Petrogenesis of two Triassic A-type intrusions in the interior of South China and their implications for tectonic transition

Abstract

The evolution of the tectonic regime that was responsible for the Indosinian granitoids in the South China Block (SCB) is still controversial. Investigations on A-type granites can provide important information regarding this tectonic evolution. A detailed study that utilizes whole-rock elemental, Sr–Nd isotopic, in situ zircon U–Pb and Lu–Hf isotopic geochemistry is conducted on the Miantuwo biotite granite in northern Guangdong Province and the Pingtian biotite monzogranite in southern Jiangxi Province, South China. The new data indicate that both the Miantuwo and Pingtian granites were emplaced at 233±2Ma and show metaluminous to slightly peraluminous A-type granite affinity. The two granites are characterized by high amounts of rare earth elements (total REEs=247ppm–557ppm and 251ppm–342ppm) and high field strength elements (Zr+Nb+Ce+Y=325ppm–605ppm and 343ppm–496ppm) and high Ga/Al ratios (10,000×Ga/Al=2.50–2.98 and 2.62–2.70). Calculations from a zircon saturation thermometer and apatite saturation thermometer indicate that the magmatic temperatures were 800°C–980°C for both granites. Both the Miantuwo and Pingtian granites show relatively high initial 87Sr/86Sr ratios (0.7151–0.7185 and 0.7170–0.7189), low εNd(t) values (−9.8 to −8.6 and −9.7 to −9.1) and low to moderate zircon εHf(t) values (−10.4 to −6.6 and −9.5 to −4.6). Based on these data, we suggest that these two A-type granites were derived from the partial melting of existing mafic to intermediate rocks in the lower crust in response to the underplating and/or intraplating of mantle-derived magma. Our study on the Miantuwo and Pingtian granites, alongside previous studies on other Triassic A-type granites in South China, indicates an extensional tectonic environment during the Late Triassic in the interior of the Cathaysia Block. Alongside existing geological observations and the tectonic evolution in the SCB, we suggest that the interior of the SCB was dominated by a compressional tectonic environment during the Late Permian–Middle Triassic in response to the collisions between the SCB and ambient blocks, and then a tectonic transition from this compressional environment to a post-collisional extension environment began at approximately 233Ma.