multiple sources of quaternary tephra layers in the mariana trough abstract tephra layers in quaternary sediments stolper and newman 1994 model of binary mixing of source components with one endmember being the nmorb source of stolper and newman 1994 and the other one being a compositionally variable slab derived h2o rich component similar to the h2o component of stolper and newman 1994 this interpretation implies that the variation in k2o content in the arc derived glasses about a factor of 3 at 60 wt sio2 reflects a magmatic diversity that is inherent to the central mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources

mariana trough basalts mtb trace element and srnd isotopic evidence for mixing between morb like and arc like melts abstract mariana trough basalt mtb glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal morb and basaltic rocks from the active northern mariana island arc miab the chemical variation is observed in trace elemental abundances and ratios such as lil lil and lil hfs mtb glasses with high lil hfs and ba sm ratios and low k rb k ba and sm nd ratios have more enriched sr and nd isotopic compositions comparison of the sr nd isotopic compositions of mtb and miab suggests that the source region within the mantle wedge is heterogeneous the diverse trace element and isotopic compositions of mtb glasses both within and between dredge sites near 18 n imply small scale source heterogeneity correlation between sm nd and143nd 144nd of the mtb glasses is interpreted as due to recent binary mixing rather than closed system evolution of a common homogeneous source mixing of melts at or near the source region between a mantle component with long term lree and lil element depletion morb like and a relatively enriched component with lower integrated143nd 144nd arc like is suggested by trends of the mtb data on ratio ratio ratio element and element element plots

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

the guerrero suspect terrane western mexico and coeval arc terranes the greater antilles and the western cordillera of colombia a late mesozoic intra oceanic arc accreted to cratonal america during the cretaceous the guerrero suspect terrane composed of late jurassic early cretaceous sequences extends from baja california to acapulco and is considered to be coeval with the late mesozoic igneous and sedimentary arc sequences of the greater antilles the west indies venezuela and the western cordillera of colombia these sequences represent the remnants of an arc which accreted to the north american and northern south american cratons at the end of the cretaceous in western mexico the arc sequences built on continental crust consist of high k calc alkaline basalts andesites and rhyolites enriched in lree with abundant siliceous pyroclastic rocks interbedded either with aptian albian reefal limestones or red beds they do not show magmatic changes during the arc development in contrast the arc sequences built on oceanic crust show an evolution with time arc activity began with the development of depleted low k tholeiitic mafic suite guanajuato igneous sequence followed first by mature tholeiitic basalts and then by calc alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of early cretaceous age at the end of the arc growth during aptian albian times calc alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc the tholeiitic and shoshonitic mafic rocks as well as the calc alkaline lavas are mildly enriched in lree y and nb and show high nd ratios typical of oceanic arcs in contrast the calc alkaline mafic suite enriched in lree y and nb exhibits lower nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by paleozoic subducted sediments or old source enrichments oib the cretaceous arc rocks of the greater antilles interbedded with and or capped by aptian albian limestones the cretaceous andesites of northern colombia the cretaceous tholeiitic and calc alkaline volcanic rocks of venezuela and the cretaceous volcano plutonic arc assemblage of tobago share a similar magmatic evolution with the western mexican oceanic arc the tholeiitic plutono volcanic assemblage of tobago depleted in lree and characterized by high nd values is similar to the guanajuato volcano plutonic sequence of mexico considered to represent the pristine stage of the arc the mature tholeiitic sequences exposed in the proto caribbean arc show flat to moderately enriched lree patterns like those of the guerrero terrane however felsic plutonic and volcanic rocks prevail in the caribbean calc alkaline suites accompanied locally by shoshonitic lavas characterize the end of arc magmatic activity in both places thus the geochemical features of the late jurassic cretaceous arc series of the guerrero terrane and the proto caribbean are consistent with the following plate tectonic model the guerrero terrane and the proto caribbean probably belonged to the same intra paleo pacific arc system the development of which was related to the subduction of oceanic basins fringing the north and northern south american cratons this subduction zone was wsw dipping while subduction was going on these magmatic arcs drifted moved closer to the north and south american cratons and finally collided with the american borderlands at different periods during the cretaceous the late mesozoic guerrero and proto caribbean arc sequences show striking similarities with the miocene calc alkaline lavas dredged from the banda ridges the north marianas seamount province and the halmahera and philippine arcs we suggest that the diverse but mostly submarine segments of this late mesozoic intra pacific arc rimmed the north and south american cratons as much as these tertiary arcs rim southeast asia

nd and sr isotopic compositions of lavas from the northern mariana and southern volcano arcs implications for the origin of island arc melts nd and sr isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern mariana and southern volcano arcs values of end range from 2 4 to 9 5 whereas 87sr 86sr ranges from 0 70319 to 0 70392 these vary systematically between and sometimes within arc segments the nd and sr isotopic compositions fall in the field of ocean island basalt oib and extend along the mantle array lavas from the volcano arc mariana central island province and the southern part of the northern seamount province have end to 10 and 87sr 86sr 0 7032 to 0 7039 these are often slightly displaced toward higher 87sr 86sr at similar end in contrast those lavas from the northern part of the mariana northern seamount province as far north as iwo jima show oib isotopic characteristics with end and 87sr 86sr 0 7035 to 0 7039 plots of 87sr 86sr and end versus ba la and la yb n support a model in which melts from the mariana and volcano arcs are derived by mixing of oib type mantle or melts therefrom and a metasomatized morb type mantle or melts therefrom an alternate interpretation is that anomalous trends on the plots of nd and sr isotopic composition versus incompatible element ratios found in some s nsp lavas suggest that the addition of a sedimentary component may be locally superimposed on the two component mixing of mantle end members

isotopic evolution of the tonga arc during lau basin rifting evidence from the volcaniclastic record new pb sr and nd isotope data from volcaniclastic sediments recovered from the tonga forearc and lau basin permit the isotopic evolution of a section of this arc system next to the modern island of ata to be traced through the backarc basin rifting process from 7 0 ma to the present the new data suggest that the isotopic character of the mantle wedge remains constant and of pacific mid ocean ridge basalt morb character during the early rifting phase the isotopic evidence supports trace element data in showing an increase in the sediment contribution to arc petrogenesis about 2 3 my after the start of lau basin rifting since 0 45 ma the sediment contribution decreased to pre rift values with the initiation of spreading in the adjacent backarc basin where the high sediment influence is not seen in the isotopes the isotopes show a relative increase in the volcaniclastic compared with pelagic sediment involvement during rifting the inferred peak in sediment subduction is probably the result of a decoupling of the two plates owing to roll back of the pacific lithosphere at the time of arc rifting

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

physical volcanology of the submarine mariana and volcano arcs narrow beam maps selected dredge samplings and surveys of the mariana and volcano arcs identify 42 submarine volcanos observed activity and sample characteristics indicate 22 of these to be active or dormant edifices in the volcano arc are larger than most of the mariana arc edifices more irregularly shaped with numerous subsidiary cones and regularly spaced at 50 70 km volcanos in the mariana arc tend to be simple cones sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 counterclockwise from that trend suggesting a strong fault control on the distribution of arc magmas volcanos in the mariana arc are generally developed west of the frontal arc ridge on rifted frontal arc crust or new back arc basin crust volcanos in the central mariana arc are usually subaerial large 500 km3 and spaced about 50 70 km apart those in the northern and southern marianas are largely submarine closer together and generally less than 500 km3 in volume there is a shoaling of the arc basement around iwo jima accompanied by the appearance of incompatible element enriched lavas with alkalic affinities the larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos if the magma supply estimated at 10 20 km3 km of arc per million years at 18 n has been relatively constant along the mariana arc we can infer a possible evolutionary sequence for arc volcanos from small irregularly spaced edifices to large over 1000 km3 edifices spaced at 50 70 km the volcano distribution and basal depths are consistent with the hypothesis of back arc propagation into the volcano arc

shoshonitic magmas in nascent arcs new evidence from submarine volcanoes in the northern marianas volcanoes in the northern mariana arc between uracas lat 20 n and minami iwo jima 24 n are very active yet entirely submarine in contrast to the predominantly low k basaltic magmas of the central mariana arc the northern mariana arc is dominated by more siliceous melts in the south and by shoshonites in the north the northern arc melts have enrichments in ba 800 ppm rb 70 ppm sr 1000 ppm ce 50 ppm and ce yb n 24 which increase to the north as far as iwo jima lavas from volcanoes north of iwo jima lack these enrichments and are indistinguishable from those of the central maranas the shoshonites are unusual in occurring along the magmatic front of a primitive intra oceanic arc we hypothesize that they represent the reconstruction of a magmatic arc following melting of enriched mantle due to the propagation of the mariana trough spreading center northward through the volcano arc shoshonites thus may characterize the initial stages of arc construction after an episode of back arc rifting and need not be restricted to the mature stages of arc evolution this situation contrasts with subduction zone initiation where first melts may be boninites or low k tholeiites these differing initial melts converge toward tholeiitic and calc alkaline compositions as arcs evolve

osmium isotope constraints on lower crustal recycling and pluton preservation at lassen volcanic center ca osmium isotope compositions of intermediate to silicic composition calc alkaline volcanic rocks from the lassen volcanic region of the cascade arc are significantly more radiogenic qos 23 to 224 than typical mantle these evolved arc rocks in the lassen region have unradiogenic sr nd and pb isotope compositions which overlap with those of contemporaneous mafic lavas crystal fractionation of mafic to intermediate composition magmas produces re os ratios that are high enough to evolve to very radiogenic os isotope compositions in only a few million years providing a potential fingerprint for detecting the involvement of such young relatively mafic crust in magmatic systems however the sr nd and pb isotope compositions will remain constant over such short time intervals due to relatively low parent daughter enrichment during magmatic evolution the radiogenic os isotope compositions in typically evolved lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic os isotope compositions beneath this section of the cascade arc large amounts of such high qos lower crust may have formed and been isolated from mash zone mixing and homogenization processes during the pliocene or late miocene the results from this study indicate that os isotopes may provide a unique glimpse into lower crustal processes such as recycling in primitive orogenic arcs 9 2002 elsevier science b v all rights reserved

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

melt dynamics of a partially molten mantle with randomly oriented veins the statistical probability of a connected network of randomly oriented melt veins in partially molten mantle suggests that the critical melting model can account for observed ra th isotope activity ratios similarly if the reactive percolation model also includes such veins then this modified model is also relevant critical melting models for the isotope activity ratios imply that material exchange between the melt and the residuum must be negligible hence any diffusive exchange between the melt in the veins and the residuum must be limited this condition suggests that the veins form a rooted system where several smaller veins supply larger veins further the veins should be situated at least some decimetres apart and the permeability constant of the residuum should be larger than 10y10 cm these conditions ensure that diffusion does not change the composition of the residuum significantly using isotope activity ratios alone it is difficult to distinguish between the percolative and critical melting models because both can account for the observed data the concentrations of incompatible elements in residual harzburgites and other evidence appear to favour the critical melting model it is suggested that veins can form as the result of overpressure caused by an interstitial melt because of the volume expansion during melting

intra oceanic arcs of the paleo asian ocean abstract the paper reviews and integrates geological geochronological geochemical and isotope data from 21 intra oceanic arcs ioa of the paleo asian ocean pao which have been identified in the central asian orogenic belt the world largest accretionary orogeny the data we discuss structural position of intra oceanic arc volcanic rocks in association with back arc terranes and accretionary complexes major periods of intra oceanic arc magmatism and related juvenile crustal growth lithologies of island arc terranes geochemical features and typical ranges of nd isotope values of volcanic rocks four groups of ioas have been recognized neoproterozoic early cambrian early paleozoic middle paleozoic and late paleozoic the neoproterozoic early cambrian or siberian group includes eleven intra oceanic arcs of eastern and western tuva sayan southern siberia russia northern and southwestern mongolia and russian altai the early paleozoic or kazakhstan group includes selety urumbai bozshakol chingiz and baydaulet aqastau arc terranes of the kazakh orocline the middle paleozoic or southern group includes six arc terranes in the tienshan orogen chinese altai east kazakhstan west junggar and southern mongoia only one late paleozoic intra oceanic arc has been reliably identified in the caob bogda in the chinese tienshan probably due to pao shrinking and termination the lithologies of the modern and fossil arcs are similar although the fossil arcs contain more calc alkaline varieties suggesting either their more evolved character or different conditions of magma generation of special importance is identification of back arc basins in old accretionary orogens because boninites may be absent in both modern and fossil ioas the three typical scenarios of back arc formation active margin rifting intra oceanic arc rifting and fore arc rifting were reconstructed in fossil intra oceanic arcs some arcs might be tectonically eroded and or directly subducted into the deep mantle therefore the structural and compositional records of fossil intra oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length life span and crust thickness

petrochemistry age and isotopic composition of alkali basalts from ponape island western pacific eleven analyzed lava samples from ponape island are alkali olivine basalt basanite and basanitoid most lavas are aphyric or sparsely phyric 10 phenocrysts and have phenocrysts of olivine fo77 80 clinopyroxene and titanomagnetite and microphenocrysts of plagioclase an53 68 in a fine grained groundmass of olivine clinopyroxene plagioclase opaques potassic oligoclase nepheline and accessary phases oxygen isotope and fe2o3feo data suggest that most samples are fresh although h2o contents are high xenoliths of chromite bearing harzburgites and dunites both with cumulate textures occur in one locality major and trace element concentrations are similar to other oceanic volcanic islands most major elements and compatible trace elements vary systematically with respect to the mg number 100mg mg fe2 in contrast the incompatible trace elements do not correlate with the mg number but do covary with other incompatible elements simple closed system shallow fractionation cannot be invoked to explain the observed chemical variation in the lavas derivation of the fractionated lavas mg number 66 48 probably involved polybaric crystal fractionation from a high mg number parental liquid in addition variable source concentration of a trace element rich minor phase is postulated however the mantle was homogeneous with respect to the ratio of 87sr86sr new k ar age data are not consistent with the hypothesis that ponape and the caroline ridge represent a simple hot spot

u pb zircon crystallization age of the muslim bagh ophiolite enigmatic remains of an extensive pre himalayan arc distinguishing between these alternatives and unraveling the tectonics of this area cannot be fully understood until the timing of ophiolite formation is known in this paper we present u pb dating of zircons from plagiogranites within the sheeted dike component of the mus lim bagh ophiolite pakistan these critical new data provide clear evidence of the mag matic age of the intra arc or backarc origin of ophiolites correlated with himalayan arc sys tems and help resolve tethyan tectonics prior to the himalayan orogeny abstract the pre himalayan evolution of tethys is fundamental in interpreting the signifi cance of india s paleogene deceleration and the timing of the india eurasia collision the archetype continental collision that governs our understanding of collisional processes here we date zircons from plagiogranite sheeted dikes in the muslim bagh ophiolite that yield a u pb age of 80 2 1 5 ma this age is interpreted as dating the crystallization of plagiogranites that occur at the top of gabbro plutons as well as within sheeted dikes these represent highly fractionated magma chamber differentiates the plagiogranite s trace element abundances are typical of subduction petrogenesis and can be explained by the addition of slab derived components to a depleted mantle wedge muslim bagh ophiolite formation was coeval with the spongtang arc in a similar tectonic environment we interpret the chain of ophiolites within western pakistan as corollaries of the spongtang arc that formed 15 ma before they were obducted on the indian passive margin these form an extensive arc system that developed as oceanic crust in tethys foundered during india s migration north latest maastrichtian ophiolite sole formation and subsequent paleocene obduction of the arc onto india was the fi rst arc india collision a herald of the collisions to come later

provenance of the north pacific sediments and process of source material transport as derived from rb sr isotopic systematics abstract rb sr isotopic systematics of 111 samples of sediments in nine cores from the north pacific of quaternary and pliocene ages have been investigated they provide information on the provenance the process of particle transport and temporal variation in the flux of source material the rb sr isotopic systematics of the core sediments show well correlated pseudo isochrons the pseudo isochrons reflect the mixing of two types of material i e the asian continental material with high 87 rb 86 sr ratios 4 5 6 5 and high 87 sr 86 sr ratios 0 723 0 727 and the volcanic material with low 87 rb 86 sr ratios and low 87 sr 86 sr ratios from island arc volcanics such as the izu ogasawara mariana and the japanese islands and oceanic islands such as the hawaiian islands 0 0 2 0 0 703 0 708 the clearness of the pseudo isochrons implies that the rb sr isotopic composition of the weathering products derived from the asian continental crust is extremely homogenized the fine fraction a few m with a high 87 rb 86 sr ratio 5 0 6 5 is widely transported into the north pacific by the middle latitude westerlies and contributes largely to pelagic sediments the coarser fraction with a lower 87 rb 86 sr ratio

granulite xenoliths from western saudi arabia the lower crust of the late precambrian arbian nubian shield mafic and intermediate granulite xenoliths collected from cenozoic alkali basalts provide samples of the lower crust in western saudi arabia the xenoliths are metaigneous two pyroxene and garnet granulites mineral and whole rock compositions are inconsistent with origin from red sea rift related basalts and are compatible with origin from island arc calc alkaline and low potassium tholeiitic basalts most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid initial87sr 86sr ratios are less than 0 7032 except for two samples at 0 7049 the sm nd data yield tdm model ages of 0 64 to 1 02 ga similar to typical arabian nubian shield upper continental crust the isotopic data indicate that the granulites formed from mantle derived magmas with little or no contamination by older continent crust calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of gettings et al 1986 based on geophysical data from western saudi arabia estimation of mean lower crustal composition using the granulite xenoliths and the gettings et al 1986 crustal model suggests a remarkably homogeneous mafic lower crust and an andesite or basaltic andesite bulk composition for pan african juvenile continental crust

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

petrology chemistry and isotopic composition of submarine volcanoes in the southern mariana arc four submarine volcanoes in the southern and central mariana arc were dredge sampled the recovered lavas were analyzed for major and trace element abundances including rare earth elements ree mineral compositions and sr and nd isotopic ratios the samples exhibit a great range of chemical and mineral compositions relative to subaerial exposures in the central arc zealandia bank produces high magnesium basalts mgo 8 9 wt enriched in ni and with forsteritic olivine as much as fo 88 this lava composition is probably parental to more fractionated basalts and basaltic andesites exposed on the adjacent islands of guguan and sarigan the southernmost seamounts in the mariana arc south of lat 15 n produce dacitic pumice a composition that is not common in the active arc ree data preclude their derivation from a basaltic parent by fractional crystallization they may be derived by direct partial mantle melting under water saturated conditions at low pressure nd and sr isotopic data 143 nd 144 nd 0 5130 0 5131 87 sr 86 sr 0 7032 0 7035 indicate a mantle source for the mariana arc lavas similar in isotopic characteristics to sources for ocean islands significant contributions to these arc magmas from subducted oceanic crust or sediment are not evident a spatial distribution in magmatic characteristics coincides with a structural break in the arc near 16 n south of this submarine seamounts produce incompatible element enriched lavas with relatively high 87 sr 86 sr 0 7034 0 7035 these lavas were produced by a relatively low degree of partial melting 5 15 of a shallow spinel bearing mantle to the north zealandia bank and the central arc islands are characterized by lower incompatible element abundances and lower 87 sr 86 sr 0 7032 0 7033 these lavas were produced by higher degrees of partial melting 15 25 of a deeper garnet bearing mantle

chromitites in ophiolites how where when why part i a review and new ideas on the origin and significance of platinum group minerals abstract platinum group minerals are the dominant control on the budget of platinum group elements in ophiolitic chromitites they vary widely in distribution morphology chemistry and os isotope compositions their distribution in different types of chromitites and different microstructural sites within them may reflect the role of chromite as a physical collector of the platinum group elements during crystallisation of both phases from melts the platinum group minerals show significant os isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains this micro scale isotopic heterogeneity and their random spatial distribution within the chromites argue strongly against an origin of the platinum group minerals via exsolution from the chromite lattice during post magmatic cooling a re interpretation of their microstructures and os isotopes analysed in situ had led us to review the traditional models and to propose several origins for the platinum group minerals in chromitites 1 the suites of platinum group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite 2 some platinum group minerals in chromitites may have been scavenged from wall rock peridotite during migration of the parental melts of the chromitite 3 other platinum group minerals may have precipitated from metasomatic fluid melts that infiltrated existing chromitites the recrystallization of pre existing platinum group minerals during polyphase metamorphism or recycling of the chromitite into deeper mantle levels can explain the presence of micrometric platinum group minerals with distinct os isotope compositions

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

lithium abundance and isotope systematics of forearc serpentinites conical seamount mariana forearc insights into the mechanics of slab mantle exchange during subduction 1 variable lithium contents and contrasting li isotopic signatures are evident in the serpentininite muds and clasts extruded at conical seamount in the mariana forearc the muds have high li contents compared to mantle values 3 7 ppm and a mean 7li value of 6 the clasts are generally lower in li and 7li varies from 6 to 10 with higher 7li correlating broadly to higher lithium contents the diverse 7li in the serpentinite clasts suggests li exchanges between mantle rocks and evolving slab derived fluids from different depths producing a subarc mantle that is zoned in terms of li abundances and 7li mariana forearc li isotopic signatures reach values as high as those reported in mariana arc lavas but neither mariana lavas nor lavas from other mature arcs show the variability observed in the serpentinites although elevated serpentinite li contents indicate forearc releases of li large differences in 7li are not evident in the slab outputs at forearc versus arc depths in the marianas despite evidence for changes in other alkaline species e g ba sr k it is thus possible that the li sampled by mariana arc volcanoes is in large part li which was released in the forearc and transported in the mantle via convection to arc depths as has been proposed to explain b isotopic variations straub and layne 2002 the uniform 7li signatures observed in mature volcanic arcs tomascak et al 2002 are consistent with higher overall fluxes of slab fluids which generate larger melting anomalies including both modified and nonmodified mantle wedge domains high or low 7li in lavas from anomalous arcs e g panama tomascak et al 2000 may indicate that the volumes of mantle sampled in these magmatic events are smaller and thus more likely to reflect the signatures of hydrated plums e g allegre and turcotte 1986 phipps morgan and morgan 1999 with extremely high or low 7li

early cenozoic saucer shaped sills of the faroe islands an example of intrusive styles in basaltic lava piles abstract the early cenozoic igneous activity of the north atlantic igneous province that generated widespread sill complexes in sedimentary basins at the nw european margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the faroe islands the faroe island basalt group comprises seven formations with a total thickness of about 6 5 km of which four major formations are built up of tholeiitic lava flows each being several hundred metres thick and three thinner formations are mostly built up of volcaniclastic lithologies each being a few metres to a few tens of metres thick the largest sills are exposed as partly saucer shaped bodies in the three uppermost formations where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows numerous subvertical and moderately inclined dykes ranging in thickness from c 0 5 to c 4 0 m intersect the areas affected by sill intrusion but only inclined dykes or sheets have been positively identified as sill feeders locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

the origin of the cretaceous gabbros in the fujian coastal region of se china implications for deformation accompanied magmatism the cretaceous gabbros in the fujian coastal region southeastern china experienced different magmatic and tectonic processes associated with deformation of the changle nanao shear zone group 1 gabbros 115 ma show marked lile e g sr ba and k and lree enrichment and hfse e g ta nb zr and hf depletion strongly suggesting an island arc affinity the parental magma of group 1 gabbros arguably derived from an arc related source mantle modified by previous subduction rose into and interacted with the coexisting syntectonic granitic magma generated during the main stage of shear zone deformation and produced the gabbro diorite granite complexes group 2 and 3 gabbros 106 95 ma possibly formed by coupling of the subduction modified source mantle with part of undepleted mantle e g nb enrichment in an extensional environment at the end of deformation their parental magmas did not undergo significant magma mixing but produced a cumulate nature all the three groups of gabbros were contaminated by crustal material during the processes of intrusion and emplacement

extraction of melt from veined mantle source regions during eruptions abstract harzburgite is the predominant type of peridotite in the mantle sections of ophiolite complexes the residual character of harzburgite suggests that the magma extraction is efficient and that harzburgite form a residual layer above a subsequent mantle source this residual layer form an upper lid for subsequent magma accumulation a theoretical estimate of the depletion rate of a veined mantle source region show that it is possible for a veined source region to become nearly completely depleted for melt within a year this period of time is comparable to the durations of abyssal and subaerial volcanic eruption episodes which varies from a few months to a few years

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

geochemistry and tectonic significance of the stony mountain gabbro north carolina implications for the early paleozoic evolution of carolinia abstract carolinia comprises a collection of neoproterozoic early paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to laurentia in the early to middle paleozoic in central north carolina mafic rocks of the stony mountain gabbro intrude sub aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the albemarle group the age of the stony mountain gabbro is constrained to the early cambrian middle ordovician field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the neoproterozoic earliest cambrian albemarle group yet the gabbro pre dates regional metamorphism and tectonism related to the late ordovician accretion of carolinia to laurentia the stony mountain gabbro has a sub alkaline basaltic composition variable tio 2 mgo and ni cr values the rocks have a geochemical signature typical of island arcs the degree of lree enrichment prominent negative nb anomalies and nb th ratios are all features of low k to medium k tholeiitic basalts in modern island arc subduction related lavas isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle the stony mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of 10 15 hydrous partial melting of variable mixtures of morb and oib like mantle sources overprinted by a minor subducted slab derived hydrous fluid component by analogy with modern settings the rocks of the stony mountain gabbro are comparable to morb like to oib type enriched rocks from the lau island and sumisu rift and are interpreted to have formed within an evolving early paleozoic island arc back arc rift basin system the presence of an early cambrian arc back arc rift system in carolinia is broadly coeval with arc back arc volcanism in other peri gondwanan blocks of the appalachians and may be related to the early paleozoic opening of the rheic ocean

isotopic characteristics of subduction fluids in an intra oceanic setting izu bonin arc japan new radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active izu bonin arc the oligocene izu arc and their associated rift basins as with many intra oceanic island arcs the pliocene recent izu bonin frontal arc lavas are highly depleted in zr nb and the rare earth elements relative to typical mid ocean ridge basalt morb indicating that the mantle wedge source has undergone a previous episode of melting ratios between these elements such as nb zr and la sm as well as 143nd 144nd do not vary significantly along the length of the frontal arc these parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge however ba zr ba rb and 87sr 86sr increase along the frontal arc to the north this leads us to propose that a variable enrichment in ba and radiogenic sr is superimposed on the mantle wedge sr nd and pb nd isotope variation indicate that both sr and pb become more radiogenic after fluid addition however pb isotope ratios do not correlate with increases in pb concentration or ratios such as ba zr and nb pb in other words the pb isotopic composition of the arc lavas appears to be independent of the amount of pb introduced by subduction fluids into the mantle source this buffering of pb isotopes along the frontal arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities despite this we are able to determine that the mantle wedge has isotopic characteristics similar to indian ocean morb and that the subduction fluid solute is primarily derived from subducted oceanic basalt with a 2 contribution from subducted sediment lavas in the oligocene izu arc and fore arc basin were derived from a mantle wedge of similar composition to the active arc despite levels of pb enrichment comparable to those of the modern arc the pb isotopes of the oligocene volcanics indicate a lower sediment input into the melting region

early jurassic volcanic rocks in the xiongcun district southern lhasa subterrane tibet implications for the tectono magmatic events associated with the early evolution of the neo tethys ocean abstract early mesozoic late triassic to middle jurassic igneous rocks in the southern lhasa subterrane tibet record important information on the early tectono magmatic evolution of the neo tethys ocean this paper presents petrological geochronological and geochemical data for the xiongcun formation ignimbrites to constrain the petrogenesis and tectonic setting of these rocks in the southern lhasa subterrane the xiongcun formation is dominated by andesitic and dacitic rocks these rocks yield early jurassic 195 176 ma zircon u pb ages and display typical arc like geochemical signatures being enriched in large ion lithophile elements e g rb ba and u and depleted in high field strength elements e g nb ta and ti sr nd pb hf isotopic compositions are highly depleted these chemical and isotopic data suggest that the xiongcun formation ignimbrites probably formed in an intra oceanic island arc setting related to the northward subduction of the neo tethys oceanic slab the parental magmas were derived from a depleted mantle source modified by fluids released from the neo tethys oceanic slab combined with previously reported data we suggest that the volcanic rocks in the early jurassic xiongcun and yeba formations of the southern lhasa subterrane were generated by a single subduction system but developed on oceanic and continental crust respectively

the isotope systematics of a juvenile intraplate volcano pb nd and sr isotope ratios of basalts from loihi seamount hawaii abstract sr nd and pb isotope ratios for a representative suite of 15 basanites alkali basalts transitional basalts and tholeiites from loihi seamount hawaii display unusually large variations for a single volcano but lie within known ranges for hawaiian basalts nd isotope ratios in alkali basalts show the largest relative variation 0 51291 0 51305 and include the nearly constant tholeiite value 0 51297 pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue tatsumoto s 31 loa trend towards higher 206pb 204pb ratios resulting in a substantial overlap with the kea trend 206pb 204pb ratios for loihi and other volcanoes along the loa and kea trends 31 are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of hawaiian tholeiites loihi lavas display no correlation of nd sr or pb isotope ratios with major element compositions or eruptive age in contrast with observations of some other hawaiian volcanoes 38 isotope data for loihi as well as average values for hawaiian volcanoes are not adequately explained by previously proposed two end member models new models for the origin and the development of hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere

evolution of the mariana convergent plate margin system the mariana convergent plate margin system of the western pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology the system s relative geologic simplicity and the well exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins the mariana convergent margin s long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the philippine sea plate the trenchward edge of the overriding plate has a relatively sparse sediment cover rocks outcropping on the trench s inner slope are typical of the early formed suprasubduction zone s lithosphere and have been subjected to various processes related to its tectonic history pervasive forearc faulting has exposed crust and upper mantle lithosphere many large serpentinized peridotite seamounts are within 100 km of the trench axis from these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids ocean drilling recovered suprasubduction zone lava sequences erupted since the eocene that suggest that the forearc region remains volcanologically dynamic seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution large portions of uplifted southern forearc are exposed at the larger islands active volcanoes at the base of the eastern boundary fault of the mariana trough vary in size and composition along strike and record regional differences in source composition their locations along strike of the arc are controlled in part by cross arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin the western boundary is the west mariana ridge the western portion of the volcanic arc active prior to formation of the mariana trough the trough evolved in a two stage extension process of rifting and subsequent seafloor spreading the back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid ocean type spreading center south of 20 n the change from initial rifting to true seafloor spreading is also evident across the mariana trough from rifted topography near the west mariana ridge to spreading ridges in the central to eastern basin south of 20 n this morphologic change indicates an early stage of extension with basin and range type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries the spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys the center of extension intersects the active volcanic front differently at either end of the mariana trough in the north extension is by rifting of arc lithosphere where it intersects the arc in the south a major strike slip fault extends from the trench axis across the forearc through the volcanic arc and into the back arc basin arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center the complexity of interrelated tectonism and magmatism in this convergent margin is daunting but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments

esmeralda bank geochemistry of an active submarine volcano in the mariana island arc esmeralda bank is the southernmost active volcano in the izu volcano mariana arc this submarine volcano is one of the most active vents in the western pacific it has a total volume of about 27 km3 rising to within 30 m of sea level two dredge hauls from esmeralda recovered fresh nearly aphyric vesicular basalts and basaltic andesites and minor basaltic vitrophyre these samples reflect uniform yet unusual major and trace element chemistries mean abundances of tio2 1 3 and feo 12 6 are higher and cao 9 2 and al2o3 15 1 are lower than rocks of similar silica content from other active mariana arc volcanoes mean incompatible element ratios k rb 488 and k ba 29 of esmeralda rocks are indistinguishable from those of other mariana arc volcanoes on a ti zr plot esmeralda samples plot in the field of oceanic basalts while other mariana arc volcanic rocks plot in the field for island arcs

shoshonitic volcanism in the northern mariana arc 1 mineralogic and major and trace element characteristics petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern mariana and southern volcano arcs define two distinct rock series the central and northern mariana arc to 23 n and the northern volcano arc north of iwo jima are characterized by plagioclase clinopyroxene orthopyroxene titantomagnetite bearing subalkaline rocks including both low k and medium k series the northern mariana arc and southern volcano arc from 23 n to iwo jima are erupting rocks of a shoshonitic series with phenocrysts of plagioclase clinopyroxene olivine and biotite these rocks are less saturated than those of the subalkaline provinces and are substantially enriched in ba 400 900 ppm sr 600 1000 ppm k2o 1 4 5 and k2o na2o 0 4 1 2 relative to the subalkaline lavas ba y and ba zr increase by a factor of 3 to 4 in the shoshonitic rocks but k rb ba sr and k ba are relatively constant throughout the arc there is no relationship between degree of enrichment and volcano volume or degree of fractionation seamounts and islands within each province have the same range of compositions much of the intra edifice variation in lava composition can be modeled by 10 70 crystallization of plagioclase and clinopyroxene with smaller amounts of olivine orthopyroxene and titanomagnetite or by accumulation of 10 40 phenocrysts dominantly plagioclase the differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources the occurrence of this enriched mantle in the northern marianas may be a consequence of the propagation of the mariana trough spreading center into the volcano arc

hf nd isotope variation in mariana trough basalts the importance of ambient mantle in the interpretation of subduction zone magmas in the study of geochemical mass balances at subduction zones the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter deconvolving the influence of ancient versus modern enrichments is particularly difficult especially when considering elements that are highly mobile here we provide an alternative approach using less mobile elements and a filter to remove the effects of recent slab additions we provide new hf isotope data for 30 mariana trough mt backarc basin lavas once filtered hf and nd isotope ratios are highly correlated of indian mid oceanic ridge basalt character and display variations similar to ocean ridges of comparable lengths the isotopic variability observed in this ambient mantle provides a new paradigm for the interpretation of the varied volcanic products of the arc thus shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source in contrast the large volcanoes of the central island province have a consistent offset in nd isotope compositions from the mt array resulting from fluid addition existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale we suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide

late paleozoic calc alkaline to shoshonitic magmatism and its geodynamic implications yuximolegai area western tianshan xinjiang article i nfo continuous late paleozoic magmatic activity in the western tianshan xinjiang china temporally overlapped the late carboniferous collision between the tarim and the kazakhstan yili plates isotopic dating whole rock geochemistry and isotopic characteristics of a suite of calc alkaline to shoshonitic rocks from yuximo legai area in the eastern end of the awulale mountains help define the tectonic evolution of this part of the tianshan zircon u pb ages of a quartz diorite and a quartz syenite porphyry are 310 8 2 1 ma and 284 4 3 6 ma respectively the geochemical and isotopic characteristics of late carboniferous igneous rocks including the quartz diorite as well as basaltic andesite and k rich trachyandesite indicate an evolu tion from calc alkaline towards shoshonitic series this was generated by the partial melting of the hydrated mantle wedge induced by changing subduction angles the younger quartz syenite porphyry however was more likely derived from the partial melting of a thickened crust in a post collisional setting or the partial melting of hot juvenile basalt in the lower crust during early permian delamination of the subcontinental lithospheric mantle therefore the generation of the calc alkaline to shoshonitic igneous rocks in yuximole gai area is genetically linked to the geodynamic evolution of the western tianshan from convergent subduc tion to collision during the late paleozoic

strontium isotopes from circum pacific intra oceanic island arcs and marginal basins regional variations and implications for magmagenesis circum pacific intra oceanic island arc lavas have a mean 87 sr 86 sr of 70355 associated marginal basins have a mean 87 sr 86 sr of 70311 significant hemispheric variations are noted for both island arcs and back arc basins in the north pacific mean 87 sr 86 sr for island arcs 70335 and back arc basins 70287 is less radiogenic than counterparts in the south pacific 70374 70336 this hemispheric variation is not related to the age of sea floor being subducted therefore partial fusion of sediments and or altered crust are not likely to be responsible back arc basin magmas are generally depleted in radiogenic strontium relative to those of the associated arc but arc basin systems are enriched or depleted systematically arcs around the western pacific show regional variations similar to that observed for alkali basalts of the pacific basin both show enrichments in radiogenic strontium near samoa and become progressively depleted away from it least radiogenic samples from continental margin volcanics indicate their sources are more radiogenic in the southern circum pacific these regional variations in 87 sr 86 sr in island arc and marginal basin systems suggest that 1 regional isotopic variations noted for the mantle beneath the pacific basin are maintained across convergent plate boundaries 2 the source regions of marginal basin basalts are isotopically similar to that of mid ocean ridge basalts while those of island arcs are similar to those of intra plate ocean island basalts 3 the sources of island arcs and marginal basin magmas lie in the mantle

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

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