The origin and crustal emplacement of orogenic peridotites in the Lower Köli- and Western Seve Nappe Complexes of the central Scandinavian Caledonides

Abstract

A remarkable feature of the Scandinavian Caledonides is the widespread occurrence of orogenic peridotites at most of its tectonostratigraphic levels. Until the present day, only a few tectonic models have been proposed that can explain their origin (protolith formation), spatial and temporal distribution and crustal emplacement, which are thus under discussion. This research aims to determine whether the current tectonic models for the Scandinavian Caledonides need to be modified and focusses on the origin and crustal emplacement of orogenic peridotites in the Western Belt of the Seve Nappe Complex (SNC) and lower part of the Köli Nappe Complex (KNC). Samples of peridotites and their immediate surrounding country rocks were collected during a one month fieldwork period in southern Västerbotten and northern Jämtland (central Sweden) for further analysis using OM, SEM, EMP, XRF and Raman spectroscopy combined with thermodynamic modelling in PerpleX. This study proposes an oceanic origin for the Lower Köli and a suboceanic mantle wedge origin for the Western Seve orogenic peridotites, based on EMP and XRF analyses. After protolith formation, the Western Seve and Lower Köli peridotites experienced a similar retrograde ocean floor serpentinization event, interpreted to be Early Finnmarkian in age (505 Ma). After serpentinization, the Western Seve belt peridotites and surrounding country rocks experienced a peak metamorphism of 610°C and 0.8 GPa (calculated in PerpleX) during Finnmarkian thrusting (490 Ma) while the Lower Köli peridotites experienced a prograde metamorphic event up to 400°C-500°C (based on the stability of antigorite and olivine in the absence of talc). The prograde metamorphism of the Western Seve and Lower Köli is overprinted by a second retrograde serpentinization event (M4). The origin and metamorphic conditions of the Western Seve peridotites and their country rocks are used to improve the current tectonic models for the Scandinavian Caledonides.