关键词: 华北北缘, 晚石炭世, 火山岩, 锆石U-Pb年龄, 锆石Hf同位素

Abstract: 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.

Key words: the northern margin of NCB, Late-Carboniferous, volcanic rocks, zircon U-Pb age, zircon Hf isotopic compositions