Petrogenesis of Permian A-type granitoids in the Cihai iron ore district, Eastern Tianshan, NW China: Constraints on the timing of iron mineralization and implications for a non-plume tectonic setting

Abstract

The geochronology and geochemistry of granitoids in the Eastern Tianshan, NW China provide important constraints on the timing of iron mineralization, as well as in understanding evolution history of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on granitoid rocks from the Cihai iron ore district in the Beishan region, southern part of the Eastern Tianshan. The granitoid rocks are composed of granodiorite, quartz monzonite, granite, and monzonite. Zircon U–Pb analyses yielded the ages of 294.1±2.2Ma, 286.5±0.7Ma, 284.3±3.3Ma, and 265.6±3.0Ma, respectively, suggesting they were formed in Early–Middle Permian. Among these granitoid rocks, the ages of quartz monzonite and granite are close to the timing of iron mineralization (~282Ma), indicating they may provide a source of iron in the Cihai ore district.Geochemically, the granodiorite, granite, and quartz monzonite samples are characterized by high FeOt/(FeOt+MgO) and Ga/Al ratios (0.84–0.94 and 2.28–3.27, respectively), as well as high zircon saturation temperatures (781–908°C), similar to those of typical A-type granitoids. Isotopically, they display consistently depleted Hf isotopic compositions (εHf(t)=+1.18 to +15.37). Geological, geochemical, and isotopic data suggest that the Cihai A-type granitoids were derived from melting of juvenile lower crust.Some Early Permian A-type granitoids were recently identified in the Tarim and Eastern Tianshan with the ages between 294 and 269Ma. The A-type granitoids in the Eastern Tianshan formed earlier between 294–284Ma and exhibit characteristics of A2 type granitoids, whereas the A-type granitoids in the Tarim formed later between 277–269Ma and show A1 granitoids affinity. We suggest that the Permian Tarim mantle plume does not account for the formation of the A-type granitoids in the Eastern Tianshan area, and the Eastern Tianshan was in a non-plume tectonic setting during Early Permian time. A slab break-off event has been suggested as the possible tectonic mechanism that triggered the formation of Early Permian A-type granitoids in the Eastern Tianshan.