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Original Article |
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark (e-mail: aag@geus.dk)
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.
