The evolution of the Baie Verte Lineament, Burlington Peninsula, Newfoundland
An area measuring about 5 km by 30 km, of the relatively well-exposed central part of the Baie Verte Lineament, has been mapped in detail. Two major rock divisions are recognised by their differing structural and metamorphic histories. A terrain of psammitic, semipelitic and mafic schists borders the western side of the Lineament. These rocks (part of the Fleur de Lys Supergroup) are in upper greenschist to epidote-amphibolite metamorphic facies, and are affected by a polyphase deformation sequence. They are intruded by post-kinematic granite and tonalite. A large, generally unfoliated, granodiorite (Burlington Granodiorite) borders the eastern side of the Lineament. Outside the map area, it is seen to be either a pre- or early syn-kinematic intrusion into Fleur de Lys schists. The Baie Verte Lineament consists, in the mapped area, of two adjacent narrow-parallel belts; to the west the Baie Verte Group of uncertain (?Arenigian) age, and to the east the early Devonian Mic Mac Lake Group. Large lensoid "alpine-type" (ophiolitic) ultramafic bodies are situated along the tectonic contact between the Fleur de Lys schists and the Baie Verte Group. A narrow discontinuous strip containing bodies of ophiolitic gabbro with parallel diabase dykes is found at the western side of the Baie Verte Group adjoining the tectonic contact. The internal structures and textures of the ophiolitic rocks are described. The Baie Verte Group consists mainly of mafic pillow lava and mafic volcaniclastic sediments, subvertical to moderately west-dipping, and facing east, for which a stratigraphy is defined. At or near the base, a conglomerate, resting on ophiolite gabbro megabreccia, contains clasts mainly referable to the mafic parts of an ophiolite complex, with some clasts identical to the Burlington Granodiorite, and rare clasts referable to previously deformed silicic volcanics and related sediments of the Fleur de Lys Supergroup found to the east of the map area. Redeposited carbonate rocks are very occasionally found near this horizon. The Mic Mac Lake Group rests with spectacular unconformity, including a significant amount of palaeotopographic relief, on the Burlington Granodiorite. It also contains a similar erosion surface within the subaerial sequence of silicic volcanics, mafic and trachyte lava flows, and mostly conglomeratic sediments. A stratigraphy is defined for the Mic Mac Lake Group. The sequence dips west at moderate angles and, with the significant exception of a narrow strip at the western side, faces west. The Baie Verte and Mic Mac Lake Groups share a single steep cleavage and low greenschist facies metamorphic grade. An autochthonous contact between Baie Verte Group and east-facing Mic Mac Lake Group in the southern part of the area indicates that the Mic Mac Lake Group is in a highly-disrupted syncline, and is unconformable on the Baie Verte Group. To the north an attenuated Mic Mac Lake Group is overthrust by the Baie Verte Group, and must eventually be cut out to the north of the map area where the Baie Verte Group directly overthrusts Burlington Granodiorite. It is shown that the deformation of the Baie Verte and Mic Mac Lake Groups, and the tectonic emplacement of the ophiolitic ultramafic bodies into their present position, was wholly later than all the regional polyphase deformation and metamorphism of the Fleur de Lys Supergroup, and that this later deformation is almost wholly localised in the belt of less complexely deformed rock. Joint mapping with J.M. Bird and J.F. Dewey at the northeast end of the Baie Verte Lineament demonstrated the presence of all members of a full ophiolite suite, overlain by mafic volcaniclastic sediments and a thick pile of pillow lava. The internal igneous-relationships of the ophiolite complex are described. These rocks are disposed in three thrust sheets (two being inverted), which are overthrust eastward above previously deformed and metamorphosed Fleur de Lys schists. One model has been proposed that coherently interprets the Palaeozoic evolution of the central and western Newfoundland Appalachians in terms of present-day continental margin / island arc sedimentation, magmatism, and tectonics. The results of this mapping are interpreted within the framework of this model. The Fleur de Lys schists and Burlington Granodiorite represent rifted continental margin sediments, and a volcanic arc built on and intruding them prior to polyphase deformation and metamorphism. The Baie Verte Group is interpreted as the remains of the oceanic crust and mantle floor and mafic volcanic fill of one of several small marginal (inter-arc) basins that developed in the northwestern Newfoundland area in lower Ordovician times. It is interpreted to be in the place where the basin formed relative to the eastern and western Fleur de Lys blocks that border it. The Mic Mac Lake Group is a proximal section to an early Devonian calc-alkaline cordilleran caldera complex which may have been related either to subduction or to continental collision processes. It was laid down over an eroded surface of Burlington Granodiorite and over little to undeformed mafic rocks filling the Baie Verte marginal basin. Both were then deformed during the Acadian continental collision orogeny by contraction of the Baie Verte basin, with medium to high angle eastward overthrusting.
Notes for the reader. Thin sections and corresponding rock samples used for this study are deposited in the Harker collection of the Department of Mineralogy and Petrology, University of Cambridge. (Catalogue no. 33, p. 251-253, 255- 283; serial numbers 110403-110456, and 110476-110990). The magnification quoted in captions to photomicrographs is the true scale of the print to the thin section. The prints are about 85 X 57 mm., and their longest side therefore represents about 8.5, 3.4, 2.1 and 0.85 mm. for magnifications of X 10, X 25, X 40 and X 100 respectively. The camera lens cap seen in photographs of outcrop surfaces is 45 mm across.