Petrology and metamorphic evolution of staurolite-bearing garnet amphibolite in Nyingchi complex of the Lhasa Block

Abstract:The newly found staurolite-bearing garnet amphibolites in Nyingchi complex of the Lhasa Block have the mineral assemblage of garnet, amphibole, staurolite, chlorite, plagioclase, mica and minor ilmenite and apatite. The cores of the garnet in the garnet amphibolite are extremely rich in Mn (XSps=0.12-0.15) and poor in Fe (XAlm=0.45-0.50), whereas their rims are relatively Mn poor (XSps=0.01-0.03) and Fe rich (XAlm=0.60-0.65), implying that the core and the rim of the garnets belong to two metamorphic generations. Xpy increases and Xgr decreases from the garnet core to the rim, indicating characteristics of prograde metamorphic zonation. The amphibole growing at different metamorphic stages have obvious composition differences. Staurolite, as an uncommen aluminium-rich mineral in the metabasite, has also recorded different metamorphic processes in their microstructures. In combination with the garnet compositional profile, it is possible to calculate the P-T evolution by phase diagram. The authors modeled the pseudosection of the staurolite-bearing garnet amphibolite under the model system of Mn-NCKMASHO, using Perple-X program with the help of Xpy and Xgr isopleths of the garnet, and obtained the peak metamorphic condition of 620oC and 12kbar. The peak mineral assemblages are garnet, amphibole, staurolite and mica. In the meanwhile, the clockwise P-T path has also been demonstrated by the microstructures in the staurolites, suggesting that the staurolite-bearing garnet amphibolites have experienced three stages of metamorphism.