Basement-cover relationships in southwest Newfoundland

Brown, Peter A. (1974) Basement-cover relationships in southwest Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In the Port aux Basques area five geological divisions are recognised. These are Cape Ray Complex, The Port aux Basques Complex, The Windsor Point Group, The Harbour Le Cou Group. The first two are separated by, and the third overlies a 1 km wide mylonite zone the Cape Ray Fault. The Harbour Le Cou and Bay du Nord Groups occur in the eastern part of the area and are deformed during the reworking of the Port aux Basques Complex. -- The Cape Ray Complex occurs to the west of the Cape Ray Fault Zone and comprises a chaotic, intensely retrogressed leucocratic gneiss intruded by granitic phases. The Windsor Point Group consists of a series of metasedimentary and metavolcanic rocks which unconformably overlie the Cape Ray Complex and the Cape Ray Fault Zone, and have been mildly deformed by late movements along the fault. -- The Port aux Basques Complex crops out to the east of the Cape Ray Fault Zone and comprises a well banded gneiss complex intruded by granitic phases. Between the fault and Isle aux Morts at least three periods of penetrative deformation are recognized. The earlier two phases are, at least in part, responsible for the development of the gneissic banding. The later phase subisoclinally folds this banding. The highest grade of metamorphism is associated with the second event and resulted in the development of garnet, staurolite, kyanite, sillimanite, and potassium feldspar. The Port aux Basques granite intruded the gneiss in post D1 - pre D2 times. -- East of Isle aux Morts the gneisses are reworked i.e., further deformed and metamorphosed. These deformations, three are recognized, overprint the gneissic fabrics and result in the development, from west to east, of shear zones, recumbent folds, and tectonic slides. During the earliest event, which is the most penetrative, the gneisses were reconstituted to a finely schistose rock such that no lithological boundary was apparent between the gneisses and a sequence of pelitic to semi-pelitic rocks, the Harbour Le Cou Group, infolded, by this event, into the gneisses. A structural and metamorphic convergence towards the basement-cover contact zone resulted in the parallel-alignment of lithological boundaries, schistosities, and intrusive rocks within it, and an apparent gradational metamorphic contact across it. Tectonic slides define the contact. -- Another sequence of predominately pelitic rocks, the Bay du Nord Group, was affected by the reworking deformations. This Group is, by correlation, lower to Middle Devonian in age, indicating that the reworking deformations are, at the oldest, an Acadian event. -- The Cape Ray Fault is a 1 km. wide zone of intense deformation which separates The Cape Ray Complex from the Port aux Basques complex. The Cape Ray Complex is correlated with the Long Range (Grenvillian) Complex of western Newfoundland and is interpreted to have formed part of the western margin of the Proto Atlantic Ocean. The Port aux Basques Complex is included in the Eastern Crystalline Belt and is interpreted to have formed part of the eastern margin of the Proto Atlantic Ocean. The Cape Ray Fault is therefore interpreted as a cryptic suture along which complete closure of the Proto Atlantic Ocean took place. -- No correlation is possible between the formation of the Cape Ray Fault and the reworking of the Port aux Basques Complex and infolding of the Harbour Le Cou Group.

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