敦煌地区古元古代(2.0 ~ 1.8 Ga)花岗质岩浆作用及其地质意义

The Dunhuang area were strongly involved into Paleozoic orogenic belt, but the Archean-Paleoproterozoic basement rocks in this area are still recognizable. However, the geological boundaries between this crystalline basement and the North China Craton (Alxa Block) on its eastern side, and the Tarim Craton on its western side are still unclear. It has always been a key scientific issue for geologists whether this Precambrian crystalline basement belongs to an independent microcontinent or is part of the Tarim Craton or North China Craton. The limited Paleoproterozoic geological data of the Dunhuang area directly restricts the understanding of this question. On the basis of previous studies, a systematic study of field investigation, petrography, zircon U-Pb geochronology, whole-rock geochemistry, minerology and zircon Hf-O isotopes on Paleoproterozoic (2.0 ~ 1.8 Ga) granitic rocks in the Dunhuang area will be conducted in this project to clarify temporal-spatial distribution, inherited zircon ages, source, petrogenesis and tectonic setting of these granitic rocks, and to investigate Paleoproterozoic geodynamic mechanism and tectonic evolution of the Dunhuang area. Finally, by summarizing the reported major early Precambrian geological events in the Dunhuang area and comparing with its adjacent areas, it is expected to provided evidences for understanding the affinity of the Precambrian crystalline basement in the Dunhuang area.

敦煌地区强烈卷入了古生代造山带，但区内尚能识别出太古宙-古元古代基底岩系。然而，该基底与东侧的华北(阿拉善地块)和西侧的塔里木古陆块之间的地质界限不清楚，属于独立微陆块还是塔里木或者华北古陆块的一部分，一直是地质学家关注的关键科学问题，而有限的古元古代地质研究资料直接制约了对该问题的理解。申请人在近年来的研究基础上，拟选择敦煌地区古元古代(2.0 ~ 1.8 Ga)花岗质岩浆岩为研究对象，开展详细的野外观察和系统的岩石学、岩相学、锆石U-Pb年代学、岩石地球化学、矿物学和锆石Hf-O同位素研究；查明花岗质岩石时空分布规律、继承年龄、源区特征、成因和产出环境等并与邻区古元古代花岗质岩石进行对比，探讨敦煌古元古代地球动力学机制和构造演化过程；综合研究区已报道的早前寒武纪重大地质事件记录，通过与邻区早前寒武纪地质演化过程作对比，为理解敦煌前寒武纪结晶基底亲缘性提供依据。

敦煌地块经历了太古宙-显生宙长期构造演化历史，其中古元古代地质过程与全球Columbia差大陆聚合事件密切相关，该研究对于理解我国西北地区前寒武纪地块形成过程有一定参考价值。项目研究表明，敦煌地区发育~2.0-1.9 Ga、1.9-1.8 Ga和1.8-1.7 Ga花岗质岩浆活动。~2.0-1.9 Ga花岗质岩浆活动的产物包括岛弧型闪长岩(2045 Ma)-同-后碰撞型英云闪长岩(1998 Ma)-碰撞后钾质花岗岩(1950 Ma)；1.9-1.8 Ga花岗质岩石包括形成于1.84-1.82 Ga的岛弧型钠质花岗岩和形成于~1.81 Ga的同碰撞型S型花岗岩；1.8-1.7 Ga花岗质岩石主要为一套形成于1.79-1.73 Ga的A2型花岗岩。结合研究区古元古代镁铁质岩浆活动、混合岩化作用和区域变质作用，敦煌古元古代~2.0-1.9 Ga和1.9-1.8 Ga构造-热事件很可能代表与Columbia超大陆聚合事件相关的两期独立的构造旋回过程，碰撞后伸展背景下形成的A2型花岗岩标志着古元古代俯冲-碰撞造山过程的结束。单从岩石学和年代学格架来看，敦煌地区古元古代地质事件与塔里木克拉通北缘和阿拉善地块具有可对比性，但若考虑到具体地质演化过程，还需要开展大量工作。