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 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 t 450 500 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

geochemistry and origin of piemontite bearing and associated manganiferous schists from arrow junction western otago new zealand abstract closely spaced analyses for major minor and trace elements have been made through 3 m of manganiferous quartzose schists and immediately overlying quartzofeldspathic schists at arrow junction otago new zealand chemically the quartzose schists correspond to pelagic siliceous muds or brown clays rather than to typical cherts whole rock oxidation ratios fe 3 100 fe 3 fe 2 increase from 16 to 99 with increasing mn apart from sio2 al2o3 is the most abundant oxide in these rocks al2o3 correlates negatively with sio2 and positively with all other components analysed in the manganiferous quartzose schists including fe2o3 mno tio2 mgo cao na2o k2o ba cr cu ni pb rb v y zn and zr parameters such as al 2 o 3 100 al 2 o 3 mno and fe ti indicate that the quartzose schists have a higher hydrothermal plus hydrogenous component than average pelagic clays but a larger terrigenous component than typical metalliferous sediments from the crest and proximal flanks of the east pacific rise close covariance of al2o3 with tio2 and zr results in ratios ti al 50 10 3 and zr al 2 10 3 conforming closely with average shale and average new zealand alpine schist metasediments but less closely with average andesite the data exclude basalt or rhyolite as the dominant aluminous detrital component al ti and zr are regarded as being of predominantly terrigenous origin normalized for constant al2o3 the following elements are enriched in grey green and in pink piemontite bearing arrow junction quartzose schists relative to average new zealand alpine metase dimentary rocks by average factors indicated mn 32 44 ni 15 15 cu 5 12 si 3 6 2 6 pb 2 4 zn 2 8 2 5 fe 1 8 1 9 y 1 7 1 8 ba 1 2 1 6 and sr 1 0 2 0 the metal enrichment trend can be regarded as being imposed on the products of a silica dilution trend in which al2o3 was reduced to 4 by dilution of end member terrigenous sediment with non terrigenous silica about 97 98 of the mn 93 of the ni 79 91 of the cu 73 61 of the sio2 and 43 48 of the fe is considered to be of a non terrigenous hydrothermal hydrogenous biogenic origin in the quartzose schists covariance of elements other than si with al2o3 within the single bed investigated is attributed firstly to accumulation of femn oxides at a rate essentially constant relative to that of clay minerals and other aluminous phases though diluted by varying amounts of largely biogenic silica and secondarily to the sorptive scavenging properties of the femn oxides and of clay mineral components of the sediment although the base metals may ultimately have been introduced to the ocean from submarine hydrothermal systems these sediments originated at a site relatively remote from a fast spreading ridge or other active submarine hydrothermal source

underthrusting and exhumation a comparison between the external hellenides and the hot cycladic and cold south aegean core complexes greece 1 after their emplacement in the course of the late mesozoic and the cenozoic the hellenic nappes became fragmented during late orogenic extension since the late eocene here we focus on the transition of underthrusting during nappe emplacement to exhumation during late orogenic extension to this end we compared previously published data on the structural geological and metamorphic history of the underthrusted parts of the tripolitza and ionian nappes which were exhumed in the cycladic and south aegean windows with newly obtained data on the sedimentary stratigraphic and structural development of the part of these nappes in the foreland in front of the subduction thrust the results allow the identification of two major events event 1 took place around the eocene oligocene transition and marks the onset of underthrusting of the tripolitza nappe below the pindos nappe and the ionian nappe below the tripolitza nappe respectively this led to the uplift and erosion of the pindos unit and the onset of deposition of the tripolitza and ionian flysch in front of the pindos thrust together with the formation of mylonites at the base of the metamorphosed portions of the pindos unit related to the underthrusting of the tripolitza unit event 2 occurred in the latest oligocene to earliest miocene and marks the decoupling of the ionian unit from the underthrusting plate the accretion of the tripolitza and ionian units to the overriding plate and the onset of late orogenic extension and exhumation in the overriding plate this led to the formation of the south aegean and cycladic core complexes and the subsidence of the klematia paramythia half graben throughout the early miocene

evolution of the blueschist and greenschist facies rocks of sifnos cyclades greece the metamorphism on the island of sifnos is characterized by the eocene development of a coherent highpressure blueschist terrane and an early miocene greenschist facies overprint this study documents the metamorphic evolution of the blueschist assemblages still preserved in the northern parts of the island and their subsequent transformation into greenschists in the central and southern parts the oxygen isotope geothermometry is based on calibrations for quartz pyroxenes and magnetite matthews et al 1983a augmented by revised calibrations for the minerals muscovite qz mu 1 55 106 t 2 epidote qz ep 1 56 1 92 ps 106 t 2 and rutile qz ru 4 54 106 t 2 oxygen isotope analyses of minerals from the blueschist unit of northern sifnos give consistent fractionations which are independent of rock type an average temperature of 455 c was obtained although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred the minerals and whole rocks of the greenschists of central sifnos are systematically enriched in 18o relative to the blueschist assemblages chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of ca2 and co2 the petrologic isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18o co2 enriched aqueous solutions it is reasonable to assume that this is connected with the miocene magmatic activity observed throughout the cyclades the marbles separating the blueschist from the greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation the pressure of the blueschist metamorphism is estimated at 14 2 kbar corresponding to a depth of ca 50 km the structural style and stratigraphy of sifnos are suggestive of the subduction of a continental margin sequence it is clear that the considerable tectonic depression may be associated with continental collision and underthrusting

gondwanan deglaciation and opening of neotethys the al khlata and saiwan formations of interior oman abstract new stratigraphic petrographic and paleontological data from the lower permian of southeastern arabia help to elucidate the still debated timing of neotethys rifting and the climatic evolution following the gondwanan glaciation the transition from the al khlata to the saiwan formation here described in detail is interpreted to record continental break up and onset of neotethyan spreading between northern gondwana and the cimmerian terranes at mid sakmarian times as constrained by brachiopod assemblages the eastern oman arm of the neotethyan rift system instead failed and sea floor spreading of the indo madagascar trough was delayed until final fragmentation of gondwana in the late jurassic petrographic composition of lower permian sandstones documents a distinct change from transitional to dissected rift shoulder provenance and widespread exposure of granitoid basement rocks at a late syn rift to early post rift stage very abundant pore filling celestite at the al khlata saiwan transition may have precipitated from circulating brines related to remobilization of proterozoic salt due to rift tectonics the sharp base of the saiwan formation records a major transgression related to the onset of neotethyan spreading final deglaciation and consequent global sea level rise as indicated by ravinement surfaces associated with lag deposits with tree trunks reworked ferruginous pedogenic concretions and phosphate and by paleoecological analyses the basal bed of the saiwan formation is characterized by the pioneer pachycyrtella paleocommunity with pachycyrtella omanensis 85 suspension feeding rapid rates of reproduction and growth r strategy early maturity high mortality rates in the juveniles the more mature secondary ecological succession developed higher in the saiwan formation records maximum flooding and significant climatic amelioration the saiwan formation at the beginning of the late sakmarian finally sutures the irregular rift related topography in the rim basin of interior oman and records a shift towards warmer temperate conditions after the end of the gondwanan glaciation

stability relations of mn fe al piemontite comparison of the stability relations of fe piemontite pm17ps17cz66 ca2al2mn0 5 fe0 5si3o12 oh with similar experimentally determined results on pm33cz67 suggests that at moderate fo2 addition of fe to piemontite raises its thermal stability and slightly decreases its sensitivity to oxygen fugacity reversal study of the reaction fe piemontite quartz garnet gr33sp27and40 anorthite fluid at pfluid 2 kbar in cold seal apparatus using solid oxygen buffers was completed the data indicated breakdown of pm17ps17cz66 at 645 10 c along the cu2o cuo buffer 477 10 c along the cu cu2o buffer and 365 10 c for the hematite magnetite buffer mn free clinozoisite and epidote have been shown to be stable to much higher temperatures than piemontite at moderate fo2 cu cu2o at very high fo2 however the presence of mn 3 in a distorted octahedral site may permit persistence of piemontite to higher temperatures than mn poor epidote minerals the compositional range of natural fe mn al piemontites supports these results cation partitioning from piemontite bearing parageneses also indicates crystallization at high fo2 but application of the cited results should be made with caution synthetic phases were too fine grained for optical or microprobe analysis and reaction was often incomplete the assumption that all fe mn was contained in piemontite and garnet respectively in the low and high temperature assemblages is supported by the cell dimensions for these synthetic minerals which are close to predicted values the reaction is pseudo univariant as solid solution in garnet and piemontite would be fo2 dependent in a more complex chemical system although bulk rock and fluid compositions may also have a marked effect on the stability of natural epidote piemontite fo2 is the major control on fe mn al in epidote minerals

geochemistry of ferromanganoan metasediments from the island of andros cycladic blueschist belt greece abstract on the island of andros cycladic blueschist belt metamorphosed ferromanganoan sediments occur in widespread lateral distribution they form concordant layers and lenses within a uniform sequence of quartz rich metapelitic schists which can be derived from pelagic siliceous mudstones three main types of ferromanganoan metasediments were geochemically investigated 1 piemontite spessartine and piemontite quartzites psq containing braunite rich lenses 2 spessartine quartzites sq and 3 napyroxene quartzites npxq textural and geochemical evidence leads to the conclusion that psq is derived from a mn nodule bearing sediment consisting of a mixture of terrigenous illitic clay with biogenic radiolarian ooze compared to recent nodule bearing sediments psq is markedly enriched in cu pb sr and as we assume that these elements were mobilized during diagenesis and or early metamorphism from the sedimentary column and were subsequently concentrated in minerals like piemontite ardennite and braunite under highly oxidizing conditions in contrast to psq sq and npxq are derived from parent sediments rich in smectite sq shows a higher al fe ratio in rock composition than npxq and may change into npxq by a decreasing content of spessartine while sq is chemically equivalent to a montmorillonitic clay mn 2 na almost spessartine free npxq can be derived from fe rich nontronitic clay na mn 2 required for the formation of sq was probably mobilized from adjacent mn nodule bearing sediments psq

breccia hosted manganese rich minerals of alpi apuane italy a marine redox generated deposit mn rich beds occur within the brecce di seravezza formation bsfm intercalated between norian metadolostone megalodontic marble and hettangian marble in the alpi apuane italy the bsfm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins the type sequence of metabreccia ends with a bed of chloritoid schist interpreted as the metamorphic equivalent of lateritic soils the four main types of mn mineral assemblages are 1 braunite piemontite and rare hausmannite 2 piemontite braunite hollandite and minor rhodochrosite and kutnahorite 3 braunite hollandite and rare piemontite the other minerals are quartz calcite muscovite phlogopite baryte and minor hematite rutile and apatite during the alpine orogeny pressures of 4 6 kb and temperatures of 350 380 c were attained mn assemblages with hematite and phlogopite indicate o2 10 8 for t 350 c and o2 10 6 when braunite appears on the mn rich rocks ti fe nb sn ta hf th ga rb pr k na nd were identified as detrital in origin and are positively correlated with al u is positively correlated with co pb bi mo the rare earth element patterns normalized to seawater show a negative slope towards the heavy rare earth elements with positive ce anomalies in some samples positive ce and eu anomalies in other samples and negative ce and positive eu anomalies in a third group the eu concentrations are nearly constant and the eu anomalies reflect differences in the concentration of other rare earth elements for ce a positive correlation with al2o3 sio2 indicate that a detrital signature prevails on that produced by duration of seawater exposure but a significant good correlation of la tm lu with mg could indicate at least for these elements a seawater signature only in some samples ba eu sr show anomalously high concentrations likely of hydrothermal origin however generally in all the chemical variation diagrams the bsfm samples fall outside the field of hydrothermal and hydrogenous deposits as defined in the literature for other mn deposits in addition metabasites and metavolcaniclastic beds are absent from the bsfm sequence and bsfm coastal basins were for the most part separated from the open ocean and related streams that must be excluded as possible mn sources owing to the arid climate of late triassic the runoff was negligible and the groundwater oxidizing and alkaline were unable to mobilize mn the bsfm basins are then the most suitable depositional environment for mn the water column was stagnant and stratified only where bottom of the basin was deeper than the oxic anoxic boundary reducing marine bottom waters percolated within the fractured basement and leached mn from disseminated minerals of metamorphic rocks and then rising along fault and fractures supplied mn to the bottom waters afterwards when tectonic uplift and sealevel drop brought the bottom into the oxic field mn precipitated and formed the mn rich matrix of the bsfm deposits

structural evolution of andros cyclades greece a key to the behaviour of a flat detachment within an extending continental crust the continental crust extends in a brittle manner in its upper part and in more distributed ductile manner in its lower part during exhumation of hp metamorphic rocks brittle features superimpose on earlier ductile ones as a result of the progressive localization of deformation the islands of tinos and andros are part of the numerous metamorphic core complexes exhumed in the aegean domain they illustrate two steps of a gradient of finite extension along a transect between mt olympos and naxos this study confirms the main role of boudinage as an initial localizing factor at the brittle ductile transition and emphasizes the continuum of strain from ductile to brittle during exhumation early low angle semi brittle shear planes superimpose onto precursory ductile shear bands whereas steeply dipping late brittle planes develop by progressive steepening of structures or sliding across en echelon arrays of veins the comparison between tinos and andros allows us to propose a complete dynamic section of the aegean extending continental crust and emphasizes that the strain localization process depends on both its rheological stratification and its compositional heterogeneity

alpine high pressure metamorphism at the almyropotamos window southern evia greece the alpine orogenic belt of the hellenides has been strongly reworked by ductile and brittle extensional tectonics extensional structures have affected the central aegean region and obliterated much of the original orogenic architecture since at least early miocene times in the area of almyropotamos on the island of evia flanking the western part of the aegean a unique remnant compressional nappe stack involving tertiary metamorphic rocks has been preserved this nappe sequence comprises a high pressure rock unit on top of a lower grade unit the upper unit south evia blueschist belt is thought to be the westward continuation of the cycladic blueschist belt metamor phosed at high pressure conditions during late cretaceous eocene times the underlying unit the almyropotamos unit is a continental margin sequence covered by a flysch and containing lutetian nummulites indicating that this unit accumulated sediments until at least late eocene times in the present study we analyse the petrology of the almyropotamos nappe stack and define the p t conditions of each of the different rock units exposed there the presence of glaucophane law sonite rimmed by epidote and jadeite 70 mol suggest that peak p t conditions in the south evia blueschist belt reached approximately 10 12 kbar and 350 450 c unlike previous studies which estimated that the underlying almyropotamos unit reached only greenschist facies conditions glau cophane relics and si rich phengites were found by us in this unit these indicate that high pressure metamorphism and crustal thickening in this part of the aegean lasted until at least the late eocene or early oligocene we note that in this respect the architecture of southern evia resembles that of north ern greece olympos ossa our structural data indicate that rock units in the almyropotamos area record different folding phases with the south evia blueschist belt having a more complex fold his tory than the underlying almyropotamos unit the entire nappe stack shares large scale folds which are e w trending and locally overturned to the south and which may represent at present coordi nates n s contraction and nappe transport

late holocene vegetation history suggests natural origin of steppes in the northern mongolian mountain taiga abstract insular occurrences of steppe vegetation are a common feature of the northern mongolian mountain taiga steppe vegetation is limited here to southern slopes whereas northern slopes and valley bottoms are principally wooded with light and dark taiga forests in a case study in the valley of the river eroo at khonin nuga research station in the western khentey mountains we searched for evidence of an anthropogenic versus natural origin of steppe vegetation on the southern slopes pollen data of three profiles covering the last 2500 years showed continuous presence of steppe throughout the late holocene with human influence restricted to the recent past virtual absence of charcoal in the soil on and beneath three steppe slopes suggested that the present steppe grasslands are not replacing former forests burnt by humans or lightning the floodplains in the center of the eroo valley were recently deforested this is suggested by the pollen analysis and by interviews with local people on landuse history steppe grasslands of the study area have probably never been used as pastures pastoral nomads traditionally avoided the eroo valley near khonin nuga because of difficult access and high densities of wolves and bears all our data suggest that the present vegetation pattern of the western khentey with steppes and single small ulmus pumila trees on south facing slopes occurring as islands in the mountain taiga is driven by climate and relief and is not the result of human activities as suggested for other regions of asia

partitioning of sr between coexisting minerals of the hollandite and piemontite groups in a quartz rich schist from the sanbagawa metamorphic belt japan coexisting minerals of the hollandite cryptomelane series piemontite group and abswurmbachite braunite series occur as a small nodule in a quartzose schist metachert from the epidote amphibolite facies area of the sanbagawa metamorphic belt central shikoku japan strontium ba k and pb contents of the hollandite cryptomelane minerals are 0 00 0 42 0 07 0 99 0 00 0 75 and 0 00 0 05 apfu atoms per formula unit o 16 respectively the piemontite group minerals can be divided into common piemontite ca sr in the ten coordinated a2 site and strontiopiemontite sr ca strontium ba and pb contents of the strontiopiemontite reach 0 71 0 26 and 0 20 apfu o 12 5 respectively suggesting the existence of two possible new end members for epidote group minerals ba piemontite baca mn3 fe3 al 3si3o12 oh and pb piemontite pbca mn3 fe3 al 3si3o12 oh the abswurmbachite braunite series minerals cu 0 30 0 53 apfu o 12 contain up to 0 34 apfu mg indicating the exchange mg mn2 cu 1 distribution coefficients for sr between the hollandite cryptomelane series and the piemontite group minerals are up to 20 implying that the hollandite type structure with its large open tunnel is more suitable for incorporation of sr than the a2 site of the piemontite group minerals a synthetic aluminosilicate analogue of the hollandite cryptomelane compound is stable under p 8 10 gpa and suggests the hollandite type aluminosilicate may be a reservoir of sr and other large ion lithophile elements in the upper mantle

mineralogy and petrology of a piemontite bearing schist western otago new zealand abstract pink piemontite spessartine bearing and grey green spessartine bearing manganiferous quartzose schists derived from siliceous pelagites and green quartzofeldspathic schists are described from the greenschist facies of the haast schist terrane near arrow junction western otago electron microprobe data are reported for sphene spessartine rich garnet manganoan epidote piemontite tourmaline phengitic muscovite chlorite albite haematite rutile manganoan calcite and chalcopyrite metamorphism occurred at about 6 4kbar 400 c xco2 was above the quartz rutile calcite sphene buffer xco2 0 02 throughout the recorded metamorphic history of the piemontite schists it dropped from above to below this critical buffering value in a spessartine rich schist and it was close to or below the buffering value in the quartzofeldspathic schists production of piemontite required high fo2 believed to be inherited from mnox in the parent pelagite substantial loss of o2 e g minimum of 0 19 by weight in one rock during diagenesis and or metamorphism is inferred in the grey green schists this inhibited piemontite formation slight loss of o2 and ca2 accompanied minor late stage replacement of piemontite by second generation spessartine observed zoning and mineral replacements indicate rise of temperature drop in pressure or invasion by solutions of lower fo2 and xco2 equilibrated with surrounding schists the detailed chemistry of the minerals studied correlates with available mn and with bulk rock fe3 x 100 fe2 fe3 the oxidation ratio ranges from 24 in average green quartzofeldspathic schist through 78 in average grey green manganiferous quartzose schist to almost 100 in some piemontite bearing schists as fe2 gives way to fe3 mg fe ratios tend to rise in chlorite phengite tourmaline spessartine and calcite mn increases and ti decreases in haematite mn increases in spessartine and calcite and fe increases in rutile available divalent cations are depleted relative to al chlorite is more aluminous and phengite more paragonitic than in typical haast schists

the geodynamic evolution of the alpine orogen in the cyclades aegean sea greece insights from diverse origins and modes of emplacement of ultramafic rocks

oxidation gradients in metamorphosed non carbonatic manganese formations this paper aims to ascertain the degree of equilibrium attained with respect to oxygen during metamorphism of non carbonatic manganiferous sediments and the nature of mobility of oxygen in rocks of adjacent beds by measuring the composition of coexisting phases through algebraic analysis of the relevant phase equilibria a difference of 17 kcal in u text o text 2 in rocks of adjacent beds is obtained this confirms the immobile nature of oxygen during metamorphism of manganiferous sediments

cymrite and celsian in manganese rich metamorphic rocks from andros island greece manganese rich rocks from the island of andros cyclades greece contain cymrite baal2si2o8 h2o and celsian the textural relationships indicate replacement of celsian by cymrite several microprobe analyses of cymrites show a solid solution of 0 7 6 1 mole kalsi3o8 h2o the relic celsians have similar k contents thus the described rocks from andros represent the first natural example for the reaction celsian water cymrite the rocks from andros underwent an eocene high pressure metamorphism lateron the high pressure parageneses were nearly effaced by a late oligocene barrovian type metamorphism it is assumed that both cymrite and celsian were formed at conditions near the reaction curve celsianss water cymritess during the barrovian type event

manganese in monoclinic members of the epidote group piemontite and related minerals according to mayo 1932 who provided a brief historical review of the names used for piemontite the first researcher who described this mineral has been cronstedt in 1758 who named it rod magnesia in 1790 chevalier napione analyzed the sample described by cronstedt and termed it manganese rouge on the basis of his chemical data hauy designated the substance as manganese oxide violet silicifere in 1801 later in his traite de mineralogie hauy 1822 adopted the name proposed by cordier 1803 who first recognized the mineral as an epidote manganesifere the name piedmontite was proposed in 1853 by kenngott the basis of the type locality and more recently transformed into piemontite according to the standard guidelines for mineral nomenclature the name piemontite should be reserved to members of the ternary solid solution ca2al2 mn fe al si2o7 sio4 o oh that basically contains mn3 dominant at one site nonetheless the use of this name for any monoclinic manganiferous epidote group members showing the characteristic strong red yellow violet pleochroism is very common and probably convenient with special regard to petrographic purposes indeed the color of manganian i e mn3 bearing epidote or clinozoisite ranges to red to pinkish while manganoan i e mn2 bearing members do not exhibit the characteristic reddish hue the discredited name withamite was used to describe poorly manganiferous piemontite hutton 1938 yoshimura and momoi 1964 but corresponds on the basis of the current nomenclature to a manganian clinozoisite the name thulite sometimes erroneously used for pinkish clinozoisite should be reserved to mn3 bearing orthorhombic members in this chapter we focus on piemontite sensu stricto with the ideal formula ca2al2mn3 si2o7 sio4 o oh but also include for the reasons explained above those members of the clinozoisite epidote piemontite

a geologic reconnaissance of the cycladic blueschist belt greece the cycladic blueschist belt consists of two distinctive segments separated by a broad zone of superposed granitic and high temperature metamorphic rocks the northern segment contains early metamorphic fold axes and parallel glaucophane lineations that trend 060 with a progressive increase in metamorphism toward the southeast the southern segment contains similar fold axes and glaucophane lineations that trend 010 with an apparent increase in metamorphism toward the northwest radiometric dating of metamorphic minerals from both segments give apparent ages of about 40 to 80 m y these data suggest the existence of late mesozoic or early cenozoic subduction zones in the aegean region that subsequently collided

the geology of the polymetamorphic complex of ios cyclades greece and its significance for the cycladic massif the geological structure of the island of ios cyclades greece is a dome consisting of an augengneiss core and a mantle of garnet mica schists that together form the basement which is overthrust by a marble schist series this series is mainly made up of meta volcanics and metamorphosed sediments presumably mesozoic in age it is a tectonically composed pile of marbles alternating with glaucophane schists actinolite schists and chlorite schists petrological relations and isotope dating indicate the polymetamorphic character of ios two alpine metamorphic phases m1 and m2 and relicts of a high grade metamorphic or magmatic phase mo that only affected the basement are demonstrated radiometric ages obtained for the basement confirm the interpretation that the mo phase is pre alpine the m1 and m2 were dated at 43 ma and 25 ma respectively the p t conditions of metamorphism are estimated as 9 11 kb and 350 to 400 c for the m1 phase and 5 7 kb and 380 420 c for the m2 phase the metamorphic history of the cyclades is discussed and it is suggested that pre alpine basement occurs also on sikinos and naxos