray analysed rocks have been made. An

PeO/PeO + PegOy ratio of 0,7 has been chosen for the intermediate and trachytic lavas. This iron-oxide ratio is typical of glassy lavas of a similar composition (Carmichael, 1962; Macdonald et al», 1970),

Oxidation state of iron

Many of the lavas have too low an PeO/PeO + PegO^ ratio indic­ ated by the appearance of hm in the norm of the basic and intermediate 175

Table 38 Tirioko basalt formation

8/81 8/122 8/l05a 8/93 8/32a 8/182 SiOg 44.00 44 • 60 44.61 44.98 45.11 45.33 TiOg 1*89 2*37 2*50 1.53 3.73 2,82 ■^^2^5 12*82 17.58 15.57 14.31 15.63 14.51 FegO) 3.90 4.50 5.18 3.98 12.90 3.95 FeO 7.47 7.25 6.49 6.61 2*68 7.16 MnO 0.18 0*14 0,17 0.10 0.09 0.20 MgO 12*74 6.10 6.64 7.46 3.45 7.93 CaO 13.27 11,05 11*07 11.54 9.36 10.41 NagO 1*51 2.79 3.71 2*80 3.09 2.86 KgO 0.59 1*09 1*24 0.63 1*10 1.65 HgO+ 0.96 2.15 1.07 1*09 2*02 2*30 HgO— 0.33 0.45 0.34 1.31 0.37 0*34 P2O5 0*19 0.31 0*55 0.18 0*80 0.56 COg 0.11 3.19 0.14 99 .8 % 100.38 99.25 99.71 100.33 100.16

s.I* 48.6 28*07 28.55 34.73 14.86 33.67

qz 3.37 or 3.49 6.44 7.33 3.72 6*50 9.75 ab 8*25 18.63 17.58 23.69 26.15 18*91 an 26*46 32.23 22*17 24.62 25.53 21.88 ne 2*45 2*70 7.48 2.87 di 30.36 16*54 22.64 9.06 12*21 20.14 hy 17.63 2.93 ol 17.87 9.50 6.86 2*24 11*28 mt 5.65 6.52 7.51 5.77 5.73 il 3.59 4.50 4.75 2.91 5.85 5.36 hm 12*90 ap 0.44 0.72 1*27 0*42 1.85 1*30 CO 0*25 7.26 0*32 1

8/81 - ankarainite, Nape it om basalts o 8/122 - feldsparphyric basalt, Kagnmnyikal basalts* 8/l05a - analcite-basalt with kaersutite xenocrysts, Lotokongolea basalts. 8/93 - basalt; Napeitom basalts* 8/32a - basalt; Napeitom basalts* 8/182 - analcite-basalt; Lotokongolea basalts*

1 includes 0*65 rutile* 179

Table 39. Tirioko basalt formation

8/360 8/119 8/85 8 /121+ 8/120 8 /l24a‘ SiOg 46.39 47.27 49.21 52.05 53.32 54.72 TiOg 2.37 2.41 2.67 2.46 2.33 1.82 AI2O3 16.70 19.38 16.51 17.02 17.26 16.77 FegO^ 4.08 2.59 10.11 11.49 10.42 10.00 FeO 8.08 7.39 2.29 0.51 MnO 0.19 0.17 0,12 0.22 0.12 0.12 MgO 6.07 3.07 1.98 1.27 0,77 1.46 CaO 9.16 9.66 7.54 4.37 4.50 3.91 Na^O 3.20 3.77 3.76 5.50 5.34 6,36 KgO 1.12 1.56 2.78 2.68 2.61 3.02 HgO+ 1,58 1.62 2.09 1.62 HgO— 0.50 0.40 0.30 0.44 P2O5 0.46 0.56 0.96 0.81 0.86 0.45 COg 0.08 0.01

100.06 100.32 97.87 100.10 98.51

8.1. 26.92 16.70 9.46 6.30 3.92 7.25

+ X.R.F, analysis

qz 2.71 4.19 or 6.62 9.22 16.43 15.84 15.42 17.85 ab 27.08 26.69 31.82 46.54 45.19 48.20 an 27.90 31.35 19.96 13.84 15.42 8.29 ne 2.83 3.05 di 11.38 10.64 8.70 2,18 1.00 6.88 hy 2.08 0.90 0.89 1.45 ol 11.19 7.47 6.92 4.83 mt 5.91 3.76 0.03 4.35 4.35 il 4.50 4.58 5.07 4,67 1.33 3.46 hm 10.09 10.42 ap 1.07 1.30 2.22 1.88 1.99 1.04 CO 0.18 0.02 2 1 2 8/360 - basalt, Kaguianyikal basalts. 8/119 - feldsparphyric basalt, Kagumnyikal basalts, 8/85 - hawaiite, Kaguranyikal basalts. 8/121 - nnigearite, Kagumnyikal basalts, 8/120 - mugearite, Kagumnyikal basalts, 8/l24a - trachyrarugearite, Napeitom mugearites,

1 includes I.63 rutile 2 norm calculated with FeO/FeO + Fe20^ ratio of 0.7 l8o

Table 40'

8 /124* 8/658 8/326 8 /26* 8/126 8/387a SiOg 55.94 61.79 43.54 51.26 56.05 59.19 TiOg 1.91 0.51 3.05 2.26 1.79 0.54 ■^^2^3 17.21 15.52 15.77 17.80 16.69 12.80 FegOg 9.78 3.32 6.49 11.31 8.29 5.46 FeO 4.55 8.09 0.93 3.46 MnO 0.19 0.24 0.22 0,17 0.12 0,35 MgO 1.58 0.19 6.27 0.91 0.68 0.55 GaO 3.74 0.70 10.42 4.44 3.55 1.38 NagO 6.15 7.04 2.43 5.17 5.82 5.08 KgO 3.06 5.23 0.64 2.76 3.15 4.41 HgO+ 0.63 1.82 1.73 4.62 HgO- 0.32 0.80 0.51 1.69 P2O5 0.50 0.01 0.47 0.93 0.47 0.04 00^ 0.21 100.06 100.05 100.22 2L01 99,78 99.57

8.1. 7.68 0.93 26.21 4.69 3.60 2.90

+ XoR oF 0 analysis qz 3.80 9.99 or 18.08 30.91 3.78 16.31 18.61 26.06 ab 51.12 50.53 20.56 43.75 49.25 41.29 an 10.32 30.23 15.95 10.11 ne 0.50 0.10 ac 7.80 1.50 ns di 4.20 3.01 13.65 3.45 5.52 hy 9.17 5.10 0.10 0.63 ol 6.71 4.86 3.43 3.88 mt 4.35 0.90 9.41 4.35 7.16 il 3.63 0.97 5.79 4.29 2.22 1.02 hm 8.29 ap 1.16 0,02 1.09 2.16 1.09 0.09 CO 0,48 2 2 1 8/124 ” trachjmrugearite, Napeitom mugearites. 8/658 - trachyte, Kafkandal. 8/326 - basalt, Nasaken. 8/26 - mugearite, Nasaken. 8/126 - trachymugearite, Nasaken. 8/387a - trachyte dyke selvage, Nasaken. 1 includes 0,46 corundum 2 norm calculated with FeO/PeO + FegO^ ratio of 0,7 l8l

Table 41» Nasaken volcano

8 /l75 a 8/387 8 /163F+ 8/1630* 8/640 8/313 SiOg 60.61 60.62 61.22 61.67 61.70 62.04 TiOg 0.52 0.52 0.48 0.48 0.56 0.78 AlgO^ 16.29 14,19 14.18 14.10 15.19 13.91 FegO^ 5.48 4.50 9.40 8.88 3.75 5.42 FeO 0.85 4.49 4.02 3.02 MnO 0.34 0.24 0.22 0.22 0.24 0.20 MgO 0.26 0.47 0.48 0.50 0.27 0.30 CaO 1.50 1.23 0.88 0.76 1.10 1.18 NagO 6.92 6.78 5.45 6.41 6.72 6.28 KgO 5.58 4.57 5.07 4.95 5.35 5.05 H2O+ 0.76 1.28 0.39 0.98 HgO— 0.18 1.32 0.49 1,16 0.10 0,08 0.01 0.08 P2O5 0.07 0.07 COg 0.83

100.22 100.28 97.46 58.04 2 9 . 7 9 100.40

8.1. 1.36 2.26 2.42 2.48 1.34 1.49

+ X.R.F, analysis

qz 1.03 4.76 1.80 5.63 or 32.97 27.00 29.96 29.25 31.61 29.84 ab 51.96 47.55 44.71 44.97 48.35 43.43 an ne 0.42 ao 5.14 8,66 1.24 7.20 7.50 8.55 ns 0.26 di 1.20 4.91 3.36 2.90 4.73 4.59 hy 5.20 8.52 10.01 3.35 0.98 ol 0.06 0.61 mt 2.34 2.19 3.21 1.68 3.57 il 0.99 0.99 0.91 0.91 1.06 1.48 hm ap 0.23 0.16 0.18 0.16 0.02 0.18 CO 1.89 2 2 8/l75a - trachyte intrusion, Kasamanang valley, Nasaken. 8/387 - trachyte, Nasaken. 8 /I63F - trachyte, Nasaken. 8 /I63G - trachyte, Nasaken. 8/640 - trachyte, Nasaken. 8/313 “ trachyte, Nasaken. 2 norm calculated with FeO/FeO + FegO^ ratio of 0.7 182

Table 42. Nasaken volcano

8/l63d* 8 /l63e+ 8/125 8 /11+ 8 /1630* 8/67 8iOg 62.24 62.28 62.34 62.49 62,64 62.99 TiOg 0.53 0,46 0.62 0.84 0.50 0.58 AlgO? 14.26 14.48 16.62 17.47 13.40 15.26 FègO^ 9.78 8.50 2.41 5.58 9.91 3.78 FeO 2.97 2.61 MnO 0.15 0.17 0.18 0.08 0,22 0.23 MgO 0.55 0,61 0.46 0.90 0.43 0.39 CaO 0.90 1.02 1.67 1.73 0.97 0.76 Na20 5.54 5.82 7.11 7.53 5.86 6.66 KgO 5.20 4.97 5.27 4.18 4.75 5.23 HgO+ 0.23 0.75 HgO- 0.34 1.07 Pg05 0.09 0.06 0.08 0.12 0,12 0.04 CO2

99.37 100,30 100,92 98,88 1 0 0 ^

S.I. 2.68 3.15 2.52 4.95 2.12 2.09

+ X.R.F, analysis qz 5.56 4.22 5.45 3-18 or 30.73 29.37 31.14 24.70 28.07 30.90 ah 44.40 46.81 51.82 60.43 42.89 49.37 an 1.52 ne 2.34 1.78 ac 2.15 3.54 5.90 6.15 ns di 3.37 4 o06 6.67 5.28 3.54 3.01 hy 7.23 7.82 10.08 2.32 ol 1.13 2.43 mt 3.98 2.37 1.72 2.90 1.07 2.40 il 1.01 0.87 1.18 1.59 0.95 1.10 hm ap 0,21 0.14 0.18 0.28 0.28 0,09 cc 2 2 2 2

8 /163d - trachyte, Nasaken, 8/l63e - trachyte, Nasaken. 8/125 - trachyte on Kaguronyikal liill, Nasaken, 8/11 - trachyte, Nasaken. 8/1630 - trachyte, Nasaken. 8/67 - trachyte, Lodukai dyke. 2 norm calculated with FeO/PeO + FegO^ ratio of 0.7 183

Table 43. Nasaken volcano

8/129 8 /5+ 8/116 8/249 8/263 8/89 Si02 63.35 63.45 63.74 63.79 64.07 64,08 TiOg 0.61 1.09 0.47 0.49 0.53 0.65 AlgOj 13.70 19.08 13.26 15.45 15.00 13.78 FegOj 4.46 2.93 4.44 3.71 2.66 4.18 FeO 3.71 3.69 2,66 3.80 3.64 MnO 0.21 0.09 0.19 0.18 0.17 0.25 MgO 0.19 0.42 0,19 0.16 0.15 0.26 CaO 0.64 1.81 1.02 0.49 1.02 0.70 NagO 6.22 6.94 6,16 6.57 6,65 6.31 KgO 5.29 4.56 4.83 5.36 5.28 4.97 H2O+ 1,01 0.75 0.87 0.53 0.77 HgO— 0.62 0.95 0.41 0,32 1,17 PgO^ 0.01 0.21 0.02 0.02 0.05 0.03 COg 0.02

100.02 100.58 99.73 100,16 100,21 100.79

s.I. 0.96 2.83 0.98 0.87 0.81 1.34

+ X •R.F. analysis

qz 6,16 1.26 8.12 4.40 2.96 7.24 or 31.26 26.95 28.54 31.67 31.20 29.37 ab 41.02 58.72 41.31 49.63 47.76 43.21 an 7.44 ne ao 10.23 9.52 5.25 7.50 8.97 ns • di 2.74 0.14 4.23 2.01 4.30 2.87 hy 4.47 2.06 3.66 2.19 4.47 4.33 ol mt 1.34 1.45 1,66 2.74 0.10 1.56 il 1.16 2.07 0.89 0.93 1.01 1.23 hm ap 0.02 0.49 0.05 0.05 0.07 0.07 cc 0.04

8/129 - quartz-trachyte o 8/5 - trachyte, 8/116 - quartz-trachyte, 8/249 - trachyte, 8/263 - trachyte, 8/89 - quartz-trachyte. 2 norm calculated using PeO/PeO + FegO^ ratio of 0.7 l84

Table 44« Nasaken volcano

8/206a 8/130 8/111 8/281+ 8 /l63b+ 8/2061 SiOg 64.54 64,67 64.82 66,88 67.00 67,02 TiOg 0.69 0.55 0,53 0.66 0.42 0,42 ■^^2^3 14.07 11.66 13.13 12,23 11.01 11,85 FegO^ 4.48 7.83 6.79 7.74 8.91 6.04 FeO 2.55 1.32 1.00 0.27 MnO 0.18 0.26 0.36 0.30 0.22 0.15 MgO 0,11 0.24 0.14 0.58 0.26 0.34 CaO 0.44 0.72 0.46 0.92 0.95 0.89 NagO 5.95 6.43 6.97 5.99 5.72 4.92 KgO • 5.15 5.19 5.00 4.91 4.04 4.44 H2O+ 1,19 1.04 0.69 1.65 EgO- 0.65 0,74 0.55 0.95 Pg05 0,00 0.04 0.03 0.06 0.08 0,00 COg 100,00 100.69 100.47 100.27 98,61 99-07 8.1. 0.60 1.14 0.70 3.02 1.41 2,12 + X.R.F, analysis qz 10.27 11,04 8.51 13.23 17.40 21.36 or 30,43 30,67 29.5 5 . 29.01 23.87 26,24 ab 43.70 31.08 39.70 35.57 34.14 36.23 an ne ac 5.86 20.55 16.98 6.71 8.68 4,76 ns 1.74 1.03 di 1.91 2.82 1.78 3.64 3.70 1.83 hy 1.13 0.56 0.31 8.96 8.27 ol mt 3.56 1.05 1.33 0.14 il 1.31 1.04 1.01 1.25 0,80 0.80 hm 4.30 ap 0.09 0.07 0.14 0.18 cc 0.30 2 2 1

8/206a - pantelleritio trachyte, 8/130 " pantelleritio trachyte, 8/111 - pantelleritio trachyte, 8/281 - pantelleritio obsidian, 8 /163b - pantelleritio trachyte. 8/206b - pantelleritio trachyte, 2 norm calculated using PeO/FeO + FegO^ ratio of 0,7 1 includes 0.52 wollastonite 185

Table 45

8/233 8/128 5/66 3/439 43/1/8/6 43/1192 SiOg 67.30 64.17 59.79 60.19 62.70 63.40 TiOg 0.73 0.25 0.59 0.70 0.69 0.73 AlgO^ 11.02 17.28 14.22 14.97 12,50 12.06 FegOj 4.58 2.89 5.98 5.29 2.04 3.50 FeO 4.13 1.16 3.46 3.16 6.90 5.70 MnO 0.18 0.13 0.25 0.27 0.35 0.40 MgO 0.03 0.20 0.48 0.42 0.18 0.31 GaO 0.74 0.37 1.43 1.42 . 1.20 0.99 NagO 5.12 7.92 6.73 6.41 8.52 7.95 KgO 4.66 5.84 4.94 5.46 4.72 4.55 HgO+ 0.86 0.27 1.37 1.03 0.19 0.24 HgO— 0.63 0.16 1.43 1.23 PgO^ 0.02 0.02 0.01 0.04 0.10 0.10 COg 100.00 100.66 100.68 100.52 100.09 99.93 s.I. 0.16 1.11 2.22 2.02 qz 18.98 0.39 0.33 6.1 7.45 or 27.54 34.51 22.19 32.26 27.8 26.7 ab 30.74 50.79 45.64 46,60 38.3 37.2 an ne 3.02 ac 11.09 8.36 9.96 6.72 6.0 5.7 ns 0.27 5.9 4.5 di 3.15 1.47 5.04 6.37 2.1 3.7 hy 4.50 1.85 0.57 11.4 8.8 ol 1.29 mt 1.08 3.68 4.29 il 1.39 0.47 1.12 1.32 1.4 1.4 hm ap 0.05 0.05 0.02 0.09 0.3 0.2 CO

8/233 - pantelleritio trachyte, Nasaken. 8/128 - pulaskite syenite, Nasaken. 5/66 “ trachyte, Ribkwo. 3/439 “ trachyte, Ribkwo. 43/1/8/6 - trachyte, Menengai 43/1192 - trachyte, Menengai. 186

rocks. Assuming that it is only the iron—oxide ratio that has been dis­

turbed (although oxidation is usually accompanied by hydration which if

only affecting the sum total of the oxides depresses the other values) re­

adjustment can be made to give a more realistic normative mineralogy. For

example 8/120, Table 39; has over 10 per cent normative haematite. Re­

calculation with an oxide ratio of 0,5 causes haematite to disappear from

the norm and = 3«2 per cent, A ratio of 0.7 produces = 1,5 per cent,

Alteration of the iron—oxide ratio alters the femic mineralogy but the

calculation of the normative feldspar is independent of such a change.

The oxidation state of iron in the trachytio lavas is such that nn just fails to appear in the norm. Changing the iron-oxide ratio from 0,46 to

0,7 in 8/89 for example, produces 0,6 per cent ns and lowers from 7*2 to 6.3 per cent. Of 20 Ribkwo trachytes9 none containing modal quartz and 13 containing modal nepheline, 16 arc quartz normative averaging 1,5 per cent. Alteration of the iron-oxide ratio which averages 0,4 to a more reduced composition causes the normative quartz to disappear in most cases

(since there is more FeO to combine with SiO^ to form hypersthene) and results in a closer correspondence between modal and normative mineralogy.

Because of the doubt as to the validity of iron-oxide ratios in the analysed rocks, graphical presentation of the data in terms of oxides rather than normative minerals is preferred thus Solidification index (Kuno et al., 1957) is used rather than Differentiation index

(Thornton and Tuttle, I960 ), However calculation of the C,I.P,¥, norm is considered useful especially as the normative feldspar compositions are unaffected by the oxidation state of iron,

Ma.ior-oxides

The mildly alkaline and undersaturated nature of the Nasaken lavas is illustrated in Fig, 56. The analyses lie well above the line separating alkalic andtholeiitic rocks of Hawaii (Macdonald and Katsmra,

1964 ). The A - F - M diagram (Fig,57) shows a gradual trend toward alkali o K

>0

9 X

>o SI s:

'o

o q o L. %» CL ■C

X 18?

enrichment 0 The curve is similar to that for the East Otago basalt-

trachyte series (Coombs and Wilkinson, I969) except that the trachytes of

Nasaken lie closer to the iron-alkalis side line.

Most of the basaltic lavas of the Tirioko formation have

nepheline in the norm (Tables ^8 and 39)» Three lavas containing norma­

tive quartz also have over 10 per cent normative haematite, A single

basalt (8/360) is ^ normative. The basalts have high alkali contents

with Na^O always greater than K^O, Compared with alkaline basalts from

other provinces the SiO^ contents are low. Silica ranges from 44<>00

weight per cent in an ankaramite to 35*94 weight per cent in a trachy­ mugearite which is just ^ normative. The ankaramite in which clino- pyroxene and olivine are probably cumulative plots below the main trend

in Fig, 56,

The lavas of the Nasaken volcano (Tables 40-45) have silica ranging from 43*54 to 67,30 weight per cent but 27 out of a total of 30 have silica contents in excess of 59 per cent. Only three trachytic lavas are nepheline normative, the remainder having normative quartz up to 21 per cent.

Major'-oxides of the Tirioko and Nasaken lavas have been plotted against Solidification index (Fig, 58)° This index (MgO x 100 / MgO+FeO+

Fe^O^+NagO+KgO) is supposed to decrease in proportion to the amount of residual liquid and to be a measure of the degree of fractionation (Kuno et al,; 1957).

The graphs in Fig, 58 show fairly smooth trends from the basaltic compositions of the Tirioko formation through those of the inter­ mediate lavas to the trachytic compositions of Nasaken volcano. Much of the scatter on these plots is due to the inclusion of porphyritic rocks.

This is especially the case for graphs of alumina and total iron-oxid*^ One point with a very high value of Al^O^ represents a highly feldsparphyric I I I I I I I I I I I I Ill'll !K* •Vi 'I I %

c . 0 0 . 0 s: 1 o O I S’ Cÿ(o > 0 1 . c o

O o O o O o’ Cl o (O O 188

basalt (8 /119 ) which has correspondingly low total iron. In general, SiO^,

NagO and KgO rise steadily with decreasing Solidification index. MgO and

CaO fall steadily although it must be noted that the linearity of the MgO

plot is due to the nature of the Solidification index, Al^O, rises 2 3 steadily and falls very rapidly for low values of the index. The amount

of this oxide is particularly variable in the trachytes due mainly to diff­

ering contents of phenocrysts. The highest silica lavas have much lower

alumina contents than the trachytes (Tables 44 and 45). Total iron rises

steadily with decreasing Solidification index and reaches a maximum in

the intermediate lavas, falling in the trachytic rocks. The tendency for

the intermediate lavas to be highly oxidised may be due in part to their

high iron contents. Moreover these rocks are rich in magnetite and it may

be that iron is more susceptible to oxidation in this form than when com­

bined with silica.

One trachyte (8 /5) has anomalously low total iron (2,93 per

cent) and has only about 6 per cent mafic minerals. This highly leuco-

cratic dyke rock is unlike the other analysed Nasaken rocks.

Included for comparison in Table 45 are analyses of two typical

Ribkwo trachytes and two glassy trachytes from Menengai volcano (Macdonald

et al,; 1970 )» The Menengai trachytes liave higher FeO/PegO^ ratios than

similar Nasaken lavas resulting in the production of ^ in the norm. They

are also slightly lower in alumina, higher in soda and have a higher soda/ potash ratio. Analyses of Ribkwo trachytes are given in McClenaghan (1971) and Webb (1971 )= In general they have lower silica than Nasaken trachytes averaging 60 per cent and slightly higher lime and alumina. On a graph of alkalis against silica, the trachytes of Ribkwo are distinct from those of

Nasaken, Also plotted are the compositions of Tirioko formation lavas from the Ribkwo area. These in general are slightly more undersaturated with respect to silica than the basalts of the Nasaken area and it is likely that the chemical differences between the trachytes of the Ribkwo 189

and Nasaken volcanoes can be tra.ced to slight compositional differences in their respective, genetically related, basaltic magmas.( Fig. 59 )*

Trace-elements

The results of the X.R.F. analysis for the elements Ra, Ce,

La, Nb, Nd, Pr, Rb, Sr and Zr of the lavas of the Tirioko formation and

Nasaken volcano are presented in Tables 46-49 . All these elements are particularly abundant in alka,line rocks and may be determined with high precision,

Ba concentration in the basaltic rocks is high, 300-700 p,p,m.

The low value of 226 p,p,m, for the ankaramite (8 /8 I), in which the mafic phenocrysts have accumulated, suggests that Ba does not enter either olivine or clinopyroxene in any great amount. Ba is concentrated in the intermediate lavas with contents in excess of 1000 p.p.m. and in one case

(8 /26) over 3000 p.p.m. The concentration falls in the trachytes and Ba is strongly depleted in the pantelleritio trachytes, The concentration of Ba in anorthoclase phenocrysts varies from 88 p.p.m. to 1734 p,p*m, the latter occurring in the feldspar least rich in Or molecule. The linked substitution of BaAl for KSi in alkali feldspars is well known.

Since the analysed alkali feldspars are typical of those of peralkaline lavas in containing significant amounts of ferric iron (Tables 36 and 37) 9 the substitution Ba Fe"^^ seems highly probable in phases precipitated from such alumina-deficient liquids •

Sr concentrations are high in basalts, particularly feldspar­ phyric ones (8 /119, 8 /122), Plagioclase phenocrysts from 8 /II9 contain

1798 p.p.m. Sr (which substitutes for Ca) and 232 p.p.m. Ba. Sr con­ centrations remain high in the intermediate lavas, but the element is strongly depleted in the trachytio lavas averaging less than 20 p.p.m.

The other trace—elements Zr, Nb, Rb and the light R.E.E. in­ crease systematically from basalts to trachytes. ••

o' CO % O

Cr >) k. tr tb k \ ^QO IÇ O Q Q O O ■O -t'­ es 'i: □o c> □ □ & ° Î -e X +- \ □ o \ II \ .‘O II V 9 V°D O \ cs o *x o S k ^ 'c: □ \ ^ K: QC k: \ • o \ □ .o X cx O > o O

‘o V) 190

Table 46» Trace—element contentsj Tirioko basalts (p«p»m.)

8/81 8/122 8/95 8/182 8/560 8/119 Ba 226 407 275 715 629 571 Ce 48 55 55 106 62 77 La 17 25 19 50 28 33 Nb 21 27 20 68 30 45 Nd 19 18 13 58 15 37 Pr 7 7 6 12 6 9 Rb 12 27 12 59 25 52 Sr 531 944 520 748 766 1227 Zr 64 105 77 195 120 157 K/Rb 408 555 456 351 372 249

8/85 8/121 8/120 8 /124A 8/124 Ba 710 960 888 1520 1290 Ce 96 156 156 146 144 La 50 69 88 79 80 Nb 54 86 89 92 95 Nd 36 51 72 62 62 Pr 11 13 15 16 15 Rb 50 62 44 85 86 Sr 740 805 774 788 775 Zr 180 507 286 516 508 K/Rb 461 559 485 295 295 191

Table 47. Trace--element contents, Nasaken volcano (p.p.m. )

8/526 8/26 8/126 8 /l65f 8/l65g 8/641 Ba 296 5511 1070 79 74 16 Ce 55 151 147 527 520 261 La 25 80 84 220 560 114 Nb 24 95 110 521 520 527 N d 6 62 66 141 226 79 Pr 7 18 20 45 65 25 Rb 19 60 71 520 261 228 Sr 625 725 766 20 19 9 Zr 115 294 521 1704 1668 975 K/Rb 280 582 562 151 157 195

8/515 8/l65d 8/l65e 8/125 8/11 8/l65( Ba 208 65 69 954 1426 75 Ce 165 628 522 165 172 616 La 94 504 204 91 86 556 N b 127 607 497 159 114 614 N d 78 186 142 81 66 227 Pr 20 59 42 16 17 66 Rb 88 555 290 124 99 521 Sr 14 21 54 15 195 25 Zr 592 1978 1646 426 451 1877 K/Rb 476 122 145 555 547 125 192

Table 48 0 Trace-■element contents, Nasaken volcano (p.p.m.)

8/67 8/129 8/116 8/249 8/265 8/89 Ba 14 22 0 39 29 17 Ce 196 269 567 5I8 131 346 La 154 56 560 162 68 190 Nb 127 558 439 287 217 288 Nd 110 40 319 129 54 142 Pr 23 16 68 32 16 34 Rb 112 214 298 141 149 172 Sr 16 13 11 21 5 14 Zr 560 1257 1590 1043 743 1007 K/Rb 588 205 134 315 292 240

8/206a 8/150 8/111 8/281 0/l65b 8/206b Ba 443 32 11 118 44 178 Ce 265 5I8 197 350 492 324 La 141 312 126 195 341 182 Nb 168 467 204 286 497 376 Nd 112 220 105 148 214 158 Pr 33 51 26 30 61 34 Rb 120 294 162 160 297 101 Sr 14 19 19 15 19 50 Zr 653 1660 734 1276 1670 1412 K/Rb 356 146 256 255 112 204 193

Table 49* Trace-element contents, Syenites (p.p.m.)

8/253 8/128 8/l28a 8/800 5/4

Ba 52 38 528 70 70 Ce 737 119 181 202 109 La 431 62 77 104 66 Nb 585 169 218 205 90 Nd 277 55 62 92 67 Pr 67 15 18 20 15 Rb 159 348 187 168 130 Sr 22 14 62 109 17 Zr 1880 431 743 580 233 K/Rb 244 139

8/128 - Nasaken syenite 8/l28a - Nasaken syenite 8/800 - Kafkandal syenite 5/4 - Ribkwo syenite 194

Table 50 0 Trace--element content of feldspars (p.p.m.)

8/119 8/89 8/129 8/206b 8/222 8/249 Ba 232 112 88 262 1734 976 Rb 6 86 90 78 48 34 Sr 1798 24 24 12 26 20

8/313 8/128 8/128a 8/800 5/4 Ba 92 84 410 76 90 Rb 82 410 203 194 128 Sr 18 8 71 16 4

8/119 plagioclase AnGl, Ab56, Or3 8/89 alkali feldspar Or35 8/129 " " Or37 8/206b H " Or34 8/222 »» » Or30 6/249 " " Or37 8/313 " " Or36 8/128 ” " Or36 8/l28a •» Or35 8/800 ÎI n Or 3 6 5/4 n " or38 o o o o

_ o

_ p

- P

(/)

Lp I4^^ CoO o

0 o # o F/g. 6! o 0

o

So «

• • e • e > * O 1 1 , , 1 * 1 t 1 1 I 1 •• 1 '1 1 1 1 1 1 *1 1

- S/'O^ • • • • 65 — - • ; • / • • % . e

> - o* 5 5 - o % * • Nasaken volcano 50 o Tirioko formation o : o°o 45—- °o : °*

40- Zr , I • I---- 1---- 1-----!-----!---- 1 1 1 t 1 I — 1 1 1 1 1 500 /OOO Z'5’0 0 p,p,m, 2000 1 MgO 0 12- F /g . 6 2

• Nosoken volcano o o o Tirioko formation

C*

o

" (g • • • # # 0 1 1 1 *1 1 * t • 1 1 1 1 # T p 1 *1 I •!

A/^Oj 20 o

0 •

% ! - : # /5- 0 0 • • . . • • # o e • • e •

/o-

Z r

l i l t 1 j 1 1 1 1 1 j _—L----1----1-----!-----1----L 1 1 500 1000 /500 p.p.m. 2 0 0 0 15-

o

o o % /O- o • • •• e

•• f\j k . • 5-

Zr

J— —J i I 1 I I 1 I I I ! I I I I I I I

12-

cP O ^ 8 + O ^ • Nasaken volcano r o o Tirioko formation 4 o o

Zr

I I I f I I f I I I I !_____ !_____ I------1------!------1------1------1------1 500 /OOO /500 2 000 p.p.m.

Fig. 6J Total iron oxide and total alkalis against Z r. 195

Although the graphical use of the Solidification index is a convenient method of representing major—oxide variations in basic and intermediate rocks, considerable variation in major-oxide composition takes place within the trachytic and pantelleritic lavas at almost con­ stant Dolidification indexc This is even moremarkcd when considering trace-elements, In Fig. 60, Zr content multiplies 5-fold with a small and non-systematio variation in S,I. The reasons for choosing zirconium as an index of fractionation are given (p,184 ) in a recent trace—element study on volcanic rocks from the East African rift system (Weaver, Sceal and Gibson, 1972). (Since the paper contains a large part of the results of the author’s geochemical study of the Nasaken area, a copy is included as part of this thesis).

In Figs. 61-659 the oxides SiO^, Al^O^, CaO, MgO, total iron as FegO^ and total alkalis are plotted against Zr content for lavas of the

Tirioko formation and Nasaken volcano. The variation diagrams obtained show continuity and overlap between the compositions of the two formations and in this respect are similar to Figs. 56-58. CaO and MgO fall rapidly as more fractionated compositions are approached. Both oxides remain nearly constant in the trachytes and pantelleritic trachytes averaging

0,8 and 0.5 per cent respectively. The AlgO^ graph shows a maximum for basic lavas rich in plagioclase phenocrysts and there is a gradual fall in

AlgO^ content from mugearites to pantelleritic trachytes. The considerable scatter on this graph is due at least in part tio the porphyritic nature of some of the lavas. Included here are five analyses from a single flow

(8/l65o-g) which appear to have anomalously high Al^O^ and SiO^ relative to Zr content compared with other trachytes. Excepting these analyses for the moment, SiO^ increases gradually with Zr content towards pantell­ eritic compositions but there is considerable scatter about the average trend. Total iron oxide is a minimum in the’least fractionated’trachytes

(5.7 per cent) but increases steadily with Zr reaching a maximum (9.9 per 196

cent) in the lavas of pantelleritic composition. Total alkalis shows a

gradual decrease throughout trachytic rocks as more fractionated compos­

itions are approached.

Graphs of the trace-elements Ba, Ce, La, Nb, Rb and Sr against

Zr are illustrated in Figl of Weaver et al. Both for the Nasaken and

Ribkwo volcanoes y lavas from the underlying Tirioko basalts are included

as they constitute the volcanic substructure. (Note that more analytical

data on the Nasaken lavas are given in Tables 46-49 than wore available at

the time of writing this paper. Plotting this extra data completes the

trends indicated in Pig. l). The graphs of Ce, La, Nb and Rb against Zr

are linear and project through the origins. This has been interpreted as

indicating (p,l88 of papep) that Ce, La, Nb, Rb and Zr have behaved

effectively as residual elements in the volcanic suites examined.

It was stated that certain residual element ratioshave different values for different volcanoes or groups of volcanoes (see Table 2 and Pig. 2).

The ratios Zr/Nb (the inverse of that previously given), Ce/La and La/Nb for the seven rift volcanoes examined are given in Table 51»

Table 51s

Zr/Nb Ce/La La/Nb Nasaken 3.41 1.67 0.60

Ribkwo 3.40 1.73 ' 0.63

Korosi 4.67 1.69 0.57

Paka 4.78 1.69 0.59

Eburru 4.42 2.00 0.67

Alutu 6.21 2.08 0.82

Pantale 5.78 2.00 0.67

The graphs of Sr and Ba against Zr have maxima at basaltic and mugearitic compositions respectively, the former being accentuated by the concentration of Sr in the plagioclase phenocrysts of highly feldspar- phyric basalts. The extreme depletion of these elements in the salic lavas 197

is apparent . •

Certain points falling off the linear trends on graphs of Ce

and La (also Nd and Pr) against Zr were thought to represent compositions

modified by the activity of a volatile phase into which the R.E.E, were

partitioned preferentially relative to Zr (p#l88 of the paper) •

4" Petrogenesis of the Pliocene volcanic rocks,

a) Relationship between basalt and trachyte

The association of minor amounts of trachyte with alkali

olivine—basalt leaves little doubt that the salic rock—type may be de­

rived from alkali basalt by some differentiation process (Turner and

Verhoogen, I960 ), The usually favoured process petrogenetically linking

basalt and trachyte is fractional crystallization. Bowen (l937) studied

the compositions of lavas actually from the Kenya rift and suggested that

they were splendid examples of the end-products of fractional crystalliz­

ation, However, because trachytic and phonolitic lavas are present in

such large volumes in the Kenya rift, many authors have followed Bailey

(1964 ) in suggesting that they might be produced by partial melting at

depth.

Within the Nasaken area, the two lava types, basalt and trach­

yte, abound. Basalts are largely confined to the Tirioko formation and

occur together with relatively small volumes of lava of composition inter­ mediate between basalt and trachyte. With no great time interval between

them, the Nasaken (and Kafkandal) trachytes followed the Tirioko basalts and occur together with very small volumes of lavas of basaltic and inter­ mediate compositions.

It was demonstrated in a trace-element study on the lavas of the Nasaken area, using lavas from the Nasaken volcano and the Tirioko formation, that graphs of Ce, La, Nb and Rb against Zr are linear and that Ce/Zr, La/Zr etc, ratios are identical for basalts, lavas of intermediate 198

compositions and traohytesfp.igg, Weaver et al.,1972). Lav.e from other volcanoes in the Kenya rift have different residual trace—element ratios

and thus the basalts and trachytes of the Nasaken area are geo chemically

related and a genetic relationship is implied. The basalts and trachytes ma j or must be related by some/petrogenetic process, either partial melting or

fractional crystallization. Moreover certain constraints are, imposed

on postulating processes involving partial melting^in particular, basaltic

and trachytic fractions must be generated from a single source or geo­

chemical ly related sources. The fusion or assimilation of Basement rocks

to generate the salic magma may be ruled out and in any case it is doubt­

ful whether the fusion of relatively alumina-rich crustal material could

generate magma of the required peralkaline composition.

It is important to note that the residual trace-element data

in no way favour either the partial melting process or that of fractional

crystallization. On the other hand various authors believe that such ex­

treme depletion of the elements Ba and Sr as is observed in the Nasaken

trachytes could only have resulted from protracted fractional crystall­

ization (Ewart et al,, I9681 Taylor et al,, 1968% Noble et al., 1969 )°

Nicholls and Carmichael (1969 ) comment that the characteristic pattern of

trace-elements in such peralkaline lavas (high Zr, Nb, Rb, R,E,E, and very

low Sr and Ba) are merely expressions of the peralkaline condition and

tell us nothing about the genetic processes involved. Until trace-element

partition coefficients are obtained from experiments involving partial melting and are compared with those obtained from well-documented natural

examples of suites inferred to ba formed by a process cf fractional- crystallization, this controversey is unlikely to be resolved •

The alkalis-silica diagram (Fig, 5&), the A E M diagram (Fig.

57) and the Solidification index variation diagrams (Fig, 58 ) each demon­ strate a compositional continuity between basaltic and trachytic lavas via intermediate lava—types, Although mugearites and trachymugearites 199

are scarce relative to basalts and trachytes, the compositional variation

is continuous , Locally in the rift, intermediate

lavas are common, Tho Kachila centre south-east of Kafkandal is composed

largely of mugearites with subordinate basalts and trachytes (Webb, 1971),

The chemical variation diagrams of the Nasaken area lavas and

their phenocrysts (Figs, 55'’58) are similar to those obtained for alkalic

lavas from Hawaii, the Hebridean province, Gough island, Nandewar volcano,

N.S.Wo and East Otago, N.Z, Trends for the lavas of these provinces are

plotted on the diagrams for comparison, those for the East Otago basalt-

traohyte series ( Coombs and Wilkins on {r I969 ) nro closest to the Kenyan

sequences .■ Study of each individual suite suggests that fractional

crystallization at low pressures is the main' process competent to explain

the chemical variation observed. The major-oxide and trace-element chem­

istry of the lavas of the Nasaken area is entirely compatible with the

hypothesis that fractional crystallization of alkali basalt is the main

differentiation process involved. The trachytic lavas from other parts

of the rift also seem to have originated by such a process. The suggest­

ion inherent in Fig, 59 that the undersaturated Ribkwo trachytes are assoc­

iated with slightly more undersaturated basalts than the Nasaken lavas is

in agreement with the view that small differences in basaltic source

composition generate chemically different salic end-products of the

fractionation process (Barth, 1936).

Development of peralkalinity,

Bowen (1945) has shown that in synthetic mixtures of NaAlSi^Og with a Ca-bearing second component, the feldspar which first forms on the liquidas is a Ca-bearing plagioclase. The preferential incorporation of

Ca and consequently AlgCy into the early crystallizing feldspar enriches the residual liquid in alkalis and generates the peralkaline condition

(molecular (Na+K)>Al), The withdrawal of large amounts of plagioclase in the 'big feldspar' basalts in the early stages of differentiation will 200

initiate a trend towards peralkalinity and this state will be achieved

all the more readily by the coprécipitation of any mineral with (Na+K)> A1

(eg. aluminous clinopyroxene). The 'plagioclase effect' will be less

efficient in peralkaline trachytic and pantelleritic liquids when the

phenocrysts contain only smll amounts of anorthite. Thus when alkali

feldspar is precipitating alone, the degree of peralkalinity will increase

very slowly indeed. The coprecipitation of a sodic ferromagnesian mineral

(sodic hedenbergite, aenigmatite or arfvedsonite) in such a case should

actually cause the degree of peralkalinity to decrease. In Fig, 64 the

ratio Na+K/Al increases systematically with increasing Zr content until

the peralkaline condition is achieved 5 the degree of peralkalinity then

remains virtually constant throughout trachytic and pantelleritic com­

positions.

Partial melting or crystal fractionation.

At present there seems to be no known way of distinguishing between the products of partial melting and fractional crystallization and

the two processes may well be reversible. The protagonists of the partial melting process cite the vast quantities of salic lavas as evidence in favour. Doubt as to whether the preponderance of salic lavas at the sur­ face is a criterion by which the crystal fractionation process can be dis­ missed has been expressed by the author (Weaver et al,, 1972) and argu­ ments have been put forward (p,191-192), based partly on the spatial and temporal distributions of the lava-types, in favour of such a process.

The experimental9 mineralogical and chemical evidence that crystal fract­ ionation at low pressures may generate trachytic compositions is manifold.

That alkali basalt, subject to relatively hi^er pressure and with or with­ out the addition of volatile constituents, is capable of melting partially to produce peralkaline salic liquid has yet to be convincingly demonstrated.

The ultimate derivation of the salic lavas and their relationship with lavas of basaltic composition remains a question for debate. (/) CD o CD CD O

— o

<\l o 00

-jOUJ /I/ / ^ 4- ZJ/vj 201

4b) Origin of the syenites

Rounded pulaskite syenite fragments are found in tuffs and

lavas from Nasaken, Kafkandal and Ribkwo volcanoes# The syenites have

cumulate textures and contain the same minerals as the trachytes except

that in addition to alkali feldspar, hedenbergitic pyroxene, magnetite and

fayalite, intergrowths of arfvedsonite-riebeckite and aegirine having an

intercumulus appearance also occur« A Nasaken syenite (8/128), the comp­

osition of which is given in Table 45$ is peralkaline, nn normative and has 12 per cent femic minerals. It is considerably more leucocratic than any other Nasaken lava. The Zr contents of the syenites vary from 273 to

743 p*p.m, (Table 49) and appear to be directly proportional to the per­

centage of mafic minerals. Such trace-element concentrations are equiv­ alent to those found in mugearites and the less fractionated trachytes,

Rb which may be as high as 348 p.p.m. in the whole rocks, is concentrated in the feldspars (Ta,ble 50)« Although the compositions of the feldspars from the syenites (Or35 “ Or38) are the same as those in the trachytes, the Rb contents are much higher in the former. High Rb syenite's which arc thou“8ht to be' cunulat-c rocks have boon described from South

Greenland by Upton (196O) , •’ .

Both pétrographie and chemical evidence suggest that the syen­ ites associated with the trachyte lavas represent accumulations of alkali feldspar crystals together with small amounts of mafic minerals. The plotted position of the Nasaken syenite slightly to the alkalis side of the feldspar point in Fig, 65 relative to the other points representing

Nasaken trachytes is consistent with the cumulate nature of this rook.

Since the syenitic feldspars have negative Ce anomalies relative to the other light R,E,E, and similar to the trachyte phenocrysts (Fig. 66) the syenites ought to have negative Ce anomalies if they represent accumu­ lations of feldspars. However the R.E,E, patterns of the syenites are (y Fig. 66 ChondritQ-normalised

logarithmic plots tor iigirt R.R.E. contents of syenites and feldspars

Aiomic Number

57 58 59 60 La Ce P r Nd 202

similar to those of the trachytes (Fig, 67)0 Thus if the negative Cc anomalies are real and the syenites are cumulates, there must he a com­ pensating effect due to positive Ce anomalies in the mafic minerals which clearly from Fig, 66 contain moderate amounts of R,E.E, That this is the case is suggested hy the small negative Ce anomaly in 5/4? the most leucocratic syenite and the small positive Ce anomaly in 8/l28a, the most mafic syenite,

4c) Differentiation within the salic lavas

The salic lavas of the Nasaken volcano range from quartz- saturated, weakly peralkaline trachytes to highly oversaturated, per­ alkaline rhyolites. Both the geology and the gradational chemical var­ iation indicate that these lavas must he petrogenetically related,

•The normative compositions of the rocks have heen plotted in the quartz-rich portion of "petrogeny's residua system’* (Bowen, 1937)?

NaAlSi^Og - KAlSi^Og - SiO^ (Fig, 68). The normative compositions lie near the line joining the alkali feldspar minimum with the minimum at

500 kg/cm PHgO on the quartz-feldspar boundary ie, along the thermal valley, Bailey and S chair er (1964) have however pointed out that the

'residua system' is only strictly valid for suhaluminous liquids. With reference to the peralkaline compositions of the Nasaken trachytes, the behaviour of the alkalis in relation to alumina and silica can he exam­ ined in two projections devised hy Bailey and Macdonald (1969). The molecular percentages of SiO^? Al^O^ and total alkalis are plotted in

Fig. 65, If the peralkaline rhyolites are derived hy feldspar fract­ ionation alone from a trachytic parent, the rocks should show a distinct trend from the feldspar point in Fig, 65, The distribution of analyses does form a trend originating from very near the feldspar point and stopping short of the quartz—alhite cotectic. It is also apparent from this diagram that the rhyolites are slightly more peralkaline than the ^ o

60 Nd 58 P r 57 LO 0./2 average^ O.JO chondn't^ P-P-"'’ 203

trachytes. The variation of alkali ratio with aliimina content is shown hy the second projection, Fig, 69. With four exceptions, the analyses fall along a hand which defines a nearly constant NagO/KgO ratio. The

compositions of alkali feldspar phenocrysts plotted in Fig, 69 indicate

that fractionation of feldspar would produce a residual liquid trend in accord with the pattern shown hy the Nasaken trachytes and rhyolites.

The cumulate origin proposed for the syenite fragments found in Nasaken tuffs lends support to this argument. However although these diagrams suggest that the dominant petrogenetic process involved is that of feld­ spar fractionation, the scatter of points especially in Fig, 69 suggests that another process or processes is involved. The phenocryst pyroxene found in the trachytes is neither rich enough in Na nor present in suff­ icient quantity to account for the scatter in Na^O/K^O ratio observed in

Fig, 69,

It has long heen recognised that peralkaline lavas are partic­ ularly susceptible to alkali loss either during crystallization (Noble,

19655 Homano, I969) or due to separation of a vapour phase (Macdonald,

1969)* Macdonald et al, (l970) demonstrated tliat pantelleritic trachytes from Menengai volcano could not he related to pantellerites from the nearby Eburru centre solely in terms of alkali feldspar - liquid equili­ bria but that a sodic vapour phase must be involved. The fractionation of the R,E,E, relative to Zr, Nb etc, in peralkaline liquids was also attributed to the activity of a volatile phase by Weaver et al, (1972).

It seems likely therefore that the scatter about the trends in Figs, 65 and 69 may be attributed to differential volatile losses involving alkalis. It can be seen from Fig, 64 that as soon as the peralkaline condition is achieved, tho plotted points become much more dispersed,,the degree of peralkalinity remaining between 1,05 and 1 .30,

Macdonald et al, (197 O) believe that progressive melting along a rising temperature path from rhyolite to trachyte is the converse of a (/) 3

oo o

O

+

o

Q o O VD O

Q O K

O 00

O 0\

f\i 204

cooling path from trachyte to rhyolite and is therefore an eqnally valid

petrogenetic process. Because of the large volumes of felsic volcanics in

the Kenya rift, they favour the melting hypothesis,

4d) Chemical variation within a single trachyte flow.

The textural and physical variation within a single trachyte

flow has heen described in Chapter 2, part 6dx. Chemical analyses of six

samples (8/l6$b-g) from a vertical section through the same flow are given

in Tables 4I9 429 44? 47 and 48 . The vertical variation of some major-

oxides and trace-elements is illustrated in Fig, 70° The thickness of

the flow at the line of section is about IO5 feet (55 metres) and the com­ position ot" the flow is uniform over 80 per cent of this thickness meas­

ured from the top of the flow. The bottom few feet are distinctly more

pantelleritic than the main body of the flow. SiO^ increases by about

5 per cent5 AlgO^ and K^O decrease by about 5«5 per cent and 1 per cent

respectively, Na^O and total iron oxide behave erratically varying from

5*5 to 6 ,4 and 8.5 to 9»9 per cent respectively. The concentrations of

the residual trace-elements are in general higher near the base of the

flow but they do not mirror the changes in the major constituents in

detail. The lowest sample (8/l63b) which is the most’ fractionated’ in

terms of major-oxide composition has lower residual element contents than

the two succeeding samples. Zirconium therefore appears in certain cir­

cumstances not to be a reliable index of fractionation.

The chemical variation within the flow appears not to have been produced in situ. Chemical changes taking place on crystallization or devitrification could hardly account for the magnitude and systematic nature of the variation described. Moreover, although three specimens

(b, 0, d) from the fine-grained, chilled zone at the base of the flow are chemically different from the main body of the flow, the topmost specimen

(g) which is glassy is the same composition as the main part of the flow. II

I N :k o 8 I

o § *o I -o

k 5

c; 0 •5 f\| 1

T o ■o- O

-o- § : u X

O K.

'O

\ 0)1 S'. Î: Cn 't) e o Ë fel 5 " 20$

The dilution of Al^O^ and K^O could be accounted for by the

addition of about 20 per cent of material perhaps precipitated from vola­

tile emanations from the underlying flow but(taking Na^O and total iron

oxide to be approximately constant throughout the flow)»this material

would have to be of composition SiO^ = 87 per cent, ha^O = 5 per cent

and total iron oxide = 8 per cent.

It seems more likely that the chemical varia cion is inherited

from differentiation processes which took place in a chemically zoned

magma chamber from which the flow was erupted. Such variation which be­

comes inverted during emplacement by laminar flowage of the lava has been

described in flows of rhyolitic composition by Smith and Bailey, 1966;

Gibson, 1970 and Lipman, 1971 « The mechanism of differentiation almost

certainly involves the slow settling of phenocryst minerals while the magma remains in the vent. Abundant alkali feldspar phenocrysts, green pyroxene and magnetite are present in the main body of the flow (zone IV and zone V) but become very scarce in the massive zone. The distribution of the phenocrysts is consistent with the crystal settling process. The erratic distribution of BhgO throughout tho flow suggests that a sodic

(and possibly iron bearing) vapour phase may also have been involved. The distribution of residual elements in the chilled zone of the flow is not well understood. There may have been some squeezing out at the base of the flow of interstitial liquid rich in residual trace-elements,

4e) Eare-earth-element distributions in feldspars.

Alkali feldspars from trachytes and syenites have been analysed for the light R.E.E, and the results are presented in diagrammatic form in Fig, 66, The alkali feldspars contain small amounts of R.E.E,*

La<40 p.p.m,, Ce< 47 p.p.m., Pr

R.E.E. content of separated plagioclase phenocrysts is much lower and at about the detection limits for these elements. A feature of all the anal- 206

yses is the negative Ce anomally with respect to the other light

The preferential incorporation of Ea into the feldspars is well known

(Taylor, 196$). Eu exists in two oxidation states Eu^^ and Eu**"^ and the +2 latter substituting for Ca gives rise to large positive Eu anomalies

with respect to other R.E.E, The lanthanide elements are all trivalent

and thus theoretically may substitute for Al^^ and Ee^^ in feldspars

although it is not valency alone which controls the ability of one cation

to substitute for another. Ce also occurs in the Ce^^ state and it may

be that under oxidising conditions less of this element enters the feld­

spars since some of it is in the higher oxidation state (Ce^^), Whether

such a process is a primary, magmatic one or a secondary effect involving

ionic exchange and redistribution is not clear. Although, if the negative

Ce anomalies of the feldspars are a primary feature, fractionation of such

feldspars would be expected to give rise to positive Ce anomalies in the

trachytes and alkali rhyolites which it does not (Fig, 67).

Vi/hat appear to be positive Ce anomalies in two trachytes, 8/658

and 8/129 (Fig, 67),are in fact negative anomalies of La, Pr and M Î

The Ce content of these trachytes is that of similar rock-types but the

concentrations of the other light R.E.E. are much too low, comparable with

those of the basalts.

The distribution of the R.E.E. in these lavas and the feld­

spars of the NasaJcen trachytes is being investigated by Prof, F.A. Frey

(M.I.T.) who has confirmed that these anomalies (which are the first to be reported) are a real feature and not a product of the X.R.F. calibrat­

ions used by the author. Although the cause of the Ce anomalies in the

feldspars and lavas is not understood, its solution must be linked with

tho property of Ce to exist in two oxidation states.

4f) Residual trace-element ratios and magma-type.

That different values of residual trace-element ratios charact­ 207

erize different volcanoes or groups of volcanoes is indicated in Table

51. In terms of Zt /Wo ratio the Kenyan volcanoes fall into two groups,

the Pliocene pair, Ribkwo and Nasaken, and the three Quaternary volcanoes,

Korosi, Paka and Eburru. The Ethopian volcanoes constitute a third group.

The Ce/La and La/Nb ratios distinguish between the volcanoes at which true pantellerites occur and those where they do not. Within the northern part of the Kenya rift such changes in trace-element ratios reflect

changes in the trace-element composition of the parental magmas in time.

The ratio Zr/Nb changes from about 2.5 for the Miocene phonolites to about

5,5 for the Pliocene lavas to 4*5 - 6,5 for the Quaternary volcanic rocks

(pig, 71)« This ratio which can be determined with a high degree of pre­ cision appears therefore to be a sensitive indicator of magma-type. In

Pig, 7^ are plotted Zr/Nb ratios for different East African magmas from carbonatitic to tholei.itic and it is apparent that the ratio is a use­ ful discriminator not only of basic magma-types but also of compositional variations within their salic differentiates. The petrogenetic processes leading to these geochemical differences are at present unknown but Zr/Nb ratio appears to correlate inversely with the depth (and pressure) in the mantle at which partial melting to produce the various magma-types takes place (Green, 19?0), O o O M

5

c: k %» 8 N ç o c 00 N-J C5 >< :d il o O o o - ‘^5 O 'O I s l

O O

ill^ Q. O O \ O <\J

O o o o 8 ° o o o

o o 00

y o o o i o • • * o o X o X o o o K\ X o o o o o o ^ o o o o o r i-i rj I r i i j i i i i p i i i i i ”i i i f\J K (\j N f\j k rs 'o rj: N N z y / v 6

/5-1

g overoge tholQ/it/'c bo^o/t 1 4 - Jebe! at Tair , Red Sea. Gass et a/.t /P7J 1 3 -

12 -

// -

1 0 -

' 9 - av. transitional basalt ^Zubair^ Red Sea . Gass et ai.^ 1973 8 - ex. alkali basalt Hanish Zukur ^ Red Sea, Gass et alj 1973 7-

(5 - alkali and trachytes x pantellerites tra n s it ion a I* Fantale JL Alutu" — basalts ^ 5 - trachytes and pantellerites . Paka ^ Korosi a Eburru

4 -

' '^T/noko tormn. J- 'plateau' phonolites

- z - nephel- inites

/ - Q CK Uganda carbonaiite , Bloomfield^ 1972

O 200 400 600 800 1000 1200 1400 1600 1800 2 0 0 0

^ r : p.p.m. Fig.72 Graph of Zr/Nb against Zr for East African voicanics 208

SUmiARY of CONCLUSIONS

Stratigraphy

Within the Nasaken area of South Turkana a 6000 feet

(1800 ffi, ) succession of Neogene volcanic rocks rests upon the sub-Miocene Basement surface. The stratigraphy of the volcanic rocks can be expressed in terms of the major lithostratigraphical groups proposed by King and Chapman (l972).

The Turkana basalt formation,consisting largely of basaltic lavas and pyroclastic rocks,is at least 1100 feet (i^Om.) thick. Age determinations on the Turkana basalts range from

18 to 16 million years b.p. and the formation which is of 'plateau' or 'flood' type correlates with the Samburu basalt Series on the eastern side of the rift.

The Kowun trachytes are part of a much eroded and faulted shield-like volcano in which one main extrusive centre has been recognized. The trachytes have a maximum thickness of about 1000 feet (3OO m. ) and are about 15 million years old.

The Kowun trachytes and the Turkana basalts are the represent- itives of group I in the Nasaken area.

The Kanitiriam and Nakasuw trachyphonolites and the

Kanitiriam microfoyaite are the local expressions of the extensive

'plateau' phonolite group II which is not well represented in

South Turkana,

After a considerable period of erosion and a phase of faulting, basaltic 'flood' lavas wore erupted in South Turkana and constitute the Tirioko basalt formation. Within the Nasaken area the formation has a maximum thickness of about 2000 feet

(600 m.) and a number of different sources and centres have been 209

recognized. Agu determinations range from about 8 to 6 million

,years b.p. and the Tirioko basalts almost certainly continue beneath Ribkwo volcano and thus may be equated with the Kaparaina basalts of the southern Kamasia (group III).

Immediately following a phase of faulting, a number

of trachytic, multicentred volcanoes were built upon a surface formed of the Tirioko basalts. The volcanoes which consist almost wholly of trachytic lavas and pyroclastic rocks, were built in rapid succession between 6 and 4 million years b.p. as the foci of activity migrated eastwards towards the centre of the rift. Most of the flank deposits of Kafkandal have been removed by erosion but Nasaken volcano has well-preserved strati­ form flank deposits passing into a less regular, source zone where air-fall pyroclastic rocks, dykes and plugs are concentrated.

Structure

The Napeitora step-faults are the dominant structural feature of the Nasaken area. Their total effect is a downward displacement towards the centre of the rift of about 2500 feet

(750 m.). Repeated movements along these faults have taken place but the main faulting is thought to have occurred at about

6 million years b.p. and may have 'triggered' the trachyte volcanisra that followed. Both major and minor faults appear to have followed the local Basement 'grain'.

The north-south trending arch just west of Kowun and the Napeitom faults are part of a major structural lineament which runs from the southern Kamasia to Lake Rudolf. West of the arch, the volcanic strata dip gently towards the Kerio; on its eastern side, progressive gentle tilting towards the centre of the rift has taken place so that the older formations 210

dip more steeply than the younger.

Petrography

The lavas of the area appear to he members of the alkaline olivine-basalt association. There is a gradational pétrographie variation between the rock types from basalt through mugearite and trachyte to alkali rhyolite. Many of the lavas, especially those of basic compositions, contain abundant phenocrysts.

Geochemistry and Petrology

The Miocene lavas are in general more undersaturated and more strongly alkaline than those of the Pliocene formations.

The basalts of the Tirioko formation are alkaline and mildly undersaturated. The Nasaken lavas range from weakly peralkaline, saturated trachytes to peralkaline rhyolites.

Studies of the major-oxide and trace-element geo­ chemistry shŒ7 that the Pliocene basalts and trachytes are genetically related. The chemical variation observed in the lavas and their phenocrysts can be best explained in terms of fractional cryst­ allization of an alkali basalt parent magma. Consideration of the relative volumes of basic and salic lavas may indicate that the trachytes have originated as partial melts of alkali basalt. However, the spatial and tenterai distributions of the lavas suggest that cupolas of trachytic magma developed above a basaltic ’reservoir* situated beneath the rift and there is some geophysical data in support of this view.

The chemical variation within the salic lavas (from trachyte to pantellerite) can be explained largely by the process of crystal fractionation of alkali feldspar but in addition a sodic vapour phase may be invoved. 211

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bearing on the origin of trachytic and pantelleritic

lavas in the East African Rift System. Contr. Mineral

and Petrol. 36, 181-194.

Webb P.K. (1971) Geology of the Tiati hills. Rift Valley Province,

Kenya. Unpub. Ph.D. thesis Univ. of London.

Williams L.A.J. (1970) The volcanics of the Gregory Rift Valley,

East Africa. Bull volcanol. 34, 439-465*

Williams L.A.J. (1972) The Kenya rift volcanics: a note on volumes

and chemical composition. In: East African rifts,

Tectonophysics l5, 83-96.

Wright J.B. (1963) A note on possible differentiation trends in

Tertiary to Recent lavas of Kenya. Geol. Mag. 100,

164-180.

Wright J.B. (1965) Pétrographie sub-provinces in the Tertiary to

Recent volcanics of Kenya. Geol. Mag. 102, 541-557.

Wright J.B. (1970) Distribution of volcanic rocks about mid-ocean

ridges and the Kenya Rift Valley. Geol. Mag. 1G7,

125-131.

Yagi K. (1966) The system acmite - diopside and its bearing on the

stability relations of natural pyroxenes of the

acmite - hedenbergite series. Am. Mineralogist 5l,

976-1000. 1 2 6

Appendix I s Analytical methods

Wet chemical analysis of the major oxides was carried out by

PIr, H. Lloyd, Bedford College, using standard procedures.

Some major element analyses and all trace element analyses were made by the author using a Phillips PW1212 automatic X-ray flouros- cence spectrometer. Samples were prepared for major element analysis by pressing fusions of rock powder and borax flux (weighed in the approx­ imate ratio Is?) into brass rings in a manner similar to that described by Norrish and Hutton (1969)0 The U.S, Geological Survey and the French standard rocks together with wet chemically analysed Kenya rift volcanics were used for calibrations, Na^O was analysed using compressed rock powder pellets, Plass absorption corrections were applied to the results using iterative computations.

Trace element analyses were carried out on compressed rock powder pellets. The elements Ba, Ce, La, ITb, Kd, Rb, Sr, Y and Zr were determined and calibration was by normal spiking procedures. Mass absorp­ tion corrections were applied to the results. For the elements Nb, Rb,

Sr and Zr, precision is approximately - 1 per cent of the amount present above the 100 p.p.m, level and for Ba, Ce and La this figure is - 2 per cent. The H.8,0,8, standard rocks were analysed in parallel to enable comparisons to be made among different laboratories. Their average trace element analyses from over 5O determinations are given below;

Table $2:

02 GSPl AGV BCR Ba 1825 1254 1161 654 Ce 161 351 76 67 La 89 144 37 28 Nb 10* 19* 13 13 Nd 70 177 42 17 Rb 173 262 69 48

(contd,) 227

Table 52 (contd.)

G2 GSPl AGV BCR Sr 487 259 665 522 Y 11 26 19 N.B. Zr 521 545 206 172

* Low due to unconnected Th interference N.L. not determined G2 - granite GSPl - granodiorite AGV - andésite BCR - basalt all determinations in p.p.m. 228

Appendix II s Bedford College C.I.P.W. norm calculation

There are some important differences from the C.I.P.W. calcu­ lation set out by Holmes in ’Pétrographie Methods’, 1921, po427-431o

These are briefly as follows?

After making ilmenite, residual TiO^ is expressed as rutile; perovskite is not made. This means that in rocks very low in CaO, unless

PpOp.^ 10 CaO (molecular proportions) apatite is always formed since there is no other oxide than CaO with which P^O^ can realistically combine. Also, excess CaO after the formation of apatite goes into the making of anorthite and/or diopside (or flourite if excess P and calcite if COg is determined) rather than perovskite.

It is possible therefore that if after allotting SiO^ to the normative minerals, both wollastonite (CaO SiO^) and rutile (TiO^) occur.

These are combined to give sphene (CaO TiO^ SiO^) plus either residual wollastonite or rutile. 229

Appendix III Localities of Nasaken area analysed rocks

Rock Number Grid reference Formation or Member

8/420 284884 Kasorogol melariephelinite 8/94 704840 Eowun trachytes 8/107 688818 Kowun trachytes 8/487 278852 Kanitiriam trachyphonolites 8/489a 269858 Kanitiriam microfoyaite

8/85 7OO8I5 Kagumnyikal basalts 8/119 696762 ?; SÎ 8/120 690755 Î! s? 8/121 688752 Çf !t 8/122 685740 tJ 1) 8/560 506652 tJ S» 8/124 685758 Napeitom mug-earites 8/l24a 684757 t! «1 8/l05a 692855 Lotokongolea basalts 8/182 555770 II II 8/52a 75O850 Napeitom basalts 8/81 7158O4 n I: 8/95 707845 :i II 8/595 716855 IT II 8/Bl 755840 II II

8/658b 707665 Kafkandal volcano 8/000 758648 II 1!

8/5 748790 Nasaken volcano 8/11 758821 II II 8/26 740827 II II 8/67 697784 " " (Lodukai dyke) 8/89 744764 il !? 8/111 766762 II II 8/116 765744 II II 8/125 682756 II II 8/126 698771 II II 8/128 708802 II II 8/l28a 7O88O2 II 1! 8/129 7O88O5 II II 8/l65b-g 751786 II I! 8/175 524704 II II 8/206a, b 761697 II II 8/222 742710 II II 8/255 757668 II II 8/249 700745 I: II 8/265 755725 II II 8/281 807705 II II 8/515 795662 II 11 8/526 762655 " " (Katirr basalt) 8/587, a 686686 II II 8/640 700705 II II Trachytic Lavas in the East African Rift System 189

Fig. 3. Plot of Ce (ppm) against Zr (ppm) for lavas of Eburru volcano 600

400

200 • obsidians • crystalline rocks

1000 2000

La/Zr (Fig. 1). Alutu also has constant residual elements ratios although the com­ positional range of lava types is more limited. The trace-element data thus indicate that the basic and saHc lavas at any one centre are geochemically related. This almost certainly impHes a genetic relation­ ship, since the values of the residual-element ratios for each suite were determined by the composition of the initial Hquid, or sohd, from which the lavas were derived. The major-element compositions of the lavas are compatible with the hypo­ thesis that the more saUc rocks were derived by a process of fractional crystallisa­ tion. During such a process, the residual-elements would remain almost completely in the liquid phase, their final concentrations being dependent on the abundance in the original hquid and the proportion of residual hquid remaining. A crystal- fractionation model is supported by the data for Sr and Ba. The sahc lavas from each of the five Kenyan volcanoes are impoverished in these elements. The lavas of Nasaken volcano, for example (Fig. 1), show maximum concentrations in the mugearitic rocks whereas concentrations in the quartz-trachytes and panteUeritic trachytes are very low, often falling below 20 ppm. Eburru trachytes have Ba values averaging about 60 ppm whereas Sr values for all the sahc lavas from Eburru fah below 15 ppm. Since Sr and Ba subsitute for the major-elements Ca and K, their depletion can be accounted for by the crystalhsation and removal of large quantities of plagioclase and alkah-feldspars. Many of the basic lavas associated with the great volumes of trachytic rocks within the Kenyan Rift are ‘ big-feldspar ’ basalts and hawahtes. This is particularly the case for the Nasaken and Ribkwo volcanoes (c.f. Abbott, 1970). It is possible that progressive or partial melting of suitable source rocks could yield the basic and sahc fractions of each volcano. The residual-element data do, however, impose an important restraint on petrogenetic schemes involving partial melting; namely that for each centre, the basic and sahc hquids be generated from a single source or geochemicaUy related sources. It fohows that partial melting of unrelated sources to generate the basic and sahc fractions of the Kenyan volcanoes is highly unlikely. The mobilization of fenitised Basement rocks (King, 1965) or the fusion of syenitic plutonic material (Harkin, 1960; Nashet al., 1969) to produce the sahc hquids, are petrogenetic processes unlikely to have operated in the examples described here. Although the residual-element data may be compatible with the generation of the sahc hquids by partial melting, the Sr and Ba data do not support this hypo­ 190 S. D. Weaveret al. : thesis. Initial salio melts formed in this way might be expected to have high con- centrations of Ba as well as K and the residual-elements. The separation of alkali- feldspar would then produce a series of liquids characterised by progressive Ba depletion but at each Kenyan centre all the salic rocks are depleted in Ba. High Ba liquids of mugearitic composition might be the initial products of perhaps 30% partial melting of basaltic material. However, in each case, the mugearites can be easily related geochemically to associated basalts by fractional crystallisation and the occurrence of ankaramites and richly feldsparphyric basalts supports a fractio­ nal crystallisation model. It thus seems unnecessary to postulate primary mugeari­ tic liquids and we suggest that all of the lavas can be related to primary basaltic hquids. Sr has a similar pattern of depletion to Ba in ah the sahc rocks and it seems hkely that the extreme Ba and Sr depletions observed can only have resulted from protracted fractional crystalhsation (Ewartet al., 1968 ; Tayloret al., 1968; Nobleet al., 1969). Although the Sr data are comparable, the Ba data for the sahc rocks from Ean- tale and Alutu show somewhat different patterns from those of the Kenyan centres. Apart from a single basalt flow erupted from a fissure on the flanks of Eantale, the most ‘basic’ rocks are plagioclase trachytes with over 1500 ppm Ba (see Table 1). Values for Ba in the most differentiated Eantale obsidians of the pre-caldera suite fah to about 700 ppm Ba. The Alutu suite, composed entirely of panteheritic obsidians, has Ba abundances from 445 to 180 ppm. Perhaps these rocks have an entirely different mode of origm. from the Kenyan lavas. Although the panteherites from the Ethiopian centres and those from Eburru are not dis- simhar, the trachytes from Eburru differ chemicahy (Table 1) and mineralogicahy from their Ethiopian counterparts. Thus the two suites of panteherites may have significantly different crystalhsation histories. It is conceivable that at Eantale the crystalhsation and fractionation of alkah-feldspar was delayed unth liquids of trachytic composition were reached, with the consequence that trachytes con­ tained large amounts of Ba. Later, crystalhsation and removal of anorthoclase produced some depletion in Ba but not as extreme as that observed in the Kenyan panteherites where the crystallisation of alkah feldspar was much earher. As noted previously, the Ribkwo trachytes are just saturated or undersaturated in composition, whereas the Nasaken trachytes usuahy contain modal quartz. It is thought that the trachytes at these two centres had separate lines of descent from an alkah basalt parent and are divergent end-products of the fractionation process. The absolute abundances of the residual elements suggest that the ne- normative trachytes are as fractionated as the gz-normative ones, and do not re­ present a stage in the evolution towards oversaturated liquids or vice-versa. In general, the extent of fractionation, as inferred from the residual-element con­ centrations at each of the volcanoes studied, is similar even though different end- products result. Thus panteherites are not necessarily more fractionated rocks than trachytes. It has been noted that differing values of the residual-element ratios, such as Nb/Zr, characterise separate volcanoes or groups of volcanoes within the Rifts. It is thought that these differences reflect changes in the trace-element content of the basaltic parents in time. Thus in Kenya, the Quaternary volcanoes of Paka and Korosi have different residual-element ratios from the Pliocene pair, Nasaken Trachytic Lavas in the East African Rift System 191 and Ribkwo. Presumably the trace-element composition of the parental material changed during the latter part of the Phocene. Preliminary work suggests that these changes in composition took place progressively during the evolution of the Rift, the Nb/Zr ratio changing from about 0.42 in the Miocene flood-phono- hte group (S. J. Lippard, pers. comm.), to 0.30 the during Phocene and to 0.21 in the Quarternary volcanoes, Paka and Korosi. From the descriptions given above of the six centres under study, it is clear that at aU of these, trachytes and alkah-rhyohtes have been erupted in large volumes compared with associated basalts. Moreover, at many centres, a scarcity of lavas of compositions intermediate between trachytes and basalts is evident. The apparent difficulty of explaining these volume relationship in terms of crystal fractionation, have led many authors to suggest that the salies might have an independent origin involving partial melting (Bailey, 1964; Wright, 1965; McCall, 1968; W right, 1970; WiUiams, 1971). During the debate on the “ Daly Gap ” initiated by Chayes (1963), various argu­ ments were put forward to account for the dearth of intermediate rock-types, including poor samphng (Harris, 1963 ; MacDonald, 1963 ; Mukherjee, 1967 ; Baker, 1968). The distributions we describe here are not however the result of poor sampling. Should there be some bias, it is possible that where they occur, inter­ mediate lavas have been oversampled in an effort to collect the less common types. Although it is doubtful whether the relative volumes of rock-types from a single volcano allow meaningful interpretation of petrogenetic processes taking place at depth (Cann, 1968), a more realistic interpretation may be made when the volcanic substructure is also included (Ridley, 1970). In northern Kenya, such considerations suggest that basaltic magma has been freely available regionally, but only rarely erupted from the main vent(s). Thus in the Phocene, basic lavas occur mainly as thick, flood-lava piles associated with the initial and closing phases of trachytic central volcanism (King, 1970; RhemtuUa, 1970). Although basic and intermediate lavas are generally poorly represented in the volcanic superstructures, they often provide the substructures on which salic centres were built although the intermediate lavas are rarely present in large proportions. Moreover, in the Quarternary axial zones in both the Kenyan and Ethiopian Rifts, basalts have been extruded from fissures on the flanks of and/or between the salic centres. The derivation of such large volumes of sahc lavas by a process of crystal- fractionation, as inferred from the trace-element data, requires that a very large amount of basic material be present at a high level beneath the Rifts. The recent gravity surveys of the Kenyan Rift (Searle, 1970; Khan and Mansfield, 1971) show a sharp axial high on which Paka, Korosi, Nasaken, Ribkwo and Eburru are situated. The spatial and temporal distribution of the lava-types suggests that the central volcanoes in the Rifts are situated above high-level sahc magma-chambers which have developed on top of a regional basaltic reservoir. The intervening areas between the sahc centres have frequently been the site of basaltic fissure eruptions which may represent the tapping of the basaltic magma during periods of tensional stress. The presence and form of this reservoir are also suggested by the linear,

13 Contr. Mineral, and Petrol., Vol. 36 192 S. D . Weaver et al. :

positive, gravity anomaly which follows the line of the salic centres. Its formation beneath the Kenya Rift possibly took place during the Mid-Pliocene (6-8 m -y) (Baker and Wohlenberg, 1971). The period of stability which followed could have allowed the development of sahc magma-chambers above this reservoir. The fractionation mechanism envisaged is similar to that proposed by Edwards (1942), to explain the differentiation trends in Tasmanian dolerite dykes. In con­ trast to the closed magma-chamber of the Skaergaard type (Wager and Deer, 1939), a dyke-like basaltic body of bathohthic proportions is inferred to be present be­ neath the Rifts. Crystallization would proceed most rapidly near the top of the magma-chamber and the crystals so formed, would sink, to accumulate at great depths or be resorbed. The residual hquid would be displaced upwards and cohect in cupolas, the crystals removed by sinking being replaced by relatively undif­ ferentiated magma. In this way, large volumes of basaltic magma could be ‘pro­ cessed ’ and correspondingly large volumes of sahc material produced. Lighter frac­ tions would float on denser fractions beneath, there being a continuous gradation from the basaltic magma through intermediate compositions to the sahc hquids above. No form of hquid immiscibihty is to be inferred. The zoned magma- chamber or cupola thus produced would be gravitationahy stable (Macdonald, 1949). It is hkely that sahc fractions within the cupola would continue to evolve by further crystal-fractionation thus accounting for the large range in concen­ trations of residual-elements observed within the sahc rocks at each centre. That the volume of magma of intermediate compositions involved in the operation, at any one time, is small relative to the accumulated volume of ‘processed’ or residual hquid, is a corohory of such a model. Eruptions at the centres are hkely to sample only the upper parts of the underlying cupolas. Ash-flow tuff eruptions might drain the whole or a large part of a zoned cupola, thus explaining the zoning observed in some extensive ash-flow tuff sheets (Smith and Bailey, 1966; Gibson, 1970). The relative paucity of lavas of intermediate compositions indicates their scarcity at depth and the fact such magmas were not available for eruption being ‘capped’ by more sahc hquids. In general terms, the lack of intermediate lavas and the over-abundance of sahc rocks are not criteria by which one can reject a crystal-fractionation model. Acknowledgements. The geology of the volcanoes Korosi, Nasaken, Ribkwo and Paka has been studied as part of the work of the East African Research Unit under the direction of Professor B. C. King. We are indebted to Mr. J. N. Camey for allowing us to include unpublished geological and geochemical data on Korosi. Dr. D. S. Sutherland kindly provided the suite of Eburru samples and notes on the geology of the volcano. Professor B. C. King kindly read earlier versions of the manuscript and made suggestions for its improvement; we are most grateful for this assistance. The financial support of the Natural Environment Research Council is gratefully acknowledged.

References Abbott, M. J.: Petrology of the Nandewar volcano, N.S.W ., Australia. Contr. Mineral, and Petrol. 20, 115— 134 (1969). Bailey, D. K. : Crustal warping — a possible control of alkaline magmatism. J. Geophys. Res. 69, 1103— 1111 (1964). Baker, B. H., Wohlenberg, J.: Structure and evolution of the Kenya Rift Valley. Nature 229, 638—542 (1971). Trachytic Lavas in the East African Rift System 1 9 3

Baker, I.: Intermediate oceanic volcanics and the “Daly Gap”. Earth Planet. Sci. Lett. 4, 103—106 (1968). Cann, J. R.: Bimodal distribution of rocks from volcanic islands. Earth Planet. Sci. Lett. 4, 479—480 (1968). Chao, E. C. T., Fleischer, M. : Abundance of zirconium in igneous rocks. Rep. 21st int. geol. Congr. Norden 1, 106— 131 (1960). Chayes, F. : Relative abundance of intermediate members of the Oceanic basalt-trachyte association. J. Geophys. Res. 68, 1519— 1534 (1963). Dakin, F. M., Gibson, I. L.: A preliminary account of Alutu, a pantelleritic volcano in the main Ethiopian Rift. Bull. Geophys. Obs. Addis Ababa 13, 110—114 (1971). Dietrich, R. V. : Behaviour of zirconium in certain artificial magmas under diverse P-T con­ ditions. Lithos 1, 20—29 (1968). Edwards, A. B.: Differentiation of the dolerites of Tasmania. 2. J. Geol. 60, 579—610 (1942). Ewart, A., Taylor, S. R., Capp, A.C. : Geochemistry of the pantellerites of Mayor Island, New Zealand. Contr. Mineral, and Petrol. 17, 116—140 (1968). Gast, P. W. : Trace element fractionation and the origin of tholeiitic and alkaline magma types. Geochim. Cosmochim. Acta 82, 1057— 1086 (1968). Gibson, I. L. : Preliminary account of the volcanic geology of Fantale, Shoa, Ethiopia. Bull. Geophys. Obs. Addis Ababa 10, 59—67 (1967). Gibson, I. L. ; A pantelleritic ash flow tuff from the Ethiopian Rift Valley. Contr. Mineral and Petrol. 28, 89-111 (1970). Harkin, D. A.: The Rungwe volcanics at the northern end of Lake Nyasa. Geol. Surv. Tan­ ganyika Mem. 2 (1960). Harris, P. G. : Relative abundance of intermediate members of the Oceanic basalt-trachyte association — A discussion. J. Geophys. Res. 68, 5103—5107 (1963). Harris, P. G.: Segregation processes in the upper mantle. In: Mantles of the Earth and ter- restial planets (ed. S. K. Runcorn). London: Interscience 1967. Herrmann, A. G.: In: Handbook of geochemistiy (ed. K. H. Wedepohl), vol. 2, part 2, sect. 39F2. Berlin-Heidelberg-New York: Springer 1970. Khan, M. A., Mansfield, J.: Gravity measurements in the Gregory Rift. Nature 229, 72—75 (1971). King, B. C. : Petrogenesis of the alkaline igneous rock suites of the volcanic and intrusive centres of Eastern Uganda. I. Petrol. 6, 67-100 (1965). King, B. C.: Vulcanity and Rift tectonics in East Africa. In: African magmatism and tec­ tonics (eds. T. N. Clifford, I. G. Gass). Edinburgh: Oliver and Boyd 1970. Kuno, H., Yamasaki, K., lida, C., Nagashima, K.: Differentiation of Hawaiian magmas. Jap. J. Geol. Geogr. 28, 179-218 (1957). Lacroix, A. : Les roches hyperalcalcines du massif du Fantale et du col de Balla. Mem. Soc. Geol. France 14, 89-102 (1930). Macdonald, G. A.: Hawaiian pétrographie province. Bull. Geol. Soc. Am. 60,1541-1596 (1949). Macdonald, G. A. : Relative abundance of intermediate members of the Oceanic basalt- trachyte association — a discussion. J. Geophys. Res. 68, 5100-5102 (1963). Macdonald, R. : The petrology of alkaline dykes from the Tugtutoq area. South Greenland. Bull. Geol. Soc. Denmark 19, 257-282 (1969). Macdonald, R., Bailey, D. K., Angell, G. R. : Trace element variation in pantellerite volcanoes of the Gregory Rift. J. Geol. Soc. Proc. 127, 411—412 (1971). McCall, G. J. H.: The five caldera volcanoes of the central Rift Valley, Kenya. Proc. Geol. Soc. Lond. 1647, 54-58 (1968). McClenaghan, M. P., Weaver, S. D., Webb, P. K. : Pliocene trachyte volcanoes of the northern Rift Valley, Kenya. J. Geol. Soc. Proc. 127, 294 (1971). Mukherjee, A. : Role of fractional crystallisation in the descent : basalt-trachyte. Contr. Mineral, and Petrol. 16, 139-148 (1967). Nash, W. P., Carmichael, I. S. E., Johnson, R.W .: The mineralogy and petrology of Mount Suswa, Kenya. J. Petrol. 10, 409^39 (1969). Nicholls, J., Carmichael, I. S. E. : Peralkaline acid liquids : a petrological study. Contr. Mineral, and Petrol. 20, 268-294 (1969). 194 S. D. Weaver, et al. : Trachytic Lavas in the East African Rift System

Noble, D. C. : Gold Elat Member of the Thirsty Canyon Tuff — a pantellerite ash-flow sheet in southern Nevada. Prof. Pap. U.S. Geol. Surv. 625-B, 85-90 (1965). Noble, D. C., Haffty, J., Hedge, C. E.: Strontium and magnesium contents of some natural peralkaline silicic glasses and their petrogenetic significance. Am. J. Sci. 267,598-608 (1969). Rhemtulla, S.: The South Turkana expedition: Scientific Papers 3. Geogr. J. 136, 61-73 (1970). Ridley, W. I.: The abundance of rock types on Tenerife, Canary Islands, and its petrogenetic significance. Bull. Volcanol. 34, 196-204 (1970). Romano, R. : Sur l’origine de l’excès de sodium dans certaines laves de l’Ile de Pantelleria. Bull. Volcanol. 83, 1-7 (1969). Rosholt, J. N., Prijana, Noble, D. C.: Mobility of uranium and thorium in glassy and crystal­ lised silicic volcanic rocks. Econ. Geol. 66, 1061-1069 (1971). Sceal, J. S. C., Weaver, S. D. : Trace-element data bearing on the origin of salic rocks from the Quaternary volcano Paka, Gregory Rift, Kenya. Earth Planet. Sci. Lett. 12, 327-331 (1971). Schilling, J. G., Winchester, J. W.: Rare-earth fractionation and magmatic processes. In: Mantles of the Earth and terrestial planets (ed. S. K. Runcorn). London : Interscience 1967. Searle, R. C. : Evidence from gravity anomalies for thinning of the lithosphere beneath the Rift Valley in Kenya. Geophys. J. Roy. Astron. Soc. 21, 13-31 (1970). Smith, R. L., Bailey, R.A. : The Bandelier tuff: a study of ash-flow eruption cycles from zoned magma chambers. Bull. Volcanol. 29, 83-103 (1966). Sutherland, D. S. : Pantelleritic volcanism in the area of Kenya. Proc. Geol. Soc. Lond. 1663, 164-165 (1970). Sutherland, D. S.: The Eburru volcano, Kenya. J. Geol. Soc. Proc. 127, 417 (1971). Taylor, S. R.: The application of trace element data to problems in petrology. In: Physics and chemistry of the Earth, vol. 6. Oxford and New York: Pergamon Press 1965. Taylor, S. R., Ewart, A., Capp, A. C.: Leucogranites and rhyolites: Trace element evidence for fractional crystallization and partial melting. Lithos 1, 179-186 (1968). Thompson, A. 0., Dodson, R. G.: Geology of the Naivasha area. Rep. Geol. Surv. Kenya 76, (1963). Thornton, C. P., Tuttle, 0. P.: Chemistry of igneous rocks. 1. Differentiation index. Am. J. Sci. 258, 664-684 (1960). Wager, L. R., Deer, W. A.; Geological investigations in East Greenland, Part 3. The petrology of the Skaergaard intrusion, Kangerdlugssuaq, East Greenland. Medd. Groenland 105, No. 4, 1-352 (1939). Williams, L. A. J. : The volcanics of the Gregory Rift Valley, East Africa. Bull. Volcanol. 34, 439-465 (1971). Wright, J. B. : Pétrographie sub-provinces in the Tertiary to Recent volcanics of Kenya. Geol. Mag. 102, 541-557 (1970). Wright, J. B. : Distribution of volcanic rocks about mid-ocean ridges and the Kenya Rift Valley. Geol. Mag. 107, 125-131 (1970).

S. D. Weaver Department of Geology Bedford College London NW l 4NS, England THE GEOLOGY OF THE NASAKEN AREA, SeptemberJ96£ Department of E SOUTH TURKANA . KENYA M arch.1973.

26______27 26______29______30______31 32

KEY TO GEOLOGICAL SYMBO STRATIGRAPHICAL COLUMN GEOLOGICAL BOUNE

GEOLOGICAL BOUNE VOLCANIC ROCKS ...... MINOR FOLD AXIS AN =^50 ...... DIP OF FOLIATION KANATIM VOLCANO DIPAND STRIKE o f : WELDED ASH-FLOW T + ...... STRATA HORIZONTAl ...... FAULT WITH DOWNTf

...... FEATURE. F LOW FRC

...... AGGLOMERATE,EXP

FAULT BRECCIA

,H, AGGLOMERATE LANDSUP

KAFKANDAL # VOLCANIC CONE ACHYTE, MORUANGITAK .. CONTOUR INTERVAL 100 FT. LCHYTE. EPONG

rHACHYTE, NGAPAWOI -

SCALE 1:50,000

BASALTIC LAPILLI TL INTRUSIVE ROCKS

WELDED ASH-FLOW T D 601/3^ MUGEARITE TIRIOKO BASALT PYROXENE PHYRÎC BA FORMATION

FELDSPARPHVRIC BA EOLOGY OF THE NASAKEN AREA, Geologically surveyed by S.D,Weaver, September,1968 - April,1969. Department of Geology,Bedford College TURKANA , KENYA M arch,1973.

36’OCfE

KEY TO GEOLOGICAL SYMBOLS MN GEOLOGICAL BOUNDARY DEFINITE

DIP AND STRIKE OF STRATA

.. FEATURE,FLOW FRONT,LAVA SCARP

VOLCANIC COMPLEX AGGLOMERATE,EXPLOSION BRECCIA

- CONTOUR INTERVAL 100 FT.

SCALE 1:50,000

INTRUSIVE ROCKS Contr. Mineral, and Petrol. 36, 181— 194 (1972) © by Springer-Verlag 1972

Trace-Element Data Relevant to the Origin of Tr achy tic and Pantelleritic Lavas in the East African Rift System

S. D. Weaver, J. S. C. Sceal, and I. L. Gibson Department of Gleology, Bedford College, London, N W l 4NS

Received May 28, 1972

Abstract. Determination of the trace-elements Ba, Ce, La, Nb, Rb, Sr and Zr have been made on lavas from six trachytic and pantelleritic volcanoes in the Kenyan and Ethiopian Rifts. Consideration of these data shows that Ce, La, Nb and Zr have behaved as truly residual elements. In the peralkaline suites examined, plotting other chemical parameters against a residual-element such as Zr appears to be informative. The individual residual-element ratios are constant at any one volcano suggesting that the salic and basic lavas are geochemically related and that the trachytes and pantellerites do not have an origin independent of the associated basalts. Different values of individual residual-element ratios characterize different groups of vol­ canic centres. In the northern part of the Kenyan Rift, progressive change in these ratios with time can be traced. In the salic lavas, Ba and Sr show patterns of extreme depletion relative to the alkali basalts and these data suggest that the salic rocks are the result of protracted crystal-fractio­ nation. The relative volumes of the extruded lava-types are discussed and it is suggested that the trachytic and pantelleritic centres developed above cupolas of salic magma situated on top of a large basaltic reservoir. It is likely that the preponderance of salic rocks and the scarcity of lavas of intermediate composition are not criteria by which one can rule out an origin of dif­ ferentiation by crystal-fractionation.

1. Introduction In many places in Kenya and Ethiopia, trachytes and pantellerites are very abundant in relation to the amount of associated basaltic flows. These salic rocks may be the product of p^ial melting of sources related to, or independent of, the associated basalts. Alternatively, the salic lavas may have been produced by crystal fractionation of a basaltic parent with other factors controlling the relative proportions of the erupted lava-types. The major-element geochemistry does not preclude either of these origins. One of the ways of approaching the problem of the origin of the salic lavas is to examine the trace-element distribution patterns in these rocks. The present account attempts to broaden an earlier trace-element study of the volcano Paka (Sceal and Weaver, 1971) by examining six other volcanoes in the East African Rift system. The six centres range in age from Pliocene to Recent and include examples in which weakly under-saturated trachytes, over-saturated trachytes, alkali-rhyo- lites or pantellerites predominate as the characteristic sahc lava-type of the vol­ cano. Basaltic lavas associated with the Kenyan centres are usually porphyritic, some containing over 25 % phenocrysts by volume. Ca-plagioclase, pyroxene and olivine TRACHYTE, NGAPAWOI —

E DOLOMITIC LIMESTONE

PYROXENE-OLIVtNE

BASALTIC LAPILLI T INTRUSIVE ROCKS

» WELDED ASH-FLOW TUFF

MUGEARITE

PYROXENE PHYRIC BASALT

FELDSPARPHYRIC BASALT

APHVRICB.SALT

■ sif-IS— "■ PYROXENE-OUVINE F PHYRIC BASALT FELDSPARPHYRIC BASALT

APHYRIC BASALT

NAKASUW TRACHYPHONOLITE

KANITIRIAM TRACHYPHONOUTE

TRACHYTE WITH MINOR

AGGLOMERATE ffl

DOLOMITIC LIMESTONE

APHYRIC MUGEARITE TURKANA BASALT BASALTIC AGGLOMERATE FORMATION s SCALE 1:50,000

INTRUSIVE ROCKS

D601/3

TIRIOKO BASALT FORMATION

SOLVSBERGITE INTRUSIONS

m > \ KANITIRIAM MICR0F0VÀ1TE

KOWUN METAMORPHIC ROCKS VOLCANO

BASEMENT SYSTEM

I 30'N_66 - TURKANA BASALT FORMATION 182 S. D. Weaveret al. : are the dominant phases but opaque minerals are also important. Intermediate lavas have relatively fewer phenocrysts, plagioclase being the most abundant. The trachytes and pantellerites commonly contain about 10% phenocrysts, the major phase being anorthoclase feldspar. A variety of dark minerals also occur in these rocks as phenocrysts ; the most common is green pyroxene. At least 16 samples from each volcano have been studied and every effort has been made to sample all the available lava-types, even at the expense of making the collection unrepresentative. In all, 173 samples have been analysed for the elements Ba, Ce, La, Nb, Rb, Sr and Zr by X-Ray fluorescence procedures.

2. Geology (a) Nasaken and Bibkwo The Pliocene centres Nasaken (1°35'N, 36°05'E) and Ribkwo (1°15'N, 36°03'E) are large trachytic strato-volcanoes associated with monochnal flexuring (Me Clenaghan et al., 1971), and form parts of the western margin of the central trough of the Gregory Rift. They are underlain and overlain by basalts but within each deeply dissected volcano, alkali basalts and intermediate rocks make up only about 5% of the total volume. The remainder are of trachytic composition, of which pyroclastics, including welded-tuffs, make up about 30%. A difference between the two volcanoes is that the Nasaken trachytes are oversaturated with up to 20% modal quartz, whereas Ribkwo trachytes are just saturated or under­ saturated with up to 10% modal nepheline.

(h) Korosi and Paka Korosi (0°47'N, 36°07'E) is a Quaternary multi-centred basalt-trachyte volcano (J. Camey, pers. comm.), lying north of Lake Baringo within the median graben of the Gregory Rift. Pétrographie and chemical studies show that mugearitic rocks are scarce in relation to the associated basalts and trachytes. However, basalts are more abundant at Paka and Korosi than at any of the other volcanoes under consideration, making up about 20-25 % by volume in either case. Paka (0°50'N, 36°10'E), lying immediately north of Korosi, has been described in an earlier account (Sceal and Weaver, 1971).

(c) Ehurru The Pleistocene to Recent massif of Ehurru, north-west of in the central part of the Kenya Rift, has been mapped in detail (Thompson and Dodson, 1963), and its lavas have subsequently been studied petrographicaUy and chemically (Sutherland, 1970, 1971). Much of the volcano is masked by pyroclastic rocks banked up against the and there is no well-defined central crater, only a summit ridge running ENE-WSW. A group of many small, late craters lies at the eastern end of this ridge. The weU exposed eastern slopes are composed of flows of glassy and crystalline pantellerites, crystalline comendites, pantelleritic trachytes and subordinate alkali ohvine-basalts. Trachytic Lavas in the East African Rift System 183

(d) Fantale This Quaternary strato-volcano is situated on the floor of the northern part of the main Ethiopian Rift at approximately 8°58'N, 39°54'E. It is composed pre­ dominantly of peralkaline rhyohtes and obsidians (Lacroix, 1930; Gibson, 1967), and these more silicic rocks overlie an earlier succession of trachytes and weakly peralkaline rhyohtes. Late in the history of the volcano a summit caldera was formed in association with the extrusion of a voluminous ash-flow tuff. This event divides hi time the earher cone-forming, pre-caldera sequence from the few recent, post-caldera obsidians. Basalts are very rare, although a single flow has been extruded very recently from a fissure cutting the south western flank of the vol-

(e) Alutu The volcano Alutu (7°46'N, 38°47'E), hke Eantale, is one of the Quaternary peralkaline süicic centres situated on the floor of the main Ethiopian Rift (Dakin and Gibson, 1971). It hes on the Quaternary fault zone between Lake Zwai and Lake Langano and is a complex mass apparently composed almst entirely of süicic lavas and pumice. No basalts, mugearites or trachytes have been found. The dome-shaped form of the volcano is controlled by a ring of very recent pantellerite vents, and lavas and pumice from these vents completely mantle the volcano.

3. Analytical Methods The trace-elements were determined using a Phillips PW1212 automatic X-Ray flourescence spectrometer. For the elements Nb, Rb, Sr and Zr, precision is approximately ± 1 % of the amount present above the 100 ppm level and for Ba, Ce and La, this figure is ± 2 %. The U. S. G. S. standard rocks were analysed in parallel to enable comparisons to be made among different laboratories and the results are given in Sceal and Weaver (1971), Table 2, together with brief notes on analytical methods. Analysis of the major oxides was by standard wet chemical procedures.

4. Statement of Results (a) Major-Element Data Table 1 gives major-element analyses of typical examples of the more abundant lava-types from the volcanoes as, in general, this information is not avaüable in the literature. Three of the centres Alutu, Eantale and Ehurru are characterised by the presence of peralkaline rhyohte flows (pantellerites), whereas at the other Kenyan centres trachytic lavas are the dominant saHc product. Mugearitic flows are uncommon at all the centres and they have not yet been found at Fantale and Alutu.

(b) Trace-Element Data The trace-element results are presented in graphical form and a complete tabula­ ted list can be obtained from the American Society for Information Science (A. S. I. S.)^. The form of the graphs is simüar in each case and shows the abundance 1 Order document NAPS (No. to be requested from the author) from ASIS-National Auxiliary Publications Service, c/o CCM-Information Corporation, 866 Third Avenue, New York 10022; remitting $2.00 for each microfiche or $5.00 for each photocopy. 184 S. D. Weaver et al. :

Table 1. Representative analyses of the more abundant lava-types from the Rift volcanoes

864 854 843 3/443 3/713 3/721 W082* W080*

SiOa 63.30 68.58 70.06 48.05 59.08 59.42 72.40 71.80 TiOj 0.71 0.48 0.53 2.05 0.43 0.46 0.31 0.19 AI2O3 13.24 12.15 9.33 16.77 16.33 15.54 9.17 8.80 FegOg 4.15 3.05 3.21 3.33 4.80 4.82 2.29 2.63 FeO 4.44 3.36 4.96 7.18 2.50 3.29 4.52 4.58 klnO 0.31 0.21 0.32 0.23 0.24 0.25 0.32 0.34 MgO 0.09 0.20 0.14 4.79 0.40 0.27 —— CaO 2.06 1.06 0.49 7.96 1.52 1.47 0.28 0.22 NagO 5.90 5.52 6.90 3.99 7.13 6.55 6.32 6.91 K 2O 5.10 4.83 4.46 2.51 5.31 4.99 4.36 4.40 P.O. 0.04 0.01 0.02 0.48 0.03 0.03 —— H2O+ 0.36 0.48 0.08 2.38 1.32 1.28 —— H2O- 0.36 0.32 0.05 0.58 1.00 , 1.47 0.04 0.02

Total 100.06 100.25 100.55 100.30 100.09 100.24 100.01 99.79

Trace-elements : ppm Ba 32 48 64 818 1 19 445 368 Ce 199 424 552 118 411 597 233 401 La 93 158 272 59 231 338 115 194 Nb 133 222 377 84 298 421 150 248 Rb 89 187 234 60 168 211 98 147 Sr 7 3 5 935 27 21 1 2 Zr 501 1163 1697 213 904 1256 850 1463

S64 = pantelleritic trachyte, Ehurru (analyst — M. Thind, Leicester). S54 = pantellerite, Eburru (analyst — M. Thind, Leicester). S43 = pantelleritic obsidian, Eburru (analyst — D. S. Sutherland, Leicester). 3/443 = olivine-basalt, Ribkwo (analyst — H. Lloyd, Bedford College). 3/713 = trachyte, Ribkwo (analyst — H. Lloyd, Bedford College). 3/721 = trachyte, Ribkwo (analyst — BE. Lloyd, Bedford College). W082 = pantelleritic obsidian, Alutu (analyst — I. L. Gibson, Leeds). W080 = pantelleritic obsidian, Alutu (analyst — I. L. Gibson, Leeds). — Not determined; * X.R.F. analysis. of one of the trace-elements plotted against Zr as the abscissa (Figs. 1 and 2). This procedure has been adopted because all the salic rocks considered here are peralkaline and it is known that Zr has a high solubility in melts of this type (Dietrich, 1968; Nicholls and Carmichael, 1969). Zircon and/or eudialjde have not been found in any of the lavas from any of the six centres under study. Neither does Zr appear to enter the major crystallising silicate phases, except perhaps in small amounts in the clino-pyroxenes (Chao and Fleischer, 1960; Taylor, 1965). Thus Zr in general behaves as a residual-element in these rocks during hquid/sohd equilibria being retamed in the liquid phase. Although Ce and La behave sumlarly, it is shown below that the abundance of these elements may be modified by late stage “volatile effects”. Zr is apparently not so affected. In addition Zr is particularly abundant in these rocks and can be determined with correspondingly high precision by the XRF procedures adopted in the present study. Trachytic Lavas in the East African Rift System 185

Table 1 (Continued)

8/182 8/313 8/111 8/130 Y802 Y756 Y379 Y335

SiOa 45.33 62.04 64.82 64.67 47.11 59.84 67.21 72.24 TiOa 2.82 0.78 0.53 0.55 3.24 1.34 0.40 0.29 AI2O3 14.51 13.91 13.13 11.66 13.65 13.39 14.05 9.54 FcgOs 3.95 5.42 6.79 7.83 5.08 2.70 1.68 2.33 FeO 7.16 3.02 1.00 1.32 10.95 7.27 3.44 3.99 MnO 0.20 0.20 0.36 0.26 0.22 0.24 0.13 0.12 MgO 7.93 0.30 0.14 0.24 4.89 1.36 0.08 0.08 CaO 10.41 1.18 0.46 0.72 9.20 3.43 1.54 0.38 NagO 2.86 6.28 6.97 6.43 3.73 6.39 6.31 6.30 KgO 1.65 5.05 5.00 5.19 1.00 2.80 3.77 4.40 P2O5 0.56 0.08 0.03 0.04 ———— HgO+ 2.30 0.98 0.69 1.04 0.04 0.11 0.14 0.12 H 2O- 0.34 1.16 0.55 0.74 0.07 0.15 0.19 0.06

Total 100.02 100.40 100.47 100.69 99.18 99.02 98.94 99.85

Trace-elements: ppm Ba 715 208 11 32 474 1577 1020 135 Ce 106 163 197 518 89 105 172 314 La 50 94 126 312 34 44 81 154 Nb 68 127 204 466 37 60 120 234 Rb 39 88 162 294 16 45 82 177 Sr 748 14 19 19 522 203 73 3 Zr 195 392 734 1660 169 305 682 1346

8/182 = olivine-basalt, Nasaken (analyst — H. Lloyd, Bedford College). 8/313 = trachyte, Nasaken (analyst — H. Lloyd, Bedford College). 8/111= quartz-trachyte, Nasaken (analyst — H. Lloyd, Bedford College). 8/130 = pantelleritic trachyte, Nasaken (analyst — H. Lloyd, Bedford College). Y802 = basalt, Fantale (analysts — F. Buckley and J. Gronow, Leeds). Y756 = trachyte, Fantale (analysts — F. BucMey and J. Gronow, Leeds). Y379 = pantelleritic obsidian, Fantale (analysts — F. Buckley and J. Gronow, Leeds). Y335 = Pantelleritic obsidian, Fantale (analysts — F. Buckley and J. Gronow, Leeds). No major-element analyses of lavas from Korosi are yet available.

In the 'present case, Zr appears to be a more useful index than the commonly used major-element indices such as Differentiation Index (Thornton and Tuttle, 1960), Solidification Index (Kunoet al., 1957) or Fractionation Index (Macdonald, 1969). However, the method will not be apphcable to rock suites in which Zr-rich phases occur. From the graphical treatment of the trace-element data it is apparent that the results for each of the six volcanoes are closely comparable, even though the suites differ in their major-element characteristics. At all the centres, the concentrations of the elements Ce, La, Nb, Rb and Zr are lowest in the basalts, higher in any mugearites and reach a maximum in the more abundant salic products, the tra­ chytes and pantellerites, there being a close relationship between the trace and major-element compositions (in contrast, see Macdonaldetal., 1971). Graphical plots of these five trace-elements against Zr are Hnear and may be projected 186 S. D. Weavereial.:

Ce La

600 300

400 150 200

Zr

0 1000 2000 0 1000 2000

Nb Rb

600 300

400 - • 150- 200

.* Zr ;• Zr 0 1000 2000 0 1000 2000

Ba Sr

1200 1200

800 . 600 400

• i • • 0 1000 Zr 2000 0 1000 Zr 2000 Fig. 1. Plots of Ce, La, Rb, Ba and Sr (ppm) against Zr (ppm) for the lavas of Nasaken volcano

through the origin. As an example of this relationship, the results for the volcano Nasaken are presented in Fig. 1, where Rb, Nb, La and Ce are plotted against Zr. At Alutu, the range of rock types sampled is more limited but the same directly proportional reahtonship of Zr to the other elements is observed. Although the trace element results for each volcano are very similar, there are some important differences in detail. In particular, the ratios of each element to Zr may have different values. This is illustrated graphically in Fig. 2 for the vary­ ing Nb/Zr ratios for the six volcanoes and statistically in Table 2 where the rele­ vant regression and correlation-coefficients are given, together with data for Paka (Sceal and Weaver, 1971). The results for Ba and Sr are very different from those for the elements discussed above ; both are present at low concentrations in most of the salic rocks at each centre and this is particularly the case for trachytes from the Kenyan centres. In contrast, in some of the Kenyan mugearites, Sr and Ba are relatively abundant and show higher values than the associated basalts (Fig. 1). Trachytic Lavas in the East African Rift System 187

Nb Nb Ebburu 400 Ribkwo 400

200 200

Zr Zr 1000 2000 1000 2000

Nb Korosi 400 Paka 400

200 200

Zr 0 1000 2000 1000 2000

Nb

Alutu 400 Fantale

200 200

Zr 0 1000 2000 1000 2000 Fig. 2. Plots of Nb (ppm) against Zr (ppm) for the lavas of six Upper Pliocene-Recent, central volcanoes from the Ethiopian and Kenyan Rifts

Table 2. Computed data for Nb v Zr graphs of seven Rift volcanoes. (Correlation and regression coefficients are all positive. The Nb/Zr ratios are given by the computed regression coefficients)

No. of Correlation Regression Error of Intercept Analyses coefficient coefficient reg. Coeff. ppm. Nb

Nasaken 24 0.993 0.304 0.008 - 6 Ribkwo 20 0.980 0.293 0.014 - 1 Korosi 18 0.992 0.212 0.007 0 Paka 49 0.988 0.209 0.005 - 4 Eburru 19 0.994 0.226 0.006 - 3 Alutu 16 0.999 0.161 0.002 + 10 Fantale 27 0.993 0.173 0.004 + 9 1 8 8 S. D. Weaver et al. :

5. Discussion (a) “ Volatile Effects” It is observed that in certain suites, on graphs of Ce and La against Zr, some points may fall off the linear trends illustrated in Fig. 1. In all cases the discrepant points (Fig. 3) represent crystalline, peralkaline lavas. Not all such rocks are aberrant and in Fig. 3, several conform to the same linear trend as the glassy specimens. It is thus possible to use glassy and crystalline rocks to define the trace-element ratios for such peraLkahne suites. It is recognised that peralkaline lavas expel a compositionally significant vola­ tile phase on crystallisation (Noble, 1965; Romano, 1969) and it is mainly for this reason that much work in recent years has concentrated on obsidians rather than their crystalline equivalents, in the belief that the former approximate as closely as possible to volcanic liquids. It has been argued (Sceal and Weaver, 1971) that providing the individual residual elements do not partition differentially to any volatile phase during the crystalhsation of salic liquids, the residual-element ratios will remain constant and the linear patterns unmodified although the abso­ lute concentrations of the elements will change. However, in the case of the Eburru samples shown in Fig. 3, it seems likely that Ce was at times partitioned preferen­ tially into a fugitive volatile phase which was lost on crystallisation, particularly as the rare-earths are known to form volatile fluorides (Herrmann, 1970). Rosholt etal. (1971) attribute the loss of TJ from crystalline, süicic, volcanic rocks both to its expulsion as a volatüe fluoride and to groundwater leaching. The latter process may have been a contributory factor in the modification of the Eburru pattern. Anomalies are only apparent on plots of Ce and La against Zr. The elements Nb, Rb and Zr behave coherently.

(b) Origin of the Salic Lavas In any solid/liquid reaction or series of reactions, the relative abundance of a pair of elements (such as Nb and Zr) will be constant if the bulk distribution-coefficients (Schüling and Winchester, 1967; Gast, 1968) between the crystalline phases and the liquid for the two elements remain approximately equal. If the abundance of any pair of such elements m the liquids which have been derived in this way are plotted against each other, the locus wül be a straight line. Such a Line wül project through the origin and its slope wül be determined by the concentrations of the two elements in the initial liquid. From the plots of the elements Nb, Rb, Ce and La, against Zr (Figs. 1 and 2), it may be inferred th at the bulk distribution- coefficients between melt and crystals, over a wide range in mineral and liquid composition, are identical and remain constant for all five elements. This is only likely when the bulk distribution-coefficients are close to zero. Those elements which are almost completely retained in the liquid phase during any solid/hquid reaction or equilibrium have been termed ' residual-elements ' (Harris, 1967). I t foUows th at in the various volcanic suites (Figs. 1 and 2), Nb, Ce, La and Zr have behaved throughout as residual elements and in most suites Rb also behaves in a similar way. Basic lavas, where present have an identical Nb/Zr ratio to th at of the associa­ ted salic rocks and this relationship holds for the other ratios Rb/Zr, Ce/Zr and