Two episodes of Late Paleozoic A-type magmatism in the Qunjisayi area, western Tianshan: Petrogenesis and tectonic implications

• The Qunjisayi rhyolites and granites were formed at 306 Ma and 296 Ma respectively.
• The rhyolites were resulted from melting of granitic source at shallow depth.
• The granites were derived from juvenile basaltic crustal source.
• The geodynamic transformation of the western Tianshan occurred in Early Permian.

A-type magmatism is an effective lithoprobe for constraining the tectonic evolution of orogenic belts. In this study, we have identified two episodes of Late Paleozoic A-type magmatism in Qunjisayi, western Tianshan. Zircon U–Pb geochronology has yielded Late Carboniferous (rhyolites, 306 ± 2 Ma)- and Early Permian (granites, 296 ± 3 Ma) ages. Both the Qunjisayi rhyolites and granites have depleted Sr–Nd–Hf isotopic compositions, moderate A/CNK ratios (0.91–1) and negative Nb and Ta anomalies. They are also characterized by high K2O + Na2O (8.72–10.6 wt.%), FeOT/(FeOT + MgO) (0.86–0.99) and Y/Nb (1.42–2.34) values, similar to those of typical A2-type granitoids. The Qunjisayi rhyolites may have been formed by partial melting of a juvenile calc-alkaline granitic crustal source induced by subduction roll-back in shallow depth. The succeeding Qunjisayi granites were likely to be derived from a lower crustal source that contains depleted mantle-derived components. Asthenospheric mantle upwelling, triggered by post-subduction slab break off, may have been important in the Qunjisayi granites formation. We conclude that the Late Paleozoic Qunjisayi A-type magmatism was genetically linked to the geodynamic transformation from late subduction to collision.