Geochronology, petrogenesis and tectonic implications of the porphyritic granodiorite related to the Cu mineralization in the Dengjitun ore district, Inner Mongolia

The Dengjitun Cu deposit, located in the central Xing'an Block, represents a key target for medium- to large-sized porphyry Cu deposit exploration. The mineralization in this area is closely associated with silicification and propylitic alteration and occurs in a distinctive sequence of quartz-bearing veinlets as well as in a disseminated form within an altered porphyritic granodiorite. In this paper, we present new precise laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon age data, geochemical data and Hf isotopic data on the porphyritic granodiorite at Dengjitun and use these data to improve our understanding of the Jurassic tectonic evolution of this region. Zircon U-Pb dating of two samples from the porphyritic granodiorite yield Early Jurassic ages of 174.2 +/- 1.1Ma and 173.9 +/- 1.1Ma, which are concordant within error. The Dengjitun porphyritic granodiorite is a high-K calc-alkaline and slightly peraluminous I-type granite. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., K, Rb, and Ba), depleted in heavy rare earth elements (HREEs; LREE/HREE=6.43-13.34) and high field strength elements (HFSE; e.g., Nb, Ti, P), and has weak positive Ce anomalies (Ce=1.15-1.25) and negligible Eu anomalies (Eu=0.85-0.99). The zircons from the porphyritic granodiorite have positive epsilon(Hf)(t) values (+ 8.7 to +11.7) and elevated Hf-176/Hf-177 ratios (0.282912-0.283000) that yield young T-DM2 ages (428-602Ma). Collectively, these data indicate that the porphyritic granodiorite formed from primitive magma that was generated by the partial melting of juvenile thickened mafic lower crust, which in turn was originally derived from depleted mantle during the Neoproterozoic. The thickened lower crustal material was metamorphosed under amphibolite- to eclogite-facies conditions at depths of >45km during the subduction of the Mongol-Okhotsk Ocean, with the resulting magmas assimilating some mantle-derived material prior to emplacement. Combining these data with the tectonic history of this area, we suggest that the Dengjitun porphyritic granodiorite formed in a post-collisional extensional tectonic setting after the Early Jurassic final closure of the Mongol-Okhotsk Ocean.