Age and tectonic implications of Paleoproterozoic granitoid intrusions within the Nain Province near Nain, Labrador

摘要

U-Pb (zircon) age determinations from threegranitoid intrusions in the Nain region, formerly assigned to thewell-known 1.35-1.29 Ga (Mesoproterozoic) Nain Plutonic Suite(NPS), indicate that they were emplaced, instead, during thePaleoproterozoic. Crystallization ages of 2109 +-3 Ma for SheetHill granite, 2052 +- 4 Ma for Loon Island granite, and 2025 +- 7Ma for Satok Island monzonite demonstrate a decreasing time ofemplacement from north to south. Leucocratic basic rocks,superficially similar to those of the NPS, occur as inclusionswithin the Satok Island monzonite and are intruded by an 1873 +-4 Ma granitic aplite dyke north of Webb's Bay. The ages clearlydemonstrate granitic magmatism in the Nain area for at least 750Ma, and anorthositic magmatism for at least 675 Ma, before thedevelopment of NPS. The earliest Paleoproterozoic magmatism(2109-2025 Ma) may have coincided with breakup of the NorthAtlantic craton, whereas the local magmatism represented by the1873 Ma aplite dyke may be related to the embryonic stages of thecontinental collision that produced the 1860-1740 Ma TorngatOrogen. Leucocratic basic rocks associated with thePaleoproterozoic granitic rocks identified here could be Archeanin age, but are more likely coeval with the granites. Other graniticand basic rocks in the Nain area may likewise be products of thisbimodal plutonism. Identification of this possible geologicalduality suggests repetitive magmatism of "anorogenic type" over asignificant time span in this part of Labrador. If the mantle-plume-related genesis accorded the Mesoproterozoic NPS is applicable tothe Paleoproterozoic rocks, it implies multiple periods of similarlithosphere-asthenosphere interaction in this area, beginning atleast in the Paleoproterozoic. The Paleoproterozoic magmas mayhave been emplaced into Nain Province crust along a linearintracontinental extension zone inboard from a rifted margin.