Abstract

The east-northeast-trending Paleoproterozoic Nagssugtoqidian Orogen of West Greenland records the response of deep crust to collision of the North Atlantic craton and a lesser known cratonic mass to the north. This paper presents conventional U–Pb ages of documented magmatic and thermotectonic events within this orogen, thus providing a precise time frame for its development and offering a test of previous spatial and temporal correlations with segments of the Trans-Hudson Orogen of northern Labrador and Quebec. Convergence of the two cratons culminated in a collision that commenced after forumla Ma, the crystallization age of the youngest known pre-Nagssugtoqidian, subduction-related intrusion. Earliest collisional deformation was thrust dominated (west-northwest vergent) and caused thickening and consequent heating to peak temperatures by ca. 1850 Ma. Subsequent north–south shortening at elevated temperatures was accommodated by a fold-dominated style of deformation dated at 1825 ± 1 Ma. Between 1821 and 1778 Ma, temperatures remained sufficiently high to generate pegmatites, metamorphic zircon, and titanite, but no major, penetrative structures are known to have formed in this interval. Further shortening between 1779 ± 6 and 1774 ± 6 Ma exploited preexisting fabrics in steep F3 fold limbs to form discrete, east-northeast – west-southwest-oriented, sinistral steep belts that are not important crustal boundaries. One of these late, steep belts is cut by 1772–1761 Ma pegmatites, indicating that deformation was waning by this time. Slow cooling followed the late shearing, with rutile closing in the central Nagssugtoqidian Orogen as late as 1676 ± 10 Ma. The timing of tectonic events in the Nagssugtoqidian Orogen is indistinguishable from that of the Quebec–Baffin and Torngat segments of the Trans-Hudson Orogen of northeastern Laurentia. Accepting an intercratonic setting for the Nagssugtoqidian Orogen, this new data require that the Quebec–Baffin, Torngat, and Nagssugtoqidian orogens meet in a triple junction offshore. This geometry implies a genetic link between the Rinkian and Nagssugtoqidian belts of West Greenland, thereby defining a middle-to deep-crustal collisional belt more than 900 km wide.

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