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

Abstract

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 °C, corresponding to granulite facies), Range 2 (550–650 °C, amphibolite facies), and Range 3 (400–500 °C, greenschist facies). Mineral separates (amphibole, muscovite, and biotite) from five samples taken from the Lushui-Pianma section were selected for 40Ar/39Ar study, yielding plateau ages of ~30 Ma (amphibole), ~17 Ma (biotite), and ~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 °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 °C, 400–500 °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.