LARGE-SCALE FLUID INFILTRATION, METASOMATISM AND RE-EQUILIBRATION OF ARCHEAN BASEMENT GRANULITES DURING PALAEOPROTEROZOIC THRUST BELT CONSTRUCTION, UNGAVA-OROGEN, CANADA

The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importa nce of structural and geohydrological controls on a retrograde amphibo lite-granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively expo sed tonalitic to dioritic metaplutonic gneisses. The older suite compr ises c. 2.7-Ga granulite facies assemblages ne-clinopyroxene-hornblend eplagioclase-ilmenite+/- biotite+/-quartz) that record moderate pressu res (less than or equal to 5 kbar) and high temperatures (greater than or equal to 800 degrees C). A younger, c.1.8-Ga suite resulted from a mphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the b asal decollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basemen t indicate that extensive hydration and re-equilibration of the Archae an granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characteriz ed by the growth of garnet-hornblende-quartz+/-cummingtonite coronas b etween plagioclase and orthopyroxene-clinopyroxene, as well as titanit e coronas on ilmenite. Multi-equilibrium thermobarometry on the coroni tic assemblages documents re-equilibration of the granulitic gneiss to 7.7 kbar at 644 degrees C in the south and 9.8 kbar at 700 degrees C in the north. The variably deformed, amphibolite facies domain sandwic hed between the coronitic garnet zone and the basal decollement is mar ked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium how of an aqueous-chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydr ating elastic rocks in the overlying thrust belt, with fluid flow prob ably focused along the basal decollement.