Petrological constraints and PT path of Devonian collapse tectonics within the Scandian mountain belt (Western Gneiss Region, Norway)

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Abstract
In Norway, post-Caledonian extension controlled the formation and sedimentation of Middle Devonian sedimentary basins. These ‘collapse basins’ were deposited along a low-angle extensional detachment fault on top of Caledonian high-grade metamorphic rocks of the Western Gneiss Region. This paper concerns the petrological analysis of eclogite and high-pressure micaschist collected within the extensional shear zone. Study of gneisses and micaschists surrounding these high-pressure rocks allows description of the complete PT path which characterized this area. Application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high-pressure conditions (ll-15kbar at 500-600°C) was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism. The shape of the PT path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift. We can observe an increase in temperature during the first stages of decompression smaller than predicted by the models. Considering the evidence for extensional tectonic in the study area, we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late-orogenic heating of the Caledonian thickened crust.

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