Petrography, petrochemistry and regional significance of the Saxvallklumpen metabasite, Köli Nappe Complex, Tännforsfältet, central Swedish Caledonides

Abstract

Abstract The Köli rocks in Tännforsfältet mainly consist of calcareous phyllite and its higher grade metamorphic equivalent – garbenschiefer. In the uppermost, northwestern part of Tännforsfältet the Saxvallklumpen metabasite occurs within the garbenschiefer. It has an elongate lensoid form, extends for 11 km along strike and is up to 500 m thick. The metabasite mainly occurs as a fine-grained amphibolite with both aphyric and plagioclase phyric texture. In its lower part the metabasite contains nodules of epidotite. It was deformed and metamorphosed to lower amphibolite facies together with the surrounding garbenschiefer, the matrix being converted to hornblende and plagioclase phenocrysts to aggregates of epidote, basic oligoclase, quartz and paragonite. Chemical analyses of the metabasite demonstrate a tholeiitic basalt composition. The metabasite follows the trend of abyssal tholeiites. The Ti-Zr-Y, Ti-Cr and Zr/Y-Zr discriminant diagrams indicate an ocean-floor-basalt origin. The calcareous phyllite-garbenschiefer of Tännforsfältet is divided by thrusts into different tectonic units, the Saxvallklumpen metabasite occurring in the uppermost one. The Köli Nappes of northern Jämtland-southern Västerbotten show a similar picture with basic rocks, gabbros, occurring within the Blåsjön calcareous phyllite. The petrochemistry of these gabbros is similar to that of the metabasite in Tännforsfältet, the Saxvallklumpen metabasite being slightly more primitive.