豫西伊川地区中元古界兵马沟组泥质岩地球化学特征及其环境与物源示踪

摘要

The Paleao-Mesoproterozoic tectonic and sedimentary evolution of southern North China craton has been debating for long time, with controversies focusing on whether it is a subduction or cracking rift. As the first sedimentary layer formed after Xiong’er Group, the Mesoproterozoic Bingmagou Formation is important to address this issue. Based on the analysis of geochemical characteristics of REE and trace element of argillaceous rocks in Bingmagou Formation, the sedimentary environment and properties of provenance and their tectonic significance were discussed in this study. The results showed that Chondrite normalized REE and trace elements have a high differention between LREE and HREE, which showed anomaly negativeδEu, anomaly insignificantδCe, and enrich Rb, Th, La, Ce, Nd, Zr, Hf, etc. Compared with the average upper crust, it was enriched in Sc, V, Cr , Co, Ni, Rb. Also we found that Ceanom index and V/V+Ni ratio reflected a reduction argillaceous sedimentary environment, and Sr/Ba and B/Ga ratios indicate its transitional environment from continent to marine. In addition, Cr/Zr, Th/U and other elements ratios prove its provenance is not deep material, and La/Yb-Ce, Co/Th-La/Sc diagrams indicate the provenance is middle-acidic volcanic rocks. Furthermore, Th-Sc-Zr/10, Th-Co-Zr/10 illustrations indicate its provenance is mainly from continental volcanic arc. Taken these results and regional geological information together, it can be concluded that Bingmagou Formation in Yichuan was formed in an extensional back-arc basin, Xiong’er Group was an arc island.%华北克拉通南缘古-中元古代构造-沉积演化的俯冲说、裂解说之争由来已久，兵马沟组作为熊耳群形成后的第一套碎屑沉积岩层，对解释上述过程有重要意义。该文通过对豫西伊川地区的兵马沟组泥质岩微量及稀土元素的地球化学特征分析，探讨了其沉积环境、物源，进而揭示了其发育的大地构造背景。结果表明：（1）兵马沟组泥质岩稀土元素含量高，球粒陨石标准化后轻重稀土分异明显，δEu负异常，δCe异常不明显，富集Rb、Th、La、Ce、Nd、Zr、Hf等，较上地壳平均值富集Sc、V、Cr、Co、Ni、Rb；（2）Ceanom指数、V/V+Ni反映了泥质岩沉积时的还原环境，Sr/Ba、B/Ga表明其为由陆相至海相的过渡型沉积；（3）Cr/Zr、Th/U等元素比值关系反应其物源无深部物质加入，La/Yb-Ce、Co/Th-La/Sc图解表明物源为中酸性火山岩及少量稳定陆壳物质；（4）Th-Sc-Zr/10、Th-Co-Zr/10图解表明其物源主要来自大陆岛弧。综合上述分析结果，结合区域地质信息，可以得出中元古界兵马沟组形成于大陆岛弧弧后盆地，熊耳群分布区为中元古代俯冲成因的大陆岛弧区。