The geology of the northern Sulitjelma area and its relationship to the Sulitjelma Ophiolite

MICHAEL F. BILLETI

Billett, M. F.: The geology of the northern Sulitjelma area and its relationship to the Sulitjelma Ophiolite, Norsk Geologisk Tidsskrift, Vol. 67, pp. 71-83. Oslo 1987. ISSN 0029-196X.

The geology of the area to the north of Sulitjelma is dominated by the Skaiti Supergroup, which occurs at the top of the Upper Allochthon beneath overthrust rocks of the Uppermost Allochthon. The regional geology comprises a locally inverted, stratified ophiolite, the Sulitjelma Ophiolite, which partly intrudes the overlying rocks of the Skaiti Supergroup. These older higher grade rocks consist of a continuous l km thick sequence of gneisses, quartzites, semipelites, graphitic and calcareous schists, which are associated with several levels of metaigneous rocks. The most important of these are the Rupsielven Amphibolites, which are geochemically analogous to within-plate oceanic island basalts and are therefore distinct from the metabasic rocks of the Sulitjelma Ophiolite. The main period of regional deformation and metamorphism occurred during the Scandian event of the Caledonian Orogeny. Evidence is presented to show that the older metasediments and metabasites of the Supergroup retain the signa ture of an earlier orogenic event of possible Finnmarkian age or older. The Skaiti Supergroup therefore represents a fragment of pre-Scandian continental crust through which the ophiolitic rocks were originally intruded and onto which they were locally erupted.

M. F. Billett, Department of Soil Science, University of Aberdeen, Meston Walk, Aberdeen, AB9 2UE, Scotland.

The Sulitjelma area (67° 10'N, 15" 21' S) is situated the Skaiti Supergroup after an area to the south approximately 70 km north of the Arctic Circle (Boyle et al. 1985). The Supergroup is intruded by straddling the international boundary bet ween the Furulund Granite and the Sulitjelma Gabbro Nor way and Sweden (Fig. 1). The cover rocks Complex, the !argest of several major intrusions occur within the Upper Allochthon of the Scandi­ at this leve !. The Skaiti Supergroup also outcrops navian Caledonides (Roberts et al. 198 1), and further south in Baldaoivve and Skaiti and in an form part of the Koli Nappe (Kulling 1972). The isolated area to the north (Fig. 1). main period of regional metamorphism and The Sulitjelma area, bounded to the north and deformation in the Sulitjelma area is related to south by the Tysfjord and Nasafjell Precambrian the Scandian phase of the Caledonian Orogeny basement culminations respectively and by the (Wilson 1972; Boyle et al. 1985). Fauske Nappe to the west and the Sjonsta Group The structurally lo west rocks are a series of to the east, has long been regarded as a classic greenschist/lower amphibolite facies metasedi­ area for research in the Scandinavian Caledon­ ments kno wn as the Sjonsta and Furulund Groups ides. Major contributions to the understanding of (Table 1). The latter, which contains fossils of the geology include: Middle- Upper Ordovician age (Vogt 1927), is (l) Sjogren's pioneer mapping in the late nine­ overlain by the Sulitjelma Amphibolites. In the teenth century, culminating in the publication area to the north of Sulitjelma, the coiltact bet ween these two units is extensively brecciated and chloritized and marks the leve! of major Cu­ Table l. The tectonostratigraphy of the Upper Allochthon in Fe- Zn-S ore-bearing horizons. Structurally over­ the Sulitjelma area. lying the Sulitjelma Amphibolites is a sequence Regional Tectonostratigraphy Kautsky's Nappe Units of upper amphibolite facies metasediments and metabasites known as the Skaiti Supergroup. In Skaiti Supergroup Gasak Nappe the past these rocks have been called the Gasak Sulitjelma Amphibolites Vasten Nappe Furulund Group Nappe (Kautsky 1953) or the Sulitjelma Schist Sjonsta Group Pieske Nappe Sequence (Wilson 1973), but have been renamed 72 M. F. Billett NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

�SKAITI E3SUPERGROUP COVER ROCKS (excluding the DSkaiti Supergroup) r::1PRECAMBRIAN L::]BASEMENT

S W E D

50 km

Fig. l. Simplified geological map of the central Scandinavian Caledonides between 66° N and 68° N (after Nicholson & Rutland 1969).

of the first geological map of the Sulitjelma turally highest unit in the area, the Skaiti region (Sjogren 1900), Supergroup, may be older than the Sulitjelma (2) the publication of a comprehensive memoir Ophiolite. of the area bet ween Baldaoivve to the south The follo wing account describes the tectono­ and Blåmannsisen to the north by Vogt stratigraphy and structural geology of the Skaiti (1927), Supergroup in the area to the north of Sulitjelma (3) Kautsky's (1953) reinterpretation of the (Fig. 2). It also highlights a number of interesting regional geology as a series of thrust sheets, relationships bet ween the Skaiti Supergroup and and the Sulitjelma Ophiolite. In recent years several (4) Nicholson & Rutland's (1969) regional syn­ workers (Henley 1968; Wilson 1968; Nicholson & thesis of the area bet ween Bodø and Rutland 1969; Cooper et al. 1979) Iiave studied Sulitjelma. speci fic parts of the Skaiti Supergroup, in areas dose to the Sulitjelma Mountains and the More recently Boyle (1980) and Boyle et al. Sulitjelma Ore Field. This contribution represents (1985) have sho wn that the Sulitjelma Amphibo­ a summary of part of a more detailed study of the lites and the Sulitjelma Gabbro Complex are a Supergroup in the area to the north of Sulitjelma, comagmatic suite of inverted ophiolitic rocks. and sho ws that it has a more complex geological These comprise a layered gabbro (the Sulitjelma history than the Sulitjelma Ophiolite, the Furu­ Gabbro Complex), which is stratigraphically lund Group and the Sjonsta Group. overlain by a sheeted dyke complex (the Mietjer­ pakte Sheeted lntrusive Complex), which in turn feeds and intrudes a thick sequence of pillo w Javas Tectonostratigraphy of the Skaiti (the Otervatn Volcanic Formation). The contact between the layered gabbros and the overlying Supergroup volcanic and sub-volcanic rocks is locally marked The distribution of the major rock types of the by the development of a unit of highly deformed, Skaiti Supergroup to the north of Sulitjelma coarse-grained FJaser Gabbro. The major inver­ sho wn in Fig. 2 is based on a more detailed sion of the stratigraphy implies that the struc- l: 16,400 geological map of the area (Billett 1984). NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 73

The lithological boundaries trend east- west gneisses comprising both leucocratic augen gneiss through most of the area, and are parallel to the and gamet-homblende gneiss. The former out­ regional schistosity. In the north and east the crops extensively to the west of the study area, contacts veer to wards north-east, south-west. where it forms typical rounded outcrops in the The outcrop pattem is largely controlled by a lo w-lying ground towards Skoffedalfjellet (Kol­ late, open, east- west trending fold known as the lung 19 80). The dominant lithology is a medium­ Røtind Synform which repeats much of the suc­ to coarse-grained quartzo-feldspathic orthogneiss cession to the north and west of Stormfjellet. The containing segregations of biotite and muscovite structurally lowest metasediments are a series of with or without garnet. Petrographically it gneisses and quartzites which are well-developed comprises quartz-plagioclase-biotite-muscovite­ in the north-east and the north-west of the area. clinozoisite ± garnet-apatite. Texturally it is They are overlain by a group of rusty and cal­ dominated by large recrystallized (5 mm) plagio­ careous schists, which in turn give way to alumi­ clase augen, which forms up to 50% of the mode. nous pelites and semi-pelites at the top of the The leucocratic gneiss is overlain by the Cal­ succession. Although the Skaiti Supergroup is careous Amphibolite, the latter being intensely primarily a sequence of metasediments, there are cross-cut by quartzo-feldspathic pegmatite veins. a number of important developments of meta­ At the contact veining is so intense that the igneous rocks. The most significant are the Rup­ amphibolite becomes volumetrically insignificant, sielven Amphibolites, which occur near the top forming rare screens amongst the pegmatites. of the succession bet ween the Rusty Schists and Veining decreases in frequency away from the the Røtind Schist. contact, and at a distance of 100 m it virtually A stratigraphy of the Skaiti Supergroup has ceases. The distribution of pegmatite veining been set up using both well-established names, sho ws that it originated from the leucocratic such as Røtind Schist (Sjogren 19 00), Lap­ gneiss, which is considered to have undergone phelleren Schist (Wilson 1968) and ne w descrip­ partial melting and remobilization. tive terms such as gamet-homblende gneiss. The The second group of gneisses, the garnet-horn­ tectonostratigraphy presented in this paper blende gneiss, comprises 1- 4 mm clots of garnet (Table 2) is a simpli fied version of that used in and quartz in a medium- to coarse-grained light Billett (1984). green matrix rich in amphibole, quartz, biotite and muscovite. It frequently develops a strong banding consisting of alternating leucocratic and darker homblende-biotite rich bands. Kyanite Metasedimentary rocks and staurolite are common porphyroblast phases. These paragneisses have sharp contacts with the Gneiss units above. The structurally lowest unit in the Skaiti Super­ group is a series of fo liated, coarse-grained Quartzite-metaconglomerate unit A sequence of micaceous quartzite and meta­ Table 2. Tectonostratigraphy of the Skaiti Supergroup to the conglomerate, which overlie the basal gneisses north of Sulitjelma, showing the main levels at which meta­ and are easily recognizable in the field by their igneous rocks occur. white, saccaroidal and often rusty appearance, Metasedimentary rocks Metaigneous rocks occurs at a number of levels in the Skaiti Super­ group (Fig. 2). The rusty colouration is due to the Lapphelleren Schist Furulund Granite weathering of up to 10% disseminated or banded Røtind Schist pyrite. Metaconglomerate horizons are a charac­ Rusty Schists (rusty pelite Rupsielven Amphibolites teristic feature of the unit, consisting of elongate, and semi-pelite, graphitic Sorjus 2 Metagabbro se hist and calcareous sehist) fiattened, grey quartzite and white quartz clasts Rusty Micaceous Quartzite in a muscovite-rich matrix. Individual meta­ and Metaconglomerate Foliated Metagabbro conglomerate bands contain a wide distribution Gneisses (garnet-homblende of clast sizes, ranging from 3 to 150 cm in length. gneiss and leucocratic gneiss) Calcareous Amphibolite The larger fragments are enclosed in a deformed matrix consisting of small quartz clasts and mica. 74 M. F. Billett NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

LEGEN O Dlp Strike of & o \ Membera of the Sulitjelma "{ regional l BLAMANNSISEN l Ophlolite schistosity GLACIER -4 Younging ,/ litjelma Amphibolites l direction of / "' Axlal ,., ," ,,/' �- ,.:.:==:::....____".,.!;/'

2km

Fig. 2. Simplified geological map of the area to the north of Sulitjelma, showing the distribution of the major units.

Petrographically the quartzite is a fine- to unit, is attributed to the oxidation of drusy pyrite coarse-grained rock with a poor schistosity and along cleavage planes. consists of quartz-muscovite-green biotite ± The calcareous schist forms a 300m thick unit kyanite - staurolite - plagioclase - pyrite - clino­ to the north of Kobbertoppen. It has a grey, zoisite-tourmaline-chlorite. Porphyroblasts are irregular weathering appearance, related to the common, particularly in the more aluminous hori­ presence of quartz- and amphibole-rich lenses zons, where kyanite and occasionally staurolite in a soft fine-grained schistose matrix. In thin­ are present. section the calcareous schist comprises quartz­ clinozoisite-biotite-muscovite-calcite-sphene ± garnet-plagioclase-hornblende-opaques. Por­ Rusty Schists phyroblasts are common, particularly of garnet, Rusty weathering pelites and semi-pelites are not muscovite and hornblende. only the most extensive and characteristic unit The structurally highest member of the Rusty of the Skaiti Supergroup, but as Sjogren (1900) Schist Unit is a 150m thick horizon of graphitic pointed out, the most widespread and important schist. In the field it appears as a fissile, rusty­ unit in northern Sulitjelma (Fig. 2). The term purple weathering, well-foliated, fine-grained Rusty Schists (Billett 1984) groups together a ph y !lite containing 1- 2 mm clasts of quartz. Petro­ large number of lithologies which include pelites, graphically the graphitic schist comprises quartz­ semi-pelites, calcareous and graphitic schists muscovite-graphite ±. biotite-garnet-chlorite­ (Table 2). clinozoisite. A unit of fragmenta!graphitic schist, The Rusty Schist sensu stricto comprises fine­ which outcrops to the north of Stormfjellet, com­ to medium-grained, well-banded, semi-pelites prises a 50 m thick graphitic phyllite containing containing the assemblage quartz-muscovite-bio­ numerous folded and broken fragments of pure tite-pyrite ± plagioclase-clinozoisite. The rusty graphite. Nicholson & Rutland (1969) have colouration, which is ubiquitous throughout the described a similar unit of graphitic schist con- NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 75 taining clasts of graphite to the north-east of of basic and acidic metaigneous rocks, which are Sorjusvann. intruded by dykes, Flaser Gabbros and layered gabbros. These later intrusions, which are anal­ ogous to the stratigraphically lo wer members of Lapphelleren Schist the Sulitjelma Ophiolite, are discussed in a later The Rusty Schists are stratigraphically overlain section. by a diverse group of pelitic, gamet-mica-quartz schists, which Wilson (1968) termed the Lap­ phelleren Schist after a locality to the north of Sulitjelma. The Lapphelleren Schist dips at 10- Calcareous Amphibolite 45 0 to the north and north-east and attains a The Calcareous Amphibolite interdigitates with maximum thickness of 270m. The Furulund the metasediments occurring close to the struc­ Granite also occurs at this leve! in the succession, tural base of the Skaiti Supergroup, and rests on where it is enclosed in an envelope of Lap­ the basal gneisses. It reaches a maximum thick­ phelleren Schist. ness of 240 m to the west of the Blåmannsisen Under the general umbrella term pelitic, the Glacier and is characterized by the occurrence Lapphelleren Schist comprises a diverse sequence of orange weathering carbonate (up to 10%) in of phyllites, semi-pelites and quartz-biotite­ veinlets, cavities or as a matrix mineral. In the amphibole schists (Billett 1984). Detailed inves­ field it outcrops as a homogeneous, poorly­ tigations of the lithological variation within the foliated, fine-grained amphibolite containing unit sho w that it represents a continuous strati­ common relict vesicles and rare pseudomorphs graphic sequence (Billett 1984). after plagioclase phenocrysts. The most common lithology of the Lap­ Petrographically it comprises the assemblage phelleren Schist comprises bands or lenses of hornblende - plagioclase - calcite - sphene ± quartz in a pink-red matrix of gamet, muscovite clinozoisite-muscovite-quartz-chlorite-Dpaques. and biotite. Porphyroblasts of kyanite and stauro­ Rare hornblende porphyroblasts retain a relict lite are common, in addition to calc-silicate knots inclusion fabric, which is oblique to the regional and minor amphibolite bands. mineral lineation de fined by the preferred orien­ tation of idiomorphic hornblende.

Røtind Schist The Røtind Schist, the structurally highest unit in the area, occurs in the core of the Røtind Foliated Metagabbro Synform, where it Iies above the Rupsielven The Calcareous Amphibolite is overlain to the Amphibolites. It is well-exposed over an area of north of Stormfjellet (Fig. 2) by a 50 m thick, 7 km2 and has a vertical thickness in excess of homogeneous, coarse-grained, moderately- to 200m. well-foliated metagabbro consisting of visible, 5-- In the field it appears as a strongly foliated 15 mm, dark-green hornblende prisms in a fine­ aluminous schist containing visible porphyro­ grained feldspar-rich matrix. A smaller body of blasts of gamet, stauro lite and kyanite in a banded Foliated Metagabbro also crops out to the west fine-grained matrix, consisting of l mm bands of of Stormfjellet. Although both metagabbros have quartz, biotite and muscovite. Petrographically it been extensively deformed and folded, the north­ is similar to the more common aluminous assem­ em intrusion still contains features such as xeno­ blages in the Lapphelleren Schist. Two of the liths of high!y deformed semi-pelitic country rocks most striking features of the Røtind Schist, ho w­ and cross-cutting metabasic dykes. ever, are its overall lithological uniformity and Both metagabbros develop a podiform struc­ the occurrence of abundant quartz lenses and ture in the field,characterized by 40-50 cm lenses boudins. This often gives it a gneissose of coarse-grained metagabbro in a fine-grained appearance. deformed or mylonitic hornblende-rich matrix. They contain the assemblage hornblende-plagio­ clase-sphene ± various minor phases. The main Metaigneous rocks textural feature is the presence of large (1.5 cm) The Skaiti Supergroup contains a large number randomly orientated poikiloblastic hornblende. 76 M. F, Billett NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

from the structurally lo wer units in the Sulitjelma Rupsielven Amphibolites area, such as the Furulund Group and the The most extensive development of metabasic Sulitjelma Amphibolites, are relatively common rocks in the Skaiti Supergroup occurs dose to the in the Skaiti Supergroup. The Furulund Granite, top of the succession, between the Rusty Schists described in detail by Vogt (1927) and Wilson and the overlying Røtind Schist. The Rupsielven (1968), is the most important of these intrusions. Amphibolites reach a maximum thickness of It is a homogeneous, coarse-grained, granite­ 700 m in the Stormfjellet area and thin pro­ gneiss which is characterized by the presence of gressive!y and die out eventually to wards the approximately l cm diameter K-feldspar augen in south-west, where they are in contact with the a foliated matrix of quartz, plagioclase, muscovite Røtind Schist (Fig. 2). Although this outcrop and biotite. A L- S fabric, which is particularly pattern appears to suggest a tectonic contact, well-developed at the margins of the intrusion, is there is no field evidence to support this and it de finedby the preferred orientation of muscovite appears that it represents an original thinning of and biotite. the volcanic succession. The metasediments surrounding the Furulund The Rupsielven Amphibolites consist of Granite contain numerous granitic apophyses up schistose coarse-grained and porphyritic meta­ to 8 min width, which have been affected by F2 basites, in which pillo wed and massive Javas, pil­ folding. This implies that the intrusion pre-dates lo w breccias, volcano-clastic rocks and dykes are or is penecontemporaneous with the regional commonly preserved and exposed over an area (Scandian) deformation. Wilson (1968, 1981) of 13 sq km. In their field relationships and pet­ obtained a Rb- Sr wholerock isochron of 424 ± rography they sho w some similarities to the 11 Ma for the Furulund Granite; this age suggests Sulitjelma Amphibolites. A more detailed com­ that the intrusion was emplaced during the Scan­ parison of both groups of metaigneous rocks will dian Orogeny. be made later (p. 23). The most common rock type (approx. 70%) of the Rupsielven Amphibolites are porphyritic Geological history of the Skaiti metabasites. These contain bet ween 30 and 40% (rare!y up to 80%) pseudomorphs after plagio­ Supergroup clase phenocrysts. At several levels an original The Caledonides in Norway involve two principal volcanic stratigraphy can be recognized in which orogenic events; an early Upper Cambrian­ individual 0.5- 2 m thick pillowed fio ws are inter­ Lower Ordovician (490-530 Ma) event known as calated with pillow breccia horizons. A similar the Finnmarkian, and a later Middle- Upper Silu­ stratigraphy has been described fr om modem pil­ rian (400-4 25 Ma) event kno wn as the Scandian low lava sequences whose formation has been (Roberts & Sturt 1980). The Finnmarkian evelit observed in Iceland and Ha waii (Wells et al. 1979; is best developed in the Finnmark region of north­ Moore et al. 1973). The Rupsielven Amphibolites em Norway (Sturt et al. 1978), whereas the Scan­ also contain intercalated pelite, semi-pelite, dol­ dian event is well developed throughout Norway ornite-magnetite and garnet-rich bands. and Sweden and represents the major tectonic The petrography of the Rupsielven Amphibo­ event throughout the Sulitjelma area. Using K­ lites is characteristic of middle- to upper Ar radiometric dating, Wilson (1972) indicates amphibolite facies conditions. They comprise the that the argon blocking temperatures of horn­ assemblage hornblende-plagioclase-clinozoisite­ blende and biotite occurred around 420 Ma; this sphene ± muscovite-garnet-opaques. Garnet concurs well with the date of the Scandian event porphyroblasts occur throughout the succession elsewhere in the Scandinavian Caledonides and commonly form porphyroblasts up to 3- 4-mm (Roberts & Sturt 1980). One of the most impor­ in size. Clinopyroxene bearing assemblages have tant features of the Scandian event in the not been observed in the Rupsielven Sulitjelma region is the development of a regional Amphibolites. penetrative schistosity, de fined by the planar gro wth of hornblende and biotite in the meta­ basites and muscovite and biotite in the meta­ Furulund Granite sediments. The Scandian event is also responsible Granitic intrusions, which are generally absent for Barrovian type regional metamorphism, NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 77

which reaches kyanite and sillimanite grade in the the Foliated Metagabbro, which itself has been Skaiti Supergroup. Boyle et al. (1985) also suggest deformed and metamorphosed during the that the Sulitjelma Fold Nappe Complex was regional Scandian event. The contacts of the formed during the Scandian event. This deform­ metagabbro cross-cut folds in the surrounding ation event will be referred to henceforth as D2. country rocks, implying that intrusion and sub­ In addition, the Skaiti Supergroup also retains sequent deformation and metamorphism took the imprint of an earlier orogenic event (Dl). place after development of the Fl folds. Tight to isoclinal folds occur elsewhere in the Skaiti Supergroup and, although comparable to Pre-Scandian deformation and many Scandian structures, the axial planes of the pre- Scandian folds still retain angular discor­ metamorphism dances with lithological contacts. The Fl fold axes Fig. 3 sho ws the development of two fabrics in are cross-cut and deformed by the regional fabric, the Rupsielven Amphibolites. The field relation­ the latter being axial planar to the F2 Scandian ships suggest that the intrusion of the dykes took structures. The axial planes of Scandian structures place after the deformation event that produced are al ways parallel to the regional foliation and a Dl fabric in the volcanic and volcano-clastic major lithological contacts. horizons. Evidence for a Dl orogenic event is augmented Although rare and difficult to recognize in the by petrographic evidence from the metabasites. field, the Skaiti Supergroup preserves evidence Large relict hornblende prisms containing a pre­ of a pre- Scandian folding event. These tight to existing fabric, form 2-4 mm, partially recry­ isoclinal folds are best preserved at the contact of stallized, altered poikiloblasts with a patchy

\ \

a b

Fig. 3. Fabrics in the Rupsielven Amphibolites. a. D2 fabric in a metabasic dyke truncating a pre-existing Dl fabric in a fragmental volcano-clastic horizon. b. Dl deformation event preserved in a porphyritic amphibolite cross-cut by a later metabasic dyke. Both rock types are overprinted by the later D2 regional deformation. 78 M. F. Billett NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

a

Fig. 4. Petrographic evidence for an early pre-Scandian period of deformation and metamorphism. a. Early Dl fabric preserved in quartz and sphene inclusion trails in a relict hornblende poikiloblast. The inclusion trails are overgrown by euhedral hornblende crystals exhibiting a D2 deformation fabric (x30). b. Euhedral hornblende prisms overprinting a large poikiloblast (x50). extinction (Figs. 4a and 4b). Many are clearly directions from pillow Javas have already been orientated at an angle to the regional (S2) schist­ successfully used to define a ma jor isoclinal fold osity and are overgrown by finer grained (1- in the Sulitjelma Amphibolites, which has an 2 mm), fresh euhedral hornblende prisms which inverted upper limb and a normal lower limb define a good regional lineation. (Boyle et al. 1979). The inverted upper limb of The evidence outlined above, therefore, sug­ the so-called Vaknahelleren Syncline includes the gests that the Skai ti Supergroup has been affected layered gabbros, sheeted dykes and pillow Javas by a pre- Scandian me tamorphic and tectonic of the Sulitjelma Ophiolite. Recognition of a event which has not been recognized in the Suli­ stratigraphic inversion at this leve! and within the tejelma Ophiolite or the younger Furulund and Furulund Group (Kirk & Mason 1984; Kekwick Sjonsta Groups. The pre- Scandian structures and in press ) suggests that the Furulund and Sjonsta me tamorphic minerals observed in the Skaiti Groups, which Iie structurally beneath the ophi­ Supergroup ma y be comparable with those pre­ olite, represent the youngest units in the area. served in xenoliths and in the contact aureole of The inversion of the stratigraphy also suggests the Sulitjelma Gabbro Complex (Mason 1971). that the Skaiti Supergroup, occurring structurally above the ophiolite, ma y in fact be older than the ophiolite. The older age of the Skaiti Supergroup is clearly indicated by the intrusive nature of the The Rupsi Antidine Sulitjelma Ophiolite (Mason 1980). Way-up criteria have been used to unravel the Pillow Javas in the Rupsielven Amphibolites structural geology of the Skaiti Supergroup and have been used in the same way to unravel the to identifya ma jor fold in the area. Younging structural geology of the Skaiti Supergroup. They NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 79

RØTIND SYNFORM l

A � B

>=Younglng direction

/

2km

Fig. 5. Northwest-southeast cross-section of the area to the north of Sulitjelma (refer to Fig. 2). show that a ma jor part of the Supergroup is right Lapphelleren Schist does contain a number of way-up, as the lavas dip and young in the same mi nor rock types not seen in the Røtind Schist. direction (Figs. 2 and 5). This implies that the The mo st likely reason for their absence is large­ stratigraphically lowest and oldest unit in the scale facies variation. The mo del also implies that Skaiti Supergroup are the gneisses, and the the Furulund Granite and the Sulitjelma youngest and highest unit the Røtind Schist. The Amphibolites and their associated ore bearing recognition of right way up strata in the Super­ horizons ma y outcrop to the north-west of the group also implies that, since the Rupsielven study area on the western side of Blåmannsisen. Amphibolites and the Sulitjelma Amphibolites The recognition of a ma jor isoclinal fold struc­ young in opposite directions, there must be a ture in the Skaiti Supergroup suggests that a rep­ structural inversion, possibly a ma jor isoclinal etition of units should occur across the axial plane fold, with an axial surface that Iies between the of the Rupsi Anticline. Fig. 2 shows that ma ny of two units in the upper part of the Skaiti the units do occur on both limbs of the Røtind Supergroup. Synform, an open, late Scandian structure, which Younging directions from graded bedding and formed after the Rupsi Anticline. The Quartzite­ cross-bedding are also summarized in Fig. 2. They Metaconglomerate unit, for example, outcrops show that the upper me mbers of the Supergroup above and below the gneisses on both limbs of the are inverted in the region to the north and north­ Røtind Synform. This suggests that the outcrop east of Kobbertoppen, as the beds dip and young pattern is caused by a structural repetition across in opposite directions. The presence of over­ a ma jor axis, with gneisses outcropping in the tumed strata dose to the contact with the core. The Rusty Schists overlie the Quartzite­ Sulitjelma Amphibolites suggests that the upper Metaconglomerate unit to the west of Stormfjellet me mbers of the Supergroup form part of the and Sjogren (1900) and Kollung (1980) have ma p­ inverted limb of the Vaknahelleren Syncline ped large areas of Rusty Schists to the north and (Boyle et al. 1985). These inverted units also help west of the study area. This suggests that they to delineate the axial trace of the ma jor isoclinal ma y underlie as well as overlie the Quartzite­ fold structure in the Skaiti Supergroup (Figs. 2 Metaconglomerate unit (Fig. 5). The outcrop pat­ and 5). This fold, which is formally named the tern produced by the folding of the Rupsi Anti­ Rupsi Antidine, has a similar stature and ampli­ eline by the Røtind Synformis similar to Ramsay's tude to the Vaknahelleren Syndine. A feature of 'Type 3' fold interference pattern, produced when this mo del is that it regards the Lapphelleren recumbent folds are refolded by later structures Schist as a structural repetition of the Røtind with upright axial surfaces (Ramsay 1967). Schist (Fig. 5). Although there are dose litho­ The repetition of ma jor lithological units across logical similarities between the two units, the the axis of the Rupsi Antidine suggests that the 80 M. F. Billett NORSK GEOLOGISK TIDSSKRIFT 67 (1987) gneisses represent the oldest exposed rocks in the cross-cut the ftaser texture. The contacts of the Skaiti Supergroup, with younger rocks out­ Flaser Gabbros with the rocks of the Skaiti Super­ cropping structurally above and belo w. The out­ group are associated with 100m thick porphyritic crop pattem of the major units also implies that sheeted dyke complexes. These are analogous to the fold doses to the south-west, as the gneisses the porphyritic metabasaltic dykes of the Mietjer­ and the Quartzite-Metaconglomerate unit pinch­ pakte Sheeted Intrusive Complex (Boyle 1980). out progressively in that direction. The contact with the Rupsielven Amphibolites is In summary, the structure of the Skaiti Super­ well exposed 2 km to the south of the Blå­ group to the north of Sulitjelma is dominated by mannsisen Glacier. Both lithologies sho w an a major isoclinal fold which is related to the increase in deformation, producing widespread formation of the Vaknahelleren Syncline. The shearing, and lenses of Flaser Gabbro occur in Rupsi Anticline may therefore represent the the amphibolites. structurally highest part of the 'Sulitjelma Fold The Skaiti Supergroup is also intruded by sev­ Nappe Complex', which Boyle et al. (1985) regard eral gabbro bodies which resemble the Sulitjelma as a single large allochthonous nappe unit. In the Gabbro Complex, both in terms of their field author's view this structural model provides a relationships and petrochemistry (Billett 1984). good working hypothesis for the geology of the The !a rgest of these satellite intrusions, the Sorjus Sulitjelma area. It also has the merit of explaining 2 Metagabbro, occurs immediately to the SSW the way-up criteria and stratigraphical anomalies of the lake of Småsorjus (Fig. 2). It comprises a in the area to the north of Sulitjelma. medium- to coarse-grained, poorly foliated meta­ basite consisting of amphiboles and plagioclase with minor amounts of clinozoisite, biotite and Relationship between the Skaiti opaques. Texturally the Sorjus 2 Metagabbro is a massive, isotropic intrusion associated with Supergroup and the Sulitjelma layered gabbros, gabbros breccias, gabbro Ophiolite pegmatites and trondhjemites. Contact meta­ Flaser Gabbros, sheeted dykes and layered morphism of the country rocks by the gabbros gabbros, which are an integral part of the has produced orthopyroxene, scapolite, silli­ Sulitjelma Ophiolite (Boyle 1980), outcrop and manite and cummingtonite bearing assemblages. intrude rocks of the Skaiti Supergroup. The Skaiti Supergroup has thus been intruded The best documented exposures of Flaser Gab­ by gabbro bodies, Flaser Gabbros and porphyritic bro occur in the Sulitjelma Mountains and consist dyke swarms, which can be directly related in time of coarse-grained, lineated, amphibolite con­ to the emplacement of the Sulitjelma Ophiolite. taining large (0.5-1 cm) porphyroblasts of horn­ Both the Flaser Gabbros and the en echelon dyke blende in a plagioclase-rich matrix. Mason (1966) swarms appear to represent zones of shearing and originally considered the Flaser Gabbro with its extension within the Skaiti Supergroup, through tectonic fabric to mark the boundary between the which the ophiolitic rocks may have been intruded Gasak Nappe and the Pieske Nappe (Table 1). In and erupted. a recent reinterpretation of the Flaser Gabbro, The Flaser Gabbros described above are anal­ Boyle (1980) recognizes within it lenses of ogous to some of the highly sheared zones of trondhjernite, layered gabbro and cross-cutting metabasic rocks described from the Chennaillet dykes, and therefore regards it as the tectonized Ophiolite in the Western Alps (Mevel et al. 1978). equivalent of the intrusive zone between the The Flaser Gabbros, which occur in 50 m wide Mietjerpakte Sheeted Intrusive Complex and the zones, are cross-cut by unfoliated dykes. These Sulitjelma Gabbro Complex. presumably post-date the shearing and meta­ Recent mapping (Billett 1984) has sho wn that morphism, which Mevel et al. believe to have the Flaser Gabbro outcrops at a lo wer stra­ occurred in situ within the oceanic crust. Strongly tigraphical level within the Skaiti Supergroup. It foliated and sheared gabbros have also been occurs as linear 300m thick bodies extending for found in the Dun Mountain Ophiolite Belt in Ne w as much as 3 km along strike (Fig. 2). The Flaser Zealand (Kidd, pers. comm.). Gabbros incorporate a number of different litho­ Metabasalts and metagabbros exhibiting logies, namely layered gabbros, trondhjernites, strongly foliated and mylonitic fabrics have been gabbro breccias and undeformed dykes which dredged frommodem fracturezones on the Mid- NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 81

Atlantic Ridge and from the FAMOUS area number of fundamental differences bet ween the (Bonatti et al. 1975). FJaser Gabbros have individual rock types. In addition, there are recently been observed in situ on the Gorringe marked differences in the abundance of ker­ Bank seamount off the Moroccan Coast (Hon­ atophyres and trondhjemites in the two groups of norez et al. 1984). They are developed along shear amphibolites. zones in coarse-grained gabbroic rocks, which are Both groups of amphibolites are associated with cross-cut by late undeformed dykes. contact mineralization, the Sulitjelma Amphibo­ These examples sho w that in situ meta­ lites with economic ore bodies which occur along morphism and deformation is common in parts its boundary with the younger Furulund Group, of the oceanic crust. TI1e shear zones contain and the Rupsielven Amphibolites with a thinner undeformed late cross-cutting dykes and are mineralized horizon along its contact with the therefore similar to the FJaser Gabbros from Røtind Schist. Table 3 sho ws that the meta­ the Rupsielven area. However, the occurrence of sediments which Iie above and below both units quartzitic sedimentary rocks within the FJaser are also fundamentally different. Gabbros and the contact relationships between In terms of metamorphic history the Rupsielven FJaser Gabbros and metasedimentary rocks, sug­ Amphibolites differ in two signi ficant ways from gest that these shear zones may relate to marginal the Sulitjelma Ophiolite. First!y they preserve the oceanic crust. imprint of a pre- Scandian phase of deformation and metamorphism and secondly they have been overprinted by a higher grade of regional (Scan­ The Rupsielven and Sulitjelma dian) metamorphism. amphibolites: a comparison These differences are also brought out in the Although both groups of amphibolites contain geochemistry of the amphibolites (Billett 1984). pillo w Javas, fragmenta! volcanic rocks, dykes and The Rupsielven Amphibolites are incompatible FJaser Gabbros, Table 3 sho ws that there are a element enriched alkali to transitional tholeiitic

Table 3. A comparison of the Rupsielven and Sulitjelma Amphibolites.

Rupsielven Amphibolites Sulitjelma Amphibolites

Field Relationships Pillow Lavas Strongly Porphyritic Aphyric or Sparsely Porphyritic Fragmenta! Rocks Pillow Breccias Hyaloclastites Dykes Fine-grained, aphyric Porphyritic FJaser Gabbro Present Present Trondhjemite Rare Widespread Keratophyre One occurrence Widespread

Contact Relationships Mineralization Thin disseminated Economic Cu-Fe-Zn-S sulphide band bodies Associated Units Stratigraphically over­ Stratigraphically lain by the Røtind Schist overlain by the Furu­ and underlain by the lund Group and under­ Rusty Schists lain by the Lapphell­ eren Schist Metamorphism Gra de Amphibolite and Garnet­ Greenschist and Lower Amphibolite Facies Amphibolite Facies Pre-Scandian Present Absent Even!

Geochemistry Chemistry Alkali to transitional Tholeiitic basalts basalts Tectonic Setting Within plate oceanic Mid-Ocean Ridge istand 82 M. F. Billett NORSK GEOLOGISK TIDSSKRIFT 67 (1987) basalts with within plate oceanic island ma gmatic resent within plate oceanic island volcanic and affinities. In contrast, the volcanic and sub-vol­ sub-volcanic rocks. These pre-date the ocean­ canic rocks of the ophiolite are characteristic of fioor ma gmatism which characterizes the ocean fioor tholeiites (Boyle et al. 1985). The Sulitjelma Ophiolite. The Skaiti Supergroup is Rupsielven Amphibolites therefore represent an intruded by me mbers of the Sulitjelma Ophiolite, early, pre-ophiolite, pre-Scandian phase of intra­ namely Flaser Gabbros, isotropic and layered plate ma gmatic activity. gabbros and sheeted dykes. The structural mo del developed for the area to the north of Sulitjelma suggests that the Sulitjelma Ophiolite and their associated base me tal deposits ma y outcrop to the Summary and discussion north-west in the area to the west of The Skaiti Supergroup represents a 0.3-1 km Blåmannsisen. thick volcano-sedimentary sequence dominated In conclusion, the Skai ti Supergroup represents by rusty weathering, pyritiferous phyllites and a continuous sequence of supracrustal rocks serni-pelites associated with a variety of basic through which the Sulitjelma Ophiolite Complex volcanic rocks. The occurrence of ma rbles, cal­ was originally intruded and erupted. The sites of careous schists and highly aluminous schists sug­ crustal extension are related to the development gests a ma rine, predominantly non-clastic of Flaser Gabbros and the intrusion of small environment of deposition for the me tasediments. sheeted dyke complexes. The oldest rocks in the Supergroup comprise a series of gneisses overlain by quartzites and Acknowledgements. - I thank Dr. A. J. Barker, Dr. A. P. Boyle and particularly Dr. R. Mason for critically reading the me taconglomerates. It could be argued that this manuscript and A/S Sulitjelma Gruber for providing logistical lower sequence is comparable to the basement­ support during fieldwork in Norway. This project was under­ cover relationships observed in ma ny parts of laken during the tenure of a N.E.R.C. research studentship at the Scandinavian Caledonides, in particular the University College, London. sparagmite-gneiss association described from Manuscript received February 1986 southern Norway (Barth 1938). The Skaiti Supergroup retains the signature of References a pre-Scandian tectonic and amphibolite facies Barth, T. F. W. 1938: Progressive metamorphism of sparagmite me tamorphic event. This earl y episode of deform­ rocks of S. Norway. Norsk geologisk Tidsskrift 18, 54-64. ation and me tamorphism has not been observed Billett, M. F. 1984: The Rupsielven Amphibolites and associ­ in the stratigraphically younger, overlying for­ ated metasediments; their relationship to the Sulitjelma Ophi­ olite, northern Norway. Unpublished Ph.D. thesis, ma tions. The Sulitjelma Ophiolite and the Furu­ University of London. lund and Sjonsta Groups were therefore Binns, R. E. 1978: Caledonian nappe correlation and orogenic emplaced and deposited onto an existing de­ history in Scandinavia north of latitude 67 N. Geological formed and me tamorphosed substratum. Society of America Bulletin 89, 1475-1490. Bonatti, G., Honnorez, J., Kirst, P. & Radicati, F. 1975: Meta­ It is now well established that parts of the Seve­ gabbros from the Mid-Atlantic Ridge at 06 N: contact­ Koli Nappe Complex, which includes the Skaiti hydrothermal-dynamic metamorphism beneath the axial Supergroup, contain the signature of both Pre­ valley. Geological Journal83, 61-78. cambrian and Finnmarkian orogenic events Boyle, A. P. 1980: The Sulitjelma Amphibolites, Norway: a (Binns 1978; Reymer et al. 1980; Dallmeyer et Lower Paleozoic ophiolite complex? In Panayiotou, A. (ed.), Ophiolites. Proceedings of the International Ophiolite Sym­ al. 1985; Dallmeyer & Gee 1986). The age of the posium, Cyprus 1979, 567-575, Cyprus Geological Survey pre-Scandian me tamorphic and tectonic event in Department. the Skaiti Supergroup could therefore be equiv­ Boyle, A. P., GriffithsA. J. & Mason, R. 1979: Stratigraphical alent to the Finnmarkian event (Sturt et al. 1978), inversion in the Sulitjelma area, Central Scandinavian Cale­ or could be considerably older. Precambrian ages donides. Geological Magazine 116, 393-402. Boyle, A. P., Hansen, T. S. &Mason, R. 1985: A new tectonic have been recorded from the Beiarn Nappe, perspective of the Sulitjelma region. In Gee, D. G. & Sturt, which overlies the Seve-Koli Nappe Complex to B. A. (eds.). The Caledonide Orogen: Scandinavia and related the west of Sulitjelma (Styles 1978) and from the areas. 529-542. Wiley Interscience, New York. Seve Nappe of central Scandinavia (Reymer et Cooper,M. A., Bliss, G.M., Ferriday, l. L. & Halls, C. 1979: The geology of the Sorjusdalen area, Nordland, Norway. al. 1980). Norges Geologiske Undersøkelse351, 31-50. The geochemistry and fieldrelationships of the Dallmeyer, R. D., Gee, D. G. & Beckholmen,M. 1985: 40Ar/ Rupsielven Amphibolites show that they rep- 39Ar mineral age record of early Caledonian tectonothermal NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Sulitjelma Ophiolite 83

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