Ca. 780 Ma OIB-like mafic dykes in the Western Jiangnan orogenic Belt, South China: evidence for large-scale upwelling of asthenosphere beneath a post-orogenic setting

摘要

Post-collisional alkaline mafic rocks record significant information regarding crust-mantle interaction in the subduction channel and provide an excellent opportunity to investigate the mechanism of transition from pre-collisional to post-collisional magmas. In this contribution, we present whole-rock major and trace element and radiogenic isotope data obtained from alkaline mafic dykes in the Guzhang-Anjiang areas in the southwestern Jiangnan orogenic belt (JOB), South China. LA-ICP-MS U-Pb zircon dating of four representative samples yields a consistent age of similar to 780 Ma. These mafic dykes are characterized by large variations in SiO2 (42.4-54.1 wt.%) and MgO (2.2-15.0 wt.%), alkalis (K2O+Na2O = 1.96-11.19 wt.%), Nb (17.55-49.62 ppm) and Ta (1.15-3.26 ppm). They exhibit ocean island basalt (OIB)-like geochemical features, which are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) relative to heavy rare earth elements (HREEs) and by positive zircon epsilon(Hf)(t) (+1.86 to +5.14) and whole-rock epsilon(Nd)(t) (+0.31 to +1.80) values. All these geochemical characteristics suggest these dykes were derived from a small degree (2-5%) of partial melting of an asthenospheric mantle source that had previously been modified by earlier subduction processes. Based on the previous reports and present study, we infer that three periods of Neoproterozoic basaltic magmatism occurred in the western Jiangnan Orogen, and constrain the time of the tectonic regime transformation at similar to 830, and at similar to 780 Ma. The source of these basaltic rocks varied in composition spatially and evolved with time. The similar to 780 Ma OIB-like rocks in this study serve as an important indicator of large-scale upwelling of the asthenospheric mantle and the transition from orogenic to post-orogenic tectonic setting.