The Scandinavian Caledonides is an orogenic belt formed when Baltica docked with Laurentia during the late Silurian to early Devonian (Scandian) collision. The thrust sheets forming the belt are divided into the Lower, Middle, Upper, and Uppermost Allochthons, derived from the Baltican margin and regions progressively farther outboard in the Iapetus Ocean. This study is of the Blåhø Nappe of the Middle Allochthon, derived from an Early Paleozoic volcanic arc off the Baltican margin. These rocks contain abundant igneous rocks that have been metamorphosed to medium P-T amphibolites (amphibole-plagioclase rocks). They contain comparatively small blocks of garnet-pyroxene eclogites, representing earlier metamorphism at high pressures and relatively low temperatures. The host amphibolites must once have been eclogites themselves, but have since equilibrated to lower-P conditions and recrystallized. Using textural and mineral analyses and thermodynamic modeling, we studied the conditions under which the host amphibolites reequilibrated from eclogites during tectonic escape from great depth.
41 rock samples from the Blåhø Nappe were made into polished thin sections. Mineral assemblages and textures were observed with a polarized light microscope. BSE imaging and standards-based EDS analyses were done on selected samples, targeting particular textures and low-variance assemblages. Analyses were modified to fit thermodynamic solution model constraints, and combined as rock compositions for the thermodynamic modeling program, Perple_X. The modal and mineral compositional results were compared across model P-T space to the mineral modes and compositions given to the program. The P-T area in which the most criteria matched (within uncertainties) was presumed to be a good estimate of the P-T conditions of last equilibration of that assemblage. Results suggest that the amphibolite facies assemblages developed at about 525°C, and 7 kbars for the recrystallized matrix, and 575-800°C and 7010 kbars for symplectites. For comparison, eclogite facies assemblages were formed at 600-800°C at 12 to 35 kbars (Hacker and Gans, 2005). Amphibolite facies reequilibration therefore seems to have occurred during isothermal decompression followed by isobaric cooling. Our results are comparable to P-T conditions determined for the Köli Nappe Complex in central Norway (Hacker and Gans, 2005).