the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

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

the granite problem as exposed in the southern snake range nevada a geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern snake range of eastern nevada the granitoids exposed range in age from jurassic through cretaceous to oligocene and include two calcic intrusions two different types of two mica granites and aplites the younger intrusions appear to have been emplaced at progressively more shallow depths all of these granitoid types are represented elsewhere in the eastern great basin but the southern snake range is distinguished by the grouping of all these types within a relatively small area

igneous and metamorphic enclaves in the s type deddick granodiorite lachlan fold belt se australia petrographic geochemical and nd sr isotopic evidence for crustal melting and magma mixing the deddick granodiorite a mafic s type pluton in the lachlan introduction fold belt contains abundant enclaves most are derived from enclaves in granites and volcanic rocks can provide high grade metasediments of pelitic psammitic composition and petrogenetic information not readily available from their migmatites are common among these discrete fragments of host rocks didier 1973 didier barbarin 1991 for melanosomes rich in cordierite and garnet are restites from partial example metasedimentary enclaves can place constraints melting however compositional and nd sr isotopic data indicate on the age composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust fleming host magma but may have formed part of a spectrum of diverse 1991 steele et al 1991 enclaves formed by acmagma source lithologies alternatively they may be accidental cumulation of early phenocrysts autoliths document xenoliths regardless of their origin data for these enclaves suggest the early evolution of the host magma whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous ordovician turbidites of the lachlan fold belt three possible role of hybridization didier 1987 types of microgranular enclaves with igneous textures are disenclave suites in the relatively mafic s type granitoids tinguished the most common type are small and rounded mafic of se australia are often diverse including abundant enclaves of tonalitic composition they carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma and some contain high mg pyroxene derived from enclaves phillips et al 1981 price 1983 chen et al a mafic magma isotopic data form an array with end from 6 to 1989 wyborn et al 1991 the metasedimentary en 12 and sr sr from 0 7130 to 0 7167 host rock 10 and claves are lithologically diverse including gneissic 0 715 respectively these enclaves formed as globules of hybrid migmatitic types and biotite rich surmicaceous types mafic magma commingled with and contaminated by the more apart from their potential as indicators of crustal structure felsic host magma other enclave types were derived from disrupted and composition they are of interest as possible fragments syn plutonic dykes having a distinct isotopic composition and from of source material and or refractory residue from which a cogenetic marginal facies of the host peraluminous melts were generated and extracted by

models for granitoid evolution and source compositions this paper critically examines the bases of various models for the chemical evolution of granitoid suites the proposed mineralogical textural and chemical criteria for the recognition of restite components are found to be equivocal and can be plausibly explained by crystal fractionation and accumulation as well as by magma mixing and assimilation of non restitic solids these processes demonstrably account for the bulk of the chemical variation in a variety of granitic suites although restite controlled variation may have played a role in some types of metasediment derived suites granitoid magmas with non minimum melt chemistry are probably rather common the petrographic geochemical and isotopic features of individual plutons should be assessed carefully to elucidate the relative importance of various mechanisms for chemical evolution conclusions as to the character of magma source materials and magma evolution paths that are based on the restite model should be treated with caution

the pb sr nd isotope geochemistry of some recent circum mediterranean granites pb sr and nd isotopic compositions have been analyzed in recent granites from northern africa northern italy and greece lead isotope compositions of k feldspars are rather homogeneous and cluster close to the modern lead of stacey and kramers 1975 but with slightly higher207pb 204pb and208pb 204pb ratios the cyclades samples however have higher206pb 204pb ratios addition of mantle derived lead was probably very limited which supports a quasi closed system evolution of this element in the continental crust the sr nd data fall in the enriched part of the143nd 144nd vs 87sr 86sr diagram and define a smooth hyperbolic mixing curve over a wide area straddling different orogens most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle like component no correlation is observed between either pb and sr or nd isotopic ratios or any isotopic ratio and major element contents quantitative modelling suggests that two cases fit the sr and nd characteristics of these granites they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes in the first case the mantle derived component may be represented by differentiated island arc type magmas and the granites result from mixing these magmas with anatectic melts in the second case mantle derived igneous rocks such as obducted ophiolites are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations

three dimensional accommodation analysis of the triassic in the paris basin a new approach in unravelling the basin evolution with time abstract the mechanisms governing the development of the paris basin throughout the triassic are regarded as being the result of superimposed and successive processes in this study the triassic succession of the paris basin was re interpreted in a sequence stratigraphic context using essentially wireline log data from this a series of isopach maps lithofacies maps and palaeobathymetric maps was produced for each sequence three dimensional accommodation analysis was then carried out sequence by sequence over the entire basin to produce a precise detailed accommodation history for the entire triassic succession previous studies have proposed that the triassic was deposited during a rift period in a transtensional stress regime with the formation of a trough superposed onto three fault systems derived from the variscan structural framework in this study scythian to ladinian sediments buntsandstein and muschelkalk record the stress regime that prevailed over much of nw europe the basin architecture at this time is in continuity with the neighbouring germanic basin our three dimensional accommodation modelling shows that the stress regime changed during the carnian and the late norian keuper the carnian events are marked by 1 the creation of a large depocentre infilled with halite and 2 a northwest migration of this depocentre during the mid late carnian along with deposition of the gres a roseaux an extensive fluvial deposit this documents renewed strike slip movement along the bray fault the norian events involved major tectonic uplift on the basin margins producting fan delta progradation into the basin rotation of the previous depocentre axis occurs on the downthrown side of the bray fault this may be viewed as a consequence of sinistral strike slip displacement along the bray fault forming a local transpressive stress regime the following liassic cycle commenced with the rhaetic sequences and illustrates a complete change in the stress regime this corresponds with the new stress regime which prevailed over northwest europe during the liassic cycle accommodation curves from the basin complement this interpretation recognising two major accommodation phases which are separated by a significant unconformity both phases record periods of accelerating accommodation followed by a more uniform phase of decelerating accommodation the accelerating phases correspond to periods of rapid accommodation space creation and result in thick evaporite deposits they correspond to rift pulses which occurred in the remote north atlantic and tethyan domain each accommodation phase is proposed to correspond to lithospheric stretching pulses followed by a relaxation period this study illustrates the importance of sequence stratigraphy coupled with three dimensional accommodation analysis in refining important stages in the basin evolution with time

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

origin of sierra nevadan granite evidence from small scale composite dikes composite dikes at hell hole meadow in the central sierra nevada contain hybrids created by small scale mixing of andesitic and rhyolitic magmas early rhyolitic injections had partially solidified when subsequent andesitic magmas arrived and mixed with small increments of remnant rhyolitic magma in major element chemistry the hybrid rocks define linear mgo variation diagrams that closely resemble those for the half dome mt givens and eagle peak granodiorites the patterns suggest that mixing of mafic and felsic magmas has been important in the evolution of these plutons

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

a thermal history of the proterozoic east alligator river terrain n t australia a fission track study abstract radiometric data indicate a major thermal event in proterozoic rocks of the east alligator river terrain at 1870 ma these data together with metamorphic mineral assemblages demonstrate peak temperatures in excess of 600 c close to the melting temperature of more deeply buried rocks a cooling rate following peak metamorphism of 3 c ma is suggested fission track dates of peak metamorphic phases however reveal a thermal event or events after 1650 ma rather than the peak metamorphic event this rise in temperature was the result of thermal blanketing of the metamorphic basement by carpentarian sediments and anomalous radiogenic heat flow from underlying granitoid gneiss the temperatures so generated 175 c were insufficient to reset rb sr and k ar systems but are clearly in excess of f t annealing temperatures for all the phases investigated a cooling history extending over 1000 m y and reflecting gradual erosion of the sedimentary cover is revealed this history is consistent with the extraordinary tectonic stability of the region the importance of f t studies in establishing a thermal history is underscored particularly when maximum temperatures experienced were less than those required to reset rb sr and k ar systems

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

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

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

chemical modification of enclave magma by post emplacement crystal fractionation diffusion and metasomatism hornblende bearing microgranitoid enclaves from the swifts creek pluton scp se australia display mineralogical and textural variations from their margins to their centers margins are fine grained and display quench textures and are enriched in amphibole biotite and in some cases magnetite relative to their coarser grained centers enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0 3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes the fine grained margins are variably enriched in ti al mg fe mg k rb ba nb y sc v ni and the ree up to a factor of three relative to the enclave centers this enrichment is compensated by a depletion of si and zr elements like ca na and sr show less coherent variation from margin to center host rocks in immediate contact with these enclaves are depleted in mg na k rb and ba relative to host rocks that are not exposed to enclaves no one single process can account for all the variations instead we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules mass transfer by diffusion and metasomatic exchange

chemistry of the ordovician and silurian greywackes of the snowy mountains southeastern australia an example of chemical evolution of sediments with time abstract the ordovician quartz rich greywackes of the snowy mountains region are essentially mixtures of two components these are quartz and clay minerals the chemistry of any rock is largely determined by its proportions of clay minerals so that there is a strong negative correlation between si and those elements other than si contained in clay minerals elements that occur only in feldspar but not in clay minerals that is ca na sr and pb have a low and variable concentration these four elements have even lower abundances in the silurian greywackes deposited in basins on a basement of the ordovician rocks this is interpreted to have resulted from a further cycle of chemical weathering that acted on ordovician source rocks to produce the silurian detritus in a similar fashion the ordovician rocks are thought to have formed by weathering of more feldspar rich sedimentary rocks at the margins of the lachlan fold belt two episodes of chemical weathering are therefore recognised the first produced the feldspar poor ordovician rocks the second acted on part of these to form the almost feldspar free silurian strata in both cases ca na sr and pb were lost as feldspars were converted to clay minerals

mafic inclusions from the glenbog and blue gum granite suites southeastern australia mafic inclusions from the i type granites of the glenbog and blue gum suites of the lachlan fold belt of southeastern australia are less than 1 m and mostly less than 10 cm in size and are generally in sharp contact with the surrounding host granites unless otherwise stated the term granite is used in this paper in the general sense to include granite senso stricto adamellite granodiorite tonalite etc although some inclusions are irregular in shape the majority have an elliptical or circular outline when viewed on exposed surfaces rounded and subrounded individuals outnumber those with an angular appearance abundances of inclusions in seven plutons of the glenbog and blue gum suites range from about 6 to less than 0 1 their abundance is negatively correlated with the sio2 content of the host rock mafic inclusions contain the same mineral species as the host granite but plagioclase and hornblende are more abundant and quartz and k feldspar are less abundant in inclusions mafic inclusions all contain more than 54 sio2 and are mostly intermediate in composition the abundances of many elements in the mafic inclusions follow those of that element in the host granites sr and nd isotopic compositions in both the granites and the inclusions are similar and relatively evolved various models for the origin of the inclusions are considered it is concluded that of those considered the only model capable of explaining all data is that the inclusions represent fragments of more refractory source material that were carried up with the magma it is suggested that this more refractory character resulted from a lesser degree of hydration of the source rocks of the magmas prior to partial melting

h type hybrid granitoids a proposed revision of the granite type classification and nomenclature abstract recent studies on calc alkaline plutonic rocks reveal field and petrographic relationships with strong implications for the processes involved in their genesis the presence of magmatic inclusions and magma mingling zones in these rocks supports an origin by magma mixing compositional variations and isotopic anomalies are a good test for such petrogenetic models most of these calc alkaline plutonic rocks which characterize both collision related and active plate margin environments have been classically identified as i igneous type granitoids following chappell and white s 1974 classification and then interpreted as derived from partial melting of older igneous rocks restite model if the magma mixing origin of a type of granitoids can be demonstrated from field and petrographic evidence and supported by chemical and isotopic variations the nomenclature must be changed in this sense the new category of h hybrid type is tentatively introduced it includes most of the so called i type granitoids and some of the s type as deduced from field relationships of magma mingling zones a mixing model is suggested that can operate on a large scale to explain the petrogenesis of the hercynian calc alkaline granodiorites of iberia finally a modified classification of common plutonic rocks in orogenic environments is proposed

chemical and isotopic evidence for major mass transfer between mafic enclaves and felsic magma abstract in both the strontian and criffell zoned granitoid plutons certain chemical elements notably k rb ca sr and pb display a correlation between their concentration in microdiorite enclaves and their concentration in the immediately enclosing host granitic rock this appears to represent the effects of post incorporation mixing and exchange between the enclave and its host magma enclave initial 87 sr 86 sr ratios are indistinguishable from those of the host while enclave initial 143 nd 144 nd ratios are systematically more radiogenic establishing that these enclaves are not melt segregations nor restite the isotopic data are in agreement with the chemical data in indicating that sr is almost entirely derived from the host despite clear evidence of major mass transfer of alkali metals alkali earths and transition metals a comparison of the concentrations and ratios of hfse and ree and nd isotopic ratios with those of a number of potential starting protoliths allows the enclave precursors to be positively discriminated in this regard enclave hfse and ree compositions may allow for models of granitoid production by the injection of hot mantle derived magmas into the crust to be tested

intracrustal detachments and implications for crustal evolution within the lachlan fold belt southeastern australia seismic reflection profiling in the southern lachlan fold belt australia indicates geometrically complex mid to lower crustal detachments possibly associated with crustal thickening by tectonic underplating of collided microplates and or oceanic plateaus low angle detachments either intersect or sole into the crust mantle boundary forming a crustal stacking wedge typical of lithospheric or a type subduction upper crustal reflections show that major moderately to steeply dipping reverse faults exposed at the surface flatten with depth and have listric form the chevron folded quartz rich turbidite sequence of the lachlan fold belt is therefore allochthonous with respect to the lower crust like the northern appalachian maritimes the lachlan fold belt has a composite crust with diverse lower crustal blocks overlain by allochthonous imbricated terranes of the surface rocks

isotope and chemical microsampling constraints on the history of an s type rhyolite san vincenzo tuscany italy fine scale heterogeneities in strontium isotope ratios 40ar 39ar ages and chemical com sition have been determined for individual mineral grains and enclaves in the rhyolites of san vincenzo italy the rhyolites are peraluminous with phenocrysts of alkali feldspar piagioclase biotite and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry group b lavas are distinguished from group a in having higher mgo and cao contents as well as containing two pyroxenes and chilled latite enclaves the group b lavas show textural evidence for d sequilib um such as resorption and sieve zones in feldspars laser fusion 40ar 39ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4 38 0 04 ma though small amounts of excess argon are present in subhedral and cloudy crystals the groundmass in the group a sample has a higher initial r7sr 86sr 0 725 than that of the group b 0 7 13 the sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within grain zonation the feldspars and coarse grained biotites have initial 87sr 8 sr that are intermediate between those of the group a and b glasses with alkali feldspar generally having higher initial 87sr 86sr ratios than plagioclase chilled latite enclaves which are present in only the group b lavas have relatively low initial 8 sr 86sr ratios in the range 0 7082 0 7088 the mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and therefore cannot be the source of the strontium isotope dis equilibrium xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts clinopyroxene orthopyroxene clots orthopyroxene plagioclase clots and plagioclase with extensive sieve textures these grains all have relatively low initial 87sr 86sr ratios other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite spinel and sillimanite and alkali feldspars with excess 4 ar the strontium isotope di quilib um in phenocrysts of plagioclase alkali feldspar and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration a mode1 for the san vincenzo magma chamber is proposed in which the sr isotope evolution of the group b rhyolites is due to magma mixing of a crustal melt with latite magma containing unra diogenic strontium the variable initial strontium isotope compositions of group a phenocrysts may be due to assimilation or crystallization along the interface between mingled group a and b magmas

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria