, ",'

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ABSTRACT

The petrographie and geochemical 'study of the suite of fenitited ... xenoliths from Dlioinyo-Lengai nephelinitic-carbonatitic volcano

(T~nzanja) shows evidence of two types of rock, metagranite and

metagabbro, in dtfferent stages of re-equilibration-with an alkali

(mainly sodie) carbonatitic fluid. The two parental compositions do

not show the same pattern and final products of reaction. ; Systematic

changes in the. distribution of ?the acmite component in the metasomatic

pyroxene ref1et changes in 6(2), during progressive fenitization, from Hm-Mt to Ni-NiD buffer conditions. The ~quilibria among mineral

< ,

phases and the disordered state ~of the newly formed alka'Ji:i felds'par

suggst reaetion in "the approximate range 500-850oC. Ultimatly, at

the highest grades, incipient fusion occurrecfand led to a mobile

mixture 'of erystal~ + melt. The composition of the partial melt (glass)

_ \..". indicates 1) a temperature of fusion near 850oC, 2) the fluxing

action of volatiles, in.cluding water, and 3) most probably,

disequili~rium conditions. Rheomorphism seems to be of only local

importance and 'cannot ~e responsible for "the generation of nephel inite- . phonolite lava sequences at Oldoinyo Lengai. The observed pattern of

fnitization of metagabbro may resemble the type of transformation

90in9 on i,n a me~agabbroicelower crust, near:.,Jhe rift zone. This

- nephel i ni zed::-carbonati zed mater~~r woul d providft, an appropriate &> ,

parental ro.ck, once fused' due to uprise of geotherms, for the Oldoinyo

Len~ai aVkali earbonatiticcompl,ex. ' , .

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(

. SOMMAIRE , .

--~-

'.

Une ~tude pHrographique et""~~och;m;que d'une suite de(e~nolithes

,f~nit;ss provenant du volcan nph~l initique-carbonatitiqu.e de

Oldoinyo Lengai (Tanzania) montre deux types de roche, mtagranite et

m~tagabbro,' ~ diffrents stades de r~-quil ibration avec un fluide

alcalin (surtout sodique) et carbonatitique. Les deux lithologies

originales ne partagent pas la mme s~quence et le mme produit final

de r~action. La distribution du ple amite dans le pyroxne mtaso-matique varie de faon~ystmatique pendant la f~nitisation avec le

changement en (OZ)' des conditions proches du tampon Hm-Mt a celles du Ni-NiO. Les quilibres' parmi les phases minrales et le degr~ de

d~sordre dans le feldspath alcal in noform~ indiquent une s~rie de

transformations dans l'intervalle SOO .. 850oC. La fusion partielle au p

stade de fnitisation intense a donn~ un m~lange mobile de cristaux

+ bain fondu. La composition du liquide (verre) indique 1) une

temp~rature de fusion pr~s de 850C, 2) un rOle de flux pour les .,y composants,..\Jolatils; y inclus l'eau, et 3) des conditions de dsqui-

,1

libre. Lji2 rhomorphisme semble avoir t~ d'importance locale se~lement.

et ne peut pas expl iquer l!. formation de squences de laves

nphl in it i que ~ phono 1 it; que ~ 01 do i nyo Lenga i. Le phnomne de /"

'fnitisation observ~ dans les roches mtagabbro'ques ressemble

peut-tre la transformation qui se passe dans une croate inf~rieure

mtagabbro~que prs de la zone de rift. Ce mat~riau nphlinis et

carbonatis pourrait former un socle appropri, une fois pass ~

l'tat de fusion ~ cause d'un rchauffement pendant 1 e bombement

rgional, pour expliquer la production du complexe de carbonatite ... alcalin Oldoinyo Lengai.

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ACKNOWLEDGEMENTS "

1 grtefully thank my thesis supervisor, Prof. R.F. Martin, for r

warm encouragement and hel pful suggestions, particularly during the

preparation of the manuscript. Special thanks to Dr. G.8. Dawson, ,

Sheffield University, who kindly provided the specimens from Oldoinyo

lengai. For the drafting of figures, l thank Messrs. R. Yates and

T. Kefa 1 as. Mr. R. Wei ss prClduced promptl y the thin sections and , ~ polished sections l required. Mr. C. Fong helped me with the photo- u. \'

graphy. Ms. Claudette Lefebvre rapidly and efficiently typed the

manuscript. l also thank my colleague and friend Dr. Karen

Stamatelopoulou-Seymour for the exchange of ideas and her continuouS"

and warm encouragement. Finally; l espec;ally thank my husband Teodore ~

for his assistence, warm encouragement and commitment to this thesis.

The present proJect was supported by a research grant from the

Natural Sciences and Engineering Research Council of Can'ada (A7721)

to R. F. Ma rt in.

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'Medium'-Grade Fenite ::-fu t :... .... ... ::.~t, ..

Gran;~e- gneiss ''''''-~:~ ,

, High-Grade .Fen;~? ~ , . '" >;o,.~\~ ~>,\ . N~phe!'ine-bearing sanidine egirine-augite fen1te

Sanidine-bearing nephe1ine aegirine-au9,fte,renite CORtact fenite . '

Discussion

Chapter 5. Compositi6n offthe Phases Present

Pyroxene.

Di scuss i ori

Feldspar

_ Information derived frqp1 X-ray diffraction , Information derived by electron micorprobe

Di scussion

- ,. Nephel ine Glass_

SiG -undersaturated pera1ka1ine glass SiG -oversaturated glass ), Di scuss i on conc~rning the ori gin of the ~)(isting melts',

Eva1yation' of the Physical Environment

Chapter 6. Geochemi stry

Discussion / ,

The Source and the Nature"...9f the Fenitizing F1uid

Chapter 7. Summary and Conclusions

Fenitization and ltimate rheomorphism , Suggs~~d petrogenesis

v

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43

,,43~, 49

49 "'55 59

67

72

72

92 95 )

95 105 . 111

115

121

122 128 130

137

141

147 1 151

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157

158 . 164

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References Li st of Figures List of Tables " Appendix 1 E1ect~on Microprobe Append i x II X- Ray F1 uorescence Appendix III Powder X-Ray Diffraction

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"1 184 189 191 192 193

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Chapter l . INTROOyCTION ~

Problems involving carbonatites ha)fe been and still figure among

the most interesting and controversial in the field of igneous yetro-

logy. The relationship of carbonatite to the large vriety of

associated alkaline rocks continues to puzzle most investigators. In

the last decages, important insights into the complexity of the

subject have been summarized by Heinrich (1966), Tutt1e & Gittins

(1966), S~rensen (1974) and Le Bas (1977,1981). Among the most

important observations are: 1) 'the inextricable 1ink between

. carbonatite and alkaline rocks; 2) the mostlij continental cratonic

or peripheral cratonic environment of emplacement; 3) the mantle

'source for primary or derivative carbonatitic magma; 4) the

occurrence of carbonatite - ijolite and carbonatite - kimberlite "

associations, considered as interdependent pairs that may have dis- _

tinctive but c10sely relatpd magmatic sources or may simply point to

two different paths of magmatic evolutlon of a common parental melt;

5) the rather exceptional occurrence of carbonatite with alkali

olivine basalt, although such a magma has been mentioned as a

possible parent to carbonatitic magma (Kapustin 1976). Normally,

alkali basalt - nephelinite associations do not inc1ude carbonatite

or ijolite. However, an exception occurs in the Tanzanian sector of

( the Gregory Rift valley, where the above associations are juxtaposed.

An almost ubiquitous feature ofcarbonatite occurrences is

the aureole of metasomatic alteration in the wall rocks,

most common1y expressed by fenitization reactions. These reactions

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transform the 'host roks into a minera' assemblage in equil ibrium

with the carbonatite intrusive complex. The purpose of thi s thesi s

is to present petrographie, mineralogfcal and geochemical .

information on blocks of fenite found in the only modern carbonatitic ,

2

volcano, at Oldoinyo Lengai,' in Tanzania. Results of this investigation

shed light on the process of ultimate rheomorphism of fenite; this

. phenomenon has been considered by sorne authors (von Eckermann 1948. >

Dawson 1962) as extremely important in the petrogenesis of alkaline

carbonatitic complexes. On the basis of the published record and

personal observations, an overall petrogene'tlc model is presented . . The volcano Oldoinyo Lengai is situated on the floor of the

Tanzanian sector of Gregory Rift valley, appr'oximately 19 km south of

Lake Natron (Fig. 1). The Gregory rift: ils part of the East African

,. , rift, a major element in the world system of rifts; it is closely

..

controlled by Precambrian structures and associated in many sectors

w;th voleanic activity. The volcano at Oldoinyo Lengai is mainly of

the explosive type, wlth poorly developed lava sequences, except for

the natrocarbonatitic episode. Its unique feature is the very

a1ka1ine nature of the carbonatitic lavas extruded in historie times.

The summit area of the vo1cano 1S occupied by two craters (Fig. 2);

on1 y the northern crater i s aet ive at present. The ma i n cone has

smaller parasitic cones, explosion craters and tuff rings on its

f1anks (Dawson 1968).

Beginning in the late nineteenth century. numerous investigators

became attracted by this very unusual vo1cano. During the first German

, /

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. . MOROTOe\

LRUDOLPH

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l' ELGON.,t f J UGANDA

TORORO ... / \ ~,~~ KENYA_ / \ ~\

[ ~URU .MENE\NGAI eMT. KE~YAr LAKE ~HILL \ -\ VICTORIA HOMA MOUNTAIN \

: KISINGIRI A\)\ l l' _ .. 1 Y JlL.NArSHA "'" . : ' ... ,.,J fJ \

fL' fTR~~. ~ t,DOINYO LENGAI 1 E~IMASli .KILIMANJARO . L.EYAS'# )oRONGOR:1 \ \

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exped'itions (Fischer lJn 1882-1883; Bauman in 1892-1893, Shller in

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:J d h a $ Mali li.

-the principal rO+e in the evo1ution of 01doinyo Lengai to the

carbonatitic magma, and stressed the importance of fenitization

reactions. In 1968, he gave a complete aCCQunt of the 1966 activity

and noted the hange in eruption type, in August 1966, from quit

5

sodium catbonate extrusions ta highly explosive eruptions of ash.

Regarding the ori-gin of the natrocarbonatitic1ava from 01doinyo Lenga i,

sever~~ferences were made. Milton (1968) suggested that the

natrocarbonatite lava represents troniferous sediments of the type

" found in Lake Natron and Lake Magadi, me1ted by nephe1initic magma. , Vinogradov et al. (1971) reported suffi.cient oxygen-isotope information

(180 8.7 0.1 per mil,) ta rule out it's 'sedimentary origin (180 36.7

0.3 per mil). The carbon-isotope data (average 13C value - 5.8 per

mil) publi,shed by Vinogradov et al. (1970) and QdNeil (1971) point to

the mant1e as, a possible source of the carbonatitic material. Bell

et al. (1973) reported re1atively primitive values of the 87Sr/86Sr ,

ratio, 0.7059 and 0.7061, supporting a truly magmatic origin of the

Oldoinyo Lengai alka1i carbontite and its'like1y source in the upper

mantle. Coopr et al. (1975)~tackled an experimental studyon the

system Na2C03-K2C03-CaC03' attempting to duplicate the exact

minera1agy of the natracarbonatite lava; theyadvacated an immiscibility

re1at!onship b~tween a sodium-rich carbonatite liquid and a nephelinitic

parental magma. Donaldson & Dawson (1978) computedthe ~omposition of -

the parental nephelinitic magma on the basis of compositions of

residuaf glass in an a.1kali-pyroxenite found at Oldo;nyo Leng'ai. They

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... also advocated an immiseibility relationship between the sodium car-

bonatite liquid"and a phono1itic magma.

The dominant1y pyroclastic character of the vo1cano has alredy - .

been mentioned. The pyrocl astic fragments. are ma inly phono 1 Hic.

nephelinitic and melanephe1initic, but a1so inc1ude a large variety

of p1utoni~ lithologies. These fragments provide direct evidence of

the rock types to be found beneath the volcano, and may enclose \

, ~ ;'

important. petrogenetic information. Dawson (1962, 1968, 1978) mentioAed .

the occurrence of xenoliths of ijolite, nepheline syenit~,

wol1astonitite. biotite pyroxenite, s6vite and fenite, and inferred

on this basis the presence of a subjacent a1ka1ine carbonatitic

complex.

This thesis is based on the study of ten xeno1ithic specimens

col1ectd by J.B. Dawson. A1Lspecimens consist otfragments of rocks

in different stages of fenitization. The presentation of the geological

environment is entire1y based on the pub1ished record. The petrographie

description of the fenite specimens is based a1most entirely on the

study of thin sections. The rock-forming minerals pyroxene, fe1dspar

ne~heline and amphibole, which are susceptible to compositional changes

during progressive fenitization, have been ana1yzed byenergy

dispersion using an electron microprobe. The partial me1t (preserved

as glass) was analyzed by the same method. Details of the analytical

procedure are listed in Appendix 1. The chemical composition of the

fenite xenoliths was obtained by the X-ray-fluorescence technique;

details concerning the analytical procedure are reported in Appendix II.

1 1 1

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The. composition and t,he structura'l state of the alkali feldf;pars have

been determined by a powder-diffraction method using the

+ __ " 'Gl.ti .. nier-,~gg camera. Relevant details are given in Appendix III.

~-- ~-----

1

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8

Chapter 2. REGIONAL GEOlOGICAl SETTING

The geologica1 evolution of the O'do~nyo Lengai vola~ic complex

cannot be discussed separately from that of the closely retated East

African rift. In the absence of personal field observations, 1 will

relyon the geological literature conce~ning the East African rift,

and especially on its Kenyan-Tanzanian sector. The East African

rift system persists for 5500 km in a NNE-SSW direction, 'from the

Afar depression to the Orange River in South Africa (Fig. 3). The

Gulf of Aden and Levant rift zones mi ght extend' it al ong its northern

extremity. The principal rifting occurred in Terti~y and Recent

times, but simi1ar patterns have been recognized further back in time

~ Most would agree tha~ the East African rift is "the ref1ection in the

continental crust of a mant1e lineament of great age and continuit~"

(McConnell 1968). As such, it undoubtedly figures among the most

spectacular phenomena in the world. Certainly, the Tertiary-tdLRecent

activity along the rift has been largeJy contro1led and directed by

older crustal nuclei and orogenic belts of the African continent

(King 1970. Harris 1970). Sinee early Paleozoie times, the African ...

e~ntinent has evolved as a stable segment of the earth's crust;

minor mobile zones were active in the southeastern, northwestern and

northern extremities duri~g 1~te Pa1eozoic to Mezozoie or even

Tertiary times (Clifford 1910). The oldest cratons (2000-3000 Ma),

as well as the younger orogenie belts (1100 200 Ma) created the

dominant structural patterns in the preSilurian Mozambique and

1 ,

1

f kEVANTINE , RIFT

50. 4!1 o 500 1000

,-' _ ...... ' _$ _..IJ km

ARABIA

Fig. 3. The configuration of the East Afri can rift system. Adapted fram Baker e.t al... (' 972)

9

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.' 10

Limpopo orogenie 'belts, and together 'control. the orientation of the

major rift systems. On a continent,l sOile,~the rift is an approx'ma-

tely N-S feature, but its structural elements seldom follow this

f

. direction. Tbe N-$ trend is emphasized in the Kenya - North

Tanzania sector, where the older structures have the same direction. , ".

An idea1 example of the close control exerted by the Precambrian structures is the bifurcation of the rift system into eastern and

western branches around -the Tanzanian craton .(Fig. 3). The N-S .. trending . ~ .

eastern branch, named the Gregory Rift valley in Kenya, approaches

a classic graben in cross-section; its width, 60-10 km, and its

general regularity possibly reflect the regu1arity of structures

within the Mozambique orogenic belt. In the Tanzanian sector, the

eastern branch degenerates into a broad zone of faul ts of differenf

trends, most probably reflecting the structural comp1exity of the

Precambrian basement.

The African rifting has general1y been accompanied by volcanic .

activity (King 1970, Harris 1970, Baker e.t al. 1972, Bailey 1974, ,

c le Bas 1980). The uplifted regions of the rift, e..g., the Ethiopia

and Kenya domes, seem ta be preferred sites of volcanism (Bai1ey 1974,

Le Bas 1980). A1so, at the intersections of rift systems, e..g., at

Lake Kivu, RJngwe, the volcanism is extensive and 1ess a1kaline than

e1~ewhere along the r;\t (King 1970, Harris 1970). The nature of the \

vo1canic activity is -variable in two directions controlled by the

factors space and time~(Tab1e 1). Along the rift system, fram north

-----~----- . - ._-------

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RECENT

PLEISTOCENE

PUOCENE

MIOCENE

EOCENE-OLIGOCENE

GULF OF

ADEN

Thole~itic basalt

Peralh-li"e volcanislll

r TABLE 1. VOLCANIC EVOLUTION OF ~ AFRICAN RIFT*

RED SEA

Thol en tic basaIt

BasaIt, trachyte basa1t

ETHIOPIA

Fissure-multicenter Central eruptions volcanos

Picrittc basalt (with tholeiitic

.. chemlstry). tra-chyte, rare cOIIIendlte (N. Afar) (Aden series) AlitaI i 01 iVlne basait

Trach~te-pan

tel1erite 1-gn 1mbr i te pitchstone a 1 ka 1 i 01 iv i ne ba~a1t.

Al kali basa lt .. basanite, pho-

nolite. ""

~ENYA

Flssure-multicenter erptlons

AlkaH basalt (east of rift)

#'

Central volcanos

Trachyte fgnlmbrite, phono 1"1 te , basalt

c

Trachyte-pantellerite Trachyte, rhyolite, Alkalt basaIt, ignimbrite and lava ignilllbrite trachyte

Alkall olivine Alkali .basalt, basalt-mugearite basanite,mela--alkal; trachyte nephelinite

(Afar Series)

Al kal i basait (rift floorJ

Phonolit~, trachyte

Alkali basaIt, trachyte, pho-nolite. (rift margi ns)

Hephelinite. phonolite AHali olivine

basalt-mugeartte Soda-rhyolite, tgnimbrite (HW Kenya)

~ (rift floar) ,

Riftini _ ~-----

AHal t olivine basa lt - h.1w, li te (Trap Series)

_______ ! i ft l ni ______ _ Rare nephell- Phonolite, phonolf-nite phonol \te tic trachyte (plateau) (Central Kenya

Alkali 01 ivine basa 1 thawa i i te + rare tholeiite (Shield Group)

plateau )

Al ka li 0 li v j ne basaIt, basanite (HW Kenya)

Nepheli n f te. phonolite, carbonat1te fKenya-Uganda

border )

* Based on data o'f Gass (1970). King (1970), Baker e.t aL. ('1972), ~ai1ey (1974) and Le Sas (1977).

\k~ ~_

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JANZAIHA

Itephel inite, phonolIte, carbOnatite

_ !i!tlni _ C Alkalt

basaIt, trachyte phonolite.

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te south, there is a continuous change in the direction of increasing

alkalinity and degree of Si02-under~aturation. T~e ~rend ;s mainly

from tholeiite in tne Red Sea sector to mildly alkaline or transitioRal

basalt in Ethiop;a to strongly alkaline Si02-oversaturated to strongly

SiOZ-undersaturated types and carbonatite in Uganda, Kenya and

Tanzania (Fig. 4). The nature of volcanism has also changed with , 1

time'. from basa1tic (Eocene-Oligocene). to phonolitic-nephelinitic ,

(Miocene). to trachytic- pantel 1 eri tic, (PHocene-Pl ei$tocene) and,

more recently. to basaltic-trachytic or nephelinitic-carbonatitic.

Basalts have appeared in the sequence severa1 times (Table 1).

earliest vo)canism related to the rift system resulted in vast

of basal tic or phonolitic ma~mas spilled over the rims of the

whereas the younger volcanism was mostly confined to the rift

itself. Volcanism and tectonic activity seem to have been increasrlngly li.

restricted to the cintral zones of the rift system. The latest phases

include ~olcanos with a large caldera and smaller ccnes along the (1

centra l graben~

The southern extremity of the Gregory rift 1S the setting of

01doinyo Lengai (Fig; 5). The Gregory rift 1s a camplex graben that

bisects the Kenya damal uplift. coinciding well with its long axis

,(Crossley 1979). 'The graben is defined bya set of major faults

named Sonjo, Sambu t Kirikiti t Haitami and Lengitoto. The main faults - . became active by late Miocene time and were rejuvenated in three

) -

phases of displacement. The Gregory rift is 60 to 70 ~m wide. but it .c.

;s reduced by the marginal stepped structures to an inner graben floor '

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...

I++l Trachytes, rhyolf{es, IQnimbrites of L:t:tJ caldera volcanoes (lote Quaternory) ln ~ 1~ Basa 11 (Quoternary) E3 Traeh~tel, rhYOlites, ionimbrites ~ (Plioelne - Pleistocene)

Phonolites (Mioeene)

Fif/J Basalts (Eocene - Pliooene) , ra'! Neph,' i n ite - phonolites L!_.l voJeanoes (Mioeene - Recenf)

o 200 400km 10.' _.L-..I' ....... ...&......J'

Fig. 4. Map illustrating the distribution of volcanism along the East African rift. Map fram Baker ~ al. (1972).

)

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Mt. Kenya

50 100km

1

2

NY~NZA

. .

Serengeti Plain

~ K

Moro Plain

1

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{\

\ Q. \~

KllimanJaro

Mo lai . Plain

_ 1 37-

1

2

J . Fig. 5. The configuration of the Gregory Rift valley (based on Baker et al.

\, 1972, Crossl ey 1979). 0

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( 17 t~5 km in width (Baker et. 01.. 1972). The approximate vertical displacement along the rift is 1600 to 2000 m on the west side and

upto 1000 m on the eastern margin, The graben floor is composed of

Pl iD-Pleistocene volcanic products dissected by a swarm of normal,

15

dip-slip type of faults, They parallel the main faults and crea te an

interna1 horst-graben structure .. Subsldence of the floor and upl ift

of the margins have contributed to the total displacements of the

boundary faults. There is a decrease in elevation of the rift floor

from Naivasha (1900 m) to the Magadi-Natron (600 m) (Fig. 5).

, Volcanism has been extensive a10n9 the Gregory rift during

Tertiary-to-Recent times, being expressed by fissure an,d multicentre

eruptions. and by major central volcanos. The early Miocene central. , volcanos, located 'near the Kenya-Uganda border region (NW sector of

the Gregory rift), have produced a large amount of nephelinitic-

phonolitic lavas; they also display intrus.ions of ijolite-carbonatite

,).0.. section (Le Bas 1977). By mid-Miocene time, the strongly 5i02-

undersatur~ted volcanic products of the northern segment of the

Gregory rift had changed to a transitional basalt-trachyte

The fissure and multicentre eruptions generated extensive association. rl~ '.

floods of basalt, trachyte and rhyolite. During l ate" Miocene - early

Pliocene times, extensive flood phonolites were erupted from the crest

of the Kenya dome and within the rift floor (Bailey 1964). After the

early Pliocene phase of rift faulting, the volcanic activity was

largely confined to the rift floor and; its margins; it was expressed

by numerous central volcanos of basalt-trachyte, basalt-trachyte-

phonoiite and phonolite-nephelinite types. From middle to late

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Pleistocene times, volcanism tended to be focused on the young axial

fracture and led to trachyte caldera vo1canos. At the same time,

basa ltic magmati sm had shi fted sorne 150-200 km to the east. In

Quaternary times, nephelinite-carbonatite volcanism became restricted

to the north Tanzanian sector.

In conclusion, sorne general features of rift-re1ated volcanism

along the Kenya-Tanzania sector might be noted: 1) alkaline and

peralkaline magmatisrn is associated with crusta1 doming fo11owed~y

rifting (Bailey 1974). More recent1y, Le Bas (1980) mentioned a

related upper mantle lithospheric doming due to the S+a olivine

inversion within the tectosphere, which wou1d al 50 cause sl i ght ~

partial melting of garnet-lherzolite mantle by decompression; 2) the

16

alkaline magmatism is represented by alkali basa1t-phono1ite and

nephelinite-carbonatite associations separated by a mixed zone

(Le Bas 1980) in the northern part of the Gregory rift, whereas in the

soutl (Tanzania), the nephel initic-carbonatiti c styl e is

superimposed on the basal tic province; 3) carbonatitic activity

migrated continuously toward east and south, causing also the easterly

shift in basal tic volcanism; 4) during the Tertiary-to-Recent

episode of volcanism along the Gregory rift, a vast amount of phonolite

and trachyte-was extruded; 5) in Recent times, the carbonatitic

vo1canisrn was extensive along the southern segment of the Gregory rift.

The younger carbonatitic volcanos are mostly aligned along the main

graben. The most recent expression of carbonatitic activity occurs at

01 do; nyo Lenga i .

1

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1

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, 1

Chapter 3. GEOlOGY OF OLDOINYO LENGAI

\> In the Tanzanian sector of the Gregory rift. site of Old'oinyo

Lengai, the main

.. N.tlllea lZUbi4 Q &; 1

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18

Highland and to the east of the Gelai and Kitumbeine volcanos (Fig. 6).

The basement rocks in these areas belong to the granitic-migmatitic

Tanzanian shie1d and to the metamorphic series of the Mozambique

orogenie belt (Guest 1954, Pickering 1961). The main li~hologies of

the Tanzanian shield include gneiss, migmatitic gneiss, nebulitic

migmatite, granite, granodiorite, metadolerite and catac1astic rocks

(Guest 1954). In the Serengeti area, 30 km west of Oldoinyo Lengai,

Guest (1953) mentioned a series of sheared rocks, e.g., quartzite,

quart'z-amphibo1ite schist, together with foliated granite and .

granodiorite. This suite of rocks may be the resu1t of a localized

shear zone between the Tanzanian s.hield and Mozambique orogenie bel t.

The only basement rocks known at 01doinyo lengai had been noted, prior

to the 1917 eruption, as ejected blocks of granitic gneiss within the

older southern crater, which was totally buried by the next

pyroclastic blanket (Reck 1914).

VOLCANIC STRATIGRAPHY

The most complete description of the evolution of 01doinyo lengai

and of its volcanic stratigraphy has been given by Dawson (1962 a, b,

1968). He noted five main stratigraphie sequences prior to the

extrusion of the a1kali carbonatit~ lavas. These units are described

in the fol 10wing paragraphs.

Y~ow l.joR.1.:ti.c. PlJlLo~:tiC. p1LOduc..t6 a.nd I:teJtbe.dde.d ta.vQ.6

The i..ja.ut..c. p;~tic. unit is the 'oldest expr;SSion of ,volcan;c activity at 01do;nyo Lengai. These first volcanic produets

1 !

, 1 -'"...... ~ ........... '

f ~,"

-,

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1--..-- S ER'NG'T 1 I--_~ '" '" '1. t--~: :PLA 1 N S: ...... '" .. '" .. '" '" t::::.--< '.' .. '" "'. '" '" . '. '"

, 1 '" '" '" "'. '" "'. '" '"

)

19

. '" .. '" '" .... '" .... t '" '" '" "'

t '" '" '" '" . , ...... '" '" . '" " . .. , .... '" ... '" ... . " .... '" ..... '" ..

Fig. 6. Geological and tectonic setting of 01doinyo Lengai volcano (based on the 1 :2,000,000 geological map of Tanzania, 1959). granitic Tanzan;an shield; f111l Mozambi que belt; 1:::::) Tertiary vo1canic rocks; , alkali basalt-olivine nephelinite volcanos (Pliocene); -0 nephelinite carbonatite volcanos (Pleistocene to Recent); ~ alka11 basalt volcanos (Pleistocene ta Recent). J.

(

~ c l , ~

-J':

(

were ejected onto the eroded surface of the older crater of Sinya

landare, created after an episode of minor NNW-SSE-trending faultfng.

20

The unit is mainly composed of pyroclastic material and phonolitic-

nephe1.1nitic lava flows. It forms a thick succession (305 m) well

exposed ,on the eastern, southern, and southeastern slopes of the

volcano; it builds the walls of the northern crater (Fig. 2). The

pyroclastic material blank~ted the area between Oldoinyo Lengai and

Kerimasi, and it is stili well preserved in the gorge of Sinya

landare. Toward the east, the pyroclastic materia1 was ejected as far

as the western flank of Lalarasi (Fig. 6), where it was found in

direct contact wit~ older basaltic tuffs. The ijolitic pyroclastic

material is exposed a1so on islands in Lake Natron (Fig. 6). The

pyroclastic material is composed of .tu.66.6 and a.gglomeJLa;(;e6.

COlllllonly, the .tu.66.6 aire massively bedded (6 m) and poorly sorted.

They are crystal-rich and consist of euhedral nepheline and rt acicular pyroxene crystals set in a fine-grained yellow matrix, r:'0w

composed of zeolites, limonite and carbonates. The agglomeJuLte6 ,are

composed of more-or-l ess rounded b10cks upto 30 cm across. The ejected

t blocks are randomly distributed, from very crowded ta very sparse, within the pyroclastic unit. They mainly consist of pi~es of

nep~linite and phonolite lavas, and of ijolite, urtite, melteigite,

jacup1rangite, bioti te pyroxenite and feni te. Certa in pyrocl astic

horizons are coated with sodium salts. "-

The interca1ated lavM associated with the ijolitic pyroclastic

unit are well exposed jlin the so-called Western Chasm as four

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-nephelinite flows and on the 10wer southeastern slope as a f1owof

feldspar nephel inite. A flow of phonolite is exposed on the

southwestern slope. Generally, the flows are massive (7.5 m thick)

on the upper slopes and thinner down the mountain. The lavs

belgng to the phonol ite-nephel inite association.

The first eruptive phase, during which a large quantity of lava

and ~c1astic material was extruded, is considered to be the most

active volcanic episode at Oldoinyo Lengai. It was characterized by

both explosive and extrusive types of volcanic activity.

GJte.y p.~Jtoc1a..&ti.c. uYlil: 06 pa!UU-Ltic eone6 and CMleJLO

This was a minor eruptive event, during which severa1 parasitic

cones were formed between 1220 and 1830 m altitude on the southern,

II. southwestern, eastern and northeastern slopes of the volcano. The

cones are composed entirely of pyroclastic material, crystal and

lithic tuffs and'ejected blocks of the older units. The L.;t}Uc. tLLU6

cons;st of lapilli of nephelinitic lava w;thinawhi~e carbonate

cement. The cty~tai tu66~ contain mica, pyroxene, nepheline and,

rarely, olivine. The elements forming the a.gg,tomelta.-teA are mainly

blocks of olivine basalt, ijolite and yellow ijolitic agglomerats.

1U.a.c.k pylLOc1o.J,c. wu..t

This sequence marks a second parQxismal volcanic ep;sode at

Oldo;ny'0 Lengai. The black pyroclastic material was deposited on the

extremely irregular surface of the eroded yellow pyroclastic blanket.

The unit is widely distributed on the lower s-lopes of the mountain,

and it extends westward against the rift margin (Fig-. 6). TO'the

21

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~ :J ~ j .' < )

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-south, it is limited by the northern slope of Kerimasi volcano. and

ta the east, by 01 der basattc hi11s.

The black pyroclastic unit grades from crystal and l1thic tuffs

ta agglomerate. The tu.66.6 consist of mica, nepheline and pyroxene

crystals Jxed with numerous lapilli of nephel inite, ijolite and

fenite, all cemented by carbonate. The very abundant nephe1initic

material imposes the characteristic black color. The a.gglomeJutte.6

have.a similar groundmass and contain crowded blocks of nephel inite, , J

phonolite, urtite, ijolite, melteigite, jacupirangite, biotite

pyroxenite, wollastonitite, fenite and s6vite. The nodules of fenite

22

that form the abject of the present study were most probably collected

from thi;p,.,.oclastic unit.

On the southern slopes of the volcano, there ;s a very unusual

horizon composed of a mica-rich tuff and numerous blacks of potassic

fenites (Dawson 1962 a). In places, thin lenticular beds of

carbonate-rich tuff are enclosed within the b1 ack tuff. The black

pyroclastic materia 1 has been ejected from the northern crater,

indicating the shifting of volcanic activity from the southern crater,

site of the ijolitic pyroclastic event. The .black pyroclastic

depositso

gradually filled up the southern crater.

Mei.a.nepheUnU:e eUJwJ,.{o n.6

This episode is mainly characterized by flows 6f melanephel1nitiG

lavas that contai" very few cogna te blocks of ijolite. The episode was

minor, leading to a flow of limited extent on the western upper s10pes

1 1

PM!) # .... JAXl! .d_ait."" ; t~""'"

~. (

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23

" and to the formati..olLof four paras; tic cones" al igned N-S on the

--~> northern lower slopes. The canes consist of scoriaceous melanephelinitic

lavas ..

GJr.elj /) em(. - .i.n.dwuLte.d .wu 04 06 :the. mode.Jr.n cycle 0 t5 'a.w..vliy The glt.elj tu-66/) overl ie the black pyroclastic unit on the -lower

slopes and the ijol itic pyroclastic unit on the upper slopes of the

Ivolcano. Usual1y, they consist of nephelinite lapilli and mica plates

cemented by carbonate. Overlying the grey tuffs, there is a

ubiquitous layer of b.ta.c.k cu.h consisting of nephelinite lapilli that is

covered by a layer of Mda a.6h. This layer, 6.1 m thick, is composed

mainly' of carbonatite, scarce nephel inite lapill i and scattered

crystals of pyroxene and nepheline. The soda ash ;s variab1y co1ored

in shades of light green. yel1ow. pink ,and white. (

The black ash and soda ash layers are the youngest products of

the vo1cano, except for the recent lavas. They contain ejectamenta

of biotite pyroxenite. biotite ijolite. ijo1ite, nephelinite and fenite.

THE RECENT LAVAS

After the explosive episodes of 1940 and 1941 (Richard 1942) and

1954 (Guest 1954), the style of volcanic activity changed fram

dominant1y gas-rich eruptions to extrusions of lava. During the ~

period 1960 - August 1966, the volcano has extruded only a1kali

carbonatite lavas. Its activity, during this time, has been described

in deta il by Dawson (1968).

, ,

(

24

..... In 1960, this type of lava was noted for the first time on the

floor of the active northern crater. The new cyc1 e began (Sept. 18 to

Oct. 8) with the extrusion of an extremely mobile lava, which

~ongealed in a "pahoehoe" type of flow. After Sept. 23, an increase

in gas content eventually led to fumarolic activity. On Oct. 8 and 9,

the change to a gas-rich lava was clearly shown by the "aa ll type of

extrusions and explosions at 5- to lO-second intervals. The surface

of the black lava flows was highly scoriaceous and blocky, and had

a rather cavernous aspect. On Oct. 10, lava disappeared from the

floor of the crater, and repeated explosions followed, during which

the ejected ash built up an almost perfect cone. According to Dawson

(1962). the change in lava type from llpo..hoe.hoe." to "a.ctll ""'eflects an

increase in gas content near the end of the volcanic episode. During

the 1960-1961 activity, the vo1cano extruded carbonatitic lavas

whose unusual minera10gy was dominated by Na-K-Ca carbonates; its

strange composition had been predicted many years before by von

Eckermann (1948) on the basis of his studies of the fenitized rocks

of Aln6 complexe The lavas from Oldoinyo lengai, referred to in the

, literature as natrocarbonatite (Dawson 1962, 196~ Cooper et d. 1975) J

or 1engaite (McKie 1966), have been considered for a long time simply

as a curiosity. The exact mineralogy of the natrocarbonatite lava has

been reported by Cooper et a..e.. (1975): microphenocrY5ts of nyerereite~a2Ca(C03)2 - fairchildite K2Ca(C03)2] 55'and (sodium ,

, , carbonate - potassium carbonate)5S enclosed within a microcrystal1ine

quenched groundmass of (NY-FC)ss' (NC.KC-CC)ss' minor fluorite. nahcolite

(

25

NaHC03 and pyrrhotite., The anhydrous l fquidus of the natural lava was

determined by the same authors as 655C at l kbar with nyerereite the ./'

1 iqufdus phase; nyerereite is jo;ned by (NC-KC-CC)ss at 620C, but the

solidus was not detennined.

During the period 1961-1966, al kali carbonatite lava was

continuous1y present on the floor of the northern crater (Dawson

1968). In August 1966, an abrupt change back to the explosive mode of

eruption occurred. The quiet extrusions of lava were replaced by

violent eruptionS' of carbonat~te ash. The crater was occupied by a

new ash cone with a sma11 double vent from which discharges of gas and

ash have continued. The recent ash layer C'onsists of euhedral

nepheline, melanite, zoned green pyroxene, minor apatite, titaAite,

wollastonite and magnetite crystals within a natrocarbonatite cement.

A few blocks of coarse ijolite and melanite-wollastonite-melteigite

were ejected.

Clearly. Oldoinyo Lengai is dominantly an explosive type of

volcano characterized by interludes of lava extrusions. The change

in eruptive style has followed a pattern: dominantlyexplosive

activity -+- ext,usion of lava ... back to explosive activity. The lava

extruded follows the trend phonolite'" nepheline phonolite .. nephelinite

... melanephel inite .. al kali carbonatite, exhibiting a continuous

increase in alkalinity and degree of Si02-undersaturation.

~ THE ROCKS OF THE PRESUMEV PLUTONIC SUITE

,Gmong the numerous blocks ejected during t}1e repeated explosive

episodes, a wide range of rocks of plutonic aspect and alkaline

-..

(

26

composition have been identified (Guest 1954, Dawson 1962, 1968, 1978). .. This fact suggests the presence of a plutonic or hypabyssal alkalfne

, -complex beneath the vol cano. The ma i n types of ejectamenta have been

attributed:to a number of alkaline series by Dawson (1962, 1968).

UlLtUe - j a.cup.Uta.ngUe .6 eJt.ieA

Blacks re1ated to this suite have been identif1ed in both

yellowand black agglomerates. The suite comprises urt1te, ijolite,

me1teigite and jacupirangite. Its main mineralogy is very simple,

consisting of variable ratios of nephe1ine and pyroxene. The

accessory minerals are: titanite, apatite, melan1te, biotite,

titanomagnetite, wol1astonite, calcite and, very rarely, 01 ivine.

The pyroxene varies in compositin from aegirine-augite a'nd aegidne

in urtite and ijolite to diopside and diopsidic augite in melteigite

and jacupirangite. The most conrnon accessory mineral is apatite,

euhedral in urtite and interstitial in melteigite and jacupirangite.

Titanite usually shows an antithetic re1atio,11ship with titanomagnetite

and melanite. Wollastonite and olivine are very rare; the kelyphitic

rim around the 01 ivine sggests its xenocrystic nature:

Wol.i.ald.o~e and woll.tutani..te ..i.jollie

These rock types have been recognized among the black

agglomerates and the blocks ejected during the eruption of

natrocarbonat1te ash. The wollastonitite consists of rad1ating masses

of wollastonfte (95% by volume) and small interstit1al crystals of

nepheline, apatite and, rarely, glass. The gradation in sorne

-, - - - --~ -----

, 1 1 :

. T-, !

1

c

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27 ),1 , instances from pure ijoli te to w.ol1astonlte-rid,' Uol ite wlth'in singl e' . .,. spet imens i s bel ieved to reflect local calcium metasomatism of . , pre-existing ijolite (Dawson 1968).

, 8.to.tUe pYltoxenUe , . Blacks of biotite pyroxenite are present in both yellow and black

agg1 ornera tes. They are ma 1nly composed of diopsidic- augite or

augite,'with biotite and magnetite as the only accessory minerals.

Ferr..Ue

The fenite 'blocks are reported to be 'ubfquitous; they have been

ejected during all pyroc1astic events, except in the 1 ast eruption of

the natrocarbonatite ash. The fenites are composed essentia 11y of

variable ratios of feldspar and al kali pyroxene. Usually, the

pyroxene occurs in veins or clots. The fenites dfsplay a

heterogeneous appearance owing to different grain sizes and textures \

in the same specimen. Nepheline, wollastonite, titani te, apa tite and

pyri te are the cOlTl11on accessory phases. From mineralogical and

compositional points of view, the rocks- (!ttributed to the fenite

,group form a suite of al kali syenite - nepheline syenite -

melanocratic nephel ine syenite; texturally, however, these metasornatic

products di ffer markedly from igneous rocks of similar compOs 1 tion.

Detailed descriptions follow.

,/

-, - -0:_- --.-..,.. .... -~-----____ ~ ____ l._t._. _"_. _ .... 1 ___ '_. _, __ ""._._._ ....... ___ ,11

1

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28

Chapter~. PETROGRAPHY

A careful petrographie study of fenfte xenoliths can yield jnfor-

mation regarding their original 1 fthology. the superimposed metasomatic

parageneses, and the fenitization reaetions active in the Oldoinyo

Lengai volcanic complex. 0 In addition, the specimens of high-grade fenite

provide data on the phenomenon of rheomorphism, which has never been

properly doeumented in such metasomatic assemblages. '

The nomenclature of fenites is rather imprecise and confusing

owing to the eomplexity of the process, ref1ected in the w;de variety

of resulting associations of minerals. The factors responsible for

this diversity are: 1) the source and, hence, the composition of the

fenitizing fluids, 2) the different types of rock affected, 3) th,e

depth and the temperature of the process. The classic scheme of .

nomenclature (von Eckermann 1948) is based on the extent of removal

of ql,lartz, which parallels the increase in grade of fenitization; this

scheme features terms such as quartz syenite fenite, alkalic syenite

fenite, nepheline syenite fenite and alkalic ultrafenite. The above

nomenclature is very adequate for fenitized rocks that initially

contained a certain amount of quartz. Other authors (e.g.~$utherland

1969) prefer to stress the rock being fenitized, using tenns such as

fenitized granite, fenitized rhyol ite, etc., which are not very ,

indicative of different grades in the overal1 conversion to a rock in

equilibrium with the incoming alkaline fluid medium. The system of

nomenclature based on the ratio of melanocratic to leucocratic minera' s

-'

1

1 j ,"

1-

r ~

t ~ , ~ F ~

~ l

, > ,

~ -- --------- - ---~----------~---~~~/r-

~

29

maireflect progressive fenitization, though evidently nct in the

case of initially mafie rocks. A system of nomenclature more directly

related to a metamorphic petro10gy approaech, based 01\. grade of

feniti zation and mineral parageneses, has been advocated recentl y

(Vartiainen & Woolley 1976, Le Bas 1977). These authors tend to

."" separate the proeess into a sodic or IInormal" trend and a potassic

trend of feni ti zatipn.

T~e scheme of nomenclature adopted in this study is based on the

increasing grade of fenitization as reflected 'by the new parageneses

and textures. The categories used are low. medium, and high grade.

Low-gltade 6erU..te6, the equi.va1ent of zone 1 to zone II of von Eckermann . . . (1948), contain identifiable primary minerals and exhibit textures

attributed largely ta deformation. In medJ..um-gltade. 6e.ni:tu (zone'TlI

of von Eckermann), the primary minera1s are searce or entirely

ob litera ted. The prevalent textures are due to replacement and

recrysta11ization. H~h-gltade 6e~~ (zones IV to V of von Eckermann)

typically contain nepheline and display textures attributed to

recrystall izationi these g;ve the rock an "igneous" appearance. The

swirl ing and f1 uxiona 1 textures in hi gh-grade fe~ites provide ev; dence

of remobil ization. Conta.c..t 6en.Uu constitute a special group dominated

by evider'lce of hi gh-temperature metamorphi sm.

'. Note that two types of rock, granitic gneiss and metagabbro'-

have been identified as starting points of fenitization reactions at

01 do i nyo Lenga i .

, , 1

M

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1$1$ 44SS

. ,

--- ~ .. ~ ~ ... ~ ~ ...... ~.

30

L~ .. GRAVE FENITf.

GJr.a.nUe. 9 Yl.W~

In view of the identifiable assemblage J)f primary alkali feldspar

+ quartz + aligo~clase + biotite, specimen BD-58 1s classified as

granitic gneiss in an inc'ipient stage of fenitization. Macroscopically,

the rock. has a very" heterogeneous appearance owing to the sinuous

veinlets of-green pyroxene developed within a leuco_cratic matrix

composed itself of grains of variab1e size (Fig. 7). An estimate of

the modal proportion of minerals is listed in Table 2. Under the micro-

scope~ the most conspicuous feature is the heteroblastic texture, 1

which was generated by deformation and different degrees of repTacement.

Large (2-3 mm) fragments of orthoclase and oligoclase are surrounded

by a very heterogeneous, fine-gra ined, feldspathic matrix, suggesti ng

a mortar texture (Fig. 8). Commonly, the contacts between grains are

sutured. The oligoclase grains display displacement of the twin

lamellae and undulatory extinction; in addition, the olgoclase i5

slightly turbid. The heterogeneity is amplified by a Juxtaposed

~work of veins, veinl ets and patches of green aegir1ne and

wollastonite (Fig. 9). The quartz grains, very fractured and granulated,

are surrounded and penetrated by small prisms of aegirine and by

fasclcul a te aggregates of woll astonite. The aeglrine forms

. ..,) pale green, aC1cular-. randomly oriented prisms within orthoclase,

or 2) cl usters j subidioblastic-xenoblastic grains enclosing quartz or pools of feldspar. Spa-rse flakes of biotite are partly replaced

by a very fine-gNined aggregate of feldspar (prsumably potassium-rich), ti

la

1 1 . 1 1 , ,

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,

Fi g. 7. Photograph of low-grade fenite of metagran1t1c ancestor. Hetwork of sinuous veins of aHal1 pyroxene and wollaston1te developed with1n a leuco-cratic matrix of feldspar and quartz .

--------------------~--~--------------------------

31

" " 4

llIUE

t (0

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,

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TAB LE 2. ESTIMATE OF THE MODE* OF LOW-JjRADE FENITE FROM GRANITIC GNEISS ANCESTOR

Mineral BD .. 58

Alkali feldspaF.~ 63 " Pl agiocl ase 4

Aegirine 20

Biotite 1

Quartz 1.5

Wollastonite 7

Ca 1 cite 2.5

Opaque phases 1

Apatite ~ trace

Anorthtte % 15

Orthoc1ase % 21 1 , 97 2

Method of optical data An, Or** estimate microprobe ana1ysis

X-ray ~iffraction**

Points 1200

* Modal--analysis (in vol. %) by a combination of visual estfmate~ n section and point count.

l Anorthoclase, 2 low anidine

32

i L 1 j

( ,

Fig. 8. Fragments of K-feldspar engulfed within a fine-grained feldspathic matrix reminiscent of a mortar texture. Gontac~ between newly formed grains are sutured. Crossed nicols. Width of field 3.3 mm.

Fig. 9. Heterogeneity;s ampl ified by a juxtaposed . network of veinlets and patches of aegirine and wollastonite. Crossed nicols. Width pf field 0.85 mm.

33 1

, f

1 l 1

J 1

1

1

1 1 1

f . ; ,

t

1

1 1 (

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,

1

iron oxides and, subsequent1y, small pri'SIlli of aegirine (Fig. 10).

Calcite occurs in ule cores of the veJnlets or forms xenomorphic grains. The relationship between aegirine and wo11astonite is ambiquous. In

some veins, the wol1astonite forms epitactic overgrowths on aegirine,

but the opposite also occurs. There are thus possible indications of

multiple metasomatic events in specimen BD-58.

The textural featur~s and the mixed assemblage of primary and

replacement minerals suggest arrested reactions, a case of

disequilibrium, and astate that characterizes an incipient stage of

feni ti za tion.

MemgabbJto

Specimen BO-44 is a homogeneous, medium-grained, subophitic,

black-and-white mottled rock (Fig. 11). The signs of deformation are

here inconspicuous. Microscopic investigation indicates a

monotonous minera1 assemblage, composed essentially of amphibole and

plagioclase, and of minor amounts of biotite and ti tanomagneti te.

Modal compositions are presented in Table 3. The olive green-to-brown

amphibole. determined as magnesian hastingsitic hornblende by

electron-microprobe analys i s (nomencl ature of amphi'bol es: leake 1978),

occurs as glomerometacrysts that presumab1y have rplaced and formed

pseudomorphs after pyroxene (Fig. 12). The subidioblastic

interstitial plagioclase, determined to be, andesine by microprobe,

exhibits a granoblastic-polygonal texture with almost planar

triple-point junctions. The blasto-ophitic or subophitic texture is

il1ustrated in Figure 13. The titanomagnetite occurs as skeletal

( ,

1 1 ( ,-

f

1

1 J

,1 , i t

(

f ! 0

Fig. 10. Biotite replaced by fine-grained K-feldspar. iran oxides and, subsequently, by small prisms of aegirine. Crossed nicols. Width of field O.85mm.

(

)

35 , 1 1 i

! 1 1

(

Fig. 11. Photograph of law-grade fenite af metagabbroic ancestor. Mottled, subophitic, gabbro;c texture.

(

36

1

J

(

Cl

TABLE 3. ESTIMATE OF THE MODE OF lOW-GRADE FENITE FROM METAGABBRO ANCESTOR

~ Mineral

Plagioclase Hornbl ende*

,Aegirine-augite

Biotite Ti tanomagnet i te

Ca lcite Apatite

Titanite

Anorthite %

Method of An estimate

Acmite %

Method of Ac estimate

points

BO-44

33

34

21 '"

, 5

3

2

l

1

25-36**

o.ptical data microprobe analysis

26-54***

microprobe analysis

1000

* Magnesian hastingsitic hornblende, as deter-mined by microprobe. **,Compositional field oligoclase-andesine. *** Variation from core to rim or between differentgra1ns.

37

()

Fig. 12. Glomerometacrysts. of magnesian hastingsitic hornblende that forms pseudomorphs after pyroxene. Plane 1 ight. Width of field 3.3 II1II.

Fig. 13. Blasto-ophitic 'or subophitic texture. The amphibole is rimmed or partly replaced by green aegirine-augite. Plagioclase 15 slightly turbl~ Plane l1ght. Width of field 8 mm.

38 ,

/

crysta l s mantl ed by rims of titanite (Fi g. 14).

In the light of compositional and textural features, specimen

80-44 has been cl ass i fied as a metagabbro. According ta Appleyard

(1981), a gabbroic rock is considered a metagabbro "when it has been

converted predominantly lnto a hornblende-plagioclase gneiss in

which aggregates of hornblende crystals occur in a matrix of

polygonal andesine". The polygonal texture of andesine is probably

the response of the feld"Spathic laths to dynamic metamorphism.

The metagabbro shows signs of very mild cataclasis and super-

imposed fenitization. The conspicuous (though mild) shattering and

mortar texture, so characteristic of the granite gneiss in an

incipient stage of fenitization, do not occur in the metagabbro.

39

The feldspar appears slightly turbid; the degree of turbidity progresses Ij

from the margins toward the core of the grain, along cracks, cleavages

and t\yin planes (Fig. 15). Small, aCicular, pale green prisms of

sadie pyroxene are developed within the andesine, along cleavages and

boundaries (Fig. 16). Calcite occurs in the core zone of the feldspar

grains and at triple-grain junctions. Generally, calcite is rimmed

by aegirine-augite. The amphibole is always rimmed by radiating prisms

of sodic pyroxene (Fig. 17). In sorne portions, it is replaced byan

agregate of brownish yellow biotite and iran oxide that later was

replaced by xenoblastic grains of aegirine-augite (Fig. 18). The

aegirine-augite, apple-green in color, forms acicuiar prisms,

xenoblastic grains or clusters of stout xenoblastic pr;sms.

,.

1

(

(

"

Fig. 14. Titanomagnetite mantled by rils of titanite. Plane light. Width of field 0.3411111.

, Fi g. 15. Turbf di ty l'rogres-ses from the-lI1argins towan.l -

the core of plagioclase, generally along fractures and cleavages. Plane light. Width of field 0.85 mm.

40

J 1

,

1 \ "

(

(

J / (

Fig. 16. Small prisms of pale green sodic pyroxene develop within the plagioclase. Plane light. Width of field 0.34 mm.

Fig. 17. Amphibole (yellowish) and biotite (dark brown} rimmed by radiating prisms of sodic pyroxene. Crossed nicols. Width of field O~85 mm.

(.

41 r ~ . ,

o

f

Fig. 18. Amphibole replaced by light brown biotite and iron oxfdes; the biotite later 15 replaced by xenoblastfc grains of apple-green aeg1r1rie .. augite. --.... Plane l1g~ Width of field 0.85 mm.

- .. ,

, ,

m

42

p

1

. .

\ " . .; , '

(

( ,

43

/

Inconspicuous dark flakes of primary biotite that pr'edate fenitization

are rimmed or entire1y rep1aced by the same acicu1ar prisms of sadie

pyroxne (Fi g. 17).

The pattern of fenitization in metagabbro, as i11ustrated by spe-

cimen 80-44, is different ~rom that in the granitic gneiss. Textures

generated by deformation are a1most absent; textures due to replacement

are prevalent. There is widespread formation bf aegirine-augite at the ,

expense of several minera1s, including the ones that possib1y indicate

early mild K-metasomatism. According to the large proportion of

identifiable primary minera1s and ta the slight modification of the

feldspar, the metagabbro is class;fi~d as a low-grade fenite.

MEVrUM-GRAVE FENITE

GJta.n.Ue. gneA.6

Specimens BD-5, 32 and 48, classified as medium-grade fenite, are

still anly slightly fractured and veined, ~nd display a heterogeneous \

appearance. Wider veins (1-2 cm) but few veinlets and clusters of ri

green pyroxene are dveloped within a fine-ta medium-grained feldspathic

groundmass (Fig. T9). With progressive fenitization, the pattern of

veins is obliterated, and the green-and-white specimens exhibit a more ,"1'

homogeneous granul ar ap'pearance (F.i g. 20).

In thin section, the higher grade of fenitization is indicated by

widespread recrysta}lization and bY-he monotonous mineral assemblage,

composed of newly'formed aeg1r1ne-aug1te and a~orthoclase. Modal

(

, q

f i

'j

1 ~ i ~

f j

f l

f 1 , 1 1

1 1 1 1

1 f 1 1

! ~ 1

. ,<

,.

i ~"1

" ~. li JI (," " f ! \.

i

t

.(

Fig. 19. Photograph of medium-grade fenite (BD-32).

Aegirine-augite forms wide veins within a fine- to medium-grained feldspathic groundmass.

Fig. 20. Photograph of medium-grade fenite (BD-48). More homogeneou$ appearance, but some veinlets ~111 occur.

44 o , j

..,..

o

, , - -~ ~ - ~ -_..... --. ............ _-...-- ~ _.I_-_ .. __ ......... ~~ ....... ~

45

. , estimtes of the m1nerals present are presented in Table 4. The quartz

in the transformed ~anitic' gneiss, almost entirely digested by

'- aegirfne-au~ite, has been identified in specimen 80-32, from the lower

limit of the scale of medium-grade fenitization. It oceurs as small

pools, enelosed within clusters of xenomorphic aegirine-augite. Relies 1

o

of the original feldspar are still present within a matrix of

recrystallized feldspar (Fig. 21). The relict oligoclase exhibits

faded twinning and slight turbidity (Fig. 22). The aggregate of new

feldspar displays a granoblastic texture with "triple-po{nt" junctions~

but still locally with sutured or curved boundaries (Fig. 23). With

1ncreasing intensity of fenitization, larger', pOikiloblastic plates of

alkali feldspar seem to form by coalescence. The outlines of the ,

smal~er grains are still identifiable. The poikiloblastic grains

contain inclusions of aegirine-augite, titanite and, rarely, prisms

of apatite. Aegirine-augite forms larger subjdioblastic and

poiki1oblastic' prisms or clusters of stout prisms that enclose pools of

feldspar (Fig. 24). Bladed prisms9 0f wollastonite display a decussate

texture. Aegirine-augite replaces, peri,pherally or zonally the

wollastonite. Replacement of aegirine-augite by wo1lastonite has a1so

occurred. Usually, idioblastic ta subidiob1astic grains of titanite

are concentrated in the pyroxene aggregates. The medium stage of

fenitization i s characteri zed by the remova 1 of the original mineral s,

leading to their disappearance. The textures might disp1ay hints of N--

cataclastic events, but almost entirely ref1ect rep1acement-~nd

r

"

\

..

. ,

TABLE 4~ ESTIMATES OF THE MODE* OF MEDIUM-GRADE FENITE OF GRANITIC GNEISS ANCESTOR

Mineral

..

Al ka 1 i feldspar Aegirine-augite

Quartz Wollastonite

Calcite l Titanite ,

Apati te .

Orthoc 1 a se %

Method of Or estimate

Acmite %

Method of . Ac estimate

Points

.

60-32 80-48

38 47 34 40

2

8 5

5 4

l 3

2 l

29** 28**

X-ray diffraction microprobe analysis

. 57 56

microprobe analysis

... 1200 1000

* 8y combination of visual estimate on th;n section and point count. ** Anorthoclase.

J

Il

-- ---_ ... -_._-46

/

- 1 i

1 , )

1

.1

~

~

t , , 7

i

i f r ,

1 t ~

1 r 1 '

1

(

( ,

Fig. 21. Searee relies of the original feldspar are still present within a matrix of recrystallized feldspar. Crossed nicols. Width of field 0.85 l11li.

Fig. 22. Relict oligoclase exhibits faded twinning and slight turbidity. Crossed nicols. 'IHdth Of field 0.85 II1II.

--_.-47

, ,

! 1

l f li

l (,

Il

______ -.. __ ~ _____ t"",.~_~lft

1.

, Fig. 23. Granoblastic texture with "trip1e point" junctions. locally, the sutured or curved boundaries are still present. Crossed nicols. Width of field 0.85 mm.

Fig. 24. Pools of rel iet feldspar enc]osed within clusters of larger prisms of aegirine-augite. Plane light. Width of field 0.85 mm.

48

1

j 1

(

r " j 1 f (;

l'

l j. } , ;

l f !

( i , f

1

recrystallization. The new paragenesis is essential1y comp?sed of

anorthoclase + aegirine-augite. The prtvalent granoblastic texture,

with triple-point junctions, the decussate texture, as wel1 as the

mineral assemblage, seem to indicate a tendency toward equilibrium in

the transformed system. However, the presence of relict miherals,

49

the sutured boundaries and common poikiloblastic textures sugg~st that

equilibrium in fact has not been attained. ,

Specimens of metagabbroic medium-grade fenite are not present

among the collection available.

H~GRAVE FENITE Jhe criteria used for the classification of a fnite as high-grade

1

are the complete obliteration of the primary minerals, the appearance

..Qf...l1ephel ine, and the coarsening of the te~ture so as ta aproach that of

an i gneous roc k.

Ne.hiline- be.aM.ng M.ni..di.tte. a.eg.iJr1ne. -augUe 6e.nUe

This type of fenite probably represents the max;um transformation

achieved by a granitic rock interacting with the feJtizing f1uid. In

hand specimen, the rock is medium-to coarse-grained and exhibits a ,

slight foliation. Metacrysts of dark green pyrox~ne, surrounded by a

glasiY film. are scattered throughout the rock in a more or less

swirling pattern (Fig. 25).

Petrographie investiga~ion indicates a very simple paragenesis

formed by idioblastic elongated crystals of sanidine and

poikiloblastic metacl"ysts of aegirine-augite. No relies of primary

- , i ~ ! , li f

1

(

Fig. 25. Rheomorphic high-grade fenlte (BO-43). Metacrysts of sodic pyrox~ne, surrounded by a glassy film. and dlsposed in a more-or-less swirling pattern. Pale olive green glass 1s scattered throughout the rock.

,.

50

(

51

minerals are present. An estimation of modal proportions is recorded

in Table 5. COll1l1only. the sanidine is clear and Carlsbad-twinned. It

forms elongate, flattened crystals or smaller laths pre~errentia"'y

oriented in a flux reminiscent of the trachytic texture (Fig. 26).

These fluxional alignments of elongate crystals might be signs of

displacement in an interstitial mel,Land crystallization while movement

was still possible under direction'1l stress. The metacrysts of

pyroxene show signs of partial melting. They exh1bit strongly embayed

margins and are surrounded by a film of pale green glass (Fig. 27).

Their sieve-like aspect is presumably due to melting around the

enclosed inclusions and extraction of the melt, leaving small holes

occasionally still filled with green glass. The aegirine-augite is

partially transformed to hematite, which may be the product of the

incongruent melting of the aegirine component (Fig. 28; Bailey &

Schairer 1969). In some cases, the laths of feldspar appear molded

against the adjacent pyroxene crystals; the pyroxene possibly behaved

as aosolid phase within the mobile phase. Nepheline is not very

cOll1l1On; on1y a feweuhedral to subhedral grains'hve been recognized.

Commonly, idioblastic grains of titanite are enclosed within pyroxene

or between two pyroxene crys'tals. Where it is not poikiloblastically

enclosed by pyroxene, the titanite is shattered and displays irregular,

embayed margins as if it also has been involved in the melting

reactions (Fig. 29). The pale green glass is ubiquitous, occurring ,

intersertally between aegrine-augite and sanidine, and along fractures

and cleavage planes in feldspar. Usually. along fractures

it is associated with wollastonite. More rarely, the glass fills small

.1

i ~

1

f

( 1

(

TABLE 5. tSTIMATE OF THE MODE* OF HIGH-GRADE 1 FENITE OF GRANITIC GNEISS ANCESTOR

Minera 1

Al kali fel dspar ., Aegfrine-augite

N~phel ine Wollastonite

Hematite Apatite

Titanite

Glass

Orthoclase %

Method of Or est1mate

Acm1te %

\

80-43

46

28

5

1

8

2

3

7

100**

4,! optfcal data X-ray diffraction

1

30-46

J

Method of Ac est1mate

microprobe analys1s

Points 400

* 8y visual estimate on thin section and point count. ** High sanidine.

Q

..

, - ~-~-------~- -----

52

t

Fig. 26. E10ngate crysta1s pr~ferrentially oriented in a flux reminiscent of the trachytic texture. Sanidine is cOl1lOOn1y Carlsbad-twinned. Crossed nicols. Width of field 3.3 RIO.

... 1 ..

, Fig. 27 1 Aeg1rine-augite exhibits strong1y embayed

marg1ns and is surroundedby a film of pale green glass. Note also the sieve-like aspect. Plane 1 ight. Width of field 0.85 ntn.

---

.'

53

:

1 ! 1

1 , , 1

, 1 1 ,

1 ,

.

-

i 1 " 1

1

1

c

Fig. 28. The aegirine-augite is partly transformed to hematite. Plane light. Width of field 3.3 mm.

Fig. 29. 'Shattered titanite di spl ays irregul ar, embayed margins, as participating in the melting reaction. Plane light. Width of field 0.85 II'1II. -.

54

" .

, . i , , ,

i 1

1

i

i 1 !

(,

holes inside the pyroxene (Fig. 30). }

Th~ poikiloblastic texture of the aegirine-augite reflects the

tedenCy of this mineral ta recrystal1ize -lnto larger porphyroblasts

during high-gr:ade fenitization. Generally, larger crysta1s represent ~

a mare stable phase. Le... have a lower free energy than countless

smaller grains, suggesting a tendenc~toward textural equilibrium;

however, inclusions increase the total free energy (Spry 1969), and

thus the paikiloblastic crystals do not mari< the most stable condi-

tians of crystall;zation. )

In summar:y, the fenite described above is a high-grade fenite,

considered to have formed at the expense of a granite gneiss; it ,

55

has undergone also partial melting and mobilization (~.[., rheomorphism).

The packing together of the e1ongate,Jeldspar crystal s in an almost

parallel alignment is probably due to flow within a partially molten

rock.

Sa~n~-b~n9 n~ph~n~ aeg~ije-~it[ 6~nite

Specimen 80-42 is rega.rded as the final product of fenitization of a metagabbroic ancestor. Macroscopically, the specimen has an

almost homogeneous appearance of a medium-grained, green-and-white

rock. rts h~mogeneity is locally disturbed by finer-grained mafic

patches (Fig. 31).

The mineral association i s aegirine-augite + nephel ine + sanidine. ,

The estimate of modal proportions 'of minerals is presented in Table 6.

The aegirine-augite is green to brown, and 'usually it forms stout,

idioblastic to subidioblastic prisms. Its poikiloblastic texture is

1

-----;------~~.- ... --~~J

i t l ~ j

i

i \ ! 1 ,

--: ,

0'

,

1

()

"". , ,

--~ .....,-;-v ...

"

"""'~ ."" -. , ~ #

,

Fig. 30. Rarely, the olive green glass f1lls smal1 holes within the pyroxene. Plane l1ght. W1dth of field 0.85 II1II.

r

56

d JI

---_._-57

o

Fig. 31. Photograph of htgh-grade fenite of metagabb~ofc , ancestor. The .homogeneous appearance of a mediam- 1

grained. dark green and white rock is local1y disturbed ~ by f1ner-grained mafie patches. ~7 _

, ~

//~ ,/

1

1

"t-.~ . ,

t

1 1 l

0

Q

-58

r

TABLE 6. ESTIMATE OF THE MOOE* O~ HIGH-GRADE FENITE OF METAGABBR01C ANCESTOR

Mi nera 1

Nephel ine Aegirine-augite

Alkali feldspar

Hemati te

Apatite

Titanite

Glass

Orthoclase %

Method of Or estimate

Acmite %

Method of - Ac estimate

Points

II

80-42

.~ 4

2

6

trace

97**

optical data X-ray diffraction

32-50

micl"oprobe analysis

700

* Visual estimate on thin section and point count. ** High sanid1,ne.

,

Il''

" Il

-) (

, ,

1 .

\ ,/ 1 t

( '."

/ given by the numerou"s inclusions of.titanite, nepheline and feldsp~r,

probably incorporated during the recrystallization of pyroxene into

larger crystals. Nephel ine is conspicuous, occur~ing' as idiablastic,

rarely xenomorphic crystals, with a slightly tu"rbid appearance owing

to minute inclusions (Fig. 32). Sanidine forms inteT'stitial r

~

xenomorphic grains or sorne larger subidioblastic plates; Carlsbad

twinning ias common. Titanite is here very abundant, and seems to '--.

belong to two generations: one type is idioblastic, with the very

characteristic wedge-shaped crystals, and the other is subidioblastic

and poil&.,iloblastic, containing ihclusfons of feldspar and aegirine-

augite (Fig. 33). "

Severa] features, such as: 1) the preferred or.ienhtion of

some crystals into an alrnost parallel alignment, 2) the embayed \

margins (Fig. 34), 3) the web-like texture (Fig. 35L 4) the

presence of two generations of titanite, migh~ suggest the remobil1-

zation of the rock in a"mOre or less plastic state. If .

Accord; ng to le Bas (1977), a spec ; al group of ra ther homo~eneous, l. ,

very leucocratic rocks that d,splay granulation to a fine to medium

grain-size and have an elevated content of feldspar (more thah ,85%), ,

have ben classified as contact feni~e or perthosite (Fig. t6). The modal proportions in representative specimens (80-35, 55) are

recorded in Table"7.

The petrQgraphic in,vestigation has stressd the obvious

difference b~twen this type and the previously described fenites.

,\

.>

-

o

, l>

j r

, !

'f

r j ,

1 ( ,

, \

~

..

-

Fig. 32. Idioblastic to subidioblastic nephel ine, rtightli turbid owing to minute inclusions. Plane light. Width of field 3.3 mm.

Fig. 33.~ Subidioblastic titanite that poikiloblastically encloses inclusions o~ aegirine-augite and feldspar. Plane 1ight: 1,t11dth of field O.85fn.n. '

.. :. =-!

! 1 Ct r t

f 1

!

(:

Fig. 34. Pyroxene displays -embayed ma~gins. Plane N light. Width of field 3.3 mm.

v

Fig. 35. Texture reminiscent of the ,web texture. Grains with rounded contours occur within the brown glass th~t 1s sl ightly deYitrified. Plane 1 ight. Width of field 0.85 l11li.

61 , f 1

o

o

')-.r

1 ..

_ ____ ..... ____ ,J , ,

-~

f:

Fig. 36. Photograph of contact fenUe. - Leucocratie, fine- to medfum-gratned rock contafnfng more than 85~ fel dspar.

-.. "

. /

.~----.... ,~ 62

/

- ,

o

"

(:

TABLE 7.

Mineral

Pl agioc1 ase Al kali fe1dspar Wo llaston i te Aegirine-augite Melanite Titanite Calcite Glass

Anorthite %

Method of An estimate

Orthoc l a se %

Method of Or estimate

Acmite %

Method of Ac estimate

Points

ESTIMATES OF THE ~tONTACT FENITE

80-35

.. 28 58

6** 3

trace l 2 2

18***

optica 1 data microprobe ana1ysis

8****

optica1 Tata microprobe analys;s

50

mtcroprobe analysis

600

f

BD-55

35 51 8** l 1

trace 2 2

15***

opt ica 1 da ta

11*****

X-ray diffraction optical data

500

* By visual estimate and point count of th;n sections, ** includes pectolite, *** o11goc1ase, **** albite, ***** anorthoclase.

/1

"

Il

fi

63

r

1

(

The main minerals here are plagioclase and alkali feldspar. Their

texture is granular-polygonal with sutured tri~le-point junctions.

Th' turbidity and exsolution lamellae in feldspar are very

conspicuous. Some of the more calcic plagioclase (oligoclase)

grains exhibits a very cloudy aspect due to turbidity (Fig. 37). In

addition, the oligoclase displays rims of albite or veins of albite

alon9 the twin planes. Orthoclase, slightly tur~id, 1S marginally

replaced by a chequer-board twinned albite (Fig. 38); a similar

r-esult of albitization has been mentioned by Sutherland (1966) .. Here

and there, the contacts between grains are dislocated and filled with

a thin film of calcite. Very few, small subidiomorphic prisms of

aegirine-augite are enclosed by orthoclase or are located a10ng the

boundaries or at triple points (Fig. 39). Wollastonite, in part

altered to pectolite, is situated in similar positions. Its

relationship with aegirine-augite, replacing it or being zonally or

even poikiloblastically replaced by it, suggests successive events of

metasomatism (Fig. 40). It seems that the minor phases ,

(aegirine-augite, wollastonite) in a granulqr-polygonal texture tend (

ta lie a10ng the grain boundaries of the major phases. Nucleation

seems favored at grain boundaries,triple points and dislocations

(Christian 1965); also, crystal growth ;s faster owing ta easier

diffusion along these discontinuities. Small ~ idioblastic ta

subid1ablastic grains of brownish melanite are enclosed within

feldspar, commonly in the albitic rims.

The albitizatiori of orthoclase in the contact fenites seems ta

indicate their formation under high-temperature and by interaction j-

:!

o

1

le r f l \

t ! ,

- . ---- - -----

Fig. 37. Cloudy aspect of. plagioclase owing ta the enhanced turbid1ty. Plane light. Wldth of field 3.3 11111.

~ Fig. 38. Slightly turbid orthoclase is replaced aiong its margins by a chequer-board tw1nned albite. Crossed nicols. Width of field 0.85 mm.

65

1

()

Fig. 39. Small -grains of aegirine-augite (yellow) are located.at triple-point junctions or along the boundaries of feldspar grains. Crossed nicols. Width of field 3.3 mm.

Fig. 40. Wollastonite replaced along its grain margins by aeg1rine-au~ite. Plane light. W1dth of field 0.85 ml. ,

66

.,

1

1

1 t )

()

(f

. with a fenitizing fluid displaying a low K/Na ratio, condHions which

have permitted the sodium felds~ar to become the stable phase. The

mineral assemblage aegirine-augite + wollastonite + melanite suggests

?lso high-temperature equilibration of the granite gneiss with

possibly alkali carbonatite, vicinal to the contact. During the

equilibration the melting also occurr~d, preserved as poorly developed

patches or films of pale green glass, intimately associated with

wollastonite in specimen~ 80-35 and BD-55. Here and there the glass

is devitrified (Fig. 41).

VISCUSSION

. The results o

t i \

t 1 1

1

1

i l 1 1

, i 1

1

( ) "-

(;

..

Fig. 41. Pale green glass associated w1th fasciculate wollastonite. Plane 11ght . W1dth of field 0.85 l11li.

1

68

1 \ \

5 -, t l

t

;

f i r ! , .'

t 1 !

( i

69

..

2) Most probably, the actual minera10gy of the xeno1iths has formed

as-a result of repeated-metasomatic events. The principal ~sode was

a "normal/l fenitization in the sense that the main metasomatic minerals

are eharacteristieally sodie; the mild K-metasomatism is not exeluded

from this episode. A superimposed ealcium-rieh metasomatit event is

suggested by the presence 6f wollastonite. In 1962, Dawson referred

to a similar event, whieh had locally transformed the ijolite into v ,

wo1lastonite-ijolite. The double relation host-metasome, metasome-host

exhibited by the pair of minerals aegirine-augite - wol1astonite might

indicate that the ea1cium-rich metasomatie event was interca1ated

betw,,~ two I/normall/, episodes of fenitization. The abundance of sanidine

in some high-grade fenites, even formed at the expense of metagabbro,

suggests a potassium-rieh episod~ which had affected~loca11y the ~

previous soaic fenites. 3) The two ancestors identi~ed for the fenite xeno1iths. granitic

gneiss and metagabbro, have reacted somewhat different1y during the

process, despite the fact that the u1timate products of fenitization

contain essentia11y the same paragenesis: sanidine + nephe1ine +

ae9iri~e-augite. Currie & Ferguson (1972) mentioned a somewhat similar

convergence concerning the fenitized felsic and mafie rocks at

Cal1ander Bay, Ontario. ~

The catalasis of the country rocks, preceding or accompanying the ..,..

early fenitization {Siemiatkowska & Martin 1975~, has affected the two

lithologies different1y. The metagabbro resisted catac1asis, whereas

the granit;c gneiss was fractured, shatter~d and granulated. ~

1 1

( f -

..

70

Note that among the few specimens studied, no examp1e of a

medium-grade fenite derived from a basic ancestor was found. Le Bas

(1977) has mentioned that in the fenitized diorite from Usaki, the

outer zone is very narrow, and zone III, which corresponds ta

medium-grade fenit, was not recognized. In fenitized mafic rocks,

the inner zone seems to be deve10ped to a greater-tha~-usual extent.

Anather distinguisning feature af-fenitization of basic and

fe1sic rocks at 01doinyo Lengai is the conspicuous desilieation of the

granitic gneiss (the quartz is ehemfcally attacked and removed) and

the slight addition in silica in the me~~bbro, required when the

sadie pyroxene replaces amphibole. In part, the required Si might be

liberated by the nephelinization of the plagioclase. Calcium would

be removed ta conform to the reactions: Na 2C03 + country rock,..s -+ CaC03 + Na-rich fenite (Vartiainen & Woolley 1976).

4) An obvious feature of fenitization at Oldoinyo Lengai is

the absence of a ~odic amphibole among the metasomatic mineral phases.

A relatively low pressure of H20 might be advocated, but most probably

the erystallization of amphibole was precluded by the high temperature

of the process. The mineralogy of jthe feldspars and the very ~light

turbidity support the above inferences. The widespread ~rystalliza

ti~n conjugated with rheomorphism in the highest grades of fenitization

" fndicate magma tic temperatures at this stage. 5) The specifie textures described above and the presence of the

glass in some specimens of high-grade fenite suggest the1r mobilization

in a more-or-less molten state. The inner part of the metasomatic-

1 1

1 (

1" \\

t 1 (J l

1

! i r !

,,'

, aureo1e must-have been under a hi9.h., thennal regime', as reflected by

/J, ... ,

the assemblage sanidine + aeg~ine-augite + l1quid.- Presumably,

the P-T conditions attained approached those of the pyroxene . 1

hornfe1s facies, possibly sanidinite facies, involving temperatures

of 700-BOOoe and variable pressures not exceeding 3-4 kbar.

Temperatures above 7000e would be compatible with the presence of

sanidine. The incipient melting could have been generated by several

factors: 1) sudden incre~se in temperature, 2) progressive increase

in P(H 20) (an event very probable during the repeated metasomatic

events, as hydrous phases were not produced) which lowered the solidus

of the newly formed assemblage, 3} a progressive increase in the

content of K-rich feldspar, which also led to ~ lowering of the

solidus. Significantly, the rock most affected by melting (largest ,

71

amount of glass) seems to be sanidine aegirine-~ugite nepheline fenite

.... . . .

:

...

"

1 , f f

1 ,. f ! " ,

.' .

, (1 j

""

, .

Chapter 5. COMPOSITION OF THE PHASES~RESENT

The newly formd minera1 phases, as well as some of the relict

minerls and the glass from speimen BO-43, have been ana1yzed with , .

72

an A~-EMX microprobe, using ~ Tracor Northern NS-880 energy-di spersion . , systf!ll1. The recrystallized. alkali 'eldspars were a1so analyzed by

1 , X-rai diffraction ta. evaluate t'heir tcompo,sit1~nal and structural states

. The analyses were performed' tb provide info'rmation on the composit1ona1

variations in eaeh major group of minerals involved in fenitizatfon . , . "', ~.. ~ ...

reactions; these variations most probably reflect the p~ysical and >

" chemical environment prevailing,during the re-equilibration of the 1 ~ , ,

invaded rocks with the fenitizing fluide The composition of the

glass provides valuable .information concerning the ,nature of the melting

reaction.

~pYROXENf

Clinopyroxene 1s the only ferromagnesian PhaseVformed during

progressive fenitization in the suitestu.died. Initial petrographie

observations suggested large compositional variations, reflected in

variable qptical properties within single crystals and betw~en crystals

of the same specimen and from different specimens. For this reaso",

severa1- crystal s were selected for analysfs if" .. each specimen; each

crystal selected was analyzed at several representative points.

Analytical resuJts, representing the full range in pyroxene composi-

tions, are presented together ~ith structural formulae in Tables. 8 ta 10.

--~,~---------

, .

____ ----".......,.,------\:,..---~"'!Sr ............ -.--\..-----

.'.

f""'\. "'" 11-~ .........,

.--. ...

1

( TABLE 8. REPRESENTATIVE PYROXENE COMPOSITIONS F~OM LOW-GRAOE FENITES

Microprobe data

-BO-44 fenitized metagabbro BD-58 fenitized gneissic granite

1 2 3 4 5 6\ 7 8 g 10 11 12 13

'~ core rim core rim core rim core rim r

S10 2 51.00 51 A7 51.77 50.03 51.64 52.58 52.56 52.30 53.28 53.31 53.46 52.64 52.25 Al ,.0 ) 1.59 0.59 0.27 1.39 0.86 0.60 0.00 . 0.00 0.00 0.00 0.00 0.00 0.00 Ti02 0.36 0.54 0.74 0.21 0.22 0.41 1.79 1.58 1.38 1.45 0.92 1: Ol 2.38 Cr 2O, 0.35 0.02 0.00 0.00 0.00 0.00 0.17 0.10 0.04 0.07 0.009 0.20 0.00 -FeO 14.16 16.83 17 .90 16.57 , 15.36 17.23 19.82 19.48 19.33 20.33 . 15.35 18.88 22.24 f Mn(t 0.46 0.42 0.55 0.44 0.36 0.64 1.00 1.28 l.22 0.60 0.64 1.661 0.92 MgO 9.25 7.79 7.24 7.22 8.27 7.58 5.11 5.24 6.47 5.86 8.30 '5.69 '.:3.08 Cao 20.07 18~00 14.49 20.11 16.90 14.9; 9.40 9.72 9.47 9.52 13.88 -13.02 14.81 ' Ha 20 2.25 3.09 5.36 ; 1.69 3.70 4.56 7.82 7.61 7.98 7.66 5.65 6'.19 10.41 K2.0 0.06 0.00 0.11 0.10 0.05 -0.18 0.03 0.17 0.16 0.05 0.40 Q.22 0.10 Total 99.55 98.76 98.43 97.75 97.35 98. 7~ 9J.70 97.47 99.33 98.86 98.33 "" 99.50 98.20

i Structural formulae based on 6 oxygen atoms

Si 1.970 2.016 2.042 1.987 2.034 2-.053 2.092 2.090 2.080 2.-094 2.076 2.n67. 2.048 A1 .070 .027 .013 1 .065 .040 .027 .000 .000 .000 .000 .000 .000 .000 Ti .011 .016 .022 ' .006 .007 .012 .058 .047 .040 .043 .027 .030 .073 Cr .011 .001 .000 .000 .000 .000 .005 .003 - - .001 .002 .002 .006 .000 Fe .457 .552 .590 .550 .506 .563 _660 .658 ~. 632 .668 .498 .620 .7,57

-Mn .015 .014 .018 " .015 .012 .021 .034 .043 .040 .020 .021_ .055 .Q32 Mg .530 .455 .425 .427 .485 .441 .303 .312 .377 .343 .480 .333 .187 Ca .830 .756 .612 .856 .713 .626 .401 .416 .397 .401 .577 .548 .210 Na .170 .235 .410 .130 .283 .345 .603 .589 .605 .583 .425 : .472 .822 K .

-~ -c . ,.' - 0' '''~ 1i.1iI tlkiJJt!.ujfi(Ua; ha r... .,;1\1" 1. 3_ h ~ At_."kmJ)'~~~~~lQO\tGj.TJQIf*U ~~"'l\4 $_""d ~""'~""r""""'Jj.,"!l;" -' ...

l'

TABLE 8 (Cont'd) C;l-

~ ~

1 2 3 4 5 6 7 8 9 10 11 12 13

Mo 1. ~ end members ...

MfA1Sh06 -- , 1.1 1.3 0.5 4.0 2.7 NaCrS1 206 1.1 0.1 " 0.5 0.3 0.1 0.2 0.2 0.6 .4 __ Na Ti FeShOs 1 1.4 . 1'.4

- NaTfMgS1 2 0 6 4.4 2.7 11.2 ' 9.4 8.0 8.6 5.4 6.0