A late-Carboniferous to early early-Permian subduction-accretion complex in Daqing pasture, southeastern Inner Mongolia: Evidence of northward subduction beneath the Siberian paleoplate southern margin

摘要

A subduction-accretion complex is identified from previously defined late-Carboniferous and early-Permian strata in Daqing pasture, southern Xiwuqi, Inner Mongolia. The subduction-accretion complex is composed of a matrix of siltstone and exotic blocks of bioclastic limestone, pillow basalt, foliated basalt, chert and asbestos. The pillow basalt possesses the geochemical characteristics of mid-ocean ridge basalt (N-MORB), whereas the foliated basalt displays the geochemical characteristics of island arc basalt (1AB), indicating that these basalts are of different origins. U-Pb (zircon) dating indicates that the foliated basalt formed in the late-Carboniferous (314.5 to 318.4 Ma) and the bioclastic limestone formed in the early early-Permian. Combined with regional geological data, the subduction-accretion complex and coeval calc-alkaline granitic belt to the north constitute the essential elements of the late-Carboniferous to early early-Permian subduction zone on the southern margin of the Siberian paleoplate. The zircon ε_(Hf)(t) values of the foliated basalt are positive (+14.4 to + 23.9), suggesting that this basalt originated directly from depleted mantle. The temporal-spatial distribution of the subduction-accretion complex and ophiolite belts in southeastern Inner Mongolia indicates that there was significant lateral crustal growth on the southern margin of the Siberian paleoplate in the late Paleozoic.