Geochronology and Petrogenesis of Ahetala Granodiorite in South Tianshan Orogenic Belt, Xinjiang: New Constraints on the Tectonic Evolution of the South Tianshan Ocean

Abstract

The Ahetala granodiorite is located in the western section of the South Tianshan Orogenic Belt (STOB), which is of great significance regarding the dispute on the closing date of the South Tianshan Ocean (STO) and the tectonic evolution of STOB. To determine the tectonic setting and petrogenesis, the study of petrography, electron probe microanalysis (EPMA), LA-ICP-MS zircon U–Pb geochronology, and major and trace elements analyses are carried out for Ahetala granodiorite. Based on LA-ICP-MS U–Pb zircon dating, the granodiorite was emplaced at 282.1 ± 1.3 Ma (MSWD = 1.11). Geochemically, Ahetala granodiorite is characterized by metaluminous (A/CNK = 0.86–0.87), rich alkali (K2O + Na2O = 6.80–7.13), which belongs to high-K calc-alkaline I-type granite. They are enriched in LREE and depleted in HREE (LREE/HREE = 9.02–13.89) and exhibit insignificant Eu anomalies (δEu = 0.94–0.97). Ahetala granodiorite is enriched in large ion lithophile elements (e.g., K, Sr, Ba) and depleted in high field-strength elements (e.g., Ta, Ti, Nb, P). The Nb/Ta values (10.97–18.10), Zr/Hf values (39.41–40.19), and Mg# (54.87–56.02) of the granodiorite and the MgO content of biotites (13.42–14.16), the M value (M = Mg/(Mg + Fe2+)) of amphiboles (0.68–0.75), suggest that granodiorite originates from the crustal contamination of the mantle-derived magmas. Combined with regional geological background, previous research, and the nature of the Ahetala granodiorite, we suggest that Ahetala granodiorite was emplaced at a transitional stage of the volcanic arc (syn-collision) to post-collision setting and the South Tianshan Ocean was closed in the Early Permian.