Petrogenesis and tectonic significance of Late Carboniferous quartz diorite in Xi Ujimqin Banner, Inner Mongolia

Late Carboniferous quartz diorites are distributed in the Xi Ujimqin Banner of Inner Mongolia. The results of LA-ICPMS zircon U-Pb dating of the quartz diorite samples indicate ages of 304.64±0.82 Ma and 309.84±0.86 Ma. In addition, new geochemical data from six quartz diorite samples are presented. All six samples show high Al₂O₃ values but low TiO₂ values, and are thus classified as calc-alkali basalt series. These samples are characterized by low REE abundance and slight LREE enrichment. They also show LILE enrichment, HFSE depletion and distinctly negative Nb and Ta anomalies. All of these characteristics resemble those of arc magmatic rocks. Furthermore, as shown in the correlation plots of La/Ba versus La/Nb, Ba/La versus Ce/Pb, and Nb/Y versus La/Yb, the magma source has experienced contamination and metasomatism from the subduction fluid. According to the Zr/Nb, La/Nb, La/Ta ratios and the diagram of Sm/Yb versus La/Sm, the magma was derived from the shallow lithospheric mantle and formed by moderate (10%-20%) partial melting of spinel-garnet lherzolites. Based on geochemical characteristics of these samples in this study, combined with regional geological data and previous research results, the authors hold that, in the late EarlyCarboniferous, there was a new limited ocean basin under an intensely extensional rift setting. Consequently, as the newly formed ocean basin was still immature or experienced subduction-related contamination and metasomatism, the Late Carboniferous quartz diorites of Xi Ujimqin Banner exhibited the continental margin-arc-like enriched signatures.