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

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 (1.42-234) values, similar to those of typical A(2)-type granitoids. The Qunjisayi rhyolites may have been formed by partial melting of a juvenile calc-alkaline granitic crustal source induced by subduction Toll-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. (c) 2015 Elsevier Ltd. All rights reserved.