Tectonothermal evolution of the Mayuan Assemblage in the Cathaysia Block; implications for Neoproterozoic collision-related assembly of the South China Craton

Abstract

The Mayuan assemblage in the Cathaysia Block, Southeast China, consists of felsic paragneiss, pelitic schist, greenschist, amphibolite, marble, calcsilicate, and quartzite that underwent three episodes of deformation (D1-D3) and four episodes of metamorphism (M1-M4) in the early Neoproterozoic. The M1 assemblage consists of mineral inclusions defining an early foliation (S1) within porphyroblasts, represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite. M2 coincides with the development of the regional schistosity (S2) and represents the formation of the porphyroblasts and growth of matrix minerals, resulting in development of prograde metamorphic zones (chlorite-biotite, garnet, staurolite, and kyanite zones). M3 is simultaneous with the third phase of deformation (D3) and produced sillimanite-bearing mineral assemblages in pelitic schist and hornblende-bearing assemblages in amphibo- lite. The last metamorphic episode M4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite. The sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near-isothermal decompression clockwise P-T path for the Mayuan pelitic schists. Using the TWEEQU software program, the garnet-biotite thermometer and garnet- muscovite-biotite-plagioclase barometer yield P-T conditions for M1 of 5.5 to 6.0 kb and 450° to 500°C and conditions for the garnet, staurolite, and kyanite zones of M2 of 6.0 to 7.0 kb and 550° to 600°C, 6.0 to 7.5 kb and 600°C and 11.0 to 11.5 kb and 600°C. The P-T conditions of M3 were estimated at 570° to 625°C and 4.0 to 4.5 kb using the muscovite-biotite thermometer and hornblende-plagioclase geothermo- barometer. The garnet-chlorite thermometer yields temperatures of 300° to 400°C for M4, but the pressures of M4 cannot be quantitatively estimated because of the lack of a suitable geobarometer. These P-T estimates also define an near- isothermal decompression clockwise P-T path, which is involved in initial crustal thickening followed by rapid exhumation and final cooling, and is related to amalgamation of the Cathaysia and Yangtze Blocks to form the South China craton.