华北北缘大青山小井沟岩体年代学、地球化学和Hf同位素特征及其地质意义

为了加深对华北北缘晚古生代构造背景以及古亚洲洋闭合时限的认识, 对华北北缘大青山地区小井沟花岗岩开展了年代学、地球化学和同位素研究。分别利用LA-MC-ICP-MS, ICP-AES和ICP-MS等方法进行锆石U-Pb测年, 主、微量和稀土元素分析以及锆石Hf同位素测试。结果表明, 小井沟岩体侵位于二叠纪, 侵位年龄为275±1 Ma (MSWD=0.93)。该花岗岩具有高硅(SiO₂=70.72%~72.64%)、高钾(K₂O=4.19%~4.23%)的特点, A/CNK均约为1.1, 属于弱过铝质的高钾钙碱性系列, 稀土元素总量(ΣREE)为87.67~101.51 μg/g, 配分曲线呈右倾型, 具有微弱的Eu负异常, 富集大离子亲石元素(LILE, 如Rb, Ba, K, Sr等), 亏损高场强元素(HFSEs, 如Nb, Ta, Y, Yb, Lu等), 应该是由下地壳部分熔融形成。该花岗岩的锆石具有负的ε_Hf(t)值(-9.56~-5.00), 模式年龄(T_DM2)在1.91~1.61 Ga之间, 表明华北陆块古元古代的地壳岩石应该是其主要物源, 可能有幔源物质的参与。结合整个华北北缘二叠纪花岗岩的岩浆演变特征, 小井沟岩体应形成于由俯冲向碰撞-后碰撞转变的过渡时期, 古亚洲洋可能在晚二叠世之前已闭合。

Geochronology,Geochemistry, Hf Isotope of Xiaojinggou Pluton in the Northern Margin of North China Craton and Its Tectonic Implications

In order to enhance the understanding of the late Paleozoic tectonic settings of northern margin of the north China craton (NNCC) and closure time of the Paleo-Asian Ocean, the zircon U-Pb age, geochemistry and zircon Hf isotope of the Xiaojinggou granite are conducted by LA-MC-ICP-MS, ICP-AES and ICP-MS. The zircon U-Pb dating results indicate that the Xiaojinggou pluton was intruded at Permian (275±1 Ma, MSWD=0.93). All the granite have high contents of SiO₂ (70.72%?to?72.64%), K₂O (4.19% to 4.23%) with moderate A/CNK values (averaged at 1.1), suggesting that Xiaojinggou pluton is slightly peraluminous and belongs to the high-K calc-alkaline series. The concentrations of rare earth elements (ΣREE) varies in the range of 87.67 to 101.51 μg/g. The Xiaojinggou pluton is characterized by enriched LREE patterns with slightly negative Eu anomalies. It is enriched in large ion lithophile elements (LILE, e.g. Rb, Ba, K, Sr), but depleted in high field strength elements (HFSEs, e.g. Nb, Ta, Y, Yb, Lu), which suggest this plutou may be derived from partial melting of the lower crust. Zircon ε_Hf(t) value varies from -9.56 to -5.00 and T_DM2 varies in the range of 1.91 to 1.61 Ga. All these characters indicates that the Xiaojinggou pluton is probably generated by partial melting of Lower Proterozoic crust of the NNCC, accompanied by the possible assimilation of mantle-derived magmas. In view of the evolution of the Permian granitic magmatism in the NNCC, middle Permian Xiaojinggou pluton formes in the transition period from subduction to collision/post-collision, and the Paleo-Asian Ocean should close before late Permian.