Combined phase diagram modelling and quartz-in-garnet barometry of HP metapelites from the Kamieniec Metamorphic Belt (NE Bohemian Massif)

Abstract

The Kamieniec Metamorphic Belt belongs to the Variscan Belt of Europe as the NE part of the Bohemian Massif. It comprises a supracrustal succession dominated by micaschists that has not been yet recognized as a high-P, low-T (HP–LT) metamorphic unit. Our work demonstrates the significance of metapelites in the study of HP metamorphism of the NE part of the Bohemian Massif. To reconstruct the P–T history of the Kamieniec Metamorphic Belt, we have investigated three samples using independent geothermobarometric techniques including phase diagram modelling, Si4+ content in white mica and quartz-in-garnet elastic barometry. Two samples contain mineral assemblages bearing a record of HP metamorphism followed by an LP event. The oldest assemblage is mostly preserved in the first generation of garnet and it comprises phengitic white mica and rutile. In one of the investigated samples, we also recognized chloritoid and inferred pseudomorphs after lawsonite composed of quartz, clinozoisite associated with margarite. The third of the investigated samples is strongly retrogressed and only contains the relics of phengitic white mica. All three samples contain a younger mineral assemblage comprising white mica with low Si4+ content and ilmenite. Mineral equilibria modelling indicates the P–T conditions of the HP event culminated at ~15.5–18 kbar and ~470–570℃, while the LP episode occurred at ~5–7 kbar and ~530–570℃. The Raman shift measured in quartz inclusions in garnet in samples with a well-preserved record of the HP stage points to their entrapment at pressures between 11 and 16 kbar. The quartz inclusions within the strongly retrogressed micaschist sample exhibit Raman shifts corresponding to the LP episode at ~5–8 kbar. Discrepant results obtained for one of the samples are discussed in detail. Our investigations show that the supracrustal succession of the Kamieniec Metamorphic Belt contains a record of HP–LT metamorphism typical for subduction systems. A recently established tectonic model for the crystalline complexes exposed in the Bohemian Massif suggests that they were formed via the collision of the Saxothuringian, Teplá–Barrandian, and Brunovistulian domains. Therefore, we interpret the Kamieniec Metamorphic Belt as representing fragments of the Saxothuringian crust that experienced cold extrusion from below the Teplá–Barrandian domain in front of the rigid Brunovistulian indenter.