The Jiangco Composite Granitoids in Central Tibet Witnessed Switching Southern Qiangtang From Middle Jurassic Accretionary Orogenesis to Late Early Cretaceous Oceanic Subduction

ABSTRACT The evolution of the Bangong Meso‐Tethys Ocean is a hot topic, not only because the Lhasa–Qiangtang collision, following its closure possibly during the Late Cretaceous, resulted in the initial elevation of the Tibetan Plateau, but also because world‐class Cu–Au polymetallic deposits discovered in central Tibet are closely related to the subduction of the Bangong Meso‐Tethys Ocean. However, many fundamental aspects regarding the Bangong Meso‐Tethys Ocean are still intensely debated. Here, we conducted a study on the Jiangco composite pluton of diorite porphyry (DP) and quartz monzonite porphyry (QMP) that was emplaced in the accreted Early Jurassic oceanic plateau on the southern margin of the Qiangtang block of central Tibet. Our research thus probes into how the Bangong Meso‐Tethys oceanic plateau interacted with the southern Qiangtang margin and how the Bangong Meso‐Tethys subduction zone evolved during and after its accretion to the continental margin in the Late Mesozoic. The DP samples were dated at ~170 Ma by the zircon U–Pb method and show low K and Rb but high Na and Fe contents. They have low Nb and Y contents and show a positive correlation between the P 2 O 5 contents and SiO 2 contents, exhibiting distinct S‐type granite characteristics. Their high Zr contents of 194–282 ppm and intermediate (Nb/Zr) PM ratios of 0.95–1.17 (averaging 1.1) are consistent with the magmatic rocks emplaced in a crust‐thickened orogen. Their high initial 87 Sr/ 86 Sr (0.7104–0.7105) and low ε Nd ( t ) (−13 to −12) and zircon ε Hf ( t ) (−12 to −8) values suggest a derivation from crustal sources. Therefore, we argue that they were produced by partial melting of the thickened southern Qiangtang lower continental crust in an accretionary orogeny triggered by the subduction of the ~185 Ma Meso‐Tethys oceanic plateau. The QMP samples have zircon U–Pb ages of 113–116 Ma. They exhibit a distinct calc‐alkaline affinity and are depleted in Nb and Ta but enriched in large‐ion lithophile elements (LILEs), Th and Pb, indicative of a continental arc environment. They have a broad ε Hf ( t ) range (−3.1 to 2.2) but low ε Nd ( t ) (−4.4 to −3.8) values, demonstrating strong Nd–Hf decoupling (Δ ε Hf ( t ) = 2.1 to 8.1). They were produced most likely by partial melting of mélange diapir raised from the Bangong Meso‐Tethys subduction channel. These results indicate that the southern margin of the Qiangtang block underwent significant crustal thickening during the Middle Jurassic accretionary orogeny and switched to normal oceanic subduction during the late Early Cretaceous.