Neogene tectonics and volcanicity in the North Tanzania sector of the Gregory Rift Valley: contrasts with the Kenya sector

Trctonophysics, Elsevier

Science

204 ( 1992) 8 l-92 Publishers

B.V., Amsterdam

Neogene tectonics and volcanicity in the North Tanzania sector of the Gregory Rift Valley: contrasts with the Kenya sector J.B. Dawson * Drpurtmmt

of Geology and Geophysics, (Received

Unirwsify

of Edinburgh,

April 21, 1989; revised version

accepted

Edinburgh

January

EHY .?JW, C/K

22. 1YYO)

ABSTRACT Dawson, J.B., 1992. Neogene tectonics and volcanicity in the North with the Kenya sector. In: R. Aitherr (Editor). The Afro-Arabian

Tanzania sector of the Gregory Rift Valley: contrasts Rift System. ~~etu~~~hy.sics , 204 (spec. sect.): Kl--92.

The Neogene tectonics and volcanism in the rift area of northern Tanzania are intimately related. A major phase of late Tertiary faulting. giving rise to a broad tectonic depression, was followed by extrusion of large amounts of basaltic to trachyte magmas from large shield volcanoes. This was separated by a second major phase of faulting at about 1.2 Ma from a Late Pleistocene-Recent phase of small volume, explosive nephelinite-phonolite-carbonatite volcanism that contrasts with the earlier phase in its volume, dominant magma type and eruptive style. In both its tectonic expression and contemporaneous magmatism, the northern Tanzania province contrasts with the southern Kenya sector of the Rift Valley. The area of tectonic disturbance is considerably broader in Tanzania where ultrabasic-basic magmatism predominates. The major episodes of basaltic magmatism. representing major thermal perturbations of the mantle, have moved southwards down the rift since the mid Tertiary.

Introduction The eastern Rift Valley in northern Tanzania is an elongate N-S depression flanked on its western side by a high, east-facing escarpment. On either side of the depression are extensive areas of Neogene volcanic rocks and, in this respect, northern Tanzania resembles Kenya and Ethiopia where there is also an area1 relationship between the rift faulting and volcanic activity. There are, nonetheless, major differences between northern Tanzania and Kenya, and it is the purpose of this paper to discuss these differences by reviewing the Neogene volcanic activity in northern Tanzania and its relationship to the structural development. The review is based upon the author’s observations, mainly in the Arusha-

Corre.~~ot&~tce

10: J.B.

Grant

of Geology,

Institute

Dawson, West

University Mains

of Edinburgh, Road,

Edinburgh

EH9 .?JW, UK.

004O- lY51/92/$05.00

0 1992 - Elsevier

Science

Publishers

Manyara-Natron area, and an analysis of the published Quarter Degree maps of the Tanzania Geological Survey. A general geological map of the area is given in Figure 1. Early structure of the rift As in Kenya to the north (Baker and Wohlenberg, 1971; Baker et al., 19721, the main evidence for the deformation of the crust derives from studies of the shape and attitude of the erosion surfaces of Miocene and Late Pliocene age. The earliest faulting in Tanzania was associated with the deformation of the mid-Tertiary land surface. In northern Tanzania there was uplift of the Lake Victoria Block and the Masai Block. The movcments gave rise to a tectonic depression (Fig. 21, bounded by faults or warps, that is considerably broader than the trough-shaped depression in Kenya to the north. The depression is bounded to the south by the Masai Biock, around which the area of tectonic disturbance bifurcates continuing

B.V. All rights reserved

J.H. DAWSON

82

to the southwest and Yaida

NW-SE-trending of the

NE

Eyasi

margin

of

as the

graben.

Kilimanjaro

as the NW-SE-trending

grabens,

and to the southeast Pangani

margin

graben.

is Mount

The structure

is speculative

(?faulted

or

downwarped) as the structure is obscured by the later Meru and Kilimanjaro volcanics. The NW

ment.

alkali basalt

runs

Oldoinyo

Nguruman present

faults Lake

up with

to the west

Natron

(Baker

the

Sonjo

and

and

north

of the

and

Wohlenberg,

Kenya

is evidence

that

some

fractures

were

influenced by the older structures. Whereas the N-S-trending Kenya graben follows the N-Strending 600-Ma-old Mozambique Belt (McConnell, 19721, and the later major rift in Tanzania also follows this grain, the fractures

bounding

the

ephelinites

and

Mosonik

Mosonik;

these

carbonatite

volcano

overlie

from

the

and

older

the

melan-

of the Bast Hills. are

and

centres

major

nephelinite-

carbonatite are both

Shombole

Es-

occurs

at

than

the

older at the

northeast

end of Lake Natron that has been dated at about 2.0 Ma (Fairhead et al., 1972). Both Essimingor and Mosonik are formed mainly of pyroclastic deposits. This contrasts with the lava-dominated shield

basalt

volcanoes

depression

and

bound-

as

as has some-

of nephelinite,

basalts

centres

basalt

northeastern

Elgon,

Pangani

volcano,

The activity at Lemagrut

and melilitites

older tectonic depression tend to cut across the Mozambique Belt. The southern boundary of the and the deduced

the

are departures

extrusions

Sambu

phonolite

There

there

association.

with

simingor

1971).

meet

is not an off-rift or Mount

However,

started

to join

depression

times been suggested (e.g., Bosworth, 1987); it is distant only from the recent (1.2 Ma) rift escarp-

margin, cutting across the eastern edge of the Serengeti plains, comprises the Eyasi Fault, which northeast

the

volcanoes,

although

differentiate

ignimbrites;

for

some

to trachytic

example,

of the tuffs

Ngorongoro

ary (beneath Kilimanjaro) run E-W. The Pangani graben reflects older shear zones in the Precambrian basement (McConnell, 1972) and the Eyasi

(GrommC et al., 1970; Hay, 1976). In addition, some of the basalt volcanoes-Ngorongoro, Olmoti, Elanairobi (collectively termed the Crater

graben cuts Mozambique

Highlands) and Ketumbeine-developed calderas in their later stages.

across the boundary between the Belt and the Archaean craton; it

follows the trend of a swarm of basic dykes Bukoban (Proterozoic) age (Vail, 1970).

of

volcanoes pression scarps

the faulting, erupted,

and, bounding

whose

a group of major shield lavas

infilled

the

basaltic

volcanoes

are contempora-

neous with the Kirikiti and Olorgesaile basalts in Kenya, extruded between 5.0 and 1.0 Ma (Fairhead et al., 1972) but the earlier Kenyan

The older volcanics Following

The older

major

de-

eventually, overstepped the fault the depression at several points

(Fig. 3). The earliest date of eruption is not known since nowhere within the depression are lavas seen overlying Basement rocks. The oldest dated rock is a 8.1 Ma nephelinite from Essimingor, and basalt from Lemagrut has a date of 5.5 Ma (Table 1). The first volcanoes produced mainly lavas of the alkali basalt-trachyte-phonolite association at Oldoinyo Sambu, Elanairobi, Olmoti, Loolmalasin, Ngorongoro, Lemagrut, Oldeani, Tarosero, Ketumbeine, Gelai, and the Shira and Mawenzi centres of Kilimanjaro. It is worthy of note that the Kilimanjaro massif stands where the E-W fractures/downwarps on the northeastern

flood phonolites, dated at about 13 Ma (Bishop et al., 1969) have no equivalent in northern Tanzania. Lava extrusions continued into the Pleistocene

at several

centres

(Table

l), together

with

lacustrine sedimentation in the Olduvai, Peninj, Amboseli (N. Kilimanjaro) and KetumbeineTarosero areas (Isaac, 196.5; Hay, 1976). There is evidence for some minor faulting in the Late Pliocene-Early Pleistocene. The Peninj Group sediments (< 2.27 Ma) were deposited in a shallow, faulted depression (Isaac, 196.5) prior to the faulting that formed the present-day Rift Valley and that also terminated the Peninj Group sedimentation. In addition, relatively minor faulting affected the volcanics in the Tarosero area 2.1 Ma ago (MacIntyre et al., 1974). The Olduvai Gorge sediments were faulted several times, beginning with faulting during the deposition of the

h

i

6

NEOGENE

TECTONICS

AND

VOLCANICITY

IN THE GREGORY

87

RIFT VALLEY

‘ONI

C

DEPRESSION

Fig. 2. Distribution

of major

mid-Tertiary

faults

Natron,

and the late Tertiary

Manyara

tectonic

and Eyasi are outlined

depression.

The position

of the present-day

for reference.

.

LateTertiary -Early Pleistocene major volcanic centres Areal extent of lava extrwons Areas of sedimentation

Fig. 3. Distribution

of Pliocene-Pleistocene

volcanic

centres

and areas of sedimentation.

Lakes

88

J.B. DAWSON

TABLE 1 Summary of radiometric dates on northern Tanzanian volcanic rocks Rock type

Volcano/Formation

Age (Ma)

Reference

Oldonyo Sambu Mosonik Humba Formation Moinik Formation >

3.50 3.18 1.55 1.33

1

Peninj Area, W Natron

Basalt Biotite lava Basalt Basalt O&i sai Gorge Trachytic ignimbrite Masek Beds Ndutu tuff Naisiusiu tuffs

Peninj Group

+ f -

0.06 0.06 2.27 1.38

1 1

1

2.0

2

1.15 0.4 - 0.6 0.32 - 0.6 0.10 - 0.17

2 2 2 2

1.51 f0.03 0.050 - 0.38 0.102 + 0.0023 0.098 + 0.0014

3 3 4 4

Amboseli Mawenzi Kibo Kibo summit Shira W. slopes of Kilimanjaro E. slopes of Kilimanjaro

0.42 0.51 0.46 0.17 0.24 2.3 0.95

-0.26 j 0.03 k 0.4 $ 0.05

5 6 6 3 4 7 I

Lemagrut area Sadiman Essimingor Essimingor Essimingor Ngorongoro Ketumbeine Ketumbiene ?Tarosero ?Tarosero Tarosero Tarosero Tarosero ?Monduli ?Loolmalasin Rift Fault between Manyara and Engaruka (?Loolmalasin)

4.3 4.5 7.35 4.78 3.22 2.45 1.80 1.60 2.12 2.16 2.40 2.22 2.02 1.70 1.19

- 5.5 + 0.4 -8.1 + 0.09 i 0.06 i 0 + 0.09 + 0.08 * 0.06 + 0.06 + 0.62 k 0.07 f 0.04 f 0.10 +0.05

7 7 7 4 4 7 4 4 4 9 9 9 9 9 9

1.4

- 3.8

7

Gelai (?late Burko Small cone, Small cone, Small cone, Kwaraha Hanang

0.97 0.97 1.15 1.13 1.10 0.7 0.9

+ 0.03 IO.03 kO.04 kO.04 5 0.08 - 1.5 - 1.5

4 4 9 9 9 7 7

?Ngorongoro Drainage Reversal ‘Kerimasi ?Kerimasi Oldoinyo Lengai

Meru

Older faulted phonolite Phonolites and nephelinites Basalt Nephelinite >

Meru West Main cone Northeast Wall

Kilimanjam

Lower olivine basalts Basalt Basalt Anorthoclase (phonolite) Trachyte Olivine basalt Olivine basalt

- 1.1

Miscellaneous older centres

Phonolite, trachybasalt Nephelinite Melaphonolites Nephelinite Nephelinite Trachyte Basalt Basalt Basalt (Monduli) Basalt (Matunginini) Olivine basalt Lower trachyte Summit trachyte Basalt (Ardai) Trachybasalt (Narabala) Olivine nephelinites, basal& trachybasalts Miscellaneous younger extmsiue centres

Nephelinite Nephelinite Trachybasalt Trachybasalt Trachybasalt Nephelinite and melilitite Nephelinites

flow) Narabala Engaruka Kitete

References: 1 = Isaac and Curtis, 1974; 2 = Hay, 1976; 3 = Wilkinson et al., 1986; 4 = Evans et al., 1971; 5 = Baker et al., 1971: 6 = Evernden and Curtis, 1965; 7 = Bagnasaryan et al., 1973; 8 = Grommt et al. 1970; 9 = MacIntyre et al., 1974.

NEOG~NtTE~~ONI(‘SANDVOI.C‘ANICI'I‘Y

earliest

(Bed

Ma (Hay,

I> sediments

INTHE

between

GREGORY

1.7 and

RIFTVAlLEY

2.0

89

occur on the floor of the rift depression

1976).

on the Mbulu escarpment

The main rift faulting

rise

main

manifestation

carpment Lake

which

runs

N-S

cutting

east-facing

es-

from Lake

Natron

the Lake

has exposed

Kenya

to

of major

of

to

Manyara

reversal

graben

(Fig. 1). Many minor

1.3-0.9

drainage

Natron,

along the foot of

which are, from north

Engaruka,

of drainage

Manyara

,

I

! I

at 1.15 Ma; the new drainage

volcanic

the metamorphic with the earlier

highlands

to the

Following

the Late Pleistocene

east

(Hay,

faulting

at 1.2

Ma B.P., there was another major phase of volcanic activity that, in eruption style, relatively

faults

Main Upper Plwstocene

I Ii

faults

: I \ \ Y?, A\

Areas of intense

minor laultlng

lnlnlll

0

Major Upper Pleistocene or Recent volcanic centres

0

Older volcanoes that continued to erupt m the U. Pleistocene or Recent Late Tertiary - Early Pleistocene major volcanic centres ---I AOA\ Kibo

.Merll

\ \ \

of main Pleistocene

rocks to drainage

The younger volcanics

--__

Fig. 4. Distribution

and

Beds there was a major

in

50milss , I , 1, : I ', -3 . 1 / 8OLcm

et al.,

was the formation

0

0

1.2 and with the area in

Ma (Fairhead

basins

escarpment,

In the Olduvai

from the 1976).

Kenya. Relatively minor faults, trending NW-SE, form the boundaries of the Engaruka Basin and the Lembolos

at between

of the faulting

was principally from the west, contrasting

older Tertiary depression; the eastern margin of the present-day Rift Valley is formed by a series of small faults and downwarps that are the southFault

south:

Balangida.

lavas rang-

of the Ngong-Turoka

inland

the east-facing

ing from 3.8 Ma at the base to 1.4 Ma at the top (Bagdasaryan et al., 1973). This major fault, which has no eastern equivalent, is superimposed on the

ern continuation

in the period

1972). A result

eastern

and the volcanoes

Between

the fault

The

is dated

0.9 Ma (MacIntyre et al., 19741, equating major faulting in the Kirikiti-Lengitoto

1.2 Ma B.P. Valley.

through

Sambu

Highlands.

and Engaruka,

Rift

is the major

of Oldoinyo

Crater

about

present-day

Balangida,

flanks the

to the

and also

to the west of the main

(Fig. 4).

The main faulting

The major phase of faulting gave

Plateau

faults and volcanic

centres.

See text regarding

the age of Sadiman.

.I.R. DAWSON

90

small

volume

of extruded

type contrasted massive

material,

with the earlier

extrusions

and magma

relatively

from the basaltic

quiet,

shield volca-

atite lava extrusions place at Oldoinyo marised

noes. The activity was highly explosive, giving rise to features varying from major steep cones domi--

olivine

nated

Lengai-

by pyroclastic

Oldoinyo

Lengai,

Hanang)

materials

Kerimasi,

Burko,

to areas of minor

sion craters. volcanic

(Meru,

Kwaraha

tuff cones

Tuffs are widespread

province

Monduli, and

and explo-

throughout

and across the Serengeti

the Plains

and tuff eruptions

the

Lengai

by Dawson,

floor

and olivine

of the

Rift

19881, and numerous in the area

to these

persisted

after

masi and Oldoinyo Lengai (Dawson, 1964; Hay, 19761. The tuff cone areas often coincide with

Tarosero,

peralkaline

of the

tuffs

in the

with eruptions

areas of intense minor faulting, for example on the Mbulu Plateau (Downie and Wilkinson, 1962) and in the Engaruka-Natron Powell, 1969).

area (Dawson

and

activity

cones

of

of Monduli

observation,

1961).

Late Pleistocene-Recent

at some the

Oldoinyo

small south

et al., 1970; personal

centres,

some

are correlated

in the

occur

In addition

flows of occur on

et al., 1985; Dawson

and Smith,

Olduvai

west;

Valley

ankaramite (Dawson

times (sum-

minor

m.elilitite

Gelai area (Dawson

of Keri-

to the

succession

in modern

1989). Recent

nephelinite

have taken

of the

Middle

older

centres

Pleistocene.

trachyte

and

At

phonolite

flows continued into the Late Pleistocene, and on the Kibo summit of Kilimanjaro (which is classified as active) sions continued Wilkinson,

phonolite and nephelinite extruuntil Recent times (Downie and

1972)

is some uncertainty as to the age of the volcano. It is a predominantly pyroclas-

Some of the minor tuff cones contain blocks of upper mantle material testifying to a heteroge-

tic cone of nephelinite-phonolite, and Pickering (1964) classifies it as one of the younger volcanoes. However, Bagdasaryan et al. (1973) give a date of 4.5 Ma for a nephelinite from the volcano and, on the basis of clasts of Sadiman lava in Bed

neous mantle beneath northern Tanzania. At Olmani Hill, near Arusha, ankaramitic scoria contain blocks of depleted harzburgite (Jones et al., 19831, whereas the ankaramitic-carbonatitic tuffs

There Sadiman

I of the Olduvai succession (1976) suggests that Sadiman

(1.7-2.0 Ma), Hay is the same age as

at Lashaine, near Monduli, contain blocks of lherzolite and garnet lherzolite, some of which derive from enriched

mantle

2.2 Ga old (Dawson

the later stages of Ngorongoro (2.0-2.5 Ma). Hence, most evidence suggests that Sadiman joins Mosonik and Essimingor as the ultrabasic-ultra-

et al., 1970; Cohen et al., 1983). metasomatised peridotite xenoliths

alkaline members of the older (pre-rift faulting) volcanic series. The magma type at most of these later centres

evidence for anomalously light, asthenospheric upper mantle beneath the Rift Valley (Dawson and Smith, 1988).

is ultrabasic-ultra-alkaline, free nephelinites plutonic ijolites

giving rise to olivine-

and phonolites, and feldspathoidal

and blocks of syenite and

fenite occur in the pyroclastic rocks at several centres. On Meru the main cone is formed of peralkaline trachytes, phonolites and nephelinites, and nephelinites also occur late in the se-

Hill,

10 km east

Minor

faulting

tocene-Recent,

of Oldoinyo

occurred

Veined and from Pello

Lengai,

in the

the pyroclastics

provide

Late

Pleis-

of Kerimasi

be-

ing faulted 0.37 Ma ago, prior to the initial eruption of Oldoinyo Lengai, the youngest centre (MacIntyre et al., 1974). Numerous modern earthquakes tinued

along

tectonic

the Rift Valley

testily

to con-

movements.

quence, forming the inner cone in the explosion caldera (Wilkinson et al., 1986). Carbonatite lavas and pyroclastics phonolites at

accompany the nephelinites and Oldoinyo Lengai, Kerimasi, Kwaraha and Hanang, and numerous minor carbonatite tuff cones are known on the Mbulu plateau (Downie and Wilkinson, 1962) and in the Monduli area (Dawson, 1964). Alkalic carbon-

Discussion

To account for the formation and geometry of the Kenya Rift Valley, Bosworth (1987) proposed that the apparent alternation of eastward thickening and westward thickening half-grabens, to-

NEOGENE

I‘tCl-ONICS

AND

VOLC‘ANICITY

IN THE

GREGORY

RIFT

gether with off-axis volcanism, are the result of two major, oppositely dipping, detachments beneath the Kenya rift. Morley (1980) has criticised the Bosworth model, mainly on the grounds of differences in the timing of initiation of allegedly synchronous boundary faults and also because of time differences between faulting and off-axis volcanism. Morley (1988) proposes that most of the Rift Valley features can be accounted for by a single, early, eastward dipping detachment. The differences between the geometries of the narrow structure in Kenya and the broad, earlier depression in northern Tanzania have been outlined above (see also Fig. 2). The Tanzania fault geometry, which is more like a minor triple junction, would be hard to reconcile with the Bosworth model, and control by basement structures is more plausibie. In the case of the major 1.2 Ma faulting, the geometry is more reconcilable. The major half-graben in Tanzania extends over a N-S distance of about 250 km, and its boundary fault is a southerly extension of the Nguruman Fault that is regarded by Bosworth (1987) as the breakaway zone of a major, east-dipping lithospheric detachment. The pattern of both the Nguruman and Natron-Balangida faults could be consistent with reactivation of an early, east-dipping detachment of the type envisaged by Morley (1988). It is worthy of note that, particularly in the Tanzania sector, the breakaway zone coincides with the boundary between the Tanzania craton and the younger Mozambique fold belt, perhaps indicating control of the detachment by east-dipping fold belt structures. Turning to the volcanic activity, overall, the Tanzania activity is younger than that in Kenya. The early major basaltic activity, representing the earliest major thermal perturbation of the mantle, occurred at 1-5 Ma in Tanzania, compared with 15-32 Ma in northern Kenya (Turkana and Samburu) and about 5 Ma in southern Kenya (Kirikiti basalts) (Baker et al., 1973). In addition, the post 1.2 Ma magmatism in Tanzania has no equivalent in Kenya, where the post 1 Ma activity consists of extrusions of trachyte or comendite within the Rift Valley, or extrusions of basalt away from the Rift in the Chiyulu Hills, the Nyambeni area to the north-

VALLEY

01

east of Mount Kenya and Marsabit (Baker et al., 1971; Fairhead et al., 19721. Furthermore, the Rift Valley trachytes are believed to result from fractionation and the comendites are the result of crustal assimilation (Macdonald et al., 1987; Davies and Macdonald, 1987). In short, the Tanzania volcanics are the results of more recent mantie melting. In particular, the xenolith-bearing olivine melilitites and ankaramites represent batches of melt very recently generated from a volatile- and alkali-rich mantIe. Thus, a major change appears to take place at a latitude of approximately 23, roughly at the north end of Lake Natron. To the north are the plateau trachytes of the Magadi area, whilst to the south are the carbonatite volcanoes Oldoinyo Lengai and Kerimasi and the minor olivine melilitites and nephelinites of the Engaruka-Natron area (Dawson et al., 1985). It is also noticeable that it is at this same latitude that the Tertiary faulting diverged from the narrow Kenya graben into the broader Tertiary depression in northern Tanzania. Both the rifting and the volcanic activity reflect major perturbation of the upper mantle. It is a matter for debate whether this perturbation, manifest in the more recent Tanzania magmatism, is giving rise to a southerly propagating lithosphere fracture system, possibly linked to easterly movement of the Somalia micro-pIate, or whether the African Plate as a whole is drifting north over a mantle plume. Conclusions From the above it can be concluded that: (11 The present-day Rift Valley in northern Tanzania developed as the result of major faulting at about 1.2 Ma and was superimposed on an earlier, wider tectonic depression that contrasted with the narrow Tertiary rift structure in Kenya to the north. (2) The Neogene volcanic rocks of the northern Tanzania province comprise an earlier (pre 1.2 Ma) group of major central volcanoes of the alkali basalt-phonolite association (dominated by basalt) together with rare nephelinite-carbonatite volcanoes, and a later (post 1.2 Ma) group

92

comprising mainly neph~linite-phonolitecarbonatite volcanoes. (3) The northern Tanzania magmatism contrasts to that in Kenya because: (a) the Pliocene-Pleistocene activity in Tanzania lacks the widespread trachytic extrusions that dominate the activity in Kenya; and (b) the post 1.2 Ma nephelinite-carbonatite activity has no contemporaneous equivalent in Kenya. (4) The change in both tectonic style and magma types between southern Kenya and northern Tanzania takes place at approximately 2% References Bagdasaryan, G.P., Gerasimovski, V.I., Polyakov, A.I. and Gukasyan, R.K.. 1973. Age of volcanic rocks in the rift zones of East Africa. Geochem. Int., 10: 66-71. Baker, B.H. and Wohlenberg, J., 1971. Structure and evolution of the Kenya Rift Valley. Nature, 229: 538-542. Baker, B.H., Williams, L.A.J., Miller, J.A. and Fitch, F.J., 1971, Sequence and geochronolgy of the Kenya rift volcarries. Tectonophysi~, II: 191-215. Baker, B.H., Mohr, P.A. and Williams, L.A.J., 1972. Geology of the Western Rift System of Africa. Geol. Sot. Am. Spec. Pap., 136: 67. Bishop, W.W., Miller, J.A. and Fitch, F.H., 1969. New potassium-argon age determinations relevant to the Miocene fossil mammal occurrences in East Africa. Am. J. Sci., 267: 669-699. Bosworth, W., 1987. Off-axis volcanism in the Gregory Rift, east Africa: Implications for models of continental rifting. Geology, 15: 397-400. Cohen, R.S., O’Nions, R.K. and Dawson, J.B., 1983. Isotope geochemists of xenoliths from East Africa: implications for the development of mantle reservoirs and their interaction Earth Planet. Sci. Lett., 68: 209-220. Davies, G.R. and Macdonald, R., 1987. Crustal influences in the petrogenesis of the Naivasha basalt-comendite complex: combined trace element and Sr-Nd-Pb isotope constraints. J. Petrol., 28: 1009-1031. Dawson, J.B., 1964. Carbonatitic volcanic ashes in northern Tanganyika. Bull. Volcanol., 27: 81-92. Dawson, J.B., 1989. Sodium carbonatite extrusions from Oldoinyo Lcngai, Tanzania: implications for carbonatite complex genesis. In: K. Bell (Editor), Carbonatites: Genesis and Evolution. Unwin Hyman, London, pp. 255-277. Dawson, J.B. and Powell, D.G., 1969. The ~ngaruka-Natron explosion crater area, northern Tanzania. Bull. Volcanol., 33: 791-817. Dawson. J.B. and Smith, J.V., 1988. Veined and metasomatised peridotites from Pello Hill, Tanzania: evidence for anomalously light mantle beneath the Tanzania sector of the eastern Rift Valley. Contrib. Mineral. Petrol., 100: 510-527. Dawson, J.B., Powell, D.G. and Reid, A.M., 1970. Ultrabasic

J.B. DAWSON

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