The Structure and P–T–t–d Evolution of the Saxothuringian/Brunovistulian Variscan Boundary Zone in the Bohemian Massif (Czech Republic, Poland)

The Variscan suture zone between the Saxothuringian and Brunovistulian terranes of the Bohemian Massif in Central Europe is characterized by a highly tectonized zone: the Stáre Město Belt (SMB). Despite extensive previous research, better understanding of some essential aspects regarding the metamorphic conditions and timing of the tectonic events within the SMB are needed to understand the structural architecture and the Variscan evolution of the Saxothuringian/Brunovistulian suture zone. The present study present systematic data from thermodynamic modelling and U–Pb LA-ICPMS dating of monazite, titanite and zircon on mica schists, felsic and mafic metavolcanic rocks, and leucocratic veins exposed in the SMB. The results indicate that progressive metamorphism related to the collision of the Saxothuringian margin and the rigid Brunovistulian crust started at c. 370–366 Ma (D1 stage). The mostly sedimentary outer units and the mostly volcanic middle unit of the SMB were buried to middle and lower crustal depths, respectively. The continued collision resulted in further burial of the outer units from 4–6 kbar at 510°C–550°C to c. 6.5–7.5 kbar at 650°C with coeval extrusion of the middle unit from 10.5 at 670°C to 6 kbar at 680°C. As a result of the juxtaposition of the units with slightly different prograde P–T histories, portions of the lower crust that were extruded in the middle of the SMB and portions of the middle crust forming the outer parts of the SMB, were transported to the same crustal level (6–7 kbar) at 640°C–680°C (D2 stage), together with formation of anatectic leucosomes at c. 342 Ma. The structure of the SMB was modified by D3 stage dextral transpression during which the former S2 planes were reactivated to form deep-seated, ductile strike-slip faults, with the coeval intrusion of tonalitic magmas along the SMB axis. The predominantly c. 340–330 Ma monazite U–Pb ages obtained from the metamorphic rocks of the SMB are interpreted as the time of the subsequent cooling due to exhumation. This study emphasizes that the long-lasting metamorphic history of the border zone between Saxothuringian and Brunovistulian terranes started at c. 370 Ma and continued to c. 330 Ma.