boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

ion exchange experiments and rbsr dating on celadonites from the troodos ophiolite cyprus abstract rb sr dating of celadonites from several geographic stratigraphic locations from the troodos ophiolite cyprus yield crystallization ages varying between 57 9 3 9 and 89 0 1 9 ma the oldest celadonite age 89 0 1 9 ma agrees well with the crystallization age of the troodos igneous complex suggesting that low temperature chemical exchange with seawater started immediately after formation of the ophiolite drillcore samples suggest a duration of low temperature alteration of ca 10 ma whereas outcrop samples yield much longer durations of 30 ma the exact reason for this discrepancy is not fully understood but may represent the difference in localized downwelling in different parts of the ophiolite or may be related to the extensive weathering of the upper extrusive suite additionally in nearly all cases rb sr age estimates are older than k ar age data from the same samples to evaluate the potential disturbances of rb sr isotope systematics from exchangeable interlayer and basal surface positions a series of ion exchange experiments was carried out these data show that significant amounts of rb and sr can be in exchangeable positions and therefore a fraction of the parent and daughter inventory of these clays is vulnerable to late stage chemical exchange however the overall agreement between the data of the whole mineral and ion exchanged clay separates suggests that the influence of this exchange on the rb sr isotopic system is minor the post crystallization chemical behavior of exchangeable k and ar was not evaluated but the loss of radiogenic argon from exchangeable sites could result in k ar ages younger than rb sr ages of the same sample

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

the uranium ore from mina fe salamanca spain as a natural analogue of processes in a spent fuel repository abstract in the frame of the enresa natural analogue programme the uranium ore from the mina fe salamanca spain has been studied as a natural analogue of radioactive spent fuel behaviour this uranium mine is hosted in highly fractured schistose rocks a geological setting that has not been envisaged in the spanish options for radioactive waste burial however some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation the pitchblende pyrite carbonate paragenesis has been studied in situ as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions similarly secondary fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals a multidisciplinary characterisation of the site has been performed in order to study these processes though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone boa fault zone the mineralogy and geochemistry of fracture fillings mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established three geochemical zones have been clearly differentiated i the oxidised zone from the surface to approximately 20 m depth ii the redox transition zone from 20 to 50 m depth and iii the reduced zone located below the transition zone the oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides the total oxidation dissolution and leaching of u iv as uranyl sulphate aqueous complexes prevailed in this oxidised zone the redox transition zone is characterised by the coexistence of the primary uranium paragenesis oxidised minerals as well as numerous secondary solid phases as a result of the physico chemical changes in the environment the optimal physico chemical conditions for the coffinitisation of pitchblende and the co precipitation of fe iii u vi took place in this zone in the reduced zone where the primary uranium paragenesis is present we currently find the necessary physico chemical conditions to stabilise pitchblende pyrite and carbonates the physico chemical conditions of the oxidised zone are not relevant to disposal conditions in the transition zone two main geochemical processes take place i the coffinitisation of pitchblende which may be an important process for the stability of spent fuel in reducing conditions and ii the co precipitation of the fe iii and u vi as oxyhydroxides another relevant mechanism for the retention of uranium the physico chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository in the same way as they have been able to preserve the 34 ma old uranium deposit of the mina fe

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

nd sr isotopic study of primitive lavas from the troodos ophiolite cyprus evidence for a subduction related setting primitive lavas from the troodos ophiolite have a large variation in initial 143 nd 144 nd ratios from nd 7 9 to 0 8 which precludes a mid ocean ridge origin initial 87 sr 86 sr ratios vary from 0 70415 to 0 70705 the range of nd values may be accounted for by a two component mixing model whereby variable amounts of a light rare earth element ree enriched component with nd between 5 and 7 has interacted with a depleted iherzolite source with nd 8 this is consistent with an arc related setting where fluids derived from a subducting slab induce further melting in an overlying iherzolite source

boninite petrogenesis and alteration history constraints from stable isotope compositions of boninites from cape vogel new caledonia and cyprus 18o values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5 8 to 7 4 and indicate that the source for most boninites is more 18o rich than morbs and other oceanic basalts the source for oxygen and other major elements is most likely a refractory portion of the mantle having a 18o value of up to 7 0 to which must be added a small amount of h2o rich fluid to induce partial melting this fluid which is derived from subducted crust is the vehicle for lrees including nd the variable normally low nd values typical of boninites do not correlate with the 18o values post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures 150 c produces 18o values of 10 in optically fresh glass hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1 2 wt to 2 4 wt and produced d values of 80 which may be lower than the original magmatic d values in contrast to most submarine pillow basalts the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater

arc continent collisional orogenesis in the sw pacific and the nature source and correlation of emplaced ophiolitic nappe components abstract southwest pacific ophiolitic nappes emplaced upon the former margin of eastern australia provide a record of cyclical episodes of arc continent collisional orogenesis that occurred throughout the paleogene cenozoic nappes in papua new guinea new caledonia and northland new zealand consist of two key components 1 a basal and apparently older late cretaceous eocene metamorphosed normal to weakly enriched morb like basaltic ocean floor terrane and 2 an uppermost supra subduction zone ssz ophiolite that represents basement of the associated magmatic forearc a detailed comparison of geochemical characteristics of unequivocal ssz basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal n and enriched e morb these slight ree enriched or transitional t morb have generally been considered as the result of seafloor spreading in a back arc basin bab or basins to the east of australia in the late cretaceous paleocene 85 55 ma the enrichment recorded by the bab basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern gondwana that left their signature on the mantle source consequently tapped by the bab basalts a subset of slightly more enriched basalts may owe their enrichment to source contamination minor assimilation of thinned gondwana derived continental crust associated lree depleted n mor basalts previously considered as part of the basaltic ocean floor terranes on the other hand are similar to ssz basalts of the northland and papuan ultramafic belt ophiolites refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying ssz ophiolites a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts nonetheless the geochemical characteristics displayed by lree depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite whether these and for that matter the majority of basalts of the basaltic ocean floor terranes indeed represent complementary forearc crust to the ssz ophiolite as opposed to a slice of late cretaceous eocene bab floor backthrust beneath the ssz ophiolite upon emplacement will have to await the acquisition of corresponding radiometric age data

superimposed tectonics of the cyprus ophiolitic massifs fault planes and slickenslides have been systematically studied in the troodos and kellaki ophiolitic sequences and in their overlying uppermost cretaceous to neogene sedimentary cover exposed in southern cyprus a relative chronology of three main deformational events has been recognized the most recent event occurring during the late neogene is characterized by a n s striking extension from late cretaceous to late miocene the deformation became compressional with a n160 strike an older tectonic event contemporaneous with the development of the cretaceous ophiolitic suite is characterized by a ese wnw extension or extensional strike slip

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

tectonic setting of high mg andesite magmatism in the sw japan arc k ar chronology of the setouchi volcanic belt summary the setouchi volcanic belt in sw japan is characterized by the occurrence of andesites with an unusually high mg concentration high mg andesites hmas such hmas may have formed under unusual tectonic settings as basaltic magmas are produced dominantly in the mantle wedge of modern subduction zones a total of 50 new k ar ages for the setouchi rocks confirmed that intermediate to felsic magmatism including the eruption of hmas took place in this volcanic belt within a short period of 13 2 0 4 ma this is synchronous with the clockwise rotation of the sw japan arc sliver that was associated with the opening of the japan sea back arc basin such rotation of an arc sliver forced the initiation of subduction and partial melting of a young and hot lithosphere of the shikoku basin beneath the arc interaction between slab melts and mantle wedge peridotites may be a possible generation process of unusual hma magmas when the sw japan arc sliver ceased to drift the setouchi magmatism was switched off by the cessation of subduction the eruption of hma magmas may also be attributed to an extensional stress field in the setouchi region in contrast to the compressional stress regime documented for other regions of the sw japan arc

mid ocean ridge or marginal basin origin of the east taiwan ophiolite chemical and isotopic evidence the east taiwan ophiolite eto occurs as allochthonous fragments embedded in the plio pleistocene lichi melange in the coastal range of eastern taiwan it is probably the youngest among the known ophiolitic complexes of the world the overall geochemical and isotopic characteristics of the representative rock types are very oceanic and strongly argue for a mid ocean or marginal basin origin of the eto the best criteria include 1 major element compositions 2 ree distribution patterns 3 nd isotopic ratios some have nd 12 4 pb isotopic ratios and 5 the presence and nature of the red shale this conclusion is supported by previous studies of metamorphic evolution of the eto

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

effects of interaction between ultramafic tectonite and mafic magma on nd pb sr isotopic systems in the neoproterozoic chaya massif baikal muya ophiolite belt sm nd rb sr and u pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the chaya massif baikal muya ophiolite belt eastern siberia in order to determine the relationship between mantle ultramafic and crustal mafic sections geological relations in the chaya massif indicate that the mafic magmas were emplaced into and interacted with older solid peridotite hand picked acid leached primary rock forming and accessory minerals olivine orthopyroxene clinopyroxene and plagioclase from the two harzburgite samples show coherent behavior and yield147sm 144nd 143nd 144nd and238u 204pb 206pb 204pb mineral isochrons corresponding to ages of 640 58 ma 95 confidence level and 620 71 ma respectively these values are indistinguishable from the crystallization age of the chaya mafic units of 627 25 ma a weighted average of internal isochron sm nd ages of four mafic cumulates the rb sr and sm nd isotopic systems in the harzburgite whole rock samples were disturbed by hydrothermal alteration these alteration related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution on the basis of initial sr and nd values ultramafic and mafic rocks of the chaya massif can be divided into two groups 1 harzburgites and the lower mafic unit gabbronorites withend 6 6 to 7 1 andesr 11 to 16 and 2 websterite of the lower unit and gabbronorites of the upper mafic unit end 4 6 to 6 1 andesr 8 to 9 initial pb isotopic ratios are identical in all rocks studied with mean values of206pb 204pb 16 994 0 023 and207pb 204pb 15 363 0 015 the similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence the isotopic data agree with a hypothesized formation of the chaya massif in a suprasubduction zone environment

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

geochemical complexities preserved in the volcanic rocks of the zambales ophiolite philippines the geochemical characteristics of the volcanic rocks from the zambales ophiolite luzon philippines indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment the volcanic rocks and dikes range in composition from magnesian basalt including mid ocean ridge basalt morb island arc tholeiite and boninite like lavas to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales the sources are mixtures of components variably depleted with respect to a morb source differences exist between respective exposures of the ophiolite from north to south the following geochemical patterns in the basaltic rocks emerge sm nd decreases with corresponding decreases in 143nd 144nd rare earth element ree patterns range from extremely light ree lree depleted with generally low abundances of all ree 1 5x chondritic to patterns which are slightly lree depleted similar to morb patterns throughout the range sr isotopic abundances are relatively high 87sr 86sr 0 7035 0 7040 perhaps more significant than the regional geochemical trends however is that all volcanic provinces display ranges of rock compositions and the spatial distribution of basalts with such variable geochemical affinities indicate superposition interfingering and possible mixing of different lavas we suggest that the array of geochemical data from the zambales ophiolite can be explained in terms of processes observed in present day convergent plate margins such as the marianas or lau basin in the western pacific complicated plate boundaries which have existed for long periods of time including closely opposing and changing subduction systems the rifting of arcs and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction related lavas some of which become extremely depleted or secondarily enriched magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics possibly similar to those observed in the zambales ophiolite indeed recent studies from present day backarc systems suggest that young arc volcanism is superimposed on morb like crust and that basalt compositions of certain backarc basins are zoned inward from arc like to morb like these studies indicate that relationships between arc and morb like crust in young subduction complexes is extremely complex hawkins and melchior 1985 volpe et al 1987 1990 we extrapolate arguments from studies of zoned backarc systems to the zambales ophiolite and in turn use data from the zambales ophiolite as a ruler for defining the scale of variation in these complex supra subduction zone systems