Late Devonian to early Carboniferous roll-back related extension setting for the Tuwu-Yandong porphyry copper metallogenic belt in the Dananhu arc of the eastern Tianshan (NW China) in the southern Altaids

Abstract

The Tuwu and Yandong deposits are the representative major porphyry Cu deposits in the eastern Tianshan orogen (NW China) of the southern Altaids. The ages of their ore-hosted volcanic rocks and their tectonic settings are controversial. To address this problem, we report new geochronological and geochemicaldata of ore-hosted volcanic rocks in the Tuwu and Yandong deposits in the middle section of the Dananhu arc. The latest-stage dacites yield U-Pb zircon ages of 363.5 ± 1.8 Ma and 344.0 ± 4.1 Ma. The ore-hosted volcanic rocks consist of basalt, andesite, dacite and volcaniclastics. The basalt and andesite are tholeiite, while the dacites are tholeiite to calc-alkaline. The geochemical characters of the basalts suggest that they derived from the partial melting of the depleted mantle which were metasomatized by fluids and/or melts released from the subducted slab. And the rocks have well correlations between SiO2 and K2O + Na2O, MgO, TiO2, Al2O3, and CaO, narrow range of La/Sm ratios, and Nb/Th ratios decrease with increasing Th/Yb ratios, suggesting fractionation and crustal contamination were also essential for their genesis. The Nb-Ta negative anomalies and enrichment in the light rare earth elements (LREEs), Rb, Ba, Th, Sr and Pb indicate that these volcanic rocks formed in the subduction zone. Integrating our data with those of previous investigations, we concluded that the protoliths of the rocks were probably derived from asthenospheric garnet-free mantle in a roll-back subduction setting in the late Devonian to early Carboniferous.