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

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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 intrusions (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 deformational sequence (Fig. 8).

7) Deformation (D«) responsible for the development of asymmetrical folds (Fc) with SE-trending axial traces and parallel pegmatite (Fig. 9). 8) Deformation (D,i) responsible for the development of asymmetrical folds (Fa) with NE-trending axial traces and parallel pegmatite veins (Fig. 10).

9) Deformation (De) responsible for the development of open folds (Ft.) with incipient shear and slight pegmatite veining in two directions on their limbs. These may prove to belong to two sets of folds (Fig. 13). 10) Deformation responsible for inclined folds which probably belongs to a position which is relatively early in the deformational sequence and as yet not located. 11) Deformation responsible for the development of more or less symmetrical open folds with subvertical north-trending axial planes and fine axial planar fracture cleavage (Fig. 14). 12) Deformation responsible for NNE- and SSE-trending conjugate shears probably causing north- trending gash veins in response to N—S compression after the late folds. 13) Slip parallel to the foliation offsetting the complementary shears. This more brittle deformation appears to be the latest to affect the migmatites. Structural development of migmatites near Skåldö, southwest Finland 49 with those of the later events, which are clearly displayed so that cross-cutting and generally more weakly expressed, and those cross-folding relationships may be deter- of uncertain position in the sequence grouped mined with a high degree of confidence. for description under the heading Dlate. The porphyritic microgranite, whose Here the degree of confidence with which emplacement appears to have accompanied, the order of events can be established both at least in part, the development of some at any one outcrop, and regionally, is less agmatites (Fig. 3), shows effects of flat- than that for the major events Da_e. Letters tening and alignment of crystals. Continued rather than numbers are used to designate flattening resulted in the development of events in the sequence as subsequent work streaky gneiss (Fig. 3 a-right). In places in the surrounding areas may reveal further agmatite blocks have resisted flattening with deformational events. Also in places where forms that indicate simple separation at the flattening and other tectonic activity have time of igneous injection (Fig. 3 c). It is in not been as extensive as is generally the case, this structural situation that the earliest further aspects of the early history may be history of the rocks is likely to be resolved. deciphered in the future. When this further However, the extent of flattening generally work is done, numbering of events should be varies from place to place, with strongly more meaningful. foliated rocks representing the effects of extreme flattening (Fig. 3 b-bottom) wrapping around more competent lens-shaped masses. Deformational sequence The pattern of zones of high deformation separating areas of much lower deformation Introduction appears to be a feature of the whole district. The alignment and flattening of the pheno- The oldest rocks comprise metasediments crysts begun with the earliest recognized (including highly deformed marbles), basic deformation phase (Da), was accentuated by intrusive igneous rocks now largely agmatiz- subsequent folding and flattening perpendic- ed, amphibolites and hornblendites (Figs. 2, ular to the dominant foliation as well by shear 3), porphyritic microgranites (Fig. 3) and pos- parallel to the gneissose foliation. With sibly basic lavas. There is a complex rela- successive emplacement of a number of tionship between folding and foliation of at igneous masses of varying composition and least four sets of basic rocks shown in Fig. 2, further agmatization (Fig. 5), the resultant as well as with gneiss, aplite and pegmatite. rocks have a thoroughly mixed (i.e. migmati- This intricate association is typical of that tic) aspect (Sederholm 1907). In places there shown by most of the migmatites in the area and the illustrations show clearly the irregu- are as many as seven different cross-cutting lar and lobate form of some of the more rock types in various states of deformation highly deformed intrusive bodies as well as and disintegration (Fig. 4). Those migmatites, the way in which the foliation cuts through showing effects of flattening, foliation forma- the folded remnants of some cross-cutting tion and igneous emplacement, were then units (e.g. the large fold in agmatised am- deformed several times and intruded by more phibolite in the bottom right hand corner of discrete masses including pale grey amphibo- Fig. 2 c). The quality of the exposure in lite (Figs. 4, 5 a), aplite and a succession of places is such that the finest details are pegmatites (Fig. 5 b).

4 50 A. M. Hopgood, D. R. Bowes and J. Addison

Amphibolite

Ouartzofeldspathic gneiss

Pale grey amphibolite

Amphibolite - minor intrusion

Aplite

Pegmatite

Tension gashes

Fig. 5 a. Relationship between amphibolites, aplite, pegmatite, foliation and folds, and cross-cutting tension gash veins. Baggöhamn near the kiosk, Skärlandet, b. Cross-cutting of post-F|, pegmatite veins. Clearing in woods west of road south of Skåldö ferry, Skärlandet.

Deformational phases axial planes are largely parallel to the foliation (Fig. 6 b). In places the latter folds con-

Da phase tain a weakly developed axial planar mineral

The earliest folds identified (Fa) are iso- growth through their hinge zones. The Fa clinal structures with attenuated limbs and folds, however, have the dominant dimen- sharp hinges. These folds, which lie wholly sional alignment of minerals cutting their within the foliation (Fig. 6), have been seen hinge zones and parallel to their axial planes. on only a small scale, usually just a few cm Wherever it has been possible to determine long. Most are isolated, either because they the axial orientation of Fa folds this has been are rootless, due to tectonic attenuation of found to be sub-horizontal, suggesting that limbs, or because of their geometry and these small scale folds may be .parasitic struc- orientation combined with the effects of later tures on the limbs of as yet unrecognized folding. In a number of localities the folds large scale isoclinal synformal and antiformal are seen to have been folded around tight, flexures with steep to vertical limbs and sub- sometimes almost symmetrical folds whose horizontal hinges. These might well have Structural development of migmatites near Skåldö, southwest Finland 51

Fig. 6a. Small Fa fold, looking south. Near the kiosk, Baggöhamn, Skärlandet, b. Parallelism of axial plane of Fa fold with that of tight Fh fold which refolds it. Both structures cut by N—S quartz filled tension gashes; looking west. West end Ängholmen, north shore, c. Parallelism of axial planes of Fa and Fb; looking east. Baggöhamn near the kiosk, Skärlandet. 52 A. M. Hopgood, D. R. Bowes and J. Addison

Fig. 7a. Isoclinal Fb folds; looking east. North shore of Skärlandet opposite Björnholmen, b. Fb folds; looking west. North shore, west end of Ängholmen.

formed in response to simple horizontal com- markedly asymmetrical (Fig. 6 c). In some pression in a direction perpendicular to the cases the structures are isoclinal (Fig. 7 a) foliation. but more usually there is an interlimb angle D|, phase of at least 10—20° especially in the hinge Folds referred to this deformational phase areas of larger folds where the smaller folds tend to be tight structures which fold the tend to be asymmetrical (Fig. 7 b). Normally dominant foliation and mineral alignment. they are readily distinguishable from Fa They have limbs more or less equally in- generation folds because of their geometry clined to their axial planes which are them- and orientation and they can often be seen selves parallel to the foliation and this rela- refolding the typically slender profiles of Fa tionship applies even where Fb folds are intrafolial folds (Fig. 6 b). In those cases Structural development of migmatites near Skåldö, southwest Finland 53

Amphibolite | * * | Pegmatite b Fig. 8a. Cross-cutting pegmatite vein folded during Db'. North shore of Skärlandet opposite Björn- holmen. b. Fb' folded pegmatite veins cutting Fi, folds. Clearing in woods west of road south of Skåldö ferry, Skärlandet. where the hinge orientation is determinable lel to the foliation appears to have affected it is seen that the axes of Fb folds are steeply the migmatites right up to the end of, or close inclined and this is consistent with develop- to the end of, the deformational sequence ment as a result of flexure on inclined axes determined and for the limited area studied probably succeeded by simple shear parallel it seems to have been a dominant feature of to the foliation which they fold. While the the style of tectonism which affected them. axial planar mineral alignment discerned in The continuance of this shear until late in the deformational sequence undoubtedly accounts Ft) folds is only faint, it may be that the more for the relatively common occurrence of obvious mineral alignment associated with coaxial-planar folds belonging to this and the Fa folds is due to the fact that the axes later generations. As yet no large scale of these folds are parallel to the outcrops examples of F folds have been recognized. which are normally more or less horizontal. b The deformational episode responsible for Thus the elongation of minerals and mineral the formation of F folds was succeeded by aggregates parallel to the F fold axes is more b a at least two intrusive episodes; the emplace- easily discernible while F axes on the other h ment of porphyritic microgranite associated hand are steep and mineral elongation paral- with agmatisation and the injection of coarse lel to them would not be seen except on steep grained pink pegmatite veins, commonly with outcrops which are less common. an approximately N—S trend. The latter

The formation of Fb folds was at least in form one of the means whereby recognition part due to slip, and deformation at this time of shear folding later than that causing Fb seems to have presaged an extensive period folds is possible and strongly discordant of folding by inclined slip parallel to the pegmatites emplaced about this time display main foliation direction. In the course of this the effects of shear folding of this type deformation successive generations of pegma- (designated Fb'). Characteristically Fb' folds tite veins and minor intrusions emplaced at have thick hinges and attenuated limbs (Fig. high angles to the foliation became folded 8 a) and can be seen to have a development while at the same time early folds were in- independent of the earlier Fb folds to whose creasingly more strongly modified. Slip paral- axial plane they are nevertheless parallel 54 A. M. Hopgood, D. R. Bowes and J. Addison

Fig. 9a. Fi, folds (axial traces trending left to right) and Fc folds with axial traces parallel to pencil; looking west. Shore north of Storängen, Ängholmen. 9 b. Fc fold cut by amphibolite. Shore northeast of Norrnäs, Skärlandet. 9 c. Fc fold in amphibolite cut by SSE pegmatite and showing the relationship between earlier shear and pegmatite emplacement. Shore northeast of Norrnäs, Skärlandet.

(Fig. 8 b). The folds grouped under Fh' prob- history of foliation shear in the migmatites ably belong to several sets initiated at dif- and they may include some of the youngest ferent times in the course of the prolonged structures in the rocks. Structural development of migmatites near Skåldö, southwest Finland 55

Dc phase ment of the quartzofeldspathic material, Folds of this generation are notably more which is mostly autochthonous or parautocht- open than any of the earlier-formed struc- honous, was directly controlled by shear or tures. They are decidedly asymmetrical (Fig. incipient shear strain more or less parallel

9 c), sometimes with quartzofeldspathic vein- to the fold axial traces. Fc fold hinge zones ing parallel to their axial traces and in places have subsequently been used as sites of con- show faint axial planar mineral growth. In siderable pegmatite injection (Fig. 9 c). isolation they are readily distinguishable In areas where Da, Db and fold sets from isolated Fb folds by virtue of the fact appear together the effects of mutual in- that their axial planar traces are inclined to terference can be considerable. In spite of the dominant foliation trend (Fig. 9 b) where- this the three sets can normally be discrimi- as Fb axial planar traces are parallel to the nated provided reasonable care is exercised in foliation (Fig. 9 a). In places where both sets applying the distinguishing criteria which in- of folds occur together the relationships clude attitude, trend, style and crossfolding between the foliation and their axial planes relationships (Fig. 10). can be seen in the one outcrop (Fig. 9 a), al- At some as yet undetermined time follow- though mutual interference where the two ing D , there was a further episode of dolerite sets of folds are of the same order of size may ( intrusion represented now by amphibolites obscure this relationship. Otherwise the two which cut across F fold structures with sets of folds are separable simply because one 0 marked discordance (Fig. 9 c). can be seen to refold the other (Fig. 10). Dd phase On horizontal outcrops F . folds appear as t The folds formed as a result of deforma- discontinuous asymmetrical structures of tion during Dj are similar in style to Fc folds variable style and wavelength, ranging from (Fig. 11 b). They are asymmetrical and their a few centimetres to a metre or more across axial traces, which are commonly parallelled (Fig. 9 b). Characteristically they have by quartzofeldspathic veins, are inclined to rounded, lobate hinges which may even be the foliation trend. They are however sepa- box-like (Figs. 9, 11) where single folds may rable from them because their axial traces show subparallel or slightly divergent axial trend at high angles to F fold axial traces traces (Figs. 9 a, b; 10 a). e and also because they tend to be more regular The acute angle between the fold axial and are frequently rectilinear. Where the two trace and the foliation trend implies a sinis- structures crop out together, F folds can be tral rotational sense inclined to the foliation d seen to cause reorientation of F folds and (Fig. 10 b) so that with the general easterly c this rules out the possibility of the two sets trend of the foliation in the Skåldö area, Fc being contemporaneous and conjugate. Often axial traces commonly trend SE with a moderately steep southwesterly dip of axial Fc fold axial traces are very strongly curved planes (Fig. 9 a). Where the foliation swings by the later folds (Fig. 11 a, c). Isolated away from the normal E—W trend, the axial examples of Fd folds can be distinguished on the grounds that they appear to be genetically trace of Fc folds correspondingly departs from the usual SE trend (Fig. 9 c). related to dextral shear couples inclined to It is probable that syntectonic or imme- the foliation. This means that because of the diately post-tectonic pegmatite veins are generally E—W strike of the foliation in the associated with this deformational episode Skåldö area, the traces of F(i axial planes, (Fig. 10 b) and it appears that the emplace- which dip steeply, almost invariably have a 56 A. M. Hopgood, D. R. Bowes and J. Addison

Streaky grey hornblende gneiss

Pegmatite

10a , - Fa and Ft, folds (axial trace parallel to foliation) and Fc folds (axial trace parallel to pencil)- looking south. Baggöhamn, near the kiosk, Skärlandet, b. F, and Fc folds; looking west. Baggöhamn near the kiosk, Skarlandet. c. Interference pattern resulting from Fa and F,i folds. North shore of ökarlandet, opposite Bjornholmen. d. Interference patterns resulting from Fa, Fi, and F,. folds North shore of Skärlandet, opposite Björnholmen. Structural development of migmatites near Skåldö, southwest Finland 57

Fig. 11a. Refolding of Fc folds by Fd; looking south. F,i fold axial trace curved by late N—S open folds. North shore of Notholmen, Skärlandet, b. Fd fold; looking south. Baggöhamn near the kiosk, Skärlandet, c. Interference between Fi>, Fc and Fd folds crossed by open N—S folds. North shore of Notholmen, Skärlandet.

NE trend, although in places this is sometimes characters that considered alone are enough altered by the effects of later folding (Fig. 12). to set them apart from Fc folds but it seems The stylistic similarities between the two that in most cases their styles are indistin- fold sets may be a consequence of subsequent guishable, apart from the fact that where the deformation, especially slip parallel to the two sets occur together and can be compared foliation, which has modified them to the (Figs. 11a, c) some Fd folds are seen to be extent of obliterating all their characteristic more open (Fig. 12). They are thus separable features, apart from the axial planar mineral- at present only on the basis of axial planar ization which appears to be confined to Fc trend or mutual cross-folding but where the folds only. Nevertheless future work may two appear together the interference patterns well show that Fd folds have stylistic produced are often extremely complicated 58 A. M. Hopgood, D. R. Bowes and J. Addison

be contemporaneous and conjugate, although on some outcrops it is possible to establish a sequential relationship where one direction is more regular than the other (Fig. 13). Whether or not a consistent cross-folding relationship exists remains to be determined and for the moment the two sets are treated together. The close similarity of style coupled with their open form and relatively weak development often tends to result in complex, highly irregular interference patterns which may be difficult to decipher (Fig. 13b). This is because mutual interference precludes the development of axial planes which continue for more than a few centimetres so that on the outcrops axial traces are intermittant and irregular, changing direction over comparatively short distances. For this reason it is quite possible that more than one or two fold sets are involved in the interference pattern so formed. In style the folds vary from open structures showing gently rounded hinges to asymmetrical kinks exhibiting sharp apices. There is a suggestion of slip parallel to one set of limbs in many cases and often these limbs Fig. 12a. Folding of F,i axial traces and Fe folds by form the locus of quartzofeldspathic material open N—S folds in quartzofeldspathic gneiss. North shore of Notholmen, Skärlandet, b. Open whose development, possibly in situ, was N S folds with fracture cleavage affecting F|„ controlled apparently by local shear stress northeast-trending F,i and Fe in quartzofeldspathic gneiss North shore of Notholmen, Skarlandet. in much the same way as comparable veining

associated with Fc folds (Fig. 13 a).

Diate (f-n) phases and then separation of the two sets requires considerable care (Fig. 11 a, c). The occasional appearance of small chev-

De phase ron-type folds whose axial traces trend Following D,i the migmatites seem to have approximately ENE where the dominant responded to deformation in a somewhat foliation strikes approximately N-S may more brittle fashion with the development represent the effects of E-W subhorizontal of two sets of relatively open folds with in- slip. This slip would normally be parallel to cipient shears parallel to their limbs. The the foliation with its general E-W trend in axial traces of these two sets of structures are the Skåldö area, and would produce folds often curved but on average trend respec- related to Fb' and subsequent generations tively NNE and SE and the folds appear to derived from later planar veins with axial Structural development of migmatites near Skåldö, southwest Finland 59

Quartzofeldspathic gneiss Pegmatite j ] Aplite Axial trace

Fig. 13a. FP folds with development of quartzofeldspathic pegmatite parallel to some limbs. Shore northwest of Furuholm, Skärlandet, b. Interference pattern produced by Fe folds and Fc. North- west shore of Flackholmen.

planes parallel to the foliation. Where, how- distinguished. Apparently associated with ever, the foliation departs from this E-W the N-S trending axial planes is a moderately trend, folds of chevron type could be formed well developed north-trending, steep fracture as a result of E-W slip-folding of the folia- cleavage which weathers out cleanly in tion, or of planar veins at high angles to the amphibolite layers as a series of fine ribs. foliation (Fig. 14 a), where this is E-W. This structure is often fanned and the fanning The only examples of structures considered may be the result of even later deformation. to have been produced by foliation folded in The axial planes of these structures, which this way are small (1—2 centimetres) folds appear to be flexural rather than shear folds, with angular to rounded hinges (Fig. 14 a) are subvertical. and limbs that in places are slightly crum- Amongst the latest structural features are pled. The interlimb angle is in the region brittle structures such as conjugate shears of 90° and the folds tend to be comparatively which trend respectively NNE and SSE. These isolated, separated by unfolded foliation for are steeply dipping surfaces apparently cutt- a distance equal to about twice their wave- ing all the folds but they are clearly not the length. latest structures in the deformational se- Fold structures demonstrably late in the quence because they are offset by shears sequence are those with axial planes trending parallel to the foliation. The relatively com- N—S (Fig. 14 b, c, d). These are mostly very mon N-trending steeply dipping, narrow, open structures but they range from no more more or less rectilinear quartz lenses (Fig. than gentle warps to moderately tight folds, 6 b) are possibly en échelon tension gash usually with very rounded hinges, but some- veins in zones parallel to the NNE and SSE times showing hinges that are relatively complementary shears (Figs. 2 a, 5 a). angular. They impart a wavy effect to the The latest deformation seems to have been banding where this would otherwise be more the continuation of the foliation shear which or less rectilinear in outcrop (Fig. 14 d, e). may have proceeded from at least the time of They may represent more than one set of D|, throughout the deformational history of folds but if this is so these have not yet been the migmatites. The progress of this move- 60 A. M. Hopgood, D. R. Bowes and J. Addison

Fig. 14a. ENE chevron folds in N—S trending foliation. North shore of Notholmen, Skärlandet, b. Complex interference pattern produced by Fa, Fc and Fd folds affected by N—S folds. Norrnäs, Skär- landet. c. Interference between open N—S folds and Fc folds. Shore north of Storängen, Ängholmen d. Open N—S warping of foliation folded by Fb. North shore of Skärlandet opposite Björnholmen

ment can be monitored throughout the se- possible to determine the relationship of these quence because of its effect on planar struc- structures to others in the sequence although tures such as dykes and veins emplaced from they are thought not to be the result of late time to time during the developmental his- deformation. The inclined folds vary in style tory of the migmatites (Figs. 5, 8) and it can between relatively tight folds with interlimb be seen that the style of deformation became angles of about 90° to comparatively open gradually more brittle, with a change from structures but their horizontal profile is in- early structures with a flow fold aspect (Fig. variably very open. 7 b) through more angular shear folds (Fig. 14 a) to fracture, with offset parallel to the foliation, of the late conjugate shear zones. Discussion and conclusions Some open waves on horizontal outcrops appear to be the expression of inclined or The sequence of events shown by the recumbent folds gently plunging either east migmatites of Skåldö is summarised in Table or west. An indication of their geometry can 1. This is considered to be only an outline of be determined on rare three dimensional what could prove to be a much more com- exposures such as low cliffs or road cuts but plicated history and further investigation is these are so few that it has not yet been likely to provide additional information and Structural development of migmatites near Skåldö, southwest Finland 61 thus increase the certainty regarding points elsewhere (cf. Menhert 1968). Hence any ex- such as the relative time of event 10 and the amination of migmatite development in this number of fold sets involved in De. Neverthe- region must not only take this into account, less, it can be stated with reasonable con- but must also take into account the time ele- fidence that the structures described and ment which has so often played a relatively figured by Sederholm (1907) nearly three minor role in many discussions of migmatite quarters of a century ago can be recognized genesis, particularly laboratory-orientated and identified with distinct sets which can studies. The fact that sufficient time must be placed in a consistent deformational se- have elapsed between successive phases of quence. Thus these migmatites record a his- intrusion to allow folding and tectonic dis- tory of polyphase deformation and meta- ruption to take place implies that time has morphic and igneous events all of which must not been insignificant and this bears out the be considered in a study of their petrogenesis. thesis of Read (1957) that time as well as The demonstration of a complex tectonic, place are important factors in considering metamorphic and igneous sequence such as the genesis of migmatites. the one shown here permits a stage by stage With the establishment of a general struc- assessment of the progressive evolution of tural framework, a study of metamorphism the deep levels of an orogenic belt. It also in relation to tectonics in these migmatites provides a sound basis for comparison with can be undertaken. This together with a the successively developed products of other study of the successively formed granitic crustal levels and environments in the Sve- veins throughout the area — their petro- cokarelian orogenic belt. Furthermore, in this graphy, chemistry, geometry and mutual re- way the validity of correlation between these lations as well as ages from isotopic studies — migmatites and the metasediments and meta- should lead to a fuller understanding of the igneous rocks of comparable age in other genesis of migmatites, the crustal environ- parts of the Baltic Shield can be tested and ments of their formation and the time spans of the processes involved. Linked with struc- in addition it will be possible to determine tural investigations it is hoped that this work how the style of tectonism, the degree of will further elucidate the crustal history of deformation, and the expression of meta- the deeper levels of orogenic belts. morphic and igneous events varies through- Acknowledgements — The authors are most out the orogenic belt. grateful to Dr. M. Härme of the Geological Survey of Finland for his advice and help in selecting the The fact that the migmatites in the Skåldö area of study and they also wish to thank the area have been shown to form a complex residents of the Skåldö area, especially Mrs. A. rock unit whose characteristic mixed aspect Berglund of Pippings Folkskola, for their hos- resulted dominantly from repeated deforma- pitality and kindness. The assistance of members of the Glasgow University Exploration Society is tion and igneous intrusion indicates that the also much appreciated. In addition the authors roles of metamorphic differentiation and par- wish to acknowledge financial assistance from the tial melting at the present level of exposure, University Courts of the Universities of Glasgow and St. Andrews and from the Carnegie Trust for do not appear to have played as important a the Universities of Scotland towards the cost of role here as that assigned to them in studies publishing the illustrations. 62 A. M. Hopgood, D. R. Bowes and J. Addison

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