High-pressure granulite-facies metamorphism and anatexis of deep continental crust: New insights from the Cenozoic Ailao Shan–Red River shear zone, Southeast Asia

Abstract

Studies of crustal anatexis have provided valuable insights into the evolution of metamorphism, deformation, and tectonic processes at convergent plate margins during the orogeny. The transition of metatexite to diatexite migmatites records crucial information about the tectonothermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites, and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high-pressure granulite-facies metamorphism, with the mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt, is first recognized from the patch of metatexite migmatites in the complex. Detailed petrographic evidence and thermodynamic modeling revealed that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained at approximately 11 kbar and 810 °C using thermodynamic modeling, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of the melts are evidenced by the presence of leucosomes within the migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircon and monazite geochronology of migmatites record the timing of the melting episode that began at approximately 36 Ma and lasted until 20 Ma. All these results are consistent with orogenic crust thickening accompanied by pervasive anatexis during the Late Eocene to the Early Oligocene in the Ailao Shan–Red River shear zone. Combined with the available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis had a significant effect on the thermal state of deep–seated shear zones, thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.