THE SAXONIAN GRANULITES - A METAMORPHIC CORE COMPLEX

The brachyanticlinal structure of the Saxonian Granulite Complex was f ormed, or at least overprinted, by large-scale crustal extension, whic h is documented in an extreme reduction of the metamorphic profile and a pressure-temperature path with nearly isothermal decompression. The granulite is situated within a south-west - northeast trending belt c haracterized by basaltic volcanism and an intrabasinal, sediment-produ cing rise, both active in Frasnian times. These features are taken to reflect at least part of the extension which has exhumed the high grad e rocks; however, Cambro-Ordovician rifting possibly initiated the pro cess. The Frasnian palaeogeography and geophysical data suggest that t he axis of the granulite dome extends at least 150 km towards the sout h-west. The overall setting strongly resembles the metamorphic core co mplexes of the western USA, but the exhumation of rocks preserving evi dence of metamorphic pressures of 10 - 12 kbar requires much higher ex tensional strain, possibly due to the longer period of stretching. Ear ly Carboniferous compression and minor post-collisional extension (Lat e Carboniferous to Permian) have not significantly modified the earlie r geometry. U - Pb and Sm - Nd data ranging between 380 and 340 Ma are interpreted to record retrogression during uplift rather than peak me tamorphism.