Early Mesozoic ferroan (A-type) and magnesian granitoids in eastern South China: Tracing the influence of flat-slab subduction at the western Pacific margin

Abstract

Zircon U–Pb ages, Hf isotopes and whole-rock geochemical data are used to constrain the petrogenesis of the Late Triassic–Early Jurassic granitoids in southeastern China and their tectonic settings. The 225Ma Qiuwang and 178Ma Xiepu granites at Ningbo, Zhejiang Province, have εHf(t) values of −15.7 to −9.9 and −17.6 to −13.4, respectively. They are both ilmenite-series granitoids derived from the Paleoproterozoic basement of the Cathaysia Block in South China. The Late Triassic Qiuwang granite is strongly peraluminous with high zircon saturation temperatures (850–900°C), and high Gd/Yb (2.7–4.8) and Ga/Al ratios (3.7–4.6×10−4). In contrast, the Early Jurassic Xiepu granite is weakly peraluminous with low zircon saturation temperatures (~670°C), low Gd/Yb (1.0–1.1) and high Ga/Al ratios (2.4–2.6×10−4). The Qiuwang granite records higher melting temperatures and pressures than the younger Xiepu granite, reflecting a change in the tectonic regime from collisional tectonics to orogenic collapse. A regional review indicates that Triassic flat-slab subduction along the western Pacific may have led to thickening of the continental crust and metasomatism-oxidation of the continental lithosphere in the regions relatively close to the trench, where Cordilleran-type magnesian granitoids were generated. However, contemporaneous magnesian granitoids were produced in the thickened crust away from the trench, where the source rocks have uniformly moderate oxygen fugacities and metasomatism-oxidation was insignificant. Such Early Mesozoic “A-type” granitoids in the coastal region of South China are related to water-deficient and reduced melting conditions rather than an anorogenic tectonic setting.