Abstract

The Paleoproterozoic Nagssugtoqidian orogen in West Greenland is largely composed of variably reworked orthogneisses with Archean protolith ages and volumetrically subordinate amounts of Paleoproterozoic metamorphosed mafic and sedimentary rocks and calc-alkaline intrusions. The dominant components of the Archean orthogneisses of the orogen are weakly deformed to foliated granodioritic to tonalitic orthogneisses that include older, polycyclic mafic and intermediate gneisses. The orthogneisses appear to contain field evidence for two Archean migmatization events and are associated with Archean metasedimentary rocks.

Samples of orthogneisses collected in a north-south transect across the orogen yield similar magmatic ages between 2.87 and 2.81 Ga, indicating that a fundamental Archean crustal boundary cannot be geochronologically identified within the orogen. Both pre–2.87 Ga gneissic inclusions and the 2.87–2.81 Ga magmatic suite were deformed and metamorphosed between 2.81 and 2.72 Ga, immediately after their emplacement. Tonalitic sheets cut at least one high-strain zone at 2.50 Ga, implying that these gneisses locally had a coherent structural grain prior to onset of the Paleoproterozoic Nagssugtoqidian orogeny. Undeformed 2.79 Ga granites in the northern Nagssugtoqidian foreland provide a northern limit for pervasive Late Archean and Nagssugtoqidian deformation and metamorphism. After the Archean deformation, granites were emplaced into both the Nagssugtoqidian orogen proper and the southern foreland between 2.72 and 2.70 Ga.

Despite strong Late Archean and Paleoproterozoic reworking, the Archean basement to the Nagssugtoqidian orogen preserves clear evidence for a cycle of magmatism, sedimentation, deformation, metamorphism, and posttectonic magmatism that is similar to the sequence of events recorded at modern convergent margins (i.e., subduction-related juvenile tonalitic-granodioritic magmatism, collision with attendant deformation and metamorphism, and posttectonic granitoid magmatism). This growth by magmatic accretion may have built upon the older Archean core of southwestern Greenland to form a coherent North Atlantic craton by the end of the Archean, an evolution comparable to that of northeastern Laurentia and Scotland.

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