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Geologiska Institutionen, Solvegatan 13, S-223 62 Lund, Sweden
The late Caledonian metamorphic evolution of the basement beneath the Caledonian allochthons of the central Scandes has been determined by studying mafic schists and phyllonites of the crystalline basement, and the overlying Vendian and/or Cambrian sedimentary rocks. The studied rocks occur along the Caledonian front and in tectonic windows within the Scandinavian Caledonides.
A gradual westwards increase in metamorphic grade is recorded, which varies from upper diagenetic conditions in the autochthonous rocks along the eastern thrust front of the Caledonides to middle amphibolite facies in the western parts of the Nasafjäll and Tømerås windows. Locally, strain-related deviations from this pattern occur. Further west the grade of Caledonian metamorphism of the basement increases and reaches the eclogite and granulite facies.
The Scandian metamorphic conditions in the basement rocks studied prevailed in the mid- to late Silurian, at pressures of 8 ± 2 kbar and temperatures of 550 ± 50°C in the Tømmerås window, and 7 ± 2 kbar at 550 ± 50 °C in the Nasafjäll window. Uplift and cooling followed thereafter, continuing into the Devonian. The final retrograde metamorphism was characterized by temperatures of 450 ± 50°C at pressures of about 3 kbar in the Nasafjäll and Tømmerås windows.
The occurence of syn-schistosity amphibolite-facies assemblages implies heating of the basement rocks during nappe emplacement. Available time constraints suggest that this was a relatively rapid process. In late Silurian to early Devonian, the prograde phase was followed by uplift and erosion. The metamorphic evolution also demonstrates that early Devonian thrust movements were associated with a decrease in the thickness of the nappe pile.
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