The Prevolcanic Basement of Thera at Athinios: Metamorphism, Plutonism and Mineralization

The petrological study reveals that these rocks were affected by an HP/LT (blue-schist facies) metamorphism. Glaucophane/crossite coexisting with Ca-amphibole, albite, pumpellyite, sphene, chlorite, phengite, stilpnomelane and calcite are the common mineral constituents of the metabasites. Phengite, muscovite, calcite, and chlorite appear in the metasediments. These rocks are a part of the Cycladic blueschist belt known from many other islands of the Cyclades. The metamorphic rocks are intruded by an I-type granite of Late Miocene age. This granite represents the southernmost part of the Miocene Cycladic granitoid province. Hornfelses and skarns were formed at the expense of blueschist rocks as a result of the granite intrusion. Contact metamorphic minerals are diopside/hedenbergite and grossular/andradite, as well as minor epidote and plagioclase. Magnetite, pyrrhotite, chalcopyrite and minor pyrite are ore minerals associated with the skarns. The Pb-Ag vein-type sulphide mineralization exploited in the past is genetically associated with the granite intrusion.

INTRODUCTION

Thera is one of the islands of the Santorini group which lies in the southernpart of the Cyclades (Fig. 1). The islands of Santorini are made up mainly of Pliocene to recent pyroclastics and lavas and are the remnants of a complex volcano, destroyed by a strong eruption resulting in the formation of a caldera 36 centuries ago. The younger volcanic islands of Palaea Kameni and Nea Kameni were formed in the centre of the caldera during the last 20 centuries.

Metamorphic rocks are exposed in the southern part of the island (Fig 2) and represent prevolcanic Thera. The largest outcrop at the south-eastern part comprises recrystallized carbonates of Triassic age (Papastamatiou 1956; Blake et al. 1981) and low-grade metamorphosed conglomerates and sandstones of lower Tertiary age (Tataris 1964). A small outcrop of metamorphic rocks, in which glaucophane as a critical metamorphic mineral was described first by Davies and Bastas (1980) are exposed at Athinios and Thermia. Part of these rocks is affected by contact metamorphism, due to a granite intrusion (Skarpelis and Liati 1987). This granite is, according to K/Ar data, of Late Miocene age (Skarpelis et al. 1988). Sulphide mineralization of vein type, mined in the past for Pb and Ag at Athinios, occurs in the glaucophane-bearing rocks (Murad and Hubberten 1975).

The aim of this paper is the petrological study of the regional and contact metamorphic rocks, as well as the study of the mineralization. The geological correlation of prevolcanic Thera with other Cycladic islands is discussed.

GEOLOGICAL SETTING

The metamorphic rocks at Athinios and Thermia areas include metapelites, metasandstones, quartzites, metaconglomerates, marbles, and metavolcanics. In the map of Fig. 3, the location of the various lithologies is given. The age of the protoliths remains unknown due to lack of fossils. Glaucophane is reported from a metadolerite (Davies and Bastas 1980; Blake et al. 1981), as well as from xenoliths found in Holocene pumice on the island of Christiana (Puchelt et al. 1977). Coarse-grained, grey-coloured, granite porphyries locally cross-cut these rocks at Athinios.

Contact metamorphic rocks were recognized in bore holes set for geothermal energy purposes by the Institute of Geology and Mineral Exploration in the areas of Megalochori and Ayia Anna. As is shown in Fig. 2, the lower part of one of these bore holes comprises contact metamorphic rocks, skarns and, at an elevation close to sea-level, a granitic body. Granite and skarns are reported also as xenoliths in the Holocene pumice of the Christiana islands (Puchelt et al. 1977) and in pyroclastic deposits in the southern part of Thera.

PETROLOGY

     1.     Regionally metamorphosed basement rocks:

Metapelites are characterized by fine layering. Two schistosities are recognized while crenulation cleavage is usually well developed. The common mineral assemblage is: quartz - phengite - chlorite - albite. Rutile and tourmaline are accessory phases.

Ca-rich metapelites (calc-schists) are abundant and consist of quartz, calcite and phengite.

Quartzitic beds intercalated in the metapelites are boudinaged and range in thickness from a few centimetres to 30 cm. Quartz textures are granoblastic. Phengite, chlorite and calcite are minor constituents.

Marbles of grey to greyish-white colour appear both as thick banks and as pebbles in metaconglomerates. They are recrystallized and contain minor amounts of quartz, K-feldspar and phengite.

Metaconglomerates consist of flattened pebbles and cobbles of calcitic marble, quartzite and metavolcanic rocks set in a pelitic to arenitic matrix. The metavolcanics correspond to hyaloclastite (Davies and Bastas 1978) and basalts. The metabasalts are, according to geochemical data, of alkaline composition. The metabasaltic fragments are characterized by the following mineral assemblage: glaucophane/crossite - Ca-amphibole - albite - chlorite - pumpellyite - phengite (-sphene) (-stilpnomelane ) (-calcite).

Relics of magmatic clinopyroxene and plagioclase phenocrysts (commonly albitized) are often found. Glaucophane/crossite commonly coexists with Ca-amphibole; in that case the two amphiboles are separated by sharp boundaries. However, in some cases Ca-amphibole has been observed to replace crossite. Finally, Ca-amphibole appears as euhedral fine-grained prisms in the matrix together with albite + chlorite + calcite.

The pelitic to arenitic matrix is characterized by the mineral assemblage: phengite - calcite - quartz - stilpnomelane - orthite - opaques.

Metasandstones are medium- to coarse-grained and consist mainly of quartz, white mica and chlorite. Two generations of white mica are distinguished: an older one of phengitic composition (see below) which is rotated and deformed and grows in pressure shadows of mineral fragments, and a new one (muscovite; see below) which is un-deformed and is arranged parallel to the schistosity planes. Chlorite appears in the form of sigmoidal aggregates.

Metabasites: in addition to the previously described mode of occurrence in the form of fragments in metaconglomerates, metabasites appear also as sills in metasediments. Three outcrops of this type have been found:

     a)      at Athinios. There, the common mineral assemblage is: clinopyroxene (magmatic) - albite - Ca-amphibole - pumpellyite - chlorite - phengite - calcite.

Clinopyroxene is a magmatic relic preserved as large crystals up to 12 mm, commonly rotated and/or broken into smaller fragments aligned along the schistosity planes. Ca-amphibole and pumpellyite form occasionally at the expense of clinopyroxene. Albitized plagioclase is also a magmatic mineral relic.

     b)     At Thermia, metabasalts and lapilli tuffs are intercalated in metapelites. The metavolcanics are massive, dark green in colour and epidotized along joint surfaces. They are characterized by the mineral assemblage: Ca-amphibole - albite - pumpellyite - sphene - calcite.

     c)     Along the road to Athinios a thin metabasaltic layer appears. The common mineral assemblage is: crossite - Ca-amphibole - albite - stilpnomelane - quartz - sphene - calcite.

Albite is partially formed at the expense of magmatic plagioclase. Albite and chlorite aggregates usually form at the expense of crossite while stilpnomelane coexists with Ca-amphibole and albite.

Granitic blocks, reaching 30 m in diameter, occur in the metapelites. They are strongly deformed in the marginal parts. Plagioclase is sericitized and biotite is chloritized.

 MINERAL CHEMISTRY

Representative microprobe analyses of minerals from the metamorphic rocks are given in Table 1. The microprobe analyses were made with a JEOL 733 SUPERPROBE and a TRACOR EDS system, TN series 2. Analysing conditions were 20 kV acceleration voltage and 4 nA probe current. A ZAF correction program was used. The structural formula calculation was made according to the computer program of Perdikatsis (1986).

Clinopyroxene is endiopside to augite in composition.

Blue amphibole: microprobe analyses of blue amphibole revealed that this is most commonly crossite and more rarely glaucophane in composition (after the nomenclature compiled by Leake 1978 and Rock and Leake 1984) (Fig. 4).

Ca-amphibole coexisting with blue amphibole in the metabasites is most commonly characterized as actinolite (always with increased Na content) and more rarely as hornblende (Fig. 5), but barroisitic amphibole was also found. In the cases where Ca-amphibole replaces Na-amphibole, it is also characterized as actinolite, but its Na content is much lower (compare analyses Nr. 17B-2 with 33-6 of Table 1).

Pumpellyite: For the characterization of the mineral, the chemical formula suggested by Coombs et al. (1976) has been used and the atomic proportions were calculated on the basis of 16 cations. Thus, most pumpellyites are of intermediate composition between Fe- and Mg-end members. In the Al - Fetot - Mg diagram of Coombs et al. (1976), the pumpellyite analyses are plotted in the low-Al part of the field for glaucophane schists of California (Ernst et al. 1970) (Fig. 6) showing partly a trend towards the pumpellyite-actinolite facies zone III of Upper Wakatipu (Kawachi 1975).

Phengite: The Si content of phengite both in the metabasites and the metasediments is quite high, ranging between 6.8-7.15 (calculated on the basis of 22 oxygen atoms). In the metasandstones muscovite (Si = 6.05-6.18) has been identified (anal. Nr. 37-5) in addition to phengite (anal. Nr. 37-1).

Chlorite of metabasites is characterized as pycnochlorite, whereas sigmoidal chlorite flakes set across the schistosity planes of metapelites are diabandite and rarely daphnite and penninite (after the nomenclature of Hey 1954).

Albite: The analysed albites of the metabasites and metapelites are almost pure (less than 3.5% An and negligible Or component).

Stilpnomelane coexisting with actinolite from a metabasalt with a Fe/Fe+Mg ratio of 0.7 is given in Table 1.

PT CONDITIONS OF METAMORPHISM

The presence of glaucophane/crossite in the metabasites indicates that the rocks were metamorphosed within the blueschist facies. This is also indicated by the presence of pumpellyite which in the diagram of Fig. 6 is plotted in the field of glaucophane schists of California (Ernst et al. 1970). High pressures of metamorphism are confirmed also by phengite with up to 7.15 Si atoms p.f.u. appearing both in the metabasites and the metasediments.

Constraints on the temperature of the blueschist facies metamorphism are placed by the occurrence of pumpellyite in the metabasites. The upper thermal stability of this mineral is 200°-280° C and 350°-380° C for the pure Fe- and Mg-variety, respectively, thus indicating that temperatures should not have exceeded 380° C.

Moreover, the presence of barroisitic amphibole in the same rocks indicates that temperatures were above 300°-350° C (Ernst 1979). Therefore, temperatures of the blueschist facies metamorphism probably ranged between 300°-380° C.

The pressures of metamorphism can be estimated by applying the geobarometer of phengite (Massone and Schreyer 1987) which for the above temperature range yields pressures in the order of 11-12 kbar. This is a minimum pressure estimate since phengite is not in the appropriate paragenesis (Massone and Schreyer 1987).

It should be noted here that the occurrence of old, deformed phengite and of neoformed, undeformed muscovite in the same sample of metasandstones leaves open the possibility of a lower pressure metamorphic overprint, already reported from other Cycladic islands.

     2.     Granite:

The granite is of I-type consisting of alkali feldspar, plagioclase, quartz, biotite and hornblende; orthite, zircon and magnetite are common accessories. Bending of twinning lamellae of feldspars indicates a slight deformation of the rock. Close to the contact with the surrounding metamorphic rocks and especially along joint planes the granite shows intense alteration of the quartz-sericite-pyrite type. Locally, thin veins of pyrite and quartz are observed. Pyrite contains 1 gr/ton Au. K/Ar dating on biotite from the granite revealed a Late Miocene age (Skarpelis et al. 1988). These data indicate that the granite of Thera belongs to the Miocene granitoid province of the Cyclades.

     3.    Contact metamorphic rocks:

Contact metamorphism due to the granite intrusion resulted in the formation of hornfelses and skarns. The distinction between hornfelses and skarns relies generally on the fact that the hornfelses form isochemically; growth of new minerals is caused only by the temperature rise. On the contrary, the skarns form anisochemically, at a later stage, in the presence of a fluid phase. Ore mineralization is associated with the skarn-forming stage, whereas hornfelses are ore-barren.

In Thera the contact metamorphic event is traced in rocks drilled by a bore hole in the areas of Megalochori and Ayia Anna.

     Petrology:      The regionally metamorphosed rocks drilled by the bore hole are more or less affected by contact metamorphism. Rocks completely converted to contact metamorphic mineral assemblages consist either of garnet + clinopyroxene ± epidote or almost entirely of garnet ('garnetites'). In these rocks ore minerals are usually present in addition. Their coarse grain size, the occurrence of part of the garnet in form of veins, the Fe-rich character of the garnet, as well as the association of these rocks with ore minerals, indicate that they can be characterized as skarns. Clinopyroxenes formed before the metasomatic (skarn-forming) stage, i.e. during hornfels formation might also participate in these rocks.

The rocks completely converted to skarns might have been initially marbles or calc-schists. The majority of the contact metamorphic rocks still preserve some mineralogical or textural characteristics of the initial regional metamorphic rock, such as the initial schistosity. Calc-schists and metasandstones are recognized as precursors of at least some of the contact metamorphic rocks.

In the skarns, garnet is euhedral to subhedral and occurs in massive aggregates. It is often anisotropic showing alternating isotropic and bi-refringent concentric lamellae and/or sector twinning. Inclusions of clinopyroxene are found in garnet but the two minerals usually coexist in equilibrium. Late-stage garnet occurs in veins, often filled with quartz, and cross-cutting the previously described garnet and clinopyroxene. Clinopyroxene occurs as equigranular aggregates. It is the most common mineral and often occurs together with ore minerals, predominantly with magnetite.

In the contact metamorphic calc-schists and metasandstones, the following mineral assemblages were identified: a) clinopyroxene - biotite - plagioclase - quartz (-garnet) (-chlorite) (-scapolite) (-calcite, secondary); b) clinopyroxene - garnet - quartz (-epidote) (-calcite, secondary). Orthite, sphene and opaque phases are occasionally present. It should be noted here that epidotization observed in metabasites at Thermia seems to be related to the skarn episode.

The initial schistosity of the rocks is still preserved. In some cases, fine-grained clinopyroxene aggregates forming monomineralic bands alternate with thinner bands of phlogopite + plagioclase. In many of these rocks, deformation after the completion of the contact metamorphic stage is recorded by secondary quartz. Quartz then shows embayed to lobate mutual crystal boundaries indicating incomplete recrystallization, while plagioclase sometimes shows bending of the twinning lamellae.

     Mineralogy:     Microprobe analyses were carried out in Kiel University, Institute of Mineralogy, using a Cameca, Camebax Microbeam microprobe.

Clinopyroxene: The pyroxene is of the diopside-hedenbergite series. Its composition varies along with the relative time of formation. Thus, the pyroxene of the initial stages of contact metamorphism (hornfelses) is rich in the diopside component (Di₇₅Hed₂₅) whereas that of the main skarn stage is rich in the hedenbergite component (Di₂₀Hed₈₀). Clinopyroxene of intermediate composition (Di₄₀Hed₆₀) was also observed. Zoning in pyroxene is generally lacking. The difference in the chemistry of the pyroxenes indicates that the skarn fluids were rich in Fe. The andraditic nature of the skarn garnet (see below) indicates that the metasomatic fluids were characterized by oxidizing conditions at a later stage, probably after pyroxene formation.

Garnet: The garnet is of the grossular-andradite series. As in the case of clinopyroxene, the garnet composition varies with the relative time of formation as well. Thus, the garnet of the hornfelses is rich in grossular and shows expressed zoning, the rim being richer in grossular than the core. Unzoned garnet of intermediate composition Andr₂₅Gros₇₀Alm₃Spes₂ has also been observed. The garnet of the main skarn stage is richer in andradite component than that of the hornfelses and shows slight zonation, the core being richer in andradite than the rim. Finally, the latest stage garnet, which occurs in the form of veins cross-cutting both earlier formed garnet and clinopyroxene, is the richest in andradite and shows expressed zoning; the rim is then richer in andradite by about 20 mol% than the core.

MINERALIZATION

Magnetite, monoclinic pyrrhotite, chalcopyrite and minor pyrite are the dominant ore minerals which form small pockets in the skarns. Chalcopyrite is closely associated with pyrrhotite but it also occurs as veins in garnetite. The sulphides postdate magnetite. Pyrite occurs in thin veins and is the last sulphide mineral formed in the skarn zone. Zoning of ore minerals is not detected in the bore hole.

Sulphide veins up to 50 cm thick, cross-cutting the metamorphic basement rocks, are recognized on the inner caldera walls by red-brown gossans. Underground mining of these veins took place mainly for Pb and Ag until the beginning of this century. The veins have a NW-SE strike and south-west dip. They cross-cut discordantly the basement rocks. The wall rocks of the veins are intensely altered to quartz-sericite assemblages. The main ore-minerals are galena and pyrite whereas sphalerite and chalcopyrite appear in small amounts. Galena contains 870 ppm Sb and 940 ppm Ag (Murad and Hubberten 1975). A sulphur isotope study of these ores was carried out by Hubberten (1974) and Hubberten et al. (1977). The δ³⁴S values of pyrite, galena, sphalerite, and chalcopyrite are close to 0. This suggests that the sulphides were formed from an isotopically homogeneous magmatic source of sulphur.

Although the conditions of formation of the skarn-type and the vein-type mineralization are not precisely known (work on this topic is in progress), their close spatial association with the skarns suggests that the mineralization formed during the cooling stage of the granite from thermally driven circulations of metasomatic fluids.

DISCUSSION

HP/LT metamorphism followed by a greenschist to amphibolite facies overprint has long been recognized in the Cycladic islands (see Schliestedt et al. 1987 and references therein). The presence of undeformed muscovite in the same thin section as the strongly deformed phengite in metasandstones of Santorini suggests a low-P metamorphic overprint after the high-P blueschist facies metamorphism. Replacement of blue-amphibole by actinolite low in Na reinforces this view. I- and S-type granitoids intruded during Miocene times resulting in the formation of contact metamorphic aureoles (Marinos and Petraschek 1956; Altherr et al. 1976; Melidonis 1980; Salemink 1985). Ca-Fe-Mg skarns, Fe-deposits (magnetite, hematite) and minor Pb, Zn and Ba mineralizations are associated with the granitoid intrusions (Marinos 1951; Economou et al. 1981; Salemink, 1985).

The recognition of an HP/LT metamorphic event in the metamorphic rocks of Athinios and Thermia, along with the discovery of the granite, the contact metamorphic rocks and the skarn-type mineralization, allow the geological correlation of Thera with the other Cycladic islands. The granite intrusion of Thera fits well in time with the Miocene granitoids of the Central Aegean. Petrologically and geochemically it can be correlated with the granites of Naxos and Mykonos/Delos (Skarpelis et al. 1988). The mineralization at Athinios represents the southernmost part of the Miocene skarn-type metallogenetic province of the central Aegean.

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(for figures and tables please refer to book)

Source: "Thera and the Aegean World III, Vol 2" (pp. 172 - 181)

Authors: N. Skarpelis (Dept. of Geology, Section of Economic Geology and Geochemistry, University of Athens, Greece) and A. Liati (Dept. of Economic Geology, Institute of Geology and Mineral Exploration, Greece)