The Scandinavian Caledonides:

Geology in the north-central Swedish Caledonides around Villa Heidelberg
Klimpfjäll (Vilhelmina, SW Västerbotten, S Lapland, Sweden)

B. Lithologic-structural sequence, age data and mineral deposits

The geological units distinguished on the maps are principally lithological or lithostratigraphic in character. These units are separated by major and minor thrusts, as demonstrated by the geological maps and cross-sections (e.g. Zachrisson 1991, Zachrisson and Sjöstrand 1990, Zachrisson and Greiling 1993). These already established thrust units were subsequently disturbed by repeated local out of-sequence thrusting, which affected, for example, the relationships between the Lower and the Middle Allochthon, the Middle and the Upper Allochthon, and between Seve subunits (Zachrisson 1993, Zachrisson and Greiling 1993).

The major units are described in ascending order:

1. Autochthon
1.1 Baltic Shield
1.2. Torneträsk Group
2. Lower Allochthon
2.1 Blaik Nappe Complex
2.2 Börgefjell Duplex
3. Middle Allochthon
3.1 Stalon Nappe and Rainesklumpen unit
3.2 Fjällfjäll Nappe, Dearka Nappe
3.3 Särv Nappe
4. Upper Allochthon
4.1 Seve Nappe
4.1.1 Eastern Belt
4.1.2 Central Belt
4.1.3 Western Belt
4.2 Köli Nappe
4.2.1 Lower Köli, Virisen Terrane
4.2.2 Middle Köli, Gjersvik Terrane
4.2.3 Upper Köli, Storfjället Terrane
4.2.4 Stratabound sulphide deposits
5. Uppermost Allochthon

1. AUTOCHTHON

1.1 Crystalline basement of the Baltic Shield

The crystalline basement at and to the east of the Caledonian margin in northern Sweden south of the Skellefte Field is built up of the coarse-grained Svecofennian Revsund granite, associated (older) acidic gneisses and minor volumes of other rock types (see Lundqvist 1979, Björk et al. 2000, Eliasson et al. 2001). It is essential to note that the typically red coloured Revsund granite is hardly deformed, in contrast to the allochthonous granitoids of the Lower Allochton. Microcline, orthoclase and plagioclase are idiomorphic and apparently undeformed. However, biotite and amphibole minerals show a weak preferred orientation and quartz mineral interfaces are characterized by suturing.

1.2 Sedimentary cover (Torneträsk Group)

The crystalline rocks are unconformably overlain by a sedimentary succession, only a few tens of metres thick and preserved from erosion by the overlying Lower Allochthon. In the Tången area, Kulling (1942) recognised a basal sedimentary sequence of mudstones and siltstones, which is part of the subsequently established Torneträsk Group (Kulling 1982). Further exposures in the Tången area and in the Gråtanliden and Sjulsberget areas were observed later (Bierlein and Greiling 1993). Local boulders south of Tresund and east of Nästansjö suggest an even wider distribution of these rocks. The Torneträsk Group comprises both the Torneträsk Formation (Thelander 1982) and overlying alum shales of the Alum Shale Formation (Gee et al. 1974). The Torneträsk Formation in the present area consists of a mixed sequence of mudstones, siltstones and sandstones of grey, dark grey or dark brown colour, commonly whith white, detrital mica flakes. Bedding is mostly poorly preserved due to intense bioturbation throughout the sequence, but, where preserved, is on a cm- to dm-scale. At the NE slope of Mount Tången the Torneträsk Formation is at least 40 m thick and overlain by the black shales of the Alum Shale Formation. The latter is characterized by black mudstones and siltstones (alum shale) with fine-grained, mainly submicroscopic minerals and variable contents of quartz and/or carbonate, sulphides (mostly pyrite) and organic matter (for a detailed lithological description, see Andersson et al. 1985). These authors also give a stratigraphic age range of the alum shale from Middle to Late Cambrian and locally even earliest Ordovician. The Fjällbränna Formation alum shale represents the highest autochthonous unit beneath the regional sole thrust of the Lower Allochthon.
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2. LOWER ALLOCHTHON

2.1 Blaik Nappe Complex

The Lower Allochthon is represented by the Blaik Nappe Complex (BNC) which was defined by Kulling (1955, 1972) as covering all the units beneath the higher allochthons in the eastern, marginal area. Subsequently, it became clear that the BNC is divided into several structural units as is outlined in section C. However, all the units of the BNC are built up of similar rock sequences and the lithology is treated here for the BNC as a whole. Farther west, the granites and gneisses of the Börgefjell massif are exposed beneath the Middle Allochthon and interpreted as part of the Lower Allochthon. Therefore the units of the Börgefjell window are described here, following the BNC.

Crystalline basement rocks form the basal parts of some units exposed in the BNC. The major part of these rocks is made up of coarse-grained granites and syenites with minor bodies (xenoliths) of mafic and intermediate composition (Greiling 1982). The granitoids are reddish and/or greenish in colour with cm-size augen of K-feldspar. The primary porphyritic texture is brecciated and mineral aggregates show a preferred orientation leading to a gneissic texture. Plagioclase and orthoclase are sericitized; biotite and hornblende are partly altered to chlorite. Undulatory extinction in quartz is common. The xenoliths are mainly amphibolites, composed of hornblende and plagioclase, with minor quartz. Biotite, chlorite, epidote and clinozoisite are secondary minerals. The crystalline rocks are discordantly overlain by arkoses of the Risbäck Group. Sometimes a primary, depositional contact is exposed.

The sedimentary succession of the Lower Allochthon is comparable with the Jämtland Supergroup in the type area further south (Gee et al. 1974, 1978, Kumpulainen 1982). The Risbäck Group (with Kalvberget Formation), the Sjoutälven Group (with Långmarkberg and Gärdsjön Formations) and the Tåsjön Group (with Fjällbränna and Norråker Formations) are represented within the present area. The thickness of the sedimentary rocks is, in general, lower than in the type area.

The Risbäck Group is contiguous with the type area further south (Kumpulainen 1982) and broadly comparable. However, a distinction between two major clastic successions, the lower Stor-Rajan Formation and the upper Mångmanberget Formation, separated by the finer clastic Tvärselet Formation, could not be made. Due to thrusting, numerous repetitions, and lack of a clear key horizon, it is as yet not possible to decide whether successions are stratigraphically coherent or structurally repeated. In the section along the road Bångnäs - Kultsjöluspen the sequence starts with coarse, often conglomeratic arkoses with dark to light reddish colours. These arkoses lie on top of the crystalline basement. The sequence gradually fines upwards and is at least several hundred metres thick. In the uppermost c. 50 m the characteristic reddish arkoses give way to an alternation of light to dark grey arkoses and dark grey, red or rarely green shales. At Trappstegsforsarna, this sequence is directly overlain by varved 'clay' and tillite of the Långmarkberg Formation. Towards the east, southeast of Hällnäs, the upper part of the Risbäck Group is dominated by red shales, a few tens of metres thick, with rare light arkose interlayers.

The arkoses and shales are locally overlain by light coloured dolomites, which belong to the Kalvberget Formation in the uppermost part of the Risbäck Group. Dolomites are most common to the northeast and east of Grytsjö and are up to 20 m thick (Kulling 1942).
The tillites of the Långmarkberg Formation vary considerably both in facies development and in thickness (up to c. 30 m). The most comprehensive description yet of the area's tillites is by Kulling (1942). Varved 'clays' with grain sizes up to silt and fine sand occur, probably beneath the tillites 'sensu stricto'. Dm-size dropstones can be observed within the varved clays. The overlying tillites may contain angular to sub-angular m-size boulders of crystalline, granitoid rocks and arkoses in a psammo-pelitic matrix. Locally, pelite or dolomite fragments also occur.

The Gärdsjön Formation is dominated by massive, often white to light grey-coloured quartzites, composed mainly of quartz (>90%), with minor amounts of feldspar, detrital mica, clay minerals and chlorite. The grain size varies between coarse and very fine-grained; particles are well rounded and graded, or poorly sorted and angular. A characteristic layer of conglomeratic and coarse-grained quartzite occurs at the base of the Gärdsjön Formation, about 10 m thick and composed of generally well-rounded pebbles of milky or bluish quartz and white feldspar, up to 1.5 cm in diameter.

Within the quartzite, irregular layers of silt- and mudstone and grey, green and sometimes red shale occur. These fine-grained layers are also composed of at least 90% quartz. Some of them are more than 10 m thick and can be followed along strike for several kilometres, as shown on the maps. Towards the top of the Gärdsjön Formation, psammites show graded bedding and a general fining upwards. The last few metres beneath the overlying Fjällbränna Formation are characterized by an alternation of fine sandstones, siltstones and dark grey, impure carbonates and marls containing minor fossils of Early Cambrian age (klöverdjur-, Kulling 1955, Asklund 1962). Two such fossil localities are indicated on the map (23 F, 0j).

The Fjällbränna Formation contains black mudstones and shales (alum shale) with fine-grained, mainly submicroscopic clay minerals and variable contents of quartz and/or carbonate, sulphides (mostly pyrite) and organic matter. Andersson et al. (1985) give a stratigraphic age range of the Swedish alum shales from Middle to Late Cambrian and locally earliest Ordovician. A trilobite of late Middle Cambrian age (Lejopyge laevigata) has been found in a limestone lens in black shale (23 F, 0j) along the main road to Stalon, close to the eastern margin of the map sheet (Gee 1972). The alum shale of the Fjällbränna Formation shows a gradational contact towards the overlying Norråker Formation, the highest stratigraphic unit of the Lower Allochthon in the area.

In places, the base of the Norråker Formation is marked by the appearance of cm to dm thick interlayers within the alum shale of dark-grey to black, impure limestones, which grade upwards into dark-grey to brownish marls. Occassionally, the limestones may form 1-2 m thick sequences, which were the aim of local quarrying (Strömnäs). Higher up, the marls, and grey mudstones, show cm- to dm-thick interlayers of greywacke beds. Most of the greywacke layers show graded bedding but are fine-grained and only rarely exceed dm-thickneses. However, at one locality (Granhöjden) coarse, faintly bedded greywackes are exposed with cm-sized grains, containing mainly quartz and rock fragments.

2.2 Börgefjell Duplex

The bedrock is composed of a basement complex including coarse granite which has been dated on the Norwegian side (Rapbekken) using the Rb/Sr whole-rock method to 1670 ± 50 Ma (Priem et al. 1967). Mafic dykes are geochemically similar to those of the adjacent Baltic Shield (Greiling et al. 1989). They do not cut the overlying, thin, mainly quartzitic, late Precambrian-Cambrian cover sequence. For details see Gustavson (1973), Greiling (1974, 1982, 1988).
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3. MIDDLE ALLOCHTHON

The Middle Allochthon comprises three major units, the Stalon Nappe and the overlying Särv Nappe in the east and, probably structurally in between, the Fjällfjäll Nappe in the west. The most important distinction between the former two is the absence (Stalon) or presence (Särv) of Caledonian igneous rocks (e.g. Gee and Zachrisson 1979, Stephens et al. 1985).

3.1 Stalon Nappe and Rainesklumpen unit

The Stalon Nappe (Kulling 1942) is widely distributed between the Lower and Upper Allochthons and also occurs as isolated klippen overlying the Lower Allochthon. It is composed of both crystalline, pre-Caledonian basement rocks, metasedimentary, generally psammitic rocks and mostly greenish mylonites. The different lithological units are separated by shear-zones and primary basement/cover relationships are not preserved (Greiling 1985). The green mylonites were probably derived mainly from crystalline pre-Caledonian rocks (Greiling 1992). The metasedimentary rocks consist of coarse meta-arkoses with subordinate pelitic interlayers. Lithological similarities suggest a correlation with the Risbäck Group.

The meta-arkoses and minor associated rocks at the northeastern and southeastern side of the Börgefiell massif, imbricated with slices of probably Precambrian schists and gneisses, are interpreted as part of the Middle Allochthon (Zachrisson 1986, Greiling 1988) and named Rainesklumpen unit (Greiling 1989). Cleaner, greenish-looking metaarkoses form an upper unit at the northeastern side of Raines(fjället). The latter rocks are probably a westwards extension of the Fjällfjäll Arkose (Zachrisson 1964a, 1969) which is exposed in a major antiform, the Fjällfjäll Window, to the east. If it is accepted that the Rainesklumpen unit is an equivalent of the Stalon Nappe, as suggested by Greiling (1989), then the Fjällfjäll Arkose belongs to a structural level on top of the Stalon Nappe (Zachrisson and Greiling 1993a, b).

3.2 Fjällfjäll and Dearka Nappes

The Fjällfjäll Arkose (Zachrisson 1964, 1969) is a fairly clean, greenish meta-arkose with a well developed banding and schistosity, often demonstrating isoclinal folding. The Fjällfjäll Arkose extends outside the type area, and can be correlated westwards with similar rocks along the northeastern side of the Börgefjellet Window (and into Norway) and also southwards with rocks of the Hetenjaure Window. The continuation of the Fjällfjäll Arkose eastwards is more problematic since no lithological equivalents appear to exist. In the area southwest of Gottern, the Fjällfjäll Arkose is overlain by lenses of highly deformed, feldspathic (K-feldspar) rocks and metagabbro/greenstone, interpreted as slices of Precambrian basement incorporated within the Middle Allochthon. Similar rocks are also present at the southern edge of Fjällfjället and at the northwestern corner of the Hetenjaure Window (Zachrisson 1991).

3.3 Särv Nappe

The Särv Nappe is most completely developed and studied at its type locality in southern Jämtland (Gee et al. 1985a). It is composed of Late Proterozoic to Early Cambrian psammitic rocks (Kumpulainen and Nystuen 1985), cut by mafic dyke swarms of predominantly tholeiitic composition (Solyom et al. 1979). Similar metapsammites with well preserved dykes occur in three small lenses to the southwest, south and southeast of Saxnäs. The geochemical composition of the mafic dykes (Eberz 1982) implies their correlation with the mafic dykes of the Särv unit further south (Greiling et al. 1984). The major part of the massive metabasic rocks with subordinate metapsammites at Mount Vinevare southeast of Saxnäs (Vinevare diabase of Kulling 1942), however, were later included in the overlying Seve unit (Zachrisson and Greiling 1993).
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4. UPPER ALLOCHTHON

The Upper Allochton comprises two major units, the Seve and overlying Köli units, respectively. Whereas the Seve unit is traditionally regarded as representing the former continental margin of Baltica and the ocean-continent transition, the Köli units represent oceanic terranes (Stephens et al. 1985, Stephens and Gee 1989).

4.1 Seve Units

The higher grade rocks in the structurally lower part of the Upper Allochthon are included in the Seve Nappes. Within the area, they are represented by a sequence of mica schist, gneiss, amphibolite and minor graphitic schist and marble, including bodies of ultramafic rocks and eclogites. The Seve rocks of the Marsfjällen area were studied and described by Trouw (1973), who distinguished three separate units, the Eastern Schist and Amphibolite Belt, the Central Seve Belt (Marsfjället Gneiss) and the Western Belt (Svartsjöbäcken schist). His subdivision was extended to the area south of Kultsjön by Brandt (1973).

To the south, the Marsfjället Gneiss and the Svartsjöbäcken Schists are correlated with the Lillfjället Gneiss and the Transition units (Sjöstrand 1978), respectively, on the 22E Frostviken map sheets (Zachrisson and Sjöstrand 1990). The major part of the Seve on these map sheets (Sjouten, Gakkafjället, Blasjöälven and eqv. units, including the Ertseke lens with abundant eclogites) are structurally related to the Eastern Schist and Amphibolite Belt on the Fatmomakke map sheets (23 F).

It should be noted that several of the eastern units within the Frostviken map area, dominated by quartzite or meta-arkose with mafic rock intercalations, were classified by Strömberg (1984) as part of the Middle Allochthon. The state of metamorphism, including the presence of eclogites and retro-eclogites in several units, the structural pattern, and regional comparisons and correlations argue for inclusion in the Upper Allochthon.

The Seve units thin markedly westwards (Zachrisson 1973), both regionally and across the area. Only minor remnants are present around the Fjällfjäll Window and, apart from a major lens along the southwestern side, south of Namsvattnet in Norway (Greiling et al. 1989), the Seve has completely disappeared in the contact zone around the Börgefjell Window.

4.1.1 The Eastern Schist and Amphibolite Belt

The Eastern Schist and Amphibolite Belt, which in this context includes the Dikanäs and Grytsjö schists, forms the lowermost unit and dominates within the eastern marginal area. A well-preserved to highly deformed gneissic granite (Nuortenjuone Gneiss) forms a recognizable unit that can be traced over a considerable distance. This Nuortenjuone Gneiss, a granitic augen-gneiss, is the first meta-igneous rock in the Seve that yielded a pre-Caledonian age (1645 +/- 4 Ma).

The Sjouten unit (Bakker 1978) is structurally the lowermost complex of the Frostviken map area. It is dominated by quartzite and feldspathic metasandstones and contains the Tjeliken eclogites and several smaller eclogite and retro-eclogite bodies. The metamorphic conditions during eclogite formation have been estimated at 14.0 ± 1.5 kb and 550 ± 70°C (Van Roermund 1985). Less metamorphosed mafic rocks also occur as dykes. Subordinate garnet-biotite-phengite schists are associated with the retro eclogites.

The Gakkafjället unit (Van Roermund 1976) is used here in an extended sense as a name for several units dominated by quartzite or meta-arkose with mafic rock intercalations; the largest of which is called the Fiskåfjället Amphibolite. The amphibolites are often garnetiferous, sometimes porphyritic with feldspar megacrysts; cross-cutting dolerites occur. On the 22E Frostviken map sheets, the Gakkafjället and Sjouten Formations are present only on the SE quadrangle.

The Blåsjöälven unit is a grouping of several amphibolite-dominated complexes, including the Blåsjöälven Formation (Sjöstrand 1978). They enclose the Ertseke Lens (see below) on all sides. Structurally underlying are the Jemesjaure 'Formation' (Van Roermund 1976), the Blerik 'Eenheit' (Biermann 1977), the Sipmesjaure Amphibolite (Sjästrand 1978) and the Sipmik Väktaren 'Formation' (Kardoes 1978); overlying are the Gipper Amphibolite (Winter 1974) and the Grultenvalle (Kardoes 1978) and Tjokkola (Van Roermund 1977) 'Formations'. The amphibolites are locally garnet-bearing and metasedimentary intercalations of garnet-biotite-muscovite schist, quartzite and marble also occur.

The Ertseke Lens occurs as a tectonic lens or a detached, recumbent, isoclinal fold-hinge within the above-mentioned amphibolites. It is composed of two lithologically distinct subunits. The Lejaren unit including the Lejaren 'Formation' (Sjöstrand 1978), the Kröneke Quartzite (Winter 1974) and the Rieksvarto 'Formation' (Kardoes 1978) are high-grade quartz-rich gneisses, often unsuitable to develop characteristic index minerals; quartz-feldspar pegmatites occur.

The Avardo unit ("Formation" acc. to Sjöstrand 1978) is a kyanite-sillimanite-K-feldspar gneiss which forms the host rock to most of the eclogites in the map area. Peak metamorphic conditions here have been estimated to be 18.0 ± 1.0 kb and 780 ± 50°C (Van Roermund 1985). Isotopic data from rocks of the Lejaren and Avardo units indicate Proterozoic elements (Reymer et al. 1980, Claesson 1987, Williams and Claesson 1987). Caledonian metamorphism of the Avardo gneisses, dated by U/Pb zircon, has been calculated at 369 ± 38 Ma with conventional (Claesson 1987) and at 423+5 Ma using ion microprobe methods (Williams and Claesson 1987).

4.1.2 The Central Seve Belt

The Central Seve Belt is represented by the migmatitic Marsfjället Gneiss, characterized by K-feldspar and kyanite, and intercalations of metabasic rocks generally with the mineral association hornblende-plagioclase-garnet-clinopyroxene, suggesting granulite facies (Trouw 1973). Metamorphic conditions have been estimated (Sillanpää et al. 1987) to represent >600°C, c. 10 kbar and a burial depth of about 35 km. The lower, eastern contact is tectonic. It is marked by an impressive blastomylonite zone which developed under amphibolite facies conditions (Zwart 1974) and was reactivated under retrograde, low-grade conditions. The Marsfjället Gneiss can be correlated with the Lillfjället Gneiss farther south.

The Lillfjället Gneiss was defined by Sjöstrand (1978) in the area south of Kvarnbergsvannet and is present as isolated lenses at a level close to the top of the Seve units across the Frostviken map area. It is a kyanite-sillimanite-K-feldspar gneiss (without eclogites or retro-eclogites), partly migmatitic with only subordinate amphibolite intercalations. Radiometric dating north of Murusjöen (Claesson 1987) has indicated a Caledonian metamorphism at 423+26 Ma and a Precambrian provenance (1512+36 Ma). The Lillfjället Gneiss also occurs as outliers ('Klippen') on top of the Ertseke Lens (see 22E NE) and is separated from this unit by amphibolite-dominated sequences which have been correlated with the Blåsjöälven 'Formation'. As a consequence, the eclogite-bearing Ertseke Lens is placed at a structurally lower level, beneath the Central Belt, intercalated within the Eastern Schist and Amphibolite Belt of Trouw (1973).

4.1.3 The Western Belt

The upper, western belt of schist and amphibolite, referred to as the Svartsjöbäcken Schists (Trouw 1973), has a gradational contact towards the structurally underlying Marsfjället Gneiss. No tectonic break has been suggested. The term Transition units was used by Sjöstrand (1978) in describing various tectonic units of uncertain affinity between the Seve and the overlying Köli rocks. Without detailed microscopical investigations it is often difficult to decide in the field whether the rocks represent the lower prograde part of the Köli, or retrograded units at the top of the Seve. The rocks are dominated by mica schist (garnet-biotite-muscovite), often with large, fresh to completely retrogressed garnets, and foliated amphibolite or actinolitic schist. Graphitic schist, calcareous Garbenschiefer and quartz schist are intercalated, and ultramafic bodies occur frequently. The position of these units on top of the Lillfjället Gneiss (Central Belt Seve) indicates that they might be correlated with the Svartsjöbäcken Schists (Trouw 1973) farther north. Immediately south of the Hetenjaure Window, a major lens of anorthosite occurs which apparently has the same tectonic position as the better documented units in the Kvikkjokk area (Kulling 1982, Greiling & Kumplainen 1989).
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4.2.KÖLI NAPPES

The greenschist facies metamorphic rocks in the structurally higher part of the Upper Allochthon are included within the Köli Nappes. The Köli rocks represent the Cambro-Silurian volcanic and sedimentary assemblages formed in oceanic westerly areas. They contrast markedly with respect to the thin platformal and passive continental margin sequences of the Autochthon and Lower and Middle Allochthons, which are part of the late Proterozoic-Silurian sequence deposited on the Baltoscandian platform.

4.2.1 Lower Köli

Lower Köli rocks in the northern part of the area can be mapped into the type area around Björkvattnet-Virisen (Kulling 1933), where the character and limits of the units have been established. The term Björkvattnet Nappe was applied as a regional name for this tectonic unit (Stephens 1982) and the Virisen terrane was introduced as a terrane concept by Stephens and Gee (1989). In a broad sense, the Köli occupies the cores of two open regional synclines, the Ransarn and Western synforms, respectively (Zachrisson 1969). The central part of the Ransarn synform is covered by a calcareous metagreywacke, the Lövfjäll Formation (Kulling 1933). The stratigraphy is generally in a right way-up position except in the northwestern and northeastern parts of 23 F NW, where very large recumbent folds have caused structurally complex patterns. The stratigraphy of the Lower Köli rocks was established by Kulling (1933) in the Björkvattnet-Virisen area (24F) with successive minor modifications over the years (e.g. Kulling 1972). The lithologies were grouped, in ascending stratigraphic order, into the Rotik (Ro), Mesket (Seima), Gilliks, Vojtja, Slätdal, Broken, Lövfjäll and Viris 'series' or 'groups'. For the present 1:50,000 bedrock maps it has not been possible, on available material, to recognize all of these formations, but general correlations can be made from the lithology and some characteristic rock types. The lower part of the sequence (pre-Lövfjäll) is composed of various types of mainly non-calcareous, often dark-coloured phyllite, quartzite, greywacke, conglomerate, limestone and metavolcanic rocks. Ultramafic rocks and serpentinite conglomerates occur at various levels. At one locality, c. 5 km north of Vardofjäll (map sheet 24F), gastropods (s. Macluritacea) of Ordovician age (Lower-Middle Ordovician boundary?) were discovered by Holmqvist (1980). The pre Vojtja part of the sequence is identical with the Tjopasi Group (Zachrisson 1964, 1969, 1991) within the map sheets 23E Sipmeke.

A U/Pb zircon dating (Claesson et al. 1983) of the subvolcanic trondhjemites of the mixed mafic and felsic volcanic rocks of the Tjopasi Group (Zachrisson 1969) has defined an age of 488 ± 5 Ma (Arenig?), which is consistent with the inferred stratigraphical age. The quartzite/marble horizon above the volcanic rocks has been correlated with the Vojtja/Slätdal Formation of Ashgillian age in the type area.

In the Frostviken map area, the tectonic contact at the top of the Björkvattnet Nappe has been demonstrated only between the lakes Ankarvattnet and Stora Blåsjön (Sundblad 1981). Further northeast and southwest, the thrust has been tentatively traced slightly above the above-mentioned quartzite/marble horizon, locally including some calcareous phyllites (Garbenschiefer) which may represent the Lövfjäll Phyllite (Kulling 1933). The presence of black phyllites (equivalents to the Broken Formation) and associated mafic volcanic rocks and intrusions makes this zone susceptible to tectonic dislocations.

The lowermost Köli units are characterized by abundant quartzite-quartzite conglomerate - serpentinite conglomerate and probably correlate with the Ro Conglomerate in the Björkvattnet-Virisen area. The klippe of Lower Köli rocks in the southwestern part of Ai 76 is dominated by these rocks. In the Autjojaure area, they are covered by felsic metavolcanic rocks (quartz keratophyre) which seem to have a maximum thickness in the Kultsjön valley and can also be followed into both limbs of the Ransarn synform. Although stratigraphic thickness as well as the proportion of felsic members diminishes northeastwards they are interpreted to be equivalents of the Mesket (Seima) Formation further north. In the type area, this unit is dominated by mafic metavolcanic rocks. The succeeding dark phyllite sequence contains coarse, fragment- and blue quartz-bearing greywackes and polymict conglomerates with granitoid rock boulders, typical of the Gilliks facies. It is overlain by a quartzite-quartzite conglomerate (Vojtja Formation) and a fossiliferous limestone (Slätdal Formation). Pelmatozoan fragments and occasional corals in these limestones within the present map sheets and particularly brachiopods and gastropods in the type area (Kulling 1933) indicate a fauna of Ashgill age. Finally, a graphitic phyllite (Broken Formation) with local, thin greenschist or tuffite layers forms the base of the very thick Lövfjäll Formation. In two restricted areas, around the northern end of lake Ransarn (8c) and east of Rotikarna (9e), coarse metasandstones and wackes represent the Viris Quartzite, the uppermost stratigraphic member of the area.

On the eastern side of the Fjällfjäll Antiform, in a north-south-extended zone in the Ransarån-Gottern area (7b-9b), Lower Köli rocks occur in an inverted position, representing the downward-facing nose of a major anticlinal fold. Such folds are early in the structural sequence and may even be synsedimentary. As demonstrated by Zachrisson (1969, sections) they are accentuated further north, originating from the Fjällfjäll antiformal zone and causing regional inversions, for example of the fossiliferous units in the Broken area (24F).

Stratigraphic overturning is even more spectacular in the Daunentjakke-Silesbaune area. Most of the quartzite conglomerate/limestone horizons, such as those forming the peak areas of mount Daunentjakke (Daunatjakko) are constantly facing downwards (Stigh 1976) and are interpreted as erosional remnants of the lower inverted limb of an extensive, recumbent anticlinal fold that originated from the Fjällfjäll antiformal zone further west (cf. section). The underlying, complementary syncline is represented by the zone of Viris Quartzite southeast of Rotikarna that passes along lake Silisen (Ai 77) and can be mapped into contact with the main Viris Quartzite unit in the surroundings of lake Virisen (24F). Here it represents the core of the large sideways-closing, east-facing Björkvattnet-Virisen synform.

Map sheet 23E, SE presents a fairly complete and systematic picture of the Lower Köli stratigraphy, especially regarding the position of various regionally-developed conglomerate horizons. The stratigraphy is in a normal-way-up position. The mixed felsic (-mafic) Tjopasi (cf. 6j: Tjatjease) metavolcanites are underlain by a metasedimentary sequence which contains conglomerates in its lower part. The lowermost member is a quartzite conglomerate which is intruded by (or contains protrusions or extrusions of) ultramafic rock (serpentinite). The latter is in many places associated with well-preserved serpentinite conglomerate. These units together form the classical Ro conglomerate. Stratigraphically on top of the Tjopasi metavolcanites, there occurs another, occasionally greenish-looking quartzite / quartzite conglomerate, which has been correlated with the Gilliks quartzite in the type area. The main part of the Gilliks in 23E is developed in shaly facies (dark, graphitic or quartz phyllites); coarser metagreywackes and polymict Gilliks conglomerates are restricted to the structurally complicated synform east of Raukasjö (23 E, 0i). The quartzite/marble/greenstone complex (Bellovare Formation) close to the stratigraphic top of the Lower Köli has been correlated with the Vojtja/Slätdal/Broken Formations (Zachrisson 1964a, 1969). Apart from pelmatozoan fragments in the limestones at Raukasjö (Svenonius 1895) and at Slengajokk, Preunttjakko and Rapstensjöarna (Högbom 1925), no macrofossils have been found to verify the Ashgillian age documented in the type area (Kulling 1933,1972). The upper contact of the Lower Köli beneath the Middle Köli is tentatively established immediately above this complex and is interpreted as an important early slide (thrust contact). The presence of graphitic phyllite (equivalent to the Broken Formation) and associated mafic volcanic rocks and intrusions render this zone susceptible to tectonic dislocation. Later folding and out-of-sequence thrusting of the tectonostratigraphy explain the structures west of Jalketsåive and at the northeastern edge of Rainesfjället where Lower Köli rocks are locally exposed.

4.2.2 Middle Köli

The Middle Köli is represented by three different tectonic units in the present area, the Stikke, Gelvenåkko and Leipikvattnet Nappes, respectively.

The Stikke Nappe derives its name from the Stekenjokk area (Stephens 1982) where the felsic-dominated Stekenjokk Ouartz-Keratophyre forms a prominent formation. This unit can be followed continuously into and through the Sipmeke-Frostviken map sheets, where, south of western Kvarnbergsvattnet, it has been named the Skogsbäcken Volcanites (Sjöstrand 1978). The stratigraphical sequence of the Stikke Nappe is inverted. Thus, the Basalt-Quartz-Keratophyre Formation (Nilsson 1964) is structurally overlain, but stratigraphically underlain, by variable, dark, often graphitic phyllites and mafic volcanites (Remdalen Group of Zachrisson 1969) and stratigraphically overlain by the underlying calcareous phyllites (Blasjö Phyllite of Nilsson 1964). U/Pb zircon dating (Claesson et al. 1988) gives a minimum age of 476 ± Ma for the Skogsbäcken Volcanites and an age of 440 ± Ma for felsic, trondhjemitic intrusions in the (stratigraphically) lower part of the Blasjö Phyllite. Thus, the age of the rocks in the Stikke Nappe is probably Ordovician.

The high content of U and V in the graphitic phyllites stratigraphically on top of the stratabound ore horizon (see section C) is geochemically correlatable with the alum shales on the platform and provides further support to this interpretation (Sundblad and Gee 1984). In map-sheet 23E, NE, the structure is complicated by the Remdalen Repetition which duplicates part of the stratigraphy and causes a cut-out of the Stekenjokk Quartz-Keratophyre in the area between Fasovardo and Rainesfjället. The quartz-keratophyre-bearing unit from Beitsetjenjunje via Raurevardo to V. Vardofjället, surrounding the Remdalen Synform, is correlated with the Stekenjokk metavolcanites. This implies that the dark quartz phyllites and the K-rich quartz porphyry in the core of the synform are amongst the lowermost units of the sequence. Attempts to date the porphyry by the Rb/Sr whole-rock method have not proven to be successful. The associated limestone contains pelmatozoan fragments (Du Rietz 1941) and is Ordovician or younger in age.

The Gelvenåkko Nappe (Zachrisson 1969) is a completely detached unit containing a rock sequence nearly identical to that of the Stikke Nappe. It is preserved in the core of the Western Synform from the culmination at Stekenjokk to the southern edge of the Sipmeke map sheet. The tectonic contact is well established at its eastern boundary and, in the Stekenjokk-Gelvenåkko area (23 E, 3g-4h), a large number of drillholes intersecting deeper levels of the Stekenjokk ore body pass through the Gelvenåkko thrust. The western contact (from 23 E, 3g and southwestwards) is more difficult to trace.

The uppermost tectonic unit of the area, along the national border to Norway, is represented by the Middle Köli Leipikvattnet Nappe (Zachrisson 1969). The thrust at its base is both geologically and geophysically distinct around lake Leipikvattnet. Although difficult to pin-point in the Frostviken map sheets, it has to be traced somewhere within the sequence of dark, often graphitic phyllites. A calcareous phyllite, the Brakkfjället Phyllite (Nilsson 1964), forms the major formation of the nappe. Characteristic rock types in the Leipikvattnet area are polymict conglomerates, coarse fragment-bearing metagreywackes and the Bjurälv limestone. An acritarch microfossil (Dactylofusa spinata) has been extracted from the limestone (Kjellström and Zachrisson 1969), while pelmatozoan fragments were reported earlier by Du Rietz (1936).

4.2.3 Upper Köli

The Upper Köli Nappe is represented by the Storfjället terrane to the north of the present area (e.g. map 24 F or northwest of the Börgefjell window).

4.2.4 Stratabound sulphide deposits

Since 1917, the Stekenjokk area has been the centre of exploration and ore investigation activities by the Geological Survey of Sweden in the Caledonides (Högbom 1925, Zachrisson 1971, 1982a, 1984a, 1986). Approximately 25 different mineralizations and more important deposits (see Table) have been investigated by drilling, ca 2/3 of these occurring within the Stekenjokk metavolcanites. The polymetallic massive sulphides, generally with a disseminated stringer zone component, have been interpreted as volcanic-exhalative in origin and related to a late rifting-stage situation in an island-arc setting (Stephens 1980b, 1982, 1986). The ore bodies are stratabound, laterally very extensive and demonstrate both a vertical and a lateral metal zonation. Although principally Cu-Zn deposits they generally exhibit an increased content of Zn, Pb, As, Sb and Ag both towards the stratigraphic top of the ore column (Zachrisson 1982a) and outwards from the exhalative centres (Zachrisson 1984a). Deposits related to mafic metavolcanites of ocean-floor affinity (e.g. Remdalen) and those in the metagreywackes of post arc (basinal) setting (e.g. Ankarvattnet) show certain chemical differences in relation to the main group of volcanite-hosted deposits. These variations, however, are even better characterized by their Pb isotope patterns (Sundblad and Stephens 1983). The volcanite-hosted deposits provide an isotope model age very close to the actual age, whereas sulphides in a more sedimentary environment demonstrate a marked increase in radiogenic lead, a pattern that is further enhanced in late veins and segregations. For details on ore deposits see geological map sheets Sipmeke and Frostviken (Zachrisson 1991, Zachrisson & Sjöstrand 1990) and Stephens (1986).
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5. UPPERMOST ALLOCHTHON

The Uppermost Allochthon is built up of exotic, active continental margin terranes with huge, batholithic granitoid intrusions, probably related to the Laurentian continents.
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Scandinavian Caledonides | Introduction | Structural and tectonic evolution | References
Autochthon | Lower Allochthon | Middle Allochthon | Upper Allochthon - Seve unit | Upper Allochthon - Köli unit | Uppermost Allochthon
Micrographs | Minerals | Fossils | Map | Overview