The Intrusive Granites of the Farsund Area, South Norway: Their Interrelations and Relations with the Precambrian Metamorphic Envelope
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The intrusive granites of the Farsund area, south Norway: Their interrelations and relations with the Precambrian metamorphic envelope TORGEIR FALKUM, J. RICHARD WILSON, JON STEEN PETERSEN & HANS DIETER ZIMMERMANN Falkum, T., Wilson, J. R., Petersen, J. S. & Zimmermann, H. D.: The intrusive granitesof the Farsund area, south Norway: Their interrelations and relations with the Precambrian metamorphic envelope. Norsk Geologisk Tidsskrift, Vol. 59, pp. 125-139. Oslo 1979. ISSN 0029- l%X. A new geological map, covering about 1500 km' of the area around Farsund, south Norway, is presented. The country rock gneisses have been subdivided into three major lithostructural units consisting of augen, banded, and granitic gneisses. The intrusive 'farsundite complex' is divided into several distinct bodies and their contact relations are described. The plutons fall mineralogically, texturally, and chemically into two groups: l. The Farsund charnockite and the strongly differentiated Kleivan granite which show anorthositic affinites. 2. The mineralogically more normal granites such as the Lyngdal hornblende granite. This paper refutes earlier interpretations considering the 'farsundite complex' as one single unit in terms of structure and. composition. T. Falkum, J. R. Wilson, J. S. Petersen & H. D. Zimmermann, Laboratoriet for Endogen Geologi, Geologisk Institut, Universitetsparken, DK-8000Aarhus C, Denmark. This paper presents a study of the field relations Earlier investigations are contradictory as re of the intrusive rocks around the town of gards the field relations of the intrusive rocks of Farsund in southern Norway. The results are the Farsund area which have been interpreted as based on geological mapping from 1965-1975 comagmatic, and the country rocks which have using aerial photographs on the scale of about been considered grossly conformable with the 1:15,000 together with topographic maps. The contacts of the plutons. Because of the pre field work has been supplemented by petrogra sumed cogenetic origin of the intrusive rocks phic, geophysical, and geochemical studies as they have been grouped together under the term well as radiometric age determinations. These 'farsundite' or 'farsundite complex' and have investigations allow the distinction of a series of been described as forming a single star-shaped independent intrusive bodies (Fig. l, A, B). intrusion. The largest is the Lyngdal homblende granite, We have re-investigated the area in detail in an an elongate E-W body with a major branch attempt to clarify the field relations as a basis for extending to the north. This northerly branch is petrological interpretations. Our studies show separated from the main body by Lyngdals that the 'farsundite body with five limbs' (Mid fjorden. The Farsund charnockite lies to the dlemost 1968a) requires considerable dissection west of the Lyngdal hornblende granite, and has and amputation, resulting in severalindependent an oval shape with the longest axis stretching granitic bodies. NW-SE. North of the Lyngdal hornblende gra nite is the smaller,elongate N-S, Kleivan granite (Petersen 1973, 1977). The peninsulas and is Previous investigations lands west of Lindesnes expose two different minor granites which are distinct from the Geological investigations have been carried out Lyngdal hornblende granite. in the area around Farsund for about 140 years, The major plutons have been separated on going back to Keilhau (1840). There have been geological and geochemical evidence (Falkum et considerable differences in interpretation of the al. 1972, Falkum & Petersen 1974) and intrusive rocks, not least because of their rela radiometric age determinations from Pedersen & tionship to the so-called 'granite controversy', Falkum (1975) and Petersen & Pedersen (1978) the 'charnockite problem', and the 'anorthosite are summarized in Table l. kindred' as discussed by Middlemost (1968a:92). 126 T. Falkum et al. NORSK GEOLOGISK TIDSSKRIFT 2 (1979) + + + + DRANGSFJORDEN + + + + + ++++ + lvVlFARSUND � AUGEN � CHARNOCKITE � CHARNOCKITE � GNEISS �GNEISS rr:;-;:::;1KLE l VAN �GRANITIC t.::::.....:::JGR ANl TE Ei.iiJ GNEISS ANORTHOSITE [l]PYRI BOLITE �FELSIC l;\ 1\l � INCLUSIONS � NORITE � BANDED � MAFIC INCLUSI � HYPERITE t:....L....;d GNEISS � ONS. AGMATITE Fig. l, A and B. Geological map of the Farsund area in south Norway. The geology of the areas a, b, c and d are shown in more detail in Fig. 3, and the area around Farsund town is shown in Fig. 4. The X near Orange is the Orange charnockite. NORSK GEOLOGISK TIDSSKRIFT 2 (1979) Intrusive granites of Farsund area 127 128 T. Falkum et al. NORSK GEOLOGISK TIDSSKRIFT 2 (1979) Table I. Rb-Sr ages and initial Sr07/Sr"" ratios for some of the intrusions in the Farsund area, south Norway. Rb-Sr age Initial ratio Reference Farsund chamockite 852±41 my. o. 7128 ± 0.0009 Pedersen & Falkum (1975) Lyngdal hornbl�nde granite 932±38 my. 0.7054 ±0.0005 Pedersen & Falkum (1975) Kleivan granite 930± 7 my. 0.7053 ±0.0002 Petersen & Pedersen (1978) We have summarized the main contributions to ally the western part of map (Fig. 2E) agrees the historical development of interpretation of with ours (Fig. 2F) except that Middlemost con the intrusive rocks in terms of geological maps in siders a part of the Farsund chamockite to be Fig. 2. intermixed with microgranite, and calls it 'B The Lyngdal homblende granite was discov type' farsundite. ered by Keilhau (1840) and the first geological In contrast to Middlemost we separate his map of southem Norway by Kjerulf & Dahll 'Kleivan limb' from the main pluton and refer to (1858-1865) includes both the Lyngdal it as the Kleivan granite, and have mapped the homblende granite and the Farsund chamockite southem boundary of the Lyngdal hornblende as separate plutons (Fig. 2A). The quality of this granite as running approximately east-west. map is excellent, particularly conceming the There has been considerable confusion as to contact relations around Farsund town, and sev the nomenclature of the rocks of the area, and a eral of the later maps show little improvement. chronological summary is presented in Table 2 The next map, produced by Kolderup (1897) which also refers to Fig. 2. Table 2 is self (Fig. 2B) in connection with a thorough explanatory in illustrating the variety of names petrographic description which includes the used for the eastern and western parts of the Egersund anorthositic rocks, seems to be largely complex, and since the use of the term based on that of Kjerulf & Dahll, though 'farsundite' has been reviewed by Middlemost Kolderup correctly interprets the granite to the (1968a, b) we will not discuss the historical north of Lyngdalsfjorden as being different from development further here. the Farsund chamockite to the west. Kolderup's (1935) later map (Fig. 2C) has undergone some alterations, especially in the northwest where the Farsund chamockite is made continuous with the country rocks enveloping the anortho Field relations site complex. Country rocks Barth's (1945) map (Fig. 2D) also shows the Farsund chamockite as merging into these gneis The plutons around Farsund have intruded a ses; the 'northem limb' of the Lyngdal high-grade metamorphic sequence of Pre homblende granite grades into the surrounding cambrian migmatitic rocks, ranging from granu granitic gneisses; the contact between the two lite to amphibolite facies. They have been di plutons is shown as gradational; the southwest vided into three major litho-structural forma contact of his birkremite is considerably further tions, established as the Flekkefjord Group (Fal south than on the earlier maps; and at the eastem kum 1966). These three formations make up end of the complex, northem and southem most of the country rock envelope around the 'limbs' are introduced. plutons and are augen gneisses, banded gneisses, The map of Middlemost (1968a) (Fig. 2E) is and granitic gneisses. Detailed accounts of these very similar to that of Kjerulf & Dahll in the area rocks from the Flekkefjord area and the area around Farsund town and none of the country northeast of Lyngdal will appear soon, and con rock contacts are drawn as gradational. Gener- sequently only a brief simplified description of NORSK GEOLOGISK TIDSSKRIFT 2 (1979) lntrusive granites of Farsund area 129 KJERULF and DAHLL KOLOERUP 1858-65 1897 A 8 BARTH KOLDERUP 1945 1935 c o MIDOLEMOST FAL KUM ET AL. 1968 1979 E F Fig. 2. Simplified geological maps of the 'Farsundite complex', showing different interpretations from 1858 to the present day. For legend and nomenclature see Table 2. In 2E Middlemost (1968a:86) considered the northwest part of the complex, shown as dots and gridiron pattem together, to be 'B-type farsundite consisting of an intermixed mass of farsundite and microgranite'. the major units will be presented here (Fig. l, A, of deformation have been recognized and the B) . first is expressed by the overall foliation and The structural evolution of the country rocks small scale isoclinal folds. A later deformation has been extremely complicated with plastic folded this foliation round large isoclinal folds, deformations acting on high grade granulite- and one of which can be seen on the map (Fig. l, A, amphibolite-facies rocks. Several major phases B) to the northwest of Åptafjorden where its 130 T. Falkum -et al. NORSK GEOLOGISK TIDSSKRIFT 2 (1979) core consists of granitic gneiss. These folds were textural variation. A few ultramafic rocks, such deformed during subsequent deformations pro as pyroxenites and