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. area has undergone extensive metasomatism with formation albitites scapolite-rich rocks numbers previously economically important deposits including the Silver Modum Cobalt mines. We demonstrate here that central part domain (Kragerø area) locally developed low-grade minerals (prehnite, pumpellyite, analcime, stilpnomelane thomsonite) belonging to prehnite-pumpellyite zeolite facies. Structurally, occur fracture fills, in alteration selvages around fractures where rock is albitized, along shear zones cataclastic zones. fill vary from millimetres scale 1 m thickness. larger systems. typically formed by both displacive (swelling) replacive reactions a combination these. Prehnite together albite, K-feldspar, quartz, epidote hydrogarnet form lenses (001) faces biotite chlorite leading bending sheet silicates through reaction mechanism. Numerous replacement earlier well occur. As talc, actinolite, titanite, calcite hydrogrossular same veins grain minerals, they probably belong assemblage some albitization region presumably occurred at conditions. Alteration olivine (Fo69) conditions results clay ferroan saponite. Reconnaissance studies east (Idefjord domain) northwest (Kongsberg sides Oslo rift reports assemblages south-western Sweden continuation into Skagerrak suggest low grade assembles adjacent its full extent. Ar-Ar dating K-feldspar gave an age 265.2 ± 0.4 Ma (MSWD = 0.514 P 0.766), suggesting metamorphism induced fluids heat magmatic activity Permian rift, which requires transport fluid over distances several kilometres. constrained stability fields prehnite, pumpellyite analcime be less than 250°C pressure 5 kbars. created micro-fractures porosity enhance flow mineral on local scale. On thin section scale, growth albite volume increase 100%. Whether opening larger, horizontally oriented systems needed distance kilometres was also remains unknown. shoulder may have contributed uplift.