variable parent magmas and recharge regimes of the parinacota magma system n chile revealed by fe mg and sr zoning in plagioclase abstract zoning patterns of an content and fe mg and sr concentrations in plagioclase phenocrysts in andesites from parinacota volcano n chile reflect alternating recharge events with two chemically distinct mafic magmas these magmas are characterized by low and high sr contents similar to two recent mafic flank eruptions one end member basaltic andesite shows large sr enrichment and heavy rare earth depletions and thus equilibrated with lower crustal rocks at depth where plagioclase high sr is unstable and garnet high hree y is stable a second end member magma is lower in sr ba contents and has ree patterns typical for parent magmas elsewhere in the central andes the number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone variations of an fe and mg contents and morphology of zones suggest also changes in water pressure including decompression under water under saturated and water saturated conditions evidence for decompression is more present in post collapse samples suggesting that the change in the volcano dynamics involves changes in magma chamber location this shows the importance of the cone collapse event in the volcano s magmatic evolution we propose that both end members are only seen at the surface of parinacota volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the central volcanic zone

the sediment hosted stratiform copper ore system sediment hosted stratiform copper deposits comprise disseminated to veinlet cu and cu fe sulfides in siliciclastic or dolomitic sedimentary rocks sediment hosted stratiform copper deposits are extremely common though economically significant deposits are rare they account for approximately 23 percent of the world s cu production and known reserves in addition to being significant sources of co and ag three sedimentary basins the paleoproterozoic kodaro udokan in siberia the neoproterozoic katangan in central africa and the permian basin of central europe contain supergiant 24 million metric tons mt contained cu deposits sediment hosted stratiform copper deposits are the products of evolving basin scale fluid flow systems that include source s of metal and s source s of metal and s transporting fluids the transport paths of these fluids a thermal and or hydraulic pump to collect and drive the fluids and the chemical and physical processes which result in precipitation of the sulfides metal sources are undoubtedly red bed sedimentary rocks containing fe oxyhydroxides capable of weakly binding metals sulfur may be derived from marine or lacustrine evaporites reduced seawater or hydrogen sulfide bearing petroleum metals appear to have been transported at low to moderate temperatures in moderately to highly saline aqueous fluids with the temperature of the fluid largely dependent on the time of fluid migration in the basin s burial history these basinal fluids were focused to potential metal precipitation sites by thinning of the red bed sequence at basin margins by faults by differentially permeable sedimentary units by paleotopography within the basin or along the margins of salt diapirs fluid movement produced widespread basin scale alteration that has commonly been overlooked but can form an important exploration guide sulfide precipitation occurred due to reduction typically caused by reaction with carbonaceous rocks or petroleum the amount of sulfides present at any deposit may be either metals or sulfide limited or could have been controlled by the amount of available reductant e g petroleum while understanding of sediment hosted stratiform copper ore genesis at the deposit scale is relatively robust there are still significant questions in regards its position in terms of basin evolution a wide variety of basin architectures and processes can lead to the formation of sediment hosted stratiform copper deposits despite general agreement that sulfides postdate sedimentation the absolute age of mineralization in many deposits has been difficult to document and the available evidence suggests that deposits can form throughout a basin s evolution from early diagenesis of ore host sediments to basin inversion and metamorphism supergiant and giant deposits formed in basins which underwent prolonged periods of fluid flow and in which unique conditions allowed for the accumulation of large amounts of metal bearing fluid sufficient reduced s and large amounts of reductants

a thermodynamic model for the plagioclase liquid hygrometer thermometer a new thermodynamic model for the plagioclase liquid exchange reaction between the albite naalsi 3 o 8 and anorthite caal 2 si 2 o 8 components is presented which can be used as a plagioclaseliquid hygrometer or thermometer the model incorporates calorimetric and volumetric data for the pure liquid and crystalline components which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition this allows a more accurate assessment of the effect of melt composition including dissolved water concentration on the exchange reaction from plagioclase liquid equilibrium experiments activity composition relations for the plagioclase solid solution are taken from holland and powell 1992 the new hygrometer is calibrated on 71 plagioclase liquid experiments of which 45 are hydrous and 26 are anhydrous three filters were applied to the phase equilibrium data 1 crystallanities 30 2 pure h 2 o fluidsaturated and 3 compositional totals including h 2 o component of 97 101 for hydrous quenched glasses the final data set spans a wide range of liquid compositions 46 74 wt sio 2 plagioclase compositions an 93 an 37 temperatures 825 1230 c pressures 0 300 mpa and dissolved melt water concentrations 0 7 wt h 2 o the standard error of estimate see for the model is 0 32 wt h 2 o and all liquid compositions are fitted equally well when the model is used as a thermometer all measured temperatures are recovered equally well within 14 c on average the model is only recommended for applications that fall within the compositional bounds of the calibration data set i e metaluminous basalts through rhyolites in equilibrium with an 95 an 35 it is not yet calibrated for rhyolites crystallizing plagioclase more sodic than an 30 owing to an absence of phase equilibrium experiments on rhyolites that pass the required filters the new plagioclase liquid hygrometer thermometer is available as a visual basic program that runs on excel 2004

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

a tin mineralized topaz rhyolite dike with coeval topaz granite enclaves at qiguling in the qitianling tin district southern china abstract the qiguling topaz rhyolite is present as a dike within the qitianling biotite granite batholith of the nanling range of southern china here the rhyolitic dike 4 5 m wide and 500 m long contains enclaves of topaz granite these rhyolites contain up to 72 wt sio 2 have alumina saturation index asi 1 1 and have groundmasses with estimated fluorine contents of approximately 1 5 wt textural relationships provide evidence of a quenched silicate melt that contains quartz k feldspar albite and zinnwaldite phenocrysts in a groundmass containing abundant topaz the rhyolites in the study area are also strongly enriched in tin 90 2700 ppm and generally have a close association between cassiterite and zinnwaldite although cassiterite is also present as sponge textured fills between rock forming minerals granite enclaves and their hosted rhyolite have similar major geochemical compositions and mineralogies to each other zircon u pb dating indicates that the topaz rhyolite 147 150 ma and topaz granite enclaves 154 ma were formed contemporaneously with ages that overlap within analytical uncertainty in addition the major and trace element compositions of the rhyolite and their granite enclaves are dissimilar to those of the hosting qitianling biotite granite this discovery of granite enclaves within rhyolite dikes suggests the presence of a topaz bearing granite body at depth that may host tin mineralization the expected hidden tin granite may be of great interest in the further exploration

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

thermal mechanical modeling of the role of thermal stresses and stoping in magma contamination mantle derived magmas emplaced in the upper crust depth 10 km are in thermal disequilibrium with the surrounding country rock as a consequence of the large temperature contrasts not only will the magma cool quite rapidly but the surrounding country rock will be heated and affected by large thermal stresses using coupled time dependent thermal mechanical models we find that a a single stage 3 km radius magma body has an eruptive life of on the order of 104 years b near the magma wall rock boundary large thermal stresses develop over short time spans 10 years and c material stoped into the chamber as a result of these thermal stresses will be assimilated either in the upper reaches of the chamber or at its base leaving a relatively uncontaminated central core these results indicate that contamination of a primary magma by stoped crustal blocks is an efficient mechanism for producing hybrid lava sequences

new insights into the processes controlling compositional zoning in plagioclase abstract although plagioclase is the most abundant mineral in crustal rocks the factors that control the magnitude of compositional zoning in plagioclase remain poorly constrained the composition of magmatic plagioclase depends upon physical parameters such as temperature t and total pressure p as well as the full range of compositional controls including the water content of the melt wt h 2 o the changes in these physical parameters can be quantified using available numerical phase equilibria models for various differentiation scenarios and the magnitude of these effects on plagioclase zoning can be calculated the role of temperature total pressure and water content on plagioclase zoning has been experimentally investigated previously in a pure albite anorthite ab an system in this study we use the melts algorithm to quantify the effects of p t and water content of the melt on compositional zoning in plagioclase in a basaltic system our first two simulations are designed to evaluate the magnitude of compositional changes that may be attributable to specific system changes exclusive of assimilation and recharge the first scenario involves a two step crystallization process in the first step magma rises from a deeper to a shallower chamber while crystallizing plagioclase at successively lower pressures in the second step the magma pools in the shallower magma chamber and continues to crystallize isobarically if these two steps are repeated the result is the development of reverse normal and oscillatory zoning in plagioclase the second scenario involves plagioclase zoning in a large convecting magma chamber where plagioclase crystallizes as the magma convects resulting in oscillatory zoning the third scenario involves a two step process where crystallization is interrupted by multiple recharge events in this case magma crystallizes isobarically as a new batch of primitive magma is injected periodically to the magma chamber and crystallization continues after mixing during the repetition of these two steps plagioclase develops normal and reverse zoning based on our results we interpret zoning in plagioclase in terms of the partitioning of na 2 o and cao between the melt and plagioclase and the changes in the total volume during dissolution reactions of albite vab dissolution and anorthite van dissolution components for the first two scenarios our calculations show that as pressure decreases vab dissolution and van dissolution decrease while as pressure increases both of them increase vab dissolution always being larger than van dissolution controls the zoning in plagioclase by compensating the larger volume change in the melt either by albite component dissolving into its oxides or by oxides reacting to produce albite component therefore at a given temperature with decreasing pressure na 2 o partitions increasingly into the melt compared to cao making the coexisting plagioclase more an rich thus ascent of magma from a deeper to a shallower chamber isentropically or convection of a magma in a large chamber polybaricaly results in the development of reverse zoning in plagioclase the magnitude of the effects is on the order of 3 mol an per kbar for both decompression driven crystallization and isothermal polybaric convection

the geotectonic environment of zambian copper cobalt mineralization suggestions that the ore shale of the lower roan group is part of a series of sedimentary fades produced by the north easterly transgression of the sea over dominantly arenaceous strata of littoral deltaic lacustrine and continental origin have been viewed with scepticism by the author the lack of proven transition facies from silty carbonaceous argillites of the type found close to the south western limits of the ore shale in the zambian copperbelt to dolomites found further to the sw has tended to accentuate this feeling the intensity of exploration drilling in these areas must imply that whatever the cause the cut out of the ore shale is rapid at the same time a 9normal9 lower roan succession of the chambishi type suddenly becomes attenuated and dolomites and dolomitic chloritic schists are found immediately overlying argillaceous gritty sandstones conglomerates and arkosic quartzites characteristic of footwall formations locally this contact may be reddened porous and brecciated amphibolite bodies of the type normally found at a much higher stratigraphic level in the upper roan group are present a short distance above this contact it is thus proposed that this feature represents a hiatus in the succession and that all rocks from the ore shale to the top of the lower roan and possibly into the base of the upper roan are missing due to non deposition and or erosion this interruption of the succession coincides with the western cut out of the ore shale from the konkola dome in the nw down to the western end of the roan basin in the s in view of various authors9 contention that the zambian copperbelt is the site of an ancient aulacogen the above observations take on an important genetic significance the present paper thus reviews the location of copper cobalt mineralization in relation to sedimentary facies changes and cut outs structural features and amphibolitic bodies and proposes a geotectonic model for the copperbelt which it is hoped provides a more satisfactory and comprehensive explanation for the facts this model also provides a source for the metals which were converted to sulphides by diagenetic processes it is believed that the complex mineral chemistry of the ore shale associated ores and in particular the presence of localized concentrations of mo co ag as bi and te bearing phases is best explained by high level seepage of magmatically enriched hydrothermal solutions along precambrian rift systems and associated fractures these metals were either dispersed by wave and current action in the sedimentary environment or were concentrated by porosity and physico chemical and bacteriogenic controls in the diagenetic environment

energy constrained open system magmatic processes iv geochemical thermal and mass consequences of energy constrained recharge assimilation and fractional crystallization ec rafc 1 a wealth of geochemical and petrological data provide evidence that the processes of fractional crystallization assimilation and magma recharge replenishment dominate the chemical signatures of many terrestrial igneous rocks previous work spera and bohrson 2001 bohrson and spera 2001 has established the importance of integrating energy species and mass conservation into simulations of complex magma chamber processes an extended version of the energy constrained formulation energy constrained recharge assimilation fractional crystallization ec rafc tracks mass and compositional variations of melt cumulates and enclaves in a magma body undergoing simultaneous recharge assimilation and fractional crystallization spera and bohrson 2002 because many ec rafc results are distinct from those predicted by extant rafc formulations the primary goal of this paper is to present a range of geochemical and mass relationships for selected cases that highlight issues relevant to modern petrology among the plethora of petrologic problems that have important well documented analogues in nature are the geochemical distinctions that arise when a magma body undergoes continuous versus episodic recharge the connection between erupted magmas and associated cumulate bodies the behavior of recharge fractionation dominated systems rfc thermodynamic conditions that promote the formation of enclaves versus cumulates and the conditions under which magma bodies may be described as chemically homogeneous investigation of the effects of continuous versus episodic recharge for mafic magma undergoing rafc in the lower crust indicates that the resulting geochemical trends for melt and solids are sensitive to the intensity and composition of recharge suggesting that ec rafc may be used as a tool to distinguish the nature of the recharge events compared to the record preserved in melts the geochemical and mass characteristics of solids associated with particular rafc events may record a more complete view of the physiochemical history of an open system magma body the capability of ec rafc to track melts and solids creates a genetic link that can be compared to natural analogues such as layered mafic intrusions and flood basalts or mafic enclaves and their intermediate composition volcanic or plutonic hosts the ability to quantify chemical and volume characteristics of solids and melts also underscores the need for integrated field petrologic and geochemical studies of igneous systems while it appears that a number of volcanic events or systems may be characterized by continuous influx or eruption of magma steady state systems reports describing compositional homogeneity for products that represent eruptions of more than one event are relatively rare in support of this ec rafc results indicate that very specific combinations of recharge conditions bulk distribution coefficients and element concentrations are required to achieve geochemical homogeneity during cooling of a magma body undergoing rafc in summary critical points are that ec rafc provides a method to quantitatively investigate complex magmatic systems in a thermodynamic context it predicts complex nonmonotonic geochemical trends for which there are natural analogues that have been difficult to model and finally ec rafc establishes the link between the chemical and physical attributes of a magmatic system application of ec rafc promises to improve our understanding of specific tectonomagmatic systems as well as enhance our grasp of the essential physiochemical principles that govern magma body evolution

shrimp zircon u pb geochronological and whole rock geochemical evidence for an early neoproterozoic sibaoan magmatic arc along the southeastern margin of the yangtze block abstract it has been generally accepted that the south china block was formed through amalgamation of the yangtze and cathaysia blocks during the proterozoic sibaoan orogenesis but the timing and kinematics of the sibao orogeny are still not well constrained we report here shrimp u pb zircon geochronological and geochemical data for the taohong and xiqiu tonalite granodiorite stocks from northeastern zhejiang southeastern margin of the yangtze block our data demonstrate that these rocks dated at 913 15 ma and 905 14 ma are typical amphibole rich calc alkaline granitoids formed in an active continental margin combined with previously reported isotopic dates for the 1 0 ga ophiolites and 0 97 ga adakitic rocks from northeastern jiangxi the timing of the sibao orogenesis is thus believed to be between 1 0 and 0 9 ga in its eastern segment it is noted that the sibao orogeny in south china is in general contemporaneous with some other early neoproterozoic 1 0 0 9 ga orogenic belts such as the eastern ghats belt of india and the rayner province in east antarctica indicating that the assembly of rodinia was not finally completed until 0 9 ga

sulfide mineral chemistry investigation of sediment hosted stratiform copper deposits nahand ivand area nw iran abstract copper mineralization in nahand ivand area nw iran appears as disseminated copper sulfides along a redox boundary between gray sandstone and microconglomerate and hematitic sandstones siltstones and shaly marl geochemical analyses of the nahand ivand deposits show as much 35 wt cu and 730 ppm ag in the gray sandstone electron probe micro analysis epma was used to determine the mineralogical composition and distribution patterns of copper and silver bearing phases mainly in pyrite and cu bearing sulfides the epma data were essential for evaluating the distribution and partitioning of potential economically valuable components between co existing minerals furthermore they contribute to a better understanding of the genesis of the nahand ivand copper deposits and will guide further exploration in the region the epma results from different types of pyrite reveal cu contents as high as 0 32 wt 1 10 wt and 2 88 wt for framboidal pyrite pyi overgrowths on framboidal pyrite pyii and diagenetic pyrite pyiii respectively this successive increase of copper from pyi to pyiii is attributed to a continuous supply of copper that replaces framboidal pyrite in turn by the more copper rich digenetic pyrite because of hydrothermal overprinting pyrite has been replaced by djurleite roxbyite and other nonstoichiometric cu s minerals that include covellite the epma study indicates that covellite contains significant concentrations of ag locally 1 wt in contrast only trace amounts of silver have been detected in pyrite and other copper sulfides indicating that covellite is the major ag carrier in the ore according to textural relationships such open space filling impregnation and replacement textures and the epma results later stage copper bearing fluids were responsible for the silver enrichment in the nahand ivand deposits

reactive bulk assimilation a model for crust mantle mixing in silicic magmas bulk assimilation of small millimeters to 1 km fragments of crust driven and ultimately masked by reactions during xenolith melting and magma crystallization is an important mechanism for crust mantle mixing xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma yielding mainly plagioclase pyroxene fe ti oxides and hydrous melt the xenolith is physically compromised by partial melting and begins to disintegrate xenolithic melt and crystals are mixed into the host magma xenocrystic zircon is liberated at this stage the cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions the reverse of dehydration melting all pyroxenes and oxides phenocrysts xenocrysts or crystals having a hybrid signature will be subject to these reactions producing feldspars amphiboles and micas that incorporate material from several sources a particularly effective mixing mechanism implicit in the model is a reduced energy penalty for bulk assimilation much of the assimilant remains in solid form compared to melt assimilation models a large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas while the assimilation of low density crust and concomitant fractionation provide the isostatic impetus for ascent the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent

origin of enclaves in s type granites of the lachlan fold belt abstract the more mafic s type granites of the lachlan fold belt contain a distinctive assemblage of lithic inclusions of deep crustal origin two types predominate the schistose enclaves and the microgranular enclaves there is also a small proportion of fragments of local country rock all the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source their ubiquity in the mafic s type granites and absence from both felsic s type granites and i type granites conform with such an origin the argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid since they represent less fertile parts of the source that had different compositions that as a result melted to a lesser extent models that derived these enclaves from regions of the crust above the zones of partial melting so that they are not restites are complex unnecessary do not conform with their ubiquity in the more mafic s type granites and are not consistent with their chemical compositions the origin of the microgranular enclaves is controversial in most s type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite in a few places such material is seen to be forming by recrystallization of material in the core of the enclave which has the assemblage quartz calcic plagioclase orthopyroxene cordierite biotite with accessory ilmenite sulfide and apatite all of the microgranular enclave cores show pseudo doleritic texture in which calcic plagioclase crystals an60 and orthopyroxene crystals project into or are enclosed by quartz large plagioclase crystals are zoned with some core compositions as calcic as an94 the low na contents of the microgranular enclaves are not consistent with an igneous origin we consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones the pseudo igneous textures result from the presence of a partial melt during metamorphism an analysis of a calc silicate lens of upper amphibolite grade from the wilsons group of victoria land antarctica is remarkably similar in composition to the microgranular enclaves from lachlan fold belt s type granites we suggest that rocks equivalent to that group which is of appropriate age and general composition were the source rocks for the s type granites of the lachlan fold belt

geology of the enterprise hydrothermal nickel deposit north western province zambia the enterprise nickel deposit 40 mt of 1 07 ni is located on the eastern edge of the kabompo dome in the north western province of zambia the deposit area contains basement schists overlain by neoproterozoic metasedimentary rocks nickel sulfides are hosted within a sequence of quartz carbonate and carbon rich metasedimentary rocks that interfinger with and overlie siliciclastic metasedimentary rocks the host rocks contain significant kyanite talc and magnesian chlorite silicification and magnesian metasomatism occurred prior to or concurrent with a regional metamorphic event 590 500 ma mineralization resulted in the precipitation of nickel and iron nickel sulfides in veins and as semimassive replacements of the host rocks nickel sulfides precipitated in two main stages a millerite vaesite pyrite assemblage that formed disseminations and semimassive replacements in vuggy textured rocks and a later millerite bravoite molybdenite assemblage in quartz kyanite veins and local semimassive replacements the deposit contains minor copper and trace amounts of cobalt and platinum group elements pges a discrete zone of copper sulfides underlies a portion of the nickel sulfide zone re os geochronology on molybdenite yields a 540 6 1 8 ma age for mineralization at enterprise the approximate age of metamorphism sulfur isotope results indicate that the sulfur at enterprise was derived largely from neoproterozoic marine sulfate by thermochemical sulfate reduction significant volumes of mafic igneous rocks are not present in the immediate area of the enterprise deposit no evidence of prealteration concentrations of nickel exists within the sedimentary rock sequence at the deposit the alteration and mineralization style of the enterprise deposit is similar to the much less metamorphosed nickel bearing shinkolobwe uranium deposit in the democratic republic of congo drc though the enterprise deposit does not contain significant uranium the sediment hosted nickel rich deposits of central african copperbelt exemplified by enterprise appear to represent a new style of hydrothermal nickel mineralization

elemental and sr nd isotope geochemistry of microgranular enclaves from peralkaline a type granitic plutons of the emeishan large igneous province sw china abstract microgranular enclaves are common within intermediate to felsic granitic rocks that have i and s type affinity however they are rare within alkaline anorogenic granitoids of a type affinity the permian 260 ma emeishan large igneous province elip of southwest china contains two peralkaline silica saturated a type granitic plutons that host microgranular enclaves the enclaves from the baima pluton are intermediate in composition and have lower sio 2 and higher tio 2 cao and mg sio 2 57 2 to 63 0 wt tio 2 0 8 to 1 8 wt cao 1 7 to 3 3 wt mg 28 to 44 than their host sio 2 62 6 to 67 8 wt tio 2 0 5 to 1 4 wt cao 0 4 to 1 8 wt mg 15 to 31 the enclaves from the taihe pluton are more felsic sio 2 63 8 to 71 3 wt tio 2 0 3 to 0 6 wt cao 0 6 to 2 3 wt mg 8 to 22 but are still less evolved than their host sio 2 69 8 to 75 1 wt tio 2 0 2 to 0 6 wt cao 0 4 to 0 8 wt mg 3 to 12 in both cases the enclaves have very similar end t values baima end t 2 8 to 3 2 taihe end t 1 0 to 2 0 as their hosts baima end t 3 0 to 3 2 taihe end t 1 5 to 1 9 the major and trace element trends of the enclave host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal the enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma

origin of the nchanga copper cobalt deposits of the zambian copperbelt the zambian copperbelt forms the southeastern part of the 900 km long neoproterozoic lufilian arc and contains one of the world s largest accumulations of sediment hosted stratiform copper mineralization the nchanga deposit is one of the most significant ore systems in the zambian copperbelt and contains two major economic concentrations of copper and cobalt hosted within the lower roan group of the katangan supergroup a lower orebody copper only and upper orebody copper and cobalt occur towards the top of arkosic units and within the base of overlying shales the sulfide mineralogy includes pyrite bornite chalcopyrite and chalcocite although in the lower orebody sulfide phases are partially or completely replaced by malachite and copper oxides carrollite is the major cobalt bearing phase and is restricted to fault propagation fold zones within a feldspathic arenite hydrothermal alteration minerals include dolomite phlogophite sericite rutile quartz tourmaline and chlorite quartz veins from the mine sequence show halite saturated fluid inclusions ranging from 31 to 38 wt equivalent nacl with homogenisation temperatures thtot ranging between 140 and 180 c diagenetic pyrites in the lower orebody show distinct relatively low 34s ranging from 1 to 17 whereas arenite and shale hosted copper and cobalt sulfides reveal distinctly different 34s from 1 to 12 for the lower orebody and 5 to 18 for the upper orebody there is also a clear distinction between the 34s mean of 12 1 3 3 n 65 for the upper orebody compared with 5 2 3 6 n 23 for the lower orebody the 13c of dolomites from units above the upper orebody give 13c values of 1 4 to 2 5 consistent with marine carbon however dolomite from the shear zones and the alteration assemblages within the upper orebody show more negative 13c values 2 9 to 4 0 and 5 6 to 8 3 respectively similarly shear zone and upper orebody dolomites give a 18o of 11 7 to 16 9 compared to lower roan dolomites which show 18o of 22 4 to 23 0 two distinct structural regimes are recognized in the nchanga area a weakly deformed zone consisting of basement and overlying footwall siliciclastics and a moderate to tightly folded zone of meta sediments of the katangan succession the fold geometry of the lower roan package is controlled by internal thrust fault propagation folds which detach at the top of the lowermost arkose or within the base of the overlying stratigraphy and show vergence towards the ne faulting and folding are considered to be synchronous as folding predominantly occurred at the tips of propagating thrust faults with local thrust breakthrough the data from nchanga suggests a strong link between ore formation and the development of structures during basin inversion as part of the lufilian orogeny sulfides tend to be concentrated within arenites or coarser grained layers within shale units suggesting that host rock porosity and possibly permeability played a role in ore formation however sulfides are also commonly orientated along but not deformed by a tectonic fabric or hosted within small fractures that suggest a significant role for deformation in the development of the mineralization the ore mineralogy hydrothermal alteration and stable isotope data lend support to models consistent with the thermochemical reduction of a sulfate and metal enriched hydrothermal fluid at the site of mineralization there is no evidence at nchanga for a contribution of bacteriogenic sulfide produced during sedimentation or early diagenesis to the ores

variable regimes in magma systems documented in plagioclase zoning patterns el misti stratovolcano and andahua monogenetic cones abstract magmas may experience closed or open system evolution during their ascent and evolution in the upper crust chemically closed system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent on the other hand newly injected magmas can mix with the residing magma and hybridize plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km3 el misti stratovolcano 5822 m and a field of relatively small monogenetic andahua scoria cones both in southern peru in particular fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed vs open system processes large and correlated anorthite fe contrasts at resorption zones identify compositional mixing related to recharge subhorizontal an fe variations record thermal or water induced effects el misti volcano a large but still relatively youthful stratovolcano is dominated during most of its approx 120 ka history by closed system effects during its evolution recharge and mixing mostly occurred cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts a small volume erupted at el misti however is affected by strong open system behaviour as reflected in strongly correlated variations in an and fe contents of plagioclase compositions its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the andahua volcanic field for the el misti magma reservoir resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies many cycles of closed system evolution heat transfer and decompression therefore occurred without resulting in an eruption plagioclase zonations from lavas of the monogenetic andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption by contrast a large magma system that feeds el misti volcano can easily accommodate a new batch without causing eruption

geodynamic and climate controls in the formation of mio pliocene world class oxidized cobalt and manganese ores in the katanga province dr congo the katanga province democratic republic of congo hosts world class cobalt deposits accounting for 50 of the world reserves they originated from sediment hosted stratiform copper and cobalt sulfide deposits within neoproterozoic metasedimentary rocks heterogenite the main oxidized cobalt mineral is concentrated as cobalt caps along the top of silicified dolomite inselbergs the supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world class manganese cap deposits in western katanga i e the black earths that are exploited by both industrial and artisanal mining here we provide constraints on the genesis and the timing of these deposits ar ar analyses of oxidized mn ore and in situ u pb sims measurements of heterogenite yield mio pliocene ages the ar ar ages suggest a multi phase process starting in the late miocene 10 5 ma when the metal rich substratum was exposed to the action of meteoric fluids due to major regional uplift further oxidation took place in the pliocene 3 7 2 3 ma and formed most of the observed deposits under humid conditions co and mn caps on metal rich substrata and coeval fe laterites on barren areas these deposits formed prior to the regional shift toward more arid conditions in central africa arid conditions still prevailed during the quaternary and resulted in erosion and valley incision which dismantled the metal bearing caps and led to ore accumulation in valleys and along foot slopes

textural analysis of magmatic enclaves from the kameni islands santorini greece abstract the kameni islands have been the focus of historic volcanic activity on santorini greece the islands comprise a series of dacite lavas the erupted compositions of which have remained approximately constant for the last 2200 years the dacite lava flows of nea kameni contain a variety of mafic enclaves distinguished by differences in texture composition and mechanical behaviour the mafic enclaves represent chilled fragments of mafic magma that crystallised rapidly on contact with the cooler more silicic host magma the absence of chilled margins on the enclaves suggests that the replenishing magma initially formed a dense layer at the base of the chamber groundmass textures are interpreted as an indication of the degree of undercooling between the enclaves and the host groundmass textural information obtained from the enclaves has been used to determine the relative volumes of replenishing magmas injected into the host magma before eruption which indicates that the volume of erupted magma is directly proportional to the volume of replenishing magma emplaced in the chamber prior to eruption which thus may act as a potential eruption trigger the variety of replenishing magmas implies the existence of a complex conduit and chamber system beneath santorini volcano andesitic enclaves with linear crystal size distributions are thought to have originated as an aphyric andesitic melt expelled from a crystal mush by filter pressing disequilibrium phenocryst assemblages in the lavas and phenocryst bearing enclaves provide evidence for the entrainment and recycling of older phenocryst populations

late triassic intrusive complex in the jidong region jiamusi khanka block ne china geochemistry zircon u pb ages lu hf isotopes and implications for magma mingling and mixing abstract whole rock major and trace element geochemistry together with zircon u pb ages and lu hf isotope compositions are reported for a late triassic intrusive complex in the jidong region jiamusi khanka block ne china zircon u pb dating yields ages between 211 and 208 ma for enclaves of microgranular diorite and quartz diorite and between 211 and 209 ma for the host granitoids these ages correlate with a previously established intensive late triassic magmatic event along the eastern asian margin field observations together with petrographic features geochemistry and zircon hf isotope data preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex the syenogranite suite has high sio 2 75 5 76 3 wt and low mgo 0 15 0 19 wt and yields enriched lile and lree patterns most of the zircons in the syenogranites have two stage model ages of 766 and 1461 ma together with positive e hf t values of 0 6 to 9 1 these results indicate that the granitoid magmas were generated by partial melting of meso to neoproterozoic lower crust the gabbro suite has a restricted range of sio 2 46 1 51 9 wt together with high mg values 49 70 and high concentrations of ni co and cr zircons from two diorite samples have single stage hf model ages of 557 787 ma and e hf t values of 1 9 to 8 3 that are consistent with the coeval gabbros previously studied in the jidong region these features together with the observation that all the gabbros are enriched in lree and lile suggest that the mafic magmas were derived from melting of depleted neoproterozoic lithospheric mantle that had been metasomatized by slab derived fluids it is concluded that the dominant igneous suites within the late triassic intrusive complex formed by mingling mixing of felsic and mafic magmas the geochemical data combined with regional geological investigations indicate that the late triassic intrusive complex formed during lithospheric extension caused by slab break off of the yuejinshan oceanic plate that was subducting westward beneath the jiamusi khanka block

quantifying fault breccia geometry dent fault nw england a geometric classification of fault breccia borrowed from the cave collapse literature has been suggested as an alternative to available genetic classifications here image analysis is used to explore geometric discrimination between the visually assigned classes of crackle breccia mosaic breccia and chaotic breccia using samples from the well understood dent fault northwest england clast sphericity and surface roughness show some correlation with the breccia classes but particle size distributions and their fractal dimension show none a more useful parameter is the percentage of sample area occupied by clasts crackle breccia has 75 clasts mosaic breccia 60 75 clasts and chaotic breccia has 60 clasts the eye is also good at judging the tessellation goodness of fit of clasts and a semi quantitative approach to assessing this parameter is explored the average degree of rotational misfit of the clasts is strongly related to breccia class crackle breccia involves less than 10 average rotation mosaic breccia 10 20 and chaotic breccia more than 20 rotation comparison charts are provided for semi quantitative classification of fault breccias

a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part ii amphibole geobarometry and gravity modeling abstract the late mesozoic geology of the lower yangtze area is characterized by extensional sedimentary basins numerous granitic plutons and several world class ore deposits regionaly the coeval tectonic and geodynamic framework remains inadequately investigated in order to provide a more comprehensive understanding of the regional tectonic context we performed a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif it consists of the granodioritic qingyang and the monzogranitic jiuhua plutons u pb dated at 142 1 0 ma and 131 2 6 ma respectively biotite and amphibole cooling ages are 5 8 myr younger results deduced from field structural observation petrographic and magnetic fabrics paleomagnetism show that this massif was probably vertically emplaced by permissive intrusion coeval with weak regional extension however detailed information on the characteristics of this tectonic event and its emplacement depth was not documented therefore we carried out an investigation of amphibole geobarometry and gravity modeling in order to address the emplacement mode of the plutons within their regional tectonic framework amphibole data show that the qingyang jiuhua massif was emplaced in the upper crust at a depth between 5 and 12 km gravity modeling indicates that the massif is laccolithic it is characterized by several ne sw striking linear thickening zones that we interpret as the feeder roots of the massif these results led us to conclude that 1 the qingyang jiuhua massif was fed by vertical ne sw striking tension gashes 2 consequently the late early cretaceous tectonics in the study area was characterized by nw se extension

plagioclase zoning as an indicator of magma processes at bezymianny volcano kamchatka back scattered electron bse derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano kamchatka based on the 2000 2007 sequence of eruptive products the erupted magmas are two pyroxene andesites which last equilibrated at 915 c temperature 77 87 mpa pressure and a water content of 1 4 wt textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core to rim sequence of oscillatory zoning an50 60 truncated by a dissolution surface followed by an abrupt increase in an content up to an85 which then gradually decreases rimward this zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas the outermost 70 to 150 m wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in an and fe contents zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

age of the zambian copperbelt the sediment hosted stratiform cu co deposits and prospects of the central african copperbelt are characterized by two intimately associated mineralization styles disseminated sulfides and sulfide bearing quartz carbonate veins and veinlets it has been widely accepted that the disseminated mineralization was introduced during sediment diagenesis in a rift setting and possibly in multiple events spanning several hundred million years in contrast the veinlet hosted mineralization is commonly thought to have been derived either by remobilization of the disseminated sulfides during the lufilian collisional orogeny or introduced at broadly the same time s as the disseminated sulfides during diagenesis and subsequent orogeny the results of 15 re os molybdenite age determinations from cu co deposits and prospects across the zambian part of the central african copperbelt suggest that both the disseminated and veinlet mineralization styles were indeed generated together but in a 50 myr cambrian window 540 490 ma during the later stages of the lufilian collisional orogeny the molybdenite ages for the disseminated stratiform mineralization at two localities do not support the notion of syndiagenetic cu introduction the molybdenite ages also show that individual deposits formed over minimum time intervals of 10 24 myr

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

assimilation of sediments embedded in the oceanic arc crust myth or reality abstract arc magmas are commonly assumed to form by melting of sub arc mantle that has been variably enriched by a component from the subducted slab although most magmas that reach the surface are not primitive the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle this jeopardises the integrity of mass balance recycling calculations here we use sr and o isotope data in minerals from a suite of volcanic rocks from st lucia lesser antilles arc to show that assimilation of oceanic arc basement can be significant analysis of 87sr 86sr in single plagioclase phenocrysts from four soufriere volcanic complex svc st lucia hand samples with similar composition 87sr 86sr 0 7089 0 7091 reveals crystal isotopic heterogeneity among hand samples ranging from 0 7083 to 0 7094 with up to 0 0008 difference within a single hand sample o 18 measurements in the svc crystals show extreme variation beyond the mantle range with 7 5 to 11 1 for plagioclase n 19 10 6 to 11 8 for quartz n 10 9 4 to 9 8 for amphibole n 2 and 9 to 9 5 for pyroxene n 3 while older lavas pre soufriere volcanic complex with less radiogenic whole rock sr composition 87sr 86sr 0 7041 0 7062 display values closer to mantle range 6 4 to 7 9 for plagioclase n 4 and 6 to 6 8 for pyroxene n 5 we argue that the 87sr 86sr isotope disequilibrium and extreme o 18 values provide compelling evidence for assimilation of material located within the arc crust positive correlations between mineral o 18 and whole rock 87sr 86sr 143nd 144nd and 206 207 208pb 204pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in st lucia but also in the whole lesser antilles since st lucia encompasses almost the whole arc range of isotopic compositions this highlights the need for detailed mineral scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes

alteration and ore distribution in the proterozoic mines series tenke fungurume cu co district democratic republic of congo two sediment hosted stratiform cu co deposits in the tenke fungurume district of the central african copperbelt were examined to evaluate the alteration history of the ore hosting mines series and its implications for ore distribution and processing core logging and petrography focused on lithology and timing relationships outlined a complex alteration sequence whose earliest features include formation of anhydrite nodules and laths followed by precipitation of dolomite later alteration episodes include at least two silica introductions accompanied by or alternating with two dolomite introductions into the existing gangue assemblages one introduction of cu co sulfides accompanied the last episode of dolomite alteration overprinting an earlier generation of ore whose gangue association was unidentifiable sulfides and some carbonates were subsequently modified by supergene oxidation transport and reprecipitation to 100 200 m depth present day ore distribution resulted from these successive processes ore is concentrated in two shale dominated units on either side of a cavernous silicified dolomite which is interpreted as the main conduit for the mineralizing fluids sulfide ores precipitated at the redox or sulfidation contacts between this dolomite and the shales later supergene fluids dissolved and moved some of the metals redepositing them as oxides and carbonates solubility differences between cu and co in supergene conditions caused them to precipitate separately thus modern ore distribution at tenke fungurume results both from original hypogene lithology and contact related precipitation and from supergene oxidation transport and cu co decoupling the supergene fluid flow also redistributed gangue minerals such as dolomite which has an economically important influence on the processing costs of supergene ores

mafic felsic magma mixing limited by reactive processes a case study of biotite rich rinds on mafic enclaves article history mafic enclaves in felsic plutons are often used to argue that intermediate magmas are formed by mafic felsic magma mixing but the extent and nature of mixing remains unclear here we examine biotite rich rinds on mafic enclaves from the cretaceous bernasconi hills pluton in the peninsular ranges batholith of southern california to gain insight into magma mixing processes rinds differ from the enclave interior and the host monzogranite in being more fine grained and more mafic and potassic rinds are also 2 5 times more enriched in rare earth elements than the host monzogranite and up to 3 times more enriched than enclave interiors these observations indicate that the rinds were not generated by isochemical quenching binary mixing between enclave and host monzogranite or in situ magmatic differentiation instead rinds appear to have been formed by chemical reaction between the solidified enclave and a hydrous k rich residual melt or fluid formed after progressive crystallization and cooling of the host magma body transforming amphibole in the enclave into biotite rich rinds field observations show snapshots of biotite rich rinds being eroded away and new rinds simultaneously forming on freshly eroded surfaces of enclaves consistent with rinds being formed by chemical reaction instead of as quenching products deformation of enclaves is accommodated primarily by ductile attenuation of the thin rind while the enclave as a whole tends to rotate as a rigid body with minimal internal deformation other than localized brittle failure a comparison of the aspect ratios and cross sectional areas of mafic bodies in the pluton shows that those with high aspect ratios indicating greater accumulated strain are systematically more biotite rich and have smaller cross sectional areas than those with lower aspect ratios which are amphibole rich these relationships not only confirm that biotite rich lithologies are more deformable but also indicate that the high aspect ratio biotite rich bodies also known as schlieren derive from small parent bodies consistent with a derivation from eroding enclave rinds rather than from the enclave itself finally geochemical and thermodynamic modeling indicates that the biotite rich rinds formed when the host felsic magma had cooled to a low melt fraction state f 0 15 0 3 700 760 c suggesting that such reactions occur late in the lifespan of a magma body thus mafic felsic mixing may not be an efficient process for making intermediate magmas unless the magma body can reside at this low temperature range long enough to permit rind formation and subsequent deformation

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

mineralization controls and source of metals in the lufilian fold belt shaba zaire zambia and angola the pan african lufilian fold belt hosts extensive and rich polymetallic mineralization dominated by cu co and zn pb sulfides u oxides and noble metals stratiform vein and skarn deposits are present in the late proterozoic rocks of the katangan supergroup mineralization is regionally widespread the sedimentary basin evolution included three phases the first two characterized by rift fault controlled rapid subsidence followed by slower thermal subsidence were associated with bimodal magmatism and probably related to two thermal events the third phase reflects the onset of tectonic deformation and uplift in the southern part of the basin the stratiform mineralization emplaced before the folding of the basin is concentrated in lithologically varied permeable rocks of the roan group that underlie the impermeable rocks of the lower kundelungu group the stratiform mineralization was emplaced by convective circulation of basinal brines forced by a thermal gradient created by a wedge shaped lithosome of lower kundelungu pelitic rocks with low thermal conductivity hypabyssal igneous intrusions and volcanic and volcaniclastic rocks were the source of metals the metals were introduced to the aquifer horizons saturated with basinal brines by hydrothermal circulation associated with the hypabyssal intrusions and by compactional dewatering of a 5 000 m thick body of pelitic rocks rich in volcaniclastic material vein mineralization located on major late faults resulted from regional metamorphism driven dewatering of the folded basin the skarn metasomatic mineralization is associated with small post katangan acid intrusions and blocks of thermally rejuvenated basement details of the distribution of mineralization are stratigraphically and structurally controlled

neoproterozoic allochthonous salt tectonics during the lufilian orogeny in the katangan copperbelt central africa the neoproterozoic katangan rocks in the copperbelt of shaba democratic republic of the congo and zambia contain the world s largest concentration of stratiform copper cobalt ores we propose that during the lufilian orogeny the katangan basin was radically transformed first by extrusion of allochthonous evaporites then by orogenic shortening salt tectonics plausibly explains spectacular breccias underlying 25 000 km2 and containing gigaclasts up to 10 km long the brecciated shaban roan supergroup the lowermost katangan unit forms regional detachments and diapirs the former existence of roan evaporites is indicated by sabkha facies crystals and pseudomorphs of gypsum and anhydrite stratigraphic gaps underlain by collapse breccias chloride inclusions in ores hydrothermally propylitized ore hosts and saline springs salt tectonics began during deposition of the roan supergroup between 1050 and 950 ma in mid roan time small walls and extrusions of evaporite gigabreccia began to be emplaced in early kundelungu time 940 850 ma evaporitic diapirs enlarged lufilian deformation began between 850 and 650 ma by laterally squeezing diapirs to form salt welds then a large sheet of commingled roan evaporite and carbonate dominated sediments and ores was extruded northward by 65 km extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits continued shortening then emplaced large thrust sheets lubricated by the preexisting salt sediment extrusive sheet restoration suggests that the lufilian foreland in this area shortened from 193 km to 85 km the depositional northern edge of the evaporite basin controlled the shape of the outer lufilian arc we infer three original salt provinces the large scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other precambrian basins

tilt and northward offset of cordilleran batholiths resolved using igneous barometry considerable controversy surrounds the suggestion based on palaeomagnetic evidence that large segments of the north american cordillera travelled long distances parallel to the coast during the latest cretaceous and early tertiary periods well after the amalgamation of exotic terranes discordant palaeomagnetic data from mid cretaceous plutonic rocks of the peninsular ranges batholith of coastal southern california and baja and the mount stuart batholith of the cascade range in washington play a pivotal role in the controversy the discordant data were originally interpreted to reflect northward transport of 1 000 km relative to cratonal north america after the batholiths cooled through their magnetic blocking temperatures1 5 more recently it has been argued that the discordances arise from local tilting of batholiths rather than northward offset6 7 here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting our results indicate that the peninsular ranges and mount stuart batholiths have undergone northward offsets of 1 000 450 and 2 900 700 km respectively and also significant tilting

magma recharge and reactive bulk assimilation in enclave bearing granitoids tonglu south china national nature science foundation of china 41530211 mlr special funds for public welfare projects 201211013 2 state administration of foreign expert affairs of china b07039 most special funds of the state key laboratory of geological processes and mineral resources msfgpmr01

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

oscillatory sr isotopic signature in plagioclase megacrysts from the damiao anorthosite complex north china implication for petrogenesis of massif type anorthosite abstract formation of proterozoic massif type anorthosites is known to be related to polybaric process involving early high pressure crystallization of plagioclase and high al orthopyroxene megacrysts at mantle crust boundary followed by emplacement of plagioclase dominated mushes to shallow level crust therefore the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites the 1 74 ga damiao complex north china comprises 90 vol anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high al orthopyroxene megacrysts haoms the plagioclase megacrysts are generally euhedral to subhedral mostly 1 to 30 cm in diameter and some of them contain very fine lamellae of orthoclase and undefined fe ti rich minerals 1 5 m the haoms occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts they contain abundant thin regular lamellae of exsolved plagioclase 15 20 vol indicative of originally high al features originally 6 0 7 3 wt al2o3 based on the al in orthopyroxene geobarometry the haoms and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9 4 11 2 kbar 33 36 km indicative of their crystallization at lower crust depths or crust mantle boundary in contrast orthopyroxene grains from late differentiation phases such as oxide apatite gabbronorites do not contain plagioclase lamellae and have much lower al2o3 contents 1 5 to 1 7 wt indicating final crystallization at

anhydrite pseudomorphs and the origin of stratiform cu co ores in the katangan copperbelt democratic republic of congo the stratiform cu co ore mineralisation in the katangan copperbelt consists of dispersed sulphides and sulphides in nodules and lenses which are often pseudomorphs after evaporites two types of pseudomorphs can be distinguished in the nodules and lenses in type 1 examples dolomite precipitated first and was subsequently replaced by cu co sulphides and authigenic quartz whereas in type 2 examples authigenic quartz and cu co sulphides precipitated prior to dolomite and are coarse grained the sulphur isotopic composition of the copper cobalt sulphides in the type 1 pseudomorphs is between 10 3 and 3 1 relative to the vienna canyon diablo troilite indicating that the sulphide component was derived from bacterial sulphate reduction bsr the generation of text hco 3 during this process caused the precipitation and replacement of anhydrite by dolomite a second product of bsr is the generation of h2s resulting in the precipitation of cu co sulphides from the mineralising fluids initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core precipitation of authigenic quartz was most likely induced by a ph decrease during sulphide precipitation fluid inclusion data from quartz indicate the presence of a high salinity 8 18 eq wt nacl fluid possibly derived from evaporated seawater which migrated through the deep subsurface 87sr 86sr ratios of dolomite in type 1 nodules range between 0 71012 and 0 73576 significantly more radiogenic than the strontium isotopic composition of neoproterozoic marine carbonates 87sr 86sr 0 7056 0 7087 this suggests intense interaction with siliciclastic sedimentary rocks and or the granitic basement the low carbon isotopic composition of the dolomite in the pseudomorphs 7 02 and 9 93 relative to the vienna pee dee belemnite v pdb compared to the host rock dolomite 4 90 and 1 31 v pdb resulted from the oxidation of organic matter during bsr

mineral textures and zoning as evidence for open system processes investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades e g milch 1905 kuno 1936 eichelberger 1975 sato 1975 anderson 1976 in fact evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by bunsen 1851 in recent years mineral studies are invigorated by the development of high precision high resolution analytical instruments and techniques through which textural information and compositional data are combined e g jerram and davidson 2007 as minerals respond texturally and compositionally to changing magmatic environments they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions cf ginibre et al 2007 on the other hand magmatic liquids melts are snapshots of current magmatic states and provide less direct evidence of the processes responsible for their evolution in addition liquids may crystallize thereby destroying the direct evidence they provide with an appropriate set of observations and measurements we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone this contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks many of which apply equally to plutonic rocks and the interpretations ascribed to these features in terms of open system magmatic processes first i will address the analytical tools that are used to acquire textural and compositional data followed by a brief review of open system processes and a summary of the relevant textures from that point on i will address some mineral specific aspects followed by a discussion

tectonic evolution of the lufilian arc central africa copper belt during neoproterozoic pan african orogenesis abstract the lufilian arc of central africa also called katangan belt or copperbelt is a zone of low to highgrade metasedimentary and subsidiary igneous rocks of neoproterozoic age hosting highgrade cucou and pbzn mineralizations the lufilian arc is located between the congo and kalahari cratons and defines a structure which is convex to the north three major phases of deformation characterize the construction of the lufilian arc the first phase d1 called the kolwezian phase developed folds and thrust sheets with a northward transport direction d1 deformation occurred in the lufilian arc between ca 800 and 710 ma with a peak in the range 790 750 ma it is here correlated with the main deformation in the zambezi belt southward verging folds with the same trends as the d 1 structures were previously linked to a second tectonic event named kundelunguian phase of the lufilian orogeny we show in this paper that they are backfolds developed during d1 along katangan ramps and especially along the kibaran foreland the second phase d2 of the lufilian orogeny is the monwezi phase including several large leftlateral strikeslip faults which have been activated successively during this deformation phase the eastern block of the belt rotated clockwise giving the present day nwse trend of d1 structures in this part of the lufilian arc and generating its convex geometry the mwembeshi dislocation the major transcurrent shear zone separating the zambezi and lufilian arc was mostly active during the d2 deformation phase d2 deformation occurred between ca 690 and 540 ma such a long time interval is attributed to the migration of strikeslip faults developed sequentially from south to north and probably to a slow convergence velocity during the collision between the congo and kalahari cratons the third phase d3 of the lufilian orogeny is a late event called the chilatembo phase marked by structures transverse to the trends of the lufilian arc this deformation and the post d 2 uppermost kundelungu sequence ks3 plateaux group are younger than 540 ma and probably early paleozoic

shocked minerals at the k t boundary explosive volcanism as a source abstract azimuthal symmetry of p waves first motion up and diminished s wave amplitude indicates an explosion to be the source of the seismicity of the 18 may 1980 eruption at mt st helens explosion coupling efficiencies and scaling from underground mining excavation and nuclear explosions imply that peak pressures could have been as high as 1000 kbar which is more than sufficient for the formation of shocked minerals in addition high load rates as well as contamination may lower phase transition pressures a region of formation of shocked minerals surrounding the magma chamber is indicated to have a thickness on the order of 100 m the outer portions of this region should be sufficiently cool so that shocked features are not annealed hence the source of shocked minerals should be the country rock surrounding the magma chamber it is unlikely any of the magmatic material would be able to retain shocked features due its high temperature the explosive mechanism may be quench supersaturation which theoretically can yield shock producing pressures but chemical processes should not be dismissed as an explosive source tektite distributions show no iridium or mass extinction association industrial and present volcanic atmospheric discharge are known to have worldwide distribution the clays present at the k t boundary may have had a volcanic origin and it is now known that the deccan traps which have been suggested by others as a source of the iridium lie athwart the k t boundary as there is common intimacy between silicic and mafic volcanism both should be anticipated at the k t boundary and their global appearance there may be a manifestation of a surge in whole mantle convection in short there is a definite possibility shocked minerals at the k t boundary have a volcanic source

geochemical and zircon u pb and hf isotopic study of the baijuhuajian metaluminous a type granite extension at 125 100 ma and its tectonic significance for south china abstract the early cretaceous baijuhuajian pluton is an a type granitic intrusion emplaced along the jiangshan shaoxing jssx fault zone in western zhejiang province se south china it intruded into a late jurassic volcanic basin bounded by proterozoic sandstone and siltstone the granite has a coarse grained porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of k feldspar quartz plagioclase and biotite the pluton has a clear a type geochemical signature e g it is metaluminous a cnk 1 0 and has high sio2 75 to 78 wt total alkalis na2o k2o 4 8 to 8 5 wt rare earth elements total ree 228 to 438 ppm hfse and fe feot feot mgo 0 87 to 0 97 on the other hand it is characterized by low cao 0 51 to 1 01 wt tio2 0 06 to 0 13 wt p2o5 below detection and sr 7 0 to 9 5 ppm the hypersolvus texture and fine grained groundmass indicate emplacement at a shallow depth shrimp u pb zircon dating yielded a weighted mean 206pb 238u age of 126 3 ma and zircons from this pluton give ehf t values between 0 52 and 4 24 suggesting the importance of juvenile material in the magma source most of the plutonic rocks in southeast china are calc alkaline subduction related intrusions of jurassic and cretaceous age previous studies suggested that the sparse a type granites in south china were emplaced in two episodes 190 155 ma in the inland area and 100 ma along the coastal area in se south china our study of the baijuhuajian granite together with the coeval suzhou a type granite 123 ma in jiangsu province defines an important extensional event between 125 and 100 ma considering the available data for the mesozoic igneous rocks in south china we suggest that subduction of the pacific plate in the mesozoic produced voluminous calc alkaline granitoids and volcanic rocks with a younging trend from west to east the a type granites were generated during periods of local extension at 190 155 ma and 125 100 ma perhaps corresponding to localized rollback of the pacific plate

li be b concentrations and δ 7 li values in plagioclase phenocrysts of dacites from nea kameni santorini greece li be b and 7li sims analyses of plagioclase phenocrysts from the 1040 1941 niki dacite lava nea kameni santorini greece exhibit varied processes from their anorthite contents alone the crystals may be segregated into four main types type n shows the normal decline in an during crystallisation an62 40 type o has only oscillatory zoning accompanied by resorption surfaces an58 39 type c is complex with high an cores subtype c1 an64 58 subtype c2 an88 73 and normal rims an55 42 type a plagioclase with high an content an92 82 is found within mafic enclaves on the basis of their li concentrations type o crystals may be subdivided into subtype o1 with flat li concentration profiles and subtype o2 with decreasing li concentration from core to rim the concentrations of be and b of all four types show a negative correlation with anorthite content an but li concentration profiles differ amongst the different plagioclase types types n and o1 and the cores of type c are equilibrated in li concentration types o2 and a and the mantles of type c display an initial enrichment in li probably from volatile influx into the melt consistent with the propensity towards equilibrium with the melt these crystals display dramatic rim ward declines in li concentration all analysed plagioclase crystals except for the xenocrystic type a have nearly the same li be and b concentrations at their rims these coincide with the composition of plagioclase microlites in the groundmass thereby affording estimates of plagioclase melt partitioning for the light elements k li 0 19 0 28 k be 0 24 0 38 and k b 0 007 0 009 7li profiles in type o2 and type a phenocrysts manifest an unmistakable inverse relation to li concentration with variations of up to 39 revealing preferential kinetic diffusion this may have been driven by li loss from the melt most likely through degassing during decompression perhaps in the course of magma ascent to subsequent eruption considering the rapid diffusion of li in plagioclase in situ phenocryst analyses may yield useful information about processes leading up to or even causing eruptions

cobalt enrichment in a paleo karstic bauxite deposit at yunfeng guizhou province sw china abstract cobalt can be enriched in a variety of different geologic settings while it is mainly produced as a by product of sediment hosted stratiform cu deposits and magmatic ni cu deposits associated with mafic and ultramafic rocks in this study we report elevated co concentrations in a paleo karstic bauxite deposit at yunfeng guizhou province whole rock chemical analyses indicate that co is concentrated in carbonaceous ferruginous clay layers 167 962 ppm average 383 ppm n 5 and siderite layers 47 439 ppm average 213 ppm n 7 within the bauxite deposit the positive correlation between co and s together with petrographic and epma analyses suggests that co occurs primarily as co rich bearing sulfides sulfoarsenides rather than cobaltiferous oxides hydroxides as in most lateritic ni co deposits many of the co concentrations measured in this study are higher than the minimum grade 300 ppm or cut off grade 200 ppm for economic exploitation of co from sulfides or arsenides basing on these results we put forward the scientific significance of co enrichment in paleo karstic environments and the potential importance of co resources as by products of aluminum production in bauxite deposits

weathering of cobalt arsenides natural assemblages and calculated stability relations among secondary ca mg co arsenates and carbonates the supergene alteration of cobalt arsenides produces various characteristic mineral assemblages including erythrite pharmacolite and other ca mg co bearing arsenates which upon precipitation remove co and as from natural waters their paragenetic relationships and stability conditions have not been investigated in detail we present a detailed study on these assemblages their successions and coexisting fluid compositions from the mining area of wittichen southwest germany where primary skutterudite and safflorite in granite hosted barite calcite veins are undergoing oxidation water analyses from the old mines a semi quantitative stability diagram and quantitative reaction path modeling are used to constrain their conditions of formation cobalt and arsenate bearing solutions invariably precipitate erythrite first and hence buffer co concentrations to very low values both during skutterudite or safflorite dissolution and erythrite precipitation the fluid s co as ratio decreases rapidly therefore spherocobaltite co carbonate is unstable in the presence of arsenate ions under most conditions the formation of various mineral assemblages precipitating after or simultaneously with erythrite strongly depends on ca 2 and mg 2 activities and ph small changes in one of these parameters lead to different mineral assemblages these small changes are partly governed by fluid host rock or fluid vein mineral reactions and partly by the precipitation of the secondary arsenate minerals themselves this complex interdependence produces the rich variety of mineral assemblages observed which effectively serves as a very sensitive monitor of fluid compositions furthermore the assemblages themselves are able to buffer the ca mg co as concentrations in the fluid to some extent and effectively immobilize both as and co in close proximity to the ore deposit

petrogenesis of late triassic granitoids and their enclaves with implications for post collisional lithospheric thinning of the liaodong peninsula north china craton abstract major and trace element sr nd and hf isotope zircon u pb and hf isotope data are reported for mafic microgranular enclaves and host granitoids from the xiuyan pluton in the liaodong peninsula of the eastern north china craton in order to investigate their sources petrogenesis and tectonic implications the zircon u pb age of the enclaves 211 1 ma is identical to that of the host granite 210 1 ma establishing that the mafic and felsic magmas were coeval the enclaves have high mgo concentrations at low silica contents and positive ehf t values they are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements which can be explained as melts derived from a depleted asthenospheric mantle with some crustal contamination the host granitoids have high sio2 contents variable initial 87sr 86sr ratios and highly negative end t and ehf t values suggesting they were mainly derived from an ancient crustal source heated by the input of asthenospheric mantle derived magmas field observations geochronology geochemistry sr and nd isotopic and zircon hf isotopic compositions point to a complex petrogenesis where asthenospheric mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between enclaves and host rocks identification of an asthenospheric mantle component in the xiuyan granitoids suggests asthenospheric mantle upwelling and decompressional melting of the mantle indicating that post collisional lithospheric thinning in the liaodong peninsula was initiated in the late triassic

petrogenesis and metallogenesis of the xinjie layered mafic ultramafic intrusion china modeling of recharge assimilation and fractional crystallization abstract the xinjie layered mafic ultramafic intrusion in the central emeishan large igneous province elip sw china hosts fe ti v oxide ore in the upper part and ni cu platinum group element pge sulfide deposits in the lower part in this study we use published sr nd isotopic data to simulate and evaluate the energy constrained recharge assimilation and fractional crystallization ec rafc model with a view to track the petrogenesis and mineralization in contrast to the energy constrained assimilation fractional crystallization ec afc model the ec rafc modeling shows that the xinjie intrusion may represent a shallow crustal magma chamber system where it experienced the rafc processes in the early stage of the magmatic process tm 1460 c a pulse of magma was injected into an actively evolving magma chamber minor melting ma 0 257 and assimilation of the wallrock underlying emeishan basalts occurred when the temperature of magma tm 1245 c decreased close to the equilibration temperature teq 1165 c the mass temperature plots indicate that fractional crystallization was significantly affected by the recharge and assimilation processes whereas the contaminated magma recharge favored the early crystallization of fe ti oxides as well as the formation of the pge bearing immiscible melt in the lower part of the intrusion in contrast the formation of the fe ti oxides ores in the upper part occurred probably through fractional crystallization and or immiscibility of the evolved fe ti rich magma resulting in the paragenesis of fe ti v oxides and ni cu pge sulfides in the xinjie layered intrusion

magmatic process recorded in plagioclase at the baogutu reduced porphyry cu deposit western junggar nw china abstract despite the fact that porphyry cu deposits contain large amounts of cu in one or more small stocks few studies have discussed evidence for significant magma recharge in porphyry cu deposits a systematic elemental and sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the baogutu reduced porphyry copper deposit western junggar nw china large compositional changes in an 12 24 mol are observed along with strong positive correlations between an and feo significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in an and feo and complex sr isotope variations three types of crystals with different core to rim sr isotope variations are recognized type i crystals have core to rim increases in 87sr 86sr i that could be explained by diffusion for example sr isotope variations recorded in bck2 1 2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100 500 years with the proximate value of 300 years type ii crystals with different variation trends in 87sr 86sr i on opposite sides of the core could be produced either by multi stage crystallization or by analytical uncertainty whereas type iii crystals with complex core to rim variations in 87sr 86sr i may record repeated magma recharge events all these results suggest repeated recharging of the magma chamber by hotter more mafic and less radiogenic sr isotope melts therefore more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber which ultimately results in cu mineralization at the baogutu reduced porphyry copper deposit

magma recharge contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks we illustrate a rapid and effective method for determining variations in 87sr 86sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer the speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized or enabling reconnaissance analysis of rocks to test for mineral scale isotopic heterogeneity our results from magmatic crystals in lavas from el chichon volcano confirm trends previously obtained by the more time consuming microdrilling method for the first time with confidence the laser data allow us to associate abrupt changes in 87sr 86sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal because the isotopic variations within crystals would be expected to equilibrate diffusively with time the width over which abrupt changes in 87sr 86sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures

origin of microgranular enclaves in granitoids equivocal sr nd evidence from hercynian rocks in the massif central france we examined the rb sr and sm nd systematics of three different high level to subvolcanic granitoid plutons and their microgranular enclaves the host granitoids are adamellites and grano diorites while the enclaves are mostly microtonalites with igneous textures the enclaves are rounded in shape and vary from 1 m across contacts with the enclosing granitoids are sharp and mostly nonlobate the microtonalitic enclaves are similar in mineralogy to their hosts initial 87sr 86sr are all 0 7070 and initial end is 3 0 to 6 6 for both the enclaves and their hosts this precludes a major input of an unmodified mantle derived component at the time of magma genesis nd isotopic contrasts between enclaves and hosts are 1 e nd unit at most these data indicate broadly cogenetic origins for enclave host systems though some enclaves have slightly less crustal signatures than the enclosing granitoids there are several possible interpretations 1 there may have been nearly complete isotopic homogenization between enclaves and host magmas derived from different starting parents 2 the enclaves and granitoids could all have purely crustal origins enclaves might represent magmas derived from slightly less evolved protoliths or parental magma or they could be concentrations of early formed crystals with interstitial melt 3 basaltic magma from the depleted mantle could have thoroughly mixed with a large volume of crustal melt in the lower crust 4 there could have been a significant input of melt derived from an enriched mantle source several of these mechanisms may have played roles in the genesis of the granitoids and their enclaves nd isotopes do not always allow unequivocal recognition of mantle components in granitoid magmatism we argue that enclaves of this type are probably not products of major mixing processes relevant to granitoid protoliths and large scale crustal evolution they commonly result from second order mingling of broadly cogenetic magma fractions that may have a variety of different origins

origin of middle triassic high k calc alkaline granitoids and their potassic microgranular enclaves from the western kunlun orogen northwest china a record of the closure of paleo tethys abstract the western kunlun orogen the northwest tibet plateau occupies a key tectonic position at the junction between the pan asia and the tethyan tectonic domains it is related to subduction of proto and paleo tethys and terrane collision from early paleozoic to early mesozoic this paper presents the first detailed shrimp zircon u pb chronology mineral chemistry major and trace element and sr nd hf isotope geochemistry of three middle triassic plutons yuqikapa muztaga and taer and their microgranular enclaves in the western kunlun orogen shrimp zircon u pb dating shows that the yuqikapa pluton was emplaced in the anisian 243 ma and the muztaga and taer plutons that contain microgranular enclaves were emplaced in the ladinian 234 227 ma the yuqikapa pluton is composed of peraluminous high k calc alkaline two mica granites monzogranite syenogranite alkali feldspar granite with initial 87sr 86sr ratios of 0 7071 0 7077 end t of 4 4 to 6 3 and ehf t in situ zircon of 3 3 elemental and isotopic data suggest that they were formed by wet anatexis of the precambrian metasedimentary and metaigneous rocks in the continental collision overthrust zone the muztaga and taer plutons consist of metaluminous to weakly peraluminous high k calc alkaline granitoids granodiorite monzogranite with initial 87sr 86sr ratios of 0 7052 0 7104 end t of 3 6 to 7 3 and ehf t in situ zircon of 3 1 to 1 0 elemental and isotopic data suggest that they were formed by partial melting of the precambrian metasedimentary and metaigneous rocks in the normal lower crust

experimental simulations of anatexis and assimilation involving metapelite and granitic melt abstract assimilation of foreign material into a granitic magma body entails mixing between two end member components the initially solid assimilant and the original magma whose composition is mostly represented by a granitic liquid in this study we assess the interactions between a haplogranite liquid made to the composition of the minimum at 200 mpa h 2 o ab 38 23 or 28 72 qtz 33 04 crn 00 01 with a schist composed of muscovite and quartz with minor biotite and garnet and traces of ilmenite plagioclase apatite and graphite from 700 to 800 c at 200 mpa aqueous carbonic fluid the primary focus of this work is to assess the changes in compositions of granitic liquids that form in each reservoir as a result of the assimilation of schist by granitic magma before doing so we evaluate the experimental database for the compositions of granitic liquids in equilibrium with peraluminous mineral assemblages most experiments that entail partial melting of peraluminous mineral assemblages begin with finely milled powders this method yields melt pools glass too small for accurate analysis by electron beam methods as a result most published analyses of glasses from such experiments are more aluminous and less sodic than their actual compositions the analytical errors are large up to 50 relative to the mean value for some elements in addition the experimental melts are commonly more hydrous than reported and vapor saturated even in experiments that are cited as vapor or fluid absent experiments that combine sand sized mineral grains and powdered granitic glass yield the best approach to chemical equilibrium and facilitate accurate analysis of the glass glasses from such experiments define the compositions of liquids that may be derived either from the anatexis of aluminous metasediments or by the assimilation of aluminous rocks into a granitic magma whose composition is approximately represented by the minimum composition in the haplogranite system ab or qtz h 2 o new experiments designed to simulate the assimilation of metapelites by granitic melt juxtaposed a solid core of anhydrous granitic glass with a solid core of mica schist both as described above at comparable conditions melting within the mica schist alone is far less extensive than when coupled with the granitic liquid this is because field diffusion the long range and highly organized migration of h na and k throughout the entire volume of interconnected melt quickly brings the composition of silicate liquid to chemical equilibrium across the metapelite granite interface with respect to these components and in regard to the asi aluminum saturation index and the h 2 o content of the melt the most significant change entails the diffusion of na from granitic liquid into the metapelite which shifts the bulk composition of the metapelite toward the minimum in the granite system and in turn promotes a greater proportion of melting within the metapelite a weaker counter flow of k back into the haplogranite raises its k molar k na k and lessens the increase in asi molar al na k due to na loss the solubility of h 2 o in granitic liquid is positively correlated with the asi value hence changes in the asi of melt that follow from field diffusion of h na and k ions also strongly affect the a h 2 o within the coupled metapelite granite system