Geochem icalJournalV ol.18,pp.95to 100,1984

N O T E

S r is ot o p e stu d y o f th e te p h rite series fr o m

N y a m u r a gira v olc a n o, Z aire

K EN-ICHIRO A OK11 and H AJIME K URASAWA2 Institute ofM in eralogy,Petrology an d Economic Geology,Tohoku University, A oba,Sendai9801 and GeologicalSurvey ofJapan,Higashi 1-chome, Yatabe-m achi ,Tsukuba-gun,Ibaraki3052,Japan

(R eceived June24,1983: Accepted Febru ary 14,198A,)

The 87Srl86Sr ratios of six representative te phr ite series rocks, including 1981-1982 Iavas, from Nyam uragira volcano, W estern Rift hav e been determi ned. Th e rang e and aver age ofthe ratios are from

0.70543 to 0.70587 and 0.70572,and agree wel with those of ugandite from the W estern Rift(0.7054 - 0.7059 and 0.7056) re ported by BELL a nd POWEL L ( 1969) and PO WEL L an d BELL (19 74 ). Petrological

and geochemical data suggest that the tephrite series, olivine basanite -tephr ite -phonolitic tephri t e= tephritic ph onoli te are prod u cts of succes sive fracti on ation o f a p arental u gandite m ag m a (A OKI etal.,

1984) . Theseisotopicresultssupportstronglythisconclusion.

(1981), N AKAMURA and A OKI (1980), V OLLM ER INTRODUCTION and N oRRY (1983) and FUJIM AKI et al. (1984). T he origin of potassic volcanic rocksis one Although m any regional studies have been re- of the m ost controversial problem s in m agm a port ed, system atic m ineralogy, petrology and genesis (TURNER and V ERHOOGEN, 1960; G UPTA geochemistry of each volcano in this regi on and Y AGI, 1980) in that it is necessary to ac- appears to have been rather neglected for a count for m any petrological and geochem ical 10ng tim e. Then, A OKI et al. (1984) report ed features. Since the large contribution by detailed m ineralogy, petrography, and m ajor H OLMES and H ARW OOD (1937), ultram afic to and trace elem ent geochem istry of the olivine m afic lavas of the W estern Rift region of the basanite-tephrite- phonolitic tephrite-tephritic E astern A frica Rift V alley, one of the notable phonolite lineage of N yam uragira volcano, and provinces of perpotassic suites in the w orld, considered that this lineage represents the have attracted attention of petrologists, ex- successive stages of olivine and clinopyroxene perim ental petrologists and geochem ists. A fractionation of a prim itive olivine ugandite fairly large variety of rock types with low Si0 2 m agm a w hich is produced by sm all degrees of contents but high concentrations of incom - partial m elting of m ica-rich garn~t Peridotite patible elem ents and rare earth elem ents (R EE) m antle at the depth of about 100km (E DGAR occurhere. et al., 1980). In order to support this con- M ore recent geochem ical studies on the clusion furt her, Srisotope ratios ofthe represen- volcanic rocks from the W estern Rift region tative rocks, including the 1976- 1977 and have been caried out by D ENAEYER (1965, 1981-1982Iavas, have been determ ined. 1972), BELL and POW ELL (1969), POWELL and This article is intended to describe the BELL (1974), FERGUSON and C UNDARI (1975), analytical results of Sr isotope ratios and to M ITCHELL and BELL (1976), E DGAR and A RIMA discuss the genetic problem s of N yam uragira

2 presentaddress: Kyushu Office, GeologicalSurvey of Japan, Shiobaru 2~hom e,M inami-ku, Fukuoko 815, Japan

95

96 K.AOKIand H. KURASAW A

magm a. FERGUSO N and CUNDARI (1975) recognized tw o

distinct series, the phonolitic tephrite (under- PETROGRAPHY OF A NALYZED R OCKS saturated and potassic) and leucite (high ly undersaturated and potassic) series from N yam uragira volcano (1'24'S, 29"12'E and B ufm bira, Virunga volcanic field. V olcanic 3,056m above sealevel)islocated in the Virunga rocks from N yam uragira belong to the form er volcanic field of the W estern Rift in Zaire near series. The revised rock nam es (STRECKEISEN, southiv est U ganda and northw est R wanda. This, l980) will be applied here except in special along with N yiragongo volvano lying to the cases. south, is one of the m ost active volcanoes in Th e petrographic features of volcanic rocks the w orld, and lava fl ow s issued out about 20 from N yam uragira are rather sim ple, and the tim es m ainly from the flank rift craters since following m inerals are identified am ong the 1900 (Fig. I). The latest started on D ecem ber tephrite series: phenocryst - olivine, Ca-rich 25, 1981 and ceased on January 14, 1982 clinopyroxene, plagioclase, titanom agentite and (U EKI,1983). chrom ian spinel, and groundm ass - olivine, Ca- H OLMES and H ARWOOD (1937) divided the rich clinopyroxene, plagioclase, Ieucite, titano- alkalic volcanic rocks from the W estern Rift m agnetite, apatite, nepheline, alkali feldspar, into seven rock series, and applied unfam iliar sodalite, titanbiotite, hornblende, ilm enite and nam es to them . In the light of m odern usage pyr hotite (A OKI et al.. 1984). based on STRECKEISEN'S classification (1967), The com m onest type of rocksis phonolitic tephrite in w hich the groundm ass seem sto have nearly the sam e m odal and chemical com posi- 29~06' 29'"18'E tions. Phonolitic tephrite grades gradually into _:~~:"~-'1: l958 tephrite and olivine basanite w ith increase of *J,~ 1"18'S phenocry stic olivine and Ca-rich clinopyroxene Ci~':;t .:rji;}!:' (up to 40 vol. % of phenocryst), or to tephritic .,,J.':*;'-L'*!! phonolite, from w hich it differs in the m ore : ~ ~ : 1981-82<~i1~ :*~''~' ' ~=" 1967 ~~j sodic and potassic natures of the average feld- ;.~~ _ Ig8O spar com position and in the sm aller percentage ..~;~:i":~'11971 6S2 1956 ~ ::'r,' 1~ 19Y5A7MelPrR5AGIR'''a~i of m afic minerals ofthe groundm ass. '" 1938-40 194S ~ ~ 2 =~;-: ~'? ~'$"~f""~:t 19~ ~{/~ Ig 4 l'~'=!:;~:'j~~.~~~:=-/i:li:'"L}I';:''j 3'- 'Y;*~:;~'i:( . '~;,i=~~i~;:s'i::~~~~"il'+_* fi EXPERIM ENTAL t_; '~:p,$;~~l~'~'~;i"1976-77 .l":;-"¥-'.~i'1977 -~"''__! i~:",:~i 4 /b o 1'30' The 87Sr/86Sr ratios w ere determ ined at O ~NbL~A~Q~~b,{ 1948 Geological Surv ey of Japan by a standard 948 * m ethod using an autom atic V. G. M icrom ass ,~ 01977 54E with double collector. The 87Sr/86Sr ;1';i '{,:s~;i+:'~. i*.･ '~ + "!{･t; '~i':'= '~:~ data w hich car y errors of i 0.01 per cent are ~l912 reported with the values for the Eim er and ¥I A m end SrC 03 and the N BS 987 standard. LAC K,V The m ean values for these standards m easured O 5km during the course of the present study are 1'42' 0.70805 i I for the E and A standard and 0.71028 :!: I forthe N B S standard. Fig. I Sketch m ap of Nyam uraeira and Nyiragongo volcanoes showing the distribution of the recent lava T he analytical results of m ajor elem ents flows and analyzed sample localitties(THONNARD and (A OKI et al., 1984) and 87Sr/86Sr ratios are D ENAEYER,1965;UEKI,1983). show n in Tables I and 2, for six representative

Srisotopestudy'ofthetephriteseries 97

Table1. M ajorelem entanalyses ofvolcanicrocksfrom Nyam uragira volcano,Zaire (A o~retal..1984)

1 2 3 4 5 6 SiO2 43.08 45.88 46.16 45.72 45.72 47.15 TiO2 3.63 3.98 3.83 3.76 3.72 3.08 Al2 0 3 1 1.37 14.60 15.01 16,OO 16.54 17.61 Cr203 0.03 Fe2 03 4.02 2.10 3.10 1.96 2.01 2.84 Fe O 9.78 9.32 9.08 9.98 9.78 7.16 M nO 0.2 2 0.1 9 0. 19 0. 19 0. 20 0. 19 M gO 1 1.21 6.3 9 4. 61 4. 47 3. 91 3. 29 C aO 1 1 .86 10.6 7 9. 99 9. 40 9. 24 8. 0 6 N a2O 2.16 3. OO 3. 19 3. 4 8 3. 75 4 . 07 K 20 2.03 3. 04 3. 33 3 . 63 3. 81 4. 47 H20+ 0.22 0 . 16 0. 34 O. 48 0. 33 0. 82 H2 0- 0.19 0 . 00 0. 00 0 . 00 0. 07 0. 57 P20 5 0.36 0.4 7 0. 66 0. 64 0. 71 0. 74 S 0.0 4 0. 05 0. 05 99.8 4 99. 54 99. 76 To t al 100.16 99.79 100.05 Analyst; K. A OKI 1. Plagioclase bearingtitanaugite olivine basanite(77102518),western partofcalderarim.

2. Plagioclase bearing olivine titanaugite tephrite (81122501), volcanic bom b, 200 m west of R ugaram biro cr ater,1981-1 9 82 erupti on . 3. A p hyric phonolitic tpp hrite glass (8112250A), Pele's hair, K unene, 2lkm west of Rugaram biro crater,

1981-19 82eru ptio n.

4. Aphyricphonolitictephrite(78060303),M urara crater,1976- 1977eru p tion. 5. Pl agioclaseolivi neti tanaugite phonolitic tephrite( 7 7102605),4km eastofcaldera.

6. Plagioclaseolivinetitanaugitetephriticphonolite(77102517),w estern partofcaldera rim.

Table2. K,R b,Sr,Baand 87Sr/86Srratios ofvolcanicrocks

ppm Ratio 87 86 Sample ( Sr/ Sr)n K Rb Sr Ba K/Rb K/Ba Rb/Sr

77102518 16800 55 649 664 305 25.3 0.0847 0.70573ｱ 3* 2 81122501 25200 80 937 985 315 25.6 0.0 8 54 0.70587ｱ 1 3 8112250A 27600 87 1100 1190 317 23.2 0 . 0 791 0.70583d:2 4 78060303 30100 90 1050 1140 334 26.4 0. 0857 0.70584:!:2 77102605 31600 97 1140 1210 326 26.1 0. 0851 0.70561i 2 25.8 6 77102517 37100 l14 1240 1440 325 O. 0919 0.70543:!:2 Average 0.70572

*internalerror E & A SrC03standard: (87Srl86Sr)n = O.708051 :!:8(1a) NBS 987 SrC03 standard: (87Srl86Sr)n = O.710278 i 9(1a) K,Rb,Srand Ba.' A OKIetal., 1984 olivine basanite, tephrite, phonolitic tephrite DISCUSSION and tephritic phonolite. D etailed Sr Isotope studies on the volcanic rocks from the Virunga volcanic field, the W estern Rift have been carried out by BELL

98 K. AOKIand H. KURASAWA

and POWELL (1969) and V OLLMER and N oRRY and phonoliticleucitite. (1983). A ccording to them ,the 87Sr/86Sr ratios BELL and POW ELL (1969) and V OLLMER

vary significantly and range betw een 0.7036 and and N oRRY (1983) reported three isotope

0.7103, and those of feldspar-bearing varieties analyses from N yarn uragira; one is kentallenite

tend to have higher values than feldspar-free (0.7050) and the othertw o are kivi te M (0 .7 05 8

ones (Fig. 2). These rocks also have a very and 0.7066). These rocks c orrespond to olivine

wide range ofchemical com position from highly basanite (HIGAZY, 1954) and phonolitic tephrite

undersaturated ultram afic to latitic, trachytic respectively. The isotope rati os of N yam uragira

and phonolitic rocks. H ow ever, the values and in this paper lies betw een 0.7 05 43 and 0.7058 7

ranges of the 87Srl86Sr ratios and rock types (average 0.70572), w hich are betw ee n the abov e

are lim ited within in dividual volcanoes in the three. N yam uragira tephrite series are charac-

Virunga field. The 87Sr/86Sr ratios and rock terized by the narrow range of the 87Sr/86 Sr,

types of som e volcanoes are as follows: K /R b, K/ Ba and R b/Sr ratios irrespective of

N yiragongo - 0.7036- 0.7058, nephelinite, rather wide chem ical com positions o r variable

leucitite and m elilitite;M uhavurango - 0.7062- rock types.

0.7076, basantite, Ieucitite, phonolitic tephrite H OL M ES (1950) recognized three im portant

and latite; Sabinyo - 0.7055- 0.7103, phono- prim itive basaltic rocks, katungite (olivine,

litic tephrite, trachyandesite and latite; m elilite with m inor perovskite, tita nom agnetite ,

K arisim bi - 0.7068- 0.7074, phonolitic tephrite chrom ite and K-rich nepheline), m afurite

and latite; M ikeno - 0.7049- 0.7062, Ieucitite (olivine, phlogopite with m inor k alsilite and

glass) and ugan dite (olivine, clinopyroxe ne,

N with m inor feldspar and leucite). R ece ntly,

V OLLMER and N oRRY (1983) dem on strated tha t

the Sr isotopic variations in such m afic roc ks 30 are attributa ble to the ancient m e tasom atis m

by fluid invasion in the Virunga m agm a source

regions about 500M yr ago. O n the bas is of

high pressure experim ental, petrographical and

geochem ical data, EDGAR et al. (19 80) and

E DGAR and A RIM A (1981) considered that 20 ugandite is not a derivative of katungite o r

m afurite m agm a, but represents a prim ary

m agm a, being produced by a sm all degree of

partial m elting of K-enriched garnet peridotite

at the depth of about 100km .

10 Petrography and m ajor and trace elem ent

geochem istry indicate the intim at e genetic

relationships betw een ugandite and N yam uragira

tephrite series, olivine basanite - tephrite - ph onolitic tephrite - tephritic phonol ite (A OKI

et al.. 1984). If so, the liq uid com posi tions

0.702 o.104 0.706 o.708 0.710 0.712 of the m ost M g-rich olivine b asanite,the recent

87Sr/86sr lavas erupted in 1976- 1977, 1980 and 19 81-

1982, an d of the m ost differenciated tephritic Fig. 2 Histogra m of the 87Srl86Sr ratios of volcanic phonolite, indicate the stages of about 5 0, 65 rocks from the Western Rift (including the data by a nd 70% solidificatio n o f th e parental ugandite BELL and (POWELL (1969)and VOLLM ER and N ORR Y (1983)). m agm a respectively, and are resulted from

FELOSPAR-BEARING ROCKS ~] NYAMURAGIRA ~lOTHERS FELDSPAR-FREE ROCKS ~l NYIRAGONGO C] OTHERS e e e e e e , e e , e e e e e e e e , , e e e , e , e , e e , e o o e o o , o e e o e , o e e o o o o

Srisotopestudy ofthetephriteseries 99

olivine and clinopyroxene-controlled fractiona- EDGAR, A. D., CONDLIFFE, E., BARNETT, R. L.,and SHIRRAN, R. J. (1980) An experimental study tion. T he range of the 87Sr/86Sr ratios of various of an olivine ugandite m agm a and m echanism s for types of ultram afic to m afic rocks such as the form ation ofitsK-enriched derivatives. J.Petrol. 21,475-497. k atungite, m afurite, nephelinite, m elilitite, FERGUSON, A. K. and CUNDARI, A.(1975) Petrol- ugandite and leucitite, w hich w ould be produced ogical aspects and evolution of the leucite bearing by partial m elting ofchem ically andisotopically lavas from Bufum bira, South Uganda. Contrib. hete rogeneous upper m antle peridotite a t M ineral.Petrol. 50,25-46. various depths, are 0.7036 to O .7071 (BELL FUJIM AKI, H., TATSUM OTO, M. and A OKI, K. (1984) Partition coefficients of Hf, Zr, and R EE between and POWELL, 1969). A m ong them , the range phenocrysts and groundm asses. Geophys. R es. 88, and average of the 87Sr/86Sr ratios of ugandite supplem ent,662-672. are 0 .7 054- 0.7059 and 0.7056 w hich are the G UPTA, A. K. and Y AGI, K.(1980) Petrology and sam e as those of N yam uragira lavas. A ccord- genesis of leucite-bearing rocks. Springer Verlag,

ingly, these isotopic data also support strongly Berlin.

th at the N yam uragira tephrite series are pro- HIGAZY, R. A. (1954) Trace elements of volcanic ultrabasic potassic rocks of south-west Uganda and d ucts of successive fractionation of parental adjoining part of the Belgian Congo. B ull. Geol. ugandite m agm a. Soc.Am. 65,39-70. HOLMES,A.(1950) Petrogenesis of katungite andits Acknowledgements- Our th anks are due to Prof. H. associates. Am.M ineral. 35,772-792. HAMAGUCHI of the Institute of Geophysics, Tohoku H OLMES, A. and H ARWOOD, H. F. (1937) The University an d Dr.N.ZANA oftheInstitut de Recherche petrology of the volcanic field of Bufum bira,south- Scientifique, Bukavu, Zaire for the recentlavasam ples west Uganda. Geol. Survey Uganda, M em. 3, I- of N yam uragira volcano. A part of expenses of this 300. study was defrayed with a Grant-in-Aid for Special MITCHELL, R. H. and BELL, K.(1976) Rare earth elem ent geochemistry of potassic lavas from the Project Research (Nos. 420901 and 520102) from the Ministry of Education, Science and Culture of Japan. Birunga and Toro-A nkole regions of Uganda, Africa. Contrib.M ineral. Petrol. 58,293-303. NAKAMURA, Y. and A OKI, K. (1980) The 1977 REFERENCES eruption of Nyiragongo volcano,eastern A frica,and chemical com position ofthe ejecta. K azan 25, 17- AOKI, K., Y OSHIDA, T., Y USA, K. and N AKAMURA, 32 (in Japanese with English abstract). Y. (1984) Petrology and geochemistry of the POWELL, J. L. and BELL, K.(1974) Isotopic com - Nyam uragira (Nyam ulagira) volcano,Zaire. Contrib. position of strontium in alkalic rocks. In: The M ineral.Petrol. (in press). alkaline rock, 412-421, ed. H. Sorensen. John BELL, L. an d POWELL, J. L. (1969) Strontium W iely & Sons,London. isotope studies of alkalic rocks. The potassium rich STRECKEISEN, A. (1967) Classification and nom en- N. Jb. Miner. Abh. 107, lavas of Birunga and Toro-Ankole regions, East and clature of igneous rocks. 144-240 . Central Equatorial Africa. J. Petrol. 10, 536- 572. DENAEYER, M. -E. (1965) Recueil d'analyses des STRECKEISEN, A. (1980) Classification and nom en-

laves du foss6 tectonique de1'Afrique Centrale(Kivu, clature ofvolcanic rocks,Iam prophyres,carbon atite s and melilititic rocks- IUGS Subcom mision on the Rwanda, Toro-Ankole). Ann. M us~e R oy. Afrique Centrale, Tervuren (B elgique), s~r. in-8, Sci, g~ol., System atics of lgneous Rocks. Geol. R undschau

n'49, 234 pp. 69,194-207. THO NNAR D, R. L. G. and DENAEYER, M .-E.(1965) D ENAEYER,M .-E.(1972) Leslaves du foss6tectoni- qu e de l'Afrique Centrale (Kivu, Rw an da, Toro- Carte volcanologique des Virunga, Afrique Centrale (Feuille N'I) , E chele: 1/50 ,00 0. P ub l.n 32, Centre A nkole). A nn.M us~eR oy.Afrique Centrale, Tervuren (Bel~ique), s~r. in-8 ',Sci. g~ol., n 72, 132 N ationalde Volcano (Bel~ique).

pp. TURNER, F.J.and V ERHOOGEN,J.(1960) Igneou s and met am orp hic p etrol ogy. McGraw-Hil Co., New EDGAR, A. D.and ARIMA, M.(1981) Geochemistry Y ork,L on do n. ofthree potassium-rich ultrabasiclavasfrom the west branch of the African Rift: inferences on their V OLLMER, R. and N ORRY, M. J. (1983) Possible o rigin o f K-rich v olcanic rock s from Virunga, E ast geneses. J.Jb.M iner.M h. 1981, 539- 552.

1OO K. AOKIand H.K URASAWA

Africa, by m etasomatism of continental crustal and Nyiragongo. In: Volcanoes Nyiragongo and m aterial: Pb, Nd an d Sr isotopic evidence, Earth Nyam uragira, Geophysical aspects (ed. H. H AMA- Planet.Sci.Lett. 64,374-386. GUCHI). Fac.Sci.,Tohoku Univ.,Sendai,Japan. UEKI, S. (1983) Recent volcan ism of Nyam uragira