中条山变质杂岩前寒武纪变质演化与原位高精度定年研究

Located at the southern section of the Trans-North China Orogen, the Zhongtiaoshan Metamorphic Complex is an ideal research target in deciphering the Precambrian tectono-metamorphic evolution of the North China Craton. Unfortunately, metamorphic evolution and petrochronology of this complex have not been extensively studied, possibly due to its lower metamorphic grade. This in turn, limits our understanding of the Precambrian tectonic evolution of the whole Trans-North China Orogen. Our preliminary investigation suggests that three stages of metamorphic mineral assemblages were formed in the sequential prograde, metamorphic peak and retrograde segments, as well as some accessary minerals. The metamorphic peak lies in the epidote amphibolite facies and is ascribed to the medium-pressure facies series. Our U-Pb dating of monazite suggests that the metamorphic peak occurred at ~1.85‒1.84 Ga. To more deeply uncover the Precambrian tectono-metamorphic evolution of the Zhongtiaoshan Metamorphic Complex, we intend to do field geologic investigation, including field reconnaissance, large-scale geologic mapping and mapping of representative geologic sections. Furthermore, we plan to rebuild the metamorphic P-T paths based on detailed micropetrographic observation and EPMA analysis. It is stated that in-situ high-resolution U-Pb dating of accessary minerals (zircon, monazite, titanite) formed at different metamorphic stages, is extremely helpful. When combined with routine U-Pb dating and 40Ar/39Ar of hornblende, biotite and muscovite, complete metamorphic P-T-t paths can be expected. Therefore, it is hopeful to decipher the tectonic background of the Zhongtiaoshan area in the Precambrian, when this research is completed.

中条山变质杂岩区位于华北中部造山带南部，是探索华北克拉通基底形成与演化过程的天然实验室，其变质演化与年代学研究亟待加强。申请者前期研究发现该地区代表性变质岩中，均有进变质、变质高峰和退变阶段形成的矿物组合，包括不同阶段的变质副矿物。变质高峰期可达绿帘角闪岩相、中压变质相系。申请者的独居石U-Pb定年研究说明，其变质高峰期可能为~1.85‒1.84 Ga。本项目拟通过详细的野外地质调查、大比例尺地质填图、地质剖面测制，查明变质地质体之间接触关系；通过显微岩相学研究，确定变质反应结构和各变质阶段矿物组合；通过矿物与岩石化学、热力学研究，恢复变质作用P-T轨迹；通过薄片状态下变质副矿物（锆石、独居石、榍石）原位微区高精度U-Pb定年，结合常规U-Pb定年和40Ar/39Ar定年，力图查明变质作用的年代学序列。可望反演出完整的变质作用P-T-t轨迹，探讨中条山地区前寒武纪大地构造背景。

通过对中条山变质杂岩中绛县群和中条群的基性变质岩和泥质变质岩开展系统的野外地质观察和岩石学、矿物化学、岩石地球化学和锆石及独居石U-Pb年代学研究，详细解析中条山变质杂岩的变质演化过程。项目取得的重要成果如下：（1）中条变质杂岩绛县群和中条群经历了角闪岩相的变质作用。角闪岩和副变质岩中识别出三个阶段矿物组合：进变质阶段矿物组合(M1)，变质峰期矿物组合(M2)，和退变质阶段矿物组合(M3)，并通过变质热力学相平衡模拟及矿物温压计方法，恢复出中条山变质杂岩记录的顺时针型变质P-T轨迹；（2）通过高精度SIMS、SHRIMP和LA-ICP-MS锆石和独居石定年方法，限定了变质杂岩的峰期变质时代为1.85-1.82 Ga，揭示了中条山变质杂岩中含有古老的继承锆石年龄信息（~2.82 Ga，2.2-2.18 Ga）；（3）认为中条山变质岩也卷入了华北克拉通在早元古代末东部陆块与西部陆块的碰撞造山过程。本项目的研究成果对于认识华北克拉通中南缘的变质演化过程具有重要意义，并在国际地学权威期刊上发表SCI论文两篇。