natural tracers for identifying the origin of the thermal fluids emerging along the aegean volcanic arc greece evidence of arc type magmatic water atmw participation abstract the aegean volcanic arc is the result of a lithosphere subduction process during the quaternary time starting from the soussaki area from west to east the arc proceeds through the islands of egina methana milos santorini the columbus bank kos and nisyros volcano tectonic activities are still pronounced at santorini and nisyros in form of seismic activity craters of hydrothermal explosions hot fumaroles and thermal springs a significant number of cold water springs emerge in the vicinity of hot waters on these islands chemical and isotopic analyses were applied on water and fumaroles samples collected in different areas of the volcanic arc in order to attempt the assessment of these fluids stable isotopes of water and carbon have been used to evaluate the origin of cold and thermal water and co 2 chemical solute concentrations and isotopic contents of waters show that the fluids emerging in egina soussaki methana and kos areas represent geothermal systems in their waning stage while the fluids from milos santorini and nisyros proceed from active geothermal systems the 2 h 18 o cl relationships suggest that the parent hydrothermal liquids of nisyros and milos are produced through mixing of seawater and arc type magmatic water atmw with negligible to nil contribution of local ground waters and with very high participation of the magmatic component which is close to 70 in both sites a very high magmatic contribution to the deep geothermal system could occur at santorini as well perhaps with a percentage similar to nisyros and milos but it cannot be calculated because of steam condensation heavily affecting the fumarolic fluids of nea kameni before the surface discharge the parent hydrothermal liquid at methana originates through mixing of local groundwaters seawater and atmw with a magmatic participation close to 19 all in all the contribution of atmw is higher in the central eastern part of the aegean volcanic arc than in the western sector this difference which is spotted in the variable isotopic composition of the sampled fluids from west to east along the arc is probably due to several causes including the tectonic regime the depth of the deep reservoir below sea level the age of volcanic activity and in general the geomorphologic state of each island

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 abstract 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

concentration of oxygen vacancies in quartz from sedimentspotential for geochronometry abstract the concentration of oxygen vacancies in 13 quartz samples from lower tertiary sandstones 24 42 ma of the liaohe basin were measured by esr it was found that the concentration of oxygen vacancies in quartz is correlated to the total absorbed dose this implies that these centres may be used as a new tool for dating sediments on a million years time scale

geochemistry and petrogenesis of late miocene granitoids cyclades southern aegean nature of source components abstract neogene igneous activity in the aegean region comprised both extrusive volcanism and intrusive granitoid plutonism granitoid plutonism in the cyclades was established in the mid miocene period and lasted until late miocene 18 to 9 ma the timing of granitoid intrusion was associated with the initiation of extensional tectonics in the aegean these intrusions form a broad belt about 200 km long running from the west lavrium and serifos to the eastern aegean both s type and i type granitoids are present the former generally being emplaced earlier than the i types 15 to 8 3 ma major and trace element variations reveal that three end member components are involved in the granitoids but the proportions of these vary in the different plutons initial isotopic compositions of all the granitoids are typical of crust derived magmas from heterogeneous metasedimentary sources i type 87 sr 86 sr 0 7091 0 712 e nd 6 4 to 10 4 s type 0 710 0715 e nd 7 5 to 10 1 three end member sources have been identified 1 one end member appears to be a metasedimentary biotite gneiss greywacke type such as that forming the metamorphic core complexes naxos and paros this is a dominant but not the only component in the s types 2a major trace element and sr nd isotopic composition correlations of the s type granitoids with basement gneiss require an extra source of sr and ca having lower initial 87 sr 86 sr indicating a more depleted metasedimentary source at depth due to possible interaction of metamorphic fluids with marbles and amphibolites and infiltration through the gneissic units at mid crustal levels 2b a possible second end member could be the marble component as indicated by the buffered values of the initial sr isotopic ratios major element variation of the mafic microgranular quartz diorites and tonalites enclaves are compatible with dehydration melting of a mafic source similar to the amphibolites island arc tholeiites at medium pressure 8 kb conditions 3 amphibolite is another end member which may contribute mostly to the source of the younger intrusions along the western flank of the arc

oxygen and strontium isotopic investigations of subduction zone volcanism the case of the volcano arc and the marianas island arc abstract recent fresh volcanic rocks of the intra oceanic mariana and volcano arcs were analyzed for o and sr isotopic compositions in order to determine the source of these magmas fresh non arc volcanic rocks from the regions surrounding the mariana volcano arcs and some dsdp sediments were also analyzed for comparison the oxygen isotopic ratios of the arc lavas 5 5 6 8 exhibited a small inter island variation that cannot be entirely explained by fractional crystallization the sr isotopic composition of the arc lavas is remarkably uniform 0 70332 0 70394 for the marianas three models are considered in order to explain the observed isotopic characteristics 1 bulk mixing and melting of morb type mantle with a subducted sediments and b subducted oceanic crust excluding sediments 2 melting of a mixture of sediment derived fluids and morb type mantle and 3 melting of a mixture of sediment derived fluids and oceanic island or hot spot type mantle the last model fits the data best the conclusion that very small and variable amounts of sediment derived fluid 1 are required to explain the observed inter island o isotopic variation is consistent with that of other workers who used different isotopic and trace element methods the generation of magmas in the mariana volcano arcs involves very little sediment and the source region of mariana lavas is isotopically indistinguishable from that of hot spot basalts

intrusive rocks in the ophiolitic melange of crete witnesses to a late cretaceous thermal event of enigmatic geological position the ophiolitic melange in the uppermost tectonic unit of the cretan nappe pile contains crystalline slices which consist of a low pressure high temperature metamorphic sequence and synmetamorphic intrusions ranging in composition from diorite to granite the plutonic rocks conform to two different igneous suites dominated by diorites in eastern and granites in central crete displaying i type and a type characters respectively some of the granites from central crete are classified as transitional i s type they are closely associated with migmatitic paragneisses based on major and trace element ree sr and nd isotope geochemistry the mafic members of both suites are derived from a depleted mantle source the higher concentrations of p and ti in the mafic members of the igneous suite in central crete and the deviant trend of the whole suite may be explained by a different mantle source or a lower degree of partial melting in both suites magmatic evolution was governed by fractional crystallization of amphibole clinopyroxene plagioclase and minor phases in addition mixing or mingling of compositionally different magmas is indicated for the intrusive suite of eastern crete whereas in central crete the magma composition was at least partially modified through assimilation of meta pelites the geochemical results suggest that the plutonic rocks formed in a supra subduction zone setting however a formation during continental lithospheric extension cannot be ruled out published and new rb sr and k ar dates on amphiboles and biotites from intrusive rocks and their metamorphic country rocks show that the peak of the low p high t metamorphism and the intrusion of the two igneous suites testify to the same thermal event of late cretaceous age a similar late cretaceous association of metamorphic and plutonic rocks has been described from the uppermost tectonic unit in the attic cycladic crystalline complex together with the cretan occurrences they form a small sector radiating ssw along a distance of 300 km across the general trend of the tectonic zones in the hellenic orogen this n s alignment is regarded as a primary feature which may delineate the frontier zone between the hellenides and the taurides the real paleogeographic position and geodynamic significance of the late cretaceous low pressure high temperature belt however remains enigmatic

petrological and geochemical characteristics of cenozoic high k calc alkaline volcanism in konya central anatolia turkey abstract late miocene to pliocene volcanic rocks outcrop west northwest and southwest of the konya area in central anatolia turkey volcanic products are lava domes nuee ardentes and ignimbrite deposits predominantly andesitic to dasitic in composition together with rare basalt basaltic andesite basaltic trachyandesite and trachyandesite 50 35 69 39 sio2 the serie exhibits high k calc alkaline affinities fractional crystallization of pyroxene plagioclase and fe ti oxides is the main process in the magmatic evolution of konya volcanic rocks volcanic units exhibit typical high k calc alkaline character their geochemical characteristics e g enrichments in lil elements such as k rb ba sr depletion in hfse such as ti nb and high ba nb and low nb y ratios are consistent with those of active continental margin regions high 87 sr 86 sr 0 704841 0 707340 and low 143 nd 144 nd 0 512390 0 512618 ratios suggest crustal involvement in their petrogenesis correlations between 86 sr 87 sr isotope with rb rb nb rb ba and rb sr also emphasize the effect of crustal contamination on the andesitic and dacitic magmas as a consequence konya volcanic rocks are products of assimilation and fractional crystallization afc processes of a magma which seems to be linked to the subduction of the african plate underneath the anatolian plate during miocene

active geothermal systems with entrained seawater as modern analogs for transitional volcanic hosted massive sulfide and continental magmato hydrothermal mineralization the example of milos island greece the paradigm for low sulphidation ls volcanic arc associated mineralization is the active geothermal systems located along the taupo volcanic zone e g broadlands however this analogue is inapt where fluid salinities are consistently in excess of 3 5 wt nacl ls mineralization on milos aegean arc records high paleofluid salinities the d and 18o data do not exemplify 18o shifted meteoric waters typical of terrestrial geothermal systems nor is a submarine origin indicated stable isotope data show mixing between meteoric seawater and volcanic arc gases strontium isotope data are comparable to a nearby active seawater entrained geothermal system these are features seen in hydrothermal systems associated with emergent volcanoes for the milos ls mineralization high salinity fluids show it cannot be explained by a broadlands type model the absence of saliferous sequences and significant intrusive rocks preclude these as salinity sources the similarities between paleo and active systems in terms of salinity d 18o and strontium isotope systematics strongly suggest that seawater is the main source for na and cl we suggest geothermal systems containing seawater associated with emergent volcanoes are an alternative analogue for ls epithermal mineralization furthermore they bridge the gap between submarine and large scale terrestrial geothermal systems the modern analogues for vhms and epithermal mineralisation in the scheme of intrusion centered hydrothermal mineralization

active tectonics of the north and central aegean sea summary in this paper we examine the connection between the westward motion of turkey relative to europe and the extension in and around the aegean sea the principal new data available since the last attempt to synthesize the tectonics of this region by mckenzie 1978 are much improved focal mechanisms of earthquakes constrained by p and sh body wave modelling as well as by first motions these mechanisms show that the faulting in the western part of the aegean region is mostly extensional in nature on normal faults with a nw to wnw strike and with slip vectors directed nnw to nne there is evidence from palaeomagnetism that this western region rotates clockwise relative to stable europe in the central and eastern aegean and in nw turkey distributed right lateral strike slip is more prevalent on faults trending ne to ene and with slip vectors directed ne palaeomagnetic data in this eastern region is more ambiguous but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations relative to europe in the south the strike slip faulting that enters the central aegean from the east appears to end abruptly in the sw against the nw trending normal faults of greece the kinematics of the deformation is controlled by three factors the westward motion of turkey relative to europe the continental collision between nw greece albania and the apulia adriatic platform in the west and the presence of the hellenic subduction zone to the south as the right lateral slip on the north anatolian fault enters the aegean region it splays out becoming distributed on several parallel faults the continental shortening in nw greece and albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed e w right lateral shear and thus leads to e w shortening in the northern aegean which is compensated by n s extension as the southern aegean margin can move easily over the hellenic subduction zone the dynamics of the system once initiated is self sustaining being driven by the high topography in eastern turkey and by the roll back of the subducted slab beneath the southern aegean the geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate in spite of its extreme simplicity a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the aegean region including the slip vectors and nature of the faulting in the eastern and western parts the senses and approximate rates of rotation the overall extensional velocity across the aegean and the distribution of strain rates as seen in the seismicity and topography or bathymetry and using geodetic measurements as part of this study we re examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 gulf of corinth earthquakes and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting an important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves

trace element geochemistry of high alumina basalt andesite dacite rhyodacite lavas of the main volcanic series of santorini volcano greece trace element systematics throughout the cal calkaline high alumina basalt basaltic andesite andesite dacite rhyodacite lavas and dyke rocks of the main volcanic series of santorini volcano greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the main series into two distinct crystallisation intervals an early basalt to andesite stage characterised by calcic plagioclase augite olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite hypersthene magnetite apatite crystallisation percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition subtraction diagrams and indicate that basaltic andesites represent 47 69 andesites 70 76 dacites ca 80 and rhyodacite ca 84 crystallisation of the initial basalt magma least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative main series lavas though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition subtraction techniques main series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine pyroxene cr spinel crystallisation on ascent through the sub aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the aegean crust residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of santorini the eruption of large volumes of basic lavas and silicic pyroclastics from santorini may have a volcanological rather than petrological explanation

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

sr isotope and trace element evidence for the role of continental crust in calc alkaline volcanism on santorini and milos aegean sea greece abstract 87 sr 86 sr major and trace element data are presented for calc alkaline lavas from santorini and milos aegean sea greece 87 sr 86 sr ratios of fresh lavas from santorini range from 0 70472 to 0 70509 whereas those of fresh lavas from milos lie in the range 0 70540 0 70620 altered lavas from both islands have higher 87 sr 86 sr ratios up to 0 70573 and 0 70662 respectively the range of 87 sr 86 sr ratios of fresh lavas from santorini is only just greater that the maximum analytical uncertainty and is thus considered to reflect that of the parental magmas from which the lava series was derived by fractional crystallization at low pressures for the analysed milos lavas dacite rhyolite there is no correlation between 87 sr 86 sr and any major or trace element concentration and this together with the fact that the 87 sr 86 sr ratios are similar to those of basaltic andesites and andesites from aegina and methana in the northwestern part of the arc likewise indicates that the 87 sr 86 sr ratios are identical to those of the parental magmas from which the lava series was derived also by fractional crystallization the relatively high 87 sr 86 sr ratios of the santorini and milos parental magmas indicate that terrigenous material has played a role in their genesis this could be either via the subduction process or by assimilation of lower crustal material during ascent of mantle derived magmas the weight of the available evidence favours the latter possibility but even so there are some indications that unusual mantle source material was involved in the genesis of the magmas

isotope geochemistry of recent magmatism in the aegean arc sr nd hf and o isotopic ratios in the lavas of milos and santorini geodynamic implications in this comparative study of variations in the isotopic compositions sr nd o and hf of the calc alkaline magmas of the largest two volcanoes milos and santorini of the aegean arc eastern mediterranean we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano on santorini the crustal contamination processes have been limited effecting the magma gradually during its differentiation the most differentiated lavas rhyodacite and pumice are also the most contaminated on milos by contrast these processes are very extensive they are expressed in the143nd 144nd vs 87sr 86sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes in contrast with santorini the least differentiated lavas on milos are the most contaminated these isotopic singularities can be correlated with the geodynamic evolution of the aegean subduction zone consisting of alternating tectonic phases of distension and compression the genesis of rhyolitic magmas can be linked to the two phases of distension and the contamination of the calc alkaline mantle derived magmas with the intermediate compressive phase the isotopic characteristics of uncontaminated calc alkaline primitive magmas of milos and santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested such as in the aleutian sunda and lesser antilles island arcs however in spite of the importance of the sediment pile in the eastern mediterranen oceanic crust 6 10 km the contribution of the subducted terrigenous materials remains of limited amplitude

sr nd and pb isotope constraints on the formation of the metalliferous sediments in the nereus deep red sea previous chemical analyses of sediments from nereus deep core 412 located in the northern part of the red sea at about 23 20 n have demonstrated the occurrence of v rich authigenic magnetites associated with fe zn and cu sulphides mostly located at the bottom of the core the evolutionary stage of nereus deep is intermediate between that of the immature shaban deep and the more evolved atlantis ii deep previous studies have proposed either morb basalts by analogy with east pacific rise sulphide deposits or sediments as possible sources of the metals in the red sea metalliferous sediments sr nd and pb isotope systematics observed at ten levels in nereus core 412 lead us to distinguish three types of sediments 1 with predominantly basaltic fragments associated with loose titanomagnetite crystals 0 70413 87sr86sr 0 70622 0 51271 143nd144nd 0 51300 18 618 206pb204pb 18 673 2 detrital or biogenic sediments with a continental isotopic imprint 0 70938 87sr86sr 0 71323 0 51242 143nd144nd 0 51262 18 848 206pb204pb 18 890 and 3 v rich metalliferous sediments 0 70872 87sr86sr 0 70895 0 51260 143nd144nd 0 51274 18 717 206pb204pb 18 851 on sr nd and pb pb isotope diagrams this last group of sediments defines a field located between the fields of the first two groups our data suggest that the sr nd and pb in the studied metalliferous sediments were derived from overlying sedimentary beds in which the two main constituent detrital components show very distinct origins red sea morb and continental arabo nubian shield these data clearly support a model of convective circulation within the nereus deep sedimentary pile this model involves leaching two main components from the upper sedimentary layers of the core and a downward circulation of a hot fluid promoting the growth of v rich magnetites and fe zn and cu sulphides when the fluid cooled by mixing with the pore water of sediments from the lower part of the core a driving force represented by magmatic activity located near the 412 core in the nereus deep is required to explain the presence of hot convective fluids these data also suggest that for sedimented ridges the isotopic imprint of a basaltic source in metalliferous sediments or a hydrothermal ore deposit may result by interaction of a hot fluid with detrital sediments that are enriched in basaltic clasts and loose magmatic crystals

mechanisms of contamination in lesser antilles island arc magmas from radiogenic and oxygen isotope relationships abstract oxygen sr and nd isotopic compositions for 29 volcanic samples from the lesser antilles are reported these span a considerable range in 18o from values similar to those of mantle derived lavas 18o 6 6 to 18o 14 0 although the highest 18o results are obtained from evolved andesites and dacites there is no clear correlation between 18o and sio2 an attempt is made to eliminate the effects of varying magma chemistry on 18o through differentiation by normalising to the chemical index of garlick 1 this enables the effects on 18o of processes other than magmatic fractionation to be more clearly assessed it is concluded that the range in 18o values measured cannot result from hydrothermal interaction or weathering alone although limited alteration of a few samples may be responsible for some scatter in the oxygen isotope data when 18o is plotted against radiogenic isotopic compositions well correlated trends are observed 18o increases with increasing87sr 86sr and decreasing143nd 144nd such behaviour is consistent with the contamination of mantle derived magmas by variable amounts of an ancient crustal component the shapes of mixing curves defining this process suggest that contamination occurs largely within the arc crust constraints from geochemical and geophysical data suggest that a terrigenous sediment intercalated within the arc crust is assimilated however subsequent fractional crystallisation at more shallow levels in the crust has obscured any chemical trends which might have resulted from combined assimilation fractional crystallisation furthermore it is suggested that the mantle derived magma has already undergone source contamination by the introduction of a slab derived fluid into the overlying mantle wedge

the pb sr nd isotope geochemistry of some recent circum mediterranean granites pb sr and nd isotopic compositions have been analyzed in recent granites from northern africa northern italy and greece lead isotope compositions of k feldspars are rather homogeneous and cluster close to the modern lead of stacey and kramers 1975 but with slightly higher207pb 204pb and208pb 204pb ratios the cyclades samples however have higher206pb 204pb ratios addition of mantle derived lead was probably very limited which supports a quasi closed system evolution of this element in the continental crust the sr nd data fall in the enriched part of the143nd 144nd vs 87sr 86sr diagram and define a smooth hyperbolic mixing curve over a wide area straddling different orogens most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle like component no correlation is observed between either pb and sr or nd isotopic ratios or any isotopic ratio and major element contents quantitative modelling suggests that two cases fit the sr and nd characteristics of these granites they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes in the first case the mantle derived component may be represented by differentiated island arc type magmas and the granites result from mixing these magmas with anatectic melts in the second case mantle derived igneous rocks such as obducted ophiolites are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations

geochemical and temporal evolution of cenozoic magmatism in western turkey mantle response to collision slab break off and lithospheric tearing in an orogenic belt post collisional magmatism in western anatolia began in the eocene and has occurred in discrete pulses throughout the cenozoic as it propagated from north to south producing volcano plutonic associations with varying chemical compositions this apparent sw migration of magma tism and accompanying extension through time was a result of the thermally induced collapse of the western anatolian orogenic belt which formed during the collision of the sakarya and tauride anatolide continental blocks in the late paleocene the thermal input and melt sources for this prolonged magmatism were provided first by slab break off generated aesthenospheric flow then by lithospheric delamination related aesthenospheric flow followed by tectonic extension driven upward aesthenospheric flow the first magmatic episode is represented by eocene granitoid plutons and their extrusive carapace that are linearly distributed along the izmir ankara suture zone south of the marmara sea these suites show moderately evolved com positions enriched in incompatible elements similar to subduction zone influenced subalkaline magmas widespread oligo miocene volcanic and plutonic rocks with medium to high k calc alkaline compositions represent the next magmatic episode partial melting and assimilation fractional crystallization of enriched subcontinental lithospheric mantle derived magmas were important processes in the genesis and evolution of the parental magmas which experienced decreasing subduction influence and increasing crustal contamination during the evolution of the eocene and oligo miocene volcano plutonic rocks collision induced lithospheric slab break off provided an influx of aesthenospheric heat and melts that resulted in partial melting of the previously subduction metasomatized mantle lithosphere beneath the suture zone producing the eocene and oligo miocene igneous suites the following magmatic phase during the middle miocene 16 14 ma developed mildly alkaline bimodal volcanic rocks that show a decreasing amount of crustal contamination and subduction influence in time both melting of a subduc tion modified lithospheric mantle and aesthenospheric mantle derived melt contribution played a significant role in the generation of the magmas of these rocks this magmatic episode was attended by region wide extension that led to the formation of metamorphic core complexes and graben systems aesthenospheric upwelling caused by partial delamination of the lithospheric root beneath the western anatolian orogenic belt was likely responsible for the melt evolution of these mildly alkaline volcanics lithospheric delamination may have been caused by peeling off during slab rollback the last major phase of magmatism in the region starting c 12 ma is represented by late miocene to quaternary alkaline to super alkaline volcanic rocks that show oib like geochemical features with progressively more potassic compositions increasing toward south in time these rocks are spatially associated with major extensional fault systems that acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface the melt source for this magmatic phase carried little or no subduction component and was pro duced by the decompressional melting of aesthenospheric mantle which flowed in beneath the atte nuated continental lithosphere in the aegean extensional province this time progressive evolution of cenozoic magmatism and extension in western anatolia has been strongly controlled by the interplay between regional plate tectonic events and the mantle dynamics and provides a realistic template for post collisional magmatism and crustal extension in many orogenic belts

magmagenesis in a subduction related post collisional volcanic arc segment the ukrainian carpathians abstract calc alkaline magmatism in the south west ukraine occurred between 13 8 and 9 1 ma and formed an integral part of the neogene subduction related post collisional carpathian volcanic arc eruptions occurred contemporaneously in two parallel arcs here termed outer arc and inner arc in the ukrainian part of the carpathians outer arc rocks mainly andesites are characterized by lile enrichment e g k and pb nb depletion low compatible trace element abundances high 87sr 86sr high 18o and low 143nd 144nd isotopic ratios 0 7085 0 7095 7 01 8 53 0 51230 0 51245 respectively inner arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas they also show low compatible element abundances but have lower 87sr 86sr 18o and higher 143nd 144nd ratios 0 7060 0 7085 6 15 6 64 0 5125 0 5126 respectively than outer arc rocks both high nb and low nb lithologies are present in the inner arc based on the lile enrichment especially pb a higher fluid flux is suggested for the outer arc magmas compared with those of the inner arc combined trace element and sr nd o isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex compositional differences between the inner and outer arcs were produced by introduction of variable proportions of slab derived sediments and fluids into a heterogeneous mantle wedge and by different extents of upper crustal contamination degrees of magmatic fractionation also differed between the two arcs the most primitive magmas belong to the inner arc isotopic modelling shows that they can be produced by adding 3 8 subducted terrigenous flysch sediments to the local mantle wedge source up to 5 upper crustal contamination has been modelled for fractionated products of the inner arc the geochemical features of outer arc rocks suggest that they were generated from mantle wedge melts similar to the inner arc primitive magmas but were strongly affected by both source enrichment and upper crustal contamination assimilation of 10 20 bulk upper crust is required in the afc modelling assuming an inner arc parental magma we suggest that magmagenesis is closely related to the complex geotectonic evolution of the carpathian area several tectonic and kinematic factors are significant 1 hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes 2 thermal disturbance caused by ascent of hot asthenospheric mantle during the back arc opening of the pannonian basin 3 clockwise translational movements of the intracarpathian terranes which facilitated eruption of the magmas

petrogenesis and time progressive evolution of the cenozoic continental volcanism in the biga peninsula nw anatolia turkey abstract the post collisional cenozoic magmatic activity in the biga peninsula nw anatolia started in the middle eocene 45 3 0 9 ma and lasted continuously until the late miocene 8 32 0 19 ma with minor punctuations producing widespread volcanoplutonic complexes these magmatic suites are characterized by calc alkaline high k calc alkaline shoshonitic mildly alkaline and alkaline series the middle eocene to middle miocene lavas have high lile lree lile hfse and 87sr 86sr i 0 7046 0 7087 and low nd i 1 2 to 6 4 values by contrast the late miocene lavas have the lowest 87sr 86sr i 0 7030 0 7033 and the highest nd i 2 6 to 6 7 values geochemical features of these rocks indicate two episodes of enrichment processes during the evolution of the eocene early miocene volcanism in the biga peninsula the first stage source enrichment metasomatism occurred within the continental lithospheric mantle as a result of the previous subduction event s and produced a heterogeneously enriched source the second stage crustal contamination resulted from interaction between the metasomatized mantle derived melts and those magmas derived from the sakarya crustal basement during their ascent to the surface slab breakoff induced asthenospheric upwelling caused the melting of metasomatized mantle lithosphere beneath the sakarya continent and produced the middle eocene calc alkaline lavas geochemical isotopic and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas from the eocene through the early miocene crustal input was peaked during the early miocene volcanism coinciding with the exhumation and rapid uplift of the kazdag metamorphic massif asthenospheric upwelling beneath the kazdag core complex contributed to the generation of hybrid magmas formed from mixing of mantle and crustal melts during this stage the middle miocene 15 2 ma mildly alkaline rocks show less pronounced enrichment in lile and lree and display intermediate major and trace element compositions between the early miocene and late miocene lavas the cenozoic volcanism in biga peninsula waned briefly during 15 11 ma and was subsequently rejuvenated late miocene 11 0 0 4 8 32 0 19 ma producing mainly oib type alkaline lavas advanced extensional tectonics throughout nw anatolia in the late miocene led to upwelling of the asthenospheric mantle and its decompressional melting that collectively resulted in the eruption of short lived episodes of mainly oib type alkaline lavas this tectonomagmatic evolution of the cenozoic continental volcanism in western anatolia presents a well documented case study of an idealized post collisional magmatism in most orogenic belts around the world

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

using the sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics the individual components of magmatic rocks are being increasingly targeted to gain insight into volcanic systems here we demonstrate the use of sr isotope microsampling to distinguish distinct populations of glass and plagioclase that represent the sources of components liquids and crystals within each erupted magma batch we analyzed glasses and plagioclase feldspar crystal cores to characterize crystal cargoes and host liquids from pumice fall units from the minoan cycle santorini greece using this approach we identify the magmatic components that interacted prior to eruption and formation of pumice clasts allowing us to interpret the magma dynamics associated with each eruption varying degrees of complexity can be identified from relatively simple cases where the crystals are in isotopic equilibrium with the glass to more complex such as the large caldera forming minoan eruption which is characterized by at least two liquid components and three different crystal provenances

the length of channelized lava flows insight from the 1859 eruption of mauna loa volcano hawai i a r t i c l e i n f o the 1859 eruption of mauna loa volcano hawai i produced paired a and p hoehoe ows of exceptional length 51 km the a ow eld is distinguished by a long n36 km and well de ned p hoehoe lined channel indicating that channelized lava remained uid to great distances from the vent the 1859 eruption was further unusual in initiating at a radial vent on the volcano s northwest ank instead of along the well de ned rift zone that has been the source of most historic activity as such it presents an opportunity both to examine controls on the emplacement of long lava channels and to assess hazards posed by future ank eruptions of mauna loa here we combine evidence from historical chronicles with analysis of bulk compositions glass geothermometry and microlite textures of samples collected along the 1859 lava ows to constrain eruption and ow emplacement conditions the bulk compositions of samples from the a and p hoehoe ow elds are bimodally distributed and indicate tapping of two discrete magma bodies during eruption samples from the p hoehoe ow eld have bulk compositions similar to those of historically erupted lavas b8 wt mgo lava that fed the a channel is more primitive n8 wt mgo nearly aphyric and was erupted at high temperatures 1194 1216 c we suggest that the physical properties of proximal channel fed lava i e high temperature low crystallinity and low bulk viscosity promoted both rapid ow advance and development of long p hoehoe lined channels critical for the latter was the large temperature decrease 50 c required to reach the point at which plagioclase and pyroxene started to crystallize the importance of phase constraints are emphasized by our dif culty in replicating patterns of cooling and crystallization recorded by high temperature eld samples using common models of ow emplacement placement of the 1859 eruption within the context of historic activity at mauna loa suggests that the formation of radial vents and eruptions of high temperature magma may not only be linked but may also be a consequence of periods of high magma supply e g 1843 1877 flank eruptions could therefore warrant special consideration in models and hazard mitigation efforts

eocene k rich adakitic rocks in the central iran implications for evaluating its cu au mo metallogenic potential abstract the kal e kafi intrusive complex is located in the central iran magmatic zone and includes a series of granitoid rocks ranging in composition from gabbro to granite these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y enrichment in lree and lile with no eu anomalies similar to products from all of which are compatible with derivation from slab melting however relatively their low end t values 0 21 to 2 35 and higher 87 sr 86 sr i 0 7041 0 7044 are inconsistent with an origin by slab melting with these rocks also showing shoshonitic affinities with high k 2 o na 2 o ratios geochemical characteristics indicate that the kal e kafi intrusive complex was probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thicknesses of 40 km generated near the boundary between garnet bearing amphibolite or amphibole bearing eclogite melting moreover the kal e kafi adakitic magmas must have interacted with the surrounding metasomatized mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity o 2 of the magma the high o 2 could have induced oxidation of metallic sulfides in the mantle resulting in the mobilization of chalcophile elements which are required to produce porphyry cu au and mo mineralization

do fluid inclusions preserve δ18o values of hydrothermal fluids in epithermal systems over geological time evidence from paleo and modern geothermal systems milos island aegean sea stable isotope compositions of quartz d 18 oquartz and fluid inclusion waters d 18 ofi and ddfi were analysed from profitis ilias a low sulphidation epithermal gold mineralisation deposit on milos island greece to establish if d 18 ofi preserve a record of paleogeothermal processes previous studies show that mineralisation at profitis ilias resulted from extreme boiling and vaporisation with a zone located at approximately 430 m above sea level asl representing the transition between liquid and vapor dominated systems miner depos 36 2001 32 the deposit is also closely associated with an active geothermal system whose waters have a well characterised stable isotope geochemistry pflumio c boulegue j liakopoulos a briqueu l 1991 source transport and deposition of metals balkema rotterdam the samples were collected over an elevation interval of 440 m 210 650 m asl to give information on the liquid and vapor dominated sections of the paleosystem the data show systematic variations with sample elevation samples from the highest elevations ca 650 m asl have the lightest d 18 ofi 7 3x and ddfi 68 0x whilst the deepest ca 210 m asl are isotopically heavier d 18 ofi 0 3x ddfi 19 0x relative changes in d 18 ofi closely parallel those in ddfi d 18 oquartz shows an opposite trend from the lightest values 13 9x at the lowest elevations to the heaviest 15 1x at the highest elevations d 18 ofi shows correlations with other parameters for example variable fluid inclusion homogenisation temperatures in the vapor dominated part of the system correlate with a rapid shift in ddfi 33 3xto 50 5x and d 18 ofi 4 1xto 6 2x gold contents also increase in the same zone up to 50 ppm au comparable correlations in d 18 oquartz or d 18 ocalculated estimated geothermal fluid from fluid inclusion homogenisation data are absent d 18 ocalculated are always 5 10xheavier than d 18 ofi comparison with the present day geothermal field shows that ddfi and d 18 ofi are similar isotope data for the modern system and fluid inclusion waters fall on linear trends subparalleling the meteoric water line and project towards seawater values numerical modelling favours kinetically controlled fractionation to explain differences in d 18 ocalculated and d 18 ofluid rather than diffusive posttrapping reequilibration the evidence suggests that in low temperature epithermal systems d 18 ofi may represent

isotopic constraints on the role of subducted sialic material in indonesian island arc magmatism the island arcs of indonesia exhibit a range of subduction related tectonic environments and consequently are of particular interest in studying the relationship between the nature of the subducted plate and volcanism the sunda arc in java manifests the underthrusting of oceanic lithosphere beneath the newly accreted margin of asia in the banda arc continental material appears to be subducting beneath a volcanic arc that is underlain by oceanic crust 87sr 86sr ratios 0 7040 to 0 7059 in holocene andesitic lavas from java are higher than those from other volcanic arcs except those arcs where sialic contamination has been inferred a group of anomalous calc alkaline lavas characterized by high 87sr 86sr ratios 0 7054 to 0 7059 and relatively high abundances of k and related elements appear to be the result of sialic contamination of a mantle derived melt high 87sr 86sr ratios as large as 0 7095 together with values of 18o as large as 9 2 smow in lavas from the banda arc imply mixing of mantle derived and sialic components because of the tectonic peculiarities of the region the most likely source of contamination is from subduction of terrigenous material derived from the northwest continental shelf of australia lead isotopic ratios in lavas from java are generally transitional to the trends defined by other arc lavas and are characterized by higher 207pb 206pb ratios than oceanic basalts the anomalous calc alkaline lavas in particular are characterized by relatively high 207pb 206pb ratios interpreted to result from subduction and melting of sediments derived from the precambrian shields of australia and india during the opening of the indian ocean neodymium isotope ratios lie within the range defined from other volcanic arcs in terms of 143nd 144nd and 87sr 86sr the banda arc samples lie on a mixing line between compositions typical of the mantle and the upper continental crust that subducted sediments appear as a component in at least some of the holocene indonesian lavas implies melting at the top of the subducted lithospheric slab and provides constraints on the thermal structure of the descending plate

geochemistry and evolution of the calc alkaline volcanic complex of santorini aegean sea greece abstract volcanic activity on santorini started in upper pliocene times and is still in progress the last eruption occurred in 1950 at least seven eruptive centers were active at various times and some centers were active contemporaneously the overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases together with variable amounts of magma mixing and assimilation comparison of the chemistry of volcanic products from different eruption centers indicates long term variations in magma chemistry progressive depletion in lil elements and enrichment in compatible elements with time that cannot be explained in terms of fractionation mixing and assimilation such variations are attributed to processes occurring within the upper mantle trace element data require that the primitive lavas erupted on santorini were derived from an upper mantle source which was relatively depleted in lil elements compared with other calc alkaline volcanics and it is proposed that the source region contained a dominant refractory component which had experienced earlier events and a component derived from the subducted slab as magmatism progressed the contribution to the source region from the slab decreased due to either 1 cessation of subduction and hence a decrease in the proportion of slab derived material in the source region 2 a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of low melting slab derived material or 3 a combination of these processes we conclude that it is possible to see through intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for are magmatism

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

miocene subduction related magmatism in southern sardinia sr nd and oxygen isotopic evidence for mantle source enrichment abstract subduction beneath southern sardinia in miocene times gave rise to a suite of basalts basaltic andesites high k andesites and rhyolitic ignimbrites the monte arcuentu volcanic complex is a typical product of this activity although the volcanic rocks display some tholeiitic affinities in terms of k 2 o sio 2 relations they resemble calc alkaline lavas of the aeolian and aegean arcs the most mafic members of the suite have moderately high ni 164 ppm and cr 550 ppm contents and high mgo 10 wt and are therefore mantle derived mantle normalised trace element patterns show high k rb ba and th but low nb and ta characteristic of subduction related magmas sr contents in almost all samples are particularly low 165 265 ppm 87 sr 86 sr i values range from 0 7054 to 0 7113 whilst nd i is between 0 9 and 8 8 the sr nd isotopic field overlaps those of the high k rocks of roccamonfina and the roman volcanic province and extends that of the aeolian islands and the aegean arc correlations of radiogenic isotope ratios with sio 2 values in bulk rocks are much stronger than with mgo conventionally analysed 18 o whole rock values range from 6 4 in basalt to 12 4 in a high k andesite however 18 o values for clinopyroxene phenocrysts analysed by a laser fluorination method range from 6 17 to 7 46 good correlations are seen between 18 o cpx values and both mgo and sio 2 differences between 18 o whole rock and 18 o cpx values are probably due to late stage alteration of the groundmass and cast doubt on the validity of petrogenetic interpretations based solely on whole rock oxygen isotope analysis the sr nd o isotopic variation can be modelled either by contamination of mantle derived magmas by hercynian continental crust or by extensive enrichment of the mantle source by subducted sediments in the 18 o cpx vs 87 sr 86 sr i diagram crustal contamination hyperbolae are concave upward because of the low sr contents of the monte arcuentu magmas so both crustal contamination trends and mantle enrichment trends can generate the same isotopic relationships however the strong correlations of radiogenic isotopic ratios with sio 2 and the weak correlations with mgo are most easily explained by extensive 2 10 mantle enrichment by subducted siliceous sediments the correlation between sio 2 and 87 sr 86 sr i for the monte arcuentu magmas is very unusual within subduction related magmatism whereas that between 18 o and 87 sr 86 sr appears to show that the suite forms an end member composition for many island arc and active continental margin volcanic suites

a geochemical study of nea kameni hyalodacites santorini volcano aegean island arc inferences concerning the origin and effects of solfataras and magmatic evolution abstract since the santorini volcano aegean arc eastern mediterranean sea collapsed volcanic activity has been located at the center of the flooded caldera over the past 800 years five lava flows have formed one of the central islets nea kameni since 1951 when the last eruption occurred a permanent fumarolic activity has remained we present chemical analyses major elements trace elements and sr isotopic ratios of ten samples from the five hyalodacitic lava flows showing different stages of alteration from a completely fresh lava up to one bearing native sulfur and other sublimates only the macroscopic aspect of these hyalodacites is affected by fumarolic activity the elements that are mobile as a result of hydrothermal processes such as the alkaline k rb or the chalcophile elements zn pb show great homogeneity the same can be said for the sr isotopic compositions which range from 0 7046 to 0 7049 none of the analyzed samples has an sr isotopic composition as high as those reported by puchelt and hoefs 1971 for rock samples collected in the same lava flows if we take into account the marine surroundings of nea kameni islet these observations put severe restraints on the different hypotheses regarding the origin of the halogens seawater or meteoric water the contamination processes of these dacitic lavas are clearly less important than assumed by other authors according to previous sr isotopic data finally the homogeneity of the elements with low partition coefficients is sufficient to show that the magma has not undergone any perceptible evolution during the last 300 years

geochemical characteristics of igneous host rocks of lubin zardeh au cu deposit nw iran abstract the lubin zardeh au cu pb zn deposit is located in the azerbaijan western alborz zone 45 km ne of zanjan iran it is situated within the eocene oligocene volcanic plutonic belt of the tarom subzone of the alborz mountains range quartz monzonite and tuffs are the main host rock of the lubin zardeh au cu pb zn deposit granitoid rocks are predominantly high k calc alkaline with shoshonitic affinities they are i type metaluminous magnesian and are characterized by lile lree and th enrichment relative to hfse depletion in nb ta ti and weak depletion in hree and y these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y and enrichment in lree and lile with weak eu anomalies their relatively low to moderate initial 87sr 86sr values 0 7047 0 7051 positive nd t 36 ma values 0 39 2 1 and tdm ages of 0 69 to 1 06 ga with pb isotopic ratios of initial 206pb 204pb 18 49 18 68 initial 207pb 204pb 15 58 15 61 and initial 208pb 204pb 38 33 38 77 are consistent with a phlogopite bearing amphibole rich and enriched lithospheric mantle source metasomatized by earlier subducted sediments the geochemical and isotopic compositions of the granitoids are similar to the host rocks of porphyry and epithermal deposits in the urumieh dokhtar zone

tectonics of the hellenic trench a synthesis of sea beam and submersible observations abstract detailed bathymetric investigations were performed in four zones of the hellenic trench system in 1978 using the multi narrow beam echo sounder sea beam on r v jean charcot in 1979 a field study using the 3000 m submersible cyana was conducted in three of the four sites covered by sea beam we summarize here the principal tectonic results of these studies and publish the corresponding sea beam maps it is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall in addition there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the mediterranean ridge we suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall we show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m y ago

subduction and vertical coastal motions in the eastern mediterranean convergence in the eastern mediterranean of oceanic nubia with anatolia and the aegean is complex and poorly understood large volumes of sediment obscure the shallow structure of the subduction zone and since much of the convergence is accommodated aseismically there are limited earthquake data to constrain its kinematics we present new source models for recent earthquakes combining these with field observations published gps velocities and reflection seismic data to investigate faulting in three areas the florence rise sw turkey and the pliny and strabo trenches the depths and locations of earthquakes reveal the geometry of the subducting nubian plate ne of the florence rise a bathymetric high that is probably formed by deformation of sediment at the surface projection of the anatolia nubia subduction interface in sw turkey the presence of a strike slip shear zone has often been inferred despite an absence of strike slip earthquakes we show that the gps derived strain rate field is consistent with extension on the orthogonal systems of normal faults observed in the region and that strike slip faulting is not required to explain observed gps velocities further sw the pliny and strabo trenches are also often interpreted as strike slip shear zones but almost all nearby earthquakes have either reverse faulting or normal faulting focal mechanisms oblique convergence across the trenches may be accommodated either by a partitioned system of strike slip and reverse faults or by oblique slip on the aegean nubia subduction interface the observed late quaternary vertical motions of coastlines close to the subduction zone are influenced by the interplay between 1 thickening of the material overriding the subduction interface associated with convergence which promotes coastal uplift and 2 subsidence due to extension and associated crustal thinning long wavelength gravity data suggest that some of the observed topographic contrasts in the eastern mediterranean are supported by mantle convection however whether the convection is time dependent and whether its pattern moves relative to nubia are uncertain and its contribution to present day rates of vertical coastal motions is therefore hard to constrain the observed extension of the overriding material in the subduction system is probably partly related to buoyancy forces arising from topographic contrasts between the aegean anatolia and the mediterranean seafloor but the reasons for regional variations are less clear

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

variation in noble gas isotopic composition of gas samples from the aegean arc greece abstract in contrast to most other arcs with oceanic plate subduction the aegean arc is characterized by continent continent subduction noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc 2 islands in the back arc region and 7 sites in the surrounding areas the 3he 4he ratios of the samples ranged from 0 027ra to 6 2ra ra denotes the atmospheric 3he 4he ratio of 1 4 10 6 demonstrating that even the maximum 3he 4he ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems which are equal to the morb value of 8 1 ra regional variations in the 3he 4he ratio were observed both along and across the arc the maximum 3he 4he ratio was obtained from nisyros volcano located in the eastern end of the arc and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano across the volcanic arc the 3he 4he ratio decreased with an increasing distance from the arc front reaching a low ratio of 0 063ra in macedonia which suggested a major contribution of radiogenic helium derived from the continental crust at nisyros a temporal increase in 3he 4he ratio due to ascending subsurface magma was observed after the seismic crisis of 1995 1998 and mantle neon was possibly detected the maximum 3he 4he ratio 6 2ra in the aegean region which is significantly lower than the morb value is not probably due to crustal assimilation at shallow depth or addition of slab derived helium to morb like mantle wedge but inherent characteristics of the subcontinental lithospheric mantle sclm beneath the aegean arc

geochemistry of mafic lavas in the early rio grande rift yarmony mountain colorado u s a abstract mafic lavas high k basalts and shoshonites erupted 21 24 ma ago early miocene near state bridge nw colorado and now exposed on yarmony mountain were related to the initial phase of extension in the northernmost section of the rio grande rift the volcanism was of small volume consisting of eleven flows on yarmony mountain although much more voluminous contemporaneous lavas occur 50 km to the west all eleven flows have been analysed comprehensively for major and trace elements and five of the flows for nd and sr isotopes some samples experienced post eruptive carbonation and leaching of alkali elements the effects of these processes on normative compositions are examined and it is suggested that major element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid lower part of the crust most of the basalts are chemically closely related and have the following characteristics 1 they are depleted in nb and ta relative to lree a feature of volcanic arc magmas 2 they have lower abundances of rb and k and to a lesser extent ba and th for a given lree content than typical magmatic arc magmas and 3 they have nd and sr isotopic ratios similar to both certain low 143 nd 144 nd oceanic basalts notably from the kerguelen area indian ocean and subduction related magmas from mature volcanic arcs notably from the andes south america we argue that the magmas erupted at yarmony mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the cenozoic the relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction before that which generated the yarmony magmas an alternative is that the mantle source was depleted in alkali elements by a dehydration event possibly involving a flow of co2 one of the lavas in the yarmony sequence is elementally and isotopically distinctive having higher k zr ba k 2 o na 2 o la ta and k la and lower 87 sr 86 sr than the rest of the samples it is argued in detail that this is not the result of contamination by any reasonable crustal composition and that this magma contained a component 10 20 of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle there is no petrographic evidence for mixing and the hybridization probably took place in magma chambers situated in the mid lower crust

vertical motions in the aegean volcanic arc evidence for rapid subsidence preceding volcanic activity on milos and aegina late orogenic extension in the aegean region has been ongoing since the late eocene or early oligocene contemporaneously numerous volcanic centres developed in the south central aegean region plio pleistocene volcanism formed a number of islands on two of these milos and aegina a sequence of late miocene and pliocene marine sediments underlie the oldest volcaniclastics to determine whether extension of the aegean lithosphere played a role in the formation and location of the early pliocene volcanoes of the aegean we aimed to reconstruct the vertical motion history as it occurred prior to the onset of volcanism to this end we reconstructed the paleobathymetry evolution using foraminifera recovered from sedimentary sections in the lower pliocene of milos and aegina age dating on the milos sections was based on bio magnetoand cyclostratigraphy from aegina only bio and magnetostratigraphy were available the results show that prior to the onset of volcanism on both islands many hundreds of metres of early pliocene subsidence occurred on milos even 900 m between 5 0 and 4 4 ma it is unlikely that extension of the aegean lithosphere led to melting of the underlying mantle however the extension probably did play a significant role in the timing of the onset of pliocene volcanic activity in the aegean the position of the volcanic centres is probably the result of the depth of the subducted slab below the aegean and the formation of a network of extensional faults in the overriding aegean lithosphere d 2004 elsevier b v all rights reserved