Magnetic Anomalies and Metamorphic Boundaries in the Southern Nagssugtoqidian Orogen, West Greenland

Magnetic anomalies and metamorphic boundaries in the southern Nagssugtoqidian orogen, West Greenland John A. Korstgård, Bo Møller Stensgaard and Thorkild M. Rasmussen Within the southern Nagssugtoqidian orogen in West Greenland metamorphic terrains of both Ar- chaean and Palaeoproterozoic ages occur with metamorphic grade varying from low amphibolite facies to granulite facies. The determination of the relative ages of the different metamorphic terrains is greatly aided by the intrusion of the 2 Ga Kangâmiut dyke swarm along a NNE trend. In Archaean areas dykes cross-cut gneiss structures, and the host gneisses are in amphibolite to granulite facies. Along Itilleq strong shearing in an E–W-oriented zone caused retrogression of surrounding gneisses to low amphibolite facies. Within this Itivdleq shear zone Kangâmiut dykes follow the E–W shear fabrics giving the impression that dykes were reoriented by the shearing. However, the dykes remain largely undeformed and unmetamorphosed, indicating that the shear zone was established prior to dyke emplacement and that the orientation of the dykes here was governed by the shear fabric. Me- tamorphism and deformation north of Itilleq involve both dykes and host gneisses, and the metamorphic grade is amphibolite facies increasing to granulite facies at the northern boundary of the southern Nagssugtoqidian orogen. Here a zone of strong deformation, the Ikertôq thrust zone, coincides roughly with the amphibolite–granulite facies transition. Total magnetic field intensity anomalies from aeromagnetic data coincide spectacularly with metamorphic boundaries and reflect changes in content of the magnetic minerals at facies transitions. Even the nature of facies transitions is apparent. Static metamorphic boundaries are gradual whereas dynamic boundaries along deformation zones are abrupt. Keywords: aeromagnetic data, magnetic anomalies, metamorphic facies, Nagssugtoqidian orogen, West Greenland __________________________________________________________________________________________________________________________________________________________ J.A.K., Department of Earth Sciences, University of Aarhus, Høegh-Guldbergsgade 2, DK-8000 Århus C, Denmark. E-mail: [email protected] B.M.S. & T.M.R., Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. The establishment of the Palaeoproterozoic Nagssugto- definition of the ‘Nagssugtoqides’. Ramberg also divided qidian orogen in West Greenland (Ramberg 1949) is based the Nagssugtoqidian orogen into three metamorphic com- on the deformation and metamorphism of the Kangâmiut plexes based on the metamorphic grade of the rocks. Thus dykes, dated at 2.04 Ga by Nutman et al. (1999). South the Egedesminde complex was the northernmost amphi- of the southern Nagssugtoqidian front (SNF in Fig. 1), in bolite facies complex, the Isortoq complex the central gran- the southern Nagssugtoqidian foreland, Kangâmiut dykes ulite facies complex, and the Ikertôq complex the south- are undeformed and cross-cut gneiss structures. North of ernmost amphibolite facies complex. The current division the front, gneisses and dykes have been metamorphosed of the orogen (Fig. 1) is based on structural criteria, and and deformed together during the Nagssugtoqidian oro- division boundaries now follow major structural features geny. Here, gneiss structures and dyke margins are con- (Marker et al. 1995). The current division therefore devi- cordant and dykes transformed into amphibolites. This is ates considerably from Ramberg’s original division for the the simple story upon which Ramberg (1949) based his northern and central Nagssugtoqidian orogen, whereas the © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 179–184. Available at: www.geus.dk/publications/bull 179 AasiaatAasiaat Fig. 1. Schematic geology of the southern Inland Ice part of the Nagssugtoqidian orogen and adjacent forelands (modified from Escher & Pulvertaft 1995 and Marker et al. 1995). Greenland 50 km SNO, southern Nagssugtoqidian orogen; NNO CNO, central Nagssugtoqidian orogen; NNO, northern Nagssugtoqidian orogen; SNF, southern Nagssugtoqidian front. The Ar locations of thrust and shear zones are fer sio 68° rfik defined from trends observed in the aeromagnetic data; note that the E–W- 500 km trending thrust zone with question marks N north of Kangerlussuaq is uncertain, as this ord re rd CNO structure has not been confirmed by Strømfjo geological mapping. Black frames show the Nordre locations of Figs 2, 3. Strømfjord shear zone Fig. 3 Nordre SNO Isortoq ? steep ? belt 67° Fig. 2 KaKangerlgerlussssuaq Sisimiut rd fjo øm Ikertôq tr S thrust zone re nd Sø Itivdleq shear zone southern Nagssugtoqidian foreland 54° SNF 51° Nagssugtoqidian orogen Quaternary Sisimiut charnockite (Palaeoproterozoic) Surficial deposits Arfersiorfik quartz diorite (Palaeoproterozoic) Orthogneiss (Archaean, reworked) Metasedimentary rocks (Palaeoproterozoic, may include Archaean components) Metasedimentary rocks (Archaean, North Atlantic craton may include Proterozoic components) Granodioritic-granitic gneiss (northern parts reworked) Amphibolite Orthogneiss Anorthosite and ultrabasic rocks (largely unreworked) southern Nagssugtoqidian orogen corresponds almost ex- and due to the fortunate timing of the intrusion of the actly to Ramberg’s original Ikertôq complex. Kangâmiut dykes it is possible to assign relative ages to The southern Nagssugtoqidian orogen (SNO in Fig. the different metamorphic terrains in the region. 1) in the coastal region between Sisimiut and Itilleq con- sists mainly of quartzofeldspathic gneisses of granodioritic to tonalitic composition. Several supracrustal layers occur, particularly in the northern part of the SNO. The Pre-dyke metamorphism and supracrustal rocks are mainly garnet-biotite schists, rusty deformation weathering biotite gneisses and amphibolites. The meta- South of and immediately north of Itilleq, the Kangâmiut morphic grade is low amphibolite facies to granulite facies, dykes are largely undeformed, unmetamorphosed and 180 Fig. 2. Correlation between metamorphic facies and 53°30' Post-dyke 53° 52°30' (Nagssugtoqidian) M aeromagnetic anomaly patterns in the Itilleq–Ikertooq G a l A i granulite facies g a a I region. White lines indicate approximate metamorphic Sisimiut q facies boundaries based on geological field work; labels H G ertooq A–J are explained in the text. A: Distribution and Ik ' H 0 relative ages of metamorphic facies. B: Total intensity 5 ° F 6 magnetic field anomaly map. Shadow of magnetic field 6 pattern modelled from a light source with inclination k F tali E qer 45° and declination 315°. Qe Post-dyke (Nagssugtoqidian) amphibolite facies arsuk erlu Kang ' E 0 4 ° Pre-dyke 6 B 6 (Archaean) granulite facies C Pre-dyke A (Archaean) amphibolite facies Pre-dyke J 10 km D D amphibolite facies D Itilleq ' 0 3 ° 6 6 53°30' 53° 52°30' M M a G a l l i B i g g a a I a Sisimiut q H G Ikertooq ' ' H 0 0 5 5 ° ° F 6 6 6 6 k F talili E qer Qe arsuk erlu Kang ' ' E 0 0 4 4 ° ° 6 6 B 6 6 C A J 10 km D D D ItilleqItilleq ' 0 3 ° 6 6 –436 –409 –398 –377 –352 –327 –305 –283 –266 –245 –222 –202 –185 –168 –150 –131 –110 –89 –68 –46 –26 118 140 164 189 216 247 282 319 365 424 526 –7 11 32 52 73 96 [nT] cross-cut gneiss structures. The main dyke direction is interpretation that prior to intrusion of the Kangâmiut NNE–SSW, and a subordinate direction is E–W to ESE– dykes the area was stabilised in amphibolite-granulite facies WNW (Fig. 1). Upon entering the Itilleq area, the dyke with a variable northerly trend of the foliation (Grocott trends are E–W, parallel to the fjord. This change in trend 1979; Korstgård 1979). At some point prior to dyke in- also corresponds to a change in foliation trend in the host trusion an E–W zone of strong deformation was estab- gneisses. However, the dykes are still largely undeformed lished along Itilleq, downgrading gneisses to low amphi- and unmetamorphosed within this E–W trend. The meta- bolite facies (epidote-muscovite). Within this Itivdleq shear morphic grade of host gneisses north and south of Itilleq zone, dykes intruded along the shear fabrics and show a is granulite facies in western parts and amphibolite facies variety of primary pinch-and-swell structures (Nash 1979). in eastern parts (Fig. 2A). However, all along the E–W Outside the shear zone, dyke margins are straight-sided trend in Itilleq, gneisses are in low amphibolite facies. indicating that dykes intruded along brittle fractures. The dyke behaviour in the Itilleq region led to the 181 Post-dyke metamorphism and Magnetisation deformation Comparing the magnetic anomaly map for the area (Fig. Farther north of Itilleq, from Kangerluarssuk and north- 2B) with the metamorphic map (Fig. 2A) a striking coin- wards (Fig. 2A), dykes are thoroughly deformed and par- cidence of magnetisation and metamorphic boundaries is allel to country rock structures. Both dykes and country evident. More information on the magnetic field data and rock structures are in amphibolite facies. Foliation trends the geological interpretations can be found in Rasmussen are variable ENE–WSW around west-plunging fold axes. & van Gool (2000), Nielsen (2004) and Nielsen & Ras- Continuing northwards the metamorphic grade increas- mussen (2004). es and reaches granulite facies north of Ikertooq fjord (Fig. Strong magnetisation in pre-dyke Archaean granulite 2A). In addition, gneiss structures and metamorphosed facies areas just north of Itilleq (A in Fig.

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