Petrogenesis and geodynamic significance of the ~850 Ma Dongling A-type granites in South China

The ongoing controversy on the timing of amalgamation of the Yangtze and Cathaysia Blocks (either prior to 890 Ma or later than 830 Ma) impedes a proper understanding of geological evolution of the South China Block and its role in the breakup of Rodinia supercontinent. In this study, we report ~850 Ma LA-ICP-MS zircon UPb ages and whole rock geochemistry of the Dongling granites from the southeastern Yangtze Block that did not receive much geoscientific attention as compared to the better-studied 830-760 Ma sedimentary and igneous rocks in the South China Block. The studied Dongling granites have high K2O + Na2O (7.44-9.09 wt%) and low MgO (0.07-0.54 wt%) and CaO (0.16-1.21 wt%) contents. Their high FeOT/(FeOT + MgO) (0.82-0.94) and Ga/Al (> 2.6) values and HFSE abundance (Zr + Nb + Ce + Y = 443-965 ppm) allow their grouping as A-type granite. In the chondrite normalized diagrams, these granites display right-inclined REE patterns and strong to moderate negative Eu anomalies (Eu/Eu* = 0.32-0.70). The Dongling granites have variable and negative whole-rock εNd(t) (−14.6 to −8.6) and zircon εHf(t) (−12.8 to −4.4) with Paleoproterozoic two-stage Nd (2.20-2.69 Ga) and Hf model ages (2.01-2.41 Ga). Zircon saturation parameters (TZr) suggest > 850 °C temperature and other geochemical characteristics reveal low melting pressure (> 350 to < 700 MPa) and 1.9-2.6 wt% H2O content. Geochemical and isotopic characteristics indicate derivation of the Dongling A-type granites by partial melting of an ancient granulitic metasedimentary source, after a previous melt extraction episode. Emplacement of the Dongling A-type granites is inferred in an extensional setting following the arc-continent collision along the southeastern Yangtze Block margin. The oceanic slab continued to subduct underneath the southeastern margin of the Yangtze Block, leading to the extensive 840-820 Ma arc-related magmatism and coeval sedimentation. These arc-related igneous and sedimentary rocks indicate that the continental arc systems started from 850 Ma, and hint at the collision between the Yangtze and Cathaysia blocks after 850 Ma.