Geochronology, geochemistry, and its geological significance of the Permian Mandula mafic rocks in Damaoqi, Inner Mongolia

The Hujierte-Chaganhadamiao volcanic-plutonic sequence is located in the Mandula area of northern Damaoqi, Inner Mongolia. It contains mainly mafic volcanics, with several ultramafic blocks in its central part and mafic-intermediate intrusions in the east. The zircon U-Pb ages of the gabbro and basalt samples are 278.5±3.0 Ma (MSWD=0.66) and 273.7±1.0 Ma (MSWD=0.36), respectively. These ages constrain the magmatism occurred in the Early Permian. The Mandula Permian mafic rocks are characterized by low abundances of REE and slightly LREE-enriched chondrite-normalized N-MORB-type REE patterns. And these mafic samples have high (143Nd/144Nd)i (0.51262–0.51270), relatively high positive ɛNd(t) (3.4–8.0), and high Mg# (49–54), suggesting derivation from the depleted asthenosphere mantle source. But they also show the enrichment in large ion lithophile elements (LILE, e.g. Rb, Ba and Sr), depletion in high field strength elements (HFSE); distinctly negative Nb, Ta anomalies and slightly negative P, Ti anomalies; high (87Sr/86Sr)i (0.70490–0.70537), low (206Pb/204Pb)i (17.39–17.93). All of these resemble the characteristics of the enriched mantle source or involvement of arc materials in the magma genesis. Furthermore, as shown in the correlation plots of ɛNd(t) vs (87Sr/86Sr)i, (206Pb/204Pb)i vs (207Pb/204Pb)i and (208Pb/204Pb)i, La/Ba La/Nb vs La/Nb, and La/Nb ratio, the magma source could have been experienced the contamination and metasomation from the previous subduction process. And the positive correlation between the selected major oxides and trace elements could be explained by the contamination from continental crust and arc materials during the magmatic emplacement. Combined with the distinct geochemistry features of Mandula mafic samples and many previous researches in the study area, the Early Permian magmatism in Mandula occurred under an extensional rift setting, and a juvenile ocean basin probably had already been formed after further development.