Petrogenesis of the Early Cretaceous granitoids and its mafic enclaves in the Northern Tengchong Terrane, southern margin of the Tibetan Plateau and its tectonic implications

摘要

Cretaceous granites are widely exposed in the Tengchong Terrane that is the southern extension of the Tibetan Plateau, resulting from the evolution of the Tethys. They, therefore, play a critical role in deciphering the Tethys evolution and the growth of the Tibetan Plateau. In this paper, we present new zircon UPb dating and Hf isotopic results, along with whole-rock elemental and SrNd isotopic data of the granitoids from the Lushui-Pianma (LP) batholith and its mafic enclaves and the strongly deformed granites from the Gaoligong shear zone in the northern Tengchong Terrane. The dating results show that both the LP granitic batholith and the strongly deformed granites in the Gaoligong shear zone formed in the Early Cretaceous of 122–110?Ma. They are composed of granodiorite, quartz monzodiorite and monzogranite with minor gabbroic enclaves. Mineralogically and geochemically, the granitoids in the LP batholith, particularly for the less evolved ones, display a metaluminous nature and an affinity to I-type granite. All the granitoids are characterized by similar rare and trace patterns and whole-rock Nd and zircon Hf isotopic compositions, indicating a common origin. The sodium-rich feature for the less evolved dioritic rocks and the predominantly negative zircon εHf(t) and whole-rock εNd(t) values for all the granitoids demonstrate that they were derived mainly from the partial melting of the Mesoproterozoic metabasic rocks (likely the basement rocks of the Gaoligong Group) in the lower crust of the Tengchong Terrane. In the case of the mafic enclaves, they share similar mineralogical assemblages and NdHf isotopic compositions to its host granodiorite, indicating a cognate origin with the latter.In combination with previous data for the granitoids in the Tengchong Terrane, our study further illustrates that the Early Cretaceous granitic rocks are dominated by metaluminous I-type granites with minor highly fractionated peraluminous I-type granites, rather than the dominant S-type granites as assumed before. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar, together with regional Early Cretaceous magmatic and sedimentation patterns, indicate that these Early Cretaceous magmatic rocks were the products of the evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab breakoff.