Report on our Excursion to Syros

in September 2005

led by Dr. John C. Schumacher

The Geology

Here is the handout given to us by John

WEGA 2005: Syros and the Cyclades
John C. Schumacher

Almost nothing in this guide is original material. It largely draws on field guides written by others, principally Excursion guide to the field trip on Seriphos, Syros and Naxos: Syros by J. E. Dixon and John Ridley, and Evolution of the Cycladic crystalline complex: Petrology, isotope geochemistry and geochronology by Schliestedt, Altherr and Matthews. Both are in Chemical Transport in Metasomatic Processes H. Helgeson (ed), D. Reidel, 1987. Most of the text was compiled by George Helffrich with a few additions by John Schumacher for past and future Bristol Excursions.

Early civilization

Historical References and Names

Regional setting

Stratigraphy

Position of the meta-igneous - serpentine belt

Significance of the Syn-metamorphic Deformation

Principal Rock Types and Mineral Assemblages in the Northern Area

             Metasediments

             Meta-igneous rocks

Eocene high-pressure metamorphism

             Geochronology

Miocene medium-pressure metamorphism

             Geochronology

Miocene granitoid plutonism

             Geochronology

Summary

References and Bibliography

Early civilization

The earliest signs of habitation on Syros are Neolithic to the Early Bronze Age (c. 4000 - 3000 BCE). Some of the earliest inhabited sites are found on the Hondras peninsula and at Koskinas both near the village of Vari (southeastern Syros). The Cycladic Civilization (aka, Syros - Keros Civilization) existed from c. 3200 - 2000 BCE, but flourished between 2800 - 2300 BCE. On Syros, the ruined fortification at Kastri and the cemetery at Chalandriam (north-eastern Syros) are important remnants of the golden age of Cycladic Civilization. The culture of Cyclades remained distinctive, but the influence of the Minoan culture, centered on Crete,- steadily increased and reached its height around 1600 BCE. The cultural center of the Aegean shifted to the mainland two hundred years later (1400 BCE) when the Mycenaean civilization reached its height.

Historical References and Names

In Homer's Odyssey, the shepherd Eumeos refers to a Syrii island, which may be one of the earliest references to the island of Syros. This narration implies the presence of a strong Phoenician (1st millennium BCE) cultural influence on the island. In fact, the island s name may be derived from two Phoenician roots: Ousyra-Ousoura meaning happy or Syr meaning rock (Happy Rock?!?). Finikas, the name of a village and a bay in the southwest of the island, may be derived from the Phoenician word for palm.

Syros is one of many islands that make up the Cyclades. These islands partially encircle the sacred island of Delos, which has a long history as a religious site that goes back to the Neolithic Age, and it is this cyclic arrangement of the other islands around Delos that give the Cyclades their name.

Figs 1a and 1b. Regional Maps

Regional setting

Syros, together with Sifnos and Tinos belongs to the belt within the Attico-Cycladic crystalline massif (Fig. 1) that shows the highest grade blueschist facies metamorphism. The common assemblage in metabasic rocks on these islands is:

glaucophane + epidote + garnet + omphacite + white mica (Fig. 2).

Figure 2. Compositional phase diagram. projection on to an AFM plane from paragonite, epidote quartz and water in the system Na-Ca-Fe-Mg-Al-Si-H-0. Grs = % grossular in the garnet. From JCS.

This high-pressure metamorphism gives consistent K-Ar mineral, and Rb-Sr whole rock ages between 40 and 45 Ma (Eocene). Later Miocene (20-25 Ma) overprinting of the Cycladic terrane with greenschist grade metamorphism coincided with development of thermal domes and preceded granitoid intrusions on a number of other islands (including nearby Mykonos). The stratigraphic age of the rocks is generally unknown, but all evidence suggests that most are Mesozoic. Sparse paleontological data based on occurrences of Triplorella cf. Remesi (Steinmann - an Upper Jurassic/Lower Cretaceous algae reported on the distant island of Schinoussa) and nummulites (on the even more distant islands of Euboea and Amorgos) suggest this (Durr et al., 1978; Dubois and Bignot, 1979; Minoux et al., 1980).

(Note by JCS: One of my students from Freiburg, J. Pohl (1999), discovered the first fossils on Syros. Daniel Vachard (CNRS) and Michael Montenari (Freiburg) determined the microfossils to be forams (Forschiidae). They give an age of Upper Tournaisian (Ivorian) to Visean (240-250 Ma). These Car­boniferous ages are significantly older than most of the protolith ages, which tend to be Permian, Triassic or Jurassic, for the sedimentary rocks in the region, as you just read above.

At this time we cannot say if all sedimentary rocks on Syros are Carboniferous, but this new discovery hints that the tectono-stratigraphy may be much more complex and difficult to unravel than previously thought - and we already thought it was difficult!).

Stratigraphy

The simplified geological map of Syros (Fig. 3) shows a continuous alternating sequence of north to north-east dipping schists, marbles and metabasites that can be traversed up section from the south-western peninsula of the island to the north coast. This pile forms the relative autochthon on the island (see Platt 1993; Ridley 1984a) and has all under­gone high-pressure metamorphism. At the top of the pile the later greenschist overprint is sporadic, and at the base it is frequently almost complete, but it is everywhere essentially post-tectonic. Nowhere in the Aegean area is there either an unmetamorphosed sedimentary sequence of the thickness seen on Syros, or sequence with repeated thick limestones and elastic units (see Fig. 4). The sequence might in part be the result of tectonic duplication of an originally thinner sedimentary sequence, possibly in an accretionary wedge. The best evidence for this comes from the sequence of repeated thick marbles seen in the north of the island. Many of the details of the stratigraphy are repeated from one marble unit to the next, and always so that "way up" is constant. There are no obviously inverted sequences. This sug­gests that the section seen is the result of repetition by thrusting of a single thick carbonate unit.

Figure 4. Cross section through the northern part of the Syros. The section is taken from north of Kini (Stonychas) through the northern meta-igneous belt (Palos). The bases of the thick marble units (mr) may mark zones where the sequences are tectonically repeated. Units: sch = "Schists of Syros," sch, = metabasic schists, sch2 = grey schists (mempelitic?), sch3 = grey schists with glaucophane ± lawsonite (pelitic?) After the map compiled by Hecht (1977).

Position of the meta-igneous - serpentine belt

The variegated suite of meta-igneous rocks seen at the top of the blueschist pile across the north of Syros is regarded as derived from a disrupted ophiolite by virtue of the range of rock types encountered. These include:

• large masses of metagabbros with an ophitic texture

• leucocratic rocks (trondjemites?) showing intrusive brecciation and net-veining textures in more gabbroic rocks

• sedimentary igneous breccias with meta-igneous clasts in a glaucophane-rich or omphacite-ankerite matrix.

The suite as a whole is formed of discrete, generally lensoid blocks within a serpentine or country rock schist matrix (see Appendix 1, Fig. Al. 1). Individual blocks bounded by meta-sediments commonly contain complex, virtually undeformed igneous textures and intrusive contact relations typical of ophiolitic high-level gabbro/dyke-complex zones. Smaller blocks with exactly comparable net-veining relationships are found as strongly metasomatized equivalents enclosed in serpentinite. These metasomatic effects, and the spectacular reaction zones at serpentinite - metasediment contacts, all mani­fested in high-grade blueschist assemblages, indicate clearly that juxtaposition of the components of the meta-igneous complex with sediments took place prior to the peak of metamorphism.

Although the boudin shape of several large blocks and the detailed fabric history are consistent with a large amount of syn-metamorphic extensional strain, there is evidence that the suite as a whole was originally an ophiolitic debris flow or olistostrome, with individual clasts perhaps originally up to 1 km or more in size. Undeformed, but metamorphosed clast-supported igneous breccias with angular to sub rounded blocks up to 1 m across are clearly of sedi­mentary origin from the diversity of lithologies and grain sizes present in the blocks. At two separate but stratigraphically equivalent localities, a thin melange unit with basic clasts in a variable carbonate ultra­mafic-rich matrix is preserved within the marble­schist sequence below the main, meta-igneous suite (near Kastri and Mega Lakkos). Field relations are the most consistent with it being a minor ophiolite debris flow precursor to the main unit above it.

Prior to the main phase of blueschist development and associated deformation the following events can be inferred:

• Deposition of a carbonate-dominated sequence, possibly redeposited calciturbidites, more distal to the N or NE.

• Influx of siliciclastics from a possibly southerly source.

• Tectonic disruption of oceanic lithosphere and possible initiation of stacking to the north.

• Southward migration of an ophiolitic debris flow, possibly down a growing imbricate wedge slope, with detrital serpentinite as a major sedimentary matrix component.

• Stacking and early layer-parallel extensional strain. Serpentinite included igneous blocks are possibly flattened into quasi-conformal horizons.

 • Deep burial during subduction.

(Note by JCS: The inferred events above are open to some interpretation. Things to think about: what if the ophiolite pieces and serpentine matrix are not detrital? What if their origin/nature is tectonic rather than sedimentary? This would require significant reinterpretation of the sequence of events you have just read.)

Significance of the Syn-metamorphic Deformation

The whole of the blueschist unit, except for the interiors of the metagabbro masses and locally protected parts of the breccia units, is intensely deformed. Small scale asymmetrical folding features indicate shortening of the entire metamorphic pile containing the blueschist unit during thrusting related to a major collisional event, possibly linked to tectonism on mainland Turkey (Ridley 1984b). There is a penetrative lithology parallel metamorphic foliation formed of minerals during the high­pressure metamorphism. This is the only penetrative fabric. Nowhere have any relics of an earlier one been found. See Appendix 2 of this guide for an annotated pictorial summary of deformation on Syros.

Principal Rock Types and Mineral Assemblages in the Northern Area

 Metasediments

Pure, coarsely crystalline calcite marble is the commonest carbonate-rich type. Dolomite marbles are much less common and occur as beds up to a few meters in thickness within the calcite marbles or interbedded with calcareous schists. Impure marbles and calcareous schists contain calcite or dolomite or both and various combinations of quartz, glaucophane, epidote, lawsonite, garnet, chlorite, phengite and paragonite. A particularly uniform, gray semi-pelitic schist occurs as the dominant metasediment between the thick marbles in the northern area. It contains the assemblage quartz + magnesian glaucophane + chlorite + garnet + muscovite ± lawsonite (as pseudomorphs of clinozoisite + albite) + rutile ± graphite. Basic schists and calcareous basic schists contain the mafic minerals of the calcareous schists but are poorer in, or devoid of, carbonates, quartz and chlorite. Omphacite and chloromelanite are locally quite abundant in these rarer rocks.

The types mentioned so far account for about 98% of the metasedimentary succession. The remaining 2% comprises quartzites with minor glaucophane, garnet, sodic pyroxene and mica or with epidote, unaltered lawsonite and muscovite, and aluminous quartzites with chloritoid and paragonite. Thin metacherts interbedded with manganiferous schists, with the assemblages spessartine + quartz and spessartine-rich almandine + aegerine/jadeite + crossite + phengite + apatite + quartz are distinctive but very minor constituents of the sequence. They occur inter­bedded with calcareous schists and thin marbles at a single highly deformed horizon now about 5 meters thick near the lower contact of a screen of metasediments partially separating two zones of meta-igneous bodies on the west side of the island (Trakhilaki).

Meta-igneous rocks

Metagabbros and metagabbroic gneisses form a large family all possessing a texture which has been strongly influenced by the original magmatic texture of discrete volumes of plagioclase feldspar and mafic minerals. This family includes completely undeformed rocks with a coarse gabbroic texture inherited by various assemblages, most commonly glaucophane + epidote pseudomorphing mafic and felsic components respectively, or actinolite ± glaucophane + omphacite ± epidote in which the omphacite occupies a textural position between mafic and felsic constituents. A great variety of small scale contact relations as for example gabbroic xenoliths in fine-grained micro-gabbroic matrices are pre­served intact. No identifiable igneous material grain has yet been found. In some quite highly deformed glaucophane + epidote + garnet + sodic pyroxene quartz gneisses, the influence of an original mafic­felsic segregation can still be clearly detected and this is also true, with some imagination in the case of striped glaucophane-epidote-garnet gneisses. A rela­tively homogeneous iron-poor metagabbro with the assemblage glaucophane ± actinolite + omphacite ± garnet + zoisite + quartz ± paragonite, the original "saussuritgabbro" of Ktenas and earlier writers, occurs in masses up to 6-700 m across that are mappable, but the other rocks in the family are not consistent enough in appearance for this to practicable. It is the critical zoisite + paragonite + quartz assem­blage in the saussurite of this gabbro, the equivalent of lawsonite + jadeite + water which constrains P-T estimates to 450 °C at > 13 kbar.

Basic gneisses containing similar assemblages to the recognizable gabbroic varieties were probably of initially finer grain or have been deformed beyond recognition subsequently. Acid gneisses, equivalent to granophyres or keratophyres, are a distinctive light coloured, usually fine grained, commonly flaggy group containing jadeite - quartz - garnet - paragonite as the most constant assemblage with glaucophane as a variable extra constituent. They occur interlayered with deformed basic gneisses and also as blocks or matrix in complex acid-basic net-veined igneous breccias.

The mappable zones of metamorphic breccias which partly surround the large metagabbro mass on the north-east coast are of three main types. One is largely undeformed and consists of an abundance of unsorted angular blocks of various meta-igneous rocks up to 1.5 m across set in a metabasic glaucophane-rich matrix.

The other type has a greater range of blocks - igne­ous and sedimentary - set in a matrix made up of about 50% ankerite with omphacite, chromium epidote and sodic amphibole in equal proportions and with minor chlorite, apatite and chromite. When the blocks are closer together than the most usual distance of about 30 cm the proportion of omphacite in the matrix reaches 50-60%. Parts of this ankerite breccia are undeformed, when they resemble the glaucophane rich matrix breccia in outcrop but more commonly the breccia has suffered severe stretching and the "blocks" are then elongated into rods up to 5 m in length and 50 cm across. Most of the blocks show compositional zoning due to reaction with the matrix either during metamorphism or prior to it or both and the quartz-rich blocks have monomineralic omphacite zones at their outer margins. The chromium-rich character of this matrix strongly suggests that prior to metamorphism this zone of breccia, now up to 40 m thick, had a matrix very rich in detrital chrome-spinel rich serpentinite.

Serpentinite is a close associate of the meta-igne­ous rocks described above and occurs as a more or less continuous ramifying sheet-like mass having both concordant and discordant contacts with the compositional layering in metasediments and meta­igneous rocks. The locally abundant metasomatic derivatives of the original antigorite - talc, dolomite, actinolite or tremolite and chlorite - are not distinguished on the map. The relative lack of com­petence of the serpentinite has been a major control on the style of deformation in the gneiss belt as a whole.

Included within the serpentinite are scattered metasomatized igneous blocks, also metamorphosed. They have spectacular blue and green outer reaction rinds rich in monomineralic glaucophane, actinolite and sodic pyroxene. Most are rounded and rarely larger then 5 m across though glaucophane eclogites as a group tend to be larger, up to 120 m across. The "monolith" is exceptional. It is glaucophane eclogite but it is attached to the country rock on one side and still contains free quartz. It stands 20 m above the land surface and was clearly the inspiration for Foullon and Goldschmidt's (1897) description of the "line of house sized blocks" stretching across the north of the island.

The most common blocks are eclogites and chlorite-eclogites; sodic pyroxenes of various types including virtually jadeitites; aegerine/jadeite + chlorite rocks with abundant magnetite and apatite; micaceous pyroxenites; the glaucophane eclogites noted earlier and occasional metagabbros. The most obvious effect of the serpentinite envelope on the mineral assemblages developed during metamorphism has been desilication resulting from the evidently low ^μSi0₂ of the antigorite-talc buffer. Other enrichment and depletion trends occur as functions of initial block composition which were 'driven' by other reactions forming chlorite, actinolite, etc. in the ultramafic envelope and yielding rocks with extremely high contents of Ti, P and Na and very low Si02. It is noteworthy that in the extremely talc-rich ultramafic sheet lying within metasediments at the west coast is a jadeite + quartz block whereas elsewhere in the main serpentinite sheet the felsic blocks are all quartz-free jadeitites (or retrograded albitized jadeitites). One block has an internal structure of shadowy angular chlorite-eclogite blocks in a jadeitite matrix, a desilicated version of otherwise comparable silicasaturated net-veining relationships in gneisses away from the serpentinite.

Eocene high-pressure metamorphism

 The assemblages of the high pressure metamor­phism are best preserved on the islands of Sifnos and Syros, where thick sequences of metasedimentary and metavolcanic rocks occur. It must be empha­sized, however, that the high pressure metamorphism was regional in extent and relict assemblages can still be found throughout the Cyclades, as, for example, on Ghiaros (Katagas 1984).

Geochronology

Rb-Sr and K-Ar geochronological data for the high pressure event on the islands of Sifnos, Syros, Tinos, Naxos, Samos, southern Euboea, and in southern Attica show a distribution of ages from 32 to 58 Ma. The most reliable ages probably are from phengite-paragonite samples in which the phengite shows the high pressure 3T polytype. These samples give concordant Rb-Sr and K-Ar ages for both micas of 40 to 42 Ma (Altherr et al. 1979). Lower ages ranging between 33 and 39 Ma most probably repre­sent partial resetting as a result of the later metamor­phic event. 40-45 Ma is Eocene in age, indicating the metamorphism is younger than the high pressure Eoalpine event (110 - 60 Ma) observed in the West­ern and Central Alps (Hunziker 1974; Bocquet et al., 1974; Delaloye and Desmons 1976; Chopin and Maluski 1980; Chopin and Monte 1984).

(Note by JCS: From Cheney et al., 2000 - Preliminary ²⁰⁶Pb/²³⁸U dates of zircons from two blueschists from the north end of Syros have been obtained from the UCLA ion microprobe. Five dates of three euhedral, crosscutting zircons in one sample average 83 ± 10 Ma. These results are consistent with the 78 Ma zircon date of Brocker & Enders (1999) and probably represent the true metamorphic age for all the rocks, as they suggested. In a second sample, 4 ages from one large (>150 microns) euhedral zircon gave a range of ages from 81 ± 2 to 54 ±4 Ma. The complexity of this zircon may be due to its occurrence in a blueschist block from the Kampos melange zone. The younger dates may reflect reequilibration that postdates transference of the Syros units to the upper plate of the subduction zone, whereas the older dates may record peak P-T conditions resulting from the slowing of the down­going slab (Schumacher et al., 2000) in the Late Cretaceous.)

Miocene medium-pressure metamorphism

The second metamorphic event in the Cycladic complex resulted in a regionally variable overprint of the earlier high pressure metamorphism. Gener­ally, greenschist to lower amphibolite facies condi­tions are developed (e.g. Sifnos, Ikaria). On Naxos, however, a thermal dome developed reaching high­grade amphibolite facies conditions (Jansen and Schuiling, 1976).

Geochronology

On Ikaria, Naxos, Tinos and Sifnos K-Ar ages of hornblendes and K-Ar and Rb-Sr ages on micas are in the range 21-25 Ma, placing the metamorphism in the early Miocene. Considerably younger mica dates from amphibolite facies areas would appear to point to a prolonged period of cooling up to the end of middle Miocene. In such areas initial cooling was possible retarded by the emplacement of granitoid magmas. Final cooling, however, was fast as indicated by nearly identical model ages for muscovites and biotites (Altherr et al. 1979; 1982; Anderissen et al. 1979). A different view has been put forward by Wijbrans and McDougall (1986) who argue, on the basis of ⁴⁰Ar/³⁹Ar dating on white micas from Naxos, that the thermal dome metamorphism occurred between 15 and 20 Ma with the 20-25 Ma ages representing mixed ages between the 40-45 Ma high pressure event and the proposed 15-20 Ma thermal event. They also suggested that the thermal metamorphism on Naxos may have been of short duration, possibly less than 1 Ma! Resolution of this difference in age interpretation is clearly of importance because of its implications concerning the question of whether the heat sources for the second metamorphic event were intruding granitoid magmas and/or regional heat flow during exhumation.

Miocene granitoid plutonism

I-type and S-type granitoids intruded after the culmination of the medium-pressure metamorphism. The I-type plutons show a systematic regional variation in composition: granodiorites occur in the (south)west (Laurium/Attica, Serifos), granites and leucogranites in the center (Tinos, Mykonos-Delos, Naxos, Keros, Ikaria), and monzonitic intrusives in the (north)east (Samos, Kos, Bodrum/Turkey). Chemically, this variation is marked by an increase in K20 at constant Si02 . S-type intrusives occur on Tinos, Paros, Naxos, Ikaria and Samos, and are thus confined to the central and (north)eastern parts of the crystalline complex (Altherr 1980; 1981 a; 1981 b; Mezger et al., 1985)

Geochronology

Despite a number of efforts to radiometrically date the different granitoids, the exact intrusion ages of most plutons are still unknown. None of the I-type plutons yielded an unequivocal whole-rock Rb-Sr isochron (Altherr et al., 1982). Anderissen et al. (1979) published a Rb-Sr whole rock age of 11. l ± 0.7 Ma for the I-type granite from Naxos. However, owing to the low spread in Rb/Sr of the samples from the granite itself, the slope of the best fit line is essentially determined by the samples from the aplitic and pegmatitic dike rocks cutting the granite and its metamorphic country rocks. The granitic samples alone do not show a correlation in the Nicolaysen diagram. Hence, the date of I 1 Ma at most can be regarded as a minimum age. K-Ar, Rb-Sr and fission track dates on hornblendes, biotites, sphenes, and apatites from all I-type plutons range from about 16 Ma to about 4 Ma (Altherr et al., 1982; Anderis­sen et al., 1979; Mezger et al, 1985; Henjes-Kunst et al., 1988). Most of these dates were interpreted as cooling ages by Altherr et al. (1982). For some plutons, however, an unequivocal interpretation of these mineral ages as cooling ages is not possible because the different mineral dates deviate slightly from an age sequence expected for a normal cooling history. In some cases the biotite K-Ar date is higher than that of hornblende from the same sample or different hornblende-biotite pairs of one intrusion show discordant model ages. Apatites from the monzonite of Kos show bimodal distribution of the lengths of spontaneous fission tracks indicating a weak thermal overprint < 2.5 Ma ago (Altherr et al., 1982).

U-Pb studies have been made on zircons and thorites from most of the I-type granitoids (Henjes­Kunst et al., 1984; 1988). Zircon dates were found to be discordant indicating that the zircons contain an old premagmatic crustal component. Thorites, however, yielded concordant dates. Except for Naxos, thorite and zircon lower intercept dates are equal to or younger than the K-Ar, Rb-Sr and fission track dates on hornblendes, biotites, sphenes, and apatites and, therefore, are not regarded to repre­sent crystallization ages. Henjes-Kunst et al. (1988) conclude that the discordia defined by the differ­ent zircon fractions were tilted due to a loss in Pb, resulting in a decrease of the upper and lower inter­cept ages. The metamict thorites also suffered a Pb loss, causing a partial resetting of their U-Pb clocks. Most probably, the postulated Pb loss occurred during the final uplift of the Cycladic crystalline complex during late Miocene times.

Summarizing, we can state that the chances of precisely dating the intrusion of the different I-type granitoids are quite slim. At the moment, the fol­lowing intrusion ages are compatible with the above date of 10 Ma for Laurium and Serifos, 12 Ma for Naxos, Kos and Samos, 15 Ma for Mykonos-Delos, and 18 Ma for Ikaria and Tinos.

For the S-type granite of Tinos Altherr et al. (1982) obtained an age of 14 ± 0.2 Ma using the Rb-Sr whole rock method. K-Ar and Rb-Sr mineral dates from the S-type granites of Paros and Ikaria range between 21 and 10 Ma.

Figure 5. Schematic P-T t paths showing the metamorphic evolution as exemplified by the islands of Sifnos and Naxos. Syros's evolutior resembles Sifnos's. In timing, nearby Mykonos's igneous intrusion resembles Naxos's thermal doming event but unlike Naxos lacks a regional metamorphic signature.

Summary

Metamorphic and igneous activities of the Cyclades have been considered in terms of three periods: (1) Eocene (possibly Cretaceous) high-pressure metamorphism, (2) Miocene medium-pressure metamorphism, (3) Miocene plutonism. The P-T paths associated with these events are schemati­cally shown in Figure 5. Pressure and temperature estimates are based on phase equilibria and oxygen isotope temperatures. Approximate age relations are shown according to the geochronological data given earlier. The Sifnos P-T path involves synkine­matic progradation through epidote-garnet-bearing blueschist to greenschist facies assemblages. The estimated pressure of 15 kbar corresponds to burial depths of more than 50 km and is similar to high pressures observed in other Alpine type collision zones (e.g. Sesia-Lanzo zone, Western Alps). The relatively broad P-T loop contrasts with more tight P-T loops deduced for the subduction of oceanic lithosphere and the associated accretionary prism (e.g. the Fransciscan (USA), Ernst 1977). Further­more, both terranes exhibit marked differences in their tectonostratigraphic history: the volcanic-sedi­mentary sequence of the Cyclades is typical for a passive continental margin and protolith formation was much older than high-pressure metamorphism during which the coherent character of the sequences appears to be preserved despite penetrative deforma­tion; the Fransciscan, however, displays a disrupted character and the metamorphic overprint occurred more or less contemporaneous with sedimentation.

The Eocene (Cretaceous?) blueschists of the Cyclades are part of a more extensive high-pressure belt running from southern Yugoslavia via Mt. Olympus in northern Greece through the Attic­Cycladic complex to Turkey. Along this belt the age of high-pressure metamorphism is constrained to late Cretaceous and Paleocene times. This meta­morphism has been related to a continental collision between the Apulian microplate in the south and the Eurasian plate in the north.

The Miocene metamorphism represents the thermal reactivation of the rising Cycladic complex. Whereas greenschist fades are widespread through­out the Cyclades, the development of thermal domes is restricted to the islands of Naxos (Figure 5) and Paros. The Miocene metamorphism and related igneous activity are contemporaneous with the development of a high-pressure metamorphic belt on the Peloponnesus and on Crete and it is possible that these phenomena can be considered as a paired metamorphic belt (Altherr et al. 1982; Seidel et al., 1982). This is also born out by the regional variation pattern in the composition of the Miocene granitoids which is similar to patterns found for the Sierra Nevada and Peninsular Ranges batholiths of western North America. It is assumed that the paired belt documents northeast-ward subduction of parts of the African plate beneath the Apulian microplate.

References and Bibliography

Altherr, R. (1980) I- and S-type granitoids of the central Aegean crystalline complex (Greece). EOS Trans. AGU 61,402.

Altherr, R. (19816) Zur Petrologic der miozanen Granitoide der Zentral-agais (Greichenland). Dr. habil. thesis, U. Braunschweig, 218pp.

Altherr, R. Variationen im Chemismus miozaner I- and S-Type Granitoide des Attisch- Kykladischen Kristallinkom­plexes (Greichenland). Fortsch. Mineral. 59, 223-224. (1981 a)

Altherr, R., Kreuzer, H., Wendt, L, Lenz, H., Wagner, G. A., Keller, J., Harre, W., Hbhndorf, A. (1982) A late Oligocene/early Miocent high temperature belt in the Attic-Cycladic crystalline complex (SE Pelagonian, Greece). Geol. Jb. E23, 97-164.

Anderissen, P. A. M., Boelrijk, N. A. I. M., Hebeda, E. H., Priem. H. N. A., Verdurmen, E. A. Th., Verschure, R. H. Dating the events of metamorphism and granitic magma­tism in the Alpine orogen of Naxos (Cyclades, Greece). Contrib. Mineral. Petrol. 69, 215-225. (1979)

Bocquet, J., Delaloye, M., Hunziker, J. C., Krummenacher, D. (1974) K-Ar and Rb-Sr dating of blue amphiboles, micas and associated minerals from the Western Alps. Contrib. Mineral. Petrol. 47, 7-46.

Bonneau, M., Blake, M. C. Jr., Geyssant, J., Kienast, J.R., Lapurier, C.M.H., Papanikolaou, D. (1980) Sur la signi­fication des series metamorphiques (schistes bleus) des Cyclades (Hellenides, Grece); file de Syros Significance of the Cyclades blueschist series, Hellenides, Greece; Syros Island as an example. Comptes Rendus Hebdoma­daires des Seances de 1'Academie des Sciences, Serie D: Sciences Naturelles 290(23): 1463-1466.

Bonneau, M., Blake, M.C., Geyssant, J., Lepvrier, C., Papan­ikolaou, D. (1980). Geology of Syros Island (Greece), type locality for glaucophane. International Geological Congress Abstract 1980: 21.

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