Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

• Published in: Journal of Metamorphic Geology
• Main Authors: BURGESS, J. L., BROWN, M., DALLMEYER, R. D., STAAL, C. R. VAN
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1995
• Subjects: Canada; Newfoundland
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1995.tb00257.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1995.tb00257.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1995.tb00257.x

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW‐directed reverse shearing. The NE‐trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright‐to‐steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3). Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast‐matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K‐feldspar, and the development of an increasing volume of anatectic migmatite in south‐eastern sectors of the area record syn‐ to late‐D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P‐T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east. Titanite from an ...