TECTONOMETAMORPHIC EVOLUTION OF THE BOHEMIAN MASSIF - EVIDENCE FROM HIGH-PRESSURE METAMORPHIC ROCKS

The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temper ature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distribut ed throughtout the Massif. Initially the high pressure rocks were cons idered to be relicts of a much older orogenic event, but the increasin g data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the t ime of the dominant low pressure event at around 320-330 Ma. Most of t he eclogite and granulite facies rocks are assigned to allochthonous n appes that arrived close to the end of the low pressure event, but bef ore final granite intrusion. The nappes contain a mixture of different units and the relationship between rocks with high pressure relicts a nd host gneisses with no apparent signs of deep burial is still proble matic. Some of the high pressure rocks retain evidence of multiple sta ges of partial re-equilibration during uplift. Moreover, it can be sho wn in certain instances that host gneisses also endured a multistage m etamorphic development but with a peak event convergent with one of th e breakdown stages in the enclosed rocks with high pressure relicts. I t thus appears that the nappe units are composite bodies probably form ed during episodic intracrustal thrusting. Fluids derived from prograd e dehydration reactions in the newly under-thrusting slab are taken to be the catalysts that drove the partial re-equilibrations. On the sca le of the whole Massif it can be seen within the units with high press ure relicts that the temperature at the peak recorded pressure and tha t during the breakdown are variable in different locations. It is inte rpreted that regional metamorphic gradients are preserved for given st ages in the history and thus the present day dismembered nappe relicts are not too far removed from their original spatial distribution in a n original coherent unit. From the temperature information alone it is highly probable that the 'refrigerating' underthrusting slab was situ ated in the north-west. However, this north-west to south-east underth rusting probably represents the major 380-370 Ma event and is no guide to the final thrusting that emplaced the much thinned nappe pile with high pressure relicts. Granite genesis is attributed to the late stag e stacking, during the final Himalayan-type collision stage, of thinne d crust covered by young, water-rich, sediments - erosion products of the earlier orogenic stages. Regional metamorphism at shallow depths a bove the voluminous granites was followed by final nappe emplacement w hich rejuvenated the granite ascent in places.