MINERALOGY AND METAMORPHIC CONDITIONS OF A GARNETIFEROUS LENS FROM THE EASTERN CHALKIDIKI PENINSULA , NORTHERN GREECE

A small lensoidal garnetiferous body, consisting of garnet + amphibole + epidote + quartz, and occuring within the gneisses, amphibolites and migmatites of the eastern part of the Chalkidiki main body, is studied in terms of petrography, mineralogy and metamorphic conditions. Garnet corresponds to almandine rich almandine-grossular solid solution and displays negligible core-rim compositional variations. Garnet grains exhibit alteration to amphibole (mainly ferropargasitic hornblende and ferropargasite) and epidote. The chemical composition of garnet and amphibole imply conditions of upper amphibolite facies. It is considered that the studied garnetiferous lens may represent a retroeclogite, lacking any relic eclogitic assemblage, which completely recrystallized and equilibrated extensively under the prevalent high-T amphibolite facies. The slight enrichment of epidote in Ce, La, Y, and consequent zoning are attributed to compositional variations of the fluid phase during the amphibolitization stage.

The present study deals with a garnetiferous lens, hosted in the metamorphic rocks of the eastern part of the Chalkidiki main body, near the village Vamvakies, in northern Greece (fig.1).The purpose of this study is the investigation of the possible metamorphic conditions of this basic lens on the basis of textural observations and mineral chemistry.

Petrographic description
The studied garnetiferous lens (about 5 x 10m in dimensions) occurs within the gneisses, amphibolites and migmatites of the area.The gneisses vary * Dept. of Mineralogy-Petrology-Economic Geology, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece.
fram biotite-gneisses (with main mineralogical assemblage: quartz + plagioclase + K-feldspar + biotite) to hornblende-gneisses (with main assemblage: quartz + plagioclase + hornblende).Hornblende + plagioclase + K-feldspar + quartz ± diopside comprise the mineralogical composition of the amphibolites, while the migmatites display a range of their leucosome component from plagioclastic to K-feldspathic.The contacts of the studied garnetiferous lens with the hosting rocks are not clear due to multiple deformation events, alteration and erosiono Details of the geological setting of these hosting lithologies are given in previous studies.Briefly, the broader area belongs to the Serbomacedonian massif.This tectonic unit is in fault contact with the Rhodope massif to the east and the Circum Rhodope belt to the west (Kauffmann et al., 1976;Kockel et al., 1977) (fig.1).The Serbomacedonian massif, composed mainly of metamorphic and minor igneous rocks, is characterized by a complex tectonometamorphic evolution, which took place from pre-Carboniferous to post-Jurassic times (e.g., Kockel et al., 1977;Kassoli-Fournaraki, 1981;Papadopoulos, 1982;Kassoli-Fournaraki et al., 1985;De Wet et al., 1989;Sakellariou, 1989;Kourou, 1991;Sidiropoulos, 1991).
The garnetiferous lens is granoblastic and contains the mineralogical assemblage: garnet, amphibole, epidote and quartz.Microscopic observation of thin sections showed that garnet constitutes nearly SS-60 % by volume of the specimens studied.The color of garnet is mostly reddish-brown and the grain size is usually l-S mm; the grains are mostly round in shape.Inclusions within the garnet grains are usually concentrated in the centers.Amphibole, epidote and quartz are commonly incorporated in this way.Spiral rotation patterns in the inclusions have not been observed.
Garnet grains are always rimmed by kelyphitic rims composed of amphibole-epidote intergrowths; the width of the rims varies from grain to grain.In most cases the same kelyphitic material replaces the inner part of the garnet grain in forms of veins or patches.The grain size of the constituent kelyphitic phases is quite often very fine and the extent of alteration varies.Backscattered electron imaging coupled with microanalyses showed that the kelyphitic products of the garnet alteration are amphibole and epidote in intergrowths.Garnet rims and kelyphitic amphibole are in textural equilibrium.
Amphibole is present in amounts of lS-30 % by volume.It either rims kelyphitically the garnet grains, or grows at the expense of garnet in the inner part of the garnet grains or even occurs as individual crystals in the matrix.In nearly all the modes of formation, it appears in intergrowths with epidote and in many cases the amphibole crystals are epidote-cored.Its colour is deep olive-green with pronounced pleochroism.
Epidote is present in amounts varying from lOto 20 % by volume.Average grain size is about 1 mm.Aggregates of euhedral or subhedral epidote crystals are abundant in the samples.Epidote occurs both as inclusions in garnet and amphilobe and as individual crystals.
Quartz occurs either as inclusions or aggregates surrounding garnets.No plagioclase, pyroxene or chlorite were observed, though sorne matrix amphibole grains display a characteristic picture of complete alteration after pyroxene.

Mineral chemistry
Minerals were analysed using a JEOL JSM-840 scanning electron microscope equipped with a LINK AN 10000 EDS microanalyzer, operated at an accelerating voltage of lS kV and a sample current of 3 nA with beam diameter 111m.Corrections were made using the ZAF-4/FLS software pravided by LINK.Natural minerals or synthetic equivalents and pure metals were used as microprobe standards.Backscattered electron images were obtained at the same operating conditions.
The observed eore-to-rim ehemical variations of the garnets in the present study eould not be considered signifieant and systematie; sorne garnet grains show a slight eore-to-rim inerease in Fe/Mg and decrease in Ca, while sorne others reveal quite the opposite.In general, the unaltered portions of the garnets in the present study, eould be eonsidered more or less homogeneous.

Epidote
Table 2 presents the ehemieal eomposition of two representative epidote grains from the garnetiferous lens.Formulae were ealculated by normalizing to 12.5 oxygen atoms and on the basis of Fe t ot =Fe 3 +.The slight excess of silicon content (3.034-3.080),aboye the ideal three atoms per formula unit, is not significant since it keeps within the acceptable range of values (2.87-3.15)given by Deer et al. (1986).
Electron microprobe analysis revealed that the studied epidotes are frequently enriched in Ce (up to 3.69 wt% Ce203).La and Y participate in lower amounts (up to 1.82 wt% La203 and up to 1.46 wt% Although under the optical microscope the crystals do not appear to be zoned, backscattered images reveal either core-rim zonation (fig. 3)or patchy domains.The bright and dark zones in backscattered images are respectively Fe, Ce-rich and Al-rich.The shape of distribution profiles suggests a complex texture of superimposed discontinuous and continuous zoning.The zoning pattern is sometimes asymmetric as opposite rims of sorne crystals differ in concentration of elements (table 2).In general, the cores and enriched in Fe, Ce and La and depleted in Y, relative to the rims (fig.4).
Electron microprobe traverses across epidote crystals, revealed that those which grew freely and slightly enriched in iron relative to the epidote

Amphibole
Amphibole is calcic-amphibole (table 3).Unitcell formulae were obtained following the method of Robinson et al. (1982) by assumig initially that all Fe is FeO and then calculating Fe 3 +from a charge balance on 13 cations exclusive of Ca 2 +, Na+ and K+.According to Leake (1978) classification, its composition corresponds mostly to ferropargasitic hornblende and ferropargasite (especially in the kelyphitic rims) and less to Mg-hastingsitic and ferrotschermakitic hornblende.It is iron-rich (23-25 wt% FeO) and contains considerable amounts of sodium (2.5-3.0 wt% Na20).Al! v is high (1.59-1.84)and the total amount of Ca + Na + K ranges between 2.54 and 2.77.
The sludied garneliferous body is charaelerized by a very Iimiled number of eomponenls.which makes the P•T estimalions very difficult.Besides, Ihe only Ihree components which are garnet, amphibole and epidole (quartz is a ubiquilous phase) displaya more 01' less conslanl chemica1 composilion making, lhus, uneffeclive a chemogr¡¡phic represenlation of the phase relmions.However, an investiga- ¡ion of the approximate metamorphie conditions is atlempled according lO the data which resulted frOl11 the examination of Ihe minerals.The homogeneily of gamel COl'es and slight ehemical variarion to Ihe gamel grain margins can be interpreled as lhe result of homogellizalion al eleva• lec! lemperalures.This homogeneity togelher wilh the large iron conlenl of garnels implies fonnalion lemperatllres >650 0 C (Miyashiro, 1973 Graham and Powell (1984) gave the sallle lemperalUre range.564-672 0 e, both fOf Ihe gamel core-amphibole inclusion as well as for the gamet rim-kelyphilic amphibole pail's.
The precise estimalion of pressure was impossible wilh lhe exisling mineral assemblage.Neverthe-1ess, Ihe amphibole's composilion (high A1 v 1 and Na COn¡Cnls), Is consislent with crystallization under high pressure conditions (see Ungareui el (1/., bolite-facies event must be responsible for the absence of characteristic zonation in garnets and consequent homogeneity (see also Yardley, 1977;1983).Furthermore, the Na/(Na + Ca) vs. Al/(Si + Al) ratios of amphiboles indicate pressure between the trends of high and medium pressure amphiboles (closer to the high pressure trend), according to Laird and Albee (1981) discrimination diagrams.
As eclogitic relics are lacking, it is not possible to establish whether this rock has undergone an earlier high-pressure metamorphism.However, both the mode of occurrence of the garnetiferous lens in field and the textural observations as well as the obtained from the gamet and amphibole compositions could not rule out the hypothesis that the present rock might be a reequilibrated retroeclogite.To this hypothesis contributes the occurrence of a limited number of small eclogitic, meta-eclogitic and completely amphibolitized eclogitic bodies in sorne other parts of the Serbomacedonian massif (Dimitriadis and Godelitsas, 1991;Kourou, 1991;Sidiropoulos, 1991).Extended periods of metamorphism and polymetamorphism should considerably reduce the probability of eclogite survival and thus account for its scarcity in the area.Sakellariou (1989) suggests an eclogite-facies metamorphic event for the rocks of the Serbomacedonian massif, while Kourou (1991) and Sidiropoulos (1991) accept a first metamorphic episode under conditions between blueschist and eclogite-facies.Equilibrium temperatures of 520-550°C and 400-500°C have been estimated for eclogitic parageneses in the Serbomacedonian massif by Dimitriadis and Godelitsas (1991) and Sidiropoulos (1991), respectively.Sidiropoulos (1991) reports a range of temperature 697-777°C with a pressure about 5 kbar, for the amphibolitefacies event which affected the eclogites, based on geothermobarometry obtained by gamet-amphibole and amphibole-plagioclase equilibrations.
The rock of the present study, displays a pervasive re-equilibration under the prevalent hight-T amphibolite facies, being completely recrystallized and not retaining any relic of high pressure asemblages.Such cases have been also previously reported (e.g., Gil Ibarguchi et al., 1990;Thelin et al., 1990;Messiga et al., 1992).
During the upper amphibolite facies stage hydration of the rock took place due to the fluids released from the surrounding meta-pelites and gneisses and primary pyroxene disappeared totally giving place to amphibole while gamet began to alter to amphibole-epidote intergrowths.The amphibole's composition varies slightly (mosaic equilibrium) depending on its position in the rock and the mineral it replaces.However, its high aluminum content, substituting for Si in the tetrahedral site, the high iron and sodium contents and the high sum of Ca + Na + K support a high metamorphic grade.Diffusion processes during the high temperature amphi- The formation of epidote is attributed to a progressive sequence of hydration reactions.In the rock studied, epidote appears to crystallize along with amphibole at the expense of garnet.During the amphibolite-facies event the fluid phase must have been slightly enriched in Ce, Y and La, which were trapped in epidote's structure.The observed zoning of epidote, with cores enriched and rims depleted in Ce and La, could be attributed to the progressively depletion of the fluid composition in these elements.The differences of Ce, La, Y contents between epidote grains could be attributed to fractionations of the fluid composition betwee different sam-pIes.Millimeter-scale compositional heterogeneities in metamorphic fluids have been also reffered by Philippot and Selverstone (1989 a, b).The variations of Ce, La, Y, within epidote grains may in part reflect local and time-dependent changes of fluid compositions.

Conclusions
The small garnetiferous lens of the present study, hosted in the gneisses, amphibolites and migmatites of the eastern part of the Chalkidiki main body, is composed of garnet, epidote, amphibole and quartz.
Its mineralogieal study revealed that gamet is an almandine rich almandine-grossular solid solution, displaying negligible compositional variation across the grains and considerable alteration in amphiboleepidote intergrowths.Amphibole (mostly ferropargasitie homblende and ferropargasite) is unzoned displaying high Al, Fe and Na contents.The homogeneous composition of amphibole implies equilibrium.Epidote is slightly enriched in Ce, La and Y. Ce and La display a prominent core-rim decrease, reflecting gradual depletion of these elements in the fluid composition of the metamorphic system in which epidote formed.Variations of Ca, Ce, La, Y and Fe within zoned grains indicate a continuous nature of many substitutions of M2+ and M3+ cations in epidote.
The chemical composition of garnet and amphibole imply conditions of upper amphibolite facies.It is considered that the studied garnetiferous lens may represent a retroeclogite, lacking any relic eclogitic assemblage, which equilibrated extensively under the prevalent high-T amphibolite facies.This is what appears to be pointed out by the data, including the relative compositional homogeneity of the phases and the absence of relicts of any earlier high-pressure assemblage.
Fig. 1.-Simplified geotectonic map of a part of northern Greece, with location of the studied area.x: Location of the garnetiferous lens studied.
Fig. 4.-Rim-to-rim profile, expressed as cations per formula unit, of one representative epidote grain from the garnetiferous lens, Chalkidiki, Greece.

Table I .
-Representative microprobe analyses of two garnets from the garnetiferous lens, Chalkidiki, Greece

Table 3 .
-Representative microprobe analyses of amphiboles from the garnetiferous lens, Chalkidiki, Greece