Formation And Evolution Of The Shada High-Grade Metamorphic Rocks From The Southern Lhasa Terrane,Tibet

In the Shada area, the southern Lhasa terrane, the high-grade metamorphic rocks, occur as a block within the Late Mesozoic Gangdese granotoid batholith. The block consists of garnet-and silimanite-bearing mica schist, marble, amphibolite and calc-silicic rock. Petrological and geochronological studies are carried out for these schsits. The mica schist commonly shows porphyroblastic texture and schistoid structure, and has the following mineral assemblages:garnet+silimanite+biotite+muscovite+plagioclase+k-feldspar+ quartz, silimanite+garnet+biotite+muscovite+quartz, garnet+biotite+plagioclase+ quartz, indicating an amphibolite-facies metamorphism. The whole-rock compositions show that the schsits have variable SiO2 (41.56%?62.96%) and FeO (7.38%?16.33%), K2O (1.08?3.44), Na2O (0.31%?1.50%), CaO (0.36%?2.58%) and MgO (0.5%?3.84%) contents, and characterized by enrichment of Al2O3 (18.57%?34.25%), indicating that their protoliths are the pelitic sedimentary rocks. Zircons from the schists commonly have an inherited magmatic core and a metamorphic overgrowth rim. By LA-ICP-MS in-situ zircon U-Pb dating, the inherited magmatic cores of zircons from these rocks give an age population of-200 Ma. The metamorphic overgrowth rims of the zircons from these rocks give a metamorphic age of?90 Ma. Rare metamorphic zircons give a metamorphic age of?37 Ma. Based on these results, we conclude that the protoliths of the Shada high-grade metamorphic rock are probably the sedimentary rocks of the Sangri Group, which has the material source derived from the Early Mesozoic igneous rocks. During the emplacement of late Mesozoic Gangdese granitoids batholith, some parts of the Sangri Group have been subjected into the thermal contact metamorphism under amphibolite-faices. The coeval magmatism and metamorphism were related to the Andean-type orogeny derived from the subduction of the Neo-Tethy beneath the Lhasa terrane during late Mesozoic before the Indian collidig with the Eurasia plate. The genesis of the Shada high-grade metamorphic rock provides a new insight to the formation and evolution of the Lhasa terrane.