MICROSTRUCTURE, METAMORPHISM, THERMOCHRONOLOGY AND P-T-T-DEFORMATION HISTORY OF THE PORT AUX BASQUES GNEISSES, SOUTH-WEST NEWFOUNDLAND, CANADA

The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Ba sques Complex; and the Harbor le Cou Group. A similar regionally devel oped polyphase history of penetrative deformation characterizes each o f these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent fol ds (F2), and transposed during development of a locally penetrative, d ifferentiated crenulation cleavage (S2). In western sectors of the are a, D2 was associated with NW-directed reverse shearing. The NE-trendin g structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upri ght-to-steeply inclined, periclinal folds (F3) and a variably penetrat ive schistosity (S3). Amphibolite facies metamorphism increases in gra de from west to east across the area. Microstructures, including porph yroblast-matrix foliation relations and internal textural unconformiti es in garnet, indicate episodic porphyroblast nucleation and growth, w hich reflect a prograde traverse sequentially across univariant reacti ons during syntectonic metamorphism. Garnet, kyanite and staurolite po rphyroblasts are wrapped by the S2 foliation, but each may contain tra ils of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequen tial reaction out of kyanite, staurolite and muscovite in favour of si llimanite, garnet, biotite and K-feldspar, and the development of an i ncreasing volume of anatectic migmatite in south-eastern sectors of th e area record syn- to late-D2 peak metamorphic conditions. Microstruct ural relationships and petrogenetic grid considerations indicate clock wise trajectories in P-T space for units of the Port aux Basques gneis ses. Peak metamorphic conditions are estimated to have been 620-650 de grees C at greater than or equal to 8kbar in the west and 700-750 degr ees C at less than or equal to 8 kbar in the east. Titanite from an up per amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in the se gneisses. Variable D3 strain was associated with some recrystalliza tion of hornblende and micas. Ar-40/Ar-39 hornblende plateau isotope c orrelation ages range from c. 419 to c. 393 Ma, from east to west acro ss the area, and are interpreted to record cooling through c. 500 degr ees C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential up lift from south-east to north-west associated with displacement on maj or shear zones. Ar-40/Ar-39 muscovite plateau ages cluster at c. 390Ma , and date cooling through c. 375 degrees C during regional exhumation . Cooling rates are moderate to fast and may indicate a component of t ectonic exhumation. The Port aux Basques gneisses are a product of Sil urian collisional tectonics. The higher grade of metamorphism in compa rison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian ma rgin) during closure of the Iapetus Ocean.