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Geol. Mag. 128 (3), 1991, pp. 207-226. Printed in Great Britain 207

Melt-enhanced deformation during emplacement of gabbroand granodiorite in the Sunnhordland Batholith,

west NorwayT O R G E I R B . A N D E R S E N , P E T E R N I E L S E N , E R L I N G R Y K K E L I D & H A N N E S 0 L N A

Geological Institute, University of Oslo, P.O. Box 1047, 0316 Blindern Oslo 3, Norway

(Received 6 September 1990; accepted 15 November 1990)

Abstract - The Caledonian Sunnhordland Batholith comprises calc-alkaline plutons that have beenassigned to three units according to their relative age and composition: a gabbro-diorite unit, agranodiorite unit and a later granodiorite-granite unit. The batholith was emplaced into an envelopeincluding ophiolite and island-arc complexes, sediments and volcanites of early Ordovician age thatwere developed in a zone of plate convergence. Continued convergence resulted in the formation ofa mature magmatic arc and a thickened crust; the late granitoids (unit 3), which commenced theircrystallization at pressures around 6 to 7 kb, rose as permitted diapiric intrusions. The ingress andascent of the magmas in this setting is considered to have been facilitated by the presence of majorshear zones developed in relation to plate convergence. In this model, plastic instabilities were formedin an otherwise elastic middle and upper crust. Non-coaxial deformation was accelerated by theemplacement of magmas and the formation of abundant partial melts in water-rich sediments of theenvelope. The deformation, which was accelerated by magma and melt lubrication in aureoles,controlled both the shape and internal structure in the gabbro and granodiorite plutons.

1. Introduction

This paper discusses the emplacement of syn-tectonicplutons in the Caledonian Sunnhordland Batholith ofwest Norway (Andersen & Jansen, 1987), and de-scribes structures which are interpreted to be the resultof syn-magmatic deformation phenomena both withinplutons as well as in their migmatitic aureoles(Andersen, 1989). It is suggested that the deformationwas enhanced during emplacement of the plutons as aresult of melt lubrication.

In a crustal segment under the influence of a majordeviatoric stress field, the presence of melts and fluidswill be an important factor in controlling thelocalization and the kinematics of shear zones (Hol-lister & Crawford, 1986). Zones of displacementinitiated at elevated temperatures, or in magmaticrocks above their solidus may, however, be difficult torecognize because of post-kinematic crystallization/recrystallization and annealing of the fabric. Hence, inorder to identify high-Tor syn-magmatic shear zones,the rock must possess an anisotropy already at anearly stage in its deformation history, which mayrecord strain by structural and/or textural modi-fication. If deformation has occurred in the presenceof melts, the rocks must record the strain aftercomplete crystallization for the strain to be recognized.Blumenfeld & Bouchez (1988) and Paterson, Vernon& Tobisch (1989) have discussed and reviewed someimportant textural criteria for recognizing pre-solidusdeformation in magmatic rocks. Phenocryst tiling(Den Tex, 1969), shape-preferred orientations (SPO)

of primary minerals and shape-controlled differentialrotation of rigid particles in a viscous matrix describedin relation to deformation of conglomerates by Gay(1968) and demonstrated experimentally by Ildefonse& Fernandez (1988), are important criteria. Wheresmall melt fractions are present, this requirement maybe satisfied as deformation will cause frequentinteraction of the solid particles, and favour asegregation of the melt from the solid (Wickham,1987). In a body of magma, or in zones where highdegrees of partial, or near complete melting haveoccurred, record of deformation will be whollydependent on the type of flow which has affected thestrain markers (Ferguson, 1979). Such sites may,however, represent zones of high strain in syn-tectonicigneous complexes, and it is likely that the strain willnot be recognized until a major part of the magma hascrystallized. In this situation, the early deformationcan be identified only from displacement of struc-tures/contacts in the envelope, or by the shape andtime-dependent spatial arrangement of plutons in amajor shear zone (Hutton, 1982, 1988a, b; Castro,1986).

High content of melt in a zone will most likely resultin a near ideal Newtonian behaviour at geologicalstrain rates (Ferguson, 1979), and the strain acrosssuch a zone will increase linearly with time given aconstant stress and viscosity and have the form:

where e = strain, cr = stress, t = time and v = vis-cosity. Van der Molen & Paterson (1979) showed on

GEO 128

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208 T. B. ANDERSEN AND OTHERS

SIMPLIFIED GEOLOGICAL MAP,SUNNHORDLAND BATHOLITH, W. NORWAY

The Batholith:

Unit I, gabbro, diorite, minor ultramafics.

Unit II, Reksteren granodiorite.

Unit III, granodiorite and granites.

AUSTEVOLL REKSTEREN

The Envelope:o O7T

«o°o o °O 00 0

Dyvikvagen Gp.Up.Ordovician toLr. Silurian in age.Partly youngerthan the batholith

UndifferentiatedCaledonian rocks

Bimodal volcanicsMid.to Lr. Ordovicianin age.Basal unconformitieswhere marked.

High-grade contactmetamorphic

sediments. Migmatitesadjacent to gabbro.

Probably Lr.to Mid. Ordovicianin age.

Ophiolite and Island-arc lithologieslocally cut by qtz-diorites.

Lr. Ordovician and older.

10km

Sunnhordland fault —•—and other late faults.

Syn-magmatic shear zonesactive during emplacement of

Unit I & II in the batholith.

ense of shear on syn-magmatic shear zones

Figure 1. Simplified geological map of the Sunnhordland Batholith, west Norway (from Andersen, 1989).



Melt-enhanced deformation 209

the basis of experimental data that the most rapidchange in the mechanical properties of a partiallymolten granite occurs at 30 to 40 % melt, referred toas the critical melt fraction (CMF). The experimentaldata were in general agreement with Arzi's (1978)definition of the rheological critical melt percentage(RCMP).

2. Regional setting of the Sunnhordland Batholith

The Sunnhordland Batholith (Andersen & Jansen,1987) is a composite, synorogenic, intrusive complexwhich was emplaced into Lower Ordovician ophio-lite-island arc lithologies, bimodal volcanites andsediments representing the one of the most completesequences of convergent margin volcanics in theNorwegian Caledonides. These envelope rocks de-scribed by Brekke et al. (1984) and Nordas et al.(1985) were formed during an extended period of plateconvergence that commenced in early Ordoviciantime, in an intra-oceanic setting (Pedersen, Furnes &Dunning, 1988). The ensimatic complexes wereaccreted to a continent or formed part of a matureisland arc prior to the development of the bimodalvolcanic and sedimentary rocks of the Siggjo andKattnakken groups (Lippard, 1976; Nordas et al.1985), and the batholith post-dates these complexes(Andersen & Jansen, 1987). The Sunnhordland Batho-lith is of I-type, and is considered to represent theproducts of a continued convergence which resulted inthe formation of a mature magmatic arc (Andersen &Jansen, 1987). The syn-deformational magma em-placement described in this paper referes only to thegabbros and granodiorites which make up the earlierunits of the Sunnhordland Batholith; the later graniteplutons are post-deformational and their Rb-Srisochron ages indicate that they were emplacedfollowing a Middle—Upper Ordovician unconformitywhich is present along the length of the NorwegianCaledonides and corresponds with a break in mag-matic activity of some 10-40 Ma.

The plutons in the Sunnhordland Batholith havebeen assigned to three units according to their relativeage and composition. The plutons of the older Unit 1(gabbros and diorites) and Unit 2 (one majorgranodiorite pluton) were emplaced syn-tectonically,while the youngest intrusions in Unit 3 (granodioriteand granite plutons) are permitted intrusions (Ander-sen & Jansen, 1987; Andersen, 1989). The age of theSunnhordland Batholith is constrained by the datingof the envelope, including precise U-Pb ages onzircons from early primitive island arc volcanites andthe bimodal volcanites of Siggjo and Kattnakkengroups, which give U-Pb ages in the range 475-495 Ma(Pedersen & Dunning, 1991). The U-Pb ages over-lap with previously published Rb-Sr whole rockages from, these rocks on B0mlo (464 +16 Ma and

535 ±45 Ma, Furnes et al. 1983). A concordant U-Pbage of 472 ± 2 Ma from the Vardafjellet Gabbro(Unit 1) in the batholith on B0mlo (Fig. 1), hasbeen reported by Pedersen & Dunning (1991).Thegeochronological studies show that the VardafjelletGabbro was emplaced shortly after the deposition ofthe sediments and volcanites of the Siggjo andKattnakken groups. Rb-Sr whole rock isochronsfrom granites of Unit 3 have given 430±10Ma(Andersen & Jansen, 1987) and 430 ± 6 Ma (Fossen &Austrheim, 1988). The structural relationships of thebatholith show that it is pre-orogenic with respect tothe nappe transport during the Scandian phase. Ayounger age, however, cannot be ruled out for theRolvsnes granodiorite on north Bomlo and the Dronigranite in Austevoll (Fig. 1), as the deformationassigned to the main thrusting event during theScandian phase cannot be shown to have affectedthese plutons. The batholith forms an integral part ofthe allochthonous outboard terranes in the UpperAllochthon of the Scandinavian Caledonides.

3. Level of crystallization of the SB

As pointed out by Zen (1989), it is important for thediscussion of emplacement models to have knowledgeof the level at which the plutons have crystallized.

3.a. Unit 1, gabbros and diorites

The oldest plutons in the Sunnhordland Batholith(Unit 1) comprise mainly gabbros and diorites. Thegabbros in the eastern part of the batholith, on Stordand Tysnesoy (Fig. 1), have not been studied in detail.The Stolmen Gabbro and the Vardafjellet Gabbro(Fig. 1), however, have been mapped in detail. Bothplutons have high-grade migmatitic aureoles wherethey intrude metasediments.

3.a.l. The Vardafjellet Gabbro

The Vardafjellet Gabbro intrudes volcanites, sedi-mentary rocks and the ophiolite-island arc complexeson Stord and Bomlo (Fig. 2). Outside the aureole ofthe Vardafjellet Gabbro, stilpnomelane co-exists withwhite mica, and the regional metamorphism affectingthese rocks did not exceed the low-J part ( « 400 °C)of the greenschist facies (K. G. Amaliksen, unpub.Cand. Real, thesis, Univ. Bergen, 1983; H. Brekke,unpub. Cand. Real, thesis, Univ. Bergen, 1983; J.Nordas, unpub. Cand. Real, thesis, Univ. Bergen,1985). The rocks on Stord and Bomlo have apparentlyremained at an elevated structural position throughoutthe Caledonian orogeny.

The Bremnes Migmatite Complex (Fig. 2) formedin the roof of the Vardafjellet Gabbro on Bomlo(Andersen, 1989; P. E. Nielsen, unpub. Cand. Scient.

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thesis, Univ. Oslo, 1990). The migmatites developedfrom sandstones, pelites and calcareous sediments andpossibly also from felsic volcanics/volcaniclastics ofthe Siggjo Group (J. Nordas, unpub. Cand. Real,thesis, Univ. Bergen, 1985). The protoliths of theBremnes Migmatite Complex are correlated with thelow-grade sedimentary rocks interlayered with vol-canites in the upper part of the Siggjo Group (Fig. 2).The precise U-Pb dating of the Vardafjellet Gabbroand the Siggjo Group (Pedersen & Dunning, 1991)suggests that the sedimentary protoliths of theBremnes Migmatite Complex were < 5 million yearsold at the time of the migmatitization. The migmatitesinclude biotite-cordierite-sillimanite-bearing diatex-ites and metatexites, which locally are garnetiferous.Calibration of the contact metamorphism in theBremnes Migmatite Complex gives temperatures ofapproximately 750 °C (P. E. Nielsen, unpub. Cand.Scient. thesis, Univ. Oslo, 1990). Cordierites from theBremnes Migmatite Complex, determined by mic-roprobe analyses, have 58 % Mg and 42 % Fe. Holda-way & Lee's (1977) geobarometer, assuming P =PH 0 and T = 750 °C, indicates pressures close to 4 kb,corresponding to a crystallization depth for theVardafjellet Gabbro in the order of 15 km. If PH 0 <Ptotal, the estimated pressure would be reduced(Holdaway & Lee, 1977). The estimate of 4 kb or lessis in good agreement with low-grade regional meta-morphic assemblages in the area.

3.a.2. The Stolmen Gabbro

The contact metamorphism in the metasedimentsadjacent to the Stolmen Gabbro in Austevoll (Fig. 1)has been studied by E. Rykkelid (unpub. Cand. Scient.thesis, Univ. Bergen, 1987). Formation of melts byanatexis was abundant in the sediments, and amigmatitic high-grade aureole formed in these litho-logies. Cordierite has not been identified in the aureolein Austevoll. The P-T estimates carried out by E.Rykkelid (unpub. Cand. Scient. thesis, Univ. Bergen,1987) on preserved early mineral assemblages in theaureole and the mineralogy of the gabbro indicatethat metamorphism associated with the emplacementof the Stolmen Gabbro occurred at temperatures inexcess of 750 °C (760-810 °C); the pressure wasestimated to be between 4 and 6 kb. The P-T path(Fig. 3) for the metasedimentary rocks in Austevollindicates that these rocks were depressed to deeperlevels after emplacement of the Stolmen Gabbro.

The metamorphism in the aureole around theStolmen Gabbro and the Vardafjellet Gabbro, as wellas the lack of corona textures developed betweenolivine and plagioclase in the presence of hydrousphases such as phlogopite and hornblende in thegabbros (Espensen, 1978; Griffin & Heier, 1973),indicate that the Unit 1 gabbros crystallized relativelyhigh in the crust. Although the P-T estimates (Fig. 3)

200 400 600 800 CFigure 3. / ' -rpath from the metasediments in the aureolesin the Mckster area, Austevoll (modified from E. Rykkelid,unpub. Cand. Scient. thesis, Univ. Bergen, 1987) and theBremnes Migmatite Complex on Bomlo, 1,2 and 3 representthe approximate P-T conditions during the intrusion ofUnits 1 to 3 in the batholith at Austevoll.

are inherently uncertain, the relative variation calcu-lated by E. Rykkelid (unpub. Cand. Scient. thesis,Univ. Bergen, 1987) between early and late mineralassemblages based on the same geothermometers andbarometers, are thought to reflect a significant trendin the P-T path for the rocks in Austevoll. In theBremnes Migmatite Complex at Bomlo, there is noevidence of a higher-P overprint on the contact-metamorphic assemblages, and this is in agreementwith the low-grade regional metamorphism of thearea (Fig. 3).

3.b. Unit 2, the Reksteren Granodiorite

Unit 2 comprises the Reksteren Granodiorite pluton,which forms an elongate body of approximately35 km exposed length and a preserved maximum widthof 10 km (Fig. 1). The northern part of the pluton iscut by a late extensional fault, while the intrusivecontact is preserved along its southern margin(Andersen & Jansen, 1987). The contact relationshipshave been described in detail by H. Solna (unpub.Cand. Scient. thesis, Univ. Oslo, 1989), and show thatthe Reksteren Granodiorite was emplaced after theStolmen Gabbro (Unit 1) and similar gabbros onTysnesoy (Fig. ip-'Contemporaneously with theintrusion of the Reksteren Granodiorite, partial meltswith a composition close to the minimum meltcomposition for a granite system formed in meta-sediments in Austevoll, and Unit 1 gabbros wereamphibolitized adjacent to the granodiorite. Thermo-barometric calibrations for this event indicate that thehigh-grade aureole around the Stolmen Gabbro had




Figure 4. (a) Photomicrograph of gabbro from Vardafjellet on Bemlo. Minerals include plagioclase, clinopyroxene and olivine,with minor biotite and hornblende. The magmatic foliation is defined by laths of plagioclase. (b) Photomicrograph of bandedmigmatite from the Bremnes Migmatite Complex. The laminae are defined by quartz-feldspathic leucosome and biotite-sillimanite aggregates. Note that the fibrolitic sillimanite overgrows the orientated biotite crystals randomly. The porphyroblastwith high relief in upper left corner is garnet.

cooled to approximately 680-700 °C, at a pressurebetween 5 and 6 kb (Fig. 3). Subsequent to theintrusion of the Reksteren Granodiorite, kyanitebecame the stable Al2SiO5 polymorph in the wall-rockenclave in Austevoll, and the P-T estimates indicatethat the depression of the area to deeper crustal levelscontinued after emplacement of the Unit 2 (Fig. 3).This is in contrast with the B0mlo-Stord area wherestratigraphical relationships show that the VardafjelletGabbro was uplifted and eroded prior to the de-position of the Ashgillian to early Llandoveriansediments of the Dyvikvagen Group (Ryan & Skeving-ton, 1976; Thon, Magnus & Breivik, 1981). Anunconformity, previously not recognized, is locallypreserved between conglomerates of the UtslettefjellFormation in the Dyvikvagen Group and invertedrocks of the Vardafjellet Gabbro. This demonstratesthat the southern part of the batholith, in contrast tothe Austevoll area (see above), underwent uplift andunroofing in middle-late Ordovician time. The meta-morphism superimposed on the aureole at Bremnes isdiaphthoretic, and has no record of a P-increase (P. E.

Nielsen, unpub. Cand. Scient. thesis, Univ. Oslo,1990).

3c. Unit 3, granodiorite and granite

Unit 3 comprises four major plutons of granodioriticto monzogranitic composition (Andersen & Jansen,1987). A common feature of the late granitic plutonsof the Sunnhordland Batholith is the occurrence ofprimary magmatic epidote (Andersen & Jansen, 1987).Magmatic epidote co-existing with a silica melt ofgranitoid composition has been suggested to bedependent on a high PH^, high oxygen fugacity and ahigh Plotal (Naney, 1983)! Zen & Hammarstrem (1984)and a recent review by Zen (1989) suggest that acrystallization pressure in excess of 6 kb is requiredfor the mineralogy of the granitoids in Unit 3(Andersen & Jansen, 1987). The crystallization of theUnit 3 granitoids in the Sunnhordland Batholith mayhave commenced at depths in the order of 20 to 25 km.This is supported by P-T estimates from a garnet-plagioclase-kyanite-bearing assemblage adjacent to




the Dreni granite in Austevoll, which gave 570 °C at7 kb (Fig. 3), and a T-P estimate of 600 ± 50 °C at9 ± 2 kb from kyanite-bearing rocks adjacent togranites north of Bjornafjorden (Fig. 1) (Fossen,1988). No record of a P increase has, however, beenfound in the metasediments adjacent to the grano-diorite on Bomlo. This suggests that the Rolvsnesgranodiorite, which also contains primary epidote,reached its solidus at lower pressures, at a depth wherea mush of early phases such as epidote, hornblendeand biotite was carried diapirically to the final site ofcrystallization.

4. Synmagmatic deformation

The margins of the plutons in Units 1 and 2 representhigh-strain zones where metasedimentary rocks formthe envelope, and locally also where the plutons makecontact with meta-igneous rocks. In many of themarginal zones, deformation was apparently enhancedby the presence of melts, either derived by partialmelting in rocks of favourable composition orintroduced from the intruding magma. Melts of bothtypes occur adjacent to Unit 1 gabbros, while enhanceddeformation due to the ingress of magma was mostsignificant during emplacement of the ReksterenGranodiorite (Unit 2).

4.a. Vardafjellet Gabbro

4.a.J. Deformation along the margin

The Vardafjellet Gabbro has a medium- to fine-grained marginal zone of non-layered gabbro andmeta-diorite (Fig. 2). The gabbro is net-veined byleucocratic, granitoid material in a zone of variablethickness (50-100 m) adjacent to the Bremnes Mig-matite Complex which forms the roof of the pluton.Deformation of the marginal zone has locally resultedin hybridization of the two melts, giving rise to dioriticand quartz-bearing gabbros. In most places, however,the differences in viscosity and solidus temperaturesprevented mixing of the two melts, and a gneissicbanding formed. Chilled margins of the gabbro incontact with granitoid veins are commonly observed.At outcrop, the gneissic structure is most prominenton E-W surfaces, where if forms a prominent elementin a L > S fabric with a subhorizontal E-W stretchinglineation. At the scale of the individual minerals andmineral aggregates, a combined LPO (lattice-preferredorientation) and SPO (shape-preferred orientation)fabric is defined by primary plagioclase crystals and ofpolygonized aggregates of amphibole in the gabbro(Fig. 4a). The fabric is most obvious at the scale ofoutcrop, because a static annealing of the texturecommonly obscures the fabric when observed in thinsections.

Intrusions of the mafic magma into anatectic meltsderived from melting of the envelope resulted in theformation of pillowy and amoeboidal, fine-grainedgabbro bodies engulfed in granitoid material. Back-veining of leucocratic material in the mafic rocks iscommonly observed. The mafic bodies usually havean elongate shape, with the long axes oriented E-W.Internally, the mafic material is without strong fabricand shows little sign of deformation. The elongation isa result of early stretching, probably developedcontemporaneously with the formation of the maficprotrusions.

Adjacent to the gabbro, the Bremnes MigmatiteComplex is characterized by irregularly bandeddiatexites formed by near complete anatexis (Fig. 2).The diatexites contain metasedimentary inclusions,most commonly metasandstones, calc-silicate schistsand mica schists, and locally an irregular anddiscontinuous banding is present. The banding ischiefly defined by variations in the content of biotiteand quartz-feldspathic material. The foliation clearlyformed at an early stage, most probably as a result ofviscous flow. A preferential segregation of neosomalmaterial with an igneous texture (Wickham, 1987),from biotite-fibrolite laminae which internally haveannealed to develop a SPO fabric (Fig. 4 b) ischaracteristic of the banding. In these zones the micais usually replaced by fibrolitic sillimanite duringsolid-state recrystallization. In zones with abundantmetasedimentary inclusions, the xenoliths are usuallydominated by one lithology, either quartzite (Fig. 5 a),knobby mica schist (Fig. 5 b) or calc-silicate schist(Fig. 5 c), indicating that they represent disrupted,melt-resistant horizons which were interlayered withmore fusible material. Disruption of such layers toproduce angular to subangular inclusions probablyoccurred during near viscous flow of the anatecticmatrix, at a time when the strain rate in the leastviscous material was sufficiently high to enable brittlefailure to take place in the more melt-resistant layers.The concentration of xenoliths of one lithology inzones probably represents a ghost stratigraphy,indicating that the flow in the migmatite complex wasdominantly laminar, with limited exchange of thesolid material vertically.

Away from the gabbro contact, the BremnesMigmatite Complex consists chiefly of metatexites(Mehnert, 1968), where transitional contacts may beobserved between zones which contained variablemelt fractions. In Figure 5 a, a gradual increase inleucocratic neosome is shown in a metatexitic mig-matite. There is a clear negative correlation betweenthe intensity of the fabric in the rock expressed by thefoliation intensity, and the amount of neosome thatwas developed. The detailed mapping by P. E. Nielsen(unpub. Cand. Scient. thesis, Univ. Oslo, 1990) showsthat the most intensely foliated material occurs inareas with a relatively low content of neosome.




Figure 5. Bremnes Migmatite Complex, (a) Preserved quartz-rich metapsammite (left) with transitional zone to highlymobilized migmatite (right). Note the intensely foliated contact zone (arrowed) where the neosome constitutes approximately20-30% of the rock. See text for discussion. Lens cap for scale, (b) Lineation in migmatitic mica schist defined by lenticularaggregates of biotite, muscovite and sillimanite. (c) Dextral shear zone (looking towards the NE) where the shear sense(arrowed) can be determined from the asymmetrical drag of calc-silicate xenoliths. Note the lack of fabric in the migmatiticmatrix parallel to the shear zone. Hammer for scale, 40 cm. (d) Shear bands (looking towards the north) parallel to melt-richzones, with asymmetrical inclusions and boudins of solid material. The asymmetry indicates a dominantly sinistral (arrowed)sense of shear. Hammer for scale, 40 cm.




Intensely banded, E-W striking, zones with shearplanes (C-planes) which are almost indistinguishablefrom planes with preferred shape-fabric orientation(S-planes) are developed in wide areas in the meta-texite, particularly in quartz-rich metapsammiteswhere little neosome has formed because of thecomposition (Fig. 5d). A variety of shear senseindicators are frequently present in the zones whichcontained abundant solid material during the syn-kinematic high-grade metamorphism (Fig. 5d), andindicate a dominantly E-W sinistral sense of shear(Nielsen & Andersen, 1990). Locally, however, zonesof dextral displacement can be observed (Fig. 5 c).

The reduction in the shear strength of a rock atincreasing melt-solid ratio suggests that under thesame deviatoric stress field, displacements with asimilar polarity and at higher strain rates would alsobe accommodated in nearly completely melted rockswith lower viscosity. In the model for the emplacementof the Vardafjellet Gabbro, the diatexites along theroof of the gabbro are thought to have constituted alayer with low shear strength in which considerabledisplacement was accommodated. The deformation,however, is not easily recognized in the diatexitesbecause of the lack of synkinematic strain markers.

The southern contact of the Vardafjellet Gabbro onBomlo (Fig. 2) represents the original floor of thepluton. The contact is inverted, and dips steeplytowards the south and southwest. A fine- to medium-grained marginal facies, usually less than 100 m wide,is developed in the gabbro. The gabbro makes contactwith igneous rocks, mainly of basic composition,belonging to the Lykling ophiolite, the Geitung islandarc and the Siggjo Group (Nordas et al. 1985). Thedevelopment of partial melts in these rocks wasinsignificant because of their composition. Locally,hornfelsed metasediments interlayered with volcanicsof the Geitung Unit (K. G. Amaliksen, unpub. Cand.Real, thesis, Univ. Bergen, 1983) occur in the aureole,and a thin basal conglomerate of the Siggjo Group ispreserved locally along the contact zone (Fig.--2).The~conglomerate comprises material derived from theophiolite and island arc lithologies. In its type section(J. Nordas, unpub. Cand. Real, thesis, Univ. Bergen,1985), the basal conglomerate and lavas of the SiggjoGroup are virtually undeformed. Adjacent to the floorof the Vardafjellet Gabbro, however, the pebbles arestrongly flattened and have an oblate shape (P. E.Nielsen, unpub. Cand. Scient. thesis, Univ. Oslo,1990). The plane of flattening is sub-parallel to thecontact of the pluton. Similarly, epidote-quartz-filledvesicles in the Siggjo volcanites are deformed anddefine oblate amygdales adjacent to the gabbro. Theincrease in the strain in the zone is ascribed todeformation during the initial phase of the emplace-ment of the gabbro, as outcrop scale fabrics in themetasediments and volcanics are overprinted by a

post-kinematic hornfels texture in which cumming-tonite is a characteristic mineral.

On Stord, the northeastern margin of the Vardaf-jellet Gabbro is preserved as an intrusive contact witha thin, usually < 5-m-wide, chilled margin along thecontact (Fig. 2). The chilled zone consists of micro-gabbro which has a transitional contact to themedium- to coarse-grained layered gabbro and is net-veined by leucocratic material. The contact dipstowards the N and NE (Fig. 2). Well preservedcumulate layering preserved in a number of localitiesalong the contact shows that the layering and thecontact to the envelope are inverted. The contactoriginally was the floor or a gently inclined wall of thepluton at the time when the density-graded cumulatesformed.

In spite of the inversion, the contact and thecumulate sequence adjacent to the contact are littledeformed. The wall rocks of the actual contact arehornfelses. A few tens of metres away from thecontact, however, fine-grained porphyritic meta-volcanites have an L > S fabric with an orientationsub-parallel to the contact, and a shallowly plungingstretching lineation with azimuth around 300° (Fig.2).

4.a.2. Deformation of the cumulate layering

The cumulates of the Vardafjellet Gabbro are wellpreserved in a number of areas, particularly on Bomloand along the northeastern margin on Stord, and theprimary mineralogy is locally unaltered. On Bomlo,the way-up in the cumulates, shown by structures suchas density grading and erosional features, is to thenorth. The strike of the layering changes graduallyfrom around 100° in the west to c. 165° nearStokksundet (Fig. 2). The layering is subvertical orsteeply inverted. At the scale of outcrop, structureswhich can be related to early, pre-solidus instabilityand deformation of the cumulate sequence arecommon.

In the type locality at Vardafjellet on Bamlo (Fig.2), the cumulate sequence has a vertical thickness ofapproximately 750 m. The layered part of the gabbrogets thinner towards the west and thickens to the east.In most of the sequence, which comprises the leastdeformed parts of the pluton, structures characteristicof pre-solidus disturbance of the layering are common.These include erosional features, with local uncon-formities (Fig. 6 a), slump structures including isoclinalfolding of layering and early faults (Fig. 6e). Slumpstructures and erosional features have been describedfrom many anorogenic layered gabbros; in theVardafjellet Gabbro, however, these structures arecommon and occur at all levels in the layered sequence.The pre-solidus faulting was associated with brec-ciation, particularly of the darker, pyroxene-rich partsof the compositional layers. The early, pre-solidifica-




Figure 6. For legend see facing page.




tion origin can be identified in the field as the faultsand breccias have undeformed gabbro as 'fault-rock'matrix (Fig. 6e). A common observation at the scaleof outcrop in the type area is a mineral SPO fabric(Fig. 7a). The fabric, which is sub-parallel to parallelwith the layering, has a pronounced sub-horizontal,linear component and is defined by polygonizedaggregates of the pyroxene, olivine and plagioclase. Indetail, the igneous texture is replaced by a polygonizedgranoblastic texture which was formed by high-7',sub- or syn-solidus crystallization under stress (Fig.7 b, c). As there is now evidence for post-emplacementhigh-71 metamorphism in the area, this can only berelated to the emplacement of the gabbro. The igneousminerals are commonly replaced by serpentine, amphi-bole, chlorite and saussurite aggregates during staticretrograde metamorphism. In relation to several ofthe early faults described above, the SPO fabric hadformed already prior to the brecciation, as it occurswith a random orientation in rotated blocks in breccia(Fig. 6e), clearly demonstrating its pre-solidus origin.

Extensional veins (000°/90) of leucogabbro, arevery common in the type area (Fig. 6a, b). The veinsare usually < 3 cm wide, and can in many cases betraced several metres along strike. The veins have anorientation normal to the L > S fabric, and both thefabric and the extensional veins are most common inthe middle and upper part of the cumulate sequence.The E-W extension is also illustrated by boudinage,which most commonly affects layers with an originalpyroxenitic composition, but also plagioclase-richlayers (Fig. 6a, b).

Mobilization of leucocratic gabbro and disruptionof mafic layers are commonly observed, and an L > Sstructure defined by primary minerals in the gabbrois locally developed. The L-component has an E-Wsub-horizontal orientation, and is usually observed inthe field as a preferred orientation of dark mineralaggregates. The S-component is generally steep tosubvertical. In many areas this structure is parallel orsub-parallel to the layering; locally, however, it can beoblique. This indicates an origin where the earlycrystallized mineral phases acquired an orientationafter the partly crystallized matrix changed its mech-

anical properties from those of a near ideal viscousfluid with suspended solids, to a solid-like behaviourwhere stress was transmitted by interaction of solidgrains (Arzi, 1978).

On Stord, the strike of the layering changes fromaround 045° to become sub-parallel with the contactalong the NW-SE trending margin of the pluton (Fig.2). The folds defined by the deformed layeringapproach a reclined orientation, and face towards thenorthwest (Fig. 2). Like the L > S structure describedabove, the folding of the cumulates was initiatedbefore the gabbro had crystallized completely. Thecoast sections along Stokksundet on Stord (Fig. 2)provide spectacular examples of pre-solidus defor-mation of the cumulates in the Vardafjellet Gabbro.In Figure 6 c, highly deformed olivine gabbro cumu-lates, with primary mineralogy, are truncated bymobilized olivine gabbro. The pyroxene-rich cumu-lates (Fig. 6d) are commonly strongly disturbed andfolded in disharmonic folds. Locally, however, asystematic sinistral sense of shear (Fig. 2) can beobserved where the deformed cumulates, providingstrain markers, are deflected into zones of highershear strain (Fig. 60- The evidence of pre-solidusdeformation of the Vardafjellet Gabbro, can besummarized in the following six points:

(1) Cumulates have been mobilized and form dykesand veins, some of which intrude parallel to the trendof the axial surface of folds near the northeasternmargin on Stord.

(2) Primary minerals, best shown by dark mineralsin a leucogabbro (Fig. 7 a), have locally acquired apreferred orientation which may be oblique to theigneous layering.

(3) Slumping and disruption of the layering iscommon, particularly in the central and eastern partsof the pluton (Fig. 6c, d, f)-

(4) Rootless veins of gabbro representing mobilizedremnant magma intrude already folded and shearedcumulates (Fig. 6 c, e). Intermediate to trondhjemiticveins and dykes which may represent late differentiatescut the already deformed layering.

(5) In areas not affected by later retrograderecrystallization, the formation of the early structures

Figure 6. Vardafjellet Gabbro. (a) Banded cumulates (looking to the west) at type locality near Vardafjellet. Note localunconformable contacts and the N-S trending vertical extensional plagioclase-rich veins. The veins are normal to the layeringand particularly common in the plagioclase-rich cumulate layers. Hammer for scale, 30 cm. (b) Detail of extensionalplagioclase veins in the deformed plagioclase-rich layers, (c) Deformed olivine gabbro with dark, highly deformed pyroxene-rich cumulates. Note the truncation and drag of the deformed layers by a more isotropic olivine gabbro. This demonstratesthe pre-solidus origin for the deformation of the cumulates. The primary mineralogy at this locality, on a small island off thecoast of Stord in Stokksundet, is preserved. Pencil for scale, arrowed, is 15 cm. (d) Highly deformed pyroxene-rich cumulatesin olivine gabbro. Note the linear fabric parallel to the elongate, white, quartzitic xenolith. Later quartz-diorite veins with darkamphibolitic alteration rims in the gabbro, truncate the earlier deformed cumulates. Same locality as Figure 6 c. Note-bookfor scale, 17 cm long, (e) Angular fragments of foliated gabbro cumulates in a matrix of undeformed gabbro. Note that theinternal fabric in the fragments is sharply truncated by the undeformed matrix. See text for discussion. From type locality nearVardafjellet. Hammer-head for scale, 13 cm. (f) Sheared cumulates of olivine gabbro, Stokksundet, Stord. Note the sinistraloffsets of the dark, pyroxene-rich layer in the upper and lower part of the photograph.



218 T.B.ANDERSEN AND OTHERS

a

e ;?Figure 7. For legend see facing page.




was not associated with retrogression or alteration ofthe primary igneous minerals (Fig. 7 b, c).

(6) The geometry of the early structures in thecumulates (Fig. 6f) is consistent with the dominantlysinistral shear that affected the aureole and the marginsof the pluton.

4.b. Stolmen GabbroThe Stolmen Gabbro and its envelope have beendescribed in detail by E. Rykkelid (unpub. Cand.Scient. thesis, Univ. Bergen, 1987). The presentdescription summarizes some of the results from thatstudy.

The Stolmen Gabbro intruded a highly dis-membered ophiolite complex and metasediments ofunknown age in the Austevoll area (Fig. 8). Thestructural and metamorphic development of themigmatitic aureole shows that it was deformedcontemporaneously with the emplacement of thegabbro (Rykkelid, 1986). E. Rykkelid (unpub. Cand.Scient. thesis, Univ. Bergen, 1987) has calculatedshear strains up to 16 based on deformed pebbles in aconglomerate adjacent to the gabbro, and argued foran average shear strain > 10 in the migmatites. It is,however, difficult to quantify the shear strains thatwere associated with the emplacement of the gabbro v.the strains that post-date the crystallization of theStolmen Gabbro. Common observations of syn-migmatization kinematic indicators in the metatexitesand a penetrative stretching lineation plunging at 20°to 30° towards 240-270° document an E-W sinistralsense of shear along the margins of the gabbro (Fig.8). The structures in the migmatites are very similar tothose described above from the Bremnes MigmatiteComplex. E. Rykkelid (unpub. Cand. Scient. thesis,Univ. Bergen, 1987), Salna & Andersen (1988) andH. Solna (unpub. Cand. Scient. thesis, Univ. Oslo,1989) have shown that the displacement on the shearzone continued with the same polarity during theemplacement of the Reksteren Granodiorite.

The internal structure of the Stolmen Gabbro issignificantly different to that described above from theVardafjellet Gabbro. The layered part of the StolmenGabbro is dominated by originally horizontal sills orsheet intrusions, of which the thicker units (max.200 m) may show internal cumulate layering. Anormal sill thickness is approximately 1.5 to 3 m, andthe total maximum thickness of the Stolmen Gabbrois approximately 2 km. The gabbro sheets and sills areseparated by thin zones of leucodiorite and granite

(Fig. 9). The present orientation of the sills and sheetsis generally E-W with steep to vertical dips (Fig. 8).The evidence for co-existence of dioritic to graniticand mafic magmas is abundant, and gravity-inducedflame and load structures occur repeatedly at the baseof the gabbro sills (Fig. 9). The mafic rocks have fine-grained chilled margins towards the leucodiorites andgranites (Fig. 9). The load structures and the density-graded cumulates show consistent way-up to thesouth, indicating that the wall-rocks in the Mogsterarea originally formed the floor of the gabbro (Fig. 9).The roof is marked by a zone of penetrativedeformation which is exposed only on the southern-most headlands of Selbjorn (Figs 1, 8). Theformation of an originally near-horizontal sheetedcomplex shows that vertical extension affected thearea during the emplacement of the Stolmen Gabbro,and a model for this will be discussed below.

4.c. The Reksteren GranodioriteThe Reksteren Granodiorite (Figs 1, 8) is an approxi-mately 30 km long, elongate pluton, where thenorthern margin is truncated by a late, south-dippingextensional fault (Andersen, 1989; H. Solna, unpub.Cand. Scient. thesis, Univ. Oslo, 1989), previouslyinterpreted as a thrust (Andersen & Jansen, 1987).Along the southern margin, which makes contact withgabbros of Unit 1, the original intrusive relationshipsare preserved.

Internally, the Reksteren Granodiorite is charac-terized by a vertical to steeply inclined compositionalbanding which is parallel to the long axis of thepluton, and the common occurrence of late, idio-morphic, K-feldspar megacrysts (Andersen & Jansen,1987). The banding has many similarities with the'regular' banding in the Main Donegal Granite(Pitcher & Berger, 1972). The origin of the banding isascribed to deformation at the crystal mush stage(Andersen & Jansen, 1987; H. Selna, unpub. Cand.Scient. thesis, Univ. Oslo, 1989), an interpretationwhich is broadly similar to that for the banding in theMain Donegal Granite (Berger, 1971; Hutton, 1982).An early L > S mineral fabric, parallel with thesteeply inclined banding, defined by SPO of primaryminerals in the granitoid, is locally preserved. Theearly fabric is, however, usually overprinted by aconcordant secondary fabric, which makes it difficultto interpret the kinematics related to the formation ofthe early fabric. The formation of the secondary fabricwas accompanied by penetrative sericitization/

Figure 7. Vardafjellet Gabbro. (a) Detail of shape-preferred orientation (SPO) fabric defined by the primary minerals in thegabbro. Ball-pen for scale is parallel to the fabric, (b) Foliation with granoblastic polygonal texture defined by bands ofplagioclase and clinopyroxene. Note that the foliation is defined by SPO fabric of the mineral aggregates. Plane-polarised light;from type locality near Vardafjellet. (c) Granoblastic polygonal texture in banded olivine gabbro. Olivine crystal is arrowed.Crossed nicols; from type-locality near Vardafjellet.




SIMPLIFIED GEOLOGICAL MAP,REKSTEREN - AUSTEVOLL,

SUNNHORDLAND, W.NORWAYLegend: Sunnhordland Batholith:

Unit 1 Unit 2 Unit 3

Gabbro Grano-diorite

Granite

Envelope:

Undillerentiated

= ..REKSTEREN

_r- _ i - j~ _r* Melt-lubricated shear zones

• * - Stretching lineation

^ ^ Sense ol shear

- $ - Dilational dykes

—y— Way-up in gabbro

Eq.area plot ol poles to extended

and lolded • dykes, SW. Reksteren. \ s t r e tching lineation

Intersection oldilational dykes

Figure 8. Simplified geological map of the Reksteren-Austevoll area. Stereogram (equal area) shows poles to late,folded and extended dykes in the contact zone of theReksteren Granodiorite on south Reksteren. See text fordiscussion of the stereogram.

saussuritization and the development of a S-Cfoliation, the asymmetry of which is consistent withan E-W sinistral shear sense (Fig. 8).

4.c.l. Deformation in the contact zone along the S-margin

The S-margin of the Reksteren Granodiorite ischaracterized by a large number of variably mega-crystic granitoid sheets interleaved with the amphi-bolitized gabbro of the country rock. Where the sheetscoalesce, they form the main body of the ReksterenGranodiorite pluton (Andersen & Jansen, 1987). Thecontact zone represents a high-strain zone, whereseveral sub-parallel, anastomosing shear zones withvariable shear strains and relative age occur. Thefoliation in the contact zone strikes E-W and isgenerally steeply inclined with dips to the north.

The evidence for syn-intrusive deformation at thescale of outcrop in the contact zone is of severalcategories. A very large number of granitoid veins anddykes were emplaced into the contact zone at variousstages in its displacement history. This provides near

ideal examples of progressive deformation where thegranitoids are successive time markers in the strainhistory (Fig. 10 a). The relationships can be studied onthe wave-washed coast exposures in a number oflocalities.

A majority of the dykes were initially intruded at ahigh angle to the sub-horizontal E-W stretchinglineation and formed dilational dykes in the com-pressional field of the incremental strain ellipsoid(Ramsay, 1967). Evidence of early shortening of thedykes may locally be preserved within the older dykes,and is very commonly observed in relation to theyoungest and least deformed dykes (see also fig. 4, p.169 in Andersen & Jansen, 1987). During progressiverotational deformation the dykes that already wereintruded underwent an anticlockwise rotation into thefield of finite and infinitesimal extension (Ramsay,1967). Examples of different stages of the progressiverotational deformation of granitoid dykes are shownin Figure 10. The youngest dykes are leucocratic andgenerally comprise composite pegmatite-aplites. In anattempt to identify the general shape of the minimumlate, finite strain ellipsoid affecting the rocks duringthe emplacement of the late dykes, the methoddescribed by Talbot (1970) was applied. The orien-tations of 23 folded, 19 boudinaged and 8 youngdilational dykes were measured on south Reksteren.The data were plotted in an equal-area stereographicprojection and the fields of extension and compressionwere defined (Talbot, 1970). The distribution of thepoles to folded and stretched dykes (Fig. 8) defines astrain ellipsoid with X/Y(a) « 1.3 and Y/Z (b) « 1.8.The intersection of the late dilational dykes is close tothe y-axis of the strain ellipsoid, and measurements ofthe stretching lineation coincide with the J-axis (Fig.8). The observed consistent anticlockwise rotationalstrain indicates that the strain in the zone was chieflyrelated to sinistral simple shear. The oblate shape ofthe strain ellipsoid (k < 1), however, indicates that thefinite strain was a result of simple and pure shear. Inaddition, the ingress of an unquantified volume ofgranitoid material into the zone from which the strainwas studied, implies a positive volume change SV =(Kj+ Vo)/Vo, suggesting that the component of pureshear was significant during the terminal stages ofstrain history in the contact zone.

H. Solna (unpub. Cand. Scient. thesis, Univ. Oslo,1989) documents in detail a large number of kinematicindicators in the S-contact zone of the ReksterenGranodiorite, and a representative selection of asym-metrical structures is shown in Figure 11. Thesegenerally indicate an E-W sinistral sense of shearthroughout the strain history, and include anti-clockwise rotation of early dykes (Fig. 11 a), asym-metrical boudinage of dykes rotated into the field ofextension (Fig. 10a, b; Fig. lla, b,d), consistentobservations of sinistral displacement on local zonesof high shear strains, fold vergence and asymmetrical




SCHEMATIC MODEL FOR THE SYN-DEFORMATIONALEMPLACEMENT OF THE STOLMEN GABBRO,

AND ITS SUB-HORIZONTAL LAYERING

DIFFERENTIALSTRESS

SCALE:1 km

APART ON RELEASING-BENDMETATEXITES

SHEAR-BANDS AND OTHERSHEAR-SENSE INDICATORS INTHE MIGMATITIC AUREOLE

MODEL FOR HORIZONTAL GABBROSHEETS. DIAMETER 2 M

LOAD \STRUCTURES

GRANITICMATERIAL

GABBROW/CHILLEDMARGINS

Figure 9. A restored vertical model (modified from the unpublished Cand. Scient. thesis of E. Rykellid, Univ. Bergen, 1987)for the emplacement and formation of the horizontal layering in the Stolmen Gabbro. A stress-% melt diagram, redrawn fromVan der Molen & Paterson (1979), illustrates that diatexites with melt contents above the critical melt fraction (CMF) in theproximal parts of the aureole will sustain smaller differential stresses than the more distal parts of the aureole. See text forfurther discussion of the model.

tails on rotated K-feldspar megacrysts (Fig. 11 c). Thetail asymmetry, however, is not completely consistent,as evidence of both sinistral and dextral shear may bepresent in the same locality. This suggests that thedeformation, already at an early stage in the strainhistory affecting porphyritic granodiorite sheets whichare cut by the later dykes (Fig. 11 e), took place in ashear zone where N-S shortening occurred normal tothe shear direction.

As mentioned above, the deformation continuedafter complete crystallization of the Reksteren Gra-

nodiorite. This produced a secondary fabric, associ-ated with saussuritization and sericitization offeldspar. The S-C relationships developed during thesolid-state deformation also indicate an E-W sinistralsense of shear (Fig. 10c). Late sinistral shears alsoaffected the metasedimentary rocks in the Austevollarea.




C.Figure 10. Southern contact zone of the Reksteren Grano-diorite. (a) A shear zone (looking to the north) along thesouthern margin of the Reksteren Granodiorite on theReksteren. Note that successive, syn-tectonic, granitic dykes,originally intruded at a high angle to the stretching direction,have been folded, rotated and progressively sheared in asinistral, anticlockwise direction according to their relativeage with respect to the deformation in the sinistral shearzone. A lens cap for scale is arrowed, (b) Strong shear fabric(looking to the north) cut by a syn-tectonic granitic dykewhich has been rotated anticlockwise into the field ofextension. Note the extensional quartz-filled veins in thedyke. Locality south Reksteren. Lens cap for scale isarrowed, (c) Detail of the strong fabric (looking to thenorth) in a sinistral shear zone on south Reksteren. Note thelate sinistral shear band which offsets the granodioriticbanding. Ball-pen for scale is 5 cm long.

5. Emplacement models for syn-tectonic plutons inthe Sunnhordland Batholith5.a. The Vardafjellet Gabbro

Above, evidence of syn-magmatic deformation bothinternally, along the margins and in the aureole of theVardafjellet Gabbro have been presented. A recon-struction of the syn-tectonic emplacement model forthe Vardafjellet Gabbro is presented in Figure 12. Itwill be noticed from the model (Fig. 12) that therestored E-W vertical section closely corresponds tothe present map of the pluton and its envelope. Thedifferences in the orientation of the bedding in theSiggjo and Kattnakken groups and the originalhorizontal cumulate layering in the pluton, show thatthe youngest rocks in the envelope already were tiltedwhen the gabbro intruded. In the model, the Vardaf-jellet Gabbro originally formed a sheet-like bodyintruded at a relatively high level ( < 4 kb) in the crust.The volcanic and sedimentary rocks of the Siggjo andKattnakken groups were tilted, and the cumulates inthe gabbro had started to form. Before the gabbro hadcrystallized completely, it was penetrated by an E-dipping reverse shear zone. The eastern part of thegabbro was transported westward onto the lessdeformed western part. The cumulates were stronglydeformed in the central and eastern parts of thepluton presently exposed on Stord and the coastalareas along Stokksundet, where the displacement wasaccommodated. At this stage, the partly crystallizedcumulates were strongly deformed and remnantmagma was mobilized, and the 'chaotic zone' wasformed (Fig. 12). The deformed cumulates providequalitative strain markers, and the preservation of aprimary igneous mineral assemblage shows that thedeformation took place before the gabbro had reachedits solidus. Deformation was also concentrated in thepartially melted metasedimentary rocks in the roof ofthe gabbro presently forming the Bremnes MigmatiteComplex. Observations of kinematic indicators fromboth the interior of the gabbro and in the migmatitesof the envelope are generally consistent with a sinistralsense of shear. Local occurrences of dextral shearzones in the Bremnes Migmatite Complex andVardafjellet Gabbro, as well as the development ofoblate pebbles and vesicles in the Siggjo Group onBemlo (see above) indicate that a component ofcontemporaneous vertical flattening affected the foot-wall of the shear zone.

The regional extent of the shear zone to which thedeformation of the Vardafjellet Gabbro can be relatedis not known, as the northwestward continuation istruncated by the late Rolvsnes Granodiorite, and inthe southeast the shear zone is cut by the late NE-SWtrending Sunnhordland Fault (Fig. 1).




Figure 11. For legend see page 224.

GEO 128




S.b. The Stolmen Gabbro

The Stolmen Gabbro comprises an originally sub-horizontal sheeted intrusion, which indicates that itwas formed in an area which underwent verticalextension. In the model, the vertical extension oc-curred by pull-apart on a flat releasing-bend segmentof an inclined reverse shear zone which also acted asa conduit for the mafic magma (Fig. 9). The magmaticpressure, together with the pull-apart geometry of theshear zone, opened the space in which the sheeted sillcomplex crystallized. With analogy to pull-apartbasins located along major wrench fault systems(Crowell, 1974), releasing-bend geometries have alsobeen envisaged to have created the space for magmasof granitoid composition in the South AmoricanShear Zone (Guineberteau, Bouchez & Vigneresse,1987) and for the Strontian Granite located along theGreat Glen Fault of Scotland (Hutton, 1988 a). In thepresent case, however, the sheeted sill complex formingthe Stolmen Gabbro indicates that the extension wasvertical. The syn-emplacement rotational strains witha sinistral sense of shear in the aureole at both the baseand the top of the gabbro are consistent with aW-dipping reverse shear zone.

5.c. The Reksteren Granodiorite

Consanguinity of deformation and emplacement ofthe Reksteren Granodiorite is documented from boththe internal structures in the pluton and the relation-ships between intrusion and deformation along thesouthern contact zone of the pluton. The pluton wasemplaced in a zone which underwent sinistral shear(Fig. 8). The shear zone was part of a relatively long-lived system of shear planes which also controlled theshape and deformation pattern of the earlier StolmenGabbro and its envelope. The deformation affectingthe partly crystallized magma resulted in segregationof potassium-rich fluids from biotite-plagioclase-richdomains and the regular, steeply inclined banding wasformed. The remnant magma crystallized as layersand lenses characterized by a higher content of K-feldspar. The idiomorphic megacrysts may locally beup to 15 cm long.

Details of the deformation in the marginal zone ofthe pluton have been outlined above. The sinistral

syn-emplacement rotational strains were accompaniedby N-S flattening. It is suggested that the regionalshear stress was accompanied by a ballooning effect(Ramsay, 1989) caused by the magma pressure fromthe late phases of the intruding granodiorite. The lateN-S shortening in the marginal zone is indicated byboth sinistral and dextral asymmetrical tails arounddeformed feldspar megacrysts and conjugate minorshear zones. The flattening is also indicated by theX—Y and Y—Z ratio of the late finite strain ellipsoiddefined by the deformation of the youngest granitoiddykes in the zone. The late, solid-state S-C texturedeveloped under retrograde metamorphism (Fig. 10 c)shows that the regional stress field resulted in limitedand localized sinistral rotational strains in the areaafter the Reksteren Granodiorite had crystallized.

An unknown factor in the development of thesystem of sinistral shear zones affecting the magmaticarc in the Austevoll-Reksteren area is the orientationof the shear zones during the emplacement of theReksteren Granodiorite. There are no availableindicators of the palaeo-horizontal during the em-placement of this pluton. Hence, it is not known whenthe steep dip of the southward younging internallayering in the Stolmen Gabbro was established, andwhether the rotation of the layering in the StolmenGabbro occurred prior to, during or after theemplacement of the Reksteren Granodiorite.

6. Conclusions

The studies in the Sunnhordland Batholith show thatthe early deformation related to the emplacement ofplutons in this complex occurred in relation to shearzones. The displacement on these shear zones wascontrolled by the increased ductility which accom-panied the introduction of magmas and partial melts(Van der Molen & Paterson, 1979) in an activetectonic region. Releasing-bend geometries on thesyn-emplacement shear zones constitute a particularlyimportant mechanism by which space for intrudingmagmas in an area under influence of horizontalcompression and vertical extension may be created, asdemonstrated from the Stolmen Gabbro in Austevoll.The detailed mapping of gabbroic and granodioriteintrusions and their envelopes in the SunnhordlandBatholith shows that the shape of the plutons as well

Figure 11. Contact zone of the Reksteren granodiorite, south Reksteren. (a) Sinistral shear zone (looking to the north) withseveral syn-tectonic granitic dykes and veins. Note the asymmetrical structures in the shear fabric and the progressive rotationaldeformation that has affected the dykes and veins. Note also the late, folded, thin vein which still has an orientation in thefield of compression, but which is sinistrally offset on local shear bands parallel to the main fabric. Lens cap for scale isarrowed, (b) Asymmetrical quartz-filled boudin neck and solid-state shear bands, both showing sinistral displacement (lookingto the north), (c) K-feldspar megacryst with asymmetrical tails indicating anticlockwise, sinistral shear (looking to the north),(d) Two examples of asymmetrical boudinage of granitic dykes on south Reksteren. The boudinage of both dykes is consistentwith a sinistral sense of shear (looking to the north). Lens cap for scale is arrowed, (e) Highly deformed bands with K-feldsparmegacrysts showing tail and augen geometry. Note that the tail asymmetry is difficult to interpret kinematically. The pencilfor scale, 14 cm, is arrowed.




PRE-SOLIDUS DEFORMATON GEOMETRY,VARDAFJELLET GABBROSUNNHORDLAND BATHOL1TH. W. NORWAY |SHEAR PLANE]

INTERNAL "CHAOTIC" ZONE,MAINLY NON-LAYERED GABBRO

1 km

A MAJOR SHEAR-2IN THE BREMNESMIGMATITE COMPLEX

MARGINAL ZONEOF THE GABBRO

BASAL CGLOF THE SIGGJO GP

NOT PRESERVEDBELOW LINE

Figure 12. A restored model for the pre-solidus deformation geometry of the Vardafjellet Gabbro. See text for discussion ofdetails.

as their internal structure can be attributed to syn-emplacement deformation. Early deformation incrystallizing magmas may be difficult to recognize andinterpret kinematically. Nevertheless, the deformationin the aureoles may provide additional evidence forhigh-T syn-emplacement deformation. Observationsof displacement in marginal parts of the ReksterenGranodiorite and in the aureole of the VardafjelletGabbro at Bremnes indicate that the strain in bothareas was chiefly the result of rotational deformation.However, observations of conjugate shears, and otherindications of flattening discussed above, suggest thatthe finite strains include a component of pure shear.The flattening is possibly an effect of the magmapressure during positive dilation (ballooning), whichresulted in shortening normal to the shear zones.

Increase in pore fluid pressures or introduction ofmagma or melts in the relatively brittle middle/uppercrust will have a profound effect on the rheology andare likely to enhance localized deformation in areassubjected to deviatoric stresses. In zones where theviscosity is significantly reduced, in relation tocrystallizing plutons and migmatitic aureoles, thestrain rate will increase accordingly, and such zoneswill form plastic instabilities (Ord & Hobbs, 1989) inan otherwise elastic crust.

Acknowledgement. Financial support for this study has beenprovided through grants (440.89/021, 440.90/007) from

the Norwegian Research Counsil for Science and theHumanities (NAVF). An unpublished field map of centralparts of Stord by H. Brekke and Chr. Stillman wasgenerously made available to the authors, and greatlyhelped the mapping of the Vardafjellet Gabbro on Stord.The first author would also express his thanks to his co-authors for all the good effort put into their theses from thearea. The manuscript has benefitted from critical reading byProf. A. Andresen, Prof. H. Furnes and Dr D. Roberts, andby a constructive review by Prof. Chr. Stillman. This studyis No. 121 in the Norwegian ILP-project.

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