Metamorphic Evolution And Tectonic Implications Of Garnet-bearing Mica-quartz Schist In Sumdo HP Belt,Lhasa Block

Garnet-bearing mica quartz schist of Sumdo HP belt in Lhasa block as country rocks of eclogite, which is mainly composed of garnet, muscovite, albite, chlorite, quartz,a few rutiles and sphenes. Garnet displays an obvious zonation where Xpy increases from the core to mantle, and then decreases in the rim, as Xsps declines gradually from the core to mantle, after rising trend of declining in the rim, suggesting garnet composition profiles from core to mantle preserve the prograde growth zoning and have been partially reset during retrogression. The model system NCKMn FMASHO is chosen to calculate P-T and P-M(H2O) pseudosections, Garnet isopleth thermobarometry involves plotting compositional isopleths of garnet as contours on a P-T pseudosection, contours of saturated H2 O content is combined, obtaining estimated peak P-T conditions of 27 kbar, 523/580 ℃ and peak mineral assemblages of(g-jd-cr-law(+phe+q/coe+H2O). The composition profile of garnet from the core to the mantle and contouring of the H2 O content saturated indicate that prograde metamorphic evolution represents a cold subduction stage with heating as pressure increasing, the rocks experience blueschist-facies to eclogite-facies metamorphism in this stage. P-M(H2O) pseudosections and isopleth of saturated H2 O content are appropriate to assess evolving mineral assemblages in terms of changes in water content during decompression, which shows that garnet-bearing mica quartz schist go into an early isothermal decompressional process and a cooling decompressional evolution in the late stage, amphibolite-facies to epidote-amphibolite-facies metamorphism occur in early stage and greenschist-facies metamorphism overprint during the later-stage decompression. Based on the LA-ICP-MS in situ zircon U-Pb chronology, the metamorphic age 230.5±3.4Ma we acquired from the garnet-bearing mica quartz schist which is recognized as the age when the eclogite and the garnet-bearing mica quartz schist went through peak P-T conditions metamorphism and oceanic crust subduction occurred in late stage and subduction got in transitional stage from ocean-continent subduction to continent-continent subduction, part of the ocean-continent broken with the others that keep go in deep of the mantle was taken to the surface by the low-density rocks, which could be recognized as the mechanism of Sumdo eclogite exposed in the earth surface. Compared to the P-T path and contact relation in the field of garnet-bearing mica quartz schist and eclogite, we can come to a reasonable conclusion that garnet-bearing mica quartz schist and eclogite have similar subduction and exhumation process.