PETROLOGY AND THERMODYNAMIC MODELING OF AMPHIBOLITE FACIES ROCKS (RETROGRADED ECLOGITES) IN THE BLÅHØ NAPPE OF THE MIDDLE ALLOCHTHON, SCANDINAVIAN CALEDONIDES IN NORWAY

The Scandinavian Caledonides are an orogenic belt formed when Baltica collided with Laurentia during the late Silurian to early Devonian (Scandian, ~426–390 Ma, Gee et al., 2008). The thrust sheets forming the belt are divided into the Lower, Middle, Upper, and Uppermost Allochthons derived, respectively, from the Baltican margin and regions progressively farther outboard. This study is of the Blåhø Nappe, Middle Allochthon, generally presumed to have been from an Early Paleozoic volcanic arc off the Baltican margin. The Nappe contains abundant igneous and sedimentary rocks, metamorphosed almost entirely at amphibolite facies, but contains scattered eclogites that indicate prior metamorphism at high pressure. The host amphibolites adjacent to eclogites retain microstructures indicating retrograde origin from eclogite, and so were once eclogites themselves. Rocks away from the eclogites, however, have no textural evidence of having been eclogites, and presumably have been retrograded from eclogite and thoroughly recrystallized.

Using textural evidence, mineral analyses, and thermodynamic modeling, we are studying the conditions under which the host amphibolites transformed from eclogites during tectonic escape from great depth. 41 rock samples, mostly amphibolites from different sites in the Blåhø Nappe, were made into polished thin sections, with mineral assemblages and textures observed with a polarized light microscope. BSE imaging and quantitative analyses (standards-based EDS) were done on selected samples, targeting particular textures and low-variance assemblages. Analyses were modified to fit thermodynamic solution model stoichiometric constraints, and combined as system compositions for thermodynamic modeling with Perple_X. The modal and mineral compositional results were compared across the model P-T space to the mineral modes and compositions given to the model. Where the modal and compositional 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 600- 650°C, and 7–11 kbars. For comparison, eclogite facies assemblages in the Middle Allochthon were metamorphosed at 600-800°C at 12 to 35 kbars (Hacker and Gans, 2005).