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

Journal of the Geological Society, May 2007 by Garde, Adam A

Abstract:

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 platetectonic 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.

The Godthâbsfjord region of the North Atlantic craton in southem West Greenland is one of the most intensely studied pieces of Archaean crust on Earth. It is well exposed, and in contrast to most other Archaean cratons, which are dominated by low-grade granite-greenstone rock associations, it mainly exposes tonalitic-trondhjemitic-granodioritic (TTG) orthogneiss and tholeiitic amphibolite metamorphosed at amphibolite to granulite grade. The central part of the region has become a renowned example of lateral tectonic terrane accretion in the late Archaean, whereby individual Archaean tectono-stratigraphic terranes of different ages were assembled into a single block (e.g. Friend et al. 1988, 1996; Friend & Nutman 2005). Based on melting experiments and geochemical studies it has been proposed that the orthogneiss precursors of the North Atlantic craton were produced by slab melting of subducted oceanic crust (e.g. Winther & Newton 1991; Garde 1997, and references therein). However, no direct field or geochronological evidence for this supposition has previously been reported; furthermore, the geotectonic relationship between the c. 3.0-2.7 Ga orthogneiss-amphibolite associations of the North Archaean craton and the lower-grade granitegreenstone associations exposed in most other Archaean cratons has remained uncertain.

This paper describes intensely deformed and metamorphosed panels of supracrustal rocks that occur within the orthogneisses close to the eastern edge of the Akia terrane in western Godthàbsfjord (Fig. 1; Bjerneeen and the peninsula east of Qussuk, 'Qussuk peninsula'). The supracrustal panels are dominated by rocks with relict volcaniclastic textures and andesitic compositions, and are interpreted as the mid-crustal remnants of a large, middle Archaean island arc complex. The poor state of preservation prevents the establishment of a proper volcanic stratigraphy or recognition of individual volcanic faciès associations, and thus hinders detailed comparison with well-preserved Archaean or modern arc environments elsewhere. Nevertheless, new age data reported here show that the arc accretion happened just prior to the emplacement of the main group of orthogneisses, and the field relationships and geochemical signature of the arc provide straightforward evidence of subduction and lateral crustal shortening. Therefore, the newly discovered arc complex represents a critical magmatic and tectonic 'bridge' between the supracrustal amphibolites of ocean-floor affinity that predate the arc and the grey, continental crustal orthogneisses of TTG affinity that postdate it. The eastern Akia terrane also provides a previously missing link between Archaean mid-crustal, highgrade orthogneiss-amphibolite associations exposed in West Greenland, and the common, lower-grade granite-greenstone associations of, for example, the Superior province in the adjacent eastern Canadian craton.

Previous work

The Akia terrane is the largest of several tectono-stratigraphic terranes in the Nuuk region and has undergone a complex history of volcanic and plutonic igneous activity, deformation and metamorphism (e.g. Berthelsen 1960; Taylor et al. 1980; Wells 1980; Riciputi et al. 1990; McGregor et al. 1991; Garde 1997, and references therein). The terrane comprises two continental crustal complexes: a dioritic core dated at c. 3220 Ma that forms most of the Akia peninsula (Fig. 1), and a larger block dated at 3.05-2.97 Ga, mainly tonalitic orthogneiss with enclaves of supracrustal rocks between Fiskefjord and Godthabsfjord and farther north. The bulk of the orthogneisses consist of tonalité that was intruded into, or was tectonically intercalated with supracrustal rocks and associated mafic intrusive complexes. The supracrustal rocks between Fiskefjord and Godthabsfjord are older than c. 3.05 Ga (the age of the oldest orthogneisses that intrude them). They consist largely of heterogeneous, compositionally banded metavolcanic amphibolite, and metagabbroic and ultramafic rocks that represent disrupted layered intrusive complexes including olivine-rich cumulate rocks. Garde (1997) also reported local leucocratic amphibolite of andesitic affinity. The supracrustal rocks were interpreted as remnants of oceanic crust, into which the precursors of the orthogneisses were intruded, presumably in a convergent plate-tectonic setting (Garde 1990, 1997; Garde et al. 2000). Komatiites or other plume-related supracrustal lithologies have not been reported from the Akia terrane.