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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
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r .......................
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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
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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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Accepted for publication March 1973
Printed July 1973
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