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

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 (D ~1~ -D ~3~ ) and four episodes of metamorphism (M ~1~ -M ~4~ ) in the early Neoproterozoic. The M ~1~ assemblage consists of mineral inclusions defining an early foliation (S ~1~ ) within porphyroblasts, represented by chlorite + muscovite + biotite + plagioclase + quartz in pelitic schist and actinolite + chlorite + epidote + albite + or - quartz enclosed in amphibolite. M ~2~ coincides with the development of the regional schistosity (S ~2~ ) 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). M ~3~ is simultaneous with the third phase of deformation (D ~3~ ) and produced sillimanite-bearing mineral assemblages in pelitic schist and hornblende-bearing assemblages in amphibolite. The last metamorphic episode M ~4~ 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 M ~1~ of 5.5 to 6.0 kb and 450 degrees to 500 degrees C and conditions for the garnet, staurolite, and kyanite zones of M2 of 6.0 to 7.0 kb and 550 degrees to 600 degrees C, 6.0 to 7.5 kb and 600 degrees C and 11.0 to 11.5 kb and 600 degrees C. The P-T conditions of M ~3~ were estimated at 570 degrees to 625 degrees 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 degrees to 400 degrees C for M4, but the pressures of M ~4~ 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.