吕梁界河口群麻粒岩的变质演化及变质过程年代学研究

In recent years, some metamorphic geochronological data, not only ~1.85 Ga but also ~1.95 Ga and ~2.5 Ga, have been reported from the metamorphic complexes in the Trans-North China Orogen (TNCO). It indicates that the forming of the TNCO was a long and complicated process. The Lüliang Metamorphic Complex is significant to understand the Precambrian tectonics and evolution of the NCC, which is located in the middle segment of the TNCO, nearest to the Ordos Block of the North China Craton (NCC). We found that there are metapelitic granulites and mafic granulites locally exposed in this complex, and these rocks preserved more than three metamorphic stages of assemblages, which were formed during the prograde, peak and post-peak decompression stages, respectively. However, less metamorphic data of the two types of granulites there has been reported by now. This project deals with metamorphism-deformation and their relationship, textures and metamorphic reactions, and the metamorphic P-T paths with the particular attention to garnet-bearing metapelitic granulites and mafic granulites in this complex. And we try to establish close to a complete metamorphism P-T-t, combined with thermodynamics analog computation and high-precision in-situ geochronological study. This project will present important information for the tectonic evolution and the amalgamation of the Precambrian basement of the NCC.

近来年，学者们报道的华北“中部造山带”变质事件的时代不再局限于~1.85Ga，出现了~1.95Ga和~2.5Ga的变质年龄记录，人们开始认识到“中部带”可能经历了一个非常复杂而漫长的演化过程。吕梁变质杂岩位于该造山带中段，是透视华北克拉通早前寒武纪构造-热演化的一个重要窗口。前期研究发现，吕梁变质杂岩区原界河口群的泥质麻粒岩和基性麻粒岩均保留了明显的变质反应结构，记录了前进变质、变质高峰期和退变质等至少三个阶段的变质矿物组合。但目前该区变质作用方面研究比较薄弱，尚未见到麻粒岩类变质作用的相关报道。本项目拟重点针对记录变质演化和年代学信息丰富的界河口群泥质、基性麻粒岩，详细查明其变质-变形期次及其相互关系、变质反应结构、反演变质作用P-T轨迹，结合热力学模拟计算、原位高精度年代学研究，反演其变质作用接近完整的PTt轨迹。本项目可为探讨华北克拉通早前寒武纪基底拼贴固化历史提供重要的科学信息。

华北中部造山带是华北克拉通内部一条重要的前寒武纪碰撞造山带，该造山带经历了一个漫长而复杂的演化过程，至少记录了三组变质年龄信息（~1.85 Ga、~1.95 Ga和~2.5 Ga）。吕梁变质杂岩位于造山带中段，是透视其构造演化的一个重要窗口。.吕梁变泥质岩中保留了三个阶段的矿物组合。进变质矿物组合由石榴石核部的石英、斜长石和黑云母等早期包裹体组成；峰期矿物组合由石榴石、黑云母、夕线石、斜长石、石英和少量钾长石组成；退变质阶段由石榴石周围交生状的堇青石、黑云母、斜长石和石英等后成合晶矿物组成，构成完美“白眼圈”。三阶段温压条件依次为：541~563 ºC / 2.0~2.3kbar、> 750 ºC / >7.0 kbar和558~620 ºC / 1.7~2.7 kbar。.基性麻粒岩同样保留了三个阶段的矿物组合。进变质阶段矿物组合由石榴石内部的普通角闪石、石英和黄铁矿等包裹体组成；峰期矿物组合由大颗粒的石榴石、斜方辉石、普通角闪石、斜长石、黑云母、石英和钛铁矿组成，形成温度>790 ºC、压力>9.4 kbar；退变质阶段由石榴石变斑晶周围发育的小颗粒石榴石、斜方辉石、普通角闪石、镁铁闪石、黑云母、斜长石和石英等后成合晶矿物组成，该阶段的变质条件为652~775 ºC / 2.8~7.5 kbar。.斜长角闪岩中保留了两个阶段矿物组合。峰期矿物组合由石榴石、角闪石、斜长石、钛铁矿和石英等组成，变质温度>750 ºC、压力>6.3 kbar；退变质阶段由石榴石周围小颗粒的角闪石、斜长石和石英等后成合晶矿物组成，变质条件约为714 ºC / 5.7 kbar。.传统矿物温压计和视剖面图模拟结果显示，吕梁变质泥质岩和变质基性岩石均记录了类似的顺时针近等温降压型的变质作用P-T轨迹，该区域变质事件与华北克拉通东西部陆块间的俯冲–碰撞–隆升作用有关。锆石年代学研究表明，该地区变质泥质岩和基性麻粒岩中多数锆石记录了1960~1920 Ma的峰期变质年龄，而少数锆石记录了1895~1815 Ma的退变质年龄。结合前人研究成果，本项目推断吕梁变质杂岩区的地壳增厚阶段可能开始于1.96 Ga，而后在约1.90 Ga开始进入构造抬升阶段。