Metamorphic P-T-t evolution of pelitic granulite from Gaozhou, western Guangdong Province and its implications for the Early Paleozoic tectonic setting in South China

The formation and evolution of the early Paleozoic tectono-thermal event in South China remains a hotly debated topic within the academic community. Studies of metamorphism can provide pivotal insight into this issue. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb geochronology have been carried out on pelitic granulite from Gaozhou, southwestern part of the Cathaysia Block, South China. This paper will compare the early Paleozoic metamorphic data in the Cathaysia block to the new data provided here in order to discuss tectonic setting in South China. The pelitic granulite contains three generations of mineral assemblage, namely the inclusion, matrix and retrograde mineral assemblages. The inclusion mineral assemblage consists of garnet core and the wrapped K-feldspar, biotite, sillimanite, quartz, rutile and ilmenite within garnet. P-T conditions of the inclusion mineral assemblage are similar to 0.95 GPa and 800 degrees C (P-max stage), obtained by phase equilibria modelling using software THERMOCALC. The matrix mineral assemblage consists of garnet porphyroblast and the matrix-type cordierite, K-feldspar, biotite, quartz, sillimanite and ilmenite. Combined with the domain of matrix mineral assemblage in the pseudosection, the Ti content of matrix-type biotite and the Ab value of matrix-type K-feldspar, P-T conditions of the matrix mineral assemblage are defined to be 0.57-0.69 GPa and 815-865 degrees C (T-max stage). The retrograde mineral assemblage is composed by late cordierite, K-feldspar, biotite, plagioclase, sillimanite, quartz and ilmenite. Mineral compositions and stabilities indicate that the retrograde metamorphism is characterized by cooling and decompression. Zircon U-Pb dating of the pelitic granulite yields a metamorphic age of 441 +/- 1.4 Ma. Ti-in-zircon thermometer applied for the metamorphic zircons gives crystallization temperatures of 693-759 degrees C, which are consistent with the temperatures of the retrograde (cooling) metamorphic stage of the rock. Therefore, this metamorphic age is interpreted to represent the timing of retrograde metamorphism. After analysing the early Paleozoic metamorphic data collected from the Cathaysia block, it is concluded that the early Paleozoic metamorphism in South China matches with medium-pressure facies series and show clockwise P-T paths, without high-pressure facies series blueschist or eclogite being found yet. Medium-pressure facies series metamorphism can be formed both in continental collision orogens and intracontinental orogenic belts. However, those collision orogens are also featured by the presence of high- to ultrahigh-pressure blueschist and eclogite. The tectonic domains that underwent the early Paleozoic metamorphism, magmatism, and deformation are most likely not the locations of subduction-collision, based on current metamorphic data and other geological features (such as the lack of compelling ophiolites and arc-related magmatic rocks) in South China. The tectonic stress causing the early Paleozoic tectono-thermal event should come from the continental margin of South China at that time.