河北矾山钾质碱性超镁铁岩-正长岩杂岩体的锆石SHRIMP U-Pb年龄

作为华北陆块北缘的碱性岩体之一,河北矾山钾质碱性超镁铁岩-正长岩杂岩体赋存着中国最大的内生磁铁矿-磷灰石矿床。关于其形成时代,已经发表的K-Ar、Rb-Sr和Sm-Nd法定年结果差别较大,因此需要采用更精确的定年方法限定岩体的形成时代。通过对矾山杂岩体的辉石正长岩样品进行SHRIMP锆石U-Pb定年,得到了218±2Ma的结果。由于定年样品取自构成矾山杂岩体主体、形成最晚的第三岩相带,这个年龄限定矾山杂岩体形成于晚三叠世。现有的锆石U-Pb年代学资料表明,晚三叠世侵入岩在华北陆块北缘、东缘、南缘普遍存在,成为中国东部晚三叠世构造-岩浆活动的显著特征之一。

Zircon SHRIMP U-Pb dating of the Fanshan potassic alkaline ultramafite-syenite complex in Hebei Province, China

As one of alkaline intrusives in northern margin of the North China block, the Fanshan potassic ultramafite-syenite complex in Hebei Province contains the largest endogenous magnetite-apatite deposit of China. However, the pre-existing K-Ar, Rb-Sr and Sm-Nd ages are so different that more precise dating is very important to constrain the timing of the Fanshan complex. This study reports a SHRIMP zircon U-Pb age of 218±2Ma from a pyroxene syenite sample. Because the dated sample was collected from the main part of the complex, which was produced by the last (third-stage) pulse of magmatism, the zircon U-Pb age provides a strong constraint on the timing of the Fanshan complex. This is consistent with the zircon U-Pb ages from the literature for other alkaline intrusives in northern, eastern, and southern margins of the North China block, indicating that the alkaline magmatism at around 220Ma is one of the most important tectonomagmatic events in eastern China.