Leucite basanites of Virunga (East African Rift): some insights into petrogenesis and source composition

Abstract

The leucite basanites of the Visoke and Muhavura volcanoes from the province of Virunga (Western Branch of the East African Rift) were investigated in detail. The major and trace element contents and Sr, Nd and Pb isotopic composition in the whole rocks were determined. The isotope ratios (87Sr/86Sr: 0.70644–0.70793; 143Nd/144Nd: 0.51233–0.51243) show that these rocks are far from the bulk Earth on the “mantle trend” and enriched in radiogenic Sr. The major element compositions reveal that they are not primary mantle melts as they have Mg# = 0.50–0.59. Moreover the leucite basanites are enriched by LILE and LREE relative to PM which may indicate an enriched mantle source. Pb isotopes of the studied leucite basanites (206Pb/204Pb: 19.112–19.438, 207Pb/204Pb: 15.737–15.783, 208Pb/204Pb: 40.509–40.866) along with the data of Karisimbi volcano form trends on isotope diagrams towards the EM II or ancient continental crust. The position of leucite basanite data on the 87Sr/86Sr - 208Pb*/206Pb* correlation line suggests that they can be a result of melt enrichment/vein metasomatism within the mantle.The phenocryst composition of leucite basanites varies in range: olivine- from Fo55 to Fo89 with NiO content 0.01–0.34% wt.; clinopyroxene - from 0.65 up to 0.86 Mg# with Cr2O3 from 0.02 to 1.5% wt., plagioclase corresponds to labradorite (50–70% An). The ore minerals present are titanomagnetite and ilmenite. The evaluation of T-fO2 based on titanomagnetite and ilmenite composition shows that phenocryst crystallization took place under QFM buffer. The presence of quartz and rutile xenocrysts in one Visoke leucite basanite lava points to the contamination of this melt by crustal material. At the same time, nonequilibrium megacrysts of high-magnesian olivine (Fo89) from another sample of Visoke leucite basanite resulted from leucite-basanite magma mixing with a more primitive melt.We identify melts from two different sources at Visoke edifice: ugandite and its possible derivatives were formed from the mantle source, whereas the leucite basanites are enriched in radiogenic strontium and could be contaminated by deep metamorphic rocks. The origin of the studied leucite basanites is the result of a combination of various processes: melting of the metasomatized mantle, fractional crystallization, magma mixing and crustal contamination.