discrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event visean niger abstract classical petrographical and geochemical approaches have been used to study visean coal seams sampled in two areas 50 km apart in northern niger t max values given by rock eval pyrolysis and individual molecular ratios determined on hopanes and steranes are in agreement with the burial history of the two sets of samples studied the difference of maturity of the two sample sets revealed by these various parameters is consistent with their current difference in burial depth 400 m only small changes of these parameters with depth were observed owing to the small depth interval sampled at both sites 15 and 40 m these variations also appear to be related to normal burial diagenesis no maturity differences between the two sample sets are revealed by vitrinite reflectance data and atomic h c ratios which are in the same range at both sites but display very large variations with depth r v 0 60 1 40 and h c at 0 80 0 90 to 0 55 these data indicate rapid entrance into the oil window this conclusion also being supported by microspectrofluorimetric observations showing oil production and expulsion and by the presence of pyrobitumens containing large devolatilization vacuoles in the deepest samples the abnormally high maturity gradient could have been caused by a hydrothermal event the present observations on coals confirm and supplement previous conclusions drawn from the study of organic materials disseminated in zn pb sulfide ore bearing sedimentary rocks the difference in sensitivity shown by the various maturity indicators is considered very likely to be due to differences in the kinetics of the transformations on which they each depend

igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

silurian orogenesis in the western penobscot bay region maine new 40ar 39ar mineral ages from rocks collected west of penobscot bay maine indicate this region was regionally deformed metamorphosed to amphibolite facies conditions and intruded by plutons in silurian times rather than in the devonian as previously assumed disturbed hornblende age spectra along with the presence of some devonian felsic plutons and extensive retrograde metamorphic textures do suggest however that these rocks were subsequently affected by low grade devonian thermal events in sharp contrast rocks west of the sennebec pond thrust fault a major tectono stratigraphic boundary in this region lack a significant silurian tectono thermal signature and instead record the effects of intense devonian deformation and high grade regional metamorphism the data suggest the two regions experienced very different pre devonian histories and were most likely juxtaposed by the sennebec pond thrust fault in latest silurian to early devonian time rocks now exposed east of the sennebec pond faul

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

chemical characteristics of migmatites accessory phase distribution and evidence for fast melt segregation rates equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration armouring slow dissolution when segregation occurs by channelised migration along high porosity pathways melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates evidence for channelised flow and deformation enhanced melt segregation into boudin necks fractures and micro shears at low melt fractions is present in the moine kirtomy migmatitie suite kms in sutherland scotland melt migration distances are on a metre to tens of metres scale concordant leucosomes in stromatic migmatities in the kms have low zr contents low lree light rare earth element and h heavy ree contents and positive eu anomalies ree patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of lree and hree bearing accessory phases in water undersaturated melts melting in the kms however occurred at or near the wet granite solidus leaving biotite as a residual phase detailed back scattered electron imaging shows that ree bearing accessory phases remained as residual phases and were concentrated in the melanosome and at the melanosome leucosome boundary irregularly shaped patches of diatexite contain a small proportion of excess zr consistent with entrainment of melanosome schlieren enriched in zircon these data indicate that deformation enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time scale less than that required to saturate the melt in zr leucosomes were thus prevented from equilibrating with accessory phases before extraction

granite ascent in convergent orogenic belts testing a model the common spatial relationship in convergent orogenic belts between a crustal scale shear zone system high grade metamorphic rocks and granites suggests a feedback relation between crustal anatexis and contractional deformation that helps granite extraction and focuses granite ascent such a feedback relation has been proposed for ascent of early devonian granites in west central maine this interpretation requires that deformation metamorphism and plutonism were synchronous we have determined precise u pb zircon and monazite ages that we interpret to record time of crystallization of syntectonic granite in metric to decametric sheets and kilometric plutons and of schlieric granite within migmatites ages are in the range ca 408 404 ma within 1 m y at 95 confidence limits these ages are similar to extant u pb monazite ages of ca 405 399 2 ma for syntectonic regional metamorphism in the same area the coincidence between the age of peak metamorphism and crystallization ages of granite shows tectonics metamorphism and magmatism were contemporaneous in support of the feedback model

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

progressive localization of deformation during exhumation of a major strike slip shear zone norumbega fault zone south central maine usa detailed structural studies along with 40ar 39ar thermochronology provide important constraints on the temporal spatial and structural evolution of a major orogen parallel strike slip shear zone the norumbega fault zone in maine detailed structural analysis in south central maine reveals two very different styles of dextral noncoaxial deformation a wide zone 25 km of heterogeneously distributed ductile shear structures and a relatively narrow zone 1 km of lower greenschist facies high strain mylonitization 40ar 39ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following middle devonian amphibolite facies metamorphism the wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature post amphibolite facies metamorphism but pre regional cooling below 320 c late devonian to early carboniferous transcurrent tectonism the narrow zone of dextral shear represents both a younger latest carboniferous and significantly cooler 320 c deformational event occurring at much higher structural levels a general model of increasingly narrow but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

geotechnical study on kordofan tropical black soils sudan republic abstract a study was made of the applicability of south kordofan tropical black clays sudan as a construction material for small earth dams in rural areas the broad task is to determine the design parameters and develop simplified design criteria for low cost small earth dams in semi arid regions the present work concentrates on the geological and basic geotechnical characteristics of kordofan tropical black clays soil samples were collected from ten locations in south kordofan province classification grain size distribution compaction tests and shear strength swelling chemical and mineralogical tests indicate that south kordofan black clays are very plastic expansive generally of low permeability and of moderate to low density most of the samples show an irregular compaction curve with peaks at very low water content 6 12 and at higher water content 19 26 probably attributable to the interaction between the physical e g fabric and chemical properties and the mineralogical composition some samples are moderately dispersive as a general conclusion these clays could be used for small earth dams successfully if attention is paid to compaction placement water content shrinkage fissures and filter design

episodic metamorphism and deformation in low pressure high temperature terranes in many cases low pressure high temperature metamorphism must be the consequence of advective heat transfer e g the ascent of granitic magma whereas such heating mechanisms are necessarily short lived the lifetime of the underlying orogenic processes is likely to be much longer and it is conceivable that during the evolution of an orogen repeated transient heating at shallow crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels the number and timing of these events would depend on such factors as critical melt segregation volume moho temperature and strain rate evolution we investigate some of the controls on this multiple event superposition using a simple thermomechanical model designed to predict simultaneously the strain rate and the isostatic and thermal evolution of convergent orogenesis subject to a constant driving force an appealing aspect of this formulation is that in contrast to models that have attempted to explain event cyclicity through episodic processes acting on the orogenic boundaries it accounts for the repeated occurrence of events through processes inherent to the orogenic system itself

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

garnet biotite geothermometry revised new margules parameters and a natural specimen data set from maine the garnet biotite geothermometer has been recalibrated using recently obtained margules parameters for iron magnesium calcium garnet mn interactions in garnet and al interactions in biotite as well as the fe oxidation state of both minerals fe mg and dwal margules parameters for biotite have been retrieved by combining experimental results on 6 al free and 6 al bearing biotite using statistical methods margules parameters per mole of biotite are bt w 5 40 719 2 30t j mole mgfe

biological origin of tetracyclic diterpanes n alkanes and other biomarkers found in lower carboniferous gondwana coals niger abstract the saturated hydrocarbons of coal samples of the anou araren mine tim mersoi basin niger republic have been studied by gc and gc ms chromatograms revealed rather unusual n alkane distributions dominated by compounds in the c 20 c 23 range and showing a rapid and irregular decrease towards higher molecular weights these signatures appear to be typical of the paleozoic gondwana swamps flora unlike typical low maturity terrestrial organic matter characterized by a dominance of odd carbon number n alkanes in the c 27 c 33 range which could be attributed to angiosperms the presence of large amounts of polycyclic diterpanes ent beyerane phyllocladanes and kauranes is unexpected in samples of that age since these compounds are usually thought to be derived from conifers which supposedly had not yet evolved at the time a more likely explanation is that these diterpanes could be derived from biological precursors of the conifers

water washing of proterozoic oil in the midcontinent rift system whole oil gas chromatography shows that petroleum inclusions trapped in 1 05 ga veins in the midcontinent rift experienced variable water washing but no significant biodegradation or phase separation by comparing our results with published experimental data we estimate that water petroleum ratios varied from 200 1 to much greater than 9000 1 extreme water washing with water petroleum ratios very much greater than 9000 1 likely produced pyrobitumen solid hydrocarbon that was locally trapped as primary inclusions in vein calcite hydrothermal ore deposits with their extremely high water rock ratios are one of the best places to search for end member water washed petroleum sedimentary basins however with their variable water rock ratios also have a wide range of water petroleum ratios which significantly affect the composition of crude oils along the flow path between source rock and trap water petroleum ratios may be a useful concept to help evaluate crude oil compositions in a variety of settings

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

experimental partitioning of rb cs sr and ba between alkali feldspar and peraluminous melt hydrous partial melting experiments performed between 650 and 750 c at 200 mpa h20 on synthetic metapelite compositions quartz albite muscovite biotite mineral mixtures doped with ba sr rb and cs yielded alkali feldspar crystals with a wide range of compositions in equilibrium at their rims with peraluminous melt measured partition coefficients for normally trace lithophile elements between feldspar and melt d m fsp gl m ba sr rb cs do not depend on either temperature or bulk composition of melt for the compositions studied values of d sr fsp gl are between 10 and 14 and appear to be independent of the albite and orthoclase contents of the feldspar crystals in contrast values of d ba fsp gl and d rb fsp glare strongly dependent on the orthoclase content of feldspar relationships that can be expressed by the following linear equations d ba fsp gl 0 07 0 25 orthoclase and d rb fsp gl 0 03 0 0 1 orthoclase where orthoclase is in mole percent these equations reproduce the range of previously reported values for d ba and d rb determined on natural and synthetic samples a single partition coefficient for cs was also determined at d cs fsp gl 0 13 these data can be used in conjunction with recently published partition coefficients for muscovite biotite and plagioclase feldspars blundy and wood 1991 icenhower and london 1995 to model quantitatively the trace element signatures of peraluminous magmas during anatexis and crystallization depending on bulk composition and pressuretemperature trajectories during episodes of partial melting ba and sr and to a lesser extent rb are retained at the source while cs and li are enriched in the first melts these results explain why peraluminous granitic magmas with s type sedimentary sources carry a distinctive enrichment in li and cs

mesozoic extensional structures of the fangshan tectonic dome and their subsequent reworking during collisional accretion of the north china block the fangshan area sw of beijing lies at the junction of the nne trending taihang mountain range and the southeastern portion of the yanshan intraplate orogenic belt and has undergone at least five stages of deformation mid to late triassic extensional deformation d 1 is represented by the formation of the fangshan tectonic dome during se directed extensional tectonics this deformation was later modified by nnw directed thrusting in the late triassic d 2 and wnw directed thrusting in the late jurassic d 3 these d 1 d 3 structures were deformed by the arcuate beiling syncline d 4 which formed during the intrusion of the 133 128 ma fangshan pluton d 5 deformation is represented by cretaceous to quaternary nne striking high angle normal faults the ages of these events demonstrate that the principal ene trending tectonic framework of the yanshan intraplate orogenic belt was established mainly in the mid to late triassic the indosinian event the jurassic to cretaceous yanshanian deformation with thick skinned thrusting was associated with intrusion of numerous plutons and strongly modified the older structures the tectonic evolution of the fangshan tectonic dome was linked genetically with the collision of the north china block and the siberian craton and the later collision of the north and south china blocks

what controls partial melting in migmatites the layers of six stromatic migmatites from northern western and central europe display small but systematic chemical and mineralogical differences at least five of these migmatites do not show any signs of largescale metamorphic differentiation metasomatism or segregation of melts it is concluded therefore that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks almost isochemical partial melting seems to be the most probable process transforming layered paragneisses metavolcanics or schists into migmatites the formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers the neosomes have less biotite and more k feldspar if k feldspar is present at all than the adjacent mesosomes these differences are small but systematic and seem to control the access of different amounts of water to the various rock portions petrographical observations chemical data and theoretical considerations indicate a close relationship between rock composition rock deformation transport of water partial melting and formation of layered migmatites

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

organic geochemical evidence for the origin of ancient anthropogenic soil deposits at tofts ness sanday orkney lipid biomarker components of soils constituting three orcadian archaeological fossil soil profiles were analysed the combined assessment of lipid distributional and compound specific stable carbon isotope data enabled the identification of grass turves as the most probable material used in the formation of the anthropogenic soil deposits appraisal of 5 stanol components indicated a faecal input to one of the soils which on considering distributional evidence was ascribed a human porcine origin additional study of polar bile acids from this profile revealed a distribution exhibiting a predominance of deoxycholic acid indicating the primary faecal input to be mainly derived from humans although the minor occurrence of hyodeoxycholic acid a characteristic component of pig faeces attested to a limited porcine input

organic geochemistry as an aid in the interpretation of the history of oil migration into different reservoirs at the hibernia k 18 and ben nevis i 45 wells jeanne d arc basin offshore eastern canada abstract the jeanne d arc basin is situated in the northeastern part of the grand banks off the east coast of newfoundland several large oil and gas fields have been discovered in this basin the largest of which is the giant hibernia field the principal source rock of most of the oils in the jeanne d arc basin is the kimmeridgian aged egret member of the rankin formation a characteristic feature of the biomarker distributions of both the oils and the egret member extracts is the very high abundance of 4 methyl steranes the high concentration of these compounds prevents the accurate measurement of maturity and other parameters based on 4 desmethyl steranes using gc ms sim the use of collision activated decomposition cad gc ms ms is demonstrated as a method of overcoming the problem of chromatographically co eluting compounds to obtain reliable biomarker data biomarker and other geochemical data are then used to help elucidate the history of oil migration into stacked reservoirs in two wells in the jeanne d arc basin all the hibernia k 18 oils are sourced from the egret member we interpret our data to indicate that all the reservoirs over a 1570 m depth interval were originally filled by a single pulse of oil that migrated vertically via faults in the lowermost hibernia reservoirs this event is being overprinted by more mature hydrocarbons leaking from the underlying overpressured zone with increasing depth at ben nevis i 45 oils show an increase in maturity and in the proportion of hydrocarbons that are derived from a source rock other than the egret member this second source is dominated by terrestrially derived organic matter oils are found in other wells in the jean d arc basin with similar characteristics all these wells are located in one small area of the basin and are in close proximity to the trinity fault

organic geochemistry of the san vicente zinc lead district eastern pucara basin peru abstract isotopic and chemical studies of organic matter of the san vicente mississippi valley type zn pb district hosted in the dolomitic units of the eastern pucara basin peru provide insight into the source of organic matter and of the organic matter ore relationship a large scatter exists in the 13c and 15n values of the highly mature kerogens these bulk data were combined with 13c covariations of the saturated and aromatic fractions of the rock extracts to indicate a contribution of continental higher plant lipids to the marine algal source of the organic matter disseminated in the host carbonate rocks an enrichment in 15n of the kerogens of mineralized samples compared to barren samples suggests that isotopically heavy nitrogen compounds were introduced to the ore fluid the fluid likely leached these compounds from the underlying clastic rocks and other detrital units of the pucara the extracts from mineralized samples are enriched in 13c compared to the associated kerogens this suggests that the rocks in the mineralized areas were stained by a mixture of extrinsic and indigenous bitumens by the use of novel instrumentation such as a gas chromatograph gc coupled to a stable isotope ratio mass spectrometer irms through a combustion c interface gc c irms stable carbon isotope analyses of individual hydrocarbons were also obtained both the molecular and isotopic signatures of the individual hydrocarbons are related to mineralization the variability in the organic molecular parameters and the 13c values of the individual n alkanes and isoprenoids indicate mixing of indigenous hydrocarbons with extrinsic 13c depleted microbially derived hydrocarbons the extrinsic source hydrocarbons were introduced to the mineralization site by the ore fluid or later fluids using the same porosity network oxidative alteration of the migrating hydrocarbons was involved in the thermochemical reduction of sulfate to h2s and precipitation of pyrobitumen large differences 12 in the carbon isotope compositions of long chain c20 n alkanes can be utilized in distinguishing sources of organics in the barren and mineralized samples

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

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

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

source rock and seep oil maturity in dorset southern england abstract analysis of the steranes and triterpanes in the oil seeps of the dorset coast of southern england mupe bay osmington mills shows both the oils to derive from a similar mid late mature source rock measured maturity of the jurassic source rocks at outcrop are uniformally immature 0 3 0 5 r despite a history of mesozoic burial and tertiary inversion and uplift evidence from oil cemented sandstone clasts within oil stained lower cretaceous fluviatile channel sands at mupe bay suggests that oil was already seeping during the lower cretaceous detailed work comparing the extracts of the oil stained clasts and the surrounding oil stained sandstones shows the clasts to contain oil derived from a slightly but significantly less mature source rock this is consistent with lower maturity oil seeping first into the sand that was then incorporated as rip up or bank collapse clasts into the channel sand at a later stage with further source rock burial more mature oil entered the matrix sandstone locally calibrated time temperature thermal geohistory modelling can just simulate source rock maturities appropriate to the seep oils if paleo geothermal gradients and surface temperatures were locally higher in the jurassic than at the present day a volumetric balance between oil in place in the wytch farm field and the available area of source rock draining into the structure suggests that an anomalously thick and or organic rich rhaeto liassic section must have developed in the bournemouth bay basin and that cretaceous palaeo geothermal gradients were higher in this specific area

nature of the crust in maine usa evidence from the sebago batholith neodymium and lead isotope and elemental data are presented for the sebago batholith 293 2 ma the largest exposed granite in new england the batholith is lithologically homogeneous yet internally heterogeneous with respect to rare earth elements ree and nd isotopic composition two mica granites in the southern central portion of the batholith group 1 are characterized by ree patterns with uniform shapes cen ybn chondrite normalized 9 4 19 and eu eu eu anomaly 0 27 0 42 and nd t 3 1 to 2 1 peripheral two mica granites group 2 spatially associated with stromatic and schlieric migmatites have a wider range of total ree contents and patterns with variable shapes cen ybn 6 1 67 eu eu 0 20 0 46 and nd t 5 6 to 2 8 the heterogeneous ree character of the group 2 granites records the effects of magmatic differentiation that involved monazite coarse grained leucogranites and aplites have kinked ree patterns and low total ree but have nd isotope systematics similar to group 2 granites with nd t 5 5 to 4 7 rare biotite granites have steep ree patterns cen ybn 51 61 eu eu 0 32 0 84 and nd t 4 6 to 3 8 the two mica granites have a restricted range in initial pb isotopic composition 206pb 204pb 18 41 18 75 207pb 204pb 15 60 15 68 208pb 204pb 38 21 38 55 requiring and old high u pb but not th u source component the nd isotope data are consistent with magma derivation from two sources avalon like crust nd 3 and central maine belt metasedimentary rocks nd 4 without material input from the mantle the variations in isotope systematics and ree patterns are inconsistent with models of disequilibrium melting which involved monazite

a mainly crustal origin for tonalitic granitoid rocks superior province canada implications for late archean tectonomagmatic processes the central wabigoon subprovince of the superior province like introduction most plutonic domains within archean cratons is dominated by over the last decade information fundamental to undergranitoid rocks of the tonalite trondhjemite granodiorite ttg standing and resolving many questions regarding arseries heterogeneous 2 74 throughout archean time documentation of alterative ga tonalite gneiss crust by younger tonalite magmas was probably petrogenetic models for archean ttg or ttd suites is an important process unlike the model of ttgs representing direct of particular relevance to archean tectonic processes see partial melts of subducting slabs in an arc setting this model de wit 1998 hamilton 1998 smithies 2000 as well requires no direct tie to subduction careful re evaluation of the as processes of archean continental crustal growth and ttg classification is required for it can mistakingly pigeon hole recycling e g ireland et al 1994 de wit 1998 hamilton temporally and genetically unrelated rocks and perhaps assume an 1998 henry et al 1998 2000 nutman et al 1999 incorrect petrogenetic or tectonic model an important role of crustal as in most archean shield terrains plutonic domains recycling processes in central wabigoon tonalite petrogenesis is in of the western superior province typically consist mainly keeping with evidence that supports substantial rates of continental of ttg or ttd compositions but also include more recycling as far back as the earliest archean mafic and more evolved granitoid compositions thurs

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

18o 16o evidence for contrasting hydrothermal regimes involving magmatic and meteoric hydrothermal waters at the valhalla metamorphic core complex british columbia modeling of published oxygen isotope analyses of 115 whole rock quartz and feldspar samples from the valhalla metamorphic core complex provides evidence for three distinct stages of hydrothermal activity affecting the detachment faults and upper plate 1 lithostatically pressured aqueous fluids having 18o 10 per mil were expelled from the lower plate and moved upward into the upper plate through an oblique sinistral transfer zone between the southern terminus of the columbia river detachment fault and the northern terminus of the slocan lake detachment fault these fluids ascended through fractures and exchanged with slocan group shales and graywackes to produce the marked zonation of vein siderite 18o 11 4 18 4 observed in previous studies of the sandon group of ag pb zn lode deposits 2 later as brittle deformation along the slocan lake detachment fault became the dominant tectonic process these magmatic and or metamorphic fluids intermittently mixed with even larger quantities of downward circulating hydrostatically pressured meteoric hydrothermal fluids in the upper plate nelson granodiorite adjacent to the slocan lake detachment fault 18o values of quartz 8 9 11 9 and feldspar 5 0 to 9 6 define a steep array in space slope 25 suggesting that synextensional meteoric hydrothermal activity along the slocan lake detachment fault initial 18owater 15 and t 300 c was relatively short lived 106 yr but very intense with w ropen system 1 05 and fluid flux 0 1 to 3 m yr the 18o values of quartz 10 8 12 4 and feldspar 6 4 10 1 from lower plate greenschist mylonites indicate that small quantities of the low 18o meteoric hydrothermal fluids penetrated into the lower plate the systematic decrease in feldspar 18o toward the slocan lake detachment fault in both the upper and lower plates shows that these low 18o waters were channeled along the detachment fault 3 meteoric hydrothermal activity along the valkyr shear zone postdates ductile deformation and was associated with the emplacement of the coryell plutons in the upper plate the 18o values of quartz 7 6 11 0 and feldspar 0 0 8 6 from the valkyr shear zone define a much shallower array in space slope 2 3 than those at the slocan lake detachment fault indicating a higher temperature and or longer lived episode of hydrothermal activity the geographic distribution of pronounced disequilibrium 18o effects in coryell quartz 5 8 8 2 and feldspar 4 4 to 7 4 indicates that meteoric hydrothermal activity was most intense along inferred coeval extensional fracture zones that lie beneath arrow lake this modeling suggests that transfer zones that accommodate deformation between detachment faults may be a very attractive exploration setting for lode type mineral deposits

late variscan emplacement and genesis of the vieira do minho composite pluton central iberian zone constraints from u pb zircon geochronology ams data and sr nd o isotope geochemistry abstract in the central iberian zone synorogenic syn and late d3 the last deformation phase composite batholiths of biotite rich granitods with peraluminous character are well represented we report here u pb zircon and monazite ages geochemical sr nd o isotopic data and anisotropy of magnetic susceptibility ams studies for the late d3 vieira do minho pluton in northern portugal ams reveals the paramagnetic behaviour of these granites and the magnetic fabric enhances the major role of nw se regional anisotropies on controlling the ascent and emplacement of the magmas magnetic anisotropy and medium temperature microstructures also point out the evolution of regional tectonics during crystallization of the magma u pb zircon and monazite analyses yield consistent ages of 310 2 ma and 312 2 ma interpreted as emplacement age the vieira do minho pluton consists of two peraluminous monzogranites the vmg and the mrg both have similar end values vmg 5 2 to 5 7 mrg 4 98 to 5 96 but the vmg is slightly enriched in 87sr 86i 0 7087 0 7098 as well as in 18o in the range of 10 6 to 11 0 than the mrg with 87sr 86i 0 7064 0 7075 and 18o 9 9 10 5 these granites are associated with coeval scarce grabroic intrusions and or mafic microgranular enclaves which are not considered as mafic precursors of the associated granitic magmatism instead a lower metaigneous crustal source at different levels is proposed based on the available data set

formation and evolution of granite magmas during crustal reworking the significance of diatexites taylor mclennan 1985 the physical processes ina continuous section through reworked archaean crust records the volved particularly how granite magma is formed remain generation of granitic magma and its subsequent development in the controversial miller et al 1988 white chappell opatica subprovince in the canadian shield there the transition 1990 a partially melted source can form a granitic from palaeosome to granite was a closed system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite the average most of their residuum understanding this process is degree of partial melting was less than 30 but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs for example melt residuum separation deformation regions that lost melt became residual diatexites could be a nearly perfect single stage process at the enriched in tio2 feot mgo cao sc cr co sr rare earth site of partial melting as described for leucosomes in elements ree and high field strength elements hsfe melt metatexite migmatites wickham 1987a sawyer 1991 accumulated to create diatexite magmas enriched in large ion 1994 brown et al 1995 alternatively the separation lithophile elements lile but contaminated with residuum macould be an imperfect multi stage process that yields a terial such diatexite magmas are parental to granites found at magma with a large residual component as is observed higher crustal levels in the terrane flow of the diatexite magma in in diatexite migmatites bea 1991 greenfield et al response to deformation separated some of its residuum into schlieren 1996 sawyer 1996 the metatexite model necessarily parautochthonous plutons were created where ascending granitic produces leucocratic residuum free granites such as magma locally ponded below impermeable layers and structures those described by le fort 1981 and montel et al magma left the anatectic region in dykes and lost its remaining residuum as it crystallized consequently the allochthonous granite 1991 and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation in contrast the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum rich granites and so reduce the residuum geochemical effect of crustal differentiation as chappell 1996 noted how anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known because few crustal sections

elemental distributions and neodymium isotopic compositions of silurian metasediments western maine usa redistribution of the rare earth elements abstract pelitic schists from the lower garnet to lower staurolite zones from the rangeley perry mountain and smalls falls formations of western maine were analyzed for major elements trace elements and neodymium isotopes these formations were derived from highlands created during the taconian orogeny deposited into a trough and metamorphosed during subsequent orogenic events most major and trace element abundances relative to al 2 o 3 were statistically identical between zones of the same formation as well as between formations although the average major element composition of these formations are the same there are systematic variations in some elements notably plots of sio 2 vs al 2 o 3 and k 2 o vs al 2 o 3 suggest that most of the variation could be produced by mixing of a fairly constant ratio of clay minerals and feldspar with varied amounts of quartz due to sorting in the sedimentary system different amounts of these minerals should not influence the shape of the ree patterns of the metapelites but higher amounts of quartz and feldspar may dilute the rees and most elemental abundances of the clay minerals and lead to lower elemental abundances the major difference between the samples within the perry mountain formation are different lree and mree abundances relative to al 2 o 3 which are not correlated to differences in major element or other trace element abundances relative to al 2 o 3 the samples in the perry mountain with higher lree and mree abundances have for example 42 3 8 3 ppm and those with low abundances have 5 6 3 6 ppm the samples with the high ree abundances of the perry mountain formation are similar in abundances and ree patterns to those of the rangeley and smalls falls formations typical of mudstones derived from granitoids another difference between the low and high ree abundance samples are the calculated t dm model ages the high ree abundance samples of the perry mountain formation show t dm similar to the samples of the rangeley formation with ages of about 1 7 1 8 ga the perry mountain samples with low ree abundances however give unrealistically old t dm s between 2 5 and 5 3 ga these unrealistically old t dm s are due to the relatively high sm nd ratios compared to crustal values which are characteristic of samples of the perry mountain formation with lower ree abundances we therefore suggest that these samples may be indicators for open system behavior of the neodymium isotopic system the timing of this disturbance of the neodymium isotope system is difficult to determine and cannot be tied to weathering or a definite postdepositional event the complexities of the data suggest more than one resetting event the most likely event that could have produced much of the movement of the lrees and mrees could have been due to small scale migration between anoxic hemipelagites and turbidite mudstones during diagenesis but some migration may have continued during metamorphism in order to reconcile the neodymium isotopic data

geotechnical characterization of lignite bearing horizons in the afsin elbistan lignite basin se turkey abstract the afsin elbistan lignite deposit with its 3 4 billion metric tons of reserves is the biggest lignite basin and one of the most important resources for electrical energy production in turkey kislakoy mining field was selected as the first opencast mine to feed four power station blocks of 300 mw each slope instability has been a continuing problem in the kislakoy opencast mine particularly complex failures along a noncircular failure surface appearing at the final slope stage and covering large areas in the mine increase the importance of slope stability this study outlines the geotechnical characteristics of the lignite bearing horizons and describes the causes and mechanisms of slope instabilities which threaten the safety of the mine quantitative x ray diffraction xrd analyses were carried out using an interactive data processing system siroquant based on rietveld interpretation methods parametric slope stability analysis and backanalysis were carried out for the failure that occurred at the northwestern final slope stage of the mine the spencer wright limiting state equilibrium method was used in order to determine with confidence the most representative values of regional shear strength parameters to explain the failure mechanism and to assess the conditions at the time of failure in the analysis phreatic and piezometric surfaces were considered site observations and numerous backanalyses of the slope failure reveal that a compound slide occurred where gyttja contact zone layers rest directly on the lignite gyttja contact zone contains the weakest material within the system the analysis showed that the main cause of the northwestern slope instability was the presence of the groundwater flow within a quaternary aquifer through buried valleys reducing the effective shear strength of the slope materials it is also noted from backanalysis that the gyttja contact zone layer presents a shear strength at or approaching the residual value at the time of failure

degradation et lessivage des hydrocarbures de la formation ordovicienne de thumb mountain encaissant le gite zn pb de polaris territoires du nord ouest canada carbonate rocks of the ordovician thumb mountain formation host to the zn pb polaris deposit contain hydrocarbons that show types of alteration classically attributed to biodegradation and water washing the hydrocarbons of the upper part of this formation and of the overlying irene bay formation indicate alterations due to water washing only the hydrocarbons of the impermeable shales of the overlying cape phillips formation display indices of only higher thermal maturity than the underlying units contrary to classical concepts of hydrocarbon biodegradation n alkanes cyclohexylalkanes even isoprenoids and perhaps also steranes seem to have been degraded simultaneously and not successively in this sequence this alteration process is mainly responsible for a log linear decrease of the amounts of n alkanes and cyclohexylalkanes with increasing depth the severe and uniform alteration of aromatic hydrocarbons throughout the interval which is opposite to the progressive alteration of associated n alkanes can be attributed solely to water washing this conclusion necessitates a reconsideration of previous interpretations attributing the loss of short side chain substituted polyaromatic compounds to microbial activity hopanes tri and tetra cyclic terpanes as well as aromatic steroids and hopanoids seem to have been unaffected by the alteration phenomena the increase in the degree of alteration of the hydrocarbons with increasing depth implies that the responsible migrating fluid circulated per ascensum

an experimental study on the compaction and collapsible behaviour of a flood defence embankment fill with renewed interest in the performance of flood embankments globally it is important that the performance of fills in conditions similar to as constructed are scrutinised the fill material investigated in this study was sampled from flood embankments located along the bengawan solo river in east java indonesia the recurrent history of overtopping and stability issues in these embankments provided the motivation for developing a better understanding of the behaviour of this compacted fill under different loading and wetting conditions the site investigation revealed that the embankment fill was compacted at low dry densities and that there was local variation in the dry densities determined a detailed study highlighted that at low compactive efforts this fill material exhibits an irregular double peak compaction curve which can be explained by the tendency of this fill to form aggregates on wetting to cover different plausible operational conditions of these embankments saturated and unsaturated compression oedometer tests and one dimensional collapse tests were performed under different initial conditions specimens compacted at conditions similar to as constructed exhibited significant collapse deformation up to 13 6 on wetting evolution of the microstructure during loading and wetting paths was investigated using mip and esem a physically based framework proposed by romero 2013 was used to explain the changes in the macroscale collapse behaviour observed in the oedometer tests based on the evolution of microporosity using this model the evolution of the microporosity with dry density and water content was presented this microstructural approach could be used as a tool for specifying appropriate compaction conditions for earthworks where fill material is susceptible to volumetric collapse

orogeny associated with anticlockwise p t t paths evidence from low p high t metamorphic terranes in the arunta inlier central australia low pressure high temperature metamorphic terranes from the proterozoic northern arunta inlier central australia are characterized by anticlockwise pressure temperature time paths the thermal peak was reached before or during the earliest folding and it was associated with abundant repeated intrusion of granite sheets we generalize our observations in the area to infer that low p high t metamorphism is induced principally by focusing of mid crustal heat from rapid repeated granite sheet intrusion which generates large metamorphic aureoles intrusion and associated metamorphism produce melting reactions which increase the fluid pressure and thermally soften the mid crust resulting in accelerated melt enhanced deformation shortening is accommodated generally by thrusting in the mid crust and by upright folding in the upper crust compression occurs during cooling of the terrane as indicated by near isobaric cooling paths and the preservation of upper crust sections the result is the production of discrete low p high t terranes that are localized around abundant granitoids show thin skinned deformation and have anticlockwise p t time paths continued granitoid intrusion produces posttectonic discordant plutons at high crustal levels within low p metamorphic belts

18o 16o homogenization of the middle crust during anatexis the thor odin metamorphic core complex british columbia the occurrence of isotopically uniform quartz 18 o 12 5 0 5 and feldspar 10 9 0 7 throughout different rock types indicates that much of a 6 km thick section of the mid crustal selkirk allochthon underwent internally buffered 18 o 16 o homogenization during paleocene melting and decompression as it moved up the monashee thrust ramp areas of uniform 18 o are those with the most leucogranite or those subjected to severe anatexis only locally in the most impermeable or refractory zones did 18 o exchange among the rocks leucogranite melts and aqueous fluids fail to go to completion i e in the deepest parts of the section in a marble rich zone around some thick amphibolites and in most garnets evidence for 18 o 16 o heterogeneity in the protoliths of these rocks is observed in stratigraphically correlative lower grade units elsewhere in british columbia as well as in garnets that coexist with isotopically homogeneous quartz in our model 18 o 16 o homogenization accompanied muscovite dehydration and partial melting of pelites with only minor influx of external h 2o followed by release of magmatic h 2o from these melts triggering further melting of adjacent feldspathic assemblages as they were uplifted 20 km during thrusting just prior to onset of detachment faulting locally low 18 o in feldspar down to 3 8 and profound quartz feldspar 18 o 16 o disequilibrium were imprinted at shallow levels during meteoric hydrothermal alteration associated with eocene detachment faulting

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

polymetamorphism in medium to high grade pelitic metamorphic rocks west central maine a regional petrologic study of pelitic metamorphic rocks has been undertaken with emphasis on the area bounded by the cities of augusta lewiston norway rangeley kingfield and madison this report involves a regional isograd map interpretation of the nature of the various reactions and discussion of petrologic problems of polymetamorphism isograd distribution and widespread occurrence of muscovite in the k feldspar sillimanite zone a future contribution will deal more specifically with mineral chemistry geotherrnometry and geobarometry of much of the area after a low grade metamorphism m 1 the region suffered a higher grade early metamorphism m 2 which in many places produced andalusite and cordierite as well as staurolite this was followed by a somewhat higher pressure metamorphism m 3 involving staurolite chlorite and sillimanite in some areas m 3 has produced extensive retrograde effects with increasing grade in ms the major mineral reactions are 1 formation of staurolite from reaction of garnet chlorite and muscovite 2 formation of sillimanite from staurolite chlorite and muscovite 3 reaction of staurolite to form sillimanite biotite and in many rocks garnet and 4 formation of k feldspar from the partial reaction of muscovite quartz and plagioclase a possible fourth metamorphism m 4 produced narrow aureoles with andalusite and staurolite around the three latest plutons analysis of the isograd pattern demonstrates that the pressure was rather uniform throughout the area at a given time but increased about 0 5 kbar or more between m 2 and m 3 metamorphism the more regional distribution of isograds to the south is consistent with higher initial temperatures and more closely spaced magmatic heat sources but not necessarily with increasing pressure to the south the pressure increase between m 2 and m 3 might have been produced by intrusion of magma above the present level and or extrusion of rhyolitic volcanics

effect of water washing on crude oil compositions crude oils from venezuela oklahoma and new mexico were water washed in the laboratory at temperatures from room temperature to 80 c and with water salinities from 0 to 300 000 mg l in experiments lasting from 7 to 338 hours the effects of water washing were determined by gas chromatographic and gas chromatography mass spectrometric analysis of the residual oil these experiments show that water washing is particularly effective for the c15 fraction and hydrocarbons are removed in the sequence aromatics then n alkanes then naphthenes in the c15 fraction no loss of pristane phytane steranes or terpanes occurs but some aromatics and sulfur compounds especially dibenzothiophene are depleted although water washing reduces api gravity water washing is unlikely to produce tar layers the effects of water washing were simulated with a numerical model based on the equation of diffusion predicted oil water ratios were in good agreement with the experimental values for the light ends and the model suggests that water washing is very fast in the subsurface comparison of oils that appeared to have been water washed in nature with the equivalent unwashed oils showed the expected compositional trends water washing is probably the dominant process affecting crude oil composition in the subsurface when water flows past oils under conditions where bacterial degradation is precluded by temperature 80 c or by lack of dissolved oxygen and the temperatures are too low for thermal cracking

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

in defense of magmatic diapirs abstract diapirism as a crustal magma ascent mechanism has been recently criticized we contend that this reflects an overly simplistic view that diapirs must resemble modeled hot stokes diapirs and the perception that magma ascent in dikes is a more problem free mechanism for the construction of plutons we describe four cordilleran plutons that have characteristics much more compatible with diapirs than dike fed chambers these plutons were emplaced at depths ranging from 10 to 30 km and record different parts of diapiric ascent paths most ascended during complex visco elastic flow of host rock during regional deformation have narrow structural aureoles indicating power law behavior of host rock and were constructed of multiple batches of magma attributes enabling them to ascend greater distances than single hot stokes diapirs some features of these plutons are not typically attributed to diapirs and thus we introduce the term visco elastic diapir for bodies consisting of one or more batches of magma rising together with length to width ratios

evolution of a transcurrent fault system in shallow crustal metasedimentary rocks the norumbega fault zone eastern maine abstract differential exhumation in maine exposes nearly half of the crustal profile of the norumbega fault zone nfz a major transcurrent fault zone of the northern appalachians a large exposure of turbidites in eastern maine records a multi stage deformation history that provides insight into the epizonal evolution of the nfz early deformation stage 1 involved episodic bed parallel faulting with fluids playing a progressively smaller role strain was broadly distributed at the exposure but was localized by lithic anisotropy in pelitic layers little stage 1 deformation occurred in the dominant wacke beds by diffusive mass transfer most took place in subordinate pelitic horizons by combined diffusive mass transfer and crystal plastic mechanisms later stage 2 activity was almost entirely brittle producing isolated ne trending faults and two deformation zones filled with cataclasite the latter comprise complexly anastomosing fault strands with multiple slickenline generations indicating episodic faulting regional scale nfz structures and history mirror those at outcrop scale late brittle faulting was superimposed on a broad region that had experienced largely ductile shearing and the regional scale brittle deformation was concentrated in three deformation zones characterized by anastomosing fault strands it is the stage 2 features that are mapped elsewhere in the northern appalachians as the norumbega fault zone

the rapakivi granites of south greenland stable isotope characteristics of their black and white facies and the nature of their protolith abstract the rapakivi granites of south greenland were emplaced into the ketilidian orogenic belt at about 1 74 ga during an episode of extensional tectonics petrographically they can be subdivided into a roughly subequal black facies and white facies distinguished by the colour turbidity of the alkali feldspars and by the associated mineralogy which is more hydrous i e amphibole and biotite compared to olivine and pyroxene in the white facies the oxygen isotope composition of these rocks is notably homogeneous from thirteen outcrops sampled over an area extending some 200 km 100 km all alkali feldspars have an average 18 o of 10 2 0 4 this lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two component protolith the high abundance of 18 o mean whole rock 18 o 9 4 argues that the major contribution perhaps 90 although originally mantle derived had undergone 18 o enrichment in a low temperature sedimentary cycle there is no oxygen isotopic distinction between alkali feldspars in the black and white facies which is argued to be a consequence of the relatively anhydrous nature of the melt hydrogen isotope compositions vary widely by 30 but are coherent on a hand specimen scale as demonstrated by approximately equal d of biotite and alkali feldspar it is not clear at which stage the d h heterogeneity was introduced

the phillips pluton maine usa evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens the phillips pluton age of 403 8 1 3 ma was assembled at a crustal level below the contemporary brittle plastic transition during regional dextral reverse transpressive deformation the pluton is composed dominantly of medium to coarse grained leucogranite sensu lato s l but within its bounds includes decametric massive outcrop of fine to medium grained granodiorite s l in places the leucogranite contains centimetric enclaves apparently of the granodiorite granodiorite is host to more biotite than muscovite and more calcic oscillatory zoned plagioclase compared to the leucogranite pegmatitic granite and composite pegmatite aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the sw magmatic fabrics defined by biotite schlieren occur locally in the leucogranite the attitude of these fabrics and layering within the leucogranite are concordant with the ne striking steeply dipping country rock foliation k2o contents rb sr ratios rb sr and ba covariations and chondrite normalized rare earth element ree patterns of leucogranite are consistent with high to moderate a h2o muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central maine belt cmb the ree patterns and rb sr ratios of granodiorite also suggest derivation from a metasedimentary source but more likely by moderate to low a h2o muscovite biotite dehydration equilibrium eutectic to non eutectic minimum melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases both granite s l types have heterogeneous initial nd isotope compositions samples of granodiorite define a range in end 404 ma of 1 8 to 0 1 0 3 2 uncertainty and samples of leucogranite define a range in end 404 ma of 8 0 to 5 3 0 3 2 uncertainty this bimodal distribution suggests that melts were derived from a minimum of two sources the data are consistent with these sources being cmb metasedimentary rocks end 404 ma 4 for the granodiorite the range of nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source these data imply that the integrity of individual melt batches was maintained during ascent and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur although centimetric enclaves have intermediate nd isotope compositions consistent with small scale interactions between magmas we infer that the phillips pluton represents the root of a larger pluton and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal scale shear zone system

effect of water washing on c15 hydrocarbon fraction of crude oils from northwest palawan philippines recognition and subsequent correlation of biodegraded and or water washed oils with their unaltered counterparts has been a long recognized problem in petroleum geochemistry because water washing and biodegradation can occur concurrently it is difficult to delineate the individual effects of these two oil altering processes especially on the c15 hydrocarbon fraction both processes can cause an oil to become enriched in sulfur and to have a lower api gravity owing to loss of the more volatile components thus the altered oil may appear to be a high sulfur crude of low maturity altered and unaltered oil fractions extracted from cores of a water washed carbonate reservoir offshore northwest palawan were studied to observe the effect of alteration on correla ion parameters water washing resulted in the following changes 1 depletion of 13c in the c15 branched cyclic alkane fraction whereas virtually no change occurred in isotope composition of the aromatic fraction 2 selective loss of dibenzothiophenes followed by loss of aromatic hydrocarbons and 3 virtually no change in steranes or tricyclic and pentacyclic terpanes with the exception of enhancement of 17 agr h norhopane and decrease of 18 agr h trisnorhopane in situations of severe water washing mild biodegradation also occurred as indicated by the loss of n alkanes and possible selective loss of n alkanes having less than 25 carbon atoms relative to branched alkanes cycloalkanes and aromatic components because cycloalkanes and aromatic hydrocarbons are not generally preferentially utilized by bacteria as long as n alkanes are present the changes observed in these two fractions most likely result from water washing because of the potential for extreme compositional changes in the n alkane and aromatic fractions the most useful correlation parameters between the altered and unaltered oils are sterane and terpane distributions stable carbon isotope ratios are also useful however more studies of oils in which water washing is known to have occurred need to be made to explain the general response or various responses of the isotopic composition of oils to water washing

the acadian thermal history of the merrimack synclinorium in new hampshire the acadian structural history of the merrimack synclinorium involves a phase of nappe emplacement and thrusting at 400 m y b p followed by a second thickening event before 370 m y b p at the end of this phase the present erosion surface was buried at pressures of 6 to 8 kbar and subsequently experienced peak metamorphic temperatures in the range of 500 c to 650 c the thermal history of the acadian metamor phism of the merrimack synclinorium is examined in a well studied area of central new hampshire to determine the conditions required for the high grade metamorphism of the region constraints are provided by metamorphic pressure estimates by information on the erosional history and by present day heat flow determinations in the synclinorium one dimensional heat transfer calculations show that the high grade metamorphism could readily have been produced during thermal relaxation of the thickened continental crust under conditions of normal continental heat flow these conditions also permit the me

novel families of tetra and hexacyclic aromatic hopanoids predominant in carbonate rocks and crude oils two novel families of monoaromatic geomarkers related to hopanoids have been separated and characterized in sediments and crude oils the benzohopanes c32 c35 1 and the ring d aromatized 8 14 secohopanoids c27 c35 4 and 5 both series are particularly abundant in many carbonate samples where they amount to approximately 100 p p m per component in some crude oils but can also be detected in other environments the benzohopanes have been separated as pure compounds by high performance liquid chromatography and identified by x ray diffraction the structure of the 8 14 secohopanoids mostly relies on spectroscopic data nmr and ms and comparison with synthetic analogs both families must be formed in the subsurface from bacterial c35 hopanoid precursors but the benzohopanes seem to occur at an earlier stage of sedimentation the two series appear in varying distributions in the different samples and may therefore be potentially useful as geochemical correlation parameters

interpretation of ages of arc magmatism metamorphism and collisional tectonics in the taconian orogen of western new england

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction