Pre-Caledonian Metamorphic Complexes Within the Southern Part of the East Greenland Caledonides

Pre-Caledonian metamorphic complexes within the southern part of the East Greenland Caledonides A. K. HIGGINS SUMMARY Isotopic dating and recent regional mapping tions of the Scoresby Sund region are extended have led to reappraisal of conventional inter- northwards into the classic areas of the central pretations of the southern part of the East metamorphic complex where the widely accept- Greenland Caledonides. The metamorphic ed concept of a deep-seated Caledonian orogeny complexes of the Scoresby Sund region pre- was developed. A similar development to that serve evidence of Archaean and Proterozoic of the Scoresby Sund region seems probable, orogenic episodes, and high grade metasedi- though statements should be treated with mentary sequences thought to be late Pre- reservation until new field work and isotopic cambrian Caledonian geosynclinal rocks have studies have been completed. Brief mention is yielded middle Proterozoic isotopic ages. The made of parallel developments in the Cale- intensity of the Caledonian orogeny is much donian fold belts of northern Europe. less than formerly imagined. The interpreta- I. Main elements of the Caledonian fold belt THE CALEDONIAN fold belt of East Greenland extends from latitudes 7°0 to 82°N, occupying most of the wide strip of land between the coast and the inland ice (Fig. i). Metamorphic complexes dominate the fold belt and consist of a variety of gneisses, migmatites, granites and metasediments. In the southern part of the fold belt from latitudes 73° to 74°N the metamorphic complexes are flanked to the east, and locally to the west, by generally non-metamorphic sequences of the late Precambrian Eleonore Bay Group (Figs. I, 3, P1. I). The lower part of the up to 14 ooo m thick Eleonore Bay Group succession consists of arenaceous and argil- laceous sediments, while the upper part includes spectacular limestone and dolo- mite units (P1. i). On the east side of the metamorphic complexes the late Pre- cambrian sequence is succeeded by the Tillite Group (3oo-I 300 m) and a largely carbonate Cambro-Ordovician sequence (c. 3000 m). These sediments exhibit for the most part a simple style of Caledonian folding and faulting. Upper Palaeozoic molasse deposits occupy large areas especially between latitudes 720 and 74°N, and narrower outcrops occur further south; their accumu- lation was disturbed by several minor phases of late Caledonian deformation. Mesozoic marine sediments succeed them, and outcrops are found along parts of the present outer coastal region from latitude 76°N southwards as far as the broad sedimentary basin of Jameson Land (Fig. I). Tertiary basalts conceal the continua- tion of the Caledonian fold belt south of latitude 7o°N. The western edge of the East Greenland Caledonides follows roughly the line of the innermost nunataks. Foreland windows believed to represent intact parts of the Greenland shield occur beneath eastward dipping major thrusts in western Jl geol. Soc. Lond. vol. x32 1976, pp. 289-305, 3 figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/289/4896931/gsjgs.132.3.0289.pdf by guest on 02 October 2021 29 o .4. K. Higgins -81 ° "" ~i , :.... ! 1 . I 0 50 100 km , ,,.°° ~.°.'* : ~..~:..:.~ -78 ° .., ~ o.... :°' .,a Caledonian Dronning fold belt Louise La / Thrust Fault -75" I ! Quatemary, ice I Tertiary baselts (& alkali intrusions) Jurassic- Cretaceous Permian - Trias (mainly marine) Devonian - Permian (molasse - type sediments) Cambrian - Ordovician •.~':.".t<.~ .=' (& Silurian in NE Greenland) Charcotf Eleonore Bay Group & Tillite Group Land ~.~ I in central East Greenland. Hagen Fjord Group in North-East Greenland Pre- Carolinidian sediments in NE Greenland Liverpool Metamorphic complexes (fold belt) Land Metamorphic complexes (foreland) Sund Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/289/4896931/gsjgs.132.3.0289.pdf by guest on 02 October 2021 Pre-Caledonian metamorphic complexes 291 G~.seland-Paul Stern Land, Charcot Land, Dronning Louise Land and Kronprins Christian Land (Fig. x). 2. History of research The East Greenland Caledonian fold belt is reasonably well known despite the difficulties of access, due largely to the expeditions led by Lauge Koch between 1926 and I958. The region between latitudes 720 and 76°N is covered by i:25oooo geological maps (Koch &Haller I97t ). The most comprehensive accounts of the fold belt are those of Hailer (t97o , I97t ). The western broad region of crystalline rocks between latitudes 7°0 and 74°N has become known as the central metamorphic complex. This complex has played a significant role in the development of ideas concerning the genesis of the fold belt. It was considered by some early workers to be an essentially Archaean massif (e.g. Parkinson & Whittard I93I , Teichert t933, Odell i944). Others, however, claimed that the stratigraphy of the late Precambrian Eleonore Bay Group could be traced into and throughout the central metamorphic complex and that it must therefore be a manifestation of Caledonian granitization and migmatization (e.g. Backlund I93O, Wegmann I935). Detailed work by members of Koch's expeditions seemed to support the latter interpretation (Hailer I955, i958 , Wenk & Hailer i953) , and the central metamorphic complex was thus widely regarded as the deep-seated mobile infrastructure of the Caledonian fold belt. High grade metasedimentary rocks within the complex have been viewed as transformed parts of the late Precambrian succession. While substantial areas of former Archaean rocks were recognized in southern parts of the central metamorphic complex, they were considered to have undergone petrogenic rejuvenation during the Caledonian orogeny (Hailer & Kulp i962 , HaUer i97i ). In the period 1968-72 the Geological Survey of Greenland undertook systematic mapping in the Scoresby Sund region (7o°-72°N), hitherto one of the least known parts of the fold belt. At first results appeared to conform with the traditional view of Caledonian orogenesis. However, as work progressed and isotopic ages became known in increasing numbers, it became clear that the metamorphic complexes of the Scoresby Sund region retain many characteristics of a complex pre-Cale- donian history. 3. Geology of the Scoresby Sund region Crystalline rocks outcrop in the western half of this region (7o°-72°N) and in the narrow strip of Liverpool Land (Fig. I). The late Palaeozoic and Mesozoic rocks of the Jameson Land basin are described by Birkelund & Perch-Nielsen (in press). The western broad area of crystalline rocks represents the southern half of the FIO. I elements of" the Caledonian fold belt in East Greenland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/289/4896931/gsjgs.132.3.0289.pdf by guest on 02 October 2021 292 A. K. Higgins central metamorphic complex, and it can be divided into several zones of dis- tinctive character limited by major thrusts or faults (Fig. 2). In the extreme west several windows cut through flat-lying thrust sheets expose outcrops of presumed foreland. The thrust sheets themselves are made up of infracrustal and supra- crustal rock units which characterize a zone running from south of Vestfjord to HinEs Land. An east dipping N-S trending thrust separates this zone from a very different zone to the east, dominated by migmatitic and granitic rocks and trace- able from G~sefjord through Milne Land and Renland to the Stauning Alper. Although late Precambrian and Lower Palaeozoic rocks are widespread between latitudes 720 and 74°N, in the Scoresby Sund region they are restricted to an isolated development in Canning Land (Fig. 3). (A) FOR ,LANV WINDOWS Two major windows through the presumed Caledonian thrusts occur, one in extreme western G~seland and Paul Stern Land, and the second around Charcot Land. The rocks exposed in the windows are assumed to represent parts of the Greenland shield. Within the G~seland--Paul Stern Land window a basement of hornblende- biotite gneisses and augen gneisses cut by discordant amphibolite dykes is uncon- formably overlain by a thin sequence of weakly metamorphosed sediments(Wenk i96i ). Mineral ages of 189o Ma (K-At on biotite) and 2290 Ma (Rb-Sr on micro- cline) were obtained on the basement rocks by HaUer & Kulp (1962). Local psephitic or tillite developments occur at the base of the sedimentary sequence, which mainly comprises cream-coloured marbles (up to 300 m thick). Overlying sericidc quartzites and chloritic schists referred to the sedimentary sequence by Wenk (I96X) have been interpreted as highly tectonised basement gneisses by Phillips ¢t al. (I 973). The age of the sediments is uncertain; Wenk considered that they represented the basal part of the Eleonore Bay Group, whereas Phillips et al. have speculated that the psephitic developments might correlate with the Tillite Group (Eocambrian) and the marble sequence with the Cambro-Ordovician carbonate succession. Most of Charcot Land, a strip of western HinES Land and some nunataEs make up the Charcot Land window, preserved beneath a major arcuate thrust which has a westward displacement of at least 4 ° km. The rock units include an infracrustal complex of banded gneisses, augen gneisses and amphibolites, overlain by at least 2000 m ofsupracrustal rocks including marbles, quartzites, semipelitic and pelitic rocks, and basic intrusives and extrusives (Steck 1971). Two late-post- kinematic intrusions are largely emplaced in the infracrustal complex; one of these, a pegmatitic muscovite granite, has yielded mineral ages of 162o-x 870 Ma as well PLATE I Steep eastward dipping strata of the upper Eleonore Bay Group in the northernmost Stauning Alper, showing a rather simple style of deformation. Dips decrease north of the fjord. Minor N.-S. trending faults are common. The boundary with the metamorphic complex is situated in the fjord just west of the left edge of the photograph. Route 653 G-NV no. o95x6, copyright Geodetic Institute, Copenhagen. Reproduced by permission A/649/72.

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