Metamorphic P-T conditions, in-situ LA-ICP-MS biotite-muscovite Rb-Sr geochronology and tectonic implications of two-mica-quartz schist from the Kuanping Group in the North Qinling Orogenic Belt

The Kuanping Group is an important lithotectonic unit in the North Qinling Tectonic Belt, which has experienced metamorphism from greenschist to high amphibolite facies and is of great significance in defining the orogenic process of the Qinling Orogenic Belt. This study presents the results of petrology, geothermo-barometry, phase equilibrium modelling, in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) biotite-muscovite Rb-Sr analysis, for the two-mica-quartz schist of the Kuanping Group in the Hongtuling and Heihe areas. These results are employed to better understand their metamorphic P-T conditions and ages, as well as geological implications. Garnet-bearing two-mica-quartz schist in the Hongtuling area records a clockwise P-T trajectory on the basis of phase equilibrium modelling, with peak P-T conditions of similar to 600 degrees C and similar to 0.85 GPa. In addition, the Ti-in-biotite thermometry yielded metamorphic temperature conditions of similar to 578 degrees C-580 degrees C and similar to 590 degrees C-596 degrees C for the garnet-bearing two-mica-quartz schist from the Hongtuling area and the two-mica-quartz schist from the Heihe area, respectively. These results indicate that metamorphic temperature conditions in both areas are higher than the closure temperature of the Rb-Sr isotopic system of muscovite and biotite. In-situ LA-ICP-MS biotite and muscovite Rb-Sr analyses yielded three groups of isochron age, i. e., ca. 410 Ma, ca. 360 Ma and ca. 320-300 Ma. On the basis of previously published data and the data in this study, the Kuanping Group underwent ca. 440 Ma regional metamorphism in the NQB, retrograde cooling at ca. 410-360 Ma due to exhumation and ca. 320-300 Ma tectonothermal overprinting.