Danish Lithosphere Centre Studies of the Nagssugtoqidian Orogen,West Greenland
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Archaean and Palaeoproterozoic orogenic processes: Danish Lithosphere Centre studies of the Nagssugtoqidian orogen,West Greenland Flemming Mengel, Jeroen A. M. van Gool, Eirik Krogstad and the 1997 field crew The Danish Lithosphere Centre (DLC) was established by Escher et al. (1970, 1975), and was viewed as a in 1994 and one of its principal objectives in the first frontal thrust, across which the southern Nagssugtoqidian five-year funding cycle is the study of Precambrian oro- orogen was translated towards the southern foreland. genic processes. This work initially focused on the ther- Figure 1 shows the main lithotectonic units of the mal and tectonic evolution of the Nagssugtoqidian Nagssugtoqidian orogen (from Marker et al. 1995; van orogen of West Greenland. Gool et al. 1996). The southern foreland consists of During the first two field seasons (1994 and 1995) Archaean granulite facies tonalitic-granodioritic gneisses most efforts were concentrated in the southern and cut by mafic dykes of the Palaeoproterozoic Kangâmiut central portions of the orogen. The 1997 field season swarm. In the SNO (southern Nagssugtoqidian orogen) was the third and final in the project in the Nagssug- gneisses and dykes from the foreland were deformed toqidian orogen and emphasis was placed on the cen- and metamorphosed in amphibolite facies and trans- tral and northern parts of the orogen in order to complete posed into NNW-dipping structures during SSE-directed the lithostructural study of the inner Nordre Strømfjord ductile thrusting. Towards the contact between the SNO area and to investigate the northern margin of the oro- and CNO (central Nagssugtoqidian orogen), the metamor- gen (NNO in Fig. 1). phic grade increases to granulite facies (Korstgård 1979). This report is partly a review of selected research The SNO–CNO boundary is defined by the Ikertooq results obtained since publication of the last Review of steep belt, a regionally pervasive high strain zone which Greenland activities (van Gool et al. 1996), and also also marks the northernmost occurrence of the Kangâmiut partly a summary of field activities in Greenland dur- dyke swarm. The CNO is characterised by a southern ing the summer of 1997. part with a generally steep and penetrative ENE-trend- ing structural grain, whereas the northern CNO comprises alternating steep and shallow-dipping structures (‘steep’ and ‘flat’ belts, respectively). Northwards, the CNO is Background for project delimited by the Nordre Strømfjord steep belt (Bak et The overall aim of DLC’s Nagssugtoqidian project is to al. 1975a, b; Hanmer et al. 1997), a regionally penetra- establish the large-scale geometry of the orogen, to tive zone of steeply dipping gneisses, including zones identify the lithotectonic components and their age and of localised high strain. Palaeoproterozoic calc-alkaline to characterise the dynamic aspects of the evolution, intrusions occur in the north-eastern CNO (c. 1.92 Ga including the structural, metamorphic and magmatic Arfersiorfik quartz diorite; Kalsbeek et al. 1987; Kalsbeek variations. & Nutman 1996) and south-western CNO (c. 1.92 Ga The southern boundary of the orogen (southern Sisimiut charnockite; Kalsbeek & Nutman 1996). The Nagssugtoqidian front, SNF in Fig. 1), was defined by northern Nagssugtoqidian orogen (NNO) is dominated Ramberg (1949) on the basis of progressive structural by variably deformed Archaean granitic to granodioritic reworking of the Palaeoproterozoic Kangâmiut dyke gneisses that include units of supracrustal sequences. swarm. South of the SNF, the dykes are discordant with Both intra- and inter-cratonic settings have been pro- respect to the structures in the Archaean granulite facies posed to explain the development of the Nagssugtoq- gneisses. North of the SNF, the dykes and their coun- idian orogen in terms of a large-scale tectonic model. try rocks are deformed and metamorphosed together. Bridgwater et al. (1973) hinted at an inter-cratonic ori- The geometry of the SNF was first described in detail gin and interpreted the ductile thrusting in the south- 100 Geology of Greenland Survey Bulletin 180, 100–110 (1998) © GEUS, 1998 Fig. 1. Lithotectonic sketch map of the 52° 50° Nagssugtoqidian orogen and its southern 50 km and northern forelands. This figure includes information from Escher (1971), Ilulissat Allaart (1982), Marker et al. (1995) and Greenland van Gool et al. (1996). NNO, CNO and Jakobshavn Isfjord SNO are the northern, central and southern Disko Bugt 69° Nagssugtoqidian orogen, respectively. SNF: southern Nagssugtoqidian front. Qasigiannguit Shading on insert map shows regional Aasiaat Inland Ice extent of the Nassugtoqidian orogen across Greenland. Lersletten Nordre Strømfjord steep belt O n CN rther ° No elt" 68 "flat b 68° Attu Ussuit rtoq re Iso NNO rd No belt steep Nordre Strømfjord Ikertooq steep belt Nordre Isortoq CNO ° 67 67° Sisimiut Ikertooq d r F jo f N Itilleq m rø S N F St e dr øn SNO S Palaeoproterozoic Sukkertoppen Syntectonic granite suite Iskappe Arfersiorfik quartz diorite Sisimiut charnockite 66° Kangâmiut dyke swarm Archaean Kangaamiut Archaean gneisses s.l. Southern Early Archaean gneisses Archaean Undifferentiated foreland Supracrustal rocks Maniitsoq Mafic dykes Thrust 52° High strain zones ern margin as indicative of a collisional origin. They had, part of the orogen led Kalsbeek et al. (1987) to revive however, insufficient data to constrain the extent of the the intercratonic model. The AQD is a calc-alkaline postulated oceanic basin between the two continents. intrusion with geochemical and isotopic signatures sim- Later workers, however, focused on the large-scale trans- ilar to subduction-generated arc magmas. Based on this current structures in the orogen, and favoured intra- data, Kalsbeek et al. (1987) suggested that the central cratonic settings to explain the geometries (e.g. Bak et part of the Nagssugtoqidian orogen could contain a al. 1975a, b). Geochemical and isotopic investigations suture as a result of convergence, subduction and col- of the Arfersiorfik quartz diorite (AQD) in the central lision between two Archaean cratonic blocks. 101 Fig. 2. Archaean basement gneisses from western Ussuit (Fig. 1). The older, mafic gneisses occur as disrupted layers in a dominantly tonalitic-granodioritic granitoid gneiss. In the foreground, a granitic pegmatite cuts both units. Notebook for scale. Recent work by DLC in the Nagssugtoqidian orogen 2. To establish the three-dimensional geometry of struc- has partly been aimed at testing the continent–conti- tures in the inner Ussuit area (Fig. 1) in order to (1) nent collision model and has been concentrated in the test the thrust stack model for the interleaving of central and southern parts of the orogen. The work has Archaean and Proterozoic gneisses (van Gool et al. involved an investigation of the AQD and its host-rocks in press); and also (2) to understand the dynamic rela- in more detail, but also a characterisation of the two tionships between ‘steep’ and ‘flat’ belts in the cen- ‘continents’ in order to verify that they represent dis- tral part of the orogen; tinct blocks that were unrelated prior to collision (sum- marised in Marker et al. 1995; van Gool et al. 1996). 3. To further investigate the compositional, geochrono- The 1997 field season focused on the central and logical and structural characteristics of the pre- northern parts of the Nagssugtoqidian orogen. The Nagssugtoqidian basement, both internal and external northern margin is poorly defined at present. There is to the orogen, in order to address these questions; no well-defined structural front similar to the southern margin, hence the overall geometry of the orogen is a. Can a northern and southern ‘continent’ be iden- asymmetric. The northward extent of Nagssugtoqidian tified? metamorphism is not known at present. Kalsbeek et al. b. What are the linkages between Archaean units (1987) reported undisturbed Rb-Sr and Pb-Pb isotope within the orogen and Archaean gneisses in the systems at Akulleq (25 km south-west of Aasiaat) and forelands, i.e. where is the suture? Aasiaat, respectively, whereas geochronological data c. What is the control of pre-Nagssugtoqidian struc- from 15 km south-east of Aasiaat (Connelly & Mengel tures on the Nagssugtoqidian structural develop- 1996) suggest minor thermal overprints of Palaeoprotero- ment? zoic age at this latitude (Figs 1, 2). With the above as background and framework, the main aims of the 1997 field season were: Archaean basement rocks in the Nag- ssugtoqidian orogen and its forelands 1. To characterise the northern margin of the Nagssug- Based on detailed field observations, a picture is emerg- toqidian orogen and to collect data and samples ing of a basement gneiss complex comprising at least necessary for outlining the northward extent of Palaeo- three components that can be recognised throughout proterozoic magmatism(?) and structural and meta- the orogen and within its marginal zones. morphic overprints; The oldest, and volumetrically least significant com- ponents are complex, polydeformed felsic to mafic 102 Fig. 3. Archaean basement gneisses from Kangersuneq (15 km south-east of Qasigiannguit; Fig. 1). The tonalitic- granodioritic component contains small, irregular inclusions of older, structurally more complex, mafic gneisses. Hammer for scale. Fig. 4. Archaean basement gneisses from Orpissooq (20 km south-south-east of Qasigiannguit; Fig. 1). In this outcrop, the youngest, granitic component is dominant. The granite, locally