Revisiting Mesozoic felsic intrusions in eastern South China: spatial and temporal variations and tectonic significance

Abstract

© 2017 Elsevier B.V. Whole-rock and mineral geochemical data are used to place new constraints on the petrogenesis and tectonic setting of Mesozoic granitoids (including syenites) in eastern South China. In the Early Mesozoic, granitoids of variable compositions were intruded in the Cathaysia Block which by this time had developed a thickened and highly differentiated Paleoproterozoic crust through the influence of subduction. Late Triassic (~ 225 Ma) syenites are significantly different from Jurassic-Cretaceous syenites in South China and from most trachytes (GEOROC database) in terms of their high Th/U, La/Nb and Gd/Yb ratios. Their low Rb contents, coupled with their high K/Rb and Nb/Ta, and low 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios suggest a source that had undergone granulite-facies metamorphism at the base of thickened ( > 45 km thick) continental crust where garnet and rutile are stable. The Late Triassic alkaline intrusions thus appear not to be related to continental rifting. Compared with the Late Triassic syenites, contemporaneous syenogranites have higher Ga/Al and Rb/K ratios and I Sr values. Their Ga/Al ratios are positively correlated with I Sr values, and their higher Ga/Al ratios likewise do not appear to be related to a rift setting but reflect the composition of the source. New Pb isotopic data from Cretaceous magmatic rocks reveal that 120–100 Ma I-type granitoids in Zhejiang Province were likely derived from mixing of three components: contemporaneous basaltic magma, an enriched crustal component and a depleted crustal component. Pb isotopes of both the I-type granitoids and the basalts become more radiogenic towards the coast, where the ca. 100 Ma intrusions dominate. Furthermore, zircon-melt partition of Ce and hornblende oxygen barometries indicate that the Early Cretaceous intrusions also became more oxidized towards the coast. In addition, the ca. 100 Ma granitoids have higher Gd/Yb and lower Fe/Mg ratios than those of the 120–110 Ma suite, implying crustal thickening resulting from 120 to 100 Ma basaltic underplating in the region.