Discovery of Late Carboniferous adakite in Manita, Inner Mongolia, and its constrains on intra-oceanic subduction in eastern Paleo-Asian Ocean

The Late Carboniferous Manita adakite distributed along the Hegenshan suture zone in the eastern part of the Xingmeng Orogenic Belt was discovered to be outcropped in the northern section of Diyanmiao SSZ ophiolite in Xiwuqi, Inner Mongolia.It consists mainly of granodiorite and plagiogranite, which belongs to low-K tholeiitic and Calc alkaline series, chemically characterized by 61.91%~75.16% of SiO₂, 13.54%~17.42% of A1₂O₃, and 0.33%~2.49% of MgO, with intensively rich in Na₂O (4.58%~5.48%), poor in K₂O (0.40%~2.08%) and high ratio of Na₂O/K₂O (2.35~12.96).The Manita adakite also manifests high Sr (309.55×10^-6~433.99×10^-6) and poor Y (3.74×10^-6~14.66×10^-6), with relatively enriched large ion lithophile elements (LILE) K, Rb, Sr, and depleted high field strength elements (HFSE) Nb, Ta, Zr, Ti and P, etc.In addition, it features low ∑REE (35.79×10^-6~70.10×10^-6) and clear REE fractionation((La/Yb)_N=1.59~10.11), without obvious Eu anomaly (δEu=0.91~1.51), and contains Yb (0.60×10^-6~1.86×10^-6).All of the geochemical characteristics indicate that the Manita pluton belongs to typical adakite.The U-Pb age of LA-ICP-MS zircon is 315.76±0.94 Ma, indicating that the formation age of Manita adakite is Late Carboniferous.The Manita adakite, Diyanmiao SSZ ophiolite, fore arc basalt and Baiyinhushu trondhjemite constitute the intra-oceanic subduction rock assemblage, which indicates that the eastern part of the Paleo-Asian Ocean was not closed in the Late Carboniferous and was in the process of subduction and extinction.