Geochemistry, geochronology and its geological implication of metamorphic rocks of the Jiehekou Group in the Lüliang complex

Abstract

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，该造山带的形成经历了一个漫长而复杂的演化过程。吕梁变质杂岩位于造山带中段，阜平杂岩西南，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。本文针对该地区的界河口（岩）群黑云斜长片麻岩和含榴斜长角闪岩开展了系统的野外地质调查、显微岩石学、地球化学和变质年代学研究。结果表明，界河口（岩）群黑云斜长片麻岩的原岩多为富铝富钾的变泥质岩石，其物源主要来自长英质火成岩，形成于有演化岛弧发育的活动大陆边缘构造背景；含榴斜长角闪岩的原岩多数是拉斑玄武岩，源区来自于富集地幔，形成于岛弧构造环境。显微岩相学研究显示，含榴夕线黑云斜长片麻岩和含榴斜长角闪岩中均保留了早期进变质、峰期变质和晚期退变质三个阶段的变质矿物组合，且石榴子石边部多发育明显的白眼圈反应结构。其中，含榴夕线黑云斜长片麻岩的峰期矿物组合包括石榴子石、黑云母、夕线石、斜长石、石英和少量钾长石等；含榴斜长角闪岩的峰期矿物组合主要由石榴子石、角闪石、斜长石、磁铁矿和石英等组成。前期研究表明，二者的变质演化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，其峰期变质条件分别为>750℃/>7.0kbar和>750℃/>6.3kbar。本文LA-ICP-MS锆石U-Pb年代学研究表明，含榴黑云斜长片麻岩中记录了1928~1920Ma和1882~1855Ma两组变质年龄。结合中部带前人研究成果，本文推断吕梁变质杂岩的区域变质事件与华北克拉通东、西部陆块间的碰撞造山作用有关，该地区与碰撞阶段同期的峰期变质作用发生在1928~1920Ma，而后于1882~1855Ma期间进入与快速构造隆升有关的退变质作用阶段。;As an important Precambrian orogenic belt in the North China Craton (NCC), the Trans-North China Orogen (TNCO) was formed by the collision between the Eastern Block and Western Block through an extremely protracted evolution process. To the southwest of the Fuping complex in the central TNCO, the Lüliang metamorphic complex provides an ideal target for studying the Precambrian tectonothermal evolution of the NCC since many upper amphibolite- to granulite-facies metamorphic rocks of the Jiehekou Group were exposed in this area. Petrographic, geochemical and metamorphic geochronological studies were carried out on the biotite plagiogneisses and garnet-bearing amphibolites in this contribution. It was indicated that the protoliths of biotite plagiogneisses were mostly metapelites with high aluminium and potassium contents whose sedimentary provenances were demonstrated to have mainly come from felsic igneous rocks formed in an active continental margin environment. The protoliths of garnet-bearing amphibolites were tholeiite basalts from the enriched mantle, and formed in the island arc tectonic setting. Microscopic petrographic studies showed that there were three stages of mineral assemblages preserved in the garnet-bearing sillimanite-biotite plagiogneisses and amphibolites, including the prograde, peak and retrograde stages. The white-eye socket reaction microstructures commonly grew surrounding the garnet rims of these rocks. The peak assemblages of garnet-bearing sillimanite-biotite plagiogneisses mainly consist of garnet, biotite, sillimanite, plagioclase, quartz and some K-feldspar. Those of garnet-bearing amphibolites include garnet, amphibole, plagioclase, magnetite and quartz. Our previous studies showed that both two types of rocks revealed clockwise P-T paths ended with a near-isothermal decompression, and their peak conditions were > 750℃/>7.0kbar and >750℃/>6.3kbar, respectively. LA-ICP-MS zircon U-Pb dating in this study indicated that two groups of metamorphic ages, including 1928~1920Ma and 1882~1855Ma, were recorded in the garnet-bearing biotite plagiogneisses. Combined with previous metamorphic and geochronological studies of other complexes in the TNCO, the regional metamorphism occurred in the Lüliang complex was thought to be related to the orogenic collision between the Eastern Block and Western Block of the NCC. During this orogenic process, the metamorphic peak occurred at 1928~1920Ma in the Lüliang area, and then followed with 1882~1855Ma retrograde stages resulted by the rapid tectonic uplift after collision.