The chronology, geochemical characteristics and tectonic setting of the volcanic rocks in the Benbatu Formation of Huolinhe area, Inner Mongolia

In this study, the authors determined a set of volcanic rocks in the Late Paleozoic strata at Huolinhe area, Inner Mongolia. The age of zircon U-Pb isotope based on LA-ICP-MS analysis is (313.7±3.5)Ma, suggesting Late Carboniferous. The characteristics of petrography and petrogeochemical indicate that the Benbatu Formation consists of andesite and andesitic basalt rocks and belongs to aluminous rocks of alkalic series. The SiO₂ values are 52.82%-54.76%, with higher Al₂O₃ content (17.43%-20.79%) and lower TiO₂ content (0.97%-1.41%); Na₂O (3.74%-4.21%)>K₂O (2.37%-3.02%). In addition, rare earth elements (ΣREE) are between 46.78×10^-6 and 124.36×10^-6, light rare earth elements (LREE) are enriched (42.57×10^-6-109.06×10^-6), and heavy rare earth elements (HREE) are relatively enriched (4.21×10^-6-15.30×10^-6). The ratios of LREE/HREE are 6.04-10.43, suggesting obvious fractionation of light and heavy rare earth elements. The ratios of (La/Yb)_N are 6.32-14.56, exhibiting right-inclined shape high on the left and low on the right on the chondrite-normalized REE patterns. Large ion lithophile elements (Rb, Ba, K, Pb, Sr)and LREE are enriched whereas high field-strength elements(Nb, Ta, Ti, P)and HREE are depleted. Magma came from mantle wedge partial melting with underthrust fluid metasomatism. These rocks are characterized by structural environment of island arc and continental edge arc and were formed along the subduction zone at the edge of the plate and related to Paleoasian Ocean subduction.