samples from the crystallising boundary layer of a zoned magma chamber the spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr b p tephra deposit of laacher see volcano the melt showed vertical chemical zonation immediately prior to eruption and throughout most of the magma volume only a small fraction of crystals were present the eruption also ejected crystal rich nodules ranging from mafic to felsic in composition which are samples of the materials crystallising at the boundaries of the chamber new data on nodule petrography and chemical compositions of whole rocks minerals and interstitial glasses are presented volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol i e the crystals interlock one exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt felsic nodules show much less pronounced differences with their interstitial melt felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor modal mineralogy glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber a broader range of mineral compositions are often found mineral assemblages were especially diverse in the upper part of the chamber it is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and or time

petrology of calc alkaline lavas at volc n ollague and the origin of compositional diversity at central andean stratovolcanoes

basalt geochemistry and tectonic settings a new approach to relate tectonic and magmatic processes abstract in recent years the effects of mantle heterogeneity crustal contamination mantle metasomatism and other non differentiation related processes on the composition of basalts have been documented in various well defined tectonic settings the fact that none of these effects are unique to a specific tectonic setting illustrates the fundamental logical error in the formulation of discrete tectonomagmatic classifications of basalts using empirical discrimination diagrams a more meaningful approach to the problem of tectonics and basalt geochemistry is careful stratigrapghc geochemical studies which document important temporal and spatial variations in melt compositions using this approach it may be possible to identify certain sequences of compositions which are characteristic of specific tectonic cycles the first problem encountered in any such study is the identification of significant compositional variations within a single tectonic province the niobium normalized spidergram presented here is a simple yet powerful tool for rapid detection of source heterogeneity or other non differentiation related compositional variations in basalts

in situ crystallization observed in the osumi granodiorite batholith abstract the osumi granodiorite is a 14 ma batholith emplaced in accreted sedimentary rocks in the forearc of southwest japan it has a chemical and mineralogical zonal structure with zones which become progressively more felsic inward feo and mgo decrease while sio2 k2o and rb increase from its margin to the interior in accord with this alkali feldspar and quartz become more abundant in inner zones and biotite and plagioclase vary oppositely the mineral assemblage however remains unvaried major bulk rock compositions form a linear variation trend which is found to be almost identical with the average trend of japanese granitic rocks these petrographic and chemical characteristics are well explained by in situ crystallization model in which the liquid returning from the solidification zone to the chamber has the granitic minimum liquid composition the composition of the liquid in the chamber and that of the cumulate lie on the straight line that connects the granitic minimum and the initial chamber composition

models for granitoid evolution and source compositions this paper critically examines the bases of various models for the chemical evolution of granitoid suites the proposed mineralogical textural and chemical criteria for the recognition of restite components are found to be equivocal and can be plausibly explained by crystal fractionation and accumulation as well as by magma mixing and assimilation of non restitic solids these processes demonstrably account for the bulk of the chemical variation in a variety of granitic suites although restite controlled variation may have played a role in some types of metasediment derived suites granitoid magmas with non minimum melt chemistry are probably rather common the petrographic geochemical and isotopic features of individual plutons should be assessed carefully to elucidate the relative importance of various mechanisms for chemical evolution conclusions as to the character of magma source materials and magma evolution paths that are based on the restite model should be treated with caution

basic volcanic rocks and tectonic setting a discussion of the zr ti y discrimination diagram and its suitability for classification purposes the zr ti y diagram cannot discriminate between normal and anomalous oceanic ridge basalts the latter tend to plot as within plate basalts this indicates that it is the mantle anomaly rather than the tectonic setting that characterizes the zr ti y interelement ratios in such rocks it is shown that rocks with sio2 56 of a tholeiitic series indicate a differentiation trend from ocean floor via within plate to calc alkaline basalts this feature shows that great care should be taken when data for altered or metamorphic rocks are selected for classification by this method

eclogites from the silvretta nappe switzerland geochemical constraints on the nature and geotectonic setting of their protoliths abstract in the austroalpine silvretta nappe of central europe eclogitic rocks of presumed paleozoic metamorphic age have recently been found their chemical composition suggests that they were derived from evolved tholeiitic magmas which have undergone variable degrees of fractionation at shallow depths involving crystallization of plagioclase plus olivine and or pyroxene the igneous protoliths probably originated at a constructive plate margin although some chemical features closely resemble those of mid ocean ridge basalts the composition of the metasedimentary country rocks suggests a tectonic setting close to a continental block perhaps in a back arc basin situation or incipient oceanic ridge

crystallization of apatite in natural magmas under high pressure hydrous conditions with particular reference to orogenic rock series two separate series of hydrous experiments involving 1 imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite and 2 crystallizing similar natural rock compositions progressively until apatite appears demonstrate a close dependence between apatite saturation and silica content of the magma and determine p2o5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite the effect of pressure on apatite solubility is not great and is most significant for silicic compositions

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

geochemistry of the miocene mount noel volcanic complex british columbia and comparison with the columbia river basalt the 19 6 ma mount noel volcanic complex mnvc is located about 150 km east of the garibaldi volcanic belt and adjacent to the chilcotin plateau basalts in southwestern british columbia the mnvc unconformably overlies jurassic to cretaceous metasedimentary rocks and lower cretaceous coast plutonic rocks the complex consists of flat lying columnar jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks basaltic flows have olivine fo 60 to fo 85 and plagioclase an 60 to an 80 phenocrysts with clinopyroxene microphenocrysts major trace and rare earth element analyses of the mnvc indicate the flows can be divided into two groups the largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt rare earth element patterns are slightly enriched in the light rare elements in the transitional group and more strongly enriched in the alkaline basalts in tectonic discriminant diagrams all basalts plot in the within plate fields or in overlap areas typical of continental flood basalts alkaline basalts found only at the base of the thick sequence of tholeiitic to transitional basalts are characterized by high p 2 o 5 sr and ba contents the mnvc tholeiitic to transitional basalts are comparable to the chilcotin group basalts and to the lowermost basalts of the columbia river group the origin of mnvc basalts is best interpreted as volcanism behind the pemberton volcanic arc

distribution of orthocumulate textures in the jimberlana intrusion the p and u contents and the texture of cumulates have been used to document variations in residual porosity in the jimberlana intrusion the results show that rocks with low residual porosity adcumulate textures occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low low residual porosity also correlates with the appearance of plagio clase as a cumulus phase and with zones of well developed rhythmic layering it is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow expelling the intercumulus magma in the process high initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled low viscosity in

evolution of late stage differentiates in the palisades sill new york and new jersey abstract the palisades sill at upper nyack ny contains evolved rocks that crystallized as ferrodiabase and ferrogranophyre and occupy 50 to 60 of the local thickness 143 nd 144 nd isotope values for rocks representing palisades diversity range between 0 512320 and 0 512331 and indicate a homogeneous source for the palisades and little or no contamination from shallow crustal sediments petrographic analysis of ferrodiabase suggests that strong iron enrichment was the result of prolonged quiescence in cycles of magmatic input ferrogranophyres in the updip northern palisades at upper nyack are members of a suite of cogenetic rocks with similar composition to sandwich horizon rocks of the southern palisades at fort lee nj but display distinct mineralogical and textural features differences in textural and mineralogical features are attributed to a updip lateral migration of residual liquid as the sill propagated closer to the surface b deformation caused by tectonic shifts and c crystallization in the presence of deuteric hydrothermal fluids resulting in varying degrees of alteration a model connecting multiple magmatic pulses compaction and mobilization of residual liquid by compositional convection closed system differentiation synchronous with tapping of the sill for extrusion of coeval basaltic subaerial flows is presented the persistence of a low temperature mushy layer represented by ferrogranophyres supports the possibility of a long lived conduit subject to reopening after periods of quiescence in magmatic input leading to the extrusion of the multiple flows of the orange mountain basalt and perhaps even subsequent preakness basalt flows depending on solidification conditions a sub newark basin network of sills subjected to similar protracted input of pulses as hypothesized for the palisades was likely responsible for 600 ka of magmatic activity required to emplace a third set of watchung flood basalts the hook mountain basalt

geochemistry and tectonic setting of the central loei volcanic rocks pak chom area loei northeastern thailand abstract the central loei volcanic rocks as evidenced by those in the pak chom area were formed in the late devonian early carboniferous and can be separated into three magmatic groups transitional tholeiitic basalt tholeiitic microgabbro and calc alkalic basalt andesite on the basis of immobile element contents and ratios of least altered samples all the tholeiitic microgabbro possibly occurred as dikes chemically the transitional tholeittic basalt and tholeiitic microgabbro have higher abundances of tio 2 ni and cr relative to the calc alkalic basalt andesite at similar values for feo mgo they also contain higher ti zr but lower zr nb the transitional tholeiitic basalt has higher concentrations of p 2 o 5 and nb relative to the tholeiitic microgabbro at similar levels of feo mgo and also has higher ratios of nb y and ti v but lower values for ti zr and zr nb in terms of chondrite normalized ree and n morb normalized patterns the transitional tholeiitic basalt tholeiitic basalt and calc alkalic basalt andesite are analogous to those from north atlantic southwest indian ridge and new britain arc on this basis the central loei volcanic rocks are comprised of morbs and oceanic island arc lavas these arc lavas may have erupted on an oceanic basement in the same ocean basin as those in the chiang rai chiang mai volcanic belt

the origin of small scale geochemical and mineralogic variations in a granite intrusion a crystallization and mixing model a post tectonic unzoned granite intrusion in the meatiq dome a late proterozoic metamorphic complex in the central eastern desert of egypt shows significant chemical and mineralogic heterogeneity on the scale of sampling 5 kg whole rock analyses of 21 samples indicate small variations in sio2 al2o3 na2o and k2o and larger systematic variations in the less abundant major elements such as feo tio2 cao mno and mgo and trace elements such as sc cr co rb sr zr la ce nd sm tb yb lu ta th and u these variations cannot be accounted for by processes such as marginal accretion assimilation alteration by late stage fluids or multiple intrusion instead we proposed a model involving 35 percent solidification of the granite magma followed by partial solid liquid segregation during emplacement resulting in rocks containing 7 71 volume percent early formed solids such randomly distributed local segregation could have been caused by filter pressing flow differentiation and possibly gravity segregation either singly or in combination thus each sample is interpreted as a mixture of two end members with nearly constant compositions an early formed solid assemblage of crystals and a complementary residual liquid early formed solids are enriched in tio2 feo cao p2o5 sc co cr sr la ce and depleted in sio2 rb yb lu ta th and u while the residual liquid has complementary enrichments and depletions this simple mixing model is consistent with field and petrographic observations experimental studies pertaining to the crystallization sequence in the system ab or qtz h2o at 1 kb and physical properties of silicic magmas furthermore it is quantitatively supported by trace element data for minerals computed endmember compositions at 35 crystallization using mineral analyses and reasonable kd values and internal consistency in the percent solid for each sample computed from each of 17 elements in the inferred end members we suggest that this model might also apply to other small epizonal granite intrusions that show small scale chemical heterogeneities

geochemistry of eclogites and associated rocks of the southeastern area of the french massif central origin of the protoliths abstract within the barrovian metamorphic series of the french massif central most of the eclogites are closely associated with metagabbros and meta ultrabasites the protoliths of the eclogites may represent basaltic or gabbroic dykes or sills of oceanic affinities erupted in either a continental domain or during early oceanization or opening up of an ensialic back arc basin both the nature and type of magma fractionation and of post magmatic evolution are consistent with a shallow crustal origin for the protoliths eclogitization postdates the low pressure magmatic differentiation and the volcano sedimentary supra crustal evolution

the role of residual melt migration in producing compositional diversity in a suite of granitic rocks abstract we present a quantitative model for in situ crystallization within a solidification zone or boundary layer based on the trace and major element compositions of plutonic igneous rocks and related geochemical data we developed the model to account for the characteristics of a suite of granitic rocks nearly uniform mineral compositions in rocks that range widely in bulk composition e g 58 76 wt sio 2 linear variation and correlation of all major and trace elements analyzed except ba large and apparently random variations in ba concentration these characteristics cannot be explained by any standard petrogenetic model involving fractionation mixing or restite unmixing but are successfully reproduced by our model of residual melt migration we do not attempt to model the entire crystallization history of a pluton but rather only that interval during which melt migration processes have recognizable geochemical effects for the granitic suite the chemical signature of residual melt migration resulted from the change in ba compatibility with the onset of orthoclase crystallization at a granite minimum our results demonstrate that plutonic rocks can develop large compositional variations comparable to those expected to result from extreme differentiation within a large magma body over short distances by melt migration under conditions of high permeability or slow crystallization rate within the solidification zone melt migration is probably a common process that could be overlooked in large plutons if sampling is sparse and if variations in some components that appear random are not considered our model equations yield estimates for parameters that describe the proportions of residual melt that crystallize within and migrate out of the solidification zone values of these parameters can be used to infer information about permeability and melt mobility within the solidification zone the model parameters derived for the granitic suite have a roughly concentric spatial pattern in the pluton suggesting that residual melt was trapped near the margins accumulated in the interior with a zone of enhanced melt mobility and possibly compositional convection in between our model may be of general usefulness because it requires no assumptions about magma chamber geometry or magma dynamics it is applicable to magmas of any composition and the equations could be formulated to include those variables best constrained by a particular suite of plutonic igneous rocks

geochemistry and significance of metavolcanic rocks from the bou azzer el graara ophiolite morocco abstract the bou azzer el graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc alkalic arc sequence or as a composite forearc and accretionary melange seventy nine new major and trace element whole rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation the results of this study indicate that the bou azzer metavolcanics comprise three petrographically and geochemically distinct groups 1 a a siliceous metavolcanic series ranging from basaltic andesites to rhyolites with calc alkalic characteristics is the principal component of the northern section of the complex it includes distinct quartz bearing rhyolite flows isolated massive pyroclastic layers are typically associated with this series these rocks are interpreted as forearc basin volcanics and pyroclastics produced during late proterozoic north dipping subduction 2 b low ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite these rocks may represent fragments of the forearc basement below the siliceous series 3 c high ti metabasalts which range from enriched tholeiitic zr y 4 compositions are the principal volcanic block in the southern melange like terrane of the complex these have high ba and sr contents 100 200 ppm average ba 260 ppm average sr they are identical to basaltic compositions from tachdamt and bleida on the southern margins of the ophiolite which have previously been interpreted as a rifted continental margin sequence these rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north dipping subduction zone or rift margin basalts which have been assembled in a tectonic melange as a consequence of a forearc continent collision both the low ti and high ti metabasalt groups include samples previously identified as ocean ridge or back arc volcanics however there is no compelling evidence for the occurrence of these lava compositions in this region the distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the bou azzer el graara complex

the cumulate paradigm reconsidered the cumulate terminology outlined by wager and coworkers provided a logical system for describing the textures of the rocks of layered intrusions but recent work shows that this model fails to accommodate all the processes responsible for the petrographic features of coarse grained rocks textures of adcumulates and orthocumulates in the skaergaard intrusion cannot be correlated with concentrations of incompatible elements representing trapped liquid compaction and open system percolative processes have altered their textures modes and even bulk compositions although a rock may be a cumulate in the absence of other evidence it can be misleading to refer to it in terms of the cumulate nomenclature

feedbacks between mantle hydration and hydrothermal convection at ocean spreading centers hydration of the oceanic lithosphere is an important and ubiquitous process which alters both the chemical and physical properties of the affected lithologies one of the most important reactions that affect the mantle is serpentinization the process of serpentinization results in a drastic decrease in the density up to 40 seismic velocity and brittle strength as well as water uptake of up to 13 wt of the ultramafic rock in this paper we use numerical models to study the amount and extent of serpentinization that may occur at mid ocean ridges and its effects on fluid flow within the lithosphere the two dimensional fem model solves three coupled time dependent equations i mass conserving darcy flow equation ii energy conserving heat transport equation and iii serpentinization rate of olivine with feedbacks to temperature exothermic reaction fluid consumption and variations in porosity and permeability volume changes the thermal structure of the ridge is strongly influenced by rock permeability in addition to the spreading velocity of the ridge increased rock permeability enhances hydrothermal convection and results in efficient heat mining from the lithosphere whereas higher spreading velocities result in a higher thermal gradient serpentinization of the oceanic mantle in turn depends on the aforementioned competing processes however serpentinization of mantle rocks is itself likely to result in strong variations of rock porosity and permeability here we explore the coupled feedbacks increasing rates of serpentinization lead to large volume changes and therefore rock fracturing thereby increasing rock porosity permeability while as serpentinization reaches completion the open pore space in the rock is reduced due to the relative dominance of mineral precipitation although variations in the relation between porosity and permeability and serpentinization before the reaction reaches completion do not significantly affect the degree of serpentinization we find that unreasonably large portions of the mantle would be serpentinized if rock closure does not occur at the final reaction stage the amount of water trapped as hydrous phases within the mantle shows a strong dependency on the spreading velocity of the ridge with water content ranging from 0 18 105 kg m2 to 2 52 105 kg m2 additionally two distinct trends are observed where the water content in the mantle at slow spreading ridges drops dramatically with an increase in spreading velocity the amount of water trapped in the mantle at fast spreading ridges on the other hand is lower and does not significantly depend on spreading velocity

crystallization dynamics of granite magma chambers in the absence of regional stress multiphysics modeling with natural examples numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection during which wall rock contamination and magma mixing are possible followed by a long period of no convective cooling during which melt segregation occurs convection is driven by the negative density gradient generated in the upper cooling zone by melt to solid phase transformation convection breaks the upper mushy zone and drags the fragments downwards with descending rayleigh taylor fingers such fragments can be preserved as microgranular enclaves the descending rayleigh taylor fingers split low aspect ratio sill like magma chambers into nearly isolated convection cells if the magma is initially heterogeneous this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios and finally results in a pluton with marked lateral compositional variations easily misinterpreted as different intrusive batches convective heat loss quickly leads most of the magma chamber to critical crystallinity independently of the vertical coordinate so that a chamber wide three dimensional skeleton of crystals with uniform initial porosity c 0 4 0 5 is formed this configuration is gravitationally unstable therefore it spontaneously compacts towards an equilibrium vertical variation of porosity approaching atty s law in the absence of regional stress the upwards migration of the inter crystalline melts as a result of compaction is the most effective way of melt solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates granite magma chambers fractionated by these mechanisms will produce short range differentiation series from a composition slightly less silicic than the initial magma to high silica segregates in the presence of regional stress tectonic squeezing and shearing during the post convective stage can expel residual fluid more efficiently and lead to wide range granite differentiation series from rocks notably less silicic than the initial magma to high silica leucogranites

a mantle heterogeneity in the southwest pacific abstract the occurrence in time and space of tertiary intraplate central volcanoes and associated lava fields in the eastern australia new zealand campbell plateau region of the southwest pacific is more consistent with derivation from a large scale mantle anomaly than from discrete plumes the basalt volcanics from the eastern part of the region show a pronounced geochemical similarity those near the indo australia pacific convergent plate boundary show some evidence of interaction with subduction derived basalts the lithospheric swells expected associated with the eruption of magma to form these volcanoes seem broadly correlative with sedimentary patterns off the east coast of australia and the west coast of new zealand at this time

early paleozoic metamorphism at two crustal levels and a tectonic model for the pennsylvania delaware piedmont two high grade metamorphic terranes of early paleozoic age are exposed in southeastern pennsylvania and northern delaware the wilmington complex in delaware was metamorphosed at high temperatures to intermediate pressure granulite facies during the early paleozoic in the west chester prong pennsylvania a granulite facies metamorphism of grenville age was overprinted by a high pressure moderate temperature metamorphism during the taconic orogeny we propose a model to account for the different metamorphic conditions in these two terranes in this model the piedmont of northern delaware and southeastern pennsylvania was the site of collision between a magmatic arc and the north american continent along an east dipping subduction zone the wilmington complex was the infrastructure of the arc whereas the grenville age gneisses of the west chester prong were basement of the north american continent between these two relatively rigid plates there are nappes of remobilized basement and highly deformed forearc basin and accretion ary prism sediments the north american continent underthrust the hot infrastructure of the volcanic arc resulting in the high pressure 9 11 kb moderate temperature 650 700 c metamorphic overprint on grenville assemblages of the west chester prong the wilmington complex was metamorphosed at moderate pressure 6 7 kb and high temperature 800 c during the early paleozoic under the conditions of the high geothermal gradient that existed at the base of the volcanic arc the intervening sediments and remobilized basement are all at amphibolite grade but the grade is highest above second sillimanite isograd everywhere adjacent to the structurally overlying wilmington complex suggesting that the isograds in the sediments are inverted beneath the wilmington complex uplift of this doubled thickness of crust was rapid enough that the gneisses of the west chester prong were never heated above 700 c

the geochemistry of late upper proterozoic volcanic groups in the red sea hills of ne sudan evolution of a late proterozoic volcanic arc system the kadawcb and homogar volcanic groups occur in the sudanese portion of the nubian shield and are late proterozoic in age 723 ma and 671 ma respectively the lavas of both groups display a calc alkaline character are broadly similar in terms of their lithology phenocryst mineralogy and major element composition and geochemically they are similar to modern arc lavas associated with converging plate margins however a clear distinction can be made between the two groups on the basis of the trace element geochemistry of the basaltic components of the suites the homogar basalts are chemically more evolved and are enriched in large ion lithophile elements lile and rare earth elements ree having high la nb ce yb n and 87 sr 86 sr ratios compared with those of the kadawcb group these chemical differences possibly follow an evolutionary trend as the less developed kadawcb lavas are some 50 ma older than those of the evolved homogar group it is suggested that the older kadawcb lavas were erupted along an intra oceanic island arc system while the younger homogar lavas were extruded along a more evolved transitional arc or possibly a continental margin the differences in light heavy ree and lil high field strength element ratios may reflect variation in the degree of alteration of an overlying mantle wedge possibly due to the introduction of lile and ree enriched fluids derived from dehydrating subducted oceanic lithosphere