petrogenesis of the laguna del maule volcanic complex chile 36 s from 33 42 s in central south chile there are numerous volcanoes which form part of the andean magmatic arc caused by subduction of the nazca plate beneath western south america the 0 3 m y old laguna del maule volcanic complex at 36 s is in a transition region between volcanoes at 33 34 s formed dominantly of hornblende bearing andesite and volcanoes south of 37 s dominantly composed of basalt and basaltic andesite the laguna del maule complex ranges in composition from basalt 0 3 m y to rhyolite post glacial although there is abundant evidence for magma mixing basalt and rhyolite have similar sr and nd isotopic ratios thereby requiring that the mixing members had the same isotopic ratios 87sr 86sr 0 70419 and 143nd 144nd 0 51274 in contrast dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite either by low pressure plagioclase dominated fractional crystallization or by partial melting of a plagioclase rich assemblage there is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics

the front speed of intrusions into a continuously stratified medium this paper examines intrusive boussinesq gravity currents propagating into a continuously stratified fluid we develop a model based on energy arguments to predict the front speed of such an intrusive gravity current from a lock release we find that the depth at which the intrusion occurs which corresponds to the level of neutral buoyancy i e the depth where the intrusion density equals the stratified fluid density affects the front speed the maximum speeds occur when the intrusion travels along the top and bottom boundaries and the minimum speed occurs at mid depth experiments and numerical simulations were conducted to compare to the theoretically predicted values and good agreement was found

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

intrusive gravity currents received 5 may 2006 and in revised form 1 august 2006 the speed of a fluid intrusion propagating along a sharp density interface is predicted using conservation of mass momentum and energy for the special case in which the intrusion density equals the depth weighted mean density of the upper and lower layers the theory of holyer huppert j fluid mech vol 100 1980 p 739 predicts that the intrusion occupies one half the total depth its speed is one half the interfacial long wave speed and the interface ahead of the intrusion remains undisturbed for all other intrusion densities the interface is deflected vertically by a long wave that travels ahead of the intrusion and thereby changes the local upstream conditions in these cases the conservation equations must be matched to an exact solution of the two layer shallow water equations which describe the spatial evolution of the nonlinear wave we obtain predictions for the intrusion speed that match closely with experiments and numerical simulations and with a global energy balance analysis by cheong keunen linden j fluid mech vol 552 2006 p 1 since the latter does not explicitly include the energetics of the upstream wave it is inferred that the energy carried by the wave is a small fraction of the intrusion energy however the new more detailed model also shows that the kinematic influence of the upstream wave in changing the level of the interface is a critical component of the flow that has previously been ignored

differentiation conditions of a basaltic magma from santorini and its bearing on the production of andesite in arc settings santorini volcano in the aegean region greece is characterized by andesitic to silicic dominated explosive activity and caldera forming eruptions sourced from magmatic reservoirs located at various structural levels beneath the volcano there is a good understanding of the silica rich magmatism of the island whereas the andesite dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood to fill this gap we have performed crystallization experiments on a representative basalt from santorini with the aim of determining the conditions of differentiation pressure temperature volatile fugacities and the parental magma relationship with the andesitic eruptive rocks experiments were carried out between 975 and 1040 c in the pressure range 100 400 mpa fo2 from qfm to nno 3 5 where qfm is quartz fayalite magnetite and nno is nickel nickel oxide with h2omelt contents varying from saturation to nominally dry conditions the results show that basalt phenocrysts within the basalt crystallized at around 1040 c in a magma storage reservoir located at a depth equivalent to 200 400 mpa pressure with 3 5 wt dissolved h2o and fo2 around qfm comparison with the xenocryst and phenocryst assemblages of the upper scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 c and 400 mpa following 60 80 wt crystallization of an ol cpx plag ti mag opx 6 pig ilm assemblage with melt water contents around 4 6 wt at santorini the andesitic low viscosity and water rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs eventually erupting upon mixing with resident magma changes in prevailing oxygen fugacity may control the tholeiitic calc alkaline character of santorini magmas explaining the compositional and mineralogical differences observed between the recent thyra and old eruptive products from akrotiri

south aegean volcanic arc geochemical variations and geotectonic implications the aegean volcanic arc is one of the most important geological structure of the mediterranean area it is a belt of volcanic centers consisting of products ranging from basaltic andesitic dacitic to rhyolitic in composition all of them displaying a typical calc alkaline chemical character

magma mixing at nisyros volcano as inferred from incompatible trace element systematics abstract rocks of the calc alkaline volcanic complex of nisyros at the eastern end of the quaternary south aegean volcanic arc range in composition from basaltic andesite to rhyodacite major and compatible trace element systematics are dominated by crystal fractionation involving olivine clinopyroxene plagioclase and magnetite in the mafic to intermediate compositional range and plagioclase clinopyroxene hypersthene fe ti oxides and apatite in the intermediate to silicic range although there is a marked paucity of lavas with 60 66 wt sio 2 the occurrence of andesitic inclusions in pre and post caldera dacites as well as reversely zoned and or partly resorbed phenocrysts exotic mineralogy e g forsteritic olivine compositionally bimodal plagioclase phenocryst populations and the observation of two distinct glass phases in melt inclusions and pumices all indicate that magma mixing processes have played a role in the petrogenesis of nisyros incompatible trace element systematics are consistent with mixing between mafic and silicic end members major element binary mixing calculations between end members successfully reproduce the dacite composition the eruptive sequence at nisyros suggests the existence of a compositionally zoned magma chamber in which mixing between underlying more mafic magma and overlying rhyodacitic melt result in the formation of dacitic magmas inherent mineral assemblages in basaltic andesites indicate that mixing between mafic liquids and between mafic and more silicic liquids played also a role in their petrogenesis

trace element geochemistry of high alumina basalt andesite dacite rhyodacite lavas of the main volcanic series of santorini volcano greece trace element systematics throughout the cal calkaline high alumina basalt basaltic andesite andesite dacite rhyodacite lavas and dyke rocks of the main volcanic series of santorini volcano greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the main series into two distinct crystallisation intervals an early basalt to andesite stage characterised by calcic plagioclase augite olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite hypersthene magnetite apatite crystallisation percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition subtraction diagrams and indicate that basaltic andesites represent 47 69 andesites 70 76 dacites ca 80 and rhyodacite ca 84 crystallisation of the initial basalt magma least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative main series lavas though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition subtraction techniques main series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine pyroxene cr spinel crystallisation on ascent through the sub aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the aegean crust residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of santorini the eruption of large volumes of basic lavas and silicic pyroclastics from santorini may have a volcanological rather than petrological explanation

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

evolution of santorini volcano dominated by episodic and rapid fluxes of melt from depth santorini volcano in greece was thought to be continually charged by small injections of magma measurements of surface deformation show that magma equivalent to 10 50 of that emitted in previous small eruptions has been injected beneath santorini since january 2011 implying that the volcano is instead charged by rapid episodic fluxes of melt

an adakitic pluton on kyushu island southwest japan arc abstract an adakitic pluton is designated the shiraishino granodiorite ca 15 km2 in outcrop area on kyushu island southwestern japanese arc sr y and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87sr 86sr material and subsequent fractional crystallization of plagioclase hornblende compiled sr and y data indicate that most cretaceous granitic rocks of southwestern japan generated by partial melting of mafic lower crust adakitic plutons derived by slab melting occur rarely and have older ages it is concluded that plutonism in the southwestern japanese arc during cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust

generation conditions of dacite and rhyodacite via the crystallization of an andesitic magma implications for the plumbing system at santorini greece and the origin of tholeiitic or calc alkaline differentiation trends in arc magmas andesitic arc volcanism is the most common type of subduction related magmatism on earth how these melts are generated and under which conditions they evolve towards silica rich liquids is still a matter of discussion we have performed crystallization experiments on a representative andesite sample from the upper scoriae 1 usc 1 eruption of santorini greece with the aim of understanding such processes experiments were performed between 1000 and 900 c in the pressure range 100 400 mpa at fo2 from qfm quartz fayalite magnetite to nno nickel nickel oxide 1 5 with h2omelt contents varying from saturation to nominally dry conditions the results show that the usc 1 andesitic magma was generated at 1000 c and 12 15 km depth 400 mpa migrated to shallower levels 8 km 200 mpa and intruded into a partially crystallized dacitic magma body the magma cooled to 975 c and generated the phenocryst assemblage and compositional zonations that characterize the products of this eruption an injection of basaltic magma problably subsequently triggered the eruption in addition to providing the pre eruptive conditions of the usc 1 magma our experiments also shed light on the generation conditions of silica rich magmas at santorini experimental of runs performed at fo2 nno 1 0 5 closely mimic the compositional evolution of magmas at santorini whereas those at reduced conditions qfm do not glasses from runs at 1000 975 c encompass the magma compositions of intermediate dominated eruptions whereas those at 950 900 c reproduce the silicic dominated eruptions altogether the comparison between our experimental results and natural data for major recent eruptions from santorini shows that different magma reservoirs located at different levels were involved during highly energetic events our results suggest that fractionation in deep reservoirs may give rise to magma series with a tholeiitic signature whereas at shallow levels calc alkaline trends are produced

isotope geochemistry of recent magmatism in the aegean arc sr nd hf and o isotopic ratios in the lavas of milos and santorini geodynamic implications in this comparative study of variations in the isotopic compositions sr nd o and hf of the calc alkaline magmas of the largest two volcanoes milos and santorini of the aegean arc eastern mediterranean we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano on santorini the crustal contamination processes have been limited effecting the magma gradually during its differentiation the most differentiated lavas rhyodacite and pumice are also the most contaminated on milos by contrast these processes are very extensive they are expressed in the143nd 144nd vs 87sr 86sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes in contrast with santorini the least differentiated lavas on milos are the most contaminated these isotopic singularities can be correlated with the geodynamic evolution of the aegean subduction zone consisting of alternating tectonic phases of distension and compression the genesis of rhyolitic magmas can be linked to the two phases of distension and the contamination of the calc alkaline mantle derived magmas with the intermediate compressive phase the isotopic characteristics of uncontaminated calc alkaline primitive magmas of milos and santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested such as in the aleutian sunda and lesser antilles island arcs however in spite of the importance of the sediment pile in the eastern mediterranen oceanic crust 6 10 km the contribution of the subducted terrigenous materials remains of limited amplitude

trace element composition of single glass shards in distal minoan tephra from sw turkey volcanic glass from the seventeenth century bc c 3600 bp minoan eruption of santorini thera has been recovered from a 4 cm thick tephra deposit at golhisar golu a small lake in sw turkey analysis of single grains of this glass by laser ablation inductively coupled plasma mass spectrometry la icp ms provides for the first time an accurate determination of the trace element composition of the juvenile glass component of this eruption free from contaminants such as phenocrysts or detrital material individual glass shards 150 m show a large degree of variation in their trace element composition indicative of substantial small scale compositional heterogeneity in the magma this is a result of the proximity of crystallizing phases and provides insights into the chemical evolution of the minoan magma the range of elemental compositions displayed by individual glass shards combined with the consistency of many trace element ratios are characteristics of the minoan glass these features provide a trace element signature for the minoan tephra which will be of value in stratigraphic correlation across the eastern mediterranean

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

continental crust melting induced by subduction initiation of the south tianshan ocean insight from the latest devonian granitic magmatism in the southern yili block nw china abstract the tianshan belt of the southwestern central asian orogenic belt was generated by paleozoic multi stage subduction and final closure of several extinct oceans including the south tianshan ocean between the kazakhstan yili and tarim blocks however the subduction initiation and polarity of the south tianshan ocean remain issues of highly debated this study presents new zircon u pb ages geochemical compositions and sr nd isotopes as well as zircon hf isotopic data of the latest devonian to early carboniferous granitic rocks in the wusun mountain of the yili paleozoic convergent margin which together with the spatial temporal distributions of regional magmatic rocks are applied to elucidate their petrogenesis and tectonic linkage to the northward subduction initiation of the south tianshan ocean our zircon u pb dating results reveal that these granites were emplaced at the time interval of 362 0 1 2 360 3 1 9 ma suggesting a marked partial melting event of the continental crust in the latest devonian to early carboniferous these granites based on their mineral compositions and textures can be categorized as monzogranites and k feldspar granites geochemically both monzogranites and k feldspar granites have characters of i type granites with high k 2 o contents 4 64 4 83 wt and the k feldspar granites are highly fractionated i type granites while the monzogranites have features of unfractionated i type granites whole rock sr nd isotopic modeling results suggest that ca 20 40 mantle derived magmas may be involved in magma mixing with continental crust partial melts to generate the parental magmas of the granites the mantle derived basaltic magmas was inferred not only to be a major component of magma mixture but also as an important heat source to fuse the continental crust in an extensional setting which is evidenced by the high zircon saturation temperatures 713 727 c and 760 782 c of the studied granites the latest devonian to early carboniferous extensional setting in the wusun mountain region of the yili paleozoic convergent margin is addressed by the subduction initiation of the south tianshan ocean and constituted a late paleozoic nascent arc back arc system in the southwestern caob

subduction related shoshonitic and ultrapotassic magmatism a study of siluro ordovician syenites from the scottish caledonides syenites are important or predominant components of several plutonic complexes emplaced between 456 and 415 ma along the nw margin of the caledonian orogenic belt adjacent to the lewisian foreland in w and nw scotland although there are in detail chemical differences between the syenites from each centre they form a well defined compositional group overall ratios amongst their trace elements especially very high values of la nb are quite different from those trachytes and syenites formed by fractional crystallisation of ocean island basalts and their continental equivalents emplaced in regions of anorogenic crustal tension instead the scottish caledonian syenites closely resemble chemically the fractional crystallisation residua of potassic subduction related magmas such as the shoshonitic series a comendite minor intrusion from a swarm associated with the loch borralan and loch ailsh syenitic complexes is remarkably similar in composition to recent obsidian from the shoshonitic volcano of lipari in the aeolian arc published sr and pb isotopic ratios preclude a significant component of either upper proterozoic moine schists or lower crust granulite facies archaean lewisian or proterozoic grenvillian gneisses in all these syenites except in local syenitic facies of the glenelg ratagain complex fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas the phases involved in this process may have included plagioclase alkali feldspar pyroxene amphibole biotite garnet fe ti oxide sphene allanite apatite zircon and zirconolite and therefore all the ratios amongst even the so called incompatible elements may have changed during the evolution of the leucocratic magmas nevertheless a detailed study of the glen dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas these appear to be picritic shoshonite mgo 15 ni 400ppm la yb 20 la nb 5 for the glen dessarry loch borralan and loch ailsh syenites and picritic ultrapotassic magmas mgo 15 ni 400 ppm la yb 60 la nb 7 such as would crystallise to minettes for the loch loyal and glenelg ratagain syenites mafic shoshonites were erupted amongst the 410 ma lome lavas of this region and also occur as widespread approximately contemporaneous volcanic feeder plugs minettes of similar age are also common as dykes in nw scotland numerous large tonalite granite complexes with minor diorites and gabbros were emplaced in w and nw scotland between 435 and 400 ma these are generally acknowledged to be mixtures of magmas from crustal and mantle sources their more mafic members show compositional features such as very high ba and sr which group them with the rare syenites and the lorne lavas of the same region into a distinctive geochemical province within which shoshonitic and allied magmas were the mantle derived component two models are presented to account for the generation of strongly potassic subduction related magmas several hundred km behind a nw directed subduction zone or during the period immediately following continental collision

a note on the structure of the head of an intrusive gravity current the head of an intrusive flow advancing along the interface between two fluids is studied experimentally when the two layers are of equal depth and the density of the intrusion is the mean of the two densities the dependence of the flow on the interface thickness and the depth of the intrusion is determined when the interface is very thin the flow is similar to the nominally inviscid gravity currents observed by britter simpson 1978

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

the record of magma chamber processes in plagioclase phenocrysts at thera volcano aegean volcanic arc greece lavas and pyroclastic rocks throughout the volcanic stratigraphy of the tertiary quaternary volcanic complex of thera in the aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures interpreted as indicative of magma mixing plagioclase zoning characteristics studied by nomarski and laser interferometry techniques establish three main categories of plagioclase i inherited plagioclase nucleated in endmember prior to initial mixing event ii in situ plagioclase nucleated in mixed or hybrid magma and iii xenocrystic plagioclase nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases depending on the composition of the lava in which they are hosted these differences reflect the contrasting effects of changes in physical chemical parameters in basic vis a vis more acidic melts during magma mixing and or influx of new magma into the subvolcanic magma chamber as well as the influence of magma chamber dynamics on plagioclase equilibration variations in bulk major and trace element abundances of thera volcanic products reflect the dominant overprint of crystal fractionation but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes consistent with compositional and textural zoning in plagioclases

lead isotopic compositions of neogene volcanic rocks from the aegean extensional area abstract lead isotope composition has been determined for a diverse suite of cenozoic igneous rocks in the central and north aegean sea these include voluminous miocene shoshonitic and calc alkali volcanic rocks and correlative plutonic phases middle miocene adakitic high mg andesite and associated rhyolite and middle miocene to quaternary ne normative basalt trachyte and k rich andesite the trace element contents of most of the studied rocks suggests that pb isotopic composition is not a result of upper crustal contamination but rather is a feature of the mantle source for k rich rocks this mantle source is represented by primitive mafic to intermediate volcanic rocks with 87 sr 86 sr 0 7095 high 207 pb 204 pb and 208 pb 204 pb 1000 ppm sr and ba and 35 ppm pb the high 207 pb 204 pb requires a metasomatic origin that lowered u pb and increased th pb followed by a second stage residence time of 1 ga this k rich magma was generated as a result of late tertiary extension of the aegean in the south aegean arc there is more evidence for crustal contamination either from present crust or from subduction of nile derived sediment

intrusive gravity currents in a stratified ambient shallow water theory and numerical results the intrusion of a fixed volume of fluid which is released from rest and then propagates horizontally at the neutral buoyancy level in a vertically stratified ambient fluid is investigated the density change is linear in a restricted layer or over the full depth of the container and locks of both rectangular and cylindrical shapes are considered a closed one layer shallow water inviscid formulation is used to obtain solutions of the initial value problem similarity solutions for the large time developed motion and an approximate box model are also presented the results are corroborated by numerical solutions of the full two dimensional navier stokes equations and comparisons with previously published experiments

petrogenesis of gympie group volcanics evidence for remnants of an early permian volcanic arc in eastern australia mafic ohenocryst rich basalt to basaltic andesite tuff breccias agglomerates and subordinate lavas together with sparsely quartz and plagiophyric dacitic tuffs and glassy flows comprise the tectonomorphically anomalous lower permian submarine volcanic sequence of the gympie group southeast queensland geochemical data imply three distinct crystallisation stages for the main volcanic series these are 1 initial olivine cr spinel clinopyroxene removal that caused rapid mgo cao ni and cr depletion in the ascending mantle derived magma 2 extensive clinopyroxene dominated olivine clinopyroxene plagioclase fractionation that gave rise to tholeiitic feo1 tio2 al2o3 p2o5 zr and y enrichment in the basalt basaltic andesite range and 3 plagioclase dominant augite magnetite apatite crystallisation that resulted in decreasing al2o3 tio2 feo1 p2o5 v nb and sr levels in the dacites identical degrees of rare earth element ree fractionation for the basalts and dacites suggest their comagmatic origin and preclude significant amphibole involvement in their petrogenesis low mg numbers mg1 ni cr ree and high field strength cation hfsc abundances in the gympie basalts along with low zry tiy and nbzr ratios are features typical of island arc tholeiites the gympie suite represents an immature submarine tholeiitic stage of development of a portion of a major intra oceanic arc that bordered gondwana but was fragmented by opening of the tasman sea the presence of coarse clinopyroxene phyric ankaramites together with highly differentiated dacites may relate to a phase of strike slip tectonics that exerted important control on magma supply and volumetric eruptive rates during the early permian evolution of the gondwana margin

a laboratory study of the velocity structure in an intrusive gravity current laboratory experiments were performed in which an intrusive gravity current was observed using shadowgraph and particle tracking methods the intrusion was generated in a two layer fluid with a sharp interface by mixing the fluid behind a vertical lock gate and then suddenly withdrawing the gate from the tank the purpose of the experiments was to determine the structure of the velocity field inside the intrusion and the stability characteristics of the interface soon after the removal of the lock gate the front of the intrusive gravity current travelled at a constant speed close to the value predicted by theory for an energy conserving gravity current the observed structure of the flow inside the intrusion can be divided into three regions at the front of the intrusion there is an energy conserving head region in which the fluid velocity is nearly uniform with speed equal to the front speed this is followed by a dissipative wake region in which large billows are present with their associated mixing and in which the fluid velocity is observed to be non uniform and have a maximum speed approximately 50 greater than the front speed behind the wake region is a tail region in which there is very little mixing and the velocity field is nearly uniform with a speed slightly faster than the front speed

geochemistry and evolution of the calc alkaline volcanic complex of santorini aegean sea greece abstract volcanic activity on santorini started in upper pliocene times and is still in progress the last eruption occurred in 1950 at least seven eruptive centers were active at various times and some centers were active contemporaneously the overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases together with variable amounts of magma mixing and assimilation comparison of the chemistry of volcanic products from different eruption centers indicates long term variations in magma chemistry progressive depletion in lil elements and enrichment in compatible elements with time that cannot be explained in terms of fractionation mixing and assimilation such variations are attributed to processes occurring within the upper mantle trace element data require that the primitive lavas erupted on santorini were derived from an upper mantle source which was relatively depleted in lil elements compared with other calc alkaline volcanics and it is proposed that the source region contained a dominant refractory component which had experienced earlier events and a component derived from the subducted slab as magmatism progressed the contribution to the source region from the slab decreased due to either 1 cessation of subduction and hence a decrease in the proportion of slab derived material in the source region 2 a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of low melting slab derived material or 3 a combination of these processes we conclude that it is possible to see through intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for are magmatism

intrusive gravity currents from finite length locks in a uniformly stratified fluid gravity currents intruding into a uniformly stratified ambient are examined in a series of finite volume full depth lock release laboratory experiments and in numerical simulations previous studies have focused on gravity currents which are denser than fluid at the bottom of the ambient or on symmetric cases in which the intrusion is the average of the ambient density here we vary the density of the intrusion between these two extremes after an initial adjustment the intrusions and the internal waves they generate travel at a constant speed for small departures from symmetry the intrusion speed depends weakly upon density relative to the ambient fluid density however the internal wave speed approximately doubles as the waves change from having a mode 2 structure when generated by symmetric intrusions to having a mode 1 structure when generated by intrusions propagating near the bottom in the latter circumstance the interactions between the intrusion and internal waves reflected from the lock end of the tank are sufficiently strong and so the intrusion stops propagating before reaching the end of the tank these observations are corroborated by the analysis of two dimensional numerical simulations of the experimental conditions these reveal a significant transfer of available potential energy to the ambient in asymmetric circumstances

geochemistry of the quaternary volcanic rocks of the northeast japan arc major and trace element data are presented for a series of lavas from 17 volcanic centres in the ne japan arc these represent a transect of the quaternary arc from its volcanic front type a volcanoes in the east across its central zone type b to its western margin type c rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase mafic silicates and titanomagnetite for comparative purposes trace element data for each volcano are presented on a 55 wt sio2 normalised basis the volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements rb sr ba k and depletion in high field strength elements zr nb hf ta using these data and derived primary magma compositions tatsumi et al 1983 mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt sio2 liquids these data together with published distribution coefficients allow the calculation of trace element abundances in the primary magmas reciprocal trace element plots indicate that irrespective of position with respect to the volcanic front the elements la ba k sr nb y and zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source however those melts close to the volcanic front type a show anomalous rb th and pb values suggesting that their mantle source was enriched in these elements further calculations suggest that the hfs depletion is not due to retention by residual mineral phases but is a feature of the source thus the data suggest a that the source of the arc volcanism was homogeneous and b that the characteristic chemical features were an inherent feature of the source since zr nb ratios in the source melts are close to those found in n type morb it is suggested that a major component of arc volcanic source rocks was a depleted mantle of the type which has given melts of n type morb composition lil elements added to such a depleted source are derived by the loss of silica rich aqueous fluids from the descending subducting slab such losses occur within the first 100 km and are fixed in the overlying mantle wedge this contaminated material is transported down by drag induced convection producing a homogeneous but metasomatised mantle within the melting zone diapiric uprise produced by small scale melting produces further melting with the major control of melt chemistry being the degree of partial melting superimposed on this chemistry is a localised addition of pb rb and th in the source of those volcanoes nearest the volcanic front

a model of reverse differentiation at dikii greben volcano kamchatka progressive basic magma vesiculation in a silicic magma chamber dikii greben volcano is the largest modern volcano with silicic rocks in the kurile kamchatka island arc it consists of many domes and lava flows of rhyodacite dacite and andesite which were erupted in a reverse differentiation sequence non equilibrium phenocryst assemblages quartz mg rich olivine an rich an poor plagioclase etc abundance of chilled mafic pillows in the dacites and andesites and linear variations of rock compositions in binary plots are considered as mineralogical textural and geochemical evidence for mixing mafic pillows in volcanics have a lower density because of high porosity and contain the same non equilibrium phenocryst assemblages as the host rocks their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water rich basaltic magma at depth they are considered as supercooled vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling the lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption the rhyodacitic and basic end members of the mixing series cannot be linked by low p fractionation though high p amphibole rich fractionation is not excluded

the front speed of intrusive gravity currents this paper describes an experimental and numerical study of an intrusion propagating along the interface of a two layer fluid in a channel we find that when the density of the intrusion is the depth weighted mean of the layer densities the interface ahead of the intrusion is undisturbed but for other densities the interface ahead of the intrusion is displaced vertically we find that this vertical displacement which takes the form of an upstream propagating long wave depends on the properties of the intrusion and not only on the relative depths of the two layers for the case when the interface is undisturbed the intrusion propagation speed is a minimum we develop an energy argument that describes the observed variation of the intrusion speed from this minimum speed as a function of the intrusion and layer densities and the ratio of the layer depths we also show that if and only if the layer depths are equal the speed of the intrusion is independent of the density of the intrusion

the relationship between adakitic calc alkaline volcanic rocks and ttgs implications for the tectonic setting of the karelian greenstone belts baltic shield abstract two types of coeval acid intermediate rocks with different petrological geochemical and isotopic features have been discovered among volcanic rocks and surrounding synkinematic tonalite trondhjemite granodiorite ttg plutons of late archaean greenstone belts in the karelian granite greenstone terrane type 1 rocks comprise trondhjemites and sub volcanic occasionally volcanic dacite rhyolite rocks they are characterized by high sr low y and hree contents high sr y ratios and strongly fractionated ree patterns with no significant positive or negative eu anomaly initial nd is positive indicating a generation from juvenile source with little or no contribution of ancient continental crust type 2 is represented by diorite granodiorites and calc alkaline basalt andesite dacite rhyolite badr series as compared to type 1 these rocks differ by their lower sr higher y and hree contents lower sr y ratios and less fractionated hree patterns with negative eu anomalies initial nd varies from negative to positive values thus indicating a variable contribution of sialic crust geochemistry of the two magmatic series suggests their formation in a convergent plate margin setting the type 1 rocks resemble phanerozoic adakites which represent slab derived melts contaminated by overlying mantle wedge the type 2 rocks resemble badr series which were derived from a mantle wedge metasomatized by slab derived fluids and melts with subsequent variable crustal contamination the spatial distribution of these two types of magmatic series defines the asymmetry of the studied granite greenstone structures which presumably reflects the primary lateral zoning of island arc formed under specific thermal conditions in the archaean mantle adakite melts upraised to the surface in the frontal part of the island arc where mantle wedge was thin showing no or little interaction with metasomatized mantle and formed adakite type plutonic and sub volcanic rocks at greater depths adakitic melts and fluids interacted with the overlying mantle wedge and caused its partial melting with generation of calc alkaline badr volcanic rocks and diorite granodiorite plutons in the rear part of the island arc our data suggest that greenstone belt volcano plutonic arcs were initiated on different types of crust which presumably determined the petrogenetic and isotope variations of the studied badr and adakite type island arc complexes

subduction and vertical coastal motions in the eastern mediterranean convergence in the eastern mediterranean of oceanic nubia with anatolia and the aegean is complex and poorly understood large volumes of sediment obscure the shallow structure of the subduction zone and since much of the convergence is accommodated aseismically there are limited earthquake data to constrain its kinematics we present new source models for recent earthquakes combining these with field observations published gps velocities and reflection seismic data to investigate faulting in three areas the florence rise sw turkey and the pliny and strabo trenches the depths and locations of earthquakes reveal the geometry of the subducting nubian plate ne of the florence rise a bathymetric high that is probably formed by deformation of sediment at the surface projection of the anatolia nubia subduction interface in sw turkey the presence of a strike slip shear zone has often been inferred despite an absence of strike slip earthquakes we show that the gps derived strain rate field is consistent with extension on the orthogonal systems of normal faults observed in the region and that strike slip faulting is not required to explain observed gps velocities further sw the pliny and strabo trenches are also often interpreted as strike slip shear zones but almost all nearby earthquakes have either reverse faulting or normal faulting focal mechanisms oblique convergence across the trenches may be accommodated either by a partitioned system of strike slip and reverse faults or by oblique slip on the aegean nubia subduction interface the observed late quaternary vertical motions of coastlines close to the subduction zone are influenced by the interplay between 1 thickening of the material overriding the subduction interface associated with convergence which promotes coastal uplift and 2 subsidence due to extension and associated crustal thinning long wavelength gravity data suggest that some of the observed topographic contrasts in the eastern mediterranean are supported by mantle convection however whether the convection is time dependent and whether its pattern moves relative to nubia are uncertain and its contribution to present day rates of vertical coastal motions is therefore hard to constrain the observed extension of the overriding material in the subduction system is probably partly related to buoyancy forces arising from topographic contrasts between the aegean anatolia and the mediterranean seafloor but the reasons for regional variations are less clear

lithology of the basement underlying the campi flegrei caldera volcanological and petrological constraints abstract a geologically reasonable working hypothesis is proposed for the lithology of the basement underlying the campi flegrei caldera in the ca 4 8 km depth range in most current geophysical modeling this portion of crust is interpreted as composed of meso cenozoic carbonate rocks underlain by a ca 1 km thick sill of partially molten rock thought to be a main magma reservoir shallower magma reservoirs likely occur in the 3 4 km depth range however the lack of carbonate lithics in any campi flegrei caldera volcanic rocks does not support the hypothesis of a limestone basement considering the major caldera forming eruptions which generated widespread and voluminous ignimbrites during late quaternary times including the campanian ignimbrite and neapolitan yellow tuff eruptions the total volume of trachytic to phonolitic ejected magma is conservatively estimated at not less than 350 km 3 results of least squared mass balance calculations suggest that this evolved magma formed through fractional crystallization from at least 2500 km 3 of parent shoshonitic magma in turn derived from even more voluminous more mafic k basaltic magma calculations suggest that shoshonitic magma likely emplaced at ca 8 km depth must have crystallized about 2100 km 3 of solid material dominated by alkali feldspar and plagioclase with a slightly lower amount of mafic minerals during its route toward shallower magma reservoirs before feeding the campi flegrei large volume eruptions the calculated volume of cumulate material likely syenitic in composition at least in its upper portions is more than enough to completely fill the basement volume in the 4 8 km depth range beneath the campi flegrei caldera estimated at ca 1250 km 3 thus it is proposed that the basement underlying the campi flegrei caldera below 4 km is composed mostly of crystalline igneous rocks as for many large calderas worldwide syenite sensu lato would meet physical properties requirements for geophysical data interpretations explain some geochemical and isotopic features of the past 15 ka volcanics and justify the carbon isotopic composition of fumaroles at the campi flegrei caldera this implies that meso cenozoic limestones if still present today beneath the campi flegrei caldera no longer constitute significant portions of its basement

a new offshore transport mechanism for shoreline released tracer induced by transient rip currents and stratification offshore transport from the shoreline across the inner shelf of early stage larvae and pathogens is poorly understood yet is critical for understanding larval fate and dilution of polluted shoreline water with a novel coupling of a transient rip current trc generating surf zone model and an ocean circulation model we show that transient rip currents ejected onto a stratified inner shelf induce a new previously unconsidered offshore transport pathway for incident waves and stratification typical for southern california in the fall this mechanism subducts surf zone origin tracers and transports them at least 800 m offshore at 1 2 km d analogous to subduction at ocean fronts this mechanism requires both trcs and stratification as trcs are ubiquitous and the inner shelf is often stratified this mechanism may have an important role in exporting early stage larvae pathogens or other tracers onto the shelf

neoproterozoic granitoids from the phan si pan belt northwest vietnam implication for the tectonic linkage between northwest vietnam and the yangtze block abstract several neoproterozoic granitic intrusions including the phin ngan intrusion and some smaller ones from the sin quyen cu mine have been identified in the phan si pan belt northwest vietnam whole rock geochemical and sm nd isotopic data and zircon u pb age and hf o isotopes have been determined for these intrusions in order to constrain their ages petrogenesis and tectonic implications the phin ngan intrusion mainly composed of syenogranite was emplaced at 824 4 ma rocks from this intrusion have high sio 2 72 1 73 5 wt and k 2 o 5 14 5 52 wt with a cnk values ranging from 1 02 to 1 06 they have negative whole rock e nd t 8 4 to 5 2 and zircon e hf t values 8 1 to 5 5 and high zircon 18 o values 9 7 10 9 these geochemical features suggest that the phin ngan intrusion was derived from ancient k rich crustal sources granitic intrusions from the sin quyen mine occur as stocks or dykes intruding ores and ore hosting rocks they are composed of monzogranite and granodiorite emplaced between 736 8 and 758 6 ma they have high sio 2 68 2 76 7 wt and k 2 o na 2 o 6 93 8 54 wt and low mgo 0 19 0 87 with a cnk values ranging from 0 91 to 1 14 their whole rock e nd t values range from 6 6 to 0 mostly clustered between 6 6 and 4 3 their zircon e hf t values vary from 6 7 to 11 1 mostly lower than 2 0 such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials coupled with subordinate involvement of mantle juvenile crust derived components the phin ngan intrusion and intrusions from the sin quyen mine are sub alkaline in nature and have arc like trace element compositions compiled with other broadly synchronous igneous rocks with arc like affinities in the region the neoproterozoic magmatism in the phan si pan belt was generated in a subduction related setting the neoproterozoic intrusions in the phan si pan belt are geochronologically and geochemically correlated with the neoproterozoic subduction related igneous rocks along the western yangtze block thus the phan si pan belt represents the southern most part of the giant arc system of the western yangtze block

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

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

rare earth element geochemistry of the island arc volcanic rocks of rabaul and talasea new britain the island arc volcanic rocks of rabaul and talasea new britain range in composition from basalt through rhyolite rare earth elements have been determined by mass spectrometric isotope dilution in 16 samples chondrite normalized rare earth element patterns are distinct for each volcanic center but all are relatively flat ce yb e f 1 1 to 3 0 within each center rare earth element concentrations increase from basalt to dacite and eu anomalies become progressively more negative from basalt to dacite eu eu 1 0 to 0 8 lavas inferred to contain cumulate phenocrysts show positive eu anomalies eu eu of up to 1 2 the observed variations are consistent with fractional crystallization of basalt or basaltic andesite by removal or accumulation of the observed phenocrysts including olivine plagioclase pyroxene and opaque minerals quantitative trace element models for talasea lavas indicate precipitation of 50 wt percent of phenocrysts from basalt to produce andesite and an additional 22 wt percent to produce dacite a total equilibrium model provides the best approximation to observed concentrations implying that the process took place at a depth sufficient to allow slow cooling of the magma as precipitation proceeded the least fractionated basalts and basaltic andesites are similar in rare earth element pattern to those of corresponding type suites in other oceanic island arcs implying a widespread uniformity of source and process in the production of each magma type

geochemistry of mafic lavas in the early rio grande rift yarmony mountain colorado u s a abstract mafic lavas high k basalts and shoshonites erupted 21 24 ma ago early miocene near state bridge nw colorado and now exposed on yarmony mountain were related to the initial phase of extension in the northernmost section of the rio grande rift the volcanism was of small volume consisting of eleven flows on yarmony mountain although much more voluminous contemporaneous lavas occur 50 km to the west all eleven flows have been analysed comprehensively for major and trace elements and five of the flows for nd and sr isotopes some samples experienced post eruptive carbonation and leaching of alkali elements the effects of these processes on normative compositions are examined and it is suggested that major element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid lower part of the crust most of the basalts are chemically closely related and have the following characteristics 1 they are depleted in nb and ta relative to lree a feature of volcanic arc magmas 2 they have lower abundances of rb and k and to a lesser extent ba and th for a given lree content than typical magmatic arc magmas and 3 they have nd and sr isotopic ratios similar to both certain low 143 nd 144 nd oceanic basalts notably from the kerguelen area indian ocean and subduction related magmas from mature volcanic arcs notably from the andes south america we argue that the magmas erupted at yarmony mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the cenozoic the relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction before that which generated the yarmony magmas an alternative is that the mantle source was depleted in alkali elements by a dehydration event possibly involving a flow of co2 one of the lavas in the yarmony sequence is elementally and isotopically distinctive having higher k zr ba k 2 o na 2 o la ta and k la and lower 87 sr 86 sr than the rest of the samples it is argued in detail that this is not the result of contamination by any reasonable crustal composition and that this magma contained a component 10 20 of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle there is no petrographic evidence for mixing and the hybridization probably took place in magma chambers situated in the mid lower crust

saline and particle driven interfacial intrusions this paper presents a theoretical and experimental investigation into saline and particle driven intrusions along the interface between two layers of dierent densities the conditions at the nose of an intrusion are described in an analysis similar to that applied by benjamin 1968 to boundary gravity currents equations for propagation velocity and front position as functions of relative density are derived these are used in an integral model for intrusions which also includes the eects of sedimentation of particles and detrainment of interstitial fluid the model describes the time evolution of the length of the intrusion and the sediment distribution it produces laboratory experiments were carried out with lock releases of a xed volume of saline or particleladen fluid into a two layer stratication measurements were taken of the intrusion propagation intrusion position and sediment distribution and are found to be in good agreement with the solutions of the integral model horizontal density gradients in a fluid may give rise to gravity currents sometimes called density currents the density gradients can be due to dierences in fluid composition phase temperature or concentration of suspended particulate in a suciently quiescent environment a sharp front may be formed across which a density dierence exists between the current and the surroundings this density dierence drives the mainly horizontal motion of the gravity current when the surrounding fluid is vertically stratied the gravity current can enter it at intermediate levels where its density is comparable with that of the ambient and is then called a gravity intrusion gravity intrusions occur frequently in the atmosphere when rising buoyant air reaches levels where its density matches the ambient density and spreads out horizontally a striking example is the outflow above volcanic plumes which carry ash high into the atmosphere and then spread laterally over many thousands of kilometres gravity intrusions are also an important phenomenon in oceanography a much quoted example is the warm but saline outflow from the mediterranean in the eastern atlantic ocean which produces a saline intrusion at about 1000 m depth particle driven intrusions in the ocean can occur when material is suspended by a river outflow by an underwater landslide from a shelf edge or even articially

pulse propagation in turbidity currents submarine turbidity currents are a key mechanism in the transportation of clastic sediments to deep seas such currents may initiate with a complex longitudinal flow structure comprising flow pulses for example by being sourced from retrogressive sea floor slope failures or acquire such structure during run out for example following flow combination downstream of confluences a key question is how far along channel pathway complex flow structure is preserved within turbidity currents as they run out and thus if flow initiation mechanism and proximity to source may be inferred from the vertical structure of their deposits to address this question physical modelling of saline flows has been conducted to investigate the dynamics of single pulsed versus multi pulsed density driven currents the data suggest that under most circumstances individual pulses within a multi pulsed flow must merge therefore initiation signatures will only be preserved in deposits upstream of the merging point and may be distorted approaching it downstream of the merging point all initiation signals will be lost this new understanding of merging phenomenon within multi pulsed gravity currents broadens our ability to interpret multi pulsed turbidites

magma cumulate mixing identified by u th disequilibrium dating magma mixing is believed to be important in magmagenesis1 particularly in volcanic arcs2 where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4 nevertheless isotope studies particularly of uranium series dis equilibria have concentrated on the analysis of bulk rock samples5 9 comprehensive u th mineral separate data from arc volcanics are scarce10 14 here we present the first u th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of santorini in the aegean arc the lava contains crystal populations from two sources of distinct thorium isotope composition one from a basic cumulate the other phenocrysts from a dacite magma the crystal populations have statistically indistinguishable crystallization ages of 79 14 12kyr and 93 29 22kyr and are thought to have physically intermingled just before eruption the crystals did not re equilibrate and hence retain the isotopic compositions and ages of the parental materials

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development