The Migmatite Complex Near Lyngdal, Southern Norway
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THE MIGMATITE COMPLEX NEA R LYNGDAL, SOUTHERN NORWAY, AND RELATED GRA NULITE METAMORPHISM JON STEEN PETERSEN Petersen, J. S.: The migmatite complex near Lyngdal, southem Norway, and related granulite metamorphism. Norsk Geologisk Tidsskrift, Vol. 57, pp. 65- 83. Oslo 1977. Geological mapping in the migmatite complex near Lyngdal has demon strated the significance of major rock units, designated lithostructural forma tions, as principal divisions of the complex. Conformable granitic gneisses and banded gneiss series form the oldest part, in which augen gneisses ap parently developed by partial anatexis and feldspar blastesis prior to deforma tion. Late granitic intrusions finally transgress the polyphase folded complex. The metamorphic environment is high-grade, showing transition between upper amphibolite and granulite facies. Three metamorphic zones are rec ognized where the isogrades Hy-in and Hbl-out indicate increasing meta morphic grade towards south. Mineral assemblages suggest total pressures in the order of 6-7 Kbar and a regional temperature between 700° and 800°C. Jon Steen Petersen, Lttboratorium for Endogen Geologi, Geologisk lnstitut, Aarhus Universitet, 8000 Aarhus C, Denmark. Present address: Mineralogisk-geologisk museum, Sars gt. l, Oslo 5, Norway. Systematic mapping in the Precambrian of southern Norway has confirmed that the migmatite complex of previously undifferentiated gneisses and granites (Barth & Dons 1960) may be subdivided into separate lithological units which often show well defined border relations. However, strong migmatitic infiltration often obscures the spatia! relations between these units, resulting in apparently very complex patterns on the map. The rocks of the Lyngdal area have, as many other basement terrains, lost all 'primary' structures back from a certain stage, due to thorough meta morphic and tectonic transformations. It is therefore obvious that simple lithological analysis of the complex has a limited scope regarding the characters and origin of the initial material. Additionally it is a common experience when mapping in migmatites that variations in lithology are often considerable both along and across the strike of foliation. To perform a structural or metamorphic analysis of such an area it is convenient to divide the complex into larger lithological 'main' units, from which further detailed study may be carried out in selected areas. Secondly, it seems convenient to consider the rock succession as conformable, although this of course does not mean that the 'initial' rocks necessarily had these characters. However, the presence or absence of non-conformities is often the limiting stage which may be reached in the structural evolution of such a gneissic terrain, evidence of earlier events being obscured by metamorphic and tectonic transformations. 66 J. S. PETERSEN Consequently geological mapping of these rocks has promoted the use of major-unit divisions, termed lithostructural formations in the initial geological analysis of the area. The term 'lithostructural formation' is here suggested for a rock unit which is characterized by a certain lithological composition or a characteristic variation and which structurally defines a restricted and continuous level in the rock series. The essential difference between this and 'lithostratigraphical formation' is the lack of lithogenetic unity in the former. In migmatites it is often not possible to indicate with any certainty the early stages of evolution of the rocks, due to lack of primary structures. However, the structural geometry may link the different parts together. At more advanced stages of the analysis it may be convenient to consider the single formation, and subdivisions of this, to reach further conclusions as regards the origin or evolution of the series. The area studied is made up of few lithostructural formations, among which pink gneiss, augen gneiss and banded gneisses are the essential (Fig. 1). However, it should be noted that within a certain formation, lithological variation occurs, but this variation should be a characteristic and integrated part of the formation. When transitional borders are found, the boundary is set on a reasonable estimate, although this should be relatively well defined, in order to make correlation between different parts of a mapped region meaningful. One purpose of this paper is therefore to describe the significance of larger lithostructural units as subdividing elements in the geological analysis of the basement complex. Field Relations Pink gneiss The field name 'pink gneiss' (Falkum 1966) actually covers a range of felsic gneisses, showing similar mineralogy and fairly good foliation, but with varying textures which characterize different types (Fig. 2). A general impression of the pink gneiss formation is that of a very homo genous series of granitic biotite gneisses where transition between several types has been observed. Inclusions are, however, generally rare; two or possibly three narrow horizons of mafic gneisses occur, often strongly migmatized, and difficult to follow over long distances. In the southeastem corner of the map banded gneiss is discordantly cut by pink gneiss, obviouly due to local mobilization of the latter. The texture in this region and along the southem part of the mapped area is characterized by reduced grainsize, and a granular texture causing a very faint foliation. The outcrops of pink gneiss are symmetrically distributed on both sides of the central augengneiss antiform, and can be viewed as the cores of two flanking synforms (Petersen 1973b), (Fig. 1). MIGMATITE COMPLEX NEAR LYNGDAL, S. NORWAY 67 90 95 00 LEGENO LYNG DA L-AREA VEST-AG O ER NORWAY PINK GNEISS FORMATION BANOEO GNEISS FORMATIONS AUGEN GNEISS FORMATION KLEIVAN GRANITE LYNG DAL HBL • GRANITE OIP AND STRIKE OF FOLIATION IREPRE SENT A Tl VEL Y SELECTED l BOUNOARIES IOBSERVEOI BOUNOARIES (INFERREO) AGMATITE MAF IC SCHLIEREN GA-SILL-CO GNE ISS o 90 95 00 Fig. l. Geological map of the Lyngdal area. The reference grid is in accordance with NGO map sheets 1:50,000, with l km spacing. Augengneiss The central part of the area is made up of a medium-grained gneiss-forma tion, designated Leland augengneiss. It is exposed in the core of a large antiform, which appears as a v-shaped body, the result of refolding. The augengneiss is a medium-grained (2-4 mm) reddish gneiss with large alkali-feldspar megacrysts forming 10--30 o/o of the rock. Megacrysts com monly measure 3-4 cm, but single crystals up to 7-9 cm have been observed in the central part. The augengneiss usually appears as a relatively homoge- 68 J. S. PETERSEN Fig. 2. Homogeneous, granitic biotite gneiss, referred to as augen-textured foliated pink gneiss from Dovreheia. neous gneiss or granite, but occasionally the rock shows a distinct layering caused by concentration of megacrysts in zones which alternate with thin melanocratic parts. Elongated mafic inclusions, commonly several hundred metres long, are seen in almost all parts of the augengneiss, becoming pro gressively enriched towards the northeast. These banded gneisses are always thoroughly migmatized. Development of augengneiss, sensu strictu, is con fined to the limbs of the structure; in general planar fabric is indistinct and resembles L=S fabric of Flinn (1965). Megacrysts in the central region are large and rectangular, their cores showing a darker purple colour than the marginal parts (Fig. 3). Pegmatites are frequently found within the formation, as well as numerous aplites which often occur as veins or larger aplo-granitic patches. The con tacts are everywhere conformable to the regional foliation. It is, however, important to note that although pink gneiss normally is the adjacent rock along the 'upper' boundary, without trace of mafic or banded rocks, an im portant exception occurs in the western part (loe. 91-53). Here banded gneiss is seen wedging in between pink and augengneiss. Reconnaisance in the area to the immediate west revealed that the augengneiss here is bordered by an extensive banded gneiss formation and thus suggests the presence of a prekinematic discordance. The culmination of the Audnedal antiform at Konsmo (loe. 03-61) reveals the structurally, lower boundary of the augengneiss, where a grada- MIGMATITE COMPLEX NEAR LYNGDAL, S. NORWAY 69 Fig. 3. Alkali feldspar megacrysts in weakly foliated Leland augen gneiss. tional transition of a very complex nature towards banded migmatitic gneiss is seen. The contact between banded and augengneiss is here observed as a thoroughly migmatized transition zone, where the leucosome contains alkali feldspar megacrysts of apparently identical mesoscopic appearance as those from more homogeneous parts of the augengneiss. Additionally, it is frequently observed that the megacrysts seem to grow into the melanosome portion (paleosome) and occasionally penetrate this. Also these megacrysts have the same appearance as those in pure augengneiss, and thus indicate that at least part of the augen gneiss may have generated by partial anatexis. Banded gneiss Banded gneiss is used as the lithostructural designation of conformable mafic gneisses, showing considerably different characters. Traditionally the term 'banded' has been applied in the literature (e.g. Dietrich 1960, Berthelsen 1960) rather than 'layered', and therefore is also adopted here. The dominating character is a 'banding' of the rock, formed by alternating dark and light layers. This appearance is, however, affected to varying degrees by more