Crust-mantle interaction triggered by oblique subduction of the Pacific plate: geochronological, geochemical, and Hf isotopic evidence from the Early Cretaceous volcanic rocks of Zhejiang Province, southeast China

摘要

Large-scale volcanism in the late Mesozoic was a prominent geological event in southeast China. The late Mesozoic volcanic sequences, named the Moshishan Group, are exposed in Zhejiang Province and are predominantly felsic in composition with subordinate mafic magma and rare andesites. To understand the late Mesozoic tectonic evolution of southeast China, we present zircon U-Pb dating, major and trace element analyses, and Hf isotopic compositions from felsic volcanic rocks of the Moshishan Group. Zircon U-Pb dating shows that the Moshishan Group formed between 145 and 129 Ma. The εHf(t) of the analysed zircons ranges from -16.58 to +6.89, and the TMD2 age ranges from 753 to 2238 Ma with a major peak at ca. 1870 Ma. Hf isotopic compositions of zircons in Early Cretaceous volcanic rocks are more radiogenic than that of the metamorphic basement rocks, indicating a juvenile component in these magmas. Major element concentrations show that the volcanic rocks mainly belong to the high-K calc-alkaline series. Both zircon saturation temperatures and the εHf(t) values of zircons gradually increased with the evolution of the magma. Trace element data indicates that neither magmatic differentiation of mantle-derived magma nor mixing of magmas from different sources were the predominant magmagenetic processes. Earlier studies suggest that contemporaneous underplating contributed to the heat source that induced crustal melting and to the material origin that inconsistently mixed with the local crustal melts. Magmatic underplating is likely to have occurred because of the southwestward subduction of the Pacific plate with episodic slab rollback. The data obtained in this study suggest that the crust-mantle interaction under the influence of slab rollback played a progressive role in the formation of Early Cretaceous felsic volcanic rocks in southeast China.