Development of Precambrian Shield in Southwestern Greenland, Labrador, and Baffin Island: ABSTRACT

The Precambrian rocks on either side of Davis Strait show a similar pattern of events and can be interpreted as having formed part of a single shield. Eight 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 migmatite and high-grade gneiss in the Archean block of eastern Labrador and southwest Greenland. (2) Deposition of greenstone belts between 2,700 and 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 part of the basement on which they lie. (4) Intrusion of numerous basic dike swarms in the general period of 2,000-2,600 m.y. ago. (5) Lower Proterozoic (Aphebian) geosynclinal rocks we e deposited on the consolidated Archean basement and were involved in an orogeny known in Canada as Hudsonian and in Greenland as Ketilidian and Nagssugtoquidian. (6) Post-orogenic magmatism, particularly prominent in areas affected by Hudsonian metamorphism, extends from southern Greenland through Labrador. This produced chiefly anorthosites, adamellitic granites, monzonites, and norites, that were probably 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. (7) Emplacement of the post-orogenic rocks was accompanied and followed, in southern Greenland and in parts of Baffin Island, by graben faulting, deposition of molasse sediments, and widespread intrusion of basic dikes. The majority of these ikes are tholeiitic; however, locally (for example, in the Gardar Province of southern Greenland), alkaline and peralkaline intrusions took place 1,100-1,300 m.y. ago. (8) The Archean and Proterozoic rocks in the southern part of the Canadian shield were metamorphosed and tectonically worked by the Grenville orogeny about 900-1,100 m.y. ago. The only effect of this orogeny in southern Greenland was a slight updating of earlier rocks from areas close to major faults so that they yield K/Ar ages of about 900-1,000 m.y. End_of_Article - Last_Page 2472------------