Fabrics and ~(40)Ar/~(39)Ar ages of metamorphic rocks in the Gaoligong tectonic belt: Implications for Cenozoic metamorphism and deformation in the SE Tibetan Plateau

摘要

As one of the major tectonic belts in the southeastern Tibetan plateau, the Gaoligong tectonic belt plays important roles in accommodating the Indian-Eurasian plate collision. In this belt, medium-high-grade metamorphic rocks are preserved. In this study, we selected three cross-sections from the northern to the southern segment of the Gaoligong tectonic belt for detailed structural (macro- and micro-) observations and quartz fabric analysis. Quartz c-axis fabrics reflect three ranges of metamorphic temperature in the metamorphic rocks: Range 1 ( > 650-700 degrees C, corresponding to granulite facies), Range 2 (550-650 degrees C, amphibolite facies), and Range 3 (400-500 degrees C, greenschist facies). Mineral separates (amphibole, muscovite, and biotite) from five samples taken from the Lushui-Pianma section were selected for Ar-40/Ar-39 study, yielding plateau ages of similar to 30 Ma (amphibole), -17 Ma (biotite), and similar to 15 Ma (muscovite), respectively. Combined with the previous geochronological data, it is suggested that there are three stages of metamorphism-deformation of the Gaoligong tectonic belt during Cenozoic. The early stage of metamorphism occurred at high temperature conditions ( > 650-700 degrees C, corresponding to granulite facies) during 50-34 Ma, coeval with the Cenozoic collision of the Indian Plate and Tengchong Block. The subsequent stage of metamorphism occurred at medium to low temperatures (550-650 degrees C, 400-500 degrees C, respectively) during 34-10 Ma and is the result of a non-coaxial progressive deformation showing dextral shear along the Gaoligong strike-slip fault. The latest stage represents rapid exhumation and cooling during 10-3 Ma.