LAUSR

Abstract Potassic igneous rocks, such as shoshonitic rocks, are an important constituent of young volcanic-intrusive suites in varied tectonic settings. The genesis of these rocks is related to tectonic environments and is thus important for tectonic reconstruction of ancient terranes. Here we report zircon U–Pb and Hf isotopes and whole-rock geochemical compositions for shoshonitic quartz syenites from Madi and Chashan in the northern Wuyi Mt Range, southeast China. LA-ICP-MS zircon U-Pb dating shows that both the Madi and Chashan quartz syenites were formed at 134.8 Ma and 135.0 Ma, respectively. They also have similar geochemical compositions and are characterized by high K2O contents and enriched in LILEs and LREEs but depleted in HFSEs, suggesting a shoshonitic affinity. Elemental and isotopic compositions indicate that they were likely formed by partial melting of metasomatised lithospheric mantle with the presence of phlogopite and potassic amphibole. It is also suggested that silicate melts derived from subducted sediments played a dominant role in mantle metasomatism. An integration of our new results with previous ones allows us to propose a modified tectonic model about the evolution of SE China in which a continental arc related to the subduction of the Palaeo-Pacific plate during the Middle Jurassic was gradually changed to southeastward-migrating, extension-related magmatic arc and back-arc basins formed in a continental arc-rifting setting as a consequence of slab rollback since the Early Cretaceous.