阜平地区前寒武纪变质作用与高精度年代学及其大地构造意义

The Fuping terrane is a key representative metamorphic terrane within the North China Craton (NCC). Various metamorphic reaction textures preserve in the garnet-bearing mafic granulite, amphibolite and metapelite, suggesting that two to four generations of metamorphic mineral assemblages survived from several metamorphic stages. Therefore, the Fuping terrane is an ideal window from which the Palaeoproterozoic tectono-metamorphic evolution of the NCC can be retrieved. In implementing of this program including the following key researches: (1) Detailed field investigations are to be done, including mapping of geology of local regions, to specify the geological contact relations of the representative tectonic units; (2) Certify the relationships of metamorphism and deformation based on field observation and micropetrography; (3) Specify the metamorphic mineral assemblages formed at different metamorphic stages, based on detailed metamorphic reaction textures; (4) Accurately retrieve the metamorphic P-T paths by integrated application of geothermobarometry and thermodynamic pseudosection computation, as well as modeling of the growth process of metamorphic zircon; and (5) Determine the metamorphic age by SIMS U-Pb dating of metamorphic zircon, titanite and monazite, and 40Ar/39Ar dating of potassic metamorphic minerals. Implementation of this program can provide reliable scientific data in understanding the Palaeoproterozoic tectono-metamorphic evolution and formation of the basement of the NCC.

阜平地区是华北克拉通典型变质地质体之一。该区含榴基性麻粒岩、含榴斜长角闪片麻岩、含榴黑云斜长片麻岩中，普遍发育各种变质反应结构，保留了两至四个阶段的变质矿物组合，是探索前寒武纪期间华北克拉通基底形成过程的理想研究窗口。本项目计划：（1）开展深入细致的野外地质调查，进行局部地区大比例尺地质填图，查明各种代表性地质体之间的接触关系；（2）结合野外地质与显微尺度研究，查明变质-变形之间的耦合关系；（3）研究代表性变质岩石中的各种变质反应结构，准确识别各个变质阶段形成的矿物组合；（4）有机结合矿物温度计与压力计和热力学视剖面图模拟计算，准确反演变质作用P-T轨迹，并模拟变质锆石形成过程；（5）通过变质锆石、榍石、独居石的高精度二次离子质谱U-Pb定年，以及含钾变质矿物的40Ar/39Ar定年，准确确定变质作用的地质时代。本项目可望为深入研究华北克拉通基底形成过程，提供第一手高质量可靠的科学资料。

阜平地区地处近南北向的古元古代华北中部造山带中段，是透视该造山带演化过程的一个理想窗口。研究表明：(1)广泛出露的基性麻粒岩和斜长角闪片麻岩，少部分与副变质岩呈“互层状”，大部分以透镜体或包体的形式被包裹于TTG片麻岩中；(2)岩石普遍经受了高角闪岩相-麻粒岩相变质作用，大多保留有三至四个变质阶段矿物组合，可见到含“双白眼圈”反应结构的含榴基性麻粒岩、含斜方辉石的半泥质高压麻粒岩；(3)岩石普遍记录了典型造山带顺时针型P-T轨迹，变质高峰属于中压变质相系。退变质阶段依次经历近等温降压和近等压冷却过程，说明构造抬升速率先快后慢。变质岩石记录了东部陆块和西部陆块于古元古代末俯冲-碰撞-构造折返的完整造山过程；(4)变质锆石和独居石SIMS U-Pb定年，以及变质角闪石、黑云母和白云母40Ar/39Ar定年表明，该区变质作用集中于1850~1802 Ma，比造山带南缘太华变质杂岩晚了~100 Ma；(5)湾子表壳岩单元中，发现有来自不同变质深度、在折返阶段才混杂在一起的变质-构造岩片；(6)龙泉关韧性剪切带的韧性剪切作用发生于~1.85–1.80 Ga，变形温度达高绿片岩相‒绿帘角闪岩相，自西而东温度有递增现象；(7)龙泉关韧性剪切带形成于东部陆块和西部陆块的碰撞过程中，于1.80–1.78 Ga期间经历了较快速冷却。

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