The larvikite plutonic rocks of the Oslo Rift

The Carboniferous-Permian Oslo Rift represents an early internal rifting on the Pangea continent that never evolved into an oceanic spreading ridge. The ancient rift valley suffered intensive volcanic activity, leaving vast deposits of alkaline lavas and pyroclastic rocks. Among the lavas, the most prominent type is latite, traditionally named “rhomb porphyry”. Larvikite is the plutonic equivalent to these porphyries. As a series of monzonitic rocks, the larvikites stands out by being primarily composed of ternary feldspars, displaying cryptoperthitic intergrowths. Interference between light waves and the intergrowth lamellae creates bright blue iridescence, which is highly appreciated in the natural stone market. Thus, larvikite is among the most attractive dimension-stone resources in the world, and it is likely to find it in a façade or kitchen top near you. The larvikite complex is a series of ring-shaped plutons becoming younger and more undersaturated from east to west. Thus, they range from quartz-bearing to nepheline-bearing. They display magmatic layering and crystal orientation patterns reflecting the overall ring structure. Larvikites also host a wide range of pegmatites, including type areas for minerals like låvenite, thorite, aegirine, mosandrite, leukophane and tritomite. Thorium was discovered in such pegmatites in the 1820’s. Along the coastline of Vestfold, the larvikites are beautifully exposed, sculptured and polished by numerous glacial episodes.