the 1989 macquarie ridge earthquake and its contribution the regional seismic moment budget the macquarie ridge earthquake 1989 may 23 mw 8 2 occurred in a region of complex tectonics that marks the southern continuation of the plate boundary pac aus that runs through new zealand the first motion focal mechanism from wwssn and regional station polarities indicates dominantly strike slip movement strike 025 dip 80 rake 180 probably dextral on the northeast striking nodal plane the plate boundary south of new zealand appears to be segmented and marked by linear bathymetric highs with distinctive trends the distribution of shallow seismicity follows these bathymetric trends the moment tensors for the largest earthquakes ms 6 0 occurring on five distinctive segments of the plate boundary for the period 1964 1989 have been summed and compared with the deformation predicted by the plate motions recurrence times of the largest earthquakes in each segment determined by assuming that all deformation occurs seismically along the macquarie ridge complex mrc generally show longer recurrence times than those determined from the historic record these results suggest that during the last 70 years there has been more seismic moment release along the mrc than predicted by the plate motions the preferred explanation is that this century is a seismically overactive period which is part of a natural episodicity of quiescence and overactivity alternative explanations include complex plate interactions resulting from proximity of the plate boundary to the instantaneous pole of rotation and or the existence of a southeast australian microplate

three s type volcanic suites from the lachlan fold belt southeast australia the concept that granitoids of the lachlan fold belt in southeast australia are derived from either igneous or sedimentary source rocks i or s type can be extended to volcanic rocks of the same age three suites of late silurian s type volcanics are described two of which can be matched quite closely with plutonic equivalents a large part of the paleozoic continental margin volcanic activity in southeast australia consisted of the magmatic recycling of old metasedimentary crust probably of late proterozoic age the three volcanic suites are moderately to strongly peraluminous with the corresponding presence of al rich minerals variation within the volcanic suites is ascribed chiefly to progressive removal of restite or source material residual from partial melting the most mafic suite the hawkins suite contains plagioclase cordierite orthopyroxene biotite and quartz as restite components with less abundant almandine these rocks are chemically equivalent to mafic biotite rich and cordierite bearing granitoids from parts of the berridale and murrumbidgee batholiths garnet is absent from the goobarragandra suite volcanics which are a little more felsic and close in compositon to granitoids of the young and maragle batholiths the s type character of the laidlaw suite is less pronounced although a sedimentary source seems to be established such a source would be less mature than those for the other two volcanic suites the laidlaw suite resembles but cannot be closely identified with felsic s type granitoids of the murrumbidgee and berridale batholiths low grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements despite this alteration unaltered rocks are present in all three suites compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters hawkins suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5 6 kbar at 800 c the laidlaw and goobarragandra suites reequilibrated at lower pressures than their source before extrusion the laidlaw suite reequilibrated at estimated temperatures of 725 730 c o2 of 10 15 5 to 10 14 bars and h2o of 2 to 2 5 kbar differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities relative biotite and orthopyroxene mg is possibly pressure dependent

origin of co2 rich fluid inclusions in leucosomes from the skagit migmatites north cascades washington usa co2 ch4 fluid inclusions are present in anatectic layer parallel leucosomes from graphite bearing metasedimentary rocks in the skagit migmatite complex north cascades washington petrological evidence and additional fluid inclusion observations indicate however that the skagit gneiss was infiltrated by a water rich fluid during high temperature metamorphism and migmatization co2 rich fluid inclusions have not been observed in skagit metasedimentary mesosomes or melanosomes meta igneous migmatites or unmigmatized rocks and are absent from subsolidus leucosomes in metasedimentary migmatites the observation that co2 rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting although some post entrapment modification of fluid inclusion composition may have occurred during decompression and deformation the generation of the co2 rich fluid is attributed to water saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite plagioclase quartz graphite al2sio5 water rich fluid garnet melt co2 ch4 the presence of co2 rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis based on the inferences that 1 an influx of fluid triggered partial melting and 2 some episodes of fluid inclusion trapping are related to migmatization by anatexis it is concluded that a free fluid was present at some time during high temperature metamorphism the infiltrating fluid was a water rich fluid that may have been derived from nearby crystallizing plutons because partial melting took place at pressures of at least 5 kbar abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km

correlation between melting deformation and fluid interaction in the continental crust of the high himalayas langtang valley nepal combined metamorphic stable isotope and structural studies from the high himalayan crystalline sequence in the langtang valley of north central nepal reveal a strong positive correlation between distance above the base of the section the main central thrust the amount of melt material and evidence of prolonged fluid and deformation histories thus suggesting that these processes are strongly interdependent kyanite grade rocks at the base of the section are unmelted and have undergone little syn or post metamorphic internal deformation and little prograde or retrograde fluid rock interaction by contrast sillimanite grade rocks higher in the section contain progressively larger volumes of melt have suffered increasingly complex syn and post metamorphic deformation and show increasing evidence for the presence of fluids although the factors that initiated these processes remain problematic it is suggested that fluid distributions within the langtang section have been passively controlled by the movement of melts these melts may have provided a primary control on deformation during both the magmatic stage and subsequently through the exsolution of exsolved aqueous fluids

phase equilibrium constraints on the viscosity of silicic magmas 1 volcanic plutonic comparison by using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at 104 5 pa s irrespective of their temperature and level of emplacement magmas crystallizing as granitic plutons are not water poor and thus not more viscous than their extrusive equivalents instead comparison between pre eruption magma viscosities of extrusive silicic intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous given the typical strain rates in silicic magma chambers magma rheological behavior is expected to be dominantly newtonian bubbles having a minor rheological influence at depth although exceptions can exist thus whether a silicic intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors but not on the rheology of the magma itself however preemptive viscosities of extrusive magmas rarely exceed 106 pa s which suggests that crystal melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes the narrow range of viscosities displayed by silicic intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas considerations of the relationships between magma crystallinities bulk sio2 and preemptive melt h2o contents show that the higher the melt h2o content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted an empirical correlation is proposed that enables us to estimate preemptive melt h2o contents of erupted magmas by knowing their crystallinity and bulk sio2

metamorphic zonation migmatization and leucogranites along the everest transect of eastern nepal and tibet record of an exhumation history

magmatic volatiles in explosive rhyolitic eruptions obsidian clasts in rhyolitic tephra deposits preserve preeruption magmatic volatile contents providing a direct means for determining the volatile content of explosively erupted magmas small to moderate volume plinian eruptions 10 sup 3 to 10 sup 1 km sup 3 appear to be driven by 0 5 1 0 wt volatiles consisting dominantly of h sub 2 o with minor co sub 2 analysis of obsidian from eruptive sequences consisting of tephra and flows indicates that this hydrous magma abruptly overlies magma with only 0 1 0 2 wt h sub 2 o

geochemistry and petrology of the galeras volcanic complex colombia abstract the galeras volcanic complex gvc has erupted lavas and pyroclastic flows ranging from basaltic andesites to dacites during the last million years the chemical composition of the volcanic products has been fairly constant characterized by calc alkaline high silica medium potassium andesites variations from basaltic andesites to dacites have occurred during individual stages but overall composition and petrographic characteristics are quite constant the guaca cinder cone is the exception being more alkaline rather than calc alkaline the trace element and ree contents of the gvc samples suggest that sediments or fluids from the subducted sediments are involved in the generation of the magmas at the source region although high sr and ba contents are also present in the upper continental crust the high b levels are consistent with the contribution of sediment related elements small degrees of partial melting of a hydrated mantle wedge are consistent with the observed distribution of ree plus the addition of highly soluble elements from the subducted sediments crystal fractionation at depths of about 36 km approximately 12 kbar of a hydrated tholeiitic basalt melt can produce andesites and basaltic andesites having a phase assemblage of amphibole plagioclase clinopyroxene magnetite ilmenite and apatite the magma then re equilibrates at higher levels reaching levels as shallow as 10 km about 2 5 to 3 kbar and crystallizing orthopyroxene and large amounts of plagioclase at these shallow levels amphibole is no longer stable and re equilibrates to oxides pyroxene and plagioclase the melts become saturated with respect to most volatiles by depressurization and crystallization in the gvc we observe that it is not necessary to have large volumes of highly differentiated magma to cause caldera forming eruptions

melt segregation and magma interactions during crustal melting breaking out of the matrix abstract differentiation of the continental crust begins with its partial melting the products of crustal melting are silicic hydrous h2o undersaturated granitic liquids that are generated within matrices of residual crystals crustal differentiation requires that felsic magmas form and escape from these solid residua an important question is whether granitic magmas collect into large batches within or near their sources which then give rise to ascent or alternatively bleed out of the sources in smaller streams or pulses we demonstrate that the physical reality is closer to the second alternative and thus question the validity of the concepts of magma segregation and source fertility as they are sometimes visualised granitic plutons contain chemically distinct sub populations formed by source level entrainment of the peritectic assemblages into the melts rapid evacuation at source levels and high ascent rates protect the magmas from wall rock interactions during their tenure at source depths and during their ascent through cool upper crust the existence of different types of granites and hence of clearly defined chemically different magmas within plutons dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move exposed deep crustal sections are typically lithologically diverse with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions if the melts were required to segregate and accumulate into large volumes within their hot sources especially by slow gravity driven melt percolation it is very likely that they would be substantially modified by reaction with diverse source rocks thus the processes mooted to occur in melting assimilation storage and homogenisation mash or deep crustal hot dch zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma perhaps carrying evidence of extensive magma mixing from various lines of evidence we conclude that mash and dch zones may not exist and even if they do they cannot form the sources of most granitic magmas the intrinsic heterogeneity in crustal source rocks the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source inherited chemical and isotopic heterogeneities rapid semi continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole rock trace element chemistry and accessory mineral chemistry will be decoupled from major element variations this also means that a large source with low nominal fertility due to low hydrous mineral content for example could still produce a substantial granitic pluton by additions of individually small increments of magma

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

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

nonlinear processes in earthquake foci the results of laboratory experiments on biaxially compressed physical models of a seismic source are presented discussed and interpreted in terms of nonlinear dynamics the relation is shown between the degree of seismic pulse coherency expressed through the amplitude frequency spectra development and energy or magnitude in a series of model experiments it has been ascertained that the degree of radiated waves coherency plays a more important role concerning the seismic energy release than the size radius of the seismic source

source complexity of small earthquakes near matsushiro japan abstract broad band seismograms from a suite of small strike slip earthquakes which occurred in the matsushiro region of honshu japan during the period august 1971 september 1974 and recorded on carnegie seismograph mat are used to study source properties for events ranging in size from 10 11 10 15 nm in an earlier study dysart et al modelled frequency spectra of these events with a single brune pulse and made estimates of moment corner frequency and stress drop they employed cepstral filtering to suppress the effects of structure and to increase the stability of parameter estimates the same 32 waveforms considered here display a notable variation in complexity which cannot be explained by size local structure or stopping phases in order to understand better the time history of the complex events within the suite the sh waveforms were divided into successive time segments and the same modelling procedure was applied to the segments as was applied to the initial sh arrivals cepstral filtering was particularly helpful in reducing the interference effects of the s coda the results indicate that corner frequency changes little from subevent to subevent this suggests that small earthquakes proceed by incremental slippage on the same fault in a sequence of partial stress releases

a tale of two plutons petrographic and mineralogic constraints on the petrogenesis of the red lake and eagle peak plutons central sierra nevada california the red lake and eagle peak plutons are similar in age k ar 87 89 m y and range in composition from granodiorite to granite the red lake pluton is equigranular has a locally greisened marginal zone and shows only minor mineralogical and chemical zoning whereas the eagle peak pluton is mineralogically compositionally and texturally zoned with an equigranular margin and a porphyritic core modal and normative trends within each pluton are consistent with separation of plagioclase and mafic minerals leading to a residual liquid enriched in quartz and alkali feldspar the formation of the prominent alkali feldspar megacrysts in the eagle peak core was promoted by plagioclase separation h 2 o undersaturation and a slower cooling rate relative to the red lake magma early loss of volatiles and a resulting increase in magma melt crystals viscosity inhibited crystal segregation within the red lake pluton and prevented the development of zoning late stage aplites associated with both plutons f

granulite facies amphibole and biotite equilibria and calculated peak metamorphic water activities samples located near the oregon dome anorthosite massif in the south central adirondack mountains new york contain the fluid buffering mineral assemblages amphibole clinopyroxene orthopyroxene quartz or biotite quartz orthopyroxene k feldspar these rocks were metamorphosed under granulite facies conditions t 725 750 c p 7 5 kbar during the grenville orogeny the mg rich nature of amphiboles micas and pyroxenes allow accurate calculation of water activities because corrections for the effects of solid solution are relatively small the activity of water was low during the peak of granulite facies metamorphism with h2o 0 15 0 14 wollastonite occurrences indicate that the co2 was low 0 3 in nearby rocks demonstrating that large quantities of co2 did not infiltrate in a pervasive manner the combination of low h2o with low co2 is consistent with the hypothesis that magmatic processes were dominant generating dry fluid absent conditions

origin and evolution of mid cretaceous garnet bearing intermediate and silicic volcanics from canterbury new zealand abstract the mt somers volcanics are part of a suite of mid cretaceous 89 2 ma intermediate to silicic volcanics erupted onto an eroded surface of torlesse sediments rock types vary from basaltic andesite to high silica rhyolite andesites are medium to high k with phenocrysts of plagioclase orthopyroxene and pigeonite dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800 c rhyolites are peraluminous with phenocrysts of quartz sanidine plagioclase biotite garnet and orthopyroxene the ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar 20 km depth at above 850 c with melt water contents of less than 3 5 major element contents trace element contents and an initial 87 sr 86 sr ratio of 0 7085 indicate that the rhyolites formed by partial melting of dominantly quartzo feldspathic torlesse sediments leaving a granulite facies residue the chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high pressure phenocryst assemblage most elements show a compositional gap between rhyolite and dacite the major element trace element and sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle derived tholeiitic magmas magmatism was the result of heat and magma flux from the mantle during the change from compressive to extensional tectonics after the culmination of the rangitata orogeny

glass bearing crustal xenoliths buchites erupted during the recent activity of stromboli aeolian islands abstract three xenoliths erupted as ejecta during recent violent explosion of stromboli volcano aeolian islands were investigated in this paper they consist of high temperature mineral association cordierite hercynite spinels sillimanite plagioclase mullite corundum and abundant glass 10 70 vol and may be classified as buchites the peraluminous composition of the xenoliths their trace element distribution and ree patterns support their origin from granulite and amphibolite facies metapelites of the calabrian continental crust which is considered the crystalline basement beneath stromboli buchites have an isotopic composition comparable to that of stromboli extrusives and significantly different from that of the calabrian basement the glass is generally colourless and has a si al alkali rich composition except for one sample where a fe mg ca rich reddish brown glass also occurs these two kinds of glass show complicated textures where patches of reddish brown glass are often surrounded by plagioclase and or cordierite or forms streaks and swirls with the colourless glass cordierite plagioclase and oxides have different compositions according to their position in the xenoliths ca rich plagioclase an72 95 mg poor cordierite mg values 47 66 and al rich spinels are in the inner portions of the xenoliths and associated with colourless glass on the contrary close to the contact with the host lava or associated with coloured glass cordierite shows higher mg values ti fe bearing oxides occur and plagioclase is chemically similar to the basalt phenocrysts an66 71 the abundant and fresh glass and the idiomorphic shape of the high temperature minerals suggest that the xenoliths were hold in the basaltic magma before its extrusion for a significant time to allow their partial or nearly complete melting and subsequent nucleation and growth of new phases during this stage the interaction between the anatectic liquid and the basaltic magma affected the original isotopic composition of the xenoliths and in some cases produced glass and mineral phases cordierite plagioclase and oxides with different composition

the volatile component of some pumice forming alkaline magmas from the azores and canary islands the abundances of pre eruptive magmatic volatile species in the system h o s may be determined by application of thermodynamic methods to phenocryst assemblages commonly found in volcanic rocks as demonstrated by rutherford and heming 1978 these methods are applied to alkaline pumice deposits of airfall and ignimbrite type from tenerife canary islands sao miguel and faial azores it is argued that reliable temperature and fo2 buffering mechanisms found in rhyolitic magmas appear not to operate in more alkaline liquids fh2o is estimated using biotite the high values found are shown to be compatible with the violently explosive nature of the magmas concerned fs2 is estimated from pyrrhotite composition fh2 fh2s fso2 fso3 are calculated from gas equilibria water fugacity may be very roughly estimated for non biotite bearing samples from data on the sulphur species abundances of these species are similar in alkaline and calc alkaline salic magmas volcanological implications relating to the release of volatiles during explosive eruptions are considered

water present eclogite melting to produce earth s early felsic crust abstract the geochemistry of well preserved archaean tonalite trondhjemite granodiorite ttg rocks such as the trondhjemites of the meso archaean barberton granitoid greenstone terrain in south africa provides insight into the origins of earth s early felsic continental crust this is particularly well demonstrated by the high pressure hp type of archaean ttg magmas where the geochemistry requires that they are formed by high pressure melting of a garnet rich eclogitic source this has consequently been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within archaean subduction zones most of the experimental data relevant to ttg genesis has been generated by studies of fluid absent melting of metabasaltic sources however water drives arc magmatism within phanerozoic subduction zones and thus understanding the behaviour of water in archaean subduction zones may have considerable value for understanding the genesis of hp type paleo to meso archaean trondhjemites consequently this study investigates the role of high pressure water present melting of an eclogite facies starting material in the production of paleo to meso archaean hp type ttg melts water saturated partial melting experiments were conducted between 1 9 and 3 0 gpa and 870 c and 900 c the melting reaction is characterised by the breakdown of the jadeite molecule in the clinopyroxene together with quartz and water to form melt in conjunction with a less sodic clinopyroxene qtz cpx 1 grt 1 h 2 o melt cpx 2 grt 2 and produced melt compositions that if allowance is made for the low mg and almost k 2 o free character of the starting material are an excellent major element match with hp type ttg compositions in two of the experimental run products melt segregated efficiently from residual crystals allowing for the measurement of a full range of trace elements via laser ablation inductively coupled plasma mass spectroscopy la icp ms the experimental glasses produced are light rare earth element lree 24 44 ppm la 50 86 ppm ce and zr 101 228 ppm enriched and heavy rare earth element hree 0 4 ppm yb 50 9 la yb n y 2 2 3 2 ppm 82 142 sr y sm 2 1 4 3 ppm 40 9 50 8 zr sm and nb 1 7 2 8 ppm depleted compared to the dse4 starting material thus the ree element compositions of the experimental glasses are also similar to hp type ttg compositions and in more particular they are strikingly similar to meso archaean barberton trondhjemites additionally we propose that due to cpx being a major reactant ni and cr contents 4 29 ppm ni and 43 ppm cr of the glasses which where analysed for these elements are within the compositional range displayed by hp type archaean ttg this study suggests that the geochemistry of hp type archaean trondhjemites may reflect high pressure water present partial melting of an eclogite facies metabasaltic source consequently we propose that water present melting of an eclogitic source is a viable mechanism for the genesis of this component of paleo to meso archaean ttg crust

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

coexisting monazite and allanite in peraluminous granitoids of the tribec mountains western carpathians monazite a typical light rare earth element lree mineral of s type granitoids in the western carpathians was found in the peraluminous biotite granodiorite tonalite in the tribec mountains commonly containing polymineralic inclusions these inclusions are dominated by anhedral allanite although allanite also occurs rarely as discrete grains not enclosed by monazite the monazite studied here is relatively homogeneous and characterized by high th contents with proportions of huttonite thsio4 and brabantite cath po4 2 up to 14 6 and 9 3 respectively the discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock however allanite included in monazite is extremely variable in lree al fe and mg contents this variation is interpreted to result from entrapment of allanite melt in monazite before local equilibrium was attained the change from allanite to monazite as the stable lree rich phase is related to an overall decrease in ca concentration caused by the onset of plagioclase crystallization the early precipitation of allanite was possible because of the high lree concentrations in the melt the crystallization temperature of allanite must have been higher than monazite saturation 856 845 c and 798 790 c for two analyzed samples the zr saturation temperature based on zircon solubility and ree thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton which coincides with an increase in the huttonite content in monazite the primary lree assemblage is accompanied by small grains of late huttonite replacing monazite and brabantite replacing allanite

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

the lithosphere of the earth as a nonlinear system with implications for earthquake prediction the lithosphere of the earth can be viewed as a hierarchy of volumes from tectonic plates to grains of rock their relative movement against the forces of friction and cohesion is realized to a large extent through earthquakes the movement is controlled by a wide variety of independent processes concentrated in the thin boundary zones between the volumes a boundary zone has a similar hierarchical structure consisting of volumes separated by boundary zones etc altogether these processes transform the lithosphere into a large nonlinear system featuring instability and deterministic chaos from this background some integral grossly averaged empirical regularities emerge indicating a wide range of similarity collective behavior and the possibility of intermediate term earthquake prediction

late variscan strike slip tectonics between the tepla barrandian and moldanubian terranes czech bohemian massif petrostructural evidence in the variscan bohemian massif czech republic no agreement has been reached on the nature of the relationship between the tepla barrandian and the moldanubian terranes however the tepla barrandian is interpreted as a central segment separating the moldanubian and saxothuringian terranes and its position is therefore essential for any reconstruction of this part of the variscan belt our petrological and structural study in the domazlice klatovy region spans the boundary between the tepla barrandian and moldanubian terranes there is no conformity of ductile structures in both domains near their interface and their p t evolution is separate in time the development of major structures and associated metamorphism in the southwestern part of the barrandian was accompanied by syntectonic intrusions of granitoid and diorite bodies dated at about 520 ma the moldanubian migmatites are characterized by a lp ht metamorphism that possibly took place at about 320 ma the metamorphic record of the barrandian rocks shows no influence of the moldanubian partial melting the contacts are marked by semi brittle to brittle structures sinistral strike slip faulting developed in the central bohemian shear zone whereas the west bohemian shear zone acted as a semi brittle normal fault with a dextral component both shear zones may be nearly contemporaneous accommodating a bulk n s compression the central bohemian shear zone was later reactivated in a dextral regime although timing is uncertain the juxtaposition of both terranes took place probably in the waning stages of the variscan orogeny earlier than the late stephanian we argue that the late orogenic semi brittle strike slip tectonics modified considerably the face of the bohemian massif and the geodynamic models of the bohemian massif have to be reviewed with reference to this essential point

ion microprobe studies of water in silicate melts concentration dependent water diffusion in obsidian abstract the temperature dependence of water diffusivity in rhyolite melts over the range 650 950 c and pt h2o 700 bars is evaluated from water concentration distance profiles measured in glass with an ion microprobe diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10 8 cm2 s at 650 c to about 10 7 cm2 s at 950 c at 2 wt water water solubility also varies with temperature at a rate of 0 14 wt per 100 c increase the avtivation energy ea appears to be constant at 19 1kal mole for 1 2 and 3 wt h2o comparison of these data with results for cation diffusion indicates that this value is a minimum ea for diffusion of any species in a rhyolite melt compensation plots of log10d0 the frequency factor versus ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts d0 increases for h2o and ca2 1 as ea decreases this suggests that these molecules may diffuse by different mechanisms than do monovalent cations and that hydration of the melt affects diffusion of ca2 and h2o differently than it does monovalent cation diffusion the results imply that dramatic increases in cation diffusivities by hydration 1 may occur with additions of less than 1 wt h2o

water contents of silicic to intermediate magmas silicic to intermediate magmas have water contents that range from 1 wt to around 7 wt depending on the character and depth of the magma source type of melting reaction and the degree of differentiation the condition of water saturation is rare implying that crustal magma genesis generally takes place under fluid absent conditions or that any fluid present must contain volatile species other than h2o the source and quantity of water present during partial melting profoundly affect the types of melting reactions residues and magmas as well as subsequent magma evolution for s type granitic magmas melting reactions involving muscovite breakdown in pelites yield low temperature wet melts that form veins pockets and small plutons in regional metamorphic terranes biotite breakdown in pelites near the amphibolite granulite facies transition yields hotter magmas with variable water contents these are emplaced at medium to high levels under granulite facies conditions biotite breakdown in quartzofeld spathic rocks yields high temperature dry magmas emplaced as subvolcanic batholiths and volcanics

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

petrogenesis of the concordia granite gneiss and its relation to wmo mineralization in western namaqualand south africa abstract the 1 1 ga concordia granite gneiss cgg is part of the late to postorogenic spektakel suite in the western namaqualand metamorphic complex south africa it intruded synkinematically with respect to the main d2 deformation event into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite facies metamorphism 800 c 5 kbar several genetically related rock types megacrystic garnet bearing granite minor aplitic leucogranites and pegmatites are distinguished all varieties are sio 2 rich 69 79 wt peraluminous granites and show subalkaline monzonitic magma characteristics geochemical differences in whole rock chemistry between megacrystic granite and aplitic leucogranites e g lower al 2 o 3 mgo cao ba zr higher k 2 o rb nb w rb sr ga al and the decrease of e g cao mgo fe 2 o 3 ba zr th u with increasing sio 2 in the megacrystic granites as well as the variation in fe mn of magmatic garnets are best explained with crystal fractionation processes fractional crystallization of plagioclase produced potassium and silica rich residual melts characterised by very high rb sr rb ba u th mn fe ratios and higher concentration of w cu and zn crystal fractionation processes also resulted in a relative lree depletion and hree enrichment megacrystic granite la lu cn 8 87 31 67 aplitic leucogranite and pegmatites la lu cn 0 71 1 44 and evolution of pronounced negative eu anomalies the crystallization sequence near solidus crystallization of biotite prior to alkali feldspar suggests that the cgg magmas were h 2 o undersaturated over a long period of their evolution water saturation during late stage crystallization is however indicated by coarse late stage eutectic mineral textures pegmatites and w mo bearing siliceous rocks furthermore the economic potential of the cgg is supported by its geochemical signature e g high u th contents which is similar to evolved high heat production hhp granites the granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated s type granites the w mo deposits represent vein type and pegmatitic deposits genetically related to a deep seated granitic system

application of catastrophe theory in earthquake hazard assessment and earthquake prediction research abstract catastrophe theory is a mathematical theory for the description of natural and social catastrophic phenomena however its application in the fields of earthquake hazard assessment and earthquake prediction research has not been reported based on real seismicity time series data in a specific region a method for earthquake hazard assessment and earthquake prediction by incorporating cusp catastrophe a 3 is presented through experimental computation of real earthquake sequences the effectiveness of the method is demonstrated it is therefore shown that the catastrophe theory has potential for application in the fields of earthquake hazard assessment and earthquake prediction research

rhyolites contaminated with metapelite and gabbro lipari aeolian islands italy products of lower crustal fusion or of assimilation plus fractional crystallization pleistocene lavas from monte s angelo and chiesa vecchia volcanoes on lipari contain two suites of inclusions a metapelitic suite consists of gneisses and granulites with combinations of cordierite garnet corundum hercynite andalusite sillimanite orthopyroxene ilmenite magnetite biotite plagioclase and quartz a gabbroic suite has cumulus texture and contains plagioclase orthopyroxene clinopyroxene and magnetite all megacryst phases in the lavas appear to be derived from rock fragments with the exception of euhedral strongly zoned calcic plagioclase and none has grown by homogeneous nucleation from liquid represented by the groundmass which is peraluminous rhyolite 70 wt sio2 6 wt k2o ground mass microcrysts were nearly all derived from disaggregated metapelites overgrowths of alkali feldspar on plagioclase and of orthopyroxene on clinopyroxene and quartz intergrown with alkali feldspar are the only phases that grew from the rhyolitic liquid euhedral cordierite hercynite and plagioclase at the margins of some rock fragments grew by reaction of metapelite with liquid

archaean ttgs as sources of younger granitic magmas melting of sodic metatonalites at 0 6 1 2 gpa two natural low k2o na2o ttg tonalitic gneisses one hornblende bearing and the other biotite bearing were partially melted at 0 8 1 2 gpa fluid absent the chief melting reactions involve the breakdown of the biotite and hornblende the hornblende tonalite is slightly less fertile than the biotite tonalite but melt volumes reach around 30 at 1 000 c this contrasts with results of most previous work on more potassic ttgs which generally showed much lower fertility though commonly producing more potassic melts garnet is formed in biotite bearing tonalitic protoliths at p 0 8 gpa and at 1 0 gpa in hornblende bearing tonalitic protoliths all fluid absent experiments produced peraluminous granitic to granodioritic melts typically with sio2 70 wt for the biotite tonalite increasing t formed progressively more melt with progressively lower k2o na2o however the compositions of melts from the hornblende tonalite do not vary significantly with t with increasing p melts from the biotite tonalite become less potassic due to the increasing thermal stability of biotite for the hornblende tonalite again there is no consistent trend fluid absent melting of sodic ttgs produces melts with insufficient k2o to model the magmas that formed the voluminous late potassic granites that are common in archaean terranes reconnaissance fluid present experiments at 0 6 gpa imply that h2o saturated partial melting of ttgs is also not a viable process for producing magmas that formed these granites the protoliths for these must have been more potassic and less silicic nevertheless at granulite facies conditions sodic ttgs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths upward abstraction of this melt would result in some lile depletion of the terrane younger k rich magmatism is unlikely to represent recycling of ttg crust on its own and it seems most likely that evolved crustal rocks and or highly enriched mantle must be involved

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

geochemical constraints on the bimodal origin of high himalayan leucogranites major and trace element and rb sr isotope systematics of the manaslu leucogranite central nepal have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis biotite and tourmaline are for the most part mutually exclusive with a predominance of two mica leucogranites over tourmaline leucogranites the 87sr86sr initial isotopic ratios sri indicate that leucogranitic melts were derived from two different sources the two mica leucogranites having a metagreywacke origin with sri 0 752 nd 15 such a bimodal nature of the source zone does not directly influence the magmatic evolution except that probably the higher initial boron content in the metapelitic rocks may increase the na2ok2o ratio in contrast the amount of water present during melting principally controls in part anatectic processes and element behaviour water saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two mica leucogranites whereas water absent conditions prevailed during melting of metapelitic layers and favoured biotite plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite

granitic melt viscosity and dike formation melt viscosity is an important variable controlling the rates of granitic melt separation and transport from its source region viscosities of granitic melts are significantly affected by changes in temperature and volatile content addition of 2 wt h2o to anhydrous peraluminous and metaluminous granitic melts reduces viscosity by six orders of magnitude at 800 c which is equivalent to the effect of raising the temperature by 500 c addition of 10 wt more h2o only decreases melt viscosity by approximately 1 5 additional orders of magnitude at higher temperatures the effect of h2o addition on melt viscosity is less significant at lower temperatures more significant the effect of pressure on hydrous granitic melt viscosities at crustal conditions is insignificant in comparison between 0 5 and 1 5 gpa the viscosity at 1000 c increases by 0 4 log units or less this effect can be ignored in most petrological calculations using newly acquired viscosity data at low h2o contents the baker model for the calculation of metaluminous and peraluminous granitic melt viscosities has been updated and expanded the new model can predict hydrous 0 3 12 3 wt h2o granitic 69 77 wt sio2 melt viscosities between 577 and 1200 c at crustal pressures the new granitic melt viscosities have been used to investigate the possibility of granitic melt transport via diking diking can be an efficient mechanism of damp granitic melt transport through the crust for granitic melts containing a minimum of 2 wt h2o if the lower crust has been sufficiently heated modeling the thermal history of basaltic intrusions and surrounding lower crust demonstrates that a sheet like intrusion 5 km thick or a suite of intrusions with the same aggregate thickness that were rapidly emplaced can provide enough heat to allow granitic magmas to migrate through much of the crust in dikes the generation of granitic dikes associated with thinner intrusions appears unlikely

controls on source and depth of emplacement of granitic magma large granitic batholiths emplaced in high grade metamorphic rocks at mid crustal levels were relatively hydrous compared with major batholiths emplaced at very shallow levels those emplaced at mid crustal levels were probably generated by partial melting of crustal rocks at only slightly deeper levels those emplaced at shallow levels are associated with penecontemporaneous volcanic ejecta and were probably generated at deep crustal or possibly mantle depths

the compositional variation of i type granites constraints from geochemical analyses and phase equilibrium calculations for granites from the qinling orogen central china abstract there is a large compositional variation of i type granites although partial melting and fractional crystallization are the two primary mechanisms for the variation it is intriguing to know how they jointly work this issue is addressed by a combined study of geochemistry and phase equilibrium modelling for a triassic pluton from the qinling orogen the pluton consists of high k calc alkaline i type granites geochemical results suggest that the granites are likely sourced from triassic mafic intermediate igneous rocks compared with experimental melts it is inferred that the natural granites are not the crystallized product from pure magmatic melts this is confirmed by phase equilibrium calculations that have utilized the triassic mafic intermediate igneous rocks as the source yielding modelled melts that contain too low feot mgo and mg to match the target granites the addition of restitic mineral assemblages to melts can relieve the discrepancy but the modelled magmas have much higher cao than the granites replacing equilibrium melting by disequilibrium melting in terms of the plagioclase behavior can lead to a satisfactory match furthermore by taking the average of the target granites as a parental magma the modelled crystalline products can well reproduce the variation trends in the high k calc alkaline i type granites from the qinling orogen in this regard the crustal anatexis would produce the parental magma at first which experienced fractional crystallization to form the granitic pluton with the compositional variation as observed therefore these two processes would jointly lead to the compositional variation of i type granites in nature

fractionation in a zoned monzonite pluton mount dromedary southeastern australia mount dromedary pluton is one of several predominantly monzonite plutons and smaller intrusive bodies which constitute the dromedary igneous complex in southeastern new south wales the pluton exhibits a striking arrangement of petrographically but not always chemically distinct zones ranging from mafic monzonite at the outside to quartz monzonite in the centre the rocks display a mineralogical and geochemical integrity which indicates a consanguineous relationship minor compositional discontinuities between zones together with observed and inferred minor intrusive zone boundaries suggest that each zone has to some extent evolved independently negative eu anomalies in ree abundance patterns show that some of the zones have been affected by fractionation of feldspar but complementary accumulates are not found at the present levels of exposure the pattern of zoning can be explained by a process of shallow fractional crystallization in which variations within zones are the result of lateral accretion of alkali feldspar as well as settling and or lateral accretion of mafic phases at lower levels in the intrusion and upward displacement of fractionated magma the parental magma of the pluton probaby originated by partial melting of an alkali basalt composition with an amphibolite mineralogy at the base of the crust

cadomian lower crustal contributions to variscan granite petrogenesis south bohemian pluton austria a reply weinsberg type granite as it is doubtful whether it should preface be named according to ima rules in view of its very the comment by finger clemens 2002 nicely concomplex origin even if we look at these rocks in a firms the statement in the introduction of our paper purely descriptive sense neglecting all genetic aspects klotzli et al 2001 on p 1622 unravelling the origin the granitoids range in composition from minor quartz of a given granite still requires a major analytical effort monzodiorites to a majority of quartz monzonites and and commonly ends in some contradictory results finger granites according to the ima nomenclature the in clemens raise a number of issues both minor and terpretation of finger clemens 2002 that all the rocks major we will discuss all of them as far as they are are quartz monzodiorites is an oversimplification and relevant to the subject we will start with the problem conflicts with the geochemical composition of the pyrof nomenclature after that we will address the problem of oxene bearing rocks their k contents 4 6 wt k2o remnants of an old crustal component versus a cumulate and the constant high amount of modal k feldspar in origin and the problem of the age of the crustal remnants addition in all norm calculations the amount of normative k feldspar has to be corrected according to the measured preserved feldspar composition of or0 78 petrology ab0 20an0 01cs0 01 f koller 2000 finger clemens 2002 in their comment accept the opxand cpx bearing weinsberg type granitoid rocks formation of the weinsberg granite sensu lato magma are widespread in an area of 15 20 km surrounding under granulite facies conditions within the stability field the villages of sarleinsbach sprinzenstein and arnreit of opx as was already postulated by koller hock in the southern bohemian massif klotzli et al 2001 1993 finger clemens interpret opx and cpx as a figs 1 and 2 this type of rock shows a wide variation product of fractional crystallization of a quartz monzoin chemical and mineralogical composition the most dioritic magma after partial melting of biotite bearing conspicuous features are large k feldspars up to 20 cm lower crustal rocks consequently in their view the pyrin size as a consequence the determination of modal oxenes are cumulate phases in contrast we interpret mineralogy must take into account the large size of the the pyroxenes as part of an older magmatic event k feldspar megacrysts and be carried out on large samples only we referred to this rock as pyroxene bearing with a superimposed granulite facies re equilibration

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

fluid present anatexis of metapelites at el joyazo se spain constraints from raman spectroscopy of graphite the garnet biotite sillimanite anatectic xenoliths in the neogene dacite dome of el joyazo also called cerro de hoyazo se spain contain four types of graphite i to iv distinguished on the basis of grain size and texture structural characterization of graphite by laser raman spectroscopy lrs shows systematic differences in the degree of ordering among the four types only type iii is fully consistent with the granulite facies conditions reached by the xenoliths during partial melting the others indicate metamorphic temperatures covering amphibolite facies conditions with only a few examples of granulite grade crystallinity all graphite crystallized before or during the anatectic event indicating that a large fraction of the graphite did not equilibrate at peak temperatures the mm scale coexistence of different types and degrees of ordering in the graphite suggests different origins i e of biogenic derivation and fluid deposited and is explained in terms of fluid melt graphite interaction during the anatectic event disequilibrium behaviour during high temperature metamorphism and anatexis is typical of types i ii and particularly of iv and is attributed to sluggish kinetics of solid state graphitization mainly owing to the limited time of the process and carbon saturation of the intergranular fluid the coexisting well ordered type iii graphite is the product of melting in the presence of a graphite saturated fluid a process that would account for the deposition of new graphite the lrs results together with petrologic observations suggest that it is possible that high melt fractions can be generated by fluid present melting of a metasedimentary protolith also in a closed system although this contradicts the commonly accepted hypothesis that due to limited rock porosities extensive fluid present melting is precluded unless infiltration occurs it is a possible end member model in anatectic settings characterized by rapid heating rates and low grade source rocks

contrasting retrograde oxygen isotope exchange behaviour and implications examples from the langtang valley nepal quantitative modelling of oxygen exchange by diffusion during slow cooling has been compared to the observed oxygen isotope distributions from high grade metamorphic and granitic rocks of the high himalayan crystallines langtang valley central nepal in order to investigate the effect of retrograde diffusional exchange on the preservation of high temperature oxygen isotope systematics modelled fractionations using water present diffusion data reported in the literature predict quartz mica fractionations to be much larger than those at peak metamorphic and igneous conditions due to low closure temperatures for micas quartz feldspar fractionations may be less than those at peak conditions and in some samples may even be slightly negative the observed oxygen isotope fractionations in the metamorphic rocks are small and largely appear to record equilibrations close to peak conditions determined by other methods hence these rocks clearly do not conform to predictions of fluid present diffusional retrograde exchange it is suggested that their retrograde history was therefore within an anhydrous closed system in which diffusion was slow and hence mineral closure temperatures were high the granitic rocks record rather larger quartz biotite fractionations approaching those predicted by the diffusion modelling however quartz feldspar fractionations are large and hence although significant retrograde exchange has clearly occurred simple diffusion alone is not sufficient to explain the observed data and open system exchange may be required the presence of fluids during the retrograde history of this part of the section is supported by petrographic evidence the different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the langtang section would appear to support the importance of water on the kinetics of such exchange and suggests that in its absence diffusional exchange may become insignificant allowing oxygen isotope thermometry to record meaningful high temperature data

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

regional spectral scaling relations of source parameters for earthquakes in the baltic shield abstract spectral analysis of regionally recorded lg waves is performed to determine source parameters such as seismic moment source radius and stress drop of earthquakes in the baltic shield and to derive regional spectral scaling relations the data consist of about 350 lg phases on short period vertical component analog seismograms from earthquakes with magnitude m l ranging from 2 to 5 2 source radii estimated from the corner frequency show only a slight increase with increasing seismic moment in the range 3 10 18 to 5 10 20 dyn cm while the increase is more rapid for larger events baltic shield earthquakes show increasing stress drops ranging from about 0 1 to 10 bars for increasing seismic moment the relatively low stress drops could in part be explained by bias due to band limited analog data the slope of the curve relating seismic moment to corner frequency is steeper than 3 which suggests a departure from a constant stress drop scaling relation proposed for large earthquakes the derived relationship between the seismic moment m o and magnitude m l is log m o 16 93 1 01 m l for 2 m l 5 2

geochemistry and origin of archean granites from the black hills south dakota the little elk granite 2549 ma and granite at bear mountain bmg 2 5 ga of the black hills formed as a result of a collisional event along the eastern margin of the wyoming province during th

fluid absent melting and the roles of fluids in the lithosphere a slanted summary abstract the presence or absence nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks despite this confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings however experimental and field investigations constrain these roles and we are able to make some valid generalizations studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments this commonly indicates internal lithological control of volatile activities under fluid present conditions crustal fluids do not appear to mix readily fluid flow is generally structurally or lithologically controlled the transition from amphibolite to granulite facies is characterized by a decrease in ah2o as well as a temperature increase in many terranes these phenomena correlate with the appearance of migmatites some granulites are not migmatitic but have chemical and mineralogical characteristics of melt depleted restites this indicates efficient extraction of melts incongruent melting reactions involving the breakdown of crystalline hydrates in crustal rocks produce granitoid melts plus denser restites that are nearly or actually anhydrous experimental and theoretical treatments of fluid absent melting show that micas and amphiboles decompose at 700 to 900 c for mid to lower crustal pressures the melts resemble the compositions of s and i type granitoids pervasive aqueous fluids could not generally have been present during this melting since exposed granulite terranes are not characterized by high melt proportions and the magmas themselves are water undersaturated likewise the general presence of very h2o poor pervasive fluids is unlikely since this would allow only subsolidus dehydration at normally attainable crustal temperatures in this case the amphibolite granulite transition is possible at relatively low temperatures and the resultant rocks would not contain evidence of in situ melting the granulite granite connexion and effective crustal differentiation seem viable only during high temperature fluid absent metamorphism data on continental geotherms show that the lower crust is not entirely granulitic granulites are produced in regions of anomalously high heat flow the mantle connexion thus inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities and probable fluid absent conditions mantle derived basaltic magmas too are volatile bearing and it is plausible that volatile fixing phases also occur in the mantle volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals as in the crust

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

controls on the mineralogy of s type volcanic and plutonic rocks abstract far from being simply two mica granites s type rocks contain highly diverse mineral assemblages the most important chemical factors influencing the mineralogy of an s type rock are peraluminosity bulk feo mgo mg mg fe k na ca and the fugacities of volatile species such as h2o hf h3bo3 and o2 however it is not just initial melt chemistry that determines the mineral assemblage the pressure and temperature of crystallization exert major effects as magmas evolve the p t conditions volatile fugacities and melt chemistry change often leading to changes in stable mineralogy many s type granites and silicic volcanic rocks contain partly or completely re equilibrated mineral assemblages recognition of these may provide important information about early crystallization conditions and the nature of the magma source terrains

diffusion controlled biotite breakdown reaction textures at the solid liquid transition in the continental crust two phase quartz intergrowths with garnet cordierite and tourmaline occur commonly in prograde high temperature migmatites granulites as well as in the last crystallization stages of biotite granites structural microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release giving quartz in silica saturated rocks biotite breakdown and growth of al rich ferromagnesian minerals occurring at the solid liquid transition in the crust early melting or final crystallization is kinetically controlled by fe and mg mass transport the network forming cations si and al being locally compensated for by feldspar dissolution crystallization this process leads to significant changes with respect to equilibrium dehydration melting reactions wherein quartz is a reactant and k feldspar a reaction product therefore quartz inclusions commonly occurring in garnets from granulite facies metapelites and metagraywackes are not simply grains passively included during garnet growth they may also correspond to newly crystallized phases resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in al than expected under equilibrium conditions hence excess alumina in granulite facies rocks is not necessarily related to initial alumina rich whole rock compositions as currently considered but may be due at least partly to kinetics of melting

beginning of melting and composition of first melts in the system qz ab or h 2 o co 2 solidus temperatures have been determined for minimum melt compositions in the system qz sio2 ab naalsi3o8 or kalsi3o8 at p fluid 2 5 and 10 kbar and at various water activities the dry solidus was investigated in a dry argon environment water activities ah2o between 0 0 and 1 0 were obtained by using h2o co2 mixtures the or ab or ratio of first melts increases considerably with decreasing water activity at 10 kbar it is 0 28 in the water saturated system and 0 56 at water activity 0 1 the qz content does not change with changing water activities the ab content of minimum melts formed at high pressures and low ah2o may remain almost constant in ascending magmas that are cooling and crystallizing qz content increases at the expense of the or component solidus temperatures decrease considerably when ah2o increases slightly from zero at 10 kbar the temperature difference between dry melting and the solidus for ah2o 0 1 is 120 c the influence of pure co2 on the solidus is very limited in the investigated p t range the solidus is approximatively 760 c at ah2o 0 5 between 2 and 10 kbar and approximatively 830 c at ah2o 0 3 this means that melting of quartz feldspar assemblages may induce dehydration reactions at p t conditions of the granulite facies

distribution of partial melt in a felsic system the importance of surface energy the texture distribution and infiltration tendency of a quartz albite melt in equilibrium with a synthetic texturally equilibrated quartzite was examined in a series of distribution and infiltration experiments at 1 250 c and 8 kbar hydrostatic pressure wetting angle measurements from melt distribution experiments show a dihedral angle of 60 degrees implying a quartz quartz interfacial energy approximately 1 7 times the quartz melt value because of this specific relationship between interfacial energies the system can achieve its lowest surface free energy state with the melt either in pools or along grain edge intersections possibly forming some interconnected channels stability of melt in pockets and along grain edge intersections was observed in a fourteen day dispersed melt experiment yet melt pools failed to disperse into the quartzite during infiltration experiments comparison of the observed dihedral angle with previously measured surface energy values for the melt and quartz shows excellent agreement and also demonstrates that an aggregate of randomly orientated anisotropic grains acts approximately isotropically

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

stratigraphy and eruptive history of the late devonian mount pleasant caldera complex canadian appalachians stratigraphic petrographic and geochemical evidence indicate that the volcano sedimentary rocks of the late devonian piskahegan group located in the northern appalachians of southwestern new brunswick represent the eroded remnants of a large epicontinental caldera complex this complex the mount pleasant caldera is one of few recognizable pre cenozoic calderas and is divisible into exocaldera intracaldera and late caldera fill sequences the intracaldera sequence comprises four formations that crop out in a triangular shaped area and includes thick ash flow tuffs thick sedimentary breccias that dip inward and stocks of intermediate to felsic composition that intrude the volcanic pile or are localized along caldera margin faults the exocaldera sequence contains ash flow tuffs mafic lavas alluvial redbeds and porphyritic felsic lavas that comprise five formations the late caldera fill sequence contains rocks that are similar to those of the outflow facies and comprises two formations and two minor intrusive units geochemical and mineralogical data support the stratigraphic subdivision and indicate that the basaltic rocks are mantle derived and have intraplate chemical affinities the andesites were probably derived from basaltic magma by fractional crystallization and assimilation of crustal material the various felsic units are related by episodes of fractional crystallization in a high level zoned magma chamber fractionation was repeatedly interrupted by eruption of material from the roof zone such that seven stages of caldera development have been identified the genesis of the caldera is related to a period of lithospheric thinning that followed the acadian orogeny in the northern appalachians

u pb geochronology and origin of granitoid rocks in the thetford mines ophiolite canadian appalachians we integrate new and existing age data from ophiolite and subophiolite rocks in the thetford mines region quebec appalachians to determine the timing of 1 oceanic crust formation 2 oceanic decoupling 3 ophiolite emplacement onto the continent and 4 postemplacement taconic orogenesis few ophiolites possess high precision temporal data for all these events this allows us to develop models to explain the origin of emplacement related granitoids that have intruded the ophiolite u pb zircon ages have been determined for four samples from the thetford mines ophiolite oceanic plagiogranite from the lac de l9est region of the thetford massif yields an age of 480 2 ma this age is identical to that of the adjacent mont ham massif thrust sheet 478 3 2 ma suggesting a close temporal and spatial association of these two units in the source oceanic domain two peridotite hosted granitoid samples yield crystallization ages of 470 5 3 ma and 469 4 ma a precambrian inherited zircon component is also present in one sample these granitoids are strongly peraluminous contain abundant inherited zircon exhibit variable but high 87 sr 86 sr initial ratios and low 208 pb 206 pb ratios in igneous zircon which suggest that they were derived by partial melting of laurentian margin sedimentary rocks the ages indicate that the ophiolite was thrust over the laurentian margin within 1 17 m y of oceanic crustal crystallization and that these obduction related granites were generated during a period of shear heating calculations indicate that shearing can generate the peridotite hosted granitoid melts and their observed contact zone mineralogies under specific conditions although margin sedimentary rock melting in response to west dipping subduction related arc magmatism cannot be excluded

combustion metamorphism of bituminous sediments and the formation of melts of granitic and sedimentary composition during combustion metamorphism i e the heating of sediments rich in organic matter by spontaneous subsurface combustion temperatures obtained are frequently high enough to cause partial or total melting of the original rocks these melts are of particular geochemical interest because at lower temperatures their composition closely simulates that of granitic and at higher temperatures that of common sedimentary rocks at the california localities studied here the parent rocks are mainly bituminous mudstones with smaller amounts of diatomites and phosphorites and lesser dolomites limestones shales and cherts it is estimated that rock melting started below 1000 c the first partial melts correspond to a melt fraction of about 5 have a constant composition which is controlled by the eutectic of the dominant mudstones and are undistinguishable in major and trace element composition from common s type granites compared with the original rocks these melts are enriched in sio2 al2o3 k2o na2o as well as in ta hf zr th and ree and are depleted in all other elements studied the only difference between these low temperature melts and granitic ones is their oxygen isotope composition which is that of the sedimentary parent rocks these melts intruded the country rocks as sills and dikes and cooled to glassy rocks simulating obsidians as temperatures rose the melts changed composition and left the magmatic field at the highest temperatures in excess of 1650 c their composition equals that of the original sediments from which they are derived except for volatile components such as h2o and co2 these melts formed intrusive bodies mainly stocks up to several kilometers across on cooling these melts formed fine grained but holocrystalline rocks the dominant minerals of which are cristobalite and tridymite calcic plagioclase cordierite wollastonite gehlenite andradite and apatite where the parent rocks included phosphorites two immiscible melts formed an emulsion in which droplets of apatitic composition are surrounded by a silicate melt almost devoid of p2o5

crystallization history of granophyric intrusives from the peruvian coastal batholith abstract in the huaura centred complex a component of the coastal batholith of peru water undersaturated granitic magma of the san jeronimo suite rose from an environment of 4 7 kb pressure to be emplaced at pressures of ca 0 5 kb within the suite different magma compositions originated from the partial melting of contrasting quartz rich and quartz poor source rocks initially marginal crystallization resulted in granitic textures with early precipitation of plagioclase quartz magnetite biotite and hornblende under relatively reducing conditions subsequent residual concentration of water and oxygen in the centre of the pluton gave rise to more oxidizing conditions and ultimately water saturation resulted in rupture of the pluton roof the consequent quenching resulted in extensive development of granophyric textures in the residual magma distribution of rb sr and major elements can be explained largely by fractionation of ca 18 of the observed phenocryst assemblage ba depletion and rb enrichment with evolution of the magma suggest an important role for diffusive transfer during the late episode of granophyre crystallization

high pressure metamorphism in the high himalayan crystallines of the stak valley northeastern nanga parbat haramosh syntaxis pakistan himalaya

fluid absent melting behavior of a two mica metapelite experimental constraints on the origin of black hills granite we have studied experimentally the vapor absent melting behavior introduction of a proterozoic metapelite hp 60 1 from the black hills south peraluminous leucogranites such as the miocene granites dakota to assess whether the high do tourmaline bearing granites of the himalaya and tourmaline granites of the harney of the harney peak suite resulted from muscovite dominated depeak suite in south dakota are widely considered to be hydration melting at 10 kbar samples were sealed with no added the product of muscovite dehydration melting yet few h2o in gold capsules then run in a 0 5 inch 1 27 cm pistonexperimental studies of muscovite dehydration melting cylinder apparatus over the temperature interval 812 975 c reactions have been performed because the low temexperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies mass balance calculations imply librium in addition melt fractions generated are typically the melting reaction muscovite biotite plagioclase quartz low resulting in small melt pockets which are very difficult melt alkali feldspar aluminosilicate garnet until to analyze with the electron microprobe in this study muscovite is exhausted 812 c then the reaction biotite we assess the possible genetic relationship between the plagioclase quartz alkali feldspar aluminosilicate tourmaline bearing granites of the harney peak suite melt garnet the inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude 812 c and produces 1 2 wt melt melt production of the although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature hydration melting reaction was hindered by the surto 32 wt in our highest temperature run the low melt prisingly low melt productivity we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid absent muscovite breakdown under the fractions by muscovite dominated reactions in our starting material conditions investigated in addition melts of hp 60 1 are considerably more mafic than isotopic and trace element studies indicate a genetic the tourmaline bearing granites with average combined tio2 relationship between s type granites and aluminous metamgo feo contents of 1 85 wt and 0 84 wt respectively sediments in the black hills of south dakota where the in particular melts of hp 60 1 are enriched in ti and would harney peak granite hpg intruded proterozoic and probably stabilize biotite rather than tourmaline upon cooling archean schists and metagreywackes undergoing regional arguing against hp 60 1 as a potential source rock for the metamorphism the timing and mode of emplacement of tourmaline granites the hpg and the deformation and regional metamorphic

the effect of feldspar on quartz h 2 o co 2 dihedral angles at 4 kbar with consequences for the behaviour of aqueous fluids in migmatites an investigation was made of the effect of trace amounts of feldspar na and or k on dihedral angles in the quartz h2o co2 system at 4 kbar and 450 1050 c quartz quartz h2o dihedral angles in feldspar bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at temperatures below 500 c above this temperature they decrease with increasing temperature until the solidus the final angle at the inception of melting is about 65 for microcline quartz h2o and microcline albite quartz h2o and much less than 60 the critical value for formation of grain edge fluid channels in an isotropic system for the albite quartz h2o system co2 was observed to produce a constant quartz quartz fluid dihedral angle of 97 in feldspar bearing quartz aggregates at all temperatures studied also examined were the dihedral angles for the two co existing supersolidus fluids in quartz aggregates in all systems the quartz volatile fluid angle is greater than 60 whereas the quartz melt angle is lower than 60 both supersolidus angles decrease with increasing temperature the transition from nonconnected to connected porosity with increasing temperature observed in the quartz albite h2o system some tens of degrees below the solidus termed a permeability transition if a common feature of rocks near their melting points will play an important role in controlling the permeability of high grade rocks to aqueous fluids

mechanics of formation of gold bearing quartz veins nova scotia canada abstract the meguma terrane of nova scotia canada is composed largely of low grade clastic metasediments swarms of structurally complex bedding concordant quartz veins are found across the whole terrane partly or wholly outlining anticlinal domes these veins exhibit spectacular features due to intermittent hydraulic fracturing and healing crack seal such as phyllosilicate inclusion bands columnar quartz grains and included sheets of host rock overprinting all the precipitation features are a suite of deformation structures which include pinch and swell tectonic stylolites and cross cutting extension and hybrid fractures the veins were formed syntectonically in folded and foliated strata the vein precipitating fluids pooling beneath less permeable beds in anticlinal domes fluid migration paths were controlled by the two fundamental mechanical anisotropies in the host rocks bedding and foliation a simple two dimensional vein model is considered using fracture propagation and quartz precipitation rates estimates of rates of vein formation are determined for example it seems possible that a quartz vein 100 m long and 175 cm wide could form in less than 10 yrs such times are effectively geologically instantaneous and have considerable implications with respect to physical and chemical processes in deformation and metamorphism

experimental study of a biotite bearing granitic system under water saturated and water undersaturated conditions phase relationships in haplogranitic melts con taining ferromagnesian minerals were investigated in the temperature range 760 850 c at ptotal 2 kbar ah2o 0 25 1 00 and at nno buffer conditions the composi tions of coexisting biotite orthopyroxene magnetite and granitic melt were determined by electron microprobe bio tite was the only fe mg mineral in the starting material it disappeared in water undersaturated melts at tempera tures above 830 c and in melts saturated with water above 810 c magnetite was present in all runs orthopyroxene became stable between 780 and 800 c the mg mg fe ratios of biotite 0 63 0 72 and orthopyroxene 0 63 0 71 are similar in water saturated and undersaturated melts at comparable temperatures and increase slightly with increas ing temperature the solubility of mg in haplogranitic melts is very low 0 07 0 24 wt mgo it increases slightly with temperature and is independent of the prevailing water ac tivity the solubility of fe is low 0 91 1 37 wt feo it also increases with increasing temperature and it is higher in water saturated than in water undersaturated melts

ordovician metamorphism and plutonism in the sierra de quilmes metamorphic complex implications for the tectonic setting of the northern sierras pampeanas nw argentina towards unravelling the geodynamic setting of the northern sierras pampeanas nw argentina we describe the tectono metamorphic and geochronologic evolution of sub greenschist to granulite facies metamorphic sediments granitoid plutons and pegmatites in the ordovician sierra de quilmes metamorphic complex the protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the neoproterozoic to cambrian puncoviscana formation the metamorphic complex consists of four zones including the 1 chlorite 2 biotite muscovite 3 garnet cordierite sillimanite and 4 orthopyroxene zones zones 3 and 4 show an increasing degree of anatexis reaching large scale diatexis in the orthopyroxene zone at p t conditions exceeding 800 c and 600 mpa at or shortly after the metamorphic peak the granitic to tonalitic cafayate pluton intruded approximately along the boundary between anatectic and non anatectic rocks retrograde near isobaric cooling of the middle crust was accompanied by non penetrative ductile shearing at granulite to amphibolite facies p t conditions evidence for significant prograde deformation is absent in the sierra de quilmes metamorphic complex monazite and titanite u pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to 470 ma retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between 470 ma and 455 ma u pb data the composite cafayate pluton intruded over a time interval of several million years between 477 ma sm nd isochron and 460 ma monazite and titanite u pb isochron followed by pegmatites a younger group of pegmatites was emplaced in the country rocks at the end of the ordovician 440 ma rb sr mineral isochrons postdating most of the retrograde shear zones resetting of the muscovite k ar and 40ar 39ar system in weakly deformed pegmatites and crystallisation of new muscovite in low grade metamorphic sediments at 400 416 ma is attributed to minor late greenschist and sub greenschist facies metamorphism and deformation the massive early ordovician heat transfer the absence of major prograde deformation and subsequent prolonged phases of non penetrative deformation in the high grade metamorphic zones during slow near isobaric cooling contradict crustal thickening and continent collision models as have been proposed for the southern sierras pampeanas we suggest continuous or stepwise extensional tectonics in a back arc or a mobile belt tectonic environment for the ordovician sierra de quilmes and the northern sierras pampeanas in general an extensional setting of the northern sierras pampeanas in the ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the sierras pampeanas in northwestern argentina and southern bolivia

opening the magmatic hydrothermal window high precision u pb geochronology of the mesoproterozoic olympic dam cu u au ag deposit south australia

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment