西藏定结高喜马拉雅石榴辉石岩-镁铁质麻粒岩的岩石特征及其地质意义

在高喜马拉雅带的定日县曲当—扎乡一带出露的高喜马拉雅结晶岩系中, 发现了高压变质的石榴辉石岩及其降压变质的镁铁质麻粒岩组合, 早期高压条件下形成的石榴辉石岩矿物组合为Grt+Cpx (富铝) +Ru+Q, 斜长石已完全消失, 形成温度为845~896℃, 压力大于1.2GPa, 已达到榴辉岩相的压力条件.中期的麻粒岩相组合为Opx+Pl±Cpx±Ga, 其中Opx、Cpx和Pl为石榴石的后成合晶, 形成温度为993~776℃, 压力为0.90~1.21GPa, 为中压麻粒岩相产物, 晚期矿物仅见普通角闪石、斜长石和石英, 是角闪岩相退变质的产物, 表明HHC经历了降压升温-降压降温的快速抬升过程, 证明其抬升作用与地幔热源的参与有关. 

Petrologic and Geologic Significance of Garnet Pyroxenite and Mafic Granulites from High Himalayan Region, Tibet

HP garnet-pyroxenite and the mafic granulites formed by decompression have been found in the high-Himalaya crystallinites (HHC) from Kada-Zaxang near Dingri County, Tibet. The mineral assemblage of the HP garnet-pyroxenite in early stage, Grt+Cpx (rich in Al₂O₃) +Ru+Q, without any plagioclase, was formed in 845-896 ℃ and 1.2 GPa, with pressure as high as that in eclogitic phase. While the mineral assemblage of mafic granulite was Opx±Cpx (Al-poor) +Pl (An high up to 90) ±Grt (rim), of which the Opx, Cpx and Pl are symplectite which replaced the early Grt and Al-rich clinopyroxene in decompression condition: t=993-776 ℃, p=1.21-0.80 GPa. The latest mineral assemblage, Hb+Pl+Q, found to have replaced the assemblage of mafic granulite, was formed by retrograde metamorphism in amphibolitic phase. All of this show that the HHC have undergone an HP metamorphism and a stage of decompression retrograde metamorphism and that in early stage the temperature rose with the decompression, but in late stage it dropped with the decompression. Therefore, it can be assumed that the uplifting of the HHC was related to the hot source of the mantle.