Geochemical and zircon U-Pb and Hf isotopic study of the Baijuhuajian metaluminous A-type granite: Extension at 125-100 Ma and its tectonic significance for South China

摘要

The Early Cretaceous Baijuhuajian pluton is an A-type granitic intrusion, emplaced along the Jiangshan-Shaoxing (JSSX) fault zone in western Zhejiang Province, SE South China. It intruded into a Late Jurassic volcanic basin bounded by Proterozoic sandstone and siltstone. The granite has a coarse-grained, porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of K-feldspar, quartz, plagiodase and biotite. The pluton has a clear A-type geochemical signature, e.g. it is metaluminous (A/CNK～1.0) and has high SiO2 (75 to 78 wt%), total alkalis (Na2O + K2O=4.8 to 8.5 wt.%), rare earth elements (total REE = 228 to 438 ppm), HFSE and Fe~* [FeO_t(FeO_t + MgO) = 0.87 to 0.97]. On the other hand, it is characterized by low CaO (0.51 to 1.01 wt%), TiO2 (0.06 to 0.13 wt%), P2O5 (below detection) and Sr (7.0 to 9.5 ppm). The hypersolvus texture and fine-grained groundmass indicate emplacement at a shallow-depth. SHRIMP U-Pb zircon dating yielded a weighted mean ~(206)Pb/~(238)U age of 126 ± 3 Ma and zircons from this pluton give εHf(T) values between -0.52 and +4.24, suggesting the importance of juvenile material in the magma source. Most of the plutonic rocks in southeast China are calc-alkaline, subduction-related intrusions of Jurassic and Cretaceous age. Previous studies suggested that the sparse A-type granites in South China were emplaced in two episodes: 190-155 Ma in the inland area and -100 Ma along the coastal area in SE South China. Our study of the Baijuhuajian granite, together with the coeval Suzhou A-type granite (123 Ma) in Jiangsu Province, defines an important extensional event between 125 and 100 Ma. Considering the available data for the Mesozoic igneous rocks in South China, we suggest that subduction of the Pacific plate in the Mesozoic produced voluminous calc-alkaline granitoids and volcanic rocks with a younging trend from west to east The A-type granites were generated during periods of local extension at 190-155 Ma and 125-100 Ma, perhaps corresponding to localized rollback of the Pacific plate.