Petrogenesis of the Mesozoic Shuikoushan peraluminous I-type granodioritic intrusion in Hunan Province, South China: Middle–lower crustal reworking in an extensional tectonic setting

Abstract

The Mesozoic Shuikoushan granitoid intrusion in the Jiangnan orogen between the Yangtze and the Cathaysia blocks in southern Hunan Province, South China, provides an opportunity to probe deep-crustal composition and structure, and to investigate the geodynamic evolution of South China. The intrusion is composed of granodiorite with I-type geochemical affinities and contains zircon with an U–Pb concordant age of 158.3±1.2Ma. Rocks from the intrusion have variable SiO2 from 58.4wt.% to 65.2wt.%, Al2O3 from 15.0wt.% to 17.2wt.%, TiO2 from 0.57wt.% to 0.76wt.%, and P2O5 from 0.27wt.% to 0.52wt.%. They have A/CNK from 0.97 to 1.23 (1.04 on average) and are dominantly peraluminous. They are characterized by enrichment in LREE ([La/Sm]N=3.47–5.19) and LIL (e.g., K, Rb) and depletion in HFSE (e.g., U, Th) in primitive mantle normalized trace element diagrams. These geochemical features indicate that the rocks are I-type granodiorites and have undergone extensive fractional crystallization of hornblende, biotite, titanite, and apatite. All samples show negative εHf(t) (−10.6 to −8.1) and εNd(t) (−5.92 to −6.13) values, with relatively high initial 87Sr/86Sr (ISr) (0.7101–0.7102) and δ18O ratios (8.4–9.7‰). These values significantly differ from mantle values, suggesting that they are generated by the amphibole-dehydration melting of a mafic source in the middle-to-lower crust beneath the Jiangnan orogen. The emplacement of the Shuikoushan granitoid intrusion is contemporaneous with the late Jurassic to early Cretaceous extension within the Shi-Hang rift zone, indicating that it may have been formed under an extensional regime in response to both the Pacific superplume activity and westward subduction of the Pacific Plate. The extensional processes along with asthenosphere upwelling and basaltic underplating may have triggered the widespread late Jurassic granitoid intrusion in southern Hunan Province and possibly in South China.