corundum orthopyroxene spinel intergrowths in an ultrahigh temperature al mg granulite from the southern marginal zone limpopo belt south africa we report the occurrence of unusual lamellar random and regular subparallel intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene in an ultrahigh temperature al mg granulite from the southern marginal zone of the limpopo belt south africa separate compositions are suggested for the homogenous phase that might have existed prior to the formation of the two types of lamellar intergrowths of corundum with orthopyroxene in the case of random lamellar corundum orthopyroxene intergrowth the estimated precursor composition is similar to fe mg garnet while although speculative an ultrahigh al orthopyroxene precursor is suggested to account for the exsolution nature of the unique regular subparallel corundum lamellae in orthopyroxene considering that the stability field of fe mg rich garnet relative to orthopyroxene corundum extends to lower pressures the estimated garnet composition x mg 0 65 is most likely stable at low pressures of 5 kbar such low pressures are further supported by the fe mg rich garnet compositional nature of the pre spinel intergrowth orthopyroxene given the rare preservation of the mineral assemblages typical of prograde metamorphism in granulite facies rocks our discovery of corundum lamellar intergrowth with orthopyroxene from an ultrahigh temperature al mg granulite is unique

tonalite granodiorite suites as cotectic systems a review of experimental studies with applications to granitoid petrogenesis abstract granodiorites and tonalites granotonalites in this review paper form large batholiths at active continental margins and collisional orogens although the relation with subduction is obvious the mechanisms of magma generation remain among the most debated topics in earth sciences today the most outstanding features of these calc alkaline batholiths are summarized in this review the problem has been addressed by laboratory experiments along decades dispersed data from experimental studies are compiled here in order to give phase equilibria constraints of general application it is apparent from chemical data that major element compositional trends follow cotectic patterns whose composition is largely dependent on intensive variables the comparison between experiments and batholith chemical trends produces an empirical thermodynamic framework useful to understand in a first approach the origin and evolution of magmas forming granite batholiths a new projection scheme of rock compositions onto the pseudoternary plane defined by the fe mg anorthite orthoclase is given this can be taken as first approach to phase equilibria model of batholiths it has been revealed useful to distinguish processes controlled by increasing melting or decreasing crystallization magma temperature from processes controlled by assimilation or magma mixing the available experimental data although scarce can be used to tentatively depict phase equilibria in the projected pseudoternary f an or diagram although important limitations are imposed by the projective procedure phase equilibria relations in equilibrium with liquids can be drafted in the diagram the stability fields of mineral in areas close to the liquid are defined according to reported phase assemblages in the experimental studies of reference the diagram is robust to identify cotectic and non cotectic relations phase equilibrium constraints and rock textures have revealed that one of the most important parameter controlling phase equilibria in granite magmas is the initial water content typically calc alkaline granitoids from qtz diorite to granotonalite and granite have crystallized from strongly undersaturated water magmas low initial water contents w i in the range 1 2 wt h 2 o may account for residual saturated melt fractions as low as 0 1 0 2 for plutons emplaced at 400 mpa the water content of these residual melt fractions is enough to stabilize hydrous minerals namely hbl and bt as predicted by experiments the initial water content is a primary feature of the magma and it is directly related to the melting reaction and more precisely to the temperature of magma generation the study of compositional trends supports magma fractionation by crystal liquid separation as a relevant role as a first approach the pressure composition relations in the melts can be seen as a response to the composition of the solid saturating assemblage and particularly related to the ratio grt pl in the solid residue

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

joint probabilistic petrophysics seismic inversion based on gaussian mixture and markov chain prior models abstractseismic reservoir characterization focuses on the prediction of reservoir properties based on the available geophysical and petrophysical data the inverse problem generally includes continuous properties such as petrophysical and elastic attributes and discrete properties such as lithology fluid classes we have developed a joint probabilistic inversion methodology for the prediction of petrophysical and elastic properties and lithology fluid classes that combined statistical rock physics and bayesian seismic inversion the elastic attributes depend on continuous petrophysical variables such as porosity and clay content and discrete lithology fluid classes through a nonlinear rock physics relationship together the seismic model relates the elastic attributes such as velocities and density to their seismic response reflectivity traveltime and amplitudes the advantage of our integrated approach is that the inversion method accounts for the uncertainty associated to each step of the mo

1 8 ga svecofennian post collisional shoshonitic magmatism in the fennoscandian shield abstract at least 14 small 1 11 km across 1 8 ga svecofennian post collisional bimodal intrusions occur in southern finland and russian karelia in a 600 km long belt from the aland islands to the nw lake ladoga region the rocks range from ultramafic calc alkaline apatite rich potassium lamprophyres to peraluminous hibasr granites and form a shoshonitic series with k 2 o na 2 o 5 k 2 o na 2 o 0 5 al 2 o 3 9 over a wide spectrum of sio 2 32 78 although strongly enriched in all rocks the lile ba and sr and the lree generally define a decreasing trend with increasing sio 2 depletion is noted for hfse ti nb and ta available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded initial magmas mg 65 in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post collisional uplift some 30 ma after the regional svecofennian metamorphic peak however prior to the melting episode the lithospheric mantle was affected by carbonatite metasomatism more extensively in the east than in the west the melts generated in the more carbonate rich mantle are extremely enriched in p 2 o 5 4 f 12 000 ppm lile ba 9000 ppm sr 7000 ppm lree la 600 ppm and ce 1000 ppm the parental magma underwent 55 60 fractionation of biotite clinopyroxene apatite magnetite sphene whereupon intermediate varieties were produced after further fractionation 60 80 of k feldspar amphibole plagioclase minor magnetite sphene and apatite leucosyenites and quartz monzonites were formed in the west where the source was less affected by carbonatite metasomatism calc alkaline lamprophyres vogesites minettes and spessartites and equivalent plutonic rocks monzonites were formed removal of about 50 of biotite amphibole plagioclase magnetite apatite and sphene produced peraluminous hibasr granites the impact of crustal assimilation is considered to be low at about 1 8 ga the post collisional shoshonitic magmatism brought juvenile material particularly enriched in alkalis lile lree and f into the crust although areally restricted the regional distribution of the post collisional intrusions may indicate that larger volumes of 1 8 ga juvenile material resides in unexposed parts of the crust

post collisional polycyclic plutonism from the zagros hinterland the shaivar dagh plutonic complex alborz belt iran the petrological and geochronological study of the cenozoic shaivar dagh composite intrusion in the alborz mountain belt nw iran reveals important clues to decipher complex relations between magmatic and tectonic processes in the central sectors of the tethyan alpine himalayan orogenic belt this pluton is formed by intrusion at different times of two main magmatic cycles the older cycle 1 is formed by calc alkaline silicic rocks which range in composition from diorites to granodiorites and biotite granites with abundant mafic microgranular enclaves the younger cycle cycle 2 is formed by k rich monzodiorite and monzonite of marked shoshonitic affinity the latter form the larger volumes of the exposed plutonic rocks in the studied complex zircon geochronology laser ablation icp ms analyses gives a concordia age of 30 8 2 1 ma for the calc alkaline rocks cycle 1 and a range from 23 3 0 5 to 25 1 0 9 ma for the shoshonitic association cycle 2 major and trace element relations strongly support distinct origins for each magmatic cycle rocks of cycle 1 have all the characteristic features of active continental margins shoshonitic rocks cycle 2 define two continuous fractionation trends one departing from a k rich basaltic composition and the other from an intermediate k rich composition a metasomatized mantle origin for the two shoshonitic series of cycle 2 is proposed on the basis of comparisons with experimental data the origin of the calc alkaline series is more controversial but it can be attributed to processes in the suprasubduction mantle wedge related to the incorporation of subducted melanges in the form of silicic cold plumes a time sequence can be established for the processes responsible of the generation of the two magmatic cycles first a calc alkaline cycle typical of active continental margins and second a k rich cycle formed by monzonites and monzodiorites this sequence precludes the younger potassic magmas as precursors of the older calc alkaline series by contrast the older calc alkaline magmas may represent the metasomatic agents that modified the mantle wedge during the last stages of subduction and cooked a fertile mantle region for late potassic magmatism after continental collision

stochastic reservoir characterization using prestack seismic data reservoir characterization must be based on information from various sources well observations seismic reflection times and seismic amplitude versus offset avo attributes are integrated in this study to predict the distribution of the reservoir variables i e facies and fluid filling the prediction problem is cast in a bayesian setting the a priori model includes spatial coupling through markov random field assumptions and intervariable dependencies through nonlinear relations based on rock physics theory including gassmann s relation the likelihood model relating observations to reservoir variables including lithology facies and pore fluids is based on approximations to zoeppritz equations the model assumptions are summarized in a bayesian network illustrating the dependencies between the reservoir variables the posterior model for the reservoir variables conditioned on the available observations is defined by the a priori and likelihood models this posterior model is not analytically tra

involvement of fluids in the metamorphic processes within different zones of the southern marginal zone of the limpopo complex south africa an oxygen isotope perspective abstract we present oxygen isotope data obtained by laser fluorination for three outcrops from the south marginal zone smz of the limpopo granulite complex south africa these outcrops are located within the granulite zone bandelierkop quarry on the retrograde anthophyllite in isograd manamead locality and within the zone of rehydrated granulite south of the retrograde isograd klipbank locality studied samples also include felsic granitoid rocks that occur as intrusive bodies veins and veinlets in the host tonalite gneisses or the metapelite the tonalite gneiss tg and the k feldspar rich segregations kfs g at klipbank have marginally different 18 whole rock wr values 7 9 and 7 3 correspondingly the kfs g may have formed by partial melting of the tg in the presence of external saline h 2 o co 2 fluid metapelites at manamead locality have 18 o values 9 5 10 3 lower than those at bandelierkop quarry 10 0 11 4 the two mineral oxygen isotope temperature estimates for metapelite minerals from manamead cluster around two values 620 30 and 730 40 c oxygen isotope temperature estimates for the metapelites from bandelierkop quarry define a narrow temperature range of 700 740 the oxygen isotope temperatures are in reasonable agreement with the results of classical geothermobarometry the 18 values of the whole rocks and individual constituent minerals of metapelites strongly negatively correlate with the rocks volatile content loi values this suggests that a fluid phase was involved in the hydration of metapelites in the vicinity of the regional anthophylite in isograd of the southern marginal zone the fluid may have been released during crystallization of leucogranite melt oxygen isotope and geochemical features of the leucogranite indicate genetic relation to the tonalitic baviaanskloof gneisses but do not support the formation of leucogranite via in situ partial melting of metapelites

selective entrainment of peritectic garnet into s type granitic magmas evidence from archaean mid crustal anatectites entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites as well as the chemical connection between granite magmas and their sources this concept has become linked to models for magma migration out of the source as restite entrainment is considered to take place when diatexitic sources mobilise en masse this is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water undersaturated magmas that ascended through narrow conduits we investigate pelitic migmatites from the mkhondo valley metamorphic suite mvms in swaziland where a mid crustal heating event produced metatexitic migmatites with minimal post anatectic recrystallisation in these rocks all the garnet is peritectic having arisen through biotite fluid absent melting which produced garnet poikiloblasts characterised by inclusions of melt quartz and biotite leucosomes that represent sites of melt transfer carry similar smaller typically 1 mm entrained garnet poikiloblasts which were capable of amalgamating to form larger composite grains in anatectic structures where melt was present for longer entrained garnet was extensively recrystallised via a dissolution precipitation process to adopt a more magmatic character the peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and hree rich accessory phases while the recrystallised garnet in the larger melt filled structures became progressively better equilibrated with these minerals thus peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk rock composition and concurrent rapid magma segregation prevented the development of diatexitic source conditions the segregated magma consisted of melt the peritectic assemblage principally garnet and the accessory minerals monazite and zircon these rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction importantly under the circumstances which produced the mvms anatectites peritectic garnet is entrained as 1 mm poikiloblasts demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic

durbachites vaugnerites a geodynamic marker in the central european variscan orogen durbachites vaugnerites are k mg rich magmatic rocks derived from an enriched mantle source observed throughout the european variscan basement their present day geographical distribution does not reveal any obvious plate tectonic context published geochronological data show that most durbachites vaugnerites formed around 335 340 ma plotted in a visean plate tectonic reconstruction the occurrences of durbachites vaugnerites are concentrated in a hotspot like cluster in the galatian superterrane featuring a distinctive regional magmatic province reviewing the existing local studies on variscan durbachite vaugnerite rocks we interpret their extensive appearance in the visean in terms of two factors i long term mantle enrichment above early variscan subduction systems and ii melting of this enriched subcontinental mantle source during the variscan collision stage due to thermal anomalies below the galatian superterrane possibly created by slab windows and and or the sinking of the subducted rheic slab into the mantle the tectonic reorganization of europe in the late palaeozoic and during the alpine orogeny has torn apart and blurred this marked domain of durbachites vaugnerites

tectonothermal evolution associated with variscan crustal extension in the tormes gneiss dome nw salamanca iberian massif spain abstract the tectonothermal variscan evolution of the tormes gneiss dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics that induced a quick exhumation of the deep parts of the metamorphic complex during crustal thinning the shearing affected a broad band of metamorphic rocks but as the temperature decreased became progressively concentrated in a low grade ductile detachment that separates two distinct units characterized by their lithology and different tectonothermal evolution kinematic indicators in non coaxial fabrics show a displacement of the hanging wall to the southeast i e parallel to the trend of the foldbelt the subhorizontal shearing is related to the extensional collapse of the variscan crust previously thickened during the collision tectonics

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

p t conditions mechanism and timing of the localized melting of metapelites from the petronella shear zone and relationships with granite intrusions in the southern marginal zone of the limpopo belt south africa

generation and emplacement of shear related highly mobile crustal melts the synkinematic leucogranites from the variscan tormes dome western spain the tormes dome consists of s type granites that intruded into ordovician augen gneisses and neoproterozoic lower cambrian metapelites metagreywackes at different extents of migmatization s type granites are mainly equigranular two mica granites occurring as 1 enclave laden subvertical feeder dykes 2 small external sill like bodies with size and shape relations indicative for self similar pluton growth and 3 as large pluton bodies emplaced at higher levels than the external ones these magmas were highly mobile as it is inferred from the high contents of fluxing components the disintegration and alignment of pelitic xenoliths in feeder dykes and at the bottom of some sill like bodies field relations relate this 311 ma magmatism u pb monazite to the regional shearing of the d3 variscan event partial melting modeling and the relatively high estimated liquidus temperatures indicate biotite dehydration partial melting 800 840 c and 400 650 mpa rather than water fluxed melting implying that there was no partial melting triggered by externally derived fluids in the shear zones instead the subvertical shear zones favored extraction of melts that formed during the regional migmatization event around 320 ma nd isotope variation among the granites might reflect disequilibrium partial melting or different protoliths mass balance and trace element partial melting modeling strongly suggest two kinds of fertile crustal protoliths augen gneisses and metapelites slight compositional variation among the leucogranites does not reflect different extent of protolith melting but is related to a small amount of fractional crystallization 13 for the equigranular granites which is generally more pronounced in shallower batholitic leucogranites than in the small and homogeneous sill like bodies the lower extent of fractional crystallization and the higher pressure emplacement conditions of the sill like bodies support a more restricted movement through the crust than for batholitic leucogranites

seismic inversion combining rock physics and multiple point geostatistics a novel inversion technique combines rock physics and multiple point geostatistics the technique is based on the formulation of the inverse problem as an inference problem and incorporates multiple point geostatistics and conditional rock physics to characterize previously known geologic information the proposed implementation combines elements of sampling from conditional probabilities and elements of optimization the technique provides multiple solutions all consistent with the expected geology well log data seismic data and the local rock physics transformations a pattern based algorithm was selected as the multiple point geostatistics component rock physics principles are incorporated at the beginning of the process defining the links between reservoir properties e g lithology saturation and physical quantities e g compressibility density making it possible to predict situations not sampled by log data results for seismic lithofacies inversion on a synthetic test and a real data application demonstrate the validity and applicability of the proposed inversion technique

the ballachulish igneous complex scotland petrography mineral chemistry and order of crystallization in the monzodiorite quartz diorite suite and in the granite

deep crustal shear zones high grade tectonites and associated metasomatic alteration in the limpopo belt south africa implications for deep crustal processes large thrust sheets of complexly folded d 1 migmatitic granulites bounded by a regional system of anastomosing ductile shear zones d 2 characterize the southern marginal zone of the limpopo belt tectonites within these shear zones are subdivided into primary straight gneisses with preserved textures that can unambiguously be referred to the d 2 exhumation stage annealed straight gneisses that recrystallized during post d 2 cooling and classic mylonites that post date the limpopo orogeny primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions this process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites the resultant asymmetric sigmoidal corona textures enhance the shear fabric in contrast pelitic granulite

petrology of an intrusion related high grade migmatite implications for partial melting of metasedimentary rocks and leucosome forming processes intrusion related migmatites comprise a substantial part of the high grade part of the southern damara orogen namibia which is dominated by al rich metasedimentary rocks and various granites migmatites consist of melanosomes with biotite sillimanite garnet cordierite hercynite and leucosomes are garnet and cordierite bearing metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies 5 6 kbar at 730 750 c field evidence petrographic observations chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites the intruding melts were significantly modified by magma mixing with in situ partial melts accumulation of mainly feldspar and contamination with garnet from the wall rocks however it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith the occurrence of lile hfse and lree enriched and depleted residues within the leucosomes implies that both quartzo feldspathic and pelitic rocks were subjected to partial melting isotope ratios of the leucosomes are rather constant 143nd 144nd 500 ma 0 511718 0 511754 e nd 500 ma 3 54 to 5 11 and sr 87sr 86sr 500 ma 0 714119 0 714686 the metasedimentary units have rather constant nd isotope ratios 143nd 144nd 500 ma 0 511622 0 511789 e nd 500 ma 3 70 to 6 93 but variable sr isotope ratios sr 87sr 86sr 500 ma 0 713527 0 722268 the most restitic melanosome mel 4 has a sr isotopic composition of 87sr 86sr 500 ma 0 729380 oxygen isotopes do not mirror the proposed contamination process due to the equally high 18o contents of metasediments and crustal melts however the most lile depleted residue mel 4 shows the lowest 18o value 10 mass balance calculations suggest high degrees of partial melting 20 40 it is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites high degrees of partial melting can be reached as long as the available h2o derived from the crystallization of the intruding granites is efficiently recycled within the rock volume due to the limited amounts of in situ melting it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies the high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas the davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures and illustrates some of the reactions that may modify leucosome compositions the area provides constraints on melting processes operating in high grade metasedimentary rocks

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

rutile saturation in hydrous siliceous melts and its bearing on ti thermometry of quartz and zircon the tio2 solubility of rutile saturated hydrous siliceous melts has been investigated at p 1 gpa and t 650 1000 c for several representative felsic compositions the dissolution of a rutile crystal into a tio2 undersaturated melt provides information on both tio2 solubility and ti diffusion results of this study confirm that tio2 solubility is strongly dependent on both temperature and melt composition but not on the amount of h2o present for a given t t io2 content decreases as the melts become more felsic the solubility of tio2 is given by log ti ppm 7 95 5305 t 0 124fm

the role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments northwest adirondacks field and petrologic studies along the adirondack lowlands highlands boundary near harrisville ny indicate that heat from the synmetamorphic intrusion of the diana syenite complex intrusion temperature of 1 050 c played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units major paragneiss of engel and engel metamorphic temperatures inferred from two feldspar and spinel quartz assemblages decrease from 850 950 c along the diana metasediment contact to 650 700 c 2 3 km away from the contact metamorphic pressures are 7 0 5 kb as determined from coexisting plagioclase garnet sillimanite quartz kyanite sillimanite and garnet rutile ilmenite sillimanite quartz grail in the paragneiss migmatites consisting of quartz microcline perthite sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite sillimanite garnet spinel plagioclase corundum cordierite qualitatively the amount of partial melt and occurrences of corundum bearing assemblages decrease away from the diana contact activity of h2o inferred from coexisting biotite sillimanite quartz garnet k feldspar ranges from 0 01 to 0 17 and is five to ten times lower in corundum bearing rocks

a linear hf isotope age array despite different granitoid sources and complex archean geodynamics example from the pietersburg block south africa abstract combined u pb and lu hf isotope data from zircon populations are widely used to constrain hadean archean crustal evolution linear hf isotope age arrays are interpreted to reflect the protracted internal reworking of crust derived from the depleted mantle during a short lived magmatic event and related 176lu 177hf ratios are used to constrain the composition of the reworked crustal reservoir results of this study however indicate that hf isotope age arrays can also result from complex geodynamic processes and crust mantle interactions as shown by u pb and lu hf isotope analyses of zircons from well characterized granitoids of the pietersburg block pb northern kaapvaal craton south africa apart from scarce remnants of paleoarchean crust most granitoids of the pb with ages between 2 94 and 2 05 ga n 32 define a straight hf isotope age array with low 176lu 177hf of 0 0022 although they show a wide compositional range were derived from various sources and emplaced successively in different geodynamic settings the crustal evolution occurred in five stages i predominately mafic crust formation in an intra oceanic environment 3 4 3 0 ga ii voluminous ttg crust formation in an early accretionary orogen 3 0 2 92 ga iii internal ttg crust reworking and subduction of ttg derived sediments in an andean type setting 2 89 2 75 ga iv post collisional high k magmatism from both mantle and crustal sources 2 71 2 67 ga and v alkaline magmatism in an intra cratonic environment 2 05 2 03 ga the inferred array results from voluminous ttg crust formation during stage ii and involvement of this crust during all subsequent stages by two different processes i internal crust reworking through both partial melting and assimilation at 2 89 2 75 ga leading to the formation of biotite granites coeval with minor ttgs and ii subduction of ttg derived sediments underneath the pb causing enrichment of the mantle that subsequently became source for high k granitoids and mafic rocks at 2 68 and 2 05 ga some scatter along the array might have resulted either from significant assimilation of ancient crust intracrustal lu hf fractionation or melting of heterogeneous mantle sources those results show that without any information about the nature and composition of zircon hosting granitoids hf isotope age data are of limited use to constrain hadean archean magmatogenesis and geodynamics

improved resolution in bayesian lithology fluid inversion from prestack seismic data and well observations part 2 real case study we have performed lithology fluid inversion based on prestack seismic data and well observations from a gas reservoir offshore norway the prior profile markov random field model captures horizontal continuity and vertical sequencing of the lithology fluid variables the prior model is also locally adjusted for spatially varying lithology fluid proportions the likelihood model is inferred from basic seismic theory and observations in wells an approximate posterior model is defined which can be simulated from by an extremely computer efficient algorithm the lithology fluid inversion results are compared to manual interpretations and evaluated by cross validation in one well moreover inversions based on simplified prior models are developed for comparative reasons both lithology fluid realizations and predictions look geologically reasonable the results seem to reflect general reservoir experience and information provided by the prestack seismic data and well observations the lithology fluid propor

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

the mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

petrology and geochemistry of shoshonitic plutons from the western kunlun orogenic belt xinjiang northwestern china implications for granitoid geneses abstract a series of granitoids from proterozoic to cenozoic age occurred in the western kunlun orogenic belt xinjiang northwestern china several intrusions such as the west datong middle caledonian age north kuda late caledonian age and kuzigan karibasheng zankan himalayan age plutons have shoshonitic affinity their rock assemblages include quartz monzodiorite quartz monzonite quartz syenite middle caledonian or monzonitic granite granite late caledonian or biotite monzonitic granite diopside granite diopside syenite himalayan generally biotite is iron phlogopite with some eastonite and high mg mg fe t and fe 3 fe 2 ratio amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende with some edenite and higher mg mg fe t and fe 3 fe 2 ratio the rocks show sio 2 contents of 52 77 71 85 and high k 2 o na 2 o mostly 8 average 9 14 k 2 o na 2 o mostly 1 average 1 50 and fe 2 o 3 feo 0 85 1 51 average 1 01 and low tio 2 contents 0 15 1 12 average 0 57 al 2 o 3 contents 13 01 19 20 are high but variable the granitoids are prominently enriched in lile lree and volatiles such as f however the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations suggesting their different geneses the origin of the west datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source the north kuda and himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late orogenic slab break off or lithospheric thinning

shoshonitic liquid line of descent from diorite to granite the late precambrian post collisional tismana pluton south carpathians romania abstract the post collision late kinematic tismana pluton belongs to the shoshonitic series it is part of a late precambrian basement within the alpine danubian nappes of the south carpathians romania this pluton displays an exceptionally complete range of compositions from ultramafic to felsic rocks granites widespread mingling mixing relationships at all scales give rise to a variety of facies a liquid line of descent from the diorites to the granites is reconstructed by considering the variation in major and trace elements ree sr rb ba nb zr hf zn v co cr u th ga pb from 33 selected samples as well as mineral melt equilibrium relationships the first step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock found in the massif a possible contamination by lower crustal mafic component takes place at this stage the second step marks the appearance of apatite and fe ti oxide minerals as liquidus phases and the third step saturation of zircon mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating endo hybridisation as depicted by nd and sr isotopes fractional crystallization was combined to an important early contamination by a mafic lower crust in a deep seated magma chamber and to a later and mild contamination by a felsic medium crust in an intermediate chamber the mingling essentially occurred during the final emplacement in the high level magma chamber the monzodioritic parent magma identified by major and trace element modelling is shown by sr and nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it the necessarily recent enrichment in k2o and associated elements of the lithospheric mantle is likely to be related to the preceding pan african subduction period the partial melting of this newly formed deep source has to be linked to a major change in the thermal state of the plate

zircon thermometry and u pb ion microprobe dating of the gabbros and associated migmatites of the variscan toledo anatectic complex central iberia in the central iberian zone there are several large thermal domes in which small bodies of ultramafic mafic and intermediate rocks appear intimately associated with crustal granites and migmatites the closest spatial association between the ultramafic mafic and intermediate rocks and migmatites is in the toledo anatectic complex where field relationships suggest that these rocks are coeval and have an age close to 340 ma this and the recent discovery in the neighbouring ossa morena zone of a large mid crustal seismic reflector interpreted as a 335 350 ma mafic sill reinforce the hypothesis that heat for crustal melting was supplied from early variscan mantle magmas emplaced in the middle crust however precise ion microprobe u pb zircon dating and ti in zircon thermometry in toledo do not support this idea whereas the mean age of four mafic bodies is 307 2 ma the migmatites are c 25 ma older the migmatites hosting ultramafic mafic and intermediate bodies have the same age and ti in zircon temperatures as migmatites far from any mafic intrusion these data reveal that ultramafic mafic and intermediate magmas are late variscan they were emplaced in already cooling anatectic zones once the extensional collapse was initiated and their thermal impact on the mid crustal variscan anatexis of central iberia was negligible

temperature and redox path of biotite bearing intrusives a method of estimation applied to s and i type granites from australia abstract estimations of the oxidation state and development of o 2 during magmatic evolution are usually not possible because o 2 is a function of temperature and pressure if two independent equations for o 2 o 2 t can be obtained o 2 and the corresponding temperature can be estimated for biotite bearing rocks an estimation of o 2 bio can be combined with an estimation of o 2 rock this latter estimation requires an extrapolation of high temperature data because low temperature data are not available the combination of the two equations provides a quadratic equation in t with the common negative solution t int 1 4 c 2 k 1 2 c 1 k 1 2 c 1 2 8 c 2 k 2 which permits back calculation of o 2 the usefulness of the method is demonstrated for typical s type ilmenite and i type magnetite granites from australia two distinct oxidation states are obtained it is suggested that the availability of h 2 o during granite emplacement largely determines o 2 conditions

numerical simulations of crystallization processes of plagioclase in complex melts the origin of major and oscillatory zoning in plagioclase numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents the simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased and that the water content in the melt has little influence on this result comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees c can be rapidly imposed on plutonic phenocrysts large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals the observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates even at large undercooling there is no significant increase in temperature at the interface caused by the release of heat of crystallization

archaean carbon reservoirs and their relevance to the fluid source for gold deposits it is commonly assumed1 2 that seawater derived carbon in altered volcanic rocks is the only major pre metamorphic carbon in carbonate reservoir in archaean greenstone belts thus carbonates from archaean gold deposits with stable isotope ratios too negative median 13c 3 to have been derived from such carbon 13c usually assumed to be near 0 have been interpreted as being derived from a local felsic magmatic source1 barley and groves3 however recognize two carbonate alteration styles that predate regional metamorphism and gold mineralization in the norseman wiluna greenstone belt of western australia evidence is summarized below these are seafloor alteration and fault controlled regional alteration which as shown below have completely different carbon isotope compositions the latter median 13c 4 8 implies a major juvenile carbon co2 flux from the mantle during greenstone belt evolution and is thus of fundamental importance in itself however here we show why the mere existence of two carbon in carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area negate some of the fundamental assumptions used to support magmatic fluid models1

the timing of successive fluid events in the southern marginal zone of the limpopo complex south africa constraints from 40ar 39ar geochronology abstract we have carried out a geochronological study in the high grade metamorphic southern marginal zone of the limpopo complex south africa aimed at providing time constraints on episodes of fluid activity this work chiefly comprises 40 ar 39 ar dating with accompanying petrography of amphibole and mica and further incudes zircon u pb geochronology on a post peak metamorphic trondhjemite intrusion the so called zone of rehydration within 30 km n of the hout river shear zone where the southern marginal zone is thrust over the kaapvaal craton was studied in particular but sampling also included areas of patchy metasomatism in the granulite zone further north where no post peak metamorphic rehydration on a regional scale occurred amphibole in a mafic granulite and an older generation of zircons in the petronella trondhjemite yielded dates in the 2700 2730 ma range corresponding to the published age of peak granulite metamorphism in the southern marginal zone a cluster of 40 ar 39 ar dates on amphiboles from the zone of rehydration indicate an age between 2660 and 2670 ma for this event in accord with results from previous work on the hout river shear zone the main population of u pb zircon dates from the trondhjemite dating its intrusion also falls in this age range as does the published age of the matok pluton these data suggest that rehydration at 2660 2670 ma was a discrete episode a further 40 ar 39 ar age cluster defined by dates on both amphiboles and micas is seen at 2010 2040 ma which is the age of a major tectonometamorphic event in the central zone of the limpopo complex and previously reported from the southern marginal zone as rb sr biotite dates events we found in the 2010 2040 ma bracket are strike slip shearing dated by fabric forming amphibole as well as localized potassic metasomatism dated by both amphiboles and micas the latter was not confined to the zone of rehydration and appears in one case to be associated with gold mineralization a number of 40 ar 39 ar dates intermediate between 2600 and 2000 ma were obtained but these are diverse and can be accounted for by mixing incomplete resetting and or excess 40 ar they do not indicate events in this intermediate age bracket

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

large volumes of anatectic melt retained in granulite facies migmatites an injection complex in northern quebec abstract the opinaca subprovince in the superior province of northern quebec is a metasedimentary belt characterized by metagreywacke rocks that record a late archean granulite facies metamorphism and contains abundant thin veins of leucogranite regional scale study of this subprovince provides insight to the net budget of anatectic melt loss versus accumulation in the lower middle continental crust during anatexis a petrological study indicates that the metagreywackes partially melted at 820 c and 7 kbar and generated 1 from the metamorphic peak at 2666 ma to the granite solidus at 2636 ma the large discrepancy about 50 between the volume of anatectic melt produced locally in situ and the observed volume of leucogranite in the terrain indicates that anatectic melt accumulated in the opinaca subprovince it is an injection complex this finding that granulite facies terranes can be enriched in melt contrasts with the general view that granulite terranes are melt depleted the presence of an injection complex in the deep crust has several broad implications for the continental crust 1 h 2 o released as the injected melt crystallized rehydrated the adjacent granulite facies rocks which is evident from orthopyroxene replaced by amphibole and biotite consequently these deep rocks would be more fertile in a subsequent anatectic event than non rehydrated residuum 2 the retention of voluminous leucogranite in the injection complex has thermal consequences for the crust first the bulk composition of the crust containing the injection complex is necessarily more felsic thus the budget of heat producing elements th u and k is locally enhanced and radiogenic heat production is higher secondly latent heat of crystallization is released at a deeper level where the leucogranite solidifies the combined effect of both maintains the lower crust several tens of degrees warmer for longer period of time

bayesian lithology fluid prediction and simulation on the basis of a markov chain prior model a technique for lithology fluid lf prediction and simulation from prestack seismic data is developed in a bayesian framework the objective is to determine the lf classes along 1d profiles through a reservoir target zone a stationary markov chain prior model is used to model vertical continuity of lf classes along the profile the likelihood relates the lf classes to the elastic properties and to the seismic data and it introduces vertical correlation because the seismic data are band limited an approximation of the likelihood model provides an approximate posterior model that is a markov chain the approximate posterior can be assessed by an exact and efficient recursive algorithm the lf inversion approach is evaluated on a synthetic 1d profile that is inspired by a north sea sandstone reservoir with a realistic wavelet colored noise model and a s n ratio of three in the seismic data the results are reliable the lf classes and the interfaces between zones are largely correct the prediction unce

granite emplacement and the retrograde p t fluid evolution of neoarchean granulites from the central zone of the limpopo complex petrological and fluid inclusion data of high grade metapelitic gneisses that occur as enclaves and in the immediate surroundings of the 2 612 ga old bulai granitoid intrusive are presented in this chapter the bulai intrusive is an important time marker in the tectono metamorphic evolution of the central zone of the limpopo complex the host rock gneisses show one generation of garnet cordierite and sillimanite whereas the enclave gneisses show two different generations of garnet grt1 2 cordierite crd1 2 and sillimanite sil1 2 the first generation defines a gneissic texture whereas the second generation shows a random mineral orientation grt1 and crd1 show a higher mg content compared with grt2 and crd2 host rock garnet and grt1 show k feldspar micro veins at the contact with quartz as a result of high temperature metasomatism host rock garnet grt1 and grt2 are zoned and participate in two simultaneously operating reactions sillimanite garnet quartz cordierite and garnet k feldspar h2o biotite sillimanite quartz the combination of petrographic geothermobarometric and fluid inclusion results shows evidence of two different pressure temperature p t paths in the enclave and a single p t path in the host rocks the decompressional cooling p t path in the host rock is typical of the country rocks throughout the central zone the high pressure part of the host rock p t path overlaps with the grt1 crd1 sil1 p t path found in the enclave rocks the second p t path is calculated from the grt2 crd2 sil2 assemblage and is found only in the enclave rocks the two p t paths in the enclave rocks can be connected by a sub isobaric heating event of 50 c at 5 5 kbar this increase in temperature is followed by decompressional cooling but with a lower p t gradient compared with that of the country rocks caused by the emplacement of the bulai pluton fluids present during granulite metamorphism include co2 and brines retrograde infiltration of water in graphite bearing country rocks under relatively reduced conditions resulted in the formation of a methane rich fluid

modelling diverse processes in the petrogenesis of a composite batholith the central bohemian pluton central european hercynides the multiple intrusions making up the central bohemian pluton in the central european hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites major element trace element and sr nd isotopic compositions are used to model their petrogenesis partial melting of metabasic locks or of a chur like mantle source are interpreted to have produced melts parental to the most primitive calc alkaline sazava suite interaction of basic with more acidic magmas followed by extensive amphibole plagioclase dominated fractionation accounts for the production of trondhjemites alternatively the trondhjemites correspond to small degree melts of a metabasic source afc assimilation fractional crystallization modelling with a paragneiss as a contaminant and increasing d nd values simulates the characteristics of the blatna suite closed system fractionation of strongly enriched mantle derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic certovo bremeno suite partial melting of a metasedimentary source followed by k feldspar dominated fractionation accounts for the granites of rite ricany suite the progression from relatively primitive calc alkaline granitoids towards evolved k rich calc alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites the evidence for hercynian subduction is equivocal and the mantle enrichment could have been significantly older

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

monzonitic series from the variscan tormes dome central iberian zone petrogenetic evolution from monzogabbro to granite magmas abstract in the iberian massif rocks of the k rich plutonic series are not abundant but towards internal parts of the belt represented by the central iberian zone there are some sectors where shoshonitic plutonism occurs over broad areas one of these areas is the anatectic tormes dome encompassing two similar studied plutons pereruela and vitigudino a monzonitic association has been defined ranging from monzogabbros to quartz monzonites or scarce monzogranites enrichment in lree p sr and ba a high water content up to 5 5 and a high degree of oxidation ni nio buffer are the main features of magmas parental to these monzonitic rocks these petrographic and geochemical features allow these granitoids to be ascribed to the shoshonitic type sh type rather than to the i type the minimum emplacement pressure range is 410 230 mpa whereas the estimated solidus temperature range is 940 765 c i e above water saturated solidus o sr and nd isotopes point to open system processes apatite cathodoluminescence suggests that magma mixing was unlikely to have occurred for the most enriched rocks assimilation fractional crystallisation afc modelling was performed for both plutons permitting assimilation crystallization rates to be estimated between 0 16 and 0 25 different contaminants have been inferred a metapelite at upper crustal level for the vitigudino pluton and a granulitic orthogneiss for the pereruela pluton a liquid line of descent linking monzogabbroic members to quartz monzonites monzogranites can be reconstructed some loss of water and oxygen can be inferred although water remained in the system below the solidus giving rise to auto metamorphism at ca 500 c on the other hand afc processes are unlikely to have been the main factor in controlling the characteristic enrichment of the monzonitic series since the least contaminated samples are the most lile and lree enriched for both plutons thus a source controlled chemical signature can be inferred the experimental data indicate that a hybrid protholith a mixture of peridotite amphibolite and metapelite can account for the enrichment and hydrated nature of the least evolved monzogabbroic magmas late orogenic slab break off and subsequent post collisional extensional events appear to be a suitable scenario to provide the heat that triggered partial melting close to the crust mantle boundary

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

accessory mineral fractionation of rare earth element ree abundances in granitoid rocks abstract accessory minerals with large distribution coefficients k d s for the ree have the potential to strongly affect the evolution of ree abundances during igneous differentiation e g crystallization melting assimilation a cogenetic suite of lavas basaltic andesites through high silica rhyolites from the batopilas area of mexico which preserve petrographic evidence for apatite and zircon saturation is used to illustrate and confirm an inverse geochemical approach to recognizing early liquidus accessory mineral saturation the transition in behavior of zr from an incompatible trace element in basaltic andesites and dacites to an essential structural constituent esc in high silica rhyolites marks the onset of zircon crystallization a previously published zircon saturation model provides liquidus temperatures that support the interpretation that three high silica rhyolites represent zircon saturated melts while one has excess zircon these results contradict the previous study in which zircon fractionation had been argued against the potential effect of zircon fractionation and accumulation on ree abundances in the samples is evaluated and contributes to an improved petrogenetic interpretation a cogenetic suite of granitoid samples from the saganaga tonalite minnesota ontario north america is characterized by apparently strongly compatible behavior of the ree apatite zircon and sphene are the observed high ree k d accessory minerals forward and inverse modelling approaches provide no evidence that accessory mineral fractionation contributed to the behavior of the ree during differentiation of the samples the data instead suggest that the ree characteristics of the samples were already established by other petrogenetic processes prior to the samples becoming saturated with the observed accessory minerals

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

the genesis of felsic mafic plutonic associations a sr and nd isotopic study of the hercynian braga granitoid massif northern portugal abstract the late tectonic braga plutonic complex in the centro iberian zone north portugal was emplaced during the hercynian orogeny within a silurian sedimentary sequence and displays an acid basic association which consists of three well defined intrusions the biotite dominant braga granite minor bodies of gabbro to granodiorite composition and the mildly peraluminous gonca leucogranite these three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a rb sr age of 310 10 ma is obtained the distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related however the rb sr and sm nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process the gabbros have chemical characters high k ba sr and light rare earth elements and low nb ti and zr contents and isotopic compositions sri 0 70497 and nd 2 5 which suggest an alkaline magmatic affinity of shoshonitic type they are probably derived from an enriched mantle source in contrast the peraluminous gonca leucogranite sri 0 70933 and nd 6 8 is more likely the result of crustal anatexis the braga granite and the evolved members of the basic series have an intermediate isotopic composition sri 0 70532 to 0 70733 and nd 3 0 to 6 2 which can be interpreted in terms of an hybridization process between the two previous end members the chemical and sr nd isotopic compositions of the braga plutonic series can be explained by an assimilation fractional crystallization afc model between a mantle derived magma equivalent to the gabbros and a crust derived magma the gonca granite

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

metamorphic segregations associated with garnet and orthopyroxene porphyroblast growth two examples from the larsemann hills east antarctica in the larsemann hills east antarctica there are two important types of zoned segregations that formed during a proterozoic granulite facies metamorphic event 1 segregations with a large garnet porphyroblast in the core potassium feldspar in the mantle and biotite cordierite and plagioclase in the matrix and 2 segregations with a large orthopyroxene porphyroblast in the core potassium feldspar in the mantle and biotite and quartz in the matrix the main difference between these and zoned metamorphic segregations with leucocratic mantles described by previous authors is that the mantle phases are not a depleted subset of the matrix assemblage but part of the product assemblage the concentrical arrangement in the garnet segregations can be explained by the sequence in which a number of garnet and potassium feldspar producing reactions were met during temperature increase with internal buffering of the activity of h2o these reactions are modelled in the system k2o feo mgo al2o3 sio2 h2o kfmash the main reaction responsible for the garnet porphyroblast bearing segregations is a kfmash divariant equilibrium quartz biotite cordierite garnet potassium feldspar melt taking place in a quartz biotite cordierite garnet assemblage potassium feldspar and melt produced by this reaction are associated spatially with garnet as a consequence of the sparse nucleation of garnet and the chemical potential gradients generated prior to the interception of this equilibrium in order to account for the mineral assemblage of the mantles and the preservation of the porphyroblasts during retrogression some melt loss from the segregation is necessary the orthopyroxene segregations can be explained in an analogous manner within this model the pt conditions in which the two types of segregations formed are consistent with the m1 pressures and temperatures suggested by previous work about 4 5 kbar and 750 c

petrogenesis of the postcollisional middle devonian monzonitic to granitic magmatism of the sierra de san luis argentina abstract middle devonian granitoids intruded the eastern sierras pampeanas basement ca 600 km east of the inferred proto pacific margin of gondwana along which a ca 390 ma collisional event developed in the sierra de san luis voluminous middle devonian 393 382 ma batholiths are composed of i to a type hybrid monzonite and granite suites shoshonite and subordinated high k series stocks synplutonic dikes and enclaves make up the monzonite suite rocks are metaluminous alkali calcic magnesian porphyritic or equigranular monzonite quartz monzonite monzodiorite and scarce monzogabbro high k and subordinated shoshonite series metaluminous to mildly peraluminous magnesian alkali calcic to calc alkalic porphyritic or equigranular quartz monzonite granodiorite monzogranite and equigranular leucomonzogranites make up the granite suite plutons and batholiths only a small group of highly evolved granites are ferroan sio 2 46 62 cr ni v sc lile lree th zr and variable sr y la yb n and tb yb n smooth eu eu moderate na 2 o ca 3 5 and troughs at nb and ta for monzonite suite rocks suggest an subduction related enriched lithospheric mantle source sm nd data t dm 0 98 1 08 ga end 380 ma 0 66 1 47 and 87 sr 86 sr i 0 703520 0 704203 are compatible with an enriched mantle source the metaluminous porphyritic quartz monzonite monzogranite and the mildly peraluminous equigranular biotite monzogranites of the granite suite are characterized by relatively moderate al 2 o 3 cao and 87 sr 86 sr i high lile cr variable sr y la yb n and eu eu and low rb sr dm 1 20 1 28 ga end 380ma 3 02 to 3 3 87 sr 86 sr i 0 706578 0 707027 and the biotite monzogranites t dm 1 31 ga end 380ma 3 3 87 sr 86 sr i 0 707782 would share a common source the equigranular alkali calcic leucomonzogranites are characterized by rb sr 1 5 asi 1 05 1 18 and ga al 2 6 3 9 end 380 ma 3 74 to 3 95 and 87 sr 86 sr i 0 710743 0 712955 which would point to metasedimentary or felsic igneous crustal sources nevertheless their t dm 1 36 1 38 ga is considerably younger than the mean 1 8 1 6 ga eastern sierras pampeanas crustal residence age and less radiogenic middle devonian magmatism would record an episode of crustal growth by enriched mantle derived magma input and variable degrees of partial melting of a lower crustal source at the waning stages of the achalian orogeny

improved resolution in bayesian lithology fluid inversion from prestack seismic data and well observations part 1 methodology the focus of our study is lithology fluid inversion with spatial coupling from prestack seismic amplitude variation with offset avo data and well observations the inversion is defined in a bayesian setting where the complete solution is the posterior model the prior model for the lithology fluid lf characteristics is defined as a profile markov random field model with lateral continuity each vertical profile is further given as an inhomogeneous markov chain model upward through the reservoir the likelihood model is defined by profile and it relates the lf characteristics to the seismic data via a set of elastic material parameters and a convolution model the likelihood model is approximated the resulting approximate posterior model is explored using an efficient block gibbs simulation algorithm the inversion approach is evaluated on a synthetic realistic 2d reservoir seismic avo data and well observations are integrated in a consistent manner to obtain predictions of the lf characteristics wi

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma

stochastic inversion by matching to large numbers of pseudo wells abstractwe have developed a 1d stochastic algorithm for estimating reservoir properties based on matching large numbers of pseudo wells to seismic angle stacks the pseudo wells are part deterministic and part stochastic 1d stratigraphic profiles with consistent elastic and reservoir properties pseudo wells are sampled from a prior distribution defined by the geological interpretation a rock physics model and a model for the vertical statistics that provides close control of the lithofacies proportions a new set of pseudo wells typically 103 104 tied to the local stratigraphy is constructed for each seismic trace synthetics derived from the pseudo wells using extended elastic impedance are matched to either one or two seismic angle stacks and the best matches are selected and averaged to provide a joint estimate of reservoir properties and impedances and the associated uncertainties the algorithm has been tested on a number of data sets and validated by blind well ties the algorithm is 1d with