Isotopic and petrological evidence for the infiltration of water-rich fluids during the Miocene M2 metamorphism on Naxos, Greece

~1~703?/89~3.~

Geocfrimica et Co~m~himic~ Acfa Vol. 53, pp. 2037-2050 Copyright &, 1989 Pegamon Fi-es Printed in U.S.A.

+ .OO

plc.

Isotopic and petrological evidence for the infiltration of water-rich fluids during the Miocene M2 metamorphism on Naxos, Greece JUDY BAKERI, M. J. BICKLE~,I, S. BUICK’, T. J. B. HOLLAND’and A. MATTHEWS’ ‘Department of Earth Sciences. University of Cambridge, Downing Street, Cambridge, Cl32 3EQ, England *Department of Geology,Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel (Received August 8, 1988; accepted in revised&m

May 5, 1989)

Abstract-A detaiied carbon and oxygen isotopic study, in conjunction with ~trolo~c~ work, has been carried out across calcite-dolomite marble layers in the high-grade metamorphic sequence on the island of Naxos, Greece. The purpose of this study is to examine fluid flow during the Miocene Barrovian metamorphic event. Individual marble bands show two dominant styles of isotopic alteration from core values of 22 to 2910 in 6t80 and 1 to 3%0 in 613C. Firstly, contacts between the marble bands and surrounding pelitic rocks have altered isotopic ratios across a metre wide boundary layer. The isotopic values of the marble along the contacts drop to 15 to 17%b0in ai80 and 1 to -5%0 in &13C.Secondly, there is a drop in the isotopic composition of the marbles along vein networks associated with the development of talc-silicates. The isotopic compositions in these veins drop to 14 to 16%~~ in 6’*0 and -3 to -4% in &13C. Cross-cutting relationships observed between these two alterations allow the relative timing of infiltration to be determined. The development ,of boundary layers is shown to result from the infiltration of fluids with isotopic signatures of 12 to 16%~in 6r8O and -3 to - 12% in ii13C,and an X,, less than 0.3. These fluids are derived from dehydration of~phite-~a~ng pelites during the prograde me~mo~hism. These conclusions are at odds with those of previous workers who suggested that the prograde metamorphism was a result of the pervasive influx of mantle-derived fluids with Xco, greater than 0.5. The alteration associated with vein networks results from peak to post-peak infiltration of fluids which drive decarbonation reactions. At least some of these fluids have an &o, less than 0.05, but this is obscured by the production of COz during the decarbonation reactions. These fluids were most probably produced from the crystallising partial melts formed during the prograde metamorphism. These infiltrating fluids are thought to have S1*OI IO%0and 613Cof -5 to -7%~

THE ISLANDOF NAXOSlies within a belt of Miocene extensional tectonics stretching from Greece to Turkey. During the Miocene, a thermal ~~urbation localised to Naxos resulted in amphibolite grade metamorphism with localised partial melting. Detailed isotopic studies of Buid inclusions and rock suites from this me~rno~hi~ complex were made by KREULEN(1977, 1980, 1988). His studies indicated that 70% of the fluid inciusions contained 60-90 moI% CQ , with Si3C values between -1 and -5%0. From these results, he concluded that the ffuid phase present during me~mo~hism had X,, = 0.5 to 0.8, and that the range of 6°C values indicated that this fluid had a mantle carbon component. RYE et ai. (1976) presented stable isotopic analyses of schists, marbles, migmatites and quartz segregations from the metamorphic complex. They documented a progressive decrease in ai80 values of silicates with increasing grade of metamorphism, and suggested that 6”O of the metamorphic fluids coexisting with the schist units decreased from about 15% in the lower grades to about 8.5%~ in the migmatite. The oxygen isotopic compositions of the marbles within the schist sequence documented interaction with metamorphic fluids along narrow boundary layers and, at higher grades, along zones of talc-silicate growth. The extent of isotopic exchange with the surrounding schists increased with metamorphic grade, and the asymmet~ of the bounds layers

of individual marble bands was taken as evidence for upwards advective transport of the fluid phase. RYE et al. (1976) suggested three possibilities for the systematic variation in the oxygen isotopic compositions of the pelitic rocks, including: 1) an original pre-metamorphic compositional profile, 2) kiiometre-scale exchange between the pelitic rocks and migmatitic rocks with igneous oxygen isotope compositions, and 3) large scale input of externally derived fluids. SCHUILING and KREULEN (1979) preferred the third alternative and proposed that the Miocene metamorphic event resulted from the influx of mantle-derived CO2 which supplied heat, buffered the fluid composition and systematically shifted 6°C values in lluid inclusions and the calcites in pelites to a range lower than the original marine sedimentary values in the marbles. Such conclusions are controversial in the light of the recent debates regarding the composition of &rids in granuhte terrains. We have studied the nature and origin of the fluid phase present during the Miocene metamorphic event through the petrography and isotopic compositions of selected calcite and ddomite marble horizons. These litholo~es are impo~nt because they contain assemblages sensitive to fluid composition and have initial isotopic compositions distinct from those of the surrounding schists. Thus, the passage of an externally derived fluid from the su~ounding pelitic sequence or from some other source should be documented by changing mineral assemblages and/or isotopic compositions within the

2038

J. Baker

marbies. This study presents petrographic observations compiled from many marble horizons and detailed isotopic profiles across five marble bands from various structural positions with respect to the leucogneiss core. These observations contradict the conclusion that the presence of C02-rkh fluid inclusions implies that C02-rich fluids were present during me~mo~hism. Instead, these obviations provide evidence that at least two episodes of infiltration of relatively Hz@ rich fluids can be documented during the Miocene metamorphism on Naxos.

et ul

0

L

REGIONAL GEOLOGY The island of Naxos iies within an arcuate belt of metamorphic rocks, the Attic Cycladic Ma&f, which extends from the Greek mainland in the east to Turkey in the west. The stratigraphic sequence within the eastern Attic Cycladic Massif, as seen on Naxos, comprises a thrust pile of Palaeozoic to Mesozoic carbonates and schists which structurally overlie a migmatitiogneissic complex. This gneiss-migmatite contains early Palaeozoic basement rocks with fabrics of Hercynian age and is exposed on the islands of 10s (e.g. VANDER MAAR el al.,1981; VAN DER MAAR and JANSEN, 1983) and Naxos (ANDREISSEN et al., 1987). The Attic Cycladic Massif underwent an early Tertiary high-pressure glaucophane-schist facies metamorphism (M 1). This was overprinted by a regional greenschist grade metamorphism (M2) which locally reached kyanite-sillimanite grade on Naxos (JANSEN and SCHUILING, 1976). WIJBRANSand MCDOUCAI_L(1988) dated this later me~mo~hism on Naxos at 15-19 Ma by 40Ar/3pAr analyses of homblendes, revising earlier K/Ar estimates of 25 2 5 Ma by ANDRIRSSEN et al. ( 1979). It is this later Miocene metamorphic event that is of interest in this study. BUICK(1988) documented two phases of folding associated with the Miocene metamorphism. The first is an isoclinal phase of folding (F,). This fold generation has axes parallel to, and is possibly part of a continuum of, peak and post-peak M2 extension. Pervasive extensional (S,) fabrics then developed in a shallowly northward dipping crustal-scale shear zone. This extension commenced prior to the peak of M2 and continued after intrusion of a post-metamorphic granodiorite at cu. 12.1-13.6 Ma (WIJBRANS,1985) along the western margin of the island. Continued extension may have been responsible for the tectonic juxtaposition of the isolated Permian to Tertiary ophiolite-bearing and sedimentary tectonic units onto both the ~n~~o~te and the me~mo~hic complex, as suggested by LISTER et nl. (1984). Both the retrograde shear fabrics and the granodiorite contacts with sedimentary and tectonic units were then gently folded about upright axial planes during F3, resulting in a structural dome exposing high-grade rocks in the centre of Naxos grading to lowgrade rocks unaffected by the M2 metamorphism in the extreme south east of the island. The geology of Naxos is shown in Fig. 1,with the positions of M2 isograds shown for reference.

GEOLOGY OF THE ISOTOPE TRAVERSES This study examines a series of detailed isotope traverses taken across marble bands at different structural positions relative to the migmatite core. These traverses allow detailed investigation of the marble-elite contacts at each of the localities, as well as examining unusual petrographic features such as talc-silicate veins. The locations of the marble bands described below are shown in Fig. 1.All traverses have been taken perpendicular to the strike of the marble contacts and are sampled as far as possible without moving along strike to allow good onedimensional control on the isotope profiles. Temperatures are estimated from the thermometry carried out by BUICK ( 1988). Traverses A to D occur at pressures of 6 * 1 kbars. Locality A crosses a 30 m marble band enclosed within a series of politic and graphitic quart&e lithologies in the kyanite-staurolite zone. Estimated peak M2 conditions are 600 + 50°C. The band consists of a coarse white calcite marble containing a boudinaged layer of a finer grained yellow dolomite. Tremolite develops within the dolomite

TECTONOlMiDIYENTARY

Et!!!! c + +

UNIT

OftANODNMiTE METATIYORIHIC COIWLEX

fzsl 0 m

p*doml6twtty

mwbh

pr~domtnwWy

schlllt

twc*(lmtM

CO,.

FIG. 1. The geoiogy of Naxos, after HANSEN and ~CHUI~II\K; ( 1976). The positions of the M2 isograds are shown in dashed lines, and labelled with the mineral phase appearing or disappearing at this position. Approximate temperatures are also given. The temperature of the commencement of melting is 7OO’C. Isotope localities A through E are indicated and labelled.

boudin necks forming a lineation consistent with formation during the top-to-north shearing. The marble band sampled at Locality I3 lies within a series of kyanite-staurolite zone pelites at estimated peak M2 conditions of 600 k 50°C. It consists of three distinct lithologies: a white coatsc calcite marble, a white calcite marble containing narrow haematitemargarite layers and a dark grey/light grey centimetre-scale layered marble, which, on weathered surfaces, develops a series of hard yellow ribs broadly parallel to layering. This third lithology may show the development of tremolite in elongate clusters parallel to layering. The resistant ribs are formed by quartz-rich bands, some of original Sedimentary origin, and some of layer parallel quartz veins which formed pre- to syn-F, . Two parallel traverses, B 1 and B2, have been sampled at this locality; they are spaced about 40 m along strike from one another to examine lateral variations in the quartz bands in the lithology 3. Locality C provides excellent exposure of a marble band in a river bed south of the leucogneiss core at peak conditions of 650 C 50°C (sillimanite zone). It lies between a layered acid gneiss and a series of pelites. The traverse crosses metre-scale interbeds of dolomite, calcite and dolomite lithologies. Calc-silicates are developed at the base of a yellow dolomite horizon. Locality D lies near the north western margin of the migmatite dome, at peak rn~o~~c temperatures of 630 + 30°C. This locality lies within the sillimanite zone. It consists of a 1.5 metre white calcite marble lying adjacent to a coarse diopside-hornblende-plagioclase lithology. A 10 cm layer within the marble and a 20 cm layer adjacent to the underlying meta”gabbro contain diopside, calcite, grossular,

2039

Isotope compositions of 0 and C in marbles Retrograde infiltration

vesuvianite and clinozoisite.. The layers have been boudinaged during the peak and post-peak extension. Locality E is a low-grade traverse in the south east of the island, lying just below the biotite zone at peak temperatures of 450 + 50°C. It consists of a layered graphitic grey marble, with occasional white calcite interlayers, overlain by 0.5 m of dolomite.

There is a later generation of talc-silicates that may be distinguished from the first in that they form well-developed lineations or develop in pull apart fractures of the first generation minerals. They have thus grown during the peak to post-peak M2 extensional phase of deformation. C&-silicates of this generation also develop along margins of late pegmatites cutting marble bands, and as cross-cutting vein networks. This retrograde generation of talc-silicates is more widespread in occurrence and volume than the first generation, and is encountered at four of the five localities in this study (A, B, C, D). It is the development ofthese talc-silicates that is discussed below. Retrograde talc-silicates from Locality C. The central yellow dolomite layer at Locality C shows development of these retrograde &c-silicate lithologies (Fig. 2). The basal contact of this layer with the underlying (higher grade) metre thick pelite interlayer is marked by the development of an irregular diopside-tremolite-calcite horizon up to 10 cm in thickness. A second generation of talc-silicates extends upwards from this basal layer as irregular narrowing-upwards fractures perpendicular to bedding. Individual veins are zoned both across and along their length. Close to the basal layer, veins contain a core of relict diopside rimmed by laths of coarse tremolite parallel to the length of the vein. This is surrounded by finer grained sprays of tremolite intergrown with calcite radiating outwards from the vein core, and finally by a zone of calcite

PETEOGICAL CONSTRAINTS ON PLUID PLOW EVENTS The talc-silicate assemblages developed within marble horizons on Naxos can be subdivided into two generations, on textural and chemical grounds. They can thus be used to divide the fluid flow into two distinct events. Prograde infiltration The first generation of talc-silicates developed during the prograde part of the M2 metamorphism. These are randomly oriented clusters of minerals, often growing around detrital quartz grains within dolomites. The common occurrence of invariant assemblages, particularly in siliceous dolomite assemblages, and the narrow pressure-temperature intervals over which new minerals appear suggest that fluid compositions are internally buffered. This result implies small infiltrating volumes of fluid. However, these assemblages do not provide any useful constraints on fluid compositions during prograde metamorphism.

cc I

dol

,rnz’

.a.

* 0

lcm

tr tremolite calcite veins \

/ /

\

im -

pelite

0

0.5cm

FIG. 2. A schematic diagram of the c&-silicate assemblages developed in the central yellow dolomite layer at Locality C. The diagram shows a vertical section through the yellow dolomite layer, with the underlying (higher grade) pelite at the base of the diagram. The diopside-bearing basal o&-silicates are indicated by a spotted area, and an enlargement of the contact between this layer and the dolomite is shown. An enlarged view of an individual tremolite calcite vein is also shown. These are described in more detail in the text. Scale is as indicated. Abbreviations used in this diagram are as follows: diopside (di), tremolite (tr), calcite (cc) and dolomite (dol); Eg. and m.g. refer to fine grained (co.1 mm) and medium grained (0.1 mm - 2 mm) minerals, respectively.

2040

J. Baker et ul TableI : Representative mineralanalysesforthec&-silicateassemblages developed at L.ocaht~es C and D. The analyseswere obtainedusing the energy dispersiveelectronmtcroproheat the Department of EarthSciences,Cambridge(conditions: 2OkV.2Opmspotsize,ZAPdatareductwn) Diopside(di)is recalculated on thebasisof 6 oxygensperformulaunit,uemolite(tr)oothebasisof 23oxygensanddolomite(dol)on thebasisof 36oxygens.Vesuvtattite (VW)ISrecalculated 011thr basis of 50 cationsper formulaunit, garnet(gr) on the basis ot 12 oxygensper formulaUIW. scapolite(scap)on thebasisof 16cationsperformulaunit,andclinozoislte(cz)on thebws ot 12.5 oxygensper formulaunit. Correctionsfor Fe3+have been made for garnet and clmozowte only. assuming a cation total of 8 for the garnets,and that all Fe is Fe?+ in the cllnozowte No Fe3+ correctionshavebeenappliedto vesuvianite.althoughHo&h (1985) suggests that as much as 75 ‘TO of all the Fe present is Fe? marked with an asterisk Lmllty di

II*

SO2 TiO2 Al203 F-&l3 Fe0 M@ Catl K20

55.18 0.W 0.00 0.00

5b 11 0.13 2.35 000 2.91 21.99 13.62 0.15

100.0497.4s Si Ti Al

2.OO 0.00 0.00

Fe3+

0.00

0.00

Fe2+

0.05 0.93 I.01 0.00

0 34 4 52 2.01 0 ,I3

3 99

I5 05

0.92 to 0.95

0.92 to 0.9b

Mg Ca K

Km mge

I .75

17.12 26.M) 0.00

7.16 0.01 0.38

The X,Q ranges of the minerals

are retrograde

observed

IS gwen. The minerals

minerals.

c

Locality II

--

doi

VW’

pr’

dl

sap’

C,’

0.00 U.00 0.00 0.00 0 12 21.32 21.26 0.00

37.27 0.92 16.27 0.M) 4.11 2.29 36.34 0.M)

39.60 0.53 18.89 3.09 2.78 0.59 35.87 0.00

53.46 0.W 2.42 0.00 2.06 16.09 26 33

44 x2 0.00 26 M 0.00 0 00 0.00 IX.63 3 39

17 hX 0 IOU 26 23 IO bb 0 00 0 Gil 24 72 0 w

97 61

s2.70

97.23

101.39

too.55

93 48

9931

90 70

7.96 0.00 0.12

0.u2 0.00 0.00

18.10 0.34 9.31

3.00 0.03 1.69

I .94 O.ou 0 IO

6.9X 0 00 4 89

2 94 0.00 2.41

I 96 0.00 0 09

0.00

0 00

000

O.lR

0.00

0.W

0.63

,,.O”

,I II 4.81 I.99 O ,I1

u 02 5.83 6 IS 0 00

1.67 1.66 18.93 (I.00

0.18 0.07 2.91 0.00 --

0.06 0.87 1.03 --.

O 00 0.00 3 II I (I2 ~_._

,I ,I 2 0

00 00 v7 IN

0. I Y 0 7h 0 99 0 00

IS WJ

12.00

50.04

8 06

4.00

IhWl

x 411

1 94

09tlto I 00

O.YY to I 00

0.49 1” O 55

0.00 to 0.20

090 to 0 95

5x.59 0.00 0 73 0.w 0.9R 23 71 13 62 0.08

showing an irregular contact against the surrounding dolomite. At a distance of 50 cm above the basal layer, the veins consist only of tremolite. No further talc-silicates are found above one metre from the basal layer. From this point the fractures can be traced up through the rest of the dolomite layer but are free of talc-silicate. Where tremolite is present, it forms a lineation oriented north-south consistent with the regional retrograde shearing deformation. The compositions of the talc-silicates from these textures are shown in Table 1. There are two distinct compositions of tremolite. One group of tremolites, characterised by XMg = 0.93, form as a retrograde rim on the diopsides of the basal diopside (XMg= 0.92) layer (Fig. 2). These tremolites form from the diopside during infiltration of a fluid: 4 diopside + dolomite + Hz0 + CO2 + tremolite + 3 calcite. The second group of finer grained tremolites is character&d by X,, = 0.98, and occurs in the veins without diopside that extend into the yellow dolomite. Textural relationships (Fig. 2) indicate that these tremolites have formed at a similar time to those rimming diopside. They are thus inferred to have formed by reaction of the dolomite with a fluid transporting aqueous silica, via: 5 dolomite + 8 silica (aq) + Hz0 + tremolite + 3 calcite + COZ.

0.00

0.00

dl’ 52

vi

,I 00

2 04 0 “,I 6 OU 13x1 24 XX 0 IJU

0.w to U x5

The differences in X,, are the result of differences in Fe-Mg partitioning between diopside-tremolite and dolomite-tremolite. The two reactions have thus occurred simultaneously as a result of infiltration of a single fluid phase. This is illustrated on a T-XCo, diagram in Fig. 3. Similar textural observations and conclusions have been reached for vein networks developed within the Bergell aureole in Switzerland by BUCHER-NURMINEN(198 1). Although this provides evidence that fluids infiltrating during retrogression introduced silica, and that such fluids were more water-rich than the prograde assemblage, it is not possible to place constraints on the actual composition of this late fluid using these assemblages. In order to do this, we need to examine talc-silicates developed at another locality. Retrograde talc-silicatesfrom Locality D. The development of talc-silicate assemblages at Locality D provides a more precise estimate of retrograde fluid compositions. Relics of a prograde assemblage containing diopside are preserved within vesuvianite and grossular. Both minerals show the development of retrograde rims of diopside and clinozoisite. Where this Lithology is boudinaged, the botulin necks contain the assemblage scapolite-diopside-gross&r with retrograde clinozoisite and plagioclase rims. A schematic diagram of the assemblages is shown in Fig. 4 and representative analyses of the minerals are given in Table 1. A T-X,, diagram for conditions appropriate to the development of the vesuvianite bearing textures is shown in

2041

Isotope compositions of 0 and C in marbles LOCALITY

C P = Skbars

basal diopside layer

1 (‘0

dl COP H2Q

xtc0,) FIG. 3. A T-X, diagram for 6 kbars for the assemblages developed at Locality C. For clarity, these are projected f dolomite. Abbreviations used are as follows: diopside (di), tremolite (tr), calcite (cc), quartz (q) and talc (ta). Arrows show the effect of the fluid infiltration upon the basal diopside layer and the dolomite as both lithologies develop tremolite and calcite. The shaded area gives the possible

The development of vesuvianite and/or grossular from a prograde diopside bearing assemblage in lithologies A and B can be seen to result from the inflation of water-rich fluids. The different assemblages developed in lithoiogies A and B result from X,, differences between the two bulk compositions. The development of clinozoisite, diopside, scapolite and plagioclase as rims on these assemblages indicates decreasing temperature, but cannot be used to constrain whether fluid compositions became more water-rich during the retrograde history. This sequence of assemblages thus suggests infiltration of water-rich (Xc% < 0.05) fluids just prior to peak temperatures. Infiltration ofwater-rich fluids continued during the retrograde history over a temperature interval of at least 100°C. Fl~id~o~ events. The development of talc-silicate assemblages provides evidence for two styles of fluid flow during the prograde and retrograde metamorphism. The composition of the fluids present during prograde metamorphism cannot be constrained. The development of retrograde c&-silicate assemblages is the consequence of infiltration of variably water-rich fluids along vein systems and into the margins of the marble bands. These fluids also transport aqueous silica complexes. At least some of these fluids, i.e. those forming the vesuvianite-bearing retrograde assemblages, can be constrained to be of composition Xco, < 0.05.

ranges in fluid composition. The positions of the individual reactions shown are for the Mg endmember system, and the diagram is produced using the the~~ynamic data and the program THERMOCALC of POWELL and HOLLAND(1985, 1988), and HOLLANDand POWELL( 1985).

Fig. 5. The bold lines are the Mg endmember reactions. The direction of movement of these invariant points with addition of Fe (domin~tly Fe3+ in this assemblage) is also shown,

STABLE ISOTOPE PROFILES

Calcite samples were ground to -lOO# then reacted with 98% phosphoric acid at 25°C for 24 hours after the method described by MCCREA (I 950). Calcite was removed from dolomite samples by reaction with dilute acetic acid, and the dolomite residue reacted with 98% hydrophosphoric acid for 96 hours. Standard samples reacted for 96 and 192 hours respectively showed no fmctionation effects outside error. Quartz veins were crushed to - 150# and cteaned

SOUTH

NORTH

0

1Ocm

I

FIG. 4. The &c-silicate assemblages developed at Locality D. A schematic diagram of a hand specimen from this locality is shown, with the orientation and scale as indicated. The mineral abbreviations used are as follows: calcite (cc), diopside (di), vesuvianite (vsv), grossular (gr), clinozoisite (cz), scapolite (SC)and plagioclase (~1). Two distinct bulk compositions, A and B are indicated; A’ is the retrograde equivalent of A. The mineral assemblages for each bulk com~sition are given with retrograde phases in parentheses. The relationships between the mineral phases are shown schematically.

2041

LOCALITY

D P = Gkbars

T

(“C) 1

\

fC

600

“““1 0

.oo

0.06

0.04

0.12

X(C0,) FIG. 5. A ~-Xcoz diagram for 6 kbars for the vesuvianite bearing assemblages shown in Fig. 5. Bold lines give the position of reactions in the Mg-endmember system, and arrows show their direction of movement with the addition of Fe (dominantly Fe-‘+in these assemblages). The shaded area gives the Yf-X,, conditions required for the development of vesuvianite-modular assemblages in bulk composition A, given that the temperature is constrained. The effect of the addition of Fe’+ to these assemblages is to create a narrow divariant field. Even for the most Fe’+-rich assemblages, this will lie at a Xco* fess than 0.1 for the temperatures at which these assembiages developed. The precursor diopside bearing ~ithoio~es lie to the left of the diagram, and extend to higher Xc%. The formula for Mgendmember vesuvianite is taken from VALLEYet (11.(1985) and HOISCH (1985), and experimental data for vesuvianite from HOCHELLA(1982). The diagram is produced using the thermodynamic data and the program THERM~AL~ of POWELLand HOLLAND ( 1985, 1988) and HOLLANDand POWELL( 1985).

using dilute hydrofluoric acid and magnetic separation. The extraction of oxygen from silicate minerals was performed by the fluorination technique described by TAYLORand EPSTEIN(1962). The analyses were carried out by Steve Wickham at the University of Chicago. Appropriate corrections have been made for fractionation of CO*hydrophosphoric acid, and errors on the analyses are +O. 15 and *0.05% for oxygen and carbon isotopic values, respectively. The 6”O values (SMOW) and 6’-‘Cvalues (PDB) are given in Table 2. Percentages of calcite, dolomite and the talc-silicates present in mixed samples have been estimated semi-quantitatively (to *IO%) using XRD and point counting and are also shown in Table 2.

Stable isotope prqjiles Oxygen and carbon isotopic profiles across the marbles at Localities A through E are presented in Figs. 6 to 10. The profiles are, in general, simiiar to those published by RYE et ul. (1976), except that more closely spaced sampling over three collecting trips has allowed correlation with petrographic features and their resolution on a much finer scale. Pure calcite or dolomite marbles away from margins, talc-silicate layers or veins are character&d by 6’*0 values of 22 to 29% (calcite) and 27 to 3 1?&I(dolomite) and 613C values between + 1 and +3%. Variations in isotopic compositions from these core values are observed in these situations:

1. In the traverses from high grades (A, B, C‘and D). lower marginal zones of the marble bands {excluding veined or other complex regions) are progressively lower in blXO over zones up to 1 m. Upper marginal zones at these localities. where exposed and free of talc-silicate veins, show littlc or no lowering of a’a0 values to within a few centimetres ofthe top contact (~.g. Localities A, B). Marginal zone? cannot be resolved in the low-grade traverse (Locality E). 2. Carbonates from talc-silicate bearing cross-cutting veins exhibit shifts in both oxygen and carbon isotopic compositions to values of L’G.6’*0 of II!‘% and 6°C of 4%. 3. The hne-grained yellow dolomite layer ar L,ocality A has 6’*0 values of 14 to 15%~ and. along the contacts with the calcite marble, 6°C values of 0%. 4. Values of 613C vary between -t-2 and -5% in different lithologies at Localities B and E. The reasons for these variations from the initial marine sedimentary values are discussed below.

The &ctx qj’gruphitr. Graphite was identihed optically in the calcite and dolomite marbles so denoted on Figs. 6 to 10 and Table 2. These ~aphite-~a~ng zones are characterised by low 6r3C values. and can be seen most particularly in traverses at Localities B and E. Similar changes in 6°C values with the appearance of graphite were found by f&XJLEN (1977). Equilib~um conditions for the exchange of 13C between calcite and graphite are not approached until temperatures above 65O’C (KREULEN, 1977). As both traverses B and E have not experienced maximum temperatures as high as this, these variations of 5% in calcite values coexisting with graphite probabiy reflect incomplete exchange between graphite and calcite. The dev&pment of’yrllow dolomite. The development of yellow dolomite at Locality A appears to result from an earlier and very localised fluid infiltration which has lowered &I80 and fi”C values. The dolomite shows prograde growth of talc-silicates during M2. suggesting it was present prior to the peak of this metamorphism. The 6’*0 values of dolomite along the dolomite/calcite contact are shifted to &‘*O values of 14 to 18%. This is 2% lower than the adjacent calcite marble. Substitution of such values into the dolomitecalcite isotopic fractionation expression of SHEPPARD and SCHWARCZ (1970) gives unre~onable tem~ratures suggesting that calcite and dolomite are out of equilibrium. Similar calcite-dolomite fractionations are found in a continuation of this marble at lower peak M2 temperatures (470°C). It is thus suggested that the in~ltration event during which these yellow dolomites formed occurred prior to the M2 metamorphic event, possibly associated with the alteration of the calcite marble to dolomite. ~ut{nd~r~ lu~er.~-pr~)~rude i~~~frat~o~l.The regular decrease in isotopic values at marble margins was noted by RYE cat ul. (I 976) and has subsequently been discussed in more detail by BICKLE and MCKENZIE (1987). These boundary layers are inferred to result from infrftration of fluid with an isotopic signature distinct from that of the marble bands. The infiltration event during which these profiles were established occurred during prograde M2 metamorphism. This may be constrained as the lower grade profiles have exchanged over much smaller length scales (Locality E). suggesting that

Isotope compositions of 0 and C in marbles

2043

Table 2 : Oxygen and carbon isotopic data from Localities A,B 1,B2,C,D and E. Distances are given in metres and are measured from the base of the individual marble band. Oxygen isotope values are given relative to the SMOW standard and carbon isotopic values relative to the PDB standard. The abbreviations used in this table are as follows : calcite (cc), dolomite(dol), marble (mbl), graphite (gr), haematite (he), margarite (ma), diopside (di), tremolite (tr), p~llogopite (phi), vesuvianite (vsv), clinozoisite (cz) and grossular (gt). Percentages of talc silicate and opaque minerals have been estimated through point counting.

it is locahsed to higher grades during M2, and because these boundary layers are subsequently cut (Figs. 7, 8) by narrow veins of retrograde calc-silicates. The isotopic composition of fluids within the pelitic sequence can be derived from the composition of the silicate assemblages within the pelite sequence. Table 3 shows a compilation of #a0 values of quartz veins from this study and from RYE et al. (.1976), and of the 613C values of fluid inclusions within these quartz veins (KREULEN, 1977). Some closed system reequilibration of quartz with the bulk rock has occurred (GRAHAM, 1981) after the peak of metamorphism, resulting in higher S’*O values for some quartz veins. These analyses are marked by asterisks in Table 3. From these data, the com~sition of the fluid in eq~lib~um with the pelitic assemblages lies between 12 and 16%~in 6r80 and between -3 and - 12% in &13C. The isotopic compositions of the lower boundary layers have been modeled for combined adv~tive~iffusive transport of’ a 6”O isotopic signature by a fluid phase flowing across the pelite-marble contact (cl BICKLEand MCKENZIE,

1987). A detailed profile across a lower boundary layer (Locality A) is illustrated in Fig. 11 together with the best fit mode1 curve. This confirms that the isotopic compositions of the marble boundary layers have been modified by fluids of the same isotopic composition as those in equilibrium with the pelitic schist. The details of the numerical fitting routines and the error estimates for the model curves will be discussed in a subsequent publication. C&-silicate assemblages developed during this prograde infiltration provide no constraints on the Xc-, of the infiltrating fluid. This XcoZ may, however, be derived from the geochemicaf transport theory. BICKLEand BAKER(1989) have shown that the loci of oxygen and carbon isotopic signatures on a 6’3~6’80 plot in such boundary layer regions are a function of only two variables, the Peclet number and fluid composition. In this case the Peclet number is of order unity. Changes in fluid composition will result in changes in isotope transport velocity. This velocity is an inverse function of the solid/fluid partition coefficient (BICKLE and MCKENZIE, 1987) for the element of interest. Oxygen solid/fluid partition

2044

J. Baker et al.

LOCALITY

A

BASE I

I

I

I

1

!

I

I

t

10

‘I

f

20

30,m

I

-2"

I

f

t

I 1 I 0

schtst

I

crl& I 10

calclte

marble

I

vein

I 20 I

I 30m

yetlow dolomite with criclte veins

I

I I

I

schlsi

5 calcite FIG.

marble

6. Isotopic traverse acrossthe marble band at Locality A. The base of the marble is shown. Isotopic compositions

of calcite are shown as open and filled circles, isotopic compositions of dolomites are shown as open and filled triangles and isotopic compositions of silicates are shown as squares. Analyses of vein calcites are indicated. The vertical dashed lines indicate ~~01~~ boundaries within the marble band. The basal boundary layer at this Locality is well developed (0 to 2 m), showing a drop from a values of 2.5% to 18% in &‘*O.The centre of the marble band has been altered during the formation of the yellow dolomites, giving steadily dropping &I80compositions centred around the dolomite. Carbon isotopic compositions are unperturbed, aside from some drops along the dolomite-calcite contact, associated with the dolomite formation.

is approximately inde~ndent of fluid CO2 content, whereas carbon ~lid/~uid partition is inversely proportional to ihtid CO1 content. BICKLEand BAKER(1989) estimate that carbon isotopic fronts will travel faster than oxygen isotopic fronts for Xc% > 0.5 and slower with Xce, < 0.5. A series of theoretical plots for varying X,, compositions for an appropriate Peclet number of one are shown in Fig. 12. All the isotopic values from the boundary layers are plotted, and the resulting trend indicates that the lowering of oxygen isotope ratios was largely accomplished before alteration of carbon isotope ratios. Such relative changes can only be produced by an infiltrating fluid with Xco,< 0.3, irrespective of Peclet number. The composition of the fluid infiltrating during prograde metamo~hism can thus be constrained. The fluid has X,O, < 0.3 with a S’*O signature of between 12 and 16460and a 613Csignature of between - 1 and - 10%. Such a combination of isotopic compositions and XCO,are considered to be characteristic of fluids in equilibrium with the pre-M2 compositions of the schists, with variations in the I~‘~Cresulting from variations in graphite contents of the dehydrating schists. The de~e~o~rn~ntof retrograde ca~c-silicate assemblages. Retrograde calosilicate assemblages are developed at Localities B, C and D. These assemblages contain calcites with 6’*0 values of 10% to 20%0.The veins show textural evidence

for retrograde talc-silicate growth and the inferred oxygen isotope com~sitions of fluids in ~ui~b~urn with these calcites (as low as +8.5%0) are substantially Lighter than those of fluids in equilibrium with the schists. Fluids with such light oxygen isotopic compositions have evolved during the crystallisation of the migmatites which have a’*0 values of ca. +%. The complex interaction ofthese retrograde fluids with the carbonate bands is well illustrated at Locality C. A 6’%:6”0 plot for the veins of retrograde talc-silicate (i.e. the tremolitecalcite assemblage) developed at Locality C is shown in Fig. 13. The locus of the trend shown on this diagram is obviously different from that in Fig. 12, indicating that other processes are operative. From the cak-silicate assemblages, it is obvious that a decarbonation reaction has taken place. The fractionation trends of oxygen and carbon isotopes resulting from this decarbonation reaction can be calculated. These C&Ulations are modeled using the “Rayleigh distillation model” (e.g., VALLEY, 1986). The fractionation trend due to the reaction 5 dolomite i- 8 quartz (aq) + Hz0 + tremolite + 3 calcite + 7 CO2

Isotope com~sitions

LOCALITY B (Bl) BASE I I

2045

of 0 and C in marbles

i

I0 I

c8lclto

marble

I.

I

cafclte marble wlth retrograde veins of margarltr

Interlayored calcIte. dolomite end quartz with minor gr8phtte and tramollte

and hanmatlte

LOCALITY

B (82) TOP

and quartz with minor graphite and trrmollte FIG. 7. Isotopic traverse across the marble band at Locality B. Two traverses Bl and B2,40 m along strike from one another, are shown. The base of the marble is indicated for Bl; traverse B2 crosses the upper margin of the marble band. Isotopic compositions of calcite are shown as open and lilled circles, isotopic compositions of dolomites are shown as open and filled triangles and isotopic compositions of silicates are shown as squares. The presence of graphite and retrograde tremolite is indicated. The vertical dashed lines represent lithological boundaries within the marble band. The general shape of the boundary layer is quite well ilhtstrated in this profile. The basal boundary layer is well developed, with drops from 26’50to 16960in S’*O. The width of the upper boundary layer is somewhat obscured by the numerous layer parallel veins containing tremolite. These veins develop during the retrograde phase of rn~rno~~. They show drops to values as low as 10% in S’*O, impiying that the infiltrating fluid had a lit (S IO?&)oxygen isotope signature. The lithologies containing graphite and margarite have carbon isotopic compositions in the range 0 to -4%. These low values are the consequence of the presence. of graphite in the former case, and the development of haematitemargarite veins in the latter case. The profile at Locality B 1 exhibits marked differences in carbon isotopic corn~~~on between graphite-bearing and graphite-absent lithologies.

204h

LOCALITY

C

(values considered typical of Igneous fluids bq. I‘AYLOKP[ a/., 1967. and DEINES, I970), we can calculate the effects of the infilt~tion of a fluid derived from such a ‘magmatic” source into the calcite marble using arguments outlined

BASE

above. A series of curves for different Xcoz of this infiltrating fluid are shown between R and 1 on Fig. 13. These curves

2-

O-

-2 -

I

A_+-t

4 layered add

gnelss

0 I

c

pIlOW dolomite

I’ !

i%l,“E

thin marble Interlayers

pass through the data, suggesting that a magmatrc source f’or the infiltmting fluid is appropriate. The data span a family of curves from .“;;o2 2~ Il.4 to ,&o, = 0.2. isotopic compositions resulting from the combined process of infiltration and decarbonation should lie along a single curve between R and I (Fig. 13), with the relative position along this curve governed by the infiitrating fluid flux. However, because CO> is produced during decarbonation, the _Xco,of the infiltrating fluid will increase along the vein length, causing the spread across curves for varying Xco, between R and I. The infiltrating fluids driving decar~nation reactions are derived from the crystallisation of migmatites formed at the peak of M2, underlying Iocaiities C and D by distances of 100-300 metres. Channelling of these fluids has resulted in the retrograde growth of talc-silicates along vein networks and in associated zones of altered isotopic composition. The isotopic values of the calcite in these retrograde veins is developed through a combination of decarbonation reactions

IOW CaIcItO with dolomlts intsttayero

:

LOCALITY D

FIG. 8. Isotopic traverse across the marble band at Locality C. The base of the marble is shown. Isotopic compositions of calcite are shown as open and filled circles and isotopic compositions of dolomites are shown as open and filled triangles. Analyses of calcite from the retrograde vein networks are indicated. The vertical dashed lines represent Iithological boundaries within the marble band. The veins at the base of the marble lower the margin values from i5k, typical of other profiles, to 12%. These drops are matched by drops in F13C isotopic compositions. Similar veins with similar isotopic compositions can be seen within the yellow dolomite layer. These veins contain retrograde cak-silicate assemblages. The general asymmetric shape of the isotopic profiies is also illustrated.

is shown on Fig. 13 as the curve OR, and has been calculated using the calcite-COz oxygen and carbon fmctionation values from BOTTINGA (1968) and assuming infiltrating fluids are in equilibrium with the preexisting dolomite assemblage. The effect of aqueous silica derived from the fluid is thus ignored

in this calculation. Isotopic com~sitions of calcites produced during retrogression are 1OL lower in d’*O and 3L lower in S13Cthan the maximum changes in calcite predicted from decarbonation viu the reaction above. These maximum changes are -2.3?&0 in 6’*0 and -2.9k in carbon, as 70% of the oxygen and 29% of the carbon are retained in solid phases at the completion of decarbonation. Similar ranges in the oxygen isotopic composition of calcites have been observed by workers in granitic aureoles. These values have been modeled as resulting from a combination of decarbonation reactions and inaction of magmatic fluids (BOWMANet al., 1985; BROWN eraA, 1985; BEBOUT and CARLSON, 1986). Using values of 6*‘0 of z~10%0and 613C of -5 to -7%0

b’3C I

2

O-

.*..I -*-

+ meta gabbro

l.Om

+

I \

cows* caloft* marble

hornbkndeplagtoc***garnet gne1ss COW** marble

oslclta

fiG. 9. Isotopic traverse across the marble band at Locality D. The base of the marble is shown. Isotopic compositions of calcite are shown as open and filled circles. The vertical dashed lines represent lithological boundaries within the ma&e band. The smooth asymmetric profiIe resulting from prograde infil~tion is cut by the basal vesuvianite-bearing layer, which shows lowered S”O and ai3C isotopic compositions.

2047

Isotope compositions of 0 and C in marbles

LOCALITY

VOLUMES AND MECHANISMS OF FLUID TRANSPORT

E

BASE

Three fluid flow events have been identified from examining the isotope traverses at Localities A through E. The earliest is a pre-metamorphic dolomitisation during which the yellow dolomite observed at Locality A and at lower grade sites was produced. The second occurs during prograde metamorphism and results in the development of asymmetric boundary layers along marble-pelite contacts, and the third is associated with the development of retrograde talc-silicate veins. The characteristics of the latter two major fluid flow events taking place during M2 metamorphism are outlined in Table 4. In terms of fluid transport processes, these two events have distinctly different characteristics. Within the basal boundary layers, advective displacements of the infiltrating 6”O signature of the fluid into the marble band lie in the range 0.1 to 1.0 metres. Using the relation between oxygen isotope transport and distance derived by BICKLE and MCKENZIE (1987, Eqn. 13), the total volume of the infiltrating fluid per unit area rock (in the direction of flow) may be derived. These values are in the range 0.2 to 2.0 m3/m2. Although such volumes are insufficiently large to completely alter entire marble bands, the effects of infiltration can be seen persistently along strike within individual marble bands. Fluid has flowed either along grain edges or in sufficiently close-spaced cracks to maintain local fluid-solid equilibrium, and, as such, the fluid flow was “pervasive”. The syn-metamorphic deformation may have assisted this fluid penetration by grain size reduction, recrystallisation and propagation of microcracks. Flow within the retrograde vein networks, on the other hand, has been channelled. Fluid volumes may be calculated for the retrograde vein networks at Locality C using a value of 1.46 wt% for the maximum solubility of quartz in water at 600°C and 6 kbars (ANDERSON and BURNHAM, 1965). These calculations give total fluid volumes per unit area of infiltrated rock of 9.0 to 0.9 m3/m2, for vein spacings of 10

I

I

I

0 0

,I I

24

b

I

1

I

““I

2

20m

10

with white Intarlsyers

calcIte

FIG. 10. Isotopic traverse across the marble band at Locality E. The base of the marble is shown. Isotopic compositions of calcite are shown as open and filled circles. The vertical dashed lines indicate lithological boundaries within the marble band. The boundary layers on this profile bear no resemblance to those at higher grade. Margin values drop to 25% in 6’sO, and the width of the basal boundary layer is certainly less than 20 cm.

driven by infiltration of fluids with 6”O of I 10% and 613C of -5 to -7%. Initially, these fluids may have had compositions as low as Xco, = 0.05, but this has increased to at least reactions have taken place. J&o, = 0.4 as decarbonation

Table 3 : Compilation

of isotopic data from silicates at WO-610°C. Data from the studies of Rye et al.

(1976) and Kreulen (1977) are also given. The composition the q-cc fractionation

of the infiltrating

of Clayton et al. (1989) and the &q-H20

fluid is calculated using

fractionation

of Matsuhisa et al.

(1979). Samples marked by asterisks are those that have undergone reaograde isotopic exchange.

Study from which isotopic values are taken

Rye et al. (1976)

This study

Kraden (1977)

Litbology born wtuch quartz vein I tluid inclusion is taken

6’80

S13C

madde(quartz) miuble(quartz)* sctdst(quutz)

17.5 25.4 13.8

schist(quartz) scJlist(quanz) s&ist(quanz) marbk(quartz)* marble(quartz)*

15.1 15.4 16.6 19.2 20.2

-----

scbist(tluid)

_.

scbia(ttuid) schisI(tluid) scbist(fluid) scbkt(tluid) scldst(fluid) scbist(tluid)

__ ._ ._ ..

-1.0 -3.7 -7.1 -2.4 -9.1 -7.4 -5.1

T(“C)

s’soofcoexlstmg tlurd

__

-

590 605 610

600 600 600 600 600

590 590 600 600 600 610 610

15.2 23.3 11.7

12.9 13.2 14.4 17.0 18.0

.. .. ._ .. _.

8’3C of c* eXiStiD Lllud

__ _.

._

..

-3.6 -6.3 -9.7 -5.0 -11.7 -10.0 -7.7

..--

LOCALITY

schist

1marble !

7

I: . 1 lo

A

LOCALITY

C

2

0

PC -2

-4

Dlatance (m) I

FIG 11. The lower boundary layer developed at Locality A. Oxygen isotopic values within the schist are calculated for calcite using the q-cc fractionation of CLAYTON et al.(1989) and the isotopic compositions of quartz veins below the marble band. This data is shown as hlled squares; calcite isotopic data is shown as circles. The curve through the data is the best fit error function curve for advectivediffusive transport of an isotopic signature by infiltration of a fluid across the schist-marble contact. This model assumes uniform flow across the schist-marble boundary (BICKLEand MCKENZIE(1987).

cm and 1 m, respectively. Boundary layers are not developed adjacent to these veins, as would be predicted from the domination of advection over diffusion in vein environments. The method of fluid transport in these veins has been one of fracture filling, with extensional veins forming as brittle fractures in the dolomite, allowing infiltration of fluid. Initial growth of tremolite within the veins has been parallel to the vein walls, and subsequent replacement of dolomite via reaction with the fluid has produced radial tremolite-calcite

-6

20

10

30

PO

FIG. 13. The effects of retrograde infiltration of fluids. The figure shows a 6”C:6’*0 plot showing isotopic compositions for calcite within retrograde veins at Locality C, and the precursor carbonate from which these calcite-bearing &c-silicates formed. The isotopic composition of the original yellow dolomite is represented by closed circles, and calcite from tremolite-calcite veins by open circles. The filled square at 0 represents the composition of the theoretical calcite that would be in equilibrium with the dolomite at 600°C calculated using the expressions from SHEPPARDand SHWARCZ (1970). Two sets of curves, labelled OR and RI, are shown. OR represents the effect of removing CO* through the simple decarbonation reaction discussed in the text. The numbers on this curve represent the mole fraction of carbon remaining in the rock. The effects of aqueous silica are neglected. The family of curves, RI (each labelled with an appropriate XcoJ, show the effect of infiltration of fluids with an isotopic composition ~10 in 6’*0 and -5 to -7 in 613C, and a variable Xco, composition. This isotopic composition is discussed in the text. The field I represents calcite compositions in equilibrium with this fluid at 600”. The curve ORI represents the combined effects of complete decarbonation and interaction with igneous fluids, and is discussed further in the text. The curves have been drawn for a Peclet number of 2. We have assumed the effects of decarbonation (i.e. removal of CO*), and infiltration are separable. As a consequence, we cannot estimate the fluid fluxes associated with this infiltration, and these are derived later from mass balance considerations.

intergrowths rimmed by calcite against unreacted dolomite. Such a sequence of minerals can be predicted given exchange of silica and MgO between fluid and rock. DISCUSSION

FIG. 12. Theoretical plots of d’3C:6’80 for infiltration of a fluid phase into a marble band. Curves for a range of fluid compositions from Xc% = 0.1 to 1.O are plotted and labelled with Xc%. These curves are drawn for a Peclet number of 1, and t’ of 1 (BICKLEand MCKENZIE, 1987). The top right of each plot represents the initial composition of the marble band. Movement along the curves towards the lower left comer represents the effects of infiltration with increasing time and distance. The composition at the lower left comer represents the infiltrating fluid composition. The lilled circles are a compilation of calcite isotopic compositions from the lower boundary layers at Localities A, B and D.

Since the late 1970s Naxos has been cited in the literature as an example where pervasive CO&h fluids are present in a terrain during metamorphism. These CO*-rich fluids were also believed to advect sufficient quantities of heat to cause the M2 metamorphism (SCHUILING and KREULEN, 1979). In this study, the petrographic data and analysis of 613C: 6”O relationships indicate that both peak metamorphic fluids and at least some of the retrograde fluids were water-rich (J&o2 < 0.4). Such fluid compositions are also consistent with general estimates for metamorphic conditions constrained by both dehydration equilibria and melting reactions. This

2049

Isotope compositions of 0 and C in marbles Table 4

: Characteristics of thetwomajorfluid50~ eventsoccurring duringM2 metamo@ism. %o

Fluid Flux* m3~m2

12 to 16

-3 to -12

0.2 to 2.0

< 10

-5 to -7

0.9 to 9.0

Fluid Flow Event

mid

derived

8’3C

from

ptogmJedeJlydlaliw < 0.3 of p&tic rocks Retrogradeintiitmtim of nuids derived from crysta5isation of the tnigmatitefomted in M2

initiallv - 0.0s

0th

Pervasive 50~ along grainedgaor micmcracks Fluid tmqoxted in vein netwodrs and along mote pmeable lithlogies. Fluid uampow Si02

* per unitarea of rock perpendiatlarto fluid 50~

raises the question as to when the COz-rich fluid inclusions were trapped and whether fluid inclusions are representative of metamorphic fluids. Work by KERRICH (1976) suggests that even small amounts of intracrystalline strain, such as would be expected in an extensional shear zone, result in leakage of fluid inclusions in quartz, and in preferential removal of HZ0 into the surrounding quartz as structural water. Similar conclusions have been reached by BAKKER and JANSEN (1989) in a study on artificial fluid inclusions. The total fluid volumes per unit area of infiltrated rock for the flow quoted in Table 4 across the marble lower boundary layers are insufficient to advect significant heat, much less the quantities required to attain peak M2 metamorphic conditions (Fig. 6 in BICIUE and MCKENZIE, 1987). However, the evidence for flow during the retrograde path along the more permeable &c-silicate bearing horizons or fractures in the marbles suggests that the marbles may have been relatively impermeable and that the calculated fluid fluxes into the lower boundaries may be substantially less than fluid flow in the schists. If dehydration of pelitic rocks yields about 12% by volume of a water-rich fluid (WALTHER and ORVILLE, 1982) then a total fluid flux of cu. 1 m3/m2 would be produced by dehydration of only cu. 8 m of pelitic rock (assuming one-dimensional flow upwards). Since the marble bands are separated by tens to hundreds of metres of pelitic or locally gneissic rocks, the recorded fluid fluxes are only part of the total flow. Assessing this fluid flow and its advective potential, however, will be problematic, as flow parallel to lithological boundaries along more permeable lithologies within the pelitic sequence will be difficult to identify and quantify. KREULEN (1988), following earlier work, argued that because the 613C values of fluid inclusions and calcites in the schists were intermediate between the positive values characteristic of sedimentary limestones and the very light values typical of graphite, a third source of fluid from the mantle was probable. The data discussed above suggest that partial (disequilibrium) mixing of 6°C from marble and graphite sources is widespread in the marbles during the metamorphic event, and that the presence of small quantities of graphite can drastically alter 6°C values. The carbon isotopic signatures reflecting the composition of the prograde metamorphic fluid could thus be explained by exchange between graphitic elastic metasediments (613Ccu. -25L; KFCEULEN, 1977) and the fluid they produce during dehydration, without invoking the presence of a mantle component.

The problem of the apparent progressive decrease in 6”O from low grade values of 15%0to high grade values of 8.50/w may also be explained. We have documented interaction of the marble bands with late fluids of 6’*0 5 10%~~If relatively limited fluid circulation occurred between the crystallising migmatite (which incorporates a previously dehydrated Hercynian basement complex) and the adjacent few kilometres of high grade schists, progressive depletion of these higher grade schists from a PO of cu. 15%0to a 6’*0 of 8L may have occurred. CONCLUSIONS The evidence for fluid interaction with calcite and dolomite horizons during the M2 metamorphism on Naxos indicates that there were two major fluid flow events during the metamorphism. During the prograde path, the upwards advection and diffusion of fluids through the relatively impermeable marble bands resulted in, at most, metre scale alteration of the isotopic signature of the marbles from core b’*O values of +22 to 29k and 613Cvalues of +2L to margin 6’*0 values of 15 to 17L and b13C values of +l to -5L. These fluids were derived by dehydration of the surrounding pelites, some of which were graphite-bearing, during prograde metamorphism. This resulted in fluids with Xco, < 0.3 and isotopic composition cu. 12 to 16% 6”O and -3 to - 12% 613C. The development of retrograde talc-silicate veins resulted from infiltration-driven decarbonation reactions. The most likely source for the fluids driving these reactions are those derived from the movement of melts produced at the peak of metamorphism and post-peak metamorphic recrystallisation of the migmatite involved in M2. The infiltrating tluids, with X,, initially as low as 0.05, have an isotopic composition ~~10k 6”O and -5 to -710 613C. Brittle fracturing and more permeable horizons within the marble bands provided channelways along which fluids could infiltrate. This infiltration resulted in the development of c&-silicate veins. Neither of the two fluid phases contain substantial amounts of CO2, and no evidence has been found for the interpretation that C02-rich fluids were pervasive during M2. We have been able to show that the presence of graphite in the marbles is a controlling factor on their carbon isotopic signature, and that calcite in such marbles will have 6°C values of0 to -5%0 in comparison with values of 1 to 36 obtained from pure calcite marbles. The presence of small quantities of graphite in the pelites, at temperatures below calcite-graphite equilib-

2050

J. Baker ef al.

rium, makes the presence of mantle carbon during prograde metamorphism unnecessary to explain the isotopic signatures observed. Acknowledgements--JB is supported by Shell Australia and thanks IGME for their permission to work on Naxos. The stable isotope work was supported by a grant to AM from the U.S.-Israel Binational Science Foundation (Jerusalem, Israel). Steve Wickham is thanked for his help with the silicate samples. We are also grateful to John Valley for a thoughtful review which has resulted in a much improved paper. This work was supported by NERC. This paper is Cambridge Dept. of Earth Sciences contribution. ES 1369. Editorial handling: J. R. O’Neil REFERENCES G. M. and BURNHAM C. W. (1965) The solubility of quartz in supercritical water. Amer. J. Sci. 263, 494-5 11. ANDREISSEN P. A. M., BOELRIJKN. A. I. M., HEBEDAE. H., PRIEM H. N. A., VERMURDENE. A. TH. and VERSCHURER. H. (1979) Dating the events of metamorphism and granitic magmatism in the Alpine Orogeny of Naxos (Cyclades, Greece). Contrib. Mineral. Petrol. 69, 2 15-225. ANDREISSEN P. A. M., BANGAG. and HEBEDAE. H. (1987) Isotopic age study of pre-Alpine rocks in the basal units on Naxos, Sikinos and 10s. Greek Cvclades. Geol. Miinbouw 66. 3- 14. BAKKERR. J. and jANSEN J. B. H.“( 1989) Experimental evidence for leakage of Hz0 from COZ-H20-rich fluid inclusions (abstr.). Terra Abstracts 1, 320. BEBOUT G. E. and CARLSONW. D. ( 1986) Fluid evolution and transport during metamorphism: Evidence from the Llano Uplift, Texas. Contrib. Mineral. Petrol. 82, 5 18-529. BICKLEM. J. and BAKERJ. (1989) Infiltration driven reaction fronts: Velocities and fractionation of oxygen and carbon isotopes by fluid flow in metamorphic rocks. Terra Abstracts 1, 329. BICKLEM. J. and MCKENZIE D. (1987) The transport of heat and matter by fluids during metamorphism. Contrib. Mineral. Petrol. 95, 384-392. BOTTINGAY. (1968) Calculation of fractionation factors for carbon and oxygen exchange in the system caicite-carbon dioxide-water. J. Phys. Chem. 72,800-808. BOWMAN J. R., O’NEIL J. R. and ESSENE E. J. (1985) Contact skam formation at Elkhom, Montana. II. Origin and evolution of C-OH &am fluids. Amer. .J. Sci. 285, 62 l-660. BROWNP. E., BOWMANJ. R. and KELLY W. C. (1985) Petrologic and stable isotope constraints on the source and evolution of skam forming fluids at Pine Creek, California. Econ. Geol. 80, 72-95. BUCHER-NURMINENK. (198 1) The formation of metasomatic reaction veins in dolomitic marble roof pendants in the Bergell Intrusion (Province Sondrio, Northern Italy). Amer. J. Sci. 281,11971222. BUICK I. S. (1988) The metamorphism and structural evolution of the Barrovian overprint, Naxos, Cyclades, Greece. Ph.D. dissertation, Univ. of Cambridge, U.K. CLAYTON R. N., &LDSMITH J. R. and MAVEDA T. K. (1989) Oxygen isotope fractionation in quartz albite anorthite and calcite. Geochim. Cosmochim. Acta 53,725-734. DEINES P. (1970) The carbon and oxygen isotopic composition of carbonates from the Oka Carbonatite Complex, Quebec, Canada. Geochim. Cosmochim. Acta 34, 1199-1225. GRAHAM C. M. (1981) Stable isotope equilibrium and the fluid phase in cooling metamorphic rocks. In Progress in Experimental Petrology NERC Series D18 5, 185-186. HOCHELLAM. F., LIOUJ. G., KESKINENM. J. and KIM H. S. (1982) Synthesis and stability relations of magnesium idocrase. Econ. Geol. 77,798-808. HOISCHT. D. (1985) The solid solution chemistry of vesuvianite. Contrib. Mineral. Petrol. 89, 205-2 14. HOLLANDT. J. B. and POWELLR. (1985) An internally consistent thermodynamic dataset with uncertainties and correlations: 2. Data and results. J. Metamorphic Geol. 3, 343-370. JANSENJ. B. H. and SCHUILINGR. D. (1976) Metamorphism on

ANDERSON

Naxos: Petrology and geothermal gradients. .Irrzcr ./ .‘+I 276, 1225-1253. KERRICHR. (1976) Some effects of tectonic recrystallisation on thud inclusions in vein quartz. Contrib. Mineral. Petrol. 59, 195-205. KREULENR. (1977) CO?-rich fluids during regional metamorphism on Naxos, a study on fluid inclusions and stable isotopes. Ph.D. dissertation, Univ. of Utrecht, Nederlands. KREULENR. (1980) COz-rich fluids during regional metamorphism on Naxos (Greece): Carbon isotopes and fluid inclusions, .-imer J. Sci. 280, 745-77 I KREULENR. (1988) High integrated fluid/rock ratios during metamorphism at Naxos: Evidence from carbon isotopes of calcite in schists and fluid inclusions. Contrib. Mineral. Petrol. 98, 28-32. LISTERG. S., BANGAG. and FEENS~RAA. (1984) Metamorphic core complexes of the Cordilleran type in the Cyclades. Aegean Sea. Greece. Geolog)l12, 22 l-225. MATSUHISA Y., GOLDSMITH J. R. and CLAYTONR. N. ( 1979)Oxygen isotope fractionation in the system quartz-albite-anorthite-water. Geochim. Cosmochim. Acta 43, 113 I - 1140. MCCREAJ. M. (1950) On the isotopic chemistry of carbonates and palaeotemperature scale. J. Chem. Phys. 18, 849-857. POWELLR. and HOLLANDT. J. B. (1985) An internally consistent thermodynamic dataset with uncertainties and correlations: I. Methods and a worked example. .J. Metamorphic Geol 3, 321342. POWELLR. and HOLLANDT. J. B. (1988) An internally consistent thermodynamic dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program. J. Metamorphic Geol. 6, 173-204. RYE R. O., SCHUILINGR. D., RYE D. M. and JANSENJ. B. H. ( 1976) Carbon, hydrogen and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece. Geochim Cosmochim. Acta 40, 1031-1049. SCHUILINGR. D. and KREULENR. (I 979) Are thermal domes heated by CO*-rich fluids from the mantle? Earth Planet. Sci. Lett. 43, 298-302. SHEPPARDS. M. F. and SCHWARCZH. P. (1970) Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contrib. Mine& Petrol. 26, 161-198. TAYLORH. P. JR. and EPSTEINS. (1962) Relationships between lKO/ I60 ratios in coexisting minerals of igneous and metamorphic rocks. II. Principles and experimental results. Bull. Geol. Sot. Amer. 73, 46 l-480. TAYLORH. P. JR., FRECHENJ. and DEGENSE. T. (1967) Oxygen and carbon isotopic studies of carbonatites from the Laacher See district, West Germany and the Alno district, Sweden. Geochim. Cosmochim. Acta 31,407. VALLEYJ. W. (1986) Stable isotope geochemistry of metamorphic rocks. In Stable Isotopes in High Temperature Geological Processes (eds. J. W. VALLEY,H. P. TAYLORand J. R. O’NEIL);Reviews in Mineralogy, Vol. 16, pp. 445-489. Mineral. Sot. Amer. VALLEYJ. W.. PEACORD. R., BOWMANJ. R., ESSENEE. J. and ALLARDM. J. (1985) Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-A1209Si02-HzO-CO*. J. Metamorphic Geol. 3, I32- 153. VAN DER MAAR P. A. and JANSENJ. B. H. (1983) The geology of the polymetamorphic complex of Ios, Cyclades, Greece and its significance for the Cycladic Massif. Geol. Rundschau 72, 283299. VAN DER MAAR P. A., FEENSTRAA., MANDERSB. and JANSEN J. B. H. (198 1) The petrology of the island of Sikinos, Cyclades, Greece, in comparison with that of the adjacent island of 10s.Neues Jahrb. Mineral. Monatsh. 10.459-469. WALTHERJ. V. and ORVILLEP. M. (1982) Volatile production and transport in regional metamorphism. Contrib. Mineral. Petrol. 79, 252-257. WIJBRANSJ. R. (1985) Geochronology of metamorphic terrains by the @Ar/“Ar age spectrum method. Ph.D. dissertation, Australian National University, Canberra. WIJBRANSJ. R. and MCDOUGALL1. (1988) Metamorphic evolution of the Attic Cycladic Massif Belt on Naxos (Cyclades, Greece) utilizing 40Ar/39Arage spectrum measurements. J. Metamorphic Geol. 6, l-23.