In situ zircon Hf–O isotopic analyses of late Mesozoic magmatic rocks in the Lower Yangtze River Belt, central eastern China: Implications for petrogenesis and geodynamic evolution

Abstract

A combined study of whole-rock major and trace elements, Sr–Nd isotopes, zircon U–Pb dating, and in situ zircon Hf–O isotopes has been carried out for late Mesozoic magmatic rocks in the Lower Yangtze River Belt. The results provide insights into the origin of mantle sources of magma above a subduction zone, and thus into the petrogenesis of high-K calc-alkaline rocks, shoshonites, and A-type granites on continental margins, and the associated tectonic transformation from a continental arc to a back-arc extensional setting. The late Mesozoic magmatism can be subdivided into three stages: high-K calc-alkaline intrusions (148–133Ma), shoshonitic rocks (133–127Ma), and A-type granitoids (127–123Ma). All the rocks have consistent arc-like trace element characteristics with positive anomalies of Rb, Th, U, Pb, and LREE, negative anomalies of Nb, Ta, and Ti, and enriched Sr–Nd–Hf isotopic signatures. The first-stage intrusions in the Tongling area usually host dark enclaves of diorite, have high Sr/Y ratios, and low Y contents, and contain zircons with relatively low εHf(t) values (−38.6 to −6.6) and high δ18O values (5.7‰ to 10.1‰). A few inherited zircons with Neoarchean to Paleoproterozoic ages and highly enriched Hf isotopic compositions were detected in both the host intrusive rocks and the enclaves. The second-stage Ningwu volcanics contain zircons with moderate εHf(t) values (−13.3 to −3.8) and elevated δ18O values (5.4‰ to 7.6‰). The third-stage intrusions can be divided into A1- and A2-type granitoids, and their zircons have relatively high δ18O values of 6.7‰ to 10.3‰ and high εHf(t) values of 0 to −7.9. Based on these geochemical data we drew the following conclusions. Before 148Ma, following metasomatism by slab-derived fluid/melts, partial melting of the lithospheric mantle produced basaltic magma in the context of a subducting paleo-Pacific plate. This basaltic magma mixed with magma derived from the Archean lower crust, and the underplated and thickened juvenile lower crust. From 148 to 133Ma, continuous production of these mixed magma resulted in their intrusions as basic rocks at shallow levels. Meanwhile, partial melting of the thickened juvenile crust formed the intermediate-acid rocks of the first stage. As subduction continued, and the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to one of back-arc extensions. Metasomatism and decompression induced intensive partial melting of the lithospheric mantle, and these magmas, together with a limited amount of assimilated crustal materials, formed the second-stage volcanics. Roll-back of the subducted slab resulted in extension, causing disassembling of the lower lithosphere and lithospheric thinning, and the upwelling of hot asthenosphere. The A2-type granites were the result of the reworking of the Mesoproterozoic juvenile crust in this tectonic setting of extension. On the other hand, the A1-type granitoids were formed from magmas that were derived both from the matasomatized mantle and from the A2-type granitic material that had its origins in the Mesoproterozoic juvenile crust. We suggest that an integrated and comparative study of the multiple stages of development of these magmatic rocks is the key to understanding the tectonic evolution and associated magmatic activities in this continental intraplate setting.