Abstract
The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kangaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are comprised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite paragneisses. Their association and geochemical compositions are consistent with a metamorphosed volcano-sedimentary basin (containing both tholeiitic and calc-alkali lavas) and is similar to other Archaean greenstone belts. The Kangaatsiaq granite forms a 15 × 3 km flat, subconcordant body of deformed, pink, porphyritic granite occupying the core of the supracrustal synform, and is demonstrably intrusive into the amphibolites. The granite displays a pronounced linear fabric (L or L > S). The post-granite deformation developed under lower amphibolite facies conditions (400 ± 50°C), and is characterised by a regular, NE–SW-trending subhorizontal lineation and an associated irregular foliation, whose poles define a great circle; together they are indicative of highly constrictional strain. The existence of a pre-granite event is attested by early isoclinal folds and a foliation within the amphibolites that is not present in the granite, and by the fact that the granite cuts earlier structures in the supracrustal rocks. This early event, preserved only in quartz-free lithologies, resulted in high-temperature fabrics being developed under upper amphibolite to granulite facies conditions.
Highlights
Introduction and regional geologyThe northern part of the Nagssugtoqidian orogen (NNO) is a domain of predominantly Archaean rocks that have been deformed and metamorphosed during Nagssugtoqidian orogenic activity at c. 1.8 Ga (Hollis et al 2006, this volume; Mazur et al 2006, this volume; Thrane & Connelly 2006, this volume; van Gool & Piazolo 2006, this volume)
It can be considered that a constrictional deformation event post-dating the granite emplacement occurred during cooling to lower amphibolite facies conditions. Since this event is apparently responsible for the regional-scale structures (Mazur 2002; van Gool et al 2002b; Piazolo et al 2004), and is of lower Proterozoic age (Willigers et al 2002), we propose that it corresponds essentially to the Nagssugtoqidian deformation proper
Since no evidence for this deformation is found in the granite, we suggest that it was pre-granite, and likely corresponds to late Archaean deformation
Summary
The northern part of the Nagssugtoqidian orogen (NNO) is a domain of predominantly Archaean rocks that have been deformed and metamorphosed during Nagssugtoqidian orogenic activity at c. 1.8 Ga (Hollis et al 2006, this volume; Mazur et al 2006, this volume; Thrane & Connelly 2006, this volume; van Gool & Piazolo 2006, this volume). Several components of the gneissic basement have been dated (Kalsbeek & Nutman 1996; Connelly & Mengel 2000; Thrane & Connelly 2002, 2006, this volume), yielding ages in the range 2.9–2.6 Ga. Supracrustal assemblages are common, and have been mapped in many places in the Kangaatsiaq, Aasiaat and Kangersuneq map sheet areas (Marker et al 1995; Mengel et al 1998; Østergaard et al 2002; van Gool et al 2002a; Garde 2004; Hollis et al 2006, this volume). The area east and north-east of the town of Kangaatsiaq (Fig. 1) is dominated by a synform of supracrustal rocks (mafic and felsic volcanic rocks associated with sediments), into which the Kangaatsiaq granite was emplaced It is, a good place to study the Archaean components and local history in the NNO. D: Detail of a small granitic apophysis which clearly cuts across the foliation of the amphibolite
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