Geological Survey of Finland

Bulletin 325

A plate tectonic interpretation of the Precambrian of the Archipelago of southwestern Finland

by Nils Edelman and Mirja Jaanus-Järkkälä

Geologinen tutkimuslaitos Espoo 1983 Geological Survey of Finland, Bulletin 325

A PLATE TECTONIC INTERPRETATION OF THE PRECAMBRIAN OF THE ARCHIPELAGO OF SOUTHWESTERN FINLAND

by NILS EDELMAN AND MIRJA JAANUS-JÄRKKÄLÄ

with 13 figures

GEOLOGINEN TUTKIMUSLAITOS ESPOO 1983 Edelman, N. & Jaanus-Järkkälä, M., 1983. A plate tectonic in· terpretation of the Precambrian of the Archipelago of southwestern Finland. Geological Survey of Finland, Bulletin 325, 33 pages, 13 figures. Remapping of parts of the archipelago of southwestern Finland has called for reinterpretation of the Precambrian geology of this area from the standpoint of plate tectonics. The region is apart of the Svecofennian fold zone with an age of about 1900 - 1800 Ma. The bed­ rock consists of sedimentary gneisses, basic volcanics and various infracrustal rocks. Transitions from mica gneisses, metamorphic clays or greywackes, in the northern part of the region, into acid gneisses poor in mica, metamorphic silts or sands, in the southern part, indi­ cate a source area south of the region. Rocks with an age of about 2400 Ma found by deep drilling in northwestern Estonia, about 100 kilometres south of the present region, indicate the existence of a continent on the shelf of which sedimentary iron ores of the present region seem to have been deposited. The basic volcanics in the central part of the region suggest remnants of an island arc. The volcanic formations are sometimes underlain by banded gabbros that often contain titanic iron ores. These rocks are interpreted as subvolcanic sills, possible magma chambers of the volcanoes. Associated with granodiorites is another type of gabbro, which intruded later, during folding. In the southern part of the region there is banded formation consisting of gneisses, amphibolites and pyroxenitic fragments. The formation is extremely deformed and is by us interpreted as a melange along a subduction zone where a southern plate carrying the continent has sunk beneath a northern plate with sediments and volcanics . This subduction zone is conspicuous on the magnetic map as the boundary between an area rich in strong anomalies in the south and an area poor in anomalies in the north. In a limited section slices of the magnetic area seem to have been pushed over the sub­ duction zone into the sediments of the northern plate. In the western part of the region an inferred transform fault permitted a protrusion of the continent to extend farther north. This part was fractured and enabled magmas, both granitic and basaltic, to intrude during and after the orogeny.

Key words: Plate tectonics, subduction, melange, stratigraphy, Precambrian, Finland

Nils Edelman, Abo Akademi, Mirja Jaanus-Järkkälä, SF-20500 Abo 50, Finland Nuolihaukantie 6 A 20 SF-90250 Oulu 25 Finland

ISBN 951-690-182-4 ISSN 0367-522X CONTENTS

Preface ...... 5

Outline of geology ...... 6

Petrology ...... 8 Acid gneisses 8 Mica gneisses ...... 8 Limestones ...... 9 Iron ores ...... 9 Amphibolites and associated gabbros ...... 10 The banded formation ...... 11 Ultramafites 14 G abbro-granodiorite series ...... 14 Microcline granites 15 Amphibolite dykes 16 Some aspects of metamorphism ...... 16 Stratigraphy ...... 17 Sequences 17 Correlation ...... 19 Tectonics ...... 20 Outline ...... 20 Structures ...... 20 F aults ...... , ...... 23 Magnetic structures ...... 26 Summary of tectonics 26

Geological evolution 27

Concluding remarks 28

Acknowledgements ...... 30

R eferences ...... 31

PREFACE

In 1960 one of us published a summary of She thereby showed that some gabbros are the geology of the eastern part of the archi­ associated with volcanic amphibolites and pelago of southwestern Finland (Edelman that there are therefore at least two genera­ 1960). Since then several new maps have been tions of gabbros. Furthermore she made a drawn and investigations undertaken in the comparison between the current concepts of archipelago (Edelman 1973b; Edelman, the stratigraphical sequences in south­ Ehlers & Suominen 1975; Ehlers & Ehlers western Finland in which she demonstrated 1978, 1981; Ehlers 1976, 1978; Suominen 1980, that the previous synthesis of the strati­ 1981; and unpublished theses). Observations graphy of the archipelago (Edelman 1960) made during the studies have shed new light was probably wrong in some important re­ upon certain problems while throwing doubt spects. It was this finding that provided the on some previous interpretations. The theory stimulus for the reinterpretation of the geol­ of plate tectonics, wh ich has altered our ideas ogy ofthe region. Under the guidance of Carl of the evolution of orogenies, is a further Ehlers, she, and later AlfLindroos, continued argument in favour of reinterpreting the geo­ the remapping of the volcanic formations on logical evolution of the archipelago. Hietanen the islands of N agu and Korpo in order to (1975) was the first to interpret the Sveco­ dü;tinguish the subvolcanic gabbros from the karelian orogeny in Finland and Sweden on later orogenie ones. Ehlers, Jaanus-Järkkälä the basis of plate tectonics. and Lindroos will publish their results sep­ One of us (Jaanus-Järkkälä), financed by arately. The new interpretation of the geology the Research Council for Natural Sciences, has been the subject of a number of lectures remapped an amphibolite syncline in the (Edelman - .Taanus 1978, 1980, 1982). commune of Nagu (Edelman 1972) in 1977. 6 Geological Survey of Finland, Bulletin 325

OUTLINE OF GE OWGY

The bedrock of the archipelago of south­ south by the Baltic Sea, and in the west and western Finland (Figs. 1 and 7) consists of north mainly by massifs of postorogenic gneisses, amphibolites and various infra­ rapakivi granites. For stratigraphical and crustal rocks, e.g. ultramafites, gabbros, dio­ tectonic reasons the region is divided into rites, granodiorites and microcline granites. four zones: the northern, central and south­ The rocks obtained their present shape dur­ ern zones, and the iron ore zone (Fig. 2). ing the Svecokarelian orogeny 1900- 1800 Ma The gneisses are principally of two types, BP. Postorogenic granites dominate in the acid gneisses poor in mica, and mica gneisses. western and northern parts ofthe archipelago All the transitions between these two rocks, and are therefore beyond the scope of the however, are also present, and different geo­ present paper. logists have used different bases for classi­ The region under discussion is bounded in fication. These minor differences do not have the east by the co ast and the western shore a crucial bearing on the general pattern of the of the large island of Kimito (Kemiö), in the regional distributions ofthe gneisses. Argilla-

I:>:»J 1

1=-=- 1 2

1== = 1 3 20 km ~ 4 r ~Mand Sweden ~ 6 0 ~" §±Hl 7

<:5 8

0' Ka

Fig. 1. Simplified geological map of the archipelago of southwestern Finland according to the map sheets A 1 Aland and B 1 Turku of the Geological Survey of Finland and Edelman (1960). 1 acid gneisses, 2 amphibolites, 3 the banded formation, 4 mica gneisses, 5 granodiorites-gabbros, 6 microcline granites, 7 postorogenie granites, 8 banded iron ores. A Attu, E Enklinge, F Föglö, G Gullkrona graben, H Hitis, Ka Kalkskär, Kf Kökarsfjärden, Ki Kimito (Kemiö), Ko Korpo, Ku Kumlinge, Kö Kökar, L Lantö, LN Lill-Nagu, M Mashaga, M-N Merimasku-Naantali area, N Nagu, Ns Nötskär, Ny Nyhamn, Nö Nötö, P Pargas, S Seglinge, So Sorpo, V Vehmaa, Va Va ttus kifte t, A Aland. Geological Survey of Finland, Bulletin 325 7 ceous mica gneisses dominate in the northern as subvolcanic rocks associated with volcanic part of the region, and arenaceous acid amphibolites and those that intruded during gneisses in the southern part, indicating that the folding. Some gneis sie rocks with a gran­ a source area for the sediments or a continent odioritic composition, especially those asso­ was located in the south, where the Precam­ ciated with the melange zone, seem to be brian bedrock in northwestern Estonia has an intensely metamorphosed supracrustal age of about 2400 Ma (Fig. 7). Amphibolites gneisses. The gneissose granodiorites of the with recognizable volcanic structures, con­ iron ore zone form larger massifs, and we stitute thick formations in the central and tentatively interpret them as parts of the old northern zones but they also occur as thin continent. Dykes with an amphibolitic com­ intercalated layers in the gneisses. Most of position are common in the iron ore zone the limestone layers are associated with but rare elsewhere. They cut the gneis ses amphibolites, but small lenses also occur and the above-mentioned magmatic rocks in the acid gneis ses. In the central zone the but are granitized when in contact with the sequence from bottom to top is: acid gneisses, microcline granites. amphibolites with limestones, mi ca gneisses. The whole region has undergone granitiza­ In the southern zone the amphibolites are tion that began as palingenesis, or the forma­ intermingled with acid gneisses to make up a tion of granitic veins in older rocks, and strongly deformed and banded formation terminated with the formation of mobile (banded series in Edelman 1960) that con­ granitic masses of intrusive behaviour. In the tains fragments and massifs of ultramafites southern zone these granites constitute two and gabbros and small lenses of limestone. rows of domes or diapirs. Locally the gran­ This banded formation is now interpreted as itization seems to have involved recrystalliza­ a melange along a subduction zone. tion, often without detectable movements. Rocks with compositions from grano­ Aceording to the older synthesis, these gran­ diorites to hornblende gabbros, sometimes ites were generated during the seeond folding with ultramafic differentiates, are encoun­ phase but many observations indieate tered as lenses and zones. There are two that the granitie veins and masses were generations of gabbros - those formed early formed at an early stage of the orogenie evo­ lution. The majority of the granites, how­ ever, are late orogenie. Aeeording to the older synthesis the tee­ tonic evolution comprised an initial folding phase with flat axes and intrusion of the gabbro-granodiorite suite; an intraorogenie quiet phase with intrusion of amphibolitic dykes; and finally a seeond folding phase with steep axes and rising granite domes. This development, however, is too schematie and detailed reinterpretations are ealled for. The here presented interpretation is as follows. The strike of the roek zones deseribe Fig. 2. Subdivision of the archipelago into zones. a large are eonvex towards the south, eorre­ H Hitis, K Kumlinge, N Nagu, P Pargas, Kr Kökarsfjärd rapakivi, Vr Vehmaa rapakivi, Ar sponding in shape with the banded forma­ Aland rapakivi. tion, i.e . the melange along the assumed sub- 8 Geological Survey of Finland, Bulletin 325 duetion zone at the line, where the southern South of the melange zone, at least in the plate was subducted under the northern west, gneissose granodiorites predominate. plate and produeed in the central zone a This zone also contains some banded iron volcanie island are: Attu- Nagu- Korpo-Enk­ ores that differ in type from t he iron ores linge. The western boundary of the are is north of the melange. These granodioritie marked by a sharp bend along a line striking areas are now interpreted as parts of the north. This line is interpreted as a transform above-mentioned continent, which was fault and some channels with the same strike conveyed northwards by the subducting farther east within the are could be due to southern plate. similar faults.

PETROLO GY

Acid gneisses

Gneisses poor in mica and consisting main­ transitions to the mica gneisses. Smallienses ly of feldspars, predominantly plagioclase, of limestone and thin amphibolite layers and quartz are termed here »acid gneisses»; occur in the acid gneiss formation. biotite and rare garnet are the principal dark Primary structures other than banding, minerals. These gneisses, which have often which is interpreted as bedding, are rare and been called »leptites» or »quartz-feldspar often dubious. Types with small feldspar schists», oeeur chiefly in the central and megacrysts are rare, and they are pro bably southern zones but only as minor lenses in acid volcanie rocks. The banded type with the northern zone. In the southern zone they alternating mica-poor and mica-rich layers alternate with amphibolites in a banded for­ suggests a varved greywacke origin, but the mation that will be treated later (p. 11). In purer acid gneis ses are interpreted principal­ some areas thin layers of gneiss poor in mica ly as m etamorphie arkosic silts or sands alternate with layers richer in mica, thus deposited in shallow water. forming banded gneisses that constitute

Mica gneisses

Mica gneisses, often veined, dominate in zones been transformed into garnet, cor­ the northern zone and cover large areas in dierite or sillimanite. Like the banded the central zone. In the southern zone, how­ gneisses, the mica gneisses commonly con­ ever, they are only met with in some cross tain layers poorer in miea indicative of pri­ synclines between granite domes, where they mary graded bedding. Layers of amphibolite are principally of the transitional type, i.e. and zones of rust-brown gneis ses containing banded gneisses. sulphides and graphite occur as intercala­ The mica gneisses consist of feldspars, tions in the mica gneisses. quartz and biotite. The mica has in many The mi ca gneisses, or kinzigites as they are Geological Survey of Finland, B ulletin 325 9 often called, have commonly been interpreted out from land and in deeper water than the as metamorphosed clays. The content of feld­ silts and sands which were transformed to spar and the rather common graded bedding the acid gneis ses. The rusty types indicate indicate that they belong to the greywacke sapropelitic sedimentation in poorly venti­ suite. These sediments were deposited farther lated basins.

Limestones

Limestones occur in the central, southern limestones lie close to or within volcanic and iron ore zones but not in the northern rocks suggests that they originated as vol­ zone. They form lenses or discontinuous canic sinters. But then, the existence of gra­ layers in the acid gneisses and amphibolites, phite in several limestones (Laitakari 1921) but the thickest occurrences lie between the indicates an organic origin (Metzger 1945). amphibolites and the overlying mica gneisses. We interpret the limestones as stromatolitic Some deposits in Pargas are quarried but reef formations (Härme and P erttunen 1964), most of the lenses are thin and impure. indicating shallow water or littoral condi­ Interpretations of the origin of the lime­ tions. stones are conflicting. The fact that many

Iron ores

A schistose magnetic iron ore with inter­ (pehrman 1927). The occurrence ofthin mag­ calations of skarn and mica-rich layers occurs netite layers together with fayalite skarn on under water west of Kalkskär (Fig. 1) . The the skerry of Skärskär (Edelman 1970) is the country rock is rich in chlorite, and the feld­ only typical sedimentary iron ore in the study spar content increases away from the ore so area north ofthe melange zone. Small banded that the rock grades into a gneissose granite iron ores occur in the environment of Ori­ (Veltheim 1971). The Jussarö ore east of the järvi east of the study area. A few sedimen­ study area (Fig. 7) is a large but poor quartz­ tary skarn ores have also been encountered banded iron ore with a few percent of man ga­ there (von Knorring 1955) but most the iron ne se (Mikkola et. al. 1966). The iron ore of Ny­ ores are associated with gabbros or amphibo­ hamn (Fig. 7) , a banded magnetic ore with lay­ lites (Härme 1960). ers of acid gneiss and skarn, lies in the same The general impression is that the large zone west of the study area. The ores are sedi­ sedimentary iron ores occur south of the mentary and similar to those in central Swe­ melange zone, that small skarn ores lie within den. Sm all skarn iron ores occur in the me­ the melange zone, and that iron ores asso­ lange zone (Härme 1960), to the north ofwhich ciated with gabbros are predominant north the iron ores of the study area lie in gabbros of the melange zone. Iron ores are absent and amphibolites as at Attu and Väsby, Korpo from the northern zone and east of it. 10 Geological SUlvey of F inland, Bulletin 325 Amphibolites and associated gabbros

Amphibolites form either thin layers in sitions to hornblende gneisses and are either the gneis ses or thick formations that are metamorphic dacites or mixtures of volcanic often deformed into large troughs. They are and sedimentary material. The origin of the also mixed with acid gneisses in the banded diopside amphibolites is more problematic. formation or melange (p. 11). Although some For chemical reasons Metzger (1945) inter­ of the amphibolite zones are a few tens of preted the diopside amphibolites in Pargas kilometres long it is doubtful whether there as metamorphic marls. Most diopside am­ has ever been a continuous amphibolite for­ phibolites, however, differ in appearance mation in the archipelago. The zones are from those in Pargas, and some exhibit de­ more likely to be remnants of separate volca­ formed pillow structures. They are therefore no es forming an island arc. interpreted as vOlcanics, either pillow lavas, Pillow lava, agglomerate, porphyrite and scoriaceous lavas or tephra layers mixed with other identifiable volcanic structures have carbonates. been found in the amphibolites (Edelman The remapping of an amphibolite syncline 1960; Ehlers 1978). In mineral composition on the islands ofNagu and Lill-Nagu showed they are plagioclase and hornblende, with that the lowermost part ofthe volcanic forma­ biotite or quartz in som types. Remnants of tion is banded gabbro (Fig. 3). The banding, augite have been found in uralite grains in which is steep and parallel to the contacts, is some porphyritic varieties. There are also regular and consists of alternating dark and thick zones of diopside amphibolites con­ light bands resembling igneous layering sisting of plagioclase, hornblende and diop­ rather than metamorphic banding. If it is in side with a pronounced content of sphene. fact igneous layering, it was primarily hori­ The amphibolites are mostly metamor­ zontal and the gabbro must have intruded be­ phic volcanics, basic to intermediate in com­ fore the folding; hence it is contemporaneous position; the quartz-bearing types show tran- with the volcanics. The gabbro then formed a lens or sill beneath the volcano and possibly acted as a magma chamber for it. Several other banded gabbros have been found in the archipelago. 11 In the island of Sorpo there is a gabbro zone which has hornblenditic lenses along the southern contact and which grades into a diorite and a granodiorite towards the north­

11 ern contact (Edelman 1949, p. 15 and Fig. 4). The gabbro is strongly sheared and mingled with granitic veins, which blur the picture. o 2km ....' --""-----', The association is suggestive of a gravita­

~r==lr=t~r:::::=l ~ r=I tively differentiated gabbro sill that intruded ~ t==..J ~ ~ L==.J ~ L::::::.J before the folding. On its eastward continua­ 1 23 45 67 tion on the island of Attu there is a banded Fig. 3. Amphibolite syncline on the island of Lili­ gabbro that contains a titanic iron ore (Pehr­ Nagu. 1 acid gneiss, 2 banded gabbro, 3 amphi­ bolite, 4 limestone, 5 mica gneiss, 6 ultramafite, man 1927; Edelman 1949, 1960). In the eastern 7 microcline granite. part of the island, north or stratigraphically Geological Survey of Finland, Bulletin 325 11 above this gabbro, there is a formation of before the folding. A genetic connection be­ uralite porphyrite. Another insignificant tween them and the overlying volcanics titanic iron ore occurs in an amphibolitic seems likely. Eskola (1914, p. 86) and Härme zone in the northwestern part of Attu (Peru:­ (1954, 1978, 1980) have earlier pointed out the man 1927, 1931 ). On the island of Korpo a connection between gabbros and basic vol­ titanic iron ore occurs in the gabbroic basal canics. Hence there seems to be at least two part of a volcanic amphibolite zone (Pehrman generations of gabbros - subvolcanic gab­ 1927 ; Hausen 1944). bros that intruded before the folding and The above facts show that some gabbros orogenic gabbros emplaced during the fold­ were intruded as horizontal sheets or sills ing stage (p. 14).

The banded formation

In the southern zone a banded formation, although some also form massifs even hun­ previously called »banded series» (Edelrnan, dreds of metres across (Stolpe 1952; Edelrnan 1954, 1956, 1960), forms a long belt of alter­ 1956,1960). Locally the massifs have a layered nating bands of acid gneisses and amphibo­ structure. Rocks of the gabbro-granodiorite lites (Fig. 4). Ultramafic rocks and associated series and smallIenses of limestone or skarn gabbros occur regularly as fragments in nar­ also occur in this formation, which is marked row bands up to tens of metres thick (Fig. 5) , by extremely strong deformation and brec-

Fig. 4. The banded formation. Acid gneis ses with broken and deformed amphi­ bolite layers and gabbroic fragments. Strong shearing and deformation Nötskär, Hitis. 12 Geologieal Survey of Finland, Bulletin 325

Fig. 5. Zone with ultramafie fragments within the banded formation. The pyroxenitie fragments have a brown eore rieh in bronzite, a greenish mantle rieh in eummingtonite and a thin blaek shell of hornblende. Nötskär, Hitis.

ciation. The amphibolitic bands are usually origin, whereas Sederholm <1926, pp. 27-30) bl'oken and deformed and the acid gneis ses has earlier discussed the transformation of have been squeezed between the fragments leptites (acid gneis ses) into gneissose granites (Fig. 4). The grade of plastic deformation through palingenesis. We are inclined to varies according to the competence of the share Sederholm's view and interpret the rocks, and even the strong ultramafic frag­ bulk of the rocks as highly recrystallized and ments have often became eyeshaped (Fig. 5). homogenized portions of the banded forma­ This formation continues for at least 80 tion. The banded structure, the bands ofbasic kilometres eastwards to the Barösund dis­ and ultrabasic fragments (Fig. 6), the inclu­ trict, from where it has been described by sions of layered gabbros and the limestone Sederholm (1926) as a polymigmatite. Kranck lenses lying like strings of beads in the (1933) has interpreted this formation in an »gneissose granodiorites» thus indicating area 40 - 50 kilometres east of the study area primary limestone marker beds, e.g. on as consisting of gabbros and gneissose gra­ Torsö, Storö and other islands farther east nites with different degrees of deformation. (see Laitala's map 1973) favour our interpre­ Laitala (1970, 1973) has mapped these rocks tation. as gabbros and gneissose granodiorites (Fig. The banded formation is at its most typical 6). Kranck and Laitala have considered that in Hitis, where it is also at its' broadest, i.e. the gneissose granodiorites have an igneous more than 10 kilometres wide. In other areas Geological Survey of Finland, Bulletin 325 13

Fig. 6. The banded formation, homogenized to some degree and turned paler; the structure is still recognizable. Western shore of Järnö about 40 kilometres east of the map area (Fig. 1). Järnö was mapped by Kranck (1933 , Fig. 1) as gneissose granite of deformation degree 1 and by Laitala (1973) as quartz diorite - granodiorite.

the formation is more compressed and homo­ tectonic, affecting consolidated rocks and genized, the gneisses and amphibolites being not a sliding of unconsolidated sediments. recrystallized to rocks resembling gneissose The banded formation is intensely deformed, granodiorites, diorites and gabbros, depend­ the amphibolite layers are broken and de­ ing on the primary bulk composition of the formed to schlieren, the ultramafites are formation. These »granodiorites-gabbros» are exotic, judging from the reaction rims against not easily distinguished from the meta­ the country rock (p. 14) and they occur morphosed igneous granodiorites-gabbros as fragments from a few decimetres to hun­ that also occur in this zone. dreds of metres across. The matrix, which is The banded formation shows characteris­ recrsytallized to a medium-grained gneiss, tics of melange. Hsü (1968) defines melanges was apparently a silt initially and not a pelite. >, as mappable bodies of deformed rocks char­ The origin of the banded formation would acterized by the inclusion of tectonically therefore be as folIo ws. Contemporaneous mixed fragments or blocks which may range sedimentation of silts and volcanic activity up to several miles long, in a pervasively produced a formation with alternating layers sheared, fine-grained, and commonly pelitic of acid gneisses and amphibolites. The silts matrix. Each melange includes both exotic indicate that the sea was shallow and small and native blocks and a matrix.» He goes on limestone re8fs were formed on shoals or to point out that the brecciation should be along shores. The formation was compressed 14 Geological Survey of Finland, Bulletin 325 and sheared in a subd uction zone and slabs tween a strongly magnetic southern area and of oceanic crust were forced into this me­ a northern area practically devoid of mag­ lange. The northern margin of the melange netic anomalies (Fig. 11). The magnetic map zone coincides with the boundary line be- will be discussed in a later section.

Ultramafites

Ultramafites of various types occur in the the ultramafites and related gabbros indicate region. The ultramafites of the banded for­ that the primary country rocks were sedi­ mation are principally pyroxenites, whereas ments or volcanics. Hence the ultramafites the ultramafites of the central zone are horn­ seem to be part of oceanic crust. blendites or peridotites. The ultramafites of the central zone are of The ultramafites of the banded formation various types. Least problematic are the occur as zolles of fragments indicating brec­ ultramafites that occur together with and as ciated bands or slabs, but also as massifs differentiates of gabbros (p. 14). They are (Stolpe 1952; Edelman 1956, 1960). They con­ commonly hornblenditic, but olivine-bearing sist mainly of bronzite, augite and minor types also occur. amounts of amphiboles. The fragments com­ Other olivine-bearing ultramafites, horn­ monly have a brownish nucleus of ortho­ blende-peridotites, and related gabbros occur pyroxenes and clinopyroxenes surrounded as lenses in some places, e.g. in the mica by a greenish mantle of cummingtonite and gneiss in the amphibolite syncline of Lill­ finally a thin outermost envelope of horn­ Nagu. They have no visible connections with blende. The same zoning even exists against other infracrustal rocks and show indistinct the joints in the fragments, showing that it pillow lava and agglomerate structures sug­ is undoubtedly due to reactions with the gestive of volcanic origin. surrounding acid material and that the pyrox­ Other ultramafic rocks probably of meta­ enes were not stable in their present environ­ morphic origin have also been encountered ment during metamorphism. The ultra­ (Edelman 1960, p. 23). Their composition is mafites seem to be exotic material that was hornblenditic or perknitic. The origin ofthese forced mechanically into and reacted with ultramafites is obscure, and they need closer the new country rock. Fragments of thin­ investigation. bedded diopside amphibolite and skarn in

Gabbro-granodiorite series

Rocks varying in composition from gab­ and often form thicker lenses than the sub­ broic to granodioritic with associated ultra­ volcanic gabbros. The composition of the mafites (p. 14 occur throughout the study basic members is simple, plagioclase and area. They differ from the above-mentioned hornblende in roughly equal amounts. In gabbros (p. 10) in several respects: They are the central and northern zones, however, not associated with volcanic rocks, they lack they often contain orthopyroxenes. There are banding, and they are more homogeneous transitions to the acid members, which con- Geological Survey of Finland, Bulletin 325 15 sist of plagioclase, q uartz, microcline, biotite granites are scarcely all true magmatic rocks; and often hornblende, too. Orthopyroxene some of them seem to be strongly meta­ is also common in acid members in the north­ morphosed and homogenized supracrustal ern and central zones (Hackman 1923; Hausen rocks, e.g. acid gneisses 01' portions of the 1944; Hietanen 1947). Simonen (1960) assign­ banded formation (p. 12 and Edelman 1949, ed these rocks to the charnockitic suite. 1960; Ehlers 1978). In a number of them the The rocks of the gabbro-granodiorite series transition can be traced from a sedimentary are commonly medium grained, seldom gneiss to a more or less homogeneous rock coarse. As a rule they exhibit a more or less with the appearance of a »gneissose granite» pronounced parallel structure. They gen er­ (Edelman 1960, p. 53). It would be an over­ ally occur as sheets 01' lenses, but bodies with whelming task to divide the »gneissose other shapes are also encountered. The con­ granites» into true magmatic rocks and meta­ tacts tend to be concordant; they are seldom morphic supracrustal rocks, and the origin cutting, and the rocks rarely form dykes. of many of these rocks will have to remain a Some of the rocks show aboundant dark in­ matter of conjecture. clusions, whereas others contain layers or The gneissose granodiorites in Fögli:i and bands of supracrustal rocks. Many are com­ Kökar (Suominen 1980, 1981) cover large paratively free [rom fragments of country areas and thus seem to be more homoge­ rocks. neous than those north of the melange zone. The acid members were formerly called They are pro bably parts of the inferred older gneissose granites even though they are gran­ continent south of the subduction zone (Fig. odioritic 01' tonalitic in composition. The 7). rocks that have been mapped as gneissose

Microcline granites

Microcline granites are common through­ more granitic as these increase in seize and out the region, in many places being even amount. More resistant rocks e .g. amphibo­ predominant. They consist of microcline, lites and gneissose granodiorites tend to be plagioclase and quartz. Biotite is the main criss-crossed by granitic veins. Finally the mafic mineral, but garnet and cordierite are various types of paleosome become trans­ locally common as derivatives of biotite. Mi­ formed more or less into granitic rocks. A crocline often forms porphyroblasts (Edel­ number of processes - recrystallizatioll, man 1972). metasomatic replacement, anatexis and in­ All the transitional types from older rocks trusion - have cooperated with or succeeded through veined gneisses and other migmatites one another to prod uce the migmatites and to more or less homogeneous granites are granites. encountered. Geologists have used different In the northern and central zones the gran­ principles on which to classify the migmatites ites occur as lenses or large irregular areas. but the ensuing discrepancies have no rele­ The old bedding and fold structures of the vance to the present study. The first sign gneisses are locally weIl preserved (Edelman of granitization are microcline porphy­ 1972). In the southern zone the granites form roblasts (Edelman 1982) or granitic veins in domes or diapirs that gene rally have a west­ the gneisses. The rocks become more and ward tilt (Edelman 1949, 1956, 1960). 16 Geological Survey of Finland, Bulletin 325

Without broaching the problem of the age of the first folding phase (Edelman 1973a). of these granites, we should like to point out It is beyond the scope of the present paper to that many observations indicate that they go into the origin and the ages of the micro­ were formed during different stages of oro­ cline granites; we believe, however, that the genie evolution and that their origin is not earlier concept of a close connection between restricted to one definite phase. This is the microcline granites and the second fold­ illustrated where one granite cuts an older ing phase was too rigid. Granites seem to pegmatite (Edelman 1979) and where peg­ form when and where the PTX conditions matite veins lie parallel to the axial plane are suitable for gran itizatio n.

Amphibolite dykes

Dykes with amphibolitic composition are The amphibolitic dykes were formerly in­ more common in Föglö and Kökar than else­ terpreted as having intruded in joints dming where in the region (Sederholm 1934, p. 17 , a quiet and brittle phase between two folding Suominen 1981). East of the study area they phases or between the intrusion of the gab­ are common on the islands west of the bro-granodiorite series and the formation of Barösundsfjärd (Sederholm 1926), and as a the microcline granites (Sederholm 1926; rule they are more common south than north Edelman 1960). As pointed out, the amphi­ of the melange zone. They are even-grained bolitic dykes do not cut, but are themselves and consist principally of hornblende and cut by the microcline granites. A possible ex­ plagioclase although biotite and garnet may planation for this is that the basic magma be present as minor constituents. These could intrude and form dykes only as long as dykes, which probably occur in several gene­ the rocks were brittle enough für jointing, rations, cut the supracrustal rocks and rocks that is, before they became plastic through belonging to the gabbro-granodiorite series, granitization. but when in granites or veined gneisses, they themselves are granitized or cut by granitic dykes.

Some aspects of metamorphism

Although systematic studies of the mineral and pyroxene in pegmatites (Edelman 1982) facies in the region are lacking, light can be point to temperatures close to those of the thrown on some aspects of the metamor­ granulite facies in large areas (Fig. 7). Schelle­ phism. The rocks of the southern zone seem kens' (1980, p. 17) investigations in the Attu to have been formed principally under con­ area in the central zone indicate »meta­ ditions of amphibolite facies. The garnet, morphism in the amphibolite and the lower cordierite, sillimanite and hypersthene of the granulite facies». His determinations show many rocks in the central and northern zones temperatures of 704°-788°C and pressures of (Hackman 1923, H ausen 1944, Hietanen 1947, 5.1 - 5.5 Kb, or a gradient of about 40 °C/kilo­ Edelman 1960), and the formation of garnet metre. Kay's values for the Merimasku-Naan- Geological Survey of Finland, Bulletin 325 17 tali area in the northern zone (Campbell1980, The above-mentioned facts indicate that p . 197) are 690°-743°C and 3.2 - 4.4 Kb, or there is a zone of high temperature or low gradients between 45°C and 65 °C/kilometre. P IT metamorphism same tens of kilometres Eastward of the present region we find char­ north of the melange, or subduction zone nockitic rocks in the Lohja region (Fig. 7, (Fig. 7). According to Miyashiro (1967), a belt Parras 1958). oflow P/T metamorphism lies on the inner or The orthopyroxene, bronzite (Stolpe 1952), concave side of subduction zones but a high has been observed in the southern zone only P /T or high pressure metamorphism zone in the ultramafic fragments. The bronzite closer to the su bd uction zones. A belt of high was, however, not stable in this environment pressure metamorphism has not been found because the brownish cores are surrounded in the study area. Erosion, however, may by shells of cummingtonite with hornblende have removed the traces of such a zone, be­ in the outermost portion (p. 14). This fact cause the study area represents a m uch deep­ adds weight to the interpretation that these er level than that in Japan. The signs of a ultramafic rocks are of exotic origin, and that granulite facies belt north of the inferred they were forced into levels characterized by subduction zone indicate metamorphic con­ conditions of amphibolite facies. ditions similar to those in Japan.

STRA TIGRAPHY

Sequences

Our discussion of the stratigraphy of the man 1960), but the resulting correlations tend study area (Table 1) begins with the central to be somewhat strained. zone, which is marked by its straightforward A trough or synform in Nagu (Fig. 3) con­ stratigraphy, simple structures and disti.nct sists ofvolcanic amphibolites (Edelman 1960, formations. This zone has been used earlier 1972; Lauren 1973) surrounded by a banded as a key area for the whole archipelago (Edel- granite and acid gneisses. The granite has

Table l. Stratigraphie scheme of the archipelago of southwestern Finland.

Iron ore zone Southern zone Central zone N orthern zone

Silts, clays, lime­ Strongly deformed mix­ Greywackes or clays Greywackes or clays stones, banded iron tures of silts and with volcanic layers with volcanic layers ores, and basic vol­ basic to intermediate Limestones canics in large volcanics, ultramafic Intermediate to basic Intermediate to basic gneissose granodiorites fragments and massifs volcanics with gabbros volcanics with with basic different­ and a few limes tone gabbros iates. lenses. Silts, sparse volcanic Greywackes, sparse layers and limestones volcanic layers and silt lenses 18 Geulogical Survey of Finland, Bulletin 325

been interpreted as a granitized acid gneiss tion; acid gneisses should be exposed in the owing to its locally spectacular banding that northern zone only in axial culminations. is conformable with the contact of the am­ This inference is, however, far-fetched and phibolites (Edelman 1972). Within the am­ the correlations with the central zone are phibolite syncline or stratigraphically over­ forced. Farther east in the Forssa area (Neu­ lying the amphibolites there are scattered vonen 1956) we find a similar sequence of limestone lenses and a thick mica gneiss for­ amphibolites and acid gneisses within a pre­ mation. Amphibolitic layers in the acid dominantly mica gneiss formation. A more gneisses emd the mica gneisses show that plausible interpretation of the stratigraphy is volcanic activity was not restricted to the as follows. Mica gneisses dominate above and period of amphibolite formation. The se­ below the vOlcanics, which probably do not quence of this syncline is: form a continuous formation but are separate lenses penecontemporaneous with the vol­ mica gneiss - greywacke clay canic formation of the central zone. The acid limes tone - reefs, probably stromatolitic gneis ses also occur as discrete lenses only amphibolite with associated gabbroes - basic occasionally associated with the amphibo­ to intermediate volcanics with magma lites. chambers or sills acid gneiss and banded granite - silts The banded formation ofthe southern zone was previously divided into two parts, the Similar sequences have been found in sev­ one with predominant acid gneisses and the eral synforms, indicating that the sequences other with predominant amphibolites (Edel­ are normal and not inverted (Edelman 1960) man 1956, 1960). This was, however, yet an­ and that the synforms are true synclines. The other strained attempt to work out a correla­ Enklinge syncline (Ehlers 1978) lacks the tion with the stratigraphy of the central zone, mica gneiss, and the uppermost formation and it is doubtful whether the amphibolites consists of volcanics. According to Metzger primarily formed a continuous formation. If (1945) there are in the Pargas synclinorium our interpretation of the banded formation two amphibolite formations within kinzigites as a melange along a subduction zone is cor­ or mica gneisses. Many of Metzger's kinzi­ rect, it is meaningless to subdivide it strati­ gites (1945, p. 22) have about 90 per cent salic graphically. Intense brecciation and deforma­ minerals so we would classifY them as acid tion have so mixed the volcanic and sedimen­ gneisses. The structure of the Pargas syn­ tary beds and intermingled them with slabs clinorium is rather complicated and it could of oceanic crust, perhaps even with mantle be interpreted in a number of ways, e.g. a material, and igneous rocks that we have no refolding of a thrust fold as suggested by earl way of reconstructing the primary sequence. Ehlers (oral communication). We can only presume that the banded forma­ Mica gneisses are predominant in the north­ tion originally formed a unit of alternating ern zone. Amphibolites form separate lenses sedimentary and volcanic layers. but they also occur as thin layers in the The iron ore zone south of the southern gneisses. Acid gneisses are rare and occur zone is mostly covered by sea in the eastern as comparatively small lenses. Limestone is part of the study area but in the western part, completely absent. In the opinion ofEdelman where the melange zone bends to northwest, (1960), the sequence is identical with that of large areas of gneissose granodiorites with the central zone. He attributed the rarity of inclusions of gneisses, amphibolites and acid gneisses to the level of the present sec- limestones lenses are exposed in Kökar and Geological Survey of Finland, Bulletin 325 19 Föglö. This zone is interpreted as the bord er the gabbros north of the melange zone (p. 9). of an old eontinent south of the subduetion Amphibolitie dykes are eommon in the ir on zone (p. 15, Fig. 7). The banded iron ores of ore zone but eomparatively rare in other this zone are similar to those of eentral Swe­ zones. No stratigraphie sequenee has hitherto den but differ from the ores assoeiated with been established for the iron ore zone.

Correlation

It is diffieult to eorrelate the stratigraphie because these sediments are deposited in sequenees of the zones in the arehipelago different environments on two separate plates. with eaeh other. The stratigraphie eolumns A summary of the stratigraphie sequenees presented here are generalizations, and closer of the region and their indications of former studies will doubtless reveal many loeal di­ environments reveals the following picture. vergeneies from the general scheme (Table 1). The iron ore zone represents the margin of a The first attempt at eorrelation was based on eontinent with shelf sediments. The banded the assumption that the basic volcanic forma­ formation is the resCllt of voleanie activity tion could be used as key horizon (Edelman and rapid sedimentation from the souree area 1960). This seems to be valid for the eentral in the south, the eontinent. This formation zone, and we are probably justified in correla­ was later mixed with slabs from the oceanic ting the amphibolites of the northern zone erust in the subduetion zone. with this formation. The main difference be­ In the eentral zone the environment tween the central zone and the northern zone changed considerably d uring evolution. The is that the sedimentary roeks below the vol­ sequenee beg ins with silts with sparse inter­ canie formation in the eentral zone are silty, ealations of volcanie layers and limestones, aeid gneisses, whereas in the northern zone indieating a shallow sea and loeally littoral they are clayey or greywacke-like mica conditions with reefs. The volcanie forma­ gneisses. What is more, limestones are absent tion with the bulk of the limestones shows from the northern zone. that volcanoes grew to sea level and formed Correlation with southern zone is impos­ islands with limestone reefs. The volcanie sible because of the strong deformation and phase was followed by sedimentation of clays mixing of the roeks there. If the amphibo­ or greywaekes, implying a pronouneed deep­ lites of the southern zone correspond to the ening of the sea. volcanic formation of the central zone, very In the northern zone the volcanie forma­ rapid eontemporaneous sedimentation must tions lie within thiek sequenees of clays and have taken plaee, as is shown by the alterna­ greywaekes, giving evidenee of almost con­ tion of aeid gneisses and amphibolites. tinuous deep-sea eonditions. Silty shallow The iron ore zone eontaining gneisses and water sediments are rare and limestone reefs banded iron ores is interpreted as the shelf are altogether lacking. of a eontinent with its sediments. The lime­ The study area seems to have been a seg­ stones of thls zone imply littoral conditions. ment of the southern part of a trough striking The sedimentation is eonsidered to have been east; its northern bord er is probably repre­ more or less eontemporaneous with the sedi­ sented by the sehist belt ofTampere (Tammer­ mentation in the basin north of the melange fors), where the stratigraphie sequenee is zone, but a clear eorrelation is not feasible entirely different (Simonen 1953): it begins 20 Geological SUIVey of FiJ'land, Bulletin 325 with grewaekes overlain by silty sediments, Evolution there began with a deep sea whieh and the uppermost formation eonsists of grew shallower till finally volcanoes reaehed basic volcanies with thieh eonglomerates. sea level.

TECTONICS

Outline

As mentioned above the region has been ? subdivided on both struetural and strat­ 1. 2. 3. 4 . 5. 6. igraphie grounds (Fig. 2) beeause of the in­ terdependenee between tectonies and stra­ tigraphy. The meehanieal properties of rocks affeet the struetures, and tectonies influenee the sedimentation and metamorphism of rocks. The predominant regional strueture is a large are eonvex against the south. It bends sharply to the west along the line Kökar• Lantö (Figs. 1 and 2). This are dominates the whole region and eontinues eastwards for ab out 80 kilometres to the Barösund area, Fig. 7. Southwestern Finland and northwestern Estonia showing the areas with high-temperature where it is terminated by another bend metamorphism and the "Archaean» (early Protero­ (Fig. 7). Ares are typieal struetures of oro­ zoic) area in Estonia. The northern border of the genie zones and often form festoons with latter is hypothetical because it is under the sea. 1 Archaean rocks (correctly early Proterozoic), rather sharp angles between the individual 2 Svecofennidic rocks, 3 the melange or subduction ares. Sinee, however, our diseussion is eon­ zone, 4 areas with high-temperature metamor­ fined mainly to the study area, we ean do phism, 5 postorogenic granites (rapakivis), 6 the banded iran ores. A Attu, B Barösundsfjärd, L no more than hint at some possible con­ Lohja, M Merimasku- Naantali area, J , K and N the nections with struetures outside it. iran ores at Jussarö, Kalkskär and Nyhamn.

Structures

The northern zone has been mapped by to be fairly simple. The folds are open and different geologists from different stand­ sinuous, often with flat-lying erests and points (Hietanen 1947; Hackman 1923; Metz­ troughs (Fig. 8). The prirnary fold axes are ger 1945; Edelman 1960); our personal knowl­ flat, and the steep axial surfaees eurve along edge of this zone is limited. The area is the large are. A seeondary folding with eurrently being remapped. With reservations steep axes has taken plaee but it seems to for misinterpretations the struetures seem be less irnportant here than farther south. Geological Survey of Finland, Bulletin 325 21

N whenever and wherever a heavy rock overlies a lighter and sufficiently plastic one, com­ monly a granite; it is therefore not limited D6b~lFl~ 1 2 3 4 5 6 to a certain phase of of the orogeny. Ac­ Fig. 8. Schematic profile through the archipelago cording to Metzger (1945) the synform of showing the tectonic style from rather open folds Pargas, which has not been studied from in the north to granitic domes with strongly com­ pressed synclines in the south. 1 acid gneisses, this standpoint, is shallow. As mentioned 2 amphibolites, 3 the banded formation, 4 mica above (p. 18) , Ehlers has tentatively inter­ gneisses, 5 granodiorite-gabbros, 6 microcline preted it as a refolded thrust fold. On the granites with migmatites. basis of an increasing grade of metamor­ phism upwards in a sequence of gneisses with moderate dips Julius Tesseire (personal On the basis of our current knowledge we communication) has proposed similar ideas can summarize the tectonics of the northern of thrusts for an area north of the Nagu zone as follows. The soft mica gneisses that syncline. The existence of large-scale thrusts dominate in the northern zone have been is feasible in this region, but they cannot compressed into large open folds by rather be confirmed without further investigation. weak orogenic stresses. Granitization in this zone has prod uced many types of rocks, from Once the layers in the central zone had veined gneis ses to large homogeneous gran­ been raised to steep or vertical attitudes they ites, which have not as a rule, however, could then be folded with steep axes. The developed into characteristic domes or dia­ same happened in the northern zone. As a pirs as in the southern zone. rule, folds like this in the central zone are small drag folds. The large Z folds of Sälsö The most pronounced structures in the and Kumlinge will be discussed later (p. 23). central zone are the amphibolite synclines An isoclinal amphibolite synform along the or troughs (Figs 3 and 8; Edelman 1960, 1972; southern border of the central zone sank Ehlers 1976, 1978; Lauren 1973; Metzger and was compressed by rising granite domes 1945), produced by the first folding, with (Fig. 8). The tectonic style changes here to flat und ulating axes. The axial surfaces are that of the southern zone where thin isoclinal steep and strike along the large arc. Some synclines of gneisses wind between granite of the synclines have accentuated their domes and diapirs. shapes by sinking into the surrounding light­ The southern zone is crucial to the inter­ er granites (Ehlers 1978; Lauren 1973). Ver­ pretation of the tectonics. The rocks of this tical movements did not playa very impor­ zone can be divided by their tectonic style ta nt role in the syncline of Nagu. The weak into granites, gneisses and banded formation. mica gneisses in the centre of the syncline, The migmatite granites form domes that where lack of space was greatest during the commonly tilt to the west; they have mod­ folding, were most strongly deformed. In the erately dipping eastern flanks and steep or syncline of Enklinge, north of Kumlinge, vertical, sometimes overturned, western the sinking was even more pronounced and flanks (Edelman 1949, 1956, 1960). The upper has produced marked deformation (Ehlers parts of the granite domes were thrust west­ 1976, 1978). The sinking of the amphibolite wards, possibly because of stretching along synclines seems to be a local phenomenon, the large arc. When the granites rose, gneisses and probably did not occur simultaneously and amphibolites were sucked down be­ throughout the region. Sinking takes place tween the domes. This is the same process 22 Geological Survey of Finland, Bulletin 325 as the sinking of the synclines in the central margin of a continent transported north­ zone, only more pronounced. wards by the subducting plate. The hypoth­ The banded formation constitutes a long esis that a continent has been carried to curved band that has been compressed be­ the subduction zone is compatible with the tween two rows of granite domes and it is fact that the sedimentary rocks are silty in broad in Hitis but narrow west of it. The the southern zone of the archipelago and broad part is most typical; in the compressed clayey farther north. parts the formation was homogenized and The rocks on both sides of the melange more thoroughly recrystallized (Figs. 3- 5). zone have not been studied in enough detail The strong brecciation and deformation and to warrant comparison; we can only point the presence of ultramafic rocks in this zone out some differences. The greater aboundance are compatible with the interpretation of it of amphibolitic dykes south of the melange as a melange in a subduction zone. The fact zone than in other zones indicates that it was that the arc is concave northwards shows easy for fractures to cut the crust there. that the southern plate was the subducting When the inferred continent approached the one. The granite domes north of the banded su bd uction zone, the margin and especially formation are much larger than the domes the protruding parts of the continent were south of it, and this assymmetry is probably bent and fractured and permitted the basic a result of northward subduction. magma to intrude and form dykes. Joints, The position of the banded formation be­ and consequently also dykes, did not develop tween granite domes does not seem to be easily in the soft sedimentary rocks north consistent with a subduction zone, in wh ich of the melange zone. one oceanic plate sinks beneath another one; The bulk ofthe continent is hidden beneath granitic material is hardly expected below the Baltic sea but its continuation is encoun­ such a zone. On the other hand, the melange tered in Estonia. Volkolakov (1974, p . 151) will probably be forced up into 01' over the has published a map showing Archaean rocks sediments, which would then later be trans­ in northwestern Estonia and on the island formed into granites. Another plausible ex­ of Dagö, about 100 kilometres south of the planation is that the subducting plate carried study area. Puura (1974, p . 49) also mentions a continent, the iron ore zone, to the sub­ Archaean, more correctly early Proterozoic, duction zone and that the melange was partly rocks with ages of 2364 and 2450 Ma in Esto­ forced upon this. Beckinsale & Mitchel (1981) nia (Fig. 7). He writes: "A hypothesis is pro­ describe a collision between a continent and posed that the Archean (?) rocks are to some aisland arc and show in a section how extent subjected to tectonic and metamor­ material of the oceanic crust occurs as slabs phic processes ofthe Proterozoic.» In Estonia between colliding blocks of sial and how the ages were determined by the K - Ar meth­ granites rise on both sides of the seam. Tha­ od but in Finland by Pb- U methods; they kur (1982, Fig. 4a) has published a section are therefore not truly comparable. Even so, from the western Himalayas where an ophi­ it is highly unlikely that an age difference olitic melange and ultrabasites lie between of 500 Ma between Estonian and Svecofen­ two granitic domes. Coleman (1971) describes nian rocks in Finland can be attributed how slabs of mantle and oceanic crust have entirely to methodical errors. The latest age been thrust over sediments in New Caledonia determinations certainly support the assump­ and N ew Guinea. tion that the crust of Estonia noticeably The iron ore zone is interpreted as the predates the Svecocarelian orogeny. Geological Survey of Finland, Bulletin 325 23

Faults

Faults or displaeements at deeper levels a b ean be inferred from eertain structures in the region on a geologieal basis. Displaee­ ments in the magnetie anomalies, on the other hand, will be deseribed and diseussed in the next ehapter. The strike makes sharp bends along a line from the sea west of Kökar northwards to Lantö (Fig. 9). An S fold has developed around Skattskär in the southern part of this line (Suominen 1972, 1980), and a few kilometres farther north we find a Z fold on 40km the island of Sälsö (Lauren 1969; Edelman 1979). Still farther north there is a mueh Fig. 9. a. Schematic map of the area along the line larger Z fold around Kumlinge (Ehlers 1976, Kökar- Lantö in the western part of the region. The dashes indicate the strike of the banding. 1978). At the northern end of this line the Ku Kumlinge, L Lantö, Sk Skattskär, S Sälsö, strikes turn from about 150° southeast of the F diabase dyke of Föglö, M and Se postorogenic island of Lantö to 90° actually on the island granites of Mäshaga and Seglinge, Kr and Ar still younger rapakivi granites of Kökarsfjärden and of (Raneken 1953; Ehlers 1978). Aland. This line is the western boundary of the b. Sketch map of the area with the subduction zone and an inferred transform fault along the line large are, and the question is whether there Kökar-Lantö. The protruding part of the southern is a eonnection between the above-men­ continent, Y-shaped signs, was apparently more or tioned folds and the boundary of the are. less crushed in the collision and offered pathways for postorogenic magmas, granitic as weil as basal- The folds on Sälsö and Kumlinge are Z tic. folds with cloekwise rotation. These areas exhibit an older folding with axes and axial planes parallel to the strike of the roek zones o bviously as a result of a stress acting at been eaused by subduetion (Fig. lOb). When right angles to the are. A seeond folding the southern plate east of the transform with steep axes and axial planes striking fault reaehed the subduetion zone and bent about 90° took plaee along the above line. downwards, the veloeity of the upper part This cloekwise rotation ean be attributed of the plate inereased eompared with that either to a stress from the north or to a ofthe western plate beeause it rotated around eouple of forees aeting along the strike of an axis at a lower level. the banding. The right-hand movement along The fault zone Kökar- Lantö seems to be the line Kökar- Lantö was probably foreed the eastern boundary of a protruding part into cloekwise rotation by the strike in the of the southern plate with a eontinental gneisses above the plates (Fig. 10e). The line erust, viz. the gneissose granodiorites of Kökar- Lantö is interpreted as a dextral Kökar and Föglö and the areas south and trans form fault (Fig. 10a). west of the rapakivi massif of Aland. The The S fold at Skattskär, with is eounter­ eontinental protrusion is obviously eonneet­ cloekwise rotation, is problematie. This eon­ ed with the transform fault and the be nd trary direetion of the movement eould have east of the fault. The intense jointing of the 24 Geological Survey of Finland, Bulletin 325

Fig. 10. a. Schematic block diagram ofthe area along the line Kökar-Lantö showing the right-hand movement along the inferred transform fault. K Kökar, L LantÖ. b. Block dia gram showing a possible explanation for the left-hand movement in the fold around Skattskär. The upper side of the subducting plate moves faster at the bend because it was running the outer track. c. Block diagram showing how the shear movement along a transform fault in the underlying plates causes in the overlying rocks shear along the strike of the ban ding.

protrusion as a result of the collision pro­ 30 kilometres long and only 3 kilometres duced channels along which the various broad. On either side of it the strike is magmas could rise. During the orogeny nu­ about 150°, whereas on the skerries within merous amphibolitic dykes were formed in the channel and on some islands closeby the the area of Kökar, Föglö, the southwestern strike is 180°. The channel could hence be Äland archipelago and the Swedish archipel­ interpreted as a right-hand fault in the und er­ ago. Later a set of diabase dykes intruded lying plate, analogous to the line Kökar­ a zone striking north-northeast from Föglö Lantö. The channel is not quite straight but, (Sederholm 1934; Ehlers & Ehlers 1977). The characteristically of transform faults, curved line Kökar- Lantö coincides with the eastern a few degrees implying rotation around a contacts of the rapakivi massifs of Kökars• pole somewhere on the western coast of fjärden and of Aland. The small postorogenic Norway. The pillow lavas that are exposed granites of Mashaga and Seglinge also lie on skerries in this channel may have erupted along this line (Fig. 9). along this fault although it has not been The other large Z folds in the region could proved. An analogous spatial connection likewise be attributed to minor faults in the between a long channel and a zone of basic underlying plates. There is a Z fold along rocks is evident farther east. the channel of Vattuskiftet, which is about ------

K A N

40 km

Fig. 11. Aeromagnetic map ofthe archipelago with a clarifyingsketchabove. The map has been reduced from maps on ascale of 1 : 100000 made by the Geological Survey ofFinland. The subduction zone marked with double dashes is the dividing line between the magnetic conti­ nental rocks in the south and the nonmagnetic geosynclinal rocks in the north. Fragments of the magnetic continent were pushed into the sediments around Nötö, where the subduction zone makes an angle. K, N and A Korpo, Nagu and Attu, parts of a volcanic island are, Nö Nötö area, A and Kö rapakivi areas of Aland and Kökarsfjärden, M and S postorogenie granites of Mäshaga and Seglinge, F diabase dyke of Föglö, Ny, Ka and J iron ores of Nyhamn, Kalkskär and Jussarö. Published with permission of the Geological Survey of Finland. 26 Geological Survey of .ti·mland, Bulletin 325 Magnetic structures

The areo-magnetic map (Fig. 11) of the present regiun shows a conspicuous border line between a southerly area crammed with strong magnetic anomalies and a northerly one poor in anomalies. The borderline co­ incides mainly with the northern limit of

...... the banded formation or the melange zone. >-"; -. . .:- West of Hitis the line follows the southern >- - border of the Gullkrona graben (Edelman 1949) but in the area of Hitis the magnetic ~ borderline continues through a dense archi­ Fig. 12. Block diagram and profiles showing the subduction zone. The upper pictures show the area pelago, showing that it is independent of around Nötö where slabs of the southern continent the topography. In the westernmost seetion were pressed in to the supracrustal rocks. The lower of Kökar- Sottunga, too, the magnetic bor­ pictures are of the area farther east where only slices of the oceanic crust were pushed up into the derline is straight and conspicuous. In an sediments. intermediate section between Nötö and Kökar the borderline is strongly disturbed by lens­ shaped anomalies that spread fan-like north­ bay dotted sparcely with skerries which may wards. A plausible explanation for this dis­ make it difficult to find geological evidence crepancy is that a protruding part of the of the faults. Since there are no signs of southern continent reached the subduction the inferred faults farther north, they can be zone and was cut into slices that were forced explained either as transform faults contem­ into the sedimentary rocks of the northern poraneous with the subduction or as a post­ plate (Fig. 12). The disturbed seetion coin­ orogenie graben. In the latter case, the non­ eides with an angle in the melange zone. magnetic rocks of the northern region that East of Hitis conspicuous breaks occur in were thrust over the southern plate were the magnetic anomalies along lines striking preserved in the graben. north. The strongly magnetic zone seems Although the explanations for the details to be displaced about 12 kilometres south­ of the magnetic map are still tentative, the wards east of Hitis; about 15 kilometres magnetic borderline along the melange zone farther east the magnetic anomalies are dis­ favour the interpretation of it as a boundary placed stepwise about 8 kilometres north­ between bedrocks of different origin and wards. Between these potential faults is a belonging to different plates.

Summary of the tectonics

When summanzmg the above concept of or basins that in some places show signs the tectonics of the region (Figs. 8 and 13) of sinking into the surrounding lighter gran­ one can state that in the northern zone the ites. Attempts to interpret some structures folding is open, sinusoidal. In the central as thrust sheets have been made but they zone there are several amphibolite synclines have still to be established. In the southern Geological Survey of Finland, Bulletin 325 27 zone vertical movements have been more intrusion of basic magma in the bent and pronounced; risen and tilted granite domes fractured continental margin close to the or diapirs surrounded by sunken gneiss or subduction zone. The iron ores sedimented amphibolite zones are the predominant forms. on the shelf of this continent were folded Hence, the mobility of the granites has in­ together with other shelf sediments du ring tensified towards the zone of exceptionally the orogeny. deformed banded formation, the melange or The structures of the area around Stock­ subduction zone. In the deeper levels around holm in Sweden, southwest of the study this zone the conditions were apparently area, were interpreted by Stälhös (1981) as suitable for granitization; density difference the result of two folding phases, the older and orogenic stress forced granite domes up one with a stress from the east and the on both sides of the subduction zone. The younger one with a stress in a north-south ultramafic breccias of the banded formation direction. The foldings could tentatively be are crushed slices of the oceanic crust, or attributed to stresses from the southern possibly of the mantle, that were mechan­ continent according to Tapponier's theory of ically forced into the melange. colliding continents with protrusions (Tap­ The exposed parts of the iron ore zone ponier & Molnar 1977). The area of collision are considered to belong to an older con­ was more or less crushed and therefore tinent in the south. They are dominated by offered suitable channels for the intrusion of gneissose granodiorites cut by numerous the amphibolitic dykes, postorogenic dia­ amphibolitic dykes that were formed through bases and granites of Aland.

GEOLOGICAL EVOLUTION

The interpretation pres~nted here gives from silts into greywackes and clays. Along the following picture of the geological evolu­ the subduction zone a melange was formed tion (Fig. 13). The study area was part of a through the intermingling of silts with vol­ large trough in southern Finland. The Tam­ canic material and ultramafic slabs from the pere (Tammerfors) schist belt probably re­ oceanic crust. In the central zone the sedi­ presented the northern border of this basin. mentation began with silty sediments in N owadays this zone is ab out 200 kilometres shallow water and was followed by the origin wide but the trough must have been much of a volcanic island arc with limestone wider before the folding. reefs. Finally the area sank into deep sea with Along the melange zone a southern plate sedimentation of greywackes and clays. In was subducted under a northern one. The the northern zone the sea was deep during southern plate carried a continent that was the whole evolution. From the deeper sec­ the source area for the sediments in the sea tions of the subduction zone magmas rose basin, which was shallow in front of the and produced the volcanoes of the central continent and became deeper northwards. and northern zones with associated subvol­ The sediments accordingly became more canic gabbros. fine grained in a northerly direction, grading The advance of the southern plate caused 28 Geological Survey of Finland, Bulletin 325

s N are therefore most common there. Later the fractured area offered a convenient pathway for postorogenie diabase and granite intru­ sions. Judging from the fan of lens-shaped mag­ netic anomalies north of the melange zone in this area, some slices of the southern con­ tinent seem to have been forced over the northern plate in the area around Nötö. Granitic veins were formed in different rocks by anatexis, granitization and intru­ sion; much of the granitic material formed 1 2 . 3. 4. 5. 6. large massifs. The best developed granitic EJ~Du0~ domes rose on both sides of the subduction zone; those north of the subduction zone Fig. 13. Profiles demonstrating the hypothetical evolution of the region. The uppe!" profile shows being larger than those south of it, probably sedimentation of coarser material closer to the because the southern plate was subducted southern continent and that of finer sediments farther north. Subduction has begun and slabs of und er the northern one. Hence the conditions the oceanic crust have been pushed up. Volcanoes were not the same on both sides of the zone. are formed r,orth ofthe subduction zone. Thelower The concepts presented here are consistent profile refers to a late stage with folded supra­ crrustal rocks and rising granite domes. 1 the with those of Hietanen (1975). She proposed southern continent, 2 oceanic crust, 3 silty and an island arc through southern Finland, the sandy sediments, 4 greywacke sediments, 5 outer margin of which she drew along the volcanic materials, 6 granites and migmatites. southern coast, in fact almost along the sub­ duction zone that we suggest. Sederholm (1930) and Metzger (959) have also drawn folding in the supracrustal rocks. The inten­ maps with the boundary between a mobilized sity of the folding and deformation decreased older basement in the south and Svecofen­ northwards from the subduction zone. In nian areas in the north coinciding with the connection with this folding gabbro- grano­ subduction zone proposed here. Metzger dioritic magmas intruded a s lenses in the writes (1959, p. 10): »Ganz in SW und S finden supracrustal rocks. Later, basic magmas rose wir denn die Gebiete der Urgranite, die along joints in the brittle rocks and formed Sederholm schon immer als Gegenstücke der amphibolitic dykes. The jointing was espe­ östlichen Granitgeisplatte angesehen hat.» cially pronounced in the protruding parts of Observations by Estonian geologists lend the southern continent, where stresses were support to the inferred continent south of the strongest as a result of the collision; dykes study area (p. 22).

CONCLUDING REMARKS

The interpretation presented here of the the final solution to the geological and tecto­ geology of the archipelago is by no means nical evolution of the region. Nevertheless, Geological Survey of Finland, Bulletin 325 29 we hope that it is a step in the right direc­ formation of granitic magmas. The am phi­ tion and that it will provide fresh ideas for bolitic dykes were earlier thought to repre­ studies, which may correct and complete sent a quiet phase that separated the two the present synthesis. Our interpretation principal folding phases. We are not sure that differs in many respects from that published the fracturing and dyke formation implies a earlier (Edelman 1960). There would seem quiet phase; fracturing mayaiso occur during to be justification for summarizing the prin­ the culmination of the orogenic movements. cipal divergencies between these two inter­ In the iron ore zone the dyke formation pretations. seems to be associated wich the ben ding of The stratigraphic correlation of the earlier the continental rand as it approaches the theory was strained by the attempt to use subduction zone. the column of the central zone as a model The earlier division of the folding into two for the whole region. The new interpretation separate phases seems to be too schematic. is based on lateral changes in the sediment­ In some areas there are signs of only one ary facies within the formations. Further­ distinct phase of folding, in others the second more, the banded formation, the origin of folding phase is rather distinct, and in yet which was not previously established, is now others still more folding phases can surely be interpreted as a melange in a subduction perceived. zone. The iron ore zone that was connected Our interpretation of the geology of the with the southern zone is now interpreted as archipelago has provided explanations for a continental margin with shelf sediments. some long recognized problems, e.g. the The zone should be correlated with the lep­ origin of the banded formation. It has also tite formation rich in iron ores in central brought to light some differences overlooked Sweden. whereas the other zones of the earlier and offered some explanations for archipelago should be correlated with the them, e.g. the two types of gabbros and the bedrock of southern Norrland in Sweden. differences between the iron ore zone and This could also solve the old controversy be­ the other zones. Our hypothesis offer many tween Swedish and Finnish geologists about new problems for further investigation, the the origin of "leptites», which arose because results of which will either confirm or deny the sedimentary acid gneisses of southern it. Some suggestions for future studies are: Finland were correlated with the volcanic 1) Classification of the gabbros into different leptites of central Sweden. suites. This study is already in progress. Gabbros, diorites and granodiorites were 2) The ultramafics in the melange and in formerly uni ted as one differentiation suite other parts of the region. that intrude during the first folding phase. 3) Metamorphism and homogenization ofthe Now these rocks are subdivided into two rocks in the melange zone. suites, a subvolcanic one associated with vol­ 4) Comparison of the granodiorites south canics and an orogenic one that intruded and north of the melange zone. during folding. The migmatic microcline 5) Remapping of the area around Nötö to granites and granitic veins do not seem to find the answer to the fan-shaped magne­ have formed exclusively during the second tic anomalies north of the melange zone. folding phase, as formerly interpreted; rather, 6) Comparison of the amphibolitic dykes they seem to have formed during much ofthe south and north of the melange zone. orogenic period. The postorogenic potassium 7) Geophysical investigations over the me­ granites are probably the result of continued lange zone. 30 Gcological Survey of Finland, Bulletin 325

ACKNOWLEDGEMENTS

Professor L K Kauranne, director of the Geo­ Mrs Merja Puumala draw the figures and typed the logical Survey of Finland, has kindly permitted manuscript several times. Mrs Gillian Häkli cor­ the publication of our paper in this series. Geo­ rected the English. To all these persons we express logical aspects were discussed with our fellow our gratitude. workers, Mr Carl Ehlers and Mr Leif Bergman. Geological Survey of Finland, Bulletin 325 31

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