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

A volcanosedimentary succession of the Mlynowiec-Stronie Group (MSG) in the Orlica-SnieZnik 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 Gniewoszow 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 degrees C/3-4 kbar, (2) 480-500 degrees C/10.5 kbar, (3) 500-530 degrees 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 degrees 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.