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Geochem Icaljournalv Ol.18,Pp.95To 100,1984 N O T E S R Is Ot O P E Stu

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Geochem Icaljournalv Ol.18,Pp.95To 100,1984 N O T E S R Is Ot O P E Stu 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 .
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