Origin and tectonic implications of Early Cretaceous high- and low-Mg series rocks and mafic enclaves in the Bomi-Chayu Fold Belt, SE Tibet

Early Cretaceous granitoids are widespread on the Tibetan Plateau and record the tectonic evolution of several Tethyan oceans. We describe Early Cretaceous high-Mg (Mg-# > 50) and low-Mg (Mg-# < 50) granitoids, minor diorites, and mafic enclaves from the Bomi-Chayu Fold Belt, SE Tibet. Zircon U-Pb dating reveals two episodes of granitoid emplacement at similar to 122 and similar to 117 Ma. High-Mg series rocks have higher Mg-# and compatible element concentrations (e.g., Cr and Ni) than low-Mg series samples. Their Mg-# and compatible element abundances are also elevated relative to an experimental melt generated under the conditions of a thickened lower crustal. Enriched whole-rock Nd and zircon Hf isotopic signatures, coupled with low Y (<18 ppm) and Yb (