Progressive structural reworking of the Uivak gneisses, Jerusalem Harbour, northern Labrador

The region around Jerusalem Harbour in northern Labrador is composed of a suite of metamorphic rocks of various ages and origins. The Uivak gneisses, containing Saglek dyke and pre-Uivak inclusions, are the deformed and metamorphosed equivalents of an igneous suite emplaced into the crust over 3.6 b.y. ago. Tectonically intercalated with the Uivak gneisses are belts of metasedimentary and basic gneisses known as the Upernavik supracrustals. -- Both the Uivak gneisses and Upernavik supracrustals have been variably migmatized during a period of crustal instability manifested by metamorphism and deformation 3.1 b.y. ago. This 3.1 b.y. event was marked by the generation of a new series of gneisses, herein termed the Iterungnek gneisses. These younger tectonites are a composite, heterogeneous, suite derived both from the remobilization of the earlier Uivak gneisses through a mechanism of structural and metamorphic reconstitution and in situ anatexis, and synkinematic granitic intrusions. In areas where structural reworking has been dominant, the Uivak gneisses can be shown to have undergone progressive transposition until the earlier layering has been completely reoriented and gradually obliterated at the expense of the imposed (Iterungnek) foliation. Where anatexis has been the dominant mechanism, the gneisses show a gradation from lit-par-lit migmatites with Uivak characteristics into Iterungnek nebulites having a granular aspect. The rejuvenation of the Uivak gneisses was synchronous with the intrusion of weakly foliated granitic bodies which gave rise to megascopic agmatite zones where they were emplaced into or adjacent to the Upernavik supracrustals. -- Detailed petrological studies of all the major rock units in the area has shown that the reworking of the Uivak gneisses occurred under granulite facies conditions, which were superimposed on rocks with upper amphibolite facies assemblages. Mineral reactions observed in thin section indicate P-T conditions for the granulite metamorphism to be in the range of 10 kb and 825-850°C, implying a crustal thickness 3.1 b.y. ago on the order of 30 km. -- Regional folds produced during the Iterungnek event are dominantly isoclinal about moderately plunging axes and nearly vertical axial planes. Mesoscopic structures indicate that shearing along the axial planes of tight folds of Uivak gneisses was an active mechanism in the production of the Iterungnek banding. The regional distribution of rock-types is suggestive of refoliation of the Uivak gneisses in synfomnal zones which have been "pinched - in" between more stable border massifs in which little reworking of the older gneiss is evident. It is postulated that this pattern resulted from "collapse" of the crust 3.1 b.y. ago along narrow parallel shear zones due to a softening effect from the build-up of radiogenic heat. -- Even though initial Sr ratios from reworked gneisses in Labrador agree with present theories on Sr systematics in crustal rocks, such is not the case in Greenland. It is suggested that more research is needed into such areas of reconstituted early Archean sialic crust like that in Labrador in order to reconcile the inconsistencies between the observations of the field geologists on one hand and the isotope geologists on the other.