Low-grade prehnite-pumpellyite facies metamorphism and metasomatism in basement rocks adjacent to the Permian Oslo rift: The importance of displacive reactions

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.