Permian−Triassic magmatism above a slab window in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Abstract

Permian−Triassic metaluminous−peraluminous granitoids, mafic−ultramafic plutons, and Ni-Cu and Au deposits are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval granitoids and mafic−ultramafic intrusions, and the geodynamics of magmatism and related mineralization, remain ambiguous. To address these ambiguities, we present petrological, geochemical, and bulk-rock Sr-Nd-Fe and zircon U-Pb-Hf isotope analyses of granitoids from the Shuangchagou Complex and gabbros from the Huangshandong Complex in the Eastern Tianshan. Zircon U-Pb ages demonstrate that the Huangshandong gabbro was emplaced at ca. 277.8 ± 1.4 Ma. In contrast, U-Pb ages determined from zircons in the granitic rocks of the Shuangchagou Complex suggest that the complex crystallized from three stages of magmatism: (1) strongly peraluminous S-type granitic magma represented by early-stage gneiss and granitic veins (ca. 289 Ma), (2) metaluminous to weakly peraluminous I-type granitic magmas represented by the intermediate-stage granitoids (ca. 283−261 Ma), and (3) late-stage granitoids (ca. 250−241 Ma). The intermediate- and late-stage granitoids (ca. 283−241 Ma) show clear enrichments in the light rare earth elements and large ion lithophile elements (e.g., Rb, Th, and U), and depletions in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, which indicates that the North Tianshan oceanic plate was still subducting during the Middle Triassic. Considering the diversity of magmatic rocks (e.g., mid-oceanic-ridge−type mafic rocks, and I-, S- and A-type igneous rocks), mineralization styles (e.g., Alaskan-type Ni-Cu sulfide deposits and orogenic gold deposits), and the dextral strike-slip faults (e.g., Kanggur Fault) that occurred concurrently in the Eastern Tianshan during the Early Permian to Middle Triassic, we suggest that splitting of the subducted portion of the North Tianshan oceanic plate created a slab window that allowed the upwelling and partial melting of asthenospheric mantle to form the mafic−ultramafic intrusions and related Ni-Cu sulfide deposits. Sustained migration of magma provided the heat necessary to induce partial melting, devolatilization, and desulfurization of crustal materials, producing the Permian−Triassic, high-K to calc-alkaline I- and S-type granitoids, and associated orogenic gold deposits. By integrating the results of this study with published work regarding the Kanggur Accretionary Complex, we suggest that the subduction of the North Tianshan Ocean may have lasted until the Late Triassic.