Magmatic evolution and formation age of the Pobei mafic-ultramafic intrusion on the northeastern margin of the Tarim plate

The Pobei mafic-ultramafic intrusion is located on the northeastern margin of the Tarim plate and has an outcrop area of 180 km². The magma differentiation was sufficient with the main lithologies consisting of such rocks as peridotite, olivine gabbro, gabbro norite, and gabbro, during the formation of the Pobei intrusion. Zircon U-Pb geochronology indicates that crystallization ages of gabbro, gabbro norite, olivine gabbro and plagioclase-bearing peridotite are 277.1±2.2 Ma, 273.5±2.9 Ma, 275.5±1.5 Ma and 280±2 Ma, respectively. The ages for the different types of rocks are concentrated at 270~280 Ma, which are temporally and spatially consistent with the age of the Tarim mantle plume. Mineral geochemical data suggest that olivine from the dunite has the highest F_o values, with maximum at 90.9%; clinopyroxene has similar concentrations in different types of rocks, with consistent negative relations between MgO and CaO, indicating similar primitive magma components. Moreover, these different rocks have similar variations for their trace and rare earth elements in spider pattern diagrams, suggesting that these rocks evolved from the same parental magma. Sr-Nd isotopic data indicate that the Pobei intrusion was sourced from depleted mantle and contaminated by 4%~15% lower crustal materials during magma ascent. Combined with published data and data obtained from this study, the authors hold that different lithofacies of the Pobei mafic-ultramafic intrusive complex have transitional or intrusive contact relation, similar formation ages and co-magmatic evolution characteristics, but they also have different emplacement sequences and degrees of crustal contamination.