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Sr Isotope Study of the Tephrite Series Nyamuragira Volcano, Zaire

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Sr Isotope Study of the Tephrite Series Nyamuragira Volcano, Zaire GeochemicalJournal Vol. 18, pp. 95 to 100, 1984 NOTE Sr isotope study of the tephrite series from Nyamuragira volcano, Zaire KEN ICH IRO AOKI1 and HAJIME KURASAWA2 Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Aoba, Sendai 9801 and Geological Survey of Japan, Higashi 1-chome, Yatabe-machi, Tsukuba-gun, Ibaraki 3052, Japan (Received June 24, 1983: Accepted February 14, 1984) The 8'Sr/"Sr ratios of six representativetephrite series rocks, including 1981-1982 lavas, from Nyamuragiravolcano, Western Rift have been determined. The range and averageof the ratios are from 0.70543 to 0.70587 and 0.70572, and agree xell with those of ugandite from the WesternRift (0.7054 0.7059 and 0.7056) reported by BELLand POWELL(1969) and POWELLand BELL(1974). Petrological and geochemical data suggest that the tephrite series, olivine basanite-tephrite-phonolitic tephrite tephritic phonolite are products of successivefractionation of a parental ugandite magma (AOKIet al., 1984). These isotopic resultssupport stronglythis conclusion. (1981), NAKAMURA and AOKI (1980), VOLLMER INTRODUCTION and NORRY (1983) and FUJIMAKI et al. (1984). The origin of potassic volcanic rocks is one Although many regional studies have been re of the most controversial problems in magma ported, systematic mineralogy, petrology and genesis (TURNER and VERHOOGEN, 1960; GUPTA geochemistry of each volcano in this region and YAGI, 1980) in that it is necessary to ac appears to have been rather neglected for a count for many petrological and geochemical long time. Then, AOKI et al. (1984) reported features. Since the large contribution by detailed mineralogy, petrography, and major HoLMES and HARWOOD (1937), ultramafic to and trace element geochemistry of the olivine mafic lavas of the Western Rift region of the basanite-tephrite-phonolitic tephrite-tephritic Eastern Africa Rift Valley, one of the notable phonolite lineage of Nyamuragira volcano, and provinces of perpotassic suites in the world, considered that this lineage represents the have attracted attention of petrologists, ex successive stages of olivine and clinopyroxene perimental petrologists and geochemists. A fractionation of a primitive olivine ugandite fairly large variety of rock types with low SiO2 magma which is produced by small degrees of contents but high concentrations of incom partial melting of mica-rich garnet peridotite patible elements and rare earth elements (REE) mantle at the depth of about 100km (EDGAR occur here. et al., 1980). In order to support this con More recent geochemical studies on the clusion further, Sr isotope ratios of the represen volcanic rocks from the Western Rift region tative rocks, including the 1976-1977 and have been carried out by DENAEYER (1965, 1981-1982 lavas, have been determined. 1972), BELL and POWELL (1969), POWELL and This article is intended to describe the BELL (1974), IERGUSON and CUNDARI (1975), analytical results of Sr isotope ratios and to MITCHELL and BELL (1976), EDGAR and ARIMA discuss the genetic problems of Nyamuragira 2 Present address: Kyushu Office, Geological Survey of Japan, Shiobaru 2-chome, Minami-ku, Fukuoka 815, Japan 95 96 K. AoKi and H. KURASAWA magma. FERGUSON and CUNDARI (1975) recognized two distinct series, the phonolitic tephrite (under PETROGRAPHY OF ANALYZED ROCKS saturated and potassic) and leucite (highly undersaturated and potassic) series from Nyamuragira volcano (1 ° 24'S, 29° 1 2'E and Bufmbira, Virunga volcanic field. Volcanic 3,056m above sea level) is located in the Virunga rocks from Nyamuragira belong to the former volcanic field of the Western Rift in Zaire near series. The revised rock names (STRECKEISEN, southwest Uganda and northwest Rwanda. This, 1980) will be applied here except in special along with Nyiragongo volvano lying to the cases. south, is one of the most active volcanoes in The petrographic features of volcanic rocks the world, and lava flows issued out about 20 from Nyamuragira are rather simple, and the times mainly from the flank rift craters since following minerals are identified among the 1900 (Fig. 1). The latest started on December tephrite series: phenocryst olivine, Ca-rich 25, 1981 and ceased on January 14, 1982 clinopyroxene, plagioclase, titanomagentite and (UEKI, 1983). chromian spinel, and groundmass olivine, Ca HOLMESand HARWOOD(1937) divided the rich clinopyroxene, plagioclase, leucite, titano alkalic volcanic rocks from the Western Rift magnetite, apatite, nepheline, alkali feldspar, into seven rock series, and applied unfamiliar sodalite, titanbiotite, hornblende, ilmenite and names to them. In the light of modern usage pyrrhotite (AOKI et al., 1984). based on STRECKEISEN'sclassification (1967), The commonest type of rocks is phonolitic tephrite in which the groundmass seems to have nearly the same modal and chemical composi 29°06' 29'18'E tions. Phonolitic tephrite grades gradually into 1958 tephrite and olivine basanite with increase of -i. f 1°18's phenocrystic olivine and Ca-rich clinopyroxene (up to 40 vol. % of phenocryst), or to tephritic phonolite, from which it differs in the more 1981-82 1111Th sodic and potassic natures of the average feld 1980 '• 195 spar composition and in the smaller percentage 1971 -52 195 8 6 I~IYAMURAGIR of mafic minerals of the groundmass. 1957*-5 ti 1938-40 195 a r2 <ti ~1938. 1914 • . 3 EXPERIMENTAL j4} 1978-77 1977 I k'? n f` ''R 4 1°30' The 87Sr/86 Sr ratios were determined at t © NYIRAGONG4 ,948 Geological Survey of Japan by a standard °1977 method using an automatic V. G. Micromass i 54E with double collector. The 87Sr/86 Sr 6r° data which carry errors of ± 0.01 per cent are r 1 reported with the values for the Eimer and Amend SrCO3 and the NBS 987 standard . LAC KlvU' The mean values for these standards measured O 5km during the course of the present study are 1°42' 0.70805 ± 1 for the E and A standard and 0.71028 ± 1 for the NBS standard. Fig. 1 Sketch map of Nyamuragira and Nyiragongo volcanoes showing the distribution of the recent lava The analytical results of major elements flows and analyzed sample localitties (THONNARDand (AoKI et al., 1984) and 87Sr/86 Sr ratios are DENAEYER,1965; UEKI, 1983). shown in Tables 1 and 2, for six representative Sr isotope study of the tephrite series 97 Table 1. Major element analyses of volcanic rocks from Nyamuragira volcano, Zaire (AOKIet al., 1984) 1 2 3 4 5 6 Si02 43.08 45.88 46.16 45.72 45.72 47.15 Ti02 3.63 3.98 3.83 3.76 3.72 3.08 A1203 11.37 14.60 15.01 16.00 16.54 17.61 Cr203 0.03 Fe203 4.02 2.10 3.10 1.96 2.01 2.84 FeO 9.78 9.32 9.08 9.98 9.78 7.16 MnO 0.22 0.19 0.19 0.19 0.20 0.19 MgO 11.21 6.39 4.61 4.47 3.91 3.29 CaO 11.86 10.67 9.99 9.40 9.24 8.06 Nat O 2.16 3.00 3.19 3.48 3.75 4.07 K20 2.03 3.04 3.33 3.63 3.81 4.47 H2O+ 0.22 0.16 0.34 0.48 0.33 0.82 H20 0.19 0.00 0.00 0.00 0.07 0.57 P205 0.36 0.47 0.66 0.64 0.71 0.74 S 0.04 0.05 0.05 Total 100.16 99.84 99.54 99.76 99.79 100.05 Analyst; K. AOKI 1. Plagioclase bearing titanaugite olivine basanite (77102518), western part of caldera rim. 2. Plagioclase bearing olivine titanaugite tephrite (81122501), volcanic bomb, 200m west of Rugarambiro crater, 1981-1982 eruption. 3. Aphyric phonolitic tephrite glass (8112250A), Pele's hair, Kunene, 21 km west of Rugarambiro crater, 1981-1982 eruption. 4. Aphyric phonolitic tephrite (78060303), Murara crater, 1976-1977 eruption. S. Plagioclase olivine titanaugite phonolitic tephrite (77102605), 4km east of caldera. 6. Plagioclase olivine titanaugite tephritic phonolite (77102517), western part of caldera rim. Table 2. K, Rb, Sr, Ba and 87Sr/86Srratios of volcanicrocks ppm Ratio Sample (87Sr/86Sr)n K Rb Sr Ba K/Rb K/Ba Rb/Sr 1 77102518 16800 55 649 664 305 25.3 0.0847 0.70573 ± 3* 2 81122501 25200 80 937 985 315 25.6 0.0854 0.70587±1 3 8112250A 27600 87 1100 1190 317 23.2 0.0791 0.70583 ± 2 4 78060303 30100 90 1050 1140 334 26.4 0.0857 0.70584 ± 2 5 77102605 31600 97 1140 1210 326 26.1 0.0851 0.70561 ± 2 6 77102517 37100 114 1240 1440 325 25.8 0.0919 0.70543 ± 2 Average 0.70572 * internal error E & A SrCO3standard: (S7Sr/86Sr)n= 0.708051 ± 8 (1 a) NBS 987 SrC03 standard: (87Sr/86Sr)n= 0.710278 ± 9 (1a) K, Rb, Sr and Ba: AOKIet al., 1984 olivine basanite, tephrite, phonolitic tephrite DISCUSSION and tephritic phonolite. Detailed Sr Isotope studies on the volcanic rocks from the Virunga volcanic field, the Western Rift have been carried out by BELL 98 K. AOKI and H. KURASAWA and POWELL(1969) and VOLLMERand NORRY and phonolitic leucitite. (1983). According to them, the 87Sr/86 Sr ratios BELL and POWELL (1969) and VOLLMER vary significantly and range between 0.7036 and and NORRY (1983) reported three isotope 0.7103, and those of feldspar-bearing varieties analyses from Nyamuragira; one is kentallenite tend to have higher values than feldspar-free (0.7050) and the other two are kivite M (0.7058 ones (Fig. 2). These rocks also have a very and 0.7066). These rocks correspond to olivine wide range of chemical composition from highly basanite (HiGAZY,1954) and phonolitic tephrite undersaturated ultramafic to latitic, trachytic respectively. The isotope ratios of Nyamuragira and phonolitic rocks.
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