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

High Resolution Image Download MS PowerPoint Slide 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.