The generation of voluminous S-type granites in the Moldanubian unit, Bohemian Massif, by rapid isothermal exhumation of the metapelitic middle crust

This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, and geochronological data to examine the link between LP–HT metamorphism and S-type granite formation in the Moldanubian unit, Bohemian Massif. We first describe the intrusive relationships of an S-type granite to its host cordierite-bearing migmatites, superbly exposed in the Rácov locality, northeastern Moldanubian batholith. The knife-sharp contacts and rectangular stoped blocks establish that the migmatites cooled and were exhumed above the brittle–ductile transition prior to the granite emplacement. The U–Pb monazite geochronology combined with P–T estimations constrain the age and depth of migmatization at ~ 329 Ma and ~ 21 km (T ≈ 730 °C). The migmatite complex was then exhumed at a rate of 6–7 mm y−1 to a depth of < 9 km where it was intruded by the granite at ~ 327 Ma. These data indicate that the hot fertile metapelitic middle crust in this part of the Moldanubian unit, newly defined as the Pelhřimov complex, underwent rapid isothermal decompression at ~ 329–327 Ma, giving rise to biotite melting and generation of large volumes of S-type granite magma. We propose that the rapid ~ 329–327 Ma exhumation of the Pelhřimov complex may have been partly assisted by the crustal-scale Přibyslav mylonite zone, which delineates the underlying western edge of the Brunia microplate underthrust beneath the eastern half of the Moldanubian unit during the early Carboniferous. The front edge of Brunia thus acted as a rigid backstop at depth, localizing the exhumation of the Pelhřimov complex and separating the hot fertile middle crust to the west from the already cooled overthrust complexes to the east. The magnetic fabric of the granite around the migmatite blocks further reveals that the Pelhřimov complex was shortened vertically and extended in the ~ WNW–ESE direction during and after its exhumation, implying that the SSE-directed underthrusting of Brunia along the eastern margin of the Bohemian Massif was replaced by vertical thinning and ~ WNW–ESE stretching of the Moldanubian crust. As a general conclusion, we suggest that even for extremely rapid crustal exhumation, S-type granite magmas can be formed at greater depths by isothermal decompression of the metapelitic host, and then ascend almost instantaneously to the already exhumed (and cooled) shallow parts of the same metamorphic core complex. This model may explain the short time spans for the extensive migmatization and associated S-type granite formation, crustal exhumation, and granite emplacement, as well as the presence of “cold”, discordant granite–migmatite contacts in once “hot” migmatite terrains.