Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

Abstract

AbstractAt the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west‐oriented compression and vertical extrusion of the high‐grade rocks into higher crustal levels. The high‐pressure mineral assemblage Grt‐Ky‐Kfs‐Pl‐Qtz‐Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high‐grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite‐bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high‐grade rocks with a cold upper‐crustal lid.