Lithos 6, 1973, 217-34 The petrology of phonc lites from the Kenya Rift STEPHEN J. L I P P A R D Lippard: S. J. 1973: T h e petrology of phonolites from the Kenya Rift. Lithos 6, 217-34. Field, petrographic and geochemical studies of a variety of phonolitic rocks of Neogene age from the Rift zone in central Kenya allow three major types to be distinguished. Trace element studies in particular enable restraints to be plac ~d on the possible interrelationships among these lava types. Stephen j~. Lippard, Geology Department, University of Naoeastle upon Tyne, Newcastle, NEI 7RU, Great Britain. It was J. W. Gregory (1921) in his pioneer geological studies of the Eastern Rift in Kenya who first drew attention to the importance of phonolites in the Tertiary volcanic successions of this area. Peuographic studies of collections of these rocks, notably by Prior (1903), Neilsen (1921), Campbell Smith (1931) and Williams (1967), led to various classificatory schemes based on distinguishing a number of modal and textural types (Table 1). Numerous reports and maps published by the Kenya Geological Survey (1940-), coupled with recent geochronological work (Baker et al. 1971), have broadly established the distribution of the Kenya phono!ites in space and time; these data have been summarized by Williams (1969, 1970). In addition, Williams (1972) has estimated the total volume of phonolite lava in Kenya as 25,000 km3 and suggests that they comprise about one sixth of the total volume of Cainozoie volcanics in the province. Although these figures may be in considerable error (see below), it is nevertheless clear that Kenya phonolites exceed the total volume of phonolite lava found elsewhere in the world by several orders of magnitude. Chapman (1971) demonstrated that in the Baringo area there is a natural sub-division of the phonolitie lavas into two petrographically and chemically distinct types; one of which he calls phonolite (sensu stricto), the other trachyphonolite. He recognizes that the more undersaturated type (phonolite s.s.) is entirely restricted to the lower part of ~:he succession (pre-7 m.y.), while the traehyphonolites occur throughout t~:e whole sequence from the mid Miecene. Where the latter type contain n,ore than 10% modal feldspathoid the name Kenya-type phonolite is c:.nsidered more appropriate and used in the present text. This is taken direclly from Prior's classification. In addition to the types recognized by Clmt,man, phonolitic rocks associated with the central volcanoes to the west of the Rift comprise at least 16 - - Lithoa 6:3 218 Stephen~. Lippard Table 1. Classifications of phonoli~es and related lava types in Kenya. Pmo~ (1903) N E t t ~ (1921) (1931) WILLIAMS(1967) Present author LO.~.~GUTA TYPE LOSAGUTA TYPE KAPITI TYPE (flinty) Intermediate type KAMASiATYPE (mottled) (Porphyritic, Kapltian type nepheline 5 nun) (introduced by Neilsen KImICHOTYPE for phonolites containing (Porphyritic, olivine and anorthoclase nepheline 5 mm, w Kenyte(?)) biotite-bearing) K~.~YA TYPE KENYA TYPE MARA TYPE (included all aphyric and sparsely porphyritic pb.onolites, including the flinty type) (As Kericho but with no bmtite) ~mmom ~'rE (l~Tepheline as KENYATYPE microphenocrysts) "• SMFI'H With groundmass sodaamphiboles Large nepheline phenccrysts Sm~dl nep~elines PLA'r~u TYPE FBURRU TYPE (Nepheline conf~ed to groundmass) Without sodaamphiboles Large nepheline phenocrysts Uimamed type 'Nephelinitoid nosean GWASZTYPE phonolite from the '(Porphyritic, Nandi district' (Smith) sphene-hearing) one further type. These nepheline-rich and sphene-bearing rocks have also been distinguished by several other authors (Table 1). Williams's name Gwasi-type is perhaps the most appropriate. During geological mapping of the Uasin Gishu-Elgeyo EscarpmentKamasia Hills area in the western part of the Kenya Rift between the Equator and one degree north the present author has studied a variety of phonolite types both in the field and by lobe:rarefy techniques. Three petrologically distinct phonolite types are recognized: the Kenya- and Gwasi-types have already been referred to; the third group is named the Plateau-type and corresponds broadly to Chapman's phonolite s.s. type (Table 1). Field characteristics, distribution and age-range Plateau-type phonolites Enormous volumes of extremely uniform phonolite lava, almost entirely of plateau-type, were erupted in central Kenya over an area broadly coincident with the 'Kenya dome' during the late Miocene (14 m.y.-10 m.y.)(Baker 1965, Williams 1965, 1969, 1970, King 1970, Baker et al. 1971). During this period several 10,000 square kilometres of the rift-bounding plateaux, including the Uasin Gishu, Kericho, Kapiti and Laikipia ar,~as (Fig. 1), were buried by these lavas to which the name 'plateau' phonoL~tes has generally been applied. In general, the maximum thicknesses occur along the plateaux O'V:AStTYPE P/m~lite, Kenya Rift 34ok ~6oE pLATEAU 219 ,. PHQNOLITES " IOOkr~ O~¢rops -- dicgo~a! shying. Pmbab~ m'kjir~, extent s~m~ by stippled anza. 2ON T~ATI" KAMA t " "_ ." ." Z ".& " . "t IKIPIA '. ..:.. , Fig. 1. Upper Miocene ~lateau phormlites in central Kenya. edges immediately adjacent to the rift margins, but even these rardy exceed 0.5 kin. Within the Rift itself a well-exposed sequence of tile plateau phonoliras is seen only in the Kamasia Range (Fig. 1). The succession here is both considerably thicker (approx. 2.5 km) and spans a greater time-range (16 m.y.-7 m.y.) than that seen on the rift shoulders (Chapman er al. in prep.). On the plateau areas these lavas characteristically occur as a limited number of thick, extremely widespread and voluminous flow units. Data on Table, 2. Phenoerysts and dimen~,ions of Uasin Gishu Plateau phonolite flows. Flow num]~,er (3) Phenocrysts Total Nepheline Sanidi~e % 30 % % 30 10 1 (15 ram)" :L5 (4 ~ ) (15 nun) (10 nun) (4-) (5) (7) 25 5 30 2 2 (2 ram) (2 ram) 10 20 • Average dimension of phenocrysts. Dimensions Total areal Thickness (m) Aspect Total volume extent (km 2) Max. Ave. ratio (kins) 19004-100 250 100 480 200-1-2.0 2400±100 270 100 610 300~:50 1300-1-100 130 60 660 100+30 1200=k100 100 45 845 60-t-10 Stephm.7. Lippard 220 I ' ' ' I • ~ '~'". ,~ ~ , .--.-~....~@~ - - so. '-,"-" ~ ! ~" ,'2 -~{.,~ i : ' / ~,;~.:;[~ ! I. ":!" -~ I i ~ ' !!, ' t~.~ r-,,;,.~o i : - ~ .. ' ; ~.'.~:" ' , r ....................... i ruling] ! P;mteaul,neno',~tes--btan~ '. Fig. 2. Plateau p h o n o l i t e s a n d central c o m p l e x e s i n the K a m a s i a H i l l s - U a s i n G i s h u area. K e y to f o r m a t i o n s : ~. S i d e k h phonolites, 2. T i i m a n d U a s i n G i s h u phonolites. 3. Ewalel phonolites. l a . T i a t i centre. 2a. B a f i n g o centre. 3a. K a p k u t centre. thickness, volume and areal extent for five of the seven flows comprising the Uasin Gishu sequence are given in Table 2. Within the known intra-rift development, complex areas consisting of greatly increased thicknesses of pyroclastic materials, with plugs, dykes and thin flows of phonolite have been located at the northern and southern ends of the Kamasia Range. These occur within the plateau phonolite sequences and are probably former eruptive centres. Suggestions that the 'flood' phonolites of the plateau areas were erupted from either rift-margin fissure zones (Wright 1965) or fissures on the crest and flanks of the 'Kenya dome' (Williams 1970, Logachev et al. 19,72) have not been substantiated by detailed investigations in these areas ~,hich have failed to identify Hnear dyke swarms or other indications of widespread fissure sources. It is considered more in keeping with the field evidence from the Kamasia area that the plateau successions are the flanks of large low-angle coalescing shield volcanoes (radii 35 km-40 kin) (Fig. 2). It is estimated that the total volume of these late Miocene-early Pliocene plateau phonolites in central Kenya is 40,000-50,000 cubic kilometres. Willivms's (1972) estimate given above is considered rather conservative and appears to have underestimated the greatly increased thicknesses within the rift. Kenya-type phtmolites In the Kenya Rift the Kenya-type phonolites are found throughout the intra-rift sequences from the Miocene to the Recent, although, perhaps, they are more characteristic of the later periods of the rift volcanicitv. The best known Miocene examples comprise the Atimet formation (Chapman et al., in prep.) which dates at about 14.5 m.y. and can be correlated from the Kamasia Hills onto the Uasin Gishu plateau. Pliocene Kenya-type phonolites occur both in localized central volcanic piles (Ribkwo Volcano, Mc.- PIumolite, Kenya Rift 221 Clenaghan 1971, Weaver et al. 1972) and as isolated, single flows (Nairobi phonolite, Saggerson 1970), (Thompson Falls phonolite, Shacldeton 1946, McCall 1967). Pleistocene-Recent examples include the Lake Hannington phonolites (McCall 196~, a complex sequex,.:e of flood lava sheet~ which infills much of the inner graben of the rift between Nakuru and Lake Baringo, and the Mount Suswa lavas (Nash et al. 1969). There are in addition numerous other scattered occurrences interbedded within local alkali olivine basalt-trachyte sequences. Gwasi-type plmaolites The Gwasi-type phonolites occur as minor imrusives (plugs, dykes) and local flows within or marginal to the giant nephelinitic central volcanoes of the area west of the Rift (Fig. 3 ) - Mount Elgon (Davies 1952), Kisingiri (McCall 1958, Findlay 1967, King et al. 1972) and Tinderet-Timboroa (Binge 1962, Jennings 1964, King et al. 1972). The last two occur at the western and eastern ends of the Kavirondo Rift (Shackleton 1950) respectively. Large numbers of scattered phonolite plugs occur in the area around the carbonatite plugs of North and South Ruri (McCall 1958). Just to the north-east of the Ruri plugs there are local cones and flows of phonolite lavas including the Nyamaji volcano (LeBas 1970). Tinderet-Timboroa is a complex area of several volcanic centres covering some 2000 square kilometres and lying between 5 km and 25 km to the west of the junction between the main Kenya Rift and the Kavirondo Rift (Shackleton 1950). Geochronological data (Baker et al. 1971) indicate a long history of eruptions in this region from the lower-middle Miocene (19 m.y.) to the middle Pliocene (5 m.y.) comprising at least three distinct phases. The development of an early nephelinite cone (Tinderet) was followed by a period of lava eruptions, mainly phonolite and phonolitic nephelinite, which covered the eastern half of the complex. The final outpourings were of basanite in the Tinderet Summit area. The phortolite specimens analysed as being representative of the Gwasitype come from the middle phase (10 m.y.-8 m.y.) of eruptions on TinderetTimboroa and the Ruri area. Petrography Plateau-type phonolites The Plateau-type phonolites show a gradation from sparsely micropo~'phyritic to strongly porphyritic varieties. The former have about 3-5°/o phenocrysts; mostly small (0.5-1 ram), scattered nepheiines and alkali feldspars. The latter types have abundant, 15-30%, phenocrysts of the same minerals, often up to several centimetres in diameter. Within the Uasin Gishu sequence it has been possible to characterize each flow by the relative abun- Stephen~ Lippard 222 MOR010 O, tr i KADAM UgondQ \ / ., / \ Kenya \ / r i -i 1 I .e / J ' , TINDEP, ET -flM B O R O / ~ r - ~ k "~z I l ;~ 'i / !l ~ is~r~I~ I 0o II~m tJl\ GWASI-TYP_.E Outcrops,P__.H0 [" JO Lj_T_~_ heevy shc~dIE Fig. 3. Nephelinite volcanoes and Gwasi-type phonolites in western Kenya. dances, sizes and compositions of the phenocrysts (Table 2). In the majority of Plateau-type phonolites, nepheline and sandine-anorthoclase occur in approximately equal abundance as phenocrysts. Opaque oxides, pale green ferroaugite, apatite and biotite are alse generally present, often in clusters Table 3. Kenya phonolite types. Distribution Space Time Mode of occurrence (tot. vol. est.) Associated lava types PLATEAU Erupted from within TYPE Miocene 'proto' rift with extensive overspill onto marginal plateaux 16-7 m.y. Plateau areas - limited number Very subordinate analof extensive lava sheets cite basanites, phonoIntra-rift areas - more numerous litic tephrites flows, pyroclastic/intrusive complexes (40,000--50,000 km 3) KEN~rA TYPE.J Rift zone. Some minor, scattered occurrences on marginal plateaux 16 m.y. - Recent 'Flood' lava sheets - central volcanoes (about 5,000 km 3) ¢;WASl TYP~ Extra..rit~ zone. Western Kenya - Eastern Uganda Northern Tanzania Peripheral dykes, plugs, local 25 re.y. -(?) flow piles (less than 100 km 3) (ma;nb" Miocer~e) 4 m.y. - Recent Trachyte, quartz trachyte phonolitic trachyte. Alkali olivir.e basalt, hawaiite, mugearite Ner~helinite, melilite nel?helinite Phonolite, Kenya Rift 223 of small micro-phenocrysts, but in total never exceed 3% of the mode. The majority of the phenocrysts in the plateau phonolites show c~Jrroded margins and other resorption effects. There is always a sharp grain-size distinction in these lavas between the phenocrysts and the groundmass. The groundma~es of the Plateau-type phonolites are fine gra'med ('flintyaphanitic') but mainly holocrystaUine (glassy type do exist but are rare). They consist largely of minate (0.05 mm long) alkali feldspar laths showing sub-parallel (pilotaxitic), radiating and spherulitic patterns. Alkali- and ironrich minerals, aegirine,augite, aenigmatite and soda-amphiboles, occur interstitially, producing a mottled textural type where they form sufficiently coarse patches to be visible with the naked eye. X-ray diffraction patterns confirm that both analcite (visible in clear interstitial patches) and nephdine (as minute groundmass grains) are important constituents of all flinty and mottled groundmasses. Reflections from sodalite have also been detected in some of the groundmass diffraction patterns. A common groundmass texture found in some flows of plateau phonolite is 'rhyolitoid' flow-banding owing to contorted bands and streaks of alternatively relatively coarse and fine grained material; in many cases the coarse bands are the more leucocratic and have a coarsely mottled texture. Kenya-type phonolites Petrographically there is a continuous range in lava types from soda-tr~chyte through intermediate varieties, variously described as phonolitic trachyte and trachyphonolite, to Kenya-type phonolite. PetrographicaUy this series is characterized by a 'trachytoid' texture, caused by the paraUel alignment of groundmass alkali feldspar plates. The more phonolitic types are recognized by a progressive increase in the abundance of groundmass feldspathoids. S~midine is the only common phenocryst mineral and many of these lavas are aphyric. The alkali feldspars frequently exhibit seriate texture and comprise 60-75% of the mode. The habit of nep!.eline (and in some cases sodalite) in these lavas is distinctive and different from that in the plateau type. It occurs as scattered groundmass grains, sometimes euhedral, 0.1 mm to 0.5 mm in diameter and often rimmed by dark haloes of alkali-iron-rich minerals. Nepheline never exceeds 15% of the mode in the Kenya-type phonolites. Petrographic and X-ray diffraction studies show that there is very little analcite in the groundmasses of the Kenya-type phonolites when compared, for example, to the plateau phonolites where it is always present as a major groundmass constituent. However, in common with the plateau type, alkall-iron-rich mafic minerals occur interstitially, producing mottled textures in some examples. Gwasi-type phonolites The majority of the Gwasi-type phonolites are aegirine-augite-rich and sphenebe~ring rocks containing about equal proportions of modal alkali reid- 224 Stephen~. Lippard spar and nepheline so that compositionally they fall on the boundary between phonolite and phonolitic nephelinite. All are strongly porphyritic rocks with phenocrysts of aegirine-augite (sometimes marginally zoned to aegirine), sphene, opaque ore, nepheline and anorthodase. These are ~et infine grained groundmass composed of minute alkali feldspar laths, small nepheline euhedra, abundant prisms of aegirine and granules of iron ore with a dull grey analcitie/zeolitic(?) base. Those rocks which show a development towards more Vpical phonolite compositions are more leucoeratic and usually contain 25-30% nepheline and alkali feldspar phenocrysts. The latter are often zoned from anorthoclase to sanidine towards the margins. The passage from phonolite to phonolitic nephelinite in this lava series is completely gradational and is marked by a diminution in the amount of alkali feldspars which become restricted to the groundmass and a complementary increase in the abundances of sphene, aegirine-rich pyroxene and nepheline. There is a notabie absence in this lawa series of the soda- and iron-rich amphiboles and aenigmatite so characteristic of the groundmasses of the other phonolite types. Geochemistry Analytical methods Trace elements were determined using a Phillips PW1212 automatic X-ray fluorescence spectrometer in the Department of Geology, Bedford College, University of London. For the elements Nb, Rb, Sr and Zr, precision is approxim~Ltely 4-1% of the amount present above the 100 ppm level and for Ba, Ce and La, tlfis figure is 4-2%. The U.S.G.S. standard rocks G2, GSP1, AGV and BCR were analysed in parallel to enable comparisons to be made with other laboratories. The results are given in Sceal and Weaver (1971, Table 2). Analysis for major element oxide wa~: by standard wet chemical procedures. Most of these were carried out by Mr. H. Lloyd at Bedford College. Results (a) Major elements All the Plateau-type phonolites analysed for major elements show a remarkable uniformity of composition (Table 5, Fig. 4). The norms contain between 15 and 30% Ne and show small amounts of either Ac or An. The Kenya-type phonolites have a different major element chemistry from that of the Plateau-type (Table 5, Fig. 4). The compositions are gradational to trachyphonolite and trachyte ~Fig. 4). The Kenya-type phonolites have less than 10% aormative Pie and almost always contain Ac in the norm (mildly peralkaline). Phonolite, Kenya Rift 225 Tab/e 4. Petrography of Kenya phonolite types. GWASI TYPE PLATEAu T Y P E Phenocrysts Essential Minor w generally less than 10% m~XA T Y P E (Gradational to phonolitic trachyte) Sanidine-anorthodase Sanid/ne-anorthoclase Nepheline Biotite Augite-ferroaugite Opaque ore Apatite (Gradational t o phonolitic nephelinite) Nepheline Aegirine-augiteaegirine Pale green clinopyroxene Sanidine-calcic Opaque ore anorthoclsse Sphene Opaque ore Apatite Phenocrysts and 'Seriate' u complete size groundmasses distinct range between pheno('hiatal') cryst and groundmas~ components Groundmass Constituents (in decreasing order of abundance) Textures Alkali feldspar Analcite Nepheline Soda-amphiboles Aenigmatite Aegirine-augite Sodalite Alkali feldspar (75 %) Nepheline Soda-amphiboles Aenigmatite Aegirine-augite Analcite Aegirinic pyr~xene Nepheline Alkali feldspar Opaque ore Weakly birefr/ngent base zeolitic(?) Aphanitic ('flinty') finely microcrystalline Spheruliti¢ Pilotaxiti,Occasionally flow-banded Coarsely microcrystalline 'Fluidal' Usually very fine grained 'Dull', greasy, clouded appearance Much ze~litisation The Gwasi-type phonolites show a much greater range in their major element compositions than the other types. At the lower end of the range of SiO~ values shown they are gradational to phonolitic nephelinites. With increasing 8iO2; CaO, MgO, TiO2, P205 and total iron oxides decrease, alkalies and alumina increase. Those examples of Gwasi-type phonolites with SiO2 contents of 54-55% have very similar major element compositions to the plateau phonolites, the most notable difference being their much higher Fe203/FeO ratio. This is reflected in the high content of modal acmite-rich pyroxene. However, the trace elements reflect much more strikingly the obvious petrological differences between these rock types. (b) Trace elements All the Plateau-type phonolites, except those flows that are appreciably porphyritic, have low abundances ( < 100 ppm) of $r and Ba, particularly 1:he 226 • • .o 10 44- NCt20 5 I" . t.:. ;, 40 .-. ,. ....... ." .~ "" , . . . . . . . . . 50 wt.O/°Si02 , . . . . . . . . . 60 70 --PLateau Gwasi ~.~7~'~--'-t,x,,, -~ - " " ~ ~ d . ~ - - " Fig. 4. Alkalies vs. silica plot of analysed volcanic rocks from the northern Kenya Rift. Phonolites indica,::ed in upper diagram by open circles. Upper dashed line separates nephdine-bearing from nepheline-free rocks in northers Tanzania (Saggerson & Williams 1964). Lower dashed live ~Jeparates alkalic and th,~leiitic suites in Hawaii (Macdonald & Katsura 1964). latter element which is freque,tly below the level of detection (about 10 ppm). The gr (450-1200 ppm) and Nb (200-450 ppm) concentrations in these rocks show considerable variation but a strong positive correlation (Fig..3). Likewise Rb, La and Ce show more or less well-defined positive trends when plotted against one another or Zr or Nb. The plateau phonolites all have gr/Nb ratios between 1.9 and 2.9. In the Kamasia succession (Fig. 2) increasing Zr and Nb contents can be correlated with increasing height in the succession (Fig. 6). The uppermost phonolites in this sequence have Zr contents of 750-900 ppm. In the Kenya-type phonolites the trace elements show similar patterns to those observed in the plateau phonolites, gr, Nb, Rb, La and Ce show similar ranges and well-defined positive correlation with one another (Fig. 5). They also show marked Ba and Sr depletions. When trace dements alone are considered, the. only outstanding difference between the Kenya-type and Plateau-type phonolites is their distinct gr/Nb ratios. The Kenya-type phonolites consistently show gr/Nb values of between 3.0 and 3.5. The Gwasi-type phonolites and phonolitic nephelinites show considerable diversity of trace element compositions. Their Zr/Nb ratios are generally less than 2.0. The phonolitie nephelinites have relatively low abundances of gr and Nb and high Sr and Ba values. The phonolites show higher Phonolite, Kenya Rift 227 Table 5. Average and typical compositions of Kenya phonol/tes. PLATEAU Typi~ KBNYA TYPE (mean of 14 analyses) (mean of 10 analyses) (95% confidence intervals indicated for major oxides) SiO~ A1203 Fe203 FeO TiO2 MnO MgO CaO NazO KzO HzO+ H20-PzOs Norms Or Ab Ne Ac An Wo Di O! Mt II Hae Ap 55.00 19.94 02.41 02.27 00.53 00.27 00.51 01.42 08.34 05,94 03.19 00.66 00.07 (0.71) (0.98) (0.85) (0.80) (1.37) (1.02) 58.22 16.58 03.97 03.11 00.65 00.26 00.56 01.47 06.87 05.21 01.99 01.45 00.06 (2.31) (1.78) (0.98) (0.96) (0.88) (1.14) GWASI TYPE Phonolltic nephelinites W]308 S/19i Phono~ite S]125 50.80 19.72 03.33 03.13 01.28 00.17 01.82 04.90 07.90 04.23 01.44 00.90 00.21 52.24 19A9 04.09 01.07 00.43 00.17 00,49 03.13 06.89 0~.32 04,,53 01,27 00.03 54.53 21.17 03.93 00.22 00.43 00.15 00.33 01.12 09.44 05.85 02.31 01.01 00.01 100.55 100.40 99.83 100.15 100.50 38.4 24.1 25.5 1.9 30.4 41.7 7.5 6.7 25.0 24.1 24.6 37.3 21.6 19.9 34.6 24.8 27.9 3.1 5.8 4.4 1.9 1.0 0.9 5.9 1.2 3.2 1.2 12.0 0,1 0.1 0.5 4.8 2.4 3.5 2.6 2.7 0.8 2.2 0.1 1.8 0.8 2.8 concentrations of Zr (up to 1000 ppm in some cases) ~md Nb and a slight, but much less marked increase in Rb. On the other hand, La and Ce are consistently lower in the phonolites than the phonolitic nephelinites. The Ba and Sr contents are also generally lower in the phonolites but the extreme depletions of these elements seen in the Plateau-type and Kenya-type phonolite are not reproduced in this rock series. Discussion It is clear that "~e geochemical sttMies reinforce the other differeno.'s noted between these phonolite types. The major element data confirm the observations that the Kenya-type and Gwasi-type phonolites are gradational tc phonolitic trachyte and phonolitic nephelinite respectively. In contrast, the Plateau-tTpe shows a marked uniformity of major oxide composition. 228 St~hen~. ~Li~a~d !~ It is however by a consideration Of the trace element abund~ncepatterns that further interesting comparisons and contrasts between the three phonolite types can be made. With regard to trace element behaviour during crystal-liquid differentiation processes (fractional crystallization or progressive partial melting) the following two assumptions are often made: (1) The relative abundance of any pair of elements will be constant if their bulk distribution factors (Schilling & Winchester 1967, Gast 1968) between the crystallizing phases and the liquid (or partial melt) are approximately equal. This will result in a straight-line plot, passing through the origin, for any series of rock analyses for which this relationship holds. Where this occurs throughout a wide range of compositions, it is likely that the bulk distribution factors for the elements concerned are close to zero (Weaver et al. 1972), i.e. they are 'residual elements' (Harris 1967). (2) Those lavas which show marked ,depletions (relative or absolute) in one or more elements have probably been derived by extensive crystal fractionation in one or more minerals in which the elements are relatively concentrated (Ewart et al. 1968, Noble et al. 1969). In particular, extreme depletion in Ba and Sr is often interpreted as the result of protracted feldspar fractionation; Sr preferentially entering plagioclase and Ba into alkali feldspars (Berlin & Henderson 1969). Zr and Nb abundance patterns and the Zr/Nb ratios Examination of the Zr/Nb graph in Fig. 5 shows that for each phonolite type the Zr/Nb ratios are significantly different throughout a wide range of Zr and Nb abundances. These elements therefore behave as a coherent pair of residual elements within each phonolite group. If Zr and Nb approximate to residual elements throughout the range of fractionation processes involved in the formation of these magmas, then it appears most unlikely, on the basis of this trace element data, that any one of the phonolite types cannot have been derived from another by simple crystal fractionation. This is because this process will only increase the abundances of both residual elements but not significantly change their ratio. Indeed, if one couples this with the field evidence, it seems most liikely that the three phonolite types were derived along quite separate petrogenetic paths from different parents (with distinct Zr/Nb rat,.'os(?)). During the later stages of fractionation at least these paths were essentially parallel d.ue to processet.~ that progressively increased Zr and Nb abundances but ~id not significantly change their relative concentration (dominantly crystal fractionation(?)). It is also interesting to note that the higher Zr/Nb ratios of the Kenyatype ~honolites are comparable to those of many of the trachytic lavas from the Kenya Rift area (Weaver et al. 1972). Phonotite, Kenya Rift 229 Ba and 8r abundancepatterns (Fig. 5) The Plateau-type and Kenya-type phonolites lmve exceptionally low abundances of Ba and Sr. Therefore it is Lrfferred that they are the prodacts of processes that have included a long history of crystal (feldspar) fractionation. The absence of extreme depletions of these elements in the Gwasi-rype phonolites points to the possibility that feldspar fractionation has not been a dominant process in their evolution. Rb abundancepatterns (Fig. 5) On the Rb/Nb (and Rb]Zr) graph the Plateau-type phonolites and Kenyatype phonolites combine to produce a straight-line plot trending towards the origin; i.e. in these two series all these elements appear to behave as 'residual elements'. ]~rt contrast, the Gwasi-type lavas show a markedly divergent trend apprommately paral.~elto the Nb axis. The differing patterns shown by the K/Rb against Zr plots show the same effect. It is clear Sat in the Gwasi-type phonolites the behaviour of Rb is not that of a true residual element. Rare-earth abundancepatterns (Fig. 5) The patterns shown by La and Ce resemble in some respects those of Rb. In the Kenya-type and plateau phonolite series their behaviour approximates to that of residual elements. There is some divergence from straight-line plots at the very highest concentrations which suggests some preferential loss of the rare earth elements relative to Zr and Nb. In contrast, in the Gwasi-type phonolites the lowest concentrations of La and Ce occur in the most differentiated rocks; i.e. those with the highest contents of the other residual elements. The conclusion to be drawn from the Rb, La and Ce data is that although these elements approximate to residual behaviour in the plateau and Kenyatype phonolites, in the Gwasi-type they do not. This implies that one cmore of the following may apply to the last-named group: (a) They are not the products of significant crystal-liquid fractionation processes. (b) Rare-earth and rubidium-bearing minerals are extensively removed during the evolution of the magmas. (c) These elements are preferentiaUy removed in volatile phases - the associated carbonatites are strongly enriched in rare earths; but also Nb, Sr and Ba(?). S u m m a r y and conclusions The trace element data presented here suggest that the three phonolite types described are formed from quite distinct source magmas. Within each type Stephen.7. Lippard 230 Rb 25o- Q) • o C¢ dD o 0 00000 00 200 ~3~i ~ 0 400" O~-,-,O0 A A O , 100 ~oA o ~ ~ ,~Ou'" ~ o 20( &k ~ oe. AA A & && A A 100- • & A 50 z~,,. A Nb 16o A0 360 ~.~o 56o 6bo Nb 300 / / A 600" AA ~/ ~ ,',,A") ~,0 d~ z / . . . . 3do 1/ 0 -" / ~o" • / " & 2000 .O @ . "T'" 600 A A 960 12()0 -. 1500 300 A A 600 goo 1200 % 400 - 0 o K/Rb o O A~ 00 3oo- A A 400 ~8ooOo A ~,~ " A. A o 2L'3 . o O II • A Zr s~o IO~O -q & 3000 ' oc~ . 12bo Ba ~ooo 200, ~o ,,/ A ,." ~,. ^./" /,.00 o~o 200- ~do Fig. 5. Trace element plots for Kenya phonolite types. Open circles - Plateau-type phonoiites; closed circles - Kenya-type phonolites; triangles - Gwasi-type phonolites (closed triangles - phonolitic ncphelinites). All scales in parts per million. Io6o 1 Phonolite, Kenya Rift 231 cor:stant Zr/Nb ratios are maintain~ 1 over awide range of compositions. These ratios may have been inherited fro.n the parent materials. "['he trace element patterns in the Gwasi-type are considerably more complex than those in the other two groups. Without further data they are difficult to interpret. The Plateau-type and Kenya-type phonolites, despite marked differences in major element compositions, havre similar trace element abundances and patterns - high contents of element exhibiting r~idual behaviour (Zr, Nb, La, Ce, Rb); very low contents of Ba and Sr. This suggests that despite initial compositional differences they have been formed by similar processes. Their trace element patterns are those that might be considered appropriate to highly differentiated magmas produced by extensive crystal fractionation. 73+_03 I 7.8+-0r 9 St'O;5 L,O0 ~ 16.0_+0.6 i ! ,~, •V ~ 300 - Fig,, 6. Zr/Nb plot of plateau phonolites from the Kama~ia Hills succession, Open circles - Sidekh phonolites (16 m.y.-14.5 m.y.); closed circles - Trim phonolites (14 m.y.-9 m.y.); triangles Ew~lel phonolites (9 m.y.-7 m.y.). Samples o v which age determinations have been carried o u t are indicated. N~ i z00 ' oO~ ° o ~ ~.s-'o~ \ 136~-03 100 - -- ! - r 200 I - v ~oo --1----t -~---'~1-~ 6oo 8oo ~ - ~ 1 " . . . . looo Zr The Kenya-type phonolites, from their associations and petrochemistry, belong to a series of mildly undersaturated trachytic lava types for which alkaline olivine basalt is an obvious parent. Some of the more peralkaline and undersaturated members of this series may have been derived from less extreme members by alkali-feldspar fractionation paths fNash et al. 1969, Carmichael 1964, Gill 1972). In the Kenya Rift it seems that this process has not produced liquids more undersaturated than Kenya-type phonolites w;:h about 10% normative nepheline. The Plateau phonolites, because of their huge bulk, restricted time-range and a lack of quantitatively significant associations of basic and intermediate rocks, pose a considerable petrogenetic problem (Williams 1970). In view of these factors writers have tended to reject crystal fractionation as an important process in the formation of these iavas. Bailey (1964) suggested the generation of these (and other) salic undersaturated lavas in East Africa by partial melting of mantle material by relief of pressure, and heat and volatile concentration, at the base of the uparched crust. However, the plateau phonolites have the trace element patterns of highly differentiated liquids. It is uncertain, but perhaps dc)ubfful, whether primary partial melts would show such extreme compositions. P:artial melting followed by fractional crystallization would produce the observed abundances however. In the writer's 232 Stephen~y. Lippard opinion the possibility of derivation by fractional crystallization from an undersaturated basan/toid parent cannot be ruled out. Although there is in general a great scarcity of basic rocks associated with the plateau phonolites, moderately undersaturated types (analcite basanites, analcite ankararnites) do occur; for example, in the Elgeyo 'basak' (Walsh 1969, Lippard 1972) and Noroyan (Chapman et al. in prep.) formations in the Elgeyo and Kamasia areas. These formations also include lava types intermediate between basanite and phonolite (analcite hawaiites, phonolitic tephrites, etc.), similar in many cases to lavas described fi'om the Otago province (Benson 1941, Coombes & Wilkinson 1969). Some of these intern,ediate types contain resorbed brown amphibole (kaersutite(?)) phenocrysts. These may indicate that if these rock types were involved in the fractionation processes leading to the production of the plateau phonolites, some of the fractionation may have taken place at relatively deep (sub-crustal) levels. The gravity evidence for large basic masses in and at the base of the crust along the rift zone (Girdler et al. 1969, Khan & Mansfield 1971) provides further support. Acknozdedgements. - This work forms part of a Ph.D. Thesis (University of London) undertaken between 1968 and 1972, and also part of a larger project to investigate the geology of the northern Kerya Rift (East African Geological Research Unit) under the direction of Professor B. C. King (Bedford College). The writer is greatly indebted to PI ofessor King (his .,~upervisor) for much helpful criticit~m and comment and also to the fol ~wing for help in different aspects of the work: Dr. L. A. J. Williams, Dr.I.L. Gibson, Dr. G. R. Chapman and Mr. S. D. Weaver. The receipt of two University of London Postgraduate Studentships (one the William Gillies Research Fellowship presented by the Clothworkcrs Company) and grams from the Central Resealch Fund for travel to and from Kenya is gratefully acknowledged. 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