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

Abstract

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 ~122 and ~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 (<1.9 ppm) concentrations in some samples, indicate that the high-Mg series was derived from thickened lower crust and underwent minor mixing with cumulate pyroxenite in the lower crust. In contrast, low-Mg series rocks record a purely crustal origin, although differences are noted between Na-rich (subgroup 1) and K-rich (subgroup 2) compositions. Mafic enclaves hosted in the granitoids yield identical formation ages and isotopic compositions to their host rocks, suggesting a common origin. The spatial–temporal distribution of Early Cretaceous granitoids in the Bomi–Chayu Fold Belt indicates that magmatism occurred in an analogous tectonic regime to that of similar rocks in the Tengchong Terrane. The Bomi–Chayu granitoids are therefore distinct from those in the northern and central Lhasa subterranes and are related to the evolution of the Myitkyina Tethys Ocean in eastern Myanmar.