8.1亿年千里山基性岩墙群的厘定及其对华北克拉通西部地质演化的启示

千里山-贺兰山地区分布着两组岩墙:一组北东走向,侵入古元古代变质基底岩系,见被晚前寒武系黄旗口组不整合截切,称为千里山岩墙群;一组北西走向,侵入基底岩系,见侵入黄旗口组,被石炭系不整合截切,称为贺兰山岩墙群。一条千里山岩墙分选出斜锆石,二次离子探针Pb-Pb定年获得813±7Ma的年龄(207Pb/206Pb平均年龄;MSWD=0.63,n=6),代表岩墙侵位时代。一条贺兰山岩墙分选出锆石,二次离子探针U-Pb定年获得最小一组年龄～370Ma(206Pb/238U年龄),近似代表岩墙侵位时代或者略大于侵位时代。千里山岩墙为拉斑玄武岩系列,以高TiO_2(2.7%～3.7%)和Fe_2OT_3(13.4%～17.0%)为特征;贺兰山岩墙为(弱)碱性系列,低TiO_2(1.0%～1.5%)和Fe_2OT_3(5.5%～12.4%)为特征。两者均显示轻稀土和大离子亲石元素富集,高场强元素相对亏损的特征;贺兰山岩墙群的富集和亏损特征均更为明显((La/Yb)N:贺兰山岩墙群2.0～5.5;千里山岩墙群1.9～2.4)。这些特征说明岩浆可能起源于交代的岩石圈地幔或者岩浆受到过地壳物质的混染。黄旗口组-王全口组-正目关组与上覆寒武系地层以及下伏千里山岩墙群的地质关系说明这些地层应该形成于新元古代晚期(810～541Ma)。千里山-贺兰山地区基底属于西华北克拉通的一部分,其以西是阿拉善地块;后者的构造归属长期存在争议。鉴于阿拉善地块发育同时期、岩浆性质基本相似的岩浆岩(狼山地区双峰式火山岩系;龙首山地区镁铁-超镁铁岩),考虑到两地的晚太古代-古元古代基底特征的相似性,我们认为阿拉善地块和千里山-贺兰山地块可能属于同一克拉通,同时经历新元古代中期伸展-裂谷事件。

Identification of the~810Ma Qianlishan mafic dyke swarm and its implication for geological evolution of the western North China Craton

Two mafic dyke swarms are identified from the Qianli-Helan Mts, one dominated with N-E-trending dykes, namely the Qianlishan swarm, intruded the Paleoproterozoic basement but was unconformably covered by the Precambrian Huangqikou Fm., while another with mostly N-W-trending ones, namely the Helanshan swarm, intruded both the basement and the Huangqikou-Wangquankou Fms. but was covered by the Carboniferous strata. Baddeleyites liberated from one Qianlishan dyke give a SIMS ²⁰⁷Pb/²⁰⁶Pb age of 813±7Ma (MSWD=0.63, n=6), representing the timing of emplacement. Zircons from one Helanshan dyke, however, give the youngest ²⁰⁶Pb/²³⁸U ages of~370Ma, representing an age of or slightly older than its intrusion. The Qianlishan dykes are tholeiitic, with high TiO₂ (2.7%~3.7%) and Fe₂O₃^T (13.4%~17.0%) contents; whereas the Helanshan dykes are (weak) alkaline basalt with low TiO₂ (1.0%~1.5%) and Fe₂O₃^T (5.5%~12.4%) contents. Both swarms show enriched light rare earth elements and large ion lithophile elements but depleted high field strength elements, which are more prominent for the Helanshan swarm than the Qianlishan (e.g., (La/Yb)_N are 2.0~5.5 for the Helanshan but 1.9~2.4 for the Qianlishan dykes). These features indicate that the parental magmas were probably both from a metasomatized lithospheric mantle or/and they were contaminated by the crust. The unconformable relationship of the Huangqikou, Wangquankou and Zhengmuguan Fms. related to the Qianlishan dykes and their relationship with the Cambrian strata indicate that these strata deposited during 810~541Ma. The basement of the Qianli-Helan Mts region belongs to the western North China Craton. It borders the Alxa block to the west, whose relationship with the craton is in debating. Based on the similarity of basement and the coeval and chemically similar igneous associations in the two regions (e.g., the 830~810Ma Langshan Group bimodal volcanics and Jinchuan mafic-ultramafic intrusions in the Alxa block), it is preferred here that the Alxa block was also a part of the craton as the basement of the Qianli-Helan Mts region, and both regions experienced extension-rifting event during the Middle Neoproterozoic.