Pressure-temperature-time evolution of a Variscan garnet-bearing micaschist from the northeastern Fichtelgebirge, NW Bohemian Massif in central Europe

Abstract

The studied micaschist from the northeastern Fichtelgebirge crystalline complex (FC) contains mm-sized garnet with a foam structure. Staurolite formed at the margin of garnet. Relics of staurolite are enclosed in mm-sized andalusite porphyroblasts. Garnet exhibits prograde concentric zonation with the pyrope component increasing from 1 to 6 mol%. Cores of potassic white-mica grains locally contain Si contents up to 3.15 per formula unit (pfu). The majority of this mica is characterized by Si contents close to 3.00 pfu. Pressure-temperature (P-T) pseudosections, calculated for the micaschist, indicate conditions of 10 kbar and 505 °C for an early metamorphic stage which was followed by a pressure decrease to 5 kbar and 535 °C. Late metamorphic P-T conditions recorded by garnet are around 4.5 kbar and 565 °C, compatible with the presence of staurolite. Andalusite might have metastably formed at this stage or somewhat lower pressures. The U-Th-Pb dating of monazite with the electron microprobe yielded subordinate ages younger than 315 Ma which were assigned to the nearby emplacement of granites in the FC. Some ages older than 395 Ma were related to detrital monazite in the sedimentary protolith of the micaschist. An age cluster at 384.5 ± 1.8 (2σ) Ma is preferentially assigned to the early high-pressure (HP) metamorphism resulting from the collision of Laurussia with a Peri-Gondwanan terrane. Exhumation to 15–20 km depths might have ended at 362.3 ± 1.0 Ma, but possibly the HP metamorphism occurred at this age and the 384.5 Ma age must be then referred to the provenance area of detrital monazite. The age cluster at 325.0 ± 0.7 Ma is related to a second collisional event that caused the overriding of the crystalline nappes of the FC onto the Laurussian crust.