The Labrador “trough” is the preserved portion of an Early Proterozoic (Aphebian) geosyncline, extending from the Grenville Front at Wabush to Payne Bay, over 600 mi farther north-northwest. Archaean gneisses constitute the foreland to the west of the geosyncline, and remetamorphosed Archaean gneisses compose much of its eastern hinterland. It is the purpose of this paper to outline and correlate the essential geological features of the Labrador trough and to propose a working hypothesis on its origin.

The basin filling represents two, or possibly three, tectonic cycles, each initiated by an orthoquartzite-limestone sequence (including iron formation in the second and third cycles), and culminating in deposition of shales and of flysch-type deposits in the external zone of the geosyncline, and in intense volcanic activity in the central and internal zones.

The paleogeography of the first cycle is briefly examined. A rapidly uplifted area situated east of the geosyncline is the probable source of much clastic material. The basin center migrated westward during the preophiolitic stage, and a geanticline emerged during the early phase of the ophiolitic stage in the center of the geosyncline.

Very large volumes of mafic volcanic and plutonic rocks erupted in the eastern and central parts of the geosyncline during the latter part of the tectonic cycles. The eugeosynclinal mafic rocks are mainly K-deficient tholeiites. Spilites and transitional types of basalts occur in subordinate volumes. Serpentinized ultra-basic sills are present.

The Aphebian rocks of the Labrador trough were folded during the Hudsonian Orogeny. A tectonic section across the geosyncline is discussed. The basement below the western part of the geosyncline has not been involved in the Hudsonian deformation, whereas the Archaean underlying the central and eastern portions of the geosyncline was folded jointly with its cover. The relations between structural style, depth of burial and metamorphic grade are briefly examined.

A late-kinematic and post-kinematic metamorphism of an intermediate pressure series overprinted the internal zones of the geosyncline. Hudsonian granites and migmatites are absent, and possible causes of their absence are discussed.

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