Mineral chemistry, P-T-t paths and exhumation processes of mafic granulites in Dinggye, Southern Tibet
Tóm tắt
The mafic granulites in Dinggye, as various scale lense-shaped enclaves within the high Himalayan crystalline rock series, occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System (STDS) and the Xainza-Dinggye normal fault system. The main lithological assemblage comprises garnet plagioclase pyroxenite, garnet two-pyroxene granulite, pyroxene garnet amphibolite and so on. The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution. The first metamorphic stage, the garnet+clinopyroxene+quart mineral assemblage (M1) was probably formed under eclogite facies, the second stage, the plagioclase+clinopyroxene symplectite mineral assemblage (M2) was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage, the third stage, the plagioclase+clinopyroxene+ hypersthene symplectite mineral assemblage (M3) was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage, pla-gioclase +hornblende mineral assemblage (M4) was formed by hydrolysis of earlier mineral assemblages during late uplifting. The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites, whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite. These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyroblasts (M1) could be formed at the eclogite facies, the early decompressive breakdown (M2) occurred at the high-pressures granulite facies of 1.35–1.48 GPa and 625–675°C, the M3 mineral assemblage recorded the granulite facies of 0.7–0.95 GPa and 775–900°C and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temperature at between 660 and 700°C These construct P-T paths from crustal subduction overthickening to tectonic uplift tectonothermal evolution. The mineral chemical characteristics and P-T condition at every metamorphic stage of these granulites indicate that these rocks experienced the eclogite facies metamorphism during the early stage. Subsequently, these mafic granulites underwent the three-stage exhumation of the eclogite facies tectonic uplift, isostatic uplift related to the transformation from eclogite/high-pressure granulite to granulite facies and extensional uplift.
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