On the protolith assemblage, the tectonostratigraphic position and metamorphic history of some orogenic peridotites in the lower Köli nappe, Southern Västerbotten, Central Swedish Caledonides.

Abstract

Orogenic peridotites are present in most of the higher nappes of the Scandinavian Caledonides. However, the origin of these orogenic peridotites is still under discussion. The Köli Nappe, subdivided in lower (eastern Iapetus) and upper Köli (western Iapetus), consists of the Iapetus related terrains and contains peridotite bodies of various sizes. During fieldwork in southern Västerbotten samples of some lower Köli peridotites were collected for further analysis using EMP, XRF and Raman spectroscopy. This work proposes an oceanic origin for the lower Köli peridotites based on the composition of protolith spinel and the magnesium number of protolith olivine grains. After protolith formation serpentinisation is inferred to have taken place at the ocean floor or after obduction onto a (micro) continent during the Finnmarkian Orogeny. The peak metamorphic conditions during the prograde metamorphism of the Scandian Orogeny are 400-500 ˚C and 5kb based on the stability of antigorite and olivine in the absence of talc. These PT conditions are subsequently used to improve earlier made tectonic models of the formation of the Scandinavian Caledonides.