Peridotite pyroxenite and granulite xenoliths from Cenozoic Shavaryn Tsaram volcanic center (Mongolia): New data

Mantle lherzolites (spinel- and garnet-bearing), megacrysts and pyroxenites from Moho and lower crust xenoliths of the Cenozoic basalts of Shavaryn-Tsaram volcanic center were newly studied by the EPMA, SEM and LA-ICP methods to define more precisely their geochemical features for genetic purposes.Thermal conditions in sampled mantle are close to the Southeastern Australian (SEA) mantle plume geotherm in 1.2–2.5 GPa divided into three intervals according to orthopyroxene-garnet barometry. The monomineral thermobarometry for ortho-, clinopyroxenes and garnets gives wider PT range to 3.0 GPa deviating to lower temperature clusters at 2–2.4 GPa. Black megacrysts show ascending from 3.0 to 1.0 GPa trend for pyroxenes and garnet accompanied by increase of Fe# for equilibrated with olivine (Ol) from 0.15 to 35 %. The intermediate Cr-bearing transitional pyroxenites are locating between the lherzolitic and black pyroxenite trends. They are referring to the primitive or slightly depleted associations according to petrographic and trace element (TRE) compositions. According to TRE of clinopyroxenes from spinel peridotites were divided into 5 groups: primitive, slightly depleted, deeply depleted; metasomatized by light rare earth (LREE); rich-Th-U (reactions with carbonatite melt) and in addition rare garnet-bearing types. Orthopyroxenes reveal decreasing to LREE straight line REE patterns and in TRE spiderdiagrams U, Ta and high Hf peaks. Some clinopyroxenes have Sr peaks and jagged left part of multicomponent spiderdiagrams (MSD). Olivines show and concaved from Eu to Tm in spider diagrams they reveal minima in Th, Nb, Pb, Zr. The Cr- bearing amphiboles reveal slightly inclined to LREE pattern at 90–60/CI (chondrite). And garnet show LREE depleted patters from Lu at 10/CI to 0.1 for La and depleted all left part at spider diagram, The pyroxenitic clinopyroxenes have flattened REE pattern or rarely depleted in heavy (HREE) and the hump from La to Sm. They reveal minima in Zr, Hf, Sr, Pb and left part starting from Nb. Garnets in black pyroxenites have higher HREE at 80/CI (chondrite) and Hf peaks and lowered jugged left part in MSD. The clinopyroxenes from black garnets pyroxenites reveal asymmetric bell- like REE with the depleted HREE to 1/CI and middle rare earth (MREE) at 12/CI. The clinopyroxenes from plagioclase-bearing varieties show flattened REE pattern at 100–80/CI and Eu minima and the MSD for megacrystic pyroxenes. Rare mantle metasomatites contain mica or amphiboles and are LREE enriched. Peridotites from Shavaryn Tsaram have no signs of essential mantle diapirism and represent ancient MORB mantle.Determined with LA-ICP-MS age of zircon in granulite 322 ± 8 Ma refer to rifting processes in Central Mongolia and be-modal volcanism while 42 ± 0.4 Ma to Early Cenozoic plume magmatism.