Contrasted Styles of Igneous Layering in the Gardar Province \ of South Greenland \ J

$Contrasted Styles of Igneous Layering in the Gardar Province \ of South Greenland \ J$ \ \ MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963 INTERNATIONAL MINERALOGICAL ASSOCIATION, PAPERS, THIRD GENERAL MEETING I \ CONTRASTED STYLES OF IGNEOUS LAYERING IN THE GARDAR PROVINCE \ OF SOUTH GREENLAND \ J. FERGUSON AND T. C. R. PULVERTAFT Geological Survey of Greenland, Copenhagen, Denmark \ ABSTRACT \ Igneous layering is developed in nearly all the larger Gardar intrusions of south Greenland in rocks varying from gabbro to granite in composition. The syenites, which are the most abundant rocks in these intrusions, display the finest \ examples of layering. Layering comprising olivine-rich bands in more feldspathic gabbro forms synclinal structures in the large (over 200 m thick) gabbro dikes. \ Layering is pronounced in the pyroxene-fayalite syenites of Kungnat and Nunarssuit. Cumulus clinopyroxene and fayalite have been concentrated in layers as a result of gravity sorting under the influence of currents. In the petrographi- cally similar syenite of the Klokken intrusion large-scale mafic banding and concordant lamination form an almost perfect saucer-shaped structure. I Most of the syenites of the Ilimaussaq Intrusion show banding, but the agpaitic rocks are the most striking. The kakortokites are laminated bottom accumulates with color banding due to rhythmic repetition of black, red and white \ bands corresponding to arfvedsonite/aegirine, eudialyte and feldspar concentrations respectively. Complementary banding occurs in the naujaite which formed simultaneously as a flotation cumulate. The residual magma crystallized to form the highly fissile lujavrites, which are locally banded as a result of alternating formation of aegirine and arfvedsonite. Fluctua- tions in pressure are thought to have caused the banding in the agpaites. The nepheline syenites of the Gr91nnedal-Ika complex are characterized by pronounced feldspar lamination, locally accompanied by mineral layering . Mineral layering is locally conspicuous in the Helene Granite where mafic minerals (including fayalite and clinopyroxene) are concentrated into layers as a result of gravity sorting assisted by magmatic currents. INTRODUCTION THE GARDAR PROVINCE Poldervaart and Taubeneck (1960) commented General. Wegmann (1938) established that the that well-layered plutons are relatively uncommon, Precambrian rocks of south Greenland belonged to and that most information concerning layered intru- two main cycles, which he termed Ketilidian and sions has come from mafic bodies. These wholly rea- Gardar. The earlier Ketilidian cycle was one of sedi- sonable remarks serve to emphasize the great interest mentation and tectogenesis. The Gardar period was of south Greenland to students of igneous geology. dominated by igneous activity: it was initiated by Igneous layering is developed in nearly all the larger the deposition of continental sandstones intercalated Gardar intrusions in the area and in rocks ranging with lavas. These strata, despite their age more or from gabbro to granite in composition. It is in the less horizontal, rest with marked unconformity on syenites, however, that the finest examples of layer- Ketilidian basement migmatites in a down-faulted ing are seen. zone north of julianehab. Over a much wider area Detailed mapping of the Gardar intrusions was of south Greenland, between latitudes 60 and 62 begun by the Geological Survey of Greenland in north, numerous dike swarms, predominantly of 1955 and has not yet been completed. This paper is basic composition, cut the basement. The intrusive centers, comprising mostly alkali rocks, are con- a review of what is known at present concerning fined to the Ivigtut- julianehab area: generally layering in these intrusions, special emphasis being speaking, with the exception of the Gr¢nnedal and placed on the Ilimaussaq and Nunarssuit syenites Igaliko complexes, these cut the dike swarms and with which the respective authors are most familiar are therefore late Gardar in age. through their own investigations. It is being pre- Faulting was widespread during the Gardar sented now in order to use the opportunity provided period. The most important faults are those running by the Symposium to draw world-wide attention to approximately east-west. These are partly respon- the remarkable wealth of layered features displayed sible for the present distribution of the sandstones in the Gardar province, even though much detailed and lavas and are believed to have influenced the laboratory work remains to be done. location of the intrusive centers. 10 CONTRASTED IGNEOUS LAYERING 11 Gardar intrusive centers. In order that the setting in which the layered rocks occur should be better under- stood, a brief summary of the Gardar intrusions and their salient features is given, in order from west to east (Fig. 1). The Kungnat Complex (Upton, 1960) consists of three pyroxene-fayalite syenite bodies which were intruded successively in order of increasing basicity with progressive easterly shift of center. Later an FIG. 1. Sketch map showing the position of the main Gardar alkali gabbro ring-dike almost surrounding the igneous centers in south Greenland. 1) Kungnat, 2) Gr91nnedal- Ika, 3) Nunarssuit, 4) Puklen, 5) Tugtutoq, 6) Narssaq, 7) I1i- syenites was emplaced. The complex is 5.4 km long maussaq, 8) Igaliko, 9) Klokken. The outcrop of the Gardar con- by 3.2 km wide. tinental series is stippled. The Gr¢nnedal-Ika Complex (Emeleus, in press) was, prior to faulting, an oval intrusion about 6 km posed in the world. The recent glaciation, during long. It comprises nepheline syenites, commonly which erosion was very much more marked than foyaitic in texture, and later carbonatite. deposition, and the harsh climate, which inhibits The Nunarssuit Complex (Harry and Pulver taft, the growth of vegetation, are responsible for this. 1963) is a very large massif, at least 45 km long The strong relief in the area provides vertical sec- by 25 km wide, dominated by two pyroxene-fayalite tions up to 1700 m in some intrusions. syenite bodies, the Nunarssuit and Kitsigsut syenites, Giant Dikes. Some of the northeasterly Gardar and the Helene Granite which has much in common dikes in south Greenland are of considerable thick- with the Scandinavian rapakivis. Other members of ness (up to a kilometer) and are referred to as giant the complex are gabbro, biotite granite and soda- dikes. The most noteworthy of these occur in granite. Tugtutoq where, although predominantly gabbroic, The Puklen Intrusion (Pulvertaft, 1961), situated they are composite, with the development of nephe- in Nunarssuit, is probably co-magmatic with the line syenite or quartz syenite centers. Anorthosite Nunarssuit Complex, being made up of pyroxene- xenoliths are a conspicuous feature of these dikes. fayalite syenite, quartz syenite and soda-granite. It Northeast of Nunarssuit similar if less persistent is of the same order of size as the Kungnat, Gr¢n- and somewhat thinner dikes have been mapped. In nedal-Ika and Tugtutcq central complexes. Nunarssuit the Eqaloqarfia dike, a 2-300 m thick The Central Complex of Tugtutoq (Upton, 1962) basic dike, connects the Nunarssuit and Puklen comprises some half-dozen ring-intrusions of related intrusions, both of which cut off the dike. syenites and granites followed by a stock of pertho- site syenite. IGNEOUS LAYERING The Narssaq Intrusion lies just west of the Gabbros. In the Alangorssuaq Gabbro of the Nunars- Ilimaussaq Intrusion. The emplacement of quartz suit Complex (Harry and Pulver taft, in press) syenite and pyroxene syenite was followed by alkali banding and concordant lamination dip at moderate granite. angles (between 30 and 50°) towards a deep focus in The Ilimaussaq Intrusion (Ussing, 1911; S¢rensen, the center of the complex, now occupied by pyroxene- 1958; Ferguson, in press) is famous for its remark- fayalite syenite and granite. able development of per-alkaline syenites rich in The mafic layers are usually between one and four chlorine and zirconium, expressed by the high con- em thick, though some measure 30 em in thickness. tent of sodalite and eudialyte respectively. Its evolu- Generally they are between 5 and 30 em apart. tion is outlined in a later section. Within a single exposure most layers, considered The Igaliko Batholith (Ussing, 1911; 0dum, 1927) individually, are of constant thickness. On a small is an enormous mass of nepheline syenites, with a scale also the banding is regular: well-defined "cur- marginal zone of augite syenite, on which work has rent-bedding," slumping and trough banding appear only just commenced. to be absent. Some of the broader layers are almost The Klokken Intrusion is a small oval mass of wholly composed of pyroxene. Local thin lenses of syenogabbro and augite syenite lying close beside the olivine-rock occur concordantly within the layering. Igaliko Batholith. The lamination, which is usually highly pro- The Gardar ·intrusions are amongst the best ex- nounced, is due to the plane parallelism of tabular 12 1. FERGUSON AND T. C. R. PULVERTAFT _ ..... The layering in the Eqaoloqarfia dike defines a synform running parallel to the length of the dyke, 0 with a maximum inward dip of the limbs of 36 • As shown in Fig. 2, the layering is developed in the lower part of a residuum of coarse gabbro in an otherwise normal dolerite dike. Homogeneous dolerite both underlies and forms a border zone to the gabbro. A perpendicular feldspar band, not more than 15 em thick, occurs in some places at the border between the dolerite and gabbro: in this band the plagioclase feldspars have grown at steep angles to the margin of the dolerite. The layering comprises bands rich in olivine set in ordinary coarse gabbro (feldspar tabulae 1-2 em wide). Augite, the remaining important mineral, is 100 m interstitial to olivine and ophitic; it is not concentrated in the bands. The individual bands are regu- E S~~~<]Gabbro ~ Layered gabbro lar and maintain a constant thickness over consider- ~1'~ Dolerite /'" Perpendicular fclspar band able distances. Passing up from the bottom of the layered succession the olivine-rich bands tend to be- FIG.

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