compositional variation and provenance of late cenozoic distal tephra beds fairbanks area alaska abstract thin fine grained discontinuous tephra beds from interior alaska can be divided into two compositional groups by their mineralogy and major and trace element glass geochemistry the source vents of these beds are unknown but are believed to be in the aleutian arc alaskan peninsula and wrangell volcanic field type i tephra has evolved trace element profiles while type ii is primitive type ii contains abundant hydrous minerals that are absent or rare in type i trace elements were used to compare the fairbanks tephra beds to volcanic rocks in the potential source areas there are very few published glass trace element analyses from these regions this necessitates comparing whole rock with glass analyses for this reason incompatible elements particularly rare earth elements are most useful whole rock analyses and petrography of lavas throughout the aleutian arc alaskan peninsula and the wrangell volcanic field are similar to type i tephra lavas and intrusive rocks similar to type ii tephra are rare and occur at one volcano in the alaskan peninsula and at three volcanoes in the wrangell volcanic field

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

possible role of amphibole in the origin of andesite some experimental and natural evidence experiments in the system high a1 basalt hab water have been conducted in the melting range at pressures between 1 atm and 10 kbar defining the amphibole stability field and the composition of liquids which coexist with this amphibole plagioclase is the anhydrous liquidus phase between 1 atm and 10 kbar but in the hydrous runs this role is taken by olivine at 7 kbar and then by clinopyroxene at higher pressures because amphibole is never on the high a1 basalt liquidus it is not likely that andesite is derived from primary basalt by pure fractional crystallisation although as we discuss other mechanisms including equilibrium crystallisation might implicate amphibole if primary basaltic magma undergoes closed system equilibrium crystallisation then the amphibole field will be intersected at between 50 and 100 c below the liquidus the compositions of melts coexisting with amphibole alone do not match those of any of the natural andesite or dacitic lavas associated with the particular high a1 basalt investigated like natural andesites they become rapidly silica enriched but they also become far more depleted in tio2 and mgo however the compositions of liquids lying directly on the divariant amphibole out reaction zone where amphibole liquid coexist with clinopyroxene or olivine plagioclase do resemble those of naturally occurring low silica andesites with increasing temperature pargasitic amphibole breaks down via incongruent melting reactions over a narrow temperature range to form a large volume of relatively low silica basaltic andesite liquid and a crystalline assemblage dominated by either clinopyroxene or olivine our important conclusion is that basaltic andesite liquid will be the product of reaction between cooling hydrous mafic liquid and anhydrous ferromagnesian phases the solid reactants could represent earlier cumulates from the same or different magma batches or they could be peridotite wall rock material because the amphibole out boundary coexisting with liquid is one of reaction it will not be traversed so long as the phases on the high temperature side remain thus the assemblage amphibole clinopyroxene olivine plagioclase liquid is one in which the liquid is buffered within limits and results reported here indicate that this buffering generates melts of low silica andesite composition when tapped to lower pressures these liquids will rise eventually to fractionate plagioclase rich assemblages yielding silicarich andesite and dacite melts conversely the partial melting of hornblende pyroxenite hornblende peridotite or hornblende gabbro can also yield basaltic andesite liquids the phase relationships suggested by these experiments are discussed in the light of naturally occurring phenocryst and xenolith assemblages from the east sunda arc primary magmatic additions to the lithosphere of volcanic arcs are basaltic and voluminous upper crustal andesite in these terranes complemented by mafic and ultramafic crystalline deposits emplaced in the lower crust or close to the moho together these components constitute total arc growth with a basaltic composition and represent the net accreted contribution to continental growth

some thermal problems associated with magma migration abstract the mechanisms by which magma migrates from the point in the earth s interior where melting occurs to the earth s surface are poorly understood in this paper several aspects of this problem are examined magma can migrate upward due to its differential buoyancy on the scale of crystalline grains or as large dispairs magma transport is an effective means of heat transport magma transport at a rate of 0 15 cm yr is equivalent to a heat flow of 10 6 cal cm 2 s if magma encounters country rock witha lower melting point the original magma is likely to solidify while melting the country rock this would be an effective mechanism of purging silicic rocks and incomparable elements from the lower crust under some circumstances magma must penetrate up to 100 km or more of cold lithospheric rock in order for this magma to reach the surface without solidification a heated path must be provided the heating of this path requires the solidification of some magma it is estimated that magma penetrates the lithosphere in about 5000 years and that the crack is lined by several hundred meters of frozen basalt

sedimentologic and geochemical evidence for middle ordovician near trench volcanism in the central appalachian orogen the jonestown volcanic rocks are a minor but important component of the greenwich slice of the hamburg klippe of eastern pennsylvania sedimentary rocks of the greenwich slice are arranged in faultbounded coarsening upward sequences of pelagic deposits early to middle ordovician overlain by hemipelagic and turbidite deposits middle ordovician petrographic and geochemical characteristics of the sedimentary rocks indicate derivation from continental rather than volcanic sources the coarsening upward sequences of the greenwich slice are inferred to record advance of a site on oceanic lithosphere toward and ultimately into a trench by middle ordovician time rocks of the jonestown volcanic suite were extruded onto and intruded into the inferred middle ordovician near trench and trench axis mudstone and sandstone immobile element concentrations of the jonestown suite are typical of mid oceanic ridge basalts and along with the lack of associated shallow water and or volcaniclastic sedimentary rocks a

mineralogy geochemistry and sr nd isotopes of the cretaceous leucogranite from karamadazi kayseri central turkey implications for their sources and geological setting the leucogranite is the major constituent of the bimodal late cretaceous karamadazi granitoid developed in relation with evolution of the inner tauride ocean along the northern margin of the taurides in central turkey new analyses of minerals major and trace elements including rare earth elements ree and of sr and nd isotopes are performed to determine the origin and geochemical characteristics of the leucogranites medium coarse grained leucogranite contains normally zoned plagioclase an12 20 mildly alkaline biotite and xenocrystic magneziohornblende actinolite and ferrohornblende it is characterized by concave up ree patterns with respect to middle heavy ree field relations mineral chemistry geochemical data and isotopic data suggest that the leucogranite could have originated from an amphibole bearing igneous source in lower to middle crust by low rate partial melting 40 under low pressure and low h2o activity conditions possibly coupled by mixing mingling with mafic magma and high level feldspar and minor biotite fractionation in contrast the quartz diorite and mafic microgranular enclave mme are probably developed from an enriched mantle with possible mingling mixing mme quartz diorite and leucogranite may represent a magmatic suite which formed in an extensional tectonic regime by bimodal magmatic activity probably because of lithospheric delamination or slab break off or after the alpine thicken within the gondwanan tauride anatolide platform initial sr data exhibit an age of 65 13 ma for the leucogranite but it does not indicate a true intrusion age of the magma due to isotopic modification of the magma

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

the 860 ma cordilleran type guandaoshan dioritic pluton in the yangtze block sw china implications for the origin of neoproterozoic magmatism abstract the tectonic setting of the widespread neoproterozoic igneous rocks in south china has long been a matter of debate situated in the western margin of the yangtze block of south china the guandaoshan pluton is composed of diorite in the core and quartz diorite in the margin both types contain abundant mafic enclaves and mafic dikes this pluton is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 858 pm 7 mathrm ma end document by the shrimp zircon u pb method the rocks have sio2 contents of 52 8 63 8

detection of gas in sandstone reservoirs using avo analysis a 3 d seismic case history using the geostack technique the geostack technique is a method of analyzing seismic amplitude variation with offset avo information one of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces the fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line the mudrock line is the line on a crossplot of p wave velocity against s wave velocity on which water saturated sandstones shales and siltstones lie some of the rock types that lie off the mudrock line are gas saturated sandstones carbonates and igneous rocks in the absence of carbonates and igneous rocks high amplitude reflections on fluid factor traces would be expected to represent gas saturated sandstones of course this relationship does not apply exactly in nature and the extent to which the mudrock line model applies varies from area to area however it is a useful model in many basins of the world including the one

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

stress fields of the overriding plate at convergent margins and beneath active volcanic arcs tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales volcanic arcs are a common element of overriding plates stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc a process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

tectonic implications from sapphirine bearing lithologies south west grenville province canada sapphirine has been found in two types of magnesian metabasic lenses from tectonite zones within the central gneiss belt of the south west grenville province canada the first type association i comes from a lenticular mafic lens within highly tectonized anorthosite the second type association ii comes from meta eclogitic pods with foliated amphibolite rims in each case the sapphirine bearing assemblages record a wealth of reaction textures the primary mineralogy in association ii is represented by high alumina clinopyroxene garnet and kyanite plagioclase and records pressures of around 14 16 kbar in association i the primary mineralogy is represented by plagioclase two pyroxenes and possibly olivine but here the equilibrium pressure is unknown the host gneisses equilibrated at approximately 8 to 10 kbar and 700 750 c by continuous cation exchange reactions during and after the culmination of the grenvillian orogeny at 1 16 1 0 ga it is unlikely that the higher pressures recorded in the meta eclogitic pods represent an earlier high pressure metamorphism as the pods are restricted to shear zones a tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely sapphirine formed by discontinuous decompression reactions in association ii this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages at the same time primary clinopyroxene became much less aluminous by evolving plagioclase pressures and temperatures from coexisting phases that are believed to have equilibrated at the same time as sapphirine formation are estimated as 11 to 12 kbar and 750 c these probably represent the peak conditions for granulite facies metamorphism in the south west grenville province

petrogenesis of the late cretaceous demirkoy igneous complex in the nw turkey implications for magma genesis in the strandja zone abstract the srednogorie zone in bulgaria and its eastern continuation in nw turkey the strandja zone consist of a late cretaceous magmatic belt comprised of plutonic volcanic and associated volcano sedimentary rocks the demirkoy igneous complex is the biggest cretaceous pluton within the strandja zone and its age is between 71 and 84 ma the demirkoy pluton and additional smaller plutons are intruded into triassic and older metamorphic basement rocks a contact metamorphic aureole with hornblende hornfels facies conditions was developed around the demirkoy pluton the wnw trending demirkoy pluton has an elliptical shape and a concentrically zoned internal structure in this structure gabbro diorites and quartz diorites are only seen in the sw corner of the pluton while granodiorites form the main body of the pluton surrounded by a zone of granite the granodiorites contain dioritic mafic microgranular enclaves small quartz monzonite lenses and or bodies and mafic dykes are also observed in the demirkoy igneous complex geochemically the demirkoy igneous complex has calc alkaline metaluminous and medium k characteristics there is a significant correlation between the lree hree ratios and sio 2 values chondrite normalized ree patterns are relatively flat la n lu n 2 in gabbros this ratio ranges from 3 to 11 for the diorite granodiorites and from 14 to 35 for the granites dioritic enclaves have ree patterns similar to those of the main body and have la n lu n values between 3 and 6 relatively enriched lile sr k rb ba and th and relatively depleted hfse ta nb and ti values indicate the classic subduction related origin of the complex the initial isotopic signatures range from e nd i 0 45 to 2 57 and 87 sr 86 sr i 0 7042 0 7064 for gabbros and e nd i 0 43 to 2 67 and 87 sr 86 sr i 0 7048 0 7059 for granodiorites and granites 18 o values range from 6 18 to 7 80 vsmow for gabbros and from 6 86 to 8 89 vsmow for granodiorites and granites based on geochemical and isotopic characteristics this magmatic complex can be subdivided into 4 groups a gabbro b diorite to alkali granite main body c syenite monzonite and d mafic dykes geochemical data and modelling results favour a typical subduction related enriched lithospheric mantle and lower crustal source for the demirkoy igneous complex slab derived fluids and crustal contamination also played a key role in the petrogenesis of this magmatic suite

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

relation of lava compositions to volcano size and structure in el salvador abstract most of the lavas at the nine volcanic centers along the volcanic front of el salvador are basalts basaltic andesites and andesites the compositional variation within and among these centers can be explained by fractionation processes within the crust cognate gabbroic inclusions found in the lavas have appropriate mineralogy plagioclase olivine magnetite and augite to be cumulates formed by fractional crystallization two main variation trends occur depending on the proportion of plagioclase removal the more common or normal trend has a high 55 proportion of plagioclase being removed a less common al rich trend has a low 40 proportion of plagioclase being removed the al rich trend is found only at volcanoes that lack large negative bouguer gravity anomalies these volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited the incompatible element na 2 o k 2 o rb ba contents of lavas vary systematically with the volume of the volcanic centers at the same level of sio 2 large volcanic centers have higher incompatible element contents than small volcanic centers this suggests that open system fractionation in a periodically refilled chamber is the controlling factor the large difference in ba contents of lavas between eastern low and western high el salvador suggests a difference in the mantle source region

high sr y magmas reflect arc maturity high magmatic water content and porphyry cu mo au potential examples from the tethyan arcs of central and eastern iran and western pakistan the temporal and geochemical evolution of arc magmatism that culminated in porphyry cu mo au deposit formation has been studied in three separate neo tethyan arc systems in central and eastern iran and western pakistan porphyry cu au deposits in the lut block of eastern iran formed in the middle eocene at the end of a period of extensive paleocene eocene volcanism porphyry cu mo deposits in the kerman belt of central iran formed in the middle miocene at the end of a period of voluminous eocene oligocene volcanism and porphyry cu au deposits in the chagai belt of western pakistan formed in four pulses during the eocene early miocene middle late miocene and late miocene pliocene after a prolonged period of arc magmatism that began in the late cretaceous and is still active in each region the late porphyry related magmas are more geochemically evolved and more hydrous as evidenced by the presence of hornblende phenocrysts than the preceding volcanic rocks we suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years leading to the generation of more evolved volatile rich magmas at later stages of the arc s history high magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore forming subvolcanic magmatic hydrothermal systems it is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites later suites are more prospective noting the common presence of hornblende or biotite phenocrysts indicating high magmatic water contents and through lithogeochemical fingerprinting of magmatic fractionation processes relatively high sr y and la yb ratios and eun eu ratios 1 indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas

petrochemistry and petrology of i type granitoids in an arc setting the composite torul pluton eastern pontides ne turkey the upper cretaceous torul pluton located in the eastern pontides is of sub alkaline affinity and displays features typical of volcanic arc granitoids it is a composite pluton consisting of granodiorite biotite hornblende monzogranite quartz monzodiorite quartz monzonite and hornblende biotite monzogranite the oldest syenogranite 77 9 0 3 ma and the youngest quartz diorite form small stocks within the pluton samples from the granodiorites biotite hornblende monzogranites quartz monzodiorites quartz monzonites and hornblende biotite monzogranites have sio2 between 57 and 68 wt and display high k calc alkaline metaluminous to peraluminous characteristics chondrite normalized ree patterns are fractionated lacn lucn 6 0 14 2 with pronounced negative eu anomalies eu eu 0 59 0 84 initial nd i values vary between 3 1 and 4 1 initial 87sr 86sr values between 0 7058 and 0 7072 and 18o values between 4 4 and 7 3 the quartz diorites are characterized by relatively high mg number of 36 38 low contents of na2o 2 3 2 5 wt and sio2 52 55 wt and medium k calc alkaline metaluminous composition chondrite normalized ree patterns are relatively flat la yb cn 2 8 3 3 tb yb cn 1 2 and show small negative eu anomalies eu eu 0 74 0 76 compared to the other rock types radiogenic isotope signatures of the quartz diorites show higher 87sr 86sr 0 7075 0 7079 and lower nd i 4 5 to 5 3 the syenogranites have high sio2 70 74 wt and display high k calc alkaline peraluminous characteristics their ree patterns are characterized by higher lacn lucn 12 9 and eu eu 0 76 0 77 values compared to the quartz diorites isotopic signatures of these rocks nd i 4 0 to 3 3 87sr 86sr i 0 7034 0 7060 18 o 4 9 to 8 2 are largely similar to the other rock types but differ from that of the quartz diorites fractionation of plagioclase hornblende pyroxene and fe ti oxides played an important role in the evolution of torul granitoids the crystallization temperatures of the melts ranged from 800 to 900 c as determined from zircon and apatite saturation thermometry all these characteristics combined with low k2o na2o low al2o3 feot mgo tio2 and low na2o k2o feot mgo tio2 ratios suggest an origin through dehydration melting of mafic lower crustal source rocks

the influence of amphibole fractionation on the evolution of calc alkaline andesite and dacite tephra from the central aleutians alaska petrologic studies of tephra from kanaga adak and great sitkin islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc alkaline andesite and dacite magmas in the central aleutians amphibole is ubiquitous in tephra from kanaga and adak islands whereas it is present only in a basaltic andesite pumice from great sitkin dacitic tephra from great sitkin do not contain amphibole hornblende dacite tephra contain hb plag ox opx cpx phenocrysts with simple zoning patterns suggesting that the dacites evolved in isolated magma chambers andesitic tephra from adak contain two pyroxene and hornbelende populations and reversely zoned plagioclase indicating a more complex history involving mixing and fractional crystallization mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole bearing cumulate xenoliths both formed during the evolution of high al basalts the presence of amphibole in andesitic and dacitic tephra implies that aleutian cale alkaline magmas evolve in the mid to lower crust under hydrous 4 wt h2o and oxidizing ni nio conditions amphibole bearing andesites and pyroxene bearing dacites from great sitkin indicates fractionation at several levels within the arc crust despite its absence in many calc alkaline andesite and dacite lavas open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on adak island neither open nor closed system fractionation involving a pyroxene bearing assemblage is capable of explaining the trace element concentrations or ratios found in the adak suite we envision a scenario where amphibole was initially a liquidus phase in many calc alkaline magmas but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust the mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma whereas the trace element chemistry provides evidence for a earlier amphibole bearing mineral assemblage

simultaneous inversion for model geometry and elastic parameters both traveltimes and amplitudes in reflection seismology are used jointly in an inversion to simultaneously invert for the interface geometry and the elastic parameters at the reflectors the inverse problem has different physical dimensions in both data and model spaces practical approaches are proposed to tackle the dimensional difficulties in using the joint inversion which may properly take care of the structural effect one potentially improves the estimates of the subsurface elastic parameters in the traditional analysis of amplitude variation with offset avo analysis of the elastic parameters estimated using the ratio of s wave to p wave velocity contrasts and the deviation of this parameter from a normal background trend promises to have application in avo analysis the inversion method is demonstrated by application to real data from the north sea

a study of rare earth element ree sio 2 variations in felsic liquids generated by basalt fractionation and amphibolite melting a potential test for discriminating between the two different processes the origin of felsic magmas 63 sio2 in intra oceanic arc settings is still a matter of debate two very different processes are currently invoked to explain their origin these include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite because both fractionation and melting can lead to similar major element trace element and isotopic characteristics in felsic magmas such lines of evidence have been generally unsuccessful in discriminating between the two processes a commonly under appreciated aspect of rare earth element ree solid liquid partitioning behavior is that d ree for most common igneous minerals especially hornblende increase significantly with increasing liquid sio2 contents for some minerals e g hornblende and augite ree partitioning can change from incomptatible d 1 at high liquid sio2 when this behavior is incorporated into carefully constrained mass balance models for mafic basaltic amphibolite melting intermediate andesitic amphibolite melting lower or mid to upper crustal hornblende present basalt fractionation and mid to upper crustal hornblende absent basalt fractionation the following general predictions emerge for felsic magmas e g 63 to 76 sio2 partial melting of either mafic or intermediate amphibolite should regardless of the type of melting equilibrium fractional accumulated fractional yield ree abundances that remain essentially constant and then decrease or steadily decrease with increasing liquid sio2 content at high liquid sio2 contents lree abundances should be slightly enriched to slightly depleted i e c l c o 2 to 0 2 while hree abundances should be slightly depleted c l c o 1 to 0 2 lower crustal hornblende bearing basalt fractionation should yield roughly constant ree abundances with increasing liquid sio2 and exhibit only slight enrichment c l c o 1 2 mid to upper crustal hornblende bearing basalt fractionation should yield steadily increasing lree abundances but constant and then decreasing hree abundances at high liquid sio2 contents lree abundances may range from non enriched to highly enriched c l c o 1 to 5 while hree abundances are generally non enriched to only slightly enriched c l c o 1 to 2 hornblende absent basalt fractionation should yield steadily increasing ree abundances with increasing liquid sio2 contents at high sio2 contents both lree and hree are highly enriched c l c o 3 to 4 it is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra oceanic arc environments where continental crust is absent

depths and water content of magma chambers in the aleutian and mariana island arcs on pseudoternary diagrams locations of experimentally produced melt compositions that coexist with plagioclase and anhydrous mafic minerals vary as functions of pressure a geobarometer and water content a geohygrometer projection of rock compositions onto diagrams allows constraints to be placed on magma chamber depths and meltwater content application of the experimentally calibrated geobarometer and gehygrometer to aleutian and mariana rock suites suggests aleutian magma chambers to be at depths of 7 to 17 km pressures from 2 to 5 kbar and meltwater content of andesites near 2 wt mariana magma chambers are at shallow depths pressure less than 2 kbar and their water content is less than 2 wt probably near 1 wt magma chamber depths can be interpreted to be controlled by density differences between crystallizing magmas and crust which in turn may be controlled by crustal thermal structure differences in water content are correlated with amounts of sediment incorporated into basaltic magmas formed near the subducting oceanic slab

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

seismic amplitude inversion for interface geometry practical approach for application summary this paper presents a practical approach for the application of real seismic amplitude data in the context of reflection seismic tomography the estimation of recorded seismic amplitudes from reflection seismic gathers is performed with the aid of pre stack time migration which enhances continuity and reflection strength and reduces reflection point dispersal and diffraction effects moreover contraction of the fresnel zone by migration brings the amplitudes closer to the ray amplitudes assumed in the inversion de migration of the amplitudes follows so that we recover a set of true observations for input to inversion to make the amplitude inversion robust the effect of noise in the amplitude data is mitigated by applying iteratively a locally reweighted regression which can efficiently reject amplitude outliers this approach is applied to a reflection seismic profile from the north sea to constrain the geometry of a stack of interfaces by using both the amplitude inversion and a joint inversion with traveltime data the application example represents a valuable contribution to the discussion of how the combined effort of imaging and inversion of seismic data should be organized when we are dealing with field data

probing the evolving andean lithosphere mid late tertiary magmatism in chile 29 30 30 s over the modern zone of subhorizontal subduction in the mid late tertiary 27 6 m y ago volcanism diminished then ceased in the main andean cordillera in chile between 29 and 30 30 s over the same time span the subduction angle shallowed under this region which today lies in the modern shallowly dipping nonvolcanic segment of the south central andes 28 33 s the trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition volcanic changes include in addition to a decreased rate of volcanism a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks and an increase in crustal assimilation these volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space time pattern of deformation and volcanism in the foreland region although the model stands alone support for the tectonic setting and magmatic source region of these tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern andean segments the older dona ana formation 27 18 m y is similar to volcanic rocks in the southern volcanic zone while the younger cerro de las tortolas 14 10 m y and vallecito 6 m y formations are similar to volcanic rocks over regions of thicker crust in the central volcanic zone and the northern part of the southern volcanic zone

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

high water contents of magmas and extensive fluid exsolution during the formation of the yulong porphyry cu mo deposit eastern tibet abstract the yulong porphyry cu mo ore district in eastern tibet features a series of eocene felsic porphyry intrusions only one of which is intensively mineralized in this study zircon u pb ages and major and trace element compositions of whole rock samples and in situ accessory minerals zircon apatite and titanite from these various intrusions were analyzed to determine the factors controlling cu mineralization new zircon u pb dating together with published zircon u pb ages suggest that these porphyry intrusions were emplaced over a relatively short period of time 43 9 0 6 ma to 40 9 0 3 ma with the mineralized yulong intrusion emplaced at a late stage of the magmatic activity 41 1 0 3 ma to 40 9 0 3 ma these intrusions have indistinguishable whole rock major and trace element compositions but the mineralized yulong intrusion has higher apatite and titanite la yb ratios average apatite la yb 115 1 28 5 n 52 average titanite la yb 22 4 4 9 n 38 than the earlier subeconomic intrusions average apatite la yb 50 4 12 3 n 72 average titanite la yb 12 4 3 5 n 58 suggesting more fractionation of amphibole from the magmas that sourced it additionally a negative relationship between la yb and cen cen ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution indistinguishable zircon eun eun values and apatite core so3 contents from the mineralized zircon eun eun 0 69 0 06 n 25 apatite so3 0 65 0 24 wt n 21 and subeconomic intrusions zircon eun eun 0 62 0 05 n 64 apatite so3 0 61 0 27 wt n 15 suggest that the entire magmatic suite was relatively oxidized hydrous and sulfur rich and was therefore fertile for ore formation however apatite crystals from the mineralized yulong intrusion have significantly lower cl contents 0 08 0 03 wt n 72 and higher f cl ratios 47 80 21 97 n 72 than those from the subeconomic intrusions cl 0 29 0 29 wt f cl 12 87 5 16 n 78 these data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing yulong magmas sudden and voluminous release of volatiles coeval with emplacement of the yulong intrusion was triggered by an as yet unidentified process but possibly including magmatic recharge

symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front abstract the regional variation of physical and geochemical characteristics of central american volcanoes occurs in two fundamentally different patterns the first pattern is symmetrical about nicaragua crustal thickness silica contents of mafic lavas and volcanic edifice heights are lowest in nicaragua and increase smoothly toward costa rica to the south and guatemala to the north magma density is maximum in nicaragua and decreases smoothly outward the regional variation in crustal thickness is just enough so that magma densities calculated at appropriate moho pressures are the same at the base of the crust throughout the region this is consistent with magma ponding at the base of the crust the bulk compositions of central american basalts show the same symmetrical variation suites of nicaraguan basalts plotted in pseudo ternary cmas projections indicate large olivine and plagioclase primary phase volumes toward costa rica and guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller which is consistent with fractionation at increasing depth the second pattern is the segmentation of the volcanic front and the plate margin in general the segmentation strongly affects the spacing and size of volcanic centers at segment boundaries volcanic centers are generally small and unusually widely spaced toward segment interiors volcano spacing and size increase systematically the lil element contents of lavas strongly reflect this pattern for lavas with similar silica contents the larger the volcano the higher the lil element contents the relationships between segmentation volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab the relationship between volcano volume and lil element content is qualitatively in agreement with an open system fractionation model

the stability of sapphirine clinopyroxene implications for phase relations in the cao mgo al2o3 sio2 system under deep crustal and upper mantle conditions reaction textures and chemographic relations in sapphirine bearing basic granulites at finero italy suggest that sapphirine and aluminous diopside were formed in mutual equilibrium from an inferred early spinel bearing assemblage finero appears to be the only known locality where this association has been found in situ although it is also known from kimberlite and breccia pipe nodules elsewhere the reactions deduced to have occurred in these rocks suggest the existence of stable invariant points involving the phases sapphirine spinel orthopyroxene clinopyroxene garnet anorthite and sapphirine two pyroxenes garnetanorthite kyanite or sillimanite in the cmas end member system p t estimates for the relevant rocks and the available experimental data suggest that these points lie at around 800 900 c 9 11 kbar a semi quantitative petrogenetic grid incorporating these invariant points with previously determined univariant reactions is proposed it is inferred that sapphirine diopside are stable relative to spinel bearing assemblages below 900 c the relatively low temperature explains why sapphirine has not to date been reported from experimental work on the cmas system it also suggests that sapphirine may be an important aluminous phase in mg rich metagabbros under conditions corresponding to the base of the continental crust despite the observed rarity of such rocks at the surface

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s

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

magma mixing in the aleutian arc evidence from cognate inclusions and composite xenoliths abstract the complexity of igneous processes in the aleutian calc alkaline magma series can be inferred from study of xenolithic fragments composite xenoliths and cognate inclusions provide direct evidence for magma magma and wall rock magma mixing processes using distributions of cr in clinopyroxene compositional endmembers involved in mixing are identified within the xenoliths the basaltic mixing endmember is more mafic than calc alkaline lavas in the arc magma mixing and wall rock assimilation within calc alkaline basaltic to andesitic magmas is identified in phenocrystic assemblages as well as in xenoliths and appears to be a widespread phenomenon in aleutian calc alkaline magmas

the petrogenesis of the neoarchean kukuluma intrusive complex nw tanzania abstract the kukuluma intrusive complex kic is a late archean igneous complex dominated by monzonite and diorite with subordinated granodiorite the monzonite and the diorite suites have low silica content sio2 62 wt moderate mg mg average 49 high sr y sr yaverage 79 and high la yb la ybaverage 56 ratios and strongly fractionated lan ybn 9 69 ree patterns their moderate ni niaverage 50 ppm cr craverage 85 ppm variable cr ni ratio 0 65 3 56 and low tio2 tio2average 0 5 wt indicate little to no interaction with the peridotitic mantle for most major elements al2o3 feot na2o tio2 and p2o5 the monzonite and the diorite suites display subparallel trends for the same sio2 content indicating they represent distinct melts intrusions belonging to the diorite suite have high na2o na2oaverage 4 2 wt dy ybn dy ybn average 1 6 a positive sr anomaly and uncorrelated nb la and zr sm ratios suggesting derivation from partial melting of garnet bearing amphibolite intrusions belonging to the monzonite suite have higher na2o na2oaverage 5 61 wt dy ybn dy ybn average 2 21 a negative sr anomaly and correlated nb la and zr sm ratios consistent with derivation from partial melting of eclogite with residual rutile small variations in the th u ratio and near chondritic morb average values th umonzonite 3 65 th udiorite 2 92 are inconsistent with a subducting slab signature and it is proposed that the monzonite and the diorite suites of the kic formed by partial melting of garnet bearing lower mafic crust of an oceanic plateau the granodiorite suite has lower mg mg average 41 moderately fractionated ree low sr y sr yaverage 20 la yb la ybaverage 15 dy ybn dyybn average 1 24 and small negative eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation near morb th u th uaverage 2 92 and zr sm zr smaverage 30 21 ratios are consistent with intracrustal melting of amphibolite archean rocks with an adakitic geochemical signature have been used to argue in favour of a plate tectonics regime in the archean the results presented here suggest that tectonic models for the tanzania craton which invoke a subduction related setting for all greenstone belts may need revision

southward migration of volcanic activity in the central mexican volcanic belt asymmetric extension within a two layer crustal stretching model abstract magmatic activity in the miocene to present mexican volcanic belt mvb is migrating to the south volcanic activity is associated with extension which is being accommodated along a major 1000 km long e w oriented continental rift in turn the mvb is cross cut by older though active n s to nnw oriented major extensional faults along which the polygenetic volcanoes are aligned a southward migration pattern is also observed for the monogenetic volcanism which however appears to be related to e w and n60 e oriented extensional faults we suggest that the southward migration of volcanic activity can be explained in terms of a two layer crustal stretching model brittle and ductile domains the layers would be separated at an upper crustal level by a zone of simple shear decoupling at the brittle ductile transition zone bdtz the overall movement above the bdtz is southward directed the only direction to which the central mvb can extend and grow our model suggests that the magmas that feed the volcanism become stored at the bdtz evidence supporting this assumption is provided by the al in hornblende thermobarometer and the ol pl sior pseudoternary diagram which indicate average pressures between 2 5 and 4 kbar the magmas feeding the monogenetic volcanism ascend rapidly along active e w and n60 e extensional faults large strain rates i e they do not have enough time to form large magma chambers the magmas feeding the polygenetic volcanism are emplaced along n s to nnw faults lower strain rates these magmas remain stored for longer periods at the bdtz of the n s to nnw faults and therefore form large magmatic chambers shaping vertical overshoots of several kilometers of height the results from geobarometry indicating magma emplacement depth at around 8 km for the polygenetic volcanism and 12 km for the monogenetic volcanism are in good agreement with the rheological constraints of a bdtz at about 10 km of depth we envisage a feedback mechanism regarding magma storage and shallowing of the bdtz i e magma emplacement shallows the bdtz in turn this shallowing controls the new zones for magma emplacement a southward directed process

magmatic evolution of semisopochnoi island alaska trace element and isotopic constraints semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas sio 2 50 65 zr 64 250 ppm la 10 60x chondrites whose bulk rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite trace element data however are not consistent with crystal fractionation operating alone instead a variety of plots involving incompatible elements or ratios among them demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace element variations and must have been operative as well we are unable to distinguish between a silicic end member represented by residual dacitic liquid like one of the analyzed lavas therefore implying magma mixing and one consisting of pre existing silicic crustal rocks therefore implying assimilation variations in sr and nd isotopic compositions are very limited 143 nd 144 nd 0 51225 0

crystallographic preferred orientation magnetic and seismic anisotropy in rocks from the finero peridotite ivrea verbano zone northern italy interplay of anisotropy contributions from different minerals abstract mineral alignment can provide valuable information on a rock s geological history many structural tectonic or geodynamic studies rely on determining the preferred orientation of rock forming minerals either by direct texture determination or using anisotropy measurements as proxies robust interpretations of anisotropy require detailed understanding of its relationship with texture this study investigates the texture magnetic and seismic anisotropies of ultramafic rocks from the ivrea verbano zone ivz northern italy with a special focus on the interplay between anisotropy contributions carried by different minerals texture was obtained from electron backscatter diffraction magnetic anisotropy from low and high field anisotropy of susceptibility measurements and seismic anisotropy from p and s wave velocities along the three principal directions of the macroscopic fabric using 22 mm diameter cores texture based models of magnetic anisotropy agree well with corresponding measurements larger variability is observed for seismic anisotropy possible explanations are 1 velocities were measured in three directions insufficient to determine the full elasticity tensor and 2 seismic anisotropy depends on grain boundaries pores and cracks in addition to crystallographic preferred orientation the contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively leading to a larger overall anisotropy or negatively resulting in a weaker anisotropy seismic anisotropy is mostly controlled by olivine whereas magnetic anisotropy can be dominated by olivine pyroxene or hornblende this study illustrates that although the relationship between texture and anisotropy can be complex it is possible to quantitatively predict magnetic and to a lesser degree seismic anisotropy from texture data even though both magnetic and seismic anisotropies are common proxies for texture they are not coaxial and no simple relationship between the degree of magnetic and seismic anisotropy was found these results underline the importance of understanding different minerals contributions to anisotropy in upper mantle rocks

hybridization of mafic microgranular enclaves mineral and whole rock chemistry evidence from the karamadazi granitoid central turkey on the eastern tauride belt the cretaceous calc alkaline karamadazi granitoid consists of quartz diorite containing mafic microgranular enclaves mme and leucocratic granite the quartz diorite consists of plagioclase an8 65 hornblende biotite k feldspar quartz epidote and titanite subrounded mme in the quartz diorite are holocrystalline fine grained quartz diorite to diorite in composition and display a similar mineral assemblage to their host large crystals in mme and quartz diorite show various disequilibrium microstructures indicative of hybridization plagioclase crystals exhibit inverse normal and oscillatory zoning with maximum core to rim an content increase up to 38 in the enclave and 40 in the quartz diorite both hornblende and augite exhibit normal and reverse zoning even in the same sample the new field textural mineral compositional and geochemical evidence leads to the conclusion that mme could have formed through injection of successive pulses of basic magma into upward mobile magma chambers containing cooler partially crystalline quartz diorite magma the quartz diorites show similarity to high al ttg tonalites trondhjemites granodiorites with their high na2o sr lree and low mg cr hree contents and are suggested to be produced by extensive interaction between the crustal and mantle derived melts through mixing at depth in contrast leucogranites have geochemical characteristics distinct from the quartz diorites and mme and are probably not involved in mme genesis

bulk composition and mineral parageneses of sapphirine bearing rocks along a gabbro lherzolite contact at finero ivrea zone n italy sapphirine occurs in a 3 5 m wide zone between amphibole lherzolite and garnetiferous metagabbro at finero in the ivrea zone nw italian alps layers consisting of plag hb sa cpx opx sp gt are interbanded with spinel pyroxenites which may contain sapphirine replacing spinel all minerals are very magnesian with xmg between 0 78 and 0 92 bulk rock analyses suggest that precursors to the sapphirine bearing rocks were igneous cumulates of plagioclase olivine hornblende spinel up to 16wt cao does not inhibit sapphirine formation and it is the unusually mg rich nature of the host rocks which allows sapphirine development the early igneous assemblage was replaced by one of cpx sa hb plag at a pressure of 9 1 kbar and temperatures of 900 50 c subsequent rapid uplift caused the instability of gt gt hb hb and sa cpx to form opx plag sp sa symplectites

numerical simulations of crystallization processes of plagioclase in complex melts the origin of major and oscillatory zoning in plagioclase numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents the simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased and that the water content in the melt has little influence on this result comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees c can be rapidly imposed on plutonic phenocrysts large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals the observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates even at large undercooling there is no significant increase in temperature at the interface caused by the release of heat of crystallization

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

crustal melting by ponding of mafic magmas a numerical model abstract we examine the possibility that an abating period of mantle related volcanic activity at continental margins triggers the production of crustal derived melts a vertical zone permeated by rising mafic melts is established below the volcanic arc if long lasting mantle derived magmatism occurs this situation can be assumed to exist at continental arcs we propose that mafic magmas release heat and water into the crustal environment if subsequent magma ascent is stopped due to establishment of a density barrier this process enhances intracrustal partial melting using a finite element program we simulate in a 2d model area the time dependent change of the crustal thermal structure the spatial distribution of potential melting zones and their degree of partial melting within the crust is computed the amount of generated melt able to segregate is calculated and compared with volume estimates given by field investigation and geochemical constraints in a natural test case we chose the altiplano puna volcanic complex apvc which is part of the central andes as an example where the process may have happened and show from simple approximations that the model can explain the volumes of crustal melts represented by the felsic volcanics in this region although the model parameters and assumptions used are specific to this geologic context the proposed model is generally applicable to any arc setting

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

sapphirine kornenipine bearing rocks and crustal uplift history of the limpopo belt southern africa sapphirine kornerupine bearing rocks occur within the anorthosites of the messina layered intrusion in the limpopo mobile belt of zimbabwe the xmg range of the major minerals is as follows cordierite 0 98 0 93 enstatite 0 97 0 86 chlorite 0 98 0 92 phlogopite 0 98 0 90 sapphirine 0 98 0 86 kornerupine 0 94 0 88 gedrite 0 96 0 85 spinel 0 92 0 78 there are four rock types the constituent minerals of which have different x m g values which decrease in the above mineral order other minerals are corundum sillimanite and relict kyanite we recognise twenty reactions without phlogopite and nine reactions involving phlogopite the textural relations and the plots of the microprobe data of coexisting minerals in the mgo al2o3 sio2 h2o system are consistent with the following sequence of main reactions 1 enstatite corundum cordierite sapphirine 4 sapphirine sillimanite cordierite corundum 8 kornerupine corundum cordierite sapphirine 13 kornerupine cordierite sapphirine enstatite 15 enstatite spinel chlorite sapphirine 18 cordierite sapphirine chlorite corundum 20 sapphirine chlorite corundum spinel the early reactions are shown by coarse grained reaction intergrowths kornerupine and gedrite breakdown is shown by finer grained symplectites and the latest reactions by very fine grained products in micro fractures these selected reactions illustrate a remarkably steep trajectory from thept peak close to 10 kbar and 800 c to the minimum observable at 3 5 4 5 kbar and 700 c as indicated by the pure mash system very rapid uplift took place under nearly isothermal conditions the protolith of this material was possibly sedimentary derived from altered volcanic rocks the bulk composition is close to the composition of kornerupine or to a mixture of alunite chlorite and pyrophyllite these texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry the 1989 1990 eruption of redoubt volcano produced medium k calc alkaline andesite and dacite of limited compositional range 58 2 63 4 sio2 and entrained quenched andesitic inclusions 55 sio2 which bear chemical similarities to the rest of the ejecta the earliest december 15 magmas are pumiceous often compositionally banded and the majority is relatively mafic 59 sio2 the most silicic magmas of the eruption are the late december to early january domes up to 63 4 sio2 subsequent magmas formed domes and rare pumices which converge on 60 sio2 chemical variations among ejecta comprise tight linear two component arrays for all elements for which the analytical uncertainty is much less than the compositional range the two component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent additionally the felsic endmember cannot be easily related to the mafic endmember by normal high temperature igneous processes e g the silicic endmember has higher zr yet lower hf than the mafic endmember also relative enrichments of highly incompatible elements are dramatically different across the arrays the mixing event must have preceded eruption by a significant yet unspecified amount of time because groundmass glass compositions are homogeneous for all post december samples swanson et al 1994 this volume in spite of the whole rock chemical diversity this implies time for additional crystallization after the mixing event swanson et al 1994 this volume discuss evidence for a potentially different mixing event recorded only in december 15 magmas cognate cumulate xenoliths composed of pl cpx opx hb mt melt were recovered from january and april deposits these blocks differ from local batholithic country rock in their low concentrations of incompatible elements e g rb 60 wt they have mg cr ni sc and v contents higher than the andesites but lower than redoubt basalts and basaltic andesites thus they may be crystallization products of andesites but do not represent the cumulate residue of basalt fractionation the xenoliths were probably derived from a shallow or intermediate crustal chamber

experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts abstract the primary goal of this investigation was to derive a set of expressions that can be used to calculate the amphibole melt partitioning behavior of the rare earth elements ree and the high field strength elements hfse in natural systems to supplement the existing data set on basaltic systems we conducted experiments on systems where amphibole was in equilibrium with dacitic tonalitic and low si rhyolitic melts these experiments doped with la sm gd lu ta nb y zr and hf were run at pressures of 2 and 5 kbar temperatures between 900 c and 945 c and oxidation conditions ranging from qfm 1 to ninio 1 the partitioning data obtained in this study were combined with published data to calculate two sets of expressions describing trace element partitioning the first set models the partitioning of trace elements into amphibole using temperature pressure and several compositional parameters including the compositionally compensated partition coefficients of ti al caand sio2 and the exchange of fe and mg between the crystal and the melt dmg dfe the second set of expressions are slightly less precise but require no specific knowledge of p t or fo2 and for application to natural systems can be constructed solely on the basis of information available from standard electron microprobe analyses these expressions predict amphibole melt partition coefficients for ree and hfse within an internal precision of 14 40 relative for alkali basalt to low si rhyolite from 850 c to 1100 c 2 20 kbar and oxygen fugacity from qfm 1 to ninio 1 partition coefficients calculated from the expressions derived in this study were used to model the partial melting and fractional crystallization of a hypothetical amphibolite and hydrous melt respectively fractionation and or melting in amphibole bearing systems produces a magma with a convex upward ree pattern a characteristic common to many hornblende bearing dacites however the removal or addition of an amphibole component cannot produce the strong hfse depletion relative to the ree observed in many arc magmas

the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

pan african volcanism petrology and geochemistry of the dokhan volcanic suite in the northern nubian shield the late proterozoic dokhan volcanic suite 620 ma of the northern nubian shield is the product of late pan african volcanism the suite covers the entire spectrum from basalt to high silica rhyolite and occurs as two units a dark coloured unit containing basalt andesite dacite and a light coloured unit encompassing dacite rhyodacite rhyolite the latter unit is made up largely of ash flow tuffs and ign imbrites that are locally interstratified with basalt and andesite lava flows the suite forms a continuum in composition with a wide range of si0 2 48 77 wt cao 0 1 8 9 wt sr 81 906 ppm zr 85 340 ppm as well as most other elements and is moderately enriched in incompatible elements including rare earth elements ree the suite exhibits fractionated subparallel ree patterns that are similar overall to andean andesites and ignimbrites well defined major and trace element trends and fractionated ree profiles are consistent with a fractionated basalt to rhyolite calc alkaline magma series it is a typical calc alka line orogenic complex and exhibits mineralogical geochemical traits of arc related volcanism the suite neither resembles products of extensional nor transitional tectonic regimes as previously thought but was produced in a subduction related tectonic environment the mafic nature of the least evolved rocks of the suite along with its relatively low initial 87 sr 86 sr ratio 0 7039 are considered to indicate a mantle source a mantle derived basaltic magma fractionated with amphibole and plagioclase dominating the fractionating assemblage to produce the more felsic varieties as suggested by major and trace element fractionation modelling

joint pp and ps seismic inversion we present a case history of joint inversion of pp and ps reflection seismic data using a weighted stacking technique our example comes from blackfoot field owned and operated by pancanadian petroleum in southeastern alberta canada the exploration target at blackfoot is a lower cretaceous channel system approximately 1 4 km deep figure 1 figure 1 the glauconitic channel system at blackfoot oil field alberta is a sequence of sand and shale filled valleys incised into lower cretaceous and mississippian carbonates the blackfoot interpretation has an upper and lower channel that are prospective and separated by a nonporous lithic channel these glauconitic channels with sand or shale fill are found throughout the region and as there were many episodes of channel formation can be stacked on top of one another at blackfoot the channel interval is about 40 m thick and 100 m wide there tends to be good porosity in an upper channel and a lower channel that are separated by a tight lithic channel the upper channel where present is usually gas prone while the lower channel is generally oil prone when the pore fluid in the channel sands is a compressible hydrocarbon instead of incompressible water the bulk compressibility is reduced and this modifies the signature of seismic reflection data because pressure waves and shear waves sense different rock and pore fluid properties joint use of pp and ps data can provide superior lithologic discrimination the conversion of one elastic wave either p or s into another upon reflection or transmission at an interface is described by the zoeppritz equations these equations are algebraically quite complex and it is not practical to reproduce them here instead we will present useful concepts and approximate forms we invite the reader to visit our web site http www crewes org and interactively examine the equations using our

forty million years of magmatic evolution in the mariana arc the tephra glass record tephra glasses retrieved from 10 deep sea drilling project dsdp cores around the mariana arc system make up a remarkable record of explosive volcanism in the marianas over the past 40 m y major element compositions for approximately 1800 tephra glasses from basalt through rhyolite are reported and used to examine the nature and history of this activity three maxima of volcanic explosivity presumably related to especially vigorous volcanism in the mariana arc are identified with the biggest maximum at around 18 11 ma and two other maxima at 35 24 and 6 0 ma the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western pacific arcs explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception no boninitic glasses were found the tephra glasses belong to the low to medium k suite except during an enigmatic phase of medium to high k explosive volcanism during the late miocene 11 7 ma even though mariana tephra glasses are largely similar in composition to mariana arc lavas the tephra show a much larger compositional range than that found for subaerial mariana arc lavas which totally lack dacitic to rhyolitic volcanic products the tephra glasses define a bimodal population in terms of silica content with a pronounced minimum or daly gap around 65 66 sio2 mariana tephra glasses are fractionated with the least evolved glass being fe rich and having a magnesium number mg 100mg mg fe 2 of 55 basaltic tephra glasses contain 16 a1203 average 14 3 a1203 and are not high alumina basalts this contrasts with the observation that modern mariana arc lavas contain 15 21 average 17 4 a1203 the high alumina basalt lavas of the mariana arc probably reflect plagioclase accumulation and not liquid compositions all tephra glasses plot near low pressure cotectics and reaction curves on the subprojection olivine clinopyroxene quartz mafic and felsic samples define distinct trends the mafic trend reflects fractional crystallization of mantle derived basaltic magma whereas the felsic trend may be due either to anatexis of mariana arc crust or to fractionation of mafic melts the tephra glass data reinforce the model that the magmatic evolution of the mariana arc has been dominated by low pressure fractionation perhaps accompanied by anatexis episodic changes in melting regime to generate miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading these episodic changes are superimposed on a long term increase in potassium that reflects progressive metasomatism of the mantle source long term increases in k20 contents for mariana arc magmas inferred from the tephra glass record are 0 004 wt m y mafic 0 011 wt m y intermediate and 0 023 wt m y felsic

geochemistry and zircon u pb geochronology of aligoodarz granitoid complex sanandaj sirjan zone iran the aligoodarz granitoid complex agc is located in the sanandaj sirjan zone ssz western iran and consists of quartz diorites granodiorites and subordinate granites whole rock major and trace element data mostly define linear trends on harker diagrams suggesting a cogenetic origin of the different rock types 87 sr 86 sr i and endt ratios are in the ranges 0 7074 0 7110 and 3 56 to 5 50 respectively the trace elements and sr nd isotopic composition suggest that the granitoids from the agc are similar to crustal derived i type granitoids of continental arcs the whole rock suite was produced by assimilation and fractional crystallization starting from a melt with intermediate composition likely possessing a mantle component in situ zircon u pb data on the granites with la icp ms yield a crystallization age of 165 ma inherited grains spanning in age from 180 ma up to 2027 ma were also found and confirm that assimilation of country rock has occurred chemical and chronological data on the agc were compared with those available for other granitoid complexes of the central ssz e g dehno boroujerd and alvand the comparison reveals that in spite of the different origins that have been proposed all these granitoid complexes are likely genetically related they share many chemical features and are derived from crustal melts with minor differences alvand granites have the most peculiar compositions most likely related to the presence of abundant pelitic component all these intrusions are coeval and reveal the presence of an extensive magmatic activity in the central sector of the ssz during middle jurassic

inversion of reflection seismic amplitude data for interface geometry summary reflection seismic tomography using only traveltime data may be unable to resolve the ambiguity caused by trade off between reflector position and velocity anomaly the inclusion of amplitude data in the inversion may help to resolve this problem because the amplitudes and traveltimes are sensitive to different features of the model therefore providing us with more accurate information about underground structures and velocity distribution the amplitude of a reflected seismic wave is determined partly by the reflection coefficients and partly by the curvature of the reflector the latter causes the spherical divergence of the seismic rays to be modified at the reflection point focused or defocused and can be represented using a simplified analytical expression we show using geologically relevant synthetic models that the information contained in amplitude versus offset data here excluding traveltime data suffices to constrain accurately the geometry of an arbitrary 2 d reflector separating constant velocity layers the most effective inversion method is a subspace gradient algorithm using a model parametrization in which the interface is described as a discrete fourier series with fixed upper and lower bounds on the wavenumber model parameters are allocated to separate subspaces first on the basis of different physical dimensionality we also found that declaring separate subspaces for those parameters defining short intermediate and long wavelength components of the interface geometry based on the magnitude of singular values of the frechet derivative matrix is very effective in accelerating convergence and obtaining a more accurate solution the inversion is robust with respect to data errors and poor initial estimates

petrology and geochemistry of the huerto andesite san juan volcanic field colorado the huerto andesite is the largest of several andesite sequences interlayered with the large volume ash flow tuffs of the san juan volcanic field colorado stratigraphically this andesite is between the region s largest tuff the 27 8 ma 3 000 km3 fish canyon tuff and the evolved product of the fish canyon tuff the 27 4 ma 1 000 km3 carpenter ridge tuff and eruption was from vents located approximately 20 30 km southwest and southeast of calderas associated with these ashflow tuffs olivine phenocrysts are present in the more mafic sio2 poor samples of andesite hence the parent magma was most likely a mantle derived basaltic magma the bulk compositions of the olivine bearing andesites compared to those containing orthopyroxene phenocrysts suggest the phenocryst assemblage equilibrated at 2 5 kbar two pyroxene geothermometry yields equilibrium temperatures consistent with near peritectic magmas at 2 5 kbar fractionation of phenocryst phases olivine or orthopyroxene clinopyroxene plagioclase ti magnetite apatite can explain most major and trace element variations of the andesites although assimilation of some crustal material may explain abundances of some highly incompatible trace elements rb ba nb ta zr hf in the most evolved lavas despite the great distance of the san juan volcanic field from the inferred oligocene destructive margin the huerto andesite is similar to typical plate margin andesites both have relatively low abundances of nb and ta and similar values for trace element ratios such as la yb and la nb

geologic implications of great interplate earthquakes along the aleutian arc the aleutian arc is one of the most seismically active regions of the world it consists of a number of segments or blocks that are rotating clockwise and translating westward along the north american pacific plate boundary two of the largest recorded earthquakes have occurred along the interplate boundary south of the andreanof islands in 1957 and the rat islands in 1965 the large magnitudes of these earthquakes implied that the probability of the recurrence of a large magnitude event at these same locations was low however the occurrence of a great earthquake south of the andreanof islands in 1986 less than 30 years after the 1957 event has prompted a reevaluation of the seismic potential of the arc we present new marine geophysical observations and synthesize previous geologic interpretations of the aleutian arc to show that the epicenters of these great thrust type earthquakes coincide with upper plate segments of the arc characterized by a coherent forearc structural fabric these segments which are associated with large plutonic masses are mechanically stronger and are related to the location of asperities positioned at depth along the main thrust zone they appear to be more strongly attached to the north american plate than adjacent blocks the tectonically controlled locations of these asperities thus appear to influence the seismicity along the arc the andreanof and rat segments of the arc are separated by the delarof segment which is considered to have a high probability of a large magnitude earthquake in the near future based on probabilistic estimates of seismic potential nishenko 1991 the delarof segment of the arc however is highly disrupted by arc transverse faults this segment is structurally weak and does not appear to be capable of accumulating the interplate strain necessary to generate a great subduction zone earthquake we propose that variations in upper plate structural strength and mobility affect the mechanical properties of the interplate thrust zone and need to be considered in localizing interplate asperities forearc tectonic segmentation associated with the partitioning of strike slip and thrust motions may exert long term controls on the rates of seismic moment release

crustal xenoliths from calbuco volcano andean southern volcanic zone implications for crustal composition and magma crust interaction crustal xenoliths in the 1961 andesite flow of calbuco volcano in the southern southern volcanic zone ssvz of the andes consist predominantly of pyroxene granulites and hornblende gabbronorites the granulites contain plagioclase pyroxene magnetite amphibole and have pronounced granoblastic textures small amounts of relict amphibole surrounded by pyroxene plagioclase magnetite glass symplectites are found in some specimens these and similar textures in the gabbronorites are interpreted as evidence of dehydration melting mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism a moderate pressure high temperature stage accompanied by melting and melt extraction from some samples followed by thermal metamorphism after entrainment in the calbuco andesite lavas high nd t values 4 0 to 8 6 nd isotope model ages of 1 7 2 0 ga and trace element characteristics like chondrite normalized la yb and la nb 1 indicate that the protoliths were oceanic basalts similar oceanic metabasalts of greenschist to amphibolite facies are found in the paleozoic metamorphic belt that underlies the chilean coastal ranges mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate crystallizing from the basaltic andesite magma at calbuco crystallization pressures for the gabbros based on total al contents in amphibole are 6 8 kbar these pressures point to middle to lower crustal storage of the calbuco magma neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of calbuco andesites although an ancient sedimentary contaminant is indicated by the lava compositions the presence of oceanic metabasaltic xenoliths together with the sedimentary isotopic imprint suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt which is an accretionary complex of intercalated basalts and sediments that formed along the paleozoic gondwanan margin if this is the case the geochemical composition of the lower and middle crust beneath the ssvz is significantly different from that of most recent ssvz volcanic rocks

aleutian terranes from nd isotopes nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the aleutian island arc the oceanic terranes are exposed in the westernmost aleutians but to the east they are completely buried by isotopically distinct arc volcanic rocks analogous oceanic terranes may be important components of the terrane collages that comprise the continents

tholeiitic aleutian arc plutonism the finger bay pluton adak alaska the earliest representatives in the sequence of tertiary to recent magmatic rocks on adak island in the central aleutians are the finger bay volcanics and associated small shallow level gabbroic intrusives the tholeiitic finger bay pluton gabbro to quartz monzodiorite is among the least altered representatives of this earliest period the field relations mineralogy and geochemistry of the finger bay pluton contrast with the more recent calcalkaline plutons in the arc e g on adak hidden bay pluton 33 m y and on kagalaska kagalaska pluton 15 m y compared with the hidden bay pluton the finger bay pluton is smaller 8 km2 versus 100 km2 has a greater proportion of gabbro 84 versus 5 has a somewhat different mineralogy and has higher whole rock and mafic mineral feo mgo ratios higher k2o and higher concentrations of incompatable elements in rocks of equivalent sio2 particularly for the more siliceous units magmatic amphibole occurs only in the most siliceous units in the finger bay pluton whereas it is common in the calc alkaline plutons except for the size these differences parallel those between tholeiitic and calc alkaline volcanic rocks from the active aleutian volcanic centers textural miner alogical and trace element analyses indicate that mixing of magmas and gabbros generated some of the rocks of intermediate monzodiorite composition in the finger bay pluton most of the differences in magmatic trends between the calc alkaline and tholeiitic plutons are attributed to the physical conditions of magmatic evolution in the crust rather than to differences in initial magmatic types this conclusion rests on the similarity of fractionation independent isotope ratios and trace element ratios in aleutian magmas of all ages in particular compared to magmas of the north pacific ocean basin morb oceanic islands aleutian magmas show excess enrichment of ba k rb and u relative to ree and high 207pb 204pb ratios for a given 206pb 204pb ratio recognition of tholeiitic series plutons in the arc provides direct evidence for magmatic conditions accompanying fractionation and serves to emphasize the diversity of magmatic trends that are found over time and space at convergent plate margins

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

magmatism of the westernmost komandorsky segment of the aleutian island arc abstract four magmatic associations are distinguished in the westernmost komandorsky segment of the aleutian island arc basalt plagiorhyolite 45 28 ma trachybasalt teschenite 27 16 ma diorite granitoid 12 5 8 5 ma and dacite rhyolite 0 7 0 ma the latter association is represented by the lavas of a recently discovered active submarine volcano piip volcano about 40 km north of the komandorsky islands and 700 km west of the quaternary center on buldir is detailed petrogeochemical data including ree and 143 nd 144 nd and 87 sr 86 sr isotopic ratios show that the parental melts of the komandorsky is were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the aleutian arc 10be concentrations in lavas of the piip volcano 2 7 106 at g indicate subduction related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non solidified sedimentary material directly beneath the volcano the komandorsky segment of the aleutian arc evolved from a depleted arc tholeiitic series through a markedly alkaline series to later calc alkaline andesites and dacites and finally to late stage calc alkaline dacites and rhyodacites erupted inboard of the older volcanic centers

weighted stacking for rock property estimation and detection of gas amplitude versus offset concepts can be used to generate weighted stacking schemes here called geo stack which can be used in an otherwise standard seismic data processing sequence to display information about rock properties the zoeppritz equations can be simplified and several different approximations appear in the literature they describe the variation of p wave reflection coefficients with the angle of incidence of a p wave as a function of the p wave velocities the s wave velocities and the densities above and below an interface using a smooth representative interval velocity model from boreholes or velocity analyses and assuming no dip the angle of incidence can be found as a function of time and offset by iterative ray tracing in particular the angle of incidence can be computed for each sample in a normal moveout corrected cmp gather the approximated zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather and certain rock properties can be estimated the estimation of the rock properties is achieved by the application of time and offset variant weights to the data samples before stacking the properties which can be displayed by geo stack are p wave reflectivity or true zero offset reflectivity s wave reflectivity and the reflectivity of p wave velocity divided by s wave velocity or pseudo poisson s ratio reflectivity if assumptions are made about the relation between p wave velocity and s wave velocity for water bearing clastic silicate rocks then it is possible to create a display which highlights the presence of gas

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

geochemical characteristics of the kuh e dom intrusion urumieh dokhtar magmatic arc iran implications for source regions and magmatic evolution abstract the kuh e dom pluton is located along the central northeastern margin of the urumieh dokhtar magmatic arc spanning a wide range of compositions from felsic rocks including granite granodiorite and quartz monzonite through to intermediate mafic rocks comprising monzonite monzodiorite diorite monzogabbro and gabbro the urumieh dokhtar magmatic arc forms a distinct linear magmatic complex that is aligned parallel with the orogenic suture of the zagros fold thrust belt most samples display characteristics of metaluminous high k calc alkaline i type granitoids the initial isotopic signatures range from end 47 ma 4 77 to 5 89 and 87 sr 86 sr i 0 7069 to 0 7074 for felsic rocks and end 47 ma 3 04 to 4 06 and 87 sr 86 sr i 0 7063 to 0 7067 for intermediate to mafic rocks this geochemical and isotopic evidence support a mixed origin for the kuh e dom hybrid granitoid with a range of contributions of both the crust and mantle most probably by the interaction between lower crust and mantle derived magmas it is seem the felsic rocks incorporate about 56 74 lower crust derived magma and about 26 44 of the enriched mantle derived mafic magma in contrast 66 84 of the enriched mantle derived mafic magma incorporates 16 34 of lower crust derived magma to generate the intermediate mafic rocks according to the differences in chemical composition the felsic rocks contain a higher proportion of crustal material than the intermediate to mafic ones enrichment in liles and depletion in hfses with marked negative nb ba and ti anomalies are consistent with subduction related magmatism in an active continental margin arc environment this suggestion is consistent with the interpretation of the urumieh dokhtar magmatic arc as an active continental margin during subduction of the neotethys oceanic crust beneath the central iranian microcontinent

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

the meta igneous granulite xenoliths from kerguelen archipelago evidence of a continent nucleation in an oceanic setting xenoliths of meta igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline lavas of the kerguelen islands these are designated type ii xenoliths and are subdivided into three mineralogical groups subtype iia and iic xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust mantle boundary during the early tholeiitic transitional magmatic activity of the kerguelen islands younger magmatism became more alkaline and subtype iib xenoliths were formed as high pressure alkaline cumulates related to the last alkaline volcanic stage subsequently the plagioclase bearing type ii rocks have been re equilibrated under granulite facies conditions this addition of mafic material around the crust mantle boundary is consistent with seismic evidence for crustal thickening to 14 20 km calculated compressional seismic velocities vp for the basic granulites are consistent with the range of observed vp in the low velocity region underlying the oceanic crust such growth in the thickness of the oceanic crust may be caused by intrusion of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the oceanic setting this study suggests that basic granulites can account for the observed seismic characteristics of oceanic plateaux and can be important components of kerguelen oceanic lithosphere where there has been large scale magma production moreover we speculate that the kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process

the mafic ultramafic complex near finero ivrea verbano zone i chemistry of morb like magmas geochemical data are presented for the meta igneous mafic ultramafic complex near finero this complex is in contact with a phlogopite bearing mantle peridotite and is subdivided into the internal gabbro unit the amphibole peridotite unit and the external gabbro unit the internal gabbro and the amphibole peridotite units consist of coarse grained chemically heterogeneous cumulates whereas the external gabbro unit is generally massive chemically more uniform and approximately representative of the residual melt with mgo contents between 6 6 and 9 1 and mg numbers between 38 and 58 both whole rock and mineral contents of ni and cr are significantly higher at similar mg numbers in the amphibole peridotite unit than in the internal gabbro unit the most straightforward interpretation of this is that the amphibole peridotite unit accumulated after the influx of fresh mafic or ultramafic magma into the magma chamber major element chemical trends are continuous from the amphibole peridotite unit to the external gabbro unit and are consistent with closed system fractionation with no further addition of magma or contamination by wall or roof rock assimilation in the external gabbro unit total feo and tio2 contents are strongly correlated with each other and with p2o5 and zr and reach values as high as 19 and 4 respectively indicating an advanced degree of crystal fractionation along a tholeftic trend the external gabbro samples have generally smooth normalized trace element patterns which are consistent with being representative of a liquid composition the residual nature of the external gabbro magma is also indicated by negative eu and sr anomalies clear evidence for prior feldspar fractionation ree patterns are otherwise indistinguishable from n type morb but th and u are significantly more depleted than in morb this th and u depletion is similar to that found in olivine basalts and picrites on iceland and hawaii its origin is not well understood no evidence is seen for any assimilation of crystal material in sharp contrast with the situation of the igneous complex in val sesia near balmuccia where the magma composition is dominated by assimilation of crust we suggest that the heat provided by at most two injections of magma near finero was insufficient to induce crystal anatexis in contrast with the excess heat supplied by multiple magma injections at balmuccia

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

high ti amphibole as a petrogenetic indicator of magma chemistry evidence for mildly alkalic hybrid melts during evolution of variscan basic ultrabasic magmatism of central iberia central iberian variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous high k calc alkaline ultrabasic to intermediate rock series despite their overall calc alkaline affinity the mafic ultramafic members contain high ti amphibole oikocrysts rimmed by lower ti amphibole cummingtonite and high ti amphibole replacing early phlogopite to understand the factors controlling the saturation of high ti amphibole in the parental magmas clinopyroxene melt phlogopite melt and amphibole melt relationships are reviewed this analysis reveals that for melts with intermediate compositions the affinity of tio2 for amphibole rises in alkalic magmas accordingly mildly alkalic trachytoid to subalkaline medium to high k andesite and dacite compositions are estimated for interstitial high ti amphibole saturated melts amphibole ce pb ratios reveal a mantle crust hybrid nature for interstitial melts with subalkaline trachytoid compositions the hydrous character of the variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of na2o with respect to silica during amphibole crystallization

simultaneous inversion of pp and ps seismic data elastic parameters derived from seismic data are valuable input for reservoir characterization because they can be related to lithology and fluid content of the reservoir through empirical relationships the relationship between physical properties of rocks and fluids and p wave seismic data is nonunique this leads to large uncertainties in reservoir models derived from p wave seismic data because s waves do not propagate through fluids the combined use of p and s wave seismic data might increase our ability to derive fluid and lithology effects from seismic data reducing the uncertainty in reservoir characterization and thereby improving 3d reservoir model building we present a joint inversion method for pp and ps seismic data by solving approximated linear expressions of pp and ps reflection coefficients simultaneously using a least squares estimation algorithm the resulting system of equations is solved by singular value decomposition svd by combining the two independent measurements pp and

the cold bay volcanic center aleutian volcanic arc the cold bay volcanic center has experienced two major stages of eruptive activity early m series acitivity produced bimodal hi alumina basalt and calc alkaline andesite lavas while later fpk series activity produced only calc alkaline andesite the spectrum of basalt compositions is believed to be due to high pressure 8 kb fractionation at or near the base of the crust abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites inspection of the mineralogical data has shown that the earliest m series andesites were produced by mixing of basalt 53 wt sio2 and silicic andesite 60 5 to 62 5 wt while later fpk series andesites resulted from the mixing of basaltic andesite 53 to 56 wt and less silicic andesite 58 5 to 60 0 wt the major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions the complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and or assimilation role in the origin of the silicic endmembers many of the features of all andesites including the important long term convergence of endmember magma compositions are consistent with the process of liquid fractionation accompanied by large scale magma mixing a deduced upper limit of 62 5 wt sio2 for the silicic endmember magmas suggests that liquid fractionation in the absence of major crustal melting cannot produce more silicic magmas a possible explanation is the presence of a rheological barrier based on the concept of critical crystallinity marsh 1981 which prohibits more silicic liquids from being extracted from a crystal liquid suspension

experimental determination of ree partition coefficients between amphibole and basaltic to andesitic liquids at high pressure abstract amphibole liquid partition coefficients for the ree damph liqree obtained from natural rocks increase systematically with bulk rock compositional change from basalt to rhyolite and are higher for the middle to heavy ree five new experimentally determined sets of dree la sm eu2 ho lu and 4 published sets are similar to these data and provide values for use in geochemical modelling of magmatic processes involving amphibole over a range of temperature pressure and oxygen fugacity the partition coefficients increase significantly with decreasing temperature and increase slightly with increasing oxygen fugacity pressure does not appear to have a major effect although one data set suggests increased pressure lowers damph liqree in a basaltic composition

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content