ORIGIN AND EVOLUTION OF NEOGENE ALKALI–BASALTIC MAGMAS IN THE SOUTHWESTERN FLANK OF THE BAIKAL RIFT ZONE (Heven lava plateau, northern Mongolia)
S.S. Tsypukova1, A.B. Perepelov1, E.I. Demonterova2, L.A. Pavlova1, A.V. Travin3, M.Yu. Puzankov4
1A.P. Vinogradov 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
3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
4Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences, bulv. Piipa 9, Petropavlovsk-Kamchatsky, 683006, Russia
Keywords: Rift zone, plume, basaltic magma, crystallization, sources
Abstract
The Heven lava plateau in the Hövsgöl field of the South Baikal igneous province formed in the Early-Middle Miocene between 20 and 15.5 Ma. It consists of Early Miocene hawaiites and trachybasalts and Middle Miocene basanites erupted, correspondingly, during two major events in its history. The Heven alkali-basaltic lavas are compositionally similar to their counterparts from other volcanic fields in the southern flank of the Baikal rift system and are richer in Ba, K, Pb, and Sr than oceanic island basalts (OIB). The basanitic, hawaiitic, and trachybasaltic magmas were generated at pressures from 25 to 15 kbar and at temperatures in the range from 1434 to 1358 ºC. The magma sources occurred at 74 to 41 km in asthenospheric and lithospheric mantle and were ~200 ºC hotter than the ambient lithospheric mantle in the surrounding areas and the continental geotherm. The crystallization history of dark-colored began with liquidus highly magnesian olivine and Cr-spinel, and then several other parageneses formed successively as pressures and temperatures decreased: Ol + Cpx and Ol + Cpx+ + TiMgt ± Pl phenocrysts and subphenocrysts, Cpx + TiMgt + Ilm + Pl microphenocrysts, and finally interstitial Ne + Kfs alkali aluminosilicates. There were two crystallization stages with different mineral chemistry trends. The chemistry of minerals changed as the rising magmas first reached the crust-mantle region and then moved to shallow depths, erupted, and solidified. The generation of the Heven hawaiite-trachybasalt and basanite magmas was controlled by the depth of the reservoirs and the melt fraction in garnet-bearing asthenospheric and lithospheric mantle associated with progressive and regressive dynamics of the lower heterogeneous mantle plume consisting of PREMA and EMI components.
DOI: http://dx.doi.org/10.1016/j.rgg.2014.01.006
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