Multi-stage early Cretaceous magmatism in the Tongling region, lower Yangtze metallogenic belt (E. China)

A combined investigation of geochemistry and geochronology was carried out for Early Cretaceous magmatic rocks in Tongling, centre of the Middle and Lower Yangtze River. The magmatic activity can be divided into two stages, peaking at ~140 Ma and ~127 Ma. The previous show high Sr and Sr/Y, low Y, and the later have low Sr, Sr/Y and La/Yb. There are correlation patterns between Sr-Nd isotopes and SiO₂, and samples show little variation of V and Cr with large variation of Rb, which suggest the effects of partial melting of the diverse source and magma mixing/crustal assimilation were more important than fractional crystallization. The broad variation of ε_Nd(t) and ε_Hf(t) values can be attributed to binary mixing of YC lower crust with slab-derived or enriched mantle melts. The T_DM2 of Nd and Hf, Zr saturation temperature, and magma oxygen fugacity display increased trend from ~140 Ma to ~127 Ma. Based on these signatures, origin model of the Early Cretaceous intrusions in Tongling was proposed. Affected by Pacific Plate subduction, following metasomatism by slab-derived fluid/melts, basaltic magma derived from lithospheric mantle underplated the lower crust. Meanwhile, partial melting of the lower crust formed the intermediate-felsic rocks of the first stage. As subduction continued, roll-back of the subducted slab resulted in extension, causing lithospheric thinning and the upwelling of hot asthenosphere. The second stage intrusions were the result of the reworking of the Meso-Neoproterozoic crust in the extension tectonic setting.