National Technical University of Athens, Heroon Polytechniou 9, Zografou, Athens, Greece *Corresponding author,

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1 Eur. J. Mineral. 2009, 21, Published online September 2008 High-pressure/low-temperature metamorphism of basalts in Lavrion (Greece): implications for the preservation of peak metamorphic assemblages in blueschists and greenschists Ioannis BAZIOTIS 1, *, Alexander PROYER 2 and Evripidis MPOSKOS 1 1 Department of Geological Sciences, School of Mining and Metallurgical Engineering, National Technical University of Athens, Heroon Polytechniou 9, Zografou, Athens, Greece *Corresponding author, baziotis@metal.ntua.gr 2 Institute of Earth Sciences, Karl-Franzens University, Universitätsplatz 2/II, 8010 Graz, Austria Abstract: The Upper Tectonic Unit of the Lavrion area is part of the Attic-Cycladic blueschist belt and was affected by high-pressure, low-temperature metamorphism. Blueschists and greenschists occur in the same outcrop and are believed to have experienced the same pressure temperature (P T) history which has been quantified using geothermobarometry and pseudosections for specific bulk-rock compositions. Calculated P T conditions indicate minimum pressure of 0.9 GPa and temperature of 370 C for the peak of metamorphism. The prograde and retrograde paths followed a very similar low geothermal gradient (10 12 C/km) with cooling during decompression. Pseudosections show that both blueschists and greenschists can exist stably at the metamorphic peak, the dominant amphibole being a function of bulk composition: the blueschists, on average, have lower Mg# than the greenschists, which results in a larger P T stability field of blue amphibole. A pseudosection analysis of the dehydration behaviour indicates that blueschists and some greenschists can preserve their peak assemblages (no dehydration along the retrograde path), whereas greenschist assemblages, in general, are rather prone to undergo dehydration and hence re-equilibration to lower P T conditions during exhumation. Key-words: blueschist, greenschist, preservation, fluid availability, Lavrion area. Introduction The spatial association of blueschists and greenschists is well known from various blueschist-facies terranes (e.g. Okrusch & Bröcker, 1990; El-Shazly et al., 1997). The preservation of blueschist-facies assemblages requires rapid exhumation or cooling during exhumation (e.g. Draper & Bone, 1981; England & Thompson, 1984; Ernst, 2006 and references therein). Numerical thermal modelling suggests that low geothermal gradients accompanied by fast exhumation (> 1 km/ma, Draper & Bone, 1981) and cooling during decompression are necessary to preserve blueschist-facies assemblages and prevent greenschist or amphibolite facies overprinting (Ernst, 1988). Fluid infiltration is a major factor causing retrogression. Variable fluid infiltration (Bröcker, 1990) results in different degrees of overprinting and hence transformation to greenschists. However, the bulk composition of the protolith can influence the degree of preservation of blueschist-facies assemblages (e.g. Barrientos & Selverstone, 1993). In the Upper Tectonic Unit (UTU) of the Lavrion area, blueschists and greenschists are exposed in the same outcrop with no tectonic contact between them. Whereas in most Cycladic Islands, the prograde and retrograde metamorphic DOI: / /2008/ conditions are well constrained by various methods, like conventional geothermobarometry, pseudosections or stable isotopes (e.g. Bröcker et al., 1993; Will et al., 1998; Trotet et al., 2001; Parra et al., 2002b). Only limited P T data are available for the Lavrion blueschists and greenschists (Baltatzis, 1996). The aim of this study is to reconstruct the prograde and exhumation P T paths for the metabasalts of the Lavrion area and understand the factors that make them a blueschist or a greenschist. Geological setting The area of Lavrion is a part of the Attic-Cycladic crystalline belt (ACCB), which was built as a stacked sequence of nappes during the Early Eocene (Dürr et al., 1978). Three major groups of units can be found within the Cycladic area: a lower group (basal unit), an intermediate (Cycladic Blueschist Unit; sometimes also called lower unit in the literature) and an upper group of units. The Lavrion area represents the westernmost continuation of the ACCB. According to Marinos and Petrascheck (1956) the area consists of two tectonic units: the Lower Tectonic Unit (LTU) and the Upper Tectonic Unit /08/ $ 7.20 Ó 2008 E. Schweizerbart sche Verlagsbuchhandlung, D Stuttgart

2 134 I. Baziotis, A. Proyer, E. Mposkos Fig. 1. (a) Simplified geological map of Lavrion area after Marinos and Petrascheck (1956) showing the areal distribution of the metabasalts. (b) Schematic columnar section showing the lithology and the order of tectonic nappes that can be found in Lavrion area. Inset: extent of Attic-Cyladic crystalline belt, ACCB (grey). (UTU, also termed phyllite nappe ), both belonging to the Cycladic Blueschist group of units. More recent studies by Photiades & Carras (2001), Photiades et al. (2004) and Photiades & Saccani (2006) propose a similar tectonostratigraphic subdivision, with a lower para-autochthonous unit (referred to as Kamariza Unit ) at the bottom, followed by a middle blueschist unit (referred to as Lavrion blueschist Unit ) and an upper unit mainly consisting of non-metamorphic rocks. The Kamariza Unit corresponds to the LTU and the Lavrion blueschist Unit to the UTU (Fig. 1b). The LTU and UTU are separated by a low-angle detachment fault (Skarpelis, 2007). The LTU, with late Triassic to early Jurassic protolith ages, comprises metaclastic rocks ( Kaesariani schists ) sandwiched between marbles ( Upper and Lower marble). According to Photiades & Saccani (2006), the LTU was metamorphosed under greenschist to amphibolite-facies conditions. A Tertiary age (pre-late Miocene) is assumed for the regional metamorphism of the LTU by analogy with well-dated rocks in the Cyclades. The UTU is dominated by low-grade high-pressure/lowtemperature (HP/LT) metapelites and metasandstones with minor intercalations of marbles. It also includes metamorphosed mafic volcanics mostly retrogressed to greenschists and late Cretaceous fossiliferous limestones, remnants of a non-metamorphic nappe (Skarpelis, unpublished data). An Eocene age was suggested for the peak of the HP/LT metamorphism (Altherr et al., 1982). By application of Raman spectroscopy to carbonaceous material method of micaschists, peak metamorphic temperatures of 320 C and 420 C have been obtained for the UTU and the LTU, respectively (Baziotis et al., 2006). The areal distribution of metabasalts in Lavrion is shown in Fig. 1. They occur as thick boudinaged layers within metapelites of the UTU, either close to the low-angle thrust plane or as massive bodies higher in the section. The thickness of individual bodies may reach up to 50 m. The nature of contacts of the metabasalts with the host metapelites is most likely primary, i.e. the metabasalts are of volcanosedimentary origin.

3 High-pressure/low-temperature metamorphism of basalts in the Lavrion 135 Analytical method Mineral chemical analyses were carried out with a JEOL JSM scanning electron microprobe at Institute of Earth Sciences, Karl-Franzens University, Graz, Austria. The operating conditions were: acceleration voltage 15 kv, beam current 5 na, interval for data acquisition 100 s and beam diameter 4 8 lm. The following natural and synthetic standards were used: quartz (Si), titanite (Ca, Ti), corundum (Al), rhodonite (Mn), garnet (Mg, Fe), adularia (K) and jadeite (Na). All samples were measured with energy dispersive system, except for Na that was analysed with wavelength dispersive system. Representative mineral compositions are listed in Table 1. Petrography and mineral chemistry Based on the dominant amphibole phase, two types of metabasalts can be distinguished: blueschists with abundant glaucophane and greenschists that are dominated by actinolite. The rocks are massive with poorly developed foliation. Primary volcanic textures are still preserved. They include fragmental textures and porphyritic fabrics with clinopyroxene phenocrysts (up to 100 lm in size). Plagioclase is replaced pseudomorphically by aggregates of pumpellyite, chlorite, calcic amphibole and albite. The fine-grained matrix consists mainly of chlorite, glaucophane, actinolite, pumpellyite and albite. The blueschists are characterized by the mineral group: glaucophane þ epidote þ chlorite þ albite þ actinolite þ pumpellyite omphacite þ phengite þ lawsonite biotite calcite quartz, whereas in the greenschists the dominant mineral group is: actinolite + chlorite + albite + epidote + pumpellyite þ glaucophane ± stilpnomelane ± calcite + quartz. In the field, the ratio of greenschists/blueschists varies between 65:35 and 75:25, with greenschist being the predominant rock type. Detailed petrographic investigation revealed the peak mineral assemblage, as well as prograde and retrograde phases (Table 2). In particular, the peak mineral assemblage is glaucophane þ epidote þ chlorite þ actinolite omphacite quartz and phengite in both blueschists and greenschists. The prograde mineral phases, that mainly occur as inclusions in epidote and albite, comprise lawsonite and pumpellyite (Pmp-I; Fe-poor), whereas the retrograde phases are albite + pumpellyite (Pmp-II; Fe-rich) ± biotite ± stilpnomelane ± calcite ± quartz as well as more actinolite and chlorite. Representative compositions of minerals are given in Table 1 and the mineral stability along the various metamorphic stages is given in Table 2. Besides, the extended glaucophane stability field is somehow overestimated for all the greenschists and clearly applied for the blueschists and for some greenschists. Clinopyroxene occurs in two varieties. Primary magmatic clinopyroxene (Fig. 2a) classified as augite (Fig. 3a) (Morimoto, 1988) is preserved in various modal proportions in both blueschists and greenschists. Augite occurs as euhedral or anhedral crystals, replaced by chlorite and amphibole. It shows normal compositional zoning with increasing Fe/Mg ratio towards, the rim. Metamorphic clinopyroxene (omphacite) is observed only in two blueschist samples (Fig. 2b). The jadeite component ranges from 23 to 35 mol% (Fig. 3b). Omphacite is replaced by glaucophane and epidote. Amphiboles are the dominant rock constituents: (ferro-) glaucophane in the blueschists and actinolite in the greenschists. The structural formulae of amphiboles (Table 1) were calculated using the PROBE-AMPH spreadsheet (Tindle & Webb, 1994). Mineral names were assigned according to the International Mineralogical Association classification, by means of proportions. Blue amphiboles occur in various modes: parallel to the foliation, replacing omphacite, rimmed by calcic amphibole, as inclusions in albite and epidote or in veinlets filled with calcite. The blue amphibole composition in the blueschists varies from glaucophane to ferroglaucophane (Fig. 4a; Table 1) with the Na B ranging from to atoms per formula unit (a.p.f.u.). The green amphiboles are colourless to greenish and classified as actinolite (Fig. 4b). In the blueschists, actinolite has higher R 3+ at a given Na B content than that in the greenschists, attributed to a higher Fe 3+ content due to the Fe-rich bulk composition of the blueschists (Fig. 4c). Mg-hornblende is observed only as inclusions in glaucophane (Fig. 2c). It probably represents a relic of a pre-hp event. Three types of zoning are observed in the amphiboles: glaucophane rimming actinolite (Fig. 2d), glaucophane with increasing Al IV,Al VI and Na B values towards the rim, and actinolite mantling glaucophane (Fig. 2c). The zoning patterns in the glaucophanes are characterized by increasing Tschermak and glaucophane components towards the rim consistent with increasing temperature and pressure along a prograde path (e.g. Laird & Albee, 1981; Triboulet, 1992). The two different modes of actinolite represent two different frames within the metamorphic P T evolution. The actinolite inclusions are interpreted as a stage of a prograde path (transition from pumpellyite actinolite to blueschist-facies conditions and continued growth of glaucophane at the expense of actinolite in the blueschist facies), whereas the actinolite overgrowths on glaucophane took place at post-peak conditions, during retrograde greenschist-facies overprinting. Chlorite is the dominant mineral phase in the matrix. The composition of chlorite is more or less constant within each sample, but varies from sample to sample depending on the bulk-rock composition. The X Mg (Mg/(Mg + Fe)) ratio ranges from 0.44 to 0.67 and the Si content from to a.p.f.u. Pumpellyite occurs as inclusions in epidote and albite, as small interstitial anhedral grains or as aggregates of needlelike crystals in the fine-grained matrix associated with chlorite, albite, epidote, actinolite and calcite (Fig. 2e). Pumpellyite aggregates pseudomorphic after plagioclase are also common. The composition of pumpellyite ranges from 0.14 to 0.27 in terms of Fe/(Fe + Al) (Fig. 5). According to formula recalculation, all Fe is ferrous. In low-grade rocks, pumpellyite commonly shows scattering in compositions mainly controlled by the local

4 136 I. Baziotis, A. Proyer, E. Mposkos Table 1. Selected representative microprobe analyses of amphiboles (Amph), clinopyroxenes (Cpx), chlorites (Chl), phengites (Phg), pumpellyites (Pmp), epidotes (Ep), lawsonite (Lws) and stilpnomelane (Stp). Sample A A Amph Amph Amph Amph Amph Amph Amph Amph Amph Amph BS BS BS BS BS BS GS GS GS GS SiO TiO Al 2 O FeOt MnO MgO CaO Na 2 O K 2 O Total Si Al IV Ti Al VI Fe Fe Mn Mg Ca Na K Sample B Amph Amph Amph Pmp Pmp Ep Ep Lws Stp GS GS GS BS BS GS GS BS SiO SiO TiO TiO Al 2 O Al 2 O FeOt FeOt MnO MnO MgO MgO CaO CaO Na 2 O Na 2 O K 2 O K 2 O Total Total Si Si Al IV Al Ti Fe Al VI Fe Fe Ti Fe Mn Mn Ca Mg Mg Ca Na Na K K environment of crystallization, especially by proximity to Fe-bearing phases (Offler et al., 1981; Cortesogno et al., 1984). Moreover, the occurrence of pumpellyites with variable Fe 2 O 3 content, within the same sample, is probably attributed to evolving f O2.Intwoofthestudied samples Fe-rich pumpellyite is associated with an Fe-poor variety. The association of both pumpellyites indicates either two generations of pumpellyite, with the Fe-poor one being a relic of the prograde stage, or the coexistence of two pumpellyites formed under pumpellyite actinolite metamorphic conditions (Coombs et al., 1976; Schiffman & Liou, 1980; Cortesogno et al., 1984). The pumpellyite inclusions in epidote and albite display more or less similar composition with that of the Fe-poor matrix

5 High-pressure/low-temperature metamorphism of basalts in the Lavrion 137 Table 1. Continued. Sample A B Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx BS BS BS BS BS GS GS GS GS GS SiO TiO Al 2 O Cr 2 O FeOt MnO MgO CaO Na 2 O K 2 O Total Si Ti Al (T) Al (M1) Fe 3+ (T) Fe 3+ (M1) Fe Mn Mg Ca K Na Sample Chl Chl Chl Chl Chl Phg Phg Phg Phg BS BS BS GS BS BS BS BS GS SiO SiO TiO TiO Al 2 O Al 2 O FeOt FeOt MnO MnO MgO MgO CaO CaO Na 2 O Na 2 O K 2 O K 2 O Total Total Si (T1+T2) Si Al (T2) Ti Al (M4) Al IV Mg (M1) Al VI Fe (M1) Fe Al (M1) Fe (M1) Mn Mg (M2+M3) Mg Fe (M2+M3) Ca Al (M2+M3) Na K (M2) Cr omitted from the structural formula of the pyroxenes. pumpellyite, probably suggesting the presence of two pumpellyite generations. Phengite flakes occur in about 3 5 % of the samples and are homogeneous in composition. The structural formulae of phengites were calculated according to the method described by Vidal & Parra (2000) and Parra et al. (2002a and b). The Si content ranges from 3.38 to 3.58 a.p.f.u. Most compositions plot below the Tschermak substitution line (Fig. 6),

6 138 I. Baziotis, A. Proyer, E. Mposkos Table 2. Mineral stability along the metamorphic path of Lavrion blueschists. Glaucophane Epidote Chlorite Albite Actinolite Pumpellyite (Pmp-I) Pumpellyite (Pmp-II) Omphacite Phengite Lawsonite Biotite Calcite Stilpnomelane Quartz Prograde stage Peak stage Retrograde stage indicating that part of the Fe is in trivalent state substituting for Al. Na contents are low (< 0.08 a.p.f.u.) and Na shows an inverse correlation with celadonite component (Table 1). Albite contains inclusions of glaucophane, actinolite, pumpellyite, epidote and lawsonite. It is almost pure albite endmember, with 1 % An content. Relic pre-hp plagioclase (An ), replaced in part by pumpellyite, is observed. Epidote is a common phase in the metabasalts. It occurs as large porphyroblasts and as clusters in the matrix. Most of the large matrix porphyroblasts exhibit a chemical zoning with increasing Fe 3+ and decreasing Al, towards the rim. The pistacite component in epidote ranges from 1.1 to 28.2 mol% for the blueschists and 13.7 to 27.7 mol% for the greenschists. However, variable bulk composition of the blueschists and greenschists seems to play an important role in the chemistry of the epidote grains, as epidote composition varies between samples. Lawsonite occurs only in some blueschists, as inclusions in albite and epidote (Fig. 2f ). It is almost pure, with minor Fe 3+ substituting for Al (Table 1). Stilpnomelane occurs as a late-stage mineral in both the blueschists and greenschists associated with chlorite and epidote in veinlets filled with calcite. The K 2 O content of stilpnomelane ranges from 1.43 to 1.71 wt.%. P T estimates Thermobarometric calculations The epidote glaucophane omphacite assemblage defines peak metamorphic conditions in the epidote blueschist facies (Evans, 1990). Lawsonite inclusions in epidote and albite probably formed at an earlier stage of the prograde P T path. Since lawsonite does not share common field boundaries with retrograde albite, late-stage albite has most likely replaced former omphacite overgrowth on lawsonite. The reaction ab! jd + qtz calculated for the maximum observed jadeite content (Jd = 35 %), using THERMO- CALC v. 3.1 of Holland & Powell (1990), constrains minimum pressure between 0.7 and 0.9 GPa for the temperature range C, assuming that clinopyroxene coexisted with albite (Fig. 7). The Si content of phengite ( a.p.f.u.), according to the method of Parra et al. (2002a and b), yields minimum pressures of GPa, whereas application of the phengite geobarometer of Massonne & Szpurka (1997) yields GPa, for the temperature range of C (Fig. 7); the pressures are considered as minimum since K-feldspar is not present and biotite is not associated with the analysed phengites. Peak and retrograde P T conditions were calculated from the compositions of mineral phases that are in close contact and considered to be stably coexisting. For the peak metamorphic conditions we have used core compositions for the retrograde stage rim compositions. The thermodynamic dataset and solid-solution properties are from the TWEEQU ver (Berman, 1991; updated database of Berman, 1988). The P T conditions are calculated using a multiequilibrium method based on the assumption of local equilibrium between the minerals chosen (Vidal & Parra, 2000; Parra et al., 2002a, b; Rimmelé et al., 2005). The methods and reactions used are the same as in Willner et al. (2004) and Willner (2005) (Table 3). The solid-solution models used are listed in Table 3. The peak metamorphic stage and a retrograde stage, both reflected by the zoning of amphiboles, were calculated (Fig. 7) using the multivariant reactions 1 4 (Table 3). The calculated invariant points define peak P T conditions at pressures ranging from 0.8 to 0.9 GPa and temperatures from 330 to 380 C, and retrograde greenschistfacies conditions at pressures of GPa and temperatures of C. From the close inspection of the equilibrated mineral assemblages, we can state that both blueschists and greenschists (only sample B) preserve peak or near-peak P T conditions (Table 3). However, most of the greenschist samples have equilibrated at P <0.7GPaandT < 345 C. The calculated P T conditions given in Table 3 come from six different samples, which are distributed in two broad localities (north of Sounion and N NE of Thorikos Bay; see Fig. 1). In the broad area of Sounion, the equilibrated conditions lie near the peak of metamorphism and range from 0.85 to 0.92 GPa and 330 to 370 C (samples B and ). Conversely, in the broad area of Thorikos Bay,

7 High-pressure/low-temperature metamorphism of basalts in the Lavrion 139 Fig. 2. (a) Phenocryst of magmatic augite. (b) Omphacite associated with glaucophane, epidote and phengite. (c) Inclusions of Mghornblende in glaucophane. Glaucophane is rimmed by actinolite. (d) Actinolite inclusion in glaucophane. (e) Pumpellyite glaucophane and actinolite inclusions in epidote. (f) Retrograde albite with lawsonite and glaucophane inclusions. the retrogression is more intense. The calculated P T conditions are mostly within the greenschist facies and range from 0.57 to 0.70 GPa and 270 to 345 C (samples , A and ); only one sample from this area (050801) reflects higher P T conditions at 0.91 GPa and 380 C. Pseudosection calculations To assess the P T stability fields of specific assemblages of metabasalts and the hydration dehydration behaviour and modal changes for blueschists and greenschists along the P T path, we calculated pseudosections for two specific bulk-rock compositions, greenschist sample and blueschist sample , in the system NCFMASH (Na 2 O CaO FeO MgO Al 2 O 3 SiO 2 H 2 O) (Fig. 8a, b). Calculations were performed with the software PERPLEX (ver. 06 July 2006). The thermodynamic database of Holland & Powell (1998, revised 2002) and solution models (Newton et al., 1980 for plagioclase; Holland & Powell, 1996 for omphacite; Holland et al., 1998 for chlorite; Wei & Powell, 2003 and White et al., 2003 for amphiboles) are incorporated into the PERPLEX software package of Connolly (1990, 2005). The fluid phase was calculated as pure H 2 O.

8 140 I. Baziotis, A. Proyer, E. Mposkos Fig. 3. Plots of magmatic (a) and metamorphic (b) clinopyroxene compositions in the classification diagrams of Morimoto (1988). Jd = jadeite, Ae = aegirine, W = wollastonite, E = enstatite, F = ferrosilite. Fig. 4. Chemical compositions of sodic (a) and calcic (b) amphiboles plotted in the classification diagrams of Mogessie et al. (2004), (c) R 3+ vs. Na B of blueschists and greenschists. The bulk compositions of the samples are given in Fig. 8a, b. Since calculations are performed in the NCFMASH system, K 2 O (phengite) was neglected. The selection of the bulk-rock compositions to fit the rule of effective bulk-rock composition is based on the very low modal amount of relic minerals (augite, Mg-hornblende and plagioclase), with their generally very fine-grained characteristics and, the low modal mineral zonation. Because of our incomplete knowledge of thermodynamic properties of Fe 3+ -endmembers and their Margules parameters in the various solid solutions, our model system is Fe 3+ -free. Hence, the stability fields calculated will shift for the true rock composition to the degree that ferric iron is relevant in the pertinent phases. Due to the above reasons, our calculations represent a first-order approximation of the model system. The characteristic difference between the two rock types concerns the mode of glaucophane and actinolite and the stability of omphacitic clinopyroxene. Inspection of Fig. 8a, b shows that the pseudosections for the blueschist and greenschist are slightly but significantly different. The main reactions, represented by narrow divariant fields and some short univariant lines, delimit the stability fields of the Ca-phases (clino-)zoisite, lawsonite, prehnite and pumpellyite, and of the sodic phases albite, omphacite and glaucophane. With increasing pressure, albite (+pumpellyite) transforms to omphacite (+lawsonite) at low temperatures and to glaucophane at higher temperatures. In the blueschist , the field of coexisting albite + omphacite is broad, and the remaining omphacite reacts to glaucophane at low T and somewhat higher P. In the greenschist , the albite + ompacite field is narrower and albite reacts out before pumpellyite, hence no glaucophane is stable in the greenschist at low T and high P. The only way glaucophane can form in the greenschist is by prograde breakdown of omphacite at relatively high P and T. Actinolite, on the other hand, is lost in both compositions with increasing pressure (at low T) during omphacite formation, but in the greenschist it re-enters in minor amounts at higher P because of pumpellyite breakdown. New (or additional)

9 High-pressure/low-temperature metamorphism of basalts in the Lavrion 141 Fig. 5. Al Fe 2+ Mg relations of pumpellyites. Two distinct groups of Fe-rich and Fe-poor pumpellyite are present. Fig. 6. Compositional variation of white K-micas from the metabasalts in terms of Al tot vs. Si. actinolite growth occurs at higher temperatures and pressures by chlorite breakdown in both rock types. Hence, actinolite could be considered as a high-pressure mineral, participating in the blueschist mineral assemblage omphacite + glaucophane + actinolite + epidote (Fig. 8a), whereas it possesses an extended stability field in the more mafic greenschist composition (Fig. 8b). Omphacite is stable along the proposed P T path in both rock types. In fact, omphacite has only been observed in the blueschist, but not in the greenschist, samples, probably due to fluid-liberating retrograde reactions in the greenschists (see below). Moreover, the divariant field chlorite omphacite glaucophane actinolite epidote, observed in both pseudosections, is shifted to lower temperatures for the blueschist bulk composition, which is another reason why more glaucophane could form in such bulk compositions. It is apparent also, consistent with our textural data, that paragonite is not stable, even on the high-pressure/high-temperature side, in either bulk composition. Preservation conditions for blueschist and greenschist assemblages Preservation of peak metamorphic assemblages and freeze-in points along the metamorphic P T path can be investigated by contouring pseudosections with mode isopleths for H 2 O(Fig.9a,b).TheH 2 O-content contours describe the direction of dehydration (towards smaller numbers). For the blueschist, they show dehydration with increasing temperature during lawsonite breakdown between points A and B in Fig. 9a and then chlorite breakdown between points B and C. The greenschists are dehydrating in a very similar fashion (Fig. 9b). The retrograde path leads through fields that would require external hydration for a renewed equilibration in the blueschist, which is why the peak blueschist assemblages are rather well preserved and only altered locally, where fluid could invade from outside during exhumation. Conversely, the greenschists dehydrate again during exhumation from B to C (Fig. 9b) as omphacite + chlorite react to form albite + actinolite + epidote. They fall dry only after crossing the epidote to pumpellyite transition (from C to D). However, along this path greenschists would never have developed glaucophane (which we have observed as inclusions in actinolite in some greenschists). This is why we assert that most of them have crossed from B to B 0, where glaucophane forms from omphacite. This is a major dehydration reaction and these greenschists also attained their driest state along the P T path and hence preserve their peak assemblage (and peak P T conditions) during exhumation to the degree that there is no external fluid ingress. The shift symbolized by the dashed path in Fig. 9b is most likely caused by a fluid with H 2 O activity below unity or expansion of the glaucophane stability field by Fe 3+ rather than by a true temperature shift. As the peak of the P T path is very close to the omphacite glaucophane transition in Fig. 8b but not in Fig. 8a, the greenschist peak assemblage is much more sensitive to changes in a H2 O than that of the blueschist. Discussion Metamorphic evolution The metabasalts (blueschists and greenschists) from the UTU of the Lavrion area experienced HP/LT metamorphism as indicated by the presence of blue amphibole, lawsonite and omphacite. The presence of pumpellyite inclusions in epidote and albite indicates a prograde path that passed through the pumpellyite actinolite stability field. Pumpellyite occurs at temperature < 320 C and pressure < 0.7 GPa (Fig. 8a, b). In the blueschist, lawsonite and glaucophane inclusions in epidote and albite suggest that the prograde path crossed the lawsonite blueschist field, following a cold geotherm. Lawsonite and glaucophane are probably formed at the expense of chlorite, pumpellyite and albite during prograde metamorphism. Their formation indicates P T conditions above the boundary between the pumpellyite actinolite and lawsonite blueschist facies field, defined by the overall transition: Chl þ pmp þ ab! law þ omph þ gl The metamorphic path entered the epidote blueschist stability field as indicated by the trivariant mineral assemblage

10 142 I. Baziotis, A. Proyer, E. Mposkos Fig. 7. P T diagram showing the prograde and retrograde paths of metamorphism for the Lavrion metabasalts. Small boxes correspond to the calculated invariant points shown in Table 2; black boxes represent the blueschists and the grey boxes the greenschists. The two large boxes correspond to average peak and retrograde metamorphic stages, derived by the calculated multivariant reactions. Dashed line refers to the reaction ab! jd + qtz calculated for the maximum observed jadeite content. Dotted dashed line corresponds to phengite isopleth for Si = 3.58 a.p.f.u. (Parra et al., 2002a and b). Dotted lines separate facies fields following Evans (1990). Facies field abbreviations: PA, pumpellyite actinolite facies; LBS, lawsonite blueschist facies; EBS, epidote blueschist facies; AEA, albite epidote amphibolite facies; GS, greenschist facies and A, amphibolite facies. For comparison, the P T paths from Syros (Trotet et al., 2001), Sifnos (Matthews & Schliestedt, 1984; Avigad et al., 1992; Schmädicke & Will, 2003), southern Evia (Klein-Helmkamp et al., 1995; Katzir et al., 2000) and a previous model for Lavrion (Baltatzis, 1996) are also included. omphacite glaucophane actinolite epidote at P T conditions > 370 C and > GPa (Fig. 7, 8a). Retrograde domains are characterized by the chlorite actinolite epidote albite assemblage, formed in the greenschist facies at P T conditions of 350 C and 0.7 GPa. The actinolite overgrowths on glaucophane indicate that the retrograde path did not exactly follow the prograde one, but occurred at slightly higher temperature for a given pressure. Retrograde actinolite formation in the blueschists started with the assemblage actinolite glaucophane epidote albite, immediately at the post-peak pressure conditions (Fig. 8a) and requires minor fluid ingress. However, in the greenschist samples the extensive actinolite overgrowths on glaucophane took place during retrograde overprinting, in the typical greenschist-facies actinolite chlorite epidote albite stability field, triggered mostly by internal dehydration. The final stage of retrogression suggests decompression and cooling at pressures < 0.6 GPa and temperatures C (Fig. 7, Fig. 8a, b) as indicated by the presence of matrix pumpellyite, epidote, actinolite and chlorite. The scatter of the P T values obtained with the multiequilibrium method is interpreted as equilibration of different samples or domains of a thin section at different times during the metamorphic cycle due to: (a) variable dehydration behaviour of the blueschists and greenschists and (b) inhomogeneous infiltration of fluid from external sources.

11 High-pressure/low-temperature metamorphism of basalts in the Lavrion 143 Table 3. Calculated invariant points of the listed reactions. Sample Type * P (GPa) T ( C) Longitude Latitude BS E N BS BS BS E N B GS E N GS E N GS GS GS E N A BS N N Calculated multivariant reactions: (1) 6czo + 7qtz + 11gln + 10Fe-cel! 22ab + 3Mg-cel + 2dph + 7ms + 6tr (2) 6czo + 7qtz + 11gln + 7Mg-cel! 22ab + 2clin + 7ms + 6tr (3) dph + 5Mg-cel! 5Fe-cel + clin (4) 30czo + 35qtz + 55gln + 35Fe-cel!110ab + 7dph + 3clin + 35ms + 30tr Mineral abbreviations: czo, clinozoisite; qtz, quartz; gln, glaucophane; Fe-cel, Fe-celadonite; Ab, albite; Mg-cel, Mg-celadonite; dph, daphnite; ms, muscovite; tr, tremolite; clin, clinochlore. Solution models and thermodynamic data: amphibole (Holland & Powell, 1998; Dale et al., 2000), chlorite (clinochlore Berman, 1988; sudoite Vidal et al., 1992; daphnite, Mg-amesite Vidal et al., 2001; Fe-amesite Vidal et al., 2005), K-white mica (muscovite, paragonite, pyrophyllite Berman, 1988; Fe-celadonite, Mg-celadonite, Parra et al., 2002a), epidote (Holland & Powell, 1998), plagioclase (Berman, 1988) and quartz (Berman, 1988). * BS, blueschists; GS, greenschists. Secondary stilpnomelane associated with chlorite in calcite veinlets were formed during retrogression at the greenschist or even at pumpellyite actinolite facies field (Miyano & Klein, 1989). Influence of bulk composition and fluid availability The association of blueschists and greenschists cannot be ascribed to different metamorphic evolution or episodes affecting the one and not the other rock type. One greenschist sample contains glaucophane as inclusion in actinolite, suggesting that it underwent HP metamorphic conditions similar to the blueschists. It is also apparent from the P T multi-equilibrium data that the greenschist sample and blueschist samples in the Lavrion region experienced HP conditions similar to one another but significantly different from other places within the ACCB. Blueschist bulk-rock compositions are best represented by sample and those of the greenschists by sample Blueschists have higher Fe 2 O 3tot and lower MgO contents, compared to the greenschists. Higher iron content of the system expands glaucophane stability (Maruyama et al., 1986). The effects of the difference in the protolith compositions on the different primary growth of glaucophane observed in our samples are explained well by the calculated pseudosections. Comparing the pseudosections of the two representative rock suites (Fig. 8a, b), it is evident that the glaucophane is present in the entire HP region of the blueschists (Fig. 8a) but only at higher T and P in the greenschists (Fig. 8b). Furthermore, pumpellyite and actinolite have reduced their stability fields in the blueschists. Similar compositional control of blueschist and greenschist formation is discussed in terms of geochemical criteria by Dungan et al. (1983). However, the majority of our greenschists are most likely retrograded blueschists, following the interpretation that all of the greenschists of the ACCB were derived from blueschist precursors (Bröcker, 1990; Schliestedt & Matthews, 1987). During retrogression, the greenschists passed through the expanded actinolite stability field and most of them have experienced the equilibration at greenschistfacies and afterwards at pumpellyite actinolite-facies conditions (e.g. sample ; see Table 3 and Fig. 7). Concluding, the more Fe-rich compositions facilitate the primary growth of lawsonite glaucophane-bearing and epidote glaucophane omphacite-bearing mineral assemblages. H 2 O-content contours show that blueschists will be preserved to the degree that no external fluid enters the rock and the rock cools during exhumation. H 2 O plays a central role as a transport medium and as a catalyst for chemical reactions in rocks. Fluid ingress was rather localized than pervasive, particularly for the blueschists. Fluid was probably attracted by minor deformation of the fluid-undersaturated rocks during exhumation, its activity buffered by the back-reaction to greenschist-facies assemblages, and fluid pathways sealed by the volume increase of the solid phases, most likely in a repetitive, multi-stage process of deformation-reaction-sealing. The chemical heterogeneity of mineral species, the retrograde phases both vein-bound and finely dispersed, and the variability of overgrowth textures indicate such a disequilibrium situation on a cm- to mm- and often smaller scale, which allows deduction of a series of P T conditions from local equilibria even within the same thin section. H 2 O-consuming reactions and the cold geotherm the rock followed during exhumation are the major factors in the preservation of the blueschists.

12 144 I. Baziotis, A. Proyer, E. Mposkos Fig. 8. Pseudosection calculated for representative bulk-rock compositions of: (a) blueschist (sample ) and (b) greenschist (sample ) in the system Na 2 O CaO FeO MgO Al 2 O 3 SiO 2 H 2 O. Dotted lines separate facies fields following Evans (1990). Facies field abbreviations as in Fig. 7. Thick continuous line represents the calculated P T path.

13 High-pressure/low-temperature metamorphism of basalts in the Lavrion 145 Fig. 9. Contours of wt.% H 2 O stored in solid phases (thin dashed lines) superimposed on the P T pseudosections for (a) blueschist (Fig. 8a) and (b) greenschist (Fig. 8b). Only the major facies fields are shown. Thick continuous line: derived P T path. Thick dashed line on (b) shifted P T path explaining the actinolite overgrowths on glaucophane. A, B, B 0, C, D, E, F: points along the P T path as discussed in the text. Abbreviations as in Fig. 7.

14 146 I. Baziotis, A. Proyer, E. Mposkos Conclusions Based on predominant amphibole phase and geochemical characteristics, the metabasalts in the Lavrion area are divided into blueschists and greenschists. Whole-rock analyses show that the samples preserving blueschist assemblages are systematically more Fe-rich than those retrogressed to greenschist. Both types of metabasalts share a common metamorphic history as they occur at the same outcrop and show unambiguous magmatic relations with no tectonic contact between them. An early stage of the prograde path involves P T conditions of pumpellyite actinolite facies followed by lawsonite blueschist facies. The peak P T conditions reached the epidote blueschist field at pressure 0.9 GPa and at temperature 370 C. The retrograde overprint passed through the greenschist-facies to the pumpellyite actinlite facies field (pressure 0.6 GPa and temperature 280 C), following cooling during decompression. Both blueschists and greenschists preserve peak or near-peak metamorphic conditions in places; however, they are more rare in the greenschists. In one greenschist sample where peak metamorphic conditions could be determined, the results are similar to the P T conditions in associated blueschists near the peak of metamorphism, suggesting that bulk composition rather than P T path controls the formation and preservation of the distinct facies assemblages. The rest of the greenschists show the typical retrogression features as in the other places within the ACCB. During exhumation, the Fe-rich bulk composition generally finished dehydration earlier than the relatively Fe-poor bulk composition, and we argue that this promotes preservation of the peak metamorphic blueschist assemblage. Limited access of an external fluid permitted the preservation of the blueschist assemblages and some HP greenschist assemblages in the metabasalts of the Lavrion area. The low geothermal gradient (10 12 C/km) of the retrograde overprint is related to exhumation during ongoing subduction in a fore-arc tectonic setting. Pseudosections and mode contours for H 2 O are valuable tools for the interpretation of the mineralogical evolution of rocks along a metamorphic path and an evaluation of preservation conditions. Relatively robust conclusions can be drawn, but a complete consistency with geothermobarometry cannot be expected due to simplification of the component set (e.g. Fe 3+, Ti and additional fluid components are disregarded). Acknowledgements: M. Bröcker and O. Vidal are thanked for their constructive comments and suggestions on an earlier version of the manuscript. We gratefully acknowledge the helpful remarks by the journal reviewers J. Brady, T. Tsujimori and an anonymous one. We want to express our sincere thanks to E. Tillmanns for his editorial handling. N. 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