High-temperature metamorphism in a major strike-slip shear zone: the Ailao Shan—Red River, People's Republic of China

Abstract

Petrographic and thermobarometric analysis provides constraints on thePT path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indochina during the Tertiary. Two different paragenesis, P1 and P2, are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies. Microprobe analysis reveals that P1 garnets bear a chemical zonation from core to rim. This zonation indicates a temperature increase during garnet growth. Conditions of formation of garnet rims (P1b), which are estimated using biotite-garnet and plagioclase-garnet thermobarometers, are close to the granitic solidus (710 ± 70°C and4.5 ± 1.5 kbar). P2 conditions are estimated to be approximately 500°C and < 3.8 kbar. Both P1b and P2 conditions correspond to much higher temperatures than expected for their depths in the continental crust, suggesting a perturbed geothermal gradient during strike-slip deformation along the Ailao Shan-Red River shear zone. Thermochronology results suggest that cooling between P1b and P2 was fast ( ≈ 100°C/Ma) and may have been associated with significant uplift. Uplift during the left-lateral shearing may have resulted from a slight reverse or, more probably normal, component of movement along the strike-slip fault. A simple numerical model suggests that the high temperatures in the shear zone at the time of deformation may be explained by shear heating in the more competent upper mantle and by advection of this heat along the shear zone by ascent of magmas and/or fluids. In this hypothesis, the medium-pressure and temperature schists bounding the mylonitic gneisses to the southwest previously interpreted as resulting from collision-related metamorphism result instead from ‘contact’ metamorphism of the shear zone at mid-crustal depths.