Albian–Cenomanian granitoid magmatism in Eastern and Central Tibet as a result of diachronous, continental collision induced slab tear propagation

Abstract

AbstractA discrete belt of Albian–Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau’s crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, whole-rock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04–0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their εHf(t) values correspond to wide ranging Hf isotope crustal model ages (TDMC) of 0.3–1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034–0.7105) and negative εNd(t) values of –8.8 to –4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145–120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian–Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.