Cenozoic retrogression and exhumation of the amphibolites in the eastern Gangdese Belt, SW China

摘要

The collision between India and Asia is marked by the Yarlung Zangbo suture zone, which can be traced from Tibet into northwest Yunnan. The suture zone provides a record of the subduction of Neotethyan oceanic lithosphere beneath the Lhasa Block, creating the Gangdese magmatic belt, prior to the India-Asia collision. This paper reports petrological, zircon U-Pb, and whole-rock elemental and isotope data for amphibolite from the extension of the Gangdese belt in northwestern Yunnan. Two metamorphic stages are identified in relict amphibolite-facies rocks preserved in granitic gneisses from the belt. They are early amphibolite-facies metamorphism (M1) and late amphibolite-facies retrograde metamorphism (M2). The assemblage of M1 comprises garnet, amphibole, and plagioclase that formed at 0.89-0.97 GPa and 658-700 degrees C. Dating of syn-metamorphic zircons assigns this event to the middle Eocene (44-38 Ma). Symplectites (corona) of amphibole, plagioclase, and quartz surrounding garnet formed during retrograde stage M2, at 30-29 Ma, under P-T conditions of 0.41-0.68 GPa and 549-716 degrees C. The amphibolites display geochemical characteristics similar to those of mid-ocean ridge basalt, with epsilon Nd(t) values of +1.8 to +8.9, and are inferred to have formed in a back-arc-basin setting, suggesting that they constitute part of the lithospheric mantle of the Tengchong terrane that lies to the east. The amphibolite-facies metamorphism was related to underplating of the Indian plate during the India-Asia collision, which led to clockwise rotation of eastern Tibet during the middle Eocene, generating amphibolite recrystallization and, subsequently, with ongoing convergence of the two plates, exhumation of the region during the middle Oligocene.