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

Abstract

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.