建平变质杂岩早前寒武纪变质演化及变质过程高精度年代学研究

Metamorphic rocks of the Jianping Metamorphic Complex, northeastern part of the North China Craton (NCC), experienced amphibolite to granulite facies metamorphism. It might be one of the few geological bodies which record two metamorphic events of both the Late Archaean and Paleoproterozoic. The “white-eye socket” symplectites are preserved in the metapelites and meta-basic rocks, and chemical zoning is found in the garnet porphyroblast. Detailed field investigation and large-scale geological mapping are to be done to clarify the geological spatial relationships of the representative tectonic units. Also, through detailed micropetrographic studies we can clarify the metamorphic reaction textures and specify the mineral assemblages formed at different metamorphic stages and events. The accurate metamorphic P-T paths will be retrieved by integrating the high-quality traditional geothermobarometry methods coupled with the thermodynamic pseudosection modeling for the representative rocks. Meanwhile, the metamorphic time sequence will be determined through SIMS U-Pb dating of metamorphic zircon, rutile and monazite, especially in-situ U-Pb dating of these accessory minerals on the thin-sections, as well as EPMA dating of monazites and 40Ar/39Ar dating of biotite and amphibole. Thus, the Late Archaean and Paleoproterozoic metamorphic P-T-t paths are expected to be retrieved，which can provide important gelogical implications in understanding the early Precambrian tectonic setting and the crustal evolution of the NCC.

出露于华北克拉通北缘东段的建平变质杂岩，变质程度达角闪岩相到麻粒岩相，是可能记录了晚太古代和古元古末期两期变质事件的为数不多的地质体。该区的变泥质岩和变基性岩普遍发育“白眼圈”等变质反应结构，石榴子石中保存有化学成分环带。本项目拟在详细的野外调查和局部大比例尺地质填图的基础上，查明该区各类岩石的野外空间关系，变质岩石组合和类型，确立变质作用期次和叠加方式，计算不同变质阶段的温度压力以及演变过程。综合采用副矿物（锆石、金红石和独居石等）的原位微区SIMS U-Pb定年和其它有效定年手段，建立变质年代学序列，构建晚太古代和古元古代两期变质过程和P-T-t轨迹。探讨晚太古代和古元古代两期变质作用的大地构造背景和地质意义，为深入理解华北克拉通早前寒武纪结晶基底的形成和演化提供依据。

建平杂岩位于华北克拉通东部陆块的西北缘，由英云闪长-奥长花岗-花岗闪长质（TTG）片麻岩、基性麻粒岩（局部退变为斜长角闪岩）、变质表壳岩和少量变质超镁铁质岩组成。TTG片麻岩在田家杖子周围大面积出露，基性麻粒岩主要以“似层状”或“透镜状”产于TTG片麻岩之中，它们发育连贯一致的片麻理，片麻岩理走向南西西-北东东，倾向南东，倾角较陡。基性麻粒岩记录了两期变质事件：早期变质事件以矿物包裹体形式极少量保存于基质中的单斜辉石和石榴子石变斑晶中，以小颗粒石榴子石含或不含后成合晶矿物、细粒斜方辉石和金红石为代表。晚期变质事件以石榴子石变斑晶以及其周围的后成合晶和基质矿物组合（粗粒单斜辉石、斜方辉石、角闪石、斜长石、钛铁矿、磁铁矿和石英。）为代表。P-T视剖面和传统地质温压计计算结果表明，基性麻粒岩记录了两条相似且独立的变质作用P-T轨迹，均包含近等温降压片段，早期和晚期变质温压条件的下限分别为~9.5–9.9 kbar/860℃和~12.5–14.7 kbar/860–880℃。锆石SIMS、LA-(HR)-ICP-MS U-Pb数据以及独居石LA-ICP-MS U-Pb数据表明，TTG片麻岩和基性麻粒岩主要记录了新太古代末期（2.48–2.53 Ga）和极少量古元古代末期（1.83–1.96 Ga）变质时代。结合矿物组合、结构关系和石榴子石与变质锆石的平衡关系，确定TTG片麻岩与基性麻粒岩记录了相同的新太古代末期和古元古代末期区域变质事件。在大多数样品中，新太古代末期形成的变质矿物组合被古元古代末期变质作用完全改造，锆石和独居石则主要记录了新太古代末期变质时代。因此，建平杂岩基性麻粒岩和TTG片麻岩中副矿物锆石和独居石记录的变质年龄与造岩矿物具有解耦关系。与建平杂岩相似的多相变质杂岩有冀东、密云、清原等。在叠加变质地体中，准确厘定矿物组合与P-T轨迹和变质年龄的对应关系应引起重视。