The Leka Ophiolite Complex, Central Norwegian Caledonides: Field Characteristics and Geotectonic Significance

Journal of the Geological Sociery, London, Vol. 145, 1988, pp. 401-412, 8 figs, 1 table. Printed in Northern Ireland

The Leka Ophiolite Complex, central Norwegian Caledonides: field characteristics and geotectonic significance

H. FURNESl,R. B. PEDERSEN’ & C. J. STILLMAN* 1 Geologisk Znstitutt, Avd. A., Allegt. 41, 5014 Bergen, Norway 2 Department of Geology, Trinity College, Dublin 2, Ireland

Abstrad: The Leka Ophiolite Complex (LOC) of Lower Ordovician age (U-Pb zircon age of 497 f 2 Ma) contains the following components: (1) A lower, strongly deformed harzburgite unit, representing the depleted upper mantle. This unit becomes progressively richer in dunite towards (2) a sequence of ultramafic cumulates, ‘unconformdbly’ overlyingthe harzburgite tectonite. The crystallization sequence in the ultramafic cumulates is: olivine-t chromite+ clinopyroxene-, orthopyroxene. In the south-western part of the ultramafic cumulate sequence, layers of metagabbro appear interlayered with banded dunite/wehrlite. The above-mentioned rock sequences may be in tectonic contact with (3) layered metagabbros which give way upwards to laminated and van-textured metagabbro. Within the laminated/vari-textured metagabbro is the first appearance of (4) metabasalt dykes, which increase upwards in abundance from scattered single dykes to a 100% sheeted dyke swarm. Associated with the van-textured metagabbro and metabasalt dykes are minor acid intrusions. Within the upper part of the dykeswarm is the first appearance of (5) pillow lavas, which are associated with thick volcaniclasticmetasediments at apparently higher stratigraphic levels. Geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite (IAT) to MORB character, and typical metaboninites are also present. This range of geochemical composition, as well as the crystallization sequence of the cumulates, strongly suggests that the first stages in the formation of the LOC wasby spreading above a subduction zone. The pillow lavas associated with volcaniclastic rocks, are of MORB or WPB type, and these probably represent the later spreading stages of the LOC in a back-arc basin, but at sufficient distance to be unaffected by the geochemical ‘fingerprints’of the subduction process.

A number of ophiolitefragments are present within the General geology Upper Allochthon of the Scandinavian Caledonides. The most complete of these is theLeka Ophiolite Complex As the LOC is nowhere seen in stratigraphic contact with (LOC). Seen only onthe island of Leka and its mainland rocks, its exact relationships are uncertain. neighbouring islets (Fig. l), the LOC comprises a coherent However, on the islet of Sendingen, some 2 km south-west complex of ultramafic rocks, gabbros, greenstones (metaba- of Leka (Fig. l),strongly sheared serpentinite of the LOC is salt dykes and pillow lavas) and minor acid intrusions. in tectonic contact with a spotted calc-silicate schist Prestvik (1972) first recognized the ultramafic complex as belonging tothe Solsemeyene Group (SoG) of metasedi- an Alpinetype, and subsequently (Prestvik 1974, 1980) ments, and a tenuous line of evidence suggests that these referred to it as an ophiolite, providing further geological rocks may representpart of a pre-526Ma autochthonous and geochemical information on the different components. sedimentary cover tothe crystalline basement onthe An indication of its age is given by the U-Pb zircon date mainland. This Solsem~yene Groupmakes up a number of of 497 f 2 Ma obtained by Dunning & Pedersen (1987) from small islands (Solsemeyene) to the south-west of Leka and quartz keratophyres of the LOC. It is thus comparable in comprises a sequence of limestones, mica schists and age with the Karmcby Ophiolite Complex (Fig. 1) which has sandstones. The dominant lithologies are light grey to white a magmatic age of 493 + 71-3 Ma (Dunning & Pedersen limestones alternating with grey to black bands of calcareous 1987), and must belong to the Group I ophiolites, which quartz-mica schist or quartz-mica-chlorite schist (with both represent the oldestophiolite complexes with associated white mica and biotite). The alternating bands which may immature island arc magmatic rocks yet foundin the grade into each other are of lO-’ to 1O-’m scale and the Norwegian Caledonides(Sturt et al. 1984; Furnes et al. proportion of limestone to schist may vary considerably. 1985). Thus limestonepredominates onJ~mflesan, quartz-mica The main purpose of this paper is to describe the schist onT~rrflesan, and Nyran is composedentirely of coherent pseudostratigraphy of the LOC based on detailed quartzite. The rocks are in the green schist facies of regional mapping, and to make a synthesis of the many components metamorphism with foliation defined by white mica, biotite of thecomplex. These, together with the substantial new and chlorite,but in some of thedark schists originally geochemical data from the metabasaltsequences, the representingcalcareous pelites, garnet, staurolite,chlorite petrographic characteristics of the ultramafic sequences and and biotite are randomly oriented and indicate a thermal general geological development, provide a sound basis for event post-dating the main foliation. evaluation of the tectonic environment in which the LOC These apparently shallow watercontinental margin formed. sediments are of uncertain stratigraphic position; however, 401

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 402 H. FURNES ET AL.

SketGP.(cgl.. sst.. mlcasch.)Sklmna ++ Hortavm Cover 6eq. 10 the LOC 1 % *2 + AUSI~B It+l lntrusyerocks (grantte. syenlte. gabbro) Leka Ophlolile Complex (LOC) Melasediments Cover seq. to the E. G. 0Basement gneosses (B. G.)

LEGEND cgl.. s.st.. mea- B m Group (schlst h lhestone) -Unconformlly Storeya Leka Ophiolite Complex ...... Plllow lava h volcanlclasllcrocks ... (Madsoya. Langdragel h Storoya) i , I Melabasall dykes mQ-keratophyre/ Plagtogranlle Varttexlured melagabbro. metabasall dykes. plagiogr. veins h meladlorile ELayered melagabbro/clmopyroxenlte 1..] Layered dunllelwehrllte

Solsemoyene Group (:F:::;

4 Dyke orientallon Foliatlon (in harzburglte A Reglonal follatlon

Fig. 1. Simplified geological map of the Leka Ophiolite Compex, Skei Groupand Sosem0yene Group. For details of the Skei Group: See Sturt et al. (1985), and of the Solsemoyene Group: See Prestvik (1974). Inset map, showingthe Leka Ophiolite Complex and adjacent rock types is modified from Gustavson & Prestvik (1979), Bering (1986) and Nordgulen & Bering (1986). they can be closely matched with abundant metasedimentary succession of shallow marine limestones, sandstones, xenoliths which arefound in an igneous complex on the greywackes and shales, now all metamorphosed to lower island of Hortavaer, some 6-12 km NW of Leka (Fig. 1). greenschist facies (Sturt et al. 1985). This complex, made up of intrusives ranging from gabbro to No detailed structural analysis of the LOC has yet been syenite and granite, has been dated by whole rock Rb-Sr made,but the generalsituation may be described. The methods to 471 f 5 Ma (Gustavson & Prestvik 1979) which earliest deformation is clearly non-orogenic; the dominant provides a minimum age forthe metasediments. The fabric of the harzburgite tectonite apparently owes its origin metasediments of the SoG are also comparable to mainland to high temperature ductile shearing in the mantle beneath rocks which, on Austra (Fig. 1) can be shown to form an an active spreading ridge (see below). Late syn-magmatic, autochthonous cover to Basement gneisses (D. Ramsay & non-orogenicdeformation is also demonstrated in the B. A.Sturt pers. comm. 1986). Mica schists which may ultramafic cumulates where folded clinopyroxenite veins are belong to the same sequence are seen in the south-eastern truncated by unfolded clinopyroxenite veins. The LOC was part of the Bindal Massif some 20-30 km east of the inset subsequently affected by severalphases of orogenic map of Fig. 1, where they areintruded by graniticand deformation, one, at least, pre-dating the sub-Skei Group granodioritic rocks which have yielded an Rb-Sr whole rock unconformity. The Skei Group itself is affected by age of 526 f 10 Ma(Nissen 1986). poly-phase deformation at greenschist facies temperatures, Unconformably overlying the LOCon the eastern side of but this deformation cannot be unequivocally distinguished the main island of Leka (Fig. 1) are the metasediments of in theLOC (Sturt et al. 1985; D. Ramsaypers. comm. the Skei Group. The basal contact is plainly seen: gabbros 1986). Detailed mapping supplemented by way-up evidence forming thesubstrate to the Skei Grouphad undergone from geochemical profiles in the dunites have shown that the subaerial weathering in semi-arid conditions which produced ultramafic cumulatesequence is affected by several a weathering profile with a development of caliche prior to large-scale, tight folds (Fig. 1). It is thought that these most the deposition of the sediments (Sturt et al. 1985). The probablybelong to a post-quartzkeratophyre (497 Ma) sediments themselves are grouped into a lower sequence of orogenic deformation. alluvial fanand braided stream deposits andan upper Widespread small and large-scale faulting is prominent.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 T HE LEKA OPHIOLITECOMPLEX,NORWAYLEKA THE 403

Themajor movements on some of these faultsclearly roots of oceanislands, aseismic ridges and island arcs pre-date the Skei Group; NW-SE faults in the gabbro do (Pearce et al. 1984). Thus the term Leka Ophiolite Complex notextend across the Skei Group, SW of Skei (Fig. l), is used toinclude all thecomponents in the magmatic although-some reactivation is shown by small offsets of development within the Leka ophiolite. lithological boundaries. These faults are truncated by much larger members of a NE-SW set which juxtapose different Ultramafic rocks of mantle affinity (mantle tectonite) levels of the ophiolite, and in particular bound the whole A complex of hanburgite with minor dunite, interpreted as high-level sequence of the ophiolite that makes up the south the depleted upper mantle,crops out along the entire length andeast of the island. The age of thesefractures is not of thewestern part of Lekaand east of the ultramafic known. cumulates of the Skria block (Fig. 1). To the east the harzburgite complex attains a thickness of about 1.5 km and is in tectonic contact with metagabbro The Leka Ophiolite Complex (LOC) of the LOC (Fig. 1). In order to provide the most complete Whilst the LOC contains all the principal components of a descriptions and illustrations we have chosen to summarize typical ophiolite (Fig. l), the geochemistryallied to field schematically thefeatures of theeastern harzburgite evidenceindicates asequence of magmaticdevelopment complex (Fig. 2), as the westem harzburgite complex only froman initial ensimatic island arcthrough an ocean shows the features of the upper 500-600m of the profile of spreading axis. Such an evolution from island arc to back- Fig. 2. arc basin is illustrated by many modem analogues in the This complex, either in transitional contact with dunites western andsouth-westem Pacific (e.g. Colley & Hindle of cumulateorigin, or tectonic contact with cumulate 1984; Leitch 1984; Sychev & Sharaskin 1984). It is apparent dunitesand wehrlites, or gabbro, comprises the following fromrecent descriptions of ophiolitesthat a variety of components: tectonicsettings arerepresented within the complexes. (l), ahost rock predominantly of hanburgite, These may include majorocean ridges, incipient ocean (2) dunite bodies of various shapes and sizes, and ridges, marginal basins, ‘leaky’ transforms and (possibly) the (3) pyroxenite to olivine pyroxenite veins.

Ha1 zburgite tectonite, eastern part (Shaded on inset map)

N 7

Olivine-rich harrburgite with pods of opx-rich harzbugite (a) and I or dunite (b) with diffuse boundary to ssrounding harzburgite Dunite layers. concordant with the tectonite fabric

Harzburgite (01 I opx 50 150) with minor dunite andlherzotite . - _--

Fig. 2. Schematic illustration of internal lithological and structural relationships within the harzburgite tectonite and its relationship to the ultramafic cumulates. Inset map (shaded) shows the area from which the illustration has been made. 1: Folded orthopyroxene vein (altered). 2: Clinopyroxene veins cutting harzburgite tectonite. 3: Irregular bodies of dunite cutting harzburgite tectonite. 4 Alternating parallel bands of dunite and harzburgite. 5: Thick bands of altered orthopyroxene in harzburgite. 6: Typical fabric in harzburgite tectonite, defined by flattened and stretched orthopyroxene (altered) crystals. 7: Folded banded harzburgite, with the development of an axial plane tectonite fabric.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 404 H. FURNES ET AL.

The harzburgite shows an imperfect foliation defined by dyke-like dunites.Within these irregular dunite bodies, bands (c. 2-10 mm thick), and aligned and/or flattened harzburgite ‘rafts’ appear.The orientation of theor- aggregates or individual crystals of serpentinized ortho- thopyroxenefabric within such ‘rafts’ is consistent and pyroxene and/or individual crystals of serpentinized parallel with that of the enclosing harzburgite. Similar orthopyroxene and/or clinopyroxene (Fig. 2). Such mineral features have been described by Nicolas & Prinzhofer (1983) foliation in the harzburgite‘tectonite’ part of ophiolite who attribute a residual origin to such bodies. We concur complexeshas been attributed to high temperature plastic with such a conclusion as a plausible one for this type of flow of themantle underneath an active spreading ridge duniticbody. Such dunites may be relatively free of (e.g. Nicolas et al. 1973; Nicolas & Prinzhofer 1983). The chromite, whereas others may contain chromitebands, or attitude of theLOC harzburgite tectonite fabric in the scattered, idiomorphic crystals up to c. 5 mm in diameter. lower part of the complex is variable, due to open folding The presence of chromite grains associated with the (Fig. 2). These folds have an axial plane cleavage of rather transitionfrom harzburgite to dunite hasbeen studied by constant attitude. Approaching the rocks of the layered Leblanc et al. (1980), who interpret such occurrences as the series, structuralelements such asbanding and foliation result of incongruent melting of orthopyroxene with con- progressively attain parallellism with the boundary tothe comitant magmatic growth of spinel. Growth of spinel grains dunite layers. The contact between the harzburgite tectonite would thus be related to the local percolation and extraction andthe layeredseries is defined by an ‘unconformity’. of melt from such areas, which would be in agreement with Based onthe relationshipsbetween primary and tectonic a residual origin of the dunite. internalstructures observed in a number of ophiolites, The other main type of dunite, commonly with chromite Nicolas & Violette (1982) distinguished two principal types: bands, is characterized by sharp boundaries with the oneformed abovea horizontally spreading asthenosphere harzburgite tectonite. Relative to the mineral foliation in the (Table Mountain type), and the other above a diapirically harzburgite, these bodies may occur as concordant (Fig. 2) spreading asthenosphere(Acoje type), i.e. distal and or cross-cutting sheets varying in thickness froma few proximal tothe spreadingaxes, respectively. Comparing centimetres up to metre size, and some are folded. Others available structuraldata from the LOC with those which are irregular in shape, with bifurcating satellite dykes (Fig. characterize these two types, it seemsthat the Table 2). Mountain type is the more appropriate. This is suggested on Bands of clinopyroxene and/or orthopyroxeneand the basis of (1) the small tomoderate angle (c. 15-30’) olivine websterite are common within the harzburgite between the layering in the ultramafic cumulatesand tectonite. Theseare unevenly distributed, being concentr- harzburgitefoliation, (2) the subparallel attitude between ated in particular areas, particularly on the western side of the harzburgite foliation and its boundary to the ultramafic Leka. Within such areas, it is the clinopyroxene bands that cumulates,and (3) the parallellism between the foliation dominate.Their thickness varies from 1 to 10 cm, anda and banding in the upper 500-600 m of the harzburgite (Fig. common spacing is 5 to 50 cm. Such bands can rarely be 2). traced continuously for more than a few metres, and they Locally lherzolite with diffuse boundariestheto may be parallel to or cross-cut the fabric of the harzburgite harzburgite tectonite is formedwhen the harzburgite tectonite (Fig. 2). Some are isoclinally folded (Fig. 2). These contains individual crystals or closely spaced (2-15 cm) thin pyroxenite to olivine websteritebands most probably bands (0.5-1 cm thick), of clinopyroxene. The clinopyro- represent dykes of partial melt in themantle, and their xene bands are parallel to the orthopyroxene fabric of the orientation relative tothe surroundingfabric of the host harzburgite, and a plausible explanation for this type of harzburgite tectonite is controlled by the partial pressure of lherzolite is that it represents a melt-impregnated harzburg- the magma (Nicolas & Jackson 1982). An interesting feature ite produced during plastic flow (Nicolas & Jackson 1982; is that in some cases the pyroxene bands are concentrated Spray 1982). and apparently rooted on one side of dunite sheets. Such The harzburgite tectonite becomes progressively more pyroxene bands may represent liquid squeezed out, during dunitic in composition towards the layered series, a common plastic flow, from a batch of olivine-crystallizing liquid in the feature of ophiolitecomplexes, attributed to aresidual ascending mantle. origin (Nicolas & Prinzhofer1983), and may contain only irregularpatches of harzburgite tectonite (Fig. 2). It is difficult, within the scale of a few metres, to determine the The plutonic rocks contact, but the appearance of chromite layers and bands, as Themajor part of the island (Fig. 1) is made up of the well as the regular variation in the Fo content of the olivine plutonic rocks of theLOC, which include layered (R. B. Pedersen & H. Furnes in prep.), clearly suggest the ultramafics, metagabbros, acid intrusions and metabasalt location of the lowest olivine cumulates. dykes. These rocks are traversed by discrete shear zones Dunite bodies of various sizes, shapes and relationship to which divide the island into anumber of large blocks the enclosing harzburgite tectonite, comprise approximately between which it is difficult to correlate. Within the blocks l-3% of this mantle-related complex. Similar features have the rocks are well exposedand preserved and large-scale been reported from the harzburgite mantle complexes from layering canbe followed forkilometres so that detailed ophiolitecomplexes elsewhere (Jackson et al. 1975; stratigraphies can be established. Browning 1982; Nicolas & Prinzhofer 1983; Girardeau et al. The number of individual magma chambers from which 1984; Gregory 1984). One principaltype of dunite is the plutonicsequence may have crystallized isdifficult to characterized by irregularly shaped bodies showing grada- deduce from field evidence alone, but at least two can be tionaltransitions fromharzburgite into dunite (Fig. 2). recognized. In the Solsem area (Fig. 1) a sharp discordant These bodiesvary in size from 10-’ to>10m. They are boundary between layered ultramafics and metagabbro can sometimes arranged in parallel rows with the same attitude be followed continuously for 3 km (Pedersen 1986). The as thetectonite fabric, andsome may beconnected by main rock type of the Western intrusion at Solsem is layered

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 THE LEKA OPHIOLITECOMPLEX, NORWAY 405

metagabbro, which, in the lower part of the intrusion, blocks (Fig. 1). The rocks exhibit layering on three different is interlayered with dunite and wehrlite. Near its intrusive scales (Fig. 3). The largest scale layering is defined by rock boundary with the layered ultramafics (Fig. l), xenoliths of sequences dominated by dunites alternating with sequences ultramafic rocks are enclosed in themetagabbro, and dominated by wehrlites and pyroxenites. The large-scale pyroxenitereaction zones are commonly developed along layers are a hundred to several hundred metres thick and the cross-cutting contacts andaround the ultramafic they can be followed laterally for several kilometres within xenoliths. Projecting this contactsouth and eastward, at the Skrla block. Halin (Fig. 1) a chilled facies of metagabbro cutslayered Macro-rhythmic units are found within the large-scale metagabbro (photograph 8 of Fig. 5). It is suggested that layers. These units vary in thickness from 10 to 50 m and can chilled metagabbro belongs to the same intrusion as that in generally betraced along the whole strike length of the the Solsem area, but represents a shallower level. large-scale layers. This lateral continuity is demonstrated by a several metre thick, 3.5 km long chromite dunite horizon. Layered ultrumufics. Cumulate ultramafic rocks are well In dunite subzones the macro-rhythmicunits begin with preserved and exposed within the Skrla and the Steinstind olivine cumulates, within which the lowest 1-2m may have

End of 85 87 89 91 23conttnuoussectlon B c-Y ,"'

-350m

Explanation:

D : Dunite W : Wehrlite

Bands 01 ohch Lh : Lherzolite Chr : Chromite

HProp.~2~~~-70130 DIW-70130 WID Prop. W10 50150 @

2m brecciated dunite

-200m / IChromhbands 1 W - 150m

Thin bands(O.l-2cn 01 cpx or chr. 1 D oh theorder I-% PWIChrT

Thin bands 01 cpx T fD7or chr. -100m

0.5 thick shear zone

-0m 9'1'3 85 ' $7 ' 8g 9'1'3 Unexposed 100 MOI (Ma + Fe)inolivine (0) A

Fig. 3. Detailed profile through partsof the ultramafic cumulate sequence, showing large-scale fayers, macro-rhythmic units (well defined in photograph 3 by F0 content in olivine of the dunite), and small-scale rhythmic layers (photographs1,2 & 4). The rank subdivision follows the scheme suggested by Irvine (1982).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 406 H. FURNES ET AL.

a high (c. 5-20%) concentration of chromite (Fig. 3). The minerals seem to havebeen modified by post-cumulus dunite becomes gradually enriched inpost-cumulus pyroxene crystal growth. Inclusions of olivine in chromite and olivine upwards, whilst some unitshave olivine-clinopyroxene and chromitein the pyroxenes confirm theorder of cumulatesat the top. From these units the sequence of crystallization deduced from the cyclic units. crystallization can bedemonstrated to be:olivine+ chromite-, clinopyroxene+ orthopyroxene, which subsequently is followed by plagioclase and hence the appearance Layered metagabbro. Within the south-westem part of of gabbros. Variation in olivine compositionthrough the the Steinstind block (Figs 1 & 4) the transition from macro-rhythmicunits shows abrupt increase in the Fo ultramafic to gabbroic cumulates is well exposed. The three contents at the baseof the units (typically from F0 86 to Fo scales of layering are still observed at this level, although 92), followed by gradual decrease towards the top (Fig. 3). they are not as distinct as within the ultramafic zone. The Boththe mineralogical andthe compositionalvariations large-scale layers are here defined by thick ultramafic units interlayered with units also containing gabbroic rocks. The between and within these units may well be explained by metagabbros are first observed, as would be expected, at the repeated influx of primitive magma followed by periods of top of cyclic units giving rise to sequences of dunite- fractionalcrystallization, and these units will therefore be referred to as cyclic units. wehrlite-gabbro. The proportion of dunite in these cyclic unitsdecreases upwards and finally disappears, while the The rocks are also rhythmically layered on a 10-30 cm, metagabbro layersbecome more dominant, finally making or even finer scale (photographs 1 & 2 of Fig. 3), and both up a sequence several hundred metres thick. In general the uniform and modally graded layers are common. The latter sequence is dominated by rhythmically layered mesocratic show the same order of crystallization as the cyclic units. metagabbro (photograph 9 of Fig. 5), though sporadic layers These exhibit a dunitic lower half, sometimes with one or of wehrlite, phroxenitic or melanocratic gabbro may occur. several chromite-enriched laminae a few centimetres above Upwards the rhythmic layering becomes less apparent until the base, whilst towards the top of individual laminae there the rock grades into finely laminatedand finally is a gradual increase in the content of both clinopyroxene van-textured metagabbro (Fig. 5). and (locally) orthopyroxene. The clinopyroxene and olivine have been totally altered Wehrlitic and pyroxenitic segregation veins are frequent, to uraliticamphibole andserpentine, respectively. The and showvariable orientations relative tothe rhythmic plagioclase is saussuritized. However the gabbroic texture is layering. MoFt areconcordant with the layering but still preserved and suggests that the gabbroic zone, similar perpendicular and conjugate sets are also common. Locally to the ultramafic zone, is dominated by adcumulates. theconcordant segregationstransgress the layering, and some places they interconnect, forming a net-vein pattern. Frequent examples of folded veins are to be seen and these Vari-textured metagabbro. The transition from layered/ are characterized by extended longlimbs with thicker hinges laminated metagabbros is seen on Mas@ya and in the area and middle limbs. The long limbs are now often parallel or SW of Skei (Fig. 1). It seems to be restricted to the crustal inclined at only a small angle tothe layering. These block south and east of a major NNE-SSW fault running deformed veins attest to a syn- or post-magmaticductile right across Leka bringing the apparent roof assemblage of deformation. the metagabbro into conjunction with deeper levels of the The minerals of the ultramafic rocks are variably altered. layered metagabbros and with the ultramafics. While the clinopyroxenes are nearlyunaltered the The relationship between the vari-textured metagabbros orthopyroxeneshave totally been replaced by bastite. and the pseudo-stratigraphically overlying dyke swarm and Olivine hasgenerally been serpentinized toan extent of pillow lavas is shown diagrammatically in Fig. 5. The 30-40%, but examples of almost unaltered as well as totally metagabbro itself is massive and its grain size varies serpentinized dunites occur. The primary textures of the irregularly from fine to coarse. In places it becomes patchily rocks suggest that they all are adcumulates. Chromite may pegmatitic; typical minor pegmatitic bodies are shown in locally exhibit euhedral crystal forms,but generally all photographs 6 & 7 of Fig. 5. At higher levels

C m Gabbro 200 m - 0 Banded dunitelwehrlote 0 Dunite

80 m-

1 00 400~~200 Fig. 4. Profiles showing the first appearance of A B D gabbro within the ultramafic cumulatesequence.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 T HE LEKA OPHIOLITECOMPLEX,NORWAYLEKA THE 407

Fig. 5. Schematic illustration showing the relationships between layered/laminated/vari-texturedmetagabbros, metabasalt dykes, microgabbro, minor acid intrusions and metadiorite, and pillow lava on Madseya. Note that the oldest dykes are approximately perpendicular to the bedding of the pillow lava. 1: Bedded chert cut by metabasalt dyke. The dark layers contain abundant garnet, minor amounts of mica (probably stilpnomelane), amphibole (probably actinolite) and iron oxide. 2: Pillow lava (note the pervasive cleavage). 3: Intersecting Metabasalt dyke. 4: Relationship between plagiogranite and metadiorite (dark). 5: Detail from the metadiorite, showing the pegmatitic development with growth of large hornblende crystals (central part of photo). 6: Van-textured metagabbro, cut by metabasalt dykes. 7: Detail from the van-textured metagabbro, showing pegmatitic development, with growth of large clinopyroxene crystals. 8: Intrusive contact at Halin. 9: Layered metagabbro.

hornblende-bearingmetadiorite patches appear, often cut phases of intrusion. There is also evidence that some of the by acid intrusions (photographs 4 & 5 of Fig. 5). earliest quartz-keratophyressuffered brecciation prior to the Within the lower parts of the van-textured metagabbro, emplacement of new batches of either quartz-keratophyre or small irregular bodies of greenstone appear (Fig. 5). These basaltmagma. The brecciation isalways restrictedto the metabasite bodies, seldom more than a few centimetres in quartz-keratophyres,and may be dueto gas breccciation diameter, are in some cases associated with a distensional (Williams & Malpas 1972). fragmentation of the metagabbro and appear to be injected Within some of the basic bodies are high concentrations into thefissures so formed. Thediscontinuous nature of these (upto 60%) of roundedtoeye-shaped, sometimes bodies, which commonly have lobate or irregular margins, ‘vesicular’ enclaves (c. 2-20cm in diameter) of quartz- suggests that the gabbros may not have been totally rigid at keratophyre, always with sharp contacts to the basaltic host. the time of emplacement, an impression reinforced by the In other cases the shape of the acid fragments is irregular, back-veining of metagabbro into greenstone. and the intimate mixtureof basic and acid material can best becharacterized as an ‘emulsion rock’, asdescribed by Minor acid intrusions. Acidintrusions of quartz- Walker (1963) and Blake et al. (1965). In the most extreme keratophyre andplagiogranite range fromas thin (millimetre cases the acid material may constitute as much as 90% of thick)veins to sizeablebodies (>100m in diameter)at the rock. various levels within thevari-textured metagabbro and Basaltic pillows arefound within a host of quartz- metabasaltdyke swarms (Figs 1 & 5). Thequartz- keratophyre,and showevidence of having been chilled keratophyres are most common in the VBtvik area, where against the felsic magma. There is no gradation between the they are associated with aboutequal amounts of basic and acid members. However, there are examples of metabasalts. Basic and acid sheets and bodies cross-cut each rocks ofintermediate compositiongrading intowhite siliceous other, forming net-veined complexes which indicate several rock. Elsewhere field relationships between basic-acid and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 408 H. FURNES ET AL.

intermediate-acid rock associations such as those described both in the lower levels of van-textured metagabbro and above, have been reported from a number of places, and higher in the dyke swarm, from irregular bodies cut by later attributed to contemporaneous development and mixing of dykes. Intense hydrothermal activity is associated with some basic and acid magma (e.g. Blake et al. 1965; Yoder 1973; of the dykes; these are full of veins filled with one or more Taylor et al. 1980; Vogel 1982). Hybridization due to mixing of the minerals epidote, calcite, quartzand K-feldspar of liquids seems to berestricted to those of similar (photograph 3 of Fig. 5). composition, and apparently does not occur between basalts andquartz-keratophyres. This is supported by recent The volcanic and volcaniclastic rocks experiments on magma mixing (Campbell & Turner 1986). Metabasaltic pillow lavas occur at three different localities Metabmalt dykes. The relationshipsbetween metagab- within the LOC: Madsejya, Langdraget and Storejya (Fig. 1). bro and dykes and the internal arrangementwithin the dyke Because of the separation their relativestratigraphic swarm can best be observed on the island of Mads~ya(Fig. relationscannot be directly observed.However, their 1). Dykes first appear in the upper part of the laminated associations with other rocks together with their geochemi- metagabbro, or at its transition to the van-textured variety cal characteristics suggest thatthe Madsejy lavas arethe (Fig. 5). The earliest dykes here dip steeply to the NW and oldest and the Storejya ones the youngest. are oriented approximately perpendicular to the laminated gabbro. They are cut by a later generation of vertical dykes The Mads@ya type. Relatively small occurrences of (Fig. 5). Passing southand east through the vari-textured non-vesicular pillow lavas and associated beddedcherts metagabbrothe number of dykesincreases, but not (photograph 1 of Fig. 5), found either as screens between uniformly. The dykes are emplaced passively by distension metabasaltdykes or surrounded by microgabbro (Fig. 5), of themetagabbros, and the percentagedilation at first are seen on the south-eastern shore ofMadsejya (Fig. 1). ranges from 10 to 50%. However, within a short distance Some of the pillows are relatively well preserved zones areencountered in which the intensity of intrusion (photograph 2 of Fig. 5), and their shapes, combined with increases sharply and narrow screens of metagabbro are all the presence of drain-outs, reliably indicate bedding which is that remain of the hostrock. These zones arenarrow, oriented approximately perpendicular to the earliest dykes seldom morethan 100m wide, andare interspersed with (Fig. 5). These dykes may well have been their feeders. areas in which the dilation is generally less than 50%. In these less intensely dyked zones the complexity of the The Langdraget type. A small occurrence of massive, intrusions can be seen. The dykes are all metabasalt, with non-vesicular, variolitic pillows, strongly deformed, appears strike varying between NNE and ENE. Some are vertical, on the westernside of the island of Langdraget (Fig. 1). others dip steeply to the SE, and they cut one another (Fig. Withinthese pillow lavas, which are associated with light 5). The change in orientation and dip may be in part due to grey todark green laminated volcaniclastic sediments, the folding which is evidenced by the distribution of stockwork-likemetal sulphide mineralization occurs. The ultramafics and metagabbrosbetween Mads~yaand the two small islands immediately west of Langdraget (Fig. l), main island,but the variable orientation and cross-cutting consist of massive metabasalt flows, associated with green relationships of dykes within the swarm suggests rather volcaniclastic rocks. unstableconditions atthe spreading centre with flexure, subsidence, internalrotation and subsequent further The Stor@ya type. The island of Stor~ya(Fig. 1) consists injection of dykes atan angle tothe earlier ones, as of a sequence of dominantly submarine volcanic rocks (Fig. proposed by Rosencrantz (1982). Early intrusions are often 6) overlain by black shales and volcaniclastic sediments, irregular,sometimes lobate, whilst later dykes are more which is in tectoniccontact with either serpentinized consistentin form and trend. As the numbers increase so ultramafics or metagabbros of the LOC. Previous published too do the numberof generations, with earlier dismembered accounts of these volcanic rocks have focussed mainly on sheets remaining as fragments with one-way chilled margins their chemical composition (Prestvik 1974, 1985; Prestvik & and later ones two-way chilling. None of the early irregular Roaldset 1978). bodies show chilled margins. The dykes do not extend over The volcanic sequence attains a thickness considerably in great distances, a few tens of metres being average, and they excess of 600 m, of which approximately half comprises often split and finger out along their length. The overall well-defined pillow lavas (Fig. 6), and the remainder roughly impression is that the earliest intrusions were emplaced into equalamounts of pillow breccias and massive lava flows, partly molten, or at least plastic gabbro in aperiod of with minor interbedded sediments. Inthe pillow lavas, general distension so that they donot form regularly features such as drain-outs in the upper half are common, oriented sheets but pods, lenses and fingers. Later injection providing reliable younging directions(Ballard & Moore was into cooler, more rigid gabbros which cracked to permit 1977). Some of the pillows are non-vesicular and variolitic, regularly oriented injection. Many of these later intrusions whereas others may contain up to 10%of amygdales, always are diorites ratherthan dolerites. Dyke thickness varies concentrated in the upper parts of the body. All gradations considerably from a few centimetresto over 10m and from well-defined pillows into pillow breccias, and from texture is also variable from dykes which are fine grained massive flows (2-20 m thick) into breccias are present. throughoutto those of comparable thickness which show The metasediments interbedded with the eruptives occur rapidgradation from chilled margin to medium-grained most frequently in the lower half of the sequence.They metagabbrocentres. From the geometry of the swarm it consist of grey to brown limestone, grey to pink chert and would seemthat the major phase of dyke injection was grey to green volcaniclastics and vary in thickness from a controlled by extensional stresses which developed a series few centimetres upto 2-3m (Fig. 6). The number of of fissures each of which was utilized by a succession of intra-volcanicsedimentary horizons, at least 20, providea injection pulses. In places microgabbrointrusions, found minimum estimate of the number of volcanic flows.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 THE LEKA OPHIOLITECOMPLEX, NORWAY 409

Well-defined Storsya volcanic rocks FA pillows mF99;,:';,";mxcias, m.pdlow breccia Pillow lava/m Melasediments I volcaniclastics Poorly defined m pillows Serperltinites -----L- -c 46 810 20 50 80 300 U - .?.*?S.?. H Massive lava H massive Brecciated lava 0 X -S Sediment or reworked 8500m-+84-LF-13 As=; volcaniclastic material E] = = Shear zone 0 X

0 X

0 X 2nrI thick reworked 0 X L.. aloclasties with ds or lenses of linated brown 0 X .. .

0 X 0 X 0 X .- .j-brown sed. 0 X 8 str .ongly epidotized 0 X 0 X 0 X 0 X

0 X 0 X

0 X

0 X X m0 46610 2-0 ZrIY Nb ppm Fig. 6. Continuous profile (A-B, B-C) through part of the Storeiya volcanic sequence, showing the relative abundanceof pillow lavas, pillow breccias, massive lava and volcaniclastic material.a: Well-defined variolitic pillows.b: Coarse volcaniclastic debris overlying pillow lava.Note the carbonate infill (darkpart in the lower left part of photo). The pronounced geochemical variations in incompatible (Nb) and compatible (Ni) elements, as well as the Zr/Y ratio for parts of the profile,is shown.

Epidotization within the metabasalts, particularly the represent only the analyses of volcanic and subvolcanic pillow lavas, is common; its intensity,however, may vary members of the complex. Data relevant to points under considerably, giving the pillow lavas a patchy appearance. discussion are presented in Figs 3,6,7, and representative Areas of strongly affected pillows may terminate against a analyses are given in Table 1. sedimentary bed,thus indicating that epidotization was The Madsoya metabasaltswere previously believed to essentially syn-volcanic. representN-type MORB,on the basis of analyses from The primary features in the pillow lavas, together with certain metabasalt dykes and pillow lavas (Prestvik 1980). beddingin the intra-volcanicsediments, show thatthe However, datapresented here from the metabasalts of sequence defines a tight, steeply plunging F2 fold (Fig. l). Madsoya, together with the chilled facies of the metagabbro In the core of this fold, a strongly deformed sedimentary at Halin (Fig. l), show a pronounced compositional sequence stratigraphically overlies the volcanics (Fig. 6, variation which, when plotted on discriminant diagrams, inset map). At the lowest stratigraphic level, a few metres of suggests formationin an island arcsetting (Fig. 7). black shalerests with a primaryconformable contact on Supporting evidence is provided by the presence of dykes of pillow lavas andon the north-westem limb of the fold; boninitic composition; apparently crystallized from a magma detailed mapping (R. Tveit pers. comm. 1986) has revealed characteristic of the early development in the fore-arc region that the volcaniclatics overstep the pillow lavas. of ensimatic island arcs (Dietrich et al. 1978; Meijer 1980). Further support is given by the acid intrusions which, with one exception,plot within the field defined by island arc Geochemistry magmatic rocks (Fig. 7, Ti0,-Zr diagram). Extensive geochemical studies (by XRF) have been made of The pillow lavas from Langdraget (Fig. 1) are, with one all rocks types inthe LOC. The data discussed here exception, generally higher in both incompatible (Ti, Zr, Y)

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 410 H. FURNES ET AL.

andcompatible (Cr) elementsthan the metabasaltsfrom Mads@ya, and plot within the MORB field (Fig. 7). The single exception is very low in Ti02, Y,Zr and V (Fig. 7) and is comparable with the Madscbya basalts. The metabasalts from Stor0ya have been divided into twogroups onthe basis of REE patterns by Prestvik & Roaldset (1978). One of thesehas a LREE-enriched pattern,the other a flat patternand this was takento indicate generation in an ocean island environment. Later Prestvik (1985) provided additional trace element data for the metabasalts which confirmed their bipartite subdivision, but suggested, on the basis of Th-Ta-Hf relations, that they 68r'Z?267Z:wB may represent 'anomalous' ridge basalts. 0000ri00000 A new,extensive and detailed geochemcal profile through the metabasalts of Storcbya confirms their chemical variability (Fig. 6a)and when plottedin discriminant diagrams (Fig. 7), the data are seen to spread from MORB into the field of within-plate basalt. This distribution of data points is unlike that of both Langdraget and Madscbya lavas. It thus seems justified to suggest that the Storcbya lavas were derived from a source different from that of the metabasalts of Madscbya and Langdraget. Whether this can be attributed to formation as an 'anomalous' ridge (Prestvik 1985) or an ocean island (Prestvik & Roaldset 1978)is equivocal, as both environments may yield comparable trace element abundances(Wood et al. 1979; Schilling et al. 1983). However, certain features of the new data favour the latter. The lavas occuras a thick pile (>a0 m) overlain by volcaniclastic sediments of similar geochemical character. The eruptiveshave high Nb contents (Fig. 6) and though some of the lava analyses fall within the restricted MORB field, all plot in the rather more extensive within-plate-field in every discriminant diagram (Fig. 7). As indicated above, the pillow lavas on two of the three areas,Langdraget and Storcbya (Fig. l), volcanics are overlain by green volcaniclastic sediments of basaltic composition. Though identical in macroscopic appearance, trace element patterns defined by Nb, Zr, Y and TiO, make it obvious thatthe sedimentsretain the geochemical character of the basalts which they overlie. They clearly derive fromthe volcanics, and retain the geochemical characteristics of their parental basalt.

Discussion and conclusions During the last few years of ophiolite research it has become evident that ophiolites may represent morethan simple It I pieces of oceanic crust (e.g. Jakes & Miyake 1984). This is well presented ina paper by Pearce et al. (1984), where ophiolite complexes have been subdivided into 'mid-ocean ridge basalt' (MORB) ophiolites and 'supra-subduction zone' (SSZ) ophiolites, of which thelatter represents ophiolites generated ina marginal basin settingand thus show the presence of a geochemical component from the underlyingsubduction zone.The SSZ ophiolites, which exhibit thestructure of oceanic crust, may thus possess geochemical affinities with island arctholeiites and boninites.However, depending upon the maturity of the adjacent subduction zone, the marginal basin ophiolites may show all transitions from N-type MORB to island arc type magmatism (Saunders & Tarney 1984), as well as alkaline type magmas (Marsh et al. 1980). A good example of relatively young to present-day island arcand associated back-arc basin evolution,where all the above-mentioned

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 THELEKA OPHIOLITE COMPLEX, NORWAY 41 1

Storeya pillow lava

o Langdraget pillow lava

~ Gabbro intrusion (chilled focies) at Halin Madseya dykes & pillow lava B Acid intrusions (Madseya) ., @I \ B .e b Metabonimtes 2; 0.1 I 10 50 100 500

Fig. 7. Geochemical variations of metabasalt dykes, pillow lava and acidintrusions of Madciya, gabbro intrusion at Halin and the pillow lavas from Langdraget and Storeya. Ti0,-Zr and Cr-Y diagrams after Pearce (1980). Ti-Cr diagram after Pearce (1975). V-Ti/1000 diagram after Shervais "b IAT (1982). MORB, Mid-oceanridge basalts; IAT, TJ1000 Cl Island arc tholeiites; WPB, within plate basalts. 0 l0 20 10 100 500 1'03

magmatic types arerepresented, is the West Philippine- 1 & 7), associated with thick volcaniclastics of similar Mariana region (Crawford er al. 1981), and this may serve as geochemical composition tothe pillow lavas, which are model for the proposed development of the LOC. probablyyounger thanthe Mads0ya pillow lavas, are Returning to the magmatic evolution of the LOC, the thought to represent partial melting of a mantle which had metabasalts at Madseya (Figs 1 & 5), which represent the suffered little or no influence from the subduction zone (Fig. earliest magmatic event, include all transitions from MORB 8b). Similarly, the Storcbya pillow lavas and associated to island arc tholeiites (IAT), as well as metaboninites with volcaniclastics of WBP affinity (Fig. 7), which may be even IAT predominant (Fig. 7). These characteristics of the younger thanthe Langdraget volcanics, represent either metabasaltdykes and pillow lavas,combined with the E-type MORB or ocean island development, and may thus arc-magmatic affinity of the acid intrusions, indicate that the have formed at a more remote position from the subduction majorpart of the LOC formed by spreadingabove a zone in a developing back-arc basin (Fig. 8c). Because of subduction zone as indicated in Fig. 8(a). This is supported the thick development of the volcanics and volcaniclastics, not only by the geochemistry of the metabasalts but also by we favour the ocean island possibility. Someage the mantletectonite which is typically aunit of residual relationships between the Storoya(WPB) and Madsaya harzburgite/dunite, as well as by the crystallization sequence (IAT) Magmatic type may be provided in the VHtvik area. of the layeredplutonic rocks (ol+ cpx+ opx+ pl), all Here the van-textured metagabbro and associated IAT-type characteristic features of the SSZ-type ophiolites (Pearce et dykes are cut by dykes of the same geochemical affinity as al. 1984). The MORB-type pillow lavas on Langdraget (Figs the Stor0ya pillow lava. This may indicate thatthe WPB-type volcanites of Stor~ya areof approximately the sameage, moreor likely, post-date the IAT-type metabasalts of the LOC.

Financial support for this study hasmainly been given through (Langdraget) l-Keratophyre Plllow Lava. grants from the Norwegian Research Council for Science and the Plag granlle melabas. dyke Humanities (NAVF), and fromNorges Geologiske Unders0kelse (1-5)+6 (e g Valvl) var~lextured b (NGU). We thank B. A. Sturt, D. M. Ramsay, J. A. Pearce and an anonymous reviewer for their comments, and B. A. Sturt and D. M. Ramsay for discussions in the field. We further thank B. A. Laugen for accommodation on Leka, and J. Ellingsen for assistance.

References BALLARD,R. D. & MOORE, J. G. 1977. Photographic Ath of the Mid-Atlantic Ridge Rift Valley. Springer-Verlag, New York. Fig. 8. Cartoon showing development of the Leka Ophiolite BEFSNG,D. 1986. Forelopige utgaver av karbladene Vikna, 16241, Nordeyan, Complex (LOC) by spreading above a subduction zone, which, at 1624 IV, Kolvereid, 1724 IV, 1:50.000. Norges geologiske Undersokeke. least in the initial stages of ophiolite genesis (a), gives it the BLAKE,D. E., ELWELL,R. W. D., GIBSON,I. L., SKELTON,R. R. & WALKER, geochemical characteristics of intraoceanic island arc tholeiites G. P. L. 1965. Some relationships resulting from the intimate association (IAT). At stage (b) the metabasalts are predominantly of MORB of acid and basic magmas. Quarterly Journal of the Geological Society of London, 121, 31-48. type with minor occurrences of IAT, and at (c) WPB are dominant. BROWNING,P. 1982. The petrology, geochemsihy and structure of the plutonic We interpret this as a progressive development of an ocean basin in rocks of the Oman ophiolite. PhD thesis, The Open University, 404 pp. which the magmatic products gradually escape from the CAMPBELL,I. H. & TURNER,J. S. 1986. Theinfluence of viscosity on geochemical influenceof the subduction zone. fountaining in magma chambers. Journal of Petrology, 27, 1-30.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021 412 H. FURNES ET AL.

COLLEY,H. & HINDLE,W. H. 1984. Volcano-tectonic evolution of Fiji and NORDGULEN,0. & BERING,D. 1986. Forelepig utgave av Kartbladet Austra, adjoining marginal basins. In: KOKELAAR,B. P. & HOWELLS,M. F. (eds) 1725 11. 1:5O.O00. Norges Geologiske Undersokelre. MarginalBasin Geology. Volcanicand associated sedimentary and PEARCE,J. A. 1975. Basalt geochemistryused to investigatepast tectonic tectonic processes in modern andancient marginal basins. Special environment in Cyprus. Tectonophysics,25, 41-67. Publication of the Geological Society, London, 16, 151-62. -1980. Geochemical evidence for the genesis and eruptive setting of lavas CRAWFORD,A. J., BECCALUVA,L. & SERRI, G. 1981.Tectono-magmatic from Tethyan ophiolites. In: PANAYIOTOU,A. (ed.) Ophiolites. evolution of the westPhilippine-marina region and the origin of Geological Survey Department, Cyprus, 261-72. boninites. Earth and Planetary Science Letters, 54, 34-56, -, LIPPARD,S. J. & ROBERTS,S. 1984. Characteristics and tectonic DI~CH,V., E~RMANN,R., OBERHANSLI,R. & PUCHELT,H. 1978. significance of supra-subduction zone ophiolites. In: KOKELAAR,B. P. & Geochemistry of basalticand gabbroic rocksfrom the West Mariana HOWELLS, M. F. (eds) Marginal Basin Geology. Volcanic and associated Basin and the Marina Trench. Earth and Planetary Science Letters, 39, sedimentary and tectonicprocesses in modern andancient marginal 127-44. basins. Special Publication of the Geological Society, London, 16,77-94. DUNNING,G. R. & PEDERSEN,R. B. 1987. Geochronology of ophiolites and PEDERSEN,R. B.1986. The nature andsignificance of magmachamber arc-related plutons of the Norwegian Caledonides. Contributions to margins in ophiolites: examples from the Norwegian Caledonides. Earth Mineralogy and Petrology (in press). and Planetary Science Letters, 77, 1W12. Fum, H., RYAN,P. D., GRENNE,T., ROBERTS,D., STURT,B. & PRESTVIK, F~ESTVIK,T. 1972. Alpine-type maficand ultramafic rocks of Leka, T. 1985. Geological and geochemical classification of the ophiolites in the Nord-TrBndelag. Norges Geologiske Undersokelre, 273, 23-34. Scandinavian Caledonides. In: GEE, D. G. & STIJRT,B. A. (eds) The - 1974. Supracrustal rocks of Leka, Nord-TrBndelag. Norges Geologiske Caledonide OrogenScandinavia and Related Areas. John Wiley, New Undersokelre,311,65587. York, 657-69. -1980. The Caledonian ophiolite complex of Leka, north central Norway. GIRARDEAU,J., MARCOLX,J., ALLEGRE,G. J., BASSOWLLET,J. P,, YOUKING, In: PANAYIOTOU,A. (eds) Ophiolifes. GeologicalSurvey Department, T.,XUCHANG, X., YOUGONG,Z. & XIBIN, W. 1984. Tectonic Cyprus, 555-66. environment and geodynamicsignificance of the Neo-Cimmerian - 1985. Origin of the volcanic StorBya Group, Leka. Results from new Donqiao ophiolite, Bangong-Nujiang suture zone, Tibet. Nafure, 307, geochemical investigations. Norsk geologivk Tidsskrift, 66, 237-9. 27-31. - & ROALDSET,E. 1978. Rare earth element abundances in Caledonian GREGORY,R. T. 1984. Melt percolation beneath a spreading ridge: evidence metavolcanics from the island of Leka, Norway. Geochemical Journal, from the Semail peridotite, Oman. In: GAS, I. G., LIPPARD,S. J. & U,89-100. SHELTON,A. W. (eds) Ophiolites and Oceanic Lithosphere. Special ROSENCRANTZ,E. 1982. Formation of uppermost oceanic crust. Tectonics, 1, Publication of the Geological Society, London, U,55-70. 471-94. GUSTAVSON,M. & PRESTVIK,T. 1979. The igneous complex of Hortavzer, SAUNDERS,A. D. & TARNEY,J. 1984. Geochemical characteristicsof basaltic Nord-TrBndelag, Central Norway. Norges geologiske Undersokelre, 348, volcanism within back-arc basins. In: KOKELAAR,B. P. & HOWELLS,M. 73-92. F. (eds) MarginalBasin Geology. Volcanic and associatedsedimentary IRWNE, T. N. 1982. Terminology of layered intrusions. Journal of Petrology, and tectonic processes in modern andancient marginal basins. Special 23, 127-62. Publication of the Geological Society, London, 16, 59-76. JACKSON,E. D., GREEN, H. W. & MOORES,E. M.1975. The Vourinos SCHILLING,J. -G., JAJAC,M., EVANS,R., JOHNSTON,T., WHITE,W., DEVINE, opholite, Greece: Cyclic units of lineated cumulates overlying J. D. & ~GSLEY,R. 1983. Petrologic and geochemical variations along harzburgite tectonite. Geological Society of America Bulletin, 86, 390-8. the Mid-Atlantic Ridge from 29"N to 73'N. American Journal of JAKES, J. P. & MIYAKE,Y.1984. Magmas in forearcs: implication for Science, 285,510-86. ophiolite generation. Tectonophysics, 106, 349-58. SHERVAIS,J. W.1982. Ti-V plots and the petrogenesis of modernand LEBLANC,M,, DUPW, C., CASSARD,D., MOUTIE, J., NICOLAS,A., ophiolitic lavas. Earth and Planetary Science Letters, 59, 101-18. PRINZHOFER, A., RABINOWI~Z,M. & ROUWIER, P. 1980. Essai sur la SPRAY,J. G. 1982. Mafic segregations in ophiolite mantle sequences. Nature, gentse des corps podiformed de chromitite dans les peridotites 299, 5248. ophtolitiques: ttude des chromite de NouveUe-Caltdonie et com- STURT,B. A., ANDERSEN,T. B. & FURNES,H. 1985. The Skei Group, Leka. paraison avec ceUe de Mtditerrant orientale. In: PANAYIOTOU,(ed.) A. An unconformable clastic sequence overlying the Leka Ophiolite. In: Ophiolites. Geological Survey Department, Cyprus, 691-701. GEE, D. G. & STURT,B. A. (eds) The Caledonide OrogenScandinavia LEITCH, E. C. 1984. Marginal basins of the SW Pacific and the preservation of and Related Areas. Wiley, New York, 395-405. their ancient analogues: a review. In: KOKELAAR,B. P. & HOWELLS,M. -, ROBERTS,D. & FURNES,H. 1984.A conspectus of Scandinavian F. (eds) MarginalBasin Geology. Volcanic and associatedsedimentary ophiolites. In: GASS,I. G., LIPPARD,S. J. & SHELTON,A. W. (eds) and tectonicbasins. SpecialPublication of the GeologicalSociety, Ophiolites and OceanicLithosphere. SpecialPublication of the London, 16,97-108. Geological Society, London, U,381-91. MARSH,N. G., SAUNDERS,A. D., TARNEY,J. & DICK, H.J. 1980. SYCHEV,P. M. & SHARASKIN,A. Y. 1984. Heat flow and magmatism in the Geochemistry of basalts from the Shikon and Daito Basins, Deep Sea NW Pacific back-arc basins. In: KOKELAAR,B. P. & HOWELLS,M. F. Drilling Project Leg 58. In: DE VRIESKLEIN, G. & KOBAYASHI, K.et al., (eds) Marginal Basin Geology. Volcanic and associated sedimentary and Initial Repottv of the Deep Sea Drilling Project. Vol. 58. Washington, tectonic processes in modern andancient marginal basins. Special D.C. (U.S. Government Printing Office), 805-42. Publication of the Geological Society, London, 16, 173-81. MEJJER, A. 1980.Primitive arc volcanismand a boninite series: examples TAYLOR,T. R., VOGEL,T. A. & WILBAND,J. T. 1980. The composite dikes at from western Pacific island arcs. In: HAYES, D. E. (ed.) The Tectonic and Mount Desert Island, Maine: an example of coexisitng acidic and basic GeologicalEvolution of Southeast Asian Seas and Islands. American magmas. Journal of Geology, 88, 433-44. Geophysical Union Monograph, 23,269-82. VOGEL, T.A. 1982.Magma mixing in the acid-basiccomplex of NICOLAS,A. & JACKSON,M. 1982. High temperature dikes in peridotites: Ardnamurchan: Implications on the evolution of shallowmagma Origin by hydraulic fracturing. Journal of Petrology, 23,268-582. chambers. Contributions to Mineralogy and Petrology, 79, 411-23. -& PRINZHOFER,A. 1983. Cumulative or residual origin for the transition WALKER,G. P. L. 1963. The Breiddalur central volcano, eastern Iceland. zone in ophiolites: structural evidence. Journal of Petrology, 24, Quarterly Journal of the Geological Society of London, 119, 29-63. 188-206. WILLIAMS,H. & MALPAS, J.1972. Sheeted dikes and brecciated dike rocks - & VIOLETIE,J. F. 1982. Mantle flowat oceanic spreading centres: within transported igneouscomplexes, Bay of Islands, Western models derived from ophiolites. Tectonophysics, 81,319-39. Newfoundland. Canadian Journal of Earth Science, 9, 1216-29. -, BOUDIER, F.& BOULLIER,A. M. 1973. Mechanisms of flow in naturally WOOD,D. A., TARNEY,J., VARET,J., SAUNDERS,A. D., BOUGAULT,H., and experimentally deformed peridotites. American Journal of Science, JORON,J. L., TREUIL,M. & CANN, J.R. 1979. Geochemistry of basalts U), 192-210. drilled in the North Atlantic by IPOD Leg 49: implications for mantle NISSEN,A. L. 1986.Rb-Sr age determination of intrusive rocks in the heterogeneity. Earth and Planetary Science Letters, 42, 77-97. southeastern part of the Bindal Massif, Nord-Tr~ndelag,Norway. Norges YODER,H. S. JR 1973. Contemporaneous basalticand rhyolitic magmas. Geologiske Undemokelre Bulletin, 406,83-92. American Mineralogy, 58, 153-71.

Received 23 April 1987; revised typescript accepted 5 October

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/3/401/4889506/gsjgs.145.3.0401.pdf by guest on 30 September 2021