Jeroen A.M. van Gool

Correlation of Archaean and Palaeoproterozoic units between northeastern Canada and western Greenland: constraining the pre-collisional upper plate accretionary history of the Trans-Hudson orogen

Abstract Based on available tectonostratigraphic, geochronological, and structural data for northeastern Canada and western Greenland, we propose that the early, upper plate history of the Trans-Hudson orogen was characterized by a number of accretionary–tectonic events, which led to the nucleation and growth of a northern composite continent (the Churchill domain), prior to terminal collision with and indentation by the lower plate Superior craton. Between 1.96 and 1.91 Ga Palaeoproterozoic deformation and magmatism along the northern margin of the Rae craton is documented both in northeastern Canada (Ellesmere–Devon terrane) and in northern West Greenland (Etah Group–metaigneous complex). The southern margin of the craton was dominated by the accumulation of a thick continental margin sequence between c. 2.16 and 1.89 Ga, whose correlative components are recognized on Baffin Island (Piling and Hoare Bay groups) and in West Greenland (Karrat and Anap nunâ groups). Initiation of north–south convergence led to accretion of the Meta Incognita microcontinent to the southern margin of the Rae craton at c. 1.88–1.865 Ga on Baffin Island. Accretion of the Aasiaat domain (microcontinental fragment?) in West Greenland to the Rae craton resulted in formation of the Rinkian fold belt at c. 1.88 Ga. Subsequent accretion–collision of the North Atlantic craton with the southern margin of the composite Rae craton and Aasiaat domain is bracketed between c. 1.86 and 1.84 Ga (Nagssugtoqidian orogen), whereas collision of the North Atlantic craton with the eastern margin of Meta Incognita microcontinent in Labrador is constrained at c. 1.87–1.85 Ga (Torngat orogen). Accretion of the intra-oceanic Narsajuaq arc terrane of northern Quebec (no correlative in Greenland) to the southern margin of the composite Churchill domain at 1.845 Ga was followed by terminal collision between the lower plate Superior craton (no correlative in Greenland) and the composite, upper plate Churchill domain in northern and eastern Quebec at c. 1.82–1.795 Ga. Taken as a set, the accretionary–tectonic events documented in Canada and Greenland prior to collision of the lower plate Superior craton constrain the key processes of crustal accretion during the growth of northeastern Laurentia and specifically those in the upper plate Churchill domain of the Trans-Hudson orogen during the Palaeoproterozoic Era. This period of crustal amalgamation can be compared directly with that of the upper plate Asian continent prior to its collision with the lower plate Indian subcontinent in the early Eocene. In both cases, terminal continental collision was preceded by several important episodes of upper plate crustal accretion and collision, which may therefore be considered as a harbinger of collisional orogenesis and a signature of the formation of supercontinents, such as Nuna (Palaeoproterozoic Era) and Amasia (Cenozoic Era).

U–Pb zircon ages of Kangâmiut dykes and detrital zircons in metasediments in the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland): Clues to the pre-collisional history of the orogen1

Abstract In the southern marginal zone of the Palaeoproterozoic Nagssugtoqidian orogen and its foreland (West Greenland), Archaean gneisses were intruded by the syn-kinematic Kangâmiut dyke swarm. Leucodioritic cores of two dykes have yielded U–Pb zircon ages of 2036±5xa0Ma and 2046±8xa0Ma (2σ). Dyke emplacement and associated crustal movements thus occurred well before the main plutonic and tectonic activity within the orogen (∼1900–1800xa0Ma). In the central Nagssugtoqidian orogen, pelitic to semi-pelitic metasedimentary rocks were intruded by 1920–1900xa0Ma quartz diorites and tonalites of the Arfersiorfik association, which have chemical and isotopic compositions comparable with igneous suites formed in arc environments. The supracrustal and intrusive rocks are separated from Archaean gneisses by decollements of high-grade mylonite and annealed marble, and are allochthonous units. Detrital zircons in two samples of metasedimentary rocks from these allochthons are mostly large, poorly rounded, 2100–1950xa0Ma old with only a few Archaean grains. Deposition of the metasediments must have taken place between ∼1950 and ∼1920xa0Ma, before the intrusion of rocks of the Arfersiorfik association. The zircons may have been shed from presently unrecognised pre-Arfersiorfik association igneous complexes (arcs?), whose emplacement coincided with intrusion of the ∼2040xa0Ma Kangâmiut dykes into deforming Archaean crust in what later became the southern margin of the orogen. Quartz-rich psammitic metasedimentary rocks are also present in the central Nagssugtoqidian orogen. Detrital zircons from one sample yielded a few 2150–2250xa0Ma grains, but are predominantly Archaean. They include a high proportion of 3300–3400xa0Ma grains (the oldest material yet found in the orogen). Thrust-bounded panels of metasedimentary rocks in Archaean gneisses occur at the boundary between the southern and central parts of the orogen coinciding with the northern limit of the Kangâmiut dykes. Of the detrital zircons in one metasediment sample from these panels a few yielded ages around 2100–2000xa0Ma, some were Archaean, and >50% were ∼2400xa0Ma. Deposition of these two sediment suites took place after ∼2100xa0Ma. The detrital zircon population shows that the sources for these sediments were different from that of the above-mentioned allochthonous rocks. Neither the 3300–3400xa0Ma nor the ∼2400xa0Ma detrital grains can be matched with any rocks in Greenland, indicating a distal source for these sediments.

Thrust stacking in the inner Nordre Strømfjord area, West Greenland: Significance for the tectonic evolution of the Palaeoproterozoic Nagssugtoqidian orogen

Abstract The Nagssugtoqidian orogen is characterised by a predominance of Archaean gneisses that were reworked at moderate to deep crustal levels during the Palaeoproterozoic, resulting in an ENE-trending structural grain. A detailed study in the central part of the orogen has shown that a lithotectonic unit containing Palaeoproterozoic supracrustal and igneous rocks, the Ussuit unit, was juxtaposed against Archaean gneisses along a tectonic contact. Subsequently, the sequence of Archaean gneisses overlain by the Ussuit unit was repeated by thrust imbrication. Tectonic contacts and thrusts are presently ductile shear zones, but some may have originated as brittle structures. Stretching lineations are consistently shallowly ESE- to E-plunging and kinematic indicators show predominantly top-to-NW movements. Significant volumes of melt in the high-strain zones indicate that shearing occurred at high temperatures. While high temperatures and partial melting still prevailed, the thrusts/shear zones were deformed in kilometre-scale, open, upright folds. The change from thrusting to folding was accompanied by rotation of the convergence direction from WNW–ESE to approximately N–S. This later deformation resulted in the presently observed ENE-trending structural grain. The observation that a period of terrane accretion and crustal thickening by thrusting and shearing predated the formation of the ENE trend of the orogen, adds further support to the hypothesis that the Nagssugtoqidian orogen is a collisional orogen.