EXTENSIONAL VERSUS COMPRESSIONAL SETTINGS FOR METAMORPHISM - GARNET CHRONOMETRY AND PRESSURE-TEMPERATURE-TIME HISTORIES IN THE MOINE SUPERGROUP, NORTHWEST SCOTLAND

Identification of the tectonic setting for metamorphism is often extre mely difficult in complex polymetamorphic terranes where individual te ctonothermal events are obscured by later thermal and structural reorg anizations. The traditional approach is to use mineral parageneses to outline the nature of the pressure-temperature-time path, but assignin g an age to that path remains a challenge. In this case study, pressur e-temperature data show that garnet in pelites of the polymetamorphic Moine Supergroup of northwest Scotland grew during compressional tecto nics. In addition, the Sm-Nd systematics of these garnets demonstrate that growth occurred in the interval 820-790 Ma and was thus coeval wi th crustal melting documented as ca, 800 Ma. The heat source for the l atter event has previously been postulated to be related to extension, but here we provide the first substantive evidence for a collisional orogeny at this time. These data further demonstrate the utility of ga rnet chronometry in identifying the timing and nature of particular te ctonothermal events in polymetamorphic settings. In addition, the data show that the period between Grenville collision and supercontinent a ssembly in the North Atlantic region ca, 1000-1100 Ma, and rifting and continental breakup ca, 750 Ma was not as quiescent as previously env isaged. Late Proterozoic orogenesis is likely to have resulted from th e closure of aborted continental rifts and/or minor oceanic tracts wit hin the Grenville supercontinent prior to final rifting and superconti nent dispersal.