Petrology And Metamorphic Evolution Of The Bashiwake Garnet Peridotite From The South Altyn Tagh

The Altyn Tagh, which is located at the northern margin of Tibet Plateau, is a typical ultra-high pressure metamorphic belt, characterized by the occurrence of eclogite, garnet peridotite and related gneiss. Garnet peridotite from the Bashiwake area in the South Altyn Tagh occurs as slices or lenses within HP mafic granulite and garnet-bearing felsic geniss.On the basis of textural relationships, mineralogical data and thermobarometer estimations, three stages of the metamorphic evolution have been identified, including peak metamorphic stage (M1), decompression retrograde stage (M2) and late amphibolite-chlorite-facies retrograde stage (M3). Stage I is defined by the assemblage garnet+olivine+orthopyroxene +clinopyroxene, which formed at 891～1054℃ and 17.2-24.7kbar. Stage II is an initial decompression stage, characterized by the formation of kelyphitic rims of orthopyroxene+ clinopyroxene+aluminous spinel around garnet, yielding 711～796℃ at 10 kbar. Stage Ⅲ is represented by the formation of amphibole+serpentine+phlogopite+chlorite+magnetite± talc. The P-T evolution of garnet peridotite is similar to that of the associated high pressure felsic granulite and mafic granulite.Geochronology of zircon U-Pb indicates that the peak metamprphic age is at 495-503Ma, which is in consistent with the time of eclogite-facies UHP metamorphism of adjacent eclogite unit. Combining with mineralogical and geochemical characteristics, we speculated that the protolith of garnet peridotite was mantle-derived mafic-ultramafic complex emplaced in the crust, which was subducted together with felsic crust material in the early Paleozoic, and they shared the subsequent metamorphic and geodynamic evolution.