Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline...Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline and shoshonitic characteristics.Relict porphyritic,sub-ophitic and poikilitic textures are noted in these rocks.Their parent magma was emplaced during the waning phase of the regional metamorphism.Geochemically,these metamafics are similar to the GroupⅢpotassic and ultrapotassic rocks of Foley et al.(1987).The magma was derived from the metasomatized subcontinental lithospheric mantle(SCLM).Subduction-related sediment melts metasomatized the SCLM.Compositionally,the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite.1%–5%batch melting of the SCLM could produce the parental magma of the mafic granulites.Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling.Peak metamorphic assemblage(M_(1):Grt-Cpx-Pl-Qz)in garnetiferous amphibolite equilibrated at 740℃and 8.7 kbar.The Cpx-Pl corona appeared around the garnet during decompression(M_(2):655℃,6 kbar).The Hbl-Pl symplectites around garnet formed during isobaric cooling(M_(3):580℃and 5.9 kbar).The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision(~990 Ma).展开更多
基金partly funded by the UGC-UPE-II given to the University of Calcutta under the Non-focus Area ProgrammeThe Research Fellowship in Sciences for meritorious students(RFSMS)of the University Grants Commission,Government of India,awarded to Susmita Das(No.F.747/2007 BSR)。
文摘Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline and shoshonitic characteristics.Relict porphyritic,sub-ophitic and poikilitic textures are noted in these rocks.Their parent magma was emplaced during the waning phase of the regional metamorphism.Geochemically,these metamafics are similar to the GroupⅢpotassic and ultrapotassic rocks of Foley et al.(1987).The magma was derived from the metasomatized subcontinental lithospheric mantle(SCLM).Subduction-related sediment melts metasomatized the SCLM.Compositionally,the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite.1%–5%batch melting of the SCLM could produce the parental magma of the mafic granulites.Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling.Peak metamorphic assemblage(M_(1):Grt-Cpx-Pl-Qz)in garnetiferous amphibolite equilibrated at 740℃and 8.7 kbar.The Cpx-Pl corona appeared around the garnet during decompression(M_(2):655℃,6 kbar).The Hbl-Pl symplectites around garnet formed during isobaric cooling(M_(3):580℃and 5.9 kbar).The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision(~990 Ma).
