dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

relations between subduction slab heating slab dehydration and continental growth abstract the hydrated altered basalts of the oceanic crust if subducted could release large volumes of water during their phase transformations and partial melting reactions if heat from shear strain recrystallizations largely heats the altered basaltic layer only a 2 km thick layer could be heated significantly before the slab reaches 80 125 km depth most of the released waters should be driven into the deeper but cooler regions at the slab and andesitic tonalitic partial melts should form in the heated altered basalt layer large volumes of water initially at the surface could be added to the mantle and tonalitic partial melts could be lost from the oceanic crust and added to the continental plate and could account for continental growth

contrasting tonalite genesis in the lyngen magmatic complex north norwegian caledonides abstract the lyngen gabbro lg defining the major part of the lyngen magmatic complex is characterised by layered gabbros of n morb affinity western suite and layered gabbronorites quartz bearing gabbros and diorites quartz diorites of iat island arc tholeiite to boninitic affinity eastern suite the boundary between the eastern and western suites is generally defined by a large scale ductile shear zone of suboceanic origin the rypdalen shear zone rsz tonalites occur within the rsz and in the eastern suite of the lg variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups tonalite intrusion the vakkas pluton up to 5 km2 large occur in the eastern suite of the lg and are characterised by high y contents average 26 ppm and high k 2 o rb ratios average 0 062 compared to tonalites on the rsz the vakkas pluton has lightly concave ree rare earth element patterns with negative eu anomalies and positive end values 3 7 to 3 9 geochemical modelling based on the ree and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma tonalites related to the rsz form irregular veins and dikes that net vein the shear zone they are characterised by low y contents average 6 ppm low k 2 o rb ratios average 0 025 and highly variable contents of na2o k2o sr and ba compared to the vakkas pluton tonalites related to the rsz show substantial variation in the content of the lrees they possess low abundances of the hrees and absence of or slightly positive eu anomalies the tonalites have highly variable end values 0 6 to 9 4 probably resulting from enrichment of nd from an external source geochemical modelling suggests that the lree rich tonalites formed by h2o rich partial melting of differentiated products from the eastern suite of the lg the presence of b in the fluid phase is suggested by the presence of tourmaline bearing tonalite pegmatites thus the anatectic tonalites of this group could have been formed by water excess melting of a variety of gabbroic cumulates of the lg in the lg lree depleted tonalites end values 5 1 also occur and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the lg

constraints on the origin of archean trondhjemites based on phase relationships of nuk gneiss with h2o at 15 kbar we report the t x h2o phase relations for the trondhjemitic nuk gneiss which comprises the principal component of the second phase of archean 3 0 2 8 by igneous activity in the godthab region of southwestern greenland a pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths under h2o saturated conditions a melting interval of 135 c separates the solidus at 610 c from the liquidus at 745 c h2o saturation at 15 kbar occurs at approximately 15 5 wt h2o the h2o undersaturated liquidus extends along a curved path from 745 c at 15 5 wt h2o to 1100 c at 2 h2o lower h2o contents were not investigated at low h2o contents 7 mafic residues consisting of some combination of hb ga ep and bi are possible and could correspond to amphibolitic source rocks at lower melt h2o contents 5 possible residues consist of na pl qz ga and could correspond to an earlier generation of tonalitic trondhjemitic rocks however such residues would not impart the highly fractionated ree patterns characteristic of archean trondhjemites if a first generation of tonalitic trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth it would crystallize to an assemblage consisting almost entirely of na pl qz with highly fractionated ree patterns these rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated ree patterns because neigher pl nor qz is capable of significantly fractionating hree from lree

the smartville intrusive complex sierra nevada california the core of a rifted volcanic arc the smartville complex is a jurassic volcanic and plutonic arc in the northwestern sierra nevada that was deformed during the late jurassic nevadan orogeny we interpret the smartville intrusive complex to have formed during the incipient rifting of an active volcanic arc this interpretation is supported by the close relationship between volcanism and plutonism and by the close association between sheeted dikes and the younger plutonic rocks further support is found in the similarities that exist between the smartville complex and modern arcs that developed on oceanic crust either at a continental margin or in the ocean basins smartville volcanic rocks consist of a lower unit of tholeiitic submarine flows and pillowed flows that grades upward into an upper unit of calc alkaline pyroclastic and volcaniclastic deposits intrusive rocks include older units of metamorphosed gabbro and massive diabase which are intruded by a unit of 100 mafic and felsic sheeted and unsheeted dikes biotite hornblende tonalite and granophyric hornblende tonalite plutons are coeval with the dike complex and both rock types occur locally as dikes within the dike unit continuously and reversely zoned gabbro diorite plutons are also coeval with the dike complex granodiorite plutons are the youngest intrusive unit and may be related to the sierra nevada batholith relative age relations suggest that the smartville complex formed in a single volcanic plutonic system of pre nevadan age the dike complex intrudes and is intruded by both the tonalite and zoned gabbro diorite plutons both the upper and lower volcanic units are intruded by all of the plutonic and hypabyssal units and were deformed prior to the intrusion of the sheeted dike complex clasts of plutonic and hypabyssal rocks resembling the younger intrusives however occur locally in some of the youngest volcaniclastic rocks suggesting that shallow plutonism and volcanism could be broadly coeval early nevadan thrust faults juxtapose volcanic and older plutonic rocks of the smartville complex and mesozoic chert argillite broken formation to the east the younger plutons and the dike complex in the smartville are deformed by steep late nevadan faults and do not intrude chert argillite broken formation the youngest granodiorite plutons in the area truncate nevadan faults intrude the chert argillite formation and appear to be unrelated to the smartville complex the smartville volcanic arc underwent pre nevadan intra arc extension the sheeted dike complex is the primary manifestation of the rifting event the elongate shapes of the younger plutons which are coeval with the dike complex reflect extensional control on their emplacement volcanic rocks in the western and southern smartville complex were deformed prior to the intrusion of the dike complex

generation of trondhjemite from partial melting of dacite under granulite facies conditions an example from the new jersey highlands usa abstract new field and geochemical data place the losee metamorphic suite a tonalite trondhjemite complex of northern new jersey into the context of a major proterozoic continental are represented by a discontinuous belt of northern appalachian metadacite samples of losee rock range from extremely leucocratic trondhjemite locally associated with amphibolite to banded biotite hornblende pyroxene and garnet bearing tonalites the major element and ree composition of the tonalite closely resembles dacite from continental are settings and model melts extracted from an eclogite residue by partial melting at 15 kbar the ree composition of most losee trondhjemite is enriched in ree particularly hree compared with losee tonalite and is interpreted as the product of local anatectic melting of losee tonalite metadacite that occurred in a granulite facies environment during the grenville orogeny

constraints on archean trondhjemite genesis from hydrous crystallization experiments on nuk gneiss at 10 17 kbar we present crystallization experiments on archean nuk gneiss from west greenland and relate these to conditions for early continental crustal genesis the studied composition is a trondhjemite from a voluminous felsic intrusive phase dated at 2 800 3 000 m y in a detailed set of experiments at 10 kbar the liquidus mineralogy is observed to change with increasing water content from plagioclase above 1000 c and 7 wt water water saturation of the melt occurs with 10 5 wt water at 800 c mafic liquidus minerals at 15 kbar differ from those at 10 kbar in that garnet replaces pyroxene and that epidote replaces hornblende below 690 c at 13 wt water and at water saturation 15 5 wt garnet crystallization is never observed at pressures below 13 kbar using additional experiments at 12 5 13 and 17 kbar the liquidus phase variation for trondhjemite with water is accurately defined between 8 18 kb

the nature and significance of magma chamber margins in ophiolites examples from the norwegian caledonides abstract large scale intrusive contacts with associated marginal series have been encountered within norwegian ophiolite complexes at karmoy solund and leka the contacts limit individual magma chambers and are found at different structural levels of the plutonic suites examples of magma chamber margins adjacent to interlayered ultramafic and gabbroic rocks modally layered gabbros high level gabbros and sheeted dykes are described the nature of the intrusive boundaries and the presence of partially resorbed xenoliths in the vicinity of the intrusive margins suggest that stoping and assimilation have been important mechanisms during the development of the magma chambers characteristic marginal series are developed along the intrusive boundaries the thicknesses and appearance of these series vary with depth in the complexes whereas the marginal series are well developed within the uppermost levels of the plutonic complexes exhibiting rock types such as microgabbro massive gabbro and magnetite gabbro the marginal series observed at lower levels are thinner and also devoid of chilled facies rocks and magnetite gabbros the marginal series may be subdivided into border and roof series the latter are characterized by an intimate relationship with sheeted dykes which comprise dyke swarms formed both prior to during and subsequent to crystallization of the roof series based on these relationships the dykes can be subdivided into rooted and rootless dykes a multiple magma chamber model with magma chambers migrating from a low to a high level within the oceanic crust is proposed on the basis of the observed features

influence of rock strength on the valley morphometry of big creek central idaho usa abstract analysis of valley morphometry and bedrock strength along big creek central idaho shows that valley floor width is strongly controlled by bedrock we performed statistical analysis of schmidt hammer rock strength as a function of lithology and aspect and of valley morphometry as a function of rock strength rock strength is significantly greater on the south side of the valley and in eocene granodiorites rock strength is weakest in eocene volcanic tuffs valley floor width depends negatively on weakest valley side rock strength and hillslope gradient on the north side of the valley depends positively on rock strength stream gradient does not depend on rock strength valley floor width appears to be controlled by bedrock strength on the weaker side of the valley which was generally the north south facing side we speculate that a higher degree of weathering via freeze thaw cycles contributes to lower strength on the north side the positive dependence of hillslope gradient on rock strength on the north side provides evidence that differential weathering across lithologies determines the gradient that can be maintained as lateral migration of the stream erodes valley walls these results suggest that in situ rock strength exerts strong influences on some measures of valley morphometry by modulating hillslope mass wasting processes and limiting lateral erosion

amphibole out phase boundary in partially melted metabasalt its control over liquid fraction and composition and source permeability a number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40 high sio2 liquids closely comparable in many geochemical respects to tonalitic trondhjemitic granitoids ttg from these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high sio2 liquids is defined by the wet basalt solidus on one side and the amphibole out phase boundary on the other the phase relations of an alkali rich tholeiitic metabasalt have been examined in the outer half of this region up to and beyond the amphibole out phase boundary results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval melt fraction increases slowly and gradually from 0 up to 20 30 between the wet basalt solidus and up to amphibole out but increases substantially from 30 to 50 across and immediately beyond it liquids are strongly to moderately peraluminous molar al2o3 cao na2o k2o a cnk 1 0 up to amphibole out neutral at the boundary a cnk 1 0 and increasingly metaluminous a cnk 1 0 beyond it up to amphibole out potassium is strongly partitioned into liquid relative to amphibole i e mineral melt kd 0 2 0 3 whereas titanium is strongly partitioned into liquid kd 2 0 6 0 physical and chemical criteria suggest that efficient melt segregation leading to ttg plutonism requires more than 20 30 batch melting of a garnet bearing basaltic protolith near or beyond the amphibole out phase boundary leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements hrees and yttrium y lower degrees of melting may involve critical melting where melt is squeezed out of the residue by deformation of the crystalline matrix resulting in disequilibrium melt compositions

the origin of low k rhyolites from the mariana frontal arc low k rhyolites and overlying 2 pyroxene andesites both of late eocene age comprise the oldest volcanic units exposed on saipan the mineralogy and geologic setting of these rocks indicate they were erupted in a volcanic arc setting the presence of andesite and lack of basalts of similar age suggest that the rhyolites are not part of a typical bimodal basalt rhyolite suite

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

controls on denudation rates in tectonically stable mediterranean carbonate terrain using cosmogenic isotopes and solute load analysis we quantify chemical weathering solutional erosion and denudation rates over variable time scales in a tectonically stable moderate relief carbonate terrain soreq drainage judea hills israel located in a semihumid mediterranean climate long term 10 4 yr denudation rates were calculated from 36 cl concentrations in 51 bedrock and sediment samples bedrock samples range in elevation 340 850 m hillslope gradient 0 30 and mean annual precipitation map 500 630 mm and vary in soil cover thickness 0 75 cm mg ca ratio 0 0 1 0 mol clay mineral contents 0 6 wt and mechanical strength 41 58 schmidt hammer rebound units soil pco2 values at a single location during the course of 1 yr range between 0 4 and 9 0 mmol mol 1 average long term denudation rate of exposed bedrock samples is 21 7 mm k y 1 field observations and 36 cl measurements indicate that soil pockets undergo cycles in the rate of deepening and that over 10 5 yr time scale average denudation rates beneath soil pockets are similar to those of exposed bedrock sediment samples yield even higher denudation rates which are probably anthropogenically induced but could also indicate that the sediment source is soil pockets long term denudation rates are decoupled from hillslope gradient elevation and rock strength denudation rates show a positive correlation with present day map values exhibit a complex relation with rock mg content and show a weak correlation with clay content annual chemical weathering rates were calculated from modern day solute load measured in waters of perched springs and the regional carbonate aquifer our results indicate that on annual decadal and 10 4 yr time scales chemical weathering and denudation are controlled by carbonate dissolution while mechanical processes are far less signifi cant overlap between the distributions of hco3 concentrations measured in runoff springs and the regional aquifer water suggests that chemical weathering focuses at the bedrock surface and therefore is comparable with solutional denudation this result is in contrast to the features of ancient fl uvial and colluvial activity steep nonconcave hillslopes and stream profi les and knickzones in the streams preserved in the present landscape such features were formed in response to mid pleistocene uplift and could have been preserved due to a decrease in stream power following the formation of subsurface drainage and the lowering in abrasive clast supply that followed the stabilization of hillslopes in the drainage long term denudation rates calculated from exposed bedrock samples are higher by factor of 1 4 relative to annual contemporary chemical weathering rates increased precipitation by a similar factor averaged over the last glacial and present interglacial can explain this difference

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

beginning of melting in the granite system qz or ab an h2o the beginning of melting in the system qz or ab an h2 o was experimentally reversed in the pressure range p text h text 2 text o 2 15 kbar using starting materials made up of mixtures of quartz and synthetic feldspars with increasing pressure the melting temperature decreases from 690 c at 2 kbar to 630 c at 17 kbar in the an free alkalifeldspar granite system qz or ab h2o in the granite system qz or ab an h2o the increase of the solidus temperature with increasing an content is only very small in comparison to the alkalifeldspar granite system the solidus temperature increases by 3 c 7 c if albite is replaced by plagioclase an 20 an 40 the difference between the solidus temperatures of the alkalifeldspar granite system and of quartz anorthite sanidine assemblages system qz or an h2o is approximately 50 c

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

chemical compositions of tourmaline in the vein type tungsten deposits of the kaneuchi mine japan chemical compositions of tourmaline from the vein type tungsten deposits of the kaneuchi mine in japan are obtained with an electron probe microanalyzer tourmaline occurring at this mine belongs to the dravite schorl series magnesium contents are high compared with tourmaline from granitic zones this suggests that ore forming solutions are not formed directly from magmatic fluid and reacted with sedimentary rocks compositions of tourmaline in quartz veins are similar to those in host rocks and the bulk compositions of host rocks do not indicate magnesium metasomatic reactions it can therefore be concluded that water rock interactions occurred at a deeper level which remains unexplored

generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

distribution of fluorine in recent marine sediments related to petrographic composition bransfield strait and northwestern weddell sea antarctica abstract the fluorine budget of 40 marine surface sediment samples from the atlantic sector of the antarctic seas has been examined in order to elucidate the relationship between granulometry mineralogy major and minor element chemistry and the fluorine distribution the concentrations of total fluorine range from 243 to 800 ppm x 423 ppm the clay and fine silt fractions contribute 60 87 to the total concentration of fluorine the clay fraction consists of clay minerals detrital quartz feldspars and an amorphous component biogenic opal and volcanic glass shards the fluorine concentrations in the crystalline components of bransfield strait and weddell sea sediments are 551 557 and 724 825 ppm respectively these variations are almost exclusively due to varying proportions of illite which is enriched in fluorine to a considerable extent montmorillonite and mixed layer clays which carry approximately as much fluorine as illite are present in insignificant amounts and are therefore of minor importance in the total fluorine concentration in these sediments biogenic opal essentially diatom skeletons which accounts for 3 77 wt of the clay fraction contains 15 ppm fluorine thus fluorine abundance in the clay fraction is negatively correlated with the content of biogenic opal volcanic glass shards contain 1688 3758 ppm fluorine they comprise only up to 3 wt of the clay fraction and are therefore of minor relevance in the total fluorine budget in these sediments

a u pb age for the shetland islands oceanic fragment scottish caledonides evidence from anatectic plagiogranites in layer 3 shear zones high precision u pb data obtained from zircons extracted from plagiogranite within the gabbro unit of the shetland islands oceanic fragment of northeast scotland yield an age of 492 3 ma field relations indicate that the plagiogranites were generated by the partial melting of amphibolitized gabbros within high temperature shear zones formed due to crustal deformation and fluid infiltration occurring in proximity to a spreading centre the u pb data therefore constrain the crystallization age of the shetland complex this age is similar to u pb ages obtained from the leka 497 2 ma karmoy 493 7 4 ma and gulfjellet 489 3 ma oceanic fragments of the norwegian caledonides and the pipestone pond 494 3 2 ma and betts cove 489 3 2 ma oceanic fragments of the canadian appalachians

beryllium and boron in subduction zone minerals an ion microprobe study secondary ion mass spectrometry was used to measure the distribution of be b rb sr y nb ba and ce among minerals in rocks from the franciscan complex and catalina schist paleosubduction zone terranes the rocks examined represent a wide range of metamorphic pressure temperature conditions 6 15 kb 300 750 c and include metasomatized and migmatitic samples white mica is the primary host of be 2 5 ppm and b 107 ppm in the rocks examined average concentrations amphibole be 0 7 b 15 ppm lawsonite be 0 9 b 37 ppm and biotite be 0 9 b 25 ppm may also contain significant amounts of both be and b omphacite may host be 4 1 ppm in veins 0 7 ppm groundmass and chlorite is a minor host of b 22 ppm minerals in these subduction zone metamorphic rocks that lack measurable be or b include albite apatite calcite clinozoisite epidote garnet ilmenite sphene stilpnomelane plagioclase quartz rutile and zoisite white mica biotite and stilpnomelane are the major mineral hosts of rb 90 300 and 30 ppm respectively and ba 2100 1000 and 4700 ppm epidote and lawsonite are the major host minerals of sr 600 and 300 ppm and ce 12 and 15 ppm garnet epidote lawsonite and sphene host y 100 30 40 and 60 ppm respectively sphene 77 ppm 1 analysis ilmenite and rutile host nb evidence of veining and metasomatism is strongly correlated with be and or b enrichment of white mica omphacite amphibole and lawsonite at the p t conditions examined both be and b appear to be transported in aqueous fluids at depths of 20 50 km in subduction zones the transfer of be b and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure dependent dehydration reactions or alternatively the be and b in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis

metamorphism of an obducted island arc example of the kohistan sequence pakistan in the himalayan collided range in northern pakistan the extraordinary 40 km thick kohistan sequence of metamorphosed mafic ultramafic and calc alkaline layered plutonic and volcanic mainly andesitic to rhyodacitic rocks has been recently reinterpreted as the only complete vertical section of an intra oceanic island arc presently exposed anywhere in the world plate scale models have been suggested to explain the origin and the tectonic evolution of this arc in the himalayan collision range despite some noticeable differences in the models there is general agreement that the kohistan sequence represents the crust of an arc obducted onto the northern edge of the indian plate before the oligocene collision of india against asia new data on the tectonometamorphic evolution of the kohistan sequence have led to the conclusion that a first major d 1 event developed with increasing metamorphic grades oriented downward the kohistan pile but also toward ensandwiched basic ultrabasic granulitic rocks rare geochronological data and petrological evidence indicate that the latter granulites pyriclasites and various plagiopyrigarnites pyrigarnites metatroctolites metawebsterites metadunites etc were parts of enormous lower to middle cretaceous layered calc alkaline plutons emplaced during the arc building stages as the d 1 metamorphic event is correlated with the upper cretaceous obduction process of the kohistan onto india it is proposed that the d 1 thermal structure was strongly controlled by remnant magmatic heat source s within the obducted arc this interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after 10 20 ma the emplacement of the former plutonic arc rocks a blueschist tectonic melange underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process the oligocene collision of india against asia was associated with a barrovian overprinting metamorphism in the kohistan arc during this second event the dynamo thermal structure was much more classical and not controlled by the occurrence of the cretaceous metaplutonic rocks a tentative model of the thermal structures during the major events which affected the kohistan arc is proposed