Zircon U-Pb ages and geochemistry of volcanic rocks from the Baishan Formation in the Yuanbaoshan-Xirehada area in Beishan orogenic collage, Inner Mongolia, NW China, and implications for the subduction history of the Paleo-Asian Ocean

Andesite and breccia-bearing granite porphyry samples were collected from the Baishan Formation in the Yuanbaoshan-Xirehada area of the northern Beishan orogenic collage in Inner Mongolia, NW China.Petrographic, LA-ICP-MS zircon U-Pb geochronology, and geochemical analyses were conducted to examine the Late Paleozoic tectonic evolution history in the Beishan area.The zircon U-Pb dating results suggest that the weighted mean ages of andesite and breccia-bearing granite porphyry are 300.0±2.5 Ma(MSWD=2.0, n=13)and 300.3±1.9 Ma(MSWD=1.9, n=23), respectively.These findings confirm that the stratigraphic age of Baishan Formation in the study area is Late Carboniferous.The volcanic rocks are relatively enriched in light rare earth elements and large ion lithophile elements(e.g.Rb, K, and Ba)and depleted in heavy rare earth elements and high field strength elements(e.g.Nb, P, and Ti).They exhibit geochemical characteristics related with active continental margin volcanic arcs, which indicate that the subduction history of the Paleo-Asian Ocean continued into the Late Carboniferous in the northern Beishan orogenic collage.Geochemical studies further suggest that the magma source of the andesite in the Carboniferous Baishan Formation is the middle-lower crust metasomatized by fluid in the subduction zone, and the breccia-bearing granite porphyry is mainly the product of the middle-lower crust melting, and the samples exhibit sediment incorporation.The volcanic rocks from the Baishan Formation changed from neutral and calc-alkalic rock series in the northern part to intermediate-acid or acid and high potassium calc-alkalic rock series in the southern part along the ophiolite mélange belt, indicating the southern subducted polarity of the Paleo-Asian Ocean plate.