Metamorphism of the Seve Nappes at Åreskutan, Swedish Caledonides

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Abstract
Two Seve Nappes occur in the Åreskutan area. The lower nappe is composed of a layered sequence of tectonized metasediments and amphibolites, metamorphosed in the amphibolite facies. The upper nappe (Åreskutan Nappe) consists of high-grade migmatitic gneisses and metabasites. The two nappes are separated by a mylonite zone. A remnant of extensively mylonitized Särv Nappe is believed to occur between the Lower Seve Nappe and the Silurian rocks of the Lower Allochthon. The Särv Nappe is metamorphosed in greenschist facies. A phyllonitic zone separates the Särv Nappe from the Lower Seve Nappe. The mineral assemblage in the amphibolite facies Lower Seve Nappe indicates recrystallization temperatures around 550°C and pressures of 200–500 MPa. The distribution coefficients for the partitioning of Mg and Fe between biotite and garnet are about K D = 0.18. Strong retrogression occurred in the mylonitic basal parts of the Särv and Seve Nappes during Caledonian nappe translation. In the gneisses of the Åreskutan Nappe, a granulite facies metamorphism at >650°C and > 500–700 MPa was overprinted by amphibolite facies metamorphism. Biotite-garnet K D-vaIues from the Åreskutan Nappe cluster around 0.30, indicative of high-grade metamorphism. Polymetamorphism is possibly reflected by a considerable spreading of the K D-values from 0.26 to 0.58. Caledonian pegmatites cut the Åreskutan Nappe. They are mylonitized at the base of the nappe. No retrogression has taken place during the formation of the mylonitic foliation, which indicates that this process took place under relatively high-grade conditions. This is confirmed by the K D-values from recrystallized garnets and biotites in the mylonite. It is believed, therefore, that this mylonite formed during Caledonian nappe translation, when the Åreskutan Nappe was superposed on the Lower Seve Nappe. Subsequent displacement of the two Seve Nappes together on to the underlying tectonic units occurred under much lower metamorphic conditions. Recent Rb-Sr age determinations suggest that a high-grade metamorphism of the Åreskutan Nappe is of Caledonian age, although the rocks of this nappe may have had a pre-Caledonian history. The few age determinations (fission track) of minerals in the Lower Seve Nappe gave Proterozoic ages.