Geochronology and geochemistry of Late Carboniferous dykes in the AqishaneYamansu belt, eastern Tianshan: Evidence for a post-collisional slab breakoff

With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB), an integrated study was conducted on the geochronological and geochemical data for dioritic, granitic and diabase dykes from the AqishaneYamansu belt in the eastern Tianshan, NW China. Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous (w311 Ma and w315 Ma). The dioritic dykes show adakitic characteristics and have high Na2O and positive εHf(t) values (þ12 to þ17), which suggest an origin from partial melts of a subducted oceanic slab. The granitic dykes have high SiO2 and K2O contents and are characterized by enriched light rare earth elements (LREE) and slightly flat heavy rare earth elements (HREE), with negative Eu and Nb eTaeTi anomalies. These dykes are alkali-calcic and show geochemical features of highly fractionated Itype granites. Their positive εHf(t) values (þ16 to þ17) suggest that they were derived from a juvenile accreted oceanic crustal source. The coeval diabase dykes have low SiO2 and K2O contents but high TiO2, MgO and Mg# (54e59). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Ba/Th, slightly low Th/Ta ratios, and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids. Thus, these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the AqishaneYamansu belt was a continental arc formed by southdipping subduction of the Kangguer oceanic plate.