Zircon U–Pb chronology and elemental and Sr–Nd–Hf isotope geochemistry of two Triassic A-type granites in South China: Implication for petrogenesis and Indosinian transtensional tectonism

Abstract

A detailed study utilizing zircon U–Pb dating, major and trace element geochemistry, and Sr–Nd–Hf isotope geochemistry has been carried out for the Caijiang granite in Jiangxi Province and the Gaoxi granite in Fujian Province, South China. The new data indicate that the Caijiang and Gaoxi granites are Triassic (228–230Ma) and have the petrographic and geochemical characteristics of A-type granites. In both granites, biotite occurs along the boundary of euhedral plagioclase and quartz, which implies that the primary magma could have been anhydrous. The two granites show high contents of total alkalis (Na2O+K2O=7.81–12.15%), high field strength elements (e.g. Zr=240–458ppm, Y=16.8–38.0ppm, Nb=13.5–33.8ppm and Zr+Nb+Ce+Y=382–604ppm) and rare earth elements (total REE=211–373ppm) as well as high Ga/Al ratios (10000×Ga/Al=2.41–3.53). The lowest magmatic temperatures estimated from zircon saturation thermometer were 800–840°C for the Caijiang granite and 820–850°C for the Gaoxi granite, respectively. The Caijiang granite has relatively high (87Sr/86Sr)i ratios of 0.71288 to 0.72009, low εNd(t) values of −9.9 to −9.3, and low zircon εHf(t) values (peak value of −7.5). Whole-rock Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.65Ga to 1.80Ga. The Gaoxi granite has also high (87Sr/86Sr)i ratios of 0.71252 to 0.71356, low εNd(t) value of −13.8 and low zircon εHf(t) values (peak value of −12.0). Whole-rock Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.95Ga to 2.10Ga. According to these data, we suggest that the two granites might have been derived from partial melting of Precambrian crustal rocks that had been granulitized during an earlier thermal event. Our study of the Caijiang and Gaoxi granites, together with previous studies on two Triassic alkaline syenites (Tieshan and Yangfang) in Fujian Province and one A-type granite (Wengshan) in Zhejiang Province in South China, indicate a wide transtensional tectonic environment in the Cathaysia Block that lasted at least from 254Ma to 225Ma. Combined with extant data for the Indosinian granites and tectonic evolution in South China, we suggest that the formation of A-type granites was related to the local NE-trending extensional faults probably caused by collision between the South China Block and the Indochina Block or the North China Block.