Variations in Isotopic Abundances of Strontium, Calcium, and Argon and Related Topics

1 H h..9 .RECEIVED - "5 1.1 -1 2 , . L -

R& ... M.I.T.-1381-17. SEVENTEENTH ANNUAL PROGRESS REPORT FOR 1969 U. S. ATOMIC ENERGY COMMISSION

CONTRACT AT(30-1)-1381

*%15 VARIATIONS IN ISOTOPIC ABUNDANCES OF STRONTIUM, CALCIUM, AND ARGON AND RELATED TOPICS

Contributions by

M.I.T. Staff Students, M.I.T. P. M. HURLEY C. M. SPOONER H. W. FAIRBAIRN J. B. REID, JR. W. H. PINSON, JR. T. M. GATES R. 1 NAYLOR B. J. FRYER J. D. AIKEN, JR.

Visiting Research Staff and

Collaborators Secretary : J. Golden - J. R. RAND V. G. POSADAS Machinist: 0. J. Annese (part-time) G. W. LEO (U. S. Geol. Survey) R. W. WHITE (U. S. Geol. Survey) W. D. MACDONALD (State U. of N. Y.) D. G. BROOKINS (Kansas State' U.) B. M. FRENCH (Goddard Space Flight Center) A. R. BERGER (U. of Toronto) R. H. REESMAN (Geochron Laboratory, Cambridge) . J. RONDOT (Ministry of Natural Resources, Quebec)

DEPARTMENT OF EARTH AND PLANETARY SCIENCES

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

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CONTRACT AT(30-1)-1381

VARIATIONS IN ISOTOPIC ABUNDANCES OF STRONTIUM, CALCIUM, AND ARGON AND RELATED TOPICS

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-7 x

L Table of Contents

CONTINENTAL EVOLUTION Page

1 Investigation of'Continental Accretion in 1 Laurasia

Northern Appalachian Geochronology as a 11 Model fur Interpreting Ages in Older Orogens

REGIONAL STUDIES

Preliminary Geochronological Studies in 19 Northeastern Newfoundland

Basement Rocks in the Northern Appalachians: 21 Preliminary Data on the Massabesic Gneiss, Manchester Area

Further Work on the Archean Complex in 23 Sierra Leone

Approximate Location of the Boundary Between 25 the Eburnean and Liberian Orogenic Provinces in Eastern Liberia

Rb-Sr Isotopic Investigation of Crater Rocks 27 of Possible Impact Origin

Ph.D. THESIS INVESTIGATIONS

Progress Report on the Isotopic Composition 31 of Strontium in Minerals from Ultramafic Inclusions in Basaltic Rocks

Radiometric Ages and Paleomagnetism of 39 Canadian Diabase Dikes

1 A Test of Rb-Sr Whole-Rock Dating of 41 Diabase Dikes Preliminary Report on Rb-Sr Whole-Rock 43 Measurements on Precambrian Iron Formations of the Canadian Shield 87 86 Sr 1 Sr Initial Ratios and Whole-Rock Ages 45 of Pyroxene Granulite

TECHNIQUES and STANDARDS Interlaboratory Comparisons and Rb and Sr 95 Isotope Dilution Analyses

Progress Report on Rock Standards 99

Iron Extraction Procedures for Rb-Sr 103 Isotopic Determinations

An Isotope Effect Suppressing Absorption 105 in Measuring Concentrations of Spike Solutions by Atomic Absorption Spectrophotometry

BIBLIOGRAPHY

Bibliography of Age Data on Continental 107 Basement Rocks

MISCELLANEOUS

Reprints of Published Reports, 1968-1969: Correlation of Radiometric Ages of Nipissing 129 Diabase and Huronian Metasediments with Proterozoic Orogenic Events in Ontario Review of Age Data in West Africa and South 131 America Relative to a Test of Continental Drift

A Rb-Sr Geochronologic Study of the 133 Pegmatites of the Middletown Area, Connecticut

Pre-Drift Continental Nuclei 137

Strontium Isotopic Compositions of Gangue 143 Minerals from Hydrothermal Vein Deposits Strontium Isotope Variation and Whole-Rock 145 Isochron Studies, Grenville Province of Ontario

Precambrian Gneisses from Northern 151 Colombia, South America Abstracts of Papers Presented at Meetings 153

Reviews 155 1t Additions zo the Project Bibliography 157 1

Investigation of Continental Accretion in Laurasia

Patrick M. Hurley

Int oduction

The concept of continental accretion has been examined therefore use this phenomenon to provide a mappable

by many investigators. In particular it has been advo- geologic unit, and propose the term "orogenic series" cated by those who have used Sr87/Sr86 initial ratios in in keeping with the usage in stratigraphy. Detailed

igneous rocks as an indication that new material has been investigations (see elsewhere in this Volume) have in-

added to the crust progressively throughout time in suc- dicated that the scatter plot in Figure 1 is actually

cessive and generally peripheral orogenies. The object made up of discrete isochron plots indicating distinct

i of this report is a more detailed examination of the hy- episodes o f plutonic and metamorphic acti.vi.ty. Names pothesis, after assuming that continental drift has oc- have been given to these individual episodes here and

curred since mid-Phanerozoic times, and that the recon- in Europe, ouch as Caiedonian, Acadian, Hercynian, and

stricted cupereontinent of Laurasia presents a more real- Taconic, and there has been much discussion about the istic test. Of particular interest is the distribution continuity of these individual episodes over great dis-

of late Precambrian and Phanerozoic orogenies, which may yances. It is proposed that the aggregate or these epi- permit further examination of the suggestion by Hurley sodes be termed an "orogenic series" in that it deline-

and Rand (1969) that there was no significant drift ates a province within which there is a limited number

prior to this last break-up of the continents. of discrete events which in obscure cases are indicated

Geological mapping requires the adoption of uniti. simply by a bounded array of whole-rock Rb-Sr points as

Since this is a study of possible continental accretion, „in Figure 1. applied we are interested in the distribution in time and space The name "Appalachian Orogenic Series" might be of some defined unit of new crust in each of several to the band in North America. The same band appears to

successive time units. The selection of such mappable be underlying much of western Europe. In view of the

units obviously depends upon carefully stated definitions. older age values currently being found in most of the Thus the present continental crust is defined as. that Pan-African of Africa, the Adelaide and Tasman of Aus- material above the moho which has typical crustal seis- tralia, the Carriri and Paraiba of South America, the mic velocities due in major part to the presence of Trans-Antarctic and Paleozoic belts of Antarctica, and feldspar; and furthermore as that material which has the belts south of the Russian and Siberian shields been piled up by undefined geological processes above and north of the alpine orogeny in central Asia, are the mean level of the earth's surface, and currently of the same age span. In addition there is good indi- resides in approximate isostatic equilibrium within cation that a similar zone of geological activity lies the boundaries of the continental This is in slopes. between older and younger belts around northern and contrast to the usage by Zartman and Wasserburg (1969) western South America. (for example), who include the lithosphere below the In this kind of map unit the consideration of the age crust in the definition of the term. Also only the of the sedimentary materials which may be caught up in

present··crust will be considered, so that any former the orogenic activity is eliminated. Thus a clear dis- crustal material which has been recycled through the tinction is made between currently used map units and

mantle will be said to have an age only equivalent to the new one proposed here. In this case there ic in- that since its last appearance at the earth'R surface. cluded only igneous and metamorphic rocks or their

This is a key part of the definition and side-steps metasomatic off-shoots at the time of their intrusion 87 the question of whether radiogenic Sr is constantly of thermal development. This decision stems from the lost by mantle recycling of crustal materials. fact that most whole-rock Rb-Sr measurements to date have It is now necessary to examine the ratio of radiogenic indicated very little difference between the apparent 87 87 Sr /Rb in total rock materials within a single oro- ages of metamorphic rocks and accompanying igneous bo- genic region. Figure 1 shows the distribution of ana- dies so that the total orogenic aggregate falls within lytical results of 200 fairly randomly selected samples the band previously described. This obviously excludes collected from the Nor·lhern Appalachians. It can be minor post-orogenic igneous activity. seen that the points fall rather uniformly within two Laurasia has been reconstructed using a conical pro-

bounding isochrons approximately 400 m.y. apart. This jection about the present pole for Eurasia, and the suggests that further sampling will not yield a signi- America of Bullard' et al. pole of rotation for North -- ficant proportion of points above and below the enve- (1965). Map units have been the following: lope that represents this orogenic province. We shall 2

1. 0 to 250 m.y., or essentially Mesozoic and Ceno- tinguish this from the lighter shading of the 0-280

zoic. m. y. group because of the reproduction process. This

2. 250 to 800, or Paleozoic and late Precambrian. latter age group is shown more clearly in the following

3. 800 to 1500 m.y., or a band that would include figure. Areas left blank belween the igneous-metamor- the Grenville of North America and the Dalslan- phic belts are mostly basins, or areas covered by un-

dian of the Baltic. folded sediments.

4. Greater than 1500 m.y., including the Hudsonian Figure 4 shows the 0-250 m.y. orogenic belts. These and Kenoran o f North America,' the Sveeofentlian include Mesozoic and Cenozoic thermo-tectonic events. dispersed generally around the peri- and Karelian of the Baltic, and the Stanovoi and . They appear to be Chara complexes of the Aldan Shield. phery of the supercontinent . In Figure 5 the major fold structures, or strikes of 2-5. When Reconstructed Laurasia is shown in Figures the folded rocks, are indicated by heavy black lineR. reduced to this scalc it is not possible to observe the Some Precambrian structural trends are also shown. In individual plutons of igneous character that were deli- general the structural directions seem to be conccntric neated in the original map sheets, 30 that the same. rather than random. stipple is used for both igneous and metamorphic rock Thc ovii·al.l. appearance of the maps definitely suggests units. The excellent new geological map of Eurasia a concentric arrangement with a peripheral development (Tectonic Map, 1966) was used as a source for the Phan- of the younger belts. Thus it seems that a case for erozoic belts in eastern Europe and Asia, and a com- continental accretion may be resurrected as a valid hy- prehensive literature survey (elsewhere in this volume) provided the information on the Precambrian. pothesis, if the continent is the reconstructed supercontinent prior to the last great drift. Figure 2 shows the locations of the late Precambrian and Phanerozoic orogenic belts in shaded gray. Grani- The same effect would appear if isograds of the ratio 87 86 of Sr / S. in the average total crust were plotted on toid rocks within these belts are in darker tones, but the same map sheet. Using the values found by Hurley may not be visible in the reproduction. The 800-1500 87 86 (1968) for the average Rb and Sr contents of the up- m.y. age span is outlined more clearly in North Ameri- per and lower crust, and the values of initial Sr87/Sr86 ca than in Europe, and is represented by the coarser found by Hurley and Rand (1969) for the mantle and the stip le. There is no present way of separating this lower and upper crusts, it is possible to estimate the age province from the > 1500 m.y. group (shown in black) present Sr87/Sr86 ratio in average total. crustal mate- because of overlap and lack of good age data. In North rial of any specific age. Figure 6 gives a plot of this America some of the stippled area in the southwestern estimate and indicates the extent to which the radioge- states is 1700 m.y. or more. The use of the .1500 m.y. 87· nic Sr would decrease towards the periphery of the boundary divides the Eurasian age data more convenient- supercontinent. ly at present, but this may change as more work is done. It is interesting to speculate on the shape, size and Basement rocks > 1500 m.y. almost certainly underly much of the areas in the Siberian and Russian platforms, and age of blocks of sial that have formed successive addi- the ice cap of Greenland, which have been left blank in tions to the accreting continent. Although the age

Figure 1. Outlines of North Africa, Arabia, and India seems generally younger proceeding outward from the center, each Orogen may cover intrinsically a range of sev- havc been 5 nr:luded in the figure for orientation, but should not be included in Laurasia as they probably have eral hundred million years. It looks as if individual been brought redently into contact with it by drift mo- belts like the Japanese Islands have been attached to tions. the continent together with adjacent basins like the Sea of In Figure 3 these outlines have been omitted, but the Japan. Seismic evidence suggests that this Sea effect of the closing of Tethys has been left in because is quite old; and there is some indication that late there is no way separating that which truly does not be- Precambrian or early Paleozoic rocks occur in the core of the Japanese Islands. However, in this report, the long to Laurasia. Also the region > 800 m.y. is shown orogeny would be classed within the 0-250 m.y. range. in cross-hatch, and it has been possible to increase its In Asia particularly the motif consisting of the arcu- size because of the known age of platform cover in the ate mountain belt and adjacent basin seems character- Siberian and Russian Shields. The orogenic series of

250-800 m.y. is shown in black. It is difficult to dis- 3

istic; and the age spread within individual units may turn were derived earlier by chemical fractiona- even include Precambrian cores (in the folded belts) tion from the mantle. The initial ratio of Sr87/ or basement (underlying the basins). Eventually it Sr86 in continental igneous rocks reflects this seems that the uplifted parts erode into the basinh, pre-history. and further compression reduces the openness of the 4. Nei.ther supercontinent was broken apart, nor re- aggregate, and new stable crust results. * distributed in any major way, prior to the last

A possible further extension of these concepts could great drift episode. be the case of a basin or trough that remained a sea- 5. The supercontinents developed and remained over way for a long period of time, and which eventually the poles of the earth with the ocean girdling filled to sufficient depth with sediment so that melting the equatorial belt. The entire process possi- and weakness occurred in its roots and it formed one of bly was the result of convective motion that was the younger orogenies in an older part of the continent, Outward at the equator and inward at the poles. like the Ul·/ls or the Caledonians of Norway. The eventual concentration of radioactive ele- The continuity of the 800-1500 m.y. and the > 1500 ments in the continents upset this pattern of m.y. regions in Figure 4, the non-random and generally mantle molion, resulting finally in a reversal concentric arrangement of structures and the non-random that caused the great continental drift. distribution of age provinces still support the additional hypothesis that there was no major large-scale drifting in variouo parls of Laurasia prior to the last great drift episode.

A similar map of the supercontinent of Gondwana is in preparation, with very much the same appearance. The midences of glaciation and paliumagnetic pole positions suggest that thio continent was over the south pole pri- or to the break-up. From these various observations the author would like to present his current views on the history of the crust:

1. The supercontinents of Laurasia and Gondwana grew separately by an accretion process in which island-

arc-like orogenic.belts and adjacent basins were

added to the periphery.

2. These orogenic belts individually developed over periods of several hundred million-years, so that

their cores and associated basin sedimentation

could well be late Precambrian even in the younger ones. However, new sialic material was continu-

ously added in episodes of igneous activity within the span of time represented by the "orogenic se- rics". 3. The centripetal motion required by the accretion

process resulted from inward- moving sea-floor.

This underthrusted the shelf of the growing continent, bringing in basaltic lithosphere (and sediments) and developing the calc-alkaline magmas that are Eharacteristic of sialic crust.

Thus the crustal additions were a mixture of 01-

der sediments and second-stage magmas resulting from the fusion of sea-floor basalts, which in 4

.- - 1 11

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Sr83/sr86 -.712 Present average Sr87/Sr86 to depth of Moho in crustal units, vs. radiometric age - .710

708 2500 2000 1500 1000 500 0 Radiometric Age of Crustal'Unit, my

References

Bullard, E. C., J. E. Everett, and A. G. Smith. Phil.

Trans. Roy. Soc. London Ser. A, 258:41, 1965.

Hurley, P. M., and J. R. Rand. Science 164:1229, 1969.

Hurley, P. M. Geochim. Acta 32:1025, 1968. Tectonic Map of Eurasia, Geol. Inst. of the Acad. of

Sci. of the USSR, Moscow, 1966. Zartman, R. E., and Wasserburg, G. J. Geochim. Acta 33:901, 1969.

Acknowledgements

Figure 1 was reproduced from another report with the kind permission of H. W. Fairbairn. The geological details of reconstructed Laurasia were transferred from

reference maps meticulously by hand by Claire Buchanan who transposed them into the conical projection. 10

,· 1, i : ' I .; . : . 5

2 :: . 1.rH IS'PAGEi t,: 1.W '. WAS INTENTIONALLY LEFT BLANK

.1 11

Nprthern Appalachian geochronology as a model for interpreting

ages in older orogens

H. W. Fairbairn and P. M. Hurley

Whole-rock Rb-Sr analyses on qranitoid rocks ' Abstract About 200 samples of granitoid rocks from the northern Isochron plots of whole-rock Rb-Sr measurements on Appalachian province have been analysed by various investigators. crystalline rocks randomly collected over ancient orogens for whole-rock Rb-Sr isotopic relationships. The generally show a scatter of points entirely within a results are plotted in Figure 1. Coordinate scales have been bounding isochron envelope. There has been controversy over chosen that wi11 include most of the analyses; a few having too the interpretation of these. A plot of isochron data from high a ratio of Rb/Sr to fall on the diagram have been reduced in the Northern Appalachians, where correlation between the scale and plotted with a tick mark. This is not a random sampling Paleozoic succession and radiometric ages has been 1 but represents a dispersed collection of samples from individual established, is remarkably similar, so -that this region rock units that are themselves scattered throughout·the province. may be taken as a model for interpreting the plots observed It can be seen that the points fall within a bounding in the more ancient cases. The interpretation states that: envelope representing isochrons at 227 m.y. and 608 m.y., or 1. The observed age band actually consists of a complex roughly a·400 m.y. spread in time. ·From these data alone it of. discrete intrusive or metasomatic events that is impossible to say anything definite about the initial ratio covers the time range within the bounding isochrons. - 87 86 87 86 of Sr /Sr 2. The initial Sr /Sr ratios given by separate because the apparent constriction to a limited range 87 86 of Sr /S'C ratio at the lower left corner of the plot may be due isochrons on granitoid rocks are similar enough so entirely to a different origin for the more basic, or less alkalic, that age values calculated on jndiviaual samples rocks in the sampling. This question has been bothersome in the using the·observed "common" initial Sr87/Sr86 ratio interprctatioi, of whole-rock Rb-Sr ages in ancient orogens arp. more likely to be true than not. where ' it is not al*ays possible to carry out the detailed work that has 3. Arid extrusive volcanic rocks frequently give age been done in the Northern values less than the time of extrusion, and have Appalachians. We have therefore been higher initial Sr87/Sr86 ratios, so that they should interested in the question of whether the limited Sr87/Sr86 ratio at the low-rubidium end of such a scatter plot can be used to be avoided in the sampling program. 4. A regional thermal rejuvenation of K-Ar age values provide a reasonable estimate of individual whole-rock ages. We have also been interested in the question of whether in the Northern Appalachians has not affected the the scatter shown by this kind of diagram is the result of later whole-rock Rb-Sr dating, so that a similar happening will not invalidate the above general interpretations migrations of Rb or Sr, or whether there are discrete episodes of plutonism and metamorphism and these scattered points in fact in the case of an ancient orogenic region.

- 1 1 1 . -

NORTHERN APPALACHIAN • PROVINCE 90- 4 .4 :.*- < 4 * * 608 m.y. .. 85- •.4 < -

. .. - Sr87 - 56 ..... , .:.. 80- .... . 0 8. ' 227 m. y :-.8 .

Ir... I · 75 - . . - - .. i: 200 PLUTONIC ROCKS .....60.... r. 77*.F - 71 93 1 1 1 1 70 10 20 30 40 R be7 Sres 12

represent individual isochrons that mark the true age of each of these Sndividual dpisodes. For example, having determined such a

scatter plot by collecting a number of samples over a wide region in an ancient orogen, can we now say that the bounding isochrons

truly represent the beginning and end of the plutonic and meta-

morphic activity in that orogenic province? An example of the kind of scatter found in Rb-Sr whole-rock

analyses on randomly collected rocks from an ancient orogenic

region is shown in Figure 2. Prior to continental drift it appears that the Imataca Complex of the northern Guayana Shield in Venezuela

was part of an extremely ancient pro/ince represented by the

basement rocks in Western Liberia and Sierra Leone. Details of

the geochronology of this region are t(, lie presented elsewhere by one of us (P. M. It. ) in collaboration wit.h G. W. Leo and R. W. White for the case of Liberia, and J. Kalliokuski for Venezuela.

We present this example as a comparisort with the NortMern Appalachian case in order to interpret the meaning of the

scattered points and their bounding isochrons. The question is whether or not the Liberian orogeny actually

started at 3400 m.y., and closed its activity at about 2700 m.y. \ Without the test of individual isochrons we can at least be encouraged to suggest from this comparison that this might be the

case. Also, as demonstrated below, we can have more confidence in

utilizing the observed common initial ratio of Sr87/Sr86 in Figure 2 | for an·age estimate on individual samples of the ancient rocks.

This statement does not necessarily apply to volcanics, as we shall note in the following examples of case studies in the'Northern

Appalachians. 11 84 - WESTERN LIBERIA PROVINCE , l -4 ='. 4- 82 - / l -

80 - i..: -

- 1= 986 78 5/./ . -

ROCK ANALYSES 76 - WHOLE - . W LIBERIA AND E. SIERRA LEONE 74 • IMATACA VENEZUELA :i,0 1 DI 72 - // 6705 .70 1 1.0 2.0 3.0 Rb87 S r 86 13

1 1

HOLYROOD GRANITE 51NEWFOUNDLAND , 1 .- 0 (GEOL. SURV. CANADA)

Sr87 Sr 86 BO

6 0 8+1 2 M.Y (Sr87/ Sr86 ) ..7040 i.0008 PF

'7O R 1 1 1 0 10 20 R b87 §786

00- 4 I MOUNT AGAMENTICUS ALKALINE INTRUSIVE , 1.00 - 1 BASIC COMPLEXES OF CAPE NEDDICK, TATNIC, , AND ALFRED l YORK COUNTY MAINE - I / / Srer.go- 1OA / 1,0 Sres fof . /

87/ 86 Rb / Sr 10 20 - 30 40 50 60 70 80 1.30 t ycl.30 NORTHBRIDGE HOPPIN HILL / , I 1.20 00 569 i 4 m.y. 514 + 17m.y. / - 1.20 0.7058+0.0005 0.7128*0.0067 , 1.10 0.90 / 0.74 / 0 E.- / %& MILFORD

1 1.00 -0.Bo Jk // 0.73 1 Q.1 & 0.90 70 / 0.72 /o DEDHAM I /t/ 591+28 m.y 0.80 - / * o ISOTOPE DILUTION 0.7054t a0010 -0·71 , / o X-RAY FLUOR 0.70' , , 0.70 0 10 20 30 0 1.0 2.0 3.0 4.0 87 / 86 Rb /Sr

Detailed geochronological work in the Northern Appalachians of the Dedham granodiorite, and with points falling on the has disclosed a series of magmatic and metamorphic episodes as isochron, appears to underlie a lower Cambrian Obolella assemblage. 1 The isochron age is therefore compatible with this geological indicated by linear isochron arrays that are compatible with observation, if the Holyrood granite is also taken to be late geological interpretations from stratigraphy. Figures 3, 4 and 5 Precambrian. Fairbairn et al (1965) have referred to this event show examples of isochron plots of analyses on samples from as the Neponbet, in a publication which slightly antedates the individual rock units. Figures 3 and 4 show the bounding idochrons indicated in Figure 1, represented by the Holyrood proposal by Lilly (1966) that this event be called the Avalonian in granite in Newfoundland and the granite of Mt. Agamenticus after the locality of the Holyrood'granite on the Avalon Penisula Maine. Figure 3 represents a study by McCartney, Poole, in Newfoundland. Wanless, Williams, and Loveridge (1966) of the Geological Survey These examples, and others unpublished, indicate that the of Canada. The Holyrood granite is clearly a late Precambrian apparent scatter of points in Figure 1 is actually made up of a

pluton as it unconformably underlies lower Cambrian fossiliferous series of discrete isochrons representing individual episodes 87 -llyr-1 shales. We have used the Rb decay constant of 1.39 x 10 of orogenic activity. The common value for the initial ratio of to obtain the value of 608 m.y. Sr87/Sr86 in Fig. 1 coincides quite well in general with the Figure 4 is a replot of analyses made at M.I.T. by J. Hoefs values found by. the individual isochron plots. This applies to extrusive volcanics. (1967) on the granite of Mt. Agamenticus.. The lower age of 223 is given the granitoid rock samples, but not to with by an isochron that excludes the three s'amples lowest Rb/Sr Analyses on volcanic rocks

ratio, which may be non-sanguinous with the alkaline Agamenticus There have been continuing problems with the dati,ng of The upper limit of age is given by utilizing these method. In intrusive. extrusive volcanic rocks by the whole-rock Rb-Sr only. In this samples and points in the lower part of the plot Figure 6 the analyticAl results by various investigators on 170

case the age of 250 m.y. would coincide with the time of a volcanic rocks from the Northern Appalachian province are plotted

Permo-friassic pervasive thermal overprinting of a large region in a single diagram. This plot differs considerably from that in in the Northern Appalachians as described by Zartman, Hurley, Figure 1. There appears to be a curvature to the general trend, Krueger and Giletti (in preparation). A K-Ar analysis by Giletti with the less alkalic samples following a steeper trend than the

(personal communication) supports the younger age value. remainder. Bounding isochrons are more difficult to draw, and A third example ib given by an isochron on granitic and there appears to be a greater range in initial ratio of Sr87/Sr86.

gneissic rocks in southeastern Massachusetts, from data presented These observations are brought out by more detailed studies. by Fairbairn, Moorbath, Ramo, Pinson, and Hurley (1967). In this the Coldbrook group of volcanics As an example, in Fig.. 7 study the Hoppin Hill·granite, which is thought to be a correlative 15

.90- - NORTHERN - APPALACHIAN PROVINCE .... /

.85 - . - 777 m.y. • .*

- .. -5.80 0 ,• 205 m.y . F. ... .:P .75 - 61. 170 VOLCANIC ROCKS

72 I-.·,' /. - 7 \ .70' i 1 1 1 1 0 10 20 30 40 R bu Tr&6

.80 1

- COLDBROOK GROUP VOLCANICS NEW BRUNSWICK • CORMIER ANALYSIS .. .78 - • M. LI ANALYSES

- A (10 ANAL.)

.76- .. „SF - 7 Sree

.74- .. 3/.. - 38-.76- A 8(25 ANAL) , 7 M.Y. .74- / \-405 1 57 /6 8-457+31

72- Se, S/h= / 777+61 ANAL.) :2'1 I '95 ( -L- C- .7055 + .0005-70 , 1 · ' · 1 1 C(25 04 8 .12 .70 l i l l i 1 1 1 1 1 1 02 4 6 8 10 1 2 1 4 16 Rbei „786 16

1 1 1 1 1.30 -

EASTPORT FORMATION VOLCANICS - MAINE (BOTTINO AND FULLAGAR) 1.10- - Sr87 Sr86

412 + 5 M: Y .90- (Sr87/Sres)0 = .707+.001 - ' - 4 -

70 0 20 40 60 80 100 Rb87 37 86 in New Brunswick, which is overlain by fossiliferous lower Cambrian we can say at the present time is that a scatteringof points, or beds, showsa plot of phints that has a marked curvature, and non-linear trend, is a danger signal in working with these kinds which could be interpreted as ranging in age from 405 to 777 m.y. of rocks. depending on which part of the plot is used. This is a plot of Conclusions sixty analyses, representing work by R. F. Cormier (1969), and Ih conclusion we have found supporting evidence by a study of Fairbairn et al (1966). The strange dropping-off of the age trend granitoid rocki in the Northern Appalachians that the bounding for rocks containing higher values of Rb/Sr, seems to be isochrons enclosing scattered Rb-Sr age data in ancient orogens accompanied by an increase in extrapolated initial ratio of 87 86 may be truly representative of the beginning and end of the Sr /Sr . This effect was first noted and clearly defined by orogenic activty. The evidence points to a rather narrow range Cormier, and has been a source of query and concern to many others for initial ratios of Sr87/Sr86, which can be used to estimate the who have attempted to date acid volcanics. ·In Figure 7 only the age value of any particular sample. It is also suggested that lower part of· the "anisochron" could fit the field relations. It is this scatter is not due to migrations of rubidium or strontium but also the only part that has a low initial ratio of Sr87/Sr86. is more likely to be due to an actual distribution of discrete Moorbath and Bell have observed (1965) in the basic igneous events within the time range of the orogenic activity. These rocks at Skye that the, low initial strontium ratios do not hold conclusions do not apply invariably to analyses on acid volcanic for the acid igneous members, and suggest partial melting of crustal rocks, which tend to show too low an age for some unexplained reason. material as a cause. Pankhurst (1969) found high Sr87/Sr86 ratios A late stage overprinting or cooling history that has reset K-Ar in gabbros in Scotland and believed that these rocks could not have age values to.250 m..y. in a large region in the Northern Appalachian assimilated enough crustal material to produce this effect. province, has npt affected the whole-rock Rb-Sr isochron age Instead he proposed isotopic equilibration with the host environment determinations. It is thus suggested that thermal rejuvenation involving no great addition of material. We shall have to leave may also not be too serious in, the case of the older orogenic this observation as an unsettled problem in the dating of regions. Phanerozoic orogenic belts, and simply state that the use of acid volcanics should involu a large program of sampling and analysis in order to assure that meaningful results may be obtained.

..An.,example of the case in which acid volcanics. appear to work well in.whple-rock Rb-Sr dating is given in Figure 8. This is a study by Bottino and Fullagar (1966) on the Eastport formation in Maine. Here the age of 412 m.y. is quite compatible with the Silurian-Devonian boundary close to which these rocks lie. In this case the analytical points are not scattered. Therefore all 17

References

Bottino, M. L., and Fullagar,. P. D., 1966, Whole-rock rubidium-

strontium age of the Silurian-Devonian boundary in

northeastern North Americd: G.S.A. Bull., v. 77, p. 1167-1175.

Cormier, R. F., 1969, Radiometric dating of the Coldbrook group of southern New Brunswick, Canada: Can. Jour. of Earth Sci., v. 6, p. 393-398

Fairbairn, H. W., Bottino, M. L. Pinson, W. H., and Hurley, P. M., 1966, Whole-rock age and initial Sr87/Sr86 of volcanics

underlying fossiliferous lower Cambrian in the Atlantic

provinces of Canada: Can. Jour. of Earth Sci., v. 3,

P. 509-521. Fairbairn, H. W., Moorbath, S., Ramo, A. 0., Pinson, W. H., and

Hurley, P. M., 1967, Rb-Sr age of granitic rocks of southeastern Massachusitts and the age of the lower Cambrian at

H ppin Hill: Earth and Planetary Sci. Letters, v. 2, p. 321-328. (Preprint in M.I.T. Thirteenth Ann. Prog. Rept. to U. S. Atomic Energy Commission, Contract AT(30-1)-1381,

1965, p. 3-10. Hoefs, J., 1967, A Rb'Sr investigation in the southern York County

area, Maine: in M.I.T. Fifteenth Ann. Prog. Rept. to U. S.

Atomic Energy Comm., Contract AT(30-1)-1381, p. 127. Lilly, H. D., 196.6, Late Precambrian and Appalachian tectonics in

the light..of submarine 'explorations on the ·Great Bank of

Newfoundland and in the Gulf of St. Lawrence-Preliminary views :

Am. Jour. Sci., v. 264, p. 569-574.

McCartney, W. D., Poole, W. H. Wanless, R. K., Williams, H., and Loveridge, W. D., 1966, Rb/Sr age and geological setting of the Holyrood granite, S6utheast Newfoundland: Can. Jour.

of Earth Sci., v. 3, p. 947-957. Moorbath, S:, and Bell, J. D., 1965, Strontium isotope abundances and the Rb-Sr age determinations on Tertiary igneous rocks from the Isle of Skye, North West Scotland: Jour. Petrol.,

v. 6, p. 37-66. Pankhurst, R. J., 1969, Strontium isotope studies related to petrogenesis in the caledonian basic igneous province of

North East Scotland: Jour. Petrol., v. 10, p. 115-143. 18

. ·. i , c#,61::I THIS PAGE t *2. ' WAS INTENTIONALLY LEFT BLANK 19

Preliminary Geochronological Studies in Northeastern Newfoundland H. W. Fairbairn and A. R. Berger (Udiversity of Toronto)

The Long Island Pluton is a roughly circular, steep- regional metamorphism and deformation. On the basis of

walled body largely hidden beneath the waters of the stratigraphic considerations and K-Ar dates in adjacent

.Bay of Exploits (Heyl, 1936). However, outcrops of areas, the Pluton had previously been considered of

several islands· and parts of the adjacent mainland Devonian Age (Williams, 1968) and the orogenic events show it to be a massive hornblende-biotite granodiorite in the country rocks as Acadian (post-Lower Devonian,

with a sharp outer contact in the north and a sheeted pre-Carboniferous). In contrast,.the Rb/Sr analyses, marginal zone in the south and southeast in which there though scattered, suggest that the age of these events

is abundant evidence of late- and post-consolidational may well be appreciably older (600 m.y.). No definitive conclusions are possible from the work penetrative strain.

A K-Ar analysis on biotite from a specimen taken from thus far . Additional field work may modify current

near the southwest contact gave a date of 440 f 30 m.y. thought on the stratigraphic relations. Further isotopic analyses may, on the other hand, resolve the (GSC 63-17) which has been considered anomalously high, puzzling discrepancy suggested from the data that are since the Pluton cuts presumably Ordovician rocks which now at hand. elsewhere pass upwards conformably into fossiliferous

Silurian (Williams, 1964a). Nevertheless, apart from REFERENCES one non-aligned sample from a locality where intimate mixing of granodforitic and rhyolitic material is Heyl, G. R.: "Geology and Mineral Deposits of the Bay expected, the Rb/Sr data *uggest an age appreciably of Exploits Area", Nfld. Dept. Natl. Res. Geol. Sect., older than that determined by K-Ar methods (.640 m.y.): Bull. 3, 1936.

The Deadman's Bay Pluton occurs over an area of some Jenness. S. R. : 'Terra Nova and Bonavista Map-Areas,

400 square miles at the northeastern end of a belt of Newfoundland°, G.S.C. Memoir 327, 1963.

granitic rocks, earlier referred to as the Ackley Williams, H.: "Notes on the Orogenic History and Batholith, which cut deformed and metamorphosed sediments Isotopic Ages of the Botwood Map-Area, Northeastern

of the lower unit of the Gander Lake Group of pre-Middle Newfoundland", Bl Age Determinations and Geological the Ordovician Age in a belt extending southwestwards to Studies, G.S.C. Paper 64-17, Part 2, pp. 22-25, 1964a.. south coast of Newfoundland (Jenness, 1963; Williams, Williams, H.: "The Appalachians in Northeastern 1964b). Although the Pluton on its eastern side appears Newfoundland - A Two-Sided Symmetrical System", Am. to grade outwards into regionally migmatized Gander Lake Jour. Sci., 262.1137-58, 1964b. Group rocks (Williams,1968), detailed studies here and Williams, H.: "Marginal Notes to G.S.C. Map 1227A, at the northwestern contact indicate that the Pluton the Wesleyville Area, 1968.

postdates both this migmatization and the bulk of the

0 73 - NEWFOUNDLAND GRANITIC ROCKS o DEADMAN'S COVE '0 /' e LONG ISLAND AVER OF 2 60//

I - 72 -

a I l S tes 1% I AERa 0 # ' .71 - I LONG ISLAND - 640 MY. / DEADMAN'S COVE 600 ? -I

.70 0 1.0 2.0 RbBY /Sr- 20

Analytical Data

A. Deadman's Cove

Sample . Rb Sr Sr"/Sr06 Rbi,/Sr. M.I.T. Field ppm Ppm computed

R7376 N204 197 216 .7306 2.68

R7377 N205 219 237 .7266 2.68

R7378 N206 175 346 .7161 1.46 2.14 R7390 Same N202A 193 262 .7240 LOCality R7391 N202B 195 260 .7235 2.18

B. Long Island

R7379 N126 70.6 174 .7179 1.17

R7380 N4 69.2 267 .7138 0.750

R7381 N106 68.8 317 .7174 0.629

R7388 N14OA 64.5 335 .7111 0.558 Same R7389 Locality N14 OB 57.1 342 .7129 0.484 21

Basement rocks in the northern Appalachians:

Preliminary data on the Massabesic Gneiss, Manchester area

New Hampshire

R. S. Naylor, T.M. Gates, and B.J. Fryer

As part of a larger program to study the history and Three of the samples lie within experimental error on a line having a slope of 300 x 106 years and an.Sre'/Sres tectonic significance of basement rocks in the northern Appalachians we have made a preliminary Rb-Sr whole-rock intercept of 0.719. A fourth sample lies above this line study of the Massabesic Gniess complex in the Manchester by several times experimental error. We feel that the data do not support a hypothesis that area, New Hampshire. these rocks are differentiated from a magma emplaced in its All of the rocks studied are massive, strongly foliated present location 300 x 106'years ago (Fitchburg Pluton) as granitic gneisses. They are intimately associated with more suggested by Billings (1956) and Sriramidas (1964). The mafic, banded gneisses locally containing sillimanite with initial Sr"/Sr" ratio of the complex is far too high for either muscovite or potassium feldspar (Sriramadas, 1964). mantle. More- These rocks are cut by massive, weakly foliated, muscovite- magmas generated in the lower crust or upper biotite quartz monzonite referred to as Concord Granite. over, this hypothesis cannot account for the data for sample

Billings (1956) mapped the gneisses together with the Concord MAS-6. The samples are more highly deformed, metamorphosed,

Granite as part of the. Fitchburg Pluton, to which he assigned and migmatized than other granites in the area which are

believed to be 300 to 350 x 10 years old , 82. Concord an Upper Devonian (7) age. D.R. Wones (oral communication, 1968) suggested to us the possibility that the gneisses might granite. A sill of muscovite-biotite (Concord "type")granite cuts the Massabesic Gneiss about thirty yards from where be part of an older, possibly Precambrian, basement as dis- The field relationships sample MAS-5.1 was collected. tinct from the Concord Granite. demonstrate that the deformation, migmatization, and meta- Experimental method and results mrphism -of the Massabesic Gneiss occurred before the In the course of several field trips to the area we emplacement of this sill, which is essentially undeformed. selected eight samples as being typical of the Massabesic A hypothesis more consistent with the field and iso- Gneiss. These came from highway cuts where we could obtain topic data is that we are dealing with a complex of older relatively fresh rock from sites suitable for continued rocks which was strongly disturbed about 300 x 106 years sampling. Preliminary X-ray analysis of these samples ago with accompanying redistribution of isotopes of Rb and indicated their Rb"/Sr" ratios ranged from 1.53 to 10.21. Sr among the whole-rock samples. .Such a hypothesis could Four samples, selected to give a spread in data on the account for the appearance of anomalously abundant radio- strontium evolution diagram, were analyzed more carefully. 87 genic. Sr in the samples, and for the scatter in the data. About 1 kg of material from each of these was crushed, The whole-rock Rb-Sr data do not permit an accurate ground, and split to about 20 grams of sand-sized material estimate of the primary age of the complex, but we favor a which was pulverized to fine powder in a Pico mill. very hypothesis that the rocks were emplaced 450 to 600 x 106

Gates, determined Rb/Sr ratios by X-ray fluorescence analysis ·MAS-9 have existed years ago. Samples MAS-5. 1 and could involving triplicate runs on an aliquot of the powder with as approximately closed systems since this time if they had G-1 used as an external standard. Naylor detei·mined Srs 7/Sr 8 6 formed with initial Sr"/Sr" of 0.705 (a feasonable magmatic ratios on unspiked aliquots of the sample powders, using an composition for rocks of this age). We note that the mean abbreviated version of the current standard sample handling Rbw/Sres ratio for eight representative samples of the technique. The twelve-inch mass "Connie" was spectrometer Massabesic Gneiss is 4.8 (analysis by reconnaissance X-ray used, and the Sr ratios were corrected to Sr86 /Sr88 equals fluorescence technique). In a rock or rock unit of this 0.1194'. The data are given in Table 1 and plotted in Figure 1. composition the Sr"/Sr86 ratio could evolve from 0.705 to Discussion 0.719 in 200 x 10'years. Assuming that the reconnaissance Data points for the preliminary analyses are scattered x-ray data indicate the average composition of the complex, on the strontium evolution diagram, giving little basis for the necessary evolution of radiogenic Srn dould have occurred a conclusion that the analyzed samples are closed, cogenetic between emplacement about.500 x 106 years ago and disturbance method. The systems as required for dating by the Rb-Sr about 300 x 106 years ago. samples are related by their occurrence in the field; eg. each of the samples is a massive, strongly foliated granitic

gniess, and all are collected from the same complex within an area of about ten square miles. From their field rela-

tionships we infer that the samples are ultimately of igneous

origin and that they may have formed at about the same time, but not necessarily from a common magma. 22

From the preliminary data we cannot exclude a hypothe- Analytical results

sis that the complex contains rocks as old as 1000 x 10 66(3) sample Rb/Sr (·1) Rb"/Sr86(2) Sr87/Sr years, but this seems less likely inasmuchas it would require the loss of very large amounts of radiogenic Sr 87 (or gain of MAS-5.1 1.18 5.23 + 0.13 0.740 Rb) in the case of samples such as MAS-5.1 and MAS-9 having MAS-6 0.51 + 0.10 0.734 MAS-8 0.51 1.53 + 0.12 0.726 relatively high Rb/Sr ratios. Extrapolating to initial MAS-9 3.51 10.21 i 0.94 0.764 Sr87/Sr" of 0.701 (k. the lowest value observed for terrestrial samples) sample MAS-5.1 cannot have existed as a closed (1) X-ray fluorescence, reconnaissance technique system for more than 540 x 106 years. The corresponding (2) X-ray fluorescence, triplicate runs with G-1 external figure for sample MAS-9 is 450 x 106 years. The samples are standard

from large bodies of granite (greater than 100 feet outcrop (3) twelve-inch mass spectrometer "Connie", all errors estimated * 0.001 length), not small aplite veins, and it seems unlikely that they could have lost all or even most of their radiogenic Description of samples 87 Sr during the dist.urbance. MAS-5.1 Massive, strongly foliated, biotite granitic 87 Wc believe the excess radiogenic Sr originated from gneiss from cut· on Everett Turnpike under bridge

within the Massabesic Gneiss complex rather than from without. -" carrying old US Route 3; one mile south o f The gneiss is imbedded in a terrain of Silurian and Devonian Manchester.

meta-shale and meta-graywacke. We consider it unlikely that MAS-6 Coarse, migmatitic, biotite-bearing, massive such a terrain could have evolved Sr as radiogenic as 0.710 granitic gneiss; NH Route 28, 100 yards north of (Sr"/Sr") in the short interval between deposition and Deer Neck Bridge, Massabesic Lake. disturbance at 300 x 106 years. We intend to seek further MAS-8 Massive, medium-grained, biotite-bearing, pink data relative to this point through analysis of the surrounding granitic gneiss; NH Route 28, s miles south of schists. 8/ Deer Neck Bridge.

MAS-9 Medium-grained, strongly foliated, biotite-bearing

greiss; NH Route 101, about 300 yards west of 0760. - bl junction with Old Cadia Road. References

- bil - Billings, Marland P., 1956, The Geology of New Hampshire, Part II Bedrock Geology, New Hampshire State Planning and Development Commission, Concord,N.H., 203. pp. 0,740 - . Sriramadas, Aluru, 1966, Geology of the Manchester Quad- + 5.1 ranqle, New Hampshire, Bulletin No. 2, New Hampshire DepartmentConcord, N.H., of Resources 78 pp. and Economic Development, 0 -4* 0' A - Acknowledgments S- 8+ Prof. D.R. Wones suggested the presence of basement rocks in the area and led several field trips. Prof. H.W.

- Fairbairn advised Gates on the X-ray procedures. Mr. J.V. 0720< (Sr Sr 890 = 0.719 Chernosky is aiding with the field and petrographic study of the complex. The research is supported in part by

- - National Science Foundation Grant GA-4480 to Naylor.

A = 1.39,10-"year-' 0700 1 1 0 2 46 8 10 Rb'>Sre« Figure 1. Preliminary strontium evolution diagram for four

samples from the Massabesic Gneiss, Manchester area, New Hampshire. 23

Further Work on the Archaan Complex in Sierra Leone

P. M. Hurley

Sierra Leone Basement Complex In our last Annual Report we outlined the general '

pro- distribution of ages in the various structural M.I.T. Latitude Longitude Sr 86 Rb87 Sr87/Sr86 vinces of Sierra Leone. Further measurements on the No. North West Ppm Ppm basement rocks of the eastern part of the country KAMBUI SCHIST has indicated that the entire complex appears to be 6550-A 8°39'40" 11°47'30" 24.09 12.06 .7225 roughly the same. age, and that we were in error in 6555-B 8°25'40" 11°40' 15.29 34.45 .8014 stating that the Kambui schist was younger. Measure- 6811-B 8°25'40" 11°40' 12.74 40.1 .8294 ments on this ancient Precambrian province are listed

in Table 1, and plotted in Figure 1. It appears that INTRUSIVE INTO KAMBUI, GRANITE the orogenic activity which produced the basement 6812-A 9°04'30" 11°45' 18.84 53.1 .8129 gneisses, the Kambui schist, and even what appeared 6812-8 n " 19.06 52.2 .8100 to be late kinematic granite that intrudes the Kam- 6812-C " " 18.36 52.5 .8172 bui, all occurs between 2750 m.y. and 3100 m.y. The 6816-A 8°26'3On 11°07'38" 20.27 53.5 .8145 Marampa schist, on the other hand, still remains at 6816-B " " 20.45 53.7 .8040 2100 to 2200 m.y. This age range for the Archean basement coincides BASEMENT GNEISS closely with measurements in western Liberia, and 6553-A 7°58'45" 11°43'15" 28.64 24.5 .7379 extends the "Liberiann orogenic province westward 6553-8 n " 27.83 22.2 .7381 into this region.

.840 SIERRA LEONE ARCHEAN COMPLEX .820 - IN 0 Basement Gneiss rap I Kambui Schist 1- .800 - 0 Intrusive into Kambui Schist .780 -

Sr877Sr86 -41 .» .760 - le -A40- 00 3/ 4,8,1 «l = 0.703 -120- = I .700 1 1 1 0 1.0 2.0 3.0 86 RbeJ/Sr 24

THIS PAGE WAS INTENTIONALLY LEFT BLANK *

Approxim-ate Location of the Boundary Between the Eburnean and Liberian Orogenic Provinces in Eastern Liberia

P. M. Hurley in collaboration with G. W. Leo and R. W. White

In the last Annual Report we presented the results of It can be seen in Figure 2 that the easternmost samples

a geochronological study of the basement crystalline rocks have dropped in age to approximately 2000 m.y., matching in western Liberia and the adjacent parts of Sierra Leone. similar age determinations on rocks throughout the extent Summary plots of whole-rock Rb-Sr age determinations are of Ivory Coast. The boundary between these provinces is repeated here in Figures 1 and 2 to indicate that this orogenic therefore set approximately as indicated by the dashed

province lies within the age span of 2700-3400 m.y. We line that lies between the Putu Range in the eastern have referred to this as the "Liberian" oro- tentatively province and the Tiehnpo Range in Maryland County, ex- genic province. During the past year an attempt has been tending from some point north of Greenville on the made to locate the boundary between this province and the coast. Although these whole-rock age determinations adjacent Eburnean province in Ivory Coast and Ghana to the are subject to an error derived from assuming an ini- Samples of basement rock were collected by R. W. 87 86 east. tial Sr /s. ratio of .705, the error will not be

White in eastern Liberia for this purpose and whole-rock significant in differentiating between these two age Rb-Sr age analyses on these are indicated in Table 1 and provinces. Figure 2. 4 .84 - - WESTERN LIBERIA PROVINCE 9 '

82 - - 4 0 e" 80 _ e. - 100 Sr87 0 Sr8678 -

'Po 76 - 5.0 - WHOLE ROCK ANALYSES ..i W LIBERIA AND E. SIERRA LEON E 74- / - /#1-.i 72 -/// 6705 .70 1.0 2.0 3.0 Rb87 Sr 86 Table 1

M. I.T. R.W.White Sr86 Rb87 Sr87/Sr86 *Whole-rock Age No. No. Ppm Ppm m.y.

R-7352-,1 WH 408-P 18.39 44.2 .7799 2340 1 60

R-7351-D WH 405-A 9.96 18.2 .7622 2250 + 80

R-7350-G WH 403-C 42.6 28.7 .7218 1800 * 210

R-7354-E WH 419-D 38.0 20.6 .7252 2650 k 260

R-7353-K WH 415-A 7.64 42.5 .8682 2100 f 25

R-7349-C WH 401-A 48.3 30.0 .7274 2570 + 230

R-7350-H WH 403-C 33.1 70.1 .7657 2060 1 70

* Assuming (Sr87/Sr86) = .7050 and normalized to Sr86/Sr88 = .1194. Rb87 A = 1.39 x 10-11 yr-1. Error given as the

equivalent of f .002.error in assumed initial ratio. 26

- to. --i-,r ,< /t .\ . SCH/SmWRAMP.4 Q .

-»r\- 1 - i:& -2 7 90 3-\ 0 0 SIERRA 2800 KAMBUI SCHSTS 1*

F,eetown · 2/0 ,%4 f) a«.9 M 60 {= '12E 60 ' #*-4.2140 6 1 LEONE f-4 4 •r2545 .4 F U. g (,1,0 A ( ri .1= 4 - -8° ) ..Ak \ .1,950 1' /.t.-1 < ./11/ J /4-, » 1 .. , F.L.YJ - 300. ,/ / 1 / .rM 4. 4 'A, f -4 2 «'« =A a .De - .3/ -41- \ tf z / / 1.-A r"-, 4 4 1 -7/ -»0 - 0 f /7 n0

2=0'PO = 5 e 0 ..t. ) f ... 14..4. - Monmvio'c· -soo- LIBERIA .... »t.» - .Sol-Z 4 VOO .6...:S'0 - - -60 /. -5. .'"'/A»'"' Or 25/0 ft.:' 6 61 / 44*44 1 ./ 40 \2 .-. #- / 2 Mo O406' S4 2100 \ 6- 9+ -50 Greenville ' C

(

13° 12° 11° 10° 9° 80 Cope po/mos

( - 2 7

Rb-Sr Isotopic Investigation of Crater Rocks

of Possible Impact Origin

P. M. Hurley

we have started an investigation of £he use of 'radio- Crater of Tenoumer, Mauritania genic Sr87/Rb87 as a possible means of determining whe- A similar test was madd on rocks associated with the

ther volcanic-like rocks within structures that have Tenoumer Crater, sent by Bevin M. French of the Goddard circular outlines are truly volcanic or result from. the Space Flight Center. Excerpts from a report by French,

impact melting of crustal materials. If the rocks are Hartung, and Short (in preparation for publication in the details. volcanic in origin, and the structure a cryptovolcanic Science· cover some of 87 86 effect, the Sr /Sr in the invading rocks should be "The Tenoumer Crater (1-3) is located in Mauritania at' in the range 0.706 i .002. If, on the other hand, the 220 55' north latitude, 10° 24' west longitude. It is about 1800 m in diameter, or about 50 percent larger volcanic-like materials are due to the melting of crustal rocks, the whole-r9ck Rb-Sr age of the material than Meteor Crater, Arizona. The crater floor is near- should reflect the age of the basement crust in which ly at the level of the surrounding plain (approximately

the impact occurred. Several test cases are under 190 m above sea level), and the maximum relief of the

examination. rim is 110 m. The crater occurs on a peneplained sur-

face of Precambrian gneisses and granites which is thin-

Malbaie Crater, Charlevoix, Quebec ly veneered by sediments of probable Pliocene or younger In October, 1968, the Meteoritical Society was taken age. The crater is younger than the sediments.

on a geological field trip to the Charlevoix Structure, "The origin of the Tenoumer Crater has been considerably debated. Richard-Molard (1) favored an origin by led by Jehan Rondot of the Ministry of Natural Resources volcanic explosion, stressing the location of the struc- 1 of Quebec. The Charlevoix Structure, represented by the ture on a line of possible tectonic activity between two Mont des Eboulments, occurs at the southern limit of the other circular structures, Richat (220 km SW) and Temim- Canadian Shield between the villages of Baie St. Paul and La Malbaie, or about 65 miles northeast of Quebec ichat (195 km NE). Allix (2) suggested a meteorite im-

City, at 47° 31' 50° north latitude and 70° 17' and pact origin. Most recently, Monod and Pomerol (3) des-

45" longitude. Figure 1 is reproduced from the guide cribed in detail the small amounts of "rhyodacite lava"

book prepared by Rondot. (4) associated with the structure and suggested that As seen in Figure 1, most of the area underlying the the crater was a small caldera subsidence. structure is made up of charnockitic rocks with a smal- "We report here the discovery of distinctive shock-

ler proportion of the area including granite gneisses metamorphic features in inclusions of the Precambrian and anorthosite. All of these rocks are within the basement rocks which occur in dikes of melt rock at the

Grenville province, and if the analyses of Krogh and Tenoumer Crater (5). Such unique petrographic features

others (1968).are correct, should have whole-rock Rb- are considered definite criteria for hypervelocity me- Sr ages in the vicinity of 1200 m.y. Geological rela- teorite impact (6-10), and their discovery supports the

tionships and K-Ar age analyses suggest that the event view that both the Tenoumer Crater and the small amount which caused the structure occurred in Devonian times, of associated melt rock were produced by meteorite im-

as described by Rondot and others in the various publi- pact. Whole-rock K-Ar age determinations on the melt cations listed in the bibliography. The presence of rock indicate a maximum age of 2.5 < 0.5 m.y. for the breccia zones, extensive shatter-coning, and the pre- Tenoumer Crater, regardless of its mode of origin.

sence of dikelets of intrusive rock phases that looked "The melt rocks at the Tenoumer Crater occur as groups like melts of the crustal material, led Rondot to the of small, nearly parallel dikelike bodies in three areas

conclusion that this structure was of meteorite impact just outside the present crater rim (1, 3). Individual

origin. This work has been well described and docu- bodies are less than 20 m in length and a few meters wide (3, p.167) and have apparently been emplaced in dis- mented by him (1968). continuous fractures oriented roughly concentric to the A sample of the melt phase sent by Rondot gave a crater. The rocks are dark grey to black in outcrop and whole-rock Rb-Sr age date of 1280 1 200 based on an hand specimen, generally vesicular or scoriaceous, and assumed initial ratio of Sr87/Sr86 - 0.705. The error assigned to the age is due to probable limits in the more rarely massive. They commonly contain altered in- range of values for the initial ratio of i .002. This clusions of the Precambrian gneisses and granites in

strong indication that the volcanic-like rocks are due which they occur (3, p.169)." to a melted basement adds substantial support to Rondot's

conclusion of an impact origin. The analytical data are

given in Tables 1 and 2. 28

Descriptions and analyses on the Tenoumer rock sam- - .=E 0 5 0 Ze .- 0 ...= ples are given in Tables 1 and 2, and in Figure 2. It . W A -1 N -,- VB R 0 , 0 0.-•... is interesting to note that the samples of typical Pre- . -r- cambrian basement rock fall on an isochron of 2440 m.y., :/ -...- 44.a 9 -.... which passes through a highly acceptable value for initial Sr87/Sr86. As far as we know, this is the first 9. r '1.. r. .. ; Fia /1 whole-rock age determination on the basement rocks in '1' i ...., 'o.... , 0 . Mauritania, and it is interesting that the age appears - k#r -,1.1.:. / to be older than much of the basement in the southern i'' - . I - BAI. Im... 9\ ' ,\\ / part of the West African Shield. The age value is al- ( I> . 4 as ·great as the typical "Liberian orogenic province" w,I:-JEW 1 ' L- 7 mostdescribed elsewhere in this report. Only onc sample of AW '. 8 24 .14 f . . -4 '4 t the melt phase has had a satisfactory analysis, and it . rff. lies so close to the isochron in Figure 2, and Go much 11' , ... , --- , ..:::.. higher than any other known continental volcanic rock of ·deep-seated origin in its Sr /Sr ratio, that we· 4I / 6921 1 87 86 have no hesitation in saying that this analysis strongly 11; I supports the conclusion of French et al. that these materials are made up of melted crust: thus supporting / ..Ill.-«--.--- /- .-- the impact origin. - I.*.-I...... '.------..../... ..-'---- .--......

...- , BM'-,

0 -I-,--I p. 0 :

T . :=- 1% »1'5-.-'...... 1»-«i

-, ..... NOUVEL IMPAa METIORITIQUE FOSSILE

.770

.760 - Tenoumer Crater Rocks 0 Typical P€ basement R-7377 0 .750 - 0 Melted crustal rock with shocked inclusions Ao ey 340 - c,50& Sr87/ /Sr86 ep. 90 .730 - eosee

- R-7375 o .720 0 R-7373

.710- / 87/ 86) \Sr /Sr 10 = 0.705 .700 0 ..2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 Rb87sr86 29

Table 1. Samples of Crater Rocks Other Craters, or Doubtful Structures Samples from the Rocheouart Crater, village of Babau-

A. Melted (or volcanic) rock from Malbaie·Crater, du, France, were sent by Bevin M. French. The impact

Charlevoix, Quebec. Collected by Jehan Rondot, breccia and melt appear to have been formerly Hercynian Minist&re des Richesses Naturelles du Quebec. basement rocks. Analytical data on the 4elt" sample are given in Table 2, and indicate the age of the pre- R-7370: Rondot sample lm-1. Darker green existing material to be about 240 m.y., which is compa- basalt-like rock-glassy or recrystal- tible with some basement age values in this region.

lized glass. Again the isotopic data support an impact origin. The Another sample from same outcrop. R-7371: Sr87/S/86 value in this glass is much too high to be R-7372: Rondot sample lS. Medium green (opaque) volcanic. with radiating crystallites possibly due to devitrification. Bibliography

Mauritania. Collected by Bevin B. Crater of Tenoumer, Krogh, T. M., and P. M. Hurley, Strontium isotope varia- M. French, Goddard Space Flight Center, Maryland. tion and whole-rock isochron studies, Grenville Pro-

Vince of Ontario. J. Geophys. Res.,73:7107-7125, 1968. R-7373: Field No.TM-2. Crystalline melt rock Robertson, P. B., La Malbaie Structure, Qu&bec - A pal&- with highly shocked inclusions separated ozoic mett6oritic impact site. Bulletin of "The Mete- Out. oritical Society", in press. R-7374:. Field No.TM-3. Ditto. Rondot, Jehan, Rapport p/6liminaire sur la region de La R-7375: Field No.TH-43. Typical Pe basement.

R-7376: Field No.TM-45. " Malbaie, Min. Rich. Nat. Qu&., R.P. 544, 1966. Rondot, Jehan, Nouvel impact m&teoritique fossile? La R-7377: Field No.TM-353 " " structure semi-circulaire de Charlevoix, Can. J. Earth

C. Rochechouart Crater, France. Collected by Bevin M. Sci., Octobre, 1968.

M. French. Rondot, Jehan, Rapport preliminalre sur la region de la

R-7485: Field No.1168-99. Impact melt (from vil- Riviere Malbaie, Min. Rich. Nat. Qu ., 576.

lage of Babaudu) containing numerous small Rondot, Jehan, Excursion gSologique structure de Charle- inclusions of quartz, probably about 90+% voix, Contribution au Congres de "The .Meteoritical

crystallized melt. Society);, Min. Rich. Nat. Qui v Octobre, 1968.

R-7486: Field No.1168-100. Strongly shocked and digested inclusion from the lava. Has

been almost entirely melted (from shock or from melt heat). Clearly not a melt. Possibly Hercynian basement rock.

D. Kofels structure. Collected by N. M. Short.

R-7487: Field No.1168-86. Off-structure basement

rock: feldspar-muscovite gneiss. R-7488: Field No.1168-87. Off-structure material

(basalt7)

R-7489: Field No.1168-74. Intensely shocked and fused Kofels nglassn.

R-7490: same as 7489, different specimen. 30

Table 2. Rb-Sr Investigation of Impact Crater Rocks

M. I.T. French Rb Sr (Rb87/Sr86)Atomic (Sr87/Sr86)N Approxititate Age No. No. Ppm Ppm m.y.

TENOUMER CRATER

Melt 7373 TM-2 52 297 0.50 .7189 2000 1 600 7374 TM-3 63 300 0.60 .7201 1800 1 600 Basement 7375 TM-43 66 422 0.45 .7203 2400 k 600 7377 TM-353 163 326 1.42 .7547 2440 i 150

MALBAIE CRATER, J. Rondot sample lm-1· Mclt 7370 147 371 1.15 .7256 1280 k 200 ROCHEOUART CRATER

Melt

7485 1168-99 156 38 11.71 .7435 240 f '20

Notes 1. Rb and Sr determined by XRF on Tenoumer samples, and by isotope dilution on Malbaie. 2. Age values calculated on basis of assuming initial Sr87/Sr86 = 0.705. 3. Errors given on basis of outside limit of error for the assumed value of initial 87 86 Sr /St = f .004 for Tenoumer samples and f .003 for Malbaie and Rocheouart. 31

Progress Report on the Isotopic Composition of Strontium in Minerals from Ultramafic Inclusions in Basaltic Rocks

John B. Reid, Jr.

INTRODUCTION this grid has placed points corresponding to the compo-

A knowledge of the histories of the ultramafic xenoliths sitions of clinopyroxenes coexisting with orthopyroxenes

found in undersaturated basalts would greatly contribute from a variety of naturally occurring four-phase alumi-

to an understanding of the processes involved in the chem- Ous peridotites. Though there may be some uncertainty ical evolution of the upper mantle in general, and in par- in the proposed conditions of pressure and temperature

ticular, to the understanding of the generation of the owing to the omission of the iron component in the de-

rocks in which the inclusions are found. The study of termination of the ac' Bc grid, it seems quite likely these inclusions has been the aim of a considerable re- that most lherzolitic inclusions in basalts have come

search effort in recent years, ahd the controversy as to from regions in the mantle with temperatures in excess what genetic relationship, if any, which exists between of 1000°C, and the garnet-bearing inclusions from re-

a given inclusion and its host rock is still a heated one. gions of somewhat higher temperatures. It might be ex- A powerful and sensitive means of determining the. pos- pected that, at these temperatures, isotopic disequili-

sible presence or absence of a genetic relationship be- brium among coexisting phases could not maintain itself, tween an inclusion and its host basalt is the study of and that all the minerals in a nodule should be expected

their isotopes of strontium whose relative abundances are to have the same Sr 87/Sr86. Before any meaningful in-

functions of time and of the relative abundance of rubi- terpretations of isotopic data from ultramafic nodules

dium, an element whose geochemical behavior differs con- can be made, it must be first be shown that some miner-

siderably from that of Sr. als, at least, from inclusions have real isotopic pat- In recent years, several studies have been made of the terns different from their host rocks. The search for

Sr isotopic compositions of lherzolites and their host such a nodule in basalts which have been extruded through basalts. Stueber and Murthy (1966) found very little continental rocks with high Sr contents and relatively difference between the Sr isotopic compositions in whole- high Sr87/S/86 ratios, runs the risk that small additions

rock ultramafic inclusions and their host basalts, while of radiogenic crustal Sr to ultramafic minerals and rocks 87 86 Leggo and Hutchinson (1968) have found that the Sr is. with considerable lower Sr contents will result in the of whole-rock lherzolites from the Massif Central, in false impression of isotopic disequilibrium between the host rock and inclusion. This seems to be a likely ex- southern France, are measurably higher than those of the rocks which contain them, and that the Rb/Sr of the xeno- planation of the results reported by Leggo and Hutchin- liths are in general too low to have been able to generate son, who found lherzolites with Sr87/Sr86 variable and up to .710, and with Rb/Sr insufficiently high to have - the observed Sr87/Sr86 ratios in times less than 4.5 b.y. generated the observed Sr87/Sr86 in the lifetime of the More recently, as part of an Sr isotopic study of alpine earth. Tatsumoto, Hedge, and Engel (1965) have shown type ultramafic rocks, Stueber (1969) determined for com- that ancient tholeiites have initial ratios which fall parison, the Rb/Sr and Sr isotopic patterns in the olivine, along the average mantle growth curve for Sr 87/Sr86, orthopyroxene, and clinopyroxene from an Australian 1herzo- whose present day value is between .702 and .704. The lite and has found an apparent isochron relationship show- residual nature of their lherzolites (i.e., their ap- ing an age of about 0.5 b.y. The intent of the present study is to investigate the strontium Botopic composi- parent ages greater than 4.5 b.y.) requires that at any tion of minerals separated from a variety of ultramafic time in the past, when they received their Sr from a more Rb-rich mantle system, that system had a Sr87/Sr86 inclusions from basalts in order to: 1. determine the ratio well above the average mantle growth curve. It ages of old mantle fragments, and 2. if possible, to elu- seems more that srnall incursions of cidate the processes involved in the basalt formation. likely relatively There is some reason to expect that such a project radiogenic crustal Sr have affected the Sr isotopic com-

stands only a moderate chance of success. O'Hara (19676) position of the lherzolite, while not noticeably changing 87 86 the Sr /SI of the more Sr-rich host basalt. - has drawn a P-T diagram showing contours for two compo- The case for a truly old xenolith, made by Stueber (1969) sitional parameters (ac = Wo/[Wo + En]) and (Bc = A1203/ is a stronger one, but is not beyond question. The fact CA1203 + Wo + En]) for clinopyroxenes coexisting with that the isochron for the Australian lherzolite is gener- orthopyroxenes from work on synthetic systems, and onto 32

ated by minerals whose Sr87/Sr86 ratios are inversely re- sample is then crushed in a boron carbide mortar, again

lated to their Sr contents, makes possible the explana- with periodic seiving, until most passes 40 mesh. Cuts tion that the same small addition of Sr with a typical of +40, -40+60, -60+80, -80+100, and -100 are taken. crustal isotopic composition, to all three minerals will Dust is removed by several washings in tap water, fol- result in an apparent isochron. The fact, on the other lowed by several rinsings in distilled water, and ace- hand, that the Rb/Sr ratio for the three minerals are di- tone drying under a sunlamp. Recently, the use of the i rectly related to their Sr87/Sr86 strengthens the case ultrasonic cleaner has become routine for far more ef- for a true age greater than that of the host rock. ficient dust removal. Initially it was planned to avoid the danger of mea- In order to reduce possible contamination to the sam- suring a crustally contaminated whole-rock lherzolite ple, and also to reduce health hazards, heavy liquids and still test for isotopic equilibration by separating are used in mineral separation only when separation can and analysing the chrome diopside from the lherzolite, not adequately be performed magnetically. In the case since the high Sr content of the clinopyroxene renders of lherzolites, magnetic separation is particularly ef- it rather invulnerable to small incursions of crustal ficient. (With a side angle of 20-25°, chrome diopside Sr. Many of the results reported here were obtained on is pure on the low side at about 1.0-1.05 amps. Enstatite this premise. It then became evident that, since the comes off on the low side nearly pure on the next pass range of Rb/Sr in average mantle material is n6t very when the current is reduced to about 0.90 amps, and fi- different from the corresponding ratios in chrome diop- nally olivine and spinel will sometimes separate with side, the determination of Sr87/Sr86 in_the clinopyrox- olivine on the low side at currents in the range of ene and host basalt could not reliably differentiate be- 0.70-0.80 amps. The use of Clerici solution is usually tween a lherzolite and host rock formed simultaneously necessary to remove spinel from olivine. This pattern and a lherzolite up to nearly a billion years older than has occurred for nearly every lherzolite examined. Mag- the basalt containing it. Hence it is necessary to anal- netic separation has not proven as effective in the case yse the orthopyroxene and olivine, though their Sr con- of garnet-bearing xenoliths in that, despite the fact tents are generally one and two orders.of magnitude, that one and perhaps two pyroxenes can be removed mag-

respectively, lower than the chrome diopside, making netically, olivine, garnet and spinel require the use them vulnerable to contamination. of heavy liquids for their separation. The problem··of contamination from country rocks con- The purification of mineral separates has been great- taining high radiogenic Sr, is eliminated by analysing ly aided in recent weeks through the use of the ultra- inclusions from oceanic island basalts such as those sonic cleaner. An electron microprobe study of the dis- from the Hawaiin Islands whose basalts have been shown tribution of potassium in minerals from an eclogite from to have Sr isotopic compositions ranging from about the Roberts-Victor mine (Erlank, 1969), has shown the .703 to about .707, but not higher (Bence, 1966). If need for a Sr87/Sr86 ratios higher than .710 or so are found in high degree of purity in certain minerals to be analyzed for their trace element abundances. Of the minerals from oceanic ultramafic material, they cannot minerals found in ultramafic rocks, garnet demands the be accounted for by calling on crustal contamination, strictest attention to the problems of purity in light and must therefore be indicative of old mantle material. of its very low alkali element abundances, and its pro- pensity to be associated with kelyphitic alteration rims SAMPLE PREPARATION AND CHEMICAL PROCEDURE containing phlogopite with very much higher alkali ele- Selected xenoliths are trimmed of their altered ex- ment abundances. teriors and host rock and cut into slabs of 40-50 grams Considerable progress has been made in the removal of each; tap water serves as the coolant for the saw. The kelyphite and zeolite alteration from garnets separated large slabs are crushed, first using a new carefully from pyroxenites from Salt Lake Crater through the com- cleaned hand sledge hammer against an acid cleaned steel bined use of acid washings and the ultrasonic vibrator. plate with several sheets of paper separating the sample The vibrator, a Branson Automatic Cleaner, made by Cole- from the steel plate, until the largest fragment is about Palmer, Chicago, consists of a water bath capable of be- 1 cm3. These fragments· are then crushed in an acid clea- ing heated, and a removable platform to support the tef- ned diamond mortar, until the largest pieces are about lon beakers containing the minerals to be cleaned. The 2mm on an edge. Very frequent seiving of the sample is agitation of the grains is sufficient to break loose a done in order to have as few fines as possible. The \ 6.8 . I % .Bilia:,28 :31 :3% 8 1.*:40,1 31**til ii., . 5I. 0.3tilit612:; . , . ¤ . „ itfi iUSY ha#i = ' '8' § ' ' o e 4 u g B. .3 . .g u l l :5 0 :i::Bilif.qi-: % : 2 · ' :2: . : :4. ¤m e . . 01 Un ro 8 Al/"2 1 Iii f lii-ti i -· d. & :8% : * ..' 0. % . : :7·26:.5:619.i :W. ¤ * li,Bagilililiga Mt /:Zes:&1,14€9 .'' i u e.e a I :S . . :t. . ii if;tjfli:1 -H I 15 , ii'mlf,:iti 5.5 0 4 4.5 4.'4 '» i 8.E a.m#3·1·a ::22.m:3 9 a l ' 5 'e i l s ' : I r : . 1 .1 fli wi i f I i : : 2 A· " 1 5 7... A.: 1 % 1 8 al :ly/liailijaita 41 ilid# * g &:m: 0.5 5 5 5 m:I m t# : ,E 5*1&:111:itguij m I:S 1:8;/:,Ii:Bal U : ... 8 5. l e(g o.K I :11112:i:i iii# i. M. / /f :e l a a· ,T m E : 1 1 & .§ . 1 3 .1 5/: & a : d m 1-: B 31 8 I6 i Ia: , E. :I.8 :31 8 9 1*' N3 Vl a,W3 2 31 H

0' m & e # - I f:,ih'dilia*,/ipi, Hi/,i-Mi/.#immi:2:2:11 ,/41,4/121 1; .a m a.M.m e; .g a m i: .0.S m. .5:3.M B) /E X. .8.3.S E#8.5 1:4: .'Ors 0 8 1 :e K E M i:&:am:£1113:B.i-M::;2:11,3,#EM;1;ilgiSaE"g:.SV·:hy::· 1,131.lillii:ia:,EN:fii'6,:1:,1'i,,ilihirliZi-iilillite 5 =*=iN·3 2 4 8 :1 8 : & .S' . · 8. g N. · : .I l· .i ·s, .: S t:3: .S .:: =0 8.g j:1.8 i :'r' .§ g 3. .5 8*0"S.Z 11*i,rit:vi:atia;;glifiallilf'imi:5141:11110"00,11%gle/,41 1111,82,4 3,11111,:3:ili:*ii:,11 f iellijmx:"::33:111#:ap 1: os:*·38. ·31:68EAXEK; i: :61%:iga :.1*:;:11.4:F. '1§E.:.5. 1 6. a':1:55:56':1::45:'::tia:igulilitilifialilitililiqi 3 A p is .p e:9 1,5 .: 2%5 g: .9*6 1: & :A m:I:3:1 2.r : : : . .s t 8 8' i m i S .t' 5 .0 1 2< 3.9 3 El, 1, B ·, i 8 . it t . 0 8 8 1 8: :1: ' .4&1 8.6 9:1:1, 8.q: #1 1*2 3 23 41 3 WO zK+ 3 'A:X1. :1 BC. E ,X.5: -0 7 :0. 0:3:2= .S:0' •0#8: ! : *9 4 211 1 Bili fii,12:·liliel''alleatilii j ilim flifilimaili·: 3e:23&230:03€83.rim f *:3.. x 3 6 6 1 i A t ij : 1 1 8 1 : 1 i 1. . 7 1 3 i A f s. 1 :E;;;i:fii1ii1i1M;A:IiiI10' M N W#l e g l.H A Q M,A:1m 8 8." 4 f R:·C ".: W-:<34.ifi:fliati:ilimhrE#mifle"i a l: & , <1 3&1 4 2 9: . .2:i j i w:2 5 8: ·8 8 34

in the pyroxene. Trains of very small voids are orthopyroxene, 5% chrome diopside, and a trace of

present in the pyroxene, and larger voids (about .3 spinel. Minor exsolution of the orthopyroxene to clinopyroxene along the mm.) are filled with calcite, a ubiquitous contami- 100 directioit is the only nant in Salt Lake Crater xenoliths, owing its origin evidence of adjustment to new physicochemical con-

to the fact that the nephelinite in which the nod- ditions subsequent to the rock's formation. Oli-

ules are found was intruded through a coral reef. vines up to 5 mm. in size occur in an interlocking fabric with smaller pyroxenes, whereas the unusual-

R7399-SLC6 ly small amount of spinel present occurs as extreme-

A garnet- and phlogopite-bearing pyroxenite with a ly finely disseminated inclusions in the olivine,

complex history. Very large garnets (more than 1 and as larger blebs along grain boundaries, especi- cm. in s6me directions), some having immediately ally in the vicinity of pyroxene grains. The rock's

adjacent books of phlogopite up to 2 mm. thick and remarkable features are its freshness, and coarse

up to 5 mm. across, are set in a matrix of pyroxene, grain size. in which at least four separate crystallizations R7416-SLC23 can be detected. Cr6ss-cutting relationships show An orthopyroxene-rich lherzolite of medium grain si 2e, that the original· orthopyroxene reacted, in part, to consisting of about 75% opx., 15% chrome diopside, 10% form clinopyroxene, which itself unmixed into an ex- olivine, and traces of spinel, which deserves the name traordinary sinuous " zebra" pattern showing no appar- 1herzolite despite its apparent websteritic modal com- ent basis in the crystallography. Alternate stripes position, in that the orthopyroxene, and clinopyroxene in the zebra pattern extinguish together, and the are the typical coffee brown and bright green respec- remnant patches of orthopyroxene·within this pattern, tively found in the more common olivine-rich lherzolite, although occurring as isolated blebs, extinguish in and not the black-gray pyroxenes of the Salt Lake py- unison over regions of up to 5 mm. in largest dimen- roxenites. Besides its low modal abundance, olivine sion. The fourth pyroxene is a clinopyroxene exsol-' is also smaller in grain size ( .S t o 1.5 mm.) than the ving along the 100 direction in the orthopyroxene. cpx. (1 to 3 mm.) and the still larger opx. (up to 8 Phlogopite occurs in the pyroxene, and seems to be enclosed by through-going pyroxene textures, and thus mIn.) . Spinel occurs mainly along fractures and grain I boundaries. to pre -date them. Garnet occurs as a crack filling,. Vulcan's Throne, Arizona often enclosing fragments of the older pyroxenes in R7469-VT2 a texture resembling reworked.xenoliths in a minia- A fresh friable lherzolite of quite variable grain ture dike. Olivine and spinel are also found enclo- size, ranging from less than .5 mm., in the case of sed in the garnet, which in all cases observed could the chrome diopside and spinel, to several mm. in not be shown to have at least one and often several the orthopyroxenes and olivines. Clusters of small obvious entry fractures which increase from very grains of cpx., opx., and olivine run as stringers much less than .1 mm. in width to as much as 1 cm. through larger monomineralic regions of enstatite in the region of the garnet. Secondary. phlogopite and olivine. Spinel is in relatively low abundance, occurs as very fine fracture filling within the gar- and concentrated in the stringers of small silicate net, and within the fragments of the xenoliths en- grains. closed by the garnet. Spinel occurs as clean to

kelyphite-rimmed inclusions within the pyroxene. R7484-VT4

The large size and the very uneven distribution Very similar in appearance to R7469-VT2, but differing mainly in being more finely fractured and in of the garnet and tha phlogopite, coupled with the

small size of the xenolith (about 100 gm.) make having a wider and more finely disseminated distri-

an estimate of the mode rather uncertain, though · bution of the spinel.

roughly it would be: pyroxene 80 , garnet 15,. phlogopite 2-3, olivine 2-3, spinel 1. Xenoliths, from which minerals have been separated, but for which no isotopic data has been taken, have been omit- descriptions. R7404-SLC11 ted from the sample A very fresh, well indurated, very coarse grained

1herzolite composed of roughly 75% olivine, 20% 9 8 :m: „ .0 =59 e i i : i a ii i i i & i E : i ; i : p i # S: i F i i 1 i i a i· i- M F ati i X i F : i 1 1 i M m : 4' : V: 3 8' 6 8, : :3 1 9 R 3 p i. i % . 1 i . i. § 1 3 E A' K .3. i : :'rE g. : XII•.0. . " -g"m s@ : -o «, W.. 9 5 5: : :B l e,m & E m a:c.M f e e:X E P E E% ,S:a l ali#%,62:''mi:%;BE,J:ili:litimiliES*Ii,Maligilit:r E· : : A & . 58151:55%,igill:e;:a,:Elliali Fi&,Fitifirfli ijilizil,em. SgeogBEWEA··,Glitillitifim,%;F''i-&:8.DOOMID m .... 0.. ... :'..'. 83'E·.'B:·:'.:Stifil,Ficii lism:.5:/g::Re:fetakE ».mcr . * : = * :2 9: # !S·g.8 8 8· g. „ : .F l la' rt re:#"SE:·M::%:94 :,awmatii,ai-;35#3":Mo. .. · : 5 ./ 6.0SIG.: ..: . 1221,i:ilis;Kilm :fli: gle:, 11. ..491:.-:W:.9.:241 .I: p 9*...'DO„ : . :4: * -5 :m- M . . :m a g a t a s'l i t,i l i m i w i i 1882.ERM P a' B„ /8 6 " = .% -, .&F R : 8 W. i : 13 .- 0 0 0 0 : 6 t i i i i : 1 5 i f i j B i i * tr i ' n mig: rto I. :428%: ( I . .. 9 2: ,9 59 8,: g 6: m=g„ = .6 :6'9 „m. 8:5. 1 5"9 5 M E N.E m e a m: # : . #2 5 5,0:2# mo ..:E: :Y:9:2 9% : .E:-„0".... # § · [ :i: = I S i g:1 9 7 5 1 Fieatigaw:Ii :Will i:lii g O&0 1' ¤ 4 5 9'M i g T a g i o n i# %1:5 *i i.W r i t i l i k1 . m 9 5i 9t i;E& 5 gL i E#i2 2 El ib f# i 0. 0 0/: . 281

W tn i i i i i i M 1 1 1 i 1 1 1 1 1 i i i i i i i i i 1 I 1 ' W i i i i i -: F : 1 i i i i i i i I 1 5£11&15:raifigisiE,9: i · 1 i 1 1 : 2 .S : i . : : *V : : . § 3 ; t f ;. ::R : 5@VV; .6'0. 1 . „ :, :F ; i.. 2& 8. . 2 :.g i, :,i''Mi 1&:·:'Bisi, ' , 115&1:,#:·-:Iii Fli li MISigilie '59:pl 1& ,4 J i l t- , ,E L:z z:0 5,3&3 8:r i 1 3 2 6 1 2:w g i E R : 5*2 3.9 3 1 12£1:Efilill IZIZE;-15 111;leggilliklieit:I,ltill;Wi ail IS t i #I; 931 8 1 0;i,i ,<51 5,5,2, & *1 5 4;1"1 88 <„.:aimi " #= 0 : . O· O A ( ,H &· m : 1 I : : I i ·E. o. 1 1 6. 9 m : I e : i : a 5. tv „ : t: „0

:5:%%1116:ilift i i;#4 0:Wl:immiI,ili-,ilile,&99951- /9 0 3 a, (D T:go·*23:4210lAPE:Wa:mmaE:3 6'2 0. *6 4.0 8% - "8 2. : - :98& 8 8 5 2*4:gw:E:E-,iz*:4* : „ 9. :-00.. 0 9 M E Oific'Og.G:Wig.:E l:L:Z A, &0*9 1:8'2 9:23.As.5 1& , 8% 8 :B GUIC E. " : 2§:*18.:&22 .m i:K I E0598#13:..054: K M E: .I „ a S„ 5 0. C -J Q C.·-'0 528Ii,#,Pti:,aisi: 5,-oxiliagiligili:Jite:gull t a.B: 4. 1 1:1 8 5:8 8 5. I. 3% .8 9 8. ,1.i: *I r:1 9"V I K E: .:727 i R e -1, „ - : G rr : .4 7 „9 9 7 9 K 36

and the basalts containing the lherzolites, which are for each gram of garnet, or, by 0.2 gm of phlogopite undetectable if they are within the uncertainty limits whose modal abundance is one-fifth that of the garnet. 87 of a single isotope ratio measurement. Average mantle Phlogopite with 150 ppm Rb can produce this Sr in 87 86 6 87 material increases its Sr /St ratio at the rate of 35 x 10 years. However, rather than absorbing Sr about 0.001/10 9 years, so that with an average uncer- in proportion to their modal abundances, it is more tainty in an unspiked Sr isotope ratio determination likely that they receive it more nearly in proportion being 0.0005, a lherzolite formed at the time of the to their Sr contents. This gives an age very much 01- basalt formation could not be safely distinguished der than 35 million years. from one formed about 10 years prior to the basalt The variability in the isotopic composition of the formation simply on the basis of the comparison of garnet is attributed to two factors. Small variable the isotopic compositions of the r.linopyroxene and amounts of CaCO3 escaping removal by the acetic acid host basalt. Conclusions as to whether the lherzolites leaching would pull the measured 87/86 ratio toward are in isotopic equilibrium with their hosts must wait 0.708, and uneven migration of radiogenic Sr from the for the results on is:.,t,·,1, i / ratio mcaourements of t,he phlogopite to the garnet would result in non-uniform enstatite and olivine from the lherzolites. Most of 87/86 values. The runs also represent powders made these necessary Sr separates have been prepared and un- from two grain sizes, which may differ in 87/86 be- ly require mass spectrometric analysis. cause of a fractionation in the crushing of grains - The present isotopic study in the case of the Salt with differing amounts of CaCO3 contamination. Frac- Lake Crater material is being augmented by a determi- tured garnets in the original rock would have more nation of the absolute and relative abundances of the surfaces coated, and would tend to crush to a fine rare earth elements in separated minerals and the ne- size more easily than would cleaner, relatively un- phelinite with the hope that if Sr isotopic analysis fractured grains which would tend to concentrate in can distinguish between residual and possible parent larger grain size fractions. material, distribution coefficients for the REE be- The other material analyzed to date which did not tween xtalline· phases in nodules, and the nephlinite show a Sr 87/Sr86 within 0.001 of its host rock value, host will elucidate the processes of trace element was R7404-SLC11 Olivine (.7084). The suspicious na- fractionation involved in the formation of oceanic ture of this result arises from two facts. The. mas s crustal material from the mantle. spectrometer run of the olivine was a weak, very high

temperature run indicating very little Sr on the filament, and the possibility of filament contamination. Table 1. Spike Calibration Results

Considerable difficulty has been encountered in getting Rb87 SPIKE. Date Spike Concentration good solutions of olivines in general, in that MgF2 Rb87 spike ID (A) 12-3-68 1.215 ygm Rb87/ml forms, and is very difficult to dissolve. Although Rb87 spike ID (E) 12-23-68 1.239 Vgm Rb87/ml the solution for this run appeared clear, it is pos- Rb87 spike·ID (F) 1-6-69 1.216 ugm Rb87/ml sible that fine MgF2 precipitate was present, and that the Sr was scavenged by the precipitate. The weak run Rb87 spike IR (A) 11-19-68 85/87 = 0.00795 experienced could thus have been made on the common Sr 84 Sr SPIKE contaminatj.on encountered in the ion exchange and subse- Sr84 spike ID (A) 11-7-68 0.8923 ygm Sr84/ml quent procedures. The possibility of contamination by Sr84 spike ID (B) 12-4-68 0.8946 ygm Sr84/ml traces of CaCO3' not leached out by the acetic acid is 84 84 Sr 12-19-68 0.8915 Ugm Sr /ml also present. Several recent attempts at solutions of spike ID (E) 84 84 Sr (F) 12-26-68 0.8901 ygm Sr /ml R7404-SLC11 Olivine have been made though they have re- spike ID sulted in large amounts of.MgF2, which seems to dissolve 84 86 Sr spike IR (A) 12-4-68 /84 = 0.04466 if allowed' to stand for several 87 with difficulty only /84 = 0.01833 weeks in HCl. 88 /84 = 0.15084 The results on inclusions from Vulcan's Throne, Ari- E & A Std. Sr 7-29-69 87/86 = 0.7084 zona, and Dish Hill, California are as yet inconclusive. Carbonate As mentioned above, there is the possibility that there can be real differences between the Sr isotopic compositions of the Sr-rich chrome diopsides from lherzolites 37

Green, D. H. and Ringwood, A. E. The genesis of basaltic Table 2. Preliminary Isotopic Results magmas. Dept. of Geophysics & Geochemistry, Australi-

SALT LAKE CRATER an Nat'l. univ. Publ. No. 444:118-205, 1966.

87 86 Green, D. H. An experimental investigation of the gabbro Sample Sr /Sr Rb ID Sr ID Date corr. to eclogite transformation and its petrological appli-

R7394-SLCl .7045 4-18-69 cations. Geochim. Cosmochim. Acta, 31:767-833, 1967. phlg Green, D. H., J. W. Morgan, and K. S. Heier. Thorium, R7404-SLC11 .7040 4-18-69 CPX Uranium and potassium abundances in peridotite inclu- R7399-SLC6 .718 5-29-69 sions and their host basalts. Earth Planet. Sci. Gar #1 Letters 4:155, 1968. .037ppm 6-11-69Rb R7399-SLC6 .740 .514ppm 6-12-69Sr Gar Jackson, E. D. Primary textures and mineral associations in the Ultramafic zone of the Stillwater Complex, Mon- R7399-SLC6 .7041 6-18-69 WR tana. U.S. Geol. Survey Prof. Paper 358:106, 1961. R7399-SLC6 0.7036 6-23-69 D. in Hawaiin basalts. U.S. Geol. phlg Jackson, E. "Eclogite" R7399-SLC6 .7105 6-24-69 Survey Prof. Paper 550-D:151-157, 1966. Gar 02 Jackion, E. D. (1967). Ultramafic cumulates in the Still- 7-15-69Rb 0.7038 R7399-SLC6 0.100ppm 130.4ppm 7-16-69Sr intrusions. In Ultra- PX water, Great Dyke, and Bushveld --

R7399-SLC6 0.7270 7-25-69 mafic Rocks, P. J. Wyllie (ed.), John Wiley & Sons, Inc., Gar #1 New York. repeat R7404-SLC11 .7082 7-31-69 Kleeman, J. D., D. H. Green, and J. F. Lovering. Uranium Olivine distribution in inclusions from Victorian basalts. R7399-SLC6 .7038 6-30-69 Px IR Earth Planet. Sci. Letters (in press).

R7416-9LC23 .7042 8-12-69 Leggo, P. J., and R. Hutchinson. A Rb-Sr study of ultra- opx IR basic xenoliths and their basaltic host rocks from the R7399-SLC6 .715 .866ppm 8-22-69 Gar Sr ID Massif Central, France. Earth Planet. Sci. Letters,

5:71, 1968. VULCAN'S THRONE, ARIZONA and DISH HILL, CALIFORNIA Macdonald, G. A. Petrology of the Island of Hawaii. U.S. Sample S/87/Sr86 Rb ID Sr ID Date corr. Geol. Survey Prof. Paper 214-D:51-96, 1949a. Macdonald, G. A. Hawaiian petrographic province. Geol. R7469-VT2 0.7047 4-7-69 Cpx Soc. Amer.,Bull. 60:1541-1596, 1949b. R7472- 0.7051 4-9-69 McDougall, Ian. Potassium-Argon aged from lavas of the Gcl-101 Hawaiian Islands. Geol. Soc. Amer., Bull. 75:107-128, R7484-VT4 0.7041 6-2-69 cpx 1964.

R7461-DH2 0.7028 6-6-69 O'Hara, M. J. Mineral paragenesis in ultrabasic rocks. cpx In Ultramafic Rocks, P. J. Wyllie (ed.), John Wiley R7479-Bal- 0.7039 6-10-69 -- 102 & Sons, Inc., New York, 1967. R7461-DH2 0.7030 7-11-69 Ross, C. S., M. D. Foster, and A. T. Myers. Origin of WR dunites and olivine-rich inclusions in basaltic rocks.

References Am. Mineraloyist, 39:693-737, 1954. Stueber, A. M., and V. Rama Murthy. S€rontium isotope Bence, A. E. The differentiation history of the earth and alkali element abundances in ultramafic rocks. by Rubidium-Strontium isotopic relationships. Ph.D. Geochim. Cosmochim. Acta, 30:1243, 1966. Dissertation, M.I.T., 1966. White, R. W. Ultramafic inclusions in basaltic rocks Erlank, A. J. Microprobe investigation of potassium from Hawaii. Beitr ge zur. Mineralogie und Petrographie, distribution in mafic and ultramafic nodules. Trans. 12:275-314, 1966. Amer. Geophys. union (Abstract) Vol. 50, April, 1969. Green, D. H. The origin of the neclogites' from Salt Yoder, H. S. Jr., and C, E. Tilley. Origin of basalt Lake Crater, Hawaii. Earth & Planetary Sci. Letters, magmas. An experimental study of natural and synthetic rock systems. J. Petrol, 3:342-532, 1962. 1:414-420, 1966.

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THIS PAGE WAS INTENTI NALLY LEFT BLANK 39

Radiometric Ages and Paleomagnetism of Canadian Diabase Dikes

Todd M. Gates

nous over a large area extending from Sault Ste Marie The diabase dikes in the Canadian Shield are a major through Elliot Lake to Gowganda Orientated sam- geologic feature. They generally occur in synchronous (1,2). ples have been obtained over approximately the same area swarms which can cover areas as large as a million square and through joint work with D. W. Strangway at the Uni- miles. A dike swarm of this magnitude must represent a versity of Toronto an attempt will be made to obtain a major geologic event, the nature of which is totally un- magnetic pole position. The known age of this swarm will known. These diabase emplacements possess the charac- be tested again both by K-Ae and Rb-Sr analyses for prac- teristic that they have recorded'the location of the tice. When satisfactory techniques for dating have been earth's magnetic pole over a wide spread of observation learned, the method will be applied to other swarms of points at singular times in Precambrian· history. There- different age, and possibly in different continents. fore a study of diabase dike swarms at different ages over different continents could possibly resolve such References questions in crust/1 his Lut·y as Ll,e early stability or motion of continents and the distribution of their com- Fairbairn, H. W., P. M. Hurley, K. D. Card, and C. J. Knight, Correlation of radiometric ages of Nipissing ponent nuclei. diabase and Huronian metasediments with Proterozoic Samples of Canadian dikes have been collected from Keweenawan, Nipissing and Matachewan swarms using mapped orogenic events in Ontario. Can. J. Earth Sci..,

occurrences as a guideline. Sample locations are shown 6:489-497, 1969.

in Figure 1 and cover an age spread roughly from 1900 .van Schmus, R., The geochronology of the Blind River- m.y. to 25nn m.y. Thi Nipissing will be investigated Bruce Mines area, Ontario. Can. J. Geol. 73:755-780, isochro- 1965. f 3.rst . It has heen shown that these dikes are 83' elI 80· 820 49' 49'

FIG. 1

*. BA

4 t a . MS . • NIPISSING

1 1 x KEWEENAWAN a MATACHEWAN

4-.-5==.2.fit MI 6 =8° KM. 47* 47.

44 - 5 83. 82• Z,2/\·--./ 8,0 80 40

·; ·'· ' :- ' ' ..·: :, 2 r... 'a <-% ;TH IS'PAGE'/l -r WAS'INTENTIONALLY LEFT BLANK 41

A Test of Rb-Sr Whole-Rock Dating of Diabase Dikes

Todd M. Gates

A test of this method is being conducted on samples of Analyses to date, using the IRIS 6" radius mass spectro-

diabase sent to M.I.T. from the Ivory Coast by U. Knopf meter are listed in Tables 2 and 3.

of the Society for the Development of Mines. The loca- Table 2. Analyses of E. and A. Standard tion of the samples is shown in Figure 1 and listed in 86/88 84/88 87/86 Table 1. Table 1 Date (87/86)N

Sample Longitude Latitude Locality Description - 7-4-69 .1198 .7066 .7077 No. West North 7-7-69 .1193 .00676 .7086 .7083 R6818 3011'00" 5°34'30" Ayamd Metadiabase (amphibolite) 7-13-69 ..1194 - .7083 ..7083

9°21'15" Kdbi . R6825 6035'45" Clinopyroxene olivine 7-14-69 .1193 .00676 .7085 .7082 diabase

R6827 4°49'10" 9052'50" Ldraba Pigeonite, micropeginatite * 87/86 values normalized to 86/88 = .1194. diabase

R6828 3031'00" 9°38'00" Kodbonou Ortho and clinopyroxene Table 3 diabase 86 87 M.I.T. Sr Sr Rb Rb (S/87/S/86)A (Rb87/S/86) R6829 7036'20" 7°50'25" Dov6 Augite and hornblende Atomic diabase No. Ppm Ppm Ppm Ppm

R6818 9.56 98.96 4.181 14.764 .7209 .4323 A plot of the analyses is shown in Figure 2. If the ini- 16.132 166.86 2.711 9.571 .7120 .1661 87 86 R6825 tial Sr /Sr ratio is taken to be .7030, it appears R6827 19.897 206.06 8.071 28.502 .7237 4010 that the Ivory Coast dike rocks are either older than R6828 18.042 186.63 4.186 14.780 .7115 .2294 2,500 m.y. or that they have not been closed systems. 8.307 85.91 1.065 3.762 .7090 .1267 87 86 R6829 If the initial Sr /Sr ratio is increased to .7040

or .7050, the ages do not change significantly. Be- cause most of the samples are known to be within the

2000 f m.y. Eburnean age province, the second alter-

native seems likely. Thus further work is obviously FIG. 1 needed before this technique can be applied to rocks so' low in radiogenic Sr87. IVORY COAST 80 -° 60 50 40 30

10°

0.90 '1 116825 R68271-'6828 . R6829

* 79

R68 8

59SCALE . t--- t= 0 7 KI. 42

l i FIG. 2 l i

DIABASE DIKES

.73 IVORY COAST

/ /-

R 6827 + " + .72- R6818

87 86 41. Sr S r 3000 0 2500 .i. 3500 R6825+ R6828 .71 R6829+

.70 11 O .1 . 2 87/ 86 .3 .4 .5 Rb / Sr 43

Preliminary Report on Rb-Sr Whole-Rock Measurements on Precembrain Iron Formations of the Canadian Shield

Brian J. Fryer

R7475-A INTRODUCTION: Iron formations of Precambrian Age are varied significantly between the two samples. required more than 100 ml. of concentrated reagent widespread throughout the ancient shield areas of the grade HF to get into solution and consequently the Sr"/ earth. Direct age determinations on these bodies, which Sr86 ratio was lowered more than that of sample R7475-B make up the majority of the world's iron ore reserves, which required only 40 ml. of HF to dissolve it. Due are essentially non-existent. The present study is to the unknown contamination in these test analyses the being undertaken to determine the primary age of these ages obtained are meaningless except that they show the chemically precipitated sediments in the Canadian Shield and consequently to determine whether they are restricted possibility of direct dating of iron formations. to a particular period in the development of the earth's SAMPLE COLLECTION: A sample collection program has been surface processes. If they are, this age might be carried out in the Labrador Trough to study the pnssi.bility correlated with the developing terrestrial atmosphere of using Rb-Sr whole-rock dating of the iron formations. and planned trace element investigations involving the Suites of samples were collected at Schefferville Quebec rare earth elements, U, and Th may help to determine and its amphibolite grade metamorphic equivalent along the possible changing conditions of sedimentation. strike at Labrador City, Newfoundland. No analyses have

PRELIMINARY RESULTS: The use of Rb-Sr whole-rock yet been completed. measurements has been made possible by the development REFERENCES of an organic extraction procedure for the removal of Fe from a concentrated HCl solution while retaining the 1. Gastil, G., and Knowles,.D. M. (1960). Geology of

Rb and Sr in the acid phase. This procedure, as given the Wabush Lake Area, Southwestern Labrador and Eastern Quebec, Canada. B.G.S.St. 71:1243-1254. elsewhere in this report, has permitted the use of large rocks samples (up to 10 grams or more) and consequently 2. Gross, G. A. (1968). Geology of Iron Deposits in with very low Rb and Sr contents can be routinely Canada: Vol. III, Iron Ranges of the Labrador analyzed. Geosyncline; Geol. Surv. Can., Econ. Geol. Rep. To determine the feasibility of using iron formations No. 22. for Rb-Sr dating a test sample, R7475, of a jasper from 3. Klein, C. Jr., (1966). Minerology and·Petrology of near the Michigamme Mine in Michigan was analyzed twice the Metamorphosed Wabush Iron Formation, Southwestern for both Rb and Sr and the results are shown in Table 1. Labrador. J. Petrol. 7:246-305, Part 2. These results show that at least this facies possess a sufficiently large Rb/Sr ratio to enable a tentative age to be calculated from the iron formation. The difference in ages calculated from the two analyses are attributed to contamination which was not strictly monitored, and

Table 1. Michiqamme Jasper

Sr87/SrBGID Sr87/Sr"IR Rb ppm Sr ppm Age (assuming Sample [sr,7/sr"10 = 0.7030)

13bo m.y. R7475-A 0.728 0.725 0.405 0.842

R7475-B 0.734 - 0.274 1.04 2300 m.y. 44

I , :t. 1 2 THIS 'PAGE WAS INTENTIONALLY LEFT BLANK 45

87 86 Sr /Sr Initial Ratios and Whole Rock Ages of Pyroxene Granulite

Charles M: Spooner

ABSTRACT knowledge of the volume percent of the mineral phase present and its

Rubidium-strontium whole-rock isotopic relationships are velocity. Christensen (1965) has found that silicic igneous rocks reported below for pyroxene granulites and charnockitic rocks from ten granulite facies terrains of world-wide distribution. The average.about 6.2 km/sec. and velocities for mafic igneous rocks range in the S/87/Sr86 initial ratios is 0.701 to 0.706, similar to that of anorthosites and continental basalts, and is lower chan average about 7.0 lan/sec. It would be expected, therefore, that values commonly enccuntered in granitic rocks. Published values for this ratio from Madras, the Nilgiri charnockites, India; the rock velocities could serve as a general guide to rock composition. Lewisian gneiss of Scotland; and the Ivory Coast, are 0.7059, 0.7023, 0.7042, and 0.7070, respectively. The velocity, however, cannot be used to distinguish igneous from

metamorphic rocks unless high pressure phase transformations are AGE (m.y.)* LOCALITY (Sr87/Sr86)0 involved. Birch (1960, in Pakiser and Robinson, 1966) has noted Kushalnagar, Mysore State, India 2618 + 46 0.7039 t 0.0005 India 1980 + 124 0.7037 + 0.0007 Pallavaram, Madras State, that anorthosites and some altered dunites have seismic velocities Salem, Madres S.ate, India 2476 E 115 0.7042 t 0.0002 similar to those of many mafic rocks despite their widely differing Oko.1.10 and Rakosi, Uganda 2629 f 117 0.7054 + 0.001 927 0.7056 * 0.0011 Pare Mountains, Tanzania $63 chemistry. This method alone has inherent difficulties since it would Labor Serrit, Tanzania 724 - 8 0.7064 i O.0001 not be correct to state simply on the basis of seismic data alone Kanuku Complex, Guyana, S.A. 2182 t 95 0.7018 i 0.0011 that the upper crust is composed of 'granite' and the lower crust Crane Mountain, New York 1336 f 71 0.7025 0.0025 and 1465 + 85 0.7014 t 0.0013 Indian Lake, Blue Mountain this technique of .'gabbro'· or 'basalt' . Interpretations based on West Canada Lakes, N.Y. must be supplemented with geochemical studies. Westport, Ontario 1338 i 47 0.7057 t 0.0009 In the recent geochemical literature, increasing attention has *1 - 1.39 x 10-lly-1 been paid to the significance of certain elemental ratios such as The results suggest that these rocks were initially of igneous origin and subsequently underwent high grade regional metamorphism K/Rb, K/Ba, K/U, K/Th, and K/Sr in crustal and oceanic rocks (Shaw, at the granulite facies level. The consisiently low initial ratios and the relatively narrow range from 0.701 to 0.708 suggest a common 1968, Lambert and Heier, 1967). In the case of the K/Rb ratio, Shaw source region with anorthosites in a deep-seated environment having a low Rb:Sr ratio. (1968) has delineated three distinct trends for igneous and quasi-

igneous rocks. Oceanic tholeiitic basalts have higher K/Rb ratios

INTRODUCTION than crustal rocks and pegmatitic and hydrothermal types are found to have even lower K/Rb ratios despite higher absolute abundances

of potassium and rubidium. For many years it has been the goal of geochemistf to sample The recent introduction of neutron activation analysis for the adequately the exposed surface of the earth's crust tc determine rare-earth elements (REE) in geological materials has revealed an average composition for material presumably common in the deeper startling differences in abundance patterns of crustal, oceanic rocks levels of the crust (for example: Goldschmidt, 1933; Reilly and and rocks presumably of mantle origin when expressed relative to a Shaw, 1967). A basic assumption underlying these studies is that chondritic meteorite REE abundance. One very important aspect in the high grade regional metamorphic rocks exposed at the surface, the understanding of crustal emlution is the REE abundance pattern especially of the granulite facies, are also abundant at depth. The of lower crustal materials. In Studies of continental basalts, there principal constraint to this assumption is the extent to which the always exists some doubt to the extent of contamination of these limited expo'sures of granulites and charnockites in the older deeply mafic rocks as they pass through the sialic crust. Studies of REE high grade metamorphic terraines represent crustal material· eroded deep distribution coefficients of coexistin8 phases (Frey, 1968, and That is, in these regions of the crust, estimates of chemical zoning Haskin et al·, 1966) and also phenocryst-matrix pairs (Schnetzler with depth must be made and this extrapolation is hazardous. and Philpotts, 1968) show a pronounced mineralogical control over Modern techniques of geophysics and geochemistry have become the fractionation of the REE. For example, it is found that some available that will do much to map out the areal extent and depth of pyroxenes concentrate the light lanthanides and that garnets· concen- deep crustal rocks and elucidate their history and origin. By means trate the heavy REE. Thus, it would be. expected that the REE dlstri-

of remote sensing methods,such a1 seismic refraction, the structure bution in a magma produced in an environment containing this mineral of the continental crust at depth has been delineated (Pakiser and pair would strongly depend upon the sequence of crys.talization of Robinson, 1966). Compressional wave velocities of rocks can be pyroxene and garnet as well as their relative proportions, total measured directly in the laboratory or they can be computed from a rare-earth content; and degree of equilibration among the mineral

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3 (b) Internal structural features common to igneous massifs are palingenesis as all the changes take place in a dry environment. also present in the Madras charnockites, such as basic fine-grained Criteria used by Groves in favor of a plutonic metamorphic rather segregative schlieren, coarse-grained, acid, contemporaneous veins, than an igneous origin are: apophyses of the basic charnockite protruding into crushed and altered (a) Geological setting of the charnocklte series among ortho- biotite-gneiss, and the occurrence of well-defined dikes with chilled and para-gneiBses· contacts with the older gneiss. (b) Lack of zircon and apatite which are common in igneous

(c) The chemical composition and mineralogy of the common rocks. Others, notably Subramaniam (1959, p. 338), have observed members of the series have their nearest equivalents in igneous apatite, zircon, and ores as occasional accessories, however. petrology· (c) Absence of observed contacts and evidence of assimilation

Variations on the magmatic theme have been proposed by Washington of the charnockites and the neighboring gneisses.

(1916), on the basis of extensive chemical analyses of the Series, (d) The abundant myrmekite and lack of twinning and clarity of

having undergone and by Goldschmidt (1922) who suggested direct crystallization from the feldspars 'are characteristic of rocks metamorphism. a primary magma. Rosenbusch (1932) grouped the charnockites with the (e) The petrography and mineralogy of the series is abnormal in anorthosites, as in eastern Canada and in the Adirondacks of New view of the normal calc-alkall chemical composition. York, proposing a "Charnocklt-Anorthositerelhe". Suter (1922) also A similar opinion has been advanced by Ramberg (1948) in the case supported this view and.believed the charnockites and anorthosites of a charnockite-mangerite-anorthosite association in a granulite to be genetically related and suggested that in localities where the facies complex in West Greenland on the coast south of Disko Island. association is lacking, it would eventually be found. Groves (1935) To the north and south of the high grade rocks, gl·· 1.sses occur belonging has noted, however, that no anorthosite has been found in any of the to the amphibolite and epidote amphibolite facies· Anorthosite is charnockite terraines anywhere in Africa. rare in this setting, as in India, Ceylon and Uganda, but, when present,

In the Adirondacks, the characteristic feature of the charnockite occurs exclusively with the charnockite suite. The main rock in this series is the presence of hyperethene throughout the acid, intermediate, area is an enderbite (hypersthene-bearing quartz diorite) which has and basic members. Although augite is present in the Adirondack approximately ·equal proportions of massive and gneissic variants. In syenite series, so also is hypersthene, though in minor amounts this vast complex of enderbitic gneisses, there are also basic norites

(Buddington, 1939, p. 229). and a few hypersthene granites. Ramberg refutes the idea that this

For the Adirondack association, Buddington (1939, p. 237) proposed suite of rocks is primary magmatic in origin because the formation of a parent magma approximating a gabbro-anorthosite. He suggested that a differentiated series requires a gradual decrease in temperature the small volume of olivine gabbro that is observed could be produced and consequent increase in acidity of the younger rocks. Instead, provided that a small amount of early olivine settled into the gabbroic the characteristic mineralogical assemblage is the result of granulite liquid following crystallization of the plagioclase (forming the facies metamorphism under the same pressure and temperature conditions anorthosite). Following consolidation of the anorthosite, the olivine- throughout the world without regard to the variations in whole rock bearing gabbroic magma could be emplaced around it. Assimilation at chemistry within an individual locality. In the Greenland locality, depth of some pre-existing basement gabbroic rock and some anorthosite the chemical and structural inhomogeneity is presumably pre-metamorphic. followed by differentiation to a more granitic composition could give Syn-metamorphic rearrangement of many of the mafic minerals due to rise to the quartz syenite magma. This type of mechanism would the differential crystalline plasticity as well as solid state diffusion explain the association of charnockites with anorthosites. between different rock types is presumed to have taken place. He points

A sudden swing to a.metamorphic approach was heralded by Stillwell out that the temperature of metamorphism was not high enough to produce

(1918) and Vrendenburg (1918) and elaborated by Groves (1935). partial melting except in the case of pegmatitic veins which fill cross-

The Uganda charnockite series described by Groves (1935) includes cutting fractures in the country rock. nearly all the rock types found in the type locality in India, and J. A. Evans is the only worker to suggesr. a direct sedimentary as in India, the sub-acid to basic variants predominate in about equal origin for the charnockite series. In a discussion of C. E. Tilley's proporcions of quartz-diorite and norite. The surrounding country paper dealing with the enderbite members of the charnockite series

rock consists of gnelss which, in the Mt. Wati area, appear to grade of Enderby Land in Antarctica (Evans, 1921) it was suggested that

into normal types in the charnockite series nearby. In the Karamojo argillite when·metamorphased would undergo mineralogical changes such district, the charnockites are associated with numerous plugs and that feldspar would combine with the excess A1203 and Si02' thus

flows of sodic lavas and masses of nepheline-syenite which are believed removing CaO from further reaction by forming anorthite. As the CaO

to be Tertiary in age. Groves appealed to plutonic metamorphism in would be insufficient for formation of diopside, hypersthene would be Tyrrell's sense for the origin of these rocks. The changes produced the only major pyroxene. in these rocks are the result of great heat and uniform pressure

involving changes in rock chemistry through diffusion in the solid 'Tyrrell, G. W. (1948) The Principles of Petrology, London, p. 313.

state. Tyrrcll did not intend thio term to include anatexis or 48

Quensel (1950) reviewed the literature pertinent to the genesis quartz, the dominant feldspar being a microperthite. (A. P. Subra-

of the charnockite series and discussed the varied suggestions in maniam, 1959, p. 328).

the light of the charnockite series of the Varberg district, Sweden. As a result of this re-interpretation of the type area, Subramaniam

He found that chemical analyses of the charnockite suite of the concluded that (A. P. Subramaniam, 1959, pp. 345-346):

district are similar to rocks of the surrounding lower metamorphic (a) Charnockite is redefined as a hypersthene quartz feldspar

grade gneiss complex. Since the basic charnockites were similar in rock with or without garnet, characterized by greenish blue feldspars

composition to the amphibolites and the felsic members to the and greyish blue quartz, the dominant feldspar being a microperthite.

orthogneiss, he concluded that the high grade assemblage was the This redefinition is justified as the paratype (Charnock's tombstone)

result of deep-seated plutonic metamorphism of the surrounding is found to be garnetiferous, and this facies is prevalent in the

heterogeneous gneiss formations. He encountered a problem with the type area.

intermediate members, however, and like Ghosh (1941), found no (b) The term "charnockite suite" is suggested for a group of

neighboring rock of appropriate composition, and concluded that these genetically related alaskites, charnockites (birkremites), enderbites are hybrid rocks formed as a result of a composite series of reactions and hypersthene-quartz syenites, all of which are partly garnetiferous. between rocks of divergent chemical composition. He postulated that This will correspond to the 'Acid' division of Holland's Charnockite

these hybrid rocks resulted from an intermingling followed by approp- Series. If it be considered desirable to retain the term Charnockite riate reactions of the orthogneiss and the basic charnockite. The Series, it should be restricted to mean only the above suite of rocks.

transformation to the intermediate rock supposedly took place ·in situ· (c) The ' Intermediate' d·-1,ision of Holland consists of an assemblage Howie (1955), in an important paper on the geochemistry of the of hybrid rocks, derived by interaction of charnockite magma on pyroxene charnockite series in Madras, presents evidence in favor of an igneous granulites of the basement. genesis which involves igneous rocks subsequently subjected to high (d) The occurrences of norite with related pyroxenitic layers grade regional metamorphism. The distinction is difficult to make, and schlieren at Pammal Hill and at a few other points in the type especially on the basis of chemistry alone, because the stability area are considered syntectonic lenses, unrelated to the charnocklte fields of the constituent phases, especially in the acid and inter- Suite. mediate members, approach or coincide under magmatic or plutonic (e) The'Basic' division of Holland is largely made up of pyroxene matamorphic conditions. The best evidence in favor of the latter granulites and variants, which are interstratified with quartzo-felds- is textural: although the order of crystallization is difficult to pathic garnetiferous sillimanite gneisses (khondalite). determine, the persistent granular texture of the rock and the embayed (f) The'Ultrabasic' division of Holland is represented by the a or corroded borders of hypersthene in the acid members point to pyroxenitic schlieren referred to in paragraph (d) above. this metamorphic alteration. The chemical analyses presented in (g) Holland's leptynite is considered a highly metamorphosed and paper are considered in detail in Chapter II. reconstituted facies of khondalite, referred to in paragraph (e) above.

In 1959 At P• Subramaniam examined the Madras type area and many (h) The charnockite suite of rocks is thought to have been emplaced of the same specimens described in Holland's original work on this as thick sheets and lenses in gently folded basement rocks; all the area. He gives a clear account of the petrology and mode of occurrence rock units have subsequently suffered intense regional deformation. of the charnockltes and their relation to the basement rocks. One (i) The charnockite suite or rocks and hybrid types are, as of the major points that he makes in this paper is the misuse and exposed at present, interstratified with the basement rocks with which resulting confusion in the literature of the term "charnockite" and they are structurally conformable. The rock units in the type area

"charnockite series". Holland anticipated this when he wrote (Holland,. represent a series'of tight isoclinal folds plunging gently to the

1900, p. 131): north, and overturned to the west.

(j) The charnockite suite of rocks. is considered of primary The much complained of burdens of petrographical nomenclature are not due to the creation of specific names for local types, igneous origin based on the following lines of evidence: but to irresponsible and unwarranted extension of such names to incl e unrelated members of different and widely separated (i) The rocks carry mesoperthitic feldspars which indicate petrogr-aphic provinces, in which the accidents of differentiation and segregative consolidation may have produced by chance similar their having consolidated at magmatic temperatures. mineral aggregates. (ii) The high barium content of the potash feldspars in

In the worst of cases, the term has been taken to include amphibolites, these rocks point to a high temperature of formation. pyroxene granulites, pyroxene gneisses, pyroxene symites, writes, (iii) The occurrence of pyrrhotite as an accessory mineral gabbros, diorites, granodiorites, and dark colored granites--all of also points to high temperatures of consolidation. which are referred to as charnockite (A. P. Subramaniam, 1959, p. (iv) The orthopyroxenes in these rocks show a systematic

328). In addition to the characteristic minerals identified by variation in composition. Some of them show lamellar

Holland, A. P. Subramaniam found almandine garnet and redefined structure while others show schiller inclusions, both charnockite as: a. hypersthene quartz feldspar rock with or without of which are regarded as characteristic of igneous garnet, characterized by greenish'blue feldspars, and greyish blue orthopyroxenes. 49

(v) The plagioclases in these rocks exhibit a rare complex Problems also arise in the interpretation of chemical analyses. twin law, which may be regarded as suggestive of a Groves (1935), for example, analyzed a specimen considered by him

magmatic origin for these rocks. representative of the acid charnockite in the West Nile District of (vi) The rocks of the charnockite suite carry inclusions of Uganda and compared it with an analysis of an orthogneiss some four

older pyroxene granulites. miles away. He concluded, on the basis of similarity of their compo- (vii) The absence of amphiboles in these rocks point to their sitions, that the two had the same origin and differed mineralogically consolidation at elevated temperatures [or low activity only because of the higher.metamorphic grade imposed upon the former.

of waterl. Quensel (1950) presents a similar line of reasoning for the origin

Naidu (1963) in a presidential address to the Indian Science of the charnockite series in the Varberg district of Sweden. Recently,

Congress has completed the circle regarding the origin of these methodo of surface trend analysis (E· H· Timothy Whitten, in Shaw, rocks. While admitting that metamorphism has taken place on a 1963) have shown the extent of compositional variability within regional scale, like Holland (1900), he places a special emphasis granite and other igneous massifs. Studies of this nature show the on the igneous nature of the series and has proposed that the extremely tenuous nature of the conclusions that may be drawn from

"charnockite problem" is not simply one relegated to high grade meta- one analysis which may not represent the outcrop from which it was morphic terraines o f India, but should include the "granite problem" taken. also, so that acid members of the series are simply hypersthene- To avoid placing a false perspective upon the specific distinc- granites rather than charnockite. tions between rock types of a particular grade of metamorphism,

On the basis of volume representation of the various rock types recent studies in geochemistry have begun to emphasize the broad in the Madras area, Naidu concludes that the intermediate and acid areal and compositional relationships of rock types to interpret group on one hand and the basic and ultrabasic group on the other their origin and the pact they have played in the evolution of the have not differentiated from the same magma. He is in agreement with crust.

Howie (1955) in choosing a norite parental magma for the hypersthene- In the present work it would seem desirable to adopt a policy eclogites, pyroxenites, basic granulites, and noritic dikes. However, regarding the meaning intended for the term "charnockite". It must for the acid and intermediate charnockites, he appeals to contamination be remembered that this term was in ·use before the introduction of and mixing through migmatisation between the very widespread potassic Eskola's facies concept of regional metamorphism and in Holland's day granites of Southern India and the older noritic rocks and Peninsular there was a tendency to propose new petrographic names for unusual gneisies. rock types. In the case of the "charnockite problem", individual

Since descriptions of rocks of charnockitic kindred are so specimens commonly do not possess all the required features, but may widespread for localities throughout the world, Naidu's suggestion qualify for inclusion in the suite if they have some of the features for dropping Fermor' s designation of a "charnockitic province" in or, more commonly, they are clearly related to OutCropS showing all

Southan India alone seems sound, especially in the context of the the defined characteristics. Thus, these rocks do not have one clearly present study. Of basic importance is the fact that these rocks defined characteristic, but a number of more or less incidental features, have remarkable constancy in mineralogy and setting that links them i.e.: a rhombic pyroxene, clouded salic minerals, and a xenomorphic to rocks forming the lowermost regions of the earth's crust. Certain granular texture. These features are easily recognized in the acid mineralogical criteria such as the presence of hypersthene fall down and intermediate types. They are so ill-defined, however, in the in many instances even in the type locality in Madras State, for basic and ultrabasic variants that Subramaniam (1959) has proposed

Holland has noted that certain horizons lack the definitive assemblage. the removal of the basic and ultrabasic from the more acid types.

This has been noted in other localities also, as for example in the Buddington (in Poldervaart, Geol· Soc· South Africa, 1966) states:

Westport map area, Ontario (Wynne-Edwards, 1967) where this writer "Who the devil knows what and when mafic rocks are associated with visited in 1967. Units mapped as pyroxene granulite and charnockite intermediate and felsic charnockitic rockst"· often lack hypersthene locally in the hand specimen, but upon exami- In terms of this investigation, the author has restricted the nation on the larger outcrop scale, a rhombic pyroxene with biotite, study to the acid and intermediate varieties. From the field descrip- or an amphibole can be distinguished. Clearly many geologists use tions for many of these localities, it is apparent that these rocks the petrological name as a field convenience using hypersthene as an have undergone high grade regional metamorphism up to granulite grade. index mineral characteristic of that grade of metamorphism. Much The development of an orthopyroxene either in hand specimen or thin confusion resulted, especially in the early literature, because of section or as an index mineral within a particular outcrop has been the mistaken identity of hypersthene for an amphibole (Holland, chosen as the prerequisite for analysis. In some cases, alaskitic

22·51&· , P· 123), especially when both are present in the same specimen gneisses were included since they were in gradational association and there is no recourse to a petrographic microscope. In this with pyroxene-bearing types (at Okollo, West Nile District, for example). study, a specimen sent to the writer identified as an "hypersthenite" In view of the dual use made of the term "charnockite", it is felt that was found to be an "amphibollte" on thin section examination. the general term "pyroxene granulite" is to be preferred. I f r WEE E Ef :05:: l i 1*9 : :ir5/ : 8 :5 II : : :8: 5 :E :ia:" N E:5 1 : 2 C - „8:02: t i 5 - 4- : 6 : /1 e 0-'ll E E I S M .*- -" E 8:E: ::0 E 8 : - a 8 8 - - - 8. . S 32 22 8 8 r , 5<:ail CO - to 9I - , - - 38 31 : :r::i£I I H . n : 2 51 „:0:-: & + 8 :8.1.: 8 /113/ 3/afF ABBEKE #35 8/F 9 2 K' 285: : B SK#:ii 0:,SH„ :8:,6 : Simw=: :10: 12: E:K: »:: 88:: :Em::m: 00 I I ...... :agi:# 8*g :32 4 8 6: 225 E i: t::t : :E A r g e:1: 1:EB E i B BSi ·1§2i 311iti i ; Ki ·§"Bl ip:8 rl'&2 ilt lil n ng i::.B:, .ir: : a a i i I i :5 6 1 i i : : 1: : 3 5 : i I i g< S E E i f i S i i i &:a i i W g i # Kg E i B i i E I i . : . i : : E . i 2 f 1Ei i tB i i f 2ti3 K: i 3 Il = 8 - : r": N N : 9 PS 1%38 : 1/: 5 8·f #59#.:R. : 1F i 1 2 .a&11 m,I»:0 m Fil 111 88 Mii :12 11#tri liti riliti ':11% LIRIP Isi 'R; a' ' g: A S= § 1&RF5b4- : 8 :S::rf 311 19; 118 11# fill jig'111 '1 il Iligil jiBI :'Dit Hi H 1 1 1 1£:11 # 1 5 5 2 3 1%

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domain of partial melting and palingenesis anQ since the temperature Experimental studies on the system KA1Si04-Si02-H20 by Luth (1967) of fusion of material of granitic composition is strongly dependent are of specific importance to the study of the charnockite problem since upon the partial pressure of water, it is not surprising to find the appropriate phase relationships approximating the acid charnockite granites associated in this terrain. assemblage differ widely in P-T space depending on whether a vapor

On the low temperature side of the granulite facies field there phase was present or whether the system consisted of condensed phases is considerable overlap with the almandine amphibolite facies via the during crystallization. If the assemblage quartz + potash feldspar + hornblende granulite subfacies, also termed the biotite granulite enstatite is an adequate representation for the natural assemblage subfacies by Hsu (1955). In many of the terrains of this study, quartz + alkali feldspar + hypersthene, then two possibilities exist there seems to be some overlap of the pyroxene granulite and horn- for the ·P-T conditions during crystallization of these rocks depending blende granulite subfacies. It is not known, however, to what extent upon whether an aqueous vapor phase is present or not (see Figure 1). polymetamorphism is responsible for the association of the two subfacies assemblages. In some localitles, the Westport area of Ph

Ontario, for example, there is a banding of the hornblende granulite *LA ;r41 and pyroxene gramlite subfacies on the scale of an outcrop which iooo _ Q\Q!0. L is reasonably attributed to variations in water activity during V metamorphism. It has been nggested by Fyfe && 81. (1958) that partial P (bars; FIGURE 1 pressures of fluorine may be related to the incidence of hornblende (schematic, after Luth, 1967) and biotite. The breakdown of blotite and hornblende is gradual and 500 - would occur over a significant temperature interval at any given pressure. The prevailing fugacity of oxygen would also affect the e:'*9 0* L ot 4 X temperature of breakdown. 9 .e 0. • • • The completely anhydrous assemblages which conform strictly to 700 800 900 1000 1100 the granulite facies are: T (Co)

1. Khondalite assemblage: quartz + perthite + garnet (+ plag- If the natural assemblage can be 'ireated as a simple quaternary system, ioclase + kyanite or sillimanite). This assemblage is recognized to then the coexistence of potash feldspar and enstatite with an aqueous ... be the result of metamorphism of aluminous sediment and is commonly vapor phase is restricted by the two pertinent 1 ,ariant reactions below the quaternary invariant point. If the existence of a vapor associated with charnockite terrains. phase petrographically, 2. Charnockite assemblage:. quartz + perthite + hypersthene (+ could be demonstrated then near surface garnet + plagioclase). This assemblage is characteristic of the crystallization (about 1.5 to 2 kilometers depth) would be indicated. acid charnockite series. In the case of a condensed system, however, no restriction can be placed upon the depth of crystallization since a near-infinite slope 3. Basic and intermediate assemblages: plagioclase + hypersthene is indicated for the appropriate invhriant reaction which continued + garnet + (quartz + perthite). Also plagioclase + hypersthene + beyond the range of 3 kb. in Luth's study. The high slope for this diopside + (quartz + orthoclase). reaction (Ph +Q=O r+E n+L)i s the result of the very small 4. Silica deficient basic assemblage: plagioclase + hypersthene change in aV for this reaction. + diopside + garnet.

5. Diverse calc-silicate assemblages: (a) diopside + plagioclase Miarolitic cavities and other evldences for a vapor phase are notably absent in rocks of the charnockite suite and, to this writer's + calcite + quartz, (b) diopside + scapolite + calcite + quartz, knowledge, there are no recorded instances in the charnockite litera- (c) calcite + diopside + forsterite, (d) dolomite + calcite + forsterite. tire of their being such cavities. In addition, there are other 6. The names blrkremite and enderbite (C. E. Tilley, 1936) have mineralogical criteria that support a deep-seated environment of been proposed for the potash-feldspar and plagioclase end-members of crystallization. Boyd and England (1964) have noted considerable the series, respectively. coupled substitution of Al-Al for Mg-Si in enstatite and monoclinic In addition, cordierite has also been recorded in some low temperature diopside with increasing temperature and pressure. The most aluminous assemblages. Among the minor accessories apatite and zircon have enstatite found under equilibrium conditions with variable abundance and are entirely absent in some localities (Groves, pyrope had 16 weight

1935). Rutile and ilmenite or ilmenomagnetite are the titanium- percent A125 maximum at about 1650'C. and 30 kb pressure. Although temperatures and pressures would not be this high during granulite bearing phases of note, apart from minor amounts of Ti in the mafia ' facies metamorphism, high alumina contents in hypersthenes have been phases. These rocks are generally silica saturated to the extent that noted by Groves (1935, p. 156-157) and Howie (1955, p. 751-756). In spinel (ulvBspinel) does not occur except possibly in the most basic the Madras Howie found bronzites from ultrabasic members of the series. Further details of the mineralogy of the Madras Charnockites, Qkl&·) charnockites and their analyses are given by Howie (1955). members to have A1203 contents ranging from 4.40 to 4.55% and in an 52

analysis of hypersthenes from two neighboring rocks from the South the enderbitic types. Howie (1955) has pointed out the similarity

Savanna complex (see Chapter III) he found a range from 2.52% to of this and the.Fe + Mg + alkali trend with those of the Crater Lake

series and the Lassen Peak volcanics. This last trend in particular 9.48% by weight A1203 (Howie, in Pitcher and Flinn, 1965, p. 319).

White (1964) presents criteria for distinguishing eclogitic and is only slightly more curved than the trend of typical calc-alkali basic granulitic clinopyroxenes on the basis of the molecular percent- igneous series. ages of the various clinopyroxene end members: Ac, Jd, Ts, Hd, and A computer program, originally written by Diness and Luth and

Di. Thompson (1947) suggested· that the amount of aluminum in four- later re-written by the author for the IBM OS/360 computer at the

fold coordination increases with temperature·while that in six-fold M.I.T. Computation Center, has been used to calculate the C.I.P.W.

norm for the'fifty-four analyses presented in Howie (1955, 1965) and coordination increases with pressure. This suggestion is evaluated Quensel (1950). The program is listed in Appendix C and is designed by plotting the percentage of Tschermak's molecule, the measure of to calculate the weight norm as originally proposed by Cross, Iddings, aluminum in four-fold coordination versus the percentage of jadeite, rirsson, and Waohington. It 13 modified only to the extent that a measure of aluminum in six-fold coordination coupled with soda.

White found a distinct separation of the two clinopyroxenes when L120' 1£ repurted in the analysis, is added with MgO and when 303

plotted on this basis,thus allowing a distinction,which is otherwise is reported it is calculated as thenardite (Na2SO4)· In addition, the MgO/FeO ratio in olivine, hypersthene, or diopside is given when difficult, between the higher pressure eclogite suite and the lower the norm contains that mineral. Also reported are the fractions of pressure basic granulite suite.

Hubbard (1966) has described the petrography and mineralogy of Or, An, and Ab and Or, Ab, and'Q. The extent of deficiency of A1203

perthite and myrmekite from charnockitic rocks of the Ikerre Group which would be required to form corundum is indicated by (-C). Ab,

of southwest Nigeria. The development of myrmekite is considered Or, Lc, Ne, and kaliophyllite (KA1SiO4) are recalculated to 1.00 in

to be the result of local exsolution processes associated and con- terms of the "granite system": Ne, Ke, and Q. The oxidation ratio is calculated according to Chinner (1960) and finally the Differentia- temporaneous with the development of perthite in the alkali feldspar.

A high temperature would allow significant solution of Ca, as the tion Index (D.I.) as proposed by Thornton and Tuttle (1960) is calculated.

fictive Schwankte's molecule: Ca(AISi308)2' to take place which would Despite the shortcomings of a weight norm compared with a molecular norm, the variation within the charnockite series as well as other exsolve to "normal" anorthite on slow cooling with the release of characteristics of the series is still apparent. excess Si02· Hubbard suggests that such local readjustments would In Figure 2, the Differentiation Index is plotted against the be in keeping with a system subjected to high confining pressures

with slow cooling in a relatively volatile-free environment. weight percentage Si02 in the rock analysis. The Differentiation Index as defined by Thornton and Tuttle is the sum of the weight 2.3 Chemical Analyses of Pyroxene Granulites percentages of normative quartz + orthoclase + albite + nepheline +

A number of papers discussed in the preceding chapter have leucite + kalsilite. Obviously only three of these minerals will

included both whole rock and mineral analyses of various members of occur depending upon the degree of silica saturation; vis: Q + Or + the charnockite series from a variety of localities. Quensel (1950) Ab or Ne + Lc + Ks. The two groups are considered the salic consti-

and Howie (1955) provide extensive compilations, the former including tuents of the C.I.P.W. norm with the exception of anorthite which in

the analytical data given by Groves (1935). Naidu (1963 has also Thoknton and Tuttle's study is considered a femic constituent. They

presented sixty-eight analyses of hypersthene-bearing rocks from also consider zircon and corundum as femic although with less justi-

Madras State. fication than in the case of anorthite. The Differentiation Indcx

Howie (1955) gives a detailed examination of the trends in the is a measure o f the degree to which a rock has approached "petrogeny ' 8

analytical results through the series. Plotting the weight percent residua system". Si02 - NaA1Si04 - KA1Si04 as proposed by N. L.

of the cation versus iti "position" given by (1/3 Si + 10 + (Ca + Mg) Bowen (1937). If a tetrahedron is constructed with Q - Ne - Ks

following the method of Larsen (1938)1 and modified by Nockolds and forming the base and the femic constituents summed to form the apex,

2 Allen (1953) , a continuous variation is observed that corresponds the distance from the apex to where the rock plots within the tetra-

very closely with the elemental variations in igneous rock series. hedron is a measure of the basicity of the rock. The Differentiation diagram, When the alkalies: Ca, K, and Na are plotted on a triangular Index may be the same whether the rock ls acid or alkaline and is not

two trends are apparent. One trend curves upward to higher potassium a measure of ailica saturation. Figure 2 does show the degree of

values, Howie (1955, p. 736), and the other has a lesser curvature silica saturation throughout the series, however. Points that fall

toward potassium, more closely approaching sodium, and.representing above the Or-An line are generally over-saturated and those analyses

falling within the Ab-An-Or triangle would be expected to be rich in 1'Larsen, E. S. (1938) 12EL· Geology, Some new variation diagrams for groups of igneous rocks, 1 , pp· 505-520. these phases and would be anorthosites, or syenites. Undersaturated 2.Nockolds, S. R. and Allen, R. (1953), The geochemistry of some rocks would fall below the Ab-An line in this figure. Superimposed igneous rock series, Geochim.et Cosmochim Acta, t, pp. 105-142. 8.# I I 8 2-5: 1 i Ifii': -ME,)M .i 4 4 :55*88 .iga 38:UMa * :4 2 : i,Er"gijme,11512:./f#iME,%6:maili: 4,0#M Wea/. ...4..:.:mo: & : 5 : S litii:8:*9:5 glf,11£:Er:3:.18551:.1.::3 :33 :glma I . 31811a.::. jii,fileii{itili: C. 1%411{ 00 • Z.: i li;142;i1%11'11'i%11 ani i ;i . JA:3:: .-WEEE,%:11//#43#MIX:9=: .... - : 4 % I ::-':3..4182:.:#58,41:.:0:wee, 33::: Eatia:lizlfBEETEMAE,alt:11:4,1117#i=:81illg i".-:itfi -0 alift,Kijigs;%:%"lifi:i:lie:91 S::r:13,M:£853#alaifirtplits#i:341:9*9:3:069. . I .::6118::1: 0

w (0 .. . -4w / - a i m a i l:f f i t .i

Z'fers.ligii-151::K=1:512119911„1 01 5 0 .. 5#.. 2.:. 8 . i: i i i s i iI a l:31.1. M , fiEN:s:/Ammi)Wgi/1;518/98:LE:: = : : I 8 13 :. 8 3 0 1 1 : 1 : 3 1 5 : : :0 F.:1 993 1821<1112;timizii:I:i,Litia:li %9311#16·&1ijfy;Ell ::0'61On C 26 : :* f ..19:3: . :811:11:5,3.:.•::all:86%39:3-4.:/11:: C: 012 ;f1a:G;aE:;f6i41i38::5a3i15%3:I . - -0 „ I = „ .53:: .111: DE,1#0:ja„83318583:g:Agga:3::&%11 :116963: 3: : : .3 : 3 0 'Mili...... I-:NAM:8:8. „ S::8 .%::8:1 0 2: : 1. C 8: €B E,E: ,5 0 6 5 i:ielifi;lil .s : / 3 M /C 54

Heier (1964)has proposed a mechanism whereby the transport of a group. Also, many of the rocks analyzed in this study contain

major and trace lithophile elements could take place during progressive small amounts of an amphibole so that if two populations really

exist, small degrees of mixmg of either end-member would only regional metamorphism. As lower grade rocks are subj ected to increas- obscure the bimodal nature of the distribution in spite of the inherent ingly higher temperatures and pressures, mineral transitions such as accuracy of each analytical point. the breakdown of micas would occur which would give rise to product Heier made a further proposal regarding the possibility of phases less favorable to the inclusion of rubidium. For example, natural fractionation of radingenic strontium as a result of regional in rocks such as granulites containing excess quartz, Heier proposes metamorphism. Heier believes that during anatectic melting of silicate the reaction: rocks containing potassic phases, these potassic phases corresponding K(Mg,Fe)3AISi3(10(OH)2 + 3Si02-KA1Si308 + 3(Mg,Fe)SiO3 + HZO to a granitic melt would be the first to break down. Since these From crystallochemical considerations, then, it would be expected are the host phases for rubidium, they will also be the sites of that rubidium would become concentrated in a volatile phase,depleting 87 8/ bullt up after the last isotopic homogenization. Since biotite the higher grade rocks in rubidium. Strontium,on the other hand, 18 fairly common in this facies, aud 18 alsu luw In cummon strontium, would not be expected tp behave in thls manner because of its ability anatectic melting could presumably lead to a residual rock with a low to proxy for calcium. During progressive high grade metamorphism, Sr87/Sr86 ratio· then, one would expect a progressive decrease in the Rb: Sr ratio. I< 18 diffkult to emisage a mechanism whereby this reaction A plot of %K vs. Rb/Sr of granulites would be expected, in light of could take place without having the radiogenic strontium, released the discussion above, to show a lower Rb/Sr ratio for a given potassium from the biotite, incorporated into the strontium acceptor phases on concentration than in amphibolites and lower grade rocks. There is a the right hand aide of the equation. In addition, it must be assumed suggestion for such a trend for amphibolites and granulites from that there be no common strontium available to mix with this radio- Langdy, Norway (Heier, 122·Sit·, Figure 1) though many more analyses genic strontium, which would render the effect immeasurable. The would be needed for statistical proof. In Figure 3, a similar plot implications of this model are considered further in Chapter IV in for the charnockitic types from a variety of localities is given, and the light of the Rb-Sr isotopic data. these show a much greater range in both ordinate and abscissa than In a study of Precambrian medium to high-grade rocks of the in Heier's work. The results of Figure 3 are not strictly comparable Australian sheild, Lambert and Heier (1967) have demonstrated distinct to Heier's results, since his work was restricted to a single locality differences in the total rock content of potassium, uranium and thorium and coexisting amphibolites and pyroxene granulites were being compared. between the gneisses of the felsic amphibolite facies and homblende On the other hand, the results show the variation in an approximate granulite subfacies and the more deeply seated pyroxene granulite sense that one would expect for the charnockite series considered as subfacies. As a result of approximately 400 analyses for U, Th, and

K vs. Rb/Sr IN PYROXENE GRANULITES 7- AND CHARNOCKITES

- 6- . 0

a &5 0 0 . .e * 0 .- 0 2 4- 91 0 E I 0+ + 0

-5 3-. + +A 82 + Amphibolites, Heier (1960) • Crane Mtn., New York 32 0 A Madras, India 0 Okollo and Rakosi, Uganda 4 2- 2 I Westport, Ontario ++ A 0 0 Indian Lake, New York 1f

0 1 2 3 4 5 6 Rb/Sr (isotope dilution)

FIGURE 3

9, 55

K the concentrations in pyroxene granulite subfacies rocks show a Heier and Rodgers (1963) have found an average (U/K) x 104 marked depletion when compared to lower grade metamorphic gneisses, ratio of about 1.2 in granitic rocks. All the ratios recorded in granites and greenstones of the sheild areas, and the granites and Lambert and Heier's work are significantly lower than this value, shales of younger mountain regions. The pyroxene granulites generally the lowest values being found in the high and medium pressure granu- have consistently low values of K and U and Th, which are much less when lites. The values for the (Th/U) x 104 ratios show a much greater compared with the lower grade rocks. In magmatic series and in sedi- degree of variability. Although values of about 4 have been recorded mentary rocks, it is common to observe an increase in the U/K and Th/K for this ratio in a variety of rock types, this ratio is generally ratios with increasing K content of the rock (Heier and Rogers, .1963); higher than 4 in lower pressure granulites, and lower than 4 in the in this respect the pyroxene granulites show a reverse trend. high ad medium pressure granulites. Lambert and Heier (1968) inter-

Compared with the common surface rocks of appropriate bulk compo- pret the depletion of lithophile elements and the higher K/Rb ratios sition, the acid and intermediate pyroxene granulite subfacies rocks and lower Rb/Sr, (Th/K).x 104, and (U/K) x 104 in high to medium are depleted in Th and U by factors of 9 and 5 respectively. pressure granulites as the result of a series of dehydration reactions

In a sequel to the paper above, Lambert and Heier (1968) consider involving partial melting under the P-T conditions of the upper amphi- these elemental variations further and also present additional data on bolite facies discussed previously in this chapter. Studies of similar

Rb, Sr, Pb, and Ba determined by X-ray fluorescence. In this more nature carried out on lower grade rpcks (up to the high amphibolite recent paper, they divide the pyroxene granulite subfacies into low, facies) have revealed that there is no change in bulk chemistry apart medium, and high pressure variants on the basis of mineralogical from loss of volatiles (Shaw, 1954, 1956; Phinney, 1963; Chinner, 1960). assemblage: Eade && . 1· (1966) found minor changes in bulk chemistry in medium pair: (i) Low pressure: Cordierite, andalusite and the mineral pressure granulite facies rocks of the Canadian shield but major olivine + plagioclase and characteristic. Also present are brownish changes in chemistry seemingly do not occur until the highest metamorphic homblende + biotite, * almandine garnet. Hypersthene is widespread grades are attained (Heier and Adams, 1965; Heier, 1965; Lambert and in the mafic types. Heier, 1967, 1968).

(ii) Medium pressure: Plagioclase + hypersthene t clinopyroxene, In the present investigation, thirty-two specimens from various + - quartz. Stable aluminosilicates are sillimanite or kyanite. localities (Table II-B) were analyzed for potassium by atomic absorption

Olivine and plagioclase are incompatible and cordierite is unstable. spectrophotometer by the addition method and were combined with rubidium

(iii) High pressure granulite subfacies: The mineral association values determined by isotope dilution. The accuracy of the potassium developed with increasing pressure is garnet + clinopyroxene + hyper- TABLE II-B sthene + plagioclase first in silica undersaturated rocks then in silica saturated rocks. At higher pressures plagioclase disappears, Rb(ppm) M. I.T. # ZK20 ZK Sr(ppm) K/Rb Rb/Sr marking the beginning of the eclogice facies. When the activity of Pallavaram Area, Madras, India. water is high enough, hydrous phases such as biotite or hornblende form. R7220 0.315 0.262 4.3 94 609 0.045 Anorthosites are exclusively developed in the medium to high pressure R7215 0.346 0.287 6.5 123 442 0.053 R7219 0.611 0.507 11.5 143 441 0.081 granulite terrains. R7216 0.561 0.466 12.6 116 370 0.109 Their results show a decrease in K, Rb, Th, U, Pb andl Si and R7221 0.624 0.518 20.2 203 256 0.100 related elements in the highest granulite grade rocks analyzed (Table II-A). R7242 1.897 1.574 210 427 75 0.492 R7241 0.487 0.404 7.6 --- 527 --- There appears to.be a corresponding increase in Ca, Mg, Fe, Mn, and Ti. R7218 0.445 0.369 11 192 335 0.056 The high grade rocks show lower Rb/Sr, (Th/K) x 104 and (U/K) x 104 R7214 0.241 0.200 6.3 110 383 0.057 ratios and higher K/Rb ratios than in the low pressure granulites and R7205 3.92 3.25 134 116 243 1.152 R7217 0.241 0.200 3.9 76 513 0.051 amphibolltes. That is, for a given potassium content. there is a R7240 1.45 1.20 146 473 82 0.309 relative depletion of Rb, Th, and U compared with the lower grade rocks. Westport, Ontario;

TABLE II-A R7070 5.09 4.23 140 466 364 0.301 R7083 4.52 3.75 85 359 532 0.236 Average K and Trace Element Compositions. Australian Shield (from Lambert and Heier, 1968) R7090 3.76 3.12 57 417 660 0.136 R7112 4.82 4.00 88 142 454 0.616 Musgrave Range Fraser Range Cape Naturaliste Eyre Peninsula SW G A G G A G A Shield* R7113 7.45 6.18 125 121 494 1.030 R7061 1.45 1.20 28 99 428 0.284 K 2.2 2.8 1.4 3.7 3.8 3.6 3.7 2.6 Rb 70 140 45 190 180 175 200 115 R7062 1.19 0.99 24 454 413 0.053 Pb 20 25 15 30 35 30 30 35

Ba 1090 990 . 420 625 780 630 610 610 Sr 340 400 170 160 190 140 120 135 * to accompany Figures 3 and 4. U 0.4 1 0.3 2.5 1.5 2.0 4.5 3.0 Th 2.1 11 1.8 35 23 23 30 20 ** Samples R7125 to R7128 inclusive: potassium analyzed by X-ray Zr 310 250 115 525 400 185 195 220 Fluorescence, (Whitney, 1967). Rubidium analyzed by isotope dilution, this study. *Average for all rocks C = granulite facies A '= amphibolite facies 56

TABLE II-C Indian Lake, West Canada Lake, New York: Summary of K/Rb Ratios of Charnockitic Rocks (Figure 4) K7321 2.70 2.24 93 220 761 0.425 R7322 4.78 3.97 135 197 354 0.682 Locality n Average K/Rb Ratio R7326 4.49 3.73 121 223 371 0.543 Madras 10 412 Madras 12 356 0.208 R7327 1.50 1.25 53 255 283 Westport 7 478 Indian Lake 4 312 Crane Mountain, New York: Crane Mtn. 5 316 Rakosi & Okollo 4 250 R7125** 5.30 4.42 151 126 293 1.195 R7123 5.58 4.65 165 140 282 1.181 Total 32 354 R7126 5.09 4.24 138 187 307 1.740 R7127 5.21 4.33 126 180 344 0.702 above having nearly identical regression coefficients described by R7128 5.21 4.33 122 187 355 0.654 the equation:

Rakosi and Okollo, West Nile District, URanda: 0.897 IK - 0.0369 (ppm Rb) 296 2.191 R7011 5.82 4.83 161 74 ** ia taken as the overall average Matt: 4.366 The line of constant R 5 263) R7012 5.88 4.88 135 31 362 147 4.879 R7018 5.24 4.35 303 62 Trend in igneous differentiation for the 12 sub-parallel suites used 207 78 196 2.668 R7020 4.81 4.00 in Shaw's study which include the following rock types: analyses was confirmed by frequent analysis of G-1 and W-1 throughout New England Granites (29) Skaergaard Complex (8) the period of analysis. The values obtained agreed to within 1.5% Slieve Gullion Granites (19) Continental Tholeiites (41) of the recommended values of Fleischer (1965). The rubidium determi- Hawaiian Basalts (35) Ultramafic rocks (23) nations by isotope dilution are of ::imilar precision. The errors in Diabases (42) Banks Peninsula Volcanics (30) the mass spectrometry are considered in Chapter III. These values, Various Composite.Igneous Rocks (9) Continental Alkalic Basalts (11) plotted in Figure 4, are compared with the trends found by Shaw (1968) Composite Basalts and Granites (5) Eclogites (19) for twenty-one suitea of igneous and quasi-igneous rocks whose potas- Number of analyses given in parentheses. sium and rubidium values have been reported with sufficient accuracy The trend in K/Rb is not constant throughout the range in. potassium in the literature. In view of the geographic diversity, there exists values. Shaw found the followidg values: a marked trend in the K/Rb ratio. For the data presented here, the mean K/Rb ratio appears to be about 300 to 400. Considering the Madras 3& charnockite suite alone, there appears to be a slightly higher K/Rb 0.01 433 ratio of about 400 (see Table II-C) for specimens having lower potas- 0.11.0 332254 < 10.0 195 sium contents. The trend for all the samples is somewhat sigmoidal with the axis of symmetry parallel to Shaw's Main Trend at about K/Rb = 300. R - (XK/ppm Rb) x 104. This Main trend is defined by 12 of the 21 suites of rocks mentioned

IC-

- "'411 1 1

'Pr .'I. ..

'·*2 *--- ,:it' 4 U ...... ". 'f. a.» f : / / 7: K-Rb TRENDS in CHARNOCKITES 0.1- I...... 1.....

Main Trend

.'...... 01- I '.'......

. --......

001 , . i'.1 ' 0.1 LO 10.0 ' ' ' ' 566.0 ' ' ' ' ' '6'Joo ' ' '6'860.0

Rb. ppm.

FIGURE 4 S7

It Is apparent from Table II-C and Figure 4 that the majority of the new (4f14) f shells as is observed to a lesser degree in the regular trans- +3 analyses fall above the Main Trend for igneous rocks. The results ition series. Thus La and Y form M ions only since the removal of

also confirm Shaw's observation that the relation is not linear for three electrons leaves the inert-gas configuration. Eu and Yb can

all values of potassium but shows a decrease in value of R with attain the f7 and £14 configurations respectively in the M+2 state.

increasing potassium: t'It should be noticed that the decrease in R Although there appear to be valid theoretical reasons for the various +3 state is thought to be prevalent in observed in these studies was accompanied either by a trend towards valence states to occur, the M 3 younger petrological age or by increased content of K." nature because fairly rigorous conditions are required (extremes of

Although this investigation was carried out in a different manner PH, for example) to attain other valence states in the laboratory.

from that of Lambert and Heier (1968), both in analytical technique TABLE II-D

and regional extent of Rampling, Phe regulti preocnted here te„d Lu SAMPLE COMPOSITE FOR RARE-EARTH ANALYSIS substantiate their results based on a regional scale. That is, chere MYSORE AND MAnRAS STATES, INDIA appears to be a tendency for slightly higher K/Rb ratios in these

M.I.T. # rocks relative to many igneous rock types that Shaw (1968) included Sample Wt. (Rms.) Sample Description and Locality

in his Main Trend. Additional restraints can be placed on the inter- Pallavaram

R7239/AS-4 10.9 pretations of these results in the light of the Rb-Sr isotopic deter- Basic Charnockite (Hb-Bio-rich), Pammal R7240/AS-5 10.4 minations in each suite of rocks. These results are considered in Basic Charnockite, Thattangannu

the last chapter. R7241/AS-7 11.6 Basic Charnockite (Hb-rich), Pammal

R7242/AS-10 10.5 Basic Charnockite (Bio-rich) Cherimalai 2.5 Rare-Earth Element Studies R7205/AS-12 11.6 Basic Charnockite, St. Thomas Mount 1 Rare-earth abundances, normalized to the chondrite abundance for Kushalnagar each element, have been published now for a variety of rock from types R7214/A-85 10.8 Charnockite, Dindgad both oceanic and continental aread. Reviews of the older literature R7215/A 95 10.6 Charnockite, Chikkamarahalli are given by Haskin 1 &1:, (1966) and more recent aspects of research R7216-A-110 10.8 Charnockite, Kudige-Kanive in this field are considered by Haskin and Schmitt (1967). Only a R7217/A-115 10.7 Charnockite, Kanive Hill (left) brief discussion of the geochemistry of the rare-earth eiements will R7218/A-117 10.7 Charnockite, Kanive-Jainkal Betta be given here. R7219/A-121 11.1 Charnockite, Marur The lanthanide elements include the fourteen elements from R7220/A-124 10.6 Chamockite, Kanive Temple Hill (right) La (Z = 57) to Lu (Z = 71) and include Y (Z = 39) because both the R7221/A-126 10.7 Charnockite, Near Adinadur Tribal atomic and M+1 ionic radii for this element lie between those values Colony R7248/A-142 10.6 Charnockite, garnetiferous, for Tb and Dy despite the fact that yttrium lies above lanthanum in Doddakamarahalli R7244/A-148 10.7 Charnockite, Cauvery River Bed, Hulse

transition element Group III. Although a rare-earth ' Pm is not 61 R7246/A-150 10.1 Charnockite, Cauvery River Bed, Shanbhoganahalli considered in this work because of its vanishingly low abundance, TABLE II-E it is found only in nature as a spontaneous fission fragment of

uranium having a maximum half-life of about 30 years for its longest- RARE-EARTH ABUNDANCES IN CHARNOCKITE COMPOSITE, lived. isotope (Pm145). Lanthanum has a ground state configuration MYSORE AND MADRAS STATES, INDIA

2 ELEMENT CHARNOCKITE of 6s 5d and the thirteen elements following it are fo'rmed by the CHARNOCKITE COMPOSITE COMPOSITE CHONDRITE* addition of up to fourteen electrons to the 4£ shell which is slightly'

(y) more stable than the 5d shell. As the·nuclear charge and number of (22.70) (11.35)

4f electrons are increased by unit increments through the series, La 12.90 39.1

there is a reduction in size of the entire 4fn shell, giving rise to Ce 16.47 18.7

the so-called lanthanide contraction. ewing to the shape of the 4f Pr 4.59 40.8

Nd 17.26 28.8 orbitals which are unab,le to shield to the same degree as · the 5d Sm 4.20 23.2 orbitals, the addition of one 4f electron increases the effective Eu nuclear charge of each electron, reducing the size of the orbital. 1.37 19.9

+3 Gd 4.63 18.6 All of the lanthanides form M ions. In addition, Nd, Sm, Eu, Tm, 0.75 16.0 and Tb form M+2 species and Pr, Nd,- Tb, and Dy form M +4 ions though Tb HO these two states are less stable than the characteristic group valency 0.97 13.9

Er 2.70 13.5 of M+3. The stability of the M+3 state may be related to a special Tm 0.37 12.3 I stability associated with empty (4£'), half-filled (4f7), or filled

1 Yb 1.86 9.3+ 3·Shaw (1968), p. 577. Lu 0.36 10.6++

* Chondrite R.E.E. values from Haskin, 81·, (1967) given in Table II-F

+ value obtained by fcounting taken because of slight 1 contamination in Yb fraction for counting by Lu.

++ identical value obtained by both r and 1 counting. 58

TABLE II-F TABLE II-H

RARE-EARTH ELEMENT ABUNDANCES IN CHONDRITES La. Yb, and Total REE Contents of Igneous Rocks (after Haskin 81·, 1967) Rock Type 92 Ilt La/Yb E REE* Gabbros and Diabases (Haskin ·, 1966, p. 224)**

Ironton, Mo. 12.8 2.7 4.7 ELEMENT ABUNDANCE 123 Bushveldt, Norite 4.8 0.54 8.9 28 Y 1.96 + 0.09 San Marcos 4.0 1.70 2.4 59 La 0.330 f 0.013 W-1 9.3 2.2 4.2 96 Ce 0.88 + 0.01 W-1 11.7 2.1 5.6 100 Pr 0.112 i O.005 Finnish, Gabbro + Diabase 1.7 0.8 2.1 22 Nd 0.60 2 0.01 Eeluxltes (Ibid·, p· 229) Sm 0.181 t. 0.006 Delegate 3.6 Z.0 1.8 82 F. 0.069 + 0.001 Roberts Victor 4.2 1.6 2.6 40 Cd 0.249 + O.011 Dutoitspan 7.4 2.3 3.2 116 Tb 0.047 + 0.001 Japan 1.8 1.13 1.6 34 Ho 0.070 i 0.001 Granites (Ibid·, P· 232) Er 0.200 i 0.005 Composite, 70% SiO 55 4.0 13.8 220 Tm 0.030 i 0.002 2 Composite, 60-70% 84 4.0 21.0 320 Yb 0.200 + 0.007 Si 2 Composite, 60% Si02 68 3.0 22.7. 250 Lu 0.034 + 0.002

Composite, Finnish 47 · 3.0 1 5.7 260

Finnish, Archean 43 1.8 23.9 200

TABLE II-G Finnish, Granite-gneiss 19 2.6 7.3 120

ELEMENT ABUNDANCES BY SPARK SOURCE MASS SPECTROMETRY, E. Tuva, Radyros 130 1.7 76.5 480 CHARNOCKITE COMPOSITE E. Tuva, Nizh-Kad 53 10 3.3 370

E. Tuva, Agash 22 Element ppm Atomic ppm Weight 5.5 4.0 230

B 2 1 Ukraine, Rapakivi 90 3.2 28.1 230 200 170 G-1 92 0.94 97.9 340 Na 2500 2500 G-1 102 0.63 161.9 350 Mg major IDBj Or Al major maj or Wisconsin, Red 31 2.3 13.5 180 Si major major Wisconsin, Ruby 36 4.3 8.4 220 P 3000 4000 Basalts (Ik&d·• P· 223) Cl 100 150

K major major Columbia Plateau 25 3.9 6.4 230 Ca major major Linz , Prussia 41 3.5 11.7 310 Sc 50 100 Jefferson Co., Colorado. 79 2.6 30.4 530 · Ti 3000 6300 V· 800 1800 Composite of 213 basalts 17 2.7 6.3 170 Cr 1000 2300 Composite Sediments (Ibid·, P· 270) Mn 3000 7200 European and Japanese Shales 18 Fe major major 2.7 6.7 148

Co 75 190 Finnish·Sediments 13.6 1.31 10.4 77 Ni 50 130 40 North American Shales 39 3.4 11.5 240 CU 100 280 Russian Platform Average 50 4.2 11.9 Zn 120 330 280 Rb 10 37 I in ppm Sr 75 290 ** All data from compilation from Haskin, · (1966), Meteoric, Ba 50 300 Solar, and Terrestrial Rare-Earth Distributions, in Physics and Chemistry of La <2 12 the Earth, Pergamon Press, Volume 7. remaining REE <2 each 10 each

* This analysis was kindly provided by Mr. T. Mariano, Ledgemont Laboratories, Kennecott Copper Company. ** the 'ppm weight' was calculated from the 'ppm atomic' by taking an'average' composition for charnockite from the literature (Mariano, personal communication).

C I 59

100 _

t 60 COMPOSITE OF 16 CHARNOCKITES E 'U-•A-- 0Z (MYSORE 8 MADRAS STATES) IREE=91(including Y)

20 0 8 •• .-

I LZE

%-4 2- 5

1 1 1 1 1 La Ce Pr Nd Pm Sm Eu Gd Tb (Y) - o Er Tm Yb Lu RARE-EARTH ATOMIC NO:

FIGURE 5

100 1,1 _ 1 1 1 1 ' .1 1 '1 1- - G-1 . 60 - La/Yb vs. IREE (including Y) - (doto from various sources, compiled in 40 - Haskin &19< 1966, Meteoric, Solar and Terrestrial Rare- Earth Distributions, - 30 Physics and Chemistry of the Earth,vi) • .. 20 - A A AA + +. 1 10 6 7- A - +.. 5- w-| r A - Charnockite composite 4. 3 + • Gabbros and Diobases - + + Eclogites A Gronites 2- • + , Bas/Its + e Sediments

10 20 30 40 60 100 200 400 600 1000 I REE

FIGURE 6

A 1 1. 8 ....M.U.„ N 3 / M #:3 /".:Wal:3: . I - „ . f : : j 7. 1 1 8 1 1 1. 1 : Egi .::0- 51 0 ..1 H. .. . = ' : : :1:1 1 #i i . ' s : : . .3 2: €1 6 :d a l f e; f E M ,#1 I· f i i i: :1,3 3: -0. . " . : 0 1:1 1; ,F i e r:j i l i j i l l i 1 1; :f:ilill i; : ,1,8 ifigibi ,1, : : ' :digi.:Mi e'l e a l: 1:4111%1:ie:ir.,1*1102:ii:ii{:fal lag:LAA &1;5:18 1 1 1#2:5 3 4 1.3 1.f A, ;i t. . . : - :2 0. :.8 1 ES#i; i;111: . · M:N 0 E : 1 . :*::Er#ficiaI I z. lilial:illillilitiligiliflill.:e.:..1 :uffilk:.a:: H cvalid:110..(na . I „ .1.:...i a:g i# ;1% A: ·i: ;1 3 3 1,4 m i E 5:::l i: : . :3/5 3:g:4·8·5 i *1. ,i i l f 1; i i· :l'i l i ;'./ :O:0.i I f; :t' *4 8; i i' :i f i l i i j i i t·i i :8: 8 8 f i j. : 2 i f;i E. *8 3 1 ] it#,MOY: · : .3:8 :8 4 3 2: .I l i A N a l:3 a:.:2 .I86·10:6*:BFRit i l i t a: .M E :e l e: § : :S t a 0.0.s s:1:guliti, : lialf /*:lia, fli ..IM:Mag:38:33:%85::8 .„ fi,·!ilfi3131118::1*lail88&52:&*139"83:0A .tii&i;:",f:i li 1:SI31 1 iti0:.4: 1;g Af:338:51 i 31 f 9,1.1·1„%431.an:g -:Od :0:0 -=.0 ... '03*48-=o.=12;:091.Bel:1311'. :31%:11:1*%88%,M Miligillflaill#tili:aqligai::::idli:#11#ii;a 4 1·* 8 M a l l s e s 23 S i· 8 8 1·1 2 1 1/i I % #1 8/ i a g i a i 1 3 5 8/1 1.942:0 5 3*t a 0 .» 1 5 8 : 1:E. g 5 -8 .3 -d , S 4111111ff Bil : 1.i,4,:im 1611/ iE:Mili iti1$#1112,1,0611 11;121 906:> :.i 4 mils,;154;Miti 13:imililitieviali;lilli:'*111::liti 811:J.-p:':':2'i:ilii.312113$Ifi 1,1:11 .: 5 i: M Y::46 %:2.:#143 :3 i g: . * 1= . ,2.6 3 m#.. :4 :*:313*111::1:/:5,11:11:1, 8:,1/31:9:8;·i;:ilitillititi 21 Elli:H/;18 91 . I „ 41:3 0 „ 3 -4 : 1 1 51 : 1 :Baillar153:*13 lfapal;Lf;ii:alaiM:snew:Ex,#83&31, i -., .1s: .4 1: : .S. : N -4.... 00 le:g::g:fi.8 :-lia :/312;1.: ilit:ligilii#lifi 11:11:ii:41;112 111:1 i :: :m:& 53::7:,Ft-:13131% : .8 . 3 1121:112;121#Hil- , 0:-:S : - . 5 . I 3 . . . g115*tilifilii:Oibliwiii l i t i, ;i q i: :i l,f l i:i l l i u i &3e,l s i i a. l l i,i..28:all l i a; 1 2 1 1 5 i j i t f i f t i i g i l l i i i:i f i i z a i l i. : :i l i#l a g i a: :M A, o. : ' : 1 5 3:l i g i s s l i f I : I i E 2 9 3.8 1 5,3 i l l a i g l i#i : 4 8 1 1 1 f & i g : 1 4 1 .3 1 i i z i : l Mi g:g i: l: ai h.w.5 · : l#j:·. 1 %i t:i. 1 0 : 8 : '.,6 g i1 .: 4.8 : -o i . i 0l -i :f 4i; W; ..,'

9·S: :8 1 : k i l i l i t a i l;N u l: ,3 & g r i n i i a l a m. 1 1 7 8,1 . Al 61

transistor preamplifier and a TC 200 main amplifier. The amplified In the table below (Table II-J) the abundances of the-individual signal was stored in a Packard 4096 channel analyzer and the spectrum rock analyses are shown in ppm unless otherwise indicated. The peaks was placed on computer compatible magnetic tape via an ADC (analog chosen for analysis are those indicated by Gordon &1&1=.(1968) which to digital converter) unit. are relatively free from interference from other elements of high

The computer program, written by Dr. P. A. Baedecker, searches abundance. The worst interference was found to be from 45d F.59 out the photopeaks, calculates their area, centroid and energy. A with a 33d = 145.43 keV. photopeak at E = 142.5 keV with Cel41 EY correction to the area is also made for detector efficiency. A Cs This interference invariably resulted in an unreasonably high yalue spectrum must be provided if the energies are to be determined and of about 200 ppm for this element. if they-ray energy of the standard photopeak is greater than 1836 keV, The abundances of Co and Sc reported here are of a similar 88 a y spectrum must also be provided. The following spectra were magnitude reported by Howie (1955) for the Madras Charnockite Series. 203 88 Hg Y used in each counting experiment: , 279.1 keV; , 1836.1 keV; In the case of Co, the values range from 100 ppm to less than 5 ppm

Na22, 1274.5 keV; Cal3; 661.6 keV; (060, 1332.4 keV. and Sc has a reported variation from 30 ppm to 10 ppm. The values

determined by Spark Source Mass Spectrometry on the charnockite

TABLE II-I composite at Co = 190 ppm and Sc = 50 ppm seem to be in rough agreement

Flux Monitors with the values found by Howie for individual specimens.

Conclusions based on so few analyses can only be tentative and Long-Lived Short-Lived u Im/01 u Im/ml though this aspect of the study is far from complete, the technique Ce 171.17 Cs tle v 35.26 of instrumental activation analysis (INAA) holds great promise for Nd 183.4 Cr 101.0 Mg 15.68 trace element analysis, especially -with the increase in resolution Gd 51.94 Co 48.88 Na 576.0 with Ge(Li).detectors. Tb 1.019 Th 102.2 Mn 256.82

Tm 1.145 Sb 24.48 Al 4.77 .6,?.S-a,Y.,of._qhrap-tgs. .9,/ml Yb 2.83 Rb 1.046 K 7.56 In this chapter, three geochemically useful parameters have Lu 5.69 Ba 1.34 La 75.55 been used to investigate the variations found in the "charnockite Sc 10.4 Hf 10.29 Sm 9.32 series". Fe 20 Eu 2.21

Dy 5.17

Ho 5.05

TABLE II-J

Some Elemental Abundances by Neutron Activation for Pyroxene Granulites

M.I.T.# Tm Gd(+Sm+Ta) g 2 111 ili &1 9 5 I&

1) R7120/ACS-40 Crane Mountain, -- 19* 19 343** 7 50 11 9 53 3.3% New York

R7121/ACS-41 Crane Mountain, -- 1 15 216** 4 28 7 9 13 2.8% New York

R7129/ACS-48a Crane Mountain, 1 17 15 200** 5 48 '11 8 -- 4.3% New York

R7091/32 Westport, Ontario 0.1 4 9 179** 3 38 25 14 -- 3.8%

* abundances in ppm unless indicated otherwise. 141 ** probable interference of Fe59 (E - 145 keV) with Ce (E - 145.43 keV).

Precision based on replicate analysis is about £ 25%. 62

The differentiation index defined by Thornton and Tuttle (1960) these elements through the geological cycle involving erosional processes

hns hep.n calculated for fifty-four analyses taken from the literature, and possibly regional metamorphism has given rise to large variations

and when plotted against the weight percent Si02 in the rock, the in certain elemental ratios. Pertinent to this discussion are the

trends outlined by these analyses coincide with those found by Thornton variations in the Rb/Sr racio in the crust and mantle. and Tuttle for igneous rocks. In magmatic processes, Rb+ (1.48 R, Pauling) and Sr + (1.13 f, The K/Rb trends determined for thirty-two specimens from the Pauling) are concentrated in the silicate melt phase during partial collections available for this study show a trend that is parallel melting or differentiation. Similarity of ionic charge and size to the Main Trend found by Shaw (1968) for igneous and quasi-igneous allows Sr to follow Ca by diadochic substitution into calcium-bearing rocks. There is a auggestion that the pyroxene granulites analyzed phases and likewise Rb follows K into potassic phases such as potash in this study may be depleted in Rb with respect to potassium when feldspar or biotite so that during magmatic crystallization there is compared to this Main Trend. The extent of depletion is discussed an increase in the Rb/Sr ratio as crystallization proceeds. in Chapter IV. The details of the pyroxene granullte trend seem to Various compilations of data for the (Sr87/Sr86) ratio have parallel Shaw' s Main Trend closely, showing a degrease in the K/Rb been given by Faure and Hurley (1963), Hedge and Walthall (1963), ratio as potassium and rubidium both increase. Tarn,Imn/0 11. (1965), and Renee (1966) fnT oceanic And Innrlnen..1 A study of the rare-earth element abundance pattern for a basalts. The measured Sr87/Sr86 ratios are remarkably uniform varying

composite of sixteen samples of pyroxene granulite (charnockite) from by less than 2%. The range in Sr87/Sr86 for oceanic basalts is rather the type localities in Madras and Mysore States of India, shows an narrow from 0.700 to 0.706 whereas continental basalts have higher enrichment of the light rare-earth elements when each element is values generally from 0.703 to 0.705 with occasional values as high normalized to its chondrite abundance. The rare-earth pattern as 0.710 (Steuber and Murthy, 1966). Ultramafic inclusions have a Coincides almost exactly, element for element, with the pattern found similar range as found in continental rocks of similar bulk chemistry for the Duluth gabbro. The total mineralogy of the composite analyzed Steuber Eli 21· (22·&11·)· Hedge and Walthall (9£.cit·) have interpreted is in agreement with the observed rare-earth pattern. older continental basalts as having come from the upper mantle and On the basis of these three aspects of total rock chemistry postulate a straight line growth of Sr87/Sr86 in this region from o f this series, an igneotis origin for these rocks is possible. The the primordial 0.698 value to a present day ratio of about 0.703. mineralogical assemblages observed in these rocks point to a later This would require a Rb/Sr - 0.03 approximately for the upper mantle metamorphic episoae that was of such intensity to give rise to basalt source region. Many oceanic tholeiitic basalts have Rb/Sr high grade assemblages consisting of garnet, hypersthene and myrmekite. ratios of about 0. 01 and Tatsumoto 32& 81· (1965) and Bence (1966)

suggest that the source region had a higher Rb/Sr ratio (to RUBIDIUM-STRONTIUM ISOTOPIC VARIATIONS IN THE PYROXENE GRANULITE FACIES give rise to the observed Sr87/Sr86 ratios) but underwent differentiation

Ca. 1 b.y. or more ago. general 3.1, Grant.tic rr,cks shnw mi,rh grpater vAriability in Sr87 /Sr 86 An hypothesis of wide acceptance in geochemical investigations

of crust-mantle systems is that the total earth is similar in initial ratios with values overlapping with basalts and as high as I compo- 0.734 and 0.741 recorded in the Heemskirk granite (Meier and Brooks,

sition to that of Type I carbonaceous chondrites (Urey, 1953; Mason, 1966). The majority of the initial ratios, however, are about 0.710

1960; Ringwood, 1961) which are more primitive in their chemical or less (Hedge and Walthall, 22·511·; Hurley 321. 1965; Fairbairi composition than any other class of meteorites. These volatile-rich 12& 81· 19648, 1964b). In view of the higher ratios encountered in and highly oxidized bodies are suggestive of a primordial material sediments (see below), Hurley &EL 81=· (1965) conclude that granitic

which accreted into a small body and was subjected to only mild thermal rocks are not formed entirely by selective fusion of reworked sedi-

metamorphism, less than 200'C, so that the more volatile elements were mentary material in a geosyncline. largely retaided. The present day differences in element abundances Sedlmentary rocks commonly have high Rb/Sr ratios so that high

and isotopic ratios in the crust are the result of an early differen- Sr87/Sr86 ratios are developed with time. Whitney (1964) and Compston

tiation of the core, mantle and crust. and Pidgeon (1962) report values between 0.710 and 0.720 for shales Cast and Hedge (1962), Faure 12& 81. (1962), and Walthall (1963) and Faure and Hurley (1963) have found a value of about 0.720 for a have suggested that the initial ratio of 0.698 + 0.001 determined composite of Paleozoic shales from both the east and west coast with for stony meteorites by Gast (1962) and Pinson .(1965), and a Rb/Sr ratio of 0.4. In recent oceanic sediments the detrital component Shields (1965) be adopted as the initial ratio for the primordial ranges from 0.709, the value of present day sea water, to about 0.730,

earth 4.5 b.y. (billion years) ago. Early development of the continenta] (Dasch t 1· , 1966). Faure &EL 181· (1963) estimate a Sr87/Sr86 ratio plates involving upward movement of strongly lithophile elements such of about 0.720 for the Canadian shield from determinations on fresh as K, Rb, U, Th, Pb, and Ba from the upper mantle has strongly depleted water calcareous shales. this region with respect to these elements. Further redistribution of : - g 0 -1Res 35 I M.iff PiT:ti. . 1. i 1 4 9 , .: :/ / •1 1 6: 8% 8.1 9: 149% 6/ii ;Pli# al#il:41. 2 : i f i i l l i l f j a M t.d, m t a l 1%81: 81138:1:8 *E F E: :9 N E: :1:t =s M #a - : :1 3 1 1 :U S E,6:3 8 : ,8 .3: '3: 4

i i i i 1 f 1s: lill#i ilm:K i i E11111 E f l i il-li l i i : d i ii lli l fi i :1, £i 'i Il l i;j li im- i i in"FIE".E 0 :AAAN 8 1 3: 3 4 4 11 1 4 - il i i. i j· 1 i ,: A E i i 4 1 #i i i 1· i : i .; * : i ' - - 4 9 # 9 ...... : .: I : : : : 6 : u : : : a A 3 : . . „:.76 mai i „ i ..5 3.. : : * : :8:8 3 8.2*1:E F E E% # :E f i l l i a 8 1 W i. :l d; : A 3 33 3 EL E % t S : 3 C 5 ' E. :t . .1 ' 3' . a 3 i t v m .; 3 . . * # .8 .m ;N E # t 6 3 3 3 .1 4 3. ,„4 3:1.2.3.E.I: : § *a)2:8: ;5" :1 5 0- : : .::0:00. .E w-1 3:2 8 0:1 5 1. :.: . 9 :J f te'Eifi:*filili Jili..2:Aligizi ':.lilI.2 ; 1.9 3 „ 3 .3.:. 2:1 1 43 : . ifi ' ' - ' ' ,i , /i s i:i 'i :1·· :- i f i· ' : : t i/f i t :i i i i , I ii , I I iii i a 3 8 5 6 0 3 N i 3 .9 : : a i# 4 # 9 1 i .i i 4 2 1 / i g · / : i / 1 ; , : . S. :E S 3 3 - ; · - 1 M 1 u M 3 1 1 41 0 i 1 1 : T . i 8 : M : 9 :8 : ' 'g 3 1 : m m ·§ M : E R : g . ; 1 F 1» - - < ' * & a 1 3 1 i J a il T . # 1 1 1 2 3 8 4 3 a ; i ; 3 4 a 8 :3 : 5 1 2 4 : 3 3 1 1 3 : a : : g :m:. ::.d:% : D Pe:Al ;Ell# #411 li#.83 :Iv:i Z:8812:: * 0 0 R WA . .44 . - ...B.. . I

m (0

1 . E ji : 2 '. E E .S : .5 „ s . . 2 -4 > W 6 :/1/4,1 Whi 5 I../.310,1 - r- . :A . S I ------562212§2 5%8 - 2---- 9 5 Z A*m gy -: ,£:MS 0' I :fs;111214:all 9 -1 00 5 6 6:4 1:6 6 1 7 6 4 8 q N; t: R : : 1.E * 1 * 1 3 : 1 1 6 1 5 6 8 4 2% M 0000 5 1 11 5 :. : 1 : : 3 3 1 8 #AlimmER Rimmi 1 *A) A e:A T E: : f i:j 3:2%:Il' 2: :3:i: " :5 1 1 4 u , .;1 . .3# . .I= ... i , iii „1 9 - . = . . . g: . : A 5.O. 1 9 8 8:0:0.0: 3 2 ... 1 : r. 8 WI : 0 9 : 2 s : . 0 0: : ; + 1 +1 + 1 IMiSM:.ENE:i:.&:5:gew.::. 1 4 I j & i i i i :3 1 - 1 2111 :3 1:3: i:% a . i 3-i : C 'IMIRRIM"In' M In - I 5 2 1 6 2 N * S i e: 8 & 1 3 3 : 8 5 1 1 i M : M i : -I H 0 ::j5ei1:E6i1 : :N. S: g .3 : 6 : 1 8 : I : 5 31 -C O 3 u / els:::i: mt jil# 1.5 51:il lilli .4 35 .1 -00.N .."'N....N . .O - BiB/J M i:i: / " :: ilifili liSMA i G E;i 41 :m a ·i j a i,I i :: 4 0 5 . : : a : : : : 3 : 1 : 1 : h N : a . 4 - : : 8 - e * E

1 .6 9/6/i m#m a g: .:::*98.:89: A . . : .6: N.M: i : 1 1 4 M a :2: f A d 3. . 3 & 4 4/ i m : Z 1 3 : 8 & i : 3, ..2 i : 1- : '3:Z: > .2 i i i i i 1 .1 : ti i ,i f ,i ·i a l= 1: ;A. §1 i w i 8 8 8: : 4.. H 2 2 3:a.:.. =i d e. . :8 - :1. 8 1* i j j 1 1 1 6 2 3 1 i latifi,14(" . . # . litilli";- . . . I I E . 1 a M i i. i N . # 1/ ' i' % /8 1 1 ::."...w.- .§ U 3 .S . E I. -i . #3 6 a i E s a / a j M I ...... 0. ... 64

rocks sucrose provided smoother emission during Evans concludes that the low (Sr87/Sr86) ratios for these slurry consisting of Ta + TaO + indicates a source region lower in Sr87/Sr86 than most igneous rocks, rubidium analyses and was used consistently throughout the investigation. and, further, if this complex ever contained normal crustal abundances Previous monitoring of .he filament for cleanliness was made prior to of Rb, it was removed within a short time (about 300 m.y.) after differ- addition of sample to the filament. Each sample, stored in a Vycor entiation from the mantle. beaker, was nitrated with 1:1 HNO3 (triply distilled) and a small

Mysore and Madras Charnockite Series. portion (on the order of 0.5 ugm) was evaporated onto the heated

Recently Crawford (1968) has presented chronological data by filament using a fine Vycor capillary. rubidium-strontium methods for the vast area of Peninsular India The calculation of the rubidium and strontium concentrations and Ceylon including several members of the charnockite suite from and the strontium isotopic ratios from spiked samples has been revised various localities in Mysore and Madras States: and brought up to date by Reesman (1968) following an earlier outline

of the method by Van Schmus (1966) and reference should be made to Tamizhagam, Nilgiri Charnockites and Gneiss: 2615 + 80(4), 0.7023 i 0.0012 papers for the actual details of the calculations· Madras City Charnockites: 2580+ 95 m.y., 0.7059 + 0.0042. these

Isotope Dllutton Analysis fur SLrutillum alid Rubldium:

The charnockitic gneisses of the Nilgiri Hills region give an age 84 During this investigation only one Sr spike was used for all slightly older than that of the Peninsular Gneisses of Bangalore determinations. The isotopic composition of this spike solution was

(2585 + 35 m.y.). The results obtained by Crawford (1968) will be measured several times by different analysts and using different discussed further in the light of the present study of Mysore and mass spectrometers. Two different shelf solutions were used in the

Madras States considered below. calibration of the rubidium and strontium spike solutions:

3.3 Analytical Procedures

All rubidium and strontium determinations were made by mass RVS CMS Sr 60.74 ugm Sr/ml 68.16 ugm Sr/ml of spectrometry in the M.I.T. Geochronology Laboratory. In the case Rb 101.8 u8m Rb/ml 80.35 ugm Rb/ml strontium, isotopic ratios were obtained on both spiked and unspiked samples, the latter in cases where no rubidium was detected by X-ray Both shelf solutions were prepared from carefully dried and accurately fluorescence (detection limit 5 ppm). weighed Johnson-Matthey 'Specpure' RbC12 and Sr(N03)2

In general, the standard analytical procedures of this laboratory The isotopically-enriched Sr 84 spike obtained from Oak Ridge were used throughout the investigation. The procedures used for sample National Laboratory was found to have the following average isotopic preparation, calibration and use of spike solutions, and method of composition: calculation of the isotope dilutlon ratios have been presented in Sr86/Sr84 = 0.04685 84 detail recently by Reesman (1968). SrB'/SK = 0.01574 5*88/5,84 = 0.13875 Mass Spectrometry:

During the course of this investigation only one mass spectrometer, The concentration of the spike was determined three times by different designated "Sally", was used for all the isotopic measurements. The analysts prior to and during the course of this study: instrument used was a 6-inch, 60° sector, solid-source, single filament, Nier-type mass spectrometer utilizing a Cary model 31 vibrating ugm Sr84/ml Analyst and Date reed electrometer (VRE) to amplify the ion beam current. The VRE 1.948 output was recorded on·a Brown strip chart recorder. The vacuum for P.M. Hurley, 6/29/67 (Connie) 1.94 C.M. Spooner, 7/12/68 (Sally) the system was attained by means of a mercury diffusion pump backed 1.941 R.S. Naylor, 1/68 by an oil-seal mechanical fore pump. A liquid nitrogen cold trap was provided for cryogenic pumping of condensibles between the diffusion Rubidium was determined by isotope dilution using three spike 87 pump and the mass spectrometer analyzer tube. Pressures of less than solutions isotopically enriched in Rb provided by Oak Ridge National -6 5 x 10 un Hg were found necessary for successful rubidium analyses Laboratory. -6 and lower pressures of less than 1 x 10 mm Hg were needed for strontium ugm Rb87/ml Analyst & Date Spike Solution ugm Rb/ml runs. Throughout the study peak-hopping was used to obtain the isotopic 65-A Rb87Spike+ 11.304 11.210 PMH 4/11/67* ratios after continuous magnetic scanning over the appropriate mass ** 65-A Rb87Spike 11.458 11.363 CMS 6/20/68 range was made to check the base line to determine whether there was ** 87 65-A Dilute Rb Spike 3.467 3.438 CMS 6/19/68 a pressure broadening effect which would seriously limit resolution. ** Rb87Spike++ 9.819 9.742 CMS This scanning also provided a check for contamination by rubidium

85. during strontium analyses since there should be no peak at m/e = +Rb85/Rb87 , 0.00841 (PMH, Connie) " Tantalum ribbon filaments 0. 020" wide and 0. 001 thick were used 4+Rb85/Rb87 . 0.00789 (PMH, Connie, 9/19/68) * analyzed on 12" mass spectrometer "Connie" in all the analyses. It was found that a thin layer of an aqueous ** analyzed on 6" mass spectrometer "Sally" 1A 3 : 31: A + ,r 5'' 92::6::*%0938:g::St:: n ir, u CON. N , #3=11'ip- i:-,3#: -:Kii, * 222222222222222222222 550:83:------. 01:89:#:ae:=81: ....-21..:i ...00.00.00....00.000 ...... 0 1 586ei:,£e:§... :a°.S:g § E S E 1 f i .1 1 8 5 1 ': .: 3 1 . . i :11315- 01 . 53 : SPRS,Efi :1431: .a .: 5 1... 1 f% 8i. S 8i:i 3.3 1 8' . . U m'i 0 0 ..... S E S 3 2 :3. 6 3. i '5#1 5 I. 8 - 2 1 1,viwialimpliffil;Aff - .13.. 2%$5251%4=:%321=5:=:m- =-»= -„.. I I -=-0-0- . -=I --,=-„9.- I .2 % --'-=* -- -' -I=-„-'-„-'I„„-'. :Zmzz=: 12 m i l: ,1 9:3* :1 1:3 1 1 2& ,1 1 a r I . .4-*..I : i:A: .:9.:89-:58:3: W: ...... 0000006 I & I w v . :1 * :3 8 1:r: - : :3:1.8: :9 8: "f : . 2 me 1 1:8% <2:a,e: : %2 1 5 5:1 4 1 1 3 > F 3:M J# #i:J E: '1: ;6 1 7:1: 5:3:0#15=Gi:GS:G:$255 : 5.- 5 5 2 5 I ------%-%------%------. 5 1 8 & : · . 5 8 : I : ' : I : I .3 : I : 2 - 8 1 3...... 3 . „ 00 : e : g 9 2 r :* 1 3 :E : : 18:::1111:2%#; lf 1 1 1 +'+' U a :.REE:::--:-:.6.:5 8:9 0:8:a e: :2:9 2 1 I a 1 :0:3/ : : -1 . ,0 .:: 3 ;P : 0-k· 0 5. i5 m f · i ai#1 g * 1#i· 0 8i =i '.i l l.1 . . 8. - 11M 3 i M S.< 52 8 3: '26 2 1:E 1"4 : C : 5:U:i . i 0i i:i:5 m; 5 f ai gt ..5.:Ca 3: Cn 9:8: .5:a s t l e m l: :r·1 - E- . . % 3 1 5 : : -:1 3 :3:3 8: O:I r m. = : :3 1 : 3:3 - 7 E. ..011:1 . g q : 9 1 m:E33/8:a:0313#j: a :a: Le I % „ 8 " . :Emnaliaral::.::8% . - . s as: liti:; {ilia . 2 2 : 3 0 3 . X. : 9 .0 90 <::.3:... iii :m::1&EiS:34E:, a .„ 3: pgrs g: : :11:21r: . 3 :Al 0 * · s: ". ila. Miulli. 8 i I i f 1 i e e. 1 # 5 i .5 e ..i . · m » : E-3. . : .9 .M:g m: 3: :5:3:1: :i/ :3 3: :2 * : 1 2 9. 3:1 j: . a. # 1 g 5 l aa i :e:0 a 2E 3 .i :1. i 1: a- 1: L i v a l a m;i 1 4 7 e i;i l i l i a,.::J rl Miffijilt,921*14111 I -0 H :11 fili:/ f i 91&;li e m i a # . m illig :. i : l i t : ; t: 1 5 21 *5 1 l a i ':Mi l a i : : i f I : 1 -9 8: . " :N E F. § . / 3:3 5 5 13 SE,353331aiNNEE 4%:0 9 - 1 .U1 E . i ui i# n. i 9 m i 1 9 3 1 i 1 1 .

E 1% . . 9 0 0. 3 3 : M e g M i .#09/2.*518'i/id. - :: ... t::Al 8 8.5 2: . :1 0: . . . "H e M'1 1- 'W: : : .2:23::: .033:ty N:j1g/g 1 :i f 2:1::ill:,imia '111338:<: Sm:i:ZEAga 15:351.4 li 911:4:i:EmiliA:jili: 1 1 8 : 5 : : 2 :. 1 : 0 : . 1 8 0 . : - I i 1 6 : : 1 · : I . 8 3 1 8 8. : El:iltii :i @:2:*116;1113:111I - . . . . .6:5' . : . MUBE:i:" 1*4:9: % *8: #8: " 2. ,f:g a i f i t: : .6 1„a g i r t# : ./.-'::48 5, :/1&51:aggle,Wi: -:d: 55:2:3:. 8 : 131:lgisili;Imemili;itil.jaildilfifilill 8:f i a a:1 9/ :1 . : : 3. : : < : .S M: . : . . : . : lizittil i ll# difiti:ijfitiligial:Mili ilitii#11:1; H C - f i l:8 9:i: : .g l e 5 1 8" -4 1. .9 8. -8 2: . .L E. K. : :··.1%:11 B 111, :ixiti i:,111111;:ili:i:tii:;11,1,&;Ilii M: .i. .S A T E 1 4#M 1:1;E : . a 6- 1: :1. ; . . 3. ; :i &3 5 1,i l m.i m. § : .: :w it ii :..f i ; i ;I i 1i# i w i i i ii f i i .1 l: i : i 1 l i i -1i)1 i i 1 a 1i : 1 i 1 i 4i 1 a i if i·i.j.1 : '; : i .i i i l ii R:f: M i.i.i a i· fI: t 5i 1f i1 i i l : i: 1 8 3 2 1 3 M:8:5 7 2 'X g l;a u: 3 ..:2... .26:a:B:6::313:5.:22:: : i i I j f i i i 1 1 1 1 8 i f i i i i j f : 3 3 ; f i i f 1 1 ; i 1 5 1 * 1 . i; i i : 5 i , g i #I l i, 1 i i 3 ::815:3:=:80*1 : : a ; : :i i i f I Y 5 1: % ,S i l i i;A l i l l' i l i i, 40 'D i - s. Za :.am i S gim e " : 8e 6f 1e 4.S * :a :8:l.a ° l.E l, l 31e: 1:8 5:g:l' :a. i: a. 8- 1 %%t : i „ i:t 3 5 3:5 1:5:1 1 1 6 9 4 1: #1 %1:1 1 -" 4:C : 2 . 4 M ; . 1 1 3 j 3 i :8 1 ; 3 3 w N i. i : 5 M : :g E : :. 1.2 U. 1 .ir WME tas MHHM :i m f i i f i l : :f i t i l i f f'm i#i l# .M m i l 5 i 3: ; I: ;3 3 3 33 3 :; 1 :+Ii 2 U 3 1 3 i i 1 ': :: Mi . a i i 1 1 1 5 2 : 3 1. 1: 2. /1 - . : =8: ,2 5 0.* .:SE:3 min,8,I:,EMa )i l i H . i 1 1 1 1 2.W %6 1 4 6: % :i m i:f:i i:t i i:i.f i l l

5 8 1 3 92 g 11 11111 1 9 1 1 : i i , -i iCO 4 #3 1 4 1 1:i w i l a : 1 1 s i t i, i; I 4i:N 2: 1 .90 11 14 61:.Af i j K.i 8 1 1 0 :S:i:t:,114:i; 1 114 !i f a. i l i; : : :i:j i t a l. 9 W5 a c i a w lag awilwil . :i m: . :4: :l a* :1:1:8: ·5: . :9·E a d e:5: I: . . 0 0 0000. m O : a 444; : ™NHnni O N:4 4,Kil 0e:dM -1 u id si K i E.Nl l i mi,5· i a" :B'U·l,8,1 §1 :a·1 5:'4.E " G:1 E;1# .1 :l 4 a.3. g i.e: i a 1. 3 eu %...=.. " 52j 3 : 3 1 : .3 1 1 : : : : : 1 3 3 :1 : ..:0 9 a 2 1 1 & 1 i : 1,1 5 1. i ji4;i i11:itif1i;iHtF'..I'll:fia,1,13; a:i 219£1 HRHE; g a l i j a:F l a a i l l i#F l i g g i e,f:i l i i l i 8 : .... * . 7 4 : 6 9 1 2 * 1: S A 1 4 3.: ::331=35.EW#,W glil 1 AS :0002:3 N e. : i :g: > 1:8:594.::,130...... 67

FIGURE 7 it strongly resembles charnockite from the type locality in Madras."

Specimens R7050 and R7051 are of this rock type. Above the charnockite RAKOSI SKETCH MAP is a coarse-grained gneissose aplo-granite having a slight foliation of OKOLLO EQUATORIAL EAST AFRICA similar to chat of the charnockite. ",4,93.t. the above lithologies is a "contact facies" (Temperley, 44, with Simple Localities 2 Between o. . . .9, 1938) where the foliation is much stronger as the concentration of Statute Miles closely-spaced biotite bands increases (R7053). HCLT Hepworth points out that tlit magmatic intrusion of the gneissose 0 * aplo-granite is cle*ly separate from the metamorphic event which 4 LAKE ia raised the charmockite to the present granulite metamorphic facies. VIC,DRIA - The later intrusion and folding of the aplo-granite produced rods and 1 ./JitVO/ "= mullions about a southeast plunging axis (see Figure 8) and imparted / s &444-

- 1/ 2 2 .3" / 27/97 3 1-1 ' PARE MTN 3.10 Discussion of Results 1 S - . 2,0# LABOR 4 4 SERR T All six samples submitted from this quarry were analyzed for Rb rf. Ff):95.. and Sr by isotope dilution methods and the results are shown in Table # III.G. The S/87/Sr86 - Rb87/Sr86 plot is given in Figure 9. 3 F 4 4... It is evident from the scatter of points in Figure 9 that the Mpw. 4 Isog. DAR specimens analyzed did not remain closed systems with respect to Rb ES SALAAM e:/% and Sr throughout their history. This is not surprising in view of

36 38 EL the extreme cataclasis and deformation to which these rocks have been

subjected.

Two previous age determinations reported in Cahen and Snelling

(1966) from this same quarry also give disparate results. Kulp and

Engels (1963) using biotite separates for both determinations obtained

3.9 Msagali Charnockite Quarry, Tanzania (6' 22' S. 36' 17' E) an age of 475 + 80 m.y. by the Rb-Sr method and an age of 3600 t 100

This locality, situated in central Tanzania between Dar es m.y. by K-Ar analysis. Salaam and Dodoma in the south central portion of Quarter Degree

Sheet 163, has been restudied by J. V. Hepworth of the Institute of FIGU RE 8 Geological Sciences, London, during the course of an investigation

into the nature of the boundary between the Mozambique Orogenic Belt

and the Granitoid Shield in Tanzania.

The term "Mozambique Belt" was introduced by Holmes (1948) to Gneissose Aplo-granite describe a younger series of biotite gneisses and migmatites containing TZ 163.11E contact facies later pegmatite phases which cut across older east-west striking rocks TZ 163.3ON 1 TZ 163.11F of the Nyanzian, Dodoman and Kauirondian Series. Holmes (1951) was 0 1 Ch ': of the opinion that the exposed rocks of this belt are the uplifted E )W and deeply eroded core of an orogenic belt. charnocklte

The disused railway ballast quarry is located in a transitional TZ 163.30.1 TZ 163.11B region between the Granitoid Shield and the generally north-south (approx.) trending Unsagaran System of the Mozambique Belt, that is, between TZ 163. R

the sheared edge of the batholithic granite and the fairly high grade . 100, metasedimentary rocks (epi(?)-amphibolite facies) of the Unsagaran

System which at rhis locality have an unusual eaot-west trend.

Within the quarry, the charnockite has been described by Hepworth MSAGALI CHARNOCKITE QIIARRY. TANZANIA

(personal communication) as a "very dark-coloured, resinous-looking after Temperley (1938) with notes by J. V. Hepworth. Folding and rodding as a ...dt of Mozambiquian deformation at top of rock, either dark brownish grey or dark blue depending to some extent section. Looking south. upon the light and the freshness of the surface.... In hand specimen 68

TABLE III-G

MSAGALI CHARNOCKITE QUARRY

Isotope Dilution Sample Rb87/Sr86 Sr86/Sr88 (S*87/Sr86) Rb(ppm) normalized Sr(ppm) Rb/Sr

R7050/ TZ 163.11B 0.4405 0.1181 0.7204 66 434 0.152,

R7051/ ' TZ 163.30J 0.5039 0.1190 0.7237 71 410 0.174

R7052/ TZ 163.3OR 0.9789 0.1195 0.7430 88 261 0.337

R7053/ TZ 163.11E 0.6056 0.1229 0.7367 78 376 0.209

R7054/ TZ 163.3ON 0.5217 0.1190 0.7308 46 253 0.180

R7055/ TZ 163.11F 0.968 0.1195 0.7361 81 242 0.334

R7057/ TZ 163.73* 63

* charnockite 5 miles west of Msagali.

FIGURE 9

0.820 TANZANIA MSAGALI 0.800 (S,87/Sre = 0.705 (assumed) 3000 my. 0.780 . 2000 m. y.

0.7,0 . .R7052 0.740 R 7053 1000 m y. R7054 0 - R7055 /R7051 0720 • R7050 -

0.700 lilli l l lilli 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Rt,BPS,86 69

From the array of time lines drawn (Figure 9), assuming an The Granulite Group has been recognized in this region on the

initial S,87/S,86 ratio of .705, a crude "age" of about 2500 m.y. basis of the typomorphic mineral assemblage peculiar to the granulite

is obtained. It is interesting to note that there is a reversal in facies defined by Eskola (1952), despite the retrogressive metamorphism

the usual trend of ages obtained by the two dating methods. The Rb- that has obscured the primary texture. As in the type locality in

Sr Whole Rock analyses confirm the extremely old age found by K-Ar Madras State, India, a wide compositional variety of chat:nockites,

during analysis and point to a disturbance of the system probably enderbites, and granodioritic charnockites are represented here.

the Mozambiquian Orogeny. The fact that a Precambrian age is hinted However, the compositional variation is expressed in the Group as a

at may indicate that these rocks are in part material of the older whole in the form of a large-scale layered series and not in a single

Precambrlan Massif to the west that have been caught up in the orogeny. outcrop, which tends to be rather massive, lacking compositional

formerly As pointed out by Holmes, these rocks may well represent bonding. Individual layers are generally several hundred yards across

deep-seated blocks brought to light as the result of orogenic activity strike. In addition to the more massive charnockites mentioned above,

and subsequent deep erosion. more magmatic and mobilized types are present, especially in the south-

3.11 Okollo and Rakosi, West Nile District, Uganda east portion of the Luku Hills (Figure 10) where there has been apparent

These localities, 2' 30' N - 3' 00' N Latitude and 31' 00' - increased mobility of the felsic fraction, forming an incipient agmacite

31' 15' E Longitude, northwest of the headwaters of the Albert Nile, mingling with angular blocks of dioritic charnockite.

are part of a highland region resulting from Pliocene and Pleistocene

rifting.

Originally assigned to the "Basement Complex" in A. W. Groves' .

(1935) original study of the crystalline gneisses of the region, a

further division of these basement rocks into three main groups

below the Kibalian Croup has been proposed by Hepwurth (1964). The

highest melwuurplilc grade present ls the Granulite Group which is

considered to be the oldest exposed, and was affected by the earliest

tectonic phase preserved now in blocks and cores unaf f ected by - JA ib W#4:,4/6 - -= 68*0''_Ic.../4/ ./Al later tectonisms. Also present are the Western Grey Gneiss t 1 =-5-/--1 /3(,3 .-- / 7./..=- (amphibolite facies) and the Eastern Grey Gneiss, the latter being

slightly lower in grade. On the basis of V 'Z-, '»'*QILL 1 AJ I Macdonald' s (1963) termi- nology in northern Uganda, Hepworth has accepted the . terms -9 8 1 4 1 _1 0 "Watian", I 1 0 "Aruan", and "Mirian" as approximately synonomous with his "granulite 11 Group", "Western Grey Gneiss Group", and "Eastern Grey Gneiss Group" - Undifferentioted -r#M in the West Nile District. By means of photogeological interpretation - -1 1-1-- 1 - - j 1 , these groups appear to extent throughout northeast and north-central -/411.tr'll-/1

:::is.th:,1::s:ejr .'::1:Eri li :e:ti :1::st::.12: : indthri olded SAMPLE LOCATIONS *osed On region. It has been suggested that this event may be correlated with ...... 1 11 the Mozambiquian event to the south. The Kibalian Group lies union- GEOLOGICAL SURVEY OF UGANDA 1 1 R.'-TO G•0*A. V H. 0.. formably Above the previous two groups and znnsists in the main of -.- 9 ...1 , 1...... t-ZZ 1 hornblende schists and amphibolite. The Kibalian Group represents •'.' ,. L_-- OKOLL...... MOSIScot.: 1:100.000 . -- 111 the lowest metamorphic grade mapped in this region (greenschist to Tert; / y r--1 'n:.rm'dill. C- rfl Undifferenlic ed LJN.-,;'. 1 11 1 -1 '., epiamphibolite facies) and is the least affected by tectonic events. 1=.....1, f 1 Gron.lite retro- =1 '.... All the groups in the region have been subjected to pervasive G.onul'. -' 1 fi

gressive metamorphism involving the unmixing of the feldspars, con-

version of the pyroxenes to biotite + garnet, amphlbolitization, and

recrystallization of the fabric of the original rock. The retro-

involving gressive event may be related to a later complex tectonism Ilill/4 - Ter,-1 7827 , northwest-southeast structural belts such as the Aswa Zone in the ..1 1.*: r 1 1) 61« 1 Nile. The two are /40'H Southeast and the Madi Belt of the northern West ,) 1 - - FLOKOLLO collectively placed in Holmes' (1951) "Chua Orogenic Belt". The ... 3-,A complex etructural relationships are discussed by Cahen and Snelling L urizz ---7,-

(1966, pp. 44-45). 70

In the north part of the region mapped, at Rakosi, an aplitic The Acid Granulites comprise the remaining suite sampled in granulite characterized by a leucocratic medium to fine-grained texture this study from the Granulite Giuup. These include rocks having a and a flesh to pink color has been included as an integral part of modal composition varying from quartz-rich microcline-oligoclase- the Cranulite Group. Similar associations of this rock type With biotite granulites and biotite-rich quartz-microcline-oligoclase more intermediate charnockites havebeen found throughout the world granulites. These rocks prpsent some difficulty in unambiguous

(i. e., the Madras State Charnockite Series) . The aplitic granulite assignment to the Granulite Group primarily because their chemistry is commonly interlayered with intermediate charnockite forming a precludes development of the granulite facies mineral assemblage so distinct junction with a zone of transition less than an inch wide, that at times it is difficult to distinguish them from the lower grade suggesting that if their juxtaposition involved intrusion, the temper- Grey Gneisses. At Goli Hill,.about two miles northwest of Okollo, atures in the adjacent bodies was not widely differant. In the Luku intermediate and basic charnockites are gradationally associated

Hills, large and small scale, apparently conformable, layering of pink with the acid granulites and thus provide justification for their aplitic granulite and intermediate charnockite has been noted by inclusion into the Granulite Group. At this locality there exists

Hepworth. Local variants of the aplitic granulite have been found a gradation from basic charnockite through the brown to grey inter- to contain the assemblage hypersthene + diopside + garnet and is felt mediaze types to acid granulites and as rh* proportion of mafia to unite the two major units within the group since it indicates that minerals decreases, the rock assumes the typical aplitic granulite the aplitic granulite also underwent metamorphism up to granulite discussed above. Hepworth has suggested that.this represents a

differentiated series although this is impossible to show when mapping grade.

Hepworth noted that the aplitic granulites become relatively at a scale of 1:50,000. scarce where the associated charnockite becomes fine-grained and ' 3.12 Geochronology banded, approaching a pyr..... granultte or "para-charnockite" following The structural complexity of the region brought to light by Parras' (1958, p. 55) nomenclature· It has been suggested that the field mapping shows that the relatively few ages available are at aplitic granulites occur in loci of higher temperature, involvin8 present insuff*cient to completely reveal the metamorphic and tectonic partial melting. The absence of biotite in the aplitic granulites events that have taken place. A characteristic of many of the radio- tends to support this since the partial pressure of water vapor in metric ages determined in this region is the surprisingly young age the high temperature regions would be lower. (see Table III-H) of about 575 m.y. for many of the intrusive rocks.

TABLE III-H

Age Determinations from the Basement of Uganda and Adjacent Parts of Kenya (from Cahen and Snelling, 1966)

SAMPLE LOCALITY AGE (m.v.)

Biotite Granite dike cutting basement acid granulites; 0.3 mi. E. of Kaabong rest camp, Uganda 515 I 18 K:Ar

Whole Rock Granite dike cutting basement acid granulite; Kaabong area 565 + 20 Rb:Sr

Whole Rock Granite dike cutting basemenE acid granulite; Kaabong area 565 t 20 Rb,Sr

Whole Rock Granite dike cutting basement acid granulite; Kaabong area 565 + 20 Rb:Sr

Biotite Pyroxene gneiss, Basement Complex; Nakothogwan Hill, Karamoja District, Uganda 575 + 20 K:Ar

Biotite Hornblende-biotite-pyroxene granulite, Basement 540 + 20 K:Ar Complex, Obongya River, E. Acholi, Uganda

Biotite Mesocratic sheared gneiss, Ogom (Basement) Complex, 1 mi. N. Gulu-Kitgum road, Acholt, Uganda 650 i 25 K:Ar

Fuschite Western Grey Gneiss quartzite, Basement Complex, Kango Hill, 2' 36' N; 30' 51" E. 540 t 20 K.Ar

Biotite Charnockite, Granulite Group, Basement Complex, 1 mi. north of Okollo 2° 41' N; 31' 09' E. 660 + 25 K:Ar

Biotite Basement Gneiss; 20 mi. E. Fort Portal, 1 mi. W. Butiti quaruite, Uganda 1040 I 40 K.Ar

Whole Rock Rakosi and Okollo Charnockltes (This Study), See Figure 10 for localities of specimens and Figure 11 for isochron. 2629 t 117 Rb:Sr 71

Both K:Ar and Rb:Sr ages along the Uganda-Kenya border from micas To the west in che Congo, Kibalian Group granulites within and amphiboles of country rock gneisses are considered anomalously greenschist to epi-amphibolite grade hornblende schists give ages young (430-660 m.y.) since reliable ages of 655 m.y. and 620 m.y. within a range of 1725. to 2075 m.y. averaging 1840 m.y. by both the have been obtained by the U:Pb method for the Morukong and Kokusan K: Ar and Rb:Sr methods. Complementary model lead ages have also been pegmatites in Kenya. These ages set an upper limit of ca. 650 m.y. obtained from two galenas from the Kibalian. The age of about for the enclosing gneisses. Although the interpretation of these 1850 m.y. is considered to correspond roughly to the post-tectonic anomalously young ages is still somewhat speculative, it appears stages of the major orogeny. likely that the ca. 650 m.y. age is the younger limit of deformation In the southern Congo, at Kasai, the Dibaya basement is and metamorphism in the area and is roughly synchronous with the considdred to have undergone granitization abwt 2700 m.y. ago

Katan8an Orogeny of Central Africa. Ages younger than about 650 m.y. (Ledent and others, 1962; in Clifford, T. N., 1968, p. 383) and a by both the K:Ar and Rb:Sr methods may represent blocks that remained Rb:Sr microcline age of ca. 3310 m.y. has been obtained from a isotopically open until emplaced by epirogenic uplift into cooler pegmatite in the Luiza-type basement further south. regions in the early Paleozolc.

TABLE III-I

OKOLLO AND RAKOSI·, WEST NILE DISTRICT. UGANDA

Isotope Dilution Sr(ppm) Sample Rb87/Sr86 Sr86/Sr86 (S/87/S/86) normalized Rb(ppm) Rb/Sr

R7042/54 0.239 0.1207 0.7159 43 521 0.082

R7011/23 6.503 0.1202 0.9068 163 74 2.191

R#012/24 13.241 0.1199 1.1969 135 31 4.366

R7018/30 14.960 0.1192 1.3158 303 62 4.879

R7020/32 8.003 0.1220 1.0081 207 78 2.668

R7038/50 0.096 0.1199 0.7082 19 68 0.033

R7049/61 0.125 0.1189 0.7117 19 434 0.043

R7027/39 0.181 0.1212 0.7119 38 611 0.062

R7039/51 0.074 0.1220 0.7134 19 730 0.026

R7040/52 0.084 0.1191 0.7066 19 642 0.029

R7019/31* 16.024 0.1196 0.8008 371 68 5.486 * this alaskitic gneiss from the Rakosi area gave an anomalous result compared with the ten analyses above. The age obtained from thls one specimen assuming the initial ratio of 0.7049 determined above gave an age of about 425 m.y.

FIGURE 11

- - 1 1 A00 l i l l i

1.300 -

RAKOSI AND OKOLLO (inset) 1.200 - WEST NILE _ CHARNOCKITIC AND GRANITIC ROCKS AGE=2629*117my

1.100 - A. 1.39 XIO-"y-' (Srs'/Srelfo.7054 + 0.001 SBPSr86 1.000- -

.720 - ' ' ' 01(OLLO

.900- :'q no-

800 - /

m. . .15 20 me,

1 1 1 .700 "'""' 2 4 6 8101214161820 Rbm/Sr C: 4 .72

3.13 Labor Serrit and Pare Mountains, Tanzania Age (Sr-87/Sr86)0 Labor Serrit and the Pare Mountains are located in the north- - Pare Mountains 927 63 m.y. 0.7056 0.0011 east corner of Tanzania near 4' S Latitude at 36' E and 38' E Longi- Labor Serrit 724 f 8 m.y. 0.7064 0.0001 tude respectively. These areas form part of a vast and poorly under- stood complex of granulites, which are suiwunded by volcanics and Thc data are enn few to come to any firm conclusion regarding the amphibolite facies biotite and hornblende gneisses. The granulites meaning of these two ages. It would appear, however, that these form gently plunging and dipp ing stratiform-like sheets that form results indicate an age older than the "Mozambiquian" orogeny that a major structural unit that appears to have undergone only a minimal took place between 450 and 600 m.y. The possibility exists t*at the amount of deformation or metamorphism. These granulites have been granulite belt studied here may be an eastern extension of the regarded as belonging to the "Mozambiquian" and ages in the range granulite and amphibolite facies rocks of the Tanganyikan Shield 450 to 600 m.y. were suspected (J. V. Hepworth, personal communication). that underwent a later episode of metamorphism. Recently, however, there hai been speculation that these rocks may be related to the older Tanganyikan Shield to the west.

In rhe presenc study, five specimens of pyroxene granulite from the Pare Mountains and three from the Labor Serrit area were provided for analysis by nr. J. V. Hepworth, Institute of Geological Sciences,

London, Thi results nf these analyses are shown in Table III-J and

Figure 12.

TABLE III-J

PARE MOUNTAINS AND LABOR SERRIT, TANZANLA

Isotope Dilution

Rb (ppm) Rb/Sr Sample Rb87/Sr86 Sr86/Sr88 (S/87/Sr86) normalized (ppm) Sr

Pare Mountains. Sheet 73:

R7224/73 1 ---- 0.1209 0.7054 nd*

R7225/73 2 1.473 0.1194 0.7259 51.7 101.8 0.5078

R7228/73 11 0.0201 0.1200 ·0.7083 2.55 366.6 0.0070

R7229/73 14 2.335 0.1192 0.7347 59.4 73.8 0.8046

' R7230/73 15 0.9397 0.1201 0.7111 42.9 132.2 0.3245

Labor Serrit:

R7231/86 24 0.1208 0.7065 721.6

R7233/86 27 1.0734 0.1202 0.7173 82.4 222.5 0.3705

R7236/86 32 0.0525 0.1201 0.7069 7.28 401.1 0.0182

*not detected at 5 ppm level by X-ray fluorescence.

FIGURE 12

0.735 - R7229 PARE MOUNTAINS and LABOR 0.730 SERRIT TANZANIA A = 1.39 x 10-"y-'

R7225 0725

< 0.720- U) E A r U) R7233 0.715-

• R 7230 • PARE MOUNTAINS, AGE = 927 f '63 m.y 0.7lo 7228 (Sr87/Sre l = 0.7056 i 0.0011 A LABOR SERRIT, AGE = 724 + Bm. y. 0.7051-R7224 (Sr 87/r850 .07064:10.0001

0.700 0 0.5 1.0 1.5 2.0 2.5 Rb87/SreS p=... 73

3.14 Adirondack Highlands, New York FIGURE 13 In the present study, suites of pyroxene granulites were collected from two localities in the Grenville province (Figure 13). Although k the rocks in each of these areas have developed the requisite mineralogy for classification in the pyroxene granulite sub-facies, totally different origins have been suggested for them. Simply stated, the anorthosites and pyroxene granulites of the central Adirondacks have --77 ... r, . T been interpreted by Buddington (see below) and others to be of 0 1' »:ZifI E- . origin, whereas the granulite terrain o f the Westport area has been 1 13 5392.4* .„ 49 ·, . interpreted by Wynne-Edwards (1967) to be of sedimentary origin, that is, the granulites are a sequence of metamorphosed greywackes. A comparison of the Sr87/Sr86 initial ratios have been made to see whether . .1 r,&Wor,7944\A„,1 67<0 r-* * 4 a distinction between the two suices can be made.

The Adirondack Highlands r which form the southern mousion of rIE) » 97 " -_,/ .„„i„v„...'„.."c .alt„.... d,„ .„„.„c.„,o. i.".„'c:I :.1 - of an anorthosite core associated with pyroxene granulite, granitic, syenitic, and gabbroic variants. 1 The gabbro and anorthosite account WESTPORT ONTAR 0 & AD WOMACK H GHLANDS N Y for about a third of the rocks which were classified as igneous by SAMPLE LOCAl TES Buddington (1939). A review of the literature pertaining to the origin of the anorthosites in the Adirondacks and other localities has been given by Heath (1967) who, in addition, has also commented 87 86 on the various hypotheses in the light of Sr /Sr initial ratios.

In close association with the anorthosite is an extensive series of syenite and granite belonging to the hornblende or pyroxene granu- 3.15 Geochronology lite sub-facies. It is generally agued (Buddington, 1939; Walton Considering the vastness and geological complexity of this area, and deWaard, 1963) that the rocks of this area are of plutonic there are relatively few age determinations available. Such infor- igneous origin but have undergone subsequent metamorphism which mation, however, may prove extremely useful in the interpretation culminated in the Grenville orogeny. The relationship of the of the pre-Grenville history as well as later metamorphic overprints "supracrustal" Grenville metasediments to the underlying igneous that may have occurred. rocks has received various interpretations. U-Pb determinations on zircons from two Grenville basement Buddington (1939, p. 197 1 EL li L.) and earlier workers (Cushing, syenites in the Adirondack Highlands (Silver, 1966) give an age of 1917, p. 507) favor an intrusive origin of the plutonic complex into 1125 m.y. the older Grenville metasediments, followed by later regional high- Hills and Gast (1964) have reported a Rb-Sr whole rock age of grade metamorphism during the Grenville orogeny. An opposing view 1092 + 20 m.y. ( A = 1. 39 x 10-lly 1) for pyroxene-hnrnblende granitc regarding the intrusive character of the plutonic mrks is considered gneisses from the Lake George Village pluton. Also included in the below. isochron were two samples from the Ticonderoga area which have a On the basis of extensive mapping of the relationships between similar mineralogy and are presumably related to one another. An the meta-igneous series and the supracrustal units, Walton and deWaard analysis of two feldspars from a pegmatite in aluminous paragneiss (1963) have observed remarkable continuity of a single marble unit gave an age of 1060 + 75 m. y. with an initial ratio of 0.7159. They in contact with the anorthosite, "charnockitic'Lquartz-syenite- noted the similarity of these ages with both K/Ar and Rb/Sr ages gneisses and granite gneisses. This continuity persists for over 80% reported by Doe (1962) from other areas of the Adirondacks, and of the exposed length where the contact between the two units is from the Grenville province of Canada (Lowden Slak·, 1963). Since exposed. They account for this remarkable continuity as the result the initial ratio of the granite gneisses is well within the range of the deposition of the marble as a basal supracrustal unit followed expected for granitic rocks (0.7058 i O.0010), Hills and Gast by the succeeding units which also show a similar stratigraphic conclude that the age represents the age of emplacement or if this coherence upon a complex older basement of the meta-igneous rocks. is indeed a metamorphic age, the body did not have an extensive Subsequently, both basement and cover rocks were involved in an intensive pre-Grenville history because on isotopic re-homogenization a higher deformation involving plastic remobilization giving rise to the complex initial ratio would be expected. structural picture now observed. It appears that this later regional

metamorphic event, the Grenville orogeny, gave rise to the pyroxene

granulite sub-facies assemblage. -

Heath (1967) has determindd a Rb-Sr whole rock isochron on Later it became apparent that within the pyroxene granulite itself seven specimens from a large pyroxene-hornblende quartz syenite there was a "dark" aid a "light" A...p which 81 ffered from each body north of the main Adirondack.anorthosite massif (Figure 16). other in chemistry as well as mineralogy (P. R. Whitney, personal

An age of 1055 + 31 m.y.(A- 1.39 x 10-lly-1) was ob'tained with communication). The present study of the Rb and Sr isotopic Fatios an initial ratio of 0.7060 t 0.0004. The age and initial ratio are confirmed the presence of · two distinct populations. The analytical in good agreement with the determination by Hills and Gast discussed results for six specimens are given in Table III-K. Based on the above, and Heath has suggested that these may be comagmatic. six analyses, an age of 1336 * 71 m.y. was obtained with an initial

In the present study, specimens from the central Adirondacks, ratio of 0.7025 I 0.0024. The close grouping of the analyses about

two points (at Rb87/Sr86 - 2 and 3.5 approximately) accounts for the Crane Mountain, Indian Lake, West Canada Lake and Blue Mountain, have been analyzed. relatively large uncertainty in the initial ratio. These values are

The Crane Mountain suite, kindly supplied by Dr. P, R. Whitney, presen,Ad in the isochron in Figure 14.

Rensselaer Polytechnic Institute, was collected from a postulated Ten specimens from the Indian Lake, Blue Mountain, and West overturned and differentiated 900' thick sill. Re]mulds &1&1• Canada Lakes QuadrAngles were collected and submitted for analysis

(1967) suggest a common origin for these pyroxene granultles and the by rroteasor Dirk d,W,4,1, Department of Geolngy, Syracuse University, adjacent Snowy Mountain anorthosite body on the basis of K/Rb ratios, Syracuse, New York. This area has been under extensive field investi-

Niggli values and the Differentiation Index (discussed in Chapter II). gation recently since it is a critical area for the recognition of

TABLE III-K

CRANE MOUNTAIN. NEW YORK

Isotope Dilution Sample Rb87/Sr86 Sr86/Sr88 (Sr87/Sr86) normalized Rb(ppm) S.(ppm) Rb/Sr

R7123/ 3.438 0.1198 0.7653 165 140 1.181 ACS 43

R7125/ 3.479 0.1205 0.7695 151 126 1.195 ACS 44

R7126/ 2.148 0.1197 0.7411 138 187 0.739 ACS 45

R7127/ 2.038 0.1198 0.7408 126 180 0.702 ACS 46

R7128/ 1.997 0.1203 0.7404 130 189 0.688 ACS 47

R7129/ 1.899 0.1201 0.7390 122 187 0.654 ACS 48

TABLE III-L

MODES OF CRANE MOUNTAIN SAMPLES ANALYZED (P. R. Whitney, personal communication)

Volume Percents Sample Number of Points Qtz Plag Orth Hornblende Pyroxene Gt Bio Other** Sections · Counced -- --

R7123/ 1 842 31.4 22.3 39.2 4.8 ------2.4 --- ACS 43

R7125/ 1 720 29.6 17.5 46.7 4.2 ------1.9 0.1 ACS 44

R7126/ 3 2857 22.7 22.2 41.3 10.6 2.6 XX* --- 0.6 ACS 45

R7127/ 2 1831 19.6 23.6 45.9 7.8 1.7 0.5 --- ACS 46 0.9

R7128/ 2 1588 16.3 30.7 42.1 -- 3.1 . 6.2 0.8 0.8 ACS 47

R7129/ 2 1818 24.0 23.2 40.9' 1.7 8.3 1.3 --- 0.7 ACS 48

* xx= garnet visible in hand specimen but not present in thin section·

** Other= apatite, zircon and ore minerals·

The "dark" group was found to contain pyroxene (hypersthene) and no biottte, whereas the "light" group contained biotite and no pyroxene. 75

FIGURE 14

1 1 1 1

0.780 - ADIRONDACK HIGHLANDS, NEW YORK / - CRANE MOUNTAIN 1336 +7 1 m. 0 770- y. R7125./ - X =1.39 x 10-"y-' AR7123 (Srer/Sres)0.0.7025+0.0024 0.760- -

0.750- - : R7127 / 0.740 /'•R7126 .- R7129/, R 7128 U) 0.730- -

0.720 - 0»*,- l - l 0.700 1 1 1 1 0 1.0 2.0 3.0 40 5.0 87 86 Rb / ST

TABLE III-M

INDIAN LAKE, BLUE MOUNTAIN . AND WEST CANADA LAKES QUADRANGLES

Isotope Dilution · Rb(ppm) Rb/Sr Sample Rb87/Sr86 Sr86/5,88 (S/87/Sr86) normalized Sr(ppm)

R7321/ 1.231 0.1207 0.7202 93 220 0.425 W-15(b)

R7322/ 1.984 0.1200 0.7417 135 197 0.682 2(a)

R7326/ 1.576 0.1195 0.7344 121 223 0.543 W-126(a)

R7327/ 0.6032 0.1178 0.7147 53 255 0.208 W-126(b)

R7329/ 0.7225 0.1198 0.7153 101 403 0.249 W-124

Specimens submitted by Dr. D. deWaard, Department of Geology, Syract,Ae Uniyermity, Syracuse, New Yurk

the basement and supracrustal sequence outlined briefly above from These ages appear to be the oldest reported for the central

Adiroddacks and suggest a period of intrusion pre-dating the events the paper by Walton and deWaard (1963).

reported by Hills and Gast and Heath at about 1100 m.y. The low Five whole rock analyses have been made and an analysis of a potash feldspar separate from specimen R7322 has also been made initial ratio for both the Crane Mountain and the Indian Lake - Blue

Mountain - West Canada Lakes isochrons indicates that there was not (Table III-M and Figure 15). Specimen R7321 from Blue Mountain and

R7322 from West Canada Lakes quadrangles have been mapped as part of an extensive pre-Grenville history and that the age determination the supreeruaL,1 sequence by deWaard (personal communication) and closely represents the age of intrusion. A later metamorphism of specimens R7326, R7327, and R7329 from Indian Lake have been interpreted lower intensity could possibly be recorded in specimen R7321 and as members of the basement complex. it would be interesting to investigate further to see if two ages

Four of the analyses define a reasonably good isochron with an can be distinguished. ' An attempt should be made to see if a distinction age of 1465 + 85 m.y. with an initial ratio of 0.7014 + O.0013. between the supracrustal sequence and the basement can be made.

Whole rock R7321 has been rather arbitrarily excluded from the regression analysis. .Assuming the same initial ratio found above, this one specimen gives a slope of 0.01527 corresponding to an age of 1087 m.y. 76

3.16 Wcotport, Ontario Map-Area

The Westport map-area is located about 30 miles northeast of 3. garnet-hypersthene-biotite-plag.-quartz garnet-hypersthene-potash feldspar-plag.-quartz Kingston, Ontario and 70 miles southwest of Ottawa, and comprises 4. hypersthene-biotite-plag.-quartz parts of Leeds, Lanark and Frontenac counties. hypersthene-potash feldspar-plag.-quartz

The most recent geological study of this area was undertaken · The textures shown in the specimens analyzed in this study show by Wynne-Edwards (1967) who mapped the area at one inch to one mile, many features in common with those from Madras State. For example, special emphasis being paid to the Precambrian rocks which outcrop in many of the specimens the characteristic blue-grey color and in two-thirds of the mapped area. greasy texture of the quartz is apparent. The potash feldspar is On a regional scale the metamorphic rocks exposed are dominantly dominantly mesoperthitic although antiperthite is also common. Biotite marbles, quartzites and quartzo-feldspathic gneisses typical of the and hornblende.,re also·present and in some cases are the predominant Grenville province of southeastern Ontario. The highest metamorphic mafic mineral present, the latter present especially in quartz-poor grade exposed in the region is attained in the Westport map-area in rocks. In the area adjacent to Devil Lake and Clear Lake the highest the Clear Lake anticline where characteristic assemblages of the mA/morphic grade wao attained and hydrmA phases are virtually absent. pyroxene granulite facies occur. This high grade core (Figure 17) It is suggested that the following.reaction took place: is surrounded by rocks of the hornblende granulite subfacies and biotite m hypersthene + potash feldspar + water these, in turn, by the amphibolite facies. The lowest grade present, The presence of these hydrous phases does not disqualify the greenschist facies, lies adjacent to the amphibolite facies and membership in the granulite facies. however. Although the use of forms the Hastings Basin to the northwest. this term is somewhat of a field convenience (Wynne-Edwards, personal The granulite horizon (Wynne-Edwards, 22·Sit; map unit 8) communication) it appears that its use is justified in that the addition forms an almost structureless unit throughout the area and is distin- of water vapor as another component must be. considered. Also the guished from the adjacent gneisses by a lower abundance of mafic partial pressure of (02 may be an important control since this component minerals and an almost complete absence of foliation. would exert a control over the activity of water in the system. Further, The mineralogical assemblages observed are characteristic of , despite the usual "definition" of charnockite from the type locality the granulite facies described by Eskola (1952): in ·India, biotite and hornblende do occur there, although in rela- 1. sillimanite-garnet-biotite-plag.-quartz sillimanite-gamet-potash feldspar-plag.-quartz tively minor abundance (Subramanian, 1959). In like fashion, the

2. garnet-biotite-plag.-quartz garnet-potash feldspar-plag.-quartz

FIGURE 15

1 1 0.760-

ADIRONDACK HIGHLANDS, NEW YORK INDIAN LAKE, BLUE MOUNTAIN, and 0.750- WEST CANADA LAKES QUADRANGLES 1465+85 m.y. X = 1.39 x 10-" y-' IR7322 0.740- (S,87/s,86)0 = 0.7014:f 0.0013 < 0 \ R7326. • R 7322 { fp) i. 0.730- CO

0.720- •R 7321 0710-R7327. R 7329

0.700 1 1 0 1.0 2.0 3.0 R b87/ S,86 77 '

FIGURE 16

....".S/,rE rm,---, ,""i'.t"#I ER. 1 4' ff

I :--1r'-r:. -t'-: -111 1--1 11 11 1111 11'fiy i' 1 'Noss ml 'MA.NOCK TIC » .....

I :..1.:·:R:.· 9. · jn.- - i / r ·:.:·.·b.-·3&\ /...... 2/.7' ,...... /-··· 1. . i. · · . ·,A,'XX··4#2:·· : . >i:;:,0.:...... :.·:..«:... :(: ...... , 4 3 ,

»2...... /2/ :...... :..7 h..lu ...... k-

VERMONT

..t 4,

/t .....7 .t'.44.2 6.-2 i.:.:2:.·'...: :! ....:.$0)41 4 S. ...d. ...Of':f-:2.'.·.·.: ·'. ·:' f · : : 4 1 97 .... 1=4..01.1=- ...... ti...... S 46 .C.ta: .t»--» ...1]61 n a: . i -,1 · 2 ...... -- , S., »--5 1- ,1 51,1:1».' )4:-1»S' 01 15 - *·le'JS«S..13'« .1 f 3973. S....3.NE' 3 \ ... /1 1..\ ' . ... f I HEATH <19671 '.,

A . »...... Ar-425*« . ''...'-1, \·'&·.1 0 SPOONERA1969 I rd OF'· '· 1 · · '· ·' . ·/:, -:rlis\ ,· M ;, '\»1 . I·.. 3, A /J ·:1 A , « ... ·'-.. 1 ; 1%·6]:;:R .'°·14 1.1, 7 5 / b-

FIGURE 17

wOLFE = == 1 44». - PALEOZOIC

44 40'N ..4 --- PALEOZ -

I « -I...... = . M-6 .7.2 1...j ..lilI.

Q 131:=,·J-··(I...... /1 El 11.-r.-F.....1. \ 1 ,411 »

621 -I . ...: -' . . . :...... ,t..'

1· .· · .· :/i· · f.1:4·9 ' - p -U . CV - . (geology *implifiem - 2 11,, 'A / 1.J A#,4 v 1...... 7/f'.:IK:''i<

' . · 447' T: p..' .r., i: -: ,: F«Vity...... , . |' 7 A)..::«·': 5''of,138:·'.. 'm ' i. 0 '1 78

same may be said about the occurrence of hypersthene. Although A least squares regression analysis (after York, 1966) on the this was an importatiL part of the definitinn, hypersthene does not eight Westport, Ontario· analyses gives an age of 1320 i 59 m.y. with occur in every hand specimen or in every outcrop. When considered an initial S/87/Sr86 ratio of 0.7059 + 0.0009. Using five points on the scale of the geological unit, however, it is a characteristic only (omitting R7070. R7071, and R7091) an age of 1334 + 24 m.y. is index mineral. obtained with an initial Sr87/Sr86 ratio of 0.7065 + 0.0004. Each

Toward the northern edge of the map-area, there are three·major analytical point used in the least squares regression was weighted plutons of quartz monzonite roughly four miles in diameter a„d usiniz an error in Sr87/Sr86 of 0.1% and in Rb87/Sr86 of 3%. concordant with the metamorphic rocks enclosing them· The cores of Krogh and Hurley (1968) have recently published whole-rock the plutons are relatively homogeneous, whereas the borders contain isochron data from various localities in the Grenville Province numerous inclusions and'in places the gneissic country rock lu altered including results on the Westport granite (quartz monzonite) and to monzonite, the original gneissic texture being only faintly preserved. the Ridge granite which has a similar geological setting and occurs

Wynne-Edwards has suggested that these three plutons were emplaced· in the greenschi:4 r. region Lo the northeast of Wcotport in the HAstings late in the tectonic sequence'of events that took place in the region area. The whole-rock age reported in their study is 1016 i 39 m.y. and that these represent remobilized pre-Gremille basement. based on seven analyses with an initial k87/Sr86 initial ratio of

Petrographic and analytical, results from the various rock units 0.704 (no error limits quoted). are discussed at length by Wynne-Edwards. It is interesting to note that the ages obtained for the pyroxene

granulites of the present study and the adjacent quartz monzonites 3.17 Geochronology

In the Fall of 1967, the writer visited the Westport map-area

and with the aid of Professor H. R. Wynne-Edwards some fifty specimens

were collected with emphasis on the charnockitic quartzo-feldspathic

gneisses. Eight specimens were selected on the basis of charact-

eristic granulite facies mineralogy and range in the rubidium-strontium

ratio,and were analyzed isotopically. The petrographic descriptions

are given in Appendix A and the analytical results are presented in

Table III-N and Figure 18.

TABLE III-N

WESTPORT MAP AREA, ONTARIO

Isotope Dilution S. Sample Rb87/Sr86 Sr86/Sr88 (Sr87/Sr &)normalized Rb(ppm) (pp.) Rb/Sr

R7085/27 0.079 0.1207 0.7078 15 530 0.027

R7083/25 0.684 0.1202 0.7203 85 359 0.236

R7070/13 0.873 0.1208 0.7193 140 466 0.301

R7090/31 0.394 0.1205 0.'7245 57 417 0.136

R7113/49(2) 2.998 0.1189 0.7628 125 ·121 1.030

R7062/5 0.154 .0.1200 0.7091 24 · 454 0.053

R7061/4 0.822 0.1195 0.7219 28 99 0.284

R7112/49(1) 1.789 0.1210 0.7393 88 142 0.616

R7071/14 1.023 0.1210 0.7234 63 180 0.353

Chondrodite Marble: Isotogic ratio

R7106/44 -- 0.1204 0..7066 .i 0.001 at 95% C.L. (2,) 79

FIGURE 18

0.780

ONTARIO _ 0.770- WESTPORT, 1338 f 47 m. y. R7113 0*,* A = 1.39 x IO-"y-1 0.760- + 0.0009 C S,87/ S,86)0 . 0.7057

0.750- . CO . C 0.740- 0 R 7112 „ . - . 0.730- R7061 R7090e R 7071 0.720- R 7083. 0 R /070

0.7101.-0 R 7062 R7085

0.700 3.0 0 1.0 2.0 Rb87/Sr 86

support the sequence of events inferred from the field relationships. initial ratio. On the basis of analytical error only, the initial

Wynne-Edwards has pointed out the distinct stratigraphic position ratios for the quartz monzonites and pyroxene granulites do not

that the granulites occupy in the sequence of rock types represented overlap, and this lends credence to Wynne-Edward's hypothesis that

in the area. The quartzo-feldspathic and dioritic pyroxene granulite the pyroxene granulites formed through high-grade regional meta-

constituting the highest grade represented in the region and the one morphism of a sequence of greywackes. Further, the Sr87/Sr86 isotopic most closely approaching the type charnockite mineralogical assemblage, ratio of a specimen of chondrodite marble (R7106/44) interpreted as

is localized in the Clear Lake anticline and is placed at the bottom being stratigraphically above the granulites yielded a value of

of the sequence. Stratigraphically above this unit are interbedded 0.7066 + 0.001 (at the 95%, 24 Confidence Limits). This value, if major units of marble, quartzite, and gneiss. taken to represent the Sr87/Sr86 ratio in sea water, lies on the

The initial Sr87/Sr86 ratio obtained for the pyroxene granulite Marine Geochron (M.I.T. Annual Report, 1965, p. 147) at about 1000 (0.7059 + 0.0009 or 0.7065.+ 0.0004) is measurably higher than the m.y. ago. Peterman 1 1· (1967) have found significant vrriations value obtained by Krogh && 111. (1968) for the combined Westport and in the isotopic composition of sea water strontium during Phanerozoic

Ridge quartz monzonite bodies. The values for the initial ratios time which may reflect the changes in isotopic composition of strontium

and ages with 3 and 5% *wrs for the standard deviation of the mean as the chemistry of sea water changed perhaps due to increased volcanism for the Rb87/Sr86 ratios of the Westport quartz monzonites are: or changes in provenance of sediment introduced into the oceanic system. (m.y.)* Error in (Rb87/Sr86) (Sr87/5086) AGE Admittedly, the difference in initial ratios for the Westport and

5% 0.7034 + 0.0005 1047 +17 Ridge quartz monzonite and the pyroxene granulites is not great. There

3% 0.1'035 i 0.0004 1044 i 17 is, however, less difference between the pyroxene granulite initial

ratio and that of the chondrodite marble which is presumably of marine * Calculated from York's (1966) method of regression analysis. origin. On the basis of the isotopic evi.dence, it ibi suggested that

This slightly higher value in initial iatio for the pyroxene granu- these high-grade rocks had an association with a marine environment

lites suggests that they were derived from a source region having a prior to metamorphism and only small additions of common strontium

slightly higher Rb/Sr ratio than in the source region for the quartz from lower crustal or upper mantle regions were made. mwonite which is intrusive into them. Krogh Il 21. (1968) suggest

an upper mantle origin for these intrusive rocks on account of the low 80

3.18 Kanuku and South Savanna Groups, Guyana, South America preserve the original ophitic texture, composition and zoning of the

The Kpnuku Complex and adjacent South Savanna Group occupy a plagioclase appropriate for an igneous origin. Apatite was also major portion of the southern half of Guyana and are part of the uoted as an acrpgory. It would appear that these rocks, originally

Guiana Shield which covers a large part of northern Brazil and igneous, subsequently underwent high-grade regional metamorphism

Venezuela. The map area and pertinent geology are presented in and metasomatism with the host gneisses (discussed below).

Figures 19 and 20. The major rock types of the Kanuku Complex are The orthopyroxene-bearing acid biotite gneisses account for a a variety of high-grade acid biotite gneisses in which are found significant portion of the map area and are characterized by assemblages enclaves of acid and basic granulite. The regional geology of this of the sillimanite-almandite subfacies of the almandine-amphibolite and adjacent groups has been presented in a report by the Geological facies of regional metamorphism. Where orthopyroxene occurs, it is

Survey of Guyana by Williams, Cannon and McConnell (1967). In a found as small anhedra in biotite flakes. On the basis of field and paper dealing primarily·with the charnockitic affinities of the petrographic criteria, Singh proposes that these rocks formed by

South Savanna-Kanuku Groups, Slugh (1966) hms, dununstrated an intrusive contamination by basic norite granulites which presently occur as relation of the presumably younger south Savanna granites with both boudins. lenses and bands within the aeld gncisses. the Kanuku Complex and the lower grade Marudi Group of metaoediment„ Though of low abundance, orthopyrorme-bearing acid granulites o f the greenschist and amphibolite facies. The emplacement also (,Hitinct from the gneisses above) occur in association with quartzo involved extensive assimilation and contact metamorphism followed feldspathic granulites, acid garnet-granulites, acid cordierite-granu- by a phase of post-crystalline shearing along northeast-southwest lites, and alaskites. This association is suggestive of the low to zones. This granite complex also contains many enclaves bearing a medium pressure assemblage suggested by Lambert and Heier (1968) for resemblance to rocks of both groups, roughly maintaining their regional rocks of a similar setting in Australia. Singh has also noted the structural trends. strained aspect of quartz with sutured margins and extensive development

' ' Though preferring to avoid the terms charnockite' and charnockitic' of myrmekite. Similar findings for the suite of specimens analyzed in his description of the Kanuku Complex, Singh draws out many points here are reported in Appendix A on petrographic descriptions. of similarity between this and the Madras type lithology. His explan- Singh ascribes their origin to the strong recrystallization of ation for the origin of some of the rocks of the Group closely paral- the acid biotite gneisses at high temperatures and pressures.

lels the proposals set forth by Howie (1955) for the Indian locality. Hybridization by norite granulites is thought to have been responsi-

In the case of the basic granulites, the dike and sill-like ble for the random occurrence of the pyroxene-rich acid granulites

attitude suggests that these were originally of igneous origin, in the acid granulite terrain.

probably gabbroic or diabasic in composition. The gabbro-granulites

Outline Map of the Guiana Shield Showing Map Area

of Kanuku Complex and South Savanna Granite

(from Records of the Geological Survey of Guyana,

Volume 5, 1967, by E. Williams, R. T. Cannon and R. B. McConnell)

i. .8. 5 4 6 Outline Map of the FIGURE 19 GUIANA SHIELD -

= en*2=00 ,.·- F-'-Ti.: :i:,:2---Fi,52.1,48:49.1. 'NW.," .1 - p.- Em amda#m, - EN'*..... Fo-'-' ts» -r=55' *w+ . $=1'%--- , - mm-

mEKE 2 ,r--'---- Hf--I 1 _ = 12/JWK _ 1_ 7_ ·.-BL-

I rFEIll H b

·.Ew'-VInLIT . - I 1 cm:/EL, 7 1-1 _ - 0"IT--1,.7 -I"'1'.. L.' 1 .,1, '.

- .9 ..I-/ T 4 11 [1- - - -1-E LID- 36= lili I VIll'A--- r r 1 _i

4:. .., ...... - z- '-

--. /# 'A V ,- 0 81

TABLE III-0 Compilation of Age Determinations by Potassium-Argon on South Savanna Group and Kanuku Group by Age Determination Unit, Institute of Geological Science, at Department of Geology and Mineralogy, Oxford University (modified from Table 3, p. 40, Williams sil 11. (1967)

Sample Rock Type and Locality K:Ar AKe (m.y.)

Biotite South Savanna Granite, strongly porphyritic type, 1 mi. SE of Shiwirtau, South Savanna, 2' 52' N, 59' 16' W. 1190 +45

Biotite South Savanna Granite, Tabtau facies, Tabtau Mountain, South Savanna, 2° 52' N, 59' 16' W. 1300 i 50

Biotite South Savanna Granite, strongly cataclased porphyritic biotite type. Biotite thought to have undergone complete recrystallization during shearing. - Sand Creed bed, 1/2 mi. S. of Cheppirariwau mouth, 2* 55' N, 59' 18' W 1256 50

Muscovite South Savanna Granite, strongly porphyritic biotite type, Rewa River, 3' 02' N, 58* 40' W 1720 i 70

Biotite South Savanna Granite, same locality 1685 70

Biotite Low-grade unmigmatized liotite schists forming an enclave in South Savanna granite: tentatively equated with Marudi Group. 3' 03' N, 58' 55' W. 1545 t 60

Biotite South Savanna Granite; near Awariwau, 2' 38' N; 59' 13' W. 1355 i 55

Biotite South Savanna Granite ?; Bat Mountain 2' 10' N, 59' 10' W. 1320 i 50

Plagioclase Fresh diabase dike cutting South Savanna Granite, near Arakwai 450 t 25 pyroxene mouth, 3 mi. S. of Dadanawa, 2' 47' N, 59' 32' W. 450 t 40

Rb-Sr isochron of South Savanna Granite based on 7 whole-rock analyses and 2 potash 1880 +. 100 m.y. feldspar mineral separates. Initial ratio: 0.7073. (Snelling and McConnell, in press).

Monazite (eluvial origin but from the South Savanna Granite), Discordant U/Th/Rb Age determined by A. G. Darnley of the Atomic Energy Di,vlsion, Geological Survey of Great Britain. 208 232 Pb /Th age=2270 1 185 m.y.

This study:

Whole Rock Rb:Sr on six samples from the Kanuku Complex, see Table III-P. Isochron age .2182 i 95 m.y.

Initial ratio: 0.7018 t 0.0011· See Figure 22 for isochron.

3.19 Geochronology

Despite the difficulties of access and the reconnaissance nature

of mapping in this geologically complex region, numerous age deter-

minations have been made by K:Ar, Rb:Sr and U:Th:Pb methods. A

sumnulry of data pertaining to the South Savanna and Kanuku Groups

is given in Table III-0. Williams fiL.al·, (1967) have placed the

Kanuku and South Savanna Groups as well as the lower metamorphic grade

Marudi Group into the Rapununi Assemblage to form a Southern structural

province. An east-west rift valley about 100 miles long and 30 wide

provides a profound structural break separating the northern structural

province from the south. This rift, which continues as a fault zone

to the east nortlleast into Surinam, precludes field correlations of

the north and south portions of Guyana so that age determinations

are essential to relate the two regions.

Snelling and McConnell (in press) have analyzed six specimens of

the South Savanna Granite by Rb:Sr techniques (see Table III-0) and

arrive at 1880 f 100 m. y. as the significant age of emplacement of

the granite. A discordant age on eluvial monazite from the South

208 232 Savanna Granite gave a Pb /Th agp of 2270 + 185 m.y. (A. G. Darnley,

in Barron, 1962a) which was later revised to 2075 m. y. by Snelling on

the basis of uranium leaching. This revised age does not differ

significantly from the Rb,Sr isochron age, Although the two have

different structural styles, the age for the South Savanna Complex approaches that of the Younger Granites of the Northern province

which have ages in the 1900-2000 m.y. range. 82

FIGURE 20

Kanuku-South Savanna Complex, Guyana

(from Records of the Geological Survey of Guyana,

Volume 5, 1967, by E. Williams, R. T. Cannon and R. B. McConnell)

- 2:1.-9,0,r /• R 7340 i "34" »--wi2 ,Niti" 1 011. ' ,1 U ,-2% . ,U N T,I N, :ly.... »t« r.2 0 9,7, ,<744-1< . JKPJ:'.-*rXLL<*fir&724 - ..0114%:f -(441'...'5.--0 .. , o

96<23AE,r-*4.2-1:„..9. . -a -.4.-: . t i.· '4).Ir•' Ic,/- - t'.iyf 5..2.356 20:-0.3..1.4.--60£2*1.2.-6.,c.:4'- h- ·' -- 4 6 Epti "·»i i. · Rmi te'*,A;47. 41'..5.f J ....t. ,h.. .t 'N.) grr-,. br._..= 4 ..':·*: #.34,51- 2 57.« ·42-«k&*11 37 A. . / : r 94

6#32*.r3.Alf·#·3*,1:)j - " Fi.. 3: ., .. /'...prgj#4442; :f , JAA. f .. -3 3...., '904.- -- 1 A.2 ..3.....:....:,e,qvt- • •_4·· c' . . r/,4' ¥i'.'.4•·-.-.1*..- '" ..'- ·: 3

1 ...... 47 litt # , ' , 3 *": --) S,il, ... .t,p .2.-9=, KANUKU - SOUTH SAVANNA COMPLEX GUYANA i EQI.Imi.-- .. 1.""N" " K Bio.·g'.One:i'*

: CAIreecki,;con,i6,/ned,or,1/ Gn•in/s , & b,/„,thene Giontie KANUKJ GP.

TABLE III-P

KANUKU COMPLEX. GUYANA,SOUTH AMERICA*

Isotnpe Dilution Rb Rb/Sr Sample Rb87/Sr86 Sr86/Sr88 (Sr87/Sr86) normalized (ppm) Sr(ppm)

R7340/ JPB 156 0.7203 0.1195 0.7217 57 230 0.248

R7341/ JPB 157 0.8849 0.1193 0.7301 68 222 0.306

R7345/ JPB 177 2.030 0.1193 0.7633 120 172 0.698

R7346/ JPB 216 1.6976 0.1200 0.7526 138 236 0.585

R7347/ 0.2272 16 0.078 JPB 300 0.2237 0.1208 0.7086 16 205 0.078

R7344/ JPB 177 0.9187 0.1194 0.7327 71 223 0.318

* Samples submitted by Dr. J. P. Berrang&, Institute of Geological Sciences, Overseas Geological Surveys; 5, Princes Gate, L6ndon S.W. 7. 83

FIGURE 21 1 1 t 10/ '60- -

.750- -

8- .740 - -

S 0 A .730- 0 KANUKU COMPLEX - GUYANA, S. A. - 0 .720 AGE=2182 + 95 m.y. (Srel/Sr86 0.0.7018 ta0011 ,710- 0 X .1.39 x 10-" y-' 1 .700

1 2 3 Rbe/Sr86

Both rock types are transected by contemporaneous charnockite 3.20 The Granulite Facies of Pallavaram and Salem, Madras State and Kushalnagar, Mysore State, India pegmatites. About three miles further south, at Pallavaram, low

rounded hills of charnockite are associated with a garnetiferous "But nothing in. India is identifiable, the mere askin8 of a question causes it to disappear or leptynite and norite. At Pammal, two miles west of Pallavaram, a to merge into something else· . hornblende-augite norite, locally biotite-rich, forms a hill rising E. M. Forster, A Passage to India about 200 feet above an alluvial plain. The regional foliation of

these rocks strikes roughly northeast-southwest parallel to the Coro- The granulite facies rocks forming the Archean of Peninsular

India have received attention continuously since the late nineteenth mandel coast. At Salem, the specimens provided by Dr. Subramanian correspond century,particularly following Holland'a (1900) descriptions of the almost exactly with Holland' s (1900, p. 181) detailed descriptions of so-called charnockite series in southern Madras State. The contro- the charnockite occurrences in this area. At Nagara Malai (malai - versy concerning the origin of the "charnockite series" is discussed hill) which forms the southwest end of the Shevaroy Hills, the coarse- in Chapter I where reference is.made to papers dealing with the field

grained garnetiferous basic· members are widely 'represented, R7336/14 relations of these high grade rocks with their neighbors. and R7337/15. Locally, the garnet is fist-sized and is associated The three areas studied in the present investigation form part with marginal lenses of pyroxenite. of the-Eastern Ghats and.Nilgiri Mountains of Peninsular India.

On the road to Trichinopoly (see Figure .22) about 3 1/2 miles Specimens from the type localities near Madras City which figure south of Salem, transgressive tongues of the charnockite series in J. prominently in Holland' s studies were kindly provided by Dr. P. R. the older biotite-gneisses have been described by Holland ( bii·, p· Naidu, Honorary Director of the Mineralogical Institute, University of 181, and p. 225). These descriptions appear to apply to specimens Mysore. He also provided the writer with an extensive. suite from 17182/7 and R7330/8 to R7335/13 inclusive. The great mass of the the Kushalnagar area, Mysore State. In addition, fifteen specimens charnockite series forming Jarugumalai protrudes into the blotite- from the Salem area of Madras State were kindly provided by Professor gneisses which form a topographic low in the region about Salem. S. Subramanian,. Government College, Salem, Madras State. The collections Holland (11$. ) discusoes several points of evidence in favor of an are liated in Appendix B. intrusive relation of the younger charnockites into the biotite gneiss· At St. Thomas' Mount, about eight miles south of Madras City,

on The specimens from the Kushalnagar area of Mysore State are a central portion of "charnockite" occurs with an augite-norite listed in Appendix B and are taken from various localities from the the northeast and southwest sides (Holland, 1900, p. 172). 84

7,00'E FIGURE 22 "9*

t GD·n

F ELO- IT NO .7,76 0 •'., 3 .7,7. I eL ™S,3 : mp 7 -.2 8 .7330 .DA'U' -/1 • 5-'ZJ 9 .733' o . R733' 67' tskill-%1. ...33 2 .73. 13 ... w OF SPECIMEN LOCALITIES. 0 : NA SALEM AREA. MADRAS STATE 173 (2.-'29 INDIA

SCALE 1: 63.36 .---1 .i t lA.Ult -IS -I.-e-.- I.-I

*- *-O,10•• I ... -*. '..... SALEW OV.r n ' Ce.-. lod-... . 24-0

OASINAYUAN PATT

.30'. 17Ur«.%<

Nilgiris Mountains and the districts of Coimbatore. These two areas to fit the same isochron equally well, it was not possible to distin- form the western extremities of the Eastern Ghats Belt which maintains guish any difference in age between the charnockitic and non-charnockitic the granulite facies grade throughout its length. gneisses. Aswathanarayana (1968a) reports an age of 2650 If 275 m.y.

The problems associated with unravelling the complex sequence for the Madras locality and suggests that these may be consanguinous of intrusion and multiple episodes of regional metamorphism in with the gneisslc charnockites of Mysore. For these rocks, located in the Nilgiri Hills area, Crawford obtained a maximum indicated age Peninsular India have been partly resolved by the use of the various of 2670 m.y. and an isochron age of 2615 It 80 m.y. with an initial techniques of radiometric dating, but there still remains the necessity of detailed investigations within each area to completely understand ratio of 0.7023 + 0.0012 based on four analyses. Although Aswathanarayana the metamorphic and tectonic sequence in these ancient rocks. suggests a derivation of the Madras and Mysore charnockites from the

Peninsular Gneisses, Crawford (22·211· , P· 145) suggests with reser- 3.21 Geochronology valluns that the Mysore (Nilgiri Hillo) rocks may be slightly older Pichamuthu (1967) gives a brief account of the general geology than the Peninsular Gneisses from the neighboring Bangalore area. and geochronology of Precambrian India in the broadest regional terms. The main contribution of this study to an understanding of these Though this is by no means the most comprehensive account of Indian 87 86 areas lies in the accurafe determination of the initial Sr /Sr ratio. geology, it clearly demonstrates the relations among the various Nine of the specimens analyzed (Tables III-Q and III-R) did not have metamorphic belts exposed. Aswathanarayana (19688, 1968b) and a sufficiently wide range in Rb/Sr ratio to construct an isochron. Crawford (1968) give possibly the most up to date account of both the Using the analytical'data presented by Crawford (1968) (Table III-S) overall age pattern in Peninsular,India and of the granulite.facies for the Pallavaram and St. Thomas' Mount areas, however, along with of Madras and Mysore States, which are of immediate concern in terms the analytical data for specimen R7205, an age of 2618 i 46 m.y. was of this investigation. The following is in no way an exhaustive attempt obtained. The initial ratio for these analyses, including the seven at an extensive compilation of age data for the whole of Peninsular specimens from·Mysore State (a total of thirteen analyses), is India, except insofar as it bears on the specimens analyzed in this 0.7039 i 0.0005.· In view of the low Rb/Sr ratio (about 0.2) for the work. seven analyses, the error in regression back to about 2600 m. y. ago Crawford (1968) presents an isochron based on six analyses from is not great and would appear to be justifiable in view of the St. Thomas' Mount and the Pallavaram area. An age of 2580 + 90 m.y. discussion above concerning the similarity in age of both the

was obtained with an initial Sr87/Sr86 ratio of 0.7056 i 0.0040, Madras and Mysore charnockites. Two specimens (R7240 and R7242) though he points out that this determination was of insufficient from Pallavaram give an apparently younger age of about 1980 t 124 m.y. accuracy to distinguish these rocks from the Peninsular Gneisses including the seven analyses having low Rb/Sr ratios, providing an which outcrop further west at Bangalore. Since the two suites appear anchor point for the initial ratios at 0.7037 0.0007· 85

FIGURE 23

1 1 1 1

0.725- PYROXENE GRANULITES (CHARNOCKITES) SALEM, MADRAS STATE X = 1.39 x 10-"y-'

0.720- -

to CD

:7 0.7 1 5- R7180

(/)

0.710- -

IR7178 AGE =2476+115 m.y. 0.705 - (S,87/Sres)0-0.7042*0.0002 - 1 :;1 I R7182 R 7336 O.700 1 1 1 1 0.1 0.2 0.3 0.4 R b 87/ S 1 86

TABLE III-Q

PYROXENE GRANULITES (CHARNOCKITES) FROM SALEM, MADRAS STATE*

Isotope Dllution Rb (ppm) Sr(ppm) Rb/Sr Sample Rb87/Sr86 Sr86/Sr88 (Sr87/Sr86) normalized

R7176/1 ---- 0.1199 0.7045 nd** ------

R7178/3 0.0898 0.1198 6.7071 14.68 473.0 0.031

R7180/5 0.2877 0.1209 0.7143 40.5 407.7 0.099

R7182/7 ---- 9.1172 0.7044 nd ------

R7336/14 ---- 0.1174 0.7037+ nd ------

R7337/15 -- 0.1188 0.7048't nd ------

* Samples provided by Dr. S. Subramaniam, Government College, Salem, Madras State, India

*. not detected at the 5 ppm level by X-ray fluorescence.

+Analyzed on mass spectrometer "Iris": value 0.003 low (from currently "accepted" value of 0.7082) for E&A 'Standard . A correction of 0.003+ applied (Dr. H. W. Fairbairn, personal communication) .

TABLE III- R

CHARNOCKITES FROM PALLAVARAM (TYPE AREA), MADRAS STATE Latitude 12' 55' N, Longicude 80° 10' E

Isotope Dilution and Rb87/S,86 Sr86/S,88 Sr(ppm) Sample (Sr87/S/86) normalized Rb(ppm) Rb/Sr Locality

Basic Charnockice St. Thomas Mount R7205/AS-12 3.373 0.1206 0.8282 134 116 1.152

Basic Charnockite Thattangannu R7240/AS-5 0.896 0.1203 0.7301 146 473 0.309

Basic Charnockite Biotite-rich, Cherimalai R7242/AS-10 1.429 0.1203 0.7430 210 427 0.492 86

TABLE III-S

CHARNOCKITES FROM KUSHALNAGAR AREA. MYSORE STATE Latitude 12' 26' 45", Longitude 75' 56' 36" Isotope Dilution and Rb87/Sr86 Sample Sr86/S,88 (S,87/5*86) normalized Rb(ppm) Sr(ppm) Rb/Sr Locality

Charnockite, Dindgad R7214/A-85 0.165 0.1185 0.7083 6.3 110 0.057

Charnockite, Chikkamarahalli 0.154 R7215/A-95 0.1200 0.7094 6.5 123 0.053

Charnockite, Kudige- Kanive 6 R7216/A-110 0.312 0.1202 0.7167 13 116 0.108

Charnockite, Kanive Hill (left) R7217/A-115 0.147 0.1200 0.7077 3.9 76 0.051

charnocklte, Kanive-Jainkal Betta R7218/A-11, 0.161 0.1205 0.7107 11 192 0.056

Charnockite, Marur R7219/A-121 0.234 0.1195 0.7203 12 143 0.081

Charnockite, Kanive Temple Hill (right) R7220/A-124 0.131 0.1205 0.7087 4.3 94 0.045

Charnockite, near Adinadur Tribal ColonY 6 R7221/A-126 0.288 0.1202. 0.7094 20· 203 0.099

Charnockite, Cauvery River Bed, Hulse R7244/A-148 0.144 0.1197 0.7068 9.5 191 0.050

TABLE III-T

ISOTOPIC ANALYSES BY A. R. CRAWFORD (1968) FOR MADRAS STATE PYROXENE GRANULITES

from A. R. Crawford (1968) Table 24 , p. 147

" Sample Number Isotope Dilution and Locality Rbal/Sr86 5,87/Sr86 Rb Dom Sr pDm

Pallavaram

Charnockite 1867, Acid 0.9051 0.7412 44.2 140:9 N side saddle

1868, Acid Charnockite 3.1046 0.8190 107.3 99.7 Porc Trust Quarry

grained 1869, Coarse 2.4519 0.7904 201.5 237.1 vein in 1868

Intermediate- 1870, 0.1742 0.7105 11.4 189.0 basic Charnockite, Port Trust Quarry

1871, Leptynite, 3.5838 0.8356 98.7 79.4 Total Rock 3.5869 0.8358 79.4 St. Thomas' Mount

1874, Acid Charnockite 5.8134 0.9182 St.Thomas' Mount proper, 127.2 63.1 Total Rock --„„ m 2 § % :8 2 1 8 8§ § 0 0 0 0 0 0 0 . 0 000 2 2:8 0· 0 · 6.... 0 0 0 0 0 0 0 0 8. a6 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 : t,+, a S § 8 h ra a i 2 Matill 313 0 0 0 . O .6 6.0 M 2 -1 9 t . 3 M 5 E :R :8 3 0: : = : - = +, +i It .1 +1 fl +, 8 +t +, .» +t +i +1 +i +t +i 11 8 MI - 9 - 0 LA .- --= - R / 8 8 12 -'e T W - I .0 2 . - - .7 - M - - - - 4 44 - IN - S S_ # S Z 2 *i - ' - ·4 0 t, -:„- g . : 2 :: : N= 5 Ce :E , E . 6 „- : E C E, § :·2 . 23 : 02 5 8: 0 8: I :4 : 0 191 3 - i -I i a Em a 34 k a - : : 1 : e: 15 :i 2 : a t h d i E l- 3 0 %# a E . 1 :9 # 11 'ia H i: i 5 1: : 10 ·3 :E C : a i .2 8/ 8 11/ 1: :4 1% :1 1 1 3: & i 1 8 3: : 4« -6 -. 4% 2 1 :E: :3 63 8· i 1 j i i : /1 i l l - 00 4 0 1\ N v 1 :4 8: 1 1.N H ag 81 3 1 1 : 31 1 .8·5 : 8 8 : H la & Ma .85 H Na 3 3 8/ 8 L3 1 3& 8 3 3 \ 1.... 0 \'. 0 0 0 4 -le S ": K j ABS. 13/"-O 1- . 9... >k.b O . A . . . . d -08 11'galigta 1,11: 1,& J:, 0 z< 4* -/ 4 :iss:e *ig:ai:izB ifilim.-=. i C '6- 3 ;i.2 »qWt . 2 E :. \ §1 #A l.S i:6·5 1 1 1 1 1/1 f# &1 1 1 8 2; 0 b--0 - 8*3 1£1 3:. I :a s:.3:: „ · .5:: . :8:Col -rli :f W 10 2 - E-_ : O 8 g a m 5/8.3 .8. m. a l=N. 8 8 1;4 1: ES S . ....:80-: -i:: .' N. 9 8 lail//1:1396:mi :5:3:Y:38:4::. - 8 3:5.1:86::8:18:. 1 1 1 . *0/2.:5&3133 imilial

8 a r : 1 #i: 1 S 1; 5 1 1 1: E M i: ,1, 4 9 1 5 1,5 1 S 6 6 . P 3,AS ES 32 7 :.:::.: 98 1 S /28 1 S a m 9856#1:95,&#::m Lialfil3:Bial, : 11=.i:ilivii=fili m i jilitilt::ilil:;,ii,121.-3-1 5 M.:.:332 314 2

FIGURE 25

INITIAL Sr 87/Sr 86 RATIOS IN CONTINENTAL ROCKS o Schists ond Gneisses - .730 • Granites , o Sed i ments • Volcanics (mostly acid) 8 Mafic Rocks * CHARNOCKITIC ROCKS ' -L.·720 - 0 0

• Sr 87

• · I o · o · o o oo " Sres 0 - 08 ..i 0-.710 0 0 ... 8*0 *0 .A.:... * N I. .. 0 b.'. S. 8*808 ... .a:* 1 8 0 * 0 0 .:- Y *. O 0 9 .700 Average Mantle Growth Line .698

3500 3000 2500 2000 1500 1000 500 0 Age, MY (after P. M. Hurley [1968] 0 part)

FIGURE 26

·0 - 4- * 00 0 8 7 8 6 22222* 6 6: O 0 0 --SI*IAFY OF (ST /Sr '13

.- -...-- -..:....1·-· .-:L ··: i.....'.1...... 1.- 1' -:. ·._.+ .,..,-... +..-·.-.-- - , , , incrt.hoti·tem (541 . He£ th' C 9671 :...:,..11:.....L.,-L.. 1-_·_1....., ...... 1- _. 1. 1.._ _ _... .i- . . _ .1. -- _ .„i-s ,a n Chainick .te S aries Alip.,-v CO 161 · · · ' 1 0

.1.:. . -...... :i:.i..-:» - 2- -0.p ... - - Cranodicritic Mifrnatite.Iv,·rv Cia: qi

:i·. i.::i ::: :.; .1: ' T W s : a, *4-p'nr 'cAA'i .'- 4' frinf :'m a .' 1 . .--2 e

:'.,imihi,0]ite . t. . .Gneisg'.·:Trve-lin.Srit...... ''.:. »:lit' . 1 :·„„::·: 1:: d:il:l' ' 1 i il Ch:,mn: 1 0.4.rl:'5 T:dia ., 1 "'.d'.'., t.y .'*"'Yk,-,B. T.d,„ '.', 't ' " 1, 1

TrPF·IFIT .Tlms

)..9.1..... lillI-t....- .1 " - -1-*71 - F,Ill,-Fum| 11 F nr/111 St/F I# 1 - -*I 1.tam p.dran 'ra'.11" .1

"ckoll•, ind'Rkr.Al. I'li,mdi 191 • I

FIr. Mointaini. Tinriria 1 1

1 1 1 ./ .: -1, 1 : 11.: 3 :U:. 1 Ah,r I#rri / T An , A/1 r £9) :i ::: r - · · ·

CrAI. M/Inta·1 n, N,w 1,rrk I f I D 3ostlea-I ke. Bluqki -dC·mul. »---/.- _38?52/J)Ew Ym k (6, . i: '..' :: :i-: 1 1 1

peR':„Ir, r„,„r,„ 'A)

: .i.:„:. „:n : :. „ .:. :.i; ····. , f ..1...

3*nik,L.CQ[DRli x._S.ixana.,i_lnld:a___ * 1 . 1 .. ·. · , ...1--h-Hij.------1 i ' 1 '1 1 i:. 89

FIGURE 27 . , ·:f.· 10,

# 1, 1 ·1 - % E ·vs:·pn Ilb - - - .- ·Bith Shaw'i (1368)_:ti.n Trend betwi:Ii":. '' H. 1% 04 FLW ->'-

:1.1. /

I. . 10'221 s. 721 0/ R721.M ,&1'- '11··P I ' -7»41 ._.17214. , 0

3- T.. - ...111'li -i'. 3.: R7220 0 R72.5.

2 r.1. I Fr'2-4

-1-!1 :1 . : E/Ib = 3 32 h o. „ • 8 910.0 15 2 25 3 4 7 £00.0 Rb, parts ncr. m:illion TABLE IV-B

M. I.T.# (Sr,ppm) (Rb/Sr) Rb +ARb (Rb/Sr) P orig di (Sr87/Sr86) (Sr87/Sr86);rig A(S/87/Sr86) 4%

R7214 110 0.057 6.6 0.600 5 0.7083 0.7087 0.0004 0.1

R7215 123 0.053 9.8 0.080 51 0.7094 0.7112 0.0028 0.4

R7216 116 0.109 16.9 0.146 34 0.7167 0.7206 0.0039 0.6

R7217 76 0.051 6.6 0.087 71 0.7077 0.7115 0.0038 0.5

R7218 192 0.056 13.4 0.070 25 0.7107 0.7122 0.0015 0.2

R7219 143 0.081 18.5 0.129 59 0.7203 0.7254 0.0051 0.7

R7220 94 0.045 9.0 0.096 113 0.7087 0.7140 0.0053 0.8

R7221 203 0.100 21.5 0.106 6 0.7094 0.7101 0.0007 0.1

p = present day measured value orig = original value of ratio before metamorphism ARb - value of increment loss in Rb assuming Shaw's relationship for K/Rb

(Sr87/Sr86) rig = value of present day Sr87/Sr86 if there had been no loss of Rb through metamorphism.

87 86 The total range in Sr /Sr initial ratios is found to vary This general survey of the initial ratios encountered in these from 0.701 to 0.708 for the pyroxene granulite facies rocks considered deep-seated crustal rocks suggests that the lowermost regions of the here. The arithmetic mean for these analyses is 0.7042. Since the continental crust are depleted in rubidium with respect to strontium; majority of the initial ratio determinations fall within the narrow further, that the consistently low values found to date suggest that, range of 0.703 to 0.706 previously established for anorthosites by if these rocks ever had higher Rb/Sr ratios and low K/Rb ratios,

Heath (1967), a common source region at depth in the crust is post- corresponding to the K/Rb trend established for crustal igneous and ulated for these rocks. Although further work would be required to quasi-igneous rocks by Shaw (1968), the rubidium must have been lost demonstrate whether the two rock types are consanguinous, the strontium soon after emplacement otherwise higher Sr87/Sr86 initial ratios would isotopic data suggest a common source in a deep-seated, low Rb/Sr ratio result. environment. 90

An approximate calculation may be made to find what the present sium-bearing phases such as biotite and potash feldspar. Daughter 87 87 day Sr87/Sr86 ratio would have been if rubidium had not been lost from Sr produced by the decay of the parent Rb in the mica would be in the system. It must be assumed that the K/Rb ratio varies linearly a less favorable site than its parent isotope and presumably would be from 332 at 0.1% K to 254 at 1.0% K on the basis of Shaw's (1968) work. easily removed from its lattice site into the melt.

To test for this effect, loss of rubidium alone is assumed. If The concentrations of rubidium and strontium in the coexisting potassium is accounted for in the migration along with rubidium, then phases of a pyroxene granulite have been determined by Howie (1965). the effect will be greater since the "tie-line" between the present-day Using these data, calculations will be made to see if such a fraction- potassium-rubldium plot and that before metamorphism will have a ation would be observable. positive slope. This calculation is carried out for the Madras and The most serious objection to the above hypothesis lies in the

Mysore State specimens. A plot of % K vs ppm Rb (Figure 27) for the nature of the reactions producing the anhydrous phases. As Heier

87 Indian specimens shows clearly a deviation from the Main Trend defined recognizes, the whole mechanism of Sr fractionation rests upon the by Shaw. For each specimen analyzed, an increment of rubidium is added assumption that there is no exchange of radiogenic strontium produced

to the present day rubidium content so that its K/Rb ratio coincides in the biotite with common strontium in the other phases. As seen with the Main Trend. Assurn-ing that strontium is fixed in the system, in Table IV-B, the other phases all have Rb/Sr<1 and an additional a new Rb/Sr ratio is obtained which is taken to represent the Rb/Sr dehydration reaction involving hornblende would also be expected in ratio after emplacement and before the high grade metamorphic event.

The average percentage change in Rb/Sr would be about 46%. Taking TABLE I V-C

Rb and Sr Distributions in an Intermediate Charnockite the initial Sr87/Sr86 ratio found for these rocks at 0.7039, and an and its Phases, Mount Wati, West Nile District, Uganda. age of approximately 2600 m.y., one can calculate -what the present-day (after R. A. Howie,1965)

Sr87/Sr86 ratio would have been had there been no migration of rubidium· Whole rock Orthoclase Andesine Hornblende Biotite

Using the pole. series expansion for eAt, the Sr87/Sr86 ratio can be Rb 75** 300 nd 10 1000

calculated approximately from the relation: Sr 800 800 1000 30 tr*

Rb/Sr 0.094 0.38 -- 0.33 1000 ca. (Sr87/Sr86)t . (Sr87/Sr86) + (Rb87/Sr86)t (Xt)

* = ppm ass,med The appropriate calculations are given in Table IV-B· The percentage ** = all values in parts per million.

change in resulting from the depletion of rubidim (Sr87/Sr86) meas view of the significant quantities of amphibole in many of these is about 0.4% and amounts to a change of about 0.003 in the strontium wcks. EAp Lessed in its simplest form, thi homblendc breakdown isotopic ratio, well within the capability of measurement. In summary, reaction could be expressed as: assuming Shaw's (1968) relationship to be valid, the episode of high

resulted grade regional metamorphism up to granulite grade in India has Ca2(Mg4,Al)(AlSi7022)(OH)2-CaA12Si206 + CaMgSi206 + 3MgSi03 + HZO in a decrease in the Rb/Sr ratio of about 46% This gives a present-day tschermakitic hornblende anorthite diopside enstatite

Sr87/S/86 ratio about 0.4% lower than would result had there been no As an example of the change in Sr87/Sr86 of the system with the depletion. breakdown of both biotite and hornblende, consider a rock containing Earlier mention was made of Heier's (1964) paper, wherein he 10% by volume of both biotite and hornblende with the above distri- proposes that mineral transitions in the deep crust,arising from bution of rubidium and strontium. Allowing a liberal increment of progressive regional metamorphism up to the granulite facies, produce time ta elapse between homogenizations of the system, say 500 m. y., phases which are increasingly less favorable to the diadochic incor- the growth in S/87/Sr86 after time (=500 x 106 y) in the biotite poration of rubidium. He suggests mineral transitions such as the would be given by: breakdown of biotite at the beginning stage of granulite development

that would be characterized, in the case of excess Si02, by the (Sr87/5*86),=soo - (Sr87/5*86)0 + Rb87/Sr86(ek t _ 1)

reaction: where:

(Sr87/Sr86)0 = 0.705 K(Mg•Fe)3A1513010(03)2 + 3Si02 -KA1Si308 + 3(Mg,Fe)Si03 + H20 from Table IV-B, = 1000, and Rb87/Sr86 = 3000 approximately. (Rb/Sr)bio Since the phases produced on the right hand side of the equation are - 1.39 x 10-11,4. less favorable hosts to rubidium, Heier proposes 1-hat it becomes so that: concentrated in a fluid phase resulting in a decrease in the Rb/Sr (Sr87/Sr86) = 21.705 bio.,t = 500 m.y. ratio in the deep-seated rocks with increasing metamorphic grade. As 87 a consequence of the concentration of rubidium in the mica, he proposes Of the 1000 ppm Rb originally present in the mica, 27.85% is Rb 87 further a mechanism for the natural fractionation of Sr87 from common and about 1000 x 0.2785 x 0.01 = 2.785 ppm Sr would be produced during t = 500 m.y. due to 87 87 strontium in a geological system. Anatectic melting under granulite the decay of Rb . Sr comprises 7.02%

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THIS PAGE WAS INTENTIONALLY LEFT BLANK 95

Interlaboratory Comparisons of Rb and Sr Isotope Dilution

Analyses

Rb and Sr elemental and isotopic analyses on the U.S. spotted with tantalum-tantalum oxide-sugar matrix material

Geological Survey standard rock samples GSP-1 and G-2 have and precleaned in the mass spectrometer before use. The been reported by Peterman, Doe and Bartel (1967), and by samples are not conditioned, but the temperature is increased

Doe, Tatsumoto, Delevaux and Peterman (1967) respectively. and the run commenced immediately. Generally the emission We wish to report here a study lasting over several months continues to increase over a period of one or more hours. in which we investigated the internal consistency of our The filament current in this case is generally slightly above various shelf solutions, spikes, and analytical results on 80 ma, but may go as high as 95 ma without an observable

these standard rocks; and to discuss particularly some change in the 85/87 ratio. The same conditions apply to both questions arising in the analysis of rubidium. Early in kinds of rubidium samples.

1969 we R.Mived a letter from Z. E. Peterman revising some Discussion of Results of his analyses downwards. His new work brought the U.S.G: S. A. Normal rubidium isotopic ratio Rb analyses closer to our findings. By using the high temperature emission we find the OMe of the problems arising with the analysis of rubidium Rk,85/Rb87 ratio :ti normal rubidium to be lower than that is that spike calibrations are made on mixtures of spike and found by Shields et al (1963). In Table 1 it is seen that shelf solutions containing pure Rb salts, whereas analyses on the ratio is about one half of one percent higher for the unknown rocks are generally made on separated rubidium that pure rubidium samples than for the rubidium in the potassium contains about 300 times i&s weight in potassium. Most matrix, and that in both cases the value is one percent or so analysts separate the alkalis from dissolved rock samples lower than Shields' value. It seems likely from a kinetic by precipitation as perchlorate and do not attempt to separate viewpoint that the true value of 85/87 in normal rubidium is potassium. the rubidium from the When the Rb sample is approached at the highest temperatures of emission, and is analyzed on the mass spectrometer the potassium-bearing sample also likely to be approached by the use of a matrix such as is generally run at a higher temperature than the pure potassium, which is also volatilizing. We therefore wonder rubidium sample because of the emission characteristics of if perhaps the true value is closer to 2.57 than 2.59 (found these materials. by Nier) or 2.60 (found by Shields). we also have found The 85/87 ratio appears to be slightly higher in natural values as high as 2.61 by using a very low temperature rubidium when analyzed as the pure oxide on the filament at a run. We are also suspicious lower temperature,than it is when analyzed at a higher tempera- emission and the first part of the that some of the new resins tend to increase the 85/87 ratio ture in a matrix of potassium oxide. We suspect that this may by particularly true when using the low temperature volatili- when eluted, in cases where pure rubidium is produced from

zation that accompanies the triple filament procedure used by rock samples. We are aware that isotopic fractionation in mass spectro- the U.S.G.S. analysts, and by Shields, Garner, Hedge, and meters is probably an overriding factor so that until an Goldich (1963). For this reason we have preferred the higher absolute isotopic standard is available it will be impossible temperature of volatilization and ionization combined, that 85 87 to establish the true absolute value of Rb /Rb in natural result from a single filament (tantalum) emission. Further- rubidium. However, from Sr analyses, we find that the mass more we have attempted to run our pure rubidium samples at spectrometer used in this work tends to fractionate in favor approximately the same temperatures as the Rb in the K matrix of the lighter isotope if at all. This suggests that our derived from rocks, by simply permitting intensive beams and 85 87 lower value for Rb /Rb is not·due to instrumental using a less sensitive amplification in the instrument. We fractionation. Fortunately this problem is of no concern in have found by experiment that the reproducibility in this procedure appears to be no less than that in the triple isotope dilution analysis, because the same bias is present in both the spiked analysis of the known shelf solution and filament procedure. We have also found by running both kinds the analysis of the unknown rock solution. The only effect of rubidium samples at high temperatures (in strong runs) that must be examined is that mentioned above, namely the that we obtain excellent reproducibility using various parts

of the run from the start to well beyond the period of difference in fractionation between pure and potassium-rich rubidium samplas due to filament temperature or matrix. increase of the emission. In these experiments we have used various scales and

scale changes and otherwise indicated that the performance

of the instrument is satisfactory. Analyses were made on a·

12" CEC analyzer with 60° sector, at 4500 volts, and vibrating

reed electrometer on the Faraday cup. The use of expanded scale eliminated most of the possible error in non-linearity

of recording system. Filaments of 1 x 20 mil tantalum are 96

B. The Analysis of Rb in Standard Rocks In·summary therefore it is seen that the revised U.S.G.S. 87 In Tables 2 and 3 the Rb spike isotope ratio and analytical procedures give values for Rb and Sr that are in calibration analyses are listed for various spike solutions. excellent agreement with the M.I.T. isotope dilution analyses

It is clear that the reproducibility is good for each on the standard rocks. G-2 seems to be the mor6 variable of particular solution. However there is some variation that the two rocks and this could well be due to inhomogeneity in

appears to be due to the preparation of the solution from the sample. X-ray work has shown clearly that the original various bottles at various times. It is not known wherein sample G-1 was also inhomogeneous, and varied from bottle to

this trouble lies: it may be the use of different pipetting bottle. It is also shown by this study that there is excellent

procedures by different persons in the laboratory, although reproducibility in the analysis of a rock sample in replicate tests on pipetting have not shown variations of this magnitude. measurements. As long as the spike calibration and the analysis

The rubidium analyses for GSP-1 and G-2 are given in of the unknown are carried out on the same instrument using

Table 4. In each repeated analysis, a new sample was weighed identical procedures and calculations there appears to be good from the bottle of standard, and the chemistry carried out in agreement between laboratories. The largest differences show

a different batch on a different date. It can be seen that up in spike calibrations involving different instruments, percent the reproducibility is considerably better than one different pipetting procedures and different operating pro- on the average. cedures. It seems clear therefore that each analyst should

calibrate his isotope dilution analyses independently. From C. Strontium Analyses the investigations we recommend that'the pure and the A value of .1200 was taken as an average value for the potassium-rich rubidium samples be analyzed at as nearly as 86/88 ratio in strontium analyzed by the 12" mass spectrometer possible the same filament temperature. in use. This value was used to correct for fractionation in 1. Normal Rb, Isotope Ratio Measurements the strontium spike isotope ratios, normalizing to .1194. Data Record Date 85/87 Remarks are presented in Table 5. This correction does not apply to the calibration runs made in 1967 indicated in Table 6, where (a) Shelf Solutions, Pure Rb

the value was assumed to be .1194 during that period. In 6033 11/4/68 2.58 Shelf solution, CMS

" Table 6 it is seen that the spike calibration reproducibility 6144 1/23/69 2.58 " , run Av. 2.8 at 81 ma was good. (b) Rock Rb, including K The strontium in GSP-1 and G-2 is indicated by the 6113 1/9/69 2.57 G-2, run at 80 ma replicate analyses in Table 7 : Each analysis was a separate 6115 1/10/69 2.56 GSP-1 " " 99 ma weighing and separate chemistry. Here again the reproducibility Av. 2.57

is a small ftaction of one It is excellent,· being percent. 2. Spike Rb87, Isotope Ratio'

noted that the 86/88 ratio in these rock analyses is generally (a) Old Spike, 2.81 gq Rb87/ml. higher than .1194, as it has been in almost all analyses with 4927 9/15/66 .0083 this instrument. 5268 5/15/67 .0084 The comparison of the rubidium and strontium in GSP-1 (b) New Spike, 9.84 ug Rb87/ml. and G-2 with the analyses from other laboratories is shown 6068 12/5/68 .0079 in Tables 8 and 9. As mentioned above the U.S.G.S. value for 87 GSP-1,, for both rubidium and strontium has been decreased 3. Rb Spike Calibrations Calib§ tion from that reported by Peterman, et al. (1967). It can be seen Record Date Ug Rb /ml.

there is good agreement between the M.I.T. and U.S.G.S. isotope (a) old Spike (1967)

dilution values for both the Rb and Sr analyses in GSP-1. 5967 1/11/67 2.81

IA·the case of G-2, the analyses were not rerun by the 5069 1/12/67 2.81 U.S.G.S. to our knowledge and are slightly higher than the 5088 1/23/67 2.80 Av. 2.81 M.I.T. values for both elements. If the same downward re- (b) Spike: Book 17, p.58 vision were applied to G-2 as was indicated by Peterman for 6954 11/22/68 11.47

the case of GSP-1, the values between the two laboratories 6148 1/24/69 11.39 Av.11.43 would be commensurate·. The X-ray values by Fairbairn and (C) Spike: Book 18, p.18 Carmichael and the isotope dilution analysis by G. Faure are 6078 12/11/68 11.10 in good agreement with both laboratories for GSP-1 in both 6154 1/29/69 11.10 Rb and Sr. They are also in good agreement with the M.I.T. Av. 11.10

values for strontium in G-2, but are in better agreement with (d) Spike: Book 18, p.29

the U.S.G.S. values for rubidium in G-2. 6087 12/18/68 49.61 97

(e) Spike: Book 18, p.9 5. Sr Spike, Isotope Ratios, Corrected for fractionation

6151 1/28/69 9.838 using average value of 86/88 = 0.1200 and normalizing this to 0.1194. 4. Rb in Standard Rocks Record Date 86/84 87/84 88/84 87 Spike Calibration Rock Record Date Rb Rb Constant Used .6052 11/20/68 .0464 .0159 .1385 87 GSP-1 5867 6/21/68 70.4 248.6 11.38 Mg Rb /il 6011 10/17/68 71.7 253.1 " 6. Sr84 Spike Calibration Value Measured " 6076 12/9/68 71.7 253.3 Record Date pg Sr84/ml 86/88 : 97 Ed in shelf Av. 251-.7 (a) Old Spike (1967) G-2 5873 6/26/68 47.2 166.8 " 5068 1/11/67 1.041 .1195 .7102 6005 10/14/68 47.5 167.6 " 5070 1/12/67 1.035 .1191 .7103 6064 12/2/68 47.5 167.8 " Av. 17. 5090 1/24/67 1.039 .1191 .7097 Av. 1.038 716'1

(b) New Spike (1968)

5379 6/29/67 1.947 .1196 . .7130

6030 10/31/68 1.947 .1203 .7105 Av. 1.947 .7117 Directly 7. Sr in Standard Rocks Calculated Measured Normalized Normalized 86 87/86 Rock Record Date Sr Sr 86/88 87/86

GSP-1 5866 6/20/68 22.5 233.7 .1201 .7682

6069 12/6/68 22.4 233.0 .1200 .7683

6153 1/28/68 .1204 .7700 ' Av. Y33 77683

G-2 5870 6/25/68 45.8 473.7 .1194 .7077

6061 11/26/68 45.9 474.6 .1202 .7097

6004 10/14/68 46.3 478.7 .1206 .7101

6149 1/27/69 .1195 .7104 Av. 475.7 .7092

8. Comparison of Rb Analyses with Other Laboratories G. Faure M.I.T. · U.S.G.S. H.W. Fairbairn Carmichael Isotope Rock values values X-ray Value x-ray Value Dilution

GSP-1 249 253 255* 260

253 253

253 251

G-2 167 173 176* 175 176

168 174

168 175

9. Comparison of Sr Analyses with Other Laboratories G. Faure M I. Rock Sr ppm Sr 87/s36 rEPGPIS. 3:8 : ittairn a chael 3 In

GSP-1 234 .7682(calc) 236 .7687 232* 230

233 .7683(calc) 236 .7686

- .7700(I.R.) 235 - 473 G-2 474 .7077(calc) 478 .7099 478* 465

475 .7097(calc) 480 .7102

479 '.7101(calc) 486 -

- .7104(I.R.)

* Relative to Rb = 220, Sr , 252 in G-1 standard. 98

' .,; . 1. , '.i 1 ... i : .,··.... :. . iTHIS''PAGE-,f:i- z ''. WAS INTENTIONALLY LEFT BLANK 99

Progress Report on Rock Standards

We have compiled in Table I all our Rb-Sr isotope dilution results for the U.S.G.S. rock standards G-2,

GSP-1, AGV-1, and BCR-1. We have not yet analyzed PCC-1 and DTS-1. Included also in the table is the

Canadian syenite standard CAAS. Based on replicate analyses, using differ6nt weighings and spectrometers as noted in Table I, our results suggest self-consistency for G-2 and GSP-1. For the remainder, analyses are confined to duplicates run on one instrument only and, except for SY-1, agreement is below par.

We have compared in Table II our data with U.S.G.S.,

Penn. State and Ohio State University results. GSP-1 shows satisfactory agreement of Rb, Sr, and, for age determination, the critcal ratio Rb/Sr. BCR-1, although based on only two analyses by us, agrees satisfactorily with U.S.G.S. results. Sr in G-2 and AGV-1 may be considered in satisfactory agreement, but Rb in both =· standards needs further investigation.

References

Goldich, S. S., C. 0. Ingamells, N. H. Suhr, and D. H. Anderson, Analyses of silicate rock and mineral stan-

dards. Can. J. Earth Sci., 4:747, 1967.

Flanagan, F. J., U.S. Geological Survey standards II. First compilation of data for the new U.S.G.S. rocks.

Geoch. Cosmo. Acta 33:81, 1969. TABLE 1 ISOTOPE DILUTION ANALYSES OF ROCK STAN DARDS Standard Sample Preparation Rubidium (Ppm) Strontium (Ppm) (Rb/Sr) wt

Lab. Book Ref. Spikes m pg/mi. Connie Sally Mean Connie Sally Mean

G-2 Granite A-Book 16,58 8 11.30 Rb 1.0365/04 166 159* 474 480 477

(U.S.G.S.) B-Book 17,49 2 11.46 Rb 1.95 Sr84 167 170 168 479 477 478

Split 37 Position 4 C-Book 18,17 7 11.46 Rb 1.95 Sr" 169 171 170 475 479 477

D-Book 18,68 3 9.90 Rb 169,176

E-Book 18,73 2 9.90 Rb 2.40 Sre' 169,170 484

Means - 167f3.9 171k2.9 16923.5 47616.5 480+5.1 478£5.8 ..3536.0.0085

(2.1%) (1.2%) (2.3%)

GSP-1 Granodiorite A-Book 16,58 7 11.30 Rb 1.036Sre, 248 248 248 234 236 235

(U.S.G.S.) B-Book 17,49 1 11.30 Rb 1.95 Sr" 252 233 236 234

Split 30 Position 19 C-Book 18,17 6 11.30 Rb 1.95 Sr84 253 249 251 236 9 0 C Means- 251f6.5 249 25012.5 - 111 236 22111.5 1.064t:0.0130 (1.0%) (0.64%) (1.2%)

AGV-1 Andesite A-Book 18,57 6 9.90 Rb 1.95 Sr84 66.1 „ 653

(U.S.G.S.) B-Book 18,68 4 9.90 Rb 2.40 Sr84 67.8 665

72 Position 23 Means - 67.0 659 Split - - 0.1017t.0020

(Assume 2%)

BCR-1 Basalt a-Book 18,57 5 9.90 Rb 1.95 Sr" 327

(U.S.G.S.) B-Book 18,68 5 9.90 Rb 47.0

Split 27 Position 17 C-Book 18,73 1 9.90 Rb 2.40 Sr84 49.3 337

Means 48.2 332 -li - - 0.145250,)29

(Assume 2%)

SY-1 Syenite A-Book 18,57 7 9.90 Rb 1.95 Sr84 145 212

(C.A.A. S.) B-Book 18,68 1 9.90 Rb 2.40 Sr,4 146 214

Means -- 145 213 0.68lt.0136

(Assume 2%)

* Omitted from calculation of mean. 101

TABLE 2

INTERLABORATORY COMPARISON OF ANALYSES

Rubidium (ppm) Strontium (ppm) (Rb/Sr) wt

M.I.T. U.S.G.S. Ohio M.I.T. U.S.G.S. Penn. Ohio M. I.T. U.S.G.S. Ohio State

G-2 169 (6) 174 (3) 176 (1) 478(7) 481(3) 479 473(1) 0.35361.0085 0.361 0.372 *3.9 f2.4 (2.3%)

GSP-1 250 (5) 252 (3) 225(5) 240(2) 214 1.064 i.0130 1.06 22.5 fl.5 (1.2%)

0.1017t.0020 0.104 AGV-1 67.0(2) 69.0(3) fill (2) 664(2) 667 (Assume 2%)

BCR-1 48.2(2) 48.6(3) 332(2) lili (5) 336 0,1452£.0029 0.144 (Assume 2%)

Y 102

' i... 1· 3., '...-, „..· . .·,· . h'

4 .". 1 ·T.H IS PAGE.t Lt. 2 < WAS·INTENTIO-NALLY LEFT BLANK 103

Iron Extraction Procedures

for Rb-Sr Isotopic Determinations

V.G. Posadas and B.J. Fryer

The present work was undertaken to study the feasibility Experimental Procedure. of using organic solvents to extract the main bulk of iron Sr Partition: Three iron ore samples (hematite jasper from iron-rich samples in order to determine their Rb and from Marquette, Michigan) were weighed and dissolved Sr content and isotopic composition. in vycor dishes with 6.6 N HCl acid. An insoluble resi- Two things had to be determined: effectiveness of Fe due consisting of a quarz (X-ray identification by Y. removal from the HCl solution and possibility of any par- Pellitier) was filtered out. The filtrates were ad- tition of Sr between the organic and inorganic phases. justed to the same volume and Sr85 tracer was added. The extraction of Fe from solutions of HCl has been ex- Filtrates plus tracer were transferred to 150 ml sep- tensively studied. In our study, three organic solvents aratory funnels and an equal volume of organic reagent were used: ethyl acetate, 2-heptanone (amyl-methyl ketone), was added. The samples were agitated for two minutes, and isopropyl ether. allowed to separate and the inorganic and organic layers The acidity of the HCl solution is an important factor, were collected in separate plastic beakers. The activity since the extraction coefficient depends strongly on it. in both phases was checked with a scintillation counter. The Fe is extracted in the form of FeC13 and HFIC14, the degree of. formation of both species depends on the concen- Fe Removal: Sample R7475 was used (jasper from near Three samples of about tration of the HCl solution, according to Irving and Wil- Michigamme Mine in Michigan). 0.250 gr were weighed, placed in vycor dishes and dis- liams the degree of formation of those species is large enough at concentrations of 4-5 N HCl to get a high per- solved with 6.6 N HCl on a hot plate. Once in solution they were filtered and the filtrate collected and ad- centage of extraction. Apparently there is no formation

of higher complexes with increasing acidity, therefore justed to 100 ml with 6.6 N HCl in volumetric flasks. the limiting factor will be the increase in solubility One 2 ml aliquot was taken from each solution and di- of the·organic phase with increasing HCl concentration. luted to 250 ml in a volumetric flask adjusting the It has been found that an acidity of about 7M will acidity to 0.2 N HCl to be used in the atomic absorp- give a maximum extraction when the organic phase is tion determination of the initial concentration of Fe. ether, a further increase in acid concentration will The remainder of the solution was quantitatively trans- increase the miscibility of the organic and inorganic ferred to beakers and evaporated to about 40-50 ml, phases decreasing the effectiveness of the extraction then quantitatively placed'in separatory funnels and

(Irving and Williams). shaken with equal volume of the organic reagent for two For the ketones there is a sharp increase in the minutes, allowed to separate and the inorganic layer

distribution coefficient with increasing HCl concen- was washed into a volumetric flask. One 2 ml aliquot

tration, reaching a maximum and then falling (Kuznetsov). was taken from each solution and diluted to 250 ml in With isopropyl ether the decrease in effectiveness volumetric flasks with 0.2 N HCl for atomic absorption of the extraction takes place at higher HCl concentra- determination of Fe. The samples were labeled 1, 2, tions than with ether, due to the lower solubulity of and 3. Sample 1 was extracted with 2-Heptanone, 2 with the former respect to the latter (Morrison and Freiser). Ethyl acetate and 3 with isopropyl ether. Following According to Kuznetsov, the ketones will remove prac- extraction, the samples were labeled la, 2a, and 3a. tically all the Fe 3in a single extraction. Under con- The atomic absorption standards were prepared from trolled conditions of acidity and Fe concentration, the Will Scientific Company standard' for Fe+3. The Melnick claims that 99.9% of the Fe will be removed in + 3/181 in concentration of the standard is 0.2 ugm Fe a single extraction with diisopropyl ether. dilute HCl. The possible partition of Sr between the inorganic

and organic phases will be determined for the three

organic solvents using Sras tracer. Table 1. Sr Partition Activity Organic Acid Organic |Background Phase Phase Phase

2-Heptanone 12,209 c/min 440 c/min 398 c/min Isopropyl 7554 ether c/30sec 231 c/30sec 240 c/30sec

ethyl 6189 acetate c/30sec 220.c/30sec 227 c/30sec

As expected, there is no evidence of significant Sr partition into the inorganic phases. 104

standard Tablc 2. Fe+3 Tablc 3. Atomic Absorption Results

Scientific Fe+3 Dilute to: Fe in Solution Will Sample % Absorption Absorbance Fe conc. in Total Fe Standard ml ugm ml ppm/ml ppm/ml in ugm

A 5 1000 2000 5 A 6.4 0.0283 5 ppm/ml - B 11.8 0.0429 10 B 10 2000 200 10 ppm/ml - C 19.27 0.0931 15 ppm/ml - C 15 3000 200 15 D 24.83 0.1238 20 ppm/ml - D 10 2000 100 20 1 12.13 0.0560 9.5 ppm/ml 118,750 2 12.46 0.0575 9.75 ppm/ml 121,875 The standards were diluted in 0.2 N HCl. 3 12.63 0.0585 9.85 ppm/ml 123,125 la .63 0.0027 1.15 ppm/ml 7187 2a .83 0.0036 1.3 ppm/ml 8125 3a 1.06 0.0046 1.5 ppm/ml 9375

The reproducibility of the results for la, 2a, and

3a was poor due to the low Fe concentration. In any case, the Fe removal was shown to be better than 90% for sam-

ples with an extremely high Fe content using any of the

three organic solvents. Isopropyl ether was the one chosen since 2-Heptanone,

despite being very effective in the removal of Fe, was very difficult to eliminate from the HCl solution due to its high boiling point (150'C) and·Ethyl Acetate

showed a solubility in the organic phase higher than

any of the others.

References

Handbook of Chemistry and Physics, 49th edition, 1968-

1969.

Hillebrand and Lundell, Applied Inorganic Analysis,

Wiley, New York, 1953.

Irving, H. and Williams, R. J. P., Liquid-Liquid Ex- traction 12. Treatise on Analytical Chemistry, Part I,

Vol.3, Kolthoff, I. M. and Elving, P. J., editors. Interscience, 1962.

Kuznetsov, V. I., Extraction.of terrie chloride by or-

ganic solvents from hydrochloric acid solution. Chem. Abs. 42: 18, 1948. Melnick, L. M., Iron i:Il Treatise on Analytical Chemistry, part II, Vol.2, Kolthoff, I. M. and Elving, P. J.,

editors, Interscience, 1962.

Morrison, G. H. and Freiser, Solvent Extraction in

- Analytical Chemistry,stry, Wiley, New York, 1957. 105

An Isotope Effect Suppressing Absorption in Measuring Concentrations of Spike Solutions by Atomic Absorption Spectrophotometry W. H. Pinson

Absorptions by the resonance lines of Rb, K and Sr are sup- pressed measurably when solutions of like concentrations (atoms/ml), but different isotopic compositions, are compared by atomic absorptiun spectrophotametry. This isotopic effect results from the overlapping of the sharp resonance lines arising from the several isotopes of a given clcment. The line profiles of the resonance line, arising in the source from an element of normal (terrestrial) isotopic composition, is slightly different from' the line profile of the absorbing line of this element, which has been enriched in one of its least abundant isotopes. Thus the absorbing atoms in the· flame miss a'small portion of the photons of the wave. 1.ength of the resonance li„e from the source. For Rb the effect is 11.3 + 1.0% which permits a measurement of the Rb85/Rb87 ratio by-phis method to within + 4%. In geochronametric work, this isotope shift effect must be taken into consideration when making spike calibrations by atomic absorption spectrophotometry. Other- wise, serious error can arise for both Rb and Sr and possibly for K spike calibrations. Table I .shows the results of this investigation

Table I

comparison of Spike Solutions by Isotope Dilution and Atomic Absorption Spectrophotometry Analyses

Concentration Concenttation of Spike of Spike Spike Solution by I.D. · Solution by A.A.S. Cmpositioh gm element/ml gm element/ml %

I 83% Sr 4.70 1 0.03 4.38 + 0.04 -6.8

98.7% K" 3.78 + 0.06 3.62 + 0.03 -4.2

7 99.2% Rb 8 8.74 + 0.13 7.74 1 0.11 -11.4

5.07 + 0.08 4.45 1 0.04 -12.2 4.60 + 0.07 4.13 1 0.02 -10.2 106

THIS PAGE WAS INTENTIONALLY LEFT BLANK r.

107

BIBLIOGRAPHY OF AGE DATA ON CONTINENTAL BASEMENT ROCKS

J. R. Rand

(1968) Potassium-argon age studies In connection with a' survey of the radiometric ages of conti- Larsen, 0. and J. Mdller nental areas, the folloUing list of references was compiled from most standard journals, privately published bulletins or laboratory in west Greenland. Canadian Jour. Earth Sciences, 1, The references define reports, and some unpublished age studies. No.3, Pt.2, 683-691. radiometric ages (predominantly K-Ar and Rb-Sr data) primarily for continental basement rocks in North and South America, Africa, Moorbath, S., H. Sigurdsson and R. Goodwin (1968) K-Ar Europe, Asia, Australia and Antarctica, as reported or otherwise rocks in Iceland. Earth made available through 1968. In a few cases, reports published ages of the 'oldest exposed early in 1969 are included for convenience. and Planetary Sci. Letters, 1, 197-205. References covering age data for continental areas in the CANADA U.S.S.R. and China are very limited in number in this compilation because most reports presenting age determinations for these areas do not describe the actual locations of samples. Since the study for which these references were collected was not particularly Baadsgaard, H., S. S. Goldich, A. 0. Nier, and J. H. Hoffman concerned with Phanerozoic folded or intruded mountain systems or (1957) The reproducibility of A40/%4 age determinations, younger intrucive rocks, references describing dates of these rocks are also very limited in number in this compilation. Trans. Amer. Geophys. Union, 11, 539-542. This compilation includes more than 500 references, and is L. Wetherill, G. W., G. L. Davis, and T.'Aldrich (1957) Age sub-divided consecutively by areas as follows: Greenland-Iceland; Canada; United States (further sub-divided by. States) ; Mexico; measurements on rocks north of Lake Huron. (Abstract) South America; Africa; Europe-British Isles and Europe-Continental, Asia-India (including Japan); Australia-New Zealand; and Antarctica. Trans. Amer. Geophys. Union, lt, 412. The references within each sub-division are listed chronologically, Gast , P. W., J. L. Kulp and L. E. Long (1958) Absolute age with the earlier works listed first and the most recent reports last.

This is believed to be a fairly complete compilation, although of early Precambrian rocks in the Bighorn Basin of certainly not definitive; there may be some inadvertent omission of Wyoming and Montana, and southeaetern Manitoba. Trans. valuable material through oversight. It is hoped that in the future the collection and compilation of accurate and complete Amer. Geophys. Union, 39, 322-334. world-wide age data can be effected by an international data center Rb-Sr and K-A ages to which all age measurements will be reported. Aldrich, L. T. and G. W. Wetherill (1960) of rocks in Ontario and northern Minnesota. Jour. Geophys. Res ., fl, 331-340. NOTE : Map plots of age data for continental rocks, as derived from the references listed here, are presented and discussed in: Fairbairn, H. W., P. M. Hurley, W. H. Pinson and R. F. Cormier (1960) Age of the granitic rocks·of Nova Scotia. Geol. Hurley, P.M. and J.R. Rand (1969) Pre-Drift Continental Nuclei, Science, 164, No. 3885, 1229-1242. Soc. Amer. .Bull., 71, 399-414.

Mair, J. A., A. D. Maynes, J. E. Patchett, and R. D. Russell

(1960) Isotopic evidence on the origin and age of the NORTH AMERICA Blind River uranium deposits. Jour. Geophys. Res., fl, GREENLAND - ICELAND 341-348. Moore, J. M., S. R. Hart, C. C. Barnett and P. M. Hurley (1960) Kulp, J. L., R. Kologrivov, J. Haller and L. Koch (1962) .in northern Manitoba. Geol. Soc. Potassium-argon ages on rocks from eastern Greenland. Potassium-argon ages 71, 225-230. Nature, 194, 953-955. Amer. Bull.,

Moorbath, S. and H. Pauly (1962) Rubidium-strontium and lead Quirke, T. T., S. S. Goldich and H. W. Krueger (1960) Composition and age of the Temiscamie iron-formation isotope studies on intrusive rocks from Ivigtut, Territory, Quebec, Greenland. Tenth Ann. Prog. Rept., Dept. of Geol. and Mistassini Canada. Econ.. Geol u it,

Geophys., Mass. Inst. Tech., 99-102. 311-326. R. netherill, G. W., G. L. Davis, and G. Tilton (1960) Age Armstrong, R. L. (1963) K-Ar dates from west Greenland. measurements on minerals from the Cutler batholith, Geol. Soc. Amer. Bull., 74, 1189-1192. Cutler, Ontario. Jour. Geophys. Res., 12, 2461-2466. Wager, L. R. and E. I. Hamilton (1964) Some radiometric Baadsgaard, H., R. E. Folinsbee and J. Lipson (1961) rock ages and the problem of the southward continuation of the east Greenlau Caledonian orogeny Nature, 204, Potassium-argon dates of biotites from Cordilleran

1079-1080. granites. Geol. Soc. Amer. Bull., 72, 689-702. The use of hornblendes and pyroxenes for Gale, N. H., S. Moorbath, J. Simons, and G. P. L. Walker Hart, S. R. (1961) K-Ar dating. Jour. Geophys. Res., 2995-3001. (1966) K-Ar ages of acid intrusive rocks from Iceland. , Earth and Planetary Sci. Letters, 1, 284-288. Smith, D. G.. W., H. Baadsgaard, R. E. Folinsbee and J.

Hamilton, E. I. (1966) The isotopic composition of lead in Lipson (1961) K/Ar age of lower Devonian Bentonites igneous rocks -,I. The origin of some Tertiary granites. of Gaspe, Quebec, Canada. Geol. Soc. Amer. Bull.,

Earth and Pldnetary Sci. Letters, 1, 30-37. 72, 171-174.

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:'. 2. z iTHIS'PAGE A WAS INTENTIONALLY LEFT BLANK 129 Correlation of radiometric ages of Nipissing diabase and Huronian metasediments with Proterozoic orogenic events in Ontariol

H. W. FAnWAIRN AND P. M. HURLEY

Massachusetts InstituteK. D. of Techno/00'.CARD Can:brid:e, Massachusetts Ontario Department of Mines, Sudbury, Ontario AND C. J. KNIGHT Carleton U iversity, Ottawa, Ontario Received February 3,1969 Accepted for publication Marcli 19, 1969

TABLEI Ficld evidence shows that tlic Huronian scction of mctaidimcnts in Ontario : younger than the Summary of Rb-Sr isochrons for scdiments and volcanics in Espanota-Sudbul section Archean and older than ihc Nipissing diabase. Previous radiometric investigations show ilic age or thc latest Ard,can granitic i/rusions lo beabout 2500 m.y. and the Nipissing diabasc ao be 2155 f SOm.y. Isochron Age' data th/ diabasc in a In this pal,cr wc present whole-rock Rt>-Sr which (1) confirm the agc of single sill al Formation (m.Y.) Initial '.Sr/.Sr No. Samples Gowganda (2162 i 27 m.y.), and (2) show the age of mc Gowianda Formation (which thesill intrudcd) to be 2288 1 87 m.y. In marked contrast me,asediments and mciavolcanics in the Huronian scction 1810+90 southwcst of Sudbury show younger ages ranging between 1800 and 2200 m.y. This is interpreted in Gowganda (tittite. argillite) Espanola (calc. silistone and limestone) 1802 i 105 .Vist .*61 Ims of known oro/nic dislurbance of the Huronian rocks south and wes, or Sudbury and ils absence 2042 t 48 .7123:E .0010 4 in the Cobak-Go/gatida Irea 50 the northcasl. Both mincials and whole rocks have responded by Rice (Conglomerate. argillitc) McKim (graywackc, argillile) 2174*160 .7072+ .0042 6 showing low and discordam agcs in the disturbed ara. The higher and accordant agcs in the undisiurbed 2005 + 125 .7048£.0014 5 environment agree with th, geologic succession and are believed to be thctrue ages of thcsc rocks. Basal volcanics (basalt, andcsite) All scdiments and volcanics 1950+100 .712 +·003 29 (Weighted) (approl.) (approx.) Introduction isochrons for a sing. diabase sill and for the The Huronian (Proterozoic) section of Ontario Huronian (Gowganda Formation), which it intruded. and adjoining Quebec (Fig. 1) has an exj'osed area of about 10000 sq. mi. (- 25 900 km ) and -=.- Age of Iluronian in the Espanola-Sudbury Area - .....'.g .... is approximately 33 000 A (- 10 060 m) thick, ...JEC .92 - ALL..ES ..1.ENI.....111< excluding basal volcanics. A wide variety of Samplcs from fivc individual horizons in the .SPANOLA +SUOetm a86- sediments is represented, dominantly quartzite, central Huronian section between Espanola and - graywacke, and congloincrate. Collins (1925) Sudbury (Fig. 1) were selecled for whole-rock C m. (m- .. made the first comprehensive study; his work, Rb-Sr analyses. Individual isochrons have been T elsewhere and that oflater investigators, has been reviewed compiled (Fairbairn e, 0/. 1967b). The - '=...... -- ' '--- N- f ... recently, with art extensive bibliography, by data are summarized in Table 1 and Figs. 2 and 3. on- ,---... --- '949." - 1800-2200 m.y. Young and Church (1966). The section overlies Ages range from with consider- 1%/%1•07,231'00 the Archean basementcomplexwith great uncon- able individual scatter. As the entire scction was -=mor,J, fomity and is everywhere inlruded by thick undoubtedly deposited within the span of time represented by the isochron .... 1 : sills and dikes of diabase usually referred to as uncertainties, a o .../.Sr composite diagram has been constructed with a Nipissing. The youngest rocks of the Archean Fio. 2. Weighed mean slope and intercept of indi- mean slope and vol- basement are granitic and wcre emplaced about representing 1950 i 100 m.y., vidual isochi·ons of Huronian mctasediments and mclascdininlFlo. 3. Sumnlary and vokinics or 29 Rb-Sr in the analyses Espanot/-Sudbury of Huronian 2500 m.y. ago 0.003. Figure 2 canics. LSC = Least Squares Cubic. NumbNs in (Fairbairn et a/. 1967a; Stockwell initial "SH"Sr of 0.712 * = number or analyzed samples. area. shows the essential (slope, parentheses 1968). The diabasc has recently been reported features centroid, old. number of analyses) of the isochrons of the by van Schmus (1965) as 2155 180 m.y. TABLE II The Huronian must therefore have been laid individual Huronian groups; Figure 3 shows all down between 2500 and 2155 m.y. ago. To tcst the analyses, undiffcrentiated as to position in Analytical data-Nipissing diabase sill this inference independently wc present (1) the section. In view of the 2150 m.y. age of the Tliffrrr r, in excess Rb-Br age data for these scdiments and for diabase and of the scatter of analytical Isompe ditution 'Sri**Sr X.ray fluorcscence error shown individual IIuronimi -Rb/··Sr .7* te;)30 voicanics at the base of the section; (2) Rb-Sr by isochrons, Sample Rb pprn Srppm RWS, .7Rw"Sr (measwed) Rblsr 1950 a 100 m.y. must bc interpreted as a meta- nd 0.7823 aM.LT. Age Studies No. 87. morphic, and therefore minimal age. R6834 nd * :tio) *1.6%2·«: 2.97 +4.2 C...... 1.....le...6,489(969) 102.7 0.978 2.85 R6835 100.5 0.79,0 *t::08 &22%

+5.2 = 0 16836 Id •d 0.462 1.34 0.7479 * · 0) 1.41 *t. R6837 55.8 120.7 14.7% SKETCH MAP 0.7456 * m (0 GEOLOGICAL \ (9 0.531 +1.7 SHOWING DISTR..... 1 .- 13.47 R6838* 74.9 0.180 0.522 0.7246 .AM + 0.54% 1· RONIAN SEDIMENTS 0.7547 1.65 -6.8 58.0 106.6 0.544 1.58 0.5680) AND R6839 0.7551 to.016 *30% +2.2 NIPISSING DIABASE 0.783 2.28 0.7748 1„ R6840 55.85 71.3 *:.= 0' 16.9% -0.7 IN -- 0.7280 0.719 29.5 118.3 0.249 0.722 ONTARIO 116841 *:fieo :1 6.0% „.P=...... ,-'... = '.I .7 +5.5 16842 70.3 86.6 0.,12 2.37 0.7790 *::=Ce *i:A „... ( 1.695 0 0.584 1.70 0.7580 - R6843 61.2 104.8 J:&0) *5.2% -2.0 0.7367 16844 37.4 107.7 0.347 1.01 *:.m) t':ri .44\ 1„.. -42 1.065 +3.5 ia 1 --- -e- •ng r 39.3 110.6 0 355 1.03 0.7370 (') -. . R6845 *rm /3.8% 0.745 +2.3 0.728 0.7290 (4) 09Lk/.; ../. R6846 32.6 130.0 0.251 129.8 f 5 5 *70% -,---4.2.1 0.366 1.63 0.7373 (4) 1.695 +3.0 R6847 70.2 124.1 a:.m *5.1% *':--ILI r ..; Average + 1.6

Flo. I. Schematic map of geological units. NOT,·ad.•motdr:r,rnfr, 1 .....lim lilI.C. oompu.... because ..1... t ceeled ..1.ic' error. TABLE III #MFOI,Ati..N ...... Analytical data-Gowganda Formation G..KI. 041.. Buor.scence Isotope ditution X.ray pr==' Pmer"" .twl/1 - -at••S, .'Sr;'6Sr (computed) (m..Ircd) RWSr 87Rb/"Sr .,87 .*M 4,11; 4.11*ILI:rl .Imple ppm Ppm •'Rbr,Sr 7 .1 , -1' -lry 76. 0 2 4.03 0.8363 .- .p . - ...0-2,"....1-4.' R7146 99.5 73.8 3.96 +1.8 -0.0016 +4.0 +0.0018 1.505 0.7553 0.7571 R7147 59.6 115.6 0.,M7 : li :; .t*5.9, :F -2.3 +0.0006 0.7232 0.7258 0:64f R7149 31.2 136.6 0.662 'i'.-/..W: +1.8 -0.0020 * : 4 1.12 0.7475 0.7455 /< R7138 64.8 72.2 2.62 0.7887 *t=0 *2& 0.7185 4.96 35.1 0.409 0.7170 ig:%%1% R7159 0.7828 2.62 It.9 R7160 93.4 103.9 2.57 0.7840 %#m- 3) -O.0009 - 0.7920 0.7911 *i:% -4.5 740 R7161 28.3 28.4 2.91 *ra 13.3% gr *m:,7-1 -- - +0.4 O.0011 1 d#85*7061•00005 Averages 700 /''t 1 „-,„ ...... 00.Lot,5202.510 .'Rt)/8 5. TABLE IV Mo. 5. Rb-Sr whole-rock isochron of diabase sm, Summaty of isochron ages Gov/ganda. Analysis at lower left of diagram (douWe circle) omitted from linal kast-squares calculation. Rb-Sr isochron Rock name, group Refeince Location - age (my.) Initial "Sr/"Sr 0 . 1 i (sce Fig.1, at 2288 + 87 0.7053&0.0019 This work GOV...... 0. 10 Imples °%2rdi-Cobalt (northeast GowgandaGov/gandi Formation, This work at 2162127 0.706110.0005 80 - MipiwingGowganda Diabm ,&, It sampres 80 0.7060 Van Schmus Sta Nipissina diabase from 2155 i /,K, Elliot Lake - Sault 9 (1965) 5 scattered localides samples Marie (western area) This work scdiments 1950&100 0.714 (excluding Espanok-Sudbury (south- Combined Huronian .- ../=-' -'m- -- and volcanics volcanics) 1 =-* central area) 1175& / r./*-I.- I 30 l i l i 0 . 20 30 40 .W'. Flo. 6. Rb-Sr whole-rock isochron of Gowganda Formation at Gow,anda. lity,lirm 31*Ii tf lEi'll:mile,%14 1, i w i s fil·imt :Elittel lir " iii:ifils,911,1,6 i:igi il M ilii 51" i i:il li i Riti tifi: i il , :91 11 liptii i tilit a ti::68:gil ti lifii i

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P. M. HURLEY J. R. RAND

JITIODUCTION

The hypothesis of continciital drift was first studied intensively by geologists who vitilizcd the matcl,ing of Phancrozoic geologic ages, ecology, and structures as their evidence. 71ic advent of radionictric agc dating tcchniqucs cxtcnds thc rang. of time ovcr which correlations can be tested, and ncw interest in the matching of pre-Cambrian age prov- inccs has arisen (Miller, 1965; Almcida et at., 1966; Hurley et al., 1966; Hart et al., 1966). In this review K have adjustcd the coordinatcs for the separate continents according Gro X to a pre-drift reconstruction of Bullard d al. (1945). We are concerned only with the 0..... basement age provinces in West Africa, Brazil, the Guianas, and Venezuela, so that ['13 •-·-· -,··- inforniation beyond thcse geographical limits, and iii the Phancrozoic, will be ignored. t,«3= 7 1 Wc have also not included lend-alph2 and model ai,d chemical Icad agcs (except in the schcniatic presentation in Fig. 1) to avoid a mixing of data that becomes difficult to VIC " ..17<9. 1+15j4--- interpret, although it is recognized that tlicsc methods gave important early indications --- \ /r\ /\'· .1\ ':'·'·:11 of the general agc provinces in Africa (Holmes and Cahen, 1957; Cahen, 1961 ). The combination of mineral ages by tha K-Ar and Rb-Sr methods and whole-rock ages by the R b-Sr nicthod is considcrcd to present the best information on mctamirphic asseni- 7-1 X » fpr. bliges of all gi ades, and these data arc indicated separately in the figures by the f Sli SOUTH, AM \\ P--- notations. All age values arc presented on the basis of A (Rbs:) = 1.39 X 10-11 yr-1, ,/0 Fi8ure 1. West Africa aid northern South America according to Bullard's predrift reconstmction, A, (K•°) = 0.584 X 10-10 yr-'; AB (K'°) - 4.72 X 10-,cyr-1 showing regional dist,ibulion of 2000 + m.yr. and 600 & m.yr. age provinces.

DIE WEST AFRICAN CRATON

The pre-Cambrian Guincan Silicld of Wast Africa is dominated by two principal age ST provinces, with a sharp boundary between them extending northerly from near Accra tllrough Ghana, Togo, Dahomcy, Upper Volta, and Nigcr (Fig. 1). To the west of this boundary, in Ghana and the Ivory Coast, the basement rocks record a period of oro- ." genesis that occurred niostly in the time interval 1800-2100 m.yr. B.P., as first noted by r 1'· A{, .AW...#I Bonhomme (1962), who gave the name Eburncan to the orogeny (Fig. 2). Other a ..: 1% , , .6 :1 a I , Al 8 114 A 6 gcochronological investigations representcd on Fifre 2 include Bonl,un„ne (1961 ). 4. 1 4-: + Vachctte (1964a), Hurley ctat. (1966 and unpublished),and Kolbc et al. (1966). The - ,. P 4:, 91 vi' A '4 ( 22 , 4694. chcma ir age data in Figuic 1 include also the measurements of Wilson (1954), Ber- 5.Er 11.·i,·· iADJ<,8 7/ /4 146. I nazeaud and Grimbert (1956), Bassot ct al. (1963), Lay and Ledcnt (1963), Eber- hardt et at. (1964), Ferrara and Gravelle (1966), Laing (1966), Picciotto ct al. (1966)· Scbnetzter et al. ( 1966), in the rcgion west of the principal age boundary. 1:.1.1 Ve- : ' Ti"A4 .' 1 4.#6641,) r. ' -'. '. -9. # wIL It can be scen (Fig. 2) that the 2000(zE) m.yr. Eburnean province is bounded on 1 -+ C/„'« +i#. gk -1 4 the west by 2700( =t=) m.yr. rocks in Westcrn Liberia, and apparently aiso to the north te- r h.y,: by similar ages in western Abaggar (Sahara). The West African Craton is therefore a W „«;*'. 7':r,·-V*4.hooe , ' L .» ..32<3 -* complcx of ancient rocks with at Icast two age groups. In this report we shall tentatively I : =.- - Libcrian for the 2700(=1=) m.yr. orogcnic episode, since the first homo- 1 i Di . '..7 1:44,%//4 i Use dic term , mi " ·.f ./&/. =d) - --....- Feous province of this age was found in Liberia. :EE=: .--- 5 / ...... ,,"...... To the mt of the principal age boundary (Fig. 1), in Togo, Dahomy, Nigeria, and - ...... , ---I Camcroun the age values fall abruptly to the range 500-650 m.yr. by both whole-rock 1:E-- Rb-Srand K-Ar analysis. Occasionally the wholc-rock method indicates an ancient retic province extends of a pre- xisting basement. RccentIy it has becn found that this age West African craton including Sierra Lcone, Libcria, Ivory Coast, Ghann, Togo, and Dahomey. cratons Figure 2.All Reported at recakulatcd and unpublished to Rb"A = 1.39 radiomet,ic X 10-" X yr.-1. m data w, and whole genmlized rock, b, gcologicabiotile; m. trendsmuscovite; in the I, 93 a great continuous belt of rejuvenated rocks around several of the Archean lepidolitc; f..K.fcldspar; p, plagioclasc; h, homblend/; Pr, pyroxene: 4 augite; na, Na.amphibol. of Africa, and has thus becn called by Kennedy ( 1964) the Pan-African 0..g.nic Cyde. 0. Rb·Sr whole rock; 0, Rb-Sr mineral; X, K Ar. Age measurements in this province (Fig. 3) include the work of Holmes and Cahen (1957), Hurley et aL (1958), reported by Cahen (1961), Snelling (1963), Jacobsen et al. (1963), Vachette (1964b), Lame (1964), Bessoles et at. (1956), Boissonnas et at. (1964), Roubault et al. (1965), Bonhomme et al. (1965, 1966), Cahen and Snelling (1966), and Hurley et al. (1966). Younger trusives were emplaccd u late as 100 m.yr., but for the most part the age range of 500-650 m.yr. covers both K-Ar and whole-rock Rb-Sr values, although the Rb-Sr ages tend to bc slightly higher than / the K-Ar ones. To the south in Gabon, thc agcs increase again to 2000 + 200 m.yr. (with scattered regions of -2700 m.yr.) in the Congo Craton (Lasserre, 1964; Vachcttc, 1964b). --Lk««74.-»--7 7 BASEMENT AGES IN Soint AMERICA 1£ : 'i 'ir - Geochronological investigations in Venezuela and the Guianas are represented in 7fla 11 -_ -.: Figure 4. Published and unpublished reports jncludc the following: Short and Stcenken (1962), McDougall et al. (1963), Sncning (1963, 1962, 1964, 1965), McConnell ct Chase Posadas «,i al. (1964), Choubert (1964), (1965), Pricm ct at. (1966), (1966), Hufley ct al. (1966). Two good whole-rock Rb-Sr dates on the gncisscs on the /31'F -- u -5 11 Complex in Venezuela give ages of 2900 and 3050 m.yr., suggesting that the northernImataca 7 - 0 13 11 part of the Guayana Shield (or the western part, in the pre-drift orientation) includes an ancient province similar to that in thc West African 2-*9- - Craton. ne coincidence of iron -·'• ··- '...·'r. · ,53.- mgcs in the Imataca Complex of Venezuela and in the Bomi and Nimba ranges of the same age in Liberia is mentioncd in passing. The remainder of thc Eastern Guayana Shield appcars to be dominatrd by the 2000( 1 ) m.yr. age value, similar to that found for thc Eburncan orogcny in West In Figure 1 it appears that the principal age boundary between the Eburncan and Pan-African age provinces in West Africa would cntcr Brazil close to the town of Sao Luis if the continents had bern together at that tinie. Recelit investigations by the .*. - 04-0 69 1 11 geochronology laboratories at the University of Sdi Paula and Africa. the Massachusetts Insti- ---„....= _.»':11 tute of Technology havc verified this age boundary in this location (Almeida et at., 1966; Hurley et al., 1966). A combined report by ,=AS»»11 these two groups has just bccn publishcd (Hurley et al., 1967). A schematic presentation representing about 150 age measurements in northeastern Brazil by the K-Ar and whole-rock Rb-Sr ti,cihods is given in r PAS Figure 1. Figure 3. Reported and unpublished radiomctric age data in the area of West Africa affected by lr, the vicinity of Sjo Luis the age results showed two groups. Saniples to 1hc cast ihe Pan African Orogenic Cycle, including Dihomcy, Ninci·ia, Cameroun, an# Gabon. of Slo Luis fell in a young age group similar to thc Pan-African agc province in West All ages mcalculated to Rbg = 1.39 X 104 X yr.-l. w, whole rock; b, biotite; rn, muscovite; 1, tepidolite: f. K.fcldspar p. ptagiociase; h, homblendc; pr, pyroxene; 4 mgitc; na, Na-amphibote. Africa. This younger provilice appears to extend to the eastern coast of has Brazil; it 0, Rb·Sr whole rock; o. Rb-Sr mineral; L K.Ar. ken called the Caririan Orogcnic Episode. Ages are mostly in the range 450-650 m.yr. by both methods. 132

REFERLNCES Almcida, r r M dc, G C McILI„r, U G Cord,nl, K Knw-shita, and P Vandoros (1966), Abso- lutc age determinations from Me/Ings, p 3 Northern Braal (Abstr ), Grol Soc Amer Program, 1966 Ann Bassot,JP,M lonhommc, M Roqi,cs, and M Vach/ie (1963), Meaures d·Ages abwlus sur les *ms prk/mb,41,# / p,Womiglics du StneB/ nrirnial, Bull Soc Ge// Fia,ice (7lh Scr ) 5 401-405 Bernaica d, J, and A Grimbell (1956),Condittons dc g,scment el tgcde I urmnitc du Has-Cavally p- .. 4.» (Cate d Ivoim), Conip, Rend Acnd Se, 242 2744 46 Bess/Ics, B.J Cossort. J Grassaud, and M Rogics ( 1956), Age apparent de la diont, quart/que .-. .3'1. 3/: AS; des Sam dw k Mayon:bc (A E F ), Conip, R//id SH Geo/ France 7 86 87 Boissonnas, J, L Duplan, J Maisonicim, M Vichclte, and Y Vialcitc (1964) Eludc gdolos'quc / gdochronotolique de roches du Compartiment Suggaricn du lioggar Central. Alatile, A,in FRIc Sa Univ Clrrinmi, Ser Ged 8 (Clildes Gtochronologiques 1) 73 90 Bonhomme, M (1961), Absolute ages in the Bouna grantle mmif In Upper Volla and in the Windcnc « .3.--/f= t granitc m Ivoly Coast, Conip, Re,id Acad Sci Pans 252 (No 25) 4016·17 Bonhonime, M (1962),Contribution A Idaude gdochronologique dclaplatefornic dc IOucst Africain, -STit Rj 9' BiL Ann r// Sci Unt, Ciermonl, Ser Ge/1 5 V Bonhomme, M,F Weber, md R ravrc Mercurcl (1965), Agc par la mahode rubkh"In strontium des 56(liments du bliSSIn de Francoville (REpubl,que G6bonaise), Bu// Sen C rie Ge// Als,*:e Y >.1% 5 Le,inine 18(4) 243 52 -fk Bonhornme, M,J Lucas, and G Millot { 1966), Sl/lification des daernlinailons isolcpiques dans la Bullard.EC,JEgdochronologic Ever/t, dis ddlments. and A GC//loq Smith /,treri, (1965) Ce,itre The N// ft of Rech the conimintsSa Par,3. aroundNo 151, the pp Allanlic, 335.59 ..- in A symposium on conlinc/71 drift, Ph,1 Tr/63 Roy Soc 1088 4 1-51 Cahen, L (1961),Review of scochron/10/cal knowlcdge in middlc and northern Africa,Aim NY Accd SM 91 535-567 Cahon. L, ind N J Snelling (1966) The geock/0,1/109 0/ Equarort/1 / Tric/, North I lollied Publ e / I Co, Amsterdam Chasc, R L (I 965), El complclo de Imataca, la anfibolla de Panamoy la tron)mit a de ./ L / 6 Prclambillas dil Luadn]ntcro dc Las Adjunt,9-Pannmo, Fdo Botivar, Vcnezield,Gun Bol G/01rocas 7 j05 Figure 4 Reported and unpublished radiometrtc age data 111 tlic Guayana Shield of South Amcrlca. Choubert, B (1964). Ages absolus dit prtcambrten guyan/s, Compt Reild Acad Sci Perl, 258 All ages recalculated to RbS = 139 X 1 0" x yr ' w, whole rock, b. biottle, m. muscovle. 1, 631 34 lepidoltte, f, K-fcldbpar, p, plag,oclase, h, homblendc, pr, pyrox/ne, a, aug//, na. Na-amph,bole Eberhardt, P,G Fermra, L Glangeaud, M Gravellc, and E Tong:orgi (1964). Stir I Age absolu des 0 Rb Sr whole rock, 0, Rb Sr mineral, X, K Ar senes met/morplgues de I Ahaggar occidental dans la re/on dc Sllet Tibchnowne (Sahara Central), CIAB Red Acid Sc: Paris 256 1 1262% Sanlples m Sao Luis and westward to Amapi generally showed ages ot abou, 2000( 1 ) Ferrari, G, and M Gravelle (1966),Radtometne ages from Western Ahoggar (Sahara) suggisting an eastern hmit lor the West An„/1 4/vii, Earxh Plm* Sci L/ter, 1 119-74 m yr, with some exceptions that are bcing mvestgated aclivcly at present 1 This 2000( + E Krogh G ) Hart, S R,T L Davis, L T Aldnch, and F Munizaga (1966). A geochronological m y. province appears to extend #cstward (with the possible gap footnoted above) into approach tothecontinental draft hypothesis, ifi: Rep D/p Te res:nni M//:chmi. Washmgton, DC,pp 57-59 the Guiinas and Venezuela and matches tlie 2000(=k) niyr Eburnean age province Holmes, A , and L Cahen (1957), 06ochronologic africninc, remitals acquls au 1 Juillet 1956, dtrec:13 opposite In West Africa. Acad Roy Sc, Colon:nts (Bri,33813). Closse Su M .Afed , 5 Fasc. 1 Hurley, P M, with LS Cahen (1958), Ale program m the Lowcr Congo, Mi T Age Smdies, North of Salvador, near the cast coast of Brazil, the rocks affected and mtruded by NYO.3939, 6/h Ann Piogrns Report for 1938, USA[C Hurley P M,H W Fairbairn, W H Pinson, Jr (1966a), ContmentolContract AT(30 dnft 1)investigalions, 1381 M / T the 600(+) m yr Cartrian orogeny becomc bounded by an undisturbcd basement com- /nn Progress R/port. 138!-14, USAEC, pp 3.15 plcx referred to as the Sao Francisco Craton This cratonic area underlies the larger part Hurley,m Northern P M,G Brazil C Melcher, coriclatcd J w:EhR Rand, ages and In WestH W Fal*alin (1966b), Rb Sr whole rock analyses of nortliern Bahm A few measurements to date indicate that tic age of gncisscs 111 the Africa (Abstr ), Gwl Soc Amer, 1966 /nn Meell,igs Prograni. pp 100 M Sao Francisco Craton is 2000(1) m yr or greater, stnittar to the findings of Vachctte Hurley, P M,F F M de Almetda, a C Melclier, U G Cordm, 1 R Rand, K Kawaslilia, P Vandoros, W H Pinson, Jr, and H W Falrbairn (1967), 1 est of continental drift by companson the 0964b) for Congo Craton directly opposite on the African side of radiomeinc ages. S/,ence 157 493-500 hobs", R R E,N I Snelling. Ind J F Truswelt (1963), Age determinations I the geology of Nigena, with special reference to the Otdc, and Younger Gratnitcs, 01 ersens Gee/ Min Res 9 SUMMARY 168-82 A reconstruction of the position of South Alnerica adjacent to Wcst Africa Kennedy, W Q ( 1964), The structural diffcrentiation of Africa in the Pan-Afncan (Bullard kitomcepisode, R„ IM Aincnn Ged (Le/ds, UK),8,h An„ Re/1, p 48 (/500 m y) et al, 1965) and a companson of reported age data on basement rocks leads to the Kolbe, P,W H Pinson, Jr,J M Saul, and E Miller (1967), Rb S, study on country rocks of de Bommtwl Crier, Ghana, Grihii,1 Cosmochin, Acia following obscrvatrons· 31 869-75 Laing, E. M (1966), Pivate commu/cation to P M Hurley on 2 lz„/r/, M (1964), Etude geochron/logique par la methode stronbum.rub,diumK-Ar dates in deSter,a Leone. 1966 que/ques 6chan 1 The ageboundal between the 2000(=t) nly, Eburnean md 600(t) m yr t,Ilons..prove.ance Pan- du Camcroun, & nit Far Sn Univ C/,i,noi,4 No 25. Ser Gcol 8 (audes G och"nologiques I) 53 68 Africal, agc provinces 9 central Wcst Africa appears to continue on line into Brazil Lay, C., and D lident (1961), Misures d Sges absolus de min6raux et de roches 2 The distnbution of age values by the K.Ar and whole-rock Rb-Sr methods on both Central),Comp: Reid /rid Sci Pai#: 257(No Ju Hoggar (Sahara. M,Connelt.RB,E Williams. R T Cannon and N J21) Snelling3188.3191 (1964), A niwinterpretation of thi sides of thls boundary m Brazil are identical to those in Africa within limits of present geology of B ntish Gulana, Nwi,re 204 1 15 )8 knowledge McDougall,I,W Com/lon, and D D Hawlcs ( 1963 ), I.eakage of =diogcic argon and stron.urn 3 The extent and age range of the region affected by the Carirlati orogeny in from minci als in Protcrozoic do]entes from British Guiana, Nature 198 56467 Brazil Miller, J A (1965) Geochronology ond contlnental drift-the North Atlantic, MA symposium on matches those of the Pan-ARican orogeny m Dahomey, Nigerm, and Cameroun contine/al dift, P/ul Tms Re k 1088 180-91 Picc,0110, E,D Ledent, and C Lay (1966), Etude gfochronologique de quelques rocha du sock 4 The 2700(1) myr L,berian age province courd be correlated with the Imataca cn/allophyllicn d/ Hoggar (Sahara Central), Co//oq hitern Cen/r0 Natl Red, Sci, No 151, complex m Venezuela of about the same age, msofar as present mdications of structural pp 277-89 Posadls, V G (1966), Rb Sr whole rock age in the Imataca Complex, Venwida. M Sc Thesis, trends will permit Dept of Gcol and Gcophys., I[ I T, Cambndge, Mass 5 There is a suggestion of another correlation between tlie Congo Craton m Gabon Pnem, H NA,N A I 1.1 Boelrllk, R H Vershure, and E H }icbcda (1966), 150:opic age diternim/tons on Stinnam rocks, Geologie /1 Mwihor,v, 43 16.19 and the Sao Franctsco Craton directly opposite m eastern Brazil, with slmilar 2000(+) Roubault, M, R Delafosse F Leutwcm, ind J Sonet (1965), Geochronotogte Prcm:*res donn&5 gtochronolog,ques sur 1/5 formillons gran/Iques /2 crisllitophyllicnnes de la Republique Centre- I yr or greater ages Af„ca,= Co¥irpt Re„d Acid SC, Pms 260 4787·92 6 The present lini,tcd data give a smal /dicalion iliat the mobile fold belt cxte,iding Schn/zler,CC,WH Pinson, Jr. and P M Bosum'/1 cracer Hurley (1966), Rubillium.siontium agc of the area, Ghana, compared w/h the age of the Ivory along the ...btal region of West Africa through Mauritania, Sencgal, and Guinea angles 817-19 Coast lektiks, Sae,ree 151 through southwestern Liberta and ts matched by a belt of sm,lar age cntenng Brazil Sho/, K C, and W E Steenlin ( 1962). A reconnalssance of Ihc Gunyana Shield from Guastpati to the RI Aro, Venezuela A V GMP Bo/ liden w:heo (Carocas) 5 189 near Biltm Snell,ng, N j ( 1963), dge of the Rorainia form,tion, Brilish Gtilana, Natuire I 98 1079 Snelling,NJ(1962-1965), Age deteimnation unit,Rep/s Oiers/as Gro/ Sune>s, 1960-1964, i Slnce th, presentalion was given Be,2 a a suggemp from recent whde rock d/eminatim //,id" (hal a narow bctt of Cal,nan admty swings westward south of Sio Luis and turns northerly /ust Vachete, M (1964a), Eisa de synthese des ddterminalons d'Ages ridlom/nqucs de form/zons cast of Bdh If so, this would math a similar Pan.Afncan belt of rejuverated rocis that appears cnslamnes de 1 Oucst Africain (Cotc (1'Ivoire, Maurit nic, Niger), An,1 Fic Sa Uni, C/ermon#, to occur m coastal bbcria ncar Monrovm and to extend through Sierra Leone to enter the West No 25, Ser Geol 8 ( Etudes Geochronologiques I) 7 30 African fold belt ,n Scnegal Vachelle, M ( 1964b), Ages radtometnques de formations cristallines d'Afrique Equatorlate. Ann Fx Sci Umi, Cle,mont, No 25, Ser G,01 8 (Ctindcs Gaochronolog,Ques I) 31-38 AcKNOWLEDGMENTS Wilson, J T (1954) Eslmates of the age for some African minents, Na/tire 174 1006

This work has been supportcd by the Division of Rebcd*,11, US Atomic Enprey Commission Although / this review we are not reporting the actual age data m Brazil (prior publication expected m Sctence), we gratefully acknowledge our partnership in the Brazillan effort wlth F F de Almetda, G C Metcher, U G Cordant, K Kawashita, and P Vandoros of the University of Sao Paulo, and our colleagues at M I T,W H Pinson, Jr, and H W Fairbairn

Departinent o! Geology (Ind Geophysics Massachusetts Insmute of Teclutology 133

A Rb-Sr Geochronologic Study of the Pegmatites of the Middletorvn Area, Connecticut D. G. BRoOKINS Department of Geology, Kansas State University, Manhattan, Kansas H. W. FAIRBAIRN, P. M. HURLEY, and W. H. PINSON Department of Geology and Geophysics, Massachusetts Institute of Technology Cambridge, Massachusetts

Received February 18, 1969

A Wric/. The pleginatitcs of Eastern Connecticut have a mineralogy consistent with a magmatic -/.7#mp - 1 origin yot mit in a non·igneousenvironment. Varioi,5 theorie8 of genesis liave been itivesti· inettiod. 073f 7£ 11 GEOLOGIC MAP OF PART OF TIE gate by the Rb·Sr geochrotiologic N MIDLE HADDAM AND GLASTONBURY Rb·Sr measummenta on early stage pcgmatile mineral, indicate an age of 258:E 1 ni.y. „·ith 284 initial S'57Sr'*=0.734 1 0.0000. Proviously reported K-Arand U. Th-Pbages for liginatik QUADRANGI-ES, CONNECTICUT minerals am 2·Wi 8 m.y. and 2601 3 m.y respectively. Rb·Sr wholc rock dati for the h it 9». I rocks rary botween 285 1 10 m.y. and 472:E 18 m.y. in age and butwoen 0.705 i 0.002 anti HA' 0.7167 :0.0016 in initial StySru. A direct genetic relatimiship betwi„ thi pegmatites and EXPLANATION their ho* rock, is thus precludcd. In addition, wlio]c rock saniptes of the Brimficld schist OUATERNARY taken at varinblo distances fro,n the Strickland Quarry pegmalite havo mmaincd essentially US closed sys... with relipect to Rband Srand thits an 14*lfuorigi„ for ll:ispegmatite is unlikely. Mizing of pegnialite alld counte rock systems hrts occurred only locally, and isotople 0 -#--- studies of theme tnixed rorks yield . date of 231 =E 4 m.y. .·ith initial &87/St' = 0.7158 1 0·001 8 Ign notincatioiliti:ti,ill, pre„ously kportcd K.Arind Rb-Sr mitiernl dates on Ilost rock F:i / PALEOZOIC minerals (approximately 220 to 240 m.y.}. Al Late Itnge elc relandites are anomalously enriched in radiogenic Sr·87. tho source of u'hich *as mit pwbably olher zonen within the crystallizing pegnintile. This is indicated by w ly,cs of Mgniatite w Job mcks u·hieh Glm,· both enrichment and depktion of radiogonic S/-87 in local s toms. Tie conclusion is dra n, that thcre u, widespread mmement of radiogenic M/mi /21" S/-87 wit]zin each pegmatite system. but tliat pegmatite-host rock reactions wore niinimal. .41 35'- 73 -7 In ,„,Ii=, ".'.. Introduction 63 Brimfield 'molion Tlte large, granitic pegmatites of the Middlelown area. Connecticut have been C. . =A Middlefown formation studied by numerous investigators (references given in the following sections) to ..£23,15.ffr E% determine age of emplacement, source rock (s), manner of emplacement, and post· formational history. They are of particular interest because of chemical and mineralogical similarity to otiler peginatites closely associated with granitic bodies, yet do not appear to be related to (and are at loast mineralogically different ed Um , Scole-I from) the quartzo·feldspathic gncisses of the Middletown area. The present investigation is confined to the Rb.Sr method. Much of this data 4. has been accumulated ainco tile mid. 1950'8 by tbe Massachusetts Institute of 2=r .... .„. S.*1 -, Technology Geochronology Laboratory (HWF, PMH, WHP, DGB); more recent cont„butioils have been made at Kans= State University (DGB). A thorough Fig. 1. Geologic Map of Part of the Middle Haddam aid Glostonbury Quadrangles, Connee· re.examination of the ticut (Modified from STUGARD, 1958). a Andrews quarry. b Half, qiinrry. c Bordonaro qui,W older data has been made. d Stridl.,id 4'ine. (Spmelli q„arty not,hown} General r.,Ingy The general geology is shown in Fig. 1. The area of interest consists of pre.Triassic T# 1. 4 data jor mittry rocks fid, ded by Nnalau dome and mliclines of quartzo-feldspatbic material flanked by aynolinally Formation Method Initial Sr (87 80) Age (m.y.) Ref. folded metasediments and metavolcanics. These rocks are bordered on the West Sample{a)

by the Triassic rocks of the Connecticut Valley and to the east by pre.Tliastlie Maromas whole rocks Rb·Sr 0.712 =EO.002 285 zi: 10 1 gnoisses and schi,ts generally correlated as part of the AIerrimac syncline (see BiL- Glastonbury whole rocks Rb·Sr 0.709 20.002 WOI 10 1 Rb·Sr 0.7167 LINGS, 1956). These pre·Triassic basement rocks have been Adied in detail by Brimfield whole rocks EO.0018 42*f41 3 Middle.In. whole rocks Rb·Sr 0.705 i 0.002 440*15 1 ROSENFELI) and EATON (1959), EAToN and ROSENFELD (1960), and EATON {in Monson whole rocks Rb-Sr 0.707 k 0.002 472*5 1 previously described preparation). T'hey find that correlation an bo made with Brimfield biotite (B3376) Rb-Sr 0.7355 227*15 2 areas in Massachusetts and New Hampshire (RoDGERS and RosENFKLD, 1959; Brimfield muscovite 913376) Rb-Sr 0.735& Mt 15 2 B,w NGS, 1956). Geochronologic studies by BROOKINS and IIUBLEY (1965) indi. Brimfield biotite (B3376) K-Ar 231 k 9 2 cate that the pre·Triassic rocks of the area (excluding pegmatites) were formed at Grnstonbitry biotite (3372) Rb-Sr 0.735& 233f 15 2 Glaitonbi'ry Bite (B3372) K-Ar 230 & 9 2 various times between 285 and 470 m.y. Pegmatites out all of the other basement rocks, but are especially comrnon in the References: 1 = BaooKINS and HURLEY (1965), 2=U. S. Atomic Energy Comm. (1961); Brimfield Formation. The schistose portions of the Atiddletown Gneiss and the 3 =This report, see Fig. 3. Glastonbury Gneiss also contain abundant peglnatites. Fewer pegmatites cut the lindioIt...... i . BnooKINs press) haa re·datt i the Ammonoogue volmnics (Middletown equivalent) been Maromas (in Monson Gneiss, and veg few pegmatites have shown to cut the from Maiachuztts / 4583z 15 m. y. Granite Gneiss. The pegmatites are of two distinct varietiee: (i) narrow leases and stringers conformable to the regional structure of their host rocks, and (2)large, semielliptical granitic masses which are mighly parallot with the regionaletrtic- Present Qeoclironologle Study tum, but are locally discordmt with the foliation of their host rocks. This latter The present study is concerned primarily with the Rb. Sr age of the pegmatite variety is the subject of this report. The mineralogy is diverse and is nmally minerals, pegmatite whole roob, pegmatite-country rock mixture systems, and characterized by microcline·perthite and/or plagioclase, with subordinate quartz, rclation of the pegmatite. to their host rocks. The period of investigation covers muscovite, tourmaline, biotite, garnet, and many rare element bearing phaas, approximately eight years, and part of the dati have been reported in preliminary including lepidolite. form earlier (see U.S. Atomic Energy Commission, 1961; BRooKINS, 1963; BRoo. Internal zoning in these larger pegmatites is common. STUGARD (1958) estiinate3 ms and HURLEY, 1965; BROOKINS, 1065; BRooms d at., 1967). This report that at least 16% of the pcgmatites in the area are well zoned; usually referred to, combi,cs all existing data and attempts an interpretation. from walts inward, as a border zone, a wall zone, an intermediate zone, and a core. Pegmatite minorals forinalysia were selected 80 as to sample both early (magmatic) Some of these zones wittlin the pegmatite cut other zones and are then designated and lato (transitional, hydrothermat) stage assemblages. According to JENKS as fictre fillings or replacement bodies. All of the large pegmatites of the area (1935), microcline (more properly microperthite) and muscovite are early stage show at least a border zono, even when internal zones are lacking. minerals; albito (transitional) and lepidolite (transitional md hydrothermol} are The following pegmatites have been sampled for this study (nomenci ture accord. late stage minerals: The albite is commonly present as trie variety Cloavelandita, ing to CAMERON d d., 1954): Strickland quarry (refs: JENI(S, 1935; CIIADWICK, md oai, 6 further divided into two apparently distinct types; a massive variety 1958; Caa,noN 0 4., 1034), nor:lutiare quarry (ref: CAMEBoN el Ld., 1954), Hale (An 5-10) and well developed cleavelandite (An 0-i). The former is presumed to and Andrews quarries (refs: STUGARD. 1958; CAMERON d d., 1954), Spinelli quarry be an early stage mineral and the latter a late stage mineral (sce CAMERoN el d., (ref: CAMERoN d at., 1954). The locations of all of these except the Spinelh quarry 1954). Such a clear division is not always possible, however. I pidolite, for are shown in Fig. 1. example, probably overlaps both stages although most inveatigators consider it a late crylitallization pro luct. Because of the difficulty i„ sampling early stage ummblages large milieral grain whole rock sample' were selected from Previous Geochronologic Studies (i.e. aizes) Numerous age dotern,in8tions by the U. Th-Pb and K-Ar methods, and to a lite stage assemblages from the Hale quarry. from Pegmatite-host rock mixture eystems were collected from dump material (Strick· lesser degree by the Rb.Sr method, have been made on pegmatite minerals land quarry) whm a clear pegmatite-Brimfield schist contact was evident. the Middletown, Colinecticut area. RoDGERS (1952) has mmniarized the pre·1950 or microscopic evidence of work, and the emphasis here will be on later work. Samples were chosen where tbet' was megascopic the Recent Uns.Pb; Un·Pb; Thm·Pb; and Pbs'1/Pbm age detcr,ninations on infiltration of Brimfield schist by pegmatitie material. whole Bamples were collected at variable distances from the uranimite and mmarskite yield an average age of 260 * 3 m.y. (refS: WASSERBUBO Brimfield achist rock hanging wall and foot wall contacts d the Strickland quarry pcgmatite. Samplea and IIAYDEN, 1955; ASSERBURO 4 4., 1956; EcKELMAN md KuLP, 1957; Nma

d d., 10:1). The agreement of the data argue for emplacement at about 260 m.y...... vith no subsequent strong thermal event affecting the Arfs 0 200 .00 600 000 1000 Seven previous K-Ar ages on pegmatite, muscovite, microoline, and lepidolite 0.75 - EARLY STAGE PEZMATIrE .*AL ISOCHAON menge 249 m.y., about 4% lower than the mean of the U and Th analyses (rofs: BRIMBE LO SCHIST U.SAtomic Energy Commission, 1961; WASSERBURa elat., 1950; BAADsount) 074 - 0 400- I *• 208*1 4 1 d d., 1957; WETHEBILL d d., 1956). Whether or not this diHerence 18 significant 1 1""51§51,•.7340*CCOM | is a matter for statistical discussion. /0.73 - the country rocks enclosing the pegmatites are . 100- reported in Table 1. BRooKINs and HURIZY (1965) found that there is a clear i Z Previousdistinction age determinations in age between the onvarious formations in the area, and that the age ..72 differences coupled with differences in initial St/St argue for different Source too - the - 1- •424*41.. 1 .U'. 0 . . // materials as well. The whole rock age data for the Middletown area agree with 0.71 (Sr//SrIZQ7167101!nll;I stratigraphic interpretation with the exception nf the Glastonbury gneiss, *Woh S.,0- may (at least in part) be older than the Brimfield schist. The reader is referred to 9 '«« i : : : BRooKINs md HURLEY (1965) md ZARTMAN et d. (1966) foradiscummof the S Rb.7/5.-- 0.80- problems involved in dating these basement rocks. 1.0. Rb.Sr and K·Ar dates on minerals from the Glastonbury gneiss and Brimfield Fig. 3. Bafield Schist, Whole data 400 0 20 40 60 . schist, host rocks to the Spinelli and Strickland quarry pegmatites, respectively, Rock Itiochron (part of the 9 200 from BaooKINs and HURLEY, ..7/." yield ages that vary between 219 and 235 m.y. (see Table although the error 1) 1905) Fig. 2. Early Stage Pegmalile aboutthe Rb-Sr data is largely due, probably, to an assumed initial S,47 Sr85 ratio Mineral Imchron of 0.734 These data do appear to be significantly lower than the 250-260 m.y. rsnge reported for the pegmatite minerals by the U. Th.Pb and K.Ar methods. 134

To test the relations, Table 2 1&.8, dul problem of host rock - pegmatita samples from the Brim- field schist were taken at variable distances from the Strickland quarry pegrnatite. Sample Qi,arry Rb (ppm) Sr {ppm RV'/Sl" SFISfI In addition, several samples from the contact zone which showed evident mixing

minerits of pegmatitic and host rock (Brimfield achist) material were studied as well, The F/rly 6tage pegmnlite data for the 13,1mfield scitiwl ate shown in Fig. 3, on,1 an age of 424 i41 m.y. with 12.19 845 3.70 B13370 Spitielli 2,732 0.0341 - 0.7167 1 0.0016 Asults. It is significant thQ (a) the age for F3371a Spile"i 219 11.46 66.4 initial Sr (87/86) 52.1 0.92)4 is within experimental ertur of the late Ordovician age to F3371b Spinolli 236 13.27 the Brimfield schist M3373 Stricklai 1 1.257 4.91 1,011 4.386$ which it is 88signcd on geologic grounds despite the much younger age of the Stricklind 992 30.85 90.8 1.072 F3374 Strickland pegmatite, (b) the &ge for the Brimfield schist lula becn determined F 1114a Bordonam 1,803 19.0 3393 1.9377 from taken near and well removed from the F'lljb Bordonaro 472 38.2 30.1 0.800 samples contact with tile pegmatite, ratio between the schist and the A4774 Strickland 6.03 15.8 122 0.7391 and (c) there is a significant difference in initial pegmatite. It is difficult to envisage any in $0,6 theory of genesis for the Strickland I=te stage peg:,latitc minerals and whole rocks pegmatite based on this information. This is consistent with the field evidence for A4725 Strickland 19.2 3.8 15 0.0454 forceful injection of the pegmatite (CHADWICK. 1958; CAMERON d at., 1954). A4992 Strickland 9.5 ..5 6.18 0.8817 a study of partial A4993 Strickland 7.8 4.2 5.4 0.8431 LuNDGIEN (1966) has reached a similar conclusion based on A1112 Strickland 4.0 3.6 3.35 1.087 melting of basement rocks in soutli-central Connecticut, and he suggests tbat the R 1043 Hale 125.8 100.6 3.63 0.7539 source material for the pegmatites of the Middletown area must le at depths not R 1014 H.le 939.3 107.1 27.80 1.1937 exposed. R5030 Andree 371-5 64.5 10.83 0.8043 presently Strickland quarry pegmatite R 5031 a Strickland - - 5.96. 0.7482 Iii samples from the immediate contid zone of the R503lb Strickland - - 15.80 0.7742 there : an obvious mixing of pegmatite.Brimfield schist material. The dita are L33755 Strickland 7,535 16.82 2,248.86 8.223 presented in Tablc 2 and in the isochron of Fig. 4 which gives an age of 231 f 4 m.y. 7-370 L3375b Strickland 8.333 19.66 2.030.29 = - and initial St/St 0.7188 i 0.0040. It is Interesting to note that this "date'* age (219-235 m.y.) for tho Brimfickl schist and Brimficid Nchist.peg„mute mixod systems is close to the wall rock mineral Clase„bury gnoins reported mrlior (see Table 1). It i8 tempting to suggest that R 4722 Strickland 69.5 321.9 0.61 0.7200 R4723 SLrickland 1,457 124.5 35.14 0.8260 this "date" may tnark the elld of thermal events it, ac ama, but meh an inter- R#718 Strickland 214.2 88·5 7.03 0.7442 prctation is not consistent with the preservat,on of older K.Ar age on early stage A '205 Strickland 8*178 12.4 131.8 1./56 pcgmatite minerals. Petrographically (and in some cases megascopically) there: R4787 Swickland - - 10.4, 0.9103 - - a clcar mixi„g uf pegmatite material (imally apli16 and some tournialine) with Brintfiold ScList (ncar thi Strickland pormalile} the normal scitistose material. The StlS# illitid ratio for tile isoell:·un of k':g. 4 R 11088 325.0 229.7 411 0.7426 is 0.7188 i 0.0010 which, wiO,ili e.1-, inicntal error, iw indistinguishable from the RlloBb 20.2 218.4 0.35 0.7162 Brimfield schist (0.7167 80.0016). 1.65 0.7271 R4717 71.0 124.5 A suite of late and whole rocks has been investigated as R 4719 181.4 183.0 2.87 0.7350 stage pegmatite minerals in 5. R472Ob 319.1 1907 4.71 0.7419 well. The data are presented Table 2 and Fig. Cleavelanditesshow pronounced R4721 129.4 252.6 1.48 0.7244 enrichment in radiogenic Srs' and are quite different from tbe early stage plagio 52.9 124.6 122 0.7263 84788 clases (see earlier discussion). In addition, attempts at "dating" intended whole A=,8;4 F=K·feld#p•r, B=biotit4 M=muxovite, L=lepidol,4 R=whole rock. rock samples have failed as some samples show enrichment of *Sr" and other ne Rbs'/St valuee marked by an a,terisk wre calculated from x.ray spectrographle dats; depletion when compared to the proposed early 64* mineral isochron of Fig. 2. an others were determined by isotope ditution. Ru.Ry (1967) and GERI.iNG d al. (1962) have also noted enrichment of *Sr87 in other late stage minerals (primarily pholph.tes, beryl, and endlichite). Unfortu- Table 3. RA-S, af catculation: 0/ eGrl, *0 Fegmala8 miner , tor di//ereni £,:itiat Se/S,8 nately the excessive size of many of the early stage crystals does not allow proper sampling, and it is not surprising that... whole rock attempts have failed, partic· Sample Age (m.y.}for ularly when it is noted tlat there has ob*iOUBly been movement of.Sp (or nor. ,/tz'&O Se/St=0.736 . , 11 mal Sr md/or Rb) during title late stage cry,tallization processes within the peg· matits. 'Wall rock.pegmatite reactions have not had an effect on the isotopic M3370 260 258 LATE STAGE PEGMATIE balance where there is In obvious direct mixing of the two systems. Perhap F3371* 286 255 MINERAL DATA except F337lb 289 255 the only true" whole rock" aample for an individual pegmatite is the entire raise

M3373 262 260 - of the pegmatite (at least for the varieties investigated). We intend to pursue this - F3374 270 251 1.1 0 CLEAVELANDITE ROCK point further. Fl 114, 268 255 0 WHOLE 275 252 Fill4b Table 4. lib·Sr agu for &:te la,e pegniatl m;nuat: and whols rocks wng,nlial A4774 293 1,716 SP'ls,a =om

'.0 -' - Age &.ple Age 1 8-,1, I : (m.Y.) (ILY·) - STRICKLAND OUARMY A#725 1,009 R5030 296 CONTACT ZONE : A4992 1,708 R503la 159 -- - LI . 0.9 - A4993 1,460 85031b 179 0 &1112 7,554 L3375a 240 Early S.. R1043 376 L3375b 235 j.. - - ...... --- Mintrot "* 81044 1,187 ; t· 0 8 - 0 Lepidolites arr considered by many pegmatite investigators to be late stage cry- two (L33758, L3375b; Table 4) :O.9- 0 stanization products. The lepidolites analyzed yield agee of 240 and 235 m.y. respectively using an initial Sr (87/86) ratio of 0.734. Bee;use of very high Rb/Sr, age Calculations for lepidolite are relatively Age • 231 t. 4al 0.8- insensitive to choice of ittitial St/St, for erample, loweIing initial SP/St from 8.4."1:07#llc)040 5 1 20 30 0.734 to 0.700 raises the age by only 0.4%. These ages are therefore definitely Rbet/SrS' - lower than the 258 m.y. age of the early stage pegmatite minerala, and the differ- /7 1 1 I lili 0 20 40 60 80 too 120 40 ence may be due either to *SF loss or to late crystallization. Rb 7/Sres- /* 8. IAte Stage Pegmatite Calculated ages for other late stage minerals and whole rocks are presented in Circles indicate 161*'61 Data. Table 4 to emphasize the anomalous nature of these systems. The excess of -Srs' in 1%. L Pegmatite·Wan Rock de*velandites, squares indicjte the late stage cleavelandites is indisputable. The early stage mineral isochron a. Ortare SystemL Strickland whole rocks. The early atage pegm,titc minent isoebron of hibits an initial ratio (Fig. 2) much higher than the surrounding country rocli 42/ shown for refereaoe (Table 1) and late stage cleavelandites are even more drastically enriched. This auggests that the parent material for the pegmatites must have undergone a lengthy crustal pre-history before melting, yet the temperature in the surrounding dich showed secondary veins and obvious replacement feature were discarded. had rocks must have also been extremely high during injection such that re- The data are presented in Table 2 aml shown graphi,ally in Figs. 2-5. Analytial distribution of Sr87 in the latter was confined to local systems and excess Sr87 in Focedures and new aampts deactiptions are presented in the Appendix. the pegmatite to 6 very late Gtage crystalli tinn products. The source of the . .*mL and ..L, Vol. excess •SIB' in late stage pegmatite minerals may well be the extremely Rb·rich crysta[lizing phases (as opposed to tate deuteric-type solutions) such as micas and Dbc.jilin with initial K-feldspars. Conversely, this excess possibly representa the last "dredges" of the minerals yield an isochron age of 258 * 1 m.y. Elzly mtage pegmatite USed intruded material. These two possibilities cannot be resolved at present, but the S#/St =0.7343;0.0096 (Fig. 2). A half We of 50 b.y. for Rbm Was Rage former is more plausible primarily became data from the wall rocka suggeat that (1 - 1.39 x 10-lly-1) for the age calculation. It is gignificant that early Spinelli, Bordonaro) thermal events continued to about 220 m.y. (Table 1 and Fig. 4). minerals from at least three different pegmatites (Strickland, the Rb·poor end by eample fall on this isocbron. The isochron is anchored at early stage lecal:Be of a higher Ca A4774, & plagioclase, which is presumed to bc Conclusions late content (An 7-10) than stage deavelandites (An 0-4). A comparison of early stage minerals minerals using initial Sr (87186) ratios Our conclusions are as follows: (1) The Rb.Sr isochron age for ages calculated for the early stage pegmatite from three = are presented in Table 3 different pdgmatites is 258 f 1 m.y. with initial Sta'/Sr 0.7340 f the isochron of Fig. 2) using of 0.710 (assumed) and 0.734 (from average 0.0096, 1 = 1.39 x 10-tl y-1, this age agrees closely with the U and Th·Pb and show clearly that the latter value yields more acceptable agea. The ages. but may be significantly higher than the mean of the K.Ar ages (249 m.y.). experimental error witk the U, and Th·Pb age of 258 k 1 m.y. also agrees within (2) The difference in slopes and initial St'/Sr" for the pegmatite minerals (Fig. 2) m.y.) would be improved by wing the ages. Agreement with the K.Ar ages (249 and the surrounding country rocks (Fig. 3) 4 believed to preclude a direct genetio alternative decay constant 1 = 1.47 x 10--'Ti. The preceeding point is also the early stage pegmatite minerals 8 much link between them (see alm LUNDGBEN, 1966). (3) The initial St/SA mtio of 0.734 for tbia supported by the whole rock isochron for the Brimfield schist which does not the initial ratios for the surrounding host rocks (see Table 1). and show any evidence *Sp higher than and for widespread migration of except in local aystems. The a direct genetic link between the pegmatites to evidence alone may preclude Piedmont resultant age of 424*41 m.y. is consistent with the geologic age which it ia host initial ratios for pegmatitos from the Maryland In their wks. Righ Middletown assigned. addition, pegrnatite-host rock mixing is confined to th. immediate have also been reported by WinLJ. 21 4. (1968). The data for the contact zones (Table 2, Fig. 4). (4) Late stage cleavelandites dow an excess of wei emplaced ata much later date (Per. pegmatites, however,show that they *St' when compared to the mineral isochron of Fig. 2. The data suggest that host rocks (with the possible exception of the MA- mian) than their Ordovician movement of *St (or normal & and Rb) waa confined to late stage processes romas gneh) and from different parent source material. operative in the pegmatite system, and that much larger samplea (perhap the entire pegmatite) must be used for whole rock requirements. l i j a i l i:1 9 1 3.j i l l'1 1: ' :1 1 1 N

li i i #11, Mil : iii fi N i 1 1 1 11 1% 1 1 i i 111 i /1 i d i 11 3 2 1 3 1 .90 1 :11 1 1 1 & F I I /7 1- ' "6-Wl /152. 41 1. /Al.124 & : ID:fqJ' vii. :1 I.1 Ni Alii i i/#r i :1 e it / 1 Sfe a: 4-1 4 MN ad*" 144 7 1*,1 ./ Hz f

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/% 1.1 1 1 1 1 8 .9 t : B H 4 8 4 1 1 123 fille),1 411#1.2 11 11 1 4 111 .illi E R.4 9 31.5 t. :1 2 1 1 321 l i 8 1 „1 1 kj iflif,ii23 53:331 ilifil . 11/15, il 14:' i#6 1 1 2 iiijii am·..5 i,i#&213# 111,11 14 30 i i /5* - Ri R#jilitililili/11 14 i j jill %*f #/3 ilfiligist;/98##14 11 & 13 61/1 # 8,11.741155.lit.3.142 28 9 r. i*Jij 9,1/a:Milf#lifliiiE.2€te°.8,A g.EE·:. #6 :'2 :, s...m 2..w.9-.2: .co 1. :1 1 . l s- 1 j H i d 13 :M-4.:ds ..wljei: jai,%,1-M ,2.,&/191; "El:,li-.d-9 1 "*feaj£:Im 1 83,311#A imi·i Ii#i//8.4 /151/1/3/1/ 1 1 /i/1/1/ 4ija=:s 331.:16111 91*112:Sj li·Fl.11311#241114 f.' 8;31/ 154:5:3.Kh %111;14: 1 0€8 1 e . S &1 9 - IL-= 3 .:. 10 3 2 B l e i g l i l i 'a f i 213 w: .:.*1 *E , i 1 : :E . . e s' . =i rl is i; e S 1 51/1:= 4 g=i: 1 i 3J :a- 51 , &1 $3 9 1i i:1 81 1.14 J 3 i 2 m s i 136

~~. 1. 1 .. ': I . ..i . .~~

~~i : . THIS PAGE, , a WAS INTENTIONALLY LEFT BLANK 137 Pre-Drift Continental Nuclei~~

~~Two ancient nuclei appear to have had a peripheral growth and no pre-drift fragmentation and dispersal.~~

~~Patrick M. Hurley and John R. Rand~~

jn a „irvey of the ages of contincntal North America The abundance of age ind within the Paraguay belt between shown on thc map$ Ibecause of the dif- areas we ha' c cimplied and plotted al- ve,ies varies rather uniformly m the the craton of G„aport and the craton ficuity of protection Louras,n, the most all of the availablc age data on range grealer than 1 300 miltion )cars of Sao Francisco Ancient age values northern nucleus, would be bounded by until, at about 3000 million years ago, continental ba",ent rocks in North havc been obt,incd by A R Crawford the Appalachian, Hercyman. and Al- the number of apparent. agc dates drops and South America. Africa, Europe, in Ce)lon. in the eastern Ghots region pine belts along the southeastern penm· indin, Australia, and Aniarctica, rrp. -sentially to zero There . a slight dip of India, and In other supposedly /cr, with the Himalayan, East Asian, resenting two-thirds of the land area of in the worldwide histogram of reported young Pirts of the WM subcontincne and western North Anicncan bals form. years, so wc the earth (sec Figs I to 7) Age datcs ages at 1700 million have i,imilarly, such agcs arc being found :n ing the remaining pcnmeter Gond- arbitrarily chosen this value as for cont,nentil areas in the U.S S R thi the #cstern part of Australia within wannland the southern nucleus, would and China m not included bccausc u e boundary between the two older age areas where potass,tim-argon doing has have peripheral belts that included the It should be could not obtain informat/n on the provinces remembered glicn ages that orc quitc young. Atlas. the Pan·Afncin of northeast actuDI location of samples In making that this refers arb.lary boundary only ·The anclent cratons arc not sca/crcd Afnca and of Arabia, the sequence of thts survcy we wished to add to some 10 apparent ages (mainly obtained by ..formly thro.ghout Ihe pre drift con- B.osy..... t. eastern A.1.1..|1[1, 1.- of the earlier colnpilations (/) and to the polassium.argon method) and 15 t:nental crust, as one might expect them clu- in Fip. 1 through 7 the age data thc recent mvestigaton of this problem for nockites. have been roughly plotted etal motion of the asthenosphcre into Phanerozoic folded and intnided moun. For thc purpose of th= article tt does or the rcconstruction of Fig 9 Wc thc continental hcm„ph/re acting over mn systems thal have becn adequately not greatly matter which of the pro- have mode no attempt to be accurate a long perod of time Recently Orowan surveyed by ordinar> geological meth- posed reconstruct:ons of india, Aus- or ahaustivc in our frarrh fir thi,o (8) 4 wggested 11,/ 11,ire may hnve odi Informatton on the nges and loco. whi. and Antar/,w . ihoose. occurrences, and have used only those been a revcrsed, or outward.moving, lions of these Phanerozotc belts is In Fig. 9 the Angara and Aldan that (nme re·,dily 10 mind. Again. a Bow of th. a,Ilic„ubplle, c from the pre- availablc in Eming,gil mnps for :hc sh & of Sib... ..' added alter those coherent pattern seems to emerge. drift continental hcnispherc, causing the various coninents The con: nenta1 regions of the Arctic Ocean that arc at 'gc Also, thS general stnkc of the basement 64 and dnfing of the continents provinces arc therefore considerably depths greater thon I 000 meters are rocks in most ofthe loCal/Ics roughly Thc great concentralion of radioawvc better closed up zo a "best.m• recons,ruction clcments n the contlnenta nuclei may known than the pointS plotted parallels a kne drawn from the upper order than for rad:omc:r:cally measured ages would (4) Only the provinces nght to the lower left in F,8· 10 In havc caused a thermal imbalanLc in thc 700 million years Inhcle 1 .re shown in Fig 9 earth that brought on th, rw crsal most cases the 8-nultic bodies occur The presentations of Figs 8 and 9 within a tectonic bat of intermediate Wc have no mdy uplanation for disclose an interesting fact The con- age, although many of them show ages thc transcurre/ bell of thermolcctonic complacs Pre Ddfl D:stnbution of Age Provinces Inc..1 *as.ment older than grealer than the age of the thermotec. acim/y th:,1 cut BL,0,5 the ancient cra- now fall showing relicl masses of 800 i ... y.ars appear to ..c activity tonic regions, For purposes of this discussion unifed of it into twa elhq closely group Thc reconstructed continental nuclei ancient rocks In zoncs yolingcr docs not matter,f one uses the age of igncous and sed:mcninry rocls, some- ings. Th oldor p *vince in the norlh. with the boundaries shown in Fig 9 a m.....phic overprint . a region m grouping Includes. the shield.areas arc surrolded by great sweeping belts ttmes uplifted after dccp burial. and rather than the primary age, the match- in Canada, Grcenland, Scandinavia, ac of younger crust These belts are not sometimes exposing rocks characteristic ing ts equally effecttvc on this basis. Ukraine. and the Aldan and Ang:tra and potasslum-argon age data arc more cratons of Stbena. These assemblages available appear £0 have been chopped up by B present *hc age data as three r.· apparent-age units ("apparent age" tiveyounger belts tran,current,It even appears geolosic:lilythat the ac- -. ' 7 < fers to ages determined from radio- o der areas within each block of crust / 21+ metric agic data) on the basis of a have snke d,rections that are nearly natural grouping of the age values in paralle (thts I not shown in Fip. 8 Figs 1 through 7 Th„ grouping ts and 9) based on the following logic Asimilar effect is seen in the south- Much of the greatest area of the ernl grouping. where the craton:c areas e continental basement rocks is Included of South Amenca, Afnia, India. Aus- ...2/14 m belts of orogenests and rejuvenation tralia. and Antarctica form a rather - I f'.':'. thit are post Grenville In Afnca and closely kn.t mosaic, again chopped up , » "A South America the ainsive regions by younger and tranwirrent active Rnctuded :n the Pan-Afncan and Can- nan nrogenict .how almit Colit:win «.Thc two regions of crust ' f ' ranges I age values from about 700 older than qj. 1700 million years can be enclosed 27% 12 4 million years 12_190 million years. and whin rather smooth ovoidal boun thus thoy ovcrrap in tit,lu the Phaneri dancs, and essentially all of the older ,, , .0 A zoic events :hal culminate in the con- age mmurcments that have been re- South tlnental margins in western ported ha,c becn oblatned within these 146}RSS'· „ r f '.4 Amcrica, eastern and western North ,/,14..,/,/: P. ... boundaries Thu ts crue not only of .·P ,L America, North Africa, and southern Irc potmium arron age measurements but Europe Similar overlaps found for also of measurements made by the most Australia and Antarctica Extensive late trus:worthy methods In other word Precambrian activity ts being found in with poss,ble exceptions that have . .f 'Fili:Alk:.t:1...E'= 0. it, . the Appalachian and Hercyman prov- escaped our attention, there arc appar· <14< Inces, in which age values :n ihc range 330$ m:mon years typical of the ently no ancient continental basement 68' ff.-3;..11 Pan- rocks outs:de these relatively small re- Afncan bclt of Central Afnca are ap. Stons on the earth's sufface- pcaring m the heart of the previously Moreover, we m now finding that within many of the younger transcur. E: -.-tk»Ii, ... lin 'gl:.,ff.. 1. «2.'St''H,i...„„'Z.,t,S, ™ rent zoncs of -/ a universal quiet interval between 700 geological activity within thce two regions. therc are nulated and 900 mithon M„ ago We have rd„t masses u/h whole rock age val- thmfore set the time boundary for the ucs comparable w those for the old- younger basement-rock age provmccs 'all. craton.. r.g.Is For ....plc, )33'.. pr=.116 . at 800 milhon years ago . <440 Selection of the lime boundary be- several ancient ages have been found • 440-800 In almost in the center of the twccn the middle and older provinces Mirta 0 800-1300 \'r ...' --r"'.'11'11 Pan-Afncon belt between the West • 1, ' . . .1 1/ , ts much more dimcult if th, age data 1300-1700 African craton and the Congo craton • 1700-2350 for Afrtca and South Amerla a. con- (& s.; g I •17 Lenscs of ancient rocks million • 2350-2700 I ..:...CS...iti J sidered together with those for North (3000 • >2700 bcci found in the belt C irFW. :-LJ 1.1 America and Europe. the previously years old) have between the West African shield and observed groupings largely disappear the Guayona shield of V/Inczucla. \'%,F -- 1 There ems to be a low point / gec>- which Cucnd, 11„uugh Ihe coastal regions of '',»»5:' |08:.al dilly In Atrica at about 1300 Sierra Lcone and Liberta Scattered milion years ago, but this :s compcn- ancient age values have been obtained Fig, 1-7 (piges 1230 to 1236) All available radtom/nc agc\t//- dola for sated by an abundance of 4. in this within the Cann orogcnic event tx:· basement rocks (mAinly data obtained by the range for a great region in centrd method) plotcd on the continental Ircas Indicated The key glvcn In Fig 1 for identincation of symbols opplms to potassium arson tw"n the craton' of northern Brazil. Data within geologic belts of known *In the Phoncrozoic hic bccn omitted nli seven 118.=

~~Dr Hu.) [s .tofessor ef g.logy al M a. chunCtS 'm.:u:c of .....logy ...btid. Dr ... I e ..... 'Ins I ..porl laine 138~~

~~of the lowcr crust. such 's lhc grant, lites nicntioncd above. Thcx bell wg ,- gest rifting ihat opened up gaps of, at -~~

most, n f,w h,Indrcd kilometers. with tk=, 744*44 scdim"sand subsequent igncous activity filling the rificd zoncs. . ' 1 -4 * '.14- The question of the possible exist- cncc ef a separate Laurasia and Gond- I q.A wanaland fanher apart than they would ... - 21„'u;"2 Z'L;; t„C Z , \ ' . ,·/34 e %0 = - without more palcomagnetic informa- lion. 11 is therefore too soon to specu- 1 <4. late on the pogibic locations of· thcse two continental nitclci, or on 11. ques- i- 4 tion of whether Ihcy wcrc o single .. t.*I .* . . P 3 0 , c,v, g

~~had not becn previously split and scat. tered over ihc surface of the earth, as Wilson believed they had bccn (9). 42. 4 4 4--2-- s .~~

~~True Crustal Ales and Apparent Ages - .1 1-~~

Wc now evaluate * usclulness 01 , · t. ... ·.'. ' 1 -- ·'1* • 9#2:4. ' & 4'• Aer£6 reconstruct™di...iii. the actual development 4. i•,9 0/ i · . ' D-k_ 1 ",' i J.Q Ats« . 3'· the crust. Such an cvaltiation requires that we define some form of absolute 1 'r,:ze'::;2:,m,;::tl,:;'d.":A ' :- 1-. .t. f..I. ' ... 24 -,73- f &086-4««',S .... .I;- . termined from radiometric age data. Before doing so wc should define a few .4 6 / 0 ,).4.·,<'. 1'.' ...... :f '. ..4.' \ AA/33 1:.,/2,#t ' " · -KI the earth's orisin: Rm = the Sti/Sr68 ratio for the source region at the time i 0. Ae.., :.2. r ...:..43 ..0 : ':3 . i Sw-, = 'SA ;Lm, ..1 0 .Se. .0 for stabilized crustal rock at the time the rock became closed to mi:rationS ; A ' 4 &/r.-9.5--:. :ff. . ofSrs;/Stl' rubidiurn andratio strontium: for thc R. = rockthe simple < -18 0, . 0 - 46' r---. analyzed; 4 = the average Rbal/Se i, = the average Rbal/SrMO -tio for the /(S ,Ihe/c,loo" n; 1' ';,2;; t /f---,-- - \ earth; T. = the age of the earth (in .,- , _L separation (millim of years ago) of crustal material, or.total crustal age.; T, = Ihe time the rock system became closed [o migrations of rubidium and strontium rwhole-rock rubidium.stron- /,\ :ium isochron age" (in millions of .,a years)]; A = the decay constant of Rb81 8·.W toc W pe million ycars. We use the term con//nad m,31 to OrN- mean the present·day material nbove the continental Mohorovitid discon- linuity. Also. we may use the extent to which the rubidium.strontium ratio for the crust exceeds th# for the whole earth os an additional criterion in de- ining average continental crust as it pertains to lhc question under eration. The forming of crustal material may involve a series of stcps of rubidium enrichment, but for the sake of - 4% consid- <6. simplicity (and because of lack of 0 knowledge) wc consider this to be a , ..f# 20 E \ \ 1 single step. Thus, che 'crustal prchis- tory" is givcn by (T. - 1-) and is dc· fined as follows. lf it takes onc or more GO. ACI) V .-25" . 3, 6- . 6 gcologic cycles for material to become 7 1- . C.»e... „ :8::that RbsT/Sr56,d,81 /:.::z:.:finally becomes·z'chz: '.• :s: and ' ' .,ii

~~Cmstal prchistolf=(T.-T,) = 1 S 0-, r-- a R,- R.~~

~~Ind, by 4 (1) , 11 + A - . 927-- 00/N -*.0 er- definition, 1 d .- -' 35 *7/ ..5 Ya%*. -Tal., I=.1 age.=Rt&+Ri- R- 1~~

or the total crustal age is the sum 0,of 90" _ t.. 1 xs the whole-rock rubidium-strontium iso- , of \.ti *.5 chron age and the crustal prehistory , 1 0 it being assumed that thI material ; Rb"/Sru equaled 4 prior to the time ' 4 1 rubidium and strontium become stabi- 0 lized in the rock unit sampled. Obvious- 'S» ly this prchistoryvaluestage for is not Rb„/St'valid for during the in- the 2( t " - dividual case, since ie is on average.

~~.... 139~~

Thus it is nccessary to restrict the 1 estimate of thc total crustal age to 60 1- Inrge units of cnist. rn which thc value . for 4 will bc reasonably meaningful. . 43.=-- Thus. sincc - 1.-: 4.-.-: ... R.=R. tr.{cxplk(T.-T.)1-1} (3) -- .d k. a large block of continctital e e t. crust having approximately thi same psWR 9.1 0 total crustal age throughout. es' · - ' 9 4/W . 1. e. . . , A...r. 4&(8.28., R. 2-1 0 0 301/ion for T= can be obfaincd if 6.- R., Rt, 4 and Ihc appropriate valuc (discussed arc determined of R. bclow) n..1,1 /6,- 1'9. V for indi,·idual rock sampics numbarcd f Wika,2 1 to n taken at uniform intervals over 6. /f b .Art--31* the block of mist.· The solution also GUA..Rf CRATON v |1| depends on a knowledge of T, A. Ro. --*.' *,1, 1 &, f ' rp Ind 4

,$ . 4, \ 1 2. I,· . EvofunCion of Paramders :/1 ':' ' L ' As noted, the total crustal agc is -/ . a. 46 :1:P . 2, bascd on thc total increase in the Sr"/ .: .9 + "' 6#44/ 20.S Srs, ratio. starting from some prccrus· tal valrie (Rm) that pertained 10 1hc 20 S f * 1= + - mantle or othor source mat. ial from which the cntst was derived. Any at- r. tcmpt to evaluate R, asa function of i timc is bound to be open to controversy. Fortunately the limits M by most reasonable models do not vary so much as to Cause I serious error in the anal- 8.- ysis that follows. We therefore select 1 the model of our choice and State that Bl , 'l the Rb,7/Sts ratio for the mantle was originally 0.10. This value 13 based on an cs,im.c (10) that the abundance of strontium and rubidium in the canh m about 69 X 101.8 and 2.4 X 1015 tons, respectively. Thc value of Sr·81/ St for the mantle therefore· would 40 S increase 81 a rate of approximately 0.(101 d fwi binion yeam The Jewy uf the parent Rb„ is included in the cal culations. Starting with a value of 0.699 (from metcoritc isochrons). wc find that the average value of SAT/St for the mantle would have been 0.700 / about 3500 million years ago, at thi time the earliest cmstal materials wcre being formed. The sudden appe,rance of early cmst in numerous pia"sat this time Cor shortly thereafter) stigges:s a niajor cvent in tlic history of diferen. tiation of the mantle (possibly core infall and first separation of hyper- fusihies from the lower mantle). In order to fit the present S#'/SF values for oceanic alialic and abyssal +-0 bailts (0.704 and 0.702, respectively) 1/), we ass,Inic that man,Ic regions \ that were partiolly depletcd in trace / '.0- components and had relatively low --/ \ \ ™bidium"im.,1.mtime ....turtly „,1-became .1the ihi,,source ity of 3 abyssal tholelitcs (/2), and that mant[6 \ regions enriched in trace elements and \ having rubidium-stronlium ratios ap. -3 proximately the same as the average ratio for ihc entire earth became the lC /// Sources of the crust. This model could 3 Vi -/ equally well be applied to the concept 6/ N uf surface cycling of a protocrust" 24.-JZ - C material. Our principal point is that·the· 3:13%5,4, .4 source materials of /hc crl. arc bc- 1 lieved not to have had a low SAT/Srss 03 -i--le: -t- ratio. like the present abys,al thol/fites, .k:., = :; 1'*. 6& 1 hi'. "'hii. w hi•ihid i., i,-I high„ - SAT/Sr" ratio than the pracot oceanic 20°N i ..«,r b Inlls. This view is supported by ob- AFER& servations on initial ratioj in those " C»" - young continental voIcanics that ap. pcar to have been directly derived from \\\\:11#30;- the mantle. in which Se./S#. values 1,1 .. in the range' 0.704 to 0.706 are com. i #fr mon (/3). Favoring the above argumcr;ts slight ly. w. arrive 01 average values for the Srs,/S,50 ratio in the source materials th/· formed the crust, a a function of time. al indicated in Ft, 11. This. i'' '61:-'4r,> would bc average values for the earth 43 -8. with . primordial rubidium·.trontium J ratio of 0.035, or a Rb87/Sr" ratio of 10'N '7.. » 0.10. Thus, when wc usc the values .* T, = 4550, 4 = 0.699, and A = 1.39 .r million yea.. the value for R. in Fig. 11 is given by Eq. 3 as fol- 44 -1# 1 lows:.10-0 70'E //1 1 90"E R. = 0.699 + 0.10 (exp Il.39 x 104 (4550 - T.)1 - 13 80'E (5) 140

~~Thc voluc of 4 the average Rhs,/~~

Stiby a combinationrotio for ofthe data crust.on the nature is determined J of the crust. Geophysical survcys involving scismic rcfroction promes and heat.now. dc/rminations. in comblna- -'3 $/0 ., m tion with geological information on rock p . , no types and densities and gcochemical , information on trace-clemcnt abund- anccs, hm yielded estimates, by vari- Ous workers. that are reasonably close. '.-,1''4 < . : Pakiser and * 4 '6 1 «." 20'S density.dcp:hRobinson distribution for typical (/4) givc a- -I4 " segments of thc contincnial crust; Birch, and .6-9 Roy, Decker (15) and Hyndman, I.nmbcrt, Hcier, Jaegcr, and Ringwood (16) demonstrate the decrease in uran. -ef X ..0 4. estimateslum, thorium. in combination andwith polawumknown .with \ b variations in the po:mium.rubidium ratio {17) and with ,Ir rmill. nf ex. pcrimcntat work on the crubial stabil O 0 ..1---\ y· 2 ;ty of gran.lite facics rocks under high 5'.pc \ \\ « , presjure (18), wc arrive at an estimate of the total didium and stront;uni ia the cnit ( m). From thic studies, the valuc of 4 is taken lo b. 0.43. ..,.9 3 --- - \·Is3.1/,, liLI . ( .1 Ra c of Developmen: of the ,-A Continental Crust - YA a....= ..'-$2 j 4- In order to investigate the history of the development of thi continental Crust it is necessary 10 obtain wide. ipread information on.total cmstal ages -1-40'S as defined above. By defining the crusi as that ellfrial which ;3 prcscnt today above

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~~Strontium Isotopic Compositions of Gangue Minerals from Hydrothermal Vein Deposits' R. H RnESMAN~~

Abstract Strontium isotopic analyses have been made on strontium-rich, rubidium-poor, gatiguc nittierals frotn se eral veiii-type hydrothermal mitieral deposits These mnetals were from deposits in which · 1) Terhary mmeralization occurs within rocks of Tertiary age, 2) Teltialy nittieralization occurs withm rocks of Precanibi mii age, and 3) the mineralization is Precambrian The results of these analyses show that the gangue strontlum was derived, at least to a very large extent, from the rocks of the host etivironment. possible source for th. Tertiary gatigti- Introducbon Tertiary Mmeralization in Tertiary Rocks ruled out as a slronlittin, t,; o possible sources of this material re- MEASUREM PhTS of the St"/Sr" isolopic con,post- Thc samples of Tertiary int,leralizat,on ocairring ""1,0"c 's tlic se„rce rigiot, of the ;oicanics, prc. lions of a series of strontium-i:ch, rub,dium-poor, 1':thin rocks of Tert:ary ige cnic from 100.tions in st„„cd lo bc citl,cr tlie lou cr crust or tlic „ppcr gailgic '11,1,crals fro'n some veili-type hydrothcrmal Colorido, Ar,iona, \tontana. wd the Philippines niantle The altcr„ali, e wirce .s the 5 olen„ics m,ncial depos,ts have been made m order to gnin The results of theaitalyses arc hsted in Table 2 The thelitsclies Daring tlic „liole of Tertwry t„ne, infoi ination conce, ning the possible or:gnis of thc St,/Sb• mos of the gangue mincr,ls ratige from about 70 ny , rocks having m a, crage Rb/Sr com- galigue materials If these materials are of primary 0703 to 0711 0,ic sample of andes:tic country positioti approvmately the same as thc crustal a, er- -Wmatic..g.s D e derived directly from deep- rock from the Telluride disirict, Colorado, has a agc i: ould KM/ 11, their St-/Srs, con,positton scited sources having Rb/Sr ratios sm,lar to those St'/St' composite 01 0709 This sample was only about 0001 0,cr their initial strot,lium conipo. iii th. sourcc Agins of basalts),and d the strontium not from the same locahly as * calc,tc sample from silloi, th„ difference „ould not bc disccrnibic in conipositions of che gangue minerals faithfully reprc- thclellitride distmt It „as analyzed solcly to gain 11'c analyses carried wl here mit the initial strolit„im con,1,ositions of the mtiter- soi,ic ide, of the presciit st ..iit:lin 1SOtOpIC Compel- Moorbath, Hurley, and rairbairn (1967) mca- aliznig flinds, then the S#,/Sr" rattos should rcflect lion of the Tertlary iolcaitics *red the #la| Strollt,Un composit ons of some the strontim con,positions of the soitrce regions The 111-lIll CoticlitslOils that can be dra„ n [roni the m„,cratied Laramidc intrusics from the soulh- 1 n a study of the stroi,tium :sotopic compositioils results of these ntialyses is that both tlic ganguc min- ; estern U,„,cd States and found that these compo- of ba,altic rocks, Faure md Htirky (1963) foind erals and the country rock sm#e show Srs'/Sr" ..IM uerc in the t.ilige Sr"/Sr" = U /115 to 0709 Iliat both the oceanic and continental types have uni- ratios that arc compatible with in origm iii a deep- formly low Sr•'/St° mtins of les:, thait .il.ut seized source regioii in Tertiary times Tlie zatigue 0:ic sa,ni,ie of fluoritc fron, the Cripplc Creek dis- 0708 tract also ga, c a strontlit,) composition wtlitti this Be ,„be these rocks are very low iii these mineral ratios arc also conipaa,ble „Ith an ortgm in mbidium, Mge Frotn these data, Moorbath et at (1967) com- rocks of Tertiary nge Unt wcre dcm cd directly ntios irc belie ed to characterize the Sr"/St° co,ictuded thit both the intri,m cs md the fuor:te posilioti of the source reg:on of these rocks at the from a deep-seated me Tile Tertiary gangue were der•vcd f: 0,11 1 deep-se.,ted so,irce region 7 he timc of thc,r for:nation The source region is stront,uni ratios are not complibic „,th a denvatton be- Arg,1111,111, Iirc .,itul al,ovc ..agut 11"* tti'Ir rn,icli· Iici cd to be either the reinoblhznlion of prc-existing itpper lower crust or thc .ppl. through the sio„ r.8.„ditig the strontium nt uic n,Im „lay 1101 matitic From the Rb and Sr ...stal mater:Its .I crcal.r thi. 1,000 m y - .ge, concentrntions mca- bc correcl In so f. r as lhL „nincd,AC SouriC of th" slircd ill thcsc samples. Fmirr and H, ricy Jl.termlned 11·ivitig ai cragc Rt>/Sr ratios iii e cess of 020 and source havit,g iveragc Srs,/Sr • con,positions iii excess of conit)011.1, 15 0011(Criled, unr Cot'Cllts,011 1, most lhA ihc Rb/Sr ratio in the region of these prohibb correct ith respect to th. 111111*L source of rocks is 0702 'rlic imphcations of this list proposition are about 0 04 and thal thc Sr"/Srs• composition this compoic"t '11 Terainry Inic of this regioti has grown to its present Vallte of about disctisscd below 0 708 from a„ :i,Itial val,ic of about 0 700 at the time Cnpple Creek Duinct, Colorado-In the Cappic 00/ma„ Disirict, Arize,ta -The mincralization in the 1 om Reed mic occurs m Tirtiary mdesites cov- of its formatioil soine 4 SOO million years ago Therc- Creek district a plug of latite-pho„01,te tiffs and n entig a basenictit of Precainbrinti gnelsses and schists fore, gaitgue mmrals conta,mng strontium demed brecaas is e,posed within a Prccninbrim terrat directly from this region should be expected to have comprised of gl·on,tr and grant tic gncisses and schists Gilett, and Damon (1961) haic obianied Rb-Sr ages of 1,200 to 1,350 iii y fron, three biolitc samples from Sr"/St° ratios of not more thai about 0708 for 7 he gnigic nill erals are associated with mieraliza- 4 more recent deposits and the ratios should tion in me Tcrtinry volcarics The gnnitic coilitry chese l'recainbri iii rocks The sanic type of argu- de- niclit can be applicd to the rocks of die Oatmati dis- crease with che increasing age of the mineralization rocks of this arci hnve yielded K-Ar and Rb-Sr agcs trict a. applied above to thI rocks of the Cripple Faure and Hurley also calculated the apprwmite of 1,000 to 1,200 m y (Ald, ich et al , 1958, Gifin Crippic Creek Rb/Sr composition of the upper continental Creek district and, as :11 the case of tlic ernst midi Kulp, 1960, Pearson ct al, 1966) This agc district, it ts very improlable thi the Precanibrian from a consideration of the avcnge Rb/Sr composi- Prccon,bria,1 repments tlic youngest rccogiticd basmelit rork, of the Oatman district could hwe lions of the various crustal rock as deteriii,ned M'it m this rcgm (IIcdge ct al, 1967) The maxi. types bccn the source of the 7 ganguc strontiuni from the anal) ses of many prev""s workers and hy n„int Rl)/Sr w,itpusitlon tliat Bitch bise,ne,it rocks ertiar> Ollic again, Ihe posoibl. alt.., Idti e sources for this weighting these nnaly= with respeCt to the approxi- could bve (020) d thy are the soitrcc rocks for gaigite strontitini are the source ..gion of t. .01- I mate ainounts of each of the rock iypes inthc rrmt the gnng,le ,l, oitllum ts Im th„, cillier the average 1 canic rocks or the volcanic rocks themselves Thrir Act estin.te fur the Rb/Sr composition of the Rb/Sr valitc of t|,C upIEr Cont,ttelit.,1 crust (025) 7 emiride Datrict, Co/orndo -In the Tellunde dis- Upi.r colitilic,itaI crust is 025 By ass,mng an (1· iurc ond 1 1,irig, 1963) or ahc a, er.,gc of rocks average age for the crust of 2,000 million years, us/1, con,positions more sialic thaii tlic a,cmgc of the trict, tlic Conclusions arc less clenr thaii Iii either of Faure and Hurley then calculated that the average tipper coiltilicittal crlist (0 25 .lid grc.itcr) ns glic: the t„ o districts Jud discussed I„ this distr,ct a series of Tcrtiary volcanics o[ md to basic compo- St'/Sr* coinposition of the upper continental crust 1,y both l-aurc atid Ilarlcy (1963) .itid 1 i,rcl init and sition 0, erlics a series of Poleozoic md Mesozoic at the prescit time is about 0725 Therefore, :f the Wcdcpohi (1961) 11 is sig„,ficantly Icss thai, tlic scdments The ore :11,tieralization occurs m the vol- ganguc miticrals contain strontlum that was derived mmgc Rb/Sr ratio of about 10 given for rocks of frot. pre-existing upper crustal materials having canics The agcs of Preainibrian rocks exposed iii gronitic conipositiotis by ]Ii„Icy, Hughes, Fat:re the area are unccrtam, but Rb-Sr and K-Ar ages Rb/Sr ratios ..mlinating the upper cristal ...r- ]rairbairn, mid Pitiso„ 1 lic niaxiniu,1 ; (1962) from Precambrian age, then the gangue stront,u,n should show Srs1/St° 51,7Sr" co„,posit,011 1& any pre-wsing dic granitic rocks in areas surroundn,g ratm s,gmfcantly grcater than 0708 If thcgangue source rocks n. this reg,0,1 ore about 1,400 m y,a n age that appears could have, if tl. pre'tously give. co to bc ubiquitous to a south„ cstern strontium was der„ cd from crust.1 materials of near ditions for agc md Rb/Sr composition n erc both nict large part of the U„,ted States (Aldrich el al, 1957, Mose md Bick basattic composition, or from very young crustal (0702), ls too lou for the bascnictit rocks of this lt would bc dificult to assess Idc- materials, the mi,ierals should show Srl,/S#° rat,os orea From tlic data given by Faurc nid Hurley, ford, 1968) of very ncar 0708, but if the gatigue stmitium „as these rocks quately the possibility that the sed,metits underly„,g should have had m Kid St'/St° or this district „cre the source derived from old sialic (Rb-rich) upper crustal Wut 0704 to 0706 if they had been derivcd from a rocks of the Tertiari gai,gue strontiutii, but this possibility be ruled matenals, thc minerals should show ratios greater, decp-seated me 1,000 my ago If thcy „cre cannot possibly considerably grealer, than 0 708 derived fron, pre-existing stalic rocks, then their out iii this district Thus, in the Tclturide district, there arc three pomble sources of the Tertiary The samples chosen for thts *·cst,gation were i,iti al should have been Sr*'/Sr I Col.positions ganguc stro„tium 1 hcse are the assumed deep- from deposits in which 1) 1 ert,ary mineralizatioil greater thai, the upi)er mititic valite at that time, lottie. regton of the volanics, the volcanics occurs within rocks of Tertiary age, 2) Teritary mil- beca„se wk crustal matermt, generally have Rb/S . seatcd cralizatioti occurs within rocks of Precambrian age, ,alues greater than that of the uppcr mantle. Iii *mavis, or the underly,ng sed,ments and 3) tile mineralization is Precambnan Fluorite Tertiary tunes, tlic Sr' /Srs° compositions of these Jud/h Aforintmi,i, A/ontana-In the Maginnis was the preferred gangul mineral as . .generally rocks would have been signifiantly greater than their mine * orc mieralization is associated with a accepted that the fluonne is of magmatic, and thus dial coti,posiDO,is, Ard chits siginificantly greater Tert:ary rhyolite port),yr> at the co:,tact with an The poss,bly deep-seated, or,gin Because of a lack of Rian 0.702 7 he granitic Picisscs and schists of this underlying Carbomferous limestone analyses of the t.0 fluorile salnples from this m te gi,e Sr'7Sr" adequate fuorite samples the mcasurclnems sere area are older than tic gratiltic rocks Just discussed compositions that arc ..hin the extended to two other strontium-rtch mt,ierals, barite -Ind arc belmed to correlate with rocks farther to rangc of ./...tlum and calate For this survey invesugatlon, samples the north that hap c been dated by Hedge, Peterman, compositions nieasured in Illnestotles of less thon 1,000 my age by Po„ eli (1962) Thus, the under- were selected from ainong those available in tic Li,id- and Braddock (1967) as 1,700my lying Carboti,ferous limestone ts n possible gren Collcct,on of Ores and Minerals in thc Depart- From thc ...... available from die rocks of source of ment of Geology and Geophysics at M I T ll,e Cripple Creek district, it appears, cry,mprobiblc the Tertinry g.ingue Itroill,ul n Three dim.rent mass spectrometen of similar tlit iny of the pie-Terllary rocks of,his district could Bag,1,0 D„in/, Ph:1:1'1,1,„s -ln dic Balatoc mne, design wrre used to make ll,c Isotopic measurements have been the soarce rocks of the 1 crliary ga,iguc on Luzon, the mineralization occurs In a volcanic In the tablcs to fo low the instrument uscd for a par- stro..m With ttic Precambrian basement rocks neck filed u ith Terttary andesite breccia, the sur- acular analysis as :dentifi Lalongside the sample rounding rocks being older diorites of early Ter- number Dun„g the period in \, htch ihcse analyses tiary (7) age These diontes cannot be ruled out were mde, the operation of each ii,slruine,it was as the source of the Tertiary gangue strontium be- inonitored by thc perlodic anat sis of a standard Te.. 2 cause of a lack of adcqu·ite in[ormat,0,1 about them Conch,stoip on th, Aiialy,fs of Terttory Milirrch- sample of SrCO, (Elmer and Aniend, lot number ..lar, Ir,*r.Ution . Te..I reck. mhon m Terfiary Rocks -The analyses of the stron- 492327) The nim„red rattos of Sr•'/Sr*• cre Sample Ne S ap -. cellt. 8&/88 87M6 87BGIC<| tium isotopic compositions of the gningue mmerals normalized to a wluc of Sr"/Sr"=01 194 The 01194 0 7076 0 7076 . 91 f:red.; -1„„- 01196 0 7069 0 70750) n,sociated with Tertiary initieralization occurnng sta,idard atial ses arc listed in Table 1 Also g,ven Illurt., Co ondo m this table are the des,galatioils of each of the instru- Mhin rocks o f Tert,ary agc have yielded Sr"/Sru ratios tliat are compatible with a derivation either ments The anal) ses made ott "Nancy" „cre the .19, 1:1 ==:::...6,-7 -. 5 :50 : IM : im Te lcrl. colon O Arst meastirenients to k mide o„ uns miclm,c after from a wirce region similar in Rb/Sr composition .299 13) .uorlt, . 1185 0 7.74 0 70.8 to that of the source region of the Terwry rocks or it „as asseti,bled As a co,ismlimicc, more stardard Mlnerst Rock M ne ...i,ses .... t.ad. 0. &...t.*ment than ...mer Crlppl Creek, Color*do from the Tprtory rocks tlic,welves tn tlic two distncts m which adequate mforniation is available of the other t„ o E,penclice has shown that the "100 (9) ...... 0 1189 0 7088 0 7073 Rt¢ I'ln 1700* 1...1 on the pre-Tertiary crtistal these rocks can prrr..ion of 11. numal.Zed .01.:cs of sbysr'• is Ees* *rt rocks, ....1...... or..0 be ruled out as possible sources of tliL gangue stron. lot % T,blIt 115301 (L) nuortte 01191 0 7077 0 7068 ...land .1- Reptlce. an*lm or mt ntte. '.Oteple.tandard Crlppl+*. Cblor•CO T ble 3

0 1199 0 7101 0 7115 Tnrt ar, alr,eral utton in Pre."brlart rock' Record No 5,86/5,88(1) 3*7/3,86(1) S.87..86(2) *5304 (8) 2:CA , *,m #-County. Arts SaRpl NO S apl-* I,OC-11. 86,88 87,86 87 86.w 9 (S)13) 0 '191 . 7094 0 70. 115305 (3) nuorite 0 1191 07103 07094 AM CLP)*tu 0 893 0 7258 0 7255 3,72 (,) 01202 0 7059 07083 L:-*t -rluortt* or* Htauthm Mlri* I.rwl. .fl Charre. County. Colo 3404 (L) 01186 0 710. 0 7000 Judith ...... r n5348 (L) Barlt* 0 1190 0 7235 0 7/2 3411 (s) . 1195 0 7077 0 707. ...'. 0 1186 0 7098 0 7075 *.cal m..- .1306 t,) :;1:r...C= 0 1, n 0* , *0 3715 (M) 01193 0 7090 O 7087 Im.. 0 2" U 710• 0 7098 115307 (L) R5349 (L) Pluerite 0 1193 0 7310 0 7298 0 1186 07109 0 7095 ...,-'..,- 3745 {N) Montana ..5 Indian .1- Jw,01th *te, Oeergeto n Cell 3770 {N) Ill. 0 7104 0 7.6 032 (L) pi Catct,I 01196 0 7.2 07.9 01197 0 7777 0 7787 Datetoc Mtne 115350 (L) Orintle(Prece."1-1 3799 (W) . 1.87 0 7110 . 7.9 Burlot. Twr el CeorK* own colo 3811 {N) Ill. 0 7100 0 7065 :SA.:.:7- m.. (11 Nomil„d to S:·86)*8 - 0 1194 (1) Nor klzed to Sr•86/5/88 , 0 119* M*urcd value 2) Sce rootr 3). Ible 1. for n t-n: 0••t .On. (2) S e footnote (3). Tabl• 1. for inst'•ming d..16,1*ticns (2) Nor•alizod to sr86Ar•88 . 0 1194 (33 m duplac te ad,m -r -de In l.od'rfemn' ...... (3) - --%'-.M% tn tht• study -¥e all bec. pi*. nal. S denot•...#.. . denote. k. end . denot . *ic,0 illmi iI; 11; f' i'31 :i·i i E9 iiit "i M M ;3: :i: iIti i i fi '1 ifi f iii Mi 'ii S:ha 'eI li : v,1 1,;ill'ii 3 ' tel l:111 r: fi· A.oit,31, i ,"U t"J 12,1 Mil '0li ME41 i Z ls' 1 1 11 ji :if11 I.#li:9, till"e

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~~Strontium Isotope Variation and Whole-Rock Isochron Studies, Grenville Province of Ontariol~~

~~T. E. KROGH2 AND P. M. HURLEY Massochuscits Institute 0/ Tcclznology, Cambi Wgc, Massachusetts 02139~~

Rb.Sr whole-rock dins wcro oblincd for live gron:lc bodics wd lr,o mctn timentary hortion, ditributed OVCr A dlitance of 100 miles I a re/on of typ:cal Grcovilk melased,- miwin mulicutem Online Ali (in mil[ionso'years ml J of plutonic :octiobtainedand their geolo&,6 agnia/M m / follon m a l m/. a bme of Imphcemell for I niaJor pretedonic mn ant m defomcd on B Mon Kalc (li I]ue Mountain nephellie 6 Hur) m *. m mb a mmi valuc for tho Imc of am mummallsm that accompanied // formation of Lho ///,/ I llc Burrigh parn8Oclo dornc 1000 ...6. tlme of emplaccmcnt for . malor .=.mae tht... deformed donng Nional dcromation (11,0 Gawnoqucsben te) I035 I 00 mb a miuimum Ima for the fomdon of a major m]1-hke gniatic body (the Kiladu gne,=c granite) 010 $ 39 rn, a Unic of cmpbcomcnt for a maJor posttecton e grantte (the Wes port ./.ton) PRecled wit/ SrS;/S-SO rmlies for (hm rock ngo from 0 704 to 0.7035, u hereas mccircd Idnes for /bbros and marbl= iN 0703 wd 0705, reapectively (vnlh ihe exception of tho Knindar gronite for which a las precue valuo of 0 711 m dotormincd) Ti:c :,dicated % of 1#0 1 N .,ul 1036 I 33 m/ for tho Mixon Allimanit, parngicin and Be mctased,- mi adtacent to the Buricl:h por*Inete dorne may =diate the luna of metamorphm o f thic mt& The low inittil stron„um ratzoi (0707 and 07 ) for die metidimmti mdiale a vco */ pro-Grenville hutow Two Brutli bodtcs 14 thi Grr nln provwoo (1 wd 9 milr• south of th: Oveuvith: frunk) in Ulc Norlb Ilay Ircs /,0 sometrhit incon- mtd :soch:ons that approach N W of tho Supmor provtncc to the norlhr/est (2000- 2700 my) Isochm dit• for mclasedmnlry roct,In thts geolo/cal selting R inco:i- mstaL Th.r litgh t...... (.... '.. 0726 in 50...ses) is .. ind..... of We pr Grenvitio h,8,0/ of ihm rocli It / u,c/fore conctudcd that Ilie rocka adjacent to Lho Grcnville front m the areas studied arc the Inclmorphic equivalents of the rocis to the northwil '1 he roct, studicd in southeastcrn Ontano wcro dcpoiled or emplaced be tween 1300 1/ 1* // 0/ Ind were not denved f/m an anclent malic basement Rch as *t found north o f the Gren„11, f/aL We refer to di Umc of plutonim ind meta. :nombim m di irca as tho 'Grcay,Ug oro//ny' hlit Reknowtodge thit . :eviam of 'amvitio' nomenetaturo 15 needed gontomcter setting A conversion factor from INTronucrtor d,ta to a hne and I unreal:st„,f the minor but ANALrrICAL MrricoDS AND RESULTS mtenslty to ..eight ratios m delennind by arb/nry adJustments of the Rb/Sr rolos d:,- - Although tho lue age of sed,mcallion and Tho results of the analyses are 811 n in anal>:in: the finely powdered X.m> standard cu/cd nboic n / co"dered The est:mated emplo)m,ent of granitic mate,ral In the Oren- Tables 1,2, and 3 The mass spectrometric and (Gl) between every two to three unt.nown" accuracy of the :sochron age includes both the * 'die Provt:,ce . conlro erGial, a rcg:o: al meta- chemical techniques arc descnbed by Faure and Esch sample uns measured /rom 4 0 101:me,, error in „ochron fit and a poss,ble calibration morphim about 1000 my ago Is DIdely recog.. 1/wil E1963] Sr motope ratlos Bero measured depending on the conentrations present A error of /3% The error ass:gned to initial .. nized Shilhbrer aid Cumming: [lgial, //uriev directly m unspiked & extractioni and pcnodic mmon m/4 either a whole-rock Ruoride or ratios (Table 5) is a Ingic *andard devition / al [1002], arld othon regarded the pro ince amlyrs of the MIT standard corbonnte were a fincy powdered rock, was used in .11 X-ray obtained from the Icast-sqi:nres analy - These made As shown in Table 4, repltcatc antlyses d/crmillations for a ungle zochron u a blod of conlittental maile litalerial tlin MI .ro som/Imes rather mill, and . PO.- m exlracted from a submatic source rcinon of thi standard gavo a mean normalized valuo For cach :sochron the Rb/Sr ratio in two or .ble.-r of .00"Ithts pr.Jectcd Sr.mr about 1 billion yan Vo Uternntnel>, SocL- for the St/Sf rolo of 07078 and a *andard more =mples Ins d/ennined by ts/topc dilu. ratio I probablymoro malistic. 9 [I964] and O:borne and Monn [1962] deviation of a single an,13= of fODOW A tion, thus a minor ad/wtment for dacrences in As a separnte tcst of tho X-ray Buorescence sundard demion of a single analyms of 0801 propose thnt no siglitficant Eed„nentation pre- matrm between the standard and the rocks technique, those somples nnalyzed by Lsotope ccdcd tlie Orennlic orogen), m thit tho rocks , probibl> a mon renltstic estimate of the Rnalyzcd could be mile. A ratio or convermon ditution Bere re:Innlyzed by using,he powdered now exposed were formed m a pre.Gren„Ic ...... y l. th. co:np.... .f Imples -th factor of mtope d]lution to X.mY Ritorrvrnre mr# m ,|1 Caxj Fur thli tet the standard orogcnic ode and are probablt more than 2000 diferent :sotopic nlios. D ,plicate cal,brations vahle m calculated for each imple nnalk zed 0-1 was :Mlf onalyzcd by :sotope ditution with Indicate my, old IMM Ind do hard [1903] a od of the electrometer and recorder used by the two methods Ench 130,hmn was then thommnip,1Muid forthou,mly,L 1 reeks 11,0 that this reported value I low by 0800£ Gren- wrj:Jited Indlviduly lf the conventon fic. reults of thls test m shown in Table 1 and Witinc-Edward, [1961], u 1,0 .•udied tlic appl,cd. villc in PrM of tht• Arlirnn/trt.: and eouthcaol· [R, wh. 1004] . 1,0 correttion has bcci tora for the tio calibration points agreed to Figum 2 In Figure 2, (wo of the three Ampli bec:,use of the small magnitude 0/ ern Ontnno, respectnely, sagge't that rocks of honcier within 1 %, the average i niue was applied to th# 3 teld a dlirerince m the two methods m thm erm The error of :nternal prcclmon 13 / in o orogcnic cycles may bc prefent Cxtenn., arl the Rb/Sr ralos obtalned by X.roy fluores- mm of 5% cont„n Im than 20 ppm Sr over.aU pluton:c orens of pre-Grrmme bicinent aod least I factor of 2 mnallcr than IIi. ence When th, converson factor, differed by Age wlculitions m boad on n Rb decay retativel> minor arens con,poER of Gren„De. reproducibility morc thi 1%, the fluorescence values were constant of 1J)9 x 10·n yro Thow Imple type scd,ments Wc shnil Orst discuss mme geo- Sr concentral,oas were detenninca by unng adju ted by m amount proportiot 1 to Ule mlyed by *Ne d:lutionaromarked ID m a spito ennchcd in both St and Sr", and hencs logical aspects of tim Grenville province Ind vahle, of the factors and the position of the Table 2 and m the mochron dingrnnts bmple then proceed to an outhlie of whole-rock Rb-Sr they 're independent of instrument frOCUO,8- mples between the * cahbraton points locations ond references to loal geology arc Im (m Appendix A for normaliI,n: pro- studies. Tho livo irca 6tudicd m mtuated (1) Tms adjuslment allorra for pomble differences gven in Appond B mmeditely south of the arena frint and' ccdure) in matnx between ihc hgher and toner points The samples collected for thu study were {2} m the region of typical Grenve, meto=ch. X.my fuorcscence techniques were uscd to 00 8 "910 'ochron hand specimen slze Pieces were 6elected fne mcnts in southeastern Ontano 4.t rocks with favorable Rb/Sr ratios and to The ad/usted Rb/Sr :atios were uscd aton: homogeneous ports of the outcrop All but a The Grenv:Ile structunt and lithologic prov- detcrmine precise values of thi ratio Precm With measured St/St Values to determine the Email chip merc ground up in a disk grindcr and Mos were masund by fixed counting tech- Ince of the Canadian ehteld m largely a Mon chron ages shon I Table 5 In this table about 30 grams were spht out for furth= to of medium. to high-grade metamorphic mcks d niques, the number of counts w. Elxted two error climate for nochron ages are shown :noding in a P,ca blender mill Snmples 4% MSO to 100 sec al ach peak and baeline sed.....= .01.nic, of plutonic Ingt. F.. * m a mngle /ondard det lation of the wo- iut for analyms mghed from 0.2 to 0.3 grams ure 1 is a gencrolized geologic map showing a povion Valm of the baseline undcr each ponk dron slopo a determined by tho test-Gqttares part of thi Grcnille province in Ontino tlet wero detennined by drawing a curved line method m which Ill of the error...s.cd to Tlts STST'EMATICS 0, TH. W,[0.2.-ROCK ba'hne rendinp on a plot includes the areas mpled 111. Grenvillc prov. through tlie three random vansion in the Rb/Sr muos Thm IsocititoN MErl" ince extends northest from the cost const of of intenslty (in counta per =cond) iermis the error 13 a mcaum of the dc:ree of at of tha Allantlc The whole-rock „ochron method provtdes an Cleorglan Bay on Lake Huron to the estimate of the time anco a mtim formed, the coast of /Abridor and m bordered on the South initial Sr =topic ratio I M n.tcm, and an and ast by Palcozo:c mer and th, St Law. evalliation of the reliability of these parameters m:Uor f.ult or, locnity, an at)rupt rencc R:ver A TABLE I Com/rison of Rlm/S/ Rabos Determined by I pical Grenulte R.5210(4) 81 (4)5 17 .1 1 +26 that y:eld incorrect agi arc of pnri,culn, con- mcbscd,ments) cro dommni ll,cse Or: mostly R5210(11) . 0 . 61 4 20 (5>1 17 /2 9 3" 0 0 124 deforrned into nnrtlimt-trending:tmcturciand R.5210(8) 734 (4}1 23 *06 08 cern References to thi d:scis on are midi R5210(0) 550 3010 0. (4)0 63 * 17 -8 5 during a latcr interpretation of the dati. For reCD'tallizcd m th, amphibolite or granulite R5179 156 0 245 0 183 (4)185 9: 00 00 olmplic,ty, putubations of tl,c Mochron ra. facies A local grecnwhit facics nres occurs ncar 0= 0 547 (4)0 382 <2 0 +1 : tiwhips are discuied Bepirately, although in Af,doc, Ontnrto, PlutoniC rn,Li of tho revert R3183-= =: 0% (4)0 647 1 0 5 nature more than one mechan,m mny be opers- di:bit compler contact rclatting,lps Mth tho Reog 143 0 107 6 3 88 (4)3 88 k 1 5 -052 77 4 03 7 235 (4)2 39 8 -0 17 tim For related discumons, the inder la re. mt-ng B,- In * st•dy pt•ton••0 - • , 2080 . 0 12 20 (4)12 29 /09 -0 24 ferrd to LSr method for agc determinations and occum follmng asmintlat:on of older ...rial, pctrogcnets has bco, developed by Schaner 46 the true =ochron 40 and a moditied (hlgh) [1058], Comp310,1 and Jegrev [1059], and \, Fairbirn c: d [1960)