Adam A. Garde

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).

A mid-Archaean island arc complex in the eastern Akia terrane, Godthåbsfjord, southern West Greenland

A relict oceanic island arc complex in the eastern Akia terrane, Godthåbsfjord, southern West Greenland, constitutes a magmatic and geotectonic link between c. 3.05–3.0 Ga tonalitic orthogneiss and enclaves of older supracrustal amphibolite. The relict arc forms isoclinally folded panels of volcaniclastic meta-andesite with major and trace element island arc signatures, intercalated with volcano-sedimentary schist, tholeiitic amphibolite and opx-rich cumulate rocks. A zircon U–Pb age of 3071 ± 1 Ma obtained from a volcano-sedimentary schist is marginally older than the main orthogneisses and is the first depositional age reported from a supracrustal rock within Akia terrane. Granite sheets were emplaced at c. 3005–2980 Ma synkinematically with the isoclinal folding, and were followed by peak metamorphism at 2990–2970 Ma with substantial recrystallization of volcanic zircon and mobility of large-ion lithophile elements. The identification of the arc complex provides new insight into mid-Archaean continental crustal accretion in West Greenland, and substantiates previous ideas that the orthogneisses are products of slab melting in convergent plate-tectonic settings. The presence of the arc complex also implies that Archaean high-grade orthogneiss–amphibolite associations may not represent plate-tectonic environments distinct from granite–greenstone associations, but expose deeper sections of the same convergent systems.

Emplacement of rapakivi granite and syenite by floor depression and roof uplift in the Palaeoproterozoic Ketilidian orogen, South Greenland

In the Palaeoproterozoic Ketilidian orogen of South Greenland, tabular intrusions of the rapakivi granite suite exposed at Graah Fjelde, Lindenow Fjord and Qernertoq, and a Mesoproterozoic syenite of the Gardar province exposed at Paatusoq, were emplaced by a combination of roof uplift and floor depression. The strain associated with the emplacement of the intrusions mainly involved redistribution of mass vertically within the lithosphere. Space for the emplacement of the rapakivi plutons was not created during regional extension on low-angle, ductile shear zones as claimed by some previous workers and there is no evidence that emplacement of the intrusions coincided with extensional collapse of the orogen following crustal thickening. The rapakivi granites post-date peak metamorphism in the orogen by 35–46 Ma and their contacts cross-cut and clearly post-date intense, flat-lying D1/D2 fabrics that formed by partitioning of deformation into arc-normal and arc-parallel components during oblique convergence.

Linking the Palaeoproterozoic Nagssugtoqidian and Rinkian orogens through the Disko Bugt region of West Greenland

The parallel Trans-Hudson-aged Nagssugtoqidian Orogen and Rinkian Fold Belt of West Greenland have long been considered distinct orogenic belts separated by a zone of low or no Palaeoproterozoic strain in the intervening Disko Bugt region. New U–Pb geochronology confirms the continuity of Palaeoproterozoic deformation and metamorphism through this region, thereby directly linking the Nagssugtoqidian Orogen and Rinkian Fold Belt. Furthermore, two newly identified Archaean blocks in this region are interpreted to represent the southern and northern extensions of the Rae and North Atlantic cratons, respectively. Their boundary comprises a 15 km wide belt of distributed, NW-vergent ductile thrusts that deform ultramafic pods and is interpreted to represent the main collisional suture of a single, variably exhumed, >1100 km wide Nagssugtoqidian–Rinkian orogenic system. We conclude that Palaeoproterozoic convergence of a northern Archaean craton carried a south-facing passive margin sedimentary sequence (Karrat Group) towards a south-dipping subduction zone along the North Atlantic Craton. Terminal collision created a high-grade core centred on the magmatic arc of the central Nagssugtoqidian Orogen and a NW-vergent fold and thrust belt in the Rinkian Fold Belt.

Mid-crustal partitioning and attachment during oblique convergence in an arc system, Palaeoproterozoic Ketilidian orogen, southern Greenland

Abstract: The Ketilidian orogen in SE Greenland contains a continental magmatic arc (Julianehåb batholith) and forearc which formed in the overriding plate of a Palaeoproterozoic oblique subduction boundary between c. 1854 and 1795 Ma. During uplift and deformation of the forearc between c. 1795 and 1780 Ma, its sedimentary sequences were invaded by arc-type magmas and metamorphosed at HT–LP upper amphibolite facies conditions which led to widespread anatexis and generation of abundant S-type granites. Transpression during early deformation (D1) was partitioned into arc-parallel, sinistral strike-slip in the magmatic arc rocks of the batholith and arc-normal contraction in the sedimentary and volcaniclastic sequences of the proximal forearc (Psammite Zone). Subsequently, D1 fabrics in the Psammite Zone were overprinted and transposed in an intense, flat-lying, D2 ductile shear zone with arc-parallel stretching lineations and top-NE kinematic indicators. The kinematic significance of the D2 shear zone is that it accommodated vertical differences in strain and displacement and permitted deformation to be partitioned differently at different levels in the crust. As such, it is an attachment or coupling zone that provided kinematic linkage between deformation at different crustal levels during NE transport of a forearc sliver. Overprinting of the upper crustal domain of D1 strike-slip partitioned transpression in the forearc during D2 is attributed to migration of the attachment zone to a higher crustal level. This migration was a consequence of an enhanced geothermal gradient in the proximal part of the forearc due to the emplacement of batholith-related plutonic suites during D2.

A buried Palaeoproterozoic spreading ridge in the northern Nagssugtoqidian orogen, West Greenland

Abstract Ophiolitic rocks on two small island groups in the northern Nagssugtoqidian orogen, West Greenland, shed new light on the eastern Laurentian Palaeoproterozoic plate-tectonic collage. The islands expose amphibolite-facies (520–550 °C, 2.6–3.0 kbar) tholeiitic pillow lava, black chloritic shale, manganiferous banded iron formation (BIF), podded chert, jasper, graded andalusite–staurolite schist with numerous sills, and terrigenous sandstone, an association that occurs today in oceanic spreading zones undergoing burial in forearc trenches. Deformation is weak, with upright folds preserving original younging. U–Pb detrital zircon ages indicate derivation from Archaean, 1890 Ma, and 1850 Ma sources. Occurring north of the Archaean Aasiaat domain that partly escaped Nagssugtoqidian deformation, the sequence pinpoints a new SSE-dipping Palaeoproterozoic subduction zone that links up in the NE with the Disko Bugt suture at the Nagssugtoqidian–Rinkian boundary. This solves an enigma of the previous plate-tectonic model, in which the Arfersiorfik magmatic arc in the central Nagssugtoqidian orogen was located within its own suture zone, and helps to explain why the Nagssugtoqidian–Rinkian system is so broad. It also explains previously recognized geochemical and Pb-isotopic compositional breaks in orthogneisses in the Kangaatsiaq region, and greatly simplifies correlation with adjacent eastern Canadian orogens in Baffin Island and Labrador.

Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic Craton

Abstract Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu–Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.

Attachment formation during partitioning of oblique convergence in the Ketilidian orogen, South Greenland.

Abstract Subhorizontal attachment zones provide coupling between lithospheric layers in orogenic belts. A mid-crustal attachment zone is exposed in the Palaeoproterozoic Ketilidian orogen, south Greenland, which formed as a result of north-directed oblique convergence at a cordilleran-type margin. Rifting (c. 2.1 Ga) and compressional deformation and magmatism (> 1850 Ma) on the continental margin was followed by an extended sinistral transpression from 1850 to 1730 Ma now separated into three episodes or peaks of activity. The first episode was focused on the back-arc region and was followed by the main arc construction phase during which transpression was partitioned into strike-slip and contraction components. Despite the longevity of this active margin system, individual tectonic events took place rapidly, e.g. development of fore-arc D1-D3 and accompanying high-temperature, low-pressure metamorphism took place over c. 12 Ma. We explain the fore-arc and batholith evolution by the upward migration of an underlying attachment structure through the upper crustal partitioned blocks. This migration may be attributed to an increase in the geothermal gradient accompanied by, or followed by, exhumation of the mid-crust. The partially molten, hence weak, attachment zone solidified and strengthened during cooling before emplacement of the post-orogenic rapakivi suite during the third distinct phase of mild sinistral transpression.

A centennial reappraisal of the Vredefort pseudotachylytes: shaken, not stirred by meteorite impact

Major pseudotachylyte zones constitute a spectacular component of the renowned c. 2.023 Ga Vredefort impact structure, South Africa, but it has always been difficult to explain how they were formed. In his original account, in 1916, Shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since: there were no associated major faults, and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on Earth. New observations show that the Vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating, not by faulting but owing to impact-induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems. Progressive cataclasis of such loose blocks by intense, high-frequency oscillations of the country rock at the beginning of the cratering process led to size reduction, rounding and comminution, and frictional melting of feldspars and biotite in the comminuted parts. Most pseudotachylyte was thus not injected from anywhere but produced in situ. The process of seismic shaking is well known from impacts on the Moon and asteroids, terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidization.

Impact-triggered, conduit-type Ni-Cu mineralisation, Norite Belt, Maniitsoq Structure, West Greenland

Veins of clay and carbonate in the nakhlite meteorite Lafayette formed by dissolution and replacement of olivine.NanoSIMS measurements record δD values up to +4725‰ in Lafayette which reveal martian waters of crustal origin are incorporated into the smectite and adjacent olivine.