1. metamorphic Geol., 1992, 10,

Transcription

1 1. metamorphic Geol., 1992, 10, Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes T. M. BOUNDY* AND D. M. FOUNTAIN Department of Geology and Geophysics, University of Wyoming, PO Box 3006, Laramie, Wyoming , USA H. AUSTRHEIM Mineralogisk-Ceologisk Museum, Sarsgate I, N-0562 Oslo 5, Norway A B S T RA C T Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsn~y show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (<2 m thick); (3) eclogite breccia consisting of <80% well-foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 f 50 C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet-rich and kyanite/zoisite-rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five-axis universal stage measurements, shows a strong b-axis maximum normal to foliation, and a c-axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [Ool] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high-grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsn~y Island, may be an orogenic-scale process in the lowermost crust of collisional orogens. Key words: eclogite; eclogite facies shear zones; granulite facies; lower crustal processes; shear zones. INTRODUCTION The Caledonides of western Scandinavia (Fig. 1) provide a window into the roots of a continent-continent collisional orogenic system that allows direct investigation of structural and lithological relationships that, in most other orogens, can only be explored by geophysical methods, particularly seismic reflection studies. An exceptionally deep exposure of lowermost Caledonian continental crust occurs on Holsn~y Island, part of the granulite facies anorthosite complex within the Bergen Arcs of western *Present address: Department of Geological Sciences, 1006 C.C. Little Building, University of Michigan, Ann Arbor, MI , USA. Norway (Fig. 2). Deep crustal rocks exposed on Holsn~y Island reveal a localized transition from the granulite facies to the eclogite facies along Caledonian shear zones (Austrheim & Griffin, 1985; Austrheim, 1987a). Austrheim (1987a) documented that the high-p eclogite facies rocks formed along the shear zones in response to fluid infiltration during deformation. Although the metamorphic processes involved in this transition have been explored in detail (Austrheim & Griffin, 1985; Austrheim, 1987a; Jamtveit et al., 1990), no detailed structural analysis of the eclogite facies shear zones has been undertaken to examine the nature of deep crustal deformational processes in collisional orogens. To gain a better understanding of the structural development and deformational mechanisms of the 127

2 im T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM Fig. 1. Regional tectonic map of the Scandinavian Caledonides and surrounding area (after Berthelsen & Marker, 1986). The map illustrates the location of the study area (Holsnay Island) in western Norway within the Caledonides. Densely stippled areas in the south and north are middle Proterozoic basement and the Kola suture belt, respectively. The Baltic shield (coarse stippled pattern) is divided by the Raahe-Ladoga megashear. eclogite facies shear zones, detailed geological mapping was undertaken in conjunction with structural and petrofabric analyses of the shear zones exposed on NW Holsn~y Island. The primary objectives of this study were: (1) description of the strain geometry and fabrics in the eclogite facies shear zones; (2) characterization of the kinematics of the shear zones; and (3) developing constraints for the deformation mechanisms that operated in the shear zones through use of petrofabric analysis. We will show that there was a sequential development of the major eclogite shear zones beginning with the formation of eclogite along fractures and culminating in development of shear zones that are rn thick, laterally continuous over several kilometres, subparallel, and which anastomose, terminate abruptly along strike, and have a strong northward-dipping foliation. A strong crystallographic preferred orientation (CPO) of omphacite within shear zone eclogites indicates deformation occurred by slip along [Ool] while the eclogites were deformed under high-p, deep crustal conditions. GEOLOGICAL AND TECTONIC FRAMEWORK Bergen Arcs, western Norway The Bergen Arcs of western Norway are situated within the mid-palaeozoic Caledonian orogen, which is charac- I EX PLANATIO N Granulite facies Anort hosite Complex Major and minor arcs Ulri kens gneiss Allocthonous basement 0... Bergsdalen Nappes kilometres 4. i Fig. 2. General geology of the Bergen Arcs, western Norway. The study area on NW Holsnay Island is situated within the granulite facies anorthosite complex between the Major and Minor Bergen Arcs. Modified from Kolderup & Kolderup (1940) and Fossen & Rykkeild (1990).

3 STRUCTURE OF ECLOCITE FACIES SHEAR ZONES 129 terized by a variety of thrust sheets displaced from west to east onto the Baltic Shield (Fig. 1; Roberts & Gee, 1985). One of the arcuate Caledonian nappes of the Bergen Arcs consists of a granulite facies anorthosite complex (Fig. 2). Interpreted to represent an exposed slice of lowermost continental crust, the Proterozoic granulite facies complex apparently originated as a layered mafic intrusion that ranged in composition from anorthosite to gabbroic anorthosite (Austrheim, 1987a). The anorthosites and gabbros were intruded by jotunites and members of the charnockite series, including the type mangerite (Kolderup & Kolderup, 1940; Griffin, 1972; Austrheim & Griffin, 1985). The meta-anorthosites contain distinctive mafic corona structures that formed by replacement of primary magmatic olivine by a series of pyroxenes that are mantled by garnet (Griffin, 1972). Local occurrences of spinel lherzolite and other ultramafic rocks appear spatially related to the metagabbro layers within the metaanorthosites (Korneliussen et al., 1990). The Bergen Arcs anorthosite complex records two main phases of deformation and metamorphism. Garnetgranulite facies metamorphism and deformation of the complex occurred at mid-crustal levels (c. 25 km depth) during the Proterozoic Grenvillian (Sveconorwegian) orogeny (Austrheim & Griffin, 1985; Cohen et al., 1988). The terrane presumably cooled isobarically over a period of Ma following the granulite facies metamorphic event (Jamtveit et al., 1990). The second phase of deformation and metamorphism occurred in response to continental collision and crustal thickening during the mid-palaeozoic Caledonian orogeny that brought the granulite facies rocks into high-p eclogite facies conditions (Austrheim & Griffin, 1985). However, not all the granulite facies rocks were converted to eclogite. Field evidence from detailed mapping (Austrheim, 1987a) shows that eclogite facies metamorphism was localized along shear zones where fluid infiltration was concurrent with metamorphism (Austrheim & Griffin, 1985; Austrheim, 1987a). The eclogite facies shear zones are approximately parallel to other Caledonian structural trends in the Bergen Arcs area, including lower grade shear zones. These lower grade shear zones, which are generally characterized by a hydrous amphibolite facies mineralogy, developed locally throughout the granulite facies anorthosite terrane during later Caledonian deformation and partially overprinted some of the eclogite facies rocks (Austrheim & Robins, 1981; Andersen et a/., 1991). Exhumation processes of the deep crustal rocks exposed within the Bergen Arcs granulite facies anorthosite complex are poorly understood. Although thrust faulting associated with tectonism during continent-continent collision almost certainly was important in transporting the deep rocks to higher levels (e.g. Austrheim & Griffin, 1985), extension of overthickened continental crust during the waning stages of the Caledonian orogeny may have been important in the final tectonic emplacement of the terrane, as postulated for the Western Gneiss Region directly north of the Bergen Arcs (SCguret et al., 1989; Andersen & Jamtveit, 1990). This hypothesis is supported by the orientation of stretching lineations and sense-ofshear data in the allochthonous gneisses immediately west of the granulite facies anorthosite complex (Kvale, 1960; Fossen & Rykkeild, 1990). Systematic regional structural analysis of lower grade shear zones in the area is necessary to test models of exhumation in the Bergen Arcs. Regional structural data (Fossen, 1988) and offshore gravity and magnetic surveys (summarized in Hurich & Kristoffersen, 1988) in the Bergen Arcs area suggest that the granulite facies anorthosite terrane may only extend to shallow crustal depths in its present tectonic position. General geology of NW Holsney Island On NW Holsncby Island the granulite facies rocks consist predominantly of metamorphosed anorthosite, gabbroic anorthosite and gabbroic layers that have been intruded by mafic mangerite and jotunite (Austrheim & M~rk, 1988). These granulite facies rocks have been locally converted to eclogite facies assemblages. Previous work on NW Holsncby Island has concentrated mainly on metamorphic processes associated with this localized Caledonian eclogitization (Austrheim & Griffin, 1985; Austrheim, 1987a; Austrheim & Mcbrk, 1988; Jamtveit et al., 1990), which affected an area on NW Holsncby Island of more than 50 km2 (Austrheim & Mqirk, 1988). The areal extent of Caledonian eclogitization of the granulites is estimated to be 30-40% (Austrheim & Merk, 1988). In an initial investigation, Austrheim & Griffin (1985) concluded that there is a strong relationship between Caledonian deformation and the formation of eclogite from the granulites. Granulite facies rocks, apparently unaffected by this deformation, were metastable under eclogite facies conditions on a scale of metres to kilometres (Austrheim & Griffin, 1985; Austrheim, 1987a). Eclogite formation involved the hydration of the granulites and is interpreted to be related to extensive fluid infiltration along deep Caledonian shear zones (Austrheim & Griffin, 1985; Austrheim, 1987a). The occurrence of hydrous eclogite facies minerals along veins and within the eclogites supports this interpretation (Austrheim, 1987a). Fluid inclusion data (Andersen et al., 1990) suggest that the eclogites formed in equilibrium with a predominantly N,-rich fluid. However, evidence from phase equilibria (Jamtveit et al., 1990) coupled with the occurrence of hydrous minerals in equilibrium with the eclogite facies assemblage (Austrheim, 1987a) indicate strongly that the eclogite formed in equilibrium with an H,O-rich fluid (XHZO> 0.75; Jamtveit et al., 1990). Although Jamtveit et al. (1990) speculated that the fluid originated from underthrust sediments, there is no evidence thus far to document the source of this fluid. Detailed geochronological investigations by Cohen et al. (1988) support relative timing relationships deduced by cross-cutting relations in the field (Austrheim & Griffin, 1985). Analysis of Sm-Nd and Rb-Sr isotope systematics from whole rock and mineral separates date the granulite facies event at 907 f 9 Ma, which was followed by the Caledonian eclogite facies event at 421 f 68 Ma (Cohen et

4 130 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHElM af., 1988). The rocks record no distinguishable evidence of the c. 500Ma history between these orogenic events. Timing of the anorthosite intrusion is not well constrained. Geobarometric and geothermometric analysis suggests that emplacement occurred at MPa and 1200 "C. Intrusion of the complex was followed by granulite facies metamorphism at 1000 MPa and 850 "C (Austrheim & Griffin, 1985). Pressure-temperature (P-T) estimates based on mineral composition and phase equilibria for eclogite facies conditions outside the major shear zones (shear zones > 30 m thick) range from 1600 to 2100 MPa and 700 to 750 C (Austrheim & Griffin, 1985; Jamtveit et af., 1990). The temperature estimate is corroborated by equilibrium temperatures based on 6l80 fractionation within the eclogite facies minerals (van Wyck et af., 1990). Although no major structural analysis of the major eclogite facies shear zones has been conducted, limited, but significant, structural data have been collected on NW HolsnGy Island (Austrheim & Griffin, 1985; Austrheim, 1987a; Austrheim & Mark, 1988). The granulite facies foliation, defined by mafic layers and flattened coronas within the meta-anorthosites, is generally oblique to the eclogite facies foliation. The granulite foliation was apparently dragged into and transposed by minor eclogite facies shear zones. The strong foliation of the eclogite, defined by alignment of omphacite, characterizes the shear zones at all scales. Austrheim & M ~rk (1988) recognized four structural settings of the eclogites: (1) eclogite around fractures (millimetre to 0.5 m thick zones); (2) eclogite in the vicinity of thin ( m thick) shear zones; (3) eclogitization breccia, where rotated blocks of granulite are supported in a well-foliated eclogite matrix; and (4) m thick zones of banded eclogite facies rock laterally continuous over distances > 1 km. Based on field-work in this study, this descriptive classification of eclogite structural types has been elaborated and refined. FIELD AND PETROLOGICAL RELATIONS, HOLSN0Y MAP AREA An area of 12 kmz that encompasses the Hundskjeften and Eldsfjell Mountains on NW Holsnay Island was mapped at a scale of 1 :SO00 using topographic maps as a base from which a generalized 1 : 20,000 geological map of the area was compiled (Fig. 3). Detailed maps of the lower Eldsfjell and Hundskjeften shear zones upon which some of the following discussion is based can be found in Boundy (1990). Rocks in the field area are generally well exposed. 5.05' I ,, Well-foliated eclogite facies rocks (>8O%) Eclogite "breccia" (80-40% eclogite w] Partially eclogitized. granulite facies anorthosite Amphibolite facies overprint 5.05' Fig : 20,000 geological map of the Hundskjeften-Eldsfjell area illustrating the distribution of the eclogite facies shear zones in the eclogite breccia and granulite facies anorthosites. Localized areas of amphibolite facies that overprint the eclogite facies shear zones are shown. For further description of the lithological units see text.

5 STRUCTURE OF ECLOCITE FACIES SHEAR ZONES 131 In addition to mapping the contacts between the various metamorphic/structural rock units (described below), key structural data (foliation, layering, and lineation orientation) were collected. A traverse of samples were collected along a north-south transect through the lower and upper Eldsfjell shear zones. A brief description of the samples is given in Table 1, where they are listed in order of their structural position from higher to lower level. The modal mineralogy in Table 1 was derived by standard point counting techniques (c. 500 points per sample). Table 2 summarizes the typical mineralogy of the granulite protoliths and corresponding eclogites for samples collected for this study. Chemical analyses of major elements from whole rock powders (Table 3), determined using inductivelycoupled plasma spectrometry and atomic absorption methods, illustrate the differences between the gabbroic anorthosite and gabbroic composition eclogites. The gabbroic anorthosite eclogites are typically distinguished by significantly higher A1 contents. Mineral compositions (Tables 4-7) were determined on Cameca CAMEBAX electron microprobes (wavelengthdispersive system) at the Mineralogisk-Geologisk Museum, Oslo, Norway, the Department of Geology and Geophysics, University of Wyoming, and the Department of Geological Sciences, University of Michigan, using natural and synthetic materials as standards. Garnet and clinopyroxene were analysed at the Mineralogisk- Geologisk Museum using Cameca s PAP program for data reduction, micas were analysed at the University of Wyoming using a ZAF correction routine and zoisites were analysed at the University of Michigan using the Cameca PAP correction routine. All samples were analysed at an accelerating voltage of 15 kv and a beam current of 10 na. The Hundskjeften-EldsfjeII area consists predominantly of mafic rocks that range in composition from anorthosite to gabbroic anorthosite, with minor layers of gabbroic rock. Although some intrusive relationships remain (see below), all the rocks in the area have been thoroughly metamorphosed under high-grade conditions. Despite the granulite and eclogite facies overprint, the igneous protoliths can be readily distinguished in the field because each protolith type has characteristic mineral assemblages (Tables 1 & 2) as well as characteristic whole rock chemistry (Table 3) and mineral composition (Tables 4-7). The eclogites exposed in the Hundskjeften-Eldsfjell area have been divided into four structural types based on field observations and we generally followed Austrheim & M~rk (1988): (1) eclogite formed along fractures; (2) minor eclogite shear zones; (3) anastomosing eclogite shear zones (eclogite breccia); and (4) major eclogite facies shear zones composed of well-foliated eclogite. The last two types constitute units large enough to be used as map units at the scale mapped, whereas the small dimension of the first two types prohibits their use as map units. In addition to these map units, a third major map unit, partially eclogitized granulite, was also recognized. The relative ages of the metamorphic rocks and associated structures can be distinguished by cross-cutting relations. Due to the gradational nature of the lithologies described here, the division of lithological units is scale-dependent. The map units and their distribution are illustrated on Fig. 3 and, in detail, in Boundy (1990). All these occurrences are described below beginning with the small-scale structural occurrences. Eclogite formed along fractures Areas within the predominantly granulite facies lithology contain systematic regularly spaced fracture systems that obliquely cross-cut the granulite foliation (Fig. 4). Commonly, a diffuse halo of eclogite resembling a metasomatic front (4-10 cm) occurs on either side of the fracture. These fractures are typically filled with a hydrous eclogite facies mineralogy of zoisite and phengitic muscovite along with minor euhedral omphacite (up to Fig. 4. Eclogite forms as a diffuse halo (dark area) along fractures that obliquely cross-cut the foliation in granulite facies anorthosite (lighter area). The fracture here is filled with kyanite, zoisite, and phengitic muscovite.

6 132 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM 4 cm in length), quartz, kyanite, and garnet. In some cases the granulite foliation is deflected into parallelism with these fractures, which indicates that they are properly interpreted as faults. Minor eclogite shear zones Shear zones composed of well-foliated eclogite (Fig. 5) form discrete zones ( m thick) that are laterally continuous over several metres within the partially eclogitized granulite unit (see below). The shear zones are discontinuous but can be traced along strike into areas characterized by weaker eclogite foliation that, in turn, terminate along fractures. In other cases, discrete eclogite shear zones terminate at intersections with other minor shear zones forming an anastomosing network. Locally, the minor shear zones cross-cut eclogite formed along fractures. The foliation in the shear zones is typically highly oblique to the orientation of the surrounding granulite foliation, which is deflected in the vicinity of and transposed by the shear zones. Offset of granulite foliation by the shear zones indicates relatively minor displacement, generally <1 m. Klaper (1990) conducted a detailed analysis of some of these minor shear zones and determined shear strains on the order of 4-5. Partially eclogitized granulite This unit consists predominantly of granulite facies rocks, typically anorthositic to gabbroic anorthositic in composition. Locally, metagabbro bodies (5-100 cm thick) cross-cut the meta-anorthosites and are interpreted as metagabbro dykes. The meta-anorthositic rocks are massive to well foliated with the foliation defined by mafic layers of garnet and pyroxene and by flattened and aligned mafic corona structures. Granulite facies gabbroic rocks tend to be massive and often lack a distinct foliation in comparison to the anorthositic granulites. There is no apparent lineation in the granulites. The granulite facies rocks show partial conversion to eclogite (up to 40% eclogite), but there are large areas over several hundred square metres where the granulite is preserved with no eclogite facies overprint. Eclogites within this unit are confined to veins, minor shear zones (<2 m thick), and diffuse areas with no apparent structural association. Granulite between the eclogite appears well preserved in outcrop and hand sample. Anorthositic granulites typically consist of plagioclase, diopside, and garnet f hypersthene f scapolite f green spinel f hornblende (Tables 1 & 2). The predominant gabbroic granulite mineralogy is antiperthitic feldspar, diopside, and garnet f orthopyroxene f Fe-Ti oxides (Tables 1 & 2). The granulites are generally coarse grained (1-3 mm) with equigranular and granoblastic textures. In thin section, the plagioclase contains zoisite needles and muscovite, and pyroxene has reacted around grain boundaries to undetermined fine-grained minerals, probably omphacite. Generally orthopyroxene shows the greatest extent of this breakdown. These textures are interpreted to result from initial reaction under eclogite facies conditions. Anastornosing shear zones (eclogite breccia) The eclogite breccia unit (Fig. 6) consists of granulite and eclogite facies rocks in roughly equal proportions. Austrheim & Mark (1988) referred to these rocks as eclogite breccia and this term is used here in a descriptive sense. The unit is characterized by anastomosing zones, typically m thick, of well-foliated eclogite (40-80%) that wrap around lens-shaped blocks of granulite that are angular, typically <5 m across. Eclogite formed along fractures in some of the granulite blocks. Foliation within the granulite facies rocks is randomly orientated, but Fig. 5. Minor eclogite facies shear zone (dark area) illustrating the strong foliation development. The granulite foliation deflected into the shear zone gives a left-lateral sense of shear displacement. The coin at the left edge is about 2 cm in diameter.

7 STRUCTURE OF ECLOGITE FACIES SHEAR ZONES 133 Table 1. Modal analyses of shear zone samples. No. Lithology Location Mineralogy BA-42 anorthositic granulite above UESZ 73.4 pl, 13.2 cpx, 4.8 bt, 0.2 op, 7.2 alt BA-2 gabbroic anorthositic eclogite BA-4 gabbroic anorthositic eclogite BA-5 gdbbroic anorthositic eclogite BA-12 anorthositic granulite/eclogite BA- I I gabbroic granulite/eclogite BA-9 gabbroic granulite/eclogite UESZ UESZ UESZ UESZ UESZ UESZ 61.0 omp, 18.4 grt, 11.6 ky, 1.2 qtz, 0.8 ms, 0.4 rt, 2.0 chl, 0.4 alt, 4.0 symp 72.2omp. 10.4qtz,6.4grt,2.0ms,0.2chl,7.6alt, 1.2symp 70.6omp,27.2grt. 1.6zo,0.4ms,0.2qtz 27.0grt,21.8cpx,12.8ms,9.4omp,5.4qtz,4.6zo,2.4amph,1.6ky,0.4pl,5.6chl, 9.0 symp 55.2cpx.28.2grt. 11.0chl. 1.6amph,0.4pl,3.4alt 77.0pl,8.4pl,2.4cpx,1.6grt,1.2zo,0.2qtz,0.4omp,0.2ehl,8.6alt BA-33 anorthositic granulite between ESZ 76.6 fsp, 17.8 grt, 3.0 cpx, 2.6 alt BA-20 anorthositic eclogite BA-39 anorthositic eclogite BA-21 anorthositic eclogite BA-40 anorthositic eclogite BA-41 anorthositic eclogite BA-22 gabbroic eclogite BA-13 anorthositic eclogite LESZ LESZ LESZ LESZ LESZ LESZ LESZ 37.0omp,15.6zo,13.6grt,9.0ms,7.8ky,7.0qtz,0.4rt,2.4chl,1.0alt,6.2symp 43.2 omp, 31.0 grt, 11.1 zo, 5.6 ms, 4.6 ky, 0.2 qtz, 0.3 amph, 0.1 bt, 1.4 alt, 2.5 symp 40.6 omp, 27.0 zo grt, 6.6 ky, 6.2 ms, 0.2 rt, 2.2 chl, 2.2 alt, 2.8 symp 39.3 omp, 26.9 zo, 10.1 grt, 7.3 ky, 6.0 ms, 1.1 amph, 0.1 chl, 2.2 alt, 7.3 symp 41.2 omp, 23.5 zo, 17.5 grt, 7.5 ky, 5.8 ms, 0.7 amph, 0.8 chl, 3.0 symp 51.4omp,40.7grt,3.8ms,1.7rt,0.2qtz,0.1zo,0.8amph,0.2alt,1.1symp 45.2 zo, 27.4 omp, 9.2 grt, 6.4 ms, 6.2 ky, 0.8 qtz, 0.6 bt, 2.2 chl, 2.6 symp BA-31 anorthositic granulite BA-32 anorthositic granulite below LESZ below LESZ 62.4 pl, 27.8 grt, 8.8 cpx, 1.0 alt 65.4 pl, 17.2 grt, cpx, 0.4 chl, 5.2 alt Abbreviations: alt, alteration; amph, amphibole; bt, biotite; chl, chlorite; cpx, clinopyroxene; ESZ, Eldsfjell shear zones; fsp, feldspar; grt, garnet; ky, kyanite; LESZ, lower Eldsfjell shear zone; ms, muscovite; omp, omphacite; op, Fe-Ti oxides; opx, orthopyroxene; pl, plagioclase, qtz, quartz; rt, rutile; symp, symplectite; UESZ, upper Eldsfjell shear zone; zo. zoisite. Table 2. Typical protolith-granulite-eclogite mineraloev. Protolith Granulite facies Eclogite facies gabbroic anorthosite PI+ grt + cpx f opx gabbro omp + grt + zo + ky + ms f amph f rt f qtz fsp+ grt + cpx f opxf op omp+ grt + ms + rt + qtz f cc See Table 1 for abbreviations except: ce, carbonate Table 3. Chemical composition of shear zone samples. No. SiO, TiO, A120, Fe,O, MnO MgO CaO Na,O K2O P*OS LO1 Total BA BA-02 BA-04 BA-05 BA- 12 BA-11 BA n.a. n.a. n.a n.a. n.a n.a. n.a. n.a. n.a. n.a n.a n.a. BA BA-20 BA-39 BA-21 BA-40 BA-41 BA-22 BA BA-31 BA Abbreviations: LOI, loss on ignition; n.a., not analysed. Determinations by inductively coupled plasma (ICP) spectrometry except K,O and Na,O by atomic absorption (AA) method

8 134 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM Fig. 6. Anastomosing zones of well-foliated eclogite (dark area) wrap around granulite facies blocks (light area) in the Hundskjeften area. Note that the orientation of foliation in these granulite blocks is oblique to the foliation in the surrounding eclogite. typically oblique to the strongly developed foliation in the eclogite matrix, suggesting that granulite blocks have been rotated with respect to each other. The more intensely strained areas have relatively more eclogite and the granulite blocks tend to be smaller and less angular than in the less strained areas (for details see Boundy, 1990). Blocks of granulite facies gabbroic rocks are restricted to areas of gabbroic eclogite. Angular blocks of pyroxenite developed by disintegration of pyroxenite layers within the banded parts of the anorthosite complex. Major eclogite shear zones (well-foliated eclogite) Anastomosing, subparallel zones m wide of well-foliated eclogite facies rocks (Fig. 7) constitute major shear zones that are subparallel to each other and are laterally continuous along strike over distances up to several kilometres (typically 1-3 km). At the map scale, the shear zones are discontinuous and terminate abruptly along strike. This unit comprises areas of >80% eclogite, contains few blocks of granulite facies rocks, and may have sharp well-defined margins. In some areas there is a transition from the major eclogite facies shear zones into the anastomosing shear zones with granulite blocks. Rod-shaped aggregates of omphacite and garnet, elongate relict corona structures, and scarce mineral lineations define a lineation within the shear zones. Schlieren of quartz and phengitic muscovite or distinct veins of quartz, omphacite and phengitic muscovite are locally common. Fig. 7. Cross-sectional view of the lower Eldsfjell shear zone looking approximately northeast showing the well-foliated character of the eclogite along the face of the cliff. The lighter areas above the shear zone are made up of anastomosing zones of eclogite facies rocks (dark) and blocks of granulite facies rocks (light). The cliff face is about 75 m high.

9 STRUCTURE OF ECLOCITE FACIES SHEAR ZONES 135 Petrography The modal mineralogy of eclogite facies rocks in this unit depends on the granulite protolith composition (Table 1). The predominant anorthositic eclogite facies mineral assemblage, estimated from modal mineralogy, consists of omphacite (Jd-,; Table 4), garnet (Pyr3XAlm4,)GrossZ,; Table 5), zoisite, kyanite along with minor Na-rich phengitic muscovite (Table 6) f rutile f quartz f amphibole. The gabbroic eclogites consist predominantly of omphacite (Jd,,; Table 5), along with minor phengitic muscovite (Table 6), rutile, quartz f carbonate. Commonly the omphacites contain inclusions of quartz. The eclogites are generally fine grained ( mm) and the omphacite, kyanite, and muscovite crystals define a strong grain-shape preferred orientation deformational fabric. All eclogite facies minerals analysed were compositionally unzoned. There is only minor variation in mineral compositions within a particular protolith type. Two generations of garnet can be distinguished in the eclogite facies rocks on the basis of textural evidence. The first-generation garnets (Pyr,,Alm,,Gross,, in anorthositic rocks; Table 5) are most abundant, are 1-3mm in diameter, with corroded grain boundaries indicating disequilibrium. Inclusions of granulite facies minerals, such as green spinel, within some of the large garnets supports the interpretation that the garnets formed during granulite facies metamorphism. Blue-green amphibole, omphacite and muscovite form along fractures in the relict garnets. The second-generation garnets (Pyr3xAlm40Gr~~~21 in Table 4. Average omphacite compositions. BA-20 BA-39 BA-21 BA-40 BA-41 BA-22 SiO, A TiO, FeO MgO MnO CaO Na,O Table 4), garnet (PyrZ6Alm4xGr~~~2h; Total Cations per 4 Si AllV AIV' Ti Fe Fe Mg Mn Ca Na cations and 6 oxygens I Jd Jd = jadeite component. anorthositic rocks; Table 5) have sharp, euhedral grain boundaries and are generally smaller ( mm in diameter). The texture of these garnets implies that they are in equilibrium with and formed during growth of the surrounding idioblastic omphacites during eclogite facies metamorphism. Table 5. Average garnet compositions. BA-20 BA-39 BA-39 BA-21 BA-40 BA-40 BA-41 BA-22 BA-22 (GI) (GI) ((32) (GI) (GI) ((32) (GI) (GI) (G2) SiOz A1203 TiO, FeO MgO MnO CaO Na@ Total Si AI Ti Fe Mg Mn Ca Na Total Cations per 12 oxygens A1 PY SP Gr Abbreviations: G1, granulite facies garnet; G2, eclogite facies garnet; Al, almandine component; Gr, grossular component; Py, pyrope component; Sp, spessartine component.

10 136 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM Table 6. Average muscovite compositions. BA-20 BA-39 BA-21 BA-40 BA-41 BA-22 SiO, A TiO, FeO MgO Na,O KZO Total Cations per 11 oxygens Si Al Ti Fe Mg Na K Si AllV Z AIV1 Ti Fe2+ Mg Y Total cations - (Na + K) = Na K A Fe/(Fe + ME) Relatively minor alteration of the eclogites along grain boundaries is characterized by the breakdown of omphacite to symplectite and of phengitic muscovite to biotite. Typically the gabbroic eclogites are better preserved than the anorthositic eclogites. Locally the eclogite facies mineralogy is overprinted by an amphibolite facies mineralogy represented by plagioclase, hornblende, biotite f epidote f quartz f muscovite f chlorite f carbonate. The amphibolite facies minerals are generally fine grained (<0.25 mm) with equilibrium textures, such as 120" dihedral angles, and typically display a strong deformational fabric. Metamorphic conditions Previous P-T estimates of NW HolsnGy Island metamorphic conditions were determined for eclogite samples collected outside the major shear zones (Austrheim & Griffin, 1985; Jamtveit et al., 1990). In this section P-T conditions for samples collected from within major eclogite facies shear zones are calculated to assess the metamorphic conditions during shear zone formation. The temperature conditions during eclogite formation in the major shear zones were determined for a gabbroic composition eclogite collected from a major shear zone (sample BA-22). The temperature was determined using the Fe-Mg exchange reaction between coexisting garnet and omphacite using the geothermometer of Krogh (1988), which is based on a reinterpretation of earlier experimental data by Ellis & Green (1979). Application of more Table 7. Representative zoisite compositions. BA-20 BA-39 BA-21 BA-40 BA-22 SiO, A TiO, Fe MgO MnO CaO SrO L%O Ca PrzO Nd203 S%O3 Total Number of cations normalized to 3 Si Si Al Fe Ti Mg 0.04 Mn Ca Sr REE 0.07 REE = total rare earth elements analysed. h (d a z v 2 3 v) v) 2 a Temperature (OC) Fig. 8. Pressure versus temperature diagram showing the range of temperatures estimated for gabbroic composition eclogite (sample BA-22; shaded area). Solid lines are jadeite isopleths (Gasparik & Lindsley, 1980) for Jd, and Jd,,, which approximate the jadeite compositions in the gabbroic and anorthositic eclogites, respectively. The P-T estimate for Holsn~y eclogites from Jamtveit et al. (1990; triangle) is shown for comparison.

11 STRUCTURE OF ECLOClTE FACIES SHEAR ZONES 137 recent experimental data on Fe-Mg exchange between garnet and clinopyroxene (e.g. Pattison & Newton, 1989) is inappropriate for the high-na clinopyroxenes in this study. At a pressure of 1500MPa the estimated temperature is 670 f 50 C (& (grt-cpx) range ; Fig. 8). A full tabulation of the microprobe data can be found in Boundy (1990). The results are consistent with the recent temperature estimates for eclogites outside the major shear zones based on heterogeneous phase equilibria (Jamtveit et al., 1990) and 6'"O equilibria between coexisting eclogite facies minerals (van Wyck et al., 1990). Although the ordered and disordered state of omphacite can be an important consideration in geobarometry (Holland, 1983), as a first approximation the experimental data of Gasparik & Lindsley (1980) and Holland (1980) are appropriate to the composition of omphacites coexisting with quartz in this study. An estimate of the minimum metamorphic pressure is given by the intersection of the Jd, isopleth (Gasparik & Lindsley, 1980) with the lower bound of the temperature estimate. The minimum pressure of about 1460 MPa determined in this manner (Fig. 8) corresponds to a depth of about 55 km in the continental crust, assuming an average crustal density of 2750 kg m-3. STRUCTURE OF MAJOR ECLOGITE FACIES SHEAR ZONE Three major eclogite shear zones were mapped on NW Holsnoy Island (Fig. 3): Hundskjeften shear zone (HSZ), lower Eldsfjell shear zone (LESZ), and upper Eldsfjell shear zone (UESZ). These are interpreted to represent zones of high shear strain because of their strong foliation and transposition of regional banding in the host granulite facies rocks that they cross-cut. We will discuss the general geometric features of the shear zones and then describe their structure, kinematic relationships and petrofabrics. Much of the detailed analysis focused on the LESZ because of its excellent exposure, and many of the observations are drawn from this example. The distribution and geometry of the eclogite facies shear zones in the Hundskjeften-Eldsfjell area on NW Holsnoy Island are illustrated in Fig. 3. The shear zones are continuous over distances of several kilometres and form an anastomosing pattern within the eclogite breccia. Their strike changes gradually from approximately E-W in the eastern part of the field area (near Eldsfjell) to about NW-SW in the western part (near Hundskjeften). On the map scale, the shear zones are discontinuous and terminate abruptly along strike. In detail the LESZ illustrates the general pattern of major eclogite facies shear zone termination; they bifurcate into multiple minor shear zones that decrease in width and apparent displacement towards the terminations. Minor shear zones bifurcate and each strand decreases in width toward the tip. Wellfoliated portions of the shear zones grade smoothly into fractures that have no visible offset and no eclogite mineralogy. The shear zones terminate at the fractures. The macroscopic structure of the major eclogite facies shear zones is exemplified by the LESZ (Fig. 9). The shear zone is laterally continuous for over 2.5 km and varies in thickness along strike, but it is typically m thick. Along its length, the shear zone bifurcates and anastomoses around lenses of relict granulite facies rocks (Figs 9 & 10). The zone is characterized by a strong foliation that has a general northward dip (Figs 7, 10 & 11). It is bounded by anastomosing 1-5-m-thick eclogite facies shear zones and blocks of granulite facies rocks along its length. The upper boundary of the zone is defined by a marked increase in mylonitic foliation intensity toward the top of the shear zone. The structural base of the shear zone is defined as a gradient through which the shear zone fabric loses intensity downward over a 10-m interval and is characterized by heterogeneous strain. The other shear zones have similar characteristics. The UESZ is characterized by a strongly mylonitic, sharply defined upper boundary. Conversion of granulite to eclogite is commonly incomplete (see samples BA-9, -11, and -12; Table 1) toward the gradational base of the shear zone. The zone is laterally continuous for a distance of >3 km. Thickness varies along strike and has a maximum of 125 m, although the apparent thickness is much greater on Fig. 3 because of topographic effects. The HSZ is laterally continuous for a distance of c1.5 km along strike and reaches a thickness of 100 m. All the shear zones have a general northward dip. The foliation within the shear zones is defined by alternating omphacite/garnet- and kyanite/zoisite-rich layers. It is subparallel to shear zone boundaries and shows a consistent orientation despite the anastomosing character (Figs 9 & 12). Porphyroclasts of relict granulite facies mineral aggregates within the shear zone typically show only minor rotation with respect to the foliation plane. There are minor folds (decimetre and centimetre wavelengths and amplitudes) of the foliation, but they are sparse. In thin section, the foliation is additionally defined by the shape-preferred orientation of omphacite (Fig. 13). Twin planes of kyanite and cleavage planes of mica tend to be parallel to the foliation in the anorthositic eclogites (Fig. 13). Porphyroclasts are symmetrical with respect to the foliation plane. Kinematic indicators The major eclogite shear zones lack unequivocal strain indicators. In general, porphyroclasts show ambiguous sense of shear and there is no discernible S-C fabric. Kinematic indicators in the shear zones are restricted to preferred alignment of linear elements, displacement along minor shear zones, and offset metagabbro dykes. Lineations Linear elements recognizable in the field are rod-shaped mineral aggregates, elongate relict corona structures, and scarce mineral lineations. The most significant of these is

12 138 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM &el,- fol oted ec oqtte (-80% ecloyitel el oqite breccia (80-40% eclog t.?) part olly ecloqit 7ed jranjlite et log te fd.01 on, laneation Fig. 9. Detailed map of the lower Eldsfjell shear zone (boxed area on Fig. 3). Eclogite foliation is shown within the shear zone and eclogite breccia. Lineations within the shear zone are defined by rod-shaded mineral aggregates that lie within the eclogite foliation plane. Sample locations are shown by dots with numbers. N20 W A Eldsfiell S 20 E A' Fig. 10. Schematic cross-section the lower Eldsfjell shear zone along A-A' (Fig. 9), looking approximately to the east. Topographic relief and projection of structures along strike allow good constraint of the data to a depth of about 225 m. The structural positions of samples from this portion of the shear zone are shown by sample numbers with arrows. For reference, the highest point in the section is the top of Eldsfjell Mountain (324 m).

13 STRUCTURE OF ECLOGITE FACIES SHEAR ZONES 139 Fig. 11. Strongly foliated eclogite within the lower Eldsfjell shear zone. The light-coloured area is a block of granulite facies rock within the shear zone eclogite (dark). A hammer (0.5 m) is shown for scale. defined by rod-shaped aggregates composed predominantly of omphacite and garnet that are surrounded by areas of predominantly zoisite and kyanite. These rods are ubiquitous throughout the shear zones, especially in the eclogites derived from gabbroic anorthosites. The symmetry axes of the rods are normal and parallel to the foliation plane. The orientations of the long axes are remarkably consistent, with some minor variation, over the length of the shear zones as illustrated by the data plotted for each shear zone in Fig. 12. Typically the axes lie in the foliation plane, have a gentle plunge, and trend NE or ENE. This preferred orientation may have resulted from rotation of the rods into parallelism with the slip direction of the shear zone as commonly observed for linear elements in mylonite zones. This interpretation is further supported by the observation that relict corona structures become elongate within the shear zones and are parallel to the preferred orientation of the rods. Minor shear zunes Discrete eclogite facies shear zones subparallel to and structurally beneath the LESZ reveal a consistent right-lateral separation. This is based on the offset N N.. LESZ UESZ HSZ 0 poles to foliation 0 lineations 0 poles to foliation 0 lineations 0 poles to foliation 0 lineations n= 229 n= 29 n=219 n= 36 n= 119 n= 3 Fig. 12. Lower hemisphere equal-area projections of poles to foliation (solid circles) and linear trends of long axes of rods (open squares) measured within the foliation plane for the major eclogite shear zones: (a) Lower Eldsfjell shear zone (LESZ); (b) Upper Eldsfjell shear zone (UESZ); (c) Hundskjeften shear zone (HSZ). The number of measurements (n) is indicated for each projection. Also shown is the average foliation plane (solid curve) for each shear zone.

14 140 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM Fig. 13. Photomicrograph of eclogite from the lower Eldsfjell shear zone (crossed polars). Gabbroic anorthositic eclogite (BA-39), where the foliation is defined by the shape-preferred orientation of omphacite and kyanite. Large relict garnets at the top of the photograph show corroded grain boundaries. The field of view is 4 mm across. (generally <0.5 m) of granulite foliation traced through the shear zones. The deflection of granulite foliation into the minor shear zones defines folds. The few hinge lines of these folds measured are approximately at right angles to the lineations (Fig. 12) within the adjacent LESZ. The slip vector could be perpendicular to the hinge line, and therefore parallel to the preferred orientation of the long axes of the rods, giving oblique right-lateral slip with the top side down to the NE. Antithetic left-lateral zones are sparse and are generally subparallel to right-lateral shear zones though several are orientated at about 45" to this strike, suggesting the minor shear zones could be a conjugate set. In comparison, minor eclogite shear zones are uncommon in the vicinity of the UESZ and HSZ. Offser of metagabbro dykes A similar analysis can be made based on the apparent offset of metagabbro dykes by the LESZ. One of the subparallel, steeply dipping granulite facies metagabbro dykes (approximately 10m thick) south of the LESZ can be traced into the shear zone where it is transposed into parallelism with the shear zone foliation and metamorphosed to an eclogite facies assemblage mineralogy. Assuming a slip line parallel to the preferred orientation of rods, we infer that the slip is oblique right lateral with the top side down to the NE. If the metagabbro bodies on either side of the shear zone were once continuous, the offset would be on the order of 250 m. Petrofabric analysis Universal stage measurements of the preferred orientation of omphacite crystallographic axes from anorthositic and gabbroic eclogite samples from the LESZ were made in an attempt to get a further constraint on the kinematic history (e.g. Nicolas & Poirier, 1976; Nicolas & Christensen, 1987) and deformation mechanisms of the shear zones. Samples BA-22, -39, -40, and -41 were selected for measurement and the mineralogy of these samples is given in Table 1. The original grain and interphase boundaries are partially obscured due to incomplete, post-eclogite facies alteration. In thin section the foliation of the gabbro eclogite (BA-22) is defined by grain-shape preferred orientation of omphacite, commonly with apparent aspect ratios of 3:l. In the gabbroic anorthosite eclogites (samples BA-39, -40 and -41) the foliation is more strongly developed and is additionally defined by compositional layering of omphacite/garnet and kyanite/zoisite (Fig. 13). Commonly, the omphacite shows a bimodal size distribution with (1-3 mm) inclusion-rich omphacites and smaller ( mm), relatively inclusion-free omphacites, which are associated with the more highly strained areas. Although the smaller omphacites may represent recrystallized omphacite grains, there is no difference in composition between the large and small grains and all the omphacites analysed are compositionally homogeneous (Jd,, for BA-22; Jd-, for BA-39, -40, -41). The larger omphacites display relatively minor undulatory extinction. Rare twins occur along (100). Methoh Orientations of crystallographic axes of omphacite were determined on a five-axis universal stage from two mutually orthogonal thin sections (one perpendicular to foliation and one perpendicular to foliation and parallel to lineation) for each sample. The optical directions of X, Y (= b-axis) and Z were measured directly and the c-axes were determined by the measurement of the {llo} cleavages and by plotting the intersection of the cleavages with the X-2 plane. One hundred b-axis measurements

15 STRUCTURE OF ECLOGITE FACIES SHEAR ZONES 141 were made for each sample (50 measurements for each thin section orientation). The number of c-axis determinations was limited because of either the lack of or the unfavourable orientation of distinct cleavages. Results The preferred orientation of omphacite crystallographic axes in all the eclogite samples analysed shows a consistent relationship with the macroscopic structure in the shear zones. The CPO patterns of the shear zone eclogites are illustrated on contour plots of omphacite crystallographic axes (Fig. 14). The diagrams for the anorthositic eclogites show strong omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane (Fig. 14). The apparent c-axis maxima within the girdles are approximately parallel to the long axes of rods within the shear zone (see Fig. 12). In contrast, a gabbroic eclogite (BA-22) shows a c-axis maximum within the foliation plane and a b-axis girdle perpendicular to the foliation plane. Previous petrofabric studies of omphacite in deformed crustal eclogites show patterns similar to those of the anorthositic eclogites from Holsn~y Island. Petrofabrics of relatively low-t, high-p eclogites from Cab0 Ortegal, NW Spain (Engels, 1972), Galicia, Spain (Baker & Carter, 1972), and foliated eclogite xenoliths from the Colorado Plateau (Helmstaedt et al., 1972) show b-axis concentrations normal to foliation with c-axis girdles parallel to the foliation. This pattern is similar to the S-type pattern of Helmstaedt et al. (1972). In some cases the c-axis orientations define a maximum in the girdle. This is a common pattern and occurs in foliated eclogites that have no apparent lineation. In contrast, heated eclogite xenoliths (Helmstaedt et al., 1972) and eclogites from different structural levels within the Scandinavian Caledonides (van Roermund, 1983) display c-axis maxima within the foliation plane, parallel to the lineation direction and resemble the gabbroic eclogite (BA-22) petrofabric pattern. This second pattern, the L-type of Helmstaedt et al. (1972), occurs in eclogites with well-defined lineations. Similar patterns have been recognized for clinopyroxenes in high-p garnet lherzolites from Alpe Arami, Switzerland (Mockel, 1969). The Naustdal eclogite from western Norway (Binns, 1967) appears to be transitional between these two patterns. In addition, Helmstaedt et al. (1972) presented petrofabrics from crustal eclogite xenolith samples that showed no preferred orientation of omphacite crystallographic axes. Interpretation of the petrofabric patterns is difficult because of the lack of experimental work on omphacite, but deformational experiments on diopside may be used to interpret the significance of the omphacite patterns. At low temperatures, diopside typically deforms by twinning on (OOl)[OOl] (Raleigh & Talbot, 1967; Kirby & Christie, 1977; KollC & Blacic, 1982, 1983; Kirby & Kronenberg, 1984). More importantly, glide occurs along the multiple slip systems (100)[001] (Griggs et al., 1960; AvC Lallemant, 1978; Kirby & Kronenberg, 1984) and also along {110}[001] and {110}(110) (Boland & Tullis, 1986). TEM observations (van Roermund & Boland, 1981) confirm that these slip systems occur in naturally deformed omphacite and indicate that (100)[001] and { 110}[001] dominate over { 110) ( 110). These observations were made on a Caledonide eclogite for which van Roermund (1983) determined a fabric similar to the L-type of Helmstaedt et al. (1972). Laboratory deformation of diopside aggregates (Carter et al., 1972) produced a preferred orientation of b-axes parallel to u, with a girdle of the c- and a-axes in the u2 = a, plane. Recognition that the dominant glide direction in diopside is parallel to the c-axis coupled with the creation of a CPO pattern in the experiments of Carter et al. (1972) indicates that preferred crystallographic orientation of omphacite characterized by a c-axis maximum should develop in eclogites (van Roermund & Boland, 1981; van Roermund, 1983). Omphacite from the LESZ eclogites show such patterns (Fig. 14), although the c-axis maxima usually lie within a girdle, implying that [001] slip was predominant. In contrast to the slip plane predicted by the experimental data, the b-axis maxima normal to the foliation plane suggest that the translation glide plane is (010). A possible interpretation of these patterns is that the c-axis maxima correspond to the flow direction within the shear zones (e.g. Nicolas & Poirier, 1976; Nicolas & Christensen, 1987). Deformation by mechanical twinning is not evident in the samples examined here. Conceivably the eclogites could have deformed by a combination of recrystallization and slip. Although the smaller omphacite grains may have experienced syntectonic recrystallization, the lack of apparent annealing textures, such as equant grains or 120" dihedral angles, suggests that the resultant LESZ CPO can be interpreted in terms of crystal plastic mechanisms. A view of the internal structure of the omphacites through transmission electron microscopy would be necessary to observe the glide plane(s) directly. The omphacite petrofabric patterns are consistent with deformation of the shear zones under eclogite facies conditions. The omphacite CPO show a direct relationship to the foliation in the shear zone. In conjunction with the metamorphic conditions estimated for the shear zones, the CPO of omphacite, the macroscopic structure of the shear zones and the association of eclogite with the high strain zones provide evidence that the deformation occurred under eclogite facies conditions. Interpretation of shear zone kinematics The structural characteristics, shear sense indicators and petrofabric data lead to a possible interpretation of the movement of the major eclogite facies shear zones. Assuming the crystallographic slip direction aligns with the shear direction in the rock the sense of shear can be deduced (Simpson & Schmid, 1983). The preferred orientation of the omphacite c-axes would represent the flow direction within the shear zones (cf. Nicolas & Poirier, 1976; Nicolas & Christensen, 1987). The preferred

16 142 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM N N b - axes c - axes N n = 100 b - axes BA-40 c - axes Fig. 14. Equal-area (lower hemisphere) contour plots showing the preferred orientation of omphacite crystallographic axes. The number of measurements (n) made for each crystallographic axis is shown for each projection. The average foliation (great circle) from this section of the lower Eldsfjell shear zone is plotted for reference or, where available, the foliation for the sample is plotted. The filled triangle represents the average lineation for this section of the shear zone. The contours are for 20 intervals. orientation of the long axes of the rods (Fig. 12) would then be a consequence of their progressive rotation into parallelism with the flow line during deformation. The apparent offset of the metagabbro dyke can be used to infer that the hanging wall moved down, parallel to c-axes concentrations, along a NE trend in the current coordinate frame. Estimated shear strain would be between 3 and 4, consistent with the small displacement. Slip along the minor shear zones with right-lateral oblique movement could be in a similar sense. The occurrence of rocks of similar composition and grade on either side of the shear zones, the termination of major shear zones along strike, and their discontinuous nature suggests the displacement is relatively small. All the kinematic indicators considered are consistent with right-lateral normal oblique movement along a NE trend with relatively small displacement. Thus, in the present coordinate framework, the eclogite shear zones studied can be characterized as extensional. The significance of the interpreted movement along the eclogite shear zones in a regional context is at present unclear and is the subject of further research. DISCUSSION AND SUMMARY The major implications and conclusions of this work are best summarized in a model illustrating the temporal evolution of the eclogite shear zones. The model, discussed below, is based not only on the observations presented here but also those of other researchers (Austrheim & Griffin, 1985; Austrheim, 1987a; Austrheim & Mark, 1988; Jamtveit et al., 1990; Klaper, 1990). Holsn~y Island provides an unusual situation in that shear zone development was arrested at various stages and at a variety of scales as illustrated by the structural types of eclogites. This permits reconstruction of an idealized model of the sequential stages that lead to the formation of the major shear zones exposed on the island.

17 STRUCTURE OF ECLOGITE FACIES SHEAR ZONES 143 N N b - axes N BA-41 c - axes N Fig. 14. (Continued) b - axes BA-22 n=32 c - axes Stage 1 The sequential development of the eclogite facies shear zones was initiated with the transformation of granulite to eclogite along fractures (Fig. 15a). Austrheim (1987a) documented that infiltration of fluids played a significant role in transition from granulite to eclogite along fractures, as evidenced by euhedral hydrous eclogite facies mineral assemblages within the fractures. Fracturing of the dry granulite under eclogite facies conditions implies high fluid pressures, at least locally (Austrheim & Mark, 1988). The fluids may play a significant role by promoting early fracturing and alteration of the rock and thereby locally weakening the rock for subsequent crystal plastic deformation (Simpson, 1986). Similar processes of shear zone development have been documented in other areas (Segall & Simpson, 1986). However, the important point here is that this process occurred under eclogite facies conditions at depths of perhaps >55 km in the continental crust in the roots of an orogenic system, at crustal depths comparable to those attained beneath young orogenic continent-continent collisional belts such as the Alps and Himalayas. Although the exhumation and preservation of the deep crustal shear zones in the Bergen Arcs may be fortuitous, the presence of deformed Caledonian-age eclogites throughout the Scandinavian Caledonides (Binns, 1967; van Roermund, 1983; Erambert, 1985; Austrheim & Mark, 1988) suggests that this could be an orogenic-scale process at depth. Stage 2 In the second stage, discontinuous minor shear zones composed of well-foliated eclogite developed along the fractures (Fig. 15b) marking a transition from predominantly brittle fracturing to predominantly crystal plastic flow. The development of the shear zones coincided with an increase in the volume of eclogite formed

18 144 T. M. BOUNDY, D. M. FOUNTAIN & H. AUSTRHEIM I' Eclogite formed along fractures Ecl ogite "breccia " well-foliated eclogite (-SO%) thot wrops oround rototed granulite blocks Eclogite in discrete shear zones (c2m thick) Major eclogite facies shear (30-100m thick) onostornosing, sub-porollel shear zones continuous over >Ikm Fig. 15. Sc..:matic drawings of the sequential development of the eclogite facies shear zones. Note that each diagram is at a different scale. (a) Eclogite formed along fractures that obliquely cross-cut the granulite foliation. (b) Minor eclogite facies shear zones that offset and deflect the granulite foliation. (c) Anastornosing eclogite shear zones wrap around granulite facies blocks. (d) Major eclogite facies shear zones up to 100 rn thick and continuous over >I km. suggesting that deformation could be reaction enhanced. Granulite foliation deflected into and traced through the shear zones reveals a sense of small-scale displacement. Field relations on Holsn~y Island suggest that the transition from brittle to crystal plastic deformation mechanisms was extremely localized and probably related to high fluid pressures in the lowermost continental crust. The deformation started in the brittle field (stage 1) and progressively gave way to crystal plastic flow (stage 2) at specific sites. These observations, when considered in the light of other studies (e.g. Simpson, 1985, 1986; Wilshire & Kirby, 1989), are important because they imply that both brittle and crystal plastic deformation can occur under the same P-T conditions in the lithosphere. In the Holsn~y study area, both types of deformation occurred in the lowermost continental crust thus illustrating that such behaviour could occur at a variety of depths in the lithosphere. Stage 3 During the third stage, as deformation and metamorphic reaction progressed, some of the minor eclogite facies shear zones anastomosed together into broader high strain zones (1-5 m thick) that dissect and wrap around blocks of granulite facies rocks (Fig. 1%). The more intensely strained areas have relatively more eclogite and the granulite blocks tend to be smaller than in the less strained areas. Granulite facies areas of various sizes were preserved even within zones of intense deformation under eclogite facies conditions. Importantly, this implies that extensive areas of lower crust, particularly areas composed of anhydrous granulite facies rocks, can survive out of their stability field where unaffected by deformation and fluid infiltration. Several similar, local occurrences of eclogite (e.g. the Gaupis locality, Austrheim, 1987b) in the 1000-km2 Bergen Arcs granulite facies anorthosite complex indicate that the entire complex was probably subjected to high-p metamorphic conditions yet retains much of its granulite facies mineralogy (Austrheim, 1987a,b). Stage 4 In the fourth stage, the anastomosing shear zones develop into major eclogite facies shear zones ( m thick), which are subparallel and continuous along strike over several kilometres (Fig. 15d). Relict granulite blocks within the shear zones are relatively few, but their presence provides evidence of the earlier stages of deformation. An interpretation of the movement history of the shear zone consistent with field and petrofabric observations assumes that the omphacite c-axis maxima are parallel to the flow direction within the shear zone. Considering that this direction is parallel to the lineation, we determined a right-lateral normal oblique slip in the present coordinate frame, indicating extensional movement along the shear zones. Displacement was probably minor. In conjunction with the estimated metamorphic conditions, the association of eclogite formation with deformation throughout each stage of shear zone development demonstrates that the shear zones deformed under eclogite facies conditions. Omphacite from the LESZ eclogites show a CPO with a b-axis maximum normal to foliation and a c-axis maximum

19 STRUCTURE OF ECLOGITE FACIES SHEAR ZONES 145 within a girdle, implying that [OOl] slip, perhaps along a [010] glide plane, was predominant during deformation of the eclogites. Similar omphacite petrofabrics have been reported for other Caledonian eclogites, and deformed Caledonian eclogites occur throughout the orogen (e.g. Binns, 1967; van Roermund, 1983; Erambert, 1985; Austrheim & MGrk, 1988) indicating that eclogite deformation in the lower Caledonian continental crust may have been an orogenic-scale process in the roots of this continent-continent collisional orogen. Concluding remarks This work demonstrates that localized faulting, accompanied by fluid infiltration, of high-grade rocks is an important mode of failure in the lower continental crust. Similar localization of strain in high-grade shear zones, although without fluid infiltration, has been documented in the Ivrea Zone, Italy, a lower continental crustal section interpreted to have deformed in an extensional environment (Brodie & Rutter, 1987). These observations are in contrast to the view that the rocks of the lower crust deform in a pervasively homogeneous and ductile manner, or that mylonite zones generally widen with depth (e.g. Sibson, 1977; Smithson et af., 1986). The field relations documented in detail in this work further emphasize that granulite facies rocks can exist in a metastable state under eclogite facies conditions. This implies that the lower continental crust can not only be compositionally heterogeneous (e.g. Fountain & Salisbury, 1981; Fountain & Christensen, 1989) but can host differing metamorphic facies. Such mixing of metamorphic facies in the deep continental crust may be an orogenic-scale process in collisional belts, as illustrated by the occurrence of deformed Caledonian-age eclogites within some of the high-grade terranes along the length of the Caledonide orogen of western Scandinavia. ACKNOWLEDGEMENTS We are particularly indebted to A. W. Snoke, B. E. John, B. R. Frost and E. J. Essene for productive discussions throughout various stages of this research. We thank H. AvC Lallemant, S. Harley and E. J. Essene for critical reviews. S. Swapp, T. Hoch and C. Henderson provided helpful assistance with microprobe analysis. S. Boese and L. Davies provided chemical analyses. S. Luhr and L. Marsten patiently drafted some of the figures. For valuable assistance with the field-work we thank B. Singer. This work was funded by NSF Grant EAR to D.M.F. 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