Tectonic significance of the meta-evaporitic magnesite and scapolite deposits in the Seve Nappes, Sarek Mts., Swedish Caledonides

ELSEVIER

Tectonophysics231(1994) 33-44

Tectonic significance of the meta-evaporitic magnesite and scapolite deposits in the Seve Nappes, Sarek Mts., Swedish Caledonides Olaf M. Svenningsen Geological Institute, Universi@ of Lund, Siilvegatan 13, S-223 62 Lund, Sweden (Received December 1,199l; revised version accepted November I,19921

Abstract Magnesite and scapolite, hosted by diabase and amphibolite derived from diabase, occur in the Sarektjikkl Nappe, Seve Nappe Complex in the Caledonides of northern Sweden. The Sarektjtil Nappe consists of a sedimentary sequence-the Favoritkammen Group-intruded by vast amounts of diabase, that constitute 70-80% of the nappe. The magnesite and scapolite layers are parts of the Spika Formation in the Favoritkammen Group. The Spika Formation formed in an evaporative setting of alternating influx ofeontinental (magnesian carbonates) and marine water (halite and anhydrite/gypsum altered to scapolite during thermal metamorphism), possibly in rift-related basins in the Late Precambrian. The stratigraphy of the Favoritkammen Group indicates a progressively deepening basin, consistent with deposition in developing rift basins. The diabase dykes that cut the Spika Formation caused regional thermal metamorphism, but are not responsible for the magnesite formation. The lithology of the SarektjlkkA Nappe thus records the evolution of a rift to the formation of a passive margin. That passive margin was detached and thrust over the Baltic Shield during the Caledonian Orogeny.

1. Introduction

Late Precambrian rifting in western Scandinavia, which preceded the continental rupture creating the Iapetus Ocean (Harland and Gayer, 1972), produced basins in which thick, mainly fluvial but also shallow-marine sedimentary sequences were deposited (Kumpulainen, 1980; Nystuen, 1980, 1987; Fayn, 1985; Knmpulainen and Nystnen, 1985). These rift-related sequences are now found mainly in the Middle Allochthon of the Scandinavian Caledonides, whereas the passive margin rock associations, connected with the initiation of seafloor spreading, are encoun-

tered mainly in the Seve Nappe Complex of the Upper Allochthon (Fig. 1). The overlying Kijli Nappe Complex, also in the Upper Allochthon (Gee and Zachrisson, 1979) contains terranes of exotic origin relative to continent Baltica (Stephens and Gee, 1985, 1989). The occurrence of a thick, presumably rift-related sequence of sedimentary rocks in the Sarektj&kl Nappe of the Seve Nappe Complex in northernmost Sweden potentially contains important implications for the terrane analysis of the transition between rocks of Baltoscandian and outboard (Iapetan) affinity in the allochthon of the Scandinavian Caledonides. In this respect the origin of the

0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved \

0.M. Svenningsen / Tectonophysics 231 (1994) 33-44

34

magnesite deposits in the Sarektjikkd Nappe is of special interest. All the magnesite deposits in the Sarek area are hosted by the amphibolite/diabase of the Sarektjdkkl Nappe in the Seve Nappe Complex (lower part of the Upper Allochthon), which represents the rifted outer passive margin of Baltoscandia (Gee, 1975; AndrCasson, 1987). This SarektjlkkA Nappe consists of 70-80% diabase, which intruded a sedimentary sequence, and represents a transitional type of crust in the rifted passive margin of Baltoscandia towards the Iapetus Ocean (Andreasson, 1987; Svenningsen, 1989). The genesis and tectonic setting of the Spika Formation, in which the magnesite deposits form lenses, thus place important constraints on the evolution of the rifting processes that led to the /!z?&

K0II Napw

Complex

\ EE3, /?!JYJ

iii@

Tsbkkok

Lens

sar.3k LW Sarekt@kb

(lem. undeformed

Mogneslte

deposit

NOppe (rIghtI

Valmok Lens

Fig. 2. Tectonostratigraphic and location map of the the Sarek National Park and adjacent areas, modified from Zachrisson and Stephens (1984). The extent of the undeformed parts of the Sarektjw Nappe has only been mapped in the northernmost parts of the Sarek National Park and is certainly larger than indicated on this map. In the northwestemmost part of the Sarek, the Sarektjiikkii Nappe constitutes the eastern 90-95% of the Sarek Lens. Al1 the magnesite deposits occur in the Sarektjikkl Nappe.

formation of the passive margin of Baltoscandia prior to Caledonian orogeny. 1.1. Magnesite deposits in the Sarek area Outboard

Seve

Kalak

Mlddte

terranes

Nappe

Nappe

Complex

CompbX

Allochthon

Fig. 1. Tectonostratigraphy of the Caledonides of Norrbotten-Trams (modified from Dallmeyer et al., 1991).

The magnesite and iron ore deposits of the Scandinavian Mountains in Norrbotten County, Sweden (Fig. 2) created expectations for the economic development of this remote and sparsely populated area around the turn of the century (Tegengren, 1910). However, the amount of useful ore was overestimated, partly because of incomplete understanding of the structure of the

O.M. Svenningsen/ Tectonophysics231 (1994) 33-44

mountain belt. Three main areas containing magnesite deposits in the Sarek area have been described (Hamberg, 1901a,b, 1910; Tegengren, 1910; Shaikh, 1974; Kulling, 19821, whereof two have been subject to prospecting and quarrying. Turrekaise is situated outside the national park proper, but geographically and geologically belongs to Sarek. The magnesite deposits were discovered in 1892 and grandiose plans were made for their exploitation (Tegengren, 1910). However, the amount of magnesite was grossly overestimated (l,OOO,OOOtons compared to the calculated 50,000 tons of Tegengren (1910)). A meticulous description, including detailed maps, of the Tarrekaise deposits is given by Tegengren (1910). The host rock is a strongly lineated black garnet amphibolite, which preserves the chilled margins and dyke-in-dyke characteristics typical for the Sarektjikkl Nappe. However, all primary structures in the magnesite have been obliterated. The occurrence at Vartastjdkki in the Sarektj&ka massif is briefly mentioned by Kulling (19821, citing Hamberg (1901a,b). The deposit consists of a ca. lo-m-wide lens of the magnesitic part of the Spika Formation, situated at 1580 m a.s.1. elevation, the major part of which is covered by blocks and perennial snow. The structural environment is similar to that at Tarrekaise and the magnesite does not preserve any primary characteristics. The purest magnesite in Sarek is found in the Apar Quarry, situated in the carbonate/calcsilicate lens on the northern slope of the Apar massif (Fig. 2). The occurrence consists of pure, sparry white magnesite and has a “visible area” of 2500 m* and has the shape of a 120 x 40 m big flat-lying horizontal cigar (Shaikh, 1974). The Apar magnesite deposit is hosted by massive grey diabase and is the only known exposure of pure magnesite in the undeformed part of the SarektjHkki Nappe.

2. Geologic setting The allochthon of the Scandinavian Caledonides is subdivided into the Autochthon/Parautochthon, Lower, Middle, Upper and Upper-

35

most allochthons (Gee and Zachrisson, 1979). The Autochthon/ Parautochthon consists of the Precambrian crystalline basement of the Baltic Shield, mainly granitic rocks, and its Upper Precambrian to Lower Palaeozoic cover sediments. The Lower Allochthon is also composed of Lower Palaeozoic miogeoclinal sediments, that have been thrust southeastwards during the Caledonian Orogeny. The Middle Allochthon in Norrbotten is dominated by the Akkajaure Nappe Complex, which is a stack of thrust sheets, composed of Precambrian plutonic rocks, which occasionally carry thin covers of sediments (Bjiirklund, 1985). Bjorklund (1985) calculated a minimum transport distance of 300 km and a plausible 600 km for the most far-travelled thrust sheet in the Akkajaure Nappe Complex. In a terranes perspective, the Autochthon/ Parautochthon, the Lower and the Middle allochthons are parts of the margin of the Baltoscandian continental shield (Stephens and Gee, 1989), whereas an important border is found in the Upper Allochthon: the Seve-Koli boundary. The Seve Nappe Complex @NC) (Zachrisson, 1973) in the lower part of the Upper Allochthon represents the rifted and dyke-intruded outer part of the Baltoscandian passive margin towards the Iapetus Ocean (Gee, 1975). The overlying Koli Nappes represent, a.o., oceanic successions and island arcs in the Iapetus Ocean and possibly units of Laurentian affinity and are therefore exotic terranes (Stephens and Gee, 1989). The SNC contains at least fifteen different characteristic lithologic associations, derived from the cratonic margin and miogeocline of Baltica and the continent-ocean transition towards Iapetus (Andreasson, 1986a,b, 1987). These lithologies almost always include psammitic and pelitic schists, with subordinate carbonates, and variable proportions of metabasic rocks (Andreasson, 1986a,b; Kullerud et al., 1990; Svenningsen, 1987, 1989, 1990). Variable and heterogeneous deformation and metamorphism, with dramatic contrasts within as well as between thrust sheets, are other typical features. In Norrbotten County, northernmost Sweden, the SNC have been subdivided into three lenses: the Vaimok, Sarek and Tsslkkok lenses (Fig. 2; Zachrisson and Stephens,

36

0. M. Suenningsen / Tectonophysics 231 (1994) 33-44

1984; Kullerud et al., 1990). Both the Vaimok and the Tslkkok lenses carry eclogites and in the TsHkkok Lens the protolith has been identified as basaltic lavas, with undeformed pillows that have survived eclogite-facies metamorphism (Kullerud et al., 1990). The Sarek Lens is composed of two thrust sheets-the Sierkavagge and the SarektjHkki nappes-separated by the distinct Sierkavagge thrust fault. This thrust has not previously been recognised and its continuation to the south is unknown. The Sierkavagge Nappe is called the KeddAive Nappe in Dallmeyer et al. (1991). However, the name “Kedd&ve” derives from a graphite-bearing dolomite (the Kedd%ive Dolomite), named after a locality far from the type section (Kulling, 1982) in the Sierkavagge Valley. This dolomite is used to identify the Surek Lens (Zachrisson and Stephens, 1984) which also contains the SarektjWH Nappe and hence the name

“Sierkavagge Nappe” is preferred. Both nappes are amphibolite/ diabase dominated, with variable proportions of basic intrusions/ sedimentary rocks.

3. The Sa~Sklrii

Nappe

The SarektjW Nappe (Andreasson, 1986b) consists to 70-80% of diabase in sheeted dyke complexes or single dykes, which intrude the Favoritkammen Group (Fig. 3; Svenningsen, 1989, 1990), which is the oldest part of the nappe. It is dominated by shallow-water sedimentary rocks, mainly quartzo-feldspathic sandstones including the magnesian carbonates and calcsilicate minerals of the Spika Formation, which hosts the magnesite deposits (Fig. 4). The core of the eastern parts of the nappe escaped Caledonian regional metamorphism and penetrative deformation and

Fig. 3. Typical sediment-dyke pattern from the central, undeformed part of the Sarektj&k& Nappe (drawing from photography of the Favoritkammen Ridge east of the Apar Quarry). Diabase dykes are stippled grey, with chilled margins indicated, and the vertical attitude of the bedding in the sediment screens is indicated. AH the screens in the picture belong to the Apar Formation.

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O.M. Svenningsen / Tectonophysics 231 (1994) 33-44

large parts display no postintrusive structures at all. The extent of the undeformed parts of the nappe has only been mapped in the northernmost part of the national park. The attitude of bedding in the Favoritkammen Group varies around vertical (Fig. 3), with an approximately WNW-ESE strike. The bedding is thus cut out by the roof and floor thrusts of the Sarektj&lcil Nappe and thus only a minor part of the original sequence is preserved. The Spika Formation can be traced along strike from the Apar massif to Niak in the Sarektj&kA massif, a distance of ca. 30 km (Fig. 2). Way-up indications are provided by unconformities, cross-lamination (Fig. 5A) and water escape structures (Fig. 5B) which all indicate up to the south-southwest in the northern part of the nappe. A metamict metaconglomerate with granitic clasts is found near the Spika Formation at Niak (Figs. 2 and 4). This conglomerate is separated from the Spika Formation by diabase dykes at this locality, but most likely rests stratigraphically on top of it.

Most of the conglomerate is sheared into the roof thrust of the nappe and the clasts are only rarely recognisable. Pre-intrusive deformational structures are dominantly of extensional character, expressed as normal faults and pull-apart structures (Fig. 6A and B). Folds are related to adjustments connected with the extension or to dyke emplacement. Early, thin fine-grained dykes have also been pulled apart, forming asymmetrical boudinages, and passively rotated into the layering in carbonate beds during simple shear extension. Dyke margins mostly cut the bedding in sediments at angles corresponding to bedding-to-normal fault angles. The main, complex-forming dyke generation is slightly affected by various pull-apart structures, but remember that the dykes do represent tremendous extension in themselves. Since the dykes constitute between 70 and 80% of the volume of the nappe, they must represent several 100% extension of the pre-intrusive crust. The Sarek diabases yield an Sm-Nd isochron

grained orthoquartzlte: rare but thick layers of metapsammlte.

m)

Metamict conglomerate. with granitic and basic clasts: occurs In the deformed part of

d h

Vuolnesskalh FormatIon: Granoblastic metapsammitic homfels, intercalations of talc-slllcate rocks near the top of the formatlon

Fig. 4. Generalised stratigraphy of the Favoritkammen Group. The sequence is dismembered by enormous quantities of diabase dykes and thus the thicknesses of the formations are difficult to estimate (cf. Fig. 3).

38

O.M. Senningsen

/ Tectonophysics 231 (1994) 33-44

of 573 f 74 Ma (Svenningsen, in press). Datings of other rift-related diabase dykes in the Scandinavian Caledonides (OttfjHllet, SIrv Nappe) yielded 665 f 10 Ma (K-Ar, Claesson and Roddick, 1983) and 745 f 37 Ma (Rb-Sr, Krill, 1983). Minimum ages on the Sarek diabases are provided by the 500 k 2.2 Ma 4oAr-3QArhornblende plateau age from the basal thrust of the Sarektj&kl Nappe, which date the first Caledonian movements that affected the basic rocks (Dallmeyer et al., 1991). The marginal parts of the Sarektjlkkl Nappe have been sheared to parallel foliated garnet amphibolite and mylonitic schist. Primary dyke complex characteristics (porphyritic central zones, chilled margins) can often be distinguished even

Fig. 6. Deformation in the Spika Formation. (A) Characteristic intercalation between scapolite and carbonate layers in the Spika Formation, deformed by normal faulting and asymmetric pull-apart boudinage. Scale bar is 10 cm long. (B) Thin section of asymmetrically boudinaged scapolite layer in dolomite. Hornfelsic and spotted texture indicate thermal metamorphism. The boudin is 1 cm wide.

Fig. 5. Sedimentary structures in the Spika Formation. (A) Cross lamination, cut by small normal faults, in psammitic bed at the base of the Spika Formation. (B) Dewatering piilars and soft-sediment convolute folds in calcarenitic part of the Spika Formation. A small normal fault cuts across the bedding and is easiest to discern above the lobe-shaped fold to the right of the centre of the picture. Scale bar is 10 cm long in both pictures.

in highly strained and isoclinally folded rocks, e.g., at Tarrekaise and VartastjWP (Fig. 2). The sediments of the Favoritkammen Group and particularly the Spika Formation generally display higher strain than the basic rocks. The transition between the undeformed and the deformed part of the nappe is gradual, but usually occurs over at most a couple of tens of metres. 4oAr-3QArdatings from the study area indicate a protracted and complex history of accretion for the Seve Nappe Complex (Dahmeyer et al., 1991). Hornblende 4oAr-3QAr datings from the basal parts of the SarektjAkki Nappe yielded plateau ages between 466.9 and 468.9 Ma (Dallmeyer et al., 1991), which date the amalgamation of the SarektjikkH Nappe with underlying Seve units. The only post-intrusive (Caledonian) fabric-forming defor-

39

0.M Svenningsen/ Tectonophysics 231 (1994) 33-44

mation in the internal part of the Sarektj&kkg Nappe is confined to narrow, a few centimetres to a couple of tens of metres wide, extension-related shear zones, yielding Scandian 4oAr-39Ar dates (Svenningsen, 1990, in press).

4. The Spika Formation The carbonate/talc-silicate rock Spika Formation constitutes a conspicuous part of the Favoritkammen Group (Fig. 4) and contains the meta-evaporitic assemblages of the Sarektj&kki Nappe. The lateral lithogical variation is considerable and single layers of carbonates or calcsilicates can rarely be correlated between the groups of screens that are separated by sheeted dyke complexes (cf. Fig. 3). The thickness of the formation is variable and hard to estimate, because it is dismembered by the dyke swarms, but appear to be a couple of tens of metres thick throughout most of the undeformed part of the nappe. The top and bottom parts of the Spika Formation intercalate with Mg- and sulphide-rich quartzo-feldspathic sandstones, identical to the underlying Vuoinesskaite Formation (Fig. 4). The upper limit of the formation has not been observed at localities where it is accessible to closeup examination.

scured or obliterated by extensional deformation and thermal metamorphism related to dyke intrusion even in the undeformed parts of the nappe. In the marginal parts of the nappe, all rocks are strained and the Spika Formation frequently displays a much higher degree of recrystallisation and deformation than other rock types in the nappe. The generally homfelsic texture of the rocks in the Spika Formation makes most mineral grains barely identifiable in thin section. Spotted textures in homfelses are ubiquitous (Fig. 6B). The colour in hand specimen varies from yellowish grey to red and green with progressive, thermal metamorphism. Main minerals are scapolite, carbonate (calcite to magnesite), plagioclase. In thermally metamorphosed parts clinopyroxene and garnet dominate the rock and cause the green-red colours. In the mixed silicate/carbonate rocks, magnesite, dolomite and calcite can occur together, whereas in the pure carbonate rocks one species dominates. Carbonate rocks also contain forsterite, Mg-rich chlorite, phlogopite, chondrodite and wollastonite. In the Apar Quarry, two horizons contain matrix-supported intraformational dolomite/ magnesite breccias (Figs. 7 and 8A) with intraclasts. The proportion between matrix and clasts as well

4.1. Mneralogy, texture and metamorphism The Spika Formation consists of alternating layers, from less than 1 cm to a couple of metres thick, of talc-silicate and carbonate rocks, which give the rock a characteristic appearance (Fig. 6A). The talc-silicate rocks are dominated by scapolite and the carbonate rocks are usually magnesian (dolomite/ magnesite), so that more or less pure scapolite and carbonate rocks are compositional end-members. The ductility of the carbonate bed and the contrast between intercalating layers of different compositions (cf. Fig. 6A) render these parts of the Spika Formation sensitive to deformation and shear zones of various ages are often localized to the formation. In large parts of the Spika Formation, primary sedimentary characteristics have therefore been ob-

Doknnltelmagneslte with pelltk layers and carbonate nodules. White warty to wary

magnesite

Scapollte-sulphlde-pyroxene

homfels

lntraformational dolomite/ magneslte breccia

Sulphlde-rich scapollte homfels

Fig. 7. Stratigraphy of the Spika Magnesite Quarry. The attitude of vertical and the bedding is cut by grey diabase dykes, indicated in the

Formation at the ipar bedding at this locality is ca. 45”s dipping massive figure.

40

0. M. Srvnningsen / Tectonophysics 231 (1994) 33-44

Fig. 8. (A) Dolomite/magnesite breccia at the Apar Quarry. Lens cap in upper right corner is 52 mm wide. (B) Carbonate pseudomorphs after anhydrite nodules, Apar Magnesite Quarry. Scale bar is 10 cm long.

as the somewhat variable clast material within the same horizons (Fig. SA) indicate that these breccias may have formed by the collapse of a carbonate crust during sub-aerial exposure in a sabkha environment (Pohl 1989, 1990). The quarried magnesite rock is sparry, coarse-grained and contains 46.8% MgO (Kulling, 1982). Stratigraphitally immediately above the magnesite, carbonate pseudomorphs after anhydrite nodules occur (Fig. SB). The magnesite rock is conformable with the underlying talc-silicate layers (Fig. 7) and the contacts are gradual; the magnesite cannot have been produced by metasomatic alteration of these

5. Discussion

rocks.

4.2. Dyke-sediment

and texture. Where diabase intrudes scapolite rocks, a carbonate-diopside-scapolite contactskarn frequently occurs. Scapolite is found in the diabase only very near contacts towards scapolite rocks, particularly in areas where back-veining or skarn occurs, otherwise exclusively as porphyroblasts in diabase altered to amphibolite. One of the least altered and metamorphosed diabase samples found in Sarek was taken from a dyke cutting across the scapolite-magnesite rock at the Apar quarry, which strongly argues against any post-intrusive metamorphic or metasomatic processes as responsible for the magnesite genesis. In the marginal parts of the nappe, the rocks of the Spika Formation were completely recrystallised during Caledonian thrusting and folding. The deformed diabase dykes preserve dyke-indyke relations even when they are altered to lineated black garnet amphiboiites, whereas the carbonate and talc-silicate rocks preserve no primary structures at all and originally cross-cutting relations to metabasic rocks are obliterated. The magnesite deposits at Tarrekaise and Vartastjhkkl are mineralogically and texturally similar and consist of magnesite impregnated by magnetite, with veins of dolomite and selvages of chlorite, tremolite, talc and serpentine. The deposits at Tarrekaise occur as three bands of cigar-shaped bodies in a pinch-and-swell or boudinage structure in Iimbs of large isoclinal folds. The repeated occurrence of the Spika Formation at the northern (Apar, Vartastj&kP) and southern parts of Sarek (and most likely also in the more or less unknown interior parts of Sarek, Fig. 2) can hardly be explained by stratigraphic repetition (which would require an unlikely total stratigraphic thickness in excess of 40 km!). Present field data are insufficient to address this problem.

relations

The wall rock of the magnesite at the Apar quarry consists of a massive grey diabase with perfectly preserved igneous mineral assemblages

The undeformed state of the easternmost parts of the Sarektj?ikki Nappe offers an excellent opportunity to study the genetical conditions of the magnesite-bearing Spika Formation in spite of the complex structural setting.

O.M. Svenningsen / Tectonophysics 231 (1994) 33-44

5.1. Previous genetic concepts Tegengren (1910) (also quoted by Shaikh, 1974) pondered the origin of the Tarrekaise magnesite deposits and concluded that they were formed “ . . . either through influence of magnesian bicarbonate and iron carbonate solutions originating from the gabbroic magma [. . .] or they are direct epigenetic precipitations from such solutions” (translation from Swedish). Hogborn (19301, quoting Holmquist (1900), favored a metasomatic origin related to “intrusive greenstones”. However, it is important to notice that none of the above cited authors studied the Apar occurrence, which is the only presently known and described major magnesite deposit in the undeformed parts of the Sarektjdkkl Nappe. 5.2. Reinterpretation The genesis of magnesite deposits in general has been subject to much controversy and theories can be divided into two main categories: hydrothermal/ metasomatic or sedimentary/ diagenetic evaporitic formation (Pohl, 1989,199O). The same can be said for scapolite; authors either advocate a deep magmatic (mantle) or a metaevaporitic origin (Hietanen, 1967; Ramsay and Davidson, 1970; Kwak, 1977; Hoefs et al., 1981). The different genetic environments seem to generate two different main types of magnesite deposits (Pohl, 1989): The Kraubath type, consisting of veins and stockworks of micro-crystalline mag nesite, hosted by ultramafic rocks; and the Veitsch type, characterized by sugary to coarse-grained magnesite in nearly monomineralic lenses within marine platform sediments (Pohl, 1989, 1990). The Kraubath type magnesites are always related to volcanic activity and are interpreted to form through hydrothermal activity. The Veitsch type deposits typically form large stratabound lenses within marine platform sediments and country rocks can be dolomite and limestones as well as siliceous elastic rocks (Pohl, 1990). The magnesite forms by direct precipitation, dewatering and diagenetic recrystallisation of, e.g., primarily precipitated hydromagnesite or from syngenetic circula-

41

tion of Mg-saturated brines (Sonnenfeld, 1984; Pohl, 1989). Scapolite in meta-evaporites are not as well studied, but scapolite deposits that have an evaporitic origin are not uncommon (Hietanen, 1967; Ramsay and Davidson, 1970; Serdyuchenko, 1975; Kwak, 1977; Sharma, 1981). These authors interpret the scapolite deposits to have formed through metamorphism of marine evaporites. The confinement to a stratigraphic level in the Favoritkammen Group and the preserved sedimentary structures in the undeformed parts of the nappe are strong indicators of a sedimentary origin for the magnesite deposits. The diabase is an unlikely source of the scapolite and magnesite: scapolite occurs in the diabase only very close to scapolite-rich layers in the Spika Formation, where diabase and sediments suffered Caledonian deformation together. Undeformed dykes show no scapolite at all. Alteration of the diabase dykes is related to the relative age of the dykes and their relative position in the dyke complexes; not to whether or not the wall rock is carbonate, scapolite, or any other rock of the Favoritkammen Group. The stratabound occurrence of dolomite and magnesite rocks can thus hardly be explained by metasomatic solutions emanating from the diabase, neither can the fine lamination of alternating scapolite and carbonate-dominated lithologies have been produced by synintrusive metasomatism. It is evident from the fresh diabase, cutting across the clearly undeformed sedimentary stratigraphy at the Apar Quarry and adjacent areas, that the magnesite was formed prior to the intrusion of the diabase dykes. Magnesite does not form as a primary evaporitic mineral in marine sequences, but can do so in continental lacustrine environments, where it can precipitate from brines that have been in contact with basic rocks (Sonnenfeld, 1984, and references therein). Recent magnesite deposition occurs in Australia (the Coorong dolomite; Von der Borch and Lock, 1979; Schroll, 1989) at 37.5’S latitude (cf. below) through precipitation from lacustrine to lagoonal water in barrier-beach complexes. The area is periodically inundated by seawater that evaporates to form marine evaporites Won der Borch and Lock, 1979). Alternating

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O.M. Suenningsen / Tectonophysics 231 (1994) 33-44

influx of continental and marine waters are postulated for other evaporite sequences, e.g., that of Sebkha el Melah in Tunisia (Busson and Perthuisot, 1977). A similar depositional environment would explain the interlayering of dolomite/ magnesite (lacustrine) and talc-silicate (marine) rocks in the Spika Formation. 5.3. Other meta-evaporitic carbonates in the Scandinavian Caledonides Presumably evaporite-derived carbonates of Late Precambrian age are widespread at different tectonostratigraphic levels in the Scandinavian Caledonides (Kumpulainen, 1980; Nystuen, 1980, 1987; Foyn, 1985; Kumpulainen and Nystuen, 1985). The Toss%sfjZllet, Risback and Engerdalen groups in the Middle Allochthon all carry magnesite-bearing dolomites, interpreted as being of evaporitic origin in a sebhka environment (Nystuen, 1980; Kumpulainen, 1980), right below the hiatus under the Vendian (ca. 650 Ma) Varanger tillites (Kumpulainen and Nystuen, 1985). The main subtropical evaporite-forming belts are situated around latitudes 28”, and palaeomagnetic data from the late Riphean give latitudes between 15 and 40”N for Baltoscandia (Vidal and Bylund, 1981).

6. Conclusions The magnesite deposits in Sarek are parts of the Spika Formation in the Favoritkammen Group. Even though it is affected by extensional deformation and thermal metamorphism, it is obvious that the Spika Formation is of sedimentary origin. The magnesite was formed prior to emplacement of the extensive dyke swarm, that is characteristic of the SarektjW Nappe. The diabase dykes are highly unlikely as sources for the scapolite/ magnesite. The following evolution is proposed for the Spika Formation: (1) Precipitation of minerals in an evaporative setting with alternating infhrx of continental (dolomite-magnesite) and marine (scapolite) wa-

ters in the Late Precambrian. The setting could be comparable to that of a sabkha (Busson and Perthuisot, 1977) or the present-day beach-barrier complexes of Coorong lagoon in Australia Won der Borch and Lock, 1979), that are periodically inundated by seawater. Circulation of syngenetic brines form dolomite/ magnesite breccias. The magnesite can have precipitated directly from a Mg-hypersaturated brine or formed by circulation of epigenetic brines. Late diagenetic recrystallization generally appears to be responsible for the sparry nature of this kind of magnesite deposits (Velasco et al.,1987; Pohl, 1990) and may be so in Sarek, too. Halite, anhydrite and gypsum provided the elements for formation of the scapolite layers during metamorphism (cf. Hietanen, 1967; Serdyuchenko, 1975). (2) Extensional deformation, mainly through normal faulting, including low-angle normal faults. The carbonates deform ductilely. Steps 1 and 2 are most likely not separated in time: the sediments may have been deposited in rift basins and the thick quartzites of the Apar Formation (Fig. 4) indicate prolonged deposition of shallow-marine sediments in a subsiding basin, in which the sediment influx balanced the subsidence rate. According to McKenzie (1978) crustal thinning associated with continental rifting can produce basin subsidence and the deposition of thick sedimentary sequences prior to the brittle rupture of the crust. This is consistent with the idea of the sedimentary sequence of the Sarektjlkki Nappe being the result of a rifting process, eventually leading to the formation of a passive margin. (3) Intrusion of a massive swarm of diabase dykes as the rifting progresses. The diabase forms sheeted dyke complexes and the sediments of the Favoritkammen Group suffer regional thermal metamorphism. This stage of extreme extension represents the initiation of seafloor spreading and what was to become the Sarektj&kfi Nappe formed a part of the transition between the Baltoscandian continental and the Iapetan oceanic crust. (41 The Caledonian Orogeny: detachment in the Late Cambrian-Early Ordovician (40Ar-39Ar datings in Dallmeyer et al., 19911, followed by a

O.M. Svenningsen/ Tectonophysics231 (1994) 33-44

complex history in the accretionary prism leading to the final eastwards thrusting of the passive margin sequence over the Baltic Shield in the Silurian during the Scandian phase of the Caledonian Orogeny.

7. Acknowledgements P.-G. AndrCasson initiated my work in the Sarek area and he and D.G. Gee held their guarding and helpful eyes on me throughout the work. M. Ripa and N.W. Svenningsen assisted me during field work concerning the Spika Formation. The field work was financed by grants from the Royal Physiographic Society in Lund, the Royal Swedish Academy of Sciences and the Swedish Natural Science Research Council (grant G-GU 1639-3081, which is gratefully acknowledged. Comments and suggestions from Drs. J.P. Nystuen and F.W. Pohl greatly improved the manuscript.

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