Age, petrogenesis and tectonic significance of the ferrobasalts in the Chagangnuoer iron deposit, western Tianshan

Abstract

The formation of large iron deposits associated with subduction and its genetic relationships with ferrobasalts are not yet well understood. Here we report a geochemical and geochronological investigation on the newly discovered ferrobasalts associated with the Chagangnuoer iron deposit, western Tianshan. The Chagangnuoer ferrobasalts are characterized by high Fe2O3T (14.55–22.68 wt.%) and MnO (0.36–0.93 wt.%) but low TiO2 (0.70–1.26 wt.%) contents. Analyses of 10 zircon grains yield a weighted zircon U–Pb age of 314 ± 8 Ma. Based on our new petrological and geochemical data, we conclude that the Chagangnuoer ferrobasalts probably have been originated from the partial melting of a spinel peridotite mantle source that has been modified by subduction related fluids. The ferrobasalts have nearly linear positive correlation between MnO and (87Sr/86Sr)i, implying the involvement of subducted Fe–Mn nodules. The mid-ocean ridge basalt (MORB)- and ocean island basalt (OIB)-like geochemical features, as well as moderate Ti/V values (18–36), indicate that the ferrobasalts may have been formed in an extensional back-arc basin setting. Combined with previous studies on the Chagangnuoer iron deposit, we propose a hypothesis that the overlying iron orebodies were likely derived from the ferrobasaltic magma.