Geochronology and geochemistry of granitoids from northern Alxa, northwest China: Petrogenesis and tectonic implications

Abstract

Multiple branch oceans existed in the Paleo-Asian Ocean (PAO), but their closure times are in dispute and unclear, which constrains our understanding of the final closure time of the PAO and the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This study focuses on the Permian plutons of the northern Alxa, which is located in the middle segment of the southern CAOB that recorded the final subduction history of the PAO. We performed the 1:50000 mapping, whole-rock geochemistry, geochronology, and Sr-Nd-Hf isotopic analysis and compiled the Sr-Nd-Hf isotopic compositions and whole-rock geochemical data of igneous rocks from the northern Alxa. LA-ICP-MS zircon U–Pb dating reveals the study plutons emplaced in the Early Permian (285–296 Ma). Whole-rock geochemical data show the intrusion belongs to medium-K calc-alkaline peraluminous highly fractionated I-type granite, enriched in Rb, K, Th, Pb, and depleted in Nb, Ta, Ti, Sr, and P elements, which suggest a subduction arc-related setting and metaluminous to weak peraluminous parental magma. The weak negative ε_Nd(t) (from −2.3 to −1.2), relatively high I_Sr (0.704772–0.708037) and depleted mantle model ages T_DM (1.14–1.49 Ga), combining with weak negative to slightly positive ε_Hf(t) (from −2.0 to +4.1) and crustal model ages T_DM^C (1.18–1.43 Ga), indicate that the parental magma might originate from remelting of the Mesoproterozoic lower crust and mixing with mantle-derived materials. The field occurrence, deformation, and geochemical features, integrating with the compiled data and regional geology, show that the igneous rocks formed before or after the late Early Permian show different features in terms of deformation, zircon saturation temperatures, crustal thickness, potassium contents, and ε_Hf(t) values. This might relate to the closure of the Yagan branch ocean of the PAO in northern Alxa.