Geochemistry of the Potassic Basalts from the Bufumbira Volcanic Field in Southwestern Uganda

GEOCHEMISTRY OF THE POTASSIC BASALTS FROM THE BUFUMBIRA VOLCANIC FIELD IN SOUTHWESTERN UGANDA

E Barifaijo, A Muwanga and A Schumann Makerere University, Department of Geology P.O. Box 7062 Kampala, UGANDA Tel. +256-41-541258, Fax. + 256-41-531061, E-mail: [email protected], [email protected]

ABSTRACT Bufumbira volcanic field is the southernmost of the four Ugandan small Pleistocene to Recent volcanic fields within the western branch of the East African rift system. The rocks consist of silica undersaturated and vesicular basalts with numerous primary structures. The rocks consist of basanites, leucitites, olivine basalts, trachytes, tephrites, trachyandesites and andesites. The basalts are picritic in the northern part of the field where they are dominated by olivine and are clinopyroxene rich in the southern part of the field. Leucite and plagioclase are common in the groundmass in varying proportions by volume for the entire field. Type 1 dunite and wehrlite upper mantle xenoliths characterize the northern part of the field whereas type II clinopyroxenite xenoliths are common in the southern part of the field. The various basalts are low in SiO2 wt %, Al2O3 wt % and Na2O wt % but high in MgO wt %, TiO2 wt %, CaO wt %, K2O wt % with K2O/Na2O = 1.08 to 2.07. These are potassic belonging to the kamafugite series. Plots discriminate two geochemical trends corresponding to the picritic and clinopyroxene rich basalts. The diagram of Na2O + K2O wt. % against SiO2 wt. % enables various rocks to plot in the designated fields for the different alkaline basalts. The field is enriched in trace, light rare earth (LREE) and high field trace elements (HFSE) where La/Yb = 31 – 55. The petrographic and geochemical studies elucidate enrichment of the upper mantle by both mineralogical (modal) and cryptic (geochemical) metasomatism.

Key Words: Basalts, Enrichment, Mantle, Metasomatism, Potassic,

INTRODUCTION metres), Mgahinga (3,420 metres) and The Bufumbira volcanic field (320 km2) is Sabinyo (3,588 metres) are aligned in an E- the southernmost of the three small fields in W direction along the boundary between Uganda that are associated with the western Uganda and Rwanda. Sabinyo sits atop the branch of the East African rift system (Fig. boundary of the three countries. The 1). It is the northeastern portion of the volcanic centres of Nyiragongo and bigger Birunga field (2600km2) which Nyamulagira (most recent eruption was extends southwards into Rwanda and January and November, 2002 for westwards into the Democratic Republic of Nyiragongo and November, 2006 for Congo. It is enclosed between longitudes Nyamulagira in the Democratic Republic of 29o 30’E and 29o 50’E and latitudes 1o 30’S Congo) are still active and the rest are and 1o 10’S. dormant (Kervyn et al. 2007; Kasereka et al. 2007, Wafula et al. 2007). The field consists of many volcanic centres with spectacular craters on top of most of The volcanic fields in the central western the conical hills. These craters give the part of Uganda (e.g. Lloyd 1972, Lloyd and entire field a hilly panoramic appearance. Bailey 1975, Lloyd 1981, Lloyd 1987, The highest mountains of Muhavura (4,127 Lloyd et al. 1987, Thomas and Nixon 1987, Ngassapa et al. – Urban dietary heavy metal intake from protein foods …

Foley et al. 1987, Link et al. 2010) and part primary structures in the lavas include of Birunga field in Rwanda and the pahoehoe, clinker and agglutinate that Democratic Republic of Congo (Vollmer resemble graded bedding. The Karagwe- and Norry 1983 a, b; Demant et al. 1994, Ankolean is the northernmost extension of Rogers et al. 1998, Platz et al. 2004, the Kibaran belt that covers most of south Rosenthal et al. 2009) have been thoroughly western Uganda. Inliers for the rocks of this investigated for upper mantle metasomatism system are sometimes found outcropping on as the responsible process for producing the some hills (e.g. Mutolere) and also as unique ultrapotassic to potassic rocks of accidental xenoliths in the basalts. those fields with their associated upper mantle xenoliths. The Bufumbira volcanic The magmas brought with them upper field was therefore investigated in this study mantle fragments which occur as xenoliths for upper mantle metasomatism to have a in the various basaltic rocks. The upper full set of results for all these volcanic mantle xenoliths are both type I, subtype Ib fields. where LREE/HREE >1. This type consists of olivine, phlogopite and clinopyroxene GEOLOGY and the xenoliths are either dunites or The volcanic rocks overlie the Karagwe- wehrlites. The xenoliths also consist of Ankolean (1000 - 1400 Ma) arenaceous and type II, subtype II (i) which have rare olivine argillaceous sedimentary rocks. The rocks most of it being replaced by clinopyroxene are mostly silica undersaturated basalts and phlogopite and subtype II (ii) with which are Pleistocene to Recent in age. The isolated and rare olivine, plenty of magmas were laden with volatiles which is clinopyroxene followed by phlogopite and envisaged in the numerous vesicles in the apatite (Table 1). lavas and a few pyroclastic rocks. The

Table 1: Xenolith types and subtypes in the three volcanic fields

Volcanic field Xenolith Xenolith Characteristics type subtype Katwe-Kikorongo II II(i), II(ii), II(i) Olivine replaced by clinopyroxene and phlogopite. and Bunyaruguru II(iiia + iiib) Diopside replaced by drark mica and augite. Diopside, After Lloyd augite and phlogopite replaced by titanomagnetite, sphene, 1972; 1981; apatite and feldspar. 1987 and Lloyd and Bailey II(ii) Rare isolated olivine, clinopyroxene, phlogopite and 1975; Lloyd et apatite. Appearance of sphene, calcite and feldspar. al. 1987 II(iia+iiib) Magmatic crystallisation of clinopyroxene (iiia) overprinted by metasomatic features in (iiib) of apatite occurring together with clinopyroxene, titanomagnetite, coexisting with apatite and clinopyroxene. Occurrence of phlogopite with sphene, calcite and feldspar. Fort Portal Lower Garnet granulite and eclogite. After Thomas crust and Nixon 1987 accidental and Nixon and xenoliths Hornung, 1973 Bufumbira I and II Ib, II(i) and I (b) Olivine, phlogopite, clinopyroxene where LREE/HREE After Barifaijo II(ii) > 1. 2000 II (i) Olivine (rarely present) replaced by clinopyroxene followed by phlogopite.

II (ii) Rare loose and isolated islands of olivine with clinopyroxene followed by phlogopite and apatite coexisting with clinopyroxene. 96 Tanz. J. Sci. Vol 36 2010

existing with clinopyroxene.

Figure 1 Geological map of Western Uganda (after Macdonald 1966) showing the small volcanic fields within the western branch of the East African rift system.

METHODS AND MATERIALS

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The fieldwork involved mapping and consist of trachytes, tephrites, samples collection. The petrographic studies trachybasanites and andesites (Fig.2). were carried out at both Makerere Olivine and clinopyroxene are ubiquitous University, Uganda and Kanazawa both as phenocrysts, xenocrysts and fine University, Japan using RPol, Zeiss grains in the groundmass (Barifaijo 2000). (491726) research microscope. The same The xenocryst orthocumulates and microscope was used for taking mesocumulates would sometimes have photomicrographs. internal voids or the voids would be filled with groundmass minerals. Olivine The microchemical studies to establish the xenocrysts may possess strain lamellae and chemistry of the various minerals were done in some instances they may react with the at Kanazawa University, Japan using a alkaline magmas to produce clinopyroxene. Scanning Electron Microprobe (SEM), The clinopyroxene xenocrysts may be both EMScope Model TB500. The results were colour and chemically zoned. The cores processed with SORD M243 computer consist of salite and rims are composed of connected to the SEM. titanaugite. The olivine phenocrysts have chromium-spinel inclusions and Major and minor element geochemistry was clinopyroxene has titanomagnetite achieved with Xray Fluorescence (XRF) inclusions. The clinopyroxene xenocrysts spectrometer at both Kanazawa University, may also have apatite in the interstices. The Japan (Model Rigoku) and University of groundmass consists of olivine, Vienna, Austria (Model Phillips PW 2400) clinopyroxene, plagioclase flakes and using bead pellets. The trace elements were microlites, round grains of leucite and analysed at both universities using powder opaque minerals which include both pellets. The precision for both results was medium to finer-grained chromium-spinel perfect. The rare earth elements (REE) were and some titanomagnetite. The accessory analysed on irradiated samples at the minerals in the groundmass are sphene and University of Vienna, Austria using a apatite. TRIGA Mark nuclear reactor. The intermediate rocks consist of PETROGRAPHY clinopyroxene among the phenocrysts and The major mafic rocks which occur in the groundmass, rarely olivine, biotite, area consist of basanites, leucitites and plagioclase, sanidine, opaque and accessory olivine basalts. The intermediate rocks minerals.

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Figure 2 Geological map of the Bufumbira volcanic field (after Barifaijo 2000)

GEOCHEMISTRY mantle source for these rocks (e.g. The rocks are generally low in SiO2 wt% Thompson 1985). The high concentration of which ranges from 41.81 to 50.46 wt% in MgO coupled with low concentrations of mafic rocks (Figs. 3&5) and 51.54 to 60.31 Al2O3 and Na2O also point to a depleted wt% in intermediate rocks and 39.86 to upper mantle which was rendered refractory 50.81 wt% in mantle xenoliths. MgO wt% by earlier eruptions which extracted magmas ranges from 8.00 to 17.16 wt% in the mafic of basaltic compositions (e.g. Mysen and rocks. It is 7.76 to 36.60 in the mantle Kushiro 1977). The high concentrations of xenoliths. Al2O3 (9.15 - 17.89) and Na2O Ni, Co and Cr especially for the picritic (1.61 - 3.00) wt% are generally low in basalts from the northern part of the field concentration. The concentrations of CaO (Mg_ = 88-90) accentuates the notion for the (4.27 - 13.35 wt%), TiO2 (1.46 - 4.18 wt% ) depletion of the upper mantle. The fact that and K2O (1.63 - 5.45wt% ) are high for such K2O/Na2O > unity for all the mafic rocks at mafic to intermediate rocks. The K2O/Na2O MgO ! 3.0 wt% (e.g. Foley et al.1987) ratio ranges from 1.08 to 2.07. suggests that the basaltic rocks are potassic. The intermediate rocks (MgO = 1.62 to 2.94 The low values of SiO2 and high values of wt%) are non-potassic (e.g. Foley et al. MgO (Fig. 3) suggest a primitive upper 1987, Platz et al. 2004). The concentrations

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of alkalis in them could have been due to xenoliths plot in the subalkaline field. The normal magmatic fractional crystallisation Cox et al. (1979) diagram (Fig. 6) places processes. the various rocks in their appropriate fields. These exactly conform with petrographic The variation diagrams, for example the plot studies. The Middlemost (1985) diagram of MgO wt% against SiO2 wt% (Fig. 3) (Fig. 7) clusters most of the mafic rocks discriminate two distinct geochemical trends between the basanite (6) and alkaline olivine corresponding to the physico-chemical basalts (8) field. The intermediate rocks processes that operated in both the picritic stretch between the trachybasalt (9) to and clinopyroxene basaltic parts of the field. andesite (19) fields. The mantle xenoliths The alkaline nature of the basalts is depicted plot in the primitive field (16 – 18) of the on the various plots of total alkalis against diagram (picrite to tholeiite basalt field). silica. The Irvine and Baragar (1971) plot The picrite field corresponds with the (Fig. 4) and Middlemost (1975) diagrams wehrlites and the tholeiite basalts correspond (Fig. 5) enable all the rock types of the field with the alkali clinopyroxenites. to plot in the alkaline field and the mantle

40 38 36 34 Wehrlites 32 30 28 26 24 22 Picritic Clinopyroxenites 20 basalt 18 MgO wt% 16 14 12 10 8 6 Clinopyroxene basalts 4 2 0 38 40 42 44 46 48 50 52 54 56 58 60 62

SiO2 wt%

Figure 3 Variation plot of MgO wt% against SiO2 wt% for the rocks from the Bufumbira volcanic field

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12 Alkali clinopyroxene Andesite 10 Basanite 8 Alkaline Basanite scoria

O wt % Leucitite 2 6 Leucitite scoria O + K 2 4 Olivine basalt Na Tephrite 2 Sub-Alkaline Trachyte Trachybasalt 0 35 40 45 50 55 60 65 70Wehrlite75 80 85

SiO2 wt %

Figure 4: The plot of total alkalis (Na2O + K2O wt%) versus SiO2 wt% (after Irvine and Baragar 1971)

5 Clinopyroxene Basalts 4 ALKALIC

3 SUB-ALKALIC BASALTS O wt% 2 Picriti K 2 Basalt

1 Mantle Xenoliths

0 LOW POTASSIUM SUB-ALKALIC 38 40 42 44 46 48 50 52 BASALTS54 56 58 60 62

SiO2 wt%

Figure 5: Binary diagram of K2O wt% against SiO2 wt% (after Middlemost 1975)

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10 Trachyte ALKALIC 9 SUB-ALKALIC Basanite 8 Leucitite

7 Andesite

6 wt% O 2 5 0+Na 2 Basalts

K 4

Mantle xenoliths 1

0 38 40 42 44 46 48 50 52 54 56 58 60 62

SiO2 wt%

Figure 6: Plot of total alkalis (K2O + N2O wt%) versus SiO2 wt% of the various rocks of the Bufumbira field (Cox et al. 1979)

The Le Maitre (1989) plot (Fig. 8) clusters These are mafic rocks from the mantle. The most of the mafic rocks between the basanite enhancement in the concentrations of (U1b) and trachybasalt (S1) fields followed potassium and the incompatible trace by the leucitite (U1c) and basaltic elements suggests enrichment of these trachyandesite (S2) fields. The intermediate elements in the upper mantle before rocks plot in the tephrite (U1a) and eruption. The variation diagrams between trachyandesite (S3) fields. The mantle major elements and trace elements (e.g. Fig. xenoliths plot in the picrobasalt (PC) for 10) and trace elements versus trace elements wehrlites and basalt (B) fields for the (e.g. Fig. 11) still discriminate the two clinopyroxenites. The Walker normative geochemical trends. plot (Fig. 9) of olivine, diopside and plagioclase define a differentiation trend for The rocks have also high concentrations of the various rocks starting from the mantle the light rare earth elements (Figs. 12a&b). xenoliths to the leucitites and olivine basalts The high concentrations could be attributed of the picritic basalts followed by basanites to enrichment of these elements in the upper and the intermediate rocks. mantle before eruption or garnet could have retained the heavy rare earth elements in the All the rock types have high concentrations mantle. The absence of garnet in the mantle of the incompatible trace elements Ba, Nb, xenoliths negates the second possibility. Rb, Sr, Y, Zr, U, Th, Ta (Figs. 11&14).

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1 2 3 4 5

11 12 13 14 15

wt% 10 Bs O 2 9 Al-cpx And O+K 2 6 Bc Sc Na Lc 5 Lc Sc 8 18 19 20 21 22 23 ol ba ol.Bs 7 Tpr 17 Tr 16 Tr. Bs Wh 0 40 50 60 70 80

SiO2 wt%

Figure 7: Variation diagram for total alkalis (Na2O + K2O wt%) against SiO2 wt% (after Middlemost 1985)

15 Bs Al-cpx Ph And Bc Sc Lc U3 Lc Sc T ol ba 10 F ol.Bs U2 R wt% Tpr O

2 Tr a S3 Tr. Bs O+K 2 Wh

Na b S2 U1 S1 5 C O3 O2 O1

B Pc

0 35 45 55 65 75

SiO2 wt%

Figure 8 Diagram of total alkalis (Na2O + K2O wt%) against SiO2 wt% of the Bufumbira basalts (after Le Maitre 1989)

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Figure 9 The Walker et al. (1989) normative ternary plot of plagioclase, olivine and diopside

700 Wehrlites

600

500

400 ppm Basalts Ni, 300 Picritic Clinopyroxenites

200

100

clinopyroxene Basalt 0 s 38 40 42 44 46 48 50 52 54 56 58 60 62

SiO2 wt%

Figure 10 The variation plot of Ni ppm versus SiO2 wt%

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700

Wehrlites 600

500

400 Picritic ppm Basalts

Ni, 300

200 Clinopyroxenites

Clinopyroxene basalts

100

0 0 20 40 60 80 100 120 140 160 180 200 220 Rb, ppm

Figure 11: The binary diagram of Ni ppm versus Rb ppm

EB4-Pr EB16-Pr EB27-Pr EB94-Pr EB102B 1000 1000 EB182 EB201 EB38-Pr EB44-Pr EB64-Pr EB218 EB222 EB65-Pr 100 100 EB231B EB243

10 1 12a 12b 1 0.1 La Ce Nd Sm Eu Yb Lu La Ce Nd Sm Eu Gd Tb Tm Yb Lu

Figure 12a&b: CI – Chondrite normalised plots for rare earth elements of some rock samples from the Bufumbira volcanic field mantle metasomatism. The Th/Yb versus The spiderdiagram plot (Fig. 13) shows the Ta/Yb (Fig.14) diagram, concentrates the level of enrichment of the incompatible trace basaltic rocks in the group II ultrapotassic to and light rare earth elements. The trough on potassic within plate basalts that were Nb could be due to a refractory accessory derived from an enriched mantle source (e.g. phase like pyrochlore retaining it in the Pearce 1982; 1983). The mantle xenoliths mantle. The trough on Cs could be also experienced moderate enrichment. attributed to its high mobility during upper

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100 0

EB65- EB18Pr EB202 10 EB211 0 EB228 EB2312 B

1 0

0. 1 R Cs Th U N L C N Sm E Gd Tb Y L b b a e d u b u

Figure 13: CI – Chondrite normalised spiderdiagram of some samples from the Bufumbira volcanic field

9 Group III 8 Ultrapotassic- potassic rocks Group 7 Ultrapotassic-II potassic Active continental rocks margins 6 Oceanic Island Th/Y Arcs b 5

3 Within plate basalts Depleted mantle 2 Enriched mantle 1

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Ta/Y b

Figure 14: Plot of Th/Yb against Ta/Yb for samples of the Bufumbira volcanic rocks (after Pearce 1982, 1983)

DISCUSSION by earlier extraction of basaltic melts which The low silica content, high magnesia and carried along the lithophile elements. Their embayed mantle xenoliths suggest a mantle replenishment in the upper mantle was a origin for the volcanic rocks of the later event. Bufumbira field. The mantle was depleted

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The proponents of the hypothesis involving mantle metasomatism proposed by Lloyd crustal assimilation (e.g. Holmes 1950; Bell (1972) was revived (e.g. Menzies and and Powell 1969; Cohen and 0' Nions 1982; Murthy 1980; Peccerillo 1992; Widom et al. Huppert and Sparks 1985; Prelevic et al. 1997, Rogers et al. 1998) to adequately 2005) to explain the enrichment of the upper explain upper mantle enrichment processes mantle were unable to account for the high and isotope variations. It was found to be isotopic variations in the rocks (e.g.Vollmer plausible in accounting for the enhancement and Norry 1983a&b, Nelson et al. 1986, in the concentrations of K2O, CaO, TiO2, Vollmer 1989; Walker et al. 1989,, Reid incompatible trace and light rare earth 1995). Very large amounts of crustal elements (e.g. Furman 1995; Rosenthal et material would be needed in order to al. 2009, Link et al. 2010). significantly modify the radiogenic signature of potassic magma (e.g. Peccerillo 1992). The earlier suggested hypothesis of upper

Table 2 Chronological order of events for both the southern and northern parts of the Bufumbira volcanic field (after Barifaijo 2000)

Order Southern Northern 1 Pervasive modal metasomatism at an Limited modal metasomatism at a depth of estimated shallow depth of 80-120 km. 80-120 km. Cryptic metasomatism is This was at low geothermal gradient. evident in the xenoliths. 2 Heat plume from greater than 200 km to trigger higher degree of partial melting. 3 Eruption of alkaline basalts for both Eruption of picritic basalts. At Rutare centre mafic and intermediate rocks. these clearly pierce through the clinopyroxene rich basalts. 4 Formation of monogenetic mostly central Formation of central volcanoes rich in volcanoes with rocks which are rich in olivine among phenocrysts and groundmass clinopyroxene both among the with limited fissure eruption. phenocrysts and the groundmass with limited fissure eruption

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Event 1 Upper mantle metasomatised 2 Plume flow 1 + 2 Plume triggers higher degree of partial melting in already moderately metasomatised upper mantle 3 Eruption of volcanoes 3a Eruption of central volcano 3b Fissure eruption

Figure 15 Chronological orders of events in the mantle beneath the Bufumbira volcanic field up to eruption. These resulted in the different rock types for the northern and southern parts of the field (after Barifaijo 2000)

Potassic volcanism in low geothermal followed by H2O and F2. Bailey (1964, gradient rift regimes is always associated 1972, 1980, 1982, 1983, 1985) developed a with volatitle activity. Large proportions of hypothesis that bending and fissuring of the volatiles include CO2, H2O, F2, Cl2 (Eggler, continental lithosphere during uplift and 1987, Foley et al. 1987). The dominant gas rifting localises mantle degassing, where the for the Bufumbira field was CO2 volatiles flow into the arches and tensional (Huntingdon 1973; Link et al. 2007) regions of the lithosphere locally

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metasomatising the mantle in those regions. tephrites, trachybasanites and andesites. The Mckenzie (1989) suggested that silicate melt various mafic rocks carry upper mantle fractions smaller than 1% may be mobile xenoliths. The xenoliths are type I, subtype within the upper mantle. The silicate melts Ib and type II, subtypes II (i) and II (ii). together with the volatiles were derived from The rocks are low in SiO2, Al2O3 and Na2O the fertile sublithospheric mantle and came wt%. They are high in MgO, CaO, TiO2, laden with the incompatible trace elements K2O wt%, Ni, Cr, Co, incompatible trace which migrated upwards. Since they were elements and the light rare earth elements. small, they could not transfer much heat and Variation diagrams discriminate the picritic they were able to freeze readily in the upper and clinopyroxene basaltic trends. The plots mantle with which they reacted (Peccerillo for K2O wt% against SiO2 wt% and total 1992). Melting of metasomatised veins alkalis against SiO2 wt% enable the basaltic which were rich in phlogopite or amphibole rocks to plot in the alkaline field and mantle and some accessory phases yielded alkaline xenoliths plot in the subalkaline field. The magmas. The Bufumbira mantle xenoliths other variation diagrams for total alkalis show evidence of modal metasomatism versus SiO2 wt% place the various rocks in where olivine was replaced by phlogopite the appropriate fields for which they were and clinopyroxene. Lack of amphibole and named. The names conform with those of preponderance of phlogopite means upper other petrographic studies. The mantle metasomatism occurred between 80 - enhancement in the concentrations of certain 120km under the normal geotherm (e.g. elements was attributed to upper mantle Lloyd and Bailey 1975; Demant et al. metasomatism. 1994). ACKNOWLEDGEMENTS The fact that the northern part of the Greatly indebted to Japan International Bufumbira field is dominated by picritic Cooperation Agency (JICA), Austrian basalts (Mg_ = 80-90) plus dunite and Academic Exchange Service (ÖAD) and wehrlite xenoliths and the southern part Makerere University (Mak) for the material consists of clinopyroxene basalts (Mg_ = support. Thanks very much to the Tanzania 69-86) and clinopyroxenite xenoliths means Journal of Science (TJS) for accepting to that upper mantle metasomatic reactions publish this work. were more pervasive in the southern part than the northern part of the field (Table 2 REFERENCES and Fig. 15). The metasomatising fluids Bailey DK 1964 Crustal warping - A fluxing in the southern part of the field were possible tectonic control of alkaline more voluminous than those from the magmatism. J. Geophys. Res. 69: 1103 - northern part (Barifaijo 2000). The picritic 1111. basalts originated from the deeper part of the Bailey DK 1972 Uplift, rifting and mantle (120-200km). magmatism in continental plates. J. Earth Sci. (Leeds) 8: 225 - 239. CONCLUSION Bailey DK 1980 Volcanism, earth degassing The Bufumbira volcanic field is the and replenished lithosphere mantle. southernmost of the three small volcanic Philos. Trans. Roy. Soc., A 297: 309 - fields found in the western branch of the 322. East African rift system. The field is the Bailey DK 1982 Mantle metasomatism - northeastern portion of the Birunga field. Continuing chemical change within the The rocks are silica undersaturated basalts earth. Nature 296: 525 - 530. which are vesicular. The mafic rocks consist Bailey DK 1983 The chemical and thermal of basanites, leucitites, olivine basalts and evolution of rifts. Tectonophysics 94: 585 - the intermediate rocks consist of trachytes, 597.

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