Geochronology and geochemistry of Permian to Early Triassic granitoids in the Alxa Terrane: Constraints on the final closure of the Paleo-Asian Ocean

Abstract

As a pivotal junction between the North China and Tarim cratons, the Alxa Terrane provides an ideal window to constrain the final closure of the middle segment of the Paleo-Asian Ocean (PAO). This study carried out petrological, whole-rock geochemical, and zircon U-Pb-Hf isotopic investigations on four major granitic plutonic complexes in the Alxa Terrane. The Bayan Nuru and Yabulai plutonic complexes are I-type granitoids that yield crystallization ages of 281-268 Ma and 277-270 Ma, respectively, with negative zircon εHf(t) values (-11.5 to -3.2), primarily sourced from the Neoproterozoic rocks in the region. The Nuergai granitoids yield crystallization ages of 281-268 Ma and show I-type affinities and positive zircon εHf(t) values (+1.6 to +6.4), indicating an origin related to magma mixing. Emplaced at ca. 241 Ma, the Oliji granitoids display varying zircon εHf(t) values from -0.9 to +11.5, which necessitate a predominant source of mantle-derived materials. A compilation of zircon εHf(t) and whole-rock εNd(t) values of the magmatic rocks in the Alxa Terrane illustrates a decreasing trend from the Late Carboniferous to the Early Permian and an increasing trend during Middle Permian to Triassic time. The marked shift with a large variation of zircon εHf(t) and whole-rock eNd(t) values at 280-265 Ma indicates a tectonic switch from subduction to post-collision tectonic regimes in the Alxa Terrane, marking the final closure of the middle segment of the PAO. Comparable isotope variations are also identified from 260 to 245 Ma magmatic counterparts on the northern margin of the North China Craton, hence suggesting a progressively eastward closure of the PAO.