Magmatic record of early Neoproterozoic subduction-accretion in the northwestern margin of the Yangtze Block, South China

Early Neoproterozoic subduction-related magmatism in South China provides an important constraint for revealing the amalgamation and accretion of marginal blocks in Rodinia. Herein, we focus on the early Neoproterozoic igneous rocks exposed in the northwestern margin of the Yangtze block, South China. After compiling the geochronological data, geochemical characteristics, and isotopic compositions of early Neoproterozoic igneous rocks, the nature of source, petrogenesis, and the tectonic settings were addressed. The results show that the igneous rocks with ages of about 1.0 to 0.9 Ga have trace element compositions similar to island arc basalts with highly incompatible elements slightly lower than continental arcs, depleted Sr-Nd-Hf isotope compositions and zircon δ¹⁸O values slightly lower than mantle values, suggesting a likely intra-oceanic subduction origin. By comparison, the magmatic rocks with ages of about 0.9 to 0.83 Ga have pronounced enrichment in LILEs and LREEs, significant depletion in HFSE, moderately enriched Sr-Nd-Hf isotopes and zircon δ¹⁸O values fall in or higher than mantle range, which suggests that they were most probably formed in a continental arc setting. The secular variation trends of whole-rock Nd and zircon Hf isotopes in mafic rocks reveal the periodic enrichment and depletion of mantle caused by repeatedly advancing and retreating subduction, which further triggers alternating regimes of contraction and extension tectonics. The results depict the early Neoproterozoic subduction accretion history in the northwestern margin of the Yangtze block.