内蒙古北山造山带圆包山—希热哈达地区白山组火山岩锆石U-Pb年龄、地球化学特征及对古亚洲洋俯冲作用的启示

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

在内蒙古北山造山带北部圆包山—希热哈达地区白山组采集了安山岩和含角砾花岗斑岩,并对其进行了岩相学、LA-ICP-MS锆石U-Pb年龄和地球化学分析,探讨北山地区晚古生代构造演化历史。锆石U-Pb定年结果显示,安山岩和含角砾花岗斑岩的年龄加权平均值分别为300.0±2.5 Ma(MSWD=2.0,n=13)和300.3±1.9 Ma(MSWD=1.9,n=23),确认研究区白山组地层时代为晚石炭世。该套火山岩相对富集轻稀土元素和Rb、K、Ba等大离子亲石元素,亏损重稀土元素和Nb、P、Ti等高场强元素,具有活动性大陆边缘弧的地球化学特征,表明晚古生代北山北部古亚洲洋板块的俯冲持续到晚石炭世。岩石地球化学研究显示,石炭纪白山组安山岩的岩浆是俯冲带流体交代中—下地壳,并使其部分熔融形成;含角砾花岗斑岩主要为中—下地壳物质熔融的产物,源区有沉积物熔体的加入。由北往南,白山组火山岩从中性钙碱性向中酸性、酸性高钾钙碱性火山岩过渡,其分布规律指示了古亚洲洋板块由北向南的俯冲极性。

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.