Early Neoproterozoic arc magmatism of the Tongmuliang Group on the northwestern margin of the Yangtze Block: Implications for Rodinia assembly

The tectonic evolution of the Yangtze Block and its position in Rodinia during Neoproterozoic remain controversial for decades. One of the important reasons for the debates is the lack of late-Mesoproterozoic to early-Neoproterozoic arc volcanic rocks on the periphery of the Yangtze Block. In this study, we firstly report the spilite-keratophyre-quartz-keratophyre associations in the Tongmuliang Group on the northwestern margin of the Yangtze Block. LA-ICP-MS zircon U-Pb dating results indicate that they formed at ca. 970 Ma and were possibly affected by Indosinian (ca. 235 Ma) metamorphism. Most samples show strong enrichments in large ion lithophile elements and Pb and evident depletions in Nb, Ta and Ti, resembling typical arc-related magmatism. Moreover, their low Th/La, Nb/La and Nb/U ratios, as well as minor crustal incorporations, further define an intra-oceanic arc environment. The highly depleted whole-rock Nd and zircon Hf isotopes indicate that these rocks were originally derived from depleted mantle sources, while the Sr isotopes and Na concentrations may have been affected by post-condensation alteration. The spilites, metabasalts and keratophyres show similar La/Sm, Zr/Nb and Nb/La ratios and arc-like geochemical features, indicating a depleted mantle source that has been metasomatized by slab-derived fluids. The keratophyres were formed through fractional crystallization of primitive mafic magmas. However, the quartz-keratophyres were derived from partial melting of juvenile materials similar to the spilites and/or keratophyres. These observations led us to conclude that there was an ocean-ocean subduction along the northwestern margin of the Yangtze Block during early Neoproterozoic, corresponding to synchronous arc magmatism along the southeastern margin of the continent. Arc magmatism of this period on different portions of margins of the Yangtze Block may have experienced different processes.