Characteristics and geological significance of Early Carboniferous high-Mg andesites in Ma'anshan area, east Inner Mongolia

The Early Carboniferous high-Mg andesites in Ma'anshan area of eastern Inner Mongolia have LA-ICP-MS zircon U-Pb age of 346.4±1.4Ma, indicating that they were formed in Early Carboniferous. Ma'anshan high-Mg andesites belong to calc-alkaline series, with SiO₂ 53.22%-54.22%, MgO 7.21%-10.03%, Al₂O₃ 14.37%-15.94%, CaO 4.81%-5.94%, rich Na, poor K (Na₂O 3.87%-4.34%, K₂O 0.49%-0.93%), low TFeO/MgO(< 1.5), high Cr(364×10^-6-429×10^-6), and Ni (204×10^-6-211×10^-6). Similar to sanukite, all samples show enrichment of LREE and LILE (e.g., Rb, Ba, Sr and K), slight Eu negative anomaly and depletion of HFSE such as Nb, Zr, Ti, Y, Yb and Lu. They are equilibrium products of Sirich melt derived from partial melting of mantle peridotite and subducted oceanic crust, indicating a subduction background and suggesting that they were formed by ocean-continent transformation during the Palaeo-Asian Ocean closure.