Terranes and tectonic boundaries in the southern Nagssugtoqudian orogen, West Greenland

Mogens Marker1, Leo M. Kriegsman2, Jeroen A.M. van Gool1 & Geoff T. Nichols3

1Danish Lithosphere Centre, Oster Voldgade 10, 1350 Copenhagen K, Denmark.

2Dept. of Applied Geology, University of New South Wales, Sydney, NSW 2052, Australia.

3GEMOC, School of Earth Sciences, Macquarie University, NSW 2109, Australia.

The Palaeoproterozoic Nagssugtoqidian orogen (NO) in W-Greenland exposes a deep crustal level of largely reworked Archaean gneisses with only minor Proterozoic crustal additions. In spite of their small volume, the Proterozoic rocks are of major significance with respect to recognition of terranes and the interpretation of the tectonic evolution. Investigations in the eastern NO of its southern (SNO) and central (CNO) segments showed that these represents different terranes that amalgamated during Nagssugtoqidian collision. While the SNO is made up by reworked, mainly retrogressed, diversified southern Archaean foreland gneisses, the southern CNO consists of uniform ca. 2.8 Ga granulite facies gneisses. The ca. 2.05 Ga Kangâmiut dyke swarm monitor the deformation. The N-dipping SNO-CNO terrane boundary marks the disappearance of the Kangâmiut dykes, interleaving of thin panels of Proterozoic metapelites with Archaean orthogneisses, and the metamorphic transition from Proterozoic amphibolite to granulite facies. Furthermore, it separates a ca.1.93 Ga charnockitic complex from SNO gneisses in the west. The highest strain recorded in Kangâmiut dykes is located along the northern edge of the SNO. The boundary is interpreted as an oblique thrust zone translating the CNO southwards. Besides a straight structural fabric, kinematic indicators are rare and presumably obliterated by subsequent thermal overprint at a deep crustal level due to thrust stack thickening, which agrees with high P and T (12-14 kbar at 700-800°C) derived from deformed Kangâmiut dykes. The panels of Proterozoic metapelites may represent remnants of a destructed sedimentary basin between the SNO and CNO. Later, generally sinistral strike-slip movement along with retrogression in localised zones overprints the thrust structure. Other tectonic zones of significance for Nagssugtoqidian evolution occur in the southern CNO. Important is a steep belt of ca. 2.8 Ga metasediments in the north with a straight structure and assumed to be an equivalent to the Nagssugtoqidian Nordre Stromfjord steep belt. A late prominent NE-SW trending strike-slip zone hosting ca.1.84-1.83 Ga sheared red granites and pegmatites transects the southern CNO.

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