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

摘要

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 225 Ma Qiuwang and 178 Ma Xiepu granites at Ningbo, Zhejiang Province, have epsilon Hf(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 degrees C), and high Gd/Yb (2.7-4.8) and Ga/Al ratios (3.7-4.6 x 10(-4)). In contrast, the Early Jurassic Xiepu granite is weakly peraluminous with low zircon saturation temperatures (similar to 670 degrees C), low Gd/Yb (1.0-1.1) and high Ga/Al ratios (2.4-2.6 x 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 fiat-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. (C) 2015 Elsevier B.V. All rights reserved.