Age, petrogenesis and tectonic implications of Late Mesoproterozoic Yuanmou Granite in the Western Yangtze Block, South China

There has always been considerable controversy in the late Mesoproterozoic geological evolution history of the western margin of the Yangtze Block. In this paper, a suite of monzonitic granites from the Yuanmou complex in the western margin of the Yangtze River has been selected as the research object for a comprehensive study of petrography, zircon U-Pb geochronology and whole-rock geochemistry, and it provides support for understanding the late-middle Proterozoic geological evolution of the western margin of the Yangtze.The LA-ICP-MS zircon U-Pb ages of two Yuanmou Monzogranite samples were 1086±10 Ma (MSWD=1.4, n=50) and 1099±10 Ma (MSWD=1.8, n=58), respectively. All the samples were characterized by high silicon (SiO₂=69.44%～73.98%), alkali-rich (K₂O+Na₂O=6.11%～7.72%), calcium-poor (0.39%～1.46%), magnesium-poor (0.52%～0.76%), and low titanium (TiO₂=0.30%～0.59%), at the same time, it shows strong peraluminous (A/CNK=1.19～1.35) and medium-k calc-alkaline-k-basalt series. They have a high total rare earth content (∑REE=211.60×10⁻⁶～349.01×10⁻⁶), a light rare earth enrichment and a heavy rare earth deficiency ((La/Yb)_N=4.32～7.36); Rb, u, Th and Zr, Th, HF were enriched, while Nb, Ta, Ba were depleted, and negative Eu anomalies were observed (δEu=0.46～0.59), the saturation temperature of zircon ranges from 827~912℃, indicating the properties of A-type granites, These monzogranite may have been formed by partial melting of intermediate-acid igneous rocks in the middle and upper crust, Combined with the research results of predecessors, they are most likely formed in the back-arc extensional environment, combined with the late-middle Proterozoic magmatic records of the margin of the Yangtze block, the formation of 1.09 Ga monzogranite in Yuanmou Complex is related to the initial participation of the Yangtze block in the Rodinia supercontinent convergence.