Geochronology, Geochemistry and Tectonic Significance of the Early Carboniferous Gabbro and Diorite Plutons in West Ujimqin, Inner Mongolia

Early Carboniferous gabbros and diorites are distributed in West Ujimqin, Inner Mongolia. The LA-ICP-MS zircon U-Pb ages of the gabbro and diorite samples are 321± 2 Ma（MSWD=0.65） and 319.4± 1.5 Ma（MSWD=0.42）, respectively. In addition, new geochemistry data from three gabbro and three diorite samples are presented. All six samples show high Al_2O_3 contents but low-TiO_2 contents, belonging to tholeiitic and calc-alkali basalt series. All of the samples have similar chondrite normalized REE patterns characterized by moderate depletion in LREE similar to normal middle oceanic ridge basalt（MORB）. The MORB and PM-normalized trace element patterns show the enrichment in large ion lithophile elements（LILE, e.g., Rb, Ba and Sr）, depletion in high field strength elements（HFSE） and distinctly negative Nb and Ta anomalies similar to volcanic arc basalt. Furthermore, as shown in the correlation plots of La/Ba vs. La/Nb, Ba/La vs. Ce/Pb, Th/La vs. Ce/Pb, Nb/La vs. Ba/Rb, and Nb/Y vs. La/Yb, the magma source has underwent contamination and metasomatism from the subduction fluid. According to the Zr/Nb, La/Nb, and La/Ta ratios and the Nb/Y vs. Zr/Y and Sm/Yb vs. La/Sm diagrams, the magma was derived from shallow depleted lithospheric mantle and formed by moderate（5%~20%） partial melting of spinel lherzolites. Tectonic setting discrimination diagrams reveal that the gabbros and diorites display both characteristics of MORB and volcanic arc basalt, which is consistent with their geochemical characteristics. On the basis of the geochemical features of these samples, combined with regional geological data and many previous researches in the study area, the Early Carboniferous gabbros and diorites of West Ujimqin are suggested to be formed in an intensely extensional rift setting, and a limited immature ocean basin probably formed after subsequent development.