Zircon U-Pb Dating and Geochemistry of Late Carboniferous Pyroxene Peridotite in Dong Ujimqi Inner Mongolia and Its Tectonic Significance

The precise chronology and tectonic significance of pyroxene peridotite in Late Paleozoic granites in Dong Ujimqi, Inner Mongolia are still unclear, which will directly restrict the discussion of the properties and tectonic evolution in the study area. In this paper, petrography analysis, geochemistry and zircon U-Pb dating for the newly discovered pyroxene peridotite were studied. The analysis results show that main rock forming minerals are olivine, hornblende, orthopyroxene, and small amounts of plagioclase, biotite and clinopyroxene. The weighted average age obtained is 317.8±1.6 Ma, indicating that the intrusion was formed in the Late Carboniferous. Geochemical data show that, samples have SiO2 content of 40.28%-44.50%, MgO content of 23.42%-29.44% and Na2O+K2O content of 1.00%-2.12%, have low m/f ratio (3.13-3.86), high FeOT content (11.18%-14.70%) and Mg# (75.60-79.26), and belong to the series of ferrous ultramafic rocks and tholeiites. These rocks have high REE (31.98×10-6-72.60×10-6),enrichment in LREE and relatively depletion in HREE, (La/Yb)N=3.56-7.72,slightly Eu anomalies (δEu=0.79-1.65).The standardized distribution pattern of REE chondrites is right-leaning. The rocks are enriched in LILE (Rb,Sr,K) and depleted in HFSE (Nb, Ta),and there are obvious negative anomalies of Nb, Ta and Ti. Thus, we consider that the pyroxene peridotite had been formed in extensional tectonics from partial melting of the Sp-peridotite, which was modified by the subducted slab-derived fluid. © 2020, Editorial Department of Earth Science. All right reserved.