Browsing by Author "Sorsa, Joona"

The Kuohatti mafic layered intrusion is located 150 kilometers northeast from the city of Nurmes. It is a part of the Kuohattijärvi suite determined earlier by the Geological Survey of Finland. The 100 meters wide and 2 kilometers long intrusion was found during the bedrock mapping related to Kuohattijärvi and its surrounding areas. Gabbros with abundant plagioclase phenocrysts and metamorphosed mafic rocks, now seen as talc-chlorite schists on the current erosion level led to this study. The plagioclase phenocrysts are visible throughout the whole intrusion and are signals of plagioclase floating - a typical feature in layered intrusions caused my magmatic differentiation. The metamorphosed mafic rocks are remnants of the peridotitic bottom of the intrusion. The intrusion is divided into multiple layers, a mafic bottom, anorthosites, gabbroic rocks and plagioclase phenocryst –rich gabbros, here abbreviated as leucocratic gabbros. The intrusion is currently thoroughly tilted 70–90 degrees, cut by diabase dykes and metamorphosed in the granulite facies. Rock compositions suggest that during the formation of the mafic bottom and the gabbroic rocks, some amounts of calcic plagioclase (An80-90) crystals subsided to the lower parts of the intrusion, whereas lighter and notably less calcic (An40-70) crystals floated upwards into the evolving gabbroic melt. Zircons from a country-rock (talc-chlorite schist) and a leucocratic gabbro from intrusion were analyzed for U-Pb isotopes. All these yield U-Pb age groups at ca. 2.72 Ga and 2.88 Ga. Monazite grains show three distinctive age groups at 2.68 Ga, 2.42 Ga and 1.90 Ga, indicating strong correlation with the formation of the diabase dykes and metamorphic events in the area. The analyses are likely from an inherited source, indicating the actual age of the surroundings. The exact crystallization age of the intrusion is possibly between 2.4 and 1.9 Ga. This would correlate with the Proterozoic layered intrusions surrounding the area and suggest that the Kuohatti layered intrusion is actually of Proterozoic rather than Archean origin. The intrusion is now located between a biotite paragneiss and an Archean tonalite, which might show a tilted lithology for the intrusions surroundings during and after its settling.