The Tectonostratigraphy and Structural Evolution of the Sitas Area, North Norway and Sweden (68 0 N)
PETER D. CROWLEY
Crowley, P.D. 1989: The tecton ostratigraphy and structural evolution of the Sitas Area, Nort h Norway and Sweden (68 0 N). Nor. geo/. unders . Bull. 416, 25-46.
The Sitas area conta ins rock s derived from the pre-Caledonian Scandinavian crato n and from an accreted Caledonian fore -arc comple x. These rocks form the par-autochthonous(?) Rombak Comp lex which can be tied stratigraphically to Baltica and three allochthons which from the base up wards are: the Storriten Complex, the Langvatn nappe and the Marko nappe . The Storr iten Comple x is a schuppen-zone composed of rock s derived from hinterland equivalents of the Rombak Comp lex while the Langvatn and Marko nappes are part of an accreted fore-arc terrane which can be correlated with parts of the Middle Koli Nappe Comple x (MKNC). The Marko nappe contains mafic rocks whose geochemical affinities suggest derivation from both MORB-Iike and arc-like sources. The MKNC was deformed by a series of five superposed Caledonian deformations (0 ,-0 ,). Only the latest two deformations that affected the MKNC, 0 , & 0 .. are recogn ized in the Storriten and Rombak Complexes. For this reason, the MKNC cannot be tied directly to the underlying domains and therefore to Baltoscand ia until D. when it was thrust onto the Storriten and Rombak Complex es. The earliest three defor mations may have occurred outboard of the lower Paleozoic margin of Baltoscandia. The two earliest defo rmations (0, & 0,) occurred under moderate pressu re amphi bolite -facies conditions (5750 C, 800 MPa) while later deformat ions occurred under lower PoT conditions. By D. the MKNC had cooled to greenschist-facies conditions (T ( 450 0 C, P ( 500 MPa). Most of the strain in the Sitas region was accommoda ted by large-scale ESE-directe d thrusting during 0 , and D•. 0 , thrusting imbricated the rocks in the Marko nappe and emplaced the Marko nappe onto the Langvatn nappe. D. thrust ing placed the MKNC onto the Storr iten Complex and the Storriten Complex on the Rombak Compl ex. In addition, it detached the sedimentary cover of the Rombak Complex from its crystalline basement, imbricated the cover and eventually delamina ted the uppermost metres to hundreds of metres of the basement form ing the Storriten Comp lex. Below this level, the crysta lline basement was not penetratively deformed by Caledonian events .
Peter D. Crow/ey, Department of Geology, Amherst College, Amherst. MA 01002, U.SA
Introduction The Sitas area (Plate 1) is located on the oceanic in origin. The continental basement southern margin of the Rombak window , a of the Rombak window was at a mid-crustal large fenster of pre-Caledonian basement be depth when this metamorph ic allochthon was neath the Caledonian nappe pile that straddles accreted to it. The Sitas area thus provides the Norwegian-Swedish border between 68° an excellent opportunity to study, by direct and 68° 30' N (Fig. 1). Precambrian granitic observation, the geometry of structures that gneisses within the Rombak window are local form in response to the accretion of an ocea ly overlain by the Tornetrask Formation, a nic terrane at an intermediate crustal level. very thin Vendian to Lower Cambrian platform This paper summarizes the results of an sedimentary sequence (Thelander 1982) that integrated geological , petrological and structu correlates with the 'Hyolithus' zone sequence ral study that was based on field work carried (Kulling 1964) of the Baltic craton . For this out during the summers of 1981-1983 and reason, the Rombak window can be conside 1985. The study concentrated on: (1) defining red to represent an extension, although per a tectonic stratigraphy within the allochthonous haps a displaced one (Tilke 1986) of the pre Caledonian rocks , (2) determining the nature Caledonian Baltic craton beneath the Caledoni of the contact between the pre-Caledonian an nappe pile. The continental basement of basement and the Caledonian nappes, and (3) the Rombak window is structurally overlain determining the pressure-temperature condi by an allochthonous terrane of medium-grade tions at which the Caledonian nappes were metamorphic rocks that are, at least in part, accreted to Scandinavian continental base- 26 Peter D. Cro wley NGU · BULl. 416. 1989
~ Upper xsu . Rod ing- ij:ill ~ ~ ~~I i dd k ~"Ii l (ult ramafic pod) ~ ~Low('rKolt :.:..: N El Sevc 1
.,':- Storriten and Akka jaure Comp lexes
~ pre-C.l1(·doni'ln basement km
o 10 zo
Fig. 1. Simplified tectonic map of the Sitas region . (Compiled from Foslie 1941. 1942. Kautsk y 1953. Kulling 1964. 1982. Gustavson 1974, & Hocqe s 1985). NGU • BULL. 416. 1989 Tectonostratigraphy and Structural Evolution 27
ment. This report emphas izes tne stratigrap Previous work hic and structural aspects of this study; the The Norweg ian portion of the study area was metamorphic petrology of the rocks in the initially mapped by Foslie (1941) and published study area has been discussed elsewhere at a scale of 1:100,000. This was included in (Crowley & Spear 1987, Crowley 1988). Gustavson's (1974) 1:250,000 map of the Nar The Sitas region can be divided into 3 tecto vik region. As part of a reconnaissance study nic domains separated by two major low of northern Norrbotten county, Kulling (1964) angle post-metamorphic thrust faults , the Fros produced the first map of the Swedish portion tisen and Mereftasfjell thrusts (Hodges 1985). of the area at a scale of 1:400,000. These Within each of the three tectonic domains the maps were valuable guides for this study ; re is a distinct tectonic stratigraphy. Beneath however, signif icant differences exist between the Frostisen thrust is the structurally lowest my map (Plate 1) and theirs , particularly in domain, the Rombak Comp lex composed of areas of difficult access . the Rombak window granitic gneisses and This study continues a northwest-southeast isolated outliers of the Tornetrask Formation transect across the Scandinavian Caledonides that are in depositional contact with the gneis from the Lofoten Islands to the Caledo nian ses. Between the Frostisen and Mereftasfjell foreland that was started by Tull (1978), Hakk i thrusts is the Storriten Complex (Hodges 1985) nen (1977), Bartley (1984) and Hodges (1985). a sliver zone of phyllonites and mylonites The 20 km transect from the Efjord culminati derived from lithologies similar to the underly on to the Rombak window , mapped at 1:50,000 ing Rombak Complex. Above the Mereftas by Hodges (1985) is cont iguous with the Sitas fjell thrust, a structurally complex sequence area. The transect has been continued to the of metasedimentary and meta-igneous rocks southeast of the Sitas area by Tilke (1986). crop out. This uppermost domain contains Part of the Rombak window to the north of rocks that I corre late with the Middle Koli this study has been mapped at 1:100,000 (Bir Nappe Complex (MKNC) of Stephens (1980) keland 1976). South of the Sitas area, Bjork and Stephens et al. (1985) exposed in the lund (1985) has mapped and described a thrust southern Norrbotten mountains. Within the complex known locally as the Akkajaure Comp Sitas area, the MKNC is composed of two lex. thrust nappes , a lower Langvatn nappe and an upper Marko nappe . To the east of the Sitas area, both the Seve and the Lower Koli nappes appear as eastward-thickening wedges Tectonic stratigraphy between the Storriten Complex and MKNC. Within the Sitas region , 29 mappable lithologi The Sitas area is located 15 km west of the cal units were recognized (Plate 1). All of the westernmost exposures of Lower Koli rocks se units have been multiply deformed and and 10 km west of the westernmost exposu metamorphosed. Contacts between many of res of Seve rocks . the lithological units are clearly tectonic, where All of the rocks in the Sitas area were regio as other contacts are gradational and are nally metamorphosed during Caledonian defor assumed to be depos itional. Still other con mation. The metamorphic grade and history tacts are of uncerta in origin ; these are sharp of each structural domain are different. The contacts, but do not appear to be zones of metamorphic grade increases structurally up concentrated strain and may be either deposi wards. Caledonian metamorph ic cond itions in tional or tecton ic. Although many contacts are the Rombak Complex never exceeded the tectonic, the lithological units appear in a regu middle greenschist facies (biotite grade). Storri lar vertical order and form a local tectonic ten Complex metamorphism reached the up stratigraphy. per greenschist facies (garnet grade) and the The stratigraphy of this region was first MKNC was metamorphosed to the amphibo described by Foslie (1941). Foslie did not re lite facies. cognize the prese nce of tectonic contacts within the Caledonian nappe pile and conside red the Rombak window basement granitoids to be Caledonian age intrusive rocks. Kulling (1964) provided the first detailed descriptions of the Vendian to Upper Cambr ian strat igraphy 28 Peter D. Crowley GU- BULl. 416. 1989
within the Rombak and Storriten Complexes . llodgev ( 1985) Th is study
In his discuss ions of the stratigraphy within Isoclinal x .w, Section the MKNC, KUlling relied heavily on the work fold S.E. Section interpretatio n S.\V. limb of IS.E. limb of of Foslie (1941, 1942) but did recogn ize the aba ndoned Haugefje ll amiform Baugefjell an tiform tecton ic nature of the cont act between the basement granitoids and the Caledonian meta I sediments and of some of the contacts within I the nappe pile. Gustavson (1 966, 1972) propo E~ sed the first detailed tecton ic stratigraphy for Il; C~era:e SJurvalnel Bauro Schis t the high-grade Caledonian metasediments at SCnosI this latitude. This tectonic stratigraphy was aao-c Gness - terrvatn c -e-ss H;ert va:n modified and subdivided into a local tectono s G-ees Ma r1o..O tratigraphy by Hodges (1985). Although this IS"'JA'j(."l1Scmst Comp1el ~'FUpe~--'--.a :\~.eorrr: I study has relied heavily on Hodges' interpreta Geppl Se SI Rep()l Scrast /~ tions, several major differences exist between lI!jel Complel Reppl Scmst I SkjAljeD$chISI S"'J.1 ·Jel SchISt S ~·"" , S , the tectonic stratigraphy presented below (Fig. Scres t I SCnosI BaugetJeI - BaugeljeM 2) and that of Hodges (1985). The tectonos tra Bauovatn Amp hloo hle AmpM >o llle RApet,aio.ka Complex - Sct-osl tigraph ic nomenclature presented below is in Bauovam Baccvatn Complel Complex N IU ~ S ~Jurva ne 1/ SCnosI formal and is presented to facilitate reference SCnosI ... • . . ~:OH llt: n to previously unnamed packages of rocks. Sl OHilen I Comp lex I Comple l
Fig. 2. Tectonic stratigraphy of the Sitas region. The tecto nic stacking of JithologicaJ units in three parts 01 the study area is comp ared with that of the adjacent Efjord-Rombak Rombak Complex transect of Hodges (1965). The Rombak Complex is composed of both Correlations with the Eljord region are not possible unless the basement gneis ses that are expo sed in Hodges' isoclinal fold is abandoned. the Rombak window and the sedimentary co ver that is in depo sltlonat contact with the Storriten Complex The Storriten Complex gneisses. It is dominated by the 1780 ± 85 is a struct urally complex zone of tectonic sli Ma (Gunner 1981) Rombak Granite-Gneiss, ces in which thin, variably mylonitized sheets first described by Vogt (1942). The Rombak of granitic gneiss and micaceous quartzite are Granite-Gneiss is lithologically identical to and interleaved in a matrix of phyllonitic schist. continuous with the Skjomen granite of Foslie Low-strain zones in slices of granitic gneiss (1 941) and Kulling's (1964) Sjangeli granite. and quartzite are lithologically similar to the The gran ite-gneiss is a foliated, coars e-grai Rombak Granite-Gneiss and basal quartzite ned, porphyritic biotite granite with accessory of the Tornetrask Formation. In fact, the ba apatite, magnetite and f1 uorite. Xenoliths and sal unconformity and quartz-pebble conglome pendants of a supracrustal sequence compo rate of the Tornetrask Formation is preserved sed of fine-grained amphibolite, amphibolitic in several thrust slices. The Storriten matrix schist, feldspathic quartzite, and muscovite phyllonite is a heterogeneous muscovite + biotite schist are common with in the gneiss. biotite ± garnet ± chlorite schist. It is commo n This lithological assemblage is called here the ly graph itic and in places calcareous. Very Skaddaive Assemblage, after the mountain graphitic beds in the matrix schist are lithologi Skadd aive where a large pendant of these litho cally similar to the Middle to Upper Cambrian logies crop s out. Alum Shale Format ion. The calcareous beds On the north slope of the mountain Rapetjak in the Storriten matrix are of uncerta in derivati ka, the Rombak Granite-Gneiss is uncon on as none are present in autochthonous formably overlain by a thin « 10 m) sedimenta sections described by Kulling (1964), Thelan ry sequence. This sequence includes quartz der (1982), or Tilke (1986) and perhaps sug pebble conglomerate, quartzite, and muscovi gest that rocks younger than those in the te-rich phyllite which correlates with the Vendi autochthonous sections (Le. Ordovician) are an lower sandstone and lower siltstone mem conta ined in the Storriten Comp lex. bers of the Torn etrask Formation of Thelander The Storriten Comp lex was mapped as the (1982). Middle Thrust Rocks by Kulling (1964) and is NGU-BULL.416,1 989 Tectonostratigraphy and Structural Evolution 29 broadly correlative with other Middle AIIoch Middle K61i Nappe Complex thon (Gee & Zachrisson 1979) rocks in the Within the Sitas area, the Middle K61 i Nappe Norrbotten mountains. It differs fro m ~
These units are recog nized by:(1) differences te. It is named here for the mountain Filfjell in schist composition, and (2) distinctive but on which it is best exposed. The Filfjell ma volumetrically minor assoc iated lithologies. The trix schist is lithologically similar to the Skjfl contacts betwee n these units are parallel to fjell Schist. Ultramafic pods within the matrix the regional mica foliation (S,) and are prob ab are common ly altered to zoned bodies with ly both depos itional and tecton ic. serpentinite cores surrounded by a rind of The Raudvatn Complex is the structurally talc-magnesite schist. Smaller ultramafic bodi lowest unit in the Langvatn nappe that crops es are common ly entirely altered to talc out in this area. It was informally named for magnesite schist. The magnesite generally exposures along the lake Raudvatn, 15 km forms distinctive large (1-5 cm) euhedral poikio north of Sitas (Hodges 1985). It is a tectonic blasts. melange composed of lenses of light-grey Kulling (1964) mapped, but did not name a calcite marble, amphibolite, and ultramafic hornblende schist or garbenscheifer unit. It is rocks in a matrix of flaggy weat hering, garnet informally named here the Rflpetjflkka Schist two-mica schist and calc-psamm ite. The tecto after the mountain Rflpetjflkka on which large nic lenses range in size from pods less than 1 hornb lende schist outcrops are found. The m in diameter to lenses that are more than Rflpetjflkk a Schist is a thick (>1m) bedded, 400 m long. The amphibolites are hornblende calcareous, hornblende ± garnet schist that plagioc lase-epidote amphibolites derived from is interbedded with garnet 2-mica schist. The both fine-grained (volcanic?) and coarse -grai coarse-grained hornb lende commonly makes ned (intrusive?) protoliths. Igneous textures garbenscheifer sprays on foliation surfaces. (euhedral clinopyroxene pseudomorphs) are Thin, orange -weathering, calcite marble inter locally preserved in the ultramafic lenses but beds are widespread. the igneous mineralogy has been altered to The Njunjas Schist is a heterogeneous sequ an amphibolite-fac ies serpentine + actinolite + ence of garnet- free calcareous psammites and forsterite ± talc assemblage. garnet two-m ica schists. Interbeds of orange The Baugefjell Amphibolite is a thin (0-50 m weathering calcite marble and graphitic garnet thick) but laterally persistent fine-grained, horn schist are common . The Iithological variation blende-plagioclase-epidote-sphene amphiboli of the Njunjas Schist is its most distinctive te. It is informally named here after the mounta characteristic. It is informally named here for in Baugefjell on which large exposures of the exposures on the mountain Njunjas. amphibolite crop out. Fine-grained amphibolite The Reppi Schist of Foslie (1941) is the is comm only interbedded with orange-weath most distinctive unit in the Langvatn nappe. It ering calcite marbles. The uniform fine grain is a dark-brown weathering calcareous schist size, intercalation of calcareous sediments, that forms flaggy outcrops due to the alterna and the local prese rvation of plagioclase micro tion of more and less calcareous bands a few lite pseudomorphs suggest derivation of the to tens of centimetres thick. Garnet porphyro amphibolite from a volcanic protolith. blasts and white clinozo isite laths are con spicu The SkjMjell Schist is the 'typical' garnet ous on weathered foliation surfaces . At the two -mica schist of the Langvatn nappe. It was southeast end of the Sitas area, on the north informally named by Hodges (1985) for ex slope of the mountain Marko, a hornblende-in, posures on the mountain SkjMjell, 20 km NW clinozoisite-out isograd crosses the Reppi of Sitas. It is silvery-grey, muscov ite-rich and Schist. To the southeast, on the low-grade(?) rhythm ically layered on a scale of centimetres side of this isograd, the Reppi Schist looks to 1O's of centimetres. Graded bedding is local quite different. There , it is no longer dark ly preserved. Thin « 1 cm) quartz stringers brown colored, but is a dark-green weathering and euneora l almandine garnet (1 mm to 5 hornb lende schist. The alternat ion of more and cm) are consp icuous on the weathered surfa less calcareous bands persists in the horn ce. Interbeds of quartzitic schist, hornb lende blende-bearing Reppi Schist, but it no longer schist, calcite marble, and small lenses of produces the distinctive flaggy outcrops. The amphibolite are present but are rare. hornb lende-bea ring Reppi schist could be con The Filfjell Complex is a unit of tectonic fused for the Rflpetjflkka Schist except that melange composed of lenses of calcite mar the Reppi Schist is generally more calcareous ble, amphibolite, and ultramafic rock s in a and thinner bedded than the Rflpetjfl kka Schist. matrix of garnet two -mica schist and psammi- NGU - BULL.416.19 89 Tectonostratigraphy and Structural Evolution 31
Fig. 3. Marko Complex lithologies a) Elongate calcareous schist xenoliths in migmatitic gneiss. b) Pillows in fine-gr ained amphibolite. cl Primary mineralogical foliation in layered gabbro. d) Alteration of igneous mineralogy to a greenschist-facies assemblag e along fracture s in layered gabbro.
Marko nappe Iy forms a coarse garbenscheifer texture . The Marko nappe has been informally divided Small (cm to 1a's of cm) amphibolite enclaves into 4 lithotectonic units: the Marko Complex , and thin amphibolitic dikes « 1 m thick) that Hjertevatn Gneiss, Bavro Gneiss, and the are locally discordant with lithological layering Bavro Schist. The bulk of the Marko nappe are common in the schist. The migmatitic is made up of the Marko Complex , a structu gneiss is a coarse-grained biotite two-feldspar rally complicated unit composed of six distinct gneiss. It is commonly garnetiferous. Elongate mappable lithologies: (1) garnet two-m ica psammitic to amphibolitic xenoliths (restite?) schist, (2) migmatitic gneiss, (3) calcareous are present in most exposures of the gneiss schist, (4) amphibolite, (5) felsite, and (6) gab (Fig. 3a). The calcareous schist unit is a thinly bro. Within the Marko Complex, these Iithologi banded (1 to 10 cm) plagioclase-epidote-clino cal units are repeated and truncated along zoisite-biotite schist. Fine-grained « 1mm) low-angle faults (Fig. 9 & Plate 2, section hornblende is everywhere present , but is not F-F'). Strain is concentrated along these con obvious in the field. The felsite unit is a fine tacts and lithological layering (relict bedding?) grained, light green-weathering, plagioclase is greatly attenuated along the contacts betwe and epidote-rich rock. It is presumed to be en different lithological units. This structural volcanic in origin , but no flow , pyroclastic or complex ity and a lack of fossils or facing indi pillow structures are preserved . cators has precluded the establishment of a A variety of amphibolitic rocks are contained strat igraphic sequence within the Marko Comp in the Marko Complex. The mineralogy of the lex. amphibolites is typically hornblende + plagioc The Marko Complex garnet schist is a grey lase + epidote + sphene ± clinozoisite ± weathering, quartz-rich, garnet two-m ica zoisite ± quartz. They vary from fine- to coar schist. Hornblende is often present, but it rare- se-grained and are common ly stro ngly linea- 32 Peter D. Crowley GU · BULL. 416.1989 ted. Locally, pillows are preserved and elonga te structures reminiscent of stretched pillow s 0.709 Q o are very common in the fine-grained amphiboli tes (Fig. 3b). Symmetrical chilled margins are 0.708 Cl') -c preserved on some small bodies of amphiboli cc te indicating an intrusive origin for some of ';::- 0.707 rY' the amphibolites. A large amphibolite sheet cc which crop s out on the NE shore of the lake 0.706 Bavrojavri (Plate 1) is compos ed of coarse grained amphibolite that has been intruded 0.705 by multiple generations of fine-grained amphi 0.704 bolite dikes. 0.0 0.6 0.8 A tectonic slice of layered olivine two-pyroxe ne gabbro crop s out on the south slope of the mountain Marko. Cyclical variations in the clivi ne/pyrox ene ratio and the planar alignment Fig. 4. Rb-Sr isocnron diagram of thin slabs fromthe Hjer tevatn Gneiss. Squares are present day values, Circles are of plagioclase laths produce a mineralogical 450 Ma values. Present day data forms an array with a (cumulate?) foliation in this gabbro (Fig. 3c). negative slope. This array cannot be interpreted .in terms of an age of formation, but could represent a mlxmg hne Assuming that this foliation represents cumula between a low " SrI" Sr, high Rb/Sr component ljuveOlle te layering and thus approximates a paleohor i magma) and a high " SrI"Sr. Iow Rb/Sr component (sea zontal surface, the gabbro represents a tecto water). The mixing line is contoured m terms of percent of sea-water component in the mixture. nic slice derived from a mafic intrusive that was at least 2.5 km thick. Throu ghout much of the body, the igneous plagioclase + clinopy chron diagram (Fig. 4) the data produce a line roxene ± olivine ± orthopyroxene mineralogy ar array with a negative slope. This array is has been preserved. Locally, along joints that interpreted to have been formed by two are perpendicular to the cumulate layering, the component mixing of a juvenile magma (" Rbl igneous mineralogy has been altered to a gre " Sr= 0.708 " Srfl6Sr.= 0.7040) with Rb and Sr enschist-facies albite-actinolite assemblage derived from sea-water (' Rb/"Sr= 0.041 " SrI (Fig 3d). The mineralogical foliation in the gab Sr.= 0.7085). bro is at a high angle to both the cont act The Bavro Gneiss is a distinctive, resistant , with the surrounding metasedimentary rock s dark-grey weathering, pelitic gneiss that is and the mica foliation in those rocks . Within informally named here for exposures on the approx imately 10 m of its contact with the north shore of Bavrojarvi. Kyanite and small metasedimentary rocks , the gabbro foliation (1 mm), round , inclusion-free, almandine gar becomes transposed so that it is approximate nets are abundant in the gneiss. Small (cm) ly parallel to the contact. Alopg this cont act, kyanite-bearing quartzo-feldspathic (migmati the gabbro has developed an amphibolite tic?) enclaves are common . facies hornblende-plagioclase mineral assem The Bavro Schist is a brown ish-weathering blage and is commonly invaded by telsic mate two-m ica schist that is informally named here rial believed to be migmatite derived from the for exposures on the north shore of the lake surroun ding metasedimentary rock s. Bavrojavri. It is a very garnetiferous pelitic The Hjertevatn Gneiss of Hodges (1985) schist. structurally overlies the Marko Complex. The gneiss is a plagioclase-rich banded gneiss compo sed of alternating tonalitic and amphibo Table 1: Rb and Sr analyses from thin slabs cut from two litic layers. Thick bands (tens of m) of garnet Hjertevatn Gneiss samples. schist are interleaved with the gneiss. Hodges GT- GT- GT- GT- GT GT (1985) suggested that the Hjertevat n Gneiss 24A 240 29A 29C 290 29E Present-day values was a migmatite derived by partial melting of Rb(ppm) 44.2 45.9 30.6 40.2 13.9 35.6 the surroun ding metasedimentary units. A Sr(ppm) 201.0 199.8 324.9 230.3 234.4 240.7 Rb-Sr isotopic study of thin slabs « 1 cm) cut rRb."Sr 0.635 0.664 0.273 0.505 0.172 0.427 from two samples of the gneiss (Table 1) sug rSri"Sr 0.7C833 0.70838 0.70899 0.70814 0.70916 0.7091 450 Ma values gests that the gneiss represe nts a sea-water rR "St, 0.635 0.663 0.273 0.504 0.172 0.427 altered juvenile igneous rock . On a Rb-Sr iso- rSr,"Sr, 0.70426 0.7041 3 0.70724 0.70491 0.70806 0.70636 NGU - BULL. 416.19 B9 Tectonostratigraphy and Structural Evolution 33
Regional Correlations continues from the Sitas area into the Efjord Structural complications and the lack of persis region and that the Reppi Schist does not sit tent, distinctive lithologies within the MKNC in the core of an isoclinal fold. make regional correlat ions difficult. However, The Langvatn nappe thins rapidly to the several of the units mapped in this area are north, pinching out between Skjomen and sufficiently similar to ones mapped in Ofoten Rombakfjord. The Marko nappe also thins to (Foslie 1941, Gustavson 1974, Hodges 1985, the north, but probably persists north of Rom Steltenpohl 1987) and southern Norrbotten bakfjord and continues to the northwest as (Foslie 1942, Kautsky 1953, Kulling 1982, Sund far as the SkAnland area studied by Stelten blad 1986, pers. comm. 1987) to propose corre pohl (1987) where he recognized similar rocks lations. These correlations are somewhat tenu as an unnamed allochthon immediately bene ous as much of the Sitas tectonostratigraphy ath the Evenes Group. cannot be traced either to the north across To the south, the Sitas MKNC is eroded the Ofoten synform or to the south across the from over the Akkajaure culmination. Where Akkajaure culmination. it reappears on the south side of the culminati Both the Langvatn and the Marko nappes on, the MKNC broadly correlates with rocks continue into the Efjord region studied by from Kautsky's (1953) Vasten and Salo nap Hodges (1985) and are part of Gustavson 's pes. Melanges similar to those seen in the (1966) Narvik Group. Gustavson (1972, 1974) Langvatn nappe are present in both the Vas placed an unnamed thrust fault within the ten and Salo nappes. However , the Langvatn Narvik Group, at the base of the Reppi Schist. nappe tectonostratigraphy is not recognized Hodges (1985) and Tull et al. (1985) disputed in the Vasten and Salo nappes, nor are the the existence of this thrust arguing that the muscovite-rich garnet schists that dominate Reppi Schist cropped out in the core of an the Langvatn nappe abundant in either the isoclinal fold. Continuing the Sitas tectonostra Vasten or the Salo nappes (Crowley, unpublis tigraphy to the northwest into the Efjord regi hed mapping 1985). Foslie (1942) mapped a on suggests that the thrust that separates the calcareous schist within Kautsky's Vasten Langvatn and Marko nappes also continues nappe that he correlated with the Reppi Schist. into the Efjord region as units from both the This calcareous schist is a thick-bedded mica Langvatn and Marko nappes are recognized ceous calc-phyllite that is unlike the thin by Hodges (1985). Within the Sitas area, the bedded Reppi Schist seen in the Sitas area fault between the two nappes is at the top of both in terms of bulk compos ition and sedimen the Reppi Schist. Therefore , I would continue tary structures. Therefore, I feel that Foslie's this fault to the northwest and place a fault (1 942) correlation is not justified and that little at the top of the Reppi Schist within the Efjord if any of the Langvatn nappe tectonostratigrap region. hy continues to the south of the Akkajaure Differences in the metamorph ic petrology culmination. between the Langvatn and Marko nappes furt Lithologies similar to those of the Marko her support the continuation of both nappes nappe are relatively common within the Vasten and the thrust that separates them into the and Salo nappes in the Stipok region studied Efjord region. Marko nappe garnets have un by Sundblad (1986). The matrix schists and zoned cores and sharply zoned rims (Fe/Mg mafic dike-intruded schists of the Marko Comp increases markedly in the outer 1DOll) while lex are quite similar to schists in the Allak the Langvatn nappe garnets have cores that Complex (Stephens et al. 1984, Sundblad 1986) are more strongly zoned than the rims (Crow that crops out on the mountain Allak 40 km ley & Spear 1987). Garnet zonation profiles to the southwest. However, the Allak Complex were analyzed from Efjord samples by Hodges contains two generations of mafic dikes , the & Royden (1984). Most of their garnets came later of which cuts across the regional folia from schists that were structura lly above the tion. Only the earliest phase of dikes, which Reppi Schist. All of these profiles are similar pre-date the development of the regional folia to the Marko garnet prof iles. One sample (79 tion, is recognized in the Sitas region. The 18A) was collected from a schist below the Hjertevatn gneiss is Iithologically very similar Reppi Schist, and its zonation profile is simi to the Middle Koli quartz keratophyre mapped lar to Langvatn garnet profiles. This is further by Kulling (1982) and Sundblad (1986). This evidence that a major fault within the MKNC correlation is consistent with the interpretation 34 Peter D. Crowley NGU · BULL. 416. 1989
Pcnctrative ESE directed mica foliation thrust faulting Amphibolitc Tectonic Mark o facies slacking (?) Stretching nappe metamorphi sm lineation Isoclinal folding W trcnding Amphibolitc ESE directed Penetrative inclined facies thrust faulting Mclangc mica foliation cross metamorphi sm formation Amphibolitc folding Langvatn Isoclinal facies nappe Isoclinal folding Tectonic metamorphism NE trcnding folding Slacking Isoclinal folding NWvcrgcm Mylonitic and back folding phyllonitic foliation Storriten Grcenschist-facics metamorphism Penetrative foliation Complex Isoclinal folding Stretching lineation
Grccnschist facies Rombak o structures form ed metamorphism Complex
Fig. 5. Summary of Caledonian struc tural events in the Sitas region.
based on Rb/Sr isotopic data that the Hjerte tions (Do to D,) that include at least 3 episodes vatn gneiss proto lith originating as a sea of low- angle faulting and 5 phases of folding water altered igneous rock. A similar explanat i (Fig. 5). One and possibly two phases of defor on was proposed by Stephens (1980) to ex mation have affected the crystalline Rombak plain chemical variations in Middle K61i vol Complex in the Precambrian prior to the depo canic rocks in the Stekenjokk region. The sition of the Tornetras k Formation. All of the Bavro gneiss is lithologically similar to a gar Caledonian deformat ions (Do to D,) affect the net schist mapped by K. Sundblad (pers. comm. rocks of the MKNC, while only the youngest 1987) and included in undifferentiated schists episode of thrusting (0,) and the two youn of his Stipok terra ne (Sundblad 1986). The gest phases of folding (0, & 0,) affect the remarkable similarities of individual lithologies Storriten and Rombak Complexes. A phase in both the Sitas and Stipok sections argues of deformation has affected the Storr iten strong ly for correlation. However, these units Complex prior to D,; however, this deformation are not in the same sequence in the two secti cannot be corr elated with any of the events ons an d m an y o f t he unit s o f t he St ip ok terra recognised in the MKNC. A ll of the deforma ne (Sundblad 1986) do not continue north into tions except D, produc ed map-scale structu the Sitas region. Unexplained structural comp li res, while all except Do produc ed regionally cations exist in the MKNC between the well extensive petrofabrics. studied Sitas and Stipok regions. Precambrian deformation Structural evolution Away from the Frostisen thrust , the Rombak The rocks in the Sitas region underwent a gneiss is weakly to strongly foliated with folia sequence of superposed Caledonian deforma - tion-parallel mylonite zones developed locally. NGU - BULL. 416, 1989 Tectonos tratigraphy and Stru ctural Evolution 35
This foliation has a very consistent orientati on, dipping steeply to the northwest (Fig. 7a) and is at a high angle to the subhorizontal Frostisen thrust. The supracrustals in the Skad calve pendant (Plate 1) are folded into tight to isoclinal, upright to inclined folds with axial surfaces and an axial planar cleavage that parallels the foliation in the Rombak Gneiss. This foliation is truncated by the basal uncon formity of the Tornetrask formation indicating that it was formed prior to Caledonian deforma- . tion (Fig. 6).
Tornetrask Formation conglomerate Do - Melange formation The first Caledonian deformation (termed here Do) to affect the rocks in the Sitas area is - - seen only in the Langvatn nappe. It produced - - - the zones of tectonic melange (the Raudvatn - - and Filfjell Complexes) found in the Langvatn Rombak Granite-Gnei ss nappe. During Do, ultramafic , mafic and car bonate rocks , lithologies typical of the ocea nic lithosphere , were chaotically intermixed b with the quartz-rich clastic sedimentary rocks that now const itute the matrix schists. Dois a Fig. 6. Tornetrask Formation basal uncomfor mity trunca ting rather crypt ic deformational phase because foliation in Rombak Granite-Gneiss). Dofabrics have been annealed by subsequent a) Photograph a) Line drawing from photograph . Dashed lines showing amphibolite-facies metamorph ism and trans pre-Caledon ian foliation. posed by subsequent deformations. The only record that remains of Dois the tectonic stac king and slicing within the Raudvatn and Fil fjell Complexes. Lithological contacts within S, varies widely across the Sitas area (Fig. the melange zones and between the melange 7b). Changes in the attitude of S, were used zones and the surrounding schists are conside to define structures produced by later deforma red to be Do faults. Other contacts within the tions. Transposition of S, to parallel S, has Langvatn nappe may also be Dofaults. All litho produced a weak southwest-dipping maximum logical contacts within the Langvatn nappe, in the attitude of S, while F, folding has formed including those interpreted to be Do faults are a very weak girdle about a southwest trending everywhere parallel to the S, regional foliation axis (Fig. 7b). D, appears to be roughly syn and are locally folded by F, folds. This demon chronous with the metamorphic peak of rocks strates that the D. tectonic mixing predated in the MKNC as both S, micas and metamor the development of D, petrofabrics. phic porphyroblasts are deformed by all subse quent deformations. Preliminary "Ar/"'Ar hornblende geochrono 0 1 - Isoclinal folding and development logy (Tilke 1986) and a detailed petrolog ical of the regional mica foliation study (Crowley & Spear 1987) suggest that the The first penetrative Caledonian fabrics were metamorphism of the MKNC in the Sitas area produced during D,. The effects of D, are recog was diachronous , with the Marko nappe rea nized in both the Langvatn and the Marko ching its thermal peak as much as 50 Ma nappes. These include small scale (cm to m before the Langvatn nappe. This strongly sug scale) isoclinal folds and a penetrative mica gests that different deformations were respon foliation (S,). No map-scale structures were sible for the development of penetrative syn produced by D,. S, is approximately parallel metamorphic foliations (S,) within the Lang to lithological layering and was deformed by vatn and Marko nappes. However, due to the all subsequent deformations. The attitude of transposition of S, in subsequent deforma- 36 Peter D. Crowley NGU -BULL. 416.1 989
N N N N
1.:/::. ' .,
a C 0 1-10% N=71 0 1-4% El 11-20% N=84 ;;: 5-8% . 21-30% . 9-12% . 31-42% . 13-17% N N N N
~ . ; . .~f';>'¥,~~j.t.\:.~.~ :',. ~ ,';.. ' . ': . . . : " '0.:: ::. b d .. .. N=48 0 2-7% N=1062 0 1-2% ;;1 3-4% ;;: 8-13% . 5-6% . 14-19% . 7-8% . 20-25%
N N N N .... ~ ..~ ~~~ & e ':" , ,;";.. .~ . . '. 9 . ~~ . N.90 0 1-4% 0 1-3% El 5-8% . 4-6% . 9-11% . 7-10% N N
. ..
/~~~;;. : . . f N=497 0 1-4% El 5-8% . 9-12% . 13-17%
Fig. 7. Equal-area stereoplots (see text for discussion). Contours: % points/1% area. a: Precambrian foliat ion in the Rombak Gneiss. b: S, mica foliation in the MKNC. c: L, Iineations in the Marko nappe. d: F, fold axes in the MKNC. e: F, fold axes. t: S. phyllonitic cleavage. g: F, fold axes. Tectonostratigraphy and Structural Evolution 37 NGU · BULL. 416. 1989
Tectonic Stratiuraphv Svmbols
><~ 1'2 thrust (major thrust. minor thrust) lZJMarko nappe ._ % ~ T4 thrust (major thrust. minor thrustL t2jungvatn nappe ~ ~ high angle fault ~ ~ I d::;;: ~ ----\---; antiforrn (fold phase indicated ) SIOrrilen Complex::' Z I F3 lill ~ synform (fold phase indicated) I ~ Rombak Complex .,...... ---ft--Fr 'enumedantiforrn(fold phase indicate
~ overturned S) nform (fold phase indicate I
Fig. B. Major Caledonian structures in the Sitas region.
tions, it was not possible to differentiate bet Within the Marko nappe, small-scale thrust ween the penetrative mica foliations of the two faults developed at a low angle to lithological nappes so they have been grouped together layering producing the lithological repetitions here. that character ize the Marko Complex (Fig. 9). Competent lithologies, particularly the mig matitic gneiss developed a mylonitic foliation
O2 - Thrusting and isoclinal folding near these thrusts. Less competent lithologi The Marko nappe was thrust onto the Lang es, particularly the calcareous schist were atte vatn nappe during Dz• However, the two nap nuated within a few tens of metres of the pes responded differently to Dz deformation. Iaults (Fig. 10a). Lithological layering (bed- 38 Peter D. Crowley NGU- BULL.416 .19 89
km o 1 2 3 4 5 Marko Complex (not in stratigraphic order Reppi Schist E"'-:'--'j migmatitic gneiss 0 calcareous schist o ~ mica schist r =· ~ '·.n mica schist w/ malicdikes [ ;Wt:;:~ amphibolite D lelsite
Fig. 9. Detailed geological map of the north slope of the mountain Marko show ing D, lithological imbrications within the Marko Complex. The faults bounding the imbricates all sole into a decolle ment at the top of the Reppi Schist.
ding?) in these units was greatly thinned near WNW and ESE trending L2 mineral and stret the thrusts. ching lineations (Fig. 7c). Small-scale thrusts The top of the Reppi Schist served as a within the Marko nappe all ramp upwards to decollement horizon for this thrusting. Many wards the ESE (Fig. 9), suggesting that the of the thrusts within the Marko nappe are lis transport is ESE directed. This is cons istent tric, merging into the decollement at the top with the vergence direction deduced from the of the Reppi Schist (Fig. 9 & Plate 2, Section asymmetry of pegmat ite boudins near the F- F'). There is no tectonic mixing between thrust (Fig. 10b). the Reppi Schist and any of the Marko nappe The displacement on the thrust is estimated lithologies; therefore, none of these thrusts to be greater than 25 km on the basis of the can continue down through the Reppi Schist. structural overlap of the Marko nappe on the The thrust faults locally deform the S, foliat ion Langvatn nappe measured parallel to the ESE in the Marko Complex, but are themselves transport direction. The amount of shorten ing folded by F, folds . This geometry indicates accommodated on the minor faults within the that the Marko nappe was emplaced during D2• Marko nappe cannot be estimated without
Small-scale D2 thrusts have not been recogn i know ing the original strat igraphy . However, zed in the Langvatn nappe. Instead , there are attempts have been made to balance and isoclinal folds that fold the S, mica foliation. palinspastically restore cross-section F-F' (Pla These folds range in wavelength from centime te 2) assuming differe nt stratigraphies for the tres to 100's of metres . Marko Complex, each required there to be The direct ion of tectonic transport during greater than 100% shortening with in the Mar
D2 is inferred to have been towards the ESE. ko nappe.
D2 transport is parallel to shallowly plunging, NGU·BULL. 416. 1989 Tectonostratigraphy and Structural Evolution 39
Fig. 10. D, structur es in the Marko Complex. a) Attenuated bedding in calcareous schist near a D, thrust fault. b) Asymmetrical pegmatite boudins in calcareous schist indicating top to the ESE transport dur ing D,.
0 3 - NW-trending folds ever, the folds and cleavage become less inten DJ produced open to tight upright to inclined, se moving structurally upwards and die out a NW-SE trend ing folds (Fig. l d). The scale of few hundred metres above the thrust. In the F folds ranges from minor folds which are J footwall of the Frostisen thrust, a zone of ultra centimetres in wavelength to map-scale folds mylonite less than one metre thick formed in kilometres in wavelength (e.g. the Baugefjell the Rombak Gneiss basement. Below this antiform, Plate 2, section A-A'). FJ folds de mylonitic zone, the basement is not penetrati form the S, foliation , F, folds and the thrust vely deformed; the effects of D, are limited to separating the Marko and Langvatn nappes the development of anastomosing shear zones (Plate 1). F folds tend to be NE vergent sug J that vary in width from less than 1 cm to 0.5 gest ing tecto nic transport to the NE during DJ. m wide. The spacing between these shear zones increases downward from the Frostisen 0 4 - Thrusting, imbrication and isocli thrust until they die out between 100 and 200 nal folding metres below the thrust. The present tectonic stacking of the Sitas The Storriten Complex is a D, duplex with area was formed during D, when the Frostisen the Frostisen thrust as its floor and the Meref and Mereftasfjell thrusts were active. D, thrus tasfjell thrust as its roof. This duplex system ting placed the MKNC onto continental base appears to have transported the MKNC en ment equivalent to the Rombak Complex , im masse as there is no tecton ic mixing between bricating the basement and its depositional the MKNC and the Storriten Complex. Faults cover to form the Storriten Complex. D, is the within the Storriten Complex ramp consistent first defo rmation to affec t all of the structural ly upwards towards the ESE (Fig. 8) sugges domains in the Sitas area. ting ESE-directed transport. This is consistent The Mereftasfjell thrust truncates lithological with the orientation of stretching lineations in units within both the MKNC and the Storriten Storriten granitic and quartzitic mylonites. The Complex while the Frostisen thrust truncates orientations of F, fold axes within the Storriten units within the Storriten and Rombak Comp Complex vary widely, but are generally shallow lexes . D, thrust ing produced isoclinal F, folds Iy plunging and roughly cluster around NE-SW and a phyllonitic spaced cleavage (S,) that are and NW-SE trends (Fig. le). F, isoclinal folds ubiquitous within the Storriten Complex probably formed with NE-SW-trending axes , schists. In addition, D, produced a mylonitic at a high angle to the ESE transport direction foliation in the granite and quartzite slices in and were subsequently rotated towards the the Storriten Complex. F, folds and an S, spa transport direction. The wide scatter in fold ced cleavage are present in the MKNC in the orientations seen in Fig. le is likely due to hangingwall of the Mereftasfjell thrust; how- varying degrees of transposition. 40 Peter D.Crowley NGU - BULL. 416, 1989
F, folds deform a pre-D, foliation in the Stor and E-E'). F, folds tend to be conce ntric and riten schists. This foliation is cut and transpo open folds. A spaced cleavage commonly for sed by S,. The pre-D, foliat ion is a synmeta med in the hinges of F, folds. morphic foliation that can be traced as an inclusion foliation in the cores of helicitic gar nets and followed into the matrix foliation. This requires the Storriten Complex to have Regional metamorphism been deformed and metamorphosed prior to Mineral assemb lages within the Sitas area 0,. However, the style of deformation and the appear to reflect one major phase of prograde timing of deformation relative to porphyroblast Caledonian metamorphism. However, both growth is different from that of the pre-D, petro logical (Crow ley & Spear 1987) and preli deformations recognized in the MKNC and minary geochronological (Tilke 1986, pers. therefore cannot be correlated with any of the comm. 1986) evidence indicate that this meta deformational events that affected the MKNC. morph ism is diachronous and may extend Many of the basement imbricates within the over a period of appro ximately 50 Ma. Move Storriten Complex are bounded by faults that ment on the post-metamorphic Frost isen and ramp from a sole thrust, the Frostisen thrust, Mereftasfjell thrusts has inverted the meta up to a roof thrust, the Mereftasfjell thrust morph ic sequence so that the highest grade (Fig. 8). Therefore, the displacement on the rocks are now found at the highest structural Frostisen-Mereftasfjell thrust system must be levels. The Rombak Gneiss and outliers of the equal to or greater than the tota l displace Tornetrask Formation exposed on the south ment on all of these smaller faults. By sum ern side of the Rombak window have been ming minimum displacements on each imbrica metamorphosed to middle greenschist facies te with in the Sitas area, the displacement on (biotite grade ). Structurally upwards in the the Frost isen-Mereftasfjell thrust system is Storriten Complex, the metamorph ic grade estimated to be greate r than 58 km. This is increases to the upper greenschist facies. The greater than the minimum displacement estima mineral assemblage garnet + biotite ± chlori ted on the basis of the structural overlap of te+ muscov ite + quartz + plagioclase is stab the MKNC on the Rombak Complex. le. Upwards into the MKNC. the metamorphic Within the Sitas area, 0, thrust fault ing appe grade increases into the amphibolite facies. ars to post-date NW-trend ing F, folding . The Pelitic and psammitic rocks generally contain limbs of the F, Baugefjell antifor m are transpo the mineral assemblage garnet + biotite + sed into sub- paralle lism with the Mereftas muscov ite + quartz + plagioclase ± horn fjell thrust (Plate 2, section B-B'). S, dips gent blende and the mineral assemblage horn ly to the SW across the entire region and blende + plagioclase + epidote + sphene is does not appear to have been folded by commo n in mafic rocks. The grade of meta NW-trend ing folds (Fig. 7f). However , on a morph ism also increases going structurally regional scale, the Sitas area appears to sit upwards from the Langvatn to the Marko nap on the NE limb of an open NW-trend ing syn pe. The pelitic mineral assemblage garnet + form that folds both the Frostisen and the biotite + muscovite + kyanite + plagioclase is Mereftasfjell thrusts (Fig. 1) . This suggests present in some Marko nappe schists . In addi the existence of a post-D. phase of NW-tren tion, quartzo-feldspathic enclaves interpreted ding folds that is not well represented within to be in-situ partial melts are present in Marko the Sitas area. nappe rocks. The P-T conditions of metamorphism within the Storriten Complex and MKNC have been estimated by garnet-biotite geot hermometry (Ferry & Spear 1978) and garnet-biotite-plagi D, - Upright NE-trending folds oclase-muscovite geobarometry (Hodges & 0, produced small- to large-scale upright folds Crow ley 1985). In calculating P-T, all phases with shallowly-plunging NE-SW-trend ing axes except biotite were considered to be non (Fig. 7g) and steep SE-dipping axial surfaces. ideal solut ions using solution models described F, folds deform all pre-existing structures and by Hodges & Crowley (1985). A more thorough form the prom inent map-scale NE-trending description of the metamorphic conditions is antiforms and synforms (Plate 2, sections 0-0" given by Crow ley & Spear (1987). NGU - BULL. 416, 1989 Tectonostratigraphy and Structural Evolution 41
Rocks from the Storriten Complex equilibra 1000 ,-----,--,..------::I:-r;-----;---.---,--70 ted at approximately 450° C and 600 MPa 900 (Fig. 11). The scatter in calculated P-T does not exceed estimated analytical error (± 20°C; ~ 800 ± 50 MPa). The calculated PoT of equilibration 0.. of the MKNC scatters about 575°C and 800 6 700 MPa. The scatter in calculated PoT exceeds ...III :l 600 analytical error , particularly for Marko nappe o: rJ:i samples. The calculated temperature for the III... 500 Marko nappe is significantly lower than that 0.. at which: (1) the staurolite-out assemblage 400 and garnet + biotite + kyanite is stable (Piggage 300 & Greenwood 1982) and (2) migmatitic partial 300 400 500 600 700 melts appear (Thompson 1982). Inasmuch as Temperature (QC) both the staurolite-out assemblage and mig matitic enclaves are present in pelitic rocks in the Marko nappe this constrains the peak Fig. 11.P-T of metamorphism of the Storrit en Complex and PoT conditions of the Marko nappe to be 625 MKNC determined by garnet-biotite -muscovite -plagio clase thermobarom etry. The symbol size appro ximates the analyti 775°C at a pressure greater than 700 MPa cal uncertainty in the poT determinations . The peak poT of (Crowley & Spear 1987). There is no evidence the Marko nappe is estimated to be higher than that deter that rocks from the Langvatn nappe ever ex mined by thermobarometry on the basis of the stability of the assemblage garnet + biotite-e kyanite + muscovite - qu perienced poT conditions in excess of those artz. (After Crowley & Spear 1987. Crowley 1988) calculated by thermobarometry and therefore did not undergo this pre-D, high-temperature heating. The P-T calculated for the MKNC by thermobarometry (575°C and 800 MPa) is in between different mafic bodies. In order to terpreted to reflect the conditions at which the determine, (1) whether all of the mafic rocks two MKNC nappes were juxtaposed during 0,. in the Marko Complex were derived from eit Rocks from both the MKNC and Storriten her a single or from multiple volcanic/plutonic Complexes had cooled to middle-greenschist sources , and (2) the tectonic nature of these facies conditions by D. time, when the Frosti sources, samples from the Marko gabbro , a sen and Mereftasfjell thrusts were active. mafic dike that intrudes the gabbro, and fine Throughout the Storriten Complex and in the grained pillowed amphibolite have been analy MKNC near the Mereftasfjell thrust , garnets zed for major and trace elements (Table 2 and have been partially to completely chloritized Figs. 12, 13). Major elements were analyzed and a greensch ist facies S, muscovite + bioti by XRF at the University of Massachusetts, te ± chlorite cleavage developed . Amherst. Trace elements were analyzed by During D., the Rombak Complex reached its XRF at the University of Massachusetts (Sr, middle greenschist facies metamorph ic peak, Zr, Ba, Ni, Cr, Co, Se, Zn, V, Nb) and by INAA with the development of a spaced muscovite (REE, Cs, Rb, Th, U, Ta, Hf) at the Massachu + biotite ± chlorite ± epidote cleavage. setts Institute of Technology. The data presen ted here represent too small a data set from which to draw conclusions about the origin of the Marko Complex; however, it is sufficient ly high-quality data to conclude that all of the Geochemistry of mafic rocks from mafic rocks cannot be cogenetic . the Marko nappe All of the gabbro samples are relatively low The Marko Complex contains a diverse range in SiO" high in MgO, and have low incompatib of mafic rocks that include: fine-grained pillo le and high compatible trace element concen wed amphibolites, fine- to coarse-gra ined trations. In general, there are good positive amphibolite dikes, coarse-grained amphibolite linear correlat ions between incompatible trace and a layered olivine-pyroxene gabbro . In elements. The dike sample tends to plot near general, the mafic rocks crop out as fault the extension of the correlat ion lines at higher bounded slices; thus, it is difficult to use field incompat ible element concentrations. Plots of relations to determine the genetic relationships incompatible elements against compatible ele- 42 Peter D. Crowley GU · BULL. 416. 1989
100 - ments do not produce linear corre lations, but rather are concave upwards and asymptotic to both the axes. Again the dike sample plots ...... along an extension of the graph at higher in :; :... 10 compatible element concentration. All of the ~ / / gabbro samples have low REE contents, light § / REE-enriched patterns and positive Eu anoma Q lies (Fig. 12). The dike sample has much high u.J er REE concentrat ions (20-100 times chondri u.J 0:: tes) and a light REE-enriched pattern that parallels the pattern of the gabbro, except that it lacks a Eu anomaly. La Ce Nd Sm Eu Tb Yb Lu The mineralogical layering, plagioclase folia tion, low incompatible element concentrations, Fig. 12.Chondrite-normalized REE plot. The Marko gabbro and positive Eu anomalies all sugges t that the and its feeder dike have light REE-enriched patterns wh ile gabbro formed by crystal accumulation. This the pillowed amph ibolite s have light REE-depleted pattern s. means that the analyzed samples do not repre sent compositions that were once liquids. However , the dike that cuts the gabbro may Table 2: Major and trace element analyse s of mal ic rocks in the Marko Com plex. be cogene tic with the gabbro and represent a liquid similar to the one from which the MT-3 MT-6 MT-l l MT-16 MT-1 ? MT-18 MT-22 MT-23 amph. dike evolved gabbro gabbro gabbro gabbro amph. gabbro crystallized. Numerical simulation of gabbro fractional crystallization (Crowley 1985) using Major Elements (wt""') trace element partition coefficients (Drake & SiO, 45.90 55.86 56.03 47.41 42.59 45.15 50.35 43.47 Weill 1975, Frey et al. 1978) supports this TiO, 1.01 2.39 0.62 0.13 0.11 0.12 1.32 0.3 1 AlP , 20.11 13.97 14.70 20.77 13.61 14.72 15.98 7.33 contention and indicates that the gabbro crys Fe,O,' 11.22 11.86 6.49 5.72 9.22 11.28 9.51 12.68 tallized from a light REE-enriched magma with MnO 0.25 0.19 0.13 0.09 0.15 0.18 0.17 0.20 incompatible element abundances greater than MgO 4.27 3.01 7.09 12.80 20.92 19.57 7.79 28.99 10 times chondrites that was chemically simi CaO 14.35 6.36 10.33 11.35 7.81 7.31 11.85 5.21 Nap 2.51 3.49 2.23 1.87 1.40 1.58 1.99 0.86 lar to the dike. KP 0.21 1.49 0.54 0.08 0.07 0.12 0.20 0.09 The fine-gr ained pillowed amphibolite samp P,O, 0.07 0.27 0.06 0.01 0.01 0.01 0.11 0.01 les have distinctly different chemistries from the gabbro. They tend to have lower comp atib Tota l 99.97 99.05 98.27 100.24 95.90 100.05 99.36 99.19 le and higher incompatible element concentrati ons. The REE patterns are light REE-depleted Trace elements (ppm) and have no Eu anomalies. The light REE Cs 0.8 1.0 depleted pillowed amphibolites cannot be rela Rb 49.0 29.0 Ba 1.5 279.1 200.0 29.2 10.4 20.0 7.5 23.3 ted to the gabbro by either fractional crystalli Th 7.4 3.1 0.1 0.2 zation or partial melting. The light REE U 2.0 0.9 enriched gabbro and light REE-depleted amphi Ta 0.8 0.3 0.2 bolites must have come from geoc hemically Nb 0.8 11.2 4.5 1.0 0.6 1.0 0.3 HI 1.6 6.8 2.9 0.1 0.1 0.1 2.4 0.3 distinct sources. Zr 55.5 270.3 101.3 4.2 3.7 4.8 95.0 10.0 The tecton ic setting of these sources can Sr 164.7 177.4 204.4 226.2 175.4 245.9 151.4 99.6 be determ ined by compa rison with modern La 1.40 25.74 13.66 0.89 0.69 0.81 3.26 1.09 mantle-derived mafic rocks of known origin Ce 6.55 66.40 29.10 1.89 1.59 1.85 12.30 2.86 Nd 6 .3 7 35 .20 13 .90 1.19 0 .85 0 .97 9 .70 1.81 (Fig. 13). On a 'spider' diagram in which incom Srn 2.28 7.86 3.22 0.33 0.26 0.26 3.16 0.60 patible trace elements are arranged from left Eu 1.03 2.14 0.98 0.35 0.25 0.29 1.21 0.31 to right in order of increasing compatibility in Tb 0.68 1.22 0.53 0.10 0.04 0.02 0.68 0.09 mafic magmas (Wood et al. 1979, Tarney et Yb 2.40 5.11 2.06 0.24 0.23 0.22 2.79 0.59 Lu 0.37 0.74 0.31 0.05 0.04 0.05 0.42 0.10 al. 1980)Ma RS, OIS and arc basalts have Ni 185 10 41 278 579 325 63 1108 distinctly different trace-element patterns. Cr 272 3 199 184 256 86 326 1444 MORS has a relatively smooth pattern that Co 69 29 30 51 82 94 34 increases in conce ntration from left to right, Sc 44 30 34 10 10 8 39 V 225 269 130 30 26 23 227 77 OIS tends to first increase and then decrea Zn 97 126 59 38 59 73 73 93 se, and arcs tend to have an overall negative NGU -BULL.416,1989 Tectonostratigraphy and Structural Evolution 43
L I II I, I I I II I in matrix of quartz-rich psammite and schist. The composition of the lenses (Le. mafic and VJ I Q) ultramafic rocks) indicates derivation from an u 100 i= ::: oceanic rather than continental source. Alt ro r= average j "0 r ARC - hough the lenses are small, lithologies repre ::: ;:j sentative of much of the oceanic crustal and ,.D t (\ '. _ " ~ upper mantle section are present. The high
sition also occurred in a fore-arc environ The basement slices in the Storriten comp ment. Therefore , both the Langvatn and the lex are thin (1 m to 0.5 km) and are commonly Marko nappes cont ain fore-arc rocks . The two in contact with or spatially associated with nappes may have been derived from the same quartzites that are the depositional cover of fore-arc; however, the metamorph ic petrology the basement gneiss. In addition, within the of rocks from the two nappes indicates a Rombak Complex, the basal part of the Tome separate thermal evolution for each nappe trask Formation is locally preserved in deposi until they were juxt aposed by O, thrusting. tional.contact on the Rombak Gneiss. Toget Therefore, we cannot tie the two nappes toget her, these two observations suggest that the her until after Dl thrusting. basement slices in the Storr iten Complex deta ched from a relatively shallow level « 0.5 km) below the basement-cover contact. This impli es that (at least for 20 km across strike) a mechanical contrast existed at a shallow level in the basement that acted as the mechanical Structural Evolution During Terrane equivalent of a decollement horizon in a sedi Accretion mentary sequence. The source of this mechani The deformation of the continental basement cal contrast is enigmatic, but may have been in the Sitas region occurr ed at a mid-crustal contro lled by the availability of downward depth (18-25 km) as a response to the accre propagating fluids from the dehydrating sedi tion of one or more exotic terranes during the mentary cover (Bartley 1982). closure of the lapetus ocean basin. Although There is little Caledonian deformation within the lithologies involved are different, the style the basement below the Frostisen thrust. Be of deformation seen in the Sitas area is quite low the thrust, there is a thin «1m thick) mylo similar to that seen in other orogens at much nite zone and for another 100-200 m there shallower levels (e.g. Bally et al. 1966, Boyer are thin anastomosing shear zones. Therefo & Elliott 1982). Most of the large-scale deforma re, beneath the 'basement decotlernent', at tion in the Sitas area is accommodated by least within the Sitas region, the basement movement along large-scale low-angle faults acted as a relatively rigid block. The structu with folds and penetrative ductile deformat ion res that formed during the accretion of the form ing second-order, albeit spectac ular struc MKNC are basement involved, but are geome tures . trically and kinematically similar to structures At shallower levels, accretion-related defor formed at shallower levels by thin-skinned mation is commonly thin-skinned , with a decol deformation. lement forming in the cover above a crystalline basement (e.g. Bally et al 1966, Elliott & John son 1980). Workers in the Scandinavian Cale Acknowledgements donides have argued over whether Caledonian This work was funded in part by National Science Founcan on (U.S.) grant EAR·8206300 to B. Clark Burchfiel and a deformation was thin-skinned (e.g. Gee 1975, Natural Sciences and Engineering Council of Canada Gee et al. 1978, Gee & Zachrisson 1979) or (NSERC) operat ing grant to P. D. Crowley. Thanks go to: Anld Andresen, Ron Boyd, David Gee. Krister Sundblad, basement involved (e.g. Gee 1975, Hodges Michael Stephens. and Ebbe Zacnrison for both logistical et al. 1982, Tilke 1986, Gee 1986). Within the support and hospitality. Discuss ions with: Kip Hedges. Peter Tilke. Clark Burchfiel. Frank Spear. John Bartley. Sitas area, 0, deformation was basement invol Dave x repackt, David Gee. Krister Sundblad. Michael Step ved. However , this deformation had a thin hens and Lennart Bjorklund greatly helped this project. The skinned-like geometry due to the presence of I.N.A.A. analyses were done by Larry McKenna and the XRF analyses were done with the help of Bart artin. An a decollement at a shallow level in the base e~r ~ ier vers io n o f th is m anuscrip t was g re atly im p ro v e d by ment. cnt!cal comments from Tekla Harms and two anonymous reviewers. NGU - BULL. 416.1989 Tectonostratigraphy and Structural Evolution 45
References rocks and Caledonian metasediments . Nor. geol. un Bally. AW., Gordy, P.L. & Stewart, G.A. 1966: Structure, ders . 239. 162 pp. seismic data and orogen ic evolution of the southern Gustavson, M. 1972: The Caledonian mountain chain of the Canadian Rocky Mountains. Bull. Can. Petrol. Geol. 14. southern Trom and Ofote n areas. Part Ill: Structures 337-381. and structur al history . Nor.geol. unders. 283. 56 pp. Bartley. J.M. 1982: Limited basement involvement in Caledo Gustavson, M. 1974: Berggrunnskort Narvik, 1:250.000. nian deformation, east Hinnoy , north Norw ay. Tectono Nor. geol. unders . physics 83. 185-203. Hakkinen. J.W. 1977: Stru ctural Geology and Metamorphic Bartley. J.M. 1984: Caledonian structural geology and tecto History of Western Hinnoy and Adjaceny parts of Eas nics of east Hinnoy . North Norway. Nor. geol. unders. tern Hinn"y. Ph. D. thesis. Rice University. Houston. 396. 1-24. TX. 161 pp. Basaltic Volcan ism Study Project 1981: Basaltic Volcanism Hodges . K.V. 1985: Tecto nic strat igraphy and structural on the Terrestrial Planets. Permagon Press, Inc., New evolution of the Efjord-Sitasjaure area. north ern Scandi York. 1286 pp. navian Caledon ides . Nor. geol. unders . 399. 41-60 . Birkeland. T. 1976: Skjomen . berggrunnsgeologisk kart Hodges , K.V. & Crow ley. P. D. 1985: Error estimation in NlO-M . 1:100,000. Nor. geo l. unders. empir ical geothermometry and geobarometry for pelitic Boyer, S.E. & Elliott, D. 1982: Thrust Systems . Bull. Am. systems. Am. Mineral. 70. 702-709. Assoc. Petrol. Geol. 66, 1196-1230. Hodges, K.V. & Royden, L. 1984: Geologic thermobarome Bjorklund. L. 1985: The Akkajaure nappe complex . nort try of retrograded metamorphic rock s: an indication of hern Scandinavian Caledon ides . In Gee. D.G. & Sturt, the uplift trajectory of a port ion of the northern Scandi BA (eds .) The Caledon ide Oroge n- Scandinavia and navian Caledonides. J. Geophys. Res. 89. 7077·70 90. Related Areas. John Wiley &Sons , Chicheste r, 515-558. Hodges . K.V.• Bartley. J.M. & Burchfiel. B.C. 1982: Structu Crowley, P.D. 1985: The stru ctural and metamorphic evoluti ral evolution of an A-type subduct ion zone, Lofo ten on of the Si/as area, northern Norway and Sweden . Rombak area, northern Scandinavian Caledonides. Tec Ph.D. thesis, M.1.T.• Cambridge MA, 253 pp. tonics 1. 431-462 . Crowley. P.D. 1988: Metamorphic break across post meta Kautsky. G. 1953: Der geologische bau des Sulitjelma morph ic faults : An unreliable indicator of structural Salojaurege bietes in den Nord skandinavischen Kaledo throw. Geology 16. 46-49. niden. Sver. geol. unaers. C 528. 228 p. Crowle y, P.D. & Spear, F.S. 1987: The PoT evolution of the Kulling. O. 1982: Oversikt over sodra Norrbottensfjallens Middle Koli Nappe Complex, Scandinavian Caledonides Kaledonberggrund. Sver. geol. unders . Ba 26. 295 pp. (68°N) and its tectonic implications. Contrib. Minera/. Kulling. O. 1964: Oversikt over norra Norrbotten sfjallens Petrol. 95, 512-522. Kaledonberggrund. Sver. geol. unders . Ba 19, 166 pp. Drake, M.J. & Weill. D.F. 1975: Partition of Sr. Ba, Ca, t.eeman, W.• Budahn. J.R.Gerlach. D.C., Smith. D.R.. & Eu++, Eu+3• and other REE between plagioclase feld Powell , B.N. 1980: Origin of Hawaiian tholeiites: trace spar and magmat ic liquid: an experimental study. Geo element constraints . Am. Jour. Sci. 280-A. 794-819 . cnem. Cosmochem. Acta 39. 689-712. Moore, G.F. & Karig, D.E. 1980: Structural geology of Nias Elliott, D. & Johnson, MW.R. 1980: Structu ral evolution of Island, Indonesia: implications for subduction zone tecto the northern part of the Moine thrust belt. NW Scot nics. Am. Jour. Sci. 280. 193-223. land. Trans. Roy. Soc. Edinb.• Earth Sci. 71. 69-96. Morris. J.D. & Hart. S.R. 1983: Isotopic and incompatible Ferry. J.M. & Spear, F.S. 1978: Experimenta l calibrat ion element constraints on the genes is of island arc vol of the part itioning of Fe and Mg between garnet and canics from Cold Bay and Amak Island, Aleutians and biotite. Contrib. Mineral. Petro l. 66. 113-117. implications for mantle structure. Geochim. et Cosmo Foslie. S. 1941: Tysfjord geologl. Nor. geol. unders. 149. cnim. Acta 47. 2015-2030. 298 pp. Page, B.M. & Suppe, J. 1981: The Pliocene Lichi Melange Foslie , S. 1942: Hellemobotn og Linnajaurr e. Nor. geol. of Taiwan. its plate tectonic and olistostromal origin. unders . 150. 119 pp. Am. Jour . Sci. 281. 193-227. Frey. FA, Green, D.H. & Roy. S.D. 1978: Integrated mo Pigage. L.C. & Greenwood. H.J. 1982: Internally cons istent dels of basalt petrogenesis: a stud y of quartz tholeiites estim ates of pressure and temperature: the staurol ite to olivine meelites from south - eastern Australia utilizing pro blem. Am. Jour. Sci. 282, 943-969. geochemical and experimental petrologi cal data. J. Speed, R.C. & Larue , D.K. 1982: Barbado s: architecture Petrol. 48. 137-1 and implications for accretion. J. Geophys. Res. 87. Gee, D.G. 1975: A tecton ic mode l for the central Scandinavi 3633-3643. an Caledonides . Am. J. Sci. 278. 468-515. Steltenpoh l, M.G. 1987: Tectonostr atigraphy and tectonic Gee. D.G. 1986: Middle and upper cru stal stru ctur e in the evolution of the skantano area, North Norway . Nor. Scandinavian Caledon ides. Geol. For. Stockh. Forh. geol. unoers. 209. 1-20. 108, 280-283. Stephens . M.B. 1980: Occurrence, nature and tectonic signi· Gee, D.G., Kumpula inen, R. & Thelander , T. 1978: The ficance of volcanic and high level intrusive rocks within Hsjon decollement central Scandinavian Caledo nides. the Swedish Caledon ides. In Wones, D.R. (ed.) The Sver. geol. unoers. C-742. 35 pp. Ca/edonides in the U.S.A.• Virginia Poly1echnic State Gee, D.G. & Zachrisson, E. 1979: The Caledon ides in Swe Univ. Dept. Geo1. Scl. Mem. 2, 289-298. den. Sver. geo l. unoers. C-769. 48pp. Steph ens, M.B., Sundb lad, K.• & Zachrisson, E. 1984: Gunner , J.D. 1981: A reconnaissance Rb-Sr study of Precam Megalens tectonic s in the Koli Nappes. southern Nor brian rocks from the Sjangeli-Rom bak window and the botten Caledonides, Sweden . Medd. frlm Stock h. Univ. pattern of initial " Srl"Sr ratios from north ern Scandina Geo/. inst . 255 . 211. via. Nor. Geol. Tidsskr. 61. 281-290. Sundblad, K. 1986: The Stipok Allochthon: an exotic terra Gustavson . M. 1966: The Caledonian mountain chain of the ne in the northern Swedish Caledon ides . Geol. For. southern Trom and Ofoten areas. Part I: Base ment Stockh. Forh. 108. 309-311. 46 Peter D. Crowley NGU - BULL. 416. 1989
Tarney. J.• Wood, D.A., Saunders. A.D., Cann, J.R. & Va the Ofoten region (68-69°N): key aspects of tectonic rets, J. 1980: Nature of mantle heterogeneity in the evolution. In Gee, D.G. & Sturt, BA (eds.) The Caledoni North Atlantic: evidence from deep sea drilling. Phi!. de oragen - Scandinavia and related areas. John Wiley Trans. Roy. Soc. London 297, 179-202. & Sons, Chichester , 515-558. Thelander, T. 1982: The Tornetrask formation of the Divi· Vogt, T. 1942: Trekk av Narvik-Ofoten traktens geologie. dal Group , northern Scandinavian Caledonides. Sver. Nor. geol. unders. 21. 198-213. geol. unaers. C 789, 1-42. Wood, D.A., Joron, J.-L, Marsh. N.G., Tarney, J., &Treuil, Thompson, A.B. 1982: Dehydration melting of pelitic rocks M. 1980: Major and trace element variations from the and the generation of H20 undersaturated granitic liqu North Phillipine Sea drilled during Deep Sea Drilling ids. Am. Jour . Sci. 282, 1567-1595. Project Leg 58: a comparative study of back-arc basin Tilke, P.G. 1986: Caledonian structure, metamorphism and basalt with lava series from Japan and mid-ocean rid tectonics of the Sitas·Singis area, Sweden. Ph.D. the ges. Initial Report s of the DSDP 58. 873-921. sis, M.I.T., Cambridge, MA , 295 pp. Wood, D.A., Jor on, J.-L. Treuil, M., Norroy, M.• & Tarney. TulI, J.F. 1978. Geology and structure of Vestvaqey. Lofo J. 1979: Elemental and Sr isotopic variations in basic ten, North Norway , Nor. geol. unaers . 333, 59 pp. lavas from Iceland and the surrounding ocean floor. Tull, J.F., Bartley, J.M., Hodges, K.V., Andressen , A., Stel Contrib. Mineral Petrol. 70. 317-339. tenpohl, M.G., & White, J.M. 1985: The Caledonides in NGU Bulletin 416 - Crowley - PI. 1
.. LANGVATN 623 Study Area. cz' Tectonic StratigraQhy ~ t Undllferenllated Quatern ary all uvial and glacial depo sits • b. Bavro Schist - Bavro Gneiss
hg Hjertevatn Gneiss W Q, N Q, mica schist )( cl Z .....W ca lcareo us psammlte Q, o ~ :lEo a: mlgmallllc gne iss u cl 100 km :lE )( I--t amp hIbollte W..... sa: l>. l el slt e cl :E m! :lE o + + U + + gabbro W Q, Q, clz • amp hlbollte ::i 00 m marble ~ ultramaflc rocks .....W C W c Reppl schist Q, Q, :iJ cl Ripet]ikka Schist Z Z NJun]as Sc hist . cl > Fltljell Complex matr ix schist Cl Z cl sks SkJafjell Schist .....
). .. 4. > < V < • Bauge lJell Amphlbo llle ~ )( ~ ca lcareous psammlte w cl> ..... c l>. mica schist J ~ :E a:uclO phy llonlllc schist
Slq quartzite gra nite mylonlte ~ ] Ul Z ~ Q mica schist .-. ~ wcl z :E quartzite a: a: J )( 00 G:=J .- ... ~w , ~"'J J cl ..... ,~, ",' Rombak Gran ite Gneis s mQ, :E:IE 00 e-c mica schist a:u ss I'i: qua rtzite sq I'i: amphlbollte s.
Symbols
Regional lollallon (0 1except In the Rombak granite, where It Is ~ Precambr lan)
___ Phyllonlllc cleavage (0 4 )
~ Cumulate lollallon
."., Minor lo ld axis (0 2to 0 5)
_ Stretching IIneallon(O -}o 0 5)
~ Contact (observed, approximate)
~ Tltr ust laull (02 )
....._-...... Thrust lau ll (04 )
i o 4
!ZSlMiddle xeu Nappe Complex ...... Mereftasfjell thrust m Storriten Complex __ Frostisen thrust t;~;,,~ Rombak Complex NGU Bulletin 416 - Crowley - PI. 2 Plate 2: Geological Cross Sections of the Sitas Region ? symbols as in Plate 1 ?' J C' ~ J / ~ A' C ~ / I A ~ ~ -, 1500 ... / -, 1500 / ~ , , -, ~ - [ - 1000 .- 1000 -., 500 500 ",' I S.L T) S.L
D' ~ o B ~ 0 " ~ B' ~ 1500 1500 ~ ~ ------1000 1000
500 500 S.L S.L.
F F' E' E 1500 fnI! / mea 1500 II I m! mm "".. -- ...... I/ I 1000 ,/ --:..-:...... - ..... ,.. -- 1000 1:------....,4'-----.,~~ ":/...... - --...... 500 500 S.L. S.L .----=---~---::-:~=-=...:_-:~=====::==-:-- -