a paleomagnetic and rock magnetic study of the manicouagan impact structure implications for crater formation and geodynamo effects we report rock magnetic and paleomagnetic data from the similar to 214ma manicouagan canada impact crater based on 25 widely distributed sites of impact melt and basement rocks collected at the surface as well as from boreholes drilled to depths 1 5km titanomagnetite and titanohematite carry the magnetic remanence in impact melts above 320m elevation and in most basement rocks impact melts below 320m contain solely titanomagnetite magnetic susceptibility and saturation magnetization proxies for titanomagnetite concentration increase more than tenfold toward the base of the thickest impact melt that underwent fractional crystallization the titanomagnetite enriched zone partially contributes to a 2000nt magnetic anomaly in the crater s center stepwise demagnetization reveals a single normal polarity magnetization component in all samples regardless of the magnetic phases present coeval lock in remanence times for titanomagnetite and titanohematite indicate that the titanohematite formed 570 degrees c during oxi exsolution the average paleomagnetic direction and intensity coincide well with 214ma reference values we find no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool a 481m thick portion of the impact melt body hence the energy released by the manicouagan impact that created one of the 10 largest known craters on earth provoked no measurable disturbance of the geodynamo magnetic anisotropy of clast free impact melts define magnetic lineation directions that are in places radially oriented with respect to the crater s center centrifugal flow of the melt within the evolving transient crater probably generated the fabric

oxygen isotope zonation and alteration in the northern noranda district quebec evidence for hydrothermal fluid flow volcanic rocks of the new vauze norbec area central noranda volcanic complex consist of andesites and rhyolites that have been chemically and isotopically altered by synvolcanic semiconformable alteration processes including spilitization chloritization silicification and epidote quartz alteration andesites have whole rock delta 18 o values of 1 9 to 9 7 per mil those with megascopic mineralogical evidence of hydrothermal alteration have ones of 5 1 to 9 7 per mil rhyolites also show a broad range in delta 18 o values 4 4 to 13 7 per mil all of the volcanic rocks are either enriched or depleted in 18 o with respect to fresh rhyolite 8ppm or fresh andesite 6ppm a marked upward stratigraphic increase in delta 18 o values and the strata bound occurrence and regional zoning of alteration minerals provide evidence of district scale hydrothermal alteration these alteration patterns formed under the conditions of an increasing thermal gradient and a downward decrease in water rock ratio controlled by the cooling of the flavrian lake synvolcanic intrusion

multi pulse cotectic evolution and in situ fractionation of calc alkaline tonalite granodiorite rocks sierra de velasco batholith famatinian belt argentina abstract the study of a magmatic series composed of qtz diorites to granites of the south part of the sierra de velasco batholith famatinian arc in argentina reveals that most rocks follow a coherent cotectic trend which is comparable to a cotectic line of liquids cll traced from experimental liquid compositions of calc alkaline andesitic systems the identification of cotectic relationships is crucial to interpret the meaning of geochemical variation trends in terms of phase equilibria we show the contrast between typical cll trends defined by rocks of the palanche pluton and rocks departing from cll array which are interpreted as resulting from local processes of bulk assimilation of a pelitic host zircon u pb age determinations with shrimp support a coeval zircon crystallization record from of about 480 to 450 ma of samples plotting on the cll the absence of intrusive contacts between these coeval cotectic granites implies that they were fractionated in situ at the level of emplacement other younger pulses 442 5 ma also fall on the cll denoting that uniform processes of melting and fractionation were operative for at least 4 0 ma at the same locus at the active continental margin of gondwana during this protracted magmatic activity sporadic pulses of mafic magma were injected e g at 456 7 ma revealing a complex process of pulse amalgamation in the building up of the batholith these mafic magma pulses may have contributed to basification of the host granitoid by enclave dissolution at the low pressure of emplacement the reported geochemical trends as true cll together with observed textures and crystallization sequences point to a parental intermediate system of andesite composition sio 2 55 wt with a water content of about 2 5 wt h 2 o similar to those formed by sediment basalt subducted melanges a sublithospheric origin for the mafic precursor is proposed in agreement with prediction by recently published thermo mechanical modeling and experimental phase equilibria

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

plutonism in the southern abitibi subprovince a tectonic and petrogenetic framework studies of plutonic rocks in archean terranes such as the southern abitibi subprovince have been an important component in the development of an accretionary tectonic model for the superior province in the southern abitibi subprovince pretectonic approximately 2 75 2 70 ga low k tholeiitic calc alkaline dioritic to tonalitic plutons e g bourlamaque batholith flavrian pluton are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc alkaline basalt andesite pretectonic synvolcanic tholeiitic intrusions e g dundonald sill kamiskotia gabbroic complex 2 72 2 70 ga show in situ fractionation trends and locally have mixed magma textures with contemporaneous calc alkaline granitoids they are believed to be derived from a midocean ridge basalt morb like depleted mantle with little evidence for contamination by an older crustal component pre to syntectonic approximately 2 72 2 69 ga porphyry and tonalite plutons e g timmins porphyries round lake batholith have geochemical characteristics typical of archean tonalite trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades late to post tectonic 2 69 ga diorite monzonite and syenite plutons e g watabeag batholith lamaque stocks otto stock have large ion lithophile element enriched calc alkaline to sho shonitic compositions these intrusions and associated timiskaming type volcanic rocks are attributed to melting of large ion lithophile element enriched mantle in the late stages of subduction late to post tectonic biotite muscovite granite intrusions contain a significant contribution of evolved crust e g s type characteristics of the preissac lacorne batholith in the southern abitibi subprovince the tectonic setting of mantle and crust derived intrusions is broadly comparable to magmatic processes in phanerozoic arcs and accreted terranes the plutonic rocks record a transition from pre to synaccretionary subduction related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources this framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern abitibi subprovince which range from ni cu sulfides in layered synvolcanic tholeiitic intrusions e g kamiskotia gabbroic complex ontario to volcanogenic sulfides in early tholeiitic and calc alkaline sequences e g flavrian pluton noranda district quebec to the spatial association of gold with late large ion lithophile element enriched intrusions e g lamaque plugs val d or district quebec

sm nd and rb sr dating of an archean massive sulfide deposit kidd creek ontario highly altered felsic metavolcanics associated with the kidd creek ontario cu zn massive sulfide deposit show a large range of sm nd ratios and yield a sm nd isochron of 2674 40 ma initial ratio nd 1 55 0 30 which represents the time of rare earth element redistribution during intense hydrothennal alteration that the sm nd age is consistent with age constraints on ore deposition provided by precise u pb zircon data indicates contemporaneity of ore deposition hydrothennal alteration and rare earth mobility the age is therefore interpreted as a minimum age of ore deposition in contrast the rb sr age of the altered rocks as well as the metavolcanic rocks outside the alteration zone has been reset at 2576 26 ma most likely as a result of widespread low temperature metasomatism unrelated to ore deposition our results suggest that sm nd dating could be a useful tool in the study of ore deposits and potentially in the study of a wide range of mineralizations initial nd t values for massive ore altered felsic volcanics and their weakly altered precursors are identical indicating derivation and redistribution of light rare earth elements within the altered footwall volcanics these data suggest that the footwall volcanics have also supplied part of the base metals to the stratiform ore

differentiation of strongly peraluminous perphosphorus granites the pedrobernardo pluton central spain the pedrobernardo pluton 295 2 ma initial 87sr 86sr 0 7122 is a layered sheet like granite body with distinct vertical zoning biotite muscovite bearing granites in the lower zone muscovite biotite leucogranites in the middle zone and muscovite bearing aplites and pegmatites in the upper zone the magma was anatectically generated intruded almost free of restite and evolved as a closed system by crystallization in situ geochemical modelling indicates that the lower and middle zones followed an equilibrium crystallization path with no separation between solids and melt and the upper zone followed a fractional crystallization path magmatic fractionation produced leucocratic differentiates with high p concentrations and a p ca ratio higher than in apatite the excess p is located in plagioclase and k feldspar monazite apatite fractionation in the lower and middle zones produced extreme depletions in th u y and the rees and chondrite normalized ree patterns with low lrees hrees and a slight discontinuity between nd and sm about 90 of the th u y and rees reside in accessory minerals especially in monazite biotite and muscovite have very low ree contents with small positive or no eu anomalies k feldspar and plagioclase in both the lower and upper zones have moderate ree contents with positive eu anomalies feldspars from the upper zone in contrast have negative eu anomalies and have very low ree contents compared to available data from literature dsr ba th u y reesbio mineral are lower by one to three orders of magnitude and dreesplag kfsp are also lower by one order of magnitude these huge discrepancies are attributed not only to contamination with ree rich inclusions in mineral concentrates used in previous studies but to real differences in geochemical behavior of trace elements in peraluminous systems with respect to subaluminous systems biotite melt and probably k feldspar melt distribution coefficients for th u y and the rees in peraluminous magmas must be much lower than previously considered

in situ crystallization observed in the osumi granodiorite batholith abstract the osumi granodiorite is a 14 ma batholith emplaced in accreted sedimentary rocks in the forearc of southwest japan it has a chemical and mineralogical zonal structure with zones which become progressively more felsic inward feo and mgo decrease while sio2 k2o and rb increase from its margin to the interior in accord with this alkali feldspar and quartz become more abundant in inner zones and biotite and plagioclase vary oppositely the mineral assemblage however remains unvaried major bulk rock compositions form a linear variation trend which is found to be almost identical with the average trend of japanese granitic rocks these petrographic and chemical characteristics are well explained by in situ crystallization model in which the liquid returning from the solidification zone to the chamber has the granitic minimum liquid composition the composition of the liquid in the chamber and that of the cumulate lie on the straight line that connects the granitic minimum and the initial chamber composition

southern abitibi greenstone belt archean crustal structure from seismic reflection profiles lithoprobe seismic reflection profiles from the 2 75 2 68 ga southern abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface the shallowest 6 12 km regional subhorizontal reflection zone is interpreted to be the base of the abitibi greenstone belt supracrustal assemblage which may be an intrusive and or tectonic contact shallowly dipping subparallel reflectors in the upper crust are interpreted as low angle faults consistent with out of sequence stratigraphy revealed by u pb geochronology truncations of reflectors suggest that regional steeply dipping gold mineralized fault zones penetrate to depths of 15 km the profiles are consistent with the kapuskasing crustal model of the central superior province wherein greenstone belts are underlain by middle crustal felsic intrusions and gneiss and a lower crustal zone of mafic gneiss and intrusions the profiles support tectonic models of deformation of the abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults

evolution of crystal sizes in the series of dissolution and precipitation events in open magma systems article i nfo we propose a model that describes the evolution of crystal sizes and crystal size distributions csd of igneous phenocrysts in a sequence of dissolution and crystallization events this model is based on the assumption that crystal dissolution is rate limited by diffusion in melt while crystal growth is controlled by the slower kinetic of new nucleation and growth as a result the dissolution rate is inversely proportional to crystal size coming into effect through the curvature of the crystal s surface but the growth rate does not depend on the crystal size closed form analytical solution of equation for csd is obtained we apply results of modeling to quartz and zircon two prime minerals in silicic igneous systems that are widely used in geochemical and isotopic investigations the time series of multiple solution reprecipitation episodes generate concave downward csds and this result fits well with experimental and natural observations on the abundant concave down csd in silicic igneous rocks we suggest that maturation of crystal populations with sizes above several micrometers can not be caused by a size effect on the solubility of the crystals ostwald ripening but is rather driven by thermal oscillations in experiments and in nature the model predicts that mean crystal size increases with time proportionally to t 0 20 which is very close to the published experimental results for quartz maturation with the exponent of 0 19 0 22 our proposed model gives an opportunity to use natural csds for interpretation of pre eruptive magma history when solubilities and diffusion data are available for constituent elements of the dissolving mineral in particular we present time estimates for maturing zircon populations in large volume ignimbrites and estimate that it takes 100 1000 yrs to mature an initially exponential csd to a lognormal csd

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

the origin of felsic microgranitoid enclaves insights from plagioclase crystal size distributions and thermodynamic models abstract magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes microgranitoid enclaves mes are considered to be remnants of such mixing processes and the term has a well established genetic implication however microgranitoid enclaves span a wide range of compositions and felsic varieties are also frequently reported nd sr isotope and textural data from felsic microgranitoid enclaves fmes mafic microgranitoid enclaves mmes and host granites from the salto pluton itu granitic province show that the cm sized mmes are dioritic have medium grained igneous textures and xenocrysts of alkali feldspar and quartz the fmes are cm to meter sized have spheric shapes show corrugated contacts with the host granites and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine grained groundmass compared to the host granites both mme and fme samples have increased feo mgo tio2 p2o5 and zr contents but their sr and nd isotope signatures are identical fme 87sr 86sri 0 7088 0 7063 endi 10 0 to 10 2 mme 87sr 86sri 0 7070 endi 10 5 host granite 87sr 86sri 0 7056 0 7060 endi 10 2 to 10 3 these indicate that the enclaves derive from a similar source although the melts from which they formed were probably hotter and chemically more primitive than their host granites crystal size distributions csds of plagioclase in samples drilled from rinds and cores of three fmes show that the rind samples are systematically finer grained than the samples from the cores which indicates that the fmes cooled inwards and contradict interpretations that the fmes are autoliths thermal modeling suggests that a slightly more primitive hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing mingling upon thermal equilibrium the fmes would have an increased crystal cargo and the resulting touching framework would impart a solid like behavior to the fme forming magma which would lead to a contrast in rheology fragmentation dragging and preservation of felsic replenishment batches as distinct enclaves

origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber experimental investigations of convecting particle laden fluids show two regimes for convection driven by cooling from above1 in very dilute suspensions convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities above a critical concentration convection is unable to keep the particles suspended so the particles settle leaving behind a layer of convecting fluid virtually free of particles here we apply these results to cooling magma chambers in which crystallization leads to an increase in suspended crystal content with time discrete sedimentation events are predicted each time the concentration exceeds the critical value for common igneous minerals critical concentrations are very small typically 0 002 0 03 wt and layers of the order of centimetres to a few metres thick will result because minerals of different density and size have different critical concentrations and settling velocities complex fluctuations in sedimentation rate and mineral proportions can occur in a multi component melt this may lead to either regular repetitive cycles or more complex fluctuations the process is confined to low viscosity magmas such as basalts in which the crystals are able to separate from the active thermal boundary layer during convection

the ages of fractures in the eye dashwa pluton atikokan canada ages of fractures in the eye dashwa granite pluton north of atikokan ontario are estimated from analyses of their filling materials pegmatite aplite hornblendeporphyry epidote chlorite gypsum diabase iron oxides carbonate and clays the analyses include radiometric age determinations of filling materials comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time and paragenetic relations derived from textures the major fractures primarily epidote and chlorite filled were produced by penetrative deformation during cooling immediately after intrusion of the pluton from 2 6 ga to 2 4 ga they transected the pluton and largely defined the present overall fracture network less severe penetrative deformation occurred about 1 1 ga when a few diabase dikes and small gypsum fractures developed a second source of fracturing appears to have existed through most of geological time and it reopened segments of old fractures within about 400 m of the present erosional surface it is unknown whether the near surface or supergene fractures formed more or less continuously through time or in distinct episodes such as during glacial periods open fractures which are most common in the near surface layer tend to transmit groundwater but fracture sealing due to the accumulation of filling materials seems to occur with time underground excavations such as nuclear fuel waste disposal vaults would be less susceptible to the incursion of groundwater if they were located below the near surface layer

recycled metaigneous crustal sources for s and i type variscan granitoids from the spanish central system batholith constraints from hf isotope zircon composition abstract the spanish central system scs is one of the largest granite batholiths in the european variscan belt zircons from five granitic intrusions from the eastern scs have been separated and analysed for hf isotopes by laser ablation mc icpms in order to evaluate the nature of granitic magma source two samples are cordierite bearing s type granites the rest are amphibole bearing i type monzogranites hf isotope composition of zircons from these granites defines a narrow range of negative hf values 1 1 to 5 8 typical of crustal protoliths the within sample variation of zircon hf isotope composition is slightly above the analytical uncertainty 1 5 units suggesting the mixing of different magma batches during pluton assembly the s and i type granites display the same range of initial hf values that is in agreement with previous data showing that both scs granite types have similar sr nd o pb isotope signatures these isotopic data suggest that they derived from similar sources while the variation in peraluminosity could be related to different partial melting conditions the presence of a lower ordovician inherited zircon population 478 462 ma at least in i type granites suggests the involvement of cambro ordovician orthogneisses as components in the source of some scs granites inherited zircon population of similar age has also been identified in the scs lower crustal granulites the hf isotope composition of the analysed granite zircons is within that of zircons from the above granulites moreover some evidence of a juvenile input at 560 595 ma is recorded by hf isotope data in zircons from both the scs granites and granulite xenoliths hf isotope signature in conjunction with other chemical isotopic sr nd o pb and geochronological evidences suggests that scs granites were probably derived from metaigneous lower crustal sources the variscan magmatism in central spain is dominantly a crustal reworking event

diverse metal sources of archaean gold deposits evidence from in situ lead isotope analysis of individual grains of galena and altaite in the ross and kirkland lake deposits abitibi greenstone belt canada lead isotope compositions for individual grains of galena and altaite pbte were determined in situ using a secondary ion mass spectrometer sims galena was collected from the ross deposit and altaite from the kirkland lake kl deposits in the southern abitibi greenstone belt superior province of canada the samples from kl are more radiogenic than those from the ross deposit isotopic compositions vary significantly between different grains in each deposit and form broad linear arrays in 207pb 204pb 206pb 204pb and 208pb 204pb 206pb 204pb diagrams the linear arrays of pb isotope data are attributed to mixing of pb from different sources at least two sources are required for individual deposits one with low u pb and th pb ratios and the other with high ratios lead minerals occurring with au are less radiogenic than those that are not obviously associated with au suggesting that au was supplied from low u pb sources such as sulphides or older ultramafic mafic rocks while most data are consistent with the derivation from local rocks highly radiogenic pb with relatively low 207pb 206pb ratios recorded at kl require post archaean mineralization or derivation of the pb from an unusual crustal source with low the latter interpretation is favored because of the lack of textural evidence and because it is difficult to dissolve and precipitate altaite at low temperatures the presence of a pb reservoir with low is also inferred from the data of archaean banded iron formations and volcanogenic massive sulphide deposits different isotopic patterns of the two deposits suggest different sources of metals in the two deposits while this conclusion does not reject the derivation of fluids from the lower crust or mantle the data are not in accord with conceptual models invoking a common source reservoir for metals the study suggests that fluids which may have a common origin leached metals and other constituents from the upper crustal rocks during their ascent the proposed model different origins for different constituents explains much of the conflicting evidence presented by archaean au deposits including provinciality of mineralogy and relatively uniform fluid inclusion and c isotope data from many au deposits

rb sr and u pb ages of volcanism and granite emplacement in the michipicoten belt wawa ontario the michipicoten greenstone belt at wawa ontario is typical of archean volcanic belts in the superior province the supracrustal rocks are divisible into lower middle and upper metavolcanic sequences which are separated by iron formation and clastic metasedimentary rocks these are intruded by granitic stocks and embayed by granitic batholiths this study reports whole rock rb sr and zircon u pb ages for the lower and upper metavolcanics for the granitic rocks that are physically within the greenstone belt internal granites and for the granitic rocks that embay the greenstone belt external granites the apparent rb sr ages for the lower metavolcanics are 2530 90 2285 70 and 2680 490 ma the u pb ages are 2749 2 and 2744 10 ma the internal granites give an rb sr age of 2560 270 ma and a u pb age of 2737 6 ma the external granite at hawk lake indicates an rb sr age of 2550 175 ma and a u pb age of 2747 7 ma it is possible that this unit contains elements older than 2812 ma as

phase field modeling of the crystallization of feox sio2 melts in contact with an oxygen containing atmosphere abstract in this work we present a phase field model to simulate the crystallization of fe 3 o 4 in feo x sio 2 melts under oxidizing conditions the diffusion of feo and fe 2 o 3 is considered because the feo fe 2 o 3 ratio in the melt can be related to the local oxygen potential of the melt a boundary condition is developed to ensure conservation of fe while the ratio of feo fe 2 o 3 is in equilibrium with the oxygen fugacity of the atmosphere two dimensional simulations are performed with different nucleation densities of fe 3 o 4 and varying oxygen fugacity in the atmosphere the results show that for the considered nucleation densities the crystallization of the melt has a larger effect on the oxidation state than the oxygen fugacity of the atmosphere furthermore for the considered composition range the growth velocities of the spinel crystals increase with decreasing oxygen fugacity

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

accessory mineral behavior during differentiation of a granite suite monazite xenotime and zircon in the sweetwater wash pluton southeastern california u s a abstract compositional and textural characteristics of the accessory minerals monazite xenotime and zircon in the sweetwater wash granites and related aplites indicate that these phases not only controlled much of the trace element geochemistry of the suite but that they also record the melt compositional changes that occurred during magmatic differentiation fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare earth element ree depletion this was accompanied by increasing heavy ree concentrations until xenotime joined the crystallizing assemblage subsequently combined monazite xenotime fractionation resulted in lowering of the entire ree budget zircon which contains inherited cores that were apparently resorbed and rounded during initial anatexis was saturated throughout the differentiation history of the sweetwater wash suite accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales the differences are best displayed by monazite on average crystals in more differentiated rocks aplites are relatively depleted in light ree s and contain higher concentrations of the substituting elements u and th zircon displays complimentary increases in hf and y while xenotime shows a slight increase in th and in gd ho ratios both phases also exhibit higher u concentrations in aplites than in granites these differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites but also within individual thin sections due to in situ fractionation on an even smaller scale strong compositional variations are present within single crystals possibly due to diffusion controlled melt compositional gradients in the regions a adjacent to growing major phases and b adjacent to the growing accessory crystal itself our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes but that the scale of observed variations must be carefully considered

are the alteration halos of massive sulfide deposits syngenetic evidence from u pb dating of hydrothermal rutile at the kidd volcanic center abitibi subprovince canada the kidd volcanic complex is composed of felsic volcanic and subvolcanic rocks of archean age the felsic rocks are intercalated with sedimentary rocks and are in contact with komatiitic ultramafic rocks large diorite plutons were emplaced into the rhyolite and the complex is overlain by younger basalts metasomatic events affecting the lithology of the kidd volcanic complex include silicification extensive co 2 metasomatism carbonate k metasomatism sericite fuchsite and chlorite and minor carbonate alterations petrographic evidence supported by stable isotope and fluid inclusion studies suggests that silicification and early carbonate alteration were synvolcanic and therefore related to ore deposition during subsequent extensive k metasomatism sericite precipitated in the rhyolite and fuchsite precipitated in the ultramafic rocks although chlorite post dates k metasomatism the micas and chlorite are both found in anastomosing microfissures commonly occupying the same set of fractures hydrothermal rutile formed by the breakdown of magnetiteilmenite during k metasomatism and chlorite alteration gives an age of 2624 62 ma 95 confidence level by pb pb isotopic measurements it is therefore approximately 100 m y younger than syngenetic massive sulfide mineralization 2717 2 ma sulfide stringers within sericite and chlorite veins suggest some remobilization of the ores during these later events this alteration assemblage generally thought to be intimately associated with mineralization is identical to that found associated with many lode gold deposits in the superior province recent dating of micas and rutile associated with gold deposits in the abitibi subprovince gives comparable ages to the rutile in the kidd volcanic complex which must therefore record a widespread late hydrothermal event affecting mineralized rocks

u pb age constraints for the magmatic and tectonic evolution of the pontiac subprovince quebec new u pb zircon titanite and monazite ages help constrain the history of magmatism and tectonism within the pontiac subprovince of western quebec the pontiac subprovince resembles other metasedimentary belts of the superior province however the stratigraphic relationships between the dominantly sedimentary rocks of the pontiac and the adjacent volcanic dominated abitibi belt to the north and west remain controversial volcanic rocks of the belleterre volcanic zone in the southern part of the pontiac subprovince have been interpreted by other workers as klippen of abitibi strata that were thrust southward onto the pontiac subprovince however volcanic rocks in the belleterre zone give crystallization ages of 2689 2682 ma which are younger than any extrusive rocks dated thus far from the abitibi belt single detrital zircon grains from pontiac sedimentary rocks give ages as young as 2683 ma indicating that the sediments are similar in age or younger than the volcanic units the volcanic rocks pro

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

testing the validity of the petrological hypothesis no phenocrysts no post emplacement differentiation an extravagant hypothesis no phenocrysts no post emplacement differentiation has been put forward by marsh in a series of papers for the development of mafic ultramafic intrusions this hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks to explain this the hypothesis postulates that phenocryst free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts producing compositionally uniform magmatic bodies layered well differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions rather than to slow magma cooling and fractional crystallization as conventional models imply such phenocryst laden magma pulses are supposed to be derived from an underlying magmatic mush column however structureless undifferentiated mafic ultramafic bodies simply do not exist in nature all wellstudied mafic ultramafic bodies with or without residual granitic rocks that crystallized from parental magmas of non eutectic composition tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences e g ol opx opx pl opx pl cpx mineral compositions e g an in plag en in cpx and compatible incompatible major and trace element geochemistry e g mg number cr rare earth elements this is especially evident in layered intrusions that represent the key evidence against the hypothesis essentially by denying the ability of magma to differentiate in intrusive bodies the hypothesis forbids magmatic differentiation in any sub chamber related to the entire magmatic mush column as a result magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions the hypothesis is thus contradictory baseless and fundamentally flawed it should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of bowen and amply confirmed over almost a century by subsequent studies of layered intrusions the classical model was is and will most probably remain the best explanation for the origin of differentiated magmatic bodies

the nature origin and thermal influence of the granite source layer of central iberia abstract the central iberian zone comprises low grade metamorphic zones alternating with variscan thermal domes apparently equilibrated at the same pressure the variscan thermal domes are composed of authochtonous to allochtonous granites and anatectic complexes that represent remnants of the granite source layer the anatectic complexes consist of 90 high grade metasediments and 10 cadomian documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 543 pm 6 end document ma felsic metaigneous rocks the latter being extremely ra

natural equivalents of thermal gradient experiments abstract crystallization experiments using the intrinsic thermal gradient in 10 mm length capsules loaded in piston cylinder assemblies were used to investigate silicic magma crystallization the application of experimental results to natural environments requires the scaling of physical parameters of petrological interest therefore we propose here a comparative study between thermal gradients and numerical simulations of natural magma chambers we use the finite element method to calculate thermal profiles across a cooling silicic magma chamber these numerical profiles are compared with the intrinsic thermal structure of half inch piston cylinder assemblies at 500 mpa it is concluded that a set of varied magma chamber geometries and or distinct stages of their cooling history can approach the intrinsic thermal structure of laboratory experiments once the thermal properties for magma and its host rock are fixed the experimental numerical approach is mostly dependent on the volume and aspect ratio of the magma chamber our results indicate for instance that a 10 mm length capsule with a thermal gradient of 40 c mm from 1100 to 700 c may represent a 150 1100 m wide portion of a cooling magma chamber of 10 20 km diameter and 2 10 km height emplaced at a depth of 18 km additional possible scenarios are represented by larger magma chambers up to 30 km diameter in which the experimental thermal gradient can represent a 150 3700 m thin section of the large igneous bodies

quantification of lower crustal synmetamorphic fluid fluxes in the kapuskasing structural zone based on oxygen isotope profiles across two paragneiss mafic gneiss contacts detailed oxygen isotope profiles were measured across two very similar amphibolite grade paragneiss mafic gneiss contacts located 2 km apart within the kapuskasing structural zone the first profile is asymmetrical about die contact 18 o values for whole rocks and minerals are homogeneous in the paragneiss whole rock 18 o 10 4 to 11 4 smoothly decrease by about 2 over 2 5 m moving into the mafic gneiss and are homogeneous in the remainder of the mafic gneiss whole rock 18 o 7 0 to 7 4 the second profile is flat with homogeneous whole rock and mineral 18 o values in bom lithologies and mafic gneiss values whole rock 18 o 8 7 to 9 8 that are higher by about 3 relative to typical mafic gneiss values mineral mineral isotopic fractioations are fairly constant in all samples and are typical of high grade metamorphic rocks despite their differences both profiles can be explained by the advection of a mixed h 2 o co 2 fluid in equilibrium with the paragneiss across the contact into the mafic gneiss coupled with diffusion of oxygen in the fluid modeling constrains the net fluid flux required to form me first profile to be 1 3 m 3 m 2 and for me second profile to be 10 m 3 m 2 thus the net fluid flux during metamorphism varied probably by at least a factor of five over 2 km these fluxes could have been generated locally within the kapuskasing structural zone during metamorphism either by metamorphic devolatilization reactions or by the release of volatiles from crystallizing magmas

an expanded non arrhenian model for silicate melt viscosity a treatment for metaluminous peraluminous and peralkaline liquids we present new viscosity measurements for melts spanning a wide range of anhydrous compositions including rhyolite trachyte moldavite andesite latite pantellerite basalt and basanite micropenetration and concentric cylinder viscometry measurements cover a viscosity range of 10 1 to 10 12 pas and a temperature range from 700 to 1650 c these new measurements combined with other published data provide a high quality database comprising 800 experimental data on 44 well characterized melt compositions this database is used to recalibrate the model proposed by giordano and dingwell giordano d dingwell d b 2003a non arrhenian multicomponent melt viscosity a model earth planet sci lett 208 337 349 for predicting the viscosity of natural silicate melts the present contribution clearly shows that 1 the viscosity temperature relationship of natural silicate liquids is very well represented by the vft equation log a b t c over the full range of viscosity considered here 2 the use of a constant high t limiting value of melt viscosity e g a is fully consistent with the experimental data 3 there are 3 different compositional suites peralkaline metaluminous and peraluminous that exhibit different patterns in viscosity 4 the viscosity of metaluminous liquids is well described by a simple mathematical expression involving the compositional parameter sm but the compositional dependence of viscosity for peralkaline and peraluminous melts is not fully controlled by sm for these extreme compositions we refitted the model using a temperature dependent parameter based on the excess of alkalies relative to alumina e g ae sm the recalibrated model reproduces the entire database to within 5 relative

k feldspar megacryst accumulations formed by mechanical instabilities in magma chamber margins asha pluton nw argentina abstract the asha pluton the oldest unit of the san blas intrusive complex early carboniferous exhibits impressive examples of magmatic structures formed by accumulation of k feldspar megacrysts enclaves and schlieren almost all recognized structures are meter scale vertically elongate bodies of variable shapes defined as fingers trails drips and blobs they preferentially developed near the external margin of the asha pluton and generally are superimposed by chamber wide magmatic fabrics they mostly have circular or sub circular transverse sections with an internal fabric defined by margin parallel inward dipping concentric foliation and steeply plunging lineation at upper parts and flat foliation at lower parts the concentration of megacrysts usually grades from upper sections where they appear in a proportion similar to the host granite to highly packed accumulations of k feldspar along with grouped flattened enclaves at lower ends these features suggest an origin by downward localized multiphase magmatic flow narrowing and log jamming and gravitational sinking of grouped crystals and enclaves with compaction and filter pressing as main mechanisms of melt removal crystal size distribution analysis supports field observations arguing for a mechanical origin of accumulations the magmatic structures of the asha pluton represent mechanical instabilities generated by thermal and compositional convection probably owing to cooling and crystallization near the pluton margins during early stages of construction of the intrusive complex

the economic significance of the bell river complex abitibi subprovince quebec the exposed part of the bell river complex on the northern edge of the abitibi subprovince represents the upper portion of a layered complex with a massive gabbroic main zone being overlain by a magnetite rich gabbroic anorthositic layered zone and a granophyre the cumulate rocks display cryptic variation in mineral and whole rock compositions towards a more evolved signature with stratigraphic height there is a decrease in an content of plagioclase from an73 et an51 and in mg of augite from mg 75 to mg 56 an increase in la yb ratio of whole rocks from 0 5 to 2 9 and a decrease in cr from 770 to 5 ppm and ni from 315 to 45 ppm contents of whole rocks concentrations of platinum group elements pge are mostly below detection limit compositional comparison with the south african bushveld complex shows that the lowest exposed levels of the complex are equivalent to the critical main zone boundary that is in the immediate hanging wall of the pge reefs based on recent geophysical studies showing that the bell river complex is approximately 5 km thick we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme a comparison of compositional data from the bell river complex and associated basalts and rhyolites of the watson lake and wabassee groups suggests that the three suites are cogenetic based on the fact that the numerous important volcanogenic massive sulphide vms deposits in the region are located around the intrusion and the similar age of the intrusion and the volcanic rocks we support earlier workers in suggesting that the intrusion and the vms deposits are genetically linked that is that the bell river complex provided the heat source that drove the hydrothermal fluids responsible for forming the vms deposits la partie expose du complexe de be11 river sur la bordure septentrionale de la sous province de l abitibi represente la portion superieure d un complexe stratifie incluant une zone principale formee d une masse gabbroique recouverte par une zone stratifiee gabbroique anorthositique riche en magnetite et une unite de granophyre les compositions obtenues sur roches totales et sur mineraux des lithologies b cumulats presentent des variations relativement subtiles neanmoins elles portent la signature d une differenciation magmatique plus prononcee en gravissant les niveaux stratigraphiques on observe une diminution de la composante an du plagioclase de an73 et an51 et du mg de l augite de mg 75 a mg 56 un accroissement pour roches totales du rapport la yb de 0 5 a 2 9 et une decroissance pour roches totales des teneurs en cr de 770 a 5 ppm et en ni de 315 a 45 ppm les concentrations des elements du groupe du platine egp sont quasiment sous leur limite de detection la comparaison de ces compositions avec celles du complexe du bushveld d afrique du sud montre que les niveaux exposes a la base du complexe sont equivalents a la frontiere des zones critique principale c est a dire du toit immediat des filons porteurs des egp les etudes geophysiques recentes ont revele que le complexe de bell river a une epaisseur d environ 5 km et a partir de cette donnee nous inferons l existence d une zone ultramafique cachee et nous proposons un site pour entreprendre un programme de forage d exploration une comparaison des compositions obtenues pour le complexe de bell river avec les basaltes et rhyolites associes des groupes de watson lake et de wabassee suggere que les trois suites sont cogenetiques sachant que de nombreux depots majeurs de sulfures massifs volcanogeniques smv dans la region sont localises tout autour d une intrusion et que l intrusion et les roches volcaniques sont sensiblement du meme ge par consequent nous adherons aux interpretations anterieures suggerant que i intrusion et les depots des smv sont lies genetiquement c est a dire que le complexe de bell river represente la source de chaleur qui activait la circulation des fluides hydrothermaux responsables de la formation des depots de smv

late archaean thrusting in the northwestern pontiac subprovince canadian shield abstract in the northwestern archaean pontiac subprovince south verging structures such as drag folds with subhorizontal axes stretched objects down dip mineral lineations low angle thrusts and allochthonous metavolcanic slabs were mainly produced by thrusting in response to n s regional compression deformation of the dominantly metasedimentary sequences occurred under barrovian type metamorphism and the horizontal shortening was accompanied by downdip stretching in the regional foliation s1 two nappes are assumed to be present their boundary being marked by an abrupt change in structural style and metamorphic facies two phases of close to isoclinal folding are evident in the upper nappe but are absent from the lower nappe in the upper nappe d2 gives and e w tectonic trend to the rock units f2 folds range from upright to recumbent but are most typically moderately southward inclined folds that plunge gently towards the east or west besides mostly retrograde recrystallization within d2 fault zones s2 is a crenulation cleavage that did not develop sufficiently to obliterate the regional schistosity s1 and its associated stretching and mineral lineations this contrasts with the regionally penetrative nature of the main deformation event d1 and suggests a narrowing or partitioning of high strain zones with time possibly due to a decrease in the regional temperature syn orogenic sedimentation in the northwestern pontiac subprovince was probably enhanced by tectonically controlled uplift resulting from early regional kenoran compression the sedimentary basin was subsequently deformed by further n s compression which led to internal stacking nappe type displacements barrovian metamorphism and thrusting of the southern abitibi over the pontiac subprovince the common style of folding and the same orientation of various structural elements such as cleavage and lineation in the transition zone between the abitibi and pontiac subprovinces indicate continuity in deformation style regional scale transcurrent displacement along the northern subprovince boundary appears as a late localized response to regional compression

melt segregation and the architecture of magmatic reservoirs insights from the muroto sill japan the separation of melt from crystals is a fundamental process driving the chemical differentiation of magmas and can lead to the formation of pockets of potentially eruptible magmas in highly crystallised magma reservoirs while geochemical and geophysical evidence exists for the presence of such isolated pockets of eruptible melt the processes that control their volume and spatial arrangement remain unclear the muroto sill in japan provides an excellent opportunity to study these processes as it is perfectly exposed and shows clear evidence for melt segregation we collected geochemical and structural data across the sill and performed thermal modelling to quantify extraction timescales and to constrain the range of crystallinity at which melt extraction occurred our data and calculations show that the middle lower portion of the sill experienced melt extraction at crystal fractions between 0 65 and 0 8 over 100 150 years until magma was too crystalline for further segregation to occur we propose a new approach that can be used to invert measured geochemical profiles and identify the range of crystallinity at which melt extraction takes place with this approach the results we obtain for the muroto sill can be generalised to magma reservoirs of different sizes and chemistries our calculations and the comparison with natural magmatic systems show that the volume of melt rich pockets in a magma reservoir is proportional to the reservoir volume while their spatial arrangement depends on the physiochemical properties of magmas the results of this study increase our understanding of the factors controlling the distribution and volume of pockets of eruptible magmas in large magma reservoirs our calculations show that eruptible magma in dacitic and rhyolitic magma reservoirs which are responsible for some of the largest eruptions on earth tend to be distributed in lenses of small volume within highly crystallised magma such architecture diminishes our capacity of identifying eruptible magma in large magma reservoirs such as yellowstone using geophysical methods and jeopardises our capacity of assessing the potential of a reservoir to feed a large eruption

magmatic processes that generate chemically distinct silicic magmas in nw costa rica and the evolution of juvenile continental crust in oceanic arcs northwestern costa rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust a major factor in converting the oceanic plateau to continental crust is the production evolution and emplacement of silicic magmas in costa rica the caribbean large igneous province clip forms the overriding plate in the subduction of the cocos plate a process that has occurred for at least the last 25 my igneous rocks in costa rica older than about 8 ma have chemical compositions typical of ocean island basalts and intra oceanic arcs in contrast younger igneous deposits contain abundant silicic rocks which are significantly enriched in sio2 alkalis and light rare earth elements and are geochemically similar to the average upper continental crust geophysical evidence high vp seismic velocities also indicates a relatively thick 40 km addition of evolved igneous rocks to the clip the silicic deposits of nw costa rica occur in two major compositional groups a high ti and a low ti group with no overlap between the two the major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes either produced by crystallization or by partial melting of plutons near their solidi in relative terms the high ti silicic liquids were extracted from a hot dry crystal mush with low oxygen fugacity where plagioclase and pyroxene were the dominant phases crystallizing along with lesser amounts of hornblende in contrast the low ti silicic liquids were extracted from a cool wet crystal mush with high oxygen fugacity where plagioclase and amphibole were the dominant phases crystallizing the hot dry reducing magmas dominate the older sequence but the youngest sequence contains only magmas from the cold wet oxidized group silicic volcanic deposits from other oceanic arcs e g izu bonin marianas have chemical characteristics distinctly different from continental crust whereas the nw costa rican silicic deposits have chemical characteristics nearly identical to the upper continental crust the transition in nw costa rica from mafic oceanic arc and intra oceanic magma to felsic upper continental crust type magma is governed by a combination of several important factors that may be absent in other arc settings 1 thermal maturation of the thick caribbean plateau 2 regional or local crustal extension and 3 establishment of an upper crustal reservoir

the lunar crater giordano bruno as seen with optical roughness imagery abstract images acquired by the lro nac allow 3d phase ratio imagery of several areas in the crater giordano bruno this is a new optical remote sensing technique that allows a determination of the optical roughness of the lunar surface our study confirms complicated impact melt movement on the flanks and the floor of the crater in many cases however flow structures seen on the inner wall can be attributed to regolith debris taluses rather than impact melt flows it was found that the whirlpool like formation seen near the western side of the crater giordano bruno has a small central depression that can be interpreted as either a vortex cavity or a feature which resembles such a vortex formed by viscous flows coming from the crater flanks we discuss several features that confirm the young age of the crater concluding however that it was not formed within human history its age is likely on the order of 1 my

heterogeneous nucleation as the predominant mode of crystallization in natural magmas numerical model and implications for crystal melt interaction crystallization of natural magmas is inherently a disequilibrium process which involves nucleation and growth kinetics melt crystal mechanical interactions and subsolidus modifications which are all recorded in the resulting rock texture we use a new high resolution three dimensional numerical model to address the significance and consequences of homogeneous versus heterogeneous crystal nucleation in silicate magmas with increasing amount of heterogeneous nuclei during crystallization initially equigranular textures evolve to porphyritic bimodal and spherulitic types the corresponding crystal size distributions csds become concave up curved the clustering index progressively decreases and the grain contact relationships record increased clustering concave up curved csds previously interpreted as resulting from multistage crystallization mixing of crystal populations grain agglomeration or size dependent growth are now predicted consistently with other size spatial and clustering parameters to form by heterogeneous crystal nucleation correlation relationships between various textural parameters and the fraction of heterogeneous nuclei are calibrated and used on representative volcanic and plutonic rocks including cumulate rocks to deduce the fraction of heterogeneous nuclei the results indicate that 60 to 99 of all nuclei are heterogeneous for plutonic and cumulate rocks the estimate of the heterogeneous nuclei fraction based on the clustering index is significantly lower than other estimates such discrepancies in general point to the occurrence of other processes and here the results imply that crystal mush compaction and interstitial melt extraction were involved during the magma solidification formation of crystals in clusters implicit for heterogeneous nucleation implies that greater efficiency of crystal melt separation is expected in these situations

the silver deposits at cobalt and gowganda ontario iii hydrothermal regimes and source reservoirs evidence from h o c and sr isotopes and fluid inclusions the ag co ni sulpharsenide deposits of the cobalt gowganda district are characterized by relatively uniform light stable isotope systematics where 18o in quartz spans 11 1 16 0 in k feldspar 10 1 12 3 in albite 8 1 14 4 in actinolite 6 0 7 6 in chlorite 3 2 5 6 and d in chlorite 42 to 35 the temperature of hydrothermal silicate deposition was 150 230 c as calculated from quartz chlorite and triple to quadruple isotopic concordancy is locally preserved amongst quartz chlorite actinolite and k feldspar or albite filling temperature modes at 230 and 330 c exist for primary inclusions in quartz and carbonates ore forming hydrothermal fluids were isotopically characterized by 18o 2 5 to 5 d 40 to 5 interpreted to reflect isotopically and chemically evolved formation brines from huronian aquifers ultimately derived from marine pore fluids and proterozoic meteoric water recharge of the sedimentary basin the restricted range of 18oquartz quartz chlorite an

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

the role of residual melt migration in producing compositional diversity in a suite of granitic rocks abstract we present a quantitative model for in situ crystallization within a solidification zone or boundary layer based on the trace and major element compositions of plutonic igneous rocks and related geochemical data we developed the model to account for the characteristics of a suite of granitic rocks nearly uniform mineral compositions in rocks that range widely in bulk composition e g 58 76 wt sio 2 linear variation and correlation of all major and trace elements analyzed except ba large and apparently random variations in ba concentration these characteristics cannot be explained by any standard petrogenetic model involving fractionation mixing or restite unmixing but are successfully reproduced by our model of residual melt migration we do not attempt to model the entire crystallization history of a pluton but rather only that interval during which melt migration processes have recognizable geochemical effects for the granitic suite the chemical signature of residual melt migration resulted from the change in ba compatibility with the onset of orthoclase crystallization at a granite minimum our results demonstrate that plutonic rocks can develop large compositional variations comparable to those expected to result from extreme differentiation within a large magma body over short distances by melt migration under conditions of high permeability or slow crystallization rate within the solidification zone melt migration is probably a common process that could be overlooked in large plutons if sampling is sparse and if variations in some components that appear random are not considered our model equations yield estimates for parameters that describe the proportions of residual melt that crystallize within and migrate out of the solidification zone values of these parameters can be used to infer information about permeability and melt mobility within the solidification zone the model parameters derived for the granitic suite have a roughly concentric spatial pattern in the pluton suggesting that residual melt was trapped near the margins accumulated in the interior with a zone of enhanced melt mobility and possibly compositional convection in between our model may be of general usefulness because it requires no assumptions about magma chamber geometry or magma dynamics it is applicable to magmas of any composition and the equations could be formulated to include those variables best constrained by a particular suite of plutonic igneous rocks

early silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system the case of the xuehuading panshanchong pluton south china block

petrogenesis of late triassic intrusive rocks in the northern liaodong peninsula related to decratonization of the north china craton zircon u pb age and hf o isotope evidence abstract major and trace element whole rock sr nd and hf isotopes and zircon u pb age and hf o isotope data have been determined for mafic to felsic intrusive rocks from the late triassic mayihe myh longtou chaxinzi xiaoweishahe lcx and nankouqian xidadingzi nx intrusions in the northern liaodong peninsula ne china in order to investigate their sources and petrogenesis related to decrationization of the north china craton the rocks include clinopyroxene diorite monzodiorite quartz diorite granodiorite monzogranite and mafic microgranular enclaves laser ablation inductively coupled plasma mass spectrometry la icpms and secondary ion mass spectrometer sims u pb analyses of zircon from myh lxc to nx intrusions yield late triassic ages of 224 218 ma establishing that the mafic and felsic magmas were coeval the clinopyroxene diorites from the myh pluton have high mgo concentrations at low silica contents with positive whole rock e nd t up to 3 8 and e hf t up to 6 2 and zircon e hf t values and some mantle like 18 o values they are enriched in large ion lithophile liles and light rare earth elements lrees and depleted in high field strength elements hfses which can be explained as melts derived from a depleted mantle with some subsequent crustal contamination however the diorites from the lxc and nx intrusions are high k rocks and have high mgo concentrations and mantle like 18 o values but with negative whole rock e nd t 12 9 to 7 6 and e hf t 14 0 to 5 6 and zircon e hf t values distinct from those of myh pyroxene diorites indicating that their parental magmas were derived from partial melting of an ancient lithospheric mantle the granitoids from three plutons have high sio 2 contents and low mgo concentrations suggesting they were mainly derived from crustal sources however the myh monzogranite and granodiorite have positive whole rock e nd t and e hf t up to 3 0 and zircon e hf t values indicating a juvenile crustal source whereas the lxc and nx granitoids have strong negative and variable whole rock e nd t and e hf t and zircon e hf t values indicating that they were derived from partial melting of ancient lower crustal materials with involvement of mantle components field observations geochronology geochemistry sr nd hf isotopic and zircon hf o isotopic compositions point to a complex petrogenesis where mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between mafic and felsic rocks identification of four components i e depleted and enriched mantle and juvenile and ancient crust in the late triassic magmatism in the northern liaodong peninsula suggests a strong mantle crust interaction process related to decratonization of the north china craton

dynamics of evolving magma chambers textural and chemical evolution of cumulates at the arrival of new liquidus phases abstract basaltic magma chambers are best exemplified by layered intrusions fossilized natural laboratories that historically constrain most fundamental principles of igneous petrology progressive fractional crystallization of basaltic melts in layered intrusions results in successive appearance of 5 10 liquidus minerals most minerals appear in the stratigraphy of layered intrusions via two distinct stages they first emerge as oikocrysts large irregularly shaped and interstitial crystals that form in a crystal liquid framework and then as primocrysts relatively small idiomorphic and cumulus crystals that crystallize from a main magma body the transition from oikocrysts to primocrysts is commonly marked by the following textural and chemical features a a similarity in chemical composition of the last forming oikocrysts and the first forming primocrysts b slow arrival modal overabundance and finer than normal size of the first primocrysts c a low abundance of trapped liquid in rocks with the first forming primocrysts d a step like increase in mineral dihedral angle in rocks slightly below the level of the primocryst arrival and e the rims on plagioclase primocrysts that are identical in composition to cores of plagioclase primocrysts at the first appearance of new liquidus phases higher up in the section the phase equilibria control on the cumulus stratigraphy of layered intrusions require large long lived and largely molten magma chambers and therefore their origin is not compatible with several recent concepts that deny the existence of such reservoirs in the earth s crust we therefore suggest that the systematic changes associated with the arrival of cumulus phases in layered intrusions are best explained by a traditional concept that considers layered intrusions as large initially crystal free melt bodies that gradually lose heat and crystallize from margins inwards through thin solidification fronts the entire magma body during its internal evolution is thought to be kinetically supercooled i e kept slightly below its liquidus temperature with respect to cumulus phases that crystallize at the margins each new cumulus phase arrives on the liquidus with some kinetic delay and nucleates heterogeneously against pre existing crystals at the floor roof and sidewalls of basaltic magma chambers this implies that internal differentiation of basaltic magma chambers mostly occurs through convective separation of evolved liquid from in situ growing crystals and its mixing with the main magma body

crystallization dynamics of granite magma chambers in the absence of regional stress multiphysics modeling with natural examples numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection during which wall rock contamination and magma mixing are possible followed by a long period of no convective cooling during which melt segregation occurs convection is driven by the negative density gradient generated in the upper cooling zone by melt to solid phase transformation convection breaks the upper mushy zone and drags the fragments downwards with descending rayleigh taylor fingers such fragments can be preserved as microgranular enclaves the descending rayleigh taylor fingers split low aspect ratio sill like magma chambers into nearly isolated convection cells if the magma is initially heterogeneous this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios and finally results in a pluton with marked lateral compositional variations easily misinterpreted as different intrusive batches convective heat loss quickly leads most of the magma chamber to critical crystallinity independently of the vertical coordinate so that a chamber wide three dimensional skeleton of crystals with uniform initial porosity c 0 4 0 5 is formed this configuration is gravitationally unstable therefore it spontaneously compacts towards an equilibrium vertical variation of porosity approaching atty s law in the absence of regional stress the upwards migration of the inter crystalline melts as a result of compaction is the most effective way of melt solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates granite magma chambers fractionated by these mechanisms will produce short range differentiation series from a composition slightly less silicic than the initial magma to high silica segregates in the presence of regional stress tectonic squeezing and shearing during the post convective stage can expel residual fluid more efficiently and lead to wide range granite differentiation series from rocks notably less silicic than the initial magma to high silica leucogranites

actinide and rare earth element characteristics of deep fracture zones in the lac du bonnet granitic batholith manitoba canada the history of major trace and actinide element mobility and fluid infiltration has been studied in two deep fractures 1 km in the lac du bonnet batholith as part of the canadian nuclear fuel waste management program core samples collected from two fracture zones fr1 and fr2 1175 m deep containing saline groundwater tds 50 g l were investigated mineralogically chemically and isotopically 238u series o and h several sequentially overprinting alteration stages were identified from early high temperature to later low temperature hydrothermal alteration k feldspar illite chlorite and later kaolinite formed during these stages subsequent infiltration of oxidizing fluids produced alteration of the chlorite to hydrous iron oxides fracture zone fr1 contains predominantly hematite coating fracture zone fr2 is characterized by the formation of a breccia and by an intense alteration of the granite in contact with this breccia to illitic clay alteration occurred during infiltration either of formation brines or of isotopically evolved meteoric water where 18o 8 to 12 and d 65 to 20 at calculated temperatures between 250 and 25 c loss of ca na and sr and to some extent ba was observed in all the altered samples relative to fresh granite a sharp increase in k mg u th and ree was observed in the illitic clay rich zone pronounced disequilibria of 234u238u 0 5 230th234u 0 7 and 226ra230th 0 9 exist in the illitic clay indicating loss of 234u 230th and 226ra to the groundwater within the last 1 5 ma in contrast an excess of 234u 230th and 226ra was measured in the brecciated samples the disequilibria are consistent with a model involving loss of 234u 230th and 226ra to groundwater by recoil from u deposited on the illitic clay surfaces these radionuclides were deposited subsequently in the nearby brecciated zone

the role of fluids during formation and evolution of the southern superior province lithosphere an overview models for fluid flow and hydrothermal alteration for the abitibi greenstone belt are reviewed in the light of lithoprobe results in the region in the abitibi greenstone belt eruption of volcanic sequences over 2750 2700 ma was accompanied by pervasive low temperature hydrothermal alteration at high water rock ratios giving systematic 18o enrichment archean ambient ocean water bottom temperatures were likely ca 30 c and 18o 0 1 chert iron formations precipitated from low temperature hydrothermal discharge base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high temperature 300 400 c fluids in convective cells sited above subvolcanic intrusions the ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high temperature low water rock exchange with the volcanic pile to nacl 3 7 wt or cacl2 nacl up to 30 wt brines of 18o 0 8 these volcanic associated hydrothermal deposits are associated with greenst

crustal velocity models for the archean abitibi greenstone belt from seismic refraction data we present velocity models for two seismic wide angle refraction profiles across the archean abitibi greenstone belt and the pontiac subprovince the seismic profiles are 210 and 220 km long traveltime inversion and amplitude forward modelling were used to obtain two dimensional velocity structure and interface geometry the main features of the velocity models include 1 three crustal layers 2 variable velocities 5 6 6 4 km s in the upper crust 0 12 km with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons 3 a relatively uniform middle crust 12 30 km with velocities ranging from 6 4 to 6 6 km s 4 a velocity increase of 0 3 km s across the middle crust lower crust boundary 5 a lower crust 30 40 km with velocities increasing from 6 9 km s at the top to 7 3 km s at the base 6 an average upper mantle velocity of 8 15 km s 7 depth to moho of about 40 km in the north central abitibi belt decreasi