Variations in the transient prograde geothermal gradient from chloritoid-staurolite equilibria: a case study from the Barrovian and Buchan-type domains in the Bohemian Massif

Thermodynamic modelling of metamorphic rocks increases the possibilities of deciphering prograde paths that provide important insights into early orogenic evolution. It is shown that the chloritoid-staurolite transition is not only an indicator of temperature on prograde P-T paths, but also a useful indicator of pressure. The approach is applied to the Moravo-Silesian eastern external belt of the Bohemian Massif, where metamorphic zones range from biotite to staurolite-sillimanite. In the staurolite zone, inclusions of chloritoid occur in garnet cores, while staurolite is included at garnet rims and is widespread in the matrix. Chloritoid X-Fe = 0.91 indicates transition to staurolite at 5 kbar and 550 degrees C and consequently, an early transient prograde geothermal gradient of 29 degrees C km(-1). The overall elevated thermal evolution is then reflected in the prograde transition of staurolite to sillimanite and in the achievement of peak temperature of 660 degrees C at a relatively low pressure of 6.5 kbar. To the south and to the west of the studied area, high-grade metamorphic zones record a prograde path evolution from staurolite to kyanite and development of sillimanite on decompression. Transition of chloritoid to staurolite was reported in two places, with chloritoid X-Fe = 0.75-0.80, occurring at 8-10 kbar and 560-580 degrees C, and indicating a transient prograde geothermal gradient of 16-18 degrees C km(-1). These data show variable barric evolutions along strike and across the Moravo-Silesian domain. Elevated prograde geothermal gradient coincides with areas of Devonian sedimentation and volcanism, and syn- to late Carboniferous intrusions. Therefore, we interpret it as a result of heat inherited from Devonian rifting, further fuelled by syntectonic Carboniferous intrusions.