ASSEMBLING THE ARCHEAN CORE OF GREENLAND: COMPARING MICROSTRUCTURE, MAP-SCALE STRUCTURE, AND METAMORPHIC ASSEMBLAGES ACROSS THE PROPOSED AKULLEQ-AKIA TERRANE BOUNDARY, GODTHABSFJORD

Major components of the Archean craton of Greenland, the Akulleq and Akia terranes in southern West Greenland, were, according to Friend et al. (1996), united at ca. 2720 Ma after undergoing contrasting tectonic histories. We are testing this accretionary model by comparing structure and metamorphism, from grain scale to map scale, on either side of the proposed tectonic boundary. We have found contrasts in structural development, hitherto unrecognized similarities in metamorphic histories, and evidence that accretion was significantly later than 2720 Ma.

Our work was on two adjacent islands, Storo and Bjorneoen. Most of Storo is within the Akulleq terrane; Bjorneoen, in the Akia terrane. On Storo, contacts parallel the sillimanite-grade S2 fabric, which rarely includes isoclinally-folded S1. Although the Akulleq terrane was previously reported to have undergone only amphibolite-facies metamorphism, we have found relict granulite-facies D1 assemblages (Cpx-Opx). S2 was folded into km-scale, S-plunging, upright F3 folds during continued high-grade (amphibolite-facies) metamorphism. The proposed terrane boundary, along the NW coast of Storo, is recognizable as a high-strain zone wherein common pegmatite dikes have been flattened into parallelism with other lithologies. However, lineations are rare in the high-strain zone, kinematic indicators are sparse and contradictory, and localized shearing took place at various grades (sillimanite grade to greenschist facies). The high-strain zone probably had a protracted history that, given that the zone is roughly parallel with F3 axial planes, may have begun during D3.

Fabrics and contacts on Bjorneoen, in contrast, predominantly dip moderately northwest. Most of the Akia terrane went through granulite-facies metamorphism, but the dominant S1 fabric on Bjorneoen was formed at amphibolite facies. Biotite-grade, top-to-the-NE (towards Storo/Akulleq terrane) thrusts and recumbent folds parallel S1.

Preliminary geochronology of a recumbently-folded Bjorneoen pegmatite and a syn-D2 Storo pegmatite indicate that late deformation on both islands, and thus juxtaposition of the two terranes, took place at the end of the Late Archean/early Paleoproterozoic.