Eo-Variscan metamorphism in the Bohemian Massif: Thermodynamic modelling and monazite geochronology of gneisses and granulites of the Gory Sowie Massif, SW Poland

In the Variscides of central Europe, the tectonothermal history of a few units jointly assigned to the Tepla-Barrandian/Bohemian microplate had already terminated in the Devonian. One of these is the Gory Sowie Massif (GSM; Sudetes, SW Poland), mainly consisting of variably migmatitic gneisses with subordinate granulite and peridotite inserts. The eo-Variscan evolution of the GSM was completed in five tectonothermal stages; however, the timing and pressure-temperature (P-T) conditions of the individual stages remain unclear because isotopic ages overlap within error, and previous P-T estimates have been determined without reference to these ages. Our thermodynamic modelling and LA-ICP-MS U-Pb and electron probe microanalysis (EPMA) U-Th-total Pb monazite geochronology provide new insights into the early Variscan multistage evolution of the GSM. The new P-T-time (t) data show that granulites were metamorphosed concurrently with gneisses but at different depths and temperatures. During subduction of Saxothuringia beneath Tepla-Barrandia at 395-390 Ma, the granulites of crustal provenance experienced high-P-high-T conditions (740-750 degrees C, 17-18 kbar) and came in contact with the overlying mantle. Slab break-off gave rise to decompression from conditions of similar to 17 kbar/840 degrees C to 11-12 kbar/850-860 degrees C, which continued at 380-370 Ma when the granulites were sheared off and extruded as tectonic slices to lower-middle crustal depths, where gneisses were undergoing metamorphism and migmatization under amphibolite facies conditions. In the gneisses, the oldest monazite date of c. 390 Ma constrains the age of the early tectonometamorphic event D-1. Two subsequent events (D-2 and D-3, sillimanite grade) occurred under conditions of 660-680 degrees C and 5-9 kbar at 380-375 Ma, when, in various parts of the future massif, the gneisses were (re)folded and migmatized due to heat arising from underplated mantle. Isostatic exhumation was accompanied by pegmatite intrusions (D-4) at 370 Ma and subsequent orogenic collapse (D-5). Concurrently, high temperature conditions (similar to 700 degrees C) promoted a disturbance of the U-Pb system in monazite in some granulites.