Slab Breakoff Model for the Triassic Post-Collisional Adakitic Granitoids in the Qinling Orogen, Central China: Zircon U-Pb Ages, Geochemistry, and Sr-Nd-Pb Isotopie Constraints

摘要

Triassic granites in the Qinling orogen of central China provide insights into regional teetono-magmatic events and tectonic evolution. Zircon LA-ICP-MS U-Pb dating shows that the Yangba, Xichahe, Wulong, and Guangtoushan plutons have magmatic crystallization ages of 209 +- 2 Ma, 212 +- 2 Ma, and 208 +- 2 Ma, respectively, slightly younger than the time of collision between the Yangtze and North China cratons (242 Ma). Major- and trace-element geochemistry indicate that the granitoids have adakitic signatures, suggesting that they were generated by partial melting of a thickened lower crust. Geochemistry and petrology of mafic microgranular enclaves in some of the plutons indicate that they were derived from partial melting of the subcontinental lithospheric mantle (SCLM) and are genetically related to the host adakitic granites. We proposed that slab-breakoff explains the spatial, age and geochemistry characters of Triassic magmatism in the Qinling orogen. In summary, Triassic collision between the Yangtze and North China cratons caused crustal thickening in the Qinling-Dabie orogen to >50 km, with breakoff of the subducting oceanic slab occuring at shallow depth. This caused: (1) upwelling of the asthenosphere along the Mianlue suture, with initial melting at the base of the lower crust, producing mafic me (2) mantle-derived melt underplating the thickened lower crust so that temperatures at the boundary between it and the mantle were high enough to trigger dehydration melting of garnet-bearing rock at a pressure of >1.5 GPa, resulting in the generation of adakitic magma with high Sr, Ba, and Sr/Y ratios, no negative Eu anomalies, and leaving eclogite as a residue. (3) Moreover, given that lithospheric mantle-derived magmas intruded the adakitic magma chamber, interaction between pristine adakitic magmas and mantle-derived magmas result in formation of the mafic enclaves, providing the adakitic magma with a mantle/ crustal component, e.g. high Mg# and Cr and Ni contents. We conclude that partial melting of lithospheric mantle played an important role in generation of the Triassic adakitic granites in the Qinling orogen, reflecting post-collisional slab-breakoff.