Petrology and thermodynamic modeling of amphibolite facies rocks of the Blåhø Nappe of the Middle Allochthon, Scandinavian Caledonides in Norway
Date of Award
Bachelor of Science
Blåhø Nappe, Norway, thermodynamic modeling, Perple_X, amphibolite, Scandinavian Caledonides
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 focuses on the Blåhø Nappe, part of the Middle Allochthon, which is generally presumed to have been from an Early Paleozoic volcanic arc off the Baltican margin. The Blåhø 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 that the amphibolites originated from retrograded eclogites, and thus 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 studied 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 T-P space to the mineral modes and compositions given to the model. Where the modal and compositional criteria matched (within uncertainties) was presumed to be the best estimate of the T-P conditions of last equilibration of that assemblage. Multiple samples giving best results at the same T-P conditions gave confidence that those T-P estimates were geologically meaningful.
Textural evidence (such as garnet-rimming symplectites and pyroxenes rich in plagioclase inclusions and exsolution lamellae) strongly indicates that the amphibolites were once eclogites themselves. Other workers show that the eclogites formed in T-P conditions of ~550 - 750°C 15 – 35 kbars. Remnant eclogite retrograde textures in Blåhø Nappe samples apparently record intermediate conditions of ~550 - 800 °C, 7 - 10 kbars. as eclogites recrystallized into amphibolites. Amphibolite matrix apparently deformed and partially reequilibrated at T-P conditions of ~475 - 525 °C, 5-8 kbars. These temperatures and pressures of metamorphism could only be determined by comparing nearby samples, because the complexity of the mineral textures and chemistry (e.g. zoning), made it very difficult to choose apparent equilibrium textures without the models.
Van Nostrand, Maria, "Petrology and thermodynamic modeling of amphibolite facies rocks of the Blåhø Nappe of the Middle Allochthon, Scandinavian Caledonides in Norway" (2018). Honors Theses. 1652.