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Abstract

The Precambrian rocks on either side of Davis Strait show a similar pattern of events and are interpreted as having formed part of a single shield. Nine major stages in the development of this shield are suggested: (1) formation of an extensive early crust before 3,000 m.y. ago, relicts of which are now preserved as migmatites and high-grade gneisses in the Archean block of eastern Labrador and southwest Greenland; (2) de-position of greenstone belts 2,700-3,000 m.y. ago; (3) plutonic activity in the period 2,500-2,900 m.y. ago, affecting both the greenstone belts and the major parts of the basement on which the greenstones lie; (4) in-trusion of numerous basic dike swarms in the general period 2,000-2,600 m.y. ago; (5) deposition of early Proterozoic (Aphebian) géosynclinal rocks on the consolidated Archean basement; (6) alteration of these rocks by orogenesis which occurred approximately 1,650-1,850 m.y. ago (the Hudsonian orogeny in Canada and the Ketilidian and Nagssugtoqidian orogenies in Greenland); (7) post-orogenic magmatism-particularly marked in areas affected by Hudsonian metamorphism-which extended from South Greenland through Labrador (This magmatism produced chiefly anorthosites, adamellitic granites, monzonites, and norites, which probably were emplaced between 1,400 and 1,700 m.y. ago, although the areas in which they occur commonly remained thermally active to 1,200 m.y. ago or later.); (8) graben faulting, deposition of molasse sediments, and widespread intrusion of basic dikes, in South Greenland and in parts of Baffin Island, accompanying and following emplacement of the post-orogenic rocks (Most of these dikes are tholeiitic; however, alkalic and peralkalic intrusions took place locally 1,100-1,300 m.y. ago.); (9) metamorphism and tectonic alteration of the Archean and Proterozoic rocks in the southern part of the Canadian shield by the Grenville orogeny about 900-1,100 m.y. ago. The only effect of the Grenville orogeny in South Greenland was a weak updating of older rocks in areas close to major faults, so that they yield K-Ar ages of about 900-1,000 m.y.

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