Hydrothermal gases offshore Milos Island, Greece

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO₂ but CH₄ concentrations up to 2% are present. The stable carbon isotopic composition of the CO₂ (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH₄ samples are enriched in the heavy species (δ¹³C = −9.4 to −17.8‰; δD = −102 to −189‰) which is believed to be characteristic for an abiogenic production of CH₄ by CO₂-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH₄ samples (to −377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures.

We show that excess helium, slightly enriched in ³He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a ³He/⁴He_excess of 3.6 · 10⁻⁶ is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface.     

Isotope Geochemistry of the Pb-Zn-Ba(-Ag-Au) Mineralization at Triades-Galana,Milos Island,Greece

The Pb-Zn-Ba(-Ag-Au) mineralization in the Triades and Galana mine areas is hosted in 2.5–1.4 Ma pyroclastic rocks, and structurally controlled mostly by NE-SW or N-S trending brittle faults. Proximal pervasive silica and distal pervasive sericite-illite alteration are the two main alteration types present at the surface. The distribution of mineralization-alteration in the district suggests at least two hydrothermal events or that hydrothermal activity lasted longer at Galana. The Sr isotope signature of sphalerite and barite (⁸⁷Sr/⁸⁶Sr = 0.709162 to 0.710214) and calculated oxygen isotope composition of a fluid in equilibrium with barite and associated quartz at temperatures of around 230°C are suggestive of a seawater hydrothermal system and fluid/rock interaction. Lead isotope ratios of galena and sphalerite (²⁰⁶Pb/²⁰⁴Pb from 18.8384 to 18.8711; ²⁰⁷Pb/²⁰⁴Pb from 15.6695 to 15.6976; ²⁰⁸Pb/²⁰⁴Pb from 38.9158 to 39.0161) are similar to those of South Aegean Arc volcanic and Aegean Miocene plutonic rocks, and compatible with Pb derived from an igneous source. Galena and sphalerite from Triades-Galana have δ³⁴S_VCDT values ranging from +1 to +3.6‰, whereas barite sulfate shows δ³⁴S_VCDT values from +22.8 to +24.4‰. The sulfur isotope signatures of these minerals are explained by seawater sulfate reduction processes. The new analytical data are consistent with a seawater-dominated hydrothermal system and interaction of the hydrothermal fluid with the country rocks, which are the source of the ore metals.     

Nature and origin of the Vani manganese deposit, Milos, Greece: an overview

The Vani manganese deposit is located in the rugged NW sector of Milos Island. It occurs within the Vani volcano-sedimentary basin, which is underlain by dacitic domes and flows of Upper Pliocene age (3.5–2.0 Ma). The end of the emplacement of the dacites was marked by the collapse of the magma chamber, which resulted in a huge pyroclastic episode and the deposition of a thick layer of pyroclastic material within a shallow submarine basin. This pyroclastic material subsequently compacted to form the volcaniclastic sandstone, which became the host for the manganese ore beds which were about 4 m thick in the two sections studied. Hydrothermal fluids penetrated these sandstone horizons via fractures and fissures to produce the manganese deposit. The permeable nature of the sandstone facilitated the retention of the hydrothermal fluids within these layers. This permitted the fluids to cool slowly and deposit the manganese oxides almost quantitatively. Formation of the hydrothermal manganese deposit took place fairly rapidly over a period of several tens of thousands of years at most. Strong tectonic activity resulted in rapid uplift of the area which elevated the deposit above sea level.Two generations of manganese oxides have been identified within this deposit; the first generation consists of pyrolusite and ramsdellite; the second generation of oxides of the isostructural series cryptomelane–hollandite–coronadite plus hydrohetaerolite characterized by high contents of K, Ba, Pb and Zn, respectively. This sequence is the result of a two-stage process of formation of the manganese-oxide minerals in which a second high-salinity hydrothermal fluid enriched in Ba, Pb and Zn as a result of the dissolution of sulphide minerals remineralized the original manganese-oxide assemblage. It is this two-stage process of formation, which was mainly responsible for the unique characteristics of this deposit. Although formed in a submarine setting, the deposit shows marked differences in mineralogy and composition from known submarine hydrothermal manganese deposits and is most analogous to the epithermal vein deposits of the southwestern United States.     

Xenoliths of lawsonite-ferroglaucophane rocks from a Quaternary volcano of Milos (Aegean Sea,Greece)

Xenoliths of metamorphic rocks with ferroglaucophane + albite + stilpnomelane + muscovite ± lawsonite ± pumpellyite are described fromn the rhylolitic tephra deposits of the Late Quaternary Phyriplaka Volcano, Milos, Aegean Island, Greece. The chemical bulk analyses reveal metabasaltic parent rocks with strongly modified chemical compositions. Unusual for common fresh basalt types is extreme Na₂O combined with low K₂O and rather low CaO. These are chemical characteristics of spilitic basalts. High iron in the bulk rock and very high Fe/Mg is probably the cause of extreme ferroglaucophane compositions. The metamorphic assemblages point to a high-p/low-T metamorphism in the pressure range 4–8 kb and with temperatures of about 350°C. Phyriplaka Volcano is likely to have only a shallow explosive center and ferroglaucophane rocks were ejected from the immediate basement of the island, which was uplifted after the metamorphic event.     

The origin and control of colour of white bentonites from the Aegean islands of Milos and Kimolos, Greece

Some of the Lower Pleistocene bentonites of Milos and Kimolos islands, Greece, are valued for their white colour and physicochemical properties. They contain opal-CT and, sometimes, zeolite along with smectite, and have been derived from the alteration of rhyolitic volcanic rocks. The Miloan white bentonites contain Tatatilla-type montmorillonite and beidellite. The Kimolian ones have Chambers-type montmorillonite. The alteration process involved removal of alkalis and uptake of Mg, probably from sea water. Si is either redistributed or partially removed. The Kimolian white bentonites have higher brightnesses, L* and whiteness index values, and lower yellowness index and ΔE*ab values compared with the Miloan ones. The variations in white colour are inversely related to the abundance of Fe oxides and anatase, the occurrence of Fe in the smectite structure and its oxidation state. The presence of silica minerals is not an important factor affecting colour, but is undesirable since it imparts high abrasiveness in commercial products. The formation of white bentonites of high quality requires the removal of alkalis and silica during alteration of acid volcanics in order to avoid crystallization of zeolites and opal-CT. Fe needs to be incorporated into the smectite structure. Such conditions are rarely attained. Received: 25 September 1996 / Accepted: 17 January 1997     

The high-pressure stability of zoisite and phase relationships of zoisite-bearing assemblages

The fluid-absent reaction 12 zoisite = 3 lawsonite + 7 grossular + 8 kyanite + 1 coesite was experimentally reversed in the model system CaO-Al₂O₃-SiO₂-H₂O (CASH) using a multi-anvil apparatus. The upper pressure stability limit for zoisite was found to extend to 5.0 GPa at 700 °C and to 6.6 GPa at 950 °C. Additional experiments both in the H₂O-SiO₂-saturated and in the H₂O-Al₂O₃-saturated portions of CASH provide further constraints on high pressure phase relationships of lawsonite, zoisite, grossular, kyanite, coesite, and an aqueous fluid. Consistency of the present experiments with the H₂O-saturated breakdown of lawsonite is demonstrated by thermodynamic analysis using linear programming techniques. Two sets of data consistent with databases of Berman (1988) and Holland and Powell (1990) were retrieved combining experimental phase relationships, calorimetric constraints, and recently measured elastic properties of solid phases. The best fits result in G _{f ,1,298} ^∘,zoisite=−6,499,400 J and S _1,298 ^∘,zoisite=302 J/K, and G _{f ,1,298} ^{∘,lawsonite}=−4,514,600 J and S _1,298 ^{∘,lawsonite}=220 J/K for the dataset of Holland and Powell, and G _{f ,1,298} ^∘,zoisite=−6,492,120 J and S _1,298 ^∘,zoisite=304 J/K, and G _{f ,1,298} ^{∘,lawsonite}=−4,513,000 J and S _1,298 ^{∘,lawsonite}= 218 J/K for the dataset of Berman. Examples of the usage of zoisite as a geohygrometer and as a geobarometer in rocks metamorphosed at eclogite facies conditions are worked, profiting from the thermodynamic properties retrieved here. Received: 23 December 1996 / Accepted: 29 August 1997     

Geochronology of high-pressure rocks on Sifnos (Cyclades,Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M ₁ phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M ₁+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M ₁ phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.     

Epithermal gold mineralisation in the active Aegean Volcanic Arc: the Profitis Ilias deposit, Milos Island, Greece

The Profitis Ilias gold deposit, located on the western part of Milos Island, Greece, is the first epithermal gold deposit discovered in the Pliocene–Pleistocene Aegean volcanic arc. Estimated ore reserves are 5 million tonnes grading 4.4 g/tonne Au and 43 g/tonne Ag. The deposit is closely associated with a horst and graben structure, and occurs in a series of steep interconnected crustiform-banded quartz veins up to 3 m wide, extending to depths of at least 300 m. The mineralisation occurs in three stages and is hosted by 3.5–2.5 Ma old silicified and sericitised rhyolitic lapilli-tuffs and ignimbrites. It consists of pyrite, galena, chalcopyrite, electrum and native gold. Additionally, adularia occurs with quartz mainly in veins. Homogenisation temperatures of primary liquid-rich inclusions vary from 145 to 399 °C for the ore stage, and 112 to 263 °C for the post-ore stage. Salinities range between 0.1 and 11.4 wt% NaCl equiv. and 0.93 to 8.5 wt% NaCl equiv. for the ore stage and the post-ore stage, respectively. Rare vapour-rich inclusions in ore stage quartz homogenise between 368 and 399 °C and estimates of eutectic melting (−25 to −38 °C) indicate the presence of Ca and Mg in the ore fluids. Sample elevation versus fluid inclusion T_h–salinity relationships show (1) a high-salinity trend, where moderate-temperature (300–250 °C) and moderate-salinity brines (∼3 wt% NaCl equiv.) trend to high-salinity (up to 15 wt% NaCl equiv.) fluids with lower (∼25–50 °C) homogenisation temperatures, and (2) a high-T_h trend where moderate-salinity and moderate-temperature brines (200–250 °C; 3 wt% NaCl equiv.) develop into low-salinity (<1 wt% NaCl equiv.), high-temperature (>350 °C) fluids. These trends are best explained by extreme boiling and vapourisation phenomena between 200 and 250 °C. The 430–450 m asl (metres above sea level) level marks the transition between a lower liquid-dominated segment of the system where only the steep high-salinity trend is seen, and an upper vapour-dominated segment where the high-T_h trend or a combination of both are seen. There is a close spatial association between mineable gold grades and the upper segment of the system. Depth-to-boiling curves suggest that the paleo-surface was ∼200 m above the present summit of Profitis Ilias. Comparison of the mineralisation and fluid geochemistry at Profitis Ilias with that of the nearby modern geothermal system indicates that the processes of metal mineralisation have probably been continuous since the Late Pliocene. Received: 24 February 2000 / Accepted: 15 July 2000     

New geochemical and mineralogical constraints on the genesis of the Vani hydrothermal manganese deposit at NW Milos island,Greece: Comparison with the Aspro Gialoudi deposit and implications for the formation of the Milos manganese mineralization

The Mn-Ba-Pb deposit at Aspro Gialoudi in NW Milos is shown to be a fossil inhalative-exhalative hydrothermal deposit that represents the deepest part of the Vani succession at the western extremity of the main Vani manganese deposit. The geology of the Vani-Aspro Gialoudi area is characterized by Upper Pliocene-Lower Pleistocene dacitic and rhyodacitic lava domes, which are overlain by the Vani volcaniclastic unit considered to be part of the 2.66–1.44 Ma magmatic event at Milos Island. The presence of in-situ and intrusive hyaloclastite breccias surrounding the coherent lava domes at Aspro Gialoudi and Vani areas indicates submarine emplacement for the domes. The dacitic-rhyodacitic domes are variously altered (mainly propylitic and/or argillic alteration, silicified and in some cases locally exhibiting adularia alteration). Both Aspro Gialoudi and main Vani deposit are located proximal to fault systems: the main Vani manganese deposit is adjacent to the NW-trending Kondaros-Katsimouti-Vani Dome fault, whereas the Aspro Gialoudi deposit is adjacent to the relatively minor NE-trending fault on the west coast of Milos. At Aspro Gialoudi, mineralization took place in a subseafloor and/or seafloor environment and is characterized by a stratabound Mn-barite-rich deposit mainly within a package of propylitized intrusive hyaloclastites and within the overlying sandstones. Banded epithermal veins trending NE-SW and composed of chalcedonic silica/quartz + barite + Mn-oxide ± sulfides crosscut the dacitic lavas, the hyaloclastites and the overlying volcaniclastic sequence at Aspro Gialoudi and are considered to represent the feeder zones of the manganese-barite mineralization. Within the veins, early sulfide (galena-sphalerite) barite and quartz deposition is followed by manganese oxides and aragonite, thus resembling the epithermal-style Pb-Zn-Ag-Mn mineralization across the NW-trending Katsimoutis-Kondaros-Vani fault. Mineralization in Aspro Gialoudi and Vani deposits seems to be controlled by alternating cycles of deposition of sulfides and hydrothermal manganese oxides within the faults. Manganese deposition in both deposits formed in a similar manner, namely by transport of hydrothermal fluids through the adjacent fault systems into a reservoir of volcanoclastic sandstone and hyaloclastites to produce a deposit initially consisting of principally of pyrolusite and occasionally ramsdellite, which were subsequently replaced by cryptomelane, hollandite, coronadite and hydrohaeterolite. Precipitation of hydrothermal manganese oxides took place very quick and under microbial Mn(II) oxidation. Compositional data show that metallic elements most enriched in the Aspro Gialoudi and Vani manganese deposits relative to the average continental crust, lie in the sequences Pb > Cd > Mn > As > Sb > Zn > W > Tl > Ba > Cu > Mo > Co > Bi and As > Sb > Pb > Mn > Tl > Cd > Zn > W > Cu > Ba > Mo > Co, respectively. Mineralogical and geochemical (e.g. REE) data from both Aspro Gialoudi and main Vani deposit are taken to indicate mainly a seawater source for the hydrothermal fluids. These two deposits are genetically and spatially related to base- and precious metal intermediate-sulfidation epithermal mineralization. They formed successively by similar processes and are considered to be integral parts of the same hydrothermal system.     

Preliminary note on deerite from high-pressure metamorphic rocks of Sifnos,Greece

A new occurrence of deerite-bearing quartzites from blueschist metamorphic rocks of the Greek island of Sifnos is described, and analytical and X-ray data on deerite are presented.     

Metamorphism of siliceous dolomites at Naxos,Greece

Siliceous dolomitic marbles at Naxos, Greece, are crystallized in a range of metamorphic temperatures, estimated from 350 to 700 °C and a total pressure of about 6 kb. A number of low variant assemblages of this rock-system have been studied. The sequence of these metamorphic assemblages is a function of increasing metamorphism, and this is in agreement with other fielddata and with experimental data for the chemical system CaO-MgO-SiO₂-H₂O-CO₂ (-HF). The composition of the associated fluid inclusions may be considered reasonably correct in the light of experimentally obtained X _CO2 values for the mineral equilibria, extrapolated for a total pressure of about 6 kb at the interpolated temperatures at the sample locations.     

Textural and isotopic development of marble assemblages during the Barrovian-style M2 metamorphic event,Naxos, Greece

A detailed petrological analysis of the marble assemblages observed within the M2 metamorphic complex on Naxos is presented. Two distinct periods of mineral growth are documented; the first is associated with prograde M2 metamorphism and the second with retrograde M2 metamorphism occurring during ductile extensional thinning of the complex. The textural and miner-alogical characteristics and the carbon and oxygen isotope compositions of each generation are described, and the P-T-X _{CO 2} conditions at which these two mineral generations were stable, and the compositions of the fluids present during metamorphism are characterised. Whereas the low variance and stable isotope compositions of prograde siliceous dolomite assemblages are consistent with internally buffered fluid evolution, the retrograde mineral generation is shown to have grown as a result of the infiltration of a water-rich fluid phase that transported silica, Al₂O₃, Na₂O and FeO into the host rocks. This observation, together with the stable isotope compositions of the retrograde calcite, and the fact that occurrences of veins of this type are limited to marbles in the highest grade areas (T>600° C) of the metamorphic complex, suggests that the fluids responsible for vein formation were generated during the crystallisation of melts as the metamorphic complex cooled from peak temperatures. The existence of this second generation of minerals has significant implications for previous studies of heat transport by fluid flow on Naxos, because many of the unusually low ¹⁸O compositions of pelites at high grades may be ascribable to the effects of interaction with retrograde M2 fluids, rather than with prograde fluids.     

Metamorphism of the variegated sequence at Kallithea,Samos, Greece

Summary The variegated sequence at Kallithea, westernmost part of the island of Samos, consists of pure dolomitic marbles, siliceous marbles, calc-silicate rocks and amphibolites. It was formed from a volcano-sedimentary series by a regional metamorphism under amphibolite facies conditions. Assuming a total fluid pressure of 5 kb, temperature may have reached 700°C. In the siliceous marbles, a retrogressive evolution is documented. The intrusion of the Kallithea igneous suite which took place about 10 Ma ago hardly affected the regional metamorphic sequence. The only exception are small veins of wollastonite in a silicate marble. In contrast to the other tectonic units which form the major part of Samos, no traces of high pressure minerals could be detected in the Kallithea metamorphics, suggesting a different geological history. The age and geotectonic position of the variegated sequence is unknown so far.

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Zusammenfassung Die bunte Serie von Kallithea (westlichster Teil der Insel Samos), bestehend aus reinen Dolomitmarmoren, Silikatmarmoren, Kalksilikatfelsen und Amphiboliten, geht auf eine vuikano-sedimentäre Folge zurück, die durch eine Regionalmetamorphose unter amphibolitfaziellen Bedingungen geprägt wurde. Hierbei dürften Temperaturen um 700°C erreicht worden sein, wenn man einen Gesamtdruck (P _fl ) von 5 kb annimmt. In den Silikatmarmoren ist eine Abfolge retrograder Mineralreaktionen dokumentiert. Vor etwa 10 Millionen Jahren intrudierten in die regionalmetamorphe Serie dioritische bis granitische Magmen ohne das Nebengestein kontaktmetamorph zu überprägen. Eine Ausnahme bilden kleine Wollastonitäderchen in einem Silikatmarmor. Im Gegensatz zu den übrigen tektonischen Einheiten, die den größten Teil der Insel aufbauen, lassen sich in den Metamorphiten von Kallithea keine Anzeichen einer Hochdruckmetamorphose nachweisen, was auf eine abweichende geologische Geschichte hindeutet. Alter und geotektonische Stellung der bunten Serie von Kallithea sind bislang noch unbekannt.

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With 5 Figures

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Herrn Professor Dr. Siegfried Matthes zum 70. Geburtstag gewidmet     

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al₂O₃ and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.

Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al₂O₃ and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.

The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.     

Hydration of orthopyroxene–cordierite-bearing assemblages at Laouni, Central Hoggar, Algeria

In the Laouni region (Central Hoggar, Algeria), retrogression of high-grade orthopyroxene–cordierite-bearing rocks led to the crystallization of orthoamphibole and garnet, and at a later stage of chlorite, from the original paragenesis. Calculated phase diagrams show that this retrogression occurred at about 3  kbar with the simplest model involving hydration at 650–700°  C and at around 500°  C, with the rocks experiencing a H₂O less than 1, except possibly in the last stages of chlorite crystallization. As the other rock types occurring in the same area as the orthopyroxene–cordierite rocks display similar features, it is concluded that regional hydration occurred, presumably related to the release of fluids during the crystallization of the Pan-African granitic and mafic magmas that are widespread in the Laouni area.     

Phase relationships of sursassite and other Mn-silicates in highly oxidized low-grade,high-pressure metamorphic rocks from Evvia and Andros islands,Greece

High-pressure, low-temperature metamorphic Mn-rich quartzites from Andros and Evvia (Euboea) islands, Greece, situated in the Eocene blueschist belt of the Hellenides, reveal different Mn-Al-Ca-Mg-silicate assemblages in response to variable metamorphic grade. On Evvia, piemontite- and/or braunite-rich quartzites which are associated with low-grade blueschists (T<400° C, P> 8 kbar) show the principle mineral assemblage quartz + montite + sursassite + braunite + Mg-chlorite + hematite + rutile + titanite. The Mn-Al-silicate sursassite, basically (Mn²⁺, Ca)₄ Al₂(Al, Fe³⁺, Mn³⁺, Mg)₄Si₆O₂₁(OH)₇, thus far reported as a rare mineral, locally occurs as a rockforming mineral in cm- to m-thick layers. On Andros, higher-grade quartzites (T450–500° C, P>10 kbar) of similar composition contain the assemblage quartz + piemontite + spessartine + braunite + Mg-chlorite+hematite + phengite+ phlogopite + rutile. Rare sursassite is present only as a relict phase. Additional, mostly accessory minerals in quartzites from Evvia and Andros are ardennite, Na-amphibole, acmitic clinopyroxene, albite, apatite, and tourmaline. The chemical composition of the main phases is characterized in detail.Disequilibrium textures and mineral compositions in some samples from Andros and Evvia imply the reactions sursassite + braunite + quartz = spessartine+clinochlore±hematite + H₂O + O₂ (1) sursassite + braunite + phengite + quartz = spessartine + phlogopite±hematite + H₂O + O₂ (2) and in braunite-free assemblages sursassite + Mn³⁺Fe _–1 ³⁺ [hematite, piemontite] + hematite + quartz = spessartine + clinochlore + H₂O+O₂ (3) Reactions (1) to (3) have positive P-T slopes. They are considered to account for the breakdown of sursassite and the formation of spessartine during prograde metamorphism of the piemontite quartzites and related rocks. P-T data from Andros and Evvia and geological data from few other occurrences reported suggest sursassite+ quartz±braunite to be stable at T<400–450° C over a considerable pressure interval at least up to 10 kbar. Theoretical phase relations among Mn³⁺-Mn²⁺-silicates in the pseudoquaternary system Al-Mn-Ca-Mg with excess quartz, H₂O, and O₂ indicate that low-grade assemblages containing sursassite (±braunite±pumpellyite±viridine±piemontite + quartz) are likely precursors of higher-grade assemblages including spessartine, Mg-chlorite, braunite, viridine, and piemontite reported from greenschist-, amphibolite-, and high-grade blueschist-facies rocks of appropriate composition.     

Soil erosion,agricultural terracing and site formation processes at Markiani,Amorgos, Greece: The micromorphological perspective

Soils and sediments of a terraced slope at an Early Bronze Age site on the Aegean island of Amorgos were examined micromorphologically to determine the nature and amount of erosion on the slope during the past 5000 years, and how this had affected the formation of the surviving archaeological record. The deposits forming representative terraces were examined, as was the postdepositional sequence overlying the site, and a palaeosol preserved beneath terrace retaining walls at the break of slope. The buried, preterrace system “red soil” was a reworked red palaeosol, much affected by downslope erosion processes, which probably commenced with clearance associated with the Early Bronze Age occupation of the site. Examination of this soil suggested that there were at least two premodern phases of use of the hillside. © 1999 John Wiley & Sons, Inc.     

Liquefaction hazard mapping at the town of Edessa,Northern Greece

The town of Edessa is located on Northern Greece at a region that is characterized as low seismicity zone due to the fact that few moderate events of M < 6 occurred during the last century. According to the Greek Seismic Code, the expected acceleration having a 10% probability of being exceeded in 50 years is equal to 0.16g. However, an amplification of ground motion is likely to occur due the local geology that is consisted of Holocene fluvio-torrential deposits. The basic aim of this paper is to evaluate the site amplification due to geological conditions and to assess the liquefaction hazard. In order to achieve this, 1-D site response analyses were performed. The data that were employed for the construction of the numerical models have been collected from borings with standard penetrations tests (SPT) that were drilled for construction purposes. Afterward, the liquefaction potential of the subsoil layers was evaluated taking into consideration two seismic scenarios. The first scenario was based on the seismic parameters, earthquake magnitude and PGA, assigned by the Greek Seismic Code. On the second seismic model, we employed the values of acceleration, resulted from the 1-D analyses and the earthquake magnitude as it was defined by the Greek Seismic Code. In order to compile the liquefaction hazard maps, we initially estimated the liquefaction potential index (LPI) of the soil columns using the parameters provided by SPT, for both seismic loadings, and afterward we correlated these values with the proposed classification of the severity of liquefaction-induced deformations. In addition, having computed the value of probability based on the LPI, liquefaction manifestations probability maps were compiled for both scenarios. The result of this study was that liquefaction-induced ground disruptions are likely to occur at the center of the city, among the branches of Voda River, only when the amplified values of acceleration are taken into account to the computation of liquefaction potential.     

Prehistoric shore environments,shellfish availability,and shellfish gathering at Franchthi,Greece

People who exploit marine resources choose among them according to technological, economic, and social considerations. Prehistoric selectivity in their exploitation has often been postulated, but can be established only by comparing the archaeological record with an estimate of the kinds and quantities available in the environments of the time. Only then can those factors be considered that might have influenced their choice, a subject which is the domain of the archaeologist. In Franchthi Cave, a thick Upper Paleolithic through Neolithic sequence (ca. 23,000–5000 yrs B.P.) of occupational deposits has been excavated. As might be expected at a site now only a few meters from the sea, this sequence has yielded ample evidence for the use of marine resources, though only from ca. 11,000 B.P. onward. Moreover, the marine molluscan record exhibits drastic changes in composition over time, implying either major variation in availability or selective collecting. A marine geophysical study supplemented by an examination of present shore environments in the area has permitted the reconstruction of the changing coastal environments since ca. 20,000 B.P., thus providing an estimate of the probably available shellfish resources. Comparison with the molluscan assemblages found in the cave demonstrates that only during the latest Paleolithic and most of the Neolithic were the collected shellfish reasonably representative of what should have been available in the area. At other times the inhabitants made highly specialized, seemingly idiosyncratic choices from the spectrum of available species.     

An example of high-pressure low-temperature metamorphic rocks from an island-arc: the Paikon Series (Innermost Hellenides, Greece)

Abstract The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn- to post-kinematic with respect to a pre-Tithonian tectonic phase and range from the lawsonite-chlorite-albite facies through transitional Na-amphibole-greenschist facies to the chlorite sub-zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.