Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north–central Norway

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Lithos 104 (2008) 177 – 198 www.elsevier.com/locate/lithos

Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north–central Norway H. Austrheim a,⁎, T. Prestvik b a

b

PGP and Department of Geosciences, University of Oslo, N-0316 Oslo, Norway Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway Received 22 May 2007; accepted 19 December 2007 Available online 11 January 2008

Abstract Ophiolite complexes in mountain chains may give supplementary information on the hydration of the oceanic lithosphere to that obtained from dredged and drilled samples from the ocean floor. The ultramafic (mantle) and the layered ultramafic to anorthositic (crustal) sequences of the Cambrian (497 Ma) Leka ophiolite are variably serpentinized and chloritized. Grossular-rodingite (rodingite s.s.) has been found over a c.500 m long and tens of meters wide zone in the layered, crustal section of the complex and is developed in both pyroxenites and gabbro/anorthosite layers. Shear zones and meter wide fracture zones, where the rock has developed a fracture cleavage, are oriented at high angel to the layering and these zones were the main conduits for transport of fluid and solute between the various lithologies. Some 5–15 cm thick layers of anorthosite (or leucogabbro) have been rodingitized around such a fractures zone, with the development of three distinct metasomatic zones along the plagioclase layer. A central grossular-dominated zone with clinopyroxene, clinozoisite, prehnite, chlorite and minor titanite (rodingite zone) extends for up to 3 m along strike and gives way to a clinozoisite-dominated zone (typically 0.5 m wide) with additional grossular, clinopyroxene and chlorite which is followed outward by a LILE-enriched zone (LILE-zone) with clinozoisite, phlogopite, K-feldspar, plagioclase and preiswerkite. The LILE-zone extends more than 3 m out from the clinozoisite-dominated zone (Clz-zone). Assuming constant volume, the rodingite formed from the plagioclase layer by addition of 20 g of CaO per 100 g of rock. All Na2O (c. 2 g) was removed from both rodingite- and Clz-zones. Ti and V increase almost 10× in the rodingite compared to its protolith. K, Ba, Rb and Cs are strongly enriched in the LILE-zone compared to the protolith and suggest interaction with sea water. The lithologies alternating with the plagioclase layers (clinopyroxenite, wehrlite, websterite and dunite) display textures indicating a number of Ca-releasing (Cpx → Chl, Cpx → Srp, Cpx → Amph) and Ca-consuming (Opx → Cpx2, Ol → Cpx2, Cpx1 → Cpx2) reactions. The replacement textures are distributed around fracture and shear zones, with the Ca-releasing reactions in the core and the Ca-consuming reactions in distal parts, forming a metasomatic column out from the fluid pathways. Serpentinization and chloritization of clinopyroxene was the main Ca-source for the rodingitization process. This first description of rodingite in a layered sequence of an ophiolite complex indicates that the hydration of the oceanic lithosphere occurred at various structural levels and was associated with Ca-metasomatism also in places where rodingite s.s. is lacking. The different lithologies exchanged elements through transport on shear and fracture zones. © 2007 Elsevier B.V. All rights reserved. Keywords: Rodingitization; Serpentinization; Chloritization; Leka ophiolite; Metasomatism; Oceanic lithosphere

1. Introduction Fluids play an important role in geological processes such as weathering, metamorphism, metasomatism and hydrothermal ore deposition. Alteration of peridotites, including serpentiniza⁎ Corresponding author. Tel.: +47 22854316; fax: +47 22 855101. E-mail addresses: [email protected] (H. Austrheim), [email protected] (T. Prestvik). 0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.12.006

tion, has a special place among fluid-rock interaction processes. Not only does it affect the volumetrically most important rock type of the lithosphere, but it also leads to the most drastic changes in rock properties such as density (reduced from 3.3 to 2.5 g/cm3), rheology, seismic velocity and oxidation state of the oceanic floor. Conversion of mantle to crustal geophysical signature may occur through serpentinization (Dean et al., 2000). The alteration of peridotites through fluid-rock interaction thus not only influences the geophysical image on which

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our models of the lithosphere are built, but also the geodynamics of subduction and collision zones where the oceanic lithosphere is involved. If the alteration occurs with addition of water and no leaching of other elements, serpentinization leads to a volume increase of c. 50% (Evans, 2004). Large-scale hydrothermal cells are active at spreading centres and lead to hydrothermal alteration of the oceanic lithosphere including serpentinization of the ultramafic parts. The geological activity at spreading ridges including black smokers, carbonate pipes and release of methane may reflect hydrothermal activity and serpentinization. In most cases, pure serpentinization is found to be isochemical except for addition of volatiles (Komor et al., 1985; Viti and Mellini, 1998; Evans, 2004). Paulick et al. (2006) found that the serpentinization of abyssal peridotite may locally be isochemical, however in other places it leads to compositional changes. Scambelluri et al. (2004) concluded that serpentinization of the oceanic mantle produces enrichment in Sr and alkalies, including Rb and Cs. Serpentinites are often associated with rodingites. Rodingite is a calcsilicate rock characterized by hydrogarnet, grossular, diopside and prehnite; minerals such as vesuvianite, titanite, chlorite, amphibole, epidote, carbonates, xonotlite, wollastonite, pectolite and zeolites may also be present. Most described rodingites are formed by Ca-metasomatism of rocks of basaltic composition, although other lithologies may also be transformed (Coleman, 1977), thus Li et al. (2007) report rodingite formed from retrograde eclogites enclosed in ultramafic rocks. This implies that there are cases where serpentinization involves metasomatism.

Dredged samples and drill cores from mid-oceanic ridges have given important information regarding serpentinization and alteration of the oceanic crust (Bach et al., 2004) and samples of rodingites have also been recovered (Aumento and Loubat, 1971; Honnerez and Kirst, 1975). However, isolated dredged and drilled samples can only give a fragmented pictures of the many important processes related to serpentinization of the ocean floor. Although complicated by later tectonism and possible metamorphism, exposed ophiolites can add important information on ocean floor alteration to that derived from the dredged samples (e.g. Coulton et al., 1995). In this paper we present field relationships, mineral chemical data and whole rock major and trace element data on a rodingite occurrence from Norway, within the Leka ophiolite complex. The field relationships demonstrate that the sites of rodingitization are connected to the source for Ca (serpentinization and chloritization of clinopyroxene) by deformation zones which acted as fluid conduits. This situation allows us to connect the reactions in the source rock with the metasomatic processes taking place during rodingitization and to better constrain the element transport during serpentinization and rodingitization. 2. Geological setting and field relationships The Leka ophiolite (Prestvik, 1980) is the best-preserved ophiolite complex within the Scandinavian Caledonides. It presently occurs in a pull-apart structure resulting from postorogenic extension (Titus et al., 2002). It originally formed

Fig. 1. Simplified map of the Leka Ophiolite Complex (LOC) and surrounding areas. Ultramafic rocks include both non-layered (mantle) and layered (crustal) sequences. Gabbroic rocks include layered and varitextured gabbros, the metabasic dike complex, and plagiogranite. The elongated rectangle represents area where grossular rodingite has been found and the star symbol shows the main rodingite location at Aune. L = Lauvhatten (serpentinized harzburgite), ● = location of analysed (reference) samples of anorthosite and pyroxenite. 1 = Pyroxenite (sample LE11-06), 2 = Anorthosite (sample LE12-06), 3 = Pyroxenite (sample 1707.04), 4 = Anorthosite (sample 1607.01), 5 = Anorthosite (sample 3006.05), 6 = Pyroxenite (sample P 3).

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Pedersen, 1988). The Leka ophiolite (Fig. 1) contains all the principal components of an ophiolite complex, including a mantle section and a layered crustal sequence. Previous studies of the Leka ophiolite have mainly focused on the primary magmatic processes in the various rock types, on the overlying sediments and of the tectonic features. However, alteration is common throughout the complex. The ultramafic part of the Leka ophiolite is variably serpentinized (Iyer et al., in press-a), ranging from 10 to 100% alteration, locally with the development of ophicarbonates (Birtel, 2002). Prestvik (1972) reported that plagioclase of the gabbro in the Aune area is completely altered to a clinozoisite–albite mixture, whereas the clinopyroxene is only weakly altered. Furthermore, Prestvik (1970) reported a thin band of speckled reddish-greengrey-white garnet rich rocks from Aune (Fig. 1). A reinvestigation of this occurrence revealed that the rock classifies as rodingite. Rodingites, recognized in the field by the presence of garnet, have been found along the transition zone of the cumulate section where ultramafics (dunite, meta-harzburgite, wherlite, websterite and clinopyroxenite) start to include thin layers (5– 15 cm across) of plagioclase-rich gabbros and anorthosite. This garnet bearing zone has been traced for a distance of c. 500 m as shown in Fig. 1. Fig. 2. Fracture cleavage as fluid conduits in clinopyroxenite. a. Field photo of fracture cleavage seen as vertical striation. Diameter of coin is 2 cm. The box shows the orientation and size of Fig. 2b relative to fracture cleavage. From NE part of the rodingitized area. b. Close up of fracture cleavage as seen on a scanned thin section. The fracture cleavage consists of alternating bands of Cpx1 (light) and Cpx2 (dark). The Cpx2 is formed from Cpx1 along mm wide fractures filled with Cpx2 and epidote. Sample LE19-07.

part of the oceanic lithosphere of the North Atlantic Iapetus Ocean (Maaløe, 2005) and was obducted during the Caledonian orogeny in Ordovican to early Silurian times (Dunning and

2.1. Structures and fluid conduits The Leka ophilite complex displays a number of structures which may have acted as channels for transport of fluid and matter. Almost layer-parallel protomylonite bands with clasts of olivine and clinopyroxene may relate to mantle flow, but are also present in the crustal section. Well defined shear zones with a high angle to the layering can be followed for at least 100 m. The shear zones may anastamose and then locally follow lithological contact. Deformation zones where the rock is broken up into rounded to angular

Fig. 3. Detailed map of the studied rodingite locality at Leka with location of analyzed samples. Fracture system (stippled) is oriented perpendicular to the layering of the complex. The fractures are locally filled with grossular garnet and represent conduits for fluid and element transport between the layers.

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fragments are present. The complex is transected by a set of fractures oriented perpendicular to the layering. The density of the fractures varies, and locally meter wide zones have developed a fracture cleavage (Fig. 2a) with a fracture density of 10 pr 5 cm, measured perpendicular to the orientation of fracture set. In most places the fractures are parallel, but the rocks may also be crisscrossed by fractures. The presence of garnet on many fractures and shear zones shows that these structures are the conduits for the metasomatizing fluids. The fracture cleavage shown in Fig. 2a is manifested on the microscopic scale as mm thin en echelon arranged epidote and clinopyroxene filled cracks along which the original pyroxene is replaced by diopside (Fig. 2b). One rodingite locality (65° 05'N, 11° 35'E) was studied in detail, mapped and sampled as shown in Fig. 3. The rodingite layers are in contact with layers of pyroxenite and are positioned

c. 7 m from a partly serpentinized dunite layer. Also in this area, shear- and fracture-zones, with cataclastic textures, transect the layering of the complex at a high angle (Fig. 3). These zones can be seen to cause a color change in the pyroxenite and locally chains of fine-grained garnet (grossular) can be observed along the fractures. Three c. 10 cm thick layers with a minimum length of 1.5 m, which are interlayered with pyroxenite, consist of garnet-rich rodingite (Fig. 3). The layering has vertical dip with a strike of N 030°. The rodingite-zones are developed along former anorthositic to leuco gabbro layers. The central pink-colored garnet-dominated zone (rodingite proper) is replaced along strike with a light colored clinozoisite-rich zone, which again gives way to a zone characterized by phlogopite (LILE-enriched zone). The LILE-zone extends beyond the end of the outcrop (4 m).

Table 1 Petrography and location of samples Sample no

Locality

Rock type

Petrography

LE03-06 LE04-06 LE05-06 L69,L69B LE07-06 LE41-06 LE01-06 LE30-06 LE12-06 LE31-06 LE32-06 LE08-07 LE09-06 LE11-06 LE02-06 LE16-06 LE17-06 LE18-06 LE19-06 LE33-06 LE34-06A LE34-06B LE42-06 LE10-06 LE20-06 LE25-06 LE43-06 LE26-06 LE27-06 LE06-05K LE19-07

Layer 2, RL Layer 1, RL Layer 3, RL RL,exact loc. unknown Layer 1,RL Layer 1,RL Layer 1, 3 m S of RL Layer 2,RL Road inters. to Vågan 60 m north of RL 30 m north of RL 4 m W of RL 4 m W of RL 1 km W of RL Betw. layer1 and 2,RL 70 m N of RL 4 m W of RL 40 m E of RL 40 m E of RL 50 m north of RL 10 m W of RL 10 m W of RL 0.8 m E of layer1,RL 10 m N of RL 5 m N of RL 10 m NE of RL 15 m NE of RL 10 m NE of RL 6 m E of RL Lauvhatten 500 m NE of RL

RD RD RD RD Clz-zone Clz-zone LILE-zone LILE-zone Plag-rich layer/protolith Plag-rich layer/protolith Plag-rich layer/protolith Cpxenite sec. cpx Cpxenite w grt veins Banded pxenite Pxenite layer Altered pxenite layer Cpxinite layer w catacl. Zones Rd pxinite w catacl. zones Pxenite tectonized w veins Cpxenite layer Pxinite-altered Pxinite w shear zone Pxinite w shear zones,DC Websterite (serpentinized) Websterite w srp shear Websterite, tectonized Dunite (reddish) Dunite-serpentinized Dunite-serpentinized Dunite-serpentinized Cpxenite w fracture cleavage

Cpx,grs,chl,VF:cpx,chl,grs Grs, pre, czo Grs, cpx, clz,pre,Amph Grs,chl,clz, pre Cpx2, prei,clz,chl,grs Clz, Amph, grt, Clz w spherulite text,intergr.alb,phl,prei Clz w spherulitic text, pholg, plag, Amph Plag, clz Clz w spherulitic text Clz w spherulitic text, Cpx1,cpx2, veins/domains of grs + chl + amp Cpx1 veined w cpx2, grs, chl Cpx1,cpx2, chl, tre Cpx1,cpx2,chl,trem,oxide,R2,R4,R5' Cpx2, grs, ves, Amph,plag,ap Cpx1, cpx2, chl, parg, grt, R2, R3, R5' Cpx1, aggr. of chl, clz, gross veins, R5 Cpx1,cpx2,chl, trem, R3,VF:trem + cpx2 + chl Cpx1, cpx2, trem, srp, chl, titanite, R5' Cpx,trem, serp Cpx, chl,srp, trem R1, R2 Cpx1,chl, srp, tit, Cpx1, cpx2, srp, chl, R1,R2,R5 Cpx1, ol, cpx2, RS, srp, R1, R4, R5 Cpx1, ol, cpx2, srp, R5, DB Ol w DB, veins of srp,chr, Ol, srp,chr Ol, srp, chr Ol, cpx2, srp, R1, R6 Cpx1, Cpx2, R4

RL = Rodingite locality-see Fig. 3, RD: rodingite, RS = riddle shear. VF = vein fill, DB = deformation band. R1: Cpx + H2O = chl + Ca2+. R2: Cpx + H2O = srp + Ca2+. R3: Cpx + Na+ + H2O = Amph + Ca2+. R4: Cpx + Ca2+ = Cpx2. R5: Opx + Ca2+ = cpx2. R5':Opx + H2O + Ca2+ = chl + trem + cpx2. R6: Ol + Ca2+ = cpx2. Locations of additional samples are shown in Figs. 1 and 3.

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3. Samples and methods In our attempt to link the rodingitization to processes in the surrounding rocks, we have sampled the various units outside the area shown on the detailed map. An overview of the studied samples and their location relative to the mapped rodingite locality is given in Table 1. Most of the samples are collected within 60 m from the mapped locality. In order to test if our observation from the Aune area applies to the rest of the Leka ophiolite we have also investigated samples from Lauvhatten, located 5 km NE of Aune, and relevant samples from several localities away from the type rodingite zone (1–6 in Fig. 1). The Lauvhatten locality is located within the mantle section of the ophiolite, where the dominant rock types are dunite and harzburgite. This area is transected by several 0.5 m thick east– west trending shear-zones, and the degree of serpentinization is higher than in similar rock types at Aune. Mineral analyses were performed by wavelength-dispersive spectrometry (WDS), using a Cameca SX100 located at the institute of Earth Sciences, Oslo University and a JEOL superprobe at Institut für Mineralogie, Münster. The microprobes were operated with an accelerating voltage of 15 kV and a beam current between 10 and 20 nA. Calibration was done on a set of natural and synthetic standards. Whole-rock major and trace elements (Y, Sr, Rb, Zn, Cu, Ni, Ba, Co, Cr, V) were determined by standard XRF-procedures at Norwegian University of Science and Technology, Trondheim and by ICP–MS (Zr, Th, Cs) at Acme Analytical Laboratories Ltd., Vancouver, using lithium borate fusion and subsequent digestion in nitric acid. 4. Petrology and mineral chemistry 4.1. Petrography of the metasomatic zones of the rodingite locality The principal minerals of the rock mapped as rodingite (Fig. 3) are grossular-rich garnet, clinopyroxene, epidote, amphibole and chlorite, with minor amounts of prehnite, titanite, and one unidentified species, probably a zeolite mineral. The rodingite is without plagioclase. A banding defined by preferred shape orientation of garnet and modal variation in garnet and pyroxene is characteristic for the rodingite. The garnet grains contain numerous fluid-inclusion bands that have an overall direction parallel to this banding. Complex zoning defined by variation in andradite component characterizes the garnet (Fig. 4a). The garnets range in composition from almost pure Gross100 to Gross80Andr15 with only minor uvarovite (Table 2). This variation in composition can be found within one zoned crystal. The total of the garnet analyses vary around 100%, suggesting a very low (if any) hydrogrossular component. This is supported by XRD analyses, which give grossular patterns. The zoned garnet displays resorption texture including discontinuity between the different zones and local porous zones, a feature typical for replacement textures (Putnis, 2002). Clinopyroxene in the rodingite is typically porous and rich in inclusions, which in most cases are too tiny to be identified, but chlorite and epidote have been identified. Several generations of

Fig. 4. Mosaic of BSE images showing textures in rodingitized samples from Leka. a. Complexly zoned and veined grossular garnet from rodingite sample L69. The brightness of the various zones reflects slight variation in andradite component. The complexity of the zoning and veining suggests that the garnet grew as a result of multiple fluid episodes with intervals of dissolution. Discontinuity between zones coinciding with porous zone suggests that the garnet evolved by dissolution reprecipitation mechanism. b. Garnet transected by a swarm of garnet filled veins. Note how the smaller veins stop on entering the porous clinopyroxene. The vein swarm is oriented perpendicular to the banding of the rodingite represented by the long direction of the garnet grain. Sample L69B. c. Veined clinopyroxene (cpx). The vertical veins contain grossular (grt), chlorite (chl) and diopside (cpx), suggesting that these three phases are formed during rodingitization. Note that the filling in the veins changes along their length. Sample LE09-06.

veins filled with garnet, pyroxene, chlorite and clinozoisite are oriented perpendicular to the foliation (Fig. 4b) and are locally displaced along planes parallel to the banding. These relationships indicate that the fluid inclusions trails, the veining, and

182

Table 2 Composition of garnet, vesuvianite and epidote from rodingites and surrounding ultramafites Sample no Garnet L69

38.90 39.28 0.30 0.01 17.66 22.89 0.00 0.00 n.a. 0.00 7.64 0.19 0.99 0.00 0.18 0.03 0.42 0.00 35.02 38.02 100.37 100.45

LE0706

LE0806

LE0806

LE0806

LE0806

LE0906

LE1606

LE1806

LE1606

LE1606

LE1606

LE1606

L69

L69

LE0406

LE0706

LE1806

38.57 1.42 20.01 0.06 0.00 3.38 0.00 0.32 0.03 36.70 100.48

38.19 0.14 22.35 0.04 n.a. 0.29 0.00 0.12 0.06 38.05 99.24

37.36 1.47 14.81 5.51 1.47 3.01 0.00 0.26 0.00 36.09 99.98

37.47 1.60 15.09 4.10 0.18 5.12 1.21 0.30 0.04 34.86 99.97

37.58 0.56 14.47 5.86 0.44 5.47 2.14 0.60 0.14 33.14 100.61

38.20 0.82 17.47 3.76 0.42 2.65 0.00 0.16 0.10 36.82 100.40

38.71 0.77 20.59 0.14 n.a. 3.66 1.82 0.56 0.16 34.59 100.92

39.27 0.66 21.31 n.a. 0.00 1.14 4.38 0.78 0.13 32.90 100.45

38.61 1.01 20.75 0.02 n.a. 3.48 0.90 0.34 0.27 35.40 100.79

35.96 1.88 15.05 n.a. 0.00 0.94 7.37 0.00 0.02 36.68 97.89

35.82 1.90 14.85 n.a. 0.00 1.84 7.22 0.09 0.00 36.68 98.40

35.35 2.72 14.18 n.a. 0.00 1.48 7.67 0.02 0.00 36.43 97.85

33.81 4.05 15.00 n.a. 0.00

38.60 0.23 28.94 0.03 n.a. 5.58

38.35 0.00 32.39 0.00 n.a. 0.92

39.46 0.00 33.19 0.00 n.a. 0.21

38.27 0.07 32.42 0.42 n.a. 1.19

38.87 0.00 33.48 0.00 n.a. 0.50

0.03 0.03 24.58 97.48

0.02 0.03 25.20 96.82

0.02 0.03 24.98 97.89

0.04 0.02 25.36 97.79

0.01 0.03 25.56 98.45

2.996 0.000 2.969 0.000 n.a. 0.012

2.934 0.004 2.930 0.025 n.a. 0.069

2.946 0.000 2.990 0.000 n.a. 0.028

0.002 0.001 0.001 0.004 0.003 0.003 2.039 2.083 2.032

0.003 0.002 2.083

0.001 0.003 2.075

0.01

0.03

0.11

Structural formula based on Si Ti Al Cr V Fe3+ Fe2+ Mn Mg Ca

2.969 0.017 1.588 0.000 0.000 0.439 0.063 0.012 0.048 2.863 8.000 Mg# 0.09 Almandine 2.11 Andradite 15.27 Goldmanite 0.00 Grossular 80.61 Pyrope 1.61 Spessartine 0.39 Uvarovite 0.00

2.946 0.000 2.023 0.000 0.000 0.011 0.000 0.002 0.000 3.054 8.037 0.03 0.00 0.36 0.00 99.55 0.01 0.07 0.00

Epidote

L69B

12(O) and 8 cations 2.926 2.911 0.081 0.008 1.789 2.008 0.003 0.002 0.000 0.000 0.193 0.017 0.000 0.000 0.021 0.008 0.003 0.007 2.983 3.107 8.000 8.068 0.02 0.28 0.00 0.00 9.11 0.79 0.00 0.00 89.98 98.67 0.11 0.22 0.68 0.25 0.11 0.08

2.917 0.086 1.363 0.340 0.092 0.177 0.000 0.017 0.000 3.019 8.011 0.00 0.00 8.80 1.54 77.71 0.00 0.58 11.38

2.928 0.094 1.390 0.253 0.011 0.301 0.079 0.020 0.005 2.919 8.000 0.01 2.62 13.09 0.19 74.89 0.15 0.66 8.40

6.14 0.00 0.01 36.74 95.75

73(O) and 50 cations 2.943 0.033 1.335 0.363 0.027 0.323 0.140 0.039 0.016 2.781 8.000 0.03 4.73 11.99 0.46 68.72 0.53 1.33 12.24

2.935 0.047 1.582 0.228 0.026 0.153 0.000 0.010 0.011 3.030 8.023 0.07 0.00 6.60 0.43 84.74 0.38 0.34 7.51

2.930 0.044 1.836 0.008 0.000 0.208 0.115 0.036 0.018 2.805 8.000 0.05 3.87 8.47 0.00 85.56 0.61 1.21 0.28

2.978 0.037 1.904 0.000 0.000 0.065 0.277 0.050 0.015 2.673 8.000 0.04 9.20 3.39 0.00 85.25 0.49 1.67 0.00

2.919 0.057 1.849 0.001 0.000 0.198 0.057 0.022 0.030 2.867 8.000 0.11 1.92 8.59 0.00 87.70 1.02 0.73 0.04

17.750 0.698 8.755 0.000 0.000 0.348 3.040 0.000 0.011 19.397 50.000 0.00

17.640 0.704 8.618 0.000 0.000 0.683 2.973 0.018 0.000 19.352 49.988 0.00

17.556 1.016 8.300 0.000 0.000 0.553 3.184 0.005 0.000 19.383 49.997 0.00

12.5(O) 17.052 1.536 8.916 0.000 0.000 0.000 2.589 0.000 0.010 19.851 49.954 0.00

2.989 0.013 2.641 0.002 n.a. 0.325

2.958 0.000 2.944 0.000 n.a. 0.054

0.06

0.22

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

SiO2 TiO2 Al2O3 Cr2O3 V2O3 Fe2O3 FeO MnO MgO CaO Total

Vesuvianite L69

Table 3 Composition of clinopyroxene from rodingites and ultramafics Sample L68B

L68B

L69B

LE03-06 LE03-06 LE04-06 LE02-06 LE02-06 LE08-06 LE08-06 LE10-06 LE10-06 LE11-06 LE11-06A LE16-06 LE17-06 LE17-06 LE18-06 LE25-06 LE06K

Rodingite n=1

n=1

n=1

n=2

n=2

n=3

n=3

n=4

n=1

n=5

n=1

n=2

n=1

n=1

n=2

n=2

n=2

n=1

Cpx2

Cpx2

Vein

Vein

non por

Vein

Cpx1

Cpx2

Cpx1

Cpx2

Cpx1

Cpx/ol

Cpx1

Cpx2

Cpx2

Cpx1

Cpx2

Cpx1

Cpx/opx Cpx/ol

50.33 0.21 5.72 5.53 0.05 13.16 24.42 0.67 0.01 100.08

52.64 0.08 1.17 6.33 0.21 14.08 24.98 0.21 0.16 99.86

50.59 0.15 5.41 4.91 0.09 13.58 24.53 0.78 0.04 100.09

48.39 0.51 4.29 12.10 0.25 9.98 24.24 0.18 0.04 99.98

50.56 0.19 5.58 4.82 0.06 13.51 24.40 0.77 0.03 99.90

48.82 0.47 3.08 11.99 0.29 10.16 24.36 0.14 0.03 99.32

51.80 0.18 3.68 3.78 0.08 15.60 23.88 0.17 n.a. 99.27

54.12 0.02 0.14 3.75 0.20 15.97 25.06 0.09 n.a. 99.64

50.12 0.30 5.77 5.08 0.12 14.27 24.61 0.10 0.07 100.44

52.80 0.00 0.05 10.35 0.64 11.56 25.11 0.16 0.00 100.67

51.30 0.29 4.05 4.15 0.11 15.60 24.14 0.15 0.35 100.13

55.52 0.00 0.03 2.63 0.14 16.76 25.69 0.10 0.36 101.23

50.46 0.23 4.86 5.06 0.19 14.62 24.40 0.12 0.21 100.14

54.01 0.05 0.60 5.68 0.17 14.92 25.47 0.06 0.04 101.00

53.57 0.04 0.67 7.90 0.03 13.05 24.98 0.33 n.a. 100.73

51.32 0.13 5.37 3.95 0.17 14.60 24.54 0.12 0.00 100.28

55.27 0.00 0.58 3.77 0.11 15.79 23.99 0.61 0.68 100.83

49.37 0.39 7.01 4.34 0.07 13.96 24.99 0.14 0.05 100.30

54.58 0.04 0.39 0.55 0.00 18.40 25.54 0.15 0.11 99.77

1.911 0.005 0.160 0.029 0.087 0.003 0.858 0.944 0.012 0.000 0.88

1.997 0.000 0.006 0.007 0.108 0.006 0.879 0.991 0.007 0.000 0.88

Structural formula based on Si 1.862 1.961 Ti 0.006 0.002 Al 0.249 0.051 Fe3+ 0.093 0.050 0.077 0.147 Fe2+ Mn 0.001 0.007 Mg 0.726 0.782 Ca 0.968 0.997 Na 0.048 0.015 Cr 0.000 0.005 Mg# 0.81 0.80

6(O) and 4 cations 1.868 1.855 1.868 0.004 0.015 0.005 0.235 0.194 0.243 0.111 0.118 0.096 0.039 0.266 0.052 0.003 0.008 0.002 0.748 0.570 0.744 0.970 0.995 0.966 0.056 0.013 0.055 0.001 0.001 0.001 0.83 0.60 0.83

1.884 0.013 0.140 0.109 0.274 0.009 0.584 1.007 0.010 0.001 0.60

1.844 0.008 0.250 0.075 0.081 0.004 0.783 0.970 0.007 0.002 0.83

1.990 0.000 0.002 0.043 0.282 0.020 0.650 1.014 0.012 0.000 0.67

1.883 0.008 0.175 0.064 0.063 0.003 0.854 0.950 0.011 0.010 0.87

2.001 0.000 0.001 0.079 0.004 0.901 0.992 0.007 0.010 0.92

1.863 0.006 0.211 0.079 0.076 0.006 0.805 0.965 0.008 0.006 0.84

1.980 0.001 0.026 0.022 0.152 0.005 0.815 1.000 0.004 0.001 0.82

1.990 0.001 0.029 0.020 0.225 0.001 0.723 0.994 0.024 0.000 0.75

1.878 0.004 0.232 0.020 0.101 0.005 0.797 0.962 0.008 0.000 0.87

2.004 0.000 0.025 0.114 0.003 0.854 0.932 0.043 0.019 0.88

1.816 0.011 0.304 0.078 0.054 0.002 0.765 0.985 0.010 0.001 0.85

n=1

54.49 0.00 0.02 0.89 0.02 18.23 26.35 0.01 0.04 100.05

1.979 0.001 0.017 0.017

1.979 0.000 0.001 0.027

0.000 0.995 0.992 0.011 0.003 0.98

0.001 0.987 1.025 0.001 0.001 0.97

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O Cr2O3 Total

Ultramafics

n=2

Cpx/opx: clinopyroxene after orthopyroxene; Cpx/ol: clinopyroxene after olivine; Non por: non porous.

183

184

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

the faulting developed contemporaneously. The veins may be monomineralic, and the mineral filling the vein may change along the length of the veins (Fig. 4b, c). Clinopyroxene from rodingitized samples (Table 3) shows a broad range in Al2O3 (1–6 wt.%) and FeOtot. (5–12 wt.%), resulting in a Mg/Mg + Fe ratio (mg # ) between 0.88–0.6. Clinopyroxenes have high CaO content (23.8–25%) and range from pure diopside to salite. Na2O ranges from 0.1 to 0.8 wt.%. The strong variation in Al2O3 and FeOtot, which is found in both vein fillings and matrix pyroxenes is in accordance with the variation found in garnet and is mainly a result of exchange of Fe3+ for Al3+. This indicates that the minerals grew from an evolving fluid or from fluid pulses with variable composition. The pinkish rodingite zone gives way to a white to greyish plagioclase-free zone enriched in clinozoisite with minor prehnite, clinopyroxene, chlorite, preiswerkite and occasionally garnet and amphibole (Clz-zone). The clinozoisite is medium grained and locally fills veins through the samples. The epidote minerals show a range of compositions varying from almost pure CaAl endmember to compositions with 0.3 Fe3+ a.f.u. (Table 2). Just like for garnet and clinopyroxene, the variation in composition is that of exchanging Fe3+ with Al3+.

Along strike the clz-zone turns into a zone with characteristic grey to brownish spots consisting of phlogopite and amphibole. Representative analyses of amphiboles are listed in Table 4. Amphibole in rodingite is pargasitic with Na and K up to 0.95 and 0.1 a.f.u., respectively for formulae calculated on 23 oxygens. Similar pargasitic amphiboles were described from the rodingite of the Zermatt–Saas ophiolites by Li et al. (2004). The phlogopite amphibole aggregates are placed in a matrix of spherulitic clinozoisite intergrown with albite; they are well defined and are likely pseudomorphs after a mafic phase, probably olivine. Intergrown preiswerkite and albite are locally present and small grains of K-feldspar have also been observed. Keusen and Peters (1980) described preiswerkite from a rodingite dike within the Geisspfad ultramafic complex of the Penninitic Alps and the very presence of preiswerkite in layers close to rodingite described in this paper indicates that this mineral is related to the rodingitization process. The presence of phlogopite and K-feldspar distinguishes this rock from the anorthosites elsewhere in the Leka complex and suggests that the anorthosite layers close to the rodingite have undergone K-metasomatism. K2O (and Na2O) enriched zones are commonly found ahead of the rodingitization front (O'Hanley,

Table 4 Composition of chlorite and amphibole from rodingites and ultramafites Sample Chlorite

Amphibole

L69

L69B LE02- LE1006 06

LE10- LE11- LE16- LE1706 06 06 06

LE18- LE1906 06

LE19- L69B 06

LE0506

LE1106

LE16- LE1707 06

LE1106

LE1906

n=1

n=1

n=1

n=1

n=1

n=1

n=1

n=4

n=2

n=1

n=1

n=1

n=1

n=1

n=1

n=2

n=1

n=1

W grt Vein

A cpx In shear A cpx A cpx Matrix A cpx

W grt Inc cpx A opx W chl Vein

A cpx

Vein

A cpx

A cpx

A cpx

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 NiO Total

26.50 0.03 21.14 20.26 0.07 18.44 0.04 0.01 0.00 0.02 n.a. 86.52

32.45 0.01 13.99 10.39 0.04 28.99 0.09 0.02 n.a. n.a. n.a. 85.98

27.30 0.02 24.50 9.92 0.04 25.66 0.13 0.01 0.03 0.04 n.a. 87.65

32.09 0.00 14.44 10.55 0.10 28.20 0.06 0.00 0.01 1.16 n.a. 86.61

31.98 0.02 14.64 10.11 0.07 28.83 0.13 0.00 0.00 0.74 n.a. 86.52

40.83 0.24 15.24 12.84 0.17 12.12 12.51 2.83 0.43 0.03 n.a. 97.24

44.12 0.71 12.33 7.41 0.06 16.46 12.46 3.13 0.01 0.75 0.01 97.45

40.54 0.17 15.43 13.76 0.13 10.94 12.35 3.39 0.50 n.a. n.a. 97.44

47.89 0.23 8.90 7.25 0.15 17.31 12.49 2.81 0.21 0.49 0.03 97.74

58.16 0.00 0.18 2.98 0.01 22.84 13.58 0.29 0.00 0.01 0.03 98.08

57.13 0.03 0.50 3.24 0.22 22.58 13.03 0.36 0.01 0.29 0.04 97.43

Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Mg#

5.498 0.005 5.168 0.000 3.514 0.012 5.701 0.009 0.004 0.000 0.003 0.62

5.281 0.003 5.586 0.000 1.605 0.007 7.400 0.027 0.004 0.002 0.006 0.82

6.323 0.000 3.353 0.000 1.738 0.017 8.284 0.013 0.000 0.003 0.181 0.83

6.289 0.003 3.393 0.000 1.663 0.012 8.452 0.027 0.000 0.000 0.115 0.84

6.084 0.027 2.676 0.000 1.600 0.021 2.692 1.997 0.817 0.082 0.004 0.63

28.01 0.03 21.07 13.27 0.08 24.24 0.09 0.00 0.01 0.03 n.a. 86.83

30.22 0.02 19.40 9.39 0.08 27.96 0.03 0.00 0.00 0.03 n.a. 87.13

32.24 0.01 15.23 8.13 0.11 30.39 0.06 0.00 0.00 0.26 n.a. 86.43

27.97 0.09 20.63 12.83 0.11 24.59 0.09 0.01 0.00 0.03 n.a. 86.35

27.29 0.00 17.95 21.86 0.16 17.20 0.20 0.02 0.02 n.a. n.a. 85.14

5.587 0.014 4.857 0.000 2.143 0.019 7.323 0.019 0.004 0.000 0.005 0.77

5.850 0.000 4.535 0.000 3.918 0.028 5.497 0.047 0.008 0.002 0.000 0.58

30.27 0.00 18.51 9.21 0.06 27.08 0.09 0.01 0.02 0.68 0.10 86.02

Structural formula based on 28(O) 5.572 0.004 4.940 0.000 2.207 0.013 7.189 0.019 0.000 0.001 0.005 0.77

6.412 0.001 3.258 0.000 1.717 0.007 8.540 0.019 0.008 0.000 0.000 0.83

5.858 0.003 4.432 0.000 1.522 0.013 8.081 0.006 0.000 0.000 0.005 0.84

6.269 0.001 3.490 0.000 1.322 0.018 8.810 0.012 0.000 0.000 0.040 0.87

39.96 0.17 17.17 8.39 0.13 14.15 12.82 3.35 0.48 0.72 0.08 97.41

Structural formula based on 23(O) 5.956 0.001 4.291 0.000 1.515 0.010 7.942 0.019 0.003 0.002 0.106 0.84

A cpx: after cpx; A opx: after opx; w grt: with garnet; In shear: in shear zone.

5.859 0.019 2.967 0.000 1.028 0.016 3.093 2.013 0.952 0.089 0.083 0.75

6.375 0.077 2.100 0.000 0.895 0.007 3.546 1.929 0.877 0.002 0.086 0.80

6.079 0.020 2.727 0.000 1.726 0.017 2.445 1.984 0.985 0.096 0.000 0.59

6.856 0.024 1.502 0.000 0.867 0.018 3.695 1.915 0.779 0.039 0.055 0.81

7.961 0.000 0.029 0.000 0.341 0.001 4.661 1.991 0.077 0.000 0.001 0.93

7.896 0.003 0.081 0.000 0.374 0.026 4.653 1.929 0.096 0.002 0.032 0.93

Table 5 Composition of olivine and serpentine from ultramafites LE26

LE26

LE27

LE27

LE10-06 LE10-06 LE10-06 LE26-06 LE27-06 LE27-06 LE27-06

n=1

n=1

n=1

n=1

n=2

n=2

n=1

n=1

n=2

n=1

n=1

n=1

n=1

n=1

n=1

n=2

n=1

n=1

n=1

Light

Dark

Light

Dark

Light

Dark

Interm

Core

Interm

Rim

Oliv2

Oliv1

A cpx

A cpx

A cpx

A oliv

A oliv

A oliv

A oliv

40.27 0.02 0.02 11.63 0.27 47.50 0.02 0.00 0.10 99.83

40.90 0.00 0.01 10.85 0.19 48.78 0.01 0.02 0.13 100.89

40.92 0.00 0.00 10.56 0.19 48.40 0.00 0.01 0.18 100.26

40.12 0.01 0.01 12.24 0.25 47.17 0.02 0.00 0.26 100.06

40.48 0.00 0.02 10.35 0.25 48.62 0.02 0.02 0.26 100.00

40.72 0.02 0.01 11.53 0.20 47.65 0.01 0.00 0.23 100.37

41.62 0.00 0.01 5.29 0.23 52.74 0.00 0.00 0.13 100.02

41.38 0.00 0.00 7.16 1.02 50.15 0.01 0.03 0.17 99.90

41.31 0.00 0.02 8.48 0.22 50.10 0.03 0.00 0.24 100.40

41.32 0.00 0.00 7.15 0.23 51.70 0.01 0.00 0.16 100.57

39.83 0.00 0.00 9.66 0.21 49.44 0.01 0.01 0.31 99.47

44.13 0.01 0.84 5.97 0.15 37.06 0.00 0.04

41.81 0.02 3.23 6.61 0.18 35.20 0.05 0.37

40.93 0.01 4.29 6.61 0.17 35.06 0.01 0.33

88.20

87.47

87.41

44.98 0.01 0.54 2.18 0.04 38.97 0.00 0.03 0.05 86.77

45.26 0.00 0.34 2.24 0.08 39.91 0.03 0.04 0.12 88.02

43.98 0.00 1.22 1.94 0.02 38.69 0.00 0.09 0.02 85.96

43.09 0.00 0.04 1.40 0.06 41.27 0.07 0.02 0.01 85.96

40.18 SiO2 TiO2 0.00 Al2O3 0.00 FeO 14.48 MnO 0.27 MgO 45.34 CaO 0.02 Cr2O3 0.00 NiO 0.15 Total 100.44

Formula based on 4(O) Si Ti Al Fe Mn Mg Ca Cr Ni Mg#

1.003 0.000 0.000 0.302 0.006 1.687 0.001 0.000 0.003 0.85

0.998 0.000 0.001 0.241 0.006 1.755 0.001 0.000 0.002 0.88

Formula based on 7(O) 0.999 0.000 0.000 0.222 0.004 1.776 0.000 0.000 0.003 0.89

1.004 0.000 0.000 0.217 0.004 1.771 0.000 0.000 0.004 0.89

0.996 0.000 0.000 0.254 0.005 1.747 0.001 0.000 0.005 0.87

0.997 0.000 0.001 0.213 0.005 1.786 0.001 0.000 0.005 0.89

1.003 0.000 0.000 0.238 0.004 1.750 0.000 0.000 0.005 0.88

1.000 0.000 0.000 0.106 0.005 1.889 0.000 0.000 0.003 0.95

1.007 0.000 0.000 0.146 0.021 1.819 0.000 0.001 0.003 0.93

1.004 0.000 0.001 0.172 0.005 1.814 0.001 0.000 0.005 0.91

0.996 0.000 0.000 0.144 0.005 1.858 0.000 0.000 0.003 0.93

0.986 0.000 0.000 0.200 0.004 1.824 0.000 0.000 0.006 0.90

2.057 0.000 0.046 0.233 0.006 2.575 0.000 0.001 0.000 0.92

1.977 0.001 0.180 0.261 0.007 2.482 0.003 0.014 0.000 0.90

1.938 0.000 0.239 0.262 0.007 2.475 0.001 0.012 0.000 0.90

2.087 0.000 0.030 0.084 0.001 2.695 0.000 0.001 0.002 0.97

2.076 0.000 0.018 0.086 0.003 2.729 0.001 0.001 0.004 0.97

2.059 0.000 0.067 0.076 0.001 2.700 0.000 0.003 0.001 0.97

2.023 0.000 0.002 0.055 0.002 2.889 0.004 0.001 0.000 0.98

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

LE20-06 LE20-06 LE24-06 LE24-06 LE25-06 LE25-06 LE25-06 LE26

Mg# = Mg2+ / (Mg2+ + Fe2+). Dark and light refer to zoned grains as observed by BSE. A oliv: after olivine.

185

186

Table 6 Gresens analyses of mineral replacements and whole rocks assuming constant volume Sample

LE1006

Cpx1

Srp

51.30 0.30 4.05 4.15 0.11 15.60 24.14 0.15 0.00 0.35 0.00 3.25

41.81 0.02 3.23 6.61 0.18 35.20 0.05 0.00 0.00 0.37 12.52 2.55

LE1106

LE1106

LE1706

LE1706

G/L

Cpx

Chl

Cpx1

Parg

− 18.50 −0.28 −1.52 1.04 0.03 12.02 − 24.10 −0.15 0.00 −0.06 9.82

50.46 0.23 4.86 5.06 0.19 14.62 24.40 0.12 0.00 0.21 0.00 3.25

27.97 0.09 20.63 12.83 0.11 24.59 0.09 0.01 0.00 0.00 13.68 2.80

51.32 0.13 5.37 3.95 0.17 14.60 24.54 0.12 0.00 0.00

47.89 0.23 8.90 7.25 0.15 17.31 12.49 2.81 0.21 0.49

3.20

3.15

Protolith Rodingite

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2 O Cr2O3 H2 O Density

Clzzone

−26.36 −0.15 12.91 5.99 −0.10 6.57 −24.32 −0.11 0.00 −0.21 11.79

LILEzone

Ave (n = 5)

Ave (n = 4)

G/L

Ave (n = 3)

G/L

44.31 0.04 30.39 1.62 0.03 2.44 18.51 2.13 0.06

40.80 0.30 16.89 5.59 0.11 2.80 32.76 0.08 0.00

3.52 0.31 − 10.59 4.93 0.10 0.84 19.90 −2.04 −0.06

41.65 0.04 26.70 1.64 0.05 3.76 25.20 0.13 0.01

0.21 0.00 − 1.85 0.13 0.02 1.58 8.43 − 1.99 − 0.05

2.90

3.40

3.10

G/L: gain/loss. Opx⁎: enstatite, peridotite, Dawros: Rothstein, (1958).

Ave (n = 3) 43.11 0.04 28.93 1.93 0.03 4.45 17.92 1.74 1.19

2.90

LE1106

LE1106

G/L

Cpx

Trem

−4.18 0.09 3.39 3.19 −0.02 2.44 − 12.25 2.65 0.21 0.48

50.46 0.23 4.86 5.06 0.19 14.62 24.40 0.12 0.00 0.21 0.00 3.30

58.16 0.00 0.18 2.98 0.01 22.84 13.58 0.29 0.01 0.01 2.50 3.05

LE 17-06

LE 02-06

G/L

W.R.

W.R.

− 0.20 0.00 − 1.50 0.30 0.00 2.00 − 0.60 − 0.60 1.10

49.95 0.15 5.75 5.65 0.12 17.27 20.65 0.97 0.03 0.01

45.91 0.13 8.08 8.58 0.11 22.88 14.25 0.15 0.01 0.01

3.00

3.00

G/L − 4.00 0.00 2.30 2.90 0.03 5.60 − 6.40 − 0.80 0.00 0.00

LE06K

LE06K

LE1006

G/L

Ol

Cpx2

G/L

Opx⁎

Cpx2

3.29 − 0.23 − 4.69 − 2.31 − 0.18 6.49 −11.85 0.15 0.01 − 0.20 2.31

40.38 0.00 0.00 9.35 0.24 49.40 0.11 0.03 0.01 0.03

54.49 0.00 0.02 0.89 0.02 18.22 26.35 0.01 0.00 0.04

12.46 0.00 0.02 − 8.49 − 0.22 − 31.73 25.44 − 0.02 − 0.01 0.01

57.10 0.17 0.70 5.80 0.17 34.52 0.62 0.07 0.03 0.27

55.52 0.00 0.03 2.63 0.14 16.76 25.69 0.10 0.00 0.36

3.30

3.20

3.20

3.20

LE1006

LE1006

G/L

Cpx1

Cpx2

G/L

−1.60 −0.20 −0.70 −3.20 0.00 −17.80 25.10 0.00 0.00 0.10

51.30 0.30 4.05 4.15 0.11 15.60 24.14 0.15 0.00 0.35

55.52 0.00 0.03 2.63 0.14 16.76 25.69 0.10 0.00 0.36

3.37 − 0.30 − 4.02 − 1.56 0.03 0.90 1.15 − 0.05 0.00 0.00

3.25

3.20

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2 O Cr2O3 H2 O Density

LE1006

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

1996), and we interpret this intense alteration of the plagioclase layer to be linked to the rodingitization process. 4.2. Petrography, Ca-consuming and Ca-releasing reactions in the ultramafics The original mineralogy of the ultramafites is variable amounts of olivine, clinopyroxene (Cpx1), orthopyroxene (inferred on textural ground as outlined below) and chromite. In the studied samples, olivine typically shows deformation bands and is variably reacted to serpentine. Olivine is altered from the grain boundary inward and also along fractures transecting the grains, resulting in a mesh texture. The olivine (Table 5) of the ultra-

187

mafites ranges in composition from Fo85 to Fo95 and high Mgolivine (Fo93) may have high MnO contents above 1 wt.%. As outlined by Iyer et al. (in press-a) olivine changes composition during serpentinization and the variation observed here is caused by secondary processes. The clinopyroxene in the unaltered ultramafites are characterized by high CaO content between 23.9 and 26.4 wt.% and is the principal Ca-carrier in the unaltered ultramafites of Leka (Table 3). Primary clinopyroxene from the ultramafites has Al2O3 contents in the 2.2–7 wt.% range, low Na2O content, and the mg# ranges from 0.83 to 0.91. Replacement textures which document mobility of Ca are ubiquitous and can be grouped as a) Ca-releasing and b) Ca-consuming reactions. Six principal

Fig. 5. BSE images showing Ca-producing replacement textures in ultramafites from Leka. a. Clinopyroxene (cpx) replaced by serpentine (srp) according to R1. Note numerous small relicts of cpx enclosed in serpentine. The degree of serpentinisation increases from the lower left corner to the upper right corner. Sample LE10-06. b. Replacement of clinopyroxene(cpx) by serpentine (srp) along vein. The center of the vein is marked by numerous small inclusions of Fe-oxide (oxide). Sample LE10-06. c. Partial replacement of clinopyroxene (cpx) by chlorite (chl). The former outline of the central clinopyroxene grain can be traced by numerous small relicts of clinopyroxene in chlorite. Bright grains are Fe-oxides. Sample LE10-06. d. Dextral shear zone with increasing chloritization of clinopyroxene towards the centre of the shear zone. The structural relationship indicates syntectonic chloritization. Sample LE10-06. e. Replacement of cpx2 by pargasite (parg). Note also that chlorite (chl) replaces both pargasite and cpx2. This indicates that cpx2 and pargasite are unstable in equilibrium with an uncharged fluid. Sample LE17-06.

188

Table 7 Major and trace element analyses, sorted according to rock type LE 01- LE 29- LE 30- LE 35- LE 36- LE 38- LE 39- LE 40- Ave 06 06 06 06 06 06 06 06 (n = 8)

LE 05- L69 06

Zone/ rock

LILE-zone

Rodingite

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O P2O5 SUM LOI Mg# Zr ICP– MS Y XRF Sr XRF Rb XRF Zn XRF Cu XRF Ni XRF Ba XRF Co XRF Cr XRF V XRF Th ICP– MS Cs ICP– MS

43.68 0.04 29.12 1.89 0.03 4.08 17.89 2.01 1.14 0.01 99.89 1.02 0.81 b0.1

41.79 0.04 29.12 1.81 0.03 4.96 18.23 1.27 1.39 0.03 98.67 2.66 0.84 b0.1

42.78 0.05 29.22 1.70 0.03 3.67 18.32 1.62 1.21 0.01 98.61 1.11 0.81 b0.1

42.92 0.03 28.84 1.74 0.03 3.96 19.54 1.15 1.06 0.02 99.30 1.56 0.82 b0.1

43.45 0.04 27.66 2.24 0.04 5.71 16.98 1.92 1.41 0.01 99.46 1.05 0.83 b0.1

43.3 0.04 29.16 1.95 0.03 4.53 17.98 1.91 0.96 0.01 99.87 1.20 0.82 b0.1

43.46 0.04 29.76 1.99 0.03 3.76 17.38 2.34 0.87 0.01 99.61 1.23 0.79 b0.1

43.49 0.04 28.57 2.13 0.03 4.91 17.03 1.73 1.50 0.01 99.43 0.92 0.82 b0.1

43.11 0.04 28.93 1.93 0.03 4.45 17.92 1.74 1.19 0.01 99.35 1.34 0.82 b0.1

40.89 0.26 16.78 6.31 0.12 2.95 32.36 0.08 0.01 0.01 99.77 0.31 0.48 b0.5

40.24 0.34 17.31 6.22 0.12 2.20 32.75 0.06 0.00 0.03 99.27 0.35 0.41

40.91 0.30 16.86 6.21 0.11 2.88 33.00 0.05 0.00 0.01 100.33 0.31 0.48 b0.5

41.17 0.28 16.59 6.07 0.09 3.16 32.94 0.12 0.00 0.05 100.47 0.28 0.51 b0.5

40.80 0.30 16.89 6.20 0.11 2.80 32.76 0.08 0.00 0.03 99.96 0.31 0.47 b0.5

41.89 0.03 26.49 1.88 0.05 3.80 25.20 0.14 0.01 0.02 99.49 1.16 0.80 b0.5

41.58 0.05 26.53 2.02 0.05 3.94 25.01 0.14 0.01 0.01 99.34 1.31 0.79 1.3

41.49 0.03 27.07 1.59 0.05 3.54 25.38 0.11 0.01 0.01 99.27 0.81 0.82 b0.5

b1 201 13 11 13 41 9 12.5 87 42 0.1

b1 374 17 12 24 50 54 16 59 48 0.1

b1 243 15 10 17 39 18 11 59 38 b0.1

b1 309 16 13 127 98 39 17 133 40 b0.1

1 194 20 14 27 70 23 21 106 52 b0.1

b1 211 13 15 38 54 18 14 156 48 0.1

b1 190 10 12 163 101 14 17 114 43 b0.1

b1 236 22 13 27 57 27 17 89 46 b0.1

0.1 245.0 16.0 12.5 55.0 64.0 25.0 16.0 100.0 45.0 0.04

2 45 1 10 21 29 2.5 13 104 260 b0.1

3 46 2 11 2 19 2 10 127 286 n.a.

2 47 1 11 2 28 b1 11 125 256 b0.1

2 24 1 11 5 38 2 13 125 253 b0.1

2.3 41.0 1.3 11.0 7.5 29.0 1.6 12.0 120.0 264.0 0.0

b1 254 1 15 2 68 bdl 10 92 35 b0.1

0.5 284 1.5 15 1 47 2 10 111 39 0.4

2.5

3.3

3.1

2.6

3.5

2.3

2

4

2.91

b0.1

n.a.

b0.1

b0.1

b0.1

b0.1

0.1

All major elements determined by XRF.

LE 03- LE 04- Ave 06 06 (n = 4)

LE 07- LE 28- LE 41- Ave 06 06 06 (n = 3)

LE 37- LE 31- LE 32- LE 1206 06 06 06

Clz-zone

Mixed

Anorthosite/protolith

41.65 0.04 26.70 1.83 0.05 3.76 25.20 0.13 0.01 0.01 99.38 1.09 0.80 1.3

41.48 0.06 28.66 2.13 0.05 4.32 22.69 0.50 0.16 0.01 100.05 1.71 0.80 b0.5

43.55 0.05 30.93 1.84 0.03 2.57 17.88 2.18 0.10 0.03 99.15 1.44 0.73 1.4

43.93 0.02 31.20 1.38 0.02 2.34 18.77 2.02 0.04 0.01 99.73 1.45 0.77 b0.5

43.89 0.03 30.54 2.05 0.03 2.47 18.06 2.13 0.02 0.01 99.23 2.09 0.70 b0.5

b1 265 1 13 1 34 5 9 83 35 b0.1

0.0 268.0 1.2 14.0 1.0 50.0 2.3 10.0 95.0 36.0 0.2

b1 376 2 16 7 59 8 13 112 64 b0.1

b1 197 1 8 73 18 3 11 86 49 b0.1

b1 194 1 9 30 46 bdl 7 115 29 b0.1

b1 208 1 15 16 27 1 13 44 20 0.1

b0.1

0.10

0.1

0.1

b0.1

b0.1

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

Sample

Table 7 (continued ) 1607.01 3006.05 Ave (n = 5)

LE 02- LE 08- LE 11- LE 16- LE 17- LE 18- LE 19- LE 20- LE 33- P 3 06 06 06 06 06 06 06 06 06

1707.04 Ave (n = 11)

LE 10- LE 25- Ave 06 06 (n = 2)

LE 26- LE 27- LE 43- Ave 06 06 06 (n = 3)

Zone/ rock

Anorthosite/protolith

Pyroxenite

Websterite

Dunite

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O P2O5 SUM LOI Mg# Zr ICP– MS Y XRF Sr XRF Rb XRF Zn XRF Cu XRF Ni XRF Ba XRF Co XRF Cr XRF V XRF Th ICP– MS Cs ICP– MS

45.28 0.02 30.95 0.91 0.03 0.71 20.11 2.16 0.02 0.01 100.20 1.02 0.61 b0.5

44.91 0.06 28.34 2.84 0.04 4.12 17.72 2.16 0.10 0.01 100.30 3.96 0.74 0.6

44.31 0.04 30.39 1.80 0.03 2.44 18.51 2.13 0.06 0.01 99.72 1.99 0.73 0.4

45.91 0.13 8.08 8.58 0.11 22.88 14.25 0.15 0.01 0.01 100.10 4.41 0.84 0.7

46.58 0.31 7.52 6.58 0.11 15.42 20.54 0.38 0.04 0.01 97.49 1.51 0.82 0.5

48.14 0.21 3.88 10.52 0.13 22.07 14.79 0.09 0.00 0.01 99.84 2.35 0.81 0.7

48.74 0.10 5.28 10.02 0.14 13.69 21.62 1.15 0.14 0.36 101.24 0.34 0.73 b.5

49.95 0.15 5.75 5.65 0.12 17.27 20.65 0.97 0.03 0.01 100.55 0.80 0.86 0.9

40.29 0.28 18.28 7.50 0.12 16.07 18.17 0.10 0.00 0.01 100.82 4.40 0.81 0.7

51.83 0.08 4.13 5.67 0.11 20.61 17.51 0.29 0.01 0.01 100.25 1.62 0.88 0.7

47.35 0.11 2.02 8.04 0.16 28.04 15.15 0.06 0.00 0.01 100.94 1.11 0.87 0.5

49.01 0.17 6.94 9.34 0.12 22.90 10.48 0.39 0.03 0.02 99.41 3.04 0.83 0.9

50.23 0.07 1.82 5.20 0.12 25.15 17.09 0.08 0.00 0.01 99.77 3.86 0.91 0

51.92 0.08 3.80 4.70 0.10 20.13 19.25 0.29 0.01 0.01 100.29 2.17 0.89 b0.5

48.61 0.14 5.77 7.40 0.12 20.66 17.19 0.38 0.02 0.05 100.34 2.19 0.85 0.5

45.83 0.13 2.16 12.67 0.15 32.15 7.63 0.00 0.00 0.01 100.73 7.11 0.83 1.2

43.42 0.05 0.65 10.26 0.20 37.50 7.73 0.00 0.00 0.01 99.82 0.56 0.88 b0.5

44.63 0.09 1.41 11.47 0.18 34.83 7.68 0.00 0.00 0.01 100.30 3.84 0.86 0.6

40.75 0.02 0.51 9.90 0.15 46.67 0.03 0.00 0.00 0.01 98.04 6.80 0.90 b0.5

39.29 0.03 0.51 10.52 0.18 47.26 0.00 0.00 0.00 0.02 97.81 8.12 0.90 b0.5

40.00 0.02 1.03 11.98 0.21 46.37 0.43 0.00 0.00 0.01 100.05 3.41 0.88 b0.5

40.01 0.02 0.68 10.80 0.18 46.77 0.15 0.00 0.00 0.01 98.62 6.11 0.90 b0.5

b1 244 1 7 bdl bdl bdl bdl bdl 19 b0.1

1 162 2 17 14 47 bdl 16 284 59 0.1

0.0 201.0 1.2 11.0 27.0 28.0 0.8 9.4 106.0 35.0 0.04

2 14 1.3 31 108 301 b1 83 1162 163 0.1

3 2 1 29 131 299 b1 48 1560 441 0.1

3 2 1 24 311 364 b1 72 1682 229 b0.1

1 36 1 42 3 99 b1 57 314 90 0.2

3 7 1 20 81 299 b1 38 2868 297 0.1

3 9 b1 87 44 195 b1 66 436 367 0.1

2 2 1 19 57 292 b1 42 3452 202 0.1

2 b1 1 20 83 519 b1 81 1778 159 b0.1

3 1 1 37 189 284 3 87 1552 192 b0.1

2 1 1 22 81 605 b1 50 4060 129 b0.1

1 3 1 19 144 400 b1 39 2059 174 b0.1

2.3 7.0 0.9 31.8 112.0 333.0 0.7 60.0 1902.0 222.0 0.1

2 b1 1 39 55 588 b1 113 1702 148 0.1

1 b1 1 33 93 1524 1 123 3311 69 b0.1

1.5 b1 1 36 74 1056 0.5 118 2507 109 0.05

b1 b1 1 37 10 2047 b1 130 7340 46 0.1

b1 b1 2 36 16 2463 b1 131 6954 46 b0.1

b1 b1 1 45 13.5 2038 b1 136 9451 75 b0.1

b1 b1 1.3 39.3 13.2 2183 b1 132 7915 56 0.03

0.1

0.2

0.1

0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

b0.1

H. Austrheim, T. Prestvik / Lithos 104 (2008) 177–198

Sample

189

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replacement textures involving clinopyroxene are outlined below, of which three are Ca-producing. The replacement reactions have been balanced by Gresens analysis (Gresens, 1966) assuming constant volume (Table 6). 4.2.1. R1: Cpx → Srp Clinopyroxene is replaced by serpentine (Fig. 5a, b). Thin serpentine veins (0.5 cm across) transect the clinopyroxene. A progressive disappearance of the clinopyroxene grains towards the center of the vein can be seen (Fig. 5b). The center of the vein is a 20 μm wide zone with small oxide inclusions. Representative analyses of serpentine are listed in Table 5. Serpentine after clinopyroxene contains higher FeO and Al2O3 than serpentine growing after olivine. XRD determination of serpentine gave spectrum of antigorite. The composition of the phases involved (Tables 3 and 5) is used to balance the following reaction for sample LE10-06: R1: 100g Cpx1 þ 12:5g H2 O þ 12g MgO þ 1g FeO→80g Srp þ 18:5g SiO2 þ 24g CaO þ 1:5g Al2 O3 þ 0:2g Na2 O 4.2.2. R2: Cpx → Chl Also chlorite is found as an alteration product after clinopyroxene. The analyses listed in Table 4 contain variable Al2O3, ranging from 14 to 24.5 wt.%. Compositionally, chlorites range from penninite to sheridanite, but most classify as clinochlore. The lowest Al2O3 is found in replacement aggregates after clinopyroxene and orthopyroxene. XRD analyses of two samples demonstrate that the aggregates contained chlorite and not septechlorite. There is a compositional gap between the chlorites and the analysed serpentines (Tables 4 and 5), and since there is complete solid solution between serpentine (lizardite) and amesite (Chernosky et al., 1988; Wicks and O'Hanley, 1988), we regard them as chlorites. The chloritization is found to attack the pyroxene crystals from the grain boundary, but chlorite also forms veins and needles transecting the clinopyroxene grain. Small remnants of clinopyroxene can be used to reconstruct the outline of the original grains (Fig. 5c). In most cases the replacement is static; however, the chloritization is also concentrated along shear zones as illustrated in Fig. 5d. The replacement of clinopyroxene by chlorite can be expressed as follows for sample LE11-06 based on the analyses in Tables 3 and 4: R2: 100g Cpx1 þ 11:8g H2 O þ 12:9g Al2 O3 þ 6g FeO þ 6:6g MgO→86g Chl þ 26:4g SiO2 þ 24:3g CaO þ 0:1g Na2 O þ 0:2g TiO2 4.2.3. R3: Cpx → Amph Replacement of clinopyroxene by amphibole and with minor chlorite and serpentine is also found (Fig. 5e). The amphibole replacing clinopyroxene is either tremolite or pargasite, and locally both phases are formed. Since tremolite and pargasite both have lower concentration of CaO (ca. 12 wt.%) than clinopyroxene (N 24 wt.%), this reaction is also Ca-releasing if near-constant volume is assumed. In sample LE 16-06 a Fe-rich pargasite formed together with vesuvianite, and in sample

LE17-06 clinopyroxene is replaced by pargasite. The pargasite contains around 3 wt.% Na2O (Table 4) and can explain the Naenrichment of some pyroxenites (Table 7). A replacement of clinopyroxene by pargasite at constant volume can be expressed as follows: R3: 100g Cpx2 þ 3:4g Al2 O3 þ 3:2g FeO þ 2:4g MgO þ 2:7g Na2 O þ 2g H2 O→99g Parg þ 12:3g CaO þ 4:2g SiO2 4.2.4. R4: Cpx1 → Cpx2 Primary clinopyroxene (Cpx1) may also be replaced by a second generation of clinopyroxene (Cpx2) and minor chlorite ± serpentine, as observed in several of the studied samples (Fig. 6a, b). The contact between the two generations of pyroxene is sharp and occurs over distances of a few µm. Then, the Cpx2, which is typically porous, makes embayment into Cpx1 (Fig. 6b). Cpx2 is richer in CaO than Cpx1 (Table 3), and it is suggested that the reaction is Ca-consuming and can be written as follows for sample LE10-06: R4: 100g Cpx1 þ 3:4g SiO2 þ 1:1g CaO þ 0:9g MgO→99g Cpx2 þ 4g Al2 O3 þ 1:6g FeO: Fig. 5f reveals that Cpx2 are also replaced by pargasite. This means that as new pulses of fluid infiltrate the rock, the secondary phases become unstable and are also being replaced. 4.2.5. R5: Opx → Cpx In several of the studied samples (Table 1), distinct aggregates of chlorite overgrown by tremolite and Cpx2 are interpreted to represent replacement of former orthopyroxene grains according to the reaction: R50: Opx þ Ca2þ →Chl þ Trem þ Cpx Locally, aggregates consisting of only fibrous clinopyroxene with small amounts of chlorite (Fig. 6c) occur together with grains of Cpx1 and may represent an almost complete Cametasomatism of the orthopyroxene. This interpretation is based on the absence of orthopyroxene from an environment that originally must have been rich in orthopyroxene. Clinopyroxene after orthopyroxene is especially low in Al2O3 and Na2O, probably reflecting the composition of the replaced mineral. The end product of this Ca-consuming replacement can be written as follows for sample LE10-06: R5: 100g Opx þ 25:1g CaO→100g Cpx2 þ 1:6g SiO2 þ 0:2g TiO2 þ 0:7g Al2 O3 þ 3:2g FeO þ 17:8g MgO: Since no orthopyroxene has been found at Leka, a composition of enstatite from Dawros (Rothstein, 1958) has been used to balance the reaction above. 4.2.6. R6: Ol → Cpx Sample LE10-06 has three distinct types of clinopyroxene. In addition to Cpx1 and Cpx2 after orthopyroxene, a third type characterized by numerous magnetite and ferrichromite

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191

Fig. 6. BSE images showing Ca-consuming replacement textures in ultramafites from Leka. a. Replacement of cpx1 by cpx2. The replacement front is sharp and the replaced part contains needles of serpentine (srp). Sample LE11-06. b. Replacement of cpx1 by cpx2. The replacement front is sharp and makes embayment into cpx1. A high porosity zone follows the replacement front. Sample LE11-06. c. Relationship between three types of clinopyroxene. Cpx1 is the primary pyroxene. The turbid cpx2 is formed from orthopyroxene according to R5. Cpx3 contains numerous inclusions of Fe-oxide and formed by replacement of olivine by R6. Sample LE10-06. d. Olivine grain replaced by clinopyroxene (cpx3) according to R6. The dark veins in olivine are filled with an alteration product probably iddingsite. Needles growing perpendicular to the veins are serpentine (srp). Note that needles of clinopyroxene grow into the iddingsite and that serpentine (srp) also overgrows the iddingsite. Cpx3 contains numerous inclusions of Fe-oxides. Sample LE06-05K.

inclusions is found (Fig. 6c). We interpret this to be clinopyroxene formed by replacement of olivine. This interpretation is substantiated by textures from Lauvhatten (in the mantle section of the ophiolite), where olivine is veined by Cpx2 and Fe-oxide and locally pervasively replaced by Cpx2 and Fe-oxide as illustrated in Fig. 5d. Like clinopyroxene after orthopyroxene, the clinopyroxene after olivine is also low in Al2O3 and Na2O (Table 3). The texture shown in Fig. 6d also indicates that serpentinization occurred pre or (syn) Cametasomatism. The replacement of olivine by clinopyroxene can be written in the following way as calculated for sample LE06K: R6: 100g Ol þ 12:5g SiO2 þ 25:4g CaO→97g Cpx2 þ 31:7g MgO þ 8:5g FeOðMtÞ: Since this requires addition of Ca, it indicates that the mantle section also experienced Ca-metasomatism at least on a local scale. 4.3. Grossular and vesuvianite formation in the ultramfites As mentioned above and shown in Table 1, garnet and vesuvianite are locally developed as thin chains and veins where mafic and ultramafic layers are transected by fault or fracture zones. There also extensive Ca metasomatism may occur. As

opposed to garnet from rodingite garnet in pyroxenite (Sample LE08-06) is rich in goldmanite and uvarovite with concentration up to 3.3 and 12%, respectively. The Leka vesuvianite is Mg-free and contains high TiO2 content, ranging from 1.8 to 4 wt.% TiO2 (Table 2). To our knowledge, this is the first report of Mg-free vesuvianite. Sample LE18-06. which represents a pyroxenite layer transected by a fault zone c. 40 m east of the mapped rodingite locality, plays a special role in understanding the relationship between the Ca-releasing reactions and rodingitization as replacement of primary clinopyroxene by chlorite and formation of garnet (grossular) occur in the same sample (Fig. 7a). This sample displays a layering defined by domains of chlorite surrounded by epidote, and extensive growth of garnet occurs where cataclastic zones cut the chlorite domains (Fig. 7b). Sample LE08-06 was collected from a pyroxenite layer 4 m W of the rodingite layers and displays a clear relationships between chloritization of clinopyroxene and garnet formation. The sample is hydrated with local domains of chlorite with inclusions of small grains of garnet (50–100 µm across) and amphibole. Extensive growth of chlorite occurred in the strained parts of the clinopyroxene grains. Sample LE16-06, which represents a fracture zone 70 m N or the rodingite locality, consists of secondary porous pyroxene, pleochroic (pargasitic) amphibole, vesuvianite and grossular, typically forming mm thick veins. The secondary pyroxene appears to be an intergrowth of

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from the vein Cpx1 is replaced by Cpx2 according to reaction R4 (Fig. 8c). We interpret these textures to have developed as Ca released from the shear zone infiltrated the wall rock and caused Ca-metasomatism. The olivine was not replaced, but changed towards a more Mg-rich composition along veins (Fig. 8c) which may have been the transport channels for the Ca-rich fluids. 5. Major and trace element whole rock geochemistry 5.1. Composition of samples from the mapped rodingite locality

Fig. 7. BSE images showing textures in rodingitized ultramafite from Leka. a. Replacement of primary clinopyroxene (cpx1) by chlorite (chl), grossular (grt) and titanite (tit). Clinopyroxene contains lamellae of amphibole (amp). The amphibole lamellae can be followed as relicts in the chloritized area and in continuation of the lamellae in cpx1, indicating static replacement at constant volume. Sample LE18-06. b. Shear zone enriched in grossular (grt). The shear zone transects a chlorite-rich area (chl) with numerous small grains of grossular (bright) and a few clinopyroxene (cpx) grains. The chlorite may have formed by replacement of former clinopyroxene as illustrated in Fig. 5c. Small grains (cpx) are diopsidic and part of the rodingite mineral assemblage. Note oriented fabric in core of the shear zone suggesting syntectonic garnet growth. Sample LE1806.

pyroxene grains with varying brightness on the BSE image. The primary mineralogy of this sample is totally altered with formation of secondary pyroxenes, minor vesuvianite and a few grains of grossular. The samples illustrate that a rodingite mineral assemblage also forms in a pyroxenitic protolith.

Major and selected trace elements from samples from the rodingite and from the anorthosite protlith are listed in Table 7, and the variation in chemistry across the metasomatic zones mapped in Fig. 3 is displayed in Fig. 9a–f. Fig. 9a–f reveals that the various zones define distinct chemical systems in most cases with abrupt transitions resembling metasomatic fronts. The rodingite of Leka is characterized by high CaO contents, averaging 32.4 wt.% CaO compared to 18.5 wt.% for the average protolith. While Al2O3 is elevated in the anorthositic rocks (30.4 wt.%), it drops to 17 wt.% in the rodingite. Na2O has an average value of 2.13 wt.% in the protolith while the rodingite and the clz-zone are virtually sodium free (Na2O = 0.08 wt.%). K2O is low in the protolith, and rodingite is devoid of K2O. A relatively high K2O concentration (1.2 wt.%) is present in the LILE-enriched zone, where phlogopite is the principal K carrier. The zone of K-enrichment extends at least 3 m out from the rodingite. Rb, Ba and Cs, which are low in the rodingite, follow K and are enriched in the LILE-zone. The rodingite contains 41 ppm Sr, compared to 201 ppm in the average protolith and 245 ppm in the LILE-enriched zone. The contrasting variations of MgO and FeO (Fig. 9c) result in a low mg# in the rodingite. The minor elements Ti and Mn are enriched by a factor of 10 and 5 respectively, in the rodingite compared to the protolith, and V is enriched from 35 ppm in the protolith to 264 ppm in the average rodingite. Compared to its protolith, the Clz-zone displays the same evolution as the rodingite with respect to CaO, Na2O and SiO2, but to a lower degree, and the typical increase in TiO2 and V and decrease in Sr observed in rodingite is not observed in the Clzzone. Sr reaches the highest level (268 ppm) in this zone, indicating that some of the Sr leached from the rodingite is deposited in the Clz-zone.

4.4. Distribution of replacement textures around deformation features in the ultramafites

5.2. Composition of the ultramafic rocks

The Ca-consuming and Ca-releasing reactions display a zonal distribution centered on veins and shear zones. Sample LE20-06 (olivine websterite) is transected by a 2–3 mm wide shear zone with strongly granulated wall-rocks (Fig. 7a). Centrally, this shear zone consists of serpentine which is surrounded by a 1 mm wide zone of ultra fine-grained aggregate of clinopyroxene and minor olivine (Fig. 8b). The wall rock is Cpxdominated with veined olivine (Fig. 8c). The wall rock also contains aggregates of fibrous diopside, which represents sites of former orthopyroxene (cf reaction R5). At the same distance

The pyroxenite, websterite and dunite samples display a range in mg# from 0.73–0.91 (Table 7), compared to 0.83–0.91 and 0.89–0.90 for the primary clinopyroxene and olivine respectively. Such a variation is difficult to relate to modal variation in the primary minerals (olivine, orthopyroxene and clinopyroxene) as these will all have mg# close to that of clinopyroxene. The samples have variable amounts of Na2O varying from (b 0.05 wt.%) to c. 1.2 wt.%. High Na2O is related to replacement of clinopyroxene by pargasitic amphibole according to R3 and reflects secondary metasomatism. The clinopyroxenites contain

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193

Fig. 8. Replacement textures relative to main fluid pathways. a. Shear zone with riddle shear cutting wehrlite. The center of the shear zone is filled with serpentine formed both from olivine and from clinopyroxene (R1). The brown grains consist of fibrous clinopyroxene that is formed after orthopyroxene. Note the cataclastic texture of the wehrlite. Sample LE20-06. b. BSE image from shear zone showing fine-grained margin and serpentine (srp) at the center. c. BSE image from wall rock. Cpx1 is replaced by cpx2 (R4) and orthopyroxene is totally replaced by an aggregate of fibrous clinopyroxene (cpx2(opx)) according to R5. Note the veined olivine grains where the darker zones have a higher Fo content. The veins may have acted as channels for the Ca-metasomatizing fluid.

varying amounts of Al2O3 ranging from 2.0 to 8.6 wt.%. Extreme values of ca. 18 wt.% are found in the rodingitized clinopyroxenite sample. We also note that this sample is low in Sr, which indicates that it must have been low in plagioclase. For the clinopyroxenites, the V content varies between 90 and 441 ppm. The two samples that contain grossular are also highest in V and TiO2 concentrations. 6. Discussion 6.1. Gresens analyses Gresens analyses of the rodingite, the Clz and the LILE zones relative to their protolith assuming constant volume are listed in (Table 6) and reveal that per 100 g of protolith 20 and 8.4 g of CaO must have been added to form the rodingite and Clz-zone respectively, while the formation of the LILE-zone does not require additional CaO. A significant amount (10.6 g) of Al2O3 must have been leached to form rodingite and this Al must have left the plagioclase layer and is a potential source for chloritization of the clinopyroxene (R2). According to O'Hanley (1996) SiO2 is typically lost during rodingitization, but in the Leka case SiO2 remains constant or is added. The same analysis shows that c. 2 g of Na2O per 100 g of protolith is leached during

rodingite formation. Similar removal of Na during rodingitization was reported from ophiolites in the Central Alps (Evans et al., 1981; Puschnig, 2002, Li et al., 2004) and is a general feature of rodingitization as outlined by Coleman (1977) and O'Hanley (1996). K2O enrichment, like that found in the LILEzone has previously been reported in association with rodingites (O'Hanley 1996) who suggested that this K2O was washed from the rodingitized layers. In the Leka case K, Ba, Rb and Cs are all enriched in the LILE-zone. As shown on Fig. 3, the rodingite and Clz zones are small compared to the LILE zone, and can not have provided the whole excess K, Rb, Ba and Cs present in the LILE zone suggesting that these elements may have been added from seawater in accordance with the results of Scambelluri et al. (2004). Na does not follow the other alkalis, as both the rodingite and the Clz-zone contain only minute amounts of Na, while the protolith and the LILE zone both have around 2 wt.%. It is unclear whether the Na from the Clz-zone and rodingite form a high Na zone (nephrite) in the plagioclase layer outside the mapped area. An alternative sink of Na is the pyroxenite layers which are locally enriched in Na and K through R3 (see below). The increase in TiO2 and V displayed by the rodingite suggest hat these elements were mobile and added during rodingitization. This result is in contrast to that of Puschnig (2002), who found that V and Ti were not mobilized during ocean floor

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Fig. 9. Compositional variation along anorthosite layer 1 from the rodingite across the Clz- and into the LILE- zone, compared to average anorthosite protolith and average rodingite (Ave Rd). a). Variation in wt.% CaO and Al2O3. CaO and Al2O3 remain constant across the LILE zone. The Clz-zone has a higher CaO and lower Al2O3 contents. On entrance to the rodingite CaO increases and Al2O3 decreases abruptly. b) Variation in wt.% Na2O and K2O. Na2O remains at a constant level equal to the protolith (c. 2 wt.%) throughout most of the LILE zones but drops to low values in the Clz- and rodingite zones. K2O is decoupled from Na2O and shows a marked jump to ca 1 wt.% on entering the LILE zone. c) Variation in wt.% FeO and MgO. FeO is constant through the LILE- and Clz-zones and at the level of the protolith. FeO shows an abrupt increase upon entering the rodingite. The MgO profile is different from that of FeO with the highest values in the LILE- and Clz-zones and the lowest values in the rodingite. d) Rb and Ba also follow K2O and show a marked increase in the LILE-zone. They are low in protolith, rodingite and Clz-zone. e) V and TiO2 remain at the level of the protolith throughout the LILE and Clz-zones and make a marked jump in the rodingite. The profiles are similar to that of FeO. f) Variation in Sr and Ni. Sr in the LILE zone is around 200 ppm and increases towards the Clz zone. In spite of its high CaO values the rodingite has relative low value of Sr. Ni displays a profile similar to that of MgO.

alteration. However, our results demonstrate that V and Ti are closely correlated, with a near constant V/Ti ratio, and their mobility will not easily be detected on a Ti vs V diagram. Gresens analyses for the main replacement reactions outlined above assuming constant volume are presented in Table 6. Replacement of clinopyroxene by serpentine (R1) releases CaO (24 g), SiO2 (18.5 g) and Al2O3 (1.5 g) and requires addition of MgO (12 g), FeO (1 g) and H2O (9.8 g). Clinopyroxene replaced by chlorite (R2) also releases CaO (24 g) and SiO2 (26 g), but requires addition of 13 g of Al2O3. The Al2O3 required to form chlorite from clinopyroxene may in part come from formation of serpentine and tremolite from clinopyroxene (R1 and R3).

Rodingitization of the plagioclase layer releases Al2O3 (10.5 g ) and this Al may be fluxed back to the ultramafite. TiO2 and V show high enrichment factors in the rodingite as compared to its protolith. TiO2 is also the element that shows the relatively strongest reduction related to reactions 1 and 2 (from 0.2 to 0.1) and (from 0.32 to 0.02 wt.%), respectively. The replacement of olivine and orthopyroxene by clinopyroxene requires addition of 25 g CaO and releases 32 and 18 g of MgO, respectively. In the olivine replacement, 12.5 g of SiO2 is required, while orthopyroxene replacement releases 1.6 g of SiO2. Reactions 5 and 6 release 8.5 and 3.2 g FeO, respectively, and the formation of magnetite during replacement of olivine by clinopyroxene

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suggests that the excess FeO from reaction R5 forms magnetite. The replacement of Cpx1 by Cpx2 has minor effects on most element budgets, but releases 4 g of Al2O3 and will be an additional Al2O3 source for chlorite formation according to reactions 2 and 5'. Importantly, the replacement of clinopyroxene with pargasitic amphibole (R3) requires addition of Na and K. Since many samples show evidence of alteration in the form of chloritization, formation of secondary minerals like Cpx2, grossular and vesuvianite, we infer that the rocks have been altered on the whole rock scale. Notably, we see that sample LE02-06 which represents pyroxenite between the rodingitized layers (Fig. 2), contains 8 wt.% Al2O3 and low SiO2 (46 wt.%) compared to 3.7 and 51.8 in the primary clinopyroxene (Table 3). Estimation of the effect that the Ca-consuming and Ca-releasing reactions had on the whole rock chemistry is difficult as all samples are altered. However, using Gresens analysis we estimated the effect of the chloritization on the whole rock composition (Table 6) by comparing sample LE17-06 with sample LE02-06 (Table 7). Sample LE02-06 is strongly chloritized while sample LE 17-06 is mainly composed of primary clinopyroxene and only a few aggregates of chlorite with tremolite after orthopyroxene. The Na2O content of sample LE17-06 is high (0.97 wt. %) related to formation of pargasite according to R3. The analysis reveals that the elements Ca and Si are released and Al, Fe and Mg are consumed. This is similar to the replacement of clinoproxene by chlorite (Table 6) and suggests that the composition of this sample was modified by chloritization. The combined chemical and textural data suggest that the composition of the ultramafites changed on the whole rock scale. They both provided the elements needed to form the rodingite and were the sink for Na and Al leached from the rodingitized plagioclase layers. 6.2. Model for alteration in the ultramafites A model for the alteration and transport of elements based on the replacement textures, their spatial relations and the above Gresens analyses is illustrated in Fig. 9a for a clinopyroxenite with minor orthopyroxene. The model assumes that fluid percolates through a shear or fracture zone and interacts to form a series of metasomatic zones (Fig. 10a). Note that the different zones are not at the same scale and that their width will depend on the fluid flow in the channel. The fluid diffuses into the sidewall along grain boundaries and microfractures as indicated by the observed textures and serpentinization of clinopyroxene will take place according to R1. This will charge the fluid with Ca, Si, Al, Ti and possibly V. The Al released by R1 may allow formation of chlorite according to R2. This will further charge the fluid by Ca, Si Ti and V. This Ca and Si charged fluid may stabilize amphibole (tremolite and pargasite) which will form according to R3. This reaction will also release Ca and increase Ca in the fluid which becomes saturated in regard to clinopyroxene, and reaction R4 will start to form diopside. This Ca-rich fluid will react with orthopyroxene according to reaction R5. The replacement of orthopyroxene takes place in two stages, and we suggest that the chlorite stage may occur in contact with a fluid before it reaches the clinopyroxene saturation level. Reactions R1, R2,

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R3 and R4 are all hydration reactions and will desiccate the fluid and lead to supersaturation. The fluid percolating through the shear zone will be enriched in the elements released by reactions R1 and R2 when leaving the clinopyroxenite layer. In a rock where orthopyroxene dominates over clinopyroxene, the consumption of Ca by reaction R5 may prevent the fluid from reaching clinopyroxene saturation. It is also to be expected that a fluid infiltrating a dunite will be charged with different elements. The position of reaction R6 in the metasomatic column is not clear. Fig. 7b from the werhlite shows that olivine remains, but is altered towards higher Fo content, where reaction R4 is taking place and R5 is completed. In the strongly serpentinized harzburgite of the mantle section, at Lauvhatten olivine is partly replaced (R6) and only small relicts after clinopyroxene are found. In sample LE10-06 both olivine and orthopyroxene are replaced and Cpx1 is partly replaced by Cpx2. The relative stability of olivine and pyroxene during serpentinization is shown by experiments to be highly temperature dependent and although this is not known for replacement textures of the type described here, we cannot exclude a temperature effect. The replacement of Cpx2 and pargasite by chlorite as documented by Fig. 5f is taken as evidence for superposition of metasomatic zones in response to infiltration of new pulses of uncharged fluid. This also indicates that not only the primary minerals, but also the secondary clinopyroxene and pargasite are unstable in contact with an uncharged fluid. 6.3. A model for the rodingitization process The geochemical profiles along the plagioclase layer (Fig. 9), with compositional plateaus separated with abrupt changes in composition suggest that the rodingites formed as part of a metasomatic column (Korzhinskii, 1968). The garnet in the rodingite displays dissolution and reprecipitation textures which supports a metasomatic origin. Notably, the filling of voids (Fig. 4a) is a characteristic texture of the rodingite garnet, and according to Korzhinskii (1968) this is an important process in infiltration metasomatism. Also according to Korzhinskii (1968), the pressure in a void will increase as it fills and the mineral which is most oversaturated will eventually fill the void. In the rodingites of Leka the voids are filled by grossular, suggesting that the incoming fluid was most oversaturated by grossular. However, the rodingites are characterized by veining which cuts the first formed garnet. This suggests that the rodingite developed metasomatic zones by several pulses of fluids and that hydrofracturing occurred in the later stages of this process. This may also explain the zoning of the clinopyroxene. A model for the rodingitization of the plagioclase layers is presented in Fig. 10b. A fluid charged with elements (Ca, Ti, V) released from the pyroxenite layer is percolating along shear zones oriented perpendicular to the plagioclase layer. A metasomatic column is developed as the fluid infiltrates along the plagioclase layer. The most oversaturated mineral (grossular) forms an almost monomineralic central zone in the column. Elements accepted by grossular (Ti, V) are enriched in this zone, while Na, K, Rb, Ba, Cs and Sr were transported with the fluid.

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Fig. 10. Models for metasomatic evolution in ultramafics (pyroxenite) and plagioclase layers. a) An uncharged fluid enters the pyroxenite layer along a main fluid channel (shear zone/fracture zone) and interacts with the wall rock. During interaction with the wall rock the fluid becomes charged with elements. The elements in excess migrate into the main fluid channel and are transported to other lithologies to form rodingites in the plagioclase layers (Fig. 9b) and serpentine in the dunite layers. R1: Cpx → Srp, R2: Cpx → Chl, R3: Cpx → Amph, R4: Cpx1 → Cpx2, R5':Opx → Cpx + Trem + Chl R5:Opx → Cpx. b) Transport during rodingitization of the plagioclase layers. Fluid from the pyroxenite layer enters the plagioclase layers through a fluid channel and infiltrates the wall rock where it causes metasomatism. The outlined transport is based on Gresens analyses (Table 6) assuming constant volume. Na may be deposited in front of the LILE zone or may have left the plagioclase layer to form Na-enriched pyroxenite.

The next zone (Clz zone) is dominated by clinozoisite, and contrary to garnet this phase accepts Sr which is enriched in the clz-zone, while K, Rb and Ba are deposited in the LILEenriched zone. The Na-sink has not been found within the altered plagioclase layer. It is unclear whether this element is deposited further away in the plagioclase layer, as indicated on Fig. 10b or if the local enrichment in the pyroxenite layer (R3) represents the main Na-sink. It is also suggested that the excess Al from the rodingite zone is fluxed back to the pyroxenite layers (see Section 6.1). 6.4. The first description of a rodingitized layered sequence Most rodingites are found as bodies enclosed in serpentinites or in contact zones between serpentinite and its wall rock. Commonly the protolith is basaltic, although rocks of variable

composition can be transformed to rodingite (O'Hanley, 1996). The rodingite described here occurs in a layered sequence of pyroxenite (websterite), dunites and plagioclase-rich layers, and the protolith for the rodingites at the mapped locality was anorthositic in composition. In this sense, the Leka occurrence represents a new type of rodingite, although the principal components i.e. spatial relationship to serpentinites is verified. The Leka rodingites display the geochemical characteristics of other rodingites, with increasing CaO and removal of Na and K. Because the protolith is low in SiO2, the reduction in SiO2 did not occur in the Leka rodingite. The situation at Leka allows us to relate the rodingitization to metasomatic changes in the surrounding ultramafites, where a number of Ca-releasing reactions are documented. Importantly, we also find that Ca-metasomatism took place in the ultramafics away from the rodingite locality by formation of grossular

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and notably by replacement of olivine and orthopyroxene by diopside. Although we have not preformed a rigorous test of the scale of element transport, we can conclude that the length of element transport along the plagioclase layers is at least 5 m and that Al was mobile at the sample scale in the ultramafites (see Section 6.1). The dual role of the ultramafites as both a source and a sink of Ca is related to multiple fluid pulses that evolve in composition as they interact with the different lithological units. 6.5. Alteration at a spreading ridge? According to Furnes et al. (1988) the Leka ophiolite formed at a spreading ridge in a supra subduction and back arc setting. The rodingites of Leka like other rodingites elsewhere are a product of serpentinization, and both processes must then take place in the same tectonic setting. Two settings seem plausible for the formation of rodingites in the Leka ophiolite. They may (1) relate to alteration at a spreading ridge shortly after formation, or (2) to the regional Caledonian metamorphism. The textures and chemical changes described in this paper are those of fluid-rock interaction and not regional metamorphism. Several authors, (Bucher and Frey, 1994; Puschnig, 2002; Li et al., 2004) view rodingitization as an ocean floor process. Our conclusion is in accordance with this view, although we cannot exclude that some hydration also occurred as a result of obduction and Caledonian metamorphism. The strongly hydrated sequences of the Leka Ophiolite may therefore reflect oceanic alteration rather than regional greenschist facies metamorphism. The numerous shear zones and fractures, locally developed as a fracture cleavage, that transect the layering of the complex at high angle were obvious channels for the transport between the layers. Oceanic serpentinization is commonly found to be related to fault and shear zones (Cannat et al., 1992; Coulton et al., 1995). The fracture zones observed at Leka may reflect an externally imposed stress systems or they may relate to reaction assisted fracturing during serpentinization as suggested by Iyer et al. (in press-b). This implies that the greenschist facies regional metamorphism – as documented by the mineral assemblages in the overlying sediments – did not obliterate older features, such as those formed in the oceanic crust. This is also in agreement with the excellent preservation of primary structures in the cover sequence (Prestvik, 1974), that was deposited after obduction but before regional metamorphism. 7. Conclusions The rodingite of Leka formed in the layered sequence of an ophiolite complex and as such represents a new type of rodingite occurrence. Grossular-containing rodingites have been found over a 500 m long zone where the layered sequence starts to include plagioclase layers. Solute was transported between the lithological units on fracture and deformation zone that cut the lithological boundaries at high angle. A metasomatic column consisting of a central grossular zone, a clz-zone and a LILEenriched zone developed where fracture zones allowed transport of fluid and elements from the hydrating ultramafic parts to the

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plagioclase-rich layers. The grossular-rich zone shares many of the geochemical characteristics of known rodingites such as elevated CaO content, low concentration of Na2O, K2O and must have formed from the plagioclase layers by addition of CaO and removal of alkalis and aluminium. The rodingite zone is enriched in V and Ti. Furthermore, K2O, Rb, Ba and Cs are enriched in the LILE zone. This enrichment cannot be balanced by the element reduction found in the rodingite and suggests interaction with sea water. The surrounding ultramafites display textures and minerals that relate to the rodingitization process, and Ca released by serpentinization and chloritization of clinopyroxene is the main Ca source for the rodingitization. And locally along the fluid conduits, the ultramafites were also rodingitized with formation of grossular and vesuvianite. This dual role of the ultramafites as both a source and a sink of Ca are related to combination of multiple fluid pulses and changing fluid composition. Replacement of olivine by clinopyroxene occurs in parts of the complex where rodingites s.s. are missing and demonstrates that Cametasomatism played an important role during hydration of the Leka ophiolite complex. Acknowledgments We thank Muriel Erambert (Oslo) and Jasper Berndt-Gerdes (Münster) for help with EMP analyses. We are grateful to Muriel Erambert, Timm John, Karthik Iyer, Björn Jamtveit and Andrew Putnis for numerous discussions on the subjects of serpentinization and rodingitization and Muriel Erambert and Calvin Barnes for a critical reading of the manuscript. Karthik Iyer kindly provided sample LE06K from Lauvhatten. Alasdair Skelton gave valuable comments and encouraged us to publish the paper. We are grateful for the valuable comments from two unknown reviewers. The work was supported by a Center of Excellence grant from the Norwegian Research Council to PGP. H.A. acknowledges a Humboldt Award. References Aumento, F., Loubat, H., 1971. The Mid-Atlantic Ridge near 45° N. Serpentinized ultramafic intrusions. Can. J. Earth Sci. 8, 631–663. Bach, W., Garrido, C.J., Paulick, H., Harvey, J., Rosner, M., 2004. Seawater– peridotite interactions: First insight from ODP Leg 209, MAR 15 degrees N. Geochem. Geophys. Geosys. 5, Q09F26. Birtel, S., 2002. Fluid-rock interaction on alpine-type ultramafic rocks from the Norwegian Caledonides. Un.Publ. Ph.D thesis University of Freiburg. 322pp. Bucher, K., Frey, M., 1994. Petrogenesis of metamorphic rocks. SpringerVerlag, Berlin. Heidelberg. Cannat, M., Bideau, D., Bougault, H., 1992. The serpentinized peridotites and gabbros in the Mid-Atlantic ridge axial valley at 15 degrees-37'N and 16degrees- 52'N. Earth Planet. Sci. Lett. 109, 87–106. Chernosky Jr., J.V., Berman, R.G., Bryndzia, L.T., 1988. Stability, phase relations, and thermodynamic properties of chlorite and serpentine minerals. In: Bailey, S.W. (Ed.), Hydrous Phyllosiclicates (Exclusive of Micas). Mineralogical Society of America, Chelsea, Mich, pp. 295–346. Coleman, R.G., 1977. Ophiolites. Springer, Berlin. Coulton, A.J., Harper, G.D., O'Hanley, D.S., 1995. Oceanic versus emplacement age serpentinization in the Josephine ophiolite — implication for the nature of the MOHO at intermediate and slow-spreading ridges. J. Geophys. Res., Solid Earth 100 (B11), 22245–22260.

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