Geological Survey of Denmark and Greenland Bulletin 11, 179-184 179 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 fab- rics 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 metamor- phic grade is amphibolite facies increasing to granulite facies at the northern boundary of the south- ern 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: john.korstgard@geo.au.dk 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- qidian orogen in West Greenland (Ramberg 1949) is based on the deformation and metamorphism of the Kangâmiut dykes, dated at 2.04 Ga by Nutman et al. (1999). South of the southern Nagssugtoqidian front (SNF in Fig. 1), in the southern Nagssugtoqidian foreland, Kangâmiut dykes are undeformed and cross-cut gneiss structures. North of the front, gneisses and dykes have been metamorphosed and deformed together during the Nagssugtoqidian oro- geny. Here, gneiss structures and dyke margins are con- cordant and dykes transformed into amphibolites. This is the simple story upon which Ramberg (1949) based his definition of the ‘Nagssugtoqides’. Ramberg also divided the Nagssugtoqidian orogen into three metamorphic com- plexes based on the metamorphic grade of the rocks. Thus the Egedesminde complex was the northernmost amphi- bolite facies complex, the Isortoq complex the central gran- ulite facies complex, and the Ikertôq complex the south- ernmost amphibolite facies complex. The current division of the orogen (Fig. 1) is based on structural criteria, and division boundaries now follow major structural features (Marker et al. 1995). The current division therefore devi- ates considerably from Ramberg’s original division for the 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 180 50 km Inland Ice ? ? SNF CNO NNO AasiaatAasiaat KangerlussuaqKangerlussuaq SNO Nordre Strømfjord shear zone Nordre Isortoq steep belt southern Nagssugtoqidian foreland Sø nd re Str øm fjo rd Nordre Strømfjo rd Aasiaat Kangerlussuaq Nordre Strømfjo rd Arfersiorfik Fig. 3 Fig. 2 Sisimiut Ikertôq thrust zone Itivdleq shear zone 54° 67° 68° 51° Surficial deposits Quaternary North Atlantic craton Granodioritic-granitic gneiss (northern parts reworked) Orthogneiss (largely unreworked) Metasedimentary rocks (Palaeoproterozoic, may include Archaean components) Nagssugtoqidian orogen Sisimiut charnockite (Palaeoproterozoic) Arfersiorfik quartz diorite (Palaeoproterozoic) Orthogneiss (Archaean, reworked) Metasedimentary rocks (Archaean, may include Proterozoic components) Amphibolite Anorthosite and ultrabasic rocks 500 km Greenland Fig. 1. Schematic geology of the southern part of the Nagssugtoqidian orogen and adjacent forelands (modified from Escher & Pulvertaft 1995 and Marker et al. 1995). SNO, southern Nagssugtoqidian orogen; CNO, central Nagssugtoqidian orogen; NNO, northern Nagssugtoqidian orogen; SNF, southern Nagssugtoqidian front. The locations of thrust and shear zones are defined from trends observed in the aeromagnetic data; note that the E–W- trending thrust zone with question marks north of Kangerlussuaq is uncertain, as this structure has not been confirmed by geological mapping. Black frames show the locations of Figs 2, 3. southern Nagssugtoqidian orogen corresponds almost ex- actly to Ramberg’s original Ikertôq complex. The southern Nagssugtoqidian orogen (SNO in Fig. 1) in the coastal region between Sisimiut and Itilleq con- sists mainly of quartzofeldspathic gneisses of granodiorit- ic to tonalitic composition. Several supracrustal layers occur, particularly in the northern part of the SNO. The supracrustal rocks are mainly garnet-biotite schists, rusty weathering biotite gneisses and amphibolites. The meta- morphic grade is low amphibolite facies to granulite facies, and due to the fortunate timing of the intrusion of the Kangâmiut dykes it is possible to assign relative ages to the different metamorphic terrains in the region. Pre-dyke metamorphism and deformation South of and immediately north of Itilleq, the Kangâmiut dykes are largely undeformed, unmetamorphosed and 181 [nT] 526 424 365 319 282 247 216 189 164 140 1189673523211–7 –26 –46 –68 –89 –110 –131 –150 –168 –185 –202 –222 –245 –266 –283 –305 –327 –352 –377 –398 –409 –436 52°30'52°30'53°30'53°30' 66 °5 0' 66 °5 0' 66 °4 0' 66 °4 0' 66 °3 0' 53°53° A B C D DD E E F F G G H H I Sisimiut 10 km Qeq erta lik Qeq erta lik ItilleqItilleq KKaanngg eerrlluuaa rrssuukk IIkkeerrttooooqq MM aalliiggaaaaqq J A B 52°30'53°30' 66 °5 0' 66 °4 0' 66 °3 0' 53° A B C D D D E E F F G G H H ISisimiut 10 km Qeq erta lik Itilleq Kang erlua rsuk Ikertooq M aligaaq Post-dyke (Nagssugtoqidian) granulite facies Post-dyke (Nagssugtoqidian) amphibolite facies Pre-dyke (Archaean) granulite facies Pre-dyke (Archaean) amphibolite facies Pre-dyke amphibolite facies J Fig. 2. Correlation between metamorphic facies and aeromagnetic anomaly patterns in the Itilleq–Ikertooq region. White lines indicate approximate metamorphic facies boundaries based on geological field work; labels A–J are explained in the text. A: Distribution and relative ages of metamorphic facies. B: Total intensity magnetic field anomaly map. Shadow of magnetic field pattern modelled from a light source with inclination 45° and declination 315°. cross-cut gneiss structures. The main dyke direction is NNE–SSW, and a subordinate direction is E–W to ESE– WNW (Fig. 1). Upon entering the Itilleq area, the dyke trends are E–W, parallel to the fjord. This change in trend also corresponds to a change in foliation trend in the host gneisses. However, the dykes are still largely undeformed and unmetamorphosed within this E–W trend. The meta- morphic grade of host gneisses north and south of Itilleq is granulite facies in western parts and amphibolite facies in eastern parts (Fig. 2A). However, all along the E–W trend in Itilleq, gneisses are in low amphibolite facies. The dyke behaviour in the Itilleq region led to the interpretation that prior to intrusion of the Kangâmiut dykes the area was stabilised in amphibolite-granulite facies with a variable northerly trend of the foliation (Grocott 1979; Korstgård 1979). At some point prior to dyke in- trusion an E–W zone of strong deformation was estab- lished along Itilleq, downgrading gneisses to low amphi- bolite facies (epidote-muscovite). Within this Itivdleq shear zone, dykes intruded along the shear fabrics and show a variety of primary pinch-and-swell structures (Nash 1979). Outside the shear zone, dyke margins are straight-sided indicating that dykes intruded along brittle fractures. 182 Post-dyke metamorphism and deformation Farther north of Itilleq, from Kangerluarssuk and north- wards (Fig. 2A), dykes are thoroughly deformed and par- allel to country rock structures. Both dykes and country rock structures are in amphibolite facies. Foliation trends are variable ENE–WSW around west-plunging fold axes. Continuing northwards the metamorphic grade increas- es and reaches granulite facies north of Ikertooq fjord (Fig. 2A). In addition, gneiss structures and metamorphosed dykes take on a pervasive E–W orientation (Ikertôq thrust zone, Fig. 1) with a steeply N-dipping foliation and N- plunging stretching lineations. The interpretation of field observations in the north- ern SNO is that the metamorphism and deformation are post-dyke, the metamorphic transition is prograde, and the Ikertôq thrust zone represents a zone of southward ductile thrusting whereby deeper-seated rocks are brought up from the north. Facies transitions Within the Itilleq–Ikertooq region four types of facies tran- sitions or boundaries are recognised. Two of these are pro- grade and two are associated with strong deformation in ductile shear zones. The amphibolite–granulite facies transition in the Ar- chaean areas around Itilleq is prograde and static in the sense that the boundary was not established as a result of a deformational event, but reflects static equilibration of the mineral assemblages to the conditions that prevailed when the rocks were at their deepest crustal level. During later uplift the rocks escaped any significant metamor- phic changes due to the absence of deformation, and the metamorphism reflects their initial Archaean state. The granulite to low amphibolite facies and amphibo- lite to low amphibolite facies transitions along Itilleq are retrograde and dynamic in the sense that they were estab- lished as a direct consequence of the deformation along the Itivdleq shear zone. Mineral assemblages in the shear zone were equilibrated to the metamorphic conditions of a higher crustal level than reflected in the surrounding gneisses, and the shearing triggered this re-equilibration. The amphibolite–granulite facies transition north of Ikertooq is both prograde and dynamic. It can be consid- ered as a displaced prograde and static transition brought up into a sub-vertical position by the overthrust move- ment along the Ikertôq thrust zone (Fig. 1). Magnetisation Comparing the magnetic anomaly map for the area (Fig. 2B) with the metamorphic map (Fig. 2A) a striking coin- cidence of magnetisation and metamorphic boundaries is evident. More information on the magnetic field data and the geological interpretations can be found in Rasmussen & van Gool (2000), Nielsen (2004) and Nielsen & Ras- mussen (2004). Strong magnetisation in pre-dyke Archaean granulite facies areas just north of Itilleq (A in Fig. 2B) is attributed to a higher content of magnetite or other magnetic min- erals. A likely explanation for this is production of mag- netite by the breakdown of hydrous (Fe, Mg)-Al-silicates (e.g. biotite, amphibole) during the transition from am- phibolites to granulite facies according to the general re- action: hydrous (Fe, Mg)-Al-silicates ± SiO 2 ± O 2 = K- feldspar + (Fe, Mg)-silicates ± magnetite + H 2 O. The low- er magnetisation in pre-dyke Archaean amphibolite facies areas (B in Fig. 2B) relative to pre-dyke Archaean granu- lite facies areas indicates no additional production of mag- netite. The gradual increase in magnetic intensity (C in Fig. 2B) marks the gradual prograde facies transition. The elongate low magnetic anomaly coincident with the Itivdleq shear zone (D in Fig. 2B) is caused by exten- sive breakdown of magnetic minerals. This may be due to chemical breakdown during metamorphic retrogression to pre-dyke amphibolite facies aided by circulating fluids in the shear zone, and mechanical destruction of the mag- netic mineral grains. The abrupt changes in anomaly pat- terns from D to A (Fig. 2B) across the metamorphic facies transition and deformation boundary are a response to the dynamic nature of this boundary. Previously suggested possible shearing south of Iker- tooq (E in Fig. 2B; Grocott 1979; Korstgård 1979) con- temporaneous with the shearing at Itilleq (D in Fig. 2B) is supported by similarities in the character of the anom- aly patterns. The post-dyke amphibolite facies areas at, and south of, Ikertooq (F in Fig. 2B) indicate the Palaeo- proterozoic retrogression to amphibolite facies and defor- mational reworking. The boundary between the pre-dyke Archaean amphibolite facies and the post-dyke amphibo- lite facies areas does not have a well-defined magnetic sig- nature (between B and F in Fig. 2B). The increase in magnetisation north of Ikertooq (G in Fig. 2B) corresponds to rocks metamorphosed under gran- ulite facies conditions after dyke intrusion and brought up by overthrusting. The offset between the mapped facies boundary north of Ikertooq (Fig. 2A) and the boundary between high and low magnetisation (H in Fig. 2B) can be explained as partially due to non-exposed post-dyke 183 granulite facies rocks, and partially to the effect of stacked thrust panels of post-dyke amphibolite and granulite facies rocks with alternating low and high magnetic intensity anomalies (I in Fig. 2B). Isolated high intensity anoma- lies can be correlated with distinct lithologies or intru- sives (e.g. an anorthosite complex at J in Fig. 2B). The presence or absence of Kangâmiut dykes is not reflected in the aeromagnetic data. The observed correlations between metamorphic facies, deformation and magnetisation can be extended to other areas of the SNO (Fig. 3) provided that the background gneisses are lithologically fairly homogeneous, as is gener- ally the case in the southern Nagssugtoqidian orogen. Where gneiss lithologies are more variable, such as in the Nordre Isortoq steep belt (Fig. 1) and the Nordre Strøm- fjord shear zone (Sørensen et al. 2006, this volume) corre- lations tend to depend on lithology rather than metamor- phic grade. Acknowledgements The authors thank Graham Leslie and Chris Pulvertaft for their concise and constructive reviews. References Escher, J.C. & Pulvertaft, T.C.R. 1995: Geological map of Greenland, 1:2 500 000. Copenhagen: Geological Survey of Greenland. Grocott, J. 1979: Controls of metamorphic grade in shear belts. In: Korstgård, J.A. (ed.): Nagssugtoqidian geology. Rapport Grønlands Geologiske Undersøgelse 89, 47–62. Korstgård, J.A. (ed.) 1979: Nagssugtoqidian geology. Rapport Grøn- lands Geologiske Undersøgelse 89, 146 pp. Marker, M., Mengel, F., van Gool, J. & field party 1995: Evolution of the Palaeoproterozoic Nagssugtoqidian orogen: DLC investigations in West Greenland. Rapport Grønlands Geologiske Undersøgelse 165, 100–105. Nash, D. 1979: An interpretation of irregular dyke forms in the Itivdleq shear zone, West Greenland. In: Korstgård, J.A. (ed.): Nagssugto- qidian geology. Rapport Grønlands Geologiske Undersøgelse 89, 77–83. Nielsen, B.M. 2004: Crustal architecture and spatial distribution of mineral occurrences in the Precambrian shield of central West Green- land based on geophysical and geological data. Danmarks og Grøn- lands Geologiske Undersøgelse Rapport 2004/26, 63 pp., 8 appen- dices. Ph.D. thesis 2004. Department of Earth Sciences, University of Aarhus, Denmark. Nielsen, B.M. & Rasmussen, T.M. 2004: Mineral resources of the Pre- cambrian shield of central West Greenland (66° to 70°15′N). Part Fig. 3. Total intensity magnetic field anomaly map of the south-eastern part of the Nagssugtoqidian orogen and its foreland, with the location of the Itilleq–Ikertooq region (white frame, Fig. 2). Abbreviations as for Fig. 1; shadow on magnetic data as for Fig. 2. The E–W-trending thrust zone with question marks north of Kangerlussuaq is uncertain, as this structure has not been confirmed by geological mapping. SN F Southern Nagssugtoqidian foreland (North Atlantic craton) N ag ss ug to qi di an o ro ge n No rdr e Is ort oq ste ep bel t sou the rn CN O Iker tôq thr ust zo ne Itiv dle q she ar z one C N O SN O 50°50°52°52° 66 °4 0' 66 °4 0' 67 °0 5' 67 °0 5' 66 °1 5' 66 °1 5' Sø nd re Str øm fjo rd Sø nd re Str øm fjo rd SisimiutSisimiut KangerlussuaqKangerlussuaq ItilleqItilleq Ikerto oq Ikerto oq QeqertalikQeqertalik KangerluarsukKangerluarsuk Fig. 2 53°53° [nT] ?? 50 km 536 407 334 284 243 209 178 151 125 76 54 32 11 –9 –30 –50 –72 –91 –110 –127 –145 –161 –176 –190 –204 –219 –234 –264 –282 –303 –326 –354 –386 –416 –461 –535 184 3. Implications of potential field data for the tectonic framework. 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