Petrogenesis of the early Permian A-type granites in eastern North Tianshan, Central Asian Orogenic Belt: Implications for the final geodynamic evolution of the Kalamaili suture zone

Abstract

The understanding of the detailed geodynamic mechanism and crustal evolution of the tectonic transition along the Kalamaili suture zone from the late Carboniferous to early Permian remains limited. We conducted an integrated study of the petrology, geochronology, and geochemistry of the early Permian syenogranites (Yingzuishan and Shihuiyao syenogranites) in eastern North Tianshan, south of the Central Asian Orogenic Belt, aiming to elucidate the aforementioned tectonic transitional processes. The zircon U-Pb dating reveals that the studied syenogranites were intruded at ca. 280 Ma. These syenogranites exhibit medium to high SiO2 (67.5–78.8 wt%), high Na2O + K2O (8.1–10.6 wt%), and low CaO (0.27–1.39 wt%) concentrations. They are enriched in Rb, Th, and light rare earth elements, while depleted in Ba, Sr, and Ti. They display positive zircon εHf(t) values ranging from + 11.2 to + 15.6, initial 87Sr/86Sr ratios between 0.512371 and 0.512643, and εNd(t) values from + 1.8 to + 7.2. Integrating with regional geological data, we propose that the studied syenogranites are A2-type granites, primarily derived from the partial melting of juvenile felsic intrusive rocks under low-pressure conditions in the shallow crust. These syenogranites were formed within a post-collisional extensional setting triggered by the slab break-off of the Kalamaili oceanic plate. Throughout the tectonic transition induced by the slab break-off, there were episodic thickening and thinning of the crustal thickness along the Kalamaili suture zone, and the concurrent crustal evolution was predominantly marked by crustal reworking.