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Structural Development of Migmatites Near Skåldö, Southwest Finland

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Structural Development of Migmatites Near Skåldö, Southwest Finland STRUCTURAL DEVELOPMENT OF MIGMATITES NEAR SKÅLDÖ, SOUTHWEST FINLAND A. M. HOPGOOD, D. R. BOWES and J. ADDISON HOPGOOD, A. M., BOWES, D, R, and ADDISON, J. 1976: Structural development of migmatites near Skåldö, southwest Finland. Bull. Geol. Soc. Finland 48, 43—62. The migmatites of the Skåldö area show the effects of an extensive polyphase deformational history at relatively deep crustal levels during the Svecokarelian episode. Despite the complexity of the fold patterns it is possible to establish a deformational sequence into which the many phases of igneous emplacement in the rocks can be placed. There are at least seven successive fold sets and a number of phases of both basic and acidic igneous activity as well as the effects of metamorphism which reached a peak, under amphibolite facies conditions, early in the deformational history. Taken together the various features comprise a complex framework which can be used as a basis for comparison and correlation with the products of the Svecokarelian orogenic episode in other parts of the Baltic Shield. The characteristics of these migmatites in this classic area, viz. their »mixed» aspect as described by J. J. Sederholm, is shown to result from their extensive deformational, metamorphic and igneous history in which tectonic and igneous activity played prominent roles. A. M. Hopgood, Department of Geology, University of St. Andrews, Fife, KY 16 9TS, Scotland. D. R. Bowes and J. Addison, Department of Geology, University of Glasgow, G12 8QQ, Scotland. Introduction it is evident that the migmatites also have undergone popyphase deformation and mul- In southwest Finland, a classic region for tiple igneous injection. migmatites (Sederholm 1967), the rocks rep- To study this deformational, metamorphic resent a relatively deep level in the Sveco- and igneous history, members of the Glasgow karelian orogenic belt which, elsewhere in University Exploration Society spent four Finland where metasedimentary schists pre- weeks in the field around Skåldö on Skär- dominate, has been shown to have an exten- landet, and some of the smaller adjacent sive polyphase deformational and meta- islands south of Tammisaari (Fig. 1; King and morphic history (Gaål et. al. 1975; Bowes 1975; Grey 1975). The work has confirmed that in press). However, from the detailed illus- the rocks, including many which are trations and descriptions of Sederholm (1967) migmatitic, preserve a consistent record of 44 A. M. Hopgood, D. R. Bowes and J. Addison Fig. 1. Map of part of the Svecokarelian belt (after Welin, 1970) showing location of area studied. a deformational, metamorphic and intrusive lated hillocks where the quality of the ex- history of considerable complexity (Figs. 2, posure is reduced in most cases by thick 3, 4). While the complexity is such that lichen cover which limits its value for the further work is needed both locally and investigation of small scale deformational regionally to complete the picture, a general features and intricate intrusive relationships.. pattern of structural development has been In some places however, it is possible to established. This is presented as a contribu- peel off the vegetation cover to expose tion to knowledge of the Svecokarelian belt particularly clean rock faces. and as a framework within which to discuss The regional structural pattern of much of the origin of the migmatites, particularly as southwestern Finland has been published by many studies relating to the genesis of this Härme (1960) together with the nature and group of rocks have been laboratory rather distribution of the various rock units. Further than field oriented (Mehnert 1968). detail is provided by the Geological Survey Exposure, which is generally very good, is of Finland 1: 100,000 Sheet 2013 — Jussarö restricted mainly to narrow and discontinu- (Laitala 1973). Amphibolite facies meta- ous strips three to four metres wide along the sediments and gneisses occur together with shore lines. These are nevertheless extremely a wide variety of igneous rocks that range clean, especially on the northern sides of the from ultrabasic to acidic in composition and islands facing the direction of Pleistocene ice which predominate over the metasedimentary movement. Inland outcrop is limited to iso- units (Härme 1965, Appendix 1). Most of the Structural development of migmatites near Skåldö, southwest Finland 45 I Amphibolite/hornblendite |;||l;!|iijj Amphibolite Rusty brown amphibolite I VV I Metagabbro I* i/1 Agmatite Feldspar augen gneiss Aplite |; * I Pegmatite |;ww/j Shear zone Fault/Fracture Fig. 2. a, b, c. Relationships between various basic to acidic intrusions and deformation. Baggöhamn near the kiosk, Skärlandet. rock types have been affected in the course Although repeated intrusion, metamor- of the deformational and metamorphic his- phism and deformation has compounded the tory and in consequence display a consider- complexity of the Skåldö migmatites, the able diversity of structural relationships. investigation has nevertheless shown that it 46 A. M. Hopgood, D. R. Bowes and J. Addison Fig. 3. Agmatites showing the development of foliation in the matrix. Road between Skåldö and Baggöhamn, Skärlandet. Structural development of migmatites near Skåldö, southwest Finland 47 0 I Amphibolite/hornblendite Weakly foliated amphibolite Coarse grained amphibolite Amphibolite Pale grey amphibolite Hl Aplite [yl Pegmatite "'i'i1 "'vlli'ii |. 1 Faults/Fractures Vi'!,11! iMiMV.' '»''i l'i'V y'P ii' it"'?1 'm /I'li'.'ii'A'V '• ii' t'"• 'SK ivl »Ä\v V a. i,I.'in,• v 1 ' 1,1 VW ,y- MM1 ÄÄIRA* w ijjjl&v.'.-s •I11 ISI •»N .•••vm'1!,'" ui In 'W" I»i in 11VA v -•In "ii' i'M'iii W.v\ ||' 'i I'M^uV J!!!."!ji: iil i iii' W'nJiii må 'iiu \'H i mi' Si w W.vW ,1 M • i» '^"Wm !' \\\ ill® IM!! ,I 11 l'.I/..WM.I1- I iU \i 'ii,m. .... f VN» "i XS." I '"I ''/liSjW, 1 HMÄß! 1 PfMilii« m W ^mm : liifillffilMi1 'l I ÄvV>\\\ii'in ' i'VI ll!i|i!ji 1 'i'i i'Ni/iij ii;i; i <•-. I ^SLILVL !" I'll i'! !! "pir 4i'i VI i jn'i il',!1,!', WI' M 1l L'i'i!"!' "i I ill liSijiifi Ii l'l'l" v 'i'ilV"'1 I1!'" i'i! !|ii| [H'lh!1/ 11 r'' " '."i' AiiliHiij vh' i :i i •I'.FK !' II IL n, w i'.iiv'ii! I'liT] "llllülSlli!!!!!! IFIT ;! 11! I'i' w lilii!""1^ i1' 'i1-'/'/^ \ 1 1 II' i I Ü vMSi'%.1 1, W! ''M', !! LL''iiii 1 \v ' " p>1 '1 i V, |'|I VVl'l'l l'l I I'I X ',|li i'i I H ' <n Ii V'lV^'lllll'll I'll, 1111 i'|ll Iii ll ii »fy! i »W'Miii'i I V,'|IIß Fig. 4. Relationship between amphibolites, aplite, pegmatite veins, foliation and folds. North shore of Ängholmen at the west end, east of Svartholm. is possible to establish a deformational se- pretation of the relationship of small-scale quence within them. Therefore they are structures to one another and to igneous and amenable to investigation based on the inter- metamorphic phenomena and this provides 48 A. M. Hopgood, D. R. Bowes and J. Addison another way in which it is possible to throw and related to metamorphic and igneous light on the history of these classic rocks. The events. approach employs techniques previously used As a result of the present preliminary successfully in highly deformed terrains in investigation a sequence involving some of the Highlands and islands of Scotland (Hop- the principal events affecting the migmatites good and Bowes 1972) and elsewhere in the has been drawn up and is presented here as world (Hopgood 1971, 1973). The method is an indication of what can be determined based on two fundamental principles of regarding the history of these rocks by using geology, namely that (1) deformed structures the relationships of small-scale structures are older than those which deform them and including minor intrusions. While the list of (2) cross-cutting features are later than those events (Table 1) is not regarded as being which they cut. Structures and igneous comprehensive, the events are considered to intrusions so found to post-date others can be be in sequence, if not necessarily consecutive. traced, using characteristic features already The deformational events producing the established such as orientation, style and structures described are designated Da_n mineral growth, to other outcrops where they, and known equivalent fold sets produced by in turn, are affected by later structures or these events are referred to as Fa_n. The cross-cut by later intrusions. Thus a struc- structures resulting from each of the major tural sequence can be built up in the rocks deformational events are described separately Table 1: Outline of sequence of events in migmatites, Skåldö 1) A series of events (sedimentary, and igneous) leading to the establishment or a lithologically layered succession (Figs. 2, 4). 2) Deformation (Da) responsible for the development of the present isoclinal, intrafolial folds (Fa). Syntectonic mineral growth with production of preferred orientation of minerals parallel to the fold axial planes. This was probably the main gneiss-forming event (Fig. 6). 3) Emplacement of basic minor intrusions, now amphibolites. Intrusion of aplitic minor intrus- ions (Figs. 2, 4). 4) Deformation (D|,) responsible for the development of tight folds with axial planes parallel to the main foliation (Fb). Both flexure and slip were involved (Fig. 7). 5) Emplacement of granite, pale cement-grey amphibolites and discordant quartzofeldspathic pegmatites with a more or less northerly trend (Fig. 3 a). 6) Onset or continuation of foliation slip (Dt/) to produce slip folds (Fb') in the cross-cutting intrusions. This or related movement appears to have continued throughout the deformat- ional sequence (Fig.
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