Reconstructions of lithospheric mantle beneath Aldan shield based on deep-seated xenocrysts from lamprophyres of Chompolo field, Russia

Lamprophyric (lamproitic) diatremes in Chompolo field, NE Aldan shield, Siberia dated 132 Ma contain abundant mantle xenocrysts studied by EPMA and LA-ICP-MS. The deepest rocks similar to 6.5 GPa determined with thermobarometry are phlogopite eclogites. Thermobarometry on pyrope garnets, Cr-diopside and chromites yield a relatively low-temperature geotherm of 35 mW/m(2) at the base to 55 mW/m(2) at the Moho, while orthopyroxenes give 55 mW/m(2). Amphiboles vary from magmatic Cr-hornblendes to metasomatic richterites. Trace element patterns for the host monchiquite are inclined with HFSE minima and are common for all Aldan lamprophyres but there are also more strongly inclined lamproitic varieties with ten times higher LREE contents. The geochemistry of Cr-diopsides, pyrope garnets and phlogopites reveal ancient multistage metasomatism of an originally depleted mantle keel related to both continental subduction and mantle plume activity. The lithospheric mantle is composed of harzburgites, roughly layered with rare dunites. Garnets appear widely at depths from 5.5 to 4.5 GPa. Trace element patterns show four groups from dunites to pyroxenites, related to low degree melting of hydrous and carbonatite melts. Clinopyroxenes reflect various melting degrees and variable trace element contents. The high-temperature orthopyroxene-based branches of the geotherm resulted from the percolation of hydrous plume-related melts. The high temperature 90 mW/m(2) geotherm from 2.5 GPa to the Moho reflects fractionation of Fe-rich plume-related siliceous melts. Eclogites and metasomatites melting at the lithosphere base produce lamprophyres rich in LREE (100/C1), LILE, Zr and Ta-Nb minima. The Mg-lamproites are more LREE-enriched to 1000/C1.