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1 Pergamon PII S (99) Geochimica et Cosmochimica Acta, Vol. 63, No. 22, pp , 1999 Copyright 1999 Elsevier Science Ltd Printed in the USA. All rights reserved /99 $ Cl-scapolite, Cl-amphibole, and plagioclase equilibria in ductile shear zones at Nusfjord, Lofoten, Norway: Implications for fluid compositional evolution during fluid-mineral interaction in the deep crust KÅRE KULLERUD 1, * and MURIEL ERAMBERT 2 1 Department of Geology, University of Tromsø, N-9037 Tromsø, Norway 2 Mineralogical-Geological Museum, Sarsgt. 1, N-0562 Oslo, Norway (Received September 25, 1998; accepted in revised form March 29, 1999) Abstract Cl-rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived Cl-bearing fluid in a gabbroanorthosite of the Flakstadøy Basic Complex, Lofoten, Norway. Amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals (orthopyroxene, clinopyroxene, biotite, and ilmenite) and plagioclase and internally in the grains of primary igneous plagioclase. The secondary minerals show large compositional variations on thin-section scale. The Cl content of scapolite varies between 0.3 and 0.95 apfu (atoms per formula unit), whereas amphibole shows Cl concentrations from 0 to 1.5 apfu. The primary igneous plagioclase (An 50 -An 60 ) underwent extensive recrystallization and alteration during the fluid rock interactions. Secondary plagioclase shows compositions in the range from An 20 to An 55. In general, plagioclase did not equilibrate with the fluid phase during alteration, due to the sluggishness of the cation exchange reactions between plagioclase and fluid. Occasionally, however, equilibrium among plagioclase, scapolite, and the fluid phase was attained. The compositional variations of amphibole, scapolite, and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios a Cl /a (CO3 ) 2 and a /a Cl OH. The large compositional variations of the minerals on thin-section scale thus indicate steep gradients in the fluid activity ratios a Cl /a (CO3 ) 2 and a /a Cl OH. The activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain-boundary fluid during the formation of hydrous silicates. Scapolite and the most Cl-enriched amphibole formed in equilibrium with an evolved fluid phase, enriched in Cl and CO 2. Copyright 1999 Elsevier Science Ltd 1. INTRODUCTION Scapolite is common in metamorphic rocks that have been altered during interactions with crustal fluids. Scapolite is stable over a wide range of pressure and temperature, and because of its ability to incorporate volatiles (e.g., Cl, CO 2, and SO 3 ), the mineral is a potential indicator of the activities of these volatile components during crustal processes. Several articles contain quantitative calculations of the activities of volatile components of metamorphic fluids on the basis of scapolite chemistry (e.g., Ellis, 1978; Moecher and Essene, 1990, 1991; Gómez-Pugnaire et al., 1994). Some problems remain, however, before the equilibrium between scapolite, fluid, and coexisting minerals is fully understood. One of the most difficult problems concerns the incorporation of anions in the large cavities of the scapolite structure (A-sites). The anion composition of scapolite commonly is described as a complete solid solution in the ternary system Cl CO 3 2 SO 4 2 (e.g., Klein and Hurlbut, 1993). Several authors (e.g., Donnay et al., 1978; Swayze and Clark, 1990), however, have pointed out the significant role of H in scapolite. HCl, HCO 3, HSO 4, and several other species may be present on the A-site, which complicate the calculations of equilibria involving scapolite. Another problem with understanding equilibria involving scapolite concerns the strong coupling between the substitutions (Na,K)Ca 1, SiAl 1, and Cl(CO 3 ) 1. Clearly, crystal chemical * Author to whom correspondence should be addressed (kaarek@ibg. uit.no) constraints play an important role on scapolite composition. At present, there are two extreme views on the partitioning of volatiles between scapolite and fluid. One suggests that the Cl content of scapolite is strongly controlled by charge-balance constraints imposed by the cation lattice (e.g., Pan et al., 1994). The second view is that the Cl content of scapolite is controlled principally by the NaCl activity of the equilibrium fluid (e.g., Jiang et al., 1994). The Cl-rich scapolites described in this paper were formed during reactions between igneous plagioclase and an externally derived Cl-bearing fluid in two 4-m-wide shear zones in a gabbroanorthosite (Kullerud, 1995, 1996; Markl and Bucher, 1998; Markl et al., 1998). Scapolite commonly occurs together with Cl-rich amphibole in fine-grained aggregates of recrystallized, and, occasionally, chemically modified igneous plagioclase. The well-defined relationship between the reactants and the products of the scapolite-forming reactions provides an excellent opportunity to study the mechanisms controlling scapolite composition. Cl- and Ba-enriched biotite and Cl-enriched amphibole have been described previously from the studied rocks (Kullerud, 1995, 1996), and recently, Markl and Bucher (1998) and Markl et al. (1998) reported metamorphic salt crystals from the shear zones. The Cl-bearing silicates show extreme compositional variations on thin-section scale, indicating the presence of large chemical potential gradients of the fluid rock system during mineral growth. Detailed control on the spatial variations of mineral compositions provides important information on the nature of the Cl-bearing fluid phase that once was present in the

2 3830 K. Kullerud and M. Erambert Fig. 1. Geological map showing the occurrence of the shear zones. Modified after Romey (1971). Mangerite is a hypersthene-bearing monzonite. rock. This paper focuses on the reactions involving scapolite, plagioclase, amphibole, and Cl-bearing fluid. 2. GEOLOGICAL SETTING The scapolite-bearing rocks analyzed in this study occur within two 4-m-wide shear zones in a gabbroanorthosite at Flakstadøy in Lofoten, northern Norway (Fig. 1). Prior to the formation of the shear zones, the gabbroanorthosite was intruded by two narrow ( 1-m thick) dikes of basaltic composition (Flaat, 1998). The subsequent deformation was focused along these dikes, which now occur as highly deformed and completely altered rocks in the central parts of the shear zones (Fig. 2a). The 1- to 1.5-m-thick marginal parts of the shear zones consist of deformed equivalents of the gabbroanorthosite (Fig. 2b). The degree of deformation of the rock decreases toward the margins of the shear zones. The gabbroanorthosite is composed of plagioclase, orthopyroxene, clinopyroxene, biotite, and ilmenite (Fig. 3a). For this study, a hand specimen of 6 kg (sample A1) representing the outermost 20 cm of the shear zones was sampled. In these parts of the shear zones, deformation resulted in moderate rotation and elongation of primary igneous mafic minerals (Opx, Cpx, Bt, and Ilm) and extensive recrystallization and alteration of igneous plagioclase (Fig. 3b). The deformation was associated with incomplete breakdown of the igneous minerals. Formation of Cl- and Ba-enriched biotite (Kullerud, 1995), Cl-enriched amphibole (Kullerud, 1996), and salt (halite-sylvite solid solution, Markl and Bucher, 1998; Markl et al., 1998) in the shear zones indicates that the deformation was associated with the infiltration of an externally derived Cl- and Ba-bearing fluid. Unfortunately, no fluid inclusions have been observed in the studied samples (see also Markl and Bucher, 1998). Abbreviations of mineral names after Bucher and Frey (1994). Fig. 2. (a) Contact between the deformed basaltic dike that occurs in the central part of the shear zone (right-hand side of the stippled line) and the deformed equivalent of the gabbroanorthosite (left-hand side). Length of chisel is 20 cm. (b) Transition from undeformed gabbroanorthosite (left-hand side) to shear-zone rock (right hand side). Length of pen is 14 cm.

3 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3831 Fig. 3. (a) Photomicrograph of the undeformed gabbroanorthosite. Long side 2.5 mm. (b) Photomicrograph of an area within the outer part of the shear zone showing the alteration of original igneous plagioclase (right-hand side) and clinopyroxene (left-hand side). Along the contact between plagioclase and clinopyroxene, amphibole has formed. Note the abundant fine-grained amphibole (gray grains) within the matrix of altered igneous plagioclase. Long side 2.5 mm. Complex reaction zones consisting of amphibole, garnet, plagioclase, and quartz have formed along the contacts between relics of igneous mafic minerals and the aggregates of recrystallized igneous plagioclase (Kullerud, 1996). The amphibole is typically zoned, with low Cl content [ 0.25 Cl apfu (atoms per formula unit)] adjacent to the relic igneous mafic minerals, and continuously increasing Cl content toward the recrystallized plagioclase (see Kullerud, 1996). Within the aggregates of recrystallized plagioclase, abundant small grains ( 1 mm) of the most Cl-enriched amphibole ( 1.0 Cl apfu) occur together with biotite, magnetite, and tiny needles ( 0.03-mm long) of kyanite (Figs. 3b, 4, and 5; see also Kullerud, 1995, 1996; Markl and Bucher, 1998; Markl et al., 1998). Kyanite was unequivocally identified by lattice constant measurements on a transmission electron microscope (G. Markl, pers. comm., 1998). Scapolite is generally rare in the shear zone; however, in some samples, Cl-rich scapolite occurs in association with plagioclase and Cl-rich amphibole. Two different textural occurrences of scapolite are observed. In the aggregates of recrystallized plagioclase, small grains of amphibole and scapolite with irregular grain boundaries can be observed (Figs. 4c and 5b). The texture suggests that amphibole and scapolite were formed during interactions between the primary igneous plagioclase and an infiltrating Cl-bearing fluid phase. In addition, scapolite occasionally occurs in association with the complex reaction zones between primary plagioclase and primary mafic minerals (Fig. 4b). Kullerud (1995, 1996) argued that the Cl-bearing minerals were formed rapidly under nearly isobaric and isothermal conditions (see also Markl and Bucher, 1998; Markl et al., 1998). Following the model of Kullerud (1995, 1996), the shear-zone rock was saturated with respect to a Cl-bearing fluid along shear surfaces, microfractures, and grain boundaries during the initial deformation. The occurrence of relics of igneous minerals and the complex reaction zones between the primary igneous minerals suggest that the presence of a free fluid phase in the rock was very transient and that the integrated fluid flux was low. Kullerud (1995, 1996) suggested that the influx of the externally derived fluid phase stopped shortly after the initial grain-boundary saturation. Subsequently, the grain-boundary fluid was consumed during amphibole- and biotite-forming reactions. Because Cl has a pronounced preference for hydrous fluids relative to amphibole and biotite, it can be assumed that the first hydrous minerals that formed during the fluid rock interactions were Cl-poor. Continued hydration reactions, however, resulted in a successive increase in the Cl content and the activity ratio a Cl /a OH of the fluid phase and a gradual development of fluid-absent domains in the rock. According to this model, the low-cl amphibole and biotite formed in domains that developed dry grain boundaries at an early stage, whereas the most Cl-enriched amphibole and biotite formed in equilibrium with salt and the last droplets of a highly saline grainboundary fluid. Furthermore, it can be assumed that the chemical zonation of the amphibole that formed along the contacts between the primary igneous minerals monitors the successive changes in fluid composition during the amphibole growth (i.e., growth zonation). This mechanism of successive chemical evolution and consumption of the free fluid phase during fluid rock interaction has later been termed the desiccation mechanism (Markl and Bucher, 1998). Garnet-biotite geothermometry (Kullerud, 1995) in combination with garnet plagioclase kyanite quartz equilibrium indicate that the Cl-enriched minerals formed under pressure temperature conditions of 550 to 620 C and 8 to 11 kbar (Markl and Bucher, 1998). Subsequent reactivation of the shear zones at temperatures above 680 C and pressures above 15 kbar resulted in the formation of eclogite facies assemblages in the central parts of the shear zones (Wade, 1985; Markl and Bucher, 1997). Apparently, the deformation associated with the formation of the high-pressure mineral assemblage was strongly focused along the central parts of the shear zones because the Cl-bearing assemblages elsewhere in the shear zones are preserved and show no evidence of a late high-p metamorphic overprint. Similar sharp transitions between highpressure shear-zone rocks and their surrounding rocks of low-p mineralogy have been reported elsewhere (e.g., Austrheim and Engvik, 1997). 3. MINERAL COMPOSITIONAL DATA For the present study, 130 analyses of scapolite, 100 analyses of amphibole, and 100 analyses of plagioclase from the

4 3832 K. Kullerud and M. Erambert Fig. 4. (a) Backscatter image of the shear-zone rock. Light gray mafic minerals, mainly amphibole and garnet (garnet occurs only along the left edge and in the lower left corner of the image). Dark gray plagioclase and scapolite. Small white grains magnetite. Boxes b and c indicate positions of b and c. Dots along the line C-D indicate positions of mineral analyses presented in Figure 9. Black spots along the upper and the right edges were caused by a marker pen. (b) Line drawing showing the texture of amphibole, scapolite, and garnet. Dots along line A-B indicate positions of mineral analyses presented in Figure 9. (c) Line drawing showing the alteration product of igneous plagioclase. b and c were drawn on the basis of backscatter images and element maps (Fe and Cl). shear-zone rock have been carried out. They include analyses of 80 pairs of scapolite and amphibole in contact and 70 pairs of scapolite and plagioclase in contact. The analyses were carried out on five thin sections prepared from one hand specimen (sample A1). In addition, 15 analyses of the igneous plagioclase from two samples of the undeformed gabbroanorthosite (samples G6 and G8) have been carried out. The quantitative mineral analyses were carried out on the CAMECA CAMEBAX MICROBEAM at the Mineralogical-Geological Museum, Oslo, Norway, by using natural and synthetic standards and CAMECA s PAP software for data reduction. To avoid migration of light elements (e.g., Na), the analyses were performed with a defocused electron beam ( m 2 ), with a probe current of 10 na and an accelerating voltage of 15 kv. The counting time was 10 s on the peaks and 5sonthe background on each side of the peaks. Light elements (Na and Fig. 5. (a) Backscatter image of the shear-zone rock. Small dark gray minerals are quartz (upper left corner) and kyanite (in the central and right part of the image). (b) Line drawing of the area shown in (a), drawn on the basis of backscatter image and element-maps (Fe and Cl). Dots along line E-F indicate positions of mineral analyses presented in Figure 9. K) were analyzed first during the analytical sequence. The size of the scan area was the smallest possible for which no significant Na loss was recorded during plagioclase analysis. Calibration for all elements and mineral analyses were performed by using the same analytical conditions. In particular, to counteract the effect of loss of X-ray intensity in areas outside the focal point, calibration was also performed in scanning mode (same raster size). The quality of the analytical procedure was checked against scapolite standards as unknowns. Selected analyses of pairs of scapolite and amphibole in contact are given in Table 1, whereas analyses of scapolite plagioclase pairs are given in Table 2, together with average analyses of plagioclase from the samples of undeformed gabbroanorthosite Scapolite Scapolite formulas were initially calculated by normalizing to Si Al 12 apfu, as recommended by Teertstra and Sherriff (1997). This resulted in contents of M-site cations above the ideal value of 4. Therefore, the formulas were recal-

5 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3833 culated by normalizing to cations 16. F and Ba were not detected, and these elements were not included during analysis. Furthermore, scapolite contains no Ti, Mg, or Mn. The contents of Fe and S are generally low ( 0.05 apfu and 0.1 apfu, respectively). The contents of Si Al relative to Na K Ca suggest that Fe is principally coordinated in tetrahedral sites as Fe 3. Some extent of vacancies on the tetrahedral sites may also be inferred. Scapolite shows a large continuous variation in Cl content, from 0.3 apfu to 0.95 apfu (Fig. 6; Tables 1 and 2). Si and Na show close positive correlations with Cl, and there are similar negative correlations between the Cl content and the contents of Al and Ca. It should also be noted that, although the K content is low, there is a clear positive correlation between K and Cl. A similar correlation between K and Cl has been reported for scapolite from calc-silicate rocks (Markl and Piazolo, 1998). The strong correlations between the variables in Figure 6 suggest that the variations in scapolite composition can be described as a mixture between two end-member components, one Cl-free and one with the A-site completely occupied by Cl. The components have been calculated by linear regression: Cl-free scapolite: Na K 1.25 Ca 2.78 Al Fe Si 7.31 O 24 A 2 (1) where A 2 denotes a divalent anion or anion group, and Cl-scapolite: Na K 3.10 Ca 0.90 Al Fe Si 8.13 O 24 Cl (2) Alternatively, the variations in scapolite composition can be approximated by the complexly coupled exchange reaction: Na K 1.9 Si 0.8 Cl 1 N Ca 1.9 Al Fe A 2 1 (3) The Cl content of the analyzed scapolites varies considerably, but none of the calculated structural formulas indicates that the A site is completely occupied by Cl. Several species have been identified in scapolite from other occurrences, e.g., S 2 and SO 4 2 (see review in Teertstra and Sherriff, 1997). For the scapolites presented here, however, the S content generally is too low to account for the rest of the A site occupancy. We therefore assume that CO 3 2 is the major divalent component on the anion site in addition to Cl. The Cl content of scapolite clearly is correlated to the textural occurrence. The most Cl-enriched scapolite ( 0.75 Cl apfu) occurs within the aggregates of recrystallized plagioclase, commonly together with amphibole (Figs. 4, 5, and 9). The scapolite associated with the complex reaction zones between the primary igneous minerals generally contains 0.75 apfu Cl Amphibole The amphibole analyses presented here range in composition from edenite to chloropotassic sadanagaite (classification after Leake et al., 1997). Amphibole shows compositional variations similar to the trends described by Kullerud (1996), with positively correlated concentrations among Cl, Fe, Al, and K but negative correlations between Cl and Si and Cl and Mg (Fig. 7). Ca and Na apparently are uncorrelated to Cl. Several of the analyses presented here show considerably higher content of Cl (and of components positively correlated with Cl) than reported by Kullerud (1996). The most Cl-rich amphiboles contain slightly above 5 wt.% Cl, which corresponds to 1.5 Cl apfu (Table 1). As pointed out by Kullerud (1996), amphibole composition is closely correlated to the textural occurrence. The most Clenriched amphibole occurs together with Cl-rich scapolite in the recrystallized plagioclase matrix, whereas the amphibole that formed along the contacts between the primary igneous minerals generally has lower Cl content (Fig. 9). Kullerud (1996) showed that the Cl contents of amphibole and biotite in contact are highly correlated. He inferred that the Cl contents of amphibole and biotite were principally controlled by the activity ratio a Cl /a OH of the equilibrium fluid during mineral growth Plagioclase Plagioclase from the undeformed gabbroanorthosite has a relatively constant composition with An content in the range from An 50 to An 60 (Fig. 8). Plagioclase from the shear-zone rock, however, shows large compositional variations, covering the range from An 20 to An 55. Figure 9 shows that the composition of the recrystallized (and apparently chemically altered) plagioclase varies between the two extreme values within a single aggregate of 1 1 mm. 4. MINERAL FLUID EQUILIBRIA AND NONEQUILIBRIA The introduction of the Cl-bearing fluid into the igneous protolith of the shear-zone rock resulted in a fluid rock system far from thermodynamic equilibrium, causing incomplete dissolution of the primary igneous minerals and precipitation of secondary minerals of highly variable compositions. The large compositional variations of scapolite, amphibole, and plagioclase within single thin sections indicate extensive chemical disequilibrium between the minerals of the shear-zone rock. Kullerud (1995, 1996), however, argued that chemical equilibrium was attained locally between the secondary minerals, e.g., between secondary minerals in contact. Following the desiccation mechanism (see above), he argued that the compositional variations of amphibole and biotite closely monitored the chemical variations of the evolving equilibrium grain-boundary fluid during mineral growth. Several lines of evidence demonstrate that the compositions of amphibole and biotite in the studied rocks were controlled by the composition of the grain-boundary fluid. The first argument is funded on the systematic textural and compositional variations of amphibole (Fig. 7; see also Kullerud, 1996) and biotite (Kullerud, 1995). Kullerud (1996) showed by principal component analyses that 93% of the total variance of the amphibole composition could be described by one complexely coupled exchange vector. He concluded that the variations in amphibole composition to a large extent were controlled by only one chemical system variable, which could be related to the activity ratio a Cl /a OH of the fluid phase. Similarly, Kullerud (1995) showed that the compositional variations of biotite could be related to two chemical system variables, namely a Cl /a OH and a Ti 4 at the biotite-fluid interface. On the basis of the strong

6 3834 K. Kullerud and M. Erambert Scapolite Table 1. Selected microprobe analyses of pairs of scapolite and amphibole in contact. Sample pair # A1-Y 1 A1-Y 2 A1-Y 3 A1-Y 4 A1-L 5 A1-L 6 A1-L 7 A1-K 8 A1-K 9 A1-K 10 A1-I 11 A1-I 12 SiO Al 2 O FeO CaO Na 2 O K 2 O Cl SO Total Total corrected for Cl Structural formula of scapolite: cations 16 Si Al Fe Ca Na K Cl S Amphibole SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cl Total Total corrected for Cl Structural formula of amphibole: cations (Ca Na K) 13 Si Ti Al Fe Mn Mg Ca Na K Cl (Continued) positive correlation between the Cl contents of amphibole and biotite in textural equilibrium (Fig. 9 in Kullerud, 1996), it was inferred that chemical equilibrium was attained along the grain boundaries of amphibole and biotite in contact. An important implication of the conclusions of Kullerud (1995, 1996) is that Si, Al, Fe, Mg, Ca, Na, K, and Ba were present in excess during growth of amphibole and biotite (i.e., the mineral compositions were not controlled by the transport rate of these components). Kullerud (1995, 1996) pointed out that the large amounts of ferro-magnesian minerals (Am, Bt, Grt, Ep, and Mag) in the matrix replacing primary igneous plagioclase (e.g., Figs. 3 5) show that Fe and Mg were efficiently mobilized on a small scale (2 3 mm) during the fluid rock interactions. The conclusions of Kullerud (1995, 1996) are supported by structural refinement studies of Cl-amphibole (Makino et al., 1993; Oberti et al., 1993). These data show that the Cl content of amphibole is controlled principally by the activity ratio a Cl /a OH of the equilibrium fluid, whereas crystal chemical

7 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3835 Table 1. (Continued) Scapolite Sample pair # A1-I SiO Al 2 O FeO CaO Na 2 O K 2 O Cl SO Total Total corrected for Cl Structural formula of scapolite: cations 16 Si Al Fe Ca Na K Cl S Amphibole SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cl Total Total corrected for Cl Structural formula of amphibole: cations (Ca Na K) 13 Si Ti Al Fe Mn Mg Ca Na K Cl constraints related to the cation composition of amphibole (e.g. the Fe/Mg ratio) are of much less importance (Kullerud, 1996, 1999). Thus, the large compositional range of amphibole and the strong correlations between the amphibole components (Fig. 7) indeed indicate the presence of steep a Cl /a OH gradients in the grain-boundary fluid. Markl et al. (1998) gave further evidence for the desiccation mechanism, including one that was based on a careful analysis of the modal distribution of amphiboles of variable Cl content in the shear-zone rock. When a fluid of a specific composition reacts with a rock following the desiccation mechanism, the modal abundances of Cl-rich relative to Cl-poor hydrous minerals will be strictly constrained. Markl et al. (1998) showed that the observed modal distributions of Cl-bearing amphibole and biotite closely matched the theoretical values. On this basis, they were able to calculate the initial composition of the fluid phase that infiltrated the rock. To include the reactions involving scapolite and plagioclase in the model, we refer to the images showing the occurrence of scapolite and amphibole in the matrix of altered igneous pla-

8 3836 K. Kullerud and M. Erambert Scapolite Table 2. Selected microprobe analyses of pairs of scapolite and plagioclase in contact. Sample pair # A1-Y 25 A1-Y 26 A1-Y 27 A1-Y 28 A1-L 29 A1-L 30 A1-L 31 A1-K 32 A1-K 33 A1-K 34 A1-I 35 A1-I 36 SiO Al 2 O FeO CaO Na 2 O K 2 O Cl SO Total Total corrected for Cl Structural formula of scapolite: cations 16 Si Al Fe Ca Na K Cl S Plagioclase SiO Al 2 O CaO Na 2 O K 2 O Total Structural formula of plagioclase: cations 5 Si Al Ca Na K (Continued) gioclase (Figs. 4 and 5). Several important conclusions can be drawn based on these textures: 1) Scapolite and amphibole formed when the primary igneous plagioclase was infiltrated by the Cl-bearing fluid phase during recrystallization. As pointed out earlier, Fe and Mg were efficiently mobilized by the fluid phase during amphibole growth. 2) In some domains, the igneous plagioclase reacted completely and was replaced by scapolite and amphibole. 3) Most commonly, however, igneous plagioclase reacted to polygranular aggregates of plagioclase with lower An content. The alteration was apparently associated with the formation of tiny grains of kyanite within the plagioclase aggregates. Below, the implications of these observations on the equilibria among scapolite, plagioclase, amphibole, and the evolving fluid phase will be discussed Plagioclase-Fluid The formation of low An plagioclase and kyanite during the alteration of the igneous plagioclase suggests that the interaction between plagioclase and the fluid phase occurred by the reaction: CaAl 2 Si 2 O 8 Na 1.5 Si OH NaAlSi 3 O Al 2 SiO 5 Ca H (4) Apparently, the re-equilibration of plagioclase was controlled by the activity ratio a Na /a Ca 2, the ph, and the activity of Si 4 of the fluid phase. Following Eqn. 4, a reduction of the

9 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3837 Table 2. (Continued) Scapolite Sample pair # A1-I 37 A1-I Average G6 (n 7) Average G8 (n 8) SiO Al 2 O FeO CaO Na 2 O K 2 O Cl SO Total Total corrected for Cl Structural formula of scapolite: cations 16 Si Al Fe Ca Na K Cl S Plagioclase SiO Al 2 O CaO Na 2 O K 2 O Total Structural formula of Plagioclase: cations 5 Si Al Ca Na K An content of 1 mol of plagioclase from An 50 to An 40 will result in the formation of 0.05 mol of kyanite. This corresponds to a modal ratio between kyanite and plagioclase of 1:50, which is in agreement with the observed modal abundances of the minerals (Figs. 4 and 5). Thus, it is inferred that Eqn. 4 gives a good description of the actual reactions that were going on during the alteration of plagioclase. This implies that Na, Ca 2, and Si 4 were dissolved in the fluid phase, whereas Al 3 behaved as an immobile component and was incorporated in kyanite. The symplectitic intergrowth of quartz and amphibole that can be observed in the complex reaction zones between primary igneous plagioclase and mafic minerals (Fig. 5; see also Kullerud, 1996), however, indicates a relatively low mobility of Si 4. Most likely, the amount of Si 4 and Na that was necessary for the progress of Eqn. 4 was acquired locally from the domains of complete plagioclase dissolution (see point 2 above). Epidote, garnet, and amphibole provided possible sinks of Ca 2 in excess from Eqn. 4. The large compositional variations of the secondary plagioclase (Fig. 8) may have two explanations: 1) as for amphibole and biotite, the compositional variations of plagioclase resulted from large activity gradients in the equilibrium grain-boundary fluid during the alteration of the primary igneous plagioclase, or 2) due to the sluggish kinetics of the coupled plagioclase fluid exchange reaction (Eqn. 4), chemical equilibrium was approached but generally not attained between plagioclase and the fluid phase. Below, it will be argued that the composition of the fluid phase that reacted with the primary igneous plagioclase varied considerably. The compositional variations of plagioclase and scapolite in contact, however, indeed suggest that plagioclase generally did not equilibrate completely with the fluid phase Scapolite Plagioclase-Fluid Figure 10 shows the variations in the equivalent anorthite content [EqAn (Al in scapolite 3)/3] and the Cl content of scapolite. Also shown is the An content of plagioclase in contact with scapolite. Scapolite plagioclase pairs in contact are indicated by tie-lines. Several studies have documented that

10 3838 K. Kullerud and M. Erambert Fig. 6. Compositional variations of scapolite. scapolite of sodic to intermediate compositions is Ca- and Al-rich relative to coexisting plagioclase (e.g., Goldsmith and Newton, 1977; Mora and Valley, 1989; Pan et al., 1994; Rebbert and Rice, 1997). For the majority of the scapolite plagioclase pairs from the shear zone, however, plagioclase shows higher contents of Ca and Al than scapolite, as indicated by the negatively sloping tie-lines in Figure 10. For some scapolite plagioclase pairs, the tie-lines are nearly vertical, whereas only a fraction of the scapolite plagioclase pairs show positively sloping tie-lines characteristic for equilibrium. The variations in the Al- and Ca-contents of plagioclase and the Al content of scapolite in contact with plagioclase are shown in Figure 11. The compositions of scapolite and plagioclase from mineral pairs with negatively sloping tie-lines in Figure 10 are apparently uncorrelated. For mineral pairs with nearly vertical and positively sloping tie-lines, however, clear correlations can be observed between scapolite and plagioclase compositions. For comparison, data of coexisting scapolite plagioclase pairs from the medium grade metamorphic rocks ( 520 C) of the Wallace Formation (Rebbert and Rice, 1997) have been included in Figure 11. The data of Rebbert and Rice (1997) clearly show that the compositions of scapolite and plagioclase in equilibrium are correlated. We therefore infer that the scapolite plagioclase pairs showing positively sloping tie-lines in Figure 10 (i.e., filled circles in Fig. 11) represent scapolite and plagioclase compositions at or close to chemical equilibrium. For the majority of the analyzed scapolite plagioclase pairs, however, equilibrium was apparently not attained between the minerals during growth of scapolite and recrystallization and alteration of plagioclase. Most likely, the apparent Fig. 7. Compositional variations of amphibole. Gray-shaded symbols are data from Kullerud (1996); open and filled circles are new data. nonequilibrium between scapolite and plagioclase reflects that plagioclase generally did not equilibrate completely with the fluid phase, due to the sluggish kinetics of the exchange reactions, as pointed out above. For the scapolite plagioclase pairs connected with positively sloping tie-lines in Figure 10, however, it can be assumed that plagioclase has approached compositions in equilibrium with the fluid phase. One important conclusion that can be drawn on the basis of the data in Figure 11 is that the composition of scapolite was not controlled by the composition (e.g., the Al/Si ratio) of the primary igneous plagioclase being replaced. This implies that the variations in scapolite chemistry principally were related to variations in the activities of the components dissolved in the infiltrating fluid phase. Furthermore, if it is correct that the scapolite plagioclase pairs showing positively sloping tie-lines in Figure 10 represent equilibrium compositions (or compositions approaching equilibrium), it can be inferred that scapolite and plagioclase compositions to a large extent were controlled by the same chemical system variable(s) during growth because the compositional variations of the mineral pairs are highly correlated (Fig. 11).

11 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3839 The exchange of NaCl and CaCO 3 between scapolite and fluid can be written: NaCl Scapolite CaCO 3 fluid CaCO 3 Scapolite NaCl fluid From this reaction it can be inferred that the variations in the composition of scapolite were related to variations in the activity ratio (a Na a Cl )/(a Ca 2 a (CO3 ) 2 ) of the equilibrium grain-boundary fluid during scapolite growth. Plagioclase, on the other hand, does not incorporate Cl or CO 2 3. This suggests that the variations in plagioclase composition principally were related to the variations in the fluid activity ratio a Na /a Ca 2. In saline solutions, however, the concentrations of the dissolved cations are strongly dependent on the anion concentrations of the fluid (e.g., Eugster and Gunter, 1981; Hanor, 1993; Yardley, 1997). This suggests that the compositional variations of scapolite and plagioclase that equilibrated with the fluid phase principally reflect variations of the activities of the volatile components of the grain-boundary fluid Scapolite Amphibole-Fluid Fig. 8. Compositional variations of plagioclase. Amphibole and scapolite in contact show clearly correlated Cl concentrations (Fig. 12). For the least Cl-enriched mineral pairs, however, the data show a relatively large scatter. It Fig. 9. Compositional variations of amphibole, scapolite, and plagioclase along the profiles A B in Figure 4b, C D in Figure 4a, and E F in Figure 5b.

12 3840 K. Kullerud and M. Erambert Fig. 10. Compositional variations of scapolite and plagioclase in the EqAn-Cl diagram. Pairs of scapolite and plagioclase in contact are indicated by tie-lines. Symbols:, scapolite plagioclase pairs with negatively sloping tie-lines suggesting chemical disequilibrium between plagioclase and scapolite; plagioclase compositions probably represent slightly modified igneous compositions. Small circles, scapolite plagioclase pairs with nearly vertical tie-lines suggesting nonequilibrium between plagioclase and scapolite; plagioclase compositions probably represent moderately altered igneous compositions. Large circles, scapolite plagioclase pairs with positively sloping tie-lines suggesting that equilibrium was attained between scapolite and plagioclase. Squares, additional scapolite analyses. cannot be ruled out that equilibrium was incompletely attained between the analyzed mineral pairs during growth or that the equilibrium compositions of the minerals were modified at a later stage. However, we believe that most of the scatter of the data in Figure 12 indicate that the compositions of scapolite and amphibole were dependent in different ways on the chemical variables of the equilibrium fluid. From the discussion above, it can be concluded that the composition of scapolite was principally controlled by the activity ratio a OH /a (CO3 ) 2 of the equilibrium fluid. Amphibole fluid equilibrium was discussed by Kullerud (1996), who emphasized the strong coupling between Cl and the cation composition of amphibole. He inferred that the cation composition of amphibole was dictated by crystal chemical constraints, induced on the crystal structure by Cl incorporated on the anion site. Thus, after the model of Kullerud (1996), the composition of Cl-amphibole (i.e., the contents of both anions and cations) is principally controlled by the fluid activity ratio a Cl /a OH. It is therefore inferred that the compositional variations of scapolite and amphibole shown in Figure 12 principally reflect variations in the activity ratios a Cl /a OH and a Cl /a (CO3)2 of the grain-boundary fluid during mineral formation. Variations in the fluid activity of Cl at constant values of a OH /a (CO3)2 [ (a Cl /a 2 )/(a /a (CO3 ) Cl OH )] resulted in correlated concentrations of the Cl contents of the minerals, corresponding to the general trend of the data in Figure 12. The scatter of the data, on the other hand, in particular for the lowermost Cl minerals, can be attributed to variations in the fluid activity ratio a OH / a (CO3 ) 2 independent on the variations of a. Cl 5. CONSTRAINTS ON FLUID COMPOSITION Based on the equilibrium between biotite, plagioclase, kyanite, garnet, and a H 2 O NaCl bearing fluid and the equilibrium between amphibole and biotite, Markl et al. (1998) formulated a set of equations that relates the Cl content of amphibole and biotite to the activity ratio a H2 O/(a NaCl ) 2 of the equilibrium fluid. From the experimental data of Aranovich and Newton (1996), they then could calculate the composition of the equilibrium fluid in terms of X H2 O ( H 2 O/(H 2 O NaCl)). The samples of the present study were collected at the same locality as the samples used by Markl et al. (1998). We could, therefore, by adopting their arguments, apply their equations directly. The calculations suggest that the most Cl-enriched amphibole of the present study (X Cl 0.75) was formed in equilibrium with a fluid phase of composition X H2 O 0.32 (calculated at P 9 kbar, T 600 C). The equations of Markl et al. (1998) were based on the assumption that the fluid was a binary H 2 O NaCl solution. The presence of scapolite in the shear-zone rock, however, suggests that CO 2 also was a constituent of the fluid phase. Equations of state for H 2 O NaCl CO 2 fluids have been published by Bowers and Helgeson (1983) and Duan et al. (1995). The accuracy of these equations of state is, however, uncertain at P 9 kbar and T 600 C, because of the lack of experimental data at high pressures. However, a qualitative description of the evolution of H 2 O NaCl CO 2 -bearing fluids during the operation of the desiccation mechanism can be carried out based on the schematic phase diagram in Figure 13. If it is correct that the fluid rock system remained closed after the initial fluid influx, it can be assumed that the CO 2 content of the fluid phase was low because of the rare occurrence of scapolite that is the only potential CO 2 -bearing mineral that has been observed in the rock. It cannot be ruled out, however, that the CO 2 content of the fluid initially was higher, because a CO 2 -enriched hydrous fluid may have separated from the Cl-enriched fluid (see path A on Fig. 13) and subsequently escaped from the rock. Studies on synthetic fluid inclusions in the system H 2 O NaCl CO 2, however, suggest that the Cl-enriched fluid would escape more

13 Cl-scapolite, Cl-amphibole, and plagioclase equilibria 3841 Fig. 11. Variations in Al and Ca in plagioclase vs. Al in scapolite. Symbols: and small open circles as in Figure 10. Large filled circles, scapolite plagioclase pairs with positively sloping tie-lines in Figure 10. Squares, data from Rebbert and Rice (1997). easily than the CO 2 -enriched hydrous fluid after unmixing, because of the differences in the wetting behavior of the two fluids (Gibert et al., 1998). Therefore, it can be assumed that the fluid rock system behaved as a closed system, also with respect to CO 2 during the fluid rock interactions. Thus, the CO 2 initially present in the infiltrating fluid phase was successively enriched in the fluid phase during the amphibole and biotite forming reactions and finally bonded in scapolite. A possible evolutionary trend for the fluid is illustrated in Figure 13 (path B), which also includes the fluid evolution trend for the binary H 2 O-NaCl system (path C). 6. THE SCAPOLITE EXCHANGE MECHANISM The scapolites of the present study show large compositional overlap with scapolites from other occurrences (Fig. 14). This Fig. 12. Variations in the Cl contents of pairs of scapolite and amphibole in contact. Fig. 13. Schematic ternary phase diagram for the system H 2 O NaCl CO 2. Three possible trends of fluid evolution during preferential extraction of H 2 O are indicated. Path A: The X CO2 /X NaCl ratio of the initial fluid phase is relatively high. When X H2 O is sufficiently lowered, the fluid will unmix and subsequently evolve to a NaCl-rich hydrous fluid phase (A )andaco 2 -rich hydrous fluid phase (A ). Path B: The X CO2 /X NaCl ratio of the initial fluid phase is relatively low. The fluid does not unmix during its evolution, but evolves toward NaCl-rich compositions. Path C: Fluid evolution in the binary system H 2 O NaCl.