冀东卢龙地区首次发现的含3.8Ga地质记录的铬云母石英岩中古老物质的研究

The firstly discovered fuchsite quartzite was located in Lulong, eastern of Hebei province. It was preserved as massive lenses (relic blocks) in an ancient Archaean basement. Its structural position was near and below the unconformity of Proterozoic Changcheng Group and Archaean basement. This situation was quite similar to the fuchsite quartzite found in Caozhuang, eastern Hebei Province. Through chronology study, we found that in Lulong fuchsite quartzite, the age scale of detrital zircons was 3200~3800Ma. This suggested the fuchsite quartzite's formation age should be less than 3200Ma. This condition was consistent with the fuchsite quartzite's detrital zircons found in Caozhuang group, deploying in Caozhuang, Qianan. Combining these tow age information, it could be concluded that this kind of fuchsite quartzite containing Eoarchaean detrital zircon, had a large dispersed area in eastern Hebei. In previous research, fuchsite quartzite's surround rock in these two different locations were considered to belong to two different structural units: Qianxi group and Luanxian group. But according to the new discovered chronological data revealed, these tow groups probably were a same layer under a same tectonic background. The chronological study of fuchsite quartzite in Lulong provided more information in understanding the Archaean geological structure of eatern Hebei area. These scientific issues could be identified by the comprehensive study including chronology, isotope tracer and inclusion analysis for ancient materials. Those researches would provide evidence for the early stage of the crust evolution in eastern of Hebei province.

冀东卢龙地区首次发现的铬云母石英岩为产自冀东太古宙基底中的巨大透镜体（残块），构造位置接近元古宙长城系地层与太古宙基底的不整合之下，与冀东曹庄地区铬云母石英岩产状类似。对其年代学研究发现，卢龙地区铬云母石英岩中碎屑锆石年龄范围为3200～3800Ma，表明铬云母石英岩形成的年代应小于3200Ma，这与迁安曹庄地区出露的曹庄组铬云母石英岩中碎屑锆石相同。说明这种含有始太古代碎屑锆石的铬云母石英岩在冀东地区可能还有更广泛地分布。此前认为两个地区铬云母石英岩的围岩分别属于两个不同的构造单元：迁西群和滦县群，但新发现的相近年代数据说明这两个岩群可能位于同一构造背景下的同一层位。本项目拟通过对古老物质展开年代学、同位素示踪和包裹体分析等研究来进一步证实这些问题，并为冀东地区早期地壳的演化提供新的证据。

对冀东卢龙地区首次发现的铬云母石英岩年代学研究，得到碎屑锆石年龄范围为3200～3800Ma，表明铬云母石英岩形成的年代应小于3200Ma，这与迁安曹庄地区出露的曹庄组铬云母石英岩中碎屑锆石相同。说明这种含有始太古代碎屑锆石的铬云母石英岩在冀东地区可能还有更广泛地分布。此前认为两个地区铬云母石英岩的围岩分别属于两个不同的构造单元：迁西群和滦县群，但新发现的相近年代数据说明这两个岩群可能位于同一构造背景下的同一层位。此次铬云母石英岩的发现, 表明在新太古代卢龙地区和曹庄地区应属于同一陆块，那么，秦皇岛-绥中古岩浆弧与冀东古陆核有可能在新太古代已经拼合到一起，因此，应将二者归于同一构造单元。本次研究发现了铬云母石英岩中存在两粒4.0Ga的锆石，而且部分锆石的Hf同位素二阶段模式年龄达到了4.4Ga，显示可能始太古代就存在地壳运动。本项目还在铬云母石英岩中发现了2.5Ga的金红石，通过年代学研究发现，其适合作为U-Pb定年参考标准物质，已经在实验室作为内部工作标样在使用。通过工作发现在坦桑尼亚的多多马群也存在相似年龄谱的古老铬云母石英岩，这为下一步的工作提供了思路。