Early Devonian Ferroan Granites Document Tectonic Switching from Advancing to Retreating Accretionary Orogen, Southern Central Asian Orogenic Belt

Accretionary orogens are sites of continental crust growth and reworking and provide an archive of magmatism in both compressional (advancing) and extensional (retreating) plate margin settings, but how this tectonic switching occurs and what the petrological indicator is remain unclear. Here, zircon U-Pb dating results reveal magma emplacement ages of 411–399 Ma for granitic rocks in the central Beishan Orogenic Belt, southern Central Asian Orogenic Belt (CAOB). They have a high proportion of K-feldspar, relatively high (K2O + Na2O)/CaO and FeOt/(MgO + FeOt) ratios, elevated zircon saturation temperatures, and diagnostic 10,000 × Ga/Al ratios (3.07–3.89). These petrological and geochemical features not only accord with typical ferroan granitoids but also imply a hot and anhydrous origin. In addition, they show relatively depleted zircon Hf isotopic signatures (εHf(t)=−2.9 to +11.1) and are typified by moderate whole-rock Nd isotopes (εNd(t)=−2.7 to +0.5). Consequently, we consider these early Devonian ferroan granitoids formed by variable mixing of crustal components (up to 30%) with juvenile magma in an back-arc basin setting, likely resulting from the slab rollback process. Since early Paleozoic magmatic rocks in this orogenic belt were largely generated by continuous slab subduction, the early Devonian granitoids likely mark the conversion from compressional to extensional settings. Combining this with other geological records, we propose that the early Devonian ferroan granites signaled a switching of the tectonic regime in the southern CAOB from an advancing to a retreating accretionary orogen.