MINERALOGY, GEOCHEMISTRY, AND SR-ND-PB ISOTOPE SYSTEMATICS OF LATE CENOZOIC BASANITES OF THE BOROZDIN BALD MOUNTAIN (Khentei Ridge, southern Transbaikalia)
A.Ya. Medvedev1, M.A. Gornova1, S.I. Dril’1, A.A. Karimov1, V.A. Belyaev1, A.V. Ivanov2, E.I. Demonterova2
1Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia
2Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
Keywords: Basalts, pressure, temperature, parental melt, crystallization sequence, plume, Pb-Nd-Sr isotopy
Abstract
We have estimated the P-T conditions of formation of basaltoid melts: P = 1.15-1.06 GPa and T = 1379-1293 ºC. Olivine pyroxenites (Ol + Cpx + Grt) are assumed to be a mantle source for nepheline-normative basanitic melts. During transition, the melt trapped mantle xenoliths, which were disintegrated into olivine and augite xenocrysts. A decrease in pressure and temperature led to the crystallization of highly magnesian (Mg# = 86) olivine and diopside and Timgt + Ilm ± Pl phenocrysts. Then, the Ol + Cpx + Timgt + Pl microlite paragenesis formed. Alkaline aluminosilicates (acid plagioclase + nepheline + leucite) were the last to crystallize in the rock interstices. The presence of residual glass indicates that the last stage of crystallization took place under subsurface conditions. The volcanic area of southern Transbaikalia (Khentei Ridge) resulted, most likely, from the mantle plume impact on the lithosphere. The age of this area is estimated at 3.51 Ma. PREMA was the main mantle source for these volcanics, and the contribution of HIMU was strongly subordinate. In geochemical features the studied volcanics correspond to mafic rocks of oceanic islands. They are similar in composition to alkali basalts of the South Baikal volcanic area.
DOI: 10.15372/RGG2019162
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