Geochronological and geochemical constraints on the petrogenesis of late Triassic aluminous A-type granites in southeast China

Abstract

The late Permian–Triassic granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Most of the intrusive bodies in the central SCB consist of S-type granites. Here, we report the first unambiguous discovery of Wengshan aluminous A-type granites in southwest Zhejiang Province, southeast China. SHRIMP and LA-ICP-MS zircon U–Pb analyses yielded similar ages about 224 Ma for two samples from the Wengshan pluton, in concordance with peak time of late Indosinian (234–205 Ma) tectonic magmatic activity in the SCB. The Wengshan granites are peraluminous with A/CNK values ranging from 1.09 to 1.24, enriched in K, Rb, Th, REE and Zr + Nb + Ce + Y, have elevated FeO tot/MgO and Ga/Al ratios, and are depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. In addition, they show high zircon saturation temperatures (820–878 °C). Their REE patterns are highly fractionated, with (La/Yb) N ratios of 36.89–101.24 and strongly negative Eu anomalies (Eu/Eu * = 0.28 − 0.53). In-situ zircon Hf isotopic analyses indicate that the Wengshan granites have ε Hf( t) values ranging from −15.9 to −12.7 and two-stage depleted mantle Hf model ages from 2.05 to 2.26 Ga. Sr–Nd isotope systematics are characterized by a high initial 87Sr/ 86Sr ratios of 0.7092–0.7218 and a low ε Nd( t) values of −15.0 to −12.1, with two-stage depleted mantle Nd model ages of 1.99–2.22 Ga, consistent with those of Paleoproterozoic basement rocks in the area. It is inferred that the Wengshan granite magma formed by partial melting of Paleoproterozoic metasedimentary sources in Cathaysia Block. Meanwhile, the underplating of mafic magma in the lower crust may provide the heat source for the formation of the Wengshan granites. The data show that the Wengshan granites, unlike other S-type granites in the SCB, are late Triassic aluminous A-type granite. Therefore they may indicate distinct geodynamic significance. Our study of the Wengshan A-type granites (224 Ma), together with Jingju A-type granites (215 Ma) in Zhejiang Province and Gaoxi A-type granites (215 Ma), Xiaotao A-type granites (222 Ma), Tieshan (254 Ma) and Yangfang (242 Ma) alkaline syenites in Fujian Province, define important extensional events in Triassic. Considering these available data, we suggest that the late Permian–Triassic A-type granites and alkaline syenites in the coastal region of the SCB probably formed under an extensional regime related to oblique subduction of the paleo-Pacific Plate.