Late Paleozoic subduction system in the southern Central Asian Orogenic Belt: Evidences from geochronology and geochemistry of the Xiaohuangshan ophiolite in the Beishan orogenic belt

The Xiaohuangshan ophiolite of the Beishan (Inner Mongolia) is located in the southern margin of the Central Asian Orogenic Belt. It consists of several blocks composed dominantly of serpentinized ultramafic rocks, cumulative gabbros and basalts. The geochemical data of gabbros and basalts obtained from the Xiaohuangshan ophiolite are similar to tholeiitic rocks. They all have low TiO₂ and high Al₂O₃ contents. Their light rare earth elements are slightly enriched, (La/Yb)_N = 3.62–6.80, similar to the typical enriched mid-ocean ridge basalts. The mafic rocks display enrichments in large ion lithophile elements and depletions in high field strength elements, as well as significant Nb–Ta–Ti negative anomalies, similar to subduction-derived rocks. All these geochemical characteritics indicate that the Xiaohuangshan ophiolite would form in a subduction zone from a slightly enriched mantle source. Ion microprobes (SHRIMP) U–Pb dating were conducted on zircons from the basalt and gabbro. The weighted mean ages are 336.4 ± 4.1 Ma and 345 ± 14 Ma, which are considered as the crystallization ages of the basalt and gabbro, respectively. Together with other two units, the Dongqiyishan arc belt and the Yueyashan–Xichangjing ophiolite, the Xiaohuangshan ophiolite forms a Late Paleozoic arc-basin system, indicating that the Paleo-Asian Ocean did not close in the early Carboniferous. Based on the geochemical characteristics of adjacent geological bodies and their settings, the Xiaohuangshan ophiolite is considered as an indicator of a suture zone between the different epicontinental belts in the Beishan region.