Magma mixing of late Early Jurassic age from Nyainrong, northern Tibet and its tectonic significance

The nature of magma source region and petrogenesis of the Nyainrong pluton in northern Tibet remain poorly constrained, consequently raising uncertainties for understanding the tectonomagmatic evolution of the Bangong-Nujiang suture zone during the Early Jurassic. To explore these issues, this paper reports the petrological, whole-rock geochemical, zircon U-Pb dating, and zircon Lu-Hf isotopic data of host granites and dioritic enclaves from the Nyainrong pluton. A dioritic enclave sample yields zircon U-Pb age of 175. 0 +/- 1. 3Ma, which is coeval with the host granitoid samples (174.5 +/- 1. 4Ma, 175. 9 +/- 0. 4Ma). The host granitoids are metaluminous to peraluminous, and shoshonitic, ranging from granodiorite to alkali-granite in rock type. They consist of I-type and S-type, and have A/CNK of 1.04 similar to 1. 16, and normative corundum (C-norm) of 0. 6% similar to 2. 1 %. and are enriched in large LILE, LREE and Pb. These rocks are characterized by negative zircon epsilon(Hf) (t) of - 9. 8 to -3.7, with Hf crustal model ages of 1.45 similar to 1. 84Ga, indicative of remelting of mature crust. The dioritic enclaves are mainly monzodioritic, metaluminous, and shoshonitic. These rocks are heterogeneous in zircon epsilon(Hf)(t) values (- 10. 3 similar to - 1. 2), with Hf mantle model ages of 0.87 similar to 1. 22Ga, indicating a derivation of partial melting of ancient lithosphere mantle materials. Magma mixing between anatexis or remelting of mature crust-derived materials (significant crustal input) and ancient lithosphere mantle-derived materials is most likely occurred in the Nyainrong region in the late Early Jurassic, in which the shift from S-type melts to I-type melts can be attributed to the enhanced contributions from mantle-derived materials. Such hybrid melts with subsequent fractional crystallization are responsible for the generation of the similar to 175Ma Nyainrong pluton. The data reported here, in combination with the data of detrital zircons in the literature, allow us to propose tentatively that the 175Ma magma mixing identified in the Nyainrong pluton could be related to the collision between the Nyainrong microcontinent and Qiangtang Terrane following the northward subduction of the Bangong-Nujiang Ocean seafloor. Our results imply in turn that the Bangong-Nujiang Ocean seafloors may have subducted southward towards beneath the Lhasa Terrane and also subducted northward towards beneath the Qiangtang Terrane.