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Indications of HP events in the volcanosedimentary succession of the Orlica–Śnieżnik Dome, NE Bohemian Massif: data from a marble-amphibolite interface

Twyrdy M., Żelaźniewicz A.

Geological Quarterly

2017

Vol. 61, No. 2

435--449

A volcanosedimentary succession of the Młynowiec-Stronie Group (MSG) in the Orlica–Śnieżnik Dome (OSD), the Sudetes, NE Bohemian Massif underwent multiple folding and shearing during the Variscan Orogeny. In the sheared domains, there are less deformed pods in which rocks preserve better records of metamorphic events prior to the regional temperature peak. In one such pod, near Gniewoszów on the western limb of the dome, marbles enclosed by massive amphibolites occur. In these rocks, zoned plagioclase with actinolite and epidote inclusions and zoned amphibole grains allowed recognition of three mineral assemblages and three P-T stages at: (1) 310°C/3–4 kbar, (2) 480–500°C/10.5 kbar, (3) 500–530°C/6–6.5 kbar, based on isopleth intersections and checked against conventional thermobarometry. These define a steep clockwise P-T path and a geothermal gradient of 17°C/km before peak conditions were attained, which suggests subduction of the metavolcano-sedimentary rocks (Stronie Formation of the MSG) on the western limb of the OSD, with a transient yet discrete higher pressure episode. Mineral relicts capable of demonstrating a higher pressure event are scarce in the supracrustal rocks of the dome, mainly because they became more thoroughly equilibrated and obliterated during the temperature peak at mid-amphibolite facies conditions and the subsequent ubiquitous greenschist facies overprint.