Geochemistry and petrology Toro Ankole kamafugite magmas: isotopic
Muravyeva N.S.*, Belyatsky B.V.**, Ivanov A.V.***
*Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia; **VNIIOkeangeologia, Antarctic Geology Department, St.-Petersburg, Russia
***Institute of the Earth Crust SB RAS, Irkutsk, Russia
Kamafugites are silica-undersaturated high-potassium volcanic rocks originally named after three petrographically-defined rock types – katungite, mafurite and ugandite from the Toro-Ankole province in the western branch of the East African rift zone. They correspond to strongly potassic types of olivine melilitite to nephelinite in the modern classifications. There are three volcanic fields: Fort Portal, Katwe-Kikorongo and Bunyaruguru in Toro-Ankole, this study is focused on the Bunyaruguru volcanic field which stretches southwards from the south shore of Lake George, partly over the floor of the western rift depression and partly over the adjacent plateau to the south and east of the rift.
The simultaneously low SiO2 (<42 wt%), high MgO (up to 21.5 wt%), low Al2O3 (<10 wt%), and high CaO contents (up to 16.45 wt%) of these volcanics express themselves in the presence of modal kalsilite, leucite, melilite and perovskite. The common minerals of phenocrysts are olivine, leucite, clinopyroxene, Cr-spinelides and phlogopite. There are perovskite and melilite phenocrysts in the katungites. Spinelides are frequently occurred as inclusions in the olivines, which also contain fluid and, sometimes, carbonate, sulfate and sulfide inclusions. The absence of plagioclase distinguishes them from the lava flows occurring in the rest of the western rift branch. These strongly silica-undersaturated potassic rocks have been little studied by modern analytical methods, particularly radiogenic isotope systems. The special features of the composition of investigated rocks are high Mg# (up to 79), high Ni content (up to 780 ppm) and Cr (up to 1170 ppm) in conjunction with the extreme incompatible trace elements enrichment: Nb up to 276 ppm; Ce/Y up to 26.38; Zr/Y up to 26.22; (La/Yb)n up to 142.69; (La/Sm)n up to 7.27. The composition of the kamafugite phenocrysts confirms its “primitive” character: forsterite content up to Fo92, NiO – up to 0.48 wt%, CaO (min) – 0.07 wt%. Spinelides are the high-chromium varieties: Cr2O3 – up to 58.4 wt%, Al2O3 – up to 10.65 wt%, the minimum of TiO2 – 2.36 wt%. The spinel composition trend for studied samples is quite similar to the kimberlite trend.
Toro-Ankole kamafugitic lavas are characterized by minimal effect of fractional crystallization (except olivine fractionation) in contrast to the lavas from volcanic fields of the western branch (Virunga, Kivu, Rungve). Some geochemical characteristics of the rocks studied – Yb and Zr minima on spidergrams – testify to the participation of garnet in residue in the process of their formation. To confirm this suggestion (the possibility of generation of investigated rock in garnet peridotite layer) some calculations were carried out. The estimation has been done on major elements, nickel and chromium kamafugite contents according to the method (Herzberg C., Asimow P. D., 2008). The results show that the large part of the rocks studied may be formed during melting of garnet peridotites as a result of removal or addition of some magnesian olivine. For two samples – katungite and mafurite, however, the model of peridotite source does not suit and it is necessary, therefore, to attract another component to peridotite melting model – pyroxenite or eclogitic.
Sr and Nd isotope signatures for kamafugites form a slightly enriched relatively to BSE cluster (87Sr/86Sr = 0.704629 – 0.705356; 143Nd/144Nd = 0.512488 – 0.512550). Some inverse correlation with major element contents is observed: 87Sr/86Sr – CaO, 143Nd/144Nd – Mg#, 87Sr/86Sr – Mg#. Sr-Nd isotope data of Bunyaruguru kamafugites suggest that its mantle source composition is nearly EM1 (Hofmann, A. W.,2003). But the range of Pb composition for investigated is 206Pb/204Pb: 18.998 – 19.566; 207Pb/204Pb: 15.686 – 15.737; 208Pb/204Pb: 39.303 – 40.264. On these data the mantle source composition for studied kamafugites is close to EM2 or Dupal characteristics. High “radiogenic” lead ratios of kamafugites are in general agreement with high Th/U (4.16 – 5.01) in comparison with primitive mantle values (3.9). Some indicator trace element ratios of kamafugites (Ce/Pb: 34.51 – 45.85 and Pb/Nd: 0.01 – 0.08) testify against participation of continental crust material in their source.
Herzberg C., Asimow P. D. Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation // Geochemistry, Geophysics, Geosystems. 2008. Vol.9. N. 9
Hofmann, A. W. Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements.// In: Treatise on Geochemistry. Editor: R.W. Carlson . 2003. Executive Editors: H. D. Holland and K.K. Turekian. pp. 568. Elsevier. Vol.2. P.61-101.