Zircon U–Pb geochronologic, geochemical and Sr–Nd–Pb isotope characteristics of the Beidaban granites in the North Qilian Orogenic Belt: Petrogenesis and tectonic implications

Abstract

The tectonic evolution and crustal accretion process of the North Qilian Orogenic Belt (NQOB) are still under debate because of a lack of integrated constraints, especially the identification of the tectonic transition from arc to initial collision. Here we present results from zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotope geochemistry of the Beidaban granites to provide crucial information for geodynamic evolution of NQOB. Zircon U–Pb dating yields an age of 468 ± 10 Ma for the Beidaban granites and most of the Beidaban samples contain amphibole, are potassium-rich, and have A/CNK values ranging from 0.7 to 0.9, illustrating that the Middle Ordovician Beidaban granites are K-rich, metaluminous, calc-alkaline granitoid. The geochemical characteristics indicate that the Beidaban granites are transitional I/S-type granitoids that formed in an arc setting. The isotopic compositions of initial (⁸⁷Sr/⁸⁶Sr)_i values ranging from 0.70545 to 0.71082 (0.70842 on average) and εNd(t) values ranging from − 10.9 to − 6.7 (− 8.8 on average) with two-stage Nd model ages (T_DM2) of 1.74–2.08 Ga suggest that the Beidaban granites originated from Paleoproterozoic crustal materials. In addition, the initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 19.14–20.26; ²⁰⁷Pb/²⁰⁴Pb = 15.71–15.77; ²⁰⁸Pb/²⁰⁴Pb = 37.70–38.26) and geochemical features, such as high Th/Ta (17.43–30.12) and Rb/Nb (6.01–15.49) values, suggest that the Beidaban granite magma source involved recycled crustal components with igneous rocks. Based on these results in combination with previously published geochronological and geochemical data from other early Paleozoic igneous rocks, we suggest that the timing of the tectonic transition from arc to the initial collision to the final closure of the North Qilian Ocean can be constrained to the Middle-Late Ordovician (ca. 468–450 Ma).