Emplacement and origin of Orogenic Peridotites, Seve Nappe Complex, Central Swedish Caledonides

Abstract

Orogenic peridotites can be found at or close by the tectonic interfaces between various nappes that make up the Seve-Köli nappe complex (SKNC) in Northern Jämtland, Sweden. These orogenic peridotites have already been found to have different origins, from suboceanic lithospheric mantle (SOLM) to sub-continental lithospheric mantle (SCLM). Several models have been proposed to address the possible crustal emplacement of these orogenic peridotites. Yet no clear model exists that explains all evidence found within the SKNC of the Swedish Caledonides. Here, in order to explain all aspects, a renewed model will be made of this part of the Scandinavian Caledonides. This will be done by looking at various orogenic peridotites from the Seve Nappe Complex. In these peridotites special attention is paid to the Mg# (Mg / Mg + Fe2+) in olivine and the Cr* (Cr / Cr + Al) in spinel. Also, chemical EMP analysis of various other minerals, such as garnet and plagioclase, were taken to better improve the subduction model of Brueckner & van Roermund, 2004, and that of Roberts, 2003. Combining the chemical data of the minerals, an origin of the orogenic peridotites could be constructed. Also, samples from the surrounding country rocks were taken to construct PTt-paths based upon thermodynamic modelling using Perple_X software along with chemical data, i.e. bulk rock chemistry derived from XRF analysis and mineral chemical data measured by using the EMP. Mg# and Cr* of the central- and eastern Seve belt peridotites show clear SCLM affinity, which suggest that the surrounding country rocks, which contain evidence of the subduction depth of the metamorphic event. Mg# and Cr* from western Seve belt peridotite show a clear oceanic lithospheric mantle affinity. Along with PTt-paths of the country rock, this results in a multi-stage subduction model.