Geochronology and geochemistry of the rocks associated with a late proterozoic ophiolite in West Pokot, NW Kenya

Journal of African Earth Sciences, Vol. 14, No. 1, pp.25-36, 1992.

0899-5362/92 $5.00+0.00 © 1992 Pergamon Press Ltd

Printed in Great Britain

Geochronology and geochemistry of the rocks associated with a Late Proterozoic ophiolite in West Pokot, NW Kenya A. c. Rms1, J. R. V~COMnE 2, R. C. PRXCL~and R.M. StuLcx.wro~ lRies-CowardAssociates Ltd., 70 GrosvenorRoad, Caversluun,Reading RG4 0ES Postgraduate ResearchInstitutefor Sedimentology,P. R. I. S. ConlributionNo. 193 The University,P. O. Box 227, Whiteknights,ReadingRG6 2AB ~ e y Centre for Teaching and Research in StrategicMineral Resources, Departmentof Geology,The Universityof Western Auslralia, Nedlands, WesternAustralia6009 3Departmentof Earth Sciences,The Open University, Milton KeynesMK7 6AA, UK WheCroft Barn,ChurchStreet,East Hendred,OxonOX12 8LA,UK (First received 11 May, 1989; revised form received 13 March, 1991) Abstract - Marie end ultramafic rocks in the W Pokot area, NW Kenya are identified as parts of a dismembered ophiolite. They lie within the late Proterozoic Mozambique Belt and are associated with metasedimentsand calc-alk~inevolcanics and intrudedby graniticrocks. All theserocks are allochthonous, du'ustwestwards towardsthe ArchaeanTanzanianCraton.The calc-alkalinevolcanics,whichare chemically similar to present-dayisland-arcvolcanicrocks, give a Rb/Sr whole-rockisochronage of 663 + 49 Ma, and the associatedmetasedimentsgive an age of 584 + 25 Ma, both ages interpretedas datinga regionalamphibolitefaciesmetamorphism.Theserocks are intrudedby theMarichGraniteftomwhicha Rb/Srwhole-rockisochron ageof 593 + 50 Ma was obtainedwithan VSrY"3rinitialratioof 0.7072 + 5 implyingsomecrustalcontamination either from the migmatitecomplex, whichslructurallyunderlies the ophioliticrocks, or from deeper crustal rocks. The age specffumis broadlysimilarto that establishedfor similarsequencesof rocksnorthwards along strike in Sudan, Egyptand Sandi Arabia.There is no support for the view that thesehigh-grademetamorphic rocks of thispart of the MozambiqueBelt are an olderseries underlyingthe lowergradeLateProterozoicrocks of NE Africa and Saudi Arabia.

INTRODUCTION

The a r e a d i s c u s s e d in t h i s p a p e r lies w i t h i n w h a t is generally k n o w n a s t h e M o z a m b i q u e Belt of NW Kenya, w i t h t h e E a s t African Rift Valley to t h e E and the Nyanzian and Kavirondian rocks and U g a n d a g n e i s s e s of t h e A r c h a e a n C r a t o n to t h e W (Fig. 1). The a r e a w a s c h o s e n for s t u d y b e c a u s e of t h e a s s o c i a t i o n of m e t a p e r i d o t i t e s , s e r p e n t i n i t e s a n d talc schists, m e t a g a b b r o s a n d epidiorltes which s e e m e d likely to r e p r e s e n t p a r t s of a n ophiolite (Shackleton, 1977). McCall (1964) originally interpreted t h e s e r o c k s a s sill-like i n t r u s i o n s w h i c h he s u g g e s t e d , m a y r e p r e s e n t a n ophiolite s u i t e emplaced in g e o s y n c l i n a l s e d i m e n t s at t h e b e g i n n i n g of a n orogeny,. S h a c k l e t o n (1977} s u g g e s t e d t h a t if s o m e of t h e s e r o c k s c o u l d be s h o w n to be ophiolitic a n d of similar age to t h o s e a l r e a d y identified as ophiolitic s e q u e n c e s in S a u d i Arabia (Bakor et a t , 1976; F r i s c h a n d AI-Shanti, 1977; S h a n t i a n d Roobol, 1979; Nasseef, 1982); in S u d a n (Fitches et a/., 1983); in E t h i o p i a (Kazmin, 1976) a n d as

25

complete s e q u e n c e s (Nasseef et al., 1980) or a s ophiolitic f r a g m e n t s in a m e l a n g e in t h e E a s t e r n Desert of Egypt (EI-Sharkawi a n d EI-Bayoumi, 1979, S h a c k l e t o n et aL, 1980, Ries et al., 1983), t h e n t h e belt of late Proterozoic r o c k s of S a u d i Arabia a n d NE Africa c o u l d be s h o w n to c o n t i n u e into t h e Mozambique Belt in E Africa, a l t h o u g h t h e plate tectonic p r o c e s s e s n e e d n o t be t h e s a m e (Fig. 2). O t h e r a u t h o r s , Vail (1976, 1979) a n d H e p w o r t h (1979) s u p p o s e d t h a t t h e low grade "Pan-African" rocks, m o s t l y volcaniclastic, a n d t h e w i d e s p r e a d a s s o c i a t e d granitic r o c k s in NE Africa a n d S a u d i Arabia were y o u n g e r t h a n t h e high g r a d e M o z a m b i q u e Belt r o c k s of E Africa w i t h a n u n c o n f o r m a b l e c o n t a c t b e t w e e n t h e two in c e n t r a l S u d a n Waft, 1976). The p a r t of West Pokot s t u d i e d b y us, t h e Marich a r e a (Fig. 1), overlaps t h e SE c o m e r of t h e S e k e r r q u a r t e r degree s h e e t (McCall, 1964) a n d t h e NE c o m e r of t h e adjoining K i t a l e - C h e r a n g a n i Hills q u a r t e r degree s h e e t (Miller, 1956). This w o r k f o r m e d p a r t of a m o r e extensive s t u d y of t h e Late Proterozoic r o c k s of NE Africa, one a i m of w h i c h w a s to t r y to e s t a b l i s h t h e r e l a t i o n s h i p

26

A.C.RIEs, J.R.VEARNCOMBE, R.C.PRICE and R.M.SHACKLETON

11o ,wah

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/ i r-~7~] Turbo 'migmatites' Granites & granodiorites

Granite

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Semor Migmatites

Lim O uarestones tzitss

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Nyanzl KavirOndi an an

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Undifferentiated-J Hornblendic schists & gneisses(ophioliteSg/ Ophiolites

Fig. I. Geological map of the West Pokot area, NW Kenya (modified from Veamcombe 1983b).

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Fig. 2. Map showing the distribution of known ophiolitic complexes in the late Precambrlan of NE Africa and the Arabian Shield.

mites described by Dickey (1975). Pyroxenites occur as screens and xenoliths between granite between the low grade Late Proterozoic terrains of sheets in the Marich Pass and as magmatic segreNE Africa (Saudi Arabia, Egypt, n o r t h e r n Sudan) gations in layered gabbros in the Ion river section. and the hlgh grade terrains of the Mozambique Belt Gabbros with well-preserved c u m u l a t e layering ( K e n y a , T a n z a n i a a n d M o z a m b i q u e ) (see are recognizable in places and can be seen in pods which merge into more deformed gabbros and Shackleton, 1986). amphibolites. Basic dykes cut the gabbros, locally forming up to 20-30% of the outcrop a n d m a y GEOLOGY OF T H E MARICH .,~f¢]B,A represent the root zone of a sheeted dyke complex. There are good pillow lavas, some with vesicular In the Marich area (Figs 1 and 3), the ultramafic tops making good way-up indicators. Manganiand marie rocks are associated with amphiboliteferous-gamet quartzites, which crop out N of the facies metasediments, the Marich-Korpu Schists of McCall (1964), which include orthoquartzites Ion river and W of Tulot, m a y have once been deepand carbonates, of possible shallow water shelf- sea manganiferous cherts, Geochemical~ the marie facies origin, a n d some turbidites (Vearncombe, lavas are similar to MORB (Price 1984) and since 1983a and b). There are also volcanic a n d acid most of the c h a r a c t e r l s t i c c o m p o n e n t s of a n ophiointrusive rocks. Although now tectonically frag- lite in the Penrose (1972) sense are present, this mented, most of the components of a n ophiollte assemblage is interpreted as a n ophiolite as sugcan be recognized, particularly in the Ion river gested by Shacldeton (1977). Associated wlth the ophiolltic fragments are section (Fig. l) (Veamcombe, 1983b). There are serpentinites, some containing podiform as well as volcaniclastic rocks, here n a m e d the Marich dismembered chromite as at Tulot; the podiform Volcanic Series, which have arc-type chemistry chromite is n o d u l a r or massive, occurs associated (see below). Structurally b e n e a t h the ophiolite is a with talc schists probably after dunite and is migmatite complex with associated pelitic, psamchemically similar to the alpine podiform chro- mitic and homblendic schists. The granitic part of

Geochronology and geochemistry of the rocks associatedwith a Late Proterozoicophiolite

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Fig. 3. Detailed geologlcal map of the Marich area, NW Kenya (from Vearncombe. 1983b}.

27

A.C.RIEs, J.R.VEARNCOMBE, R.C,PRicE and R.M.SHACKLETON

28

ages in NE Africa (Harris et al., 1984}. Both the Marich Volcanic Series a n d the Marich metasedlmerits have been m e t a m o r p h o s e d in amphibiolite facies; there is no conclusive field evidence to show their relative ages. The Marich Granite is clearly later t h a n both, b u t it is itself deformed and metamorphosed.

this complex is petrographicaUy similar to the Marich Granite which intrudes the ophiolite and granitic dykes from the mlgrnatite complex intrude up into the ophiolitic rocks. The fragmented ophiolite and associated calcalkaline volcanic rocks a n d m e t a s e d i m e n t s are all heterogeneously deformed a n d part of a n easterlydipping imbricate stack with folds verging to the W. The metamorphic grade is amphibolite facies with kyanite in m e t a s e d i m e n t a r y rocks b e n e a t h and, at Marich, above the ophiolite belt (Vearncombe, 1983 a and b).

T h e Mm4¢~ V o l e ~ n l c S e r i e s These rocks, part of the amphibollte schists of McCall (1964), which are well exposed at the confluence of the Ion and M a r u n rivers (Loc. 31, Fig. 3), are mostly agglomerates often with large GEOCHRONOLOGY (up to 15 c m across) pale angular blocks, with a metaporphyritic appearance, in a d a r k matrix. Prior to this study, there were no radiometric Some of the units are bedded. ages available for the Mozambique Belt in NW Seven samples of these rocks gave a R b / S r Kenya with the exception of a deformed A r c h a e a n whole-rock isochron age of 663 + 49 Ma with an granite in the marginal mylonite zone, a few k m E ' ~ S r / ~ S r tnitt~l ratio of 0.7035 + 1 (MSWD = 18) of Webuye (Fig. 1). Three suites of rocks were (Fig. 4, Table 1). The large MSWD and consequent collected during the present s t u d y for dating by the large errors are attributed partly to the low R b / S r R b / S r method. They represent the calc-~lkAllne ratios, the small range of composition of the series Marich Volcanic Series associated with the ophio- and the altered n a t u r e of the samples. From field lites, the Marich m e t a s e d i m e n t s and the Marich relations it is obvious t h a t the two points (31F and Granite. Samples from each of these suites were 31G) at the u p p e r end of the line are part of the analyzed as part of a separate s t u d y of model Nd same volcanic series a n d so there is noj ustification for treating the points at the lower a n d u p p e r ends 0 Z204 MARICH METASEDIMENTS separately. In thin section the analyzed samples contain large h o m b l e n d e porphyroblasts in a & matrix of flne grained plagioclase a n d some quartz. / , < The hornblendes contain s u b - r o u n d e d grains of // J plagioclase and quartz and overgrow the main biotite flattening fabric. Most of the original 7 / ..4Y igneous minerals have been replaced during meta9morphism. Because of the extent of recrystallizaQ411 tion in these rocks, it is not obvious w h e t h e r the MSWO 6 4 663 _+49 Ma age represents a n extrusive age or has Aqe 5 8 4 * 2 5 M a ( 2 slgrr,.]) been reset by the amphibolite-facies metamor(87Sr/B6Srb O 7042 " 2 phism. The age is j u s t signlficanfly older t h a n the age obtained from the m e t a s e d i m e n t s which al87Rb/86Sr most certainly represents a m e t a m o r p h i c age. It 1 (}

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Fig. 4. Rb/Sr whole-rock Isochron plots for the West Pokot area, NW Kenya.

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Geochronology and geochemistry of the rocks associated with a Late Proterozoic ophiolite Table I. RblSr data for the Marich area Sample No.

Rb ppm

Sr ppm "1~bl"Sr

sTSr/~Sr

278 358 248 273 225 157 305

0.062 0.040 0.026 0.025 0.195 0.629 0.085

0.70408 0.70396 0.70397 0.71384 0.70513 0.70962 0,70431

a. Marich Volcanic Series 3 IA 31AA 3 IB 31C 31F 31G 3 IH

6.24 5.06 2.20 2.38 15.45 35.05 9.17

b. Marlch metasediments 32 33D 33E 33B 33

61.48 115 65.75 125 197

573 538 542 514 456

0.3 I0 0.619 0.351 0.702 1.259

0.70676 0.70951 0.70705 0.71003 0.71458

57A 57B 41

24.65 50.52 9.12

509 248 419

0.140 0.589 0.063

0.70476 0.71115 0.70380

287 577 635 748 379 . 428

1.382 0.692 0.356 O. 195 1.260 0.780

0.71890 0.71251 0.71058 0.70872 0.71804 0.71385

r,.i~aW, h_9.ma~ 34 35 36 37A 81118 81121

237 136 78.23 50.27 164 116

Rb/Sr ratios obtained by automatic Phillips 1450 spectrometer at the British Geological Survery. STSr/~Sr ratios, norvmli~,ed to NBS 987 value of 0.71014 + 3, obtained on an automated 54E mass spectrometer at the O p e n UniversiW. Data regressed using the method of York {1969) and ~ b = 1.42 x I0 -n xyr -L All errors quoted are 2 sigma; where M S W D > I, the errors on the ages and initialratios have been multiplied by M S W D .

c o u l d therefore b e a r g u e d t h a t t h e age of 6 6 3 + 4 9 Ma is t h e age of eruption. However, u s i n g a m e a n R b / S r v a l u e of 0.15, it c a n b e s e e n from a Sr evolution d i a g r a m t h a t a m a x i m u m age of 1015 Ma is p o s s i b l e for t h e s e rocks. A s s u m i n g n o c h a n g e in t h e R b / S r ratio, t h e age of e r u p t i o n of t h e s e v o l c a n i c s c o u l d therefore b e a s m u c h a s ca. I 0 0 0 Ma. The

Marich

metasediments

This g r o u p of m e t a s e d i m e n t s i n c l u d e s ferrugin o u s c h e r t s a n d quartzites, garnet-biotite schists, calc-silicate b a n d s a n d p u r e m a r b l e s . The distrib u t i o n of t h e s e d i m e n t a r y facies s u g g e s t s a n eastw a r d d e e p e n i n g s e q u e n c e with c o n t i n e n t a l m a r g i n a n d s h e l f f a c i e s q u a r t z l t e s in t h e W (in t h e C h e r a n g a n i Hills), l i m e s t o n e s thickening e a s t w a r d s and deep sea turbidites and cherts farther E (Vearncombe, 1983a}. Eight s a m p l e s from t h i s g r o u p were collected for dating. Five garnet-biotite s c h i s t s gave a R b / S r

29

w h o l e - r o c k age of 5 8 4 + 2 5 Ma with a n S~Sr/S6Sr initial ratio of 0. 7 0 4 2 + 2 (MSWD = 6.4) (Fig. 4, Table 1). T h e s e five s a m p l e s are finely l a m i n a t e d (layers 1-2 c m thick), with r h y t h m i c a l l y r e p e a t e d u n i t s with pale f e l d s p a t h i c s a n d y b a s e s grading u p into d a r k m l c a c e o u s tops. T h e y w e r e p r o b a b l y originally turbidites. T h e y v o w c o n t a i n large r e d g a r n e t s , s o m e e u h e d r a l , w h i c h in p l a c e s a p p e a r to h a v e overgrown t h e biotite flattening fabric, while o t h e r s have t h e s a m e fabric w r a p p e d r o u n d t h e m with quartz-filled p r e s s u r e s h a d o w s . Most of t h e g a r n e t s c o n t a i n i n c l u s i o n s of q u a r t z g r a i n s w h i c h s h o w only a w e a k s h a p e fabric. T h e age o b t a i n e d is i n t e r p r e t e d a s b e i n g t h e age of t h e amphlbolitefacies m e t a m o r p h i s m . The t h r e e s a m p l e s , a n a m p h i b o l i t e (41} a n d t w o calc-sflicates {57A & B) plotted o n Fig. 3 b u t n o t i n c l u d e d in t h e regression, a p p r a x i m a t e to a line equivalent to a n age of 9 8 2 + 4 0 M a with a n STSr/ S6Sr initial ratio of 0 . 7 0 2 9 + 2 (MSWD = 5.8). The r o c k s in localities 41 a n d 57 a r e m o s t l y h o m o g e n e o u s m a s s i v e d a r k a m p h i b o l l t e s a s s o c i a t e d with s o m e i m p u r e d a r k c a l c a r e o u s b a n d s a n d are t h u s lithologically distinguishable from t h e garnet- biotite s c h i s t s in localities 31 a n d 32. The a m p h i b o l i t e s are chemically similar to t h e pillow lavas of t h e ophiolite s e q u e n c e a n d h a v e M O R B t y p e c h e m i s try. It m a y b e t h a t t h i s older d a t e reflects a m e t a m o r p h i s m a s s o c i a t e d with t h e e m p l a c e m e n t of t h e ophiolite. Nd i s o t o p e s t u d i e s o n s a m p l e s 32, 3 3 a n d 5 7 B indicate t h a t t h e s e s e d i m e n t s m a y h a v e b e e n s o u r c e d from a t e r r a i n a s old a s 1 2 0 0 - 1 5 0 0 Ma (Harris et a t , 1984). The

Ms, r i c h

Grnnlte

McCall (1964) d e s c r i b e d t h e Marich Granite a s a c o m p l e x of " c o n c o r d a n t a n d slightly c r o s s - c u t t i n g s h e e t s of foliated granite c h a r a c t e r / z e d b y a b u n d a n t pegmatitic material". X e n o l i t h s of pyroxenite, h o m b l e n d i t e a n d g a b b r o , n o t e d b y McCall (1964), indicate t h a t t h e i n t r u s i o n of t h e granite postd a t e d t h e e m p l a c e m e n t of t h e ophiolite. Veins of t h e granite a p p e a r to h a v e locally c u t early tectonic c o n t a c t s a n d b e e n i n t r u d e d after t h e m a i n t h r u s t ing e v e n t b u t before, or s y n c h r o n o u s l y with, a later deformation during w h i c h c o n c o r d a n t granite veins a n d s h e e t s were folded a n d b o u d i n a g e d . The s a m p l e s d a t e d were collected from t h e Marich P a s s where, a l t h o u g h m u c h of t h e granite is s h e e t e d , t h e central zone, w i t h o u t s c r e e n s , h a s a plutonic aspect. Petrographically the samples analyzed show m o s t l y quartz, microcline a n d s o m e plagioclase (and. - olig.). Small flakes of biotite p i c k o u t t h e tectonic fabric a n d g a r n e t s are often present. The t e x t u r e s h o w s s o m e recrystallization; m o s t of t h e quartz grains are strain-free and the grain b o u n d a r i e s s h o w triple point relationships.

30

A.C.RIEs, J.R.VEARNCOMBE, R.C.PRICE and R.M.SHACKLETON

The R b / S r whole-rock isochron age of 593 + 50 Ma could represent the age of intrusion of the granite. Field evidence suggests that it was int r u d e d Just before, or during, the m a i n deformation a n d associated m e t a m o r p h i s m dated at ca. 584 Ma. The large error on the age is probably due to the strong deformation a n d metamorphic grade of these rocks. The 87Sr/~Sr initial ratio of 0.7072 + 5 for this granite is higher t h a n that found in granitic rocks in the Pan-African terrains of Saudi Arabia, Egypt a n d S u d a n where, apart from late within-plate alkali granites, initial r a t i o s a r e mostly ca. 0.703 (Duyverman and Harris, 1982). This higher initial Sr ratio implies the Marich Granite h a s a m a x i m u m age of co_ 1090 Ma. The field evidence suggests t h a t this granite is a component of the underlying Samor (para)gneisses t h u s suggesting that the higher STSr/S6Sr initial ratio and high m a x i m u m age is attributable to melting of sediments which almost certainly include clastic material eroded from the Archaean Tanzanian craton, 100 k m to the SW. Nd isotope studies indicate a crustal precursor of 1200-1500 Ma (Harris et aL, 1984). The ages presented above fall within the range of ages which h a s been obtained from the PanAfrican domains of Ethiopia, Sudan, Egypt and Saudi Arabia. In all of these regions the R b / S r whole-rock ages cluster between 500 a n d 800 Ma with s o m e extending b a c k to 1100 Ma (e.g. Stoesser a n d Elliott, 1979; Stem, 1979; Kr6ner et al., 1979; Hashad, 1980; Fleck et al., 1980; Duyverman et al., 1982; Fitches et aL, 1983; Ries eta/., 1985). In the Mozambique Belt to the S, a similar pattern emerges from the few published R b / S r whole-rock ages. In central Kenya, Shibata and Suwa (1979) obtained a R b / S r whole-rock isochron age (4 points) of 766 + Ma with a n 87Sr/S6Sr initial ratio of 0.7041 + 11 from granitoid gneisses (see addendum). From granulite-facies rocks in northern Tanzania, Spooner et a t (1970) obtained rather poorly constrained R b / S r whole-rock isochron ages scattering between 731 and 936 Ma with 87Sr/~Sr initial ratios between 0.705 and 0.708. Coolen etal. (1982), however, obtained a U/ Pb zircon age from the F u r u a g r a n u l i t e s in s o u t h e r n Tanzania which gave a lower intercept of 652 + 10 Ma a n d an upper intercept of 2-3 Ga. The significance of the older age was not understood but it was thought that the younger, rather t h a n the older, age represented that of the granulitefacies metamorphism. The Late Proterozoic dates with low "7Sr/~Sr initial ratios from the Mozambique Belt and the Pan-African domains in NE Africa and Arabia contrast with the Archaean craton to the W in Uganda, where R b / S r whole-rock isochron ages

back to co_ 2.7 Ga and U/Pb zircon ages to co_ 2.9 Ga have been recorded (Leggo, 1974; Bell and Dodson, 1981}. Also in Malagasy, to the E, m a n y Archaean ages have been recorded, as well as y o u n g e r ages including Pan-African g r a n i t e s giving Rb / Sr whole- rock ages of 447-736 Ma (CaenVachette, 1979). Here, however, Pan-African Rb/ Sr whole-rock ages are characterized by high ~ S r / S6Sr initial ratios (>0.720) similar to those obtained by Priem et aL (1979) in s o u t h e r n Tanzania. The low S?Sr/~Sr initial ratios (see references above) and more recently 14ZNd/144Ndisotope data (Bokhari and Kramers, 1981; D u y v e r m a n et a t , 1982) obtained from both the Pan-African domains to the N and the Mozambique Belt to the S imply that the crust in these areas was generated during the Late Proterozoic (< 1500 Ma) and there is a s y e t no radiometric or isotopic evidence for a n y contribution of older continental crust (see addendum). Nor does the present data give any support to the proposal that the high grade Mozambique Belt is older t h a n the low grade Pan-African domains to the N. In fact the ages reported here are somewhat younger, rather t h a n older, t h a n the bulk of the ages reported from the low-grade terrains. GEOCHEMISTRY T h e Marlch V o l c a n i c S e r i e s The volcanic series comprises basic to basicintermediate rocks, forming the basic end ofa calcalkaline sequence. They are m e t a m o r p h o s e d in amphibolite facies and few, if any, original minerals are preserved. Because of this secondary alteration little significance can be attached to the major element analyses, but plots of the less mobile trace elements (Table 2) can be used to discriminate between different tectonic settings Ti/100

Zr

3y

Fig. 5. Ti/100-Zr-3y plot for the Marich Volcanic Series.

Geochronologyand geochemistry of the rocks associated with a Late Proterozoic ophiolite (Pearce, 1082). On a TI/100-Zr-3Y plot (Pearce a n d Cann, 1973), the data plot In the field of calcalkaline rocks typical of those occurring above a destructive plate margin (Fig, 5). On Tt-Zr (Fig. 6} and C r Y (Fig. 7) plots, the data fall in the field of arc volcanlcs. Further indication of their arc source is given by theirgcochemlcal patterns {Fig.8} which m a y also be used to discriminate between differenttectonic settings(Pearce, 1982}. In Fig.8 low ionicpotential elements, In particularTi, Y and Yb, show strong depletion. The high Cr levels probably indicate little pyroxene or olivine fractionation. Similar geochemical patterns are found in other Late Protemzoic calc-alkalinevolcanic sequences, for example A b u H a m e d in central Sudan {Ries et oL

/ Ti

I

f

\

\

\

10 '

I I

l

.4~

%%~,,SoS~'

""~"~'%%%

l

i

1985), Sol Hamed in NE S u d a n (Fitches et al., 1983}, in the E a s t e r n Desert of Egypt (Stem, 1979) and S a u d i A r a b i a (Duyverman, 1981). The REE patterns show slight LREE enrichment, HREE depletion and no Eu anomaly. This too is similar to other Late Proterozoic arc volcanics b u t overall RRE a b u n d a n c i e s are lower t h a n those of the Central E a s t e r n Desert of Egypt (Stem, 1979). The

MArlch

Granite

The major element geochemistry (Table 3) shows little variaUon for most elements implyLng little or no fractionaUon. On a normative Ab-An-Or diag r a m the data plot in the granite field. Trace element values for Rb, Sr a n d Ba show a wide variation, related to modal a b u n d a n c e s of the feldspars a n d biotite. Other elements e.g. Nb a n d Y show a m u c h smaller range, a n d are also very low in abundance, characteristic of graniUc m a g m a s from arc or collision settings. Dietrich and Gansser (1981) u s e d geochemical data for discriminating certain granite rocks in the Himalayas a n d following this, the Marich granite appears to be transitional between a n arc a n d collision setting. In addition to Nb and Y, Zr shows values characteristic of high S i O 2 a r c magmas. However, Al, Ti a n d the alkalis have a range which overlaps both settings. These observations c a n only be regarded as tentative due to the limited n a t u r e of the data.

ARC %

i

31

=

DISCUSSION

• |

=

5060 70

The whole of the western part of the Mozambique Belt in NW Kenya, between the Rift Valley to the E a n d the Tanzanian craton to the W (-100 k m across), appears to consist of imbricate slices striking about NNW/SSE with moderate dips to the E. The geometry of this t h r u s t belt is indicated from published m a p s a n d sections (Sanders, 1963, 1965; Miller, 1956; McCall, 1964; Veamcombe,

i

100

500

Zr Fig. 6. Tt-Zr plot for the Marich Volcanic Series. i r -- ,~% !I t

Cr 1000

! ! !

°

!

i

-

|

•

! !

el

\ 100

"

/i ,

"

~

~

31F

I

ARC - ~ l 10

%

I "

I I/

' 10

2'0

Y

Fig. 7. Cr-Y p l o t for t h e M a r i c h V o l c a n i c Series.

100

Sr

K Rb Ba Th Ta

Nb Ce P Zr Hf Sm

Ti

Y

Yb Sc Cr

Fig. 8. T r a c e e l e m e n t d a t a for t h e M a r l e h V o l c a n i c Series.

32

A.C.RIES, J.R.VEARNCOMBE, R.C.PRICE and R.M.SHACKLETON T a b l e 2. Major a n d t r a c e e l e m e n t d a t a for t h e S e k e r r V o l c a n i c S e r i e s 81145 81146 81147 81148 81149 81150 81151 81152 31A

31AA 31B

31c

31D

31F

31G

31M

SiO 2 5 5 . 1 7 4 9 . 6 1 4 6 . 5 6 55.21 5 3 . 2 6 51.11 4 5 . 6 9 5 3 . 3 8 5 3 . 4 6 4 8 . 4 5 5 0 . 0 0 4 8 . 2 0 5 0 . 9 7 4 5 . 6 0 5 1 . 7 7 4 9 . 2 4 TIO2 0.53 0.51 0.46 0.60 0.57 0.53 0.38 0.54 0.66 0.62 0.55 0.57 0.52 0.48 0.54 0.54 Ai20 s 17.27 13.22 12.10 16.19 15.01 13.76 11.71 14.35 15.38 14.21 13.35 13.09 1 3 . 3 0 12.66 14.12 13.40 Fe20 s 8.39 9.39 9.02 8.81 9.63 9.07 9 . 1 2 10.05 7.30 9.38 8.57 8.39 6.89 9.04 9.12 9.02 MnO 0. I0 0.16 0.17 0.14 0.14 0.16 0.16 0.15 0.12 0.16 0.18 0.19 0.13 0.17 0.14 0.17 MgO 5.77 6.09 7.04 5.65 7.43 5.12 8.07 7.64 3.93 5.55 5.66 5.15 4.46 6.70 4.81 6.34 CaO 9 . 1 4 13.46 13.72 9.66 8 . 9 4 11.65 13.53 9.61 11.27 12.55 12.08 13,99 13.08 14.75 10.45 12,70 Na=O 3.16 1.43 1.03 2.61 3.94 1.79 1.66 2.81 2.25 1.68 1.89 1.79 1.94 0.89 2.57 1.38 I~O 0.35 0.21 0.22 0.31 0.13 0.60 0.09 0.22 0.28 0.32 0.23 0.25 0, I0 0.52 1'22 0.40 P2Os 0.14 0.12 0.11 0.13 0.13 0.10 0. I0 0.14 0.13 0.12 0. II 0.16 0.08 0.14 0.08 0.14 LOI 1.20 5,81 6.83 0.62 I. 18 4 . 6 8 8.51 2.25 3.78 4.78 6.29 7,25 6.50 8.03 4.34 5.19 TotaI l O I . 2 2 I00.01

97.26

9 9 . 9 3 105.36 9 8 . 5 7 9 9 . 0 2

101.1598.57

98.14

98.91

99.03

97.96

98.96

99.16

98.49

Rb Sr Ba Nb Zr Y Cr Ni V

2 172 80 4 53 15 408 72 211

4 407 188 6 72 18 244 57 229

5 214 III 3 63 15 366 63 346

5 358 95 7 76 18 175 33

2 246 117 7 66 16 326 54

2 273 147 7 63 19 249 41

l 381 66 5 73 16 375 73

16 225 254 3 62 16 313 62

35 157 442 4 55 17 281 55

9 305 132 6 62 17 305 62

4 383 279 3 67 18 212 89 297

2 272 56 3 58 15 286 62 212

ND 194 67 4 67 17 312 59 228

16 245 266 6 63 14 278 59 211

2 190 28 3 46 13 495 102 195

6 278 112 8 73 19 210 44

Major e l e m e n t s analyzed at the Open University, traces analyzed at Nottingham University ] ~ r e earth e l e m e n t s for Marich Volearl|¢ Serlgs. La Ce Nd Sm Eu Gd Tb Tm Yb Lu Th Ta Hf Sc

9.3 13.4 ND 1.97 0.70 ND 0.42 ND 1.60 0.25 1.2 0.3 2.1 38

7.3 15.0 ND 1.98 0.67 ND 0.43 ND 1.38 ND 1.00 0.24 1.36 37

A n a l y s e s by INAA at the O p e n University.

1983a and b). The thrust sheets are particularly well seen in the southernmost Cherangani Hills, NE of Kitale (Fig. I) where numerous repetitions of thick-bedded orthoquartzites and thin impure limestones form prominent N / S trending ridges (see Sanders, 1963). In the northern Cherangani Hills, S of Ortum, limestones with thinner quartzites are similarly repeated (Miller, 1956) and farther NE in the Marich region, imbricate slices of rocks with distinctly different origins, i.e. oceanic crust, arc-related volcanic rocks, shaUow water and deeper water sediments must have been Juxtaposed by thrusting. The deformation state in the rocks changes from ductile to brittle westwards and structurally downwards in the thrust belt so that at Webuye, at the

western margin of the Mozambique Belt, (Fig. 1), there is a zone c a . 2 km wide of mylonlte and crushed granodiorite on thrusts, which dip about 45 ° to the NE (Sanders, 1965). Immediately W of the mylonite belt, on the foreland, this deformation dies out suddenly, where little deformed lowgrade Kavirondian sediments, striking E/W, are cut by granodiorites dated at 2.45 Ga (Bell and Dodson, 1981). In the mylonitic rocks of the Webuye area, the linear fabric plunges gently NW or SE (Sanders, 1965). In the Marich area the main extension direction is WSW/ENE. A N / S extension lineation, locally developed in the Marich area, is parallel to the main stretching direction in central Kenya (see addendum) but whether the N / S lineations in the

Geochronology and geochemistry of the rocks associated with a Late Proterozoic ophiolite

33

Table 3. Major and trace element data for the Marlch Granite 34

35

36

37A

81118

81121

75.33 0.04 10.70

74.80 0.01 14.85

74.17 0.17 15.30

73.88 0.07 15.51

74.36 0.09 14.48

74.31 0.07

0.37

0.43

1.22

0.34

0.74

14.99 0.47

0.03 ND 1.07 5.46 4.24 ND

0.01 0.21 I.I0 4.57 5.35 ND 0.21

0.02 0.26 1.43 4.46 3.63 ND 0.32

ND ND 1.70 4.75 2.93 ND 0.15

0.04 0.15 1.00 5.28 4.68 ND 0.11

0.04 0.19 1.16 4.54 4.26 ND 0.15

TOTAL

101.45

101.61

100.99

99.32

100.93

100.16

Rb Sr Ba Nb Zr Y Cr

137 287 463 6 79 6 9

136 577 1513 9 99 7 ND

50 746 1279 3 88 2 ND

164 379 915 8 I00 6 4

166 428 732 6 81 II 8

65 8

52 I0

40 9

66 15

57 13

SiO 2 TIO 2

Al20s Fe2Os~°T MnO MgO CaO Na~O KsO P2Os L.O.I

N|

Pb Th

78 635 1191 5 123 5 2

-

-

41 13

ND = not detected two regions are t h e s a m e is not yet k n o w n (Shackleton a n d Ries, 1984). The a m o u n t of shortening in the t h r u s t belt cannot at present b e calculated b u t m u s t be considerable. Presumably, to account for the ophioliUc and arc-related rocks in the Marich area, there m u s t at one time have b e e n a destructive margin with a n island arc building u p above a s u b d u c t i o n zone. B e c a u s e all the s t r u c t u r e s dip east, it is a s s u m e d t h a t w e s t of t h e s u b d u c t i o n zone shallow-water sediments accumulated onArchaean continental c r u s t (see addendum). Closure of the ocean c a u s e d the plate of u n k n o w n dimensions a n d probably mostly of arc-related material to collide with t h e c o n t i n e n t a l plate to t h e W (Shackleton, 1986). Amphibolite-facies metamorphism a n d crustal melting is associated with the collision. Extrapolating n o r t h w a r d s along strike, this Late Proterozoic ophiolite zone c a n be traced through the Karapokot (Karasuk) area (Walsh, 1966) and probably into KaramoJa area of NE Uganda (Fleuty, 1961). This in t u r n m a y connect with the extensive zone ofophiolites traced through Western Ethiopia (Kazmin, 1976), a n d thence p e r h a p s to others in NE Africa (Fig. 2). The ophiolitic a n d associated rocks of the Marich area differ from the ophiolites of Saudi Arabia, Egypt and northern S u d a n in having b e e n meta-

m o r p h o s e d to amphibolite facies a n d complexly deformed. This higher metamorphic grade and absence of high level graniUc rocks in Kenya relative to Saudi Arabia, Egypt and north and central S u d a n m u s t b e a function of depth of erosion, itself d e p e n d a n t on the isostatic uplift of thickened crust. It is not yet k n o w n w h e t h e r this thickening is the result of c o n t i n e n t / c o n t i n e n t collision. The p r e s e n t w o r k does not s u p p o r t the view (Vail, 1976, 1979; Hepworth, 1979) t h a t the high grade m e t a m o r p h i s m of the Mozambique Belt rocks in Kenya represent a n older b a s e m e n t sequence to the lower grade m e t a m o r p h i c rocks of NE Africa. On the contrary, the age of the m a i n metamorphism in the part of W Kenya dealt with in this s t u d y is slightly younger, although the m a x i m u m calculated ages go b a c k to ca. 1000 Ma (see Fig. 6), t h a n the ages of the m a i n m e t a m o r p h i s m in the parts of NE Africa for which reliable data are available. This evidence is s u m m a r i z e d in Table 4. There still remains the problem of obtaining absolute ages from m a n y of the ophiolitic rocks in NE a n d E Africa including those in West Pokot. Published ages from ophiolitic rocks in the PanAfrican belt of NE Africa include those from the J a b a l a I W a s k ophiolite in S a u d i A r a b i a from which De la Boisse e t al. (1980) obtained a U / P b age of

A.C.RIEs, J.R.VEARNCOMBE, R.C.PRICE and R.M.SHACKLETON

34

T a b l e 4: S u m m a r y o f P r e c a m b r i a n r o c k s in N E Africa. NW Kenya

Abu H a m e d C. Northern Sudan

NE S u d a n

E a s t e r n Desert of Egypt

None

549 + 12 tz2~

633 + 19 t3)

615-570 ~.4~

Arc-related volcanlcs/ intrusives

663 + 49 ~n

678 + 43 ul 800 + 83

712-649 u . a l

618-602 u.sl

Metasediments (metamorphic ages)

584 ± 25 {i)

761 ± 22 uj

593 ± 49 c~

898 ± 51 ~

Younger granites

Syn-tectonic plutonic rocks

987-700 t~

Sources : (I) Present study; (2) Ries et al. u n p u b , data; (3) C a v a n a g h (1979); (4) Fullager & Greenberg (1978); (5) Stern (1979); (6) H a s h a d (1980); (7) Dixon (1980). 880 Ma from zircons in the gabbroic component and a Rb/Sr whole-rock age of 778 + 88 Ma from ophiolitic p l a g i o g r a n i t e c l a s t s i n t h e E a s t e m D e s e r t o f E g y p t i n t e r p r e t e d a s t h e a g e o f c r y s t a l l i z a t i o n of t h e p l a g i o g r a n i t e (Ries u n p u b l , d a t a ) . M o s t o f t h e ophiolitic r o c k s i n v e s t i g a t e d in E g y p t a n d N E S u d a n are too altered for dating by the Sm/Nd method and devoid of the trondhJemitic fraction for zircon d a t i n g . H o w e v e r , a s in t h e o t h e r a r e a s f a r t h e r n o r t h , it is r e a s o n a b l e t o a s s u m e t h a t t h e o p h i o l i t i c r o c k s a r e g e n e t i c a l l y r e l a t e d to, a n d n o t c o n s i d e r ably older than, the arc volcanic and intrusive r o c k s w h i c h give a g e s b e t w e e n 9 0 0 a n d 5 0 0 Ma. A c k n o w l ~ I g t m u m t a - This w o r k formed part of a project s u p p o r t e d b y NERC on 'The Pan-African evolution of NE Africa" (GR 3/3692). This s u p p o r t is gratefully acknowledged. Invaluable help a n d a s s i s t a n c e were given to the a u t h o r s b y J . Wachira a n d R. Cannon, D e p a r t m e n t of Mines a n d Geological Resources, Nalrobi a n d B. D. H a c k m a n , R. Key, a n d T. Charsley, British Geological Survey (Nalrobi) whilst in Kenya. The University of Nalrobi is t h a n k e d for the use of safari equipment. Permission to w o r k in Kenya w a s granted by the Office of the President, Nalrobi. J. R. V. acknowledges the receipt of a NERC Postdoctoral Fellowship. REFERENCES AI-Shanti, A. M. S. a n d Mitchell, A. H. G. 1976. Late P r e c a m b r t a n s u b d u c t i o n and collision in the AI-AmarIdsas region, Arabian Shield, Kingdom of SaudiArabia. Tectonophyslcs, 31, 41-7. Bakor, A. R., Gass, I. G. a n d Neary, C. R. 1976. J a b a l al Wask, n o r t h w e s t Saudi Arabia, a n E o c a m b r l a n Backarc ophiollte. Earth Planet. S c t Lett., 30, 1-9. Bell, IC a n d Dodson, M. H, 1981. The geochronology of the T a n z a n l a n Shield. Jour. Geology, 89, 109-128. Bokhari, F. Y. a n d Kramers, J. D, 1981. Island arc c h a r a c t e r a n d Late P r e c a m b r l a n age of volcanlcs at

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