Low-grade prehnite-pumpellyite facies metamorphism and metasomatism in basement rocks adjacent to the Permian Oslo rift: The importance of displacive reactions
Håkon Austrheim, Ane K. Engvik, Morgan Ganerød, Kristina G. Dunkel, Mari Roen Velo
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引用次数: 3
Abstract
The Kongsberg and Bamble lithotectonic domains of SE-Norway are known as classical Precambrian high-grade metamorphic terrains. The area has undergone extensive metasomatism with formation of albitites and scapolite-rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low-grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite-pumpellyite and zeolite facies. Structurally, the low-grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low-grade minerals are part of larger fracture systems. The low-grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K-feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low-grade minerals occur. As albite, K-feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low-grade minerals, they probably belong to the low-grade assemblage and some of the albitization in the region presumably occurred at low-grade conditions. Alteration of olivine (Fo69) at low-grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low-grade assemblages in south-western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar-Ar dating of K-feldspar from the low-grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low-grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro-fractures and porosity in the adjacent minerals that enhance fluid flow and low-grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the opening of the larger, horizontally oriented fracture systems needed to transport the fluid over a distance of several kilometres was also the results of displacive reactions remains unknown. The low-grade metamorphism and metasomatism formed in the shoulder of the Oslo rift and may have contributed to its uplift.
挪威东南部的Kongsberg和Bamble岩石构造域是典型的前寒武纪高变质构造域。该地区经历了广泛的交代作用,形成了富含钠长石和角长石的岩石,以及许多以前具有重要经济意义的矿床,包括Kongsberg银矿和Modum钴矿。本研究表明,班布尔岩体构造域中部(Kragerø地区)局部发育低品位变质矿物(前黝岩、泵柱岩、铝镁石、铁镁石和thomsonite),属于前黝岩-泵柱岩和沸石相。在构造上,低品位矿物以裂隙填充物、裂隙周围的蚀变边缘以及剪切带和碎裂带的形式赋存。裂缝充填体和蚀变边缘厚度从毫米级到1米级不等。含低品位矿物的裂缝是较大裂缝系统的一部分。低品位矿物,通常由驱替(溶胀)和替代反应以及两者的结合形成。前辉石与钠长石、钾长石、石英、绿帘石和水榴石沿黑云母和绿泥石的(001)面形成透镜,通过位移反应机制导致硅酸片弯曲。发生了许多替代反应,包括早期矿物和低品位矿物。由于钠长石、钾长石、滑石、石英、放线石、钛矿、方解石和水镁石与经典低品位矿物在同一脉脉和同一黑云母颗粒中形成,它们可能属于低品位组合,该地区的部分钠长石化可能发生在低品位条件下。在低品位条件下,橄榄石(Fo69)蚀变形成了包括铁皂石在内的粘土矿物。在奥斯陆裂谷东侧(Idefjord岩石构造域)和西北侧(Kongsberg岩石构造域)进行的勘测研究,以及在瑞典西南部沿着裂谷延伸至Skagerrak的低品位组合的报道表明,低品位组合出现在奥斯陆裂谷完整范围内邻近的岩石中。低品位组合钾长石的Ar-Ar年龄为265.2±0.4 Ma (MSWD = 0.514, P = 0.766),表明低品位变质作用和部分交代作用是由二叠纪奥斯陆裂谷岩浆活动的流体和热量引起的,这需要流体在几公里的距离上进行输送。变质条件受前辉石、泵辉石和铝矿稳定场的限制,在小于250℃和小于5 kbar的压力下。驱替代性反应在邻近矿物中产生微裂缝和孔隙,从而增强了局部范围内的流体流动和低品位矿物形成。在薄片尺度上,钠长石在黑云母中的置换生长导致局部体积增加数100%。是否需要打开更大的、水平导向的裂缝系统,将流体输送到几公里的距离,这也是驱替反应的结果,目前尚不清楚。低变质作用和交代作用形成于奥斯陆裂谷肩部,可能对其隆升起了作用。
期刊介绍:
The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.