Reassessment of the north-western border zone of the Palaeoproterozoic Ketilidian orogen, South Greenland

Abstract

As part of ongoing research into the plate tectonic setting of the Palaeoproterozoic Ketilidian orogen led by the Geological Survey of Greenland and Denmark, four geologists from Denmark and the U.K. re-examined parts of the north-western border zone in July–August 1997. The field work was generously supported by the Danish Natural Science Research Council and the Carlsberg Foundation. One team studied the Proterozoic (Ketilidian) sedimentary and volcanic rocks and the regional structure, working from six inland camps along the variably deformed Archaean–Proterozoic unconformity between Midternæs and Qoornoq and on Arsuk Ø (Fig. 1). A second team investigated the plutonic and kinematic evolution of the Kobberminebugt area at the northwestern margin of the Julianehåb batholith (Fig. 1); the latter forms the central part of the Ketilidian orogen (Chadwick & Garde 1996). In addition, samples of volcanic and granitic rocks were collected for geochemical studies and dating of depositional and tectonic events. The first systematic study of the Ketilidian orogen took place in the 1960s and was largely concentrated in its western and southern parts (Allaart 1976). Essential new data from the central and eastern parts of the orogen were acquired during the Survey’s SUPRASYD project (1992–1996; e.g. Garde & Schønwandt 1994, 1995; Garde et al. 1997; Stendal et al. 1997), which was initiated with the aim of assessing the potential for mineral resources in supracrustal sequences (Nielsen et al. 1993). In the course of the SUPRASYD project a new plate-tectonic model for the entire orogen was also published (Chadwick & Garde 1996), in which the orogen is viewed as the result of oblique convergence between the Archaean craton of southern Greenland and a supposed oceanic plate located south of the present orogen, which was subducted towards the north. Chadwick & Garde (1996) also suggested a new division of the Ketilidian orogen into a ‘Border Zone’ adjacent to the Archaean craton, the ‘Julianehåb batholith’ (formerly the Julianehåb granite) in the central part of the orogen, and the ‘Psammite Zone’ and ‘Pelite Zone’ to the south-east, which largely consist of deformed and metamorphosed erosion products derived from the evolving batholith. The north-western border zone and the Ketilidian supracrustal sequences were mapped in the 1960s by Harry & Oen Ing Soen (1964), Watterson (1965), Bondesen (1970), Higgins (1970), Muller (1974), Berthelsen & Henriksen (1975) and Pulvertaft (1977). It was shown that an Archaean gneiss complex (part of the Archaean craton of southern West Greenland) and Palaeoproterozoic basic igneous rocks (the so-called ‘MD’ (metadolerite) dyke swarms and related intrusions), and the unconformably overlying Ketilidian supracrustal rocks are progressively affected by Ketilidian deformation and metamorphism towards the Julianehåb batholith in the south and south-east. Boundary relationships were reviewed by Henriksen (1969). Where the Ketilidian supracrustal rocks are best preserved at Midternæs and Grænseland, Bondesen (1970) and Higgins (1970) divided them into the Vallen Group, which largely consists of sedimentary rocks (Table 1), and the overlying Sortis Group, in which basic pillow lavas and related doleritic to gabbroic sills predominate. Supracrustal rocks of presumed Palaeoproterozoic age further south have previously been referred to as the Qipisarqo and Ilordleq Groups (Berthelsen & NoeNygaard 1965; Allaart et al. 1969). Based on data collected in the 1960s, the earliest plate-tectonic interpretation of the Ketilidian orogen included a prominent suture in Kobberminebugt (Windley 1991). However, other critical aspects of Windley’s model were not substantiated during the Survey’s studies in 1992–1996 (Chadwick et al. 1994; Chadwick & Garde 1996), and a re-evaluation of the north-west border zone was therefore a natural focus of subsequent investigations.