Newly Identified Early Cretaceous High-Mg Diorites in the Southern Dayaoshan Area of South China: Petrogenesis and Geodynamic Implications.

The geodynamic mechanism responsible for Late Mesozoic magmatism in South China remains highly debated, particularly with regard to the relative contributions of the Paleo-Pacific and Neo-Tethyan subduction systems. This study documents newly identified high-Mg diorites (∼102 Ma) from the southern Dayaoshan area. These rocks exhibit high Mg# (up to 64), elevated Cr and Ni concentrations, and decoupled zircon Hf (εHf-(t) = -9.95 to -5.66) and whole-rock Nd (εNd-(t) = -8.74 to -8.71) isotopic signatures. Such geochemical and isotopic features are inconsistent with simple crustal melting or fluid-metasomatized mantle-wedge melting. Instead, Hf-Nd isotopic and trace-element modeling indicates that their petrogenesis involved the partial melting of a subduction-related mélange source, defined as a physical mixture of metasediments and depleted mantle peridotite. The E-W-trending spatial distribution of the magmatic belt (including the Cancun pluton) stands in sharp contrast to the regional NE-striking Paleo-Pacific magmatic belt. This geometric decoupling implies that the Southern Dayaoshan area was governed by far-field tectonic processes tied to Neo-Tethyan ridge subduction or slab window formation during the Early-to-Late Cretaceous transition. These results provide new constraints on the operation of a multislab geodynamic system during the Mesozoic evolution of South China.