Multiple origins for the Middle Jurassic to Early Cretaceous high-K calc-alkaline I-type granites in northwestern Fujian province, SE China and tectonic implications

Abstract

A comprehensive study of zircon U–Pb dating and in situ Hf isotopes, whole-rock major and trace element geochemistry and Sr–Nd isotopes was carried out for three late Mesozoic granitic plutons (Waitun, Shipi and Taiyuan) in northwestern Fujian province, SE China. We assess the origin of the granites and their relationship to the evolution of the late Mesozoic volcanic–intrusive complex belt in SE China. LA-ICP-MS zircon U–Pb dating shows that three plutons were emplaced in the Middle Jurassic to Early Cretaceous (168–109Ma), in which the Waitun and Shipi plutons are intrusive complexes. All the plutons are composed of high-K calc-alkaline I-type granites with a great diversity in elemental and isotopic compositions. The granites have SiO2 contents of 68.3–78.5wt.%, showing a gradual decrease in initial 87Sr/86Sr (0.7181 to 0.7091) and increase in εNd (T) (−16.7 to −8.1) and εHf (T) (in-situ zircon) (−20.6 to −6.9) with decreasing emplacement ages. Geochemical data suggest that the Middle Jurassic (~168Ma) Waitun granites are of purely crustal origin, derived by partial melting of a mixed source of Paleoproterozoic metaigneous (~78%) and metasedimentary (~22%) rocks at a depth of 30–40km triggered by underplating of basaltic magma. Mixing of such crustal melts with about 10% basaltic magma could account for the origin of the Late Jurassic (~161Ma) Waitun granites. The Late Jurassic (~156Ma) Shipi and Early Cretaceous (~134Ma) Taiyuan granites were produced by extensive fractional crystallization of primary crustal melts, the source of which show relatively high proportion (~82%) of metaigneous rocks. The Early Cretaceous (~109Ma) Shipi granites were generated by partial melting of a mixed source of Paleoproterozoic metaigneous (~92%) and metasedimentary (~8%) rocks at a depth of ~30km plus additional (~15%) input from coeval basaltic magma. The granites were formed in a continental arc setting induced by northwestward subduction of the Paleo-Pacific plate. Progressive slab rollback since the Middle Jurassic from inland to the coastal area is the most likely mechanism for the generation of the granites.