the petrogenesis of the lac guyer komatiites and basalts and the nature of the komatiite komatiitic basalt compositional gap a sequence of ultramafic rocks in the lac guyer archean greenstone belt exhibit brecciated flow tops pillow structures and spinifex textures testifying to their volcanic origin massive spinifex textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite with mgo ranging from 19 25 wt associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with mgo contents ranging from 16 to 19 wt mgo and are best termed pyroxenitic komatiites unlike other komatiitic occurrences the peridotitic and pyroxenitic komatiites at lac guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt mgo the contrasting behaviour of highly compatible elements such as ni and cr versus incompatible elements such as zr indicate that this compositional spectrum was produced by a variation in the extent of partial melting 10 40 of a garnet lherzolite source in the archean mantle the pyroxenitic komatiites represent liquids produced during lower 10 20 degrees of melting during which garnet remained in the mantle residue however a change in slope in the distribution of zr vs y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites the lac guyer volcanic rocks display a population minimum at 15 wt mgo separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation the population minimum near 18 wt mgo which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin

chronology and mechanism of depletion in lewisian granulites the pattern of th u and rb depletion has been investigated in a suite of contrasting lithologies from the classic lewisian granulite terrain at scourie n w scotland the study is based on lithologies ranging from ultramafic to tonalitic gneisses together with calcsilicate and pelitic metasediments sampled on the metre scale because of their close proximity all of these lithologies have experienced identical p t t histories since depletion however their expected dehydration and melting characteristics are markedly different pb isotope analyses of the samples define the latest time of u pb fractionation as 2 665 0 026 ga and indicate substantial u and th depletion relative to pb in tonalitic and mafic lithologies and in some metasediments in contrast ultramafic samples show no u and th depletion sm nd isotope analyses of the mafic and ultramafic samples define an age of 2 707 0 052 ga which is interpreted as the time of igneous differentiation of the mafic ultramafic bodies this time is indistinguishable from the latest time of th pb and u pb fractionation overall there is a straightforward relationship between the magnitude of depletion lithology and the stability of mineral phases tonalitic and mafic gneisses are depleted in th and u relative to pb whereas ultramafic gneisses which retain amphibole show no evidence for depletion the observations are consistent with loss of u th and rb in fluids produced by metamorphic dehydration and melting a role for externally derived co2 in the depletion process is not excluded by the data but there is no evidence for it having existed limits are placed on the length scale of transport for several elements u th and rb are depleted regionally from mafic and tonalitic lithologies transport distances must have been on the kilometre scale in contrast movements of sm and nd have been more restricted

sm nd evidence for diachronous crustal accretion in the lewisian complex of northwest scotland sm nd whole rock analyses of several cogenetic rock suites from the late archaean lewisian complex of northwest scotland yield chronological data for a crustal accretion differentiation superevent cads in this terrane three layered mafic ultramafic bodies predate badcallian high grade metamorphism 2660 ma but their temporal relationship to the igneous precursors of the host quartzofeldspathic gneiss complex remains unclear regression of data from these samples yield the following achiltibuie 2850 95 ma with nd t 1 93 0 52 drumbeg 2910 55 ma with nd t 1 70 0 32 scouriemore 2670 110 ma with nd t 0 98 0 73 these data suggest that contemporaneous sub lewisian mantle had a long term light rare earth element depleted history relative to a chondritic reservoir composition three suites of quartzofeldspathic gneisses the pyroxene granulites of scourie the hornblende granulites of gruinard bay and the amphibolites of the northern region north of loch laxford have small ranges of sm nd ratios and do not yield precise regression results if these gneisses were derived from a relatively homogeneous source region perhaps represented by the mafic ultramafic bodies then sm nd model ages suggest a diachronous sequence of crustal generation decreasing in age from scourie terrane average tdm 2930 ma through gruinard bay 2860 ma to the northern region 2780 ma a diachronous sequence such as this suggests that for a cads in which separate accretion and differentiation episodes may be resolved final high grade metamorphism may not be related to magmatic crustal generation processes

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

mineralogical changes occurring during the retrogression of archaean gneisses from the lewisian complex of nw scotland abstract gneisses metamorphosed at granulite facies ca 2 7 ga were subsequently retrogressed to amphibolite facies during a prolonged period of retrogression perhaps lasting as long as 200 m y the scourie dykes were emplaced towards the end of this event localised laxfordian shear zones further modified the mineral assemblages the retrogression caused the production of a uniform plagioclase hornblende quartz biotite assemblage a study of hornblende composition shows that it depends on metamorphic grade host rock composition and paragenesis the sequence of mineral assemblages suggests that retrogression took place on a falling temperature path beginning at about 650 50 c post tectonic muscovite indicates that temperatures were still in excess of 500 c after the formation of laxfordian shear zones this indicates that the lewisian complex was uplifted and cooled extremely slowly

is there a link between faulting and magmatism in the south central aegean sea a distinct spatial relationship between surface faulting magmatic intrusions and volcanic activity exists in the aegean continental crust in this paper we provide detailed structural observations from key onshore areas as well as compilations of lineament maps and earthquake locations with focal plane solutions from offshore areas to support such a relationship although pluton emplacement was associated with low angle extensional detachments the nne to ne trending strike slip faults also played an important role in localizing the middle miocene plutonism providing ready pathways to deeper magma batches and controlling the late stage emplacement and deformation of granites in the upper crust additionally the linear arrangements of volcanic centres from the quaternary volcanoes along the active south aegean volcanic arc are controlled primarily by ne trending faults and secondarily by nw trending faults these volcanic features are located at several extensional settings which are associated with the main ne trending faults such as i in the extensional steps or relay zones between strike slip and oblique normal fault segments ii at the overlap zones between oblique normal faults associated with an extensional strike slip duplex and iii at the tip zone of a ne trending divergent dextral strike slip zone the ne trend of volcano tectonic features such as volcanic cone alignments concentration of eruptive centres hydrothermal activity and fractures indicates the significant role of tectonics in controlling fluid and magma pathways in the aegean upper crust furthermore microseismicity and focal mechanisms of earthquakes in the area confirm the activity and present kinematics of these ne trending faults

water rock interaction in the magmatic hydrothermal system of nisyros island greece abstract in this work we investigated the water rock interaction processes taking place in the hydrothermal reservoir of nisyros through both 1 a review of the hydrothermal mineralogy encountered in the deep geothermal borehole nisyros 2 and 2 a comparison of the analytically derived redox potentials and acidities of fumarolic related liquids with those controlled by redox buffers and ph buffers involving hydrothermal mineral phases the propylitic zone met in the deep geothermal borehole nisyros 2 from 950 to 1547 m total depth is characterised by abundant well crystallised epidote adularia albite quartz pyrite chlorite and sericite muscovite accompanied by less abundant anhydrite stilpnomelane wairakite garnet tremolite and pyroxene these hydrothermal minerals were produced in a comparatively wide temperature range from 230 to 300 c approximately hydrothermal assemblages are well developed from 950 to 1360 m whereas they are less developed below this depth probably due to low permeability based on the r h values calculated for fumarolic gases and for the deep geothermal fluids of nisyros 1 and nisyros 2 wells redox equilibrium with the feo feo 1 5 rock buffer appears to be closely attained throughout the hydrothermal reservoir of nisyros this conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite since r h values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation the ph of the liquids feeding the fumarolic vents of stephanos and polybote micros craters was computed by means of the eq3 code based on the cl d relationship which is constrained by the seawater magmatic water mixing occurring at depth in the hydrothermal magmatic system of nisyros the temperature dependence of analytically derived ph values for the reservoir liquids feeding the fumarolic vents of stephanos and polybote micros craters suggests that some unspecified ph buffer fixes the acidity of these reservoir liquids at values of 4 72 4 85 and 4 88 5 23 respectively many of these ph values are lower than those expected for the full equilibrium condition although they are close to those of the reservoir liquids of nisyros 1 5 16 and nisyros 2 4 87 it is likely that this excess of acidity producing species chiefly co 2 promotes release of fe ii and fe iii to the reservoir liquids through rock dissolution permitting the attainment of redox equilibrium with the feo feo 1 5 rock buffer as already suggested by the late werner giggenbach

pre eruption history of phyric basalts from dsdp legs 45 and 46 evidence from morphology and zoning patterns in plagioclase phyric basalts recovered from dsdp legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains megacrysts or aggregates glomerocrysts and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition almost all the plagioclase crystals are complexly zoned in most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts core mantle and rim the cores exhibit two major types of morphology tabular with a euhedral to slightly rounded outline or a skeletal inner core wrapped by a slightly rounded homogeneous outer core the mantle region is characterized by a zoning pattern composed of one to several spikes plateaus superimposed on a gently zoned base line with one large plateau always at the outside of the mantle and by in most cases a rounded internal morphology the inner rim is typically oscillatory zoned the width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow the presence of a skeletal inner core and the concentration of glass inclusions in low an zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount the resorption features at the outer margins of low an zones indicate superheating of the liquid with respect to the crystal

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

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

seismic properties in the eastern part of the south aegean volcanic arc the south aegean active volcanic arc lies along the 150 km seismic isodepth of a benioffzone and consists in andesitic dacitic and rhyolitic volcanoes of an orogenic calc alkaline type in the eastern part of the arc there are two main volcanic sites in the nisyros and kos islands high shallow and intermediate depth seismic activity occurs in this volcanic area seismological data concerning the 1911 1980 period have been used to investigate seismic properties in that area two distinct regions with different seismotectonic features have been defined the internal region nisyros active volcano and its proximity is characterized by a shallow thin seismogenetic layer with abnormally highb value locally concentrated stresses low seismicity and highly heterogeneous structure these features are probably due to a magmatic body intruded in shallow depths within the crust the possibility of magma formation in the upper surface of the descending slab or within the upper mantle wedge overlying this slab in the nisyros kos area is herein discussed on the contrary the remaining external region including the non active volcanic island of kos is characterized by normal for tectonic shocksb value thick seismogenetic layer and high seismicity it seems that in this region there is not a magmatic intrusion at least in shallow depths within the crust

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

paleomagnetic stratigraphy and radiometric dating of the pliocene volcanic rocks of aegina greece the island of aegma includes some of the oldest volcanic rocks in the south hellenic arc previous radiometric dates range from 3 87 to 4 4 ma the volcanic sequence is divided into nine units on the basis of field relations petrography and geochemistry and the characteristic paleomagnetic polarity of each unit has been determined two new radiometric dates 2 1 and 3 9 ma show that the volcanism spans the pliocene a detailed chronology is proposed using the paleomagnetic data which places the most voluminous volcanism in the mid pliocene

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

physical properties and state of the crust and upper mantle of the eastern mediterranean sea deduced from geophysical data abstract by evaluating the geophysical information available for the eastern mediterranean sea ems a crustal thickness map and some conclusions as to the nature of the crustal type were drawn it was found that the ionian abyssal plain is floored by an oceanic type of crust which is 14 km thick and covered by sediments of the order of 8 km thick the herodotus abyssal plain in the levantine sea is floored by a crust 22 km thick and covered by sediments ranging between 10 and 15 km thick the eastern mediterranean rise extending more or less continuously from the apulian plateau to rhodes and the south of the antalya basin is of a thickness ranging between 28 and 34 km the thickness of its sedimentary cover is of the order of 10 km the borders of the eastern mediterranean sea are built up of thick continental crust for example the hellenides of western greece range between 40 and 44 km thick and western turkey has a similar crustal thickness the crust of sicily also exceeds 40 km whereas the southern and eastern borders of the ems are approximately 30 32 km thick tectonically the northern border of the ems is built up of active continental margins defined by the calabrian hellenic and cyprus antalya arcs whereas the southern border represents a passive continental margin controlled mainly by subsidence and stretching to the east the dead sea megashear limits the area the deep basins of the eastern mediterranean sea are tectonically controlled by subsidence and partial stretching the mediterranean ridge or rise is still an area of dispute regarded by some authors as a subsiding continental fragment and separated from the hellenic arc by the hellenic trench whereas others consider it to be a zone of crustal shortening thickened mainly by thrusting both the western and southwestern borders of the mediterranean rise and the hellenides are controlled by compressional processes similar to those deforming the calabrian arc the stravo and pliny trenches to the east southeast border of the hellenic arc are partly affected by strike slip movements various plate tectonic models of the area have been proposed by several authors though the complexity of the deformation hardly permits reliable reconstructions the lack of reliable paleomagnetic information and the absence of linear magnetic anomalies do not favour the application of plate tectonic concepts vertical processes due to the uprising of lithothermal systems below the hellenic and calabrian arcs may also be regarded as the driving force controlling the present day deformation

the application of a multichannel laser raman microprobe microdil 28 to the analysis of fluid inclusions abstract the application to the analysis of fluid inclusions of laser excited raman microspectrometry with a multichannel detection system has two important features connected with the very short time needed to acquire one single spectrum 1 5 s the accumulation of a great number of the same spectral range results in an averaged spectrum with a very high signal to noise ratio with corresponding low detection limits and good counting statistics on the other hand the rapid acquisition of spectra enables the analysis of many individual inclusions in a relatively short time

petrology and evolution of transitional alkaline sub alkaline lavas from patmos dodecanesos greece evidence for fractional crystallization magma mixing and assimilation petrographic mineral chemical and whole rock major oxide data are presented for the lavas of the main volcanic series of patmos dodecanesos greece these lavas were erupted about 7 m y ago and range in composition from ne trachybasalts through hy trachybasalts and trachyandesites to q trachytes to some extent the ne trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc alkaline lavas of destructive plate margins major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz olivine plagioclase clinopyroxene and ti magnetite in the mafic lavas plagioclase clinopyroxene mica and ti magnetite in the evolved lavas apatite which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed however mass balance calculations indicate that the chemistry of the hy trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80 77 ne trachybasalt with 20 23 trachyandesite it is estimated that the mixing event preceded eruption by a period of 12 h 2 weeks suggesting that mixing triggered eruption combined fractionation and mixing cannot explain the relatively low mgo contents of the hy trachybasalts and it is concluded that assimilation also occurred assimilation and especially addition of volatiles to the magmas may be responsible for the evolutionary trend from ne normative to hy normative magmas and was probably facilitated by intensified convection resulting from mixing a model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids influx of hot primitive magma into the base of the chamber facilitates assimilation but eventually mixing yields the hy trachybasalts and finally the ne trachybasalts are erupted

lateral variability of subtidal flow at the mid reaches of a macrotidal estuary transverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary this was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid reaches of the gironde estuary france nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides subtidal flows were driven by a combination of internal asymmetry tidal advective accelerations nonlinear effects of water level variations quadratic friction and river discharge the quarter diurnal overtide band d4 in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide the ratio of quarter diurnal to squared semidiurnal bands d4 d22 was largest 0 3 in sections of the channel showing subtidal outflow river discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents during spring tide d4 was generated by tidal advective accelerations and quadratic friction combined with river discharge rather than by internal asymmetry the sixth diurnal overtide d6 in the flow was comparable to d4 for both neap and spring tides largest d6 d23 ratios were found in the shallowest cross channel locations during both neap and spring tides

a new analytical framework for assessing the effect of sea level rise and dredging on tidal damping in estuaries this paper explores different analytical solutions of the tidal hydraulic equations in convergent estuaries linear and quasi nonlinear models are compared for given geometry friction and tidal amplitude at the seaward boundary proposing a common theoretical framework and showing that the main difference between the examined models lies in the treatment of the friction term a general solution procedure is proposed for the set of governing analytical equations expressed in dimensionless form and a new analytical expression for the tidal damping is derived as a weighted average of two solutions characterized by the usual linearized formulation and the quasi nonlinear lagrangean treatment of the friction term the different analytical solutions are tested against fully nonlinear numerical results for a wide range of parameters and compared with observations in the scheldt estuary overall the new method compares best with the numerical solution and field data the new accurate relationship for the tidal damping is then exploited for a classification of estuaries based on the distance of the tidally averaged depth from the ideal depth relative to vanishing amplification and the critical depth condition for maximum amplification finally the new model is used to investigate the effect of depth variations on the tidal dynamics in 23 real estuaries highlighting the usefulness of the analytical method to assess the influence of human interventions e g by dredging and global sea level rise on the estuarine environment

the trace element geochemistry of clinopyroxenes from pyroxenites in the lewisian of nw scotland insights into light rare earth element mobility during granulite facies metamorphism clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the lewisian granulite complex nw scotland have distinctive chemistries suggestive of different origins clinopyroxenes in the layered pyroxenites crystallised from mafic melts in a magma chamber located in the middle to shallow crust whereas clinopyroxenes in pods in the felsic gneisses crystallised from the tonalitic protolith to the felsic gneisses in detail clinopyroxenes in the layered pyroxenites are variably enriched in the light ree inversion modelling shows that this is not a primary feature inherited from their parent magmas rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the localised partial melting of the hornblende pyroxenites during granulite facies metamorphism published isotopic evidence suggests that the light ree mobilisation took place at ca 2 7 ga about 200 ma after the time of crust formation this observation provides an explanation for the scattered pattern of whole rock isochron ages from the lewsian granulites

the loch scridain xenolithic sill complex isle of mull scotland fractional crystallization assimilation magma mixing and crustal anatexis in subvolcanic conduits

granulite fades nd isotopic homogenization in the lewisian complex of northwest scotland a published sm nd whole rock isochron of 2 920 50 myr obtained from a wide range of lithologies in the lewisian complex of north west scotland was interpreted1 as the time of protolith formation this date is 260 myr older than estimates for the timing of high grade metamorphism in the complex at 2 660 myr2 3 and this period is considered to represent the duration of the lewisian crustal accretion differentiation superevent cads 4 here we give new sm nd data obtained specifically from granulite facies tonalitic gneisses that yield a date of 2 600 155 myr although depleted mantle model ages tdm suggest 200 myr of premetamorphic crustal residence the regression date and its associated initial nd isotopic parameters demonstrate nd isotopic homogenization during the high grade event as well as the probability of general rare earth element ree mobility models for selective element depletion in the complex have previously assumed ree immobility since 2 920 myr but the data presented here suggest that a reappraisal of the depletion mechanism is required

origins of stratiform and stratabound fe cu zn horizons in the lower proterozoic moran lake group labrador central mineral belt zn cu and fe are concentrated as stratiform and stratabound sulphide rich beds in the lower proterozoic warren creek formation moran lake group central labrador upper member sedimentary rocks have a hydrothermal like fe enrichment but a dominantly hydrogenous signature as indicated by high al2o3 relative to sio2 and high al and fe relative to mn the upper member shales and sulphide rich beds were deposited as fe rich pelagic sediments the paucity of mn and abundance of fe in typical shale samples and lack of cu pb and zn fractionation in stratiform massive sulphide beds that contain up to 4702 ppm zn 533 ppm cu and 15 ppm pb suggest that deposition occurred in restricted brine pools i e cu and zn were precipitated rapidly and were not fractionated stratabound sphalerite mineralization containing 3 7 zn and 121 ppm cu but no pb was deposited in a porous lithology at the top of the warren creek formation and represents a unique style of metal concentration the stratiform deposits proba

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

inverse differentiation pathway by multiple mafic magma refilling in the last magmatic activity of nisyros volcano greece based on detailed field petrographic chemical and isotopic data this paper shows that the youngest magmas of the active nisyros volcano south aegean arc greece are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts triggering complex magma interaction processes the final magmatic activity of nisyros was characterized by sub plinian caldera forming eruption 40 ka emplacing the upper pumice up rhyolitic deposits followed by the extrusion of rhyodacitic post caldera domes about 31 10 ka the latter are rich in magmatic enclaves with textural and compositional basaltic andesite to andesite characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt dome lavas have different chemical isotopic and mineralogical characteristics from the enclaves the latter have lower 87sr 86sr and higher 143nd 144nd values than dome lavas silica contents and 87sr 86sr values decrease with time among dome lavas and enclaves micro scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes together with enclave content we demonstrate that the dome lavas are multi component magmas formed by progressive mingling mixing processes between a rhyolitic component post up and the enclave forming mafic magmas refilling the felsic reservoir from 15 wt to 40 wt of mafic component with time we recognize that only the more evolved enclave magmas contribute to this process in which recycling of cumulate plagioclase crystals is also involved the post up end member derives by fractional crystallization from the magmas leftover after the previous up eruptions the enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time a time related picture of the relationships between dome lavas and relative enclaves is proposed suggesting a delay between a mafic magma input and the relative dome outpouring we also infer that the magma viscosity reduction by re heating allows dome extrusion without explosive activity

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

interpretation of nisyros volcanic terrain using land surface parameters generated from the aster global digital elevation model abstract to model the morphotectonic evolution of nisyros stratovolcano in the aegean volcanic arc 36 35 n 27 10 e a 30 m resolution aster gdem was used nisyros is characterized by a relative pristine volcanic terrain elevation slope and aspect images the corresponding frequency distributions and rose diagrams enabled the geomorphometric analysis of nisyros revealing the major geomorphological structures that are associated to both endogenetic and exogenetic processes acting on the island either new or previously reported in the literature new elements include the number loci of issue relative age ogive structures of the voluminous precalderan nikia flows and their contact relationships with the avlaki flows the tectonic control fine feature morphology and flow paths of lavas and smaller domes associated with the main postcalderan domes become visually apparent particularities of the hydrographic network accentuate and bring forward non mapped radial faults intense landslide scarring and the volcanic stratigraphy of the intact units were revealed in the northeastern quadrant of nisyros major new volcano tectonic features include the division of the island into three northwesterly trending sectors and the dipping of nisyros towards the southeast as a result of segmentation by two major ring faults the kos ring fault krf and perigussa ring trapdoor fault prf which represent ring faults of the kos sagging caldera the aster gdem has provided suitable thematic information content in the geomorphometric analysis of nisyros and therefore it offers a reconnaissance tool in the geomorphological analysis of a volcanic landscape

a note on tidal asymmetry in the severn estuary a one dimensional non linear numerical model is used to investigate the tidally averaged frictional stress and set up of water level due to tidal asymmetry in the severn estuary these quantities depend on the overtides in the region a linearized model of the overtides is applied to calculations of the m4 currents in order to delineate the mechanisms responsible for their generation the relative importance of individual non linear mechanisms to the tidally averaged stress and set up is determined these mechanisms are interactions between tidal flow and changes in depth or breadth over a cross section frictional interaction between the tidal flow and stokes drift interaction between the tidal fluctuations in water depth and frictional retardation and non linear advection

tidal dynamics and substrate response in a salt marsh estuary abstract the tidal wave in the duplin river a well mixed salt marsh estuary on the georgia coast displays distinctive forms of asymmetry in the horizontal and vertical tides ebb current speeds reach a maximum of up to 30 higher than corresponding flood currents and maximum ebb and flood discharges are both displaced toward the time of high water duration of the flooding tide can be more than one hour longer than duration of the corresponding ebbing tide tidal asymmetry is clearly reflected in the tidally varying dynamic balance of the estuary estimated from a simplified equation of motion the ebb related pressure gradient term and corresponding shear stress term at the bed are an order of magnitude greater than flood related pressure gradient and bed shear ebb dominated asymmetry in the duplin river is attributed to interaction of the principal lunar m2 tide with its first harmonic overtide m4 generation of the m4 tide is thought to be from non linearities that arise from continuity of the salt marsh estuary which is best described by a changing basin area model substrate response to ebb dominated tidal dynamics is the development of largescale bedforms that maintain ebb oriented geometry through both ebb and flood tide bedforms are in equilibrium with maximum ebb related bed shear values which result in sand transport rates that are an order of magnitude greater during ebb compared with flood tide

wave driven flow over shallow reefs long term 1 month observations of waves and currents over a natural reef are presented which show a strong correlation between offshore rms incident wave height and cross reef currents at subtidal frequencies the energy spectrum of the cross reef currents shows a significant peak at twice the semidiurnal tidal frequency while the spectrum of sea surface elevation over the reef flat shows no corresponding peak furthermore experimental results reported by gourlay 1993 show setup over the reef occurs in the absence of a beach and the cross reef transport decreases with an increase in the sea surface slope across the reef flat due to an increase in setup at the top of the reef face analytic solutions for flow forced by wave breaking over an idealized reef explain the above features of cross reef flows in the absence of a beach through the surf zone on the reef face the cross reef gradient in the radiation stress due to wave breaking is partitioned between balancing an offshore pressure gradient associated with setup over the reef and forcing a mean flow across the reef over the reef flat where the depth is constant there is no forcing due to wave breaking and the flow is driven by a pressure gradient which results from the setup through the surf zone the magnitude of the setup through the surf zone is such that the transport across the reef flat matches the transport through the surf zone which is forced by the gradient in the radiation stress solutions are presented for general reef geometry defined by the reef width and slope of the seaward reef face and incident wave forcing defined by the depth at the breakpoint and the depth of water over the reef as the depth over the reef goes to zero the solutions converge to the plane beach solutions described by longuet higgins and stewart 1964 wave setup is maximized and the cross reef transport is zero in other cases the relative magnitudes of the setup and the cross reef transport depend on the geometry of the reef and the incident wave forcing

laser interferometry study of oscillatory zoning in plagioclase the record of magma mixing and phenocryst recycling in calc alkaline magma chambers iztaccihuatl volcano mexico

conflicting mineral and whole rock isochron ages from the late archaean lewisian complex of northwestern scotland implications for geochronology in polymetamorphic high grade terrains we report major and trace element including rare earth element ree geochemistry and whole rock smnd geochronology on two suites of mafic ultramafic inclusions in an agmatite complex within a trondhjemitic host at gruinard bay in the late archaean lewisian complex of northwest scotland a suite of medium grained equigranular amphibolites main amphibolite suite mas has broadly chondritic high field strength element hfse ratios ti zr ca 115 ti y ca 318 y cr ca 0 39 flat heavy ree patterns at five to fifteen times chondrites usually with mild light ree enrichment lan 50 and small eu anomalies this is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite trondhjemite granodiorite ttg suite magmas as argued previously by rollinson and fowler 1987 a smnd regression line mswd 3 4 obtained from the mas yields an age of 2 943 0 091 ga with an initial end value of 2 69 0 62 a suite of coarser grained hornblendites and metagabbros hornblendite metagabbro suite hms displays significant geochemical differences from the mas exemplified by significantly light ree enriched patterns in which the hornblendites have the highest total ree with lan ndn 1 and large negative eu anomalies and the most felsic metagabbros have low total ree and large positive eu anomalies samples from the hms yield a smnd isochron mswd 0 52 age of 2 846 0 073 ga with an initial end value of 1 10 0 72 the hms geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local ttg magmas combining the new data with previously published data from the host trondhjemites at gruinard bay whitehouse 1989a yields a smnd isochron mswd 1 09 age of 2 795 0 028 ga with an initial end value of 0 66 0 38 the relatively low initial end values for the hms and combined hms trondhjemite isochrons compared to contemporaneous depleted mantle end 2 8 ca 2 1 to 3 3 is consistent with derivation of a parental magma from light ree enriched components of a mafic basaltic precursor which was itself derived from a depleted mantle source approximately 100 200 ma previously the mas suite at gruinard bay may be representative of such a mafic precursor our new geochronological data differ significantly from the ca 3 3 ga mineral smnd and pbpb regression ages presented recently for two members of the hms by burton et al 1994 which if correct require radical revision of the early geological history of the lewisian complex comparison of the relatively old mineral ages with our new whole rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole rock systematics however we argue that extensive retrogression evident in all lithologies at gruinard bay makes them less than ideal samples for a mineral isotopic study and it is likely that the ca 3 3 ga ages may be spurious the product of disturbance of isotope systematics during the laxfordian on the basis of the geochronology presented here evolution of the lewisian closely follows generally accepted models for late archaean crust formation

petrology of scourian supracrustal rocks and orthogneisses from stoer nw scotland implications for the geological evolution of the lewisian complex

late holocene pelean style eruption at tacana volcano mexico and guatemala past present and future hazards tacana volcano located on the border between mexico and guatemala marks the northern extent of the central american volcanic chain composed of three volcanic structures it is a volcanic complex that has had periodic explosive eruptions for at least the past 40 k y the most recent major eruption occurred at the san antonio volcano the youngest volcanic edifice forming the complex about 1950 yr ago the pelean style eruption issued from the southwest part of the dome and swept a 30 sector with a hot block and ash flow that traveled about 14 km along the cahoacan ravine deposits from this event are well exposed around the town of mixcun and were therefore given the name of that town the mixcun flow deposit the mixcun flow deposit is in the channel facies a light gray massive thick 10 m matrix supported unit with dispersed lithic clasts of gravel to boulder size divisible in some sections into a variable number of flow units the overbank facies is represented by a thin 1 m massive matrix supported unit in both of these facies the deposit has disseminated charcoal fumarolic pipes and juvenile lithics with cooling joints the mixcun flow deposit contains clasts of 1 light gray dense andesite 2 dark gray glassy and banded andesite and 3 minor altered red andesite from the edifice set in a matrix of sand and silt the mixcun flow deposit covers an area of at least 25 km2 and has a minimum estimated volume of 0 12 km3 basaltic andesite inclusions 54 sio2 and various signs of disequilibrium in the mineral assemblage of the two pyroxene andesitic products 60 63 sio2 suggest that magma mixing may have triggered the eruption following deposition of the mixcun flow deposit andesitic to dacitic 62 64 sio2 lava flows were extruded and a dacitic dome 64 4 sio2 at the san antonio summit formed syn eruptive and posteruptive lahars flooded the main drainages of the cahoacan and izapa mixcun valleys in the area of the present city of tapachula population 250000 and the pre hispanic center of izapa three radiocarbon ages date this event between a d 25 and 72 range 1 38 b c a d 216 which correlates with a halt in construction at izapa hato phase of ca 50 b c a d 100 probably due to temporary abandonment of the city caused by lahars another similar event would produce extensive damage to the towns population of about 68 000 people now built upon the mixcun flow deposit the main summit of tacana volcano continues to show signs of fumarolic activity the most recent period of activity in 1985 1986 culminated in a minor phreatic explosion

observed fluxes of water salt and suspended sediment in a partly mixed estuary abstract observations of the residual fluxes of water salt and suspended sediment are presented for seven stations along the tamar estuary the data include measurements over single spring and neap tidal cycles and are generally applicable to medium or high run off conditions surface to bed differences in salinity are typically of the order of several parts per thousand gravitational circulation is an important component of residual flow in the deep lower reaches of the estuary here stokes drift is insignificant in the shallow upper reaches the major residual currents are generated by stokes drift and freshwater inputs data are compared with predictions from hansen and rattray s 1966 model of estuarine circulation salt fluxes due to tidal pumping and vertical shear are directed up estuary at spring tides tidal pumping being dominant tidal pumping of salt is also directed up estuary at neap tides although it is insignificant in the lower reaches where vertical shear dominates tidal pumping of suspended sediment is directed up estuary near the head at spring tides and probably contributes to the formation of the turbidity maximum the existence of the turbidity maximum is predicted using a simplified model of the transport of water and sediment the model shows that an additional mechanism for the existence of the turbidity maximum is an up estuary maximum in the tidal current speeds and thus resuspension in the lower reaches transport of suspended sediment is directed down estuary at both spring and neap tides and sediment is essentially flushed to sea with the fresh water

tidal velocity asymmetries and bedload transport in shallow embayments abstract tidal circulation can cause a net transport of sediment when the tidal velocity is asymmetric about a zero mean flood or ebb dominant and the sediment transport rate is related nonlinearly to velocity the relationship between tidal elevation and velocity is elucidated here to permit determination from tide gauge data and sediment transport relations whether tidal asymmetry needs to be considered as a mechanism for net sediment transport in the embayment of interest a relationship between elevation and velocity in a shallow water nonlinear system is derived through the continuity equation and shown to be significantly different than the linear relation finite difference numerical solutions of the one dimensional shallow water nonlinear equations are compared to the continuity relation and are in agreement especially toward the landward end of the channel tide gauge data collected at the landward end of the embayment are most useful for predicting velocity asymmetries throughout a major portion of the embayment channel the ratio of flood to ebb bedload transport and its relation to an asymmetric tidal elevation has been determined for both the linear relation between elevation and velocity and the nonlinear relation results show that the ratio of flood to ebb bedload transport as calculated from the nonlinear relation between elevation and velocity is similar to the flood to ebb ratio calculated from the linear relation because of offsetting effects

laser interference and nomarski interference imaging of zoning profiles in plagioclase phenocrysts from the may 18 1980 eruption of mount st helens washington

magmatic interaction between mantle and crust during the evolution of the archean lac guyer greenstone belt new quebec the lac guyer greenstone belt was one of a series of volcanic troughs active during the archean in the james bay territory of the superior province of quebec the belt consists of a succession of i

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

polymetamorphic evolution of the lewisian complex nw scotland as recorded by u pb isotopic compositions of zircon titanite and rutile u pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the lewisian complex and bordering laxford front reveal complex discordance patterns indicating multiple late archean and early proterozoic crystallization overgrowth and pb loss events the earliest stages in the evolution of the complex remain poorly resolved zircon ages of 2710 ma date high grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting a distinct event at 2490 2480 ma possibly initiated by metamorphism and deformation at high grade conditions caused amphibolitization of the granulites and emplacement of granitic pegmatites this event can be correlated with development of inverian shear zones and formation of granitoid layers along the laxford front the emplacement of a younger generation of granitoid sheets during the laxfordian event fromed hydrothermal titanite at 1754 ma in gneisses south of the laxford bridge and partially reset older titanite at scourie growth of secondary titanite and rutile also occurred during subsequent low grade metamorphism at 1690 1670 ma

is there a compositional gap at nisyros volcano a comment on magma generation at the easternmost section of the hellenic arc hf nd pb and sr isotope geochemistry of nisyros and yali volcanoes greece lithos 83 2005 29 46

new evidence for protolith ages of lewisian granulites northwest scotland the granulite facies scourian rocks in the central region of the lewisian complex of northwest scotland play an important role in the understanding of the development of mid archean high grade gneiss complexes whereas the later largely proterozoic history of the scourian has been clarified in a detailed u pb study unraveling the archean ages of the protoliths using conventional isotope techniques has proved impossible in order to advance our understanding of the deep crustal processes responsible for these granulites it is essential to establish the age s of accretion of the protoliths and their subsequent tectonothermal history a combined cathodoluminescence cl and sensitive high mass resolution ion microprobe shrimp single zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites use of cl allowed identification of relict oscillatory igneous zoning metamorphic overgrowths and irregular areas of recrystallization from the shrimp data an age of ca 2960 ma is inferred for the gneiss protoliths which were altered considerably during an important metamorphic event ca 2490 ma

n2 and co2 in deep crustal fluids evidence from the caledonides of norway abstract high pressure metamorphic rocks eclogites and high pressure granulites occur along the entire length of the norwegian caledonides and have formed from a variety of protoliths in some cases the relationship between protoliths high pressure rocks and their later retrogression products have been preserved in situ fluid inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition 1 eclogites and high pressure granulites contain n 2 bearing fluids pure n 2 or mixtures with co 2 or h 2 o with x n 2 5 in some eclogite facies rocks co 2 1bn 2 inclusions are associated with aqueous brine inclusions ca 30 wt nacl the two compositions representing immiscible fluids at peak metamorphic conditions 2 granulite facies protoliths and eclogites reworked in the granulite facies contain pure co 2 or co 2 dominated fluids with less than 2 5 n 2 3 rocks retrograded in the amphibolite facies contain h 2 o 1bnacl fluids immiscibility between brine and anhydrous n 2 1bco 2 fluid and between anhydrous fluid and waterbearing aluminosilicate melt have taken place in some eclogites during high grade metamorphism nitrogen may be incorporated in minerals as nh 4 substituting for k or it may occur as n 2 in the free fluid phase the partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism low a h 2 o and or high f o 2 partitioning nitrogen to the fluid phase a rock interacting with a carbonic fluid at granulite facies pt conditions will be depleted in mineralogically bound nitrogen in cases where the protoliths of high pressure rocks have been through a previous granulite facies event a local source for the nitrogen contained in high pressure fluid is therefore unlikely

the potential for future explosive volcanism associated with dome growth at nisyros aegean volcanic arc greece monitoring of the explosivity controlling parameters in magmas associated with dome growth in the quaternary volcano at nisyros aegean sea involves determining the chemical composition including dissolved volatiles h2o co2 f cl and s of melt inclusions in phenocrysts and interstitial matrices in pre and post caldera volcanic products our results indicate that the volatile content of the pre caldera magma was a dominant factor responsible for the catastrophic explosivity of the caldera forming episode the relatively high estimated volatile contents and apparent interstitial viscosities of the post caldera magmas as well as the geophysical evidence for the existence of an active magma chamber beneath nisyros all suggest that this volcano is presently dormant but has a high potential for future explosive volcanism

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

the upper and lower nisyros pumices revisions to the mediterranean tephrostratigraphic record based on micron beam glass geochemistry abstract the greek island of nisyros is the source of tephra found in distal settings throughout the aegean in this contribution we define geochemical fingerprints for the lower and upper pumice eruptions of nisyros based on micron beam major empa and trace la icp ms element data these two eruptions are compositionally distinct from each other the high phenocryst content of proximal lower and upper pumice tephra means that the differences between the two magmas are masked in whole rock analyses we demonstrate that all previous reports of distal marine and continental tephras correspond to the younger upper pumice not the older lower pumice this has important implications for the marine tephrochronology of the mediterranean and for the age of the upper and lower pumice eruptions we suggest an age of at least 47 ka for the upper pumice and an older unconstrained age for the lower pumice

re evaluating ambiguous age relationships in archean cratons implications for the origin of ultramafic mafic complexes in the lewisian gneiss complex abstract archean ultramafic mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations however their age relative to the other components of archean cratons are often poorly constrained introducing significant ambiguity when interpreting their origin and geodynamic significance the lewisian gneiss complex lgc of the northwest scottish mainland a high grade tonalite trondhjemite granodiorite ttg terrane that forms part of the north atlantic craton nac contains a number of ultramafic mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described previous studies have interpreted these complexes as representing a wide range of geological environments from oceanic crust to the sagducted remnants of archean greenstone belts these interpretations which are often critically dependent upon the ages of the complexes relative to the surrounding rocks have disparate implications for archean geodynamic regimes in the nac and globally most previous authors have inferred that the ultramafic mafic complexes of the lgc pre date the ttg magmas this fundamental age relationship is re evaluated in this investigation through re mapping of the geodh nan sgadan complex where tonalitic gneiss reportedly cross cuts mafic rocks and new mapping of the 7 km2 ben strome complex the largest ultramafic mafic complex in the lgc alongside detailed petrography and spinel mineral chemistry this new study reveals that despite their close proximity in the lgc 12 km the ben strome and geodh nan sgadan complexes are petrogenetically unrelated indicating that the lgc and thus nac records multiple temporally and or petrogenetically distinct phases of ultramafic mafic archean magmatism that has been masked by subsequent high grade metamorphism moreover field observations and spinel mineral chemistry demonstrate that the ben strome complex represents a layered intrusion that was emplaced into a ttg dominated crust further to representing a significant re evaluation of the lgc s magmatic evolution these findings have important implications for the methodologies utilised in deciphering the origin of archean ultramafic mafic complexes globally where material suitable for dating is often unavailable and field relationships are commonly ambiguous

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

monitoring oxygen fugacity conditions in pre syn and postcaldera magma chamber of nisyros volcano aegean island arc greece abstract the magmatic oxygen fugacities of the pre syn and postcaldera products of the quaternary volcano of nisyros in the aegean volcanic arc eastern mediterranean sea have been measured from analyzed compositions of titaniferous magnetite ilmenite equilibrium pairs quench temperatures in the precaldera rhyodacitic flows range from 877 to 836 c and oxygen fugacities from 11 4 to 11 8 log units one sample of the climactic pumice displays average quench temperatures of 853 c with f o 2 of 11 7 log units crystallization temperatures of the dacitic domes that intercept the nisyros caldera range from 843 to 804 c while oxygen fugacities range from 11 4 to 12 1 log units the rocks of the nisyros volcano record no significant change in oxygen fugacity relative to buffer curves immediately prior during and after the caldera forming event this is surprising given the considerable potential for interaction between the magmas and atmospheric oxygen during caldera collapse and the concomitant massive exsolution of volatiles however the nisyros fugacity data may not be atypical of stratovolcanoes with products of similar composition and histories of caldera forming events we speculate that only in the volcanoes where caldera collapse coincides with magma meteoric water interaction do the climactic or early postcaldera units show anomalously high oxygen fugacities

an analytical and experimental study of zoning in plagioclase a detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars discontinuous sector and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends the relative accuracy of 3 element na ca k microprobe traverses was found statistically to be 2 mole percent comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals

petrograph a new software to visualize model and present geochemical data in igneous petrology a new software petrograph has been developed to visualize elaborate and model geochemical data for igneous petrology purposes the software is able to plot data on several different diagrams including a large number of classification and petrotectonic plots petrograph gives the opportunity to handle large geochemical data sets in a single program without the need of passing from one software to the other as usually happens in petrologic data handling along with these basic functions petrograph contains a wide choice of modeling possibilities from major element mass balance calculations to the most common partial melting and magma evolution models based on trace element and isotopic data results and graphs can be exported as vector graphics in publication quality form or they can be copied and pasted within the most common graphics programs for further modifications all these features make petrograph one of the most complete software presently available for igneous petrology research

generation of harmonics by sea breeze in nontidal water bodies abstract the sea breeze introduces harmonics of its diurnal cycle into water bodies which can be notable for strong sea breeze and microtidal and nontidal conditions the harmonics can be of the same magnitude as those produced by bottom friction and dominate those produced by nonlinear terms in the equations of motion a closed form analytic solution of the linearized depth averaged equations of motion including friction is discussed for the situation of a sea breeze blowing on an idealized one dimensional basin of constant depth the solution reveals the generation of odd harmonics introduced by the quadratic wind stress and role of linearized friction sea breeze forcing on the idealized basin is numerically modeled and agreement with the analytic solution is found the numerical model is then run with quadratic bottom friction and nonlinear terms to compare relative contributions to the generation of harmonics harmonics of the water motion are distinguished as forced or arising from the wind for

oxygen isotope geochemistry of the scourian complex northwest scotland the scourian complex of northwest scotland underwent granulite facies metamorphism at 2 7 ga granulite facies orthogneisses have whole rock 18o values similar to those of igneous rocks of comparable compositions ultrabasic gneisses 5 5 5 9 basic gneisses 6 4 6 8 tonalitic gneisses 9 5 9 7 and steep 18o gradients of up to 3 in 10cm are recorded at two lithological contacts these data suggest that large volumes of pervasive metamorphic fluids have not infiltrated the rocks but do not preclude localized fluid rock interaction scourian paragneisses have lower 18o values 8 5 9 9 than typical metasediments and may have undergone fluid hosted isotopic exchange with the surrounding orthogneisses felsic sheets within tonalitic and basic gneisses have similar 18o values to their hosts these data combined with petrological and geochemical information suggest that the sheets were formed by local anatexis during granulite facies metamorphism fluid infiltration during the 2 5 ga amphibolite facies inverian retrogression caused homogenization of 18o values of the scourian gneisses unaltered tholeiitic scourie dykes have low 18o values of 2 0 it is unlikely that heated surface derived fluids infiltrated the scourian complex and these dykes were probably emplaced as low 18o magmas

multielement geochemistry of offshore sediments in the southeastern yellow sea and implications for sediment origin and dispersal abstract in total 116 surface sediments were collected from an offshore area of the southeastern yellow sea including a large mud belt southeastern yellow sea mud seysm and analyzed for grain size and geochemical composition organic carbon and 42 elements principal component analysis revealed that elemental compositions of the offshore sediments were mostly controlled by common aluminosilicate clay minerals and quartz most elements heavy minerals hf zr u th rare earth elements rees biogenic calcium carbonates ca sr feldspar and mica k ba pb and mn oxides mn oxide coatings on sand grains notably total ree concentrations may be associated with heavy minerals possibly zircon and monazite especially in silty grains even though heavy ree hrees concentrations are more strongly controlled by clay minerals in muddy sediments the diagnostic discrimination plots of geochemical compositions e g fe vs al mg vs al light ree vs hree and la lu ucc vs la y ucc suggested that the offshore sediments especially in seysm deposits were composed of a mixture of korean river derived silts and chinese river derived clays this demonstrated that chinese river sediments especially clay particles largely contributed to the formation of the giant mud belt in the korean coastal area furthermore the observed geochemical end members provide new markers of sediment provenance which can be used to track dispersal patterns and sediment limits from chinese and or korean rivers

carbonic fluid inclusions in south indian granulites evidence for entrapment during charnockite formation field evidence and fluid inclusion studies on south indian incipient charnockites suggest that charnockite formation occurred during a decompressional brittle regime following the peak of metamorphism and regional deformation the most abundant type of inclusions in quartz and garnet grains in these charnockites contain high density carbonic fluids although lower density fluids occur in younger arrays of inclusions discrete fluid inclusion generations optically are observed to decrepitate over well defined temperature intervals and quantitative measurements of co2 abundance released from these inclusions by stepped thermal decrepitation show up to a four fold increase by volume in the incipient charnockites relative to the adjacent gneisses from which they are derived studies based on optical thermometry visual decrepitation and stepped heating inclusion release together indicate that entrapment of carbonic fluids coincided with charnockite formation we confirm that an influx of carbon dioxide rich fluids is associated with the amphibolite granulite transition as recorded by the incipient charnockites the remnants of which are commonly preserved as the earliest generation of high density fluid inclusions

fluid inclusions in scourian granulites from the lewisian complex of nw scotland evidence for co2 rich fluid in late archaean high grade metamorphism in this paper the first fluid inclusion data are presented from late archaean scourian granulites of the lewisian complex of mainland northwest scotland pure co2 or co2 dominated fluid inclusions are moderately abundant in pristine granulites these inclusions show homogenization temperatures ranging from 54 to 10 c with a very prominent histogram peak at 16 to 32 c isochores corresponding to this main histogram peak agree with p t estimates for granulite facies recrystallization during the badcallian 750 800 c 7 8 kbar as well as with inverian p t conditions 550 600 c 5 kbar the maximum densities encountered could correspond to fluids trapped during an early higher p t phase of the badcallian metamorphism 900 1000 c 11 12 kbar homogenization temperatures substantially higher than the main histogram peak may represent laxfordian reworking 500 c 4 kbar in the pristine granulites aqueous fluid inclusions are of very subordinate importance and occur only along late secondary healed fractures in rocks which have been retrograded to amphibolite facies from inverian and or laxfordian shear zones co2 inclusions are conspicuously absent only secondary aqueous inclusions are present presumably related to post granulite hydration processes these data illustrate the importance of co2 rich fluids for the petrogenesis of late archaean granulites and demonstrate that early fluid inclusions may survive subsequent metamorphic processes as long as no new fluid is introduced into the system