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    • Keywords

       

      Mantle lithosphere; pyrope garnet; chrome-diopsides; thermobarometry; trace element geochemistry; lamprophyre.

    • Abstract

       

      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 ${\sim}$6.5 GPa determined with thermobarometry are phlogopite eclogites. Thermobarometry on pyrope garnets, Cr-diopside and chromites yield a relatively low-temperature geotherm of 35 mW/m$^2$ at the base to 55 mW/m$^2$ at the Moho, while orthopyroxenes give 55 mW/m2. 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 reCect various melting degrees and variable trace element contents. The hightemperature 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.

    • Author Affiliations

       

      IGOR V ASHCHEPKOV1 NIKOLAY V VLADYKIN2 NIKOLAY S MEDVEDEV3 EVGENY I NIKOLENKO1 4 DENIS S YUDIN1 HILARY DOWNES5

      1. Sobolev N.V. Institute of Geology and Mineralogy SD RAS, Koptyug ave. 3, Novosibirsk, Russia.
      2. Vinogradov A.P. Institute of Geochemistry SD RAS, Favorski str.1a, Irkutsk, Russia.
      3. Nikolaev Institute of Inorganic Chemistry SD RAS, Novosibirsk, Russia.
      4. ALROSA (Zimbabwe) Limited, Van Praagh Avenue 19, Harare, Zimbabwe.
      5. Department of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK.
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  • Journal of Earth System Science | News

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