The Lillevik dyke complex, Narvik: geochemistry and tectonic implications of a probable ophiolite fragment in the Caledonides of the Ofoten region, North Norway ROGNVALD BOYD Boyd, R.: The Lillevik dyke complex, Narvik: geochemistry and tectonic implications of a probable ophiolite fragment in the Caledonides of the Ofoten region, North Norway. Norsk Geologisk Tidsskrift, Vol. 63, pp. 39-54. Oslo 1983, ISSN 0029-196X. The Lillevik dyke complex occurs in an allochthonous unit and shows field relationships indicative of a transition from the mafic cumulate to the sheeted dyke zone in a segment of an ophiolite. Major and trace element chemistry confirm the MORB character of most of the diabases. Certain diabase, gabbro and trondhjemite dykes have REE patterns suggesting a later stage of ocean-island volcanism. The Lillevik complex and equivalent bodies along strike on the eastern limb of the Ofoten synform are a probable source for the mafic facies of the overlying Elvenes Conglomerate. Analogies with other areas suggest that the Lillevik complex was obducted during the Finnmarkian orogeny. R. Boyd, Norges geologiske undersøkelse, Postboks 3006, N-7001 Trondheim, Norway. The topic of this paper is a tectonically bounded gen Groups is marked by a conglomerate hori­ lens, consisting of gabbro cut by diabase and zon, the Elvenes Conglomerate, which consists gabbroic dykes and by leucocratic veins, which is mainly of matrix-supported cobbles of meta­ exposed on a shore section within the town of trondhjemite, quartzite and dolomitic marble in Narvik in North Norway. The section Iies in the a matrix of calcareous mica schist (Foslie 1941, upperrnost part of the Narvik Group of Gustav­ Gustavson 1966); this unit is currently being son (1966, 1972) (Fig. 1), in an area last mapped studied by A. Andresen and J. TulL Locally it by T. Vogt and his students, mainly in the 1930s contains a basal mafic facies (Foslie 1941, Tull et (Vogt 1950). Gustavson (1966, 1969 and 1972) aL in press) consisting of cobbles of greenstone, synthesized his own and Vogt's data from north metagabbro and metatrondhjemite in a matrix of of Ofotfjord with that of Foslie (1941, 1949) from greenschist. Gustavson (1972) interpreted the south of the fjord and described the rocks of the conglomerate as a whole as signifying 'early Ofoten area as a sequence of nappes emplaced epeirogenic movements' and the inversion of at on a granitic Precambrian basement and refolded !east the structurally lowermost part of the Salan­ by a major upright fold, the Ofoten Synform, gen Group (because of the paucity of carbonate during the Caledonian orogeny. The Narvik units in the upper part of the Narvik Group). Group of Gustavson (1966) consists of an upper Tull et aL (in press) conclude that the Elvenes portion, mainly of two-mica gneisses and a lower Conglomerate represents a regional unconfor­ portion of gneisses with marble and amphibolite mity and that it is right way up. Along the east­ bands, while the overlying Salangen Group con­ em limb of the Ofoten Synforrn south of Ofot­ sists of marbles and mica schist with bands of fjord, the conglomerate is underlain by a mafic iron forrnation and quartzite. The upper portion unit described by Foslie (1941) as strongly de­ of the Narvik Group, above a unit of calcareous forrnedhornblende schist and amphibolite cut by mica schist - called the Reppi Schist by Foslie trondhjemitic veins, some of which postdate the (1941) - and the Salangen Group were thought earliest deformation of the host rocks. by Gustavson (1972) to form a part of a single One of the keys to improved understanding of nappe. Gustavson (1972) considered the possibil­ the Norwegian Caledonides in recent years has ity of a thrust boundary between the Salangen been the recognition of obducted ophiolite as­ Group and the overlying Niingen Group, and semblages or parts thereof (Gale & Roberts this thrust was included on the 1:250000 map 1974, Prestvik 1974, Sturt & Thon 1978). In sheet, Narvik (Gustavson 1974). North Norway, Boyle (1980) has suggested an The boundary between the Narvik and Salan- ophiolitic origin for the Sulitjelma gabbro and 40 R. Boyd NORSK GEOLOGISK TIDSSKRIFT l (1983) Field relationships The Lillevik dyke complex is exposed on a penin­ sula immediately east of the bay, Lillevik, on the southern shore of Ofotfjord, within the town of Narvik (Figs. l & 2); The peninsula itself is unnamed on the l: 10 000 topographic map of the area. Exposures are few along the presumed in­ land continuation of the dyke complex (an area which is also quite densely built up) and are totally lacking on certain parts of the coastal section to the east and west. The dyke complex forms a lens, possibly a megaboudin, within a + + .. + .. + sequence of sulphide- and graphite-bearing phyl­ ...+ ....... .. .. + + .. + + • + .....+ + lites and phyllonites and highly deformed mafic + + + + + + + + + + + + .... + + + + + + + + +.+ + ...... rocks, some of which are recognizably similar to .. + .. + + + + + + ....+ ..... + + + + + .. + + + + + + ....+ + + + + .. + + components in the dyke complex. These rocks, + + + + + + .. + + + + - .. + + + + + + + + + + + + + + + + + + .. + on both sides of the dyke complex, are affected by tight to isodinal folds, now in an upright c::::J SALANGEN GROUP AND HIGHER UNITS c:::::J NARVIK GROUP AND LOWER CALEDONIOE UNITS position, and locally appear to have been affect­ � ROMBAK BASEMENT WINDOW ed by a later phase of more open folding. West of • AREA DESCRIBED IN THIS PAPER the break in exposure in Lillevik there is a head­ land (Fig. 2} consisting of metre-scale bands of Fig. l. Location map after Gustavson (1974). feldspathic grit with centimetre- to decimetre­ scale bands of chloritic schist. These rocks are deformed by upright open to dose folds which associated volcanic rocks, though certain aspects plunge gently northwards, and show no dear of this model have been questioned by Stephens indication of earlier falding. The field relation­ et al. (in press). Mapping by the Geological Sur­ ships permit the condusion that the boundary vey of Norway (Boyd & Minsaas, in prep. ) sug­ between the grit and the underlying unit which gests that the Lyngen mafic complex is an ob­ contains the dyke complex is probably an uncon­ ducted and disrupted ophiolite. Minsaas (1981) formity. and Minsaas & Sturt (in press) have demonstrat­ The Lillevik complex consists of a central body ed that the Lyngen ophiolite was obducted dur­ of layered horn blende gabbro cut by several gen­ ing the late Cambrian/early Ordovician Finnmar­ erations of mafic and ultramafic (hornblenditic) kian orogeny. While ophiolitic assemblages have dykes and leucocratic veins, and with marginal not been proven to exist between Lyngen and zones in which the dykes are dominant and gab­ Sulitjelma (e.g. by the presence of diagnostic bro is locally absent. Certain of the leucocratic field relationships and the appropriate trace ele­ veins postdate some of the mafic dykes. The ment geochemistry}, it is significant that Gustav­ mafic and ultramaficdykes cut the layering in the son (1969) conduded that the older (pre-meta­ gabbro at a high angle. The field relationships morphic) mafic and ultramafic rocks in the Ofo­ are thus consistent with those dose to the transi­ ten and S. Troms area correspond in major ele­ tion from the mafic cumulate zone to the sheeted ment chemistry to the dassical ophiolites of the dyke zone in a segment of an ophiolite (see Alps and the Mediterranean area as described by Moores (1982) for a recent review of ophiolite Burri & Niggli (1945). stratigraphy). The field relationships of the com­ This paper will consider a body of mafic rocks plex will be described in terms of nine shear­ with field relationships typical of the lowermost bounded blocks (Fig. 2}, which are reasonably part of a sheeted dyke complex. It contains rock distinctive on the well-exposed coastal section. types similar to those found as cobbles in the The dyke complex shows a crude zonation mafic facies of the Elvenes Conglomerate and with a core, Blocks 2 to 6, in which gabbro/ Iies at a tectonic level which makes it, or equiv­ metagabbro is dominant while the remaining alent bodies along strike, a possible source for blocks contain a higher proportion of mafic these cobbles. dykes, over 80% in Blocks 8 and 9. The degree NORSK GEOLOGISK TIDSSKRIFf l (1983) The Lillevik dyke camp/ex 41 N l �""""""T' CM SCALE BANDED SEDIMENT (INCL. CHLORITE R\\ir;''il GABBRo. DIABASE oYKEs suBSIDIARY LilliliillJJ SCHIST, GARBEN SCHIST, CARBONATE, SEMI-PELITEl - PHYLLITE, GREENSCHIST, GREENSTONE D NO EXPOSURE ON COAST lt::\\\1 FELDSPATHIC GRIT WITH THIN GREENSCHIST BANDS 1 ro9 BLOCK NUM BER IN DYKE COMPLEX ISEE TEXT) •, •, • DIA BASE DYKES, SCREENS AND LENSES CONTACT IOBSERVED, APPROXIMATE, INFERREDl "" OF GABBRO SUBSIDIARY 50 100m ________._ ______ � Fig. 2. Map of the Lillevik shore section, Narvik. of preservation of primary textures also follows a ly (Blocks 3 and 4) there are dykes of microgab­ rough zonation; primary textures in the gabbroic bro/gabbro and also dykes consisting almost en­ rocks have been almost obliterated by the effects tirely of amphibole as well as a later generation of deformation in the marginal blocks and are of diabases. The gabbro dykes cut all leucocratic best preserved in Blocks 3 and 4. The presence of veins in their immediate vicinity and therefore zones of pronounced shearing between the probably postdate the early diabase dykes. The blocks suggests that they may represent different