白云鄂博裂谷带中元古代多期幔源岩浆岩及其大地构造背景研究

Bayan Obo Rift Belt, as a Mesoproterozoic-Neoproterozoic rift belt in the northern margin of the North China Craton, was generated during the breakup of Columbia supercontinent, recording the Mesoproterozoic-Neoproterozoic evolution processes of the North China Craton. However, the magma events in the Bayan Obo Rift Belt are poorly constrained, as a result, the Mesoproterozoic-Neoproterozoic tectonic evolution of the rift belt and North China Craton remain unclear. On the basis of our multiple-round regional geological and mineral surveys and project researches in this belt during the last decade, we had made several primary achievements in Stratigraphic Sequence and Geochronology of Bayan Obo Group, especially new discoveries of the three-stage mantle-derived magmatic rocks including the Jishengtai gabbro (1670Ma), kimberlite (1552Ma?), Dajingpo gabbro and Xiaonan Shan hornblende-gabbro (1342Ma). Based on these findings, this project will further studies on the geochronology of Halahuoqite Formation in the Bayan Obo Rift Belt, tectonic setting analysis according to the whole-rock and isotope geochemistry of these igneous rocks. The results will provide the basis for further research on the Mesoproterozoic-Neoproterozoic tectonic evolution of the Bayan Obo Rift Belt and North China Craton.

白云鄂博裂谷带是哥伦比亚超大陆裂解过程中在华北克拉通北部边缘形成的中-新元古代裂谷带，其间保存了丰富的华北克拉通中-新元古代地质演化信息。但由于裂谷带内中-新元古代岩浆岩发现不多且研究薄弱，制约了白云鄂博裂谷带与华北克拉通中-新元古代地质演化的研究。十多年来，申请人在四子王旗地区进行了多轮区域地质矿产调查和专题研究工作，对白云鄂博群的地层层序和岩浆岩同位素年龄取得了初步研究成果，发现了白云鄂博群中发育的吉生太辉长岩（1670 Ma）、金伯利岩（1552 Ma?）、大井坡辉长岩及小南山角闪辉石岩（1342 Ma）等3期幔源岩浆岩。本项目将调查这3期岩浆岩与围岩的接触关系，研究其年代学特征，进一步约束白云鄂博群哈拉霍圪特组地层的时代。采用岩石和同位素地球化学研究方法，分析其大地构造背景，为研究白云鄂博裂谷带与华北克拉通中元古代的地质演化提供依据。

项目以白云鄂博裂谷带及华北克拉通北缘发育的古-中元古代幔源岩浆岩为研究对象，围绕白云鄂博裂谷带形成的构造背景、地层时代、裂谷演化及后期改造等科学问题开展研究。在扎实的野外地质调查的基础上，通过区域构造与变形解析，结合岩石学、同位素地质年代学和岩石地球化学等方法，新发现华北克拉通汇聚到哥伦比亚超大陆过程中形成的1.9 Ga构造混杂岩，是目前世界上发现的最古老的沉积混杂岩；依据东沟子超基性岩体的研究提出华北克拉通1.85 Ga经历了造山后伸展构造事件；采用碎屑锆石的侵入岩同位素年代学方法很好地约束了白云鄂博群的形成时代为中元古代，论证了尖山组微晶丘为水下垮塌热水晶洞的产物，提出白音宝拉格组可能为格林威尔造山后伸展的产物；依据裂谷带发育的3期幔源岩浆岩的大地构造背景研究，结同位素地球化学研究，提出小南山铜镍矿的形成与1.35-1.32 Ga哥伦比亚超大陆裂解作用形成基性-超基性岩有关；通过详细的构造变形解析，查明白云鄂博群受中亚造山带造山作用的改造，向北逆冲于白乃庙岛弧带之上，位移距离大于14 km。项目研究系统地论证了白云鄂博裂带形成的大地构造背景与演化、地层时代及其与成矿的关系，圆满完成了项目的目标任务。