Adjacent terranes with ca. 2715 and 2650 Ma high-pressure metamorphic assemblages in the Nuuk region of the North Atlantic Craton, southern West Greenland: Complexities of Neoarchaean collisional orogeny

Abstract

In the gneiss complex of the Nuuk region of the North Atlantic Craton in southern West Greenland, terrane juxtaposition was followed by Neoarchaean folding under amphibolite facies conditions, with widespread low-pressure recrystallisation (5 kbar and 550-700 °C). The complex metamorphic overprinting requires that the P-T history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the U/Pb dating, petrology and geochemistry of metamorphic zircons. In the south of the region, the Færingehavn terrane (Eoarchaean orthogneisses) is tectonically overlain by a supracrustal package of amphibolites and paragneisses (ca. 2840 Ma felsic volcano-sedimentary protoliths). This package is juxtaposed against a higher tectonic level represented by the Tre Brødre terrane (2825 Ma orthogneisses) and the Tasiusarsuaq terrane (2920-2810 Ma orthogneisses). The terranes were assembled by 2710-2720 Ma, as shown by dating of granitic sheets intruded along the terrane boundary mylonites. In the Færingehavn terrane and in the overlying 2840 Ma supracrustal package, relict early high-pressure assemblages (12-8 kbar, 700-750 °C) are clinopyroxene + garnet + plagioclase + quartz ± hornblende in mafic rocks and garnet + kyanite + rutile bearing assemblages in paragneisses. These are commonly replaced by lower pressure assemblages (7-5 kbar) such as cordierite ± sillimanite ± garnet in paragneisses and hornblende + plagioclase + quartz ± garnet in mafic rocks. In situ partial melting took place during both low- and high-pressure regimes. Metamorphic zircon in the high- and low-pressure assemblages yields dates of ca. 2715 Ma, mostly with errors of <±5 Ma, thereby demonstrating rapid decompression at high temperatures. Zircons in the overlying Tre Brødre and Tasiusarsuaq terranes show little response to the ca. 2715 Ma event supporting structural interpretations that they were at a higher structural level at ca. 2715 Ma. High temperature recrystallisation continued after ca. 2715 Ma, as demonstrated by intergrowth of sillimanite with 2680 Ma metamorphic zircon. The Kapisilik terrane (3050-2960 Ma orthogneisses) and another supracrustal assemblage of amphibolites and ca. 2800 Ma quartzo-feldspathic metasedimentary rocks are exposed north of the Færingehavn terrane and within fold cores along its western margin, are bounded by folded Neoarchaean mylonites. The Kapisilik terrane and this supracrustal assemblage include high-pressure metamorphic remnants in amphibolites and metasediments (metamorphic segregations with garnet + clinopyroxene and kyanite, respectively) that formed at ca. 2650 Ma. These remnants are overprinted by high temperature, but lower pressure metamorphic events at ca. 2630, 2610 and 2580 Ma. Thus remnants of early metamorphism reveal mutually exclusive ca. 2715 and 2650 Ma high-pressure events in adjacent tectonically juxtaposed terranes. Repeated high-pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens.