内蒙古大青山地区同盛茂晚石炭世火山岩的年代学、地球化学和Hf同位素研究及其地质意义

华北北缘作为古亚洲洋闭合和演化的重要研究地区,一直受到学者们广泛关注。已有研究表明其晚古生代岩浆作用强烈,但以侵入岩为主,缺乏同期火山岩的报道。文章对华北北缘内蒙古大青山地区同盛茂拴马桩组上部的火山岩进行详细的LA-ICP-MS锆石U-Pb年代学、地球化学和Hf同位素研究。结果表明,同盛茂火山岩形成于晚石炭世末期(305 Ma),火山岩岩性为流纹质弱熔结凝灰岩,岩石发生强的高岭石化,具有相对低的REE总量、弱的Eu负异常以及低的重稀土(Y、Yb)含量,样品中原位锆石的εHf(t)值为-8.3~-4.0。综合资料表明,火山岩可能是由华北克拉通加厚的基底经部分熔融形成,并有幔源物质参与。结合最近报道的区域晚古生代火山岩研究资料,推测华北北缘晚古生代至少发生过两期火山活动:分别为305~290 Ma和278~261 Ma,且峰期在272 Ma。

Geochronology,Geochemistry, and Hf isotopic Compositions of the Late-Carboniferous Volcanic Rocks in Tongshengmao of Daqinshan Area,Inner Mongolia and Their Geological Implications

The northern margin of North China Block (NCB) is a key region for exploring the closure and evolution of the Paleo-Asian Ocean, which has attracted much attention. Previous research has shown that there exists intensive late-Paleozoic magmatism which is composed almost entirely of intrusives and lack of coeval volcanic rocks. In this paper, we present detailed LA-ICP-MS zircon U-Pb ages, major and trace elements, and Hf isotopic compositions of the volcanic rocks from the upper part of the Shuanmazhuang Formation, Tongshengmao of Daqinshan district in Inner Mongolia. LA-ICP-MS zircon U-Pb dating shows that the volcanic rocks of the upper part of the Shuanmazhuang Formation were generated at 305 Ma. The volcanic rocks consist of weakly-welded rhyolitic tuffs that suffered from intense kaolinization. These volcanic rocks have low REE contents with subtle negative Eu anomalies and show low HREE (e.g., Y, Yb) contents. They also have negative εHf(t) values ranging from -8.3 to -4.0. These characteristics suggest that they were likely generated from partial melting of the thickened NCB crustal basements, and with some involvement of mantle-derived magmas. In combination with the latest reports about the distribution of late Paleozoic volcanic rocks inthe northern margin of NCB, it is inferred that there likely existed two stages of volcanism at least during the late-Paleozoic, with ages of 305~290 Ma and 278~261 Ma, and a peak volcanism at 272 Ma.