Geochronological and Petrological Constraints for Tectonic Evolution of the Central Greater Himalayan Sequence in the Kharta Area,Southern Tibet

摘要

Within the Kharta area,east of Mount Qomolangma(Everest),garnet sillimanite gneisses and granites including mafic lenses that form the Greater Himalayan Sequence(GHS)were displaced beneath the North Col Formation by the lower South Tibetan Fault(LSTF)and above the Lesser Himalayan Sequence by the Main Central Thrust(MCT).Zircons from mafic lenses,probably former dikes,were dated by thermal ionization mass spectrometry to yield discordant U-Pb results.One sample gave a discordia line with an upper intercept age of 971 Ma,suggesting a late Proterozoic age for emplacement of the dike.Zircon SHRIMP analyses show that the garnet sillimanite gneisses and granites were derived mainly from early Paleozoic rocks produced by high-temperature metamorphism and/or partial melting of Neoproterozoic(meta)sedimentary rocks.These crystalline rocks were buried beneath southern Tibet and experienced metamorphic conditions of 750°-800°C and 14 kbar at 33 ± 2 Ma.After exhumation they were transformed to low-pressure rocks at 23 ± 2 Ma.Structural analyses have revealed that further exhumation of the GHS is attributed to the LSTF and MCT faults beginning at 12-13 Ma,according to monazite U-Th-Pb dating.Finally,these rocks underwent north-south-trending folding at shallow depths after 12 Ma.