isotopic and fluid inclusion constraints on the formation of polymetallic vein deposits in the central argentinian patagonia the lead isotope compositions of galena and the fluid inclusion systematics of nine barite bearing polymetallic au ag pb zn deposits of the central argentinian patagonia chubut and rio negro provinces have been investigated to constrain the compositions and sources of the mineralizing fluids most of the deposits occur as veins with less common wall rock disseminations and or stockworks and are low sulfidation epithermal deposits hosted in jurassic volcanic rocks fluid inclusion homogenization temperatures th from quartz and sphalerite from the deposits fall within the range of 100 300 c with the highest measured average temperatures for the most eastern deposits mina angela 298 c canadon bagual 343 c the salinities of the hydrothermal fluids at all deposits were low to moderate 10 4 equiv wt nacl three groups of ore deposits can be defined on the basis of 206pb 204pb ratios for galena and these show a general decrease from west to east from 18 506 to 18 000 the central argentinian patagonia deposits have distinctly less radiogenic lead isotope compositions than similar deposits from peru and chile except for the porphyry copper deposits of central and southern peru galena from the mina angela deposit is characterized by very low radiogenic lead isotope compositions 18 000 206pb 204pb 18 037 and 38 03 208pb 204pb 38 09 and reflects interaction with precambrian basement the geographic trend in lead isotope compositions of both galena and whole rocks indicates a crustal contribution which increases eastwards also reflected in the strontium neodymium isotope systematics of the host lavas finally due to the lack of precise age determinations for the central patagonian polymetallic deposits a potential link with andean porphyry copper systems remains an open question

microgranular enclaves in island arc andesites a possible link between known epithermal au and potential porphyry cu au deposits in the tulasu ore cluster western tianshan xinjiang china abstract the successful exploration for porphyry copper deposit in western tianshan xinjiang faces great challenge tulasu basin is an important epithermal gold ore cluster in western tianshan which was formed in a southwest pacific type island arc setting during the late paleozoic by the southward subduction of the north tianshan ocean beneath the yili plate porphyry cu au deposits are possibly to be found at depth or adjacent to these epithermal gold deposits some sulfide mineralized microgranular enclaves of monzonite porphyry and microdiorite were found in andesites of the tawuerbieke gold district tulasu basin the enclaves are randomly distributed with generally round or subangular shape and commonly clearly defined within their host andesite and have a chilled surrounding margin of andesite the monzonite porphyry enclaves mpe exhibit porphyritic texture with the phenocrysts of plagioclase and k feldspar the microdiorite enclaves mde are mainly composed of plagioclase and hornblende with an aplitic texture and massive structure the host andesites show porphyritic texture with the phenocrysts major of plagioclase minor of hornblende and clinopyroxene the groundmass consists of short column plagioclase and minor clinopyroxene with a hyalopilitic texture zircon grains from a mpe sample yield a weighted 206pb 238u age of 356 2 4 3 ma n 13 mswd 1 11 which is effectively coincident with the 360 5 3 4 ma n 20 mswd 0 61 of an andesite sample within analytical error indicating that they were coeval in addition the mpe mde and the andesite samples share similar normalized incompatible element and rare earth element patterns that are characterized by a pronounced enrichment of large ion lithophile elements and a deficit of high field strength elements moreover the samples show similar nd isotope compositions to the contemporary andesites and basaltic andesites detailed petrology geochronology and geochemistry studies suggest that these enclaves were captured from an underlying body during the eruption of island arc magma thus unmapped cognate porphyry intrusions associated with cu au mineralization may exist under the andesite strata this evidence in combination with the low and high sulfidation epithermal gold deposits acidic hydrothermal alterations and copper mineralization evidence suggests that a porphyry epithermal cu au metallogenic system might occur in the tulasu basin of western tianshan and that the tawuerbieke district should be an important target for porphyry cu au exploration

fractal analysis of mingled mixed magmas an example from the upper pollara eruption salina island southern tyrrhenian sea italy abstract upper pollara eruption products 13 ka salina island italy include both homogeneous and heterogeneous pumices resulting from mixing mingling processes between an hk andesite and a high sio2 rhyolite representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling mixing structures image analysis techniques are applied and eight grey color ranges classes are extracted from high resolution scans of pumice class 1 lighter colors and class 8 darker colors show end member glass compositions i e hk andesite and high sio2 rhyolite respectively these two classes show spot to cluster like morphological structures intermediate classes show an hk dacitic to rhyolitic composition and a banding to fold like morphology fractal analysis by box counting of the boundary pattern of eight grey classified images is performed over a length scale of 0 028 1 8 cm fractal dimension d is between 1 01 and 1 84 coupled fractal analysis and geochemical data reveal that d increases as the degree of magma interaction homogenization increases this feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection we conclude that the increase of d observed in the upper pollara samples reflects the transition from fractal mixing to homogenization end member magmas hk andesite and high sio2 rhyolite represent isolated mixing regions while homogenized magmas represent active mixing regions in the analyzed pumices isolated and active mixing regions coexist at scales between 10 4 and 10 2 m morphological and compositional features of the upper pollara pumices result from turbulence

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

abrupt change in magma liquidus temperature because of volatile loss or magma mixing effects on nucleation crystal growth and thermal history of the magma the liquidus temperature of magma that reaches shallow levels introduction during its ascent may change abruptly as a result of the release of on its way from the source region into the earth s volatiles or through magma mixing or a combination of both crust or even onto the earth s surface magma travelling immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves and one of the occurs a disequilibrium is observed in the melt and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re equilibrates to its thermodynamic equilibrium that occur during this passage of special interest in the a quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes such two component model melt is used to investigate such re equilibration as magma crust interaction magma mixing deprocesses the relaxation time defined as the time required for the volatilization and depressurization that change the geosystem to re equilibrate after varying degrees of liquidus temperature chemical signature of the melt its crystal content its perturbations is found to be 3 or significantly less than the liquidus temperature its flow behaviour and its erupttime it takes to crystallize 40 of the melt regardless of the ability whereas crustal contamination of magma can be amplitude of the perturbation associated with the re equilibration traced through for example trace element and isotope process is an increase in crystal fraction that can easily reach studies e g wilson 1989 and is discussed extensively 10 depending on the amplitude of the perturbation during the in the other papers of this special volume cooling compensation of the liquidus temperature perturbation the temdepressurization and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly depending on the latent heat budget possibly heat of mixing affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution which suggests and crust for example any pressure decrease in volatilecrystallization of the melt without cooling saturated systems not uncommon in natural systems especially for more evolved compositions will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range one of the major consequences of such behaviour is isothermal crystallization which was recognized almost 40 years ago by tuttle bowen 1958 pp 67 and 68 and the

early cretaceous subvolcanic calc alkaline granitoid magmatism in the nubra shyok valley of the shyok suture zone ladakh himalaya india evidence from geochemistry and u pb shrimp zircon geochronology abstract the lithounits constituting the ladakh himalaya are exposed along the indus and shyok sutures zones of northwest himalaya the shyok suture zone ssz in northern ladakh represents a highly tectonized zone of a back arc basin which is mainly composed of volcano sedimentary formations shyok and khardung formations intimately associated with intrusive granitoids in the nubra shyok valley of the ssz calc alkaline granitoids of batholithic dimension are exposed in the tirit region referred herein as the tirit granitoids which are intrusive evidently into the shyok volcanic rhyolite rocks belonging to the shyok formation in this valley the northern margins of granitoids of the ladakh batholith can also be found intrusive into the metasediments shale slate and metavolcanics of the shyok formation the compositions and crystallization pressures 66 to 91 mpa of amphiboles in the intrusive tirit granitoid corroborate a calc alkaline nature and solidification of tirit granitoid melt at subvolcanic level equivalent to a minimum of 2 5 km to a maximum of 3 5 km thick overburden of shyok volcanics u pb shrimp zircons from the tirit granitoids have yielded mean crystallization ages of 109 4 1 1 ma and 105 30 0 80 ma which strengthen the idea of early cretaceous subduction beneath the karakoram terrain inherited older zircon cores 278 393 ma 476 519 713 952 ma and 1933 ma suggest a contribution from heterogeneous palaeozoic and proterozoic sources in the generation of the tirit granitoids similar to those observed elsewhere in the karakoram kohistan region a mean crystallization age 105 30 0 80 of zircons in the tirit granitoid hosting xenoliths of porphyritic volcanics places a minimum eruption age of ca 105 ma for the shyok volcanics the ladakh granitoid tirit granitoids and porphyritic volcanic xenolith belong to a calc alkaline series a mean crystallization age 67 32 0 66 ma for zircon in the ladakh granitoid implies that the shyok metasediments shale slate were deposited and metamorphosed prior to ca 68 ma present and earlier determined ages on tirit granitoids revealed that the subduction related calc alkaline magmatism in the nubra shyok valley of the ssz prevailed between 110 and 68 ma and hence a minimum age of early cretaceous can be suggested for development of the ssz

multiple sources of the early mesozoic gouli batholith eastern kunlun orogenic belt northern tibetan plateau linking continental crustal growth with oceanic subduction orogenic belts have been among the most important locations to investigate the growth of continental crust cc the eastern kunlun orogenic belt ekob which contains widespread permian triassic granitoids is volumetrically comparable to the cenozoic gangdese magmatic belt in the tibetan plateau and is an ideal region to investigate the mechanism of the paleozoic mesozoic cc growth in this region the gouli batholith at the eastern end of the ekob consists of the synchronous xiangride granodiorite asiha quartz diorite ca 242 ma and adamellite the granodiorite and quartz diorite both of which contain magmatic enclaves show medium high k calc alkalic and metaluminous signatures and have similar rare earth element and trace element patterns to those of bulk cc besides the xiangride granodiorite displays distinct adakitic signatures average sr y of 47 the sr nd isotopic values for the different types of rocks are roughly similar 87sr 86sr i 0 708167 0 713553 end t 6 8 to 5 3 while hf isotopes are distinguishable with ehf t granodiorite 0 3 to 5 1 and ehf t diorite 1 6 to 0 7 these geochemical and petrographic signatures suggest that the granodiorite originated from the partial melting of subducting oceanic crust and terrigenous sediments and the quartz diorite and enclaves formed via the mixing of slab derived magma and enriched mantle derived melt further comprehensive analyses of the spatial and temporal distribution of regional magmatic rocks metamorphism and sedimentary facies reveal that the gouli batholith and most of the permian triassic granitoids in the ekob formed during the subduction of the paleo tethys ocean instead of subsequent syn collision setting thus we contend that the permian triassic cc growth of the ekob occurred in a slab subduction setting and that both oceanic slab and subcontinental mantle significantly contributed to this process

crystallization and hybridism in microgranitoid enclave magmas microstructural evidence microstructural evidence indicates that many microgranitoid enclaves mafic inclusions in granitoids crystallize from globules of magma that have undergone previous magma mixing hybridism the evidence includes 1 rounded xenocrysts ocelli of quartz rimmed with fine grained aggregates of early formed minerals especially mafic minerals 2 k feldspar megacrysts commonly rimmed with plagioclase rapakivi structure and 3 corroded cores dendritic overgrowths and calcic spikes in plagioclase initial quenching of a magma globule in felsic magma involves rapid temperature equilibration which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time followed by a period of slow cooling during which the nucleation and growth rates of later crystallizing minerals are similar to the much slower rates prevailing in the granitic melt this may result in markedly poikilitic microstructures involving inclusion of the earlier formed finer grained minerals in quartz and or k feldspar which are common in microgranitoid enclaves

late holocene atmospheric lead deposition in the peruvian and bolivian andes the analysis of lead pb in lake sediment cores is a useful method to reconstruct local histories of pb pollution here we use pb concentration profiles from lake sediments to reconstruct local trajectories of pre colonial smelting from three metallurgical centres in the south american andes the morococha mining district peru the bolivian altiplano around lake titicaca and the silver mining centre of potosi bolivia the earliest evidence for pb enrichment from smelting is on the altiplano beginning ad 400 coincident with the rise of the pre incan tiwanaku and wari empires coeval with the collapse of these empires ad 1000 there is a dramatic decrease in pb pollution on the altiplano suggesting metallurgical activity was closely tied to the tiwanaku state in contrast metallurgy at morococha peru and potosi bolivia began ad 1000 1200 likely as the result of the diaspora generated by the collapses of tiwanaku and wari the independent chronologies of these records suggest asynchronous metallurgical activity between mining centres and local scale control of mineral resources following inca conquest of the andes ad 1450 strong increases in pb are noted at all three study sites suggesting an increase in silver production to meet inca imperial demand following hispanic conquest ad 1532 large increases in pb pollution are noted at morococha and potosi only to be superseded by industrial development the records presented here have implications for the reconstruction of andean prehistory and demonstrate the sensitivity of lake sediment geochemistry to pre colonial smelting activity the technique has much potential for exploring the timing and magnitude of pre industrial metallurgy in the new world

mineralogy and geochemistry of microgranular enclaves in palaeoproterozoic malanjkhand granitoids central india evidence of magma mixing mingling and chemical equilibration palaeoproterozoic ca 2 480 ma felsic magmatism of malanjkhand region of central indian precambrian shield referred to as malanjkhand granitoids mg contain xenoliths of country rocks and mesocratic to melanocratic fine grained porphyritic microgranular enclaves me the shape of me is spheroidal ellipsoidal discoidal elongated and lenticular varying in size from a few centimeters to about 2 m across the contact of me with the host mg is commonly sharp crenulate and occasionally diffuse which we attribute to the undercooling and disaggregation of me globules within the cooler host mg the me as well as mg show hypidiomorphic texture with common mineral hbl bt kfs pl qtz assemblage but differ in modal proportions the variation in minerals composition presence of apatite needles elongated biotites resorbed plagiclase ocellar quartz and other mafic felsic xenocrysts strongly oppose the restite and cognate origins of me compositions of plagioclases an3 an29 amphiboles mg mg fe2 0 55 0 69 and biotites mg mg fe2 0 46 0 60 of me are slightly distinct or similar to those of mg which suggest partial to complete equilibration during mafic felsic magma interactions al in amphibole estimates the mg pluton emplacement at ca 3 4 0 5 kbar and therefore magma mixing and mingling must have occurred at or below this level the hbox fe rightleftharpoons hbox mg substitution in biotites of me and mg largely suggests subduction related calc alkaline metaluminous i type nature of felsic melts most major and trace elements against sio2 produce near linear variation trends for me and mg probably generated by the mixing of mafic and felsic magmas in various proportions trace including rare earth elements patterns of me mg pairs however show partial to complete equilibration most likely governed by different degrees of elemental diffusion the available evidence supports the model of me origin that coeval mafic enclave and felsic mg magmas produced a hybrid me magma layer which injected into cooler partly crystalline mg and dispersed mingled and undercooled as me globules in a convectively dynamic magma chamber

geology geochronology and hf and pb isotope data of the raul condestable iron oxide copper gold deposit central coast of peru raul condestable is a 32 million metric ton mt iron oxide copper gold iocg deposit located on the peruvian coast 90 km south of lima the ore occurs as veins replacement mantos and disseminations consisting of a chalcopyrite pyrite pyrrhotite magnetite amphibole mineral association the geology of the studied area comprises a series of superposed volcanic edifices of late jurassic to early cretaceous age which are part of a larger volcanic island to continental arc system particularly good exposures of the tilted host sequence allow the mapping of the raul condestable iocg deposit in a nearly complete oblique cross section from its associated volcanic edifice down to a paleodepth of about 6 km u pb zircon ages indicate that in the deposit area felsic magmatic activity took place between 116 7 0 4 and 114 5 1 ma defining a new raul condestable superunit the oldest so far of the peruvian coastal batholith this superunit is located west of the main part of the batholith and includes a dacite andesite volcanic dome and a subvolcanic quartz diorite porphyry sill dike complex that were emplaced at 116 7 0 4 and 116 4 0 3 ma respectively followed by tonalite stocks and dikes emplaced between 115 1 0 4 and 114 5 1 ma all these rocks contain hornblende and or biotite but no pyroxene and correspond to silica and water rich magmas following a calcic differentiation trend hf isotope data on zircons ehf 115 ma 5 2 7 5 and pb isotope data on whole rock combined with lithogeochemical results suggest that magmas were generated by partial melting of the upper mantle enriched through hydrous metasomatism and or melting of subducted pelagic sediments the lack of zircon inheritance suggests that there was no direct involvement of continental crust the raul condestable iocg deposit is connected in space and time with the magmatism of the raul condestable superunit the mineralization was emplaced in the core of the dacite andesite volcanic dome at a paleodepth of 2 to 3 km surrounding two tonalitic intrusions formed at 115 1 0 4 and 114 8 0 4 ma the u pb age of hydrothermal titanite from iocg veins at 115 2 0 3 ma indicates that the mineralization was coeval with or more probably just followed the emplacement of the tonalites re os geochronology on molybdenite did not yield reliable ages due to apparent re loss copper ore is associated with a zoned alteration pattern which surrounds the tonalite intrusions it consists of a core of biotite alteration and quartz stockwork grading outward to actinolite magnetite chlorite titanite scapolite albite epidote and upward to sericite fe chlorite alteration an upper distal alteration halo consisting of hematite chlorite surrounds the sericite fe chlorite and actinolite alterations laterally most of the ore is spatially associated with the actinolite alteration and to a lesser extent with the sericite fe chlorite alteration the results of this study confirm that the 110 to 120 ma age range was a productive time period for andean iocg deposits the characterization of the hydrous intermediate magmatism related to the mineralization as well as the subvolcanic position of the deposit and its related alteration pattern provides further criteria that may be used when exploring for iocg deposits in a convergent plate tectonic setting

upb geochronologic constraints on deposition of the campbellrand subgroup transvaal supergroup south africa abstract a volcanic ash bed that crops out 40 50 m below the contact between the campbellrand subgroup and the kuruman iron formation transvaal supergroup south africa yields an u pb zircon age of 2521 3 ma this new age demonstrates that the campbellrand malmani carbonate platform was deposited during late archean time and is the oldest extensively preserved carbonate platform known in addition the age constrains deposition of the kuruman griquatown and penge iron formations to latest archean to earliest paleoproterozoic time although a previously reported pb pb age of 2557 49 ma for dolomites at the base of the campbellrand subgroup is broadly consistent with this u pb zircon age new pb isotopic data for carbonate cements and massive sulfides lie along the same pb pb array as the dolomites this relationship implies that the isochron is the result of post depositional resetting of the u pb system in dolomite by fluids responsible for sulfide mineralization and that the age which corresponds to its slope has no geological significance

magma mixing in the subvolcanic environment petrology of the gerena interaction zone near seville spain the mechanisms by which felsic and mafic magmas interact and approach a uniform hybrid composition through the processes of mingling and mixing have been studied in a high level subvolcanic setting in the spanish hercynian at gerena near seville the compositions involved are calc alkaline and the situation is one of tonalite quartz diorite synplutonic dykes injected into a granitic magma chamber the resulting hybrids include dykes pillows and globules of tonalite with chilled margins which are variously disrupted and homogenised with the host granite the present investigation is based on field and petrographic observations of hybridization textures the identification of different stages in the crystallisation history of the tonalite through mineral textures and the characterization of mineral compositions at these various stages proportions of the end member magmas involved were obtained by major oxide mixing models and tested satisfactorily with trace elements a mechanistic model is presented to account for these observations which involves the early quenching of the tonalite when it was emplaced into the granite magma chamber after high temperature crystallization had occurred the two magmas attained thermal equilibrium and disruption of the tonalite in the high energy regime of this subvolcanic complex resulted in dispersion of fragments and crystals through the granite giving rise to hybrid granodiorite compositions it is argued that such high energy flow conditions are a necessary requirement for effective hybridization in this environment in contrast to most large scale magma chamber settings where mixing is driven by thermal and buoyancy contrasts

the origin and implications of undercooled andesitic inclusions in rhyolites clear lake volcanics california at clear lake california undercooled andesitic inclusions are common in rhyolitic to dacitic lava flows and pyroclastic deposits one group of rhyolite lava flows contains about 1 vol undercooled andesitic inclusions 2 cm in size the inclusions are evenly distributed in several flows and do not vary significantly in size or abundance they consist of plagioclase orthopyroxene ilmenite apatite needles and vesicular rhyolitic glass olivine clinopyroxene microphenocryst size in these small inclusions correlates positively with inclusion size suggesting that inclusions have not undergone extensive disaggregation since formation texturally similar inclusions up to 50 cm in size occur in associated pyroclastic deposits none of the inclusions represent the mafic end member of the system rather they represent intermediate magmas formed by liquid crystal fractionation and hybridization of distinct andesitic magmas at or near the base of a silicic magma chamber the large inclusions are transitional in composition and mineralogy between clear lake basaltic andesites and smaller inclusions which show more evidence of hybridization these differences suggest similar but distinct sources and mechanisms of formation for the large and small inclusions the small inclusions probably formed by episodic mingling across an interface or hybrid zone separating rhyolitic and andesitic magmas we speculate that earthquakes generated along the san andreas fault system may have triggered mingling events this occurrence demonstrates that dispersion of undercooled intermediate to mafic inclusions in silicic magma chambers is the dominant mode of mafic felsic magma interaction in some systems

a type granitoids from the eastern pontides ne turkey records for generation of hybrid a type rocks in a subduction related environment the eastern pontides is characterized by the many of intrusive bodies formed throughout the late mesozoic early cenozoic most of these are i type granitoids but here we present for the first time an a type pluton from the region to assess source characteristics and geodynamic implications the a type pirnalli pluton has a shrimp zircon u pb age of 81 2 1 ma it is composed of granite syenite and quartz monzonite and its enclaves are monzonitic in composition with elevated ga al ratios and low mg 42 the pluton is dominantly metaluminous a cnk 0 88 to 1 00 and belong to shoshonitic and ultra potassic series the samples are highly enriched in lree and show significant negative eu eu eu 0 33 to 0 92 ba nb sr and ti anomalies in the spidergrams the host rocks and their enclaves posses quite similar sr nd isotopic compositions isr 0 70693 to 0 70736 end 81 ma 2 6 to 2 0 tdm 0 94 to 1 12 ga pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle we hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed melting of lower crust was provided by the underplating of this basic magma mixing between lower crust and mantle derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton sr nd isotope modeling suggests mixing of 82 90 of the lower crustal derived melt with 10 18 of the mantle derived melt then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types our data also suggest that the a type pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late cretaceous ongoing extension then led to opening of black sea as a back arc basin further north of the eastern pontides

assimilation and partial melting of continental crust evidence from the mineralogy and geochemistry of autoliths and xenoliths this paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the loch doon granitic intrusion of southern scotland the melting of quartz diorite was modelled using autoliths which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma compared to their quartz diorite parents the autoliths are enriched to varying degrees in some elements notably rb nb ta sm y yb and depleted in others sr and ba eu and p are also depleted in the more assimitated autoliths the compositions of melts that could be derived from assimilation of the autoliths have also been calculated their ree patterns reveal a light ree enrichment low concentrations of heavy rees 1 3 x chondrite and a positive eu anomaly the calculated degrees of melting vary from 35 in the least assimilated to 84 in the most assimilated autolith assuming a bulk distribution coefficient of 10 for the most compatible element results from modelling of xenolith compositions derived from metasediments are also reported but because of uncertainties in the composition of the parental sediment these data are subject to larger errors they do however indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths in particular the ree pattern of a greywacke derived melt shows a slight enrichment in light rees greater concentrations of heavy rees 10 x chondrite and a small negative eu anomaly the calculated degrees of melting of the xenoliths fall in the range of 66 88 assuming a bulk distribution coefficient of 10 for the most compatible element the results have direct implications for assimilation and melting of the upper crust by taking into account how the nature of residual phases is likely to change with depth it can be demonstrated that some archaean tonalite gneisses could represent liquids derived by partial melting of igneous material

magma mixing versus xenocryst assimilation the genesis of trachyandesites in sancy volcano massif central france sr and nd isotopic compositions have been determined on basaltic and acid trachyandesites bta ata from the sancy volcano mont dore massif france these represent more than 80 of the lavas erupted during its activity between 0 9 and 0 2 ma these lavas have been recently interpreted as the result of two component magma mixing during and after repeated injections of basaltic magmas in trachytic reservoirs magmatic heterogeneities in the ata s large to small enclaves banded lavas megacrysts testify to the mingling event complete mixing is supposed to have been achieved in the hybrid bta s which contain sanidine plagioclase and clinopyroxene megacrysts in disequilibrium with their host the megacrysts are interpreted as relicts of the trachytic end member isotopic data on basic comagmatic enclaves and host ata matrix samples from three different cycles of mingling succession of heterogeneous pyroclastics heterogeneous ata lava flows or domes and occasionally homogeneous bta lava flows are not incompatible with two component mixing but could just reflect the heterogeneity of the analysed samples however the bta s have sr contents and sr isotopic ratios which are too high to be simple binary mixing products between the postulated end members three hypotheses are considered to explain this discrepancy 1 the analysed end members are not those involved in bta genesis 2 some crustal contamination occurred during and after the mixing event 3 sr rich sanidine xenocrysts with radiogenic 87sr86sr have been assimilated and digested in the bta s in this third hypothesis that i favour it is not necessary to resort to magma mixing to explain the genesis of the bta s assimilation of xenocrysts by basaltic hawaiitic or mugearitic magmas accounts for both mineralogical disequilibria and isotopic characteristics of these lavas

the cristal zinc prospect amazonas region northern peru part i new insights on the sulfide mineralization in the bongara province the cristal zn prospect consists of a mixed sulfide and nonsulfide mineralization located in the bongara province amazonas region northern peru the mineralization is hosted by carbonate rocks of the pucara group deposited in a mesozoic extensional basin on the western margin of the brazilian guyana shield zinc sulfides at cristal occur in the roots of the nonsulfide concentrations and are locally present also nearer to surface the sulfide mineralization postdates two hydrothermal dolomitization phases dol1 and dol2 and the sulfides occur mainly in veins cavity fillings or as disseminated mineralization within sparry to saddle dolomite they mostly consist of dark brown sphalerite intergrown with smaller amounts of pyrite sphalerite shows a distinct fe zonation with average ca 7 wt fe max ca 12 wt fe and is ge rich mean concentration of 142 ppm galena is rare the cristal sphalerite has sulfur isotopic compositions of 34s 14 to 15 vcdt oxygen isotopic compositions of dolomites are 18o 24 4 to 24 7 vsmow for dol1 and 18 4 to 22 for dol2 34s and 18o values of cristal sulfides and dolomites are similar to those observed in two mississippi valley type mvt deposits located ca 20 30 km south of the cristal prospect namely the florida canyon and florcita deposits this could be consistent with one or more events within a same mvt mineralizing system acting at the district scale the pb isotope compositions of sphalerite from two different areas of the prospect named cristal s s and charlita north define two distinct data point clusters centered around averages of 206pb 204pb 18 850 0 002 207pb 204pb 15 685 0 002 208pb 204pb 38 752 0 004 and 206pb 204pb 19 042 0 002 207pb 204pb 15 712 0 002 208pb 204pb 39 080 0 004 respectively this difference requires distinct metal bearing hydrothermal pulses in the mineralized area and or distinct pb sources the pb isotopic compositions of the cristal s s and charlita north sulfides are intermediate between the compositions of galena from the san vicente and shalipayco mvt deposits and record a contribution from an old crustal component the paleozoic basement which has pb isotopic ratios roughly matching those of dolomites and sulfides from cristal and charlita north areas represents the most reliable candidate to be an end member source of the metals of the cristal sulfide mineralization the second end member could be an igneous source isotopically identical to the late paleozoic to early mesozoic intrusives of the peruvian eastern cordillera or the triassic volcanic rocks occurring within the mitu group

pb isotopic discrimination of crustal domains within the highgrade basement of sri lanka pb isotope data confirm the recognition of two isotopically distinct crustal domains originally proposed on the basis of nd model age data within the central belt of granulite grade rocks in sri lanka one domain the highland complex comprise rocks that have late archaean palaeoproterozoic nd model ages and unusually high 207pb204pb compositions they plot well above the 207pb204pb plumbotectonic growth curves for average upper crust and orogeny of zartman and doe 1981 and have isolated from plumbotectonic exchange more than 1 ga prior to metamorphism 600 ma ago two highland samples suggest that in some cases this isolation occurred some 2 ga ago or earlier this isolation age is considered to date the time the pre granulite highland terrain was cratonised i e incorporated into a stable shield environment pb isotopic compositions of rocks from the second domain the wanni complex straddle the upper crust and orogeny plumbotectonic curves in contrast to the highland granulites there is no evidence for an extended plumbotectonic isolation period prior to metamorphism the available u pb zircon nd and pb isotope data indicate that the wanni complex represents a mid neoproterozoic crustal package 0 7ga t 1 3 ga metamorphosed some 600 ma ago a time gap exceeding 0 5 ga separates the age of cratonisation 1 8 2 0 ga of the pre granulite highland terrain from the times of formation of the earliest wanni protoliths 1 3 ga the distinctive isotopic characteristics and the unrelated geological evolution reconstructed from these data indicate that the two now adjacent highland and wanni complexes were geologically unrelated crustal packages prior to high grade metamorphism and as such represent suspect terranes brought together during a collision event

some additional observations on inclusions in the granitic rocks of the sierra nevada microgranular quartz diorite and diorite inclusions are widespread in central sierra nevada granitoid rocks and are almost exclusively restricted to hornblende bearing rocks most commonly felsic tonalites and mafic granodiorites strontium 87 strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages neodymium 143 neodymium 144 values of five inclusions ranged from 0 51229 to 0 51248 and yield end values that correlate closely with values of their respective hosts the nd sm and rb sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation silica contents of inclusions and granitoids are contiguous but inclusions generally contain less than and granitoids more than 60 sio2 linear continuity of the femic oxide and h2o trends relative to sio2 suggests many inclusions formed as concentrations of hydrous mafic minerals variation of other major element oxides and trace elements support this inference the aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle derived basalt however many initial sierra nevada magmas were produced by mixing of mafic mantle derived melts and felsic crustal materials resulting in isotopically varied tonalitic and granodioritic magmas most inclusions thus represent fragmented crystal accumulations of early formed near liquidus minerals generated from these previously mixed magmas

progressive hybridisation between late devonian mafic intermediate and felsic magmas in the meguma zone of nova scotia canada two felsic plutons of late devonian 385 370 ma age in the meguma zone of southwestern nova scotia contain three circa 376 ma synplutonic mafic intermediate intrusions that collectively record progressive stages of in situ hybridisation a 5 m wide spessartite dyke in the port mouton pluton probably underwent rapid cooling and crystallisation immediately after intrusion which heated and coarsened the adjacent tonalite an 85 m long sheet of pillowed kersantite also in tonalite of the port mouton pluton presumably contained residual magma after quenching and obtained k p ba rb more radiogenic sr zr nb and light ree from the tonalite during magma mingling the third synplutonic body a 100 m wide diorite sheet intrudes granodiorite of the adjacent shelburne pluton and has a circa 45 m wide gradational contact of metaluminous hornblende tonalite this tonalite dominantly records magma mixing by the transfer of ti mg fe ca and v in hornblende biotite plagioclase and at least in part apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite probably in response to convection scattered data points unusual concave down variation trends for al2o3 p2o5 and sr and non hyperbolic sr nd isotopic characteristics in the tonalite apparently reflect syn or post mixing fractionation and accumulation of xenocrysts from residual magma phosphorus may have assisted diffusion of sr zr nb and light ree and caused premature quenching of the hybrids at mcleods cove and birchtown during magma mingling and mixing

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

geochemistry and u pb shrimp zircon chronology of granitoids and microgranular enclaves from jhirgadandi pluton of mahakoshal belt central india tectonic zone india abstract the northern part of central india tectonic zone citz is delineated by an arc shaped supracrustal belt commonly referred to as mahakoshal belt which is considered as a product of intense rifting of sialic crust that occurred at ca 2400 2600 ma several granitoid plutons intrude the parsoi formation of mahakoshal belt among these an elliptical small stock like granitoid body trending e w is exposed in and around jhirgadandi region of mahakoshal belt referred herein as jhirgadandi pluton it is composed of minor amount of mafic rocks diorite and predominant granitoids country rock pelitic xenoliths and microgranular enclaves me are commonly hosted in granitoids but are absent in diorite the me exhibit typical magmatic texture with a bt cpx hbl pl kf qtz mag ap assemblage similar to that in host granitoids but with contrasting mineral proportions whole rock molar al 2 o 3 cao na 2 o k 2 o a cnk ratios of diorite 0 63 0 72 me 0 69 1 21 and granitoids 0 83 1 05 suggest their nature largely metaluminous i type to rarely peraluminous s type granitoids on most binary plots involving silica two distinct compositional paths can be recognized one formed by an array of differentiating diorite and me and another by fractionating granitoids gradually depleting in compatible elements it is most likely that me were generated by progressive and concurrent mixing of coeval pristine mafic diorite and granitoid magmas and fractionation processes however coherent and identical trace elements except for sr th y and ni and ree patterns for me granitoid pairs most likely suggest partial to near complete chemical equilibration through varying degrees of diffusion process across the me partly crystalline host granitoid boundary high precision u pb shrimp zircon 206 pb 238 u ages for me 1758 19 ma and host granitoid 1753 9 1 ma from jhirgadandi pluton further support the notion that they were coeval the obtained age 1750 ma of jhirgadandi pluton also points to the existence and role of super columbian continental component in the evolution of mahakoshal belt of the citz

provenance of quaternary and modern alluvial deposits of the amazonian floodplain brazil inferred from major and trace elements and pb nd sr isotopes abstract chemical and pb nd and sr isotope data from floodplain sedimentary deposits in the solimoes river northwest brazil constrain the variability of the deposits through the quaternary and help reconstruct the paleogeography of amazonia since the uplift of the andes compared to the modern alluvial deposits in the active channel the quaternary terrace deposits in the solimoes river have a higher chemical index of alteration cia higher sio2 al2o3 ratios higher concentrations of immobile elements higher radiogenic sr isotopic compositions more negative end t 0 values and older nd tdm ages these geochemical characteristics indicate that the quaternary sediment sources were composed by more felsic and or recycled old continental crust derived rock materials than the younger source areas which contributed major arc volcanic materials to the flood plain deposits this shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the solimoes river has occurred since the uplift of the andes

geochemistry of enclaves and host granites from the nelas area central portugal abstract tonalitic granodioritic and monzogranitic enclaves occur in the hercynian peraluminous porphyritic biotite granite and biotite muscovite granite from the nelas region some variation diagrams show linear trends but others show dispersion the enclaves generally have closely similar isotopic signatures to those of the host granites they contain xenocrystic plagioclase with the same composition as phenocrysts of the host granite and have biotite as the sole ferromagnesian phase with a composition typically similar to that of the host granite however some cores of enclaves have biotite slightly chemically distinct from that of rims the modelling of major and trace elements of granodioritic enclaves and hybrid host granite indicate that they result from simple mingling mixing between a tonalitic magma and the host granite magma microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched mantle source partial equilibration has been achieved between these enclaves and the host granite

plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the sierra nevada batholith california plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the sierra nevada batholith are complexly zoned and commonly divided into three neatly distinct parts an oscillatory locally patchy zoned andesine or more calcic core a ring with dusty calcic plagioclase and a normally zoned rim of sodic plagioclase although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts cores of both phenocrysts and large plagioclase crystals show similar zoning and composition the andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma respectively and been incorporated into the coeval magma when the two magmas mixed introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution development of abundant patchy zoning and coating with dusty calcic plagioclase in both granitoids and mafic magmatic enclaves composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves this hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc alkaline granitoids and mafic magmatic enclaves of the sierra nevada batholith plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons

enclaves in the cadillac mountain granite coastal maine samples of hybrid magma from the base of the chamber the cadillac mountain intrusive complex is dominated by the introduction cadillac mountain granite and a 2 3 km thick section of interlayered enclaves occur in nearly all granites and have recently gabbroic dioritic and granitic rocks which occurs near the base of been the focus of many studies see reviews by vernon the granite the layered rocks record hundreds of injections of basaltic 1983 and didier barbarin 1991 although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma magmatic enclaves ranging in different sources there has been a growing consensus composition from 55 to 78 wt sio2 are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks the most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments their compositions can be best explained by periodic vernon 1984 1990 these can appropriately be termed replenishment mixing and fractional crystallization of basaltic magmatic enclaves their compositions are commonly magma at the base of the chamber the intermediate to silicic intermediate but can vary from basaltic to highly silicic enclaves formed by hybridization between the evolved basaltic magma magmatic enclaves occur in all types of granite and are and resident silicic magma there is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite instead alkaline i type granites in spite of partial reequilibration hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have nd isotopic commagmas at the base of the chamber enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics suggesting support their formation from mantle derived magmas they were derived from a discrete batch of hybrid magma enclaves holden et al 1987 were probably dispersed into a localized portion of the granitic the compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton and they can only layer rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma there is ample petrographic and chemical evidence in many enclaves for hybridization mixing of liquids and

nd and pb isotope signatures on the southeastern south american upper margin implications for sediment transport and source rocks abstract neodymium and lead isotope values in sediment samples were used to interpret sediment transport and source rocks on the southeastern south american upper margin the sediments of the argentinian margin exhibit an average nd value of 1 9 indicating the influence of the andean rocks as sediment sources sediments from the rio de la plata estuary show an average nd value of 9 6 which is similar to that of the southern brazilian upper margin finally sediments of southeastern brazil which are associated with the transport of the brazil current exhibit an average nd of 13 0 the pb isotope signatures also confirm the differentiation of source rocks in the sedimentation of the study area in addition pb isotopes helped to establish the extent of the influence of the rio de la plata on the sedimentation of the southern brazilian margin in terms of pb isotopes the sediments from the rio de la plata estuary and southern brazil are more radiogenic than those of southeastern brazil and the argentinian margin

lead isotope systematics of epigenetic lead zinc mineralization in the western part of the rheinisches schiefergebirge germany in the western part of the rheinisches schiefergebirge three different types of epigenetic lead zinc vein mineralization in paleozoic rocks can be distinguished 1 syntectonic variscan veins 2 post variscan saxonian veins mainly in paleozoic siliceous clastic rocks 3 post variscan carbonate hosted mineralization in the aachen stolberg area ore impregnations occur in the triassic buntsandstein bunter sandstone and muschelkalk of the maubach mechernich triangular depression covering the paleozoic rocks data points define two distinct populations in both lead isotope diagrams and thus indicate two major variscan and post variscan mineralization events the lead isotope pattern of the paleozoic sedimentary rocks bears good evidence that this thick rock sequence may be the main source of metals for the mineralization the metals were mobilized at deeper levels by saline fluids at different times there are no direct indications for remobilization of the older variscan vein ore deposits the sandstone and carbonate hosted ore impregnations of maubach mechernich have a similar lead isotope pattern to the post variscan mineralization in the paleozoic sedimentary basement

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

geochemical investigations of microgranitoid enclaves in the s type cowra granodiorite lachlan fold belt se australia the cowra granodiorite is a relatively mafic enclave rich s type pluton in the lachlan fold belt which has been cited as a type example of a restitic origin for all varieties of enclaves in lachlan fold belt s type granites microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage pyroxene microtonalites and biotite microgranites no geochemical or isotopic distinction can be made between the two varieties petrographic evidence acicular apatites xenocrysts from the host granite suggests an origin as mingled more mafic magmas which have been variably contaminated by the more felsic host magma this is supported by the fact that the microgranitoid enclaves have isotopic compositions 87sr 86sr i 0 7095 to 0 7144 end i 9 2 to 6 9 that are generally more primitive than or similar to those of the host granite 87sr 86sr i 0 7142 end i 8 8 the spread in isotopic compositions like their trace element compositions is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite instead they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement

small scale disequilibrium in a magmatic inclusion and its more silicic host basaltic andesite inclusions and their host dacite from the purico chascon complex in northern chile are isotopically distinct textural characteristics of the inclusions are typical of those resulting from magma mingling serial sectioning across the interface of an inclusion and its host dacite complemented by microdrill sampling and detailed microprobe work has enabled us to examine the scale of mixing and chemical isotopic and trace element disequilibrium the results of this work show that 1 the composition of the inclusion is relatively homogeneous 2 the dacite host is generally higher in 87sr 86sr and lower in 143nd 144nd than the enclave but it is heterogeneous on a small scale and probably a hybrid 3 the isotopic composition in the marginal zone apparently on both host and inclusion sides of the weakly chilled interface actually shows the highest 87sr 86sr and lowest 143nd 144nd 4 large plagioclase crystals in the inclusions and host are xenocrystic with higher 87sr 86sr than any of the other samples these observations are reconciled with a model of magma evolution in a crustal magma chamber in such a scenario the mafic magma is overlain by a cap of rhyolite a partial melt of the dacitic ignimbrites which now underlie the purico chascon complex overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

separate lead isotope analyses of leachate and residue rock fractions implications for metal source tracing in ore deposit studies separate lead isotope analyses of leachate and residue fractions are applied to a broad spectrum of rocks commonly investigated in metallogenic studies resulting data highlight a systematic behavior of leachate and residue fractions with respect to lead isotope compositions which essentially depends on the mineralogical composition of the rock granitoid and high grade metamorphic rocks have residue compositions virtually identical to common lead in contrast low grade metasedimentary rocks may have residue compositions swamped by radiogenic lead of leach resistant zircons mafic magmatic rocks have residues that are often more radiogenic than leachates depending on the ratio of leach refractory zircons to common lead in the residual fraction of these rocks separate leachate and residue analyses of source rocks provide two lead isotope end members whose mixture may represent lead with the appropriate ore fluid composition our leaching experiments indicate that hot acid solutions and by inference hydrothermal fluids preferentially leach radiogenic lead from medium to high grade metamorphic and granitoid rocks whereas they preferentially leach common lead from low grade metasedimentary and mafic magmatic rocks the method presented in this study provides a reliable alternative to other methods i e age correction of bulk rock compositions for the determination of the common lead signature of felsic to intermediate magmatic rocks this may be preferable to age corrected bulk rock analyses where ages to apply for corrections of bulk rock data are not known or where moderately to highly altered rocks must be used case studies of orogenic gold and mvt districts of peru pataz and san vicente respectively show that separate leachate and residue lead isotope analyses carried out systematically on whole rocks allow a more thorough evaluation of metal source reservoirs than does the standard method of age corrected or uncorrected bulk rock analyses

microgranitoid enclaves from the swifts creek pluton se australia textural and physical constraints on the nature of magma mingling processes in the plutonic environment the early devonian swifts creek pluton with an areal extent of 35 km2 intruded ordovician meta flysch sequences along the southwestern margin of the omeo metamorphic belt southeastern australia

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

book review enclaves and granite petrology developments in petrology 13 edited by j didier and b barbarin elsevier science publishers amsterdam the netherlands 1991 hardcover xxiv 600 pp price us 143 50 dfl 280 isbn 9 444 89145 5

felsic enclave swarms in the gouldsboro granite coastal maine a record of eruption through the roof of a silicic magma chamber prominent zones of large globular fine grained felsic enclaves occur at the highest exposed levels of the gouldsboro granite along the coast of maine the physical characteristics of these enclaves and their chemical relations to the host granite indicate that they represent globules of resident magma that were trapped probably during eruption in the crystal mush at the top of the magma chamber these zones can appropriately be termed eruption trails enclave compositions provide a record of magma compositions within the chamber at the time of eruption they fall into two groups high k and low k with similar wt sio2 suggesting that magma in the chamber consisted of discrete compositional batches the distribution of these two types of enclaves within the swarms is consistent with the low k source magma residing as a layer beneath the high k source magma chilled and vesiculated inclusions of basalt occur only in the low k enclaves their presence there suggests that basaltic magma was injected

chaotic dynamics and fractals in magmatic interaction processes a different approach to the interpretation of mafic microgranular enclaves abstract fractals have been used for describing a wide range of natural processes however applications of fractal geometry to the study of magma mingling mixing processes is almost completely lacking in spite of the fact that such an approach could give new insights into the nature of magmatic interactions fractal analyses of two geochemically linked groups of mafic microgranular enclaves mme from the sithonia plutonic complex northern greece have been performed because of the difficulties involved in using contours of enclaves to estimate their fractal dimensions a different approach is used in this paper fractal dimension is estimated using distribution maps of chemical elements inside enclaves the results obtained indicate that a clear correlation between fractal dimensions of enclaves and geochemical evidences of interaction exists and hence the two groups of enclaves reflect different degrees of interaction from a dynamical point of view stretching and folding processes between two magmas with different rheologies are considered representative of magmatic interaction this dynamic system can easily generate chaotic patterns recognized by the presence of fractal structures the contemporaneous occurrence of well mixed regions together with poorly mixed regions in the same system is thought to be similar to the host enclave system found in many granitoid plutons in particular mme are believed to represent islands of partially mixed mafic fluid subjected to different degrees of interaction with the host granitoid whereas host rocks are believed to represent mixed regions where the mixing process acted more efficiently producing almost completely hybrids it is shown that fractal geometry and chaotic dynamics can be considered suitable techniques in studying complex petrological phenomena and they can represent helpful methods in the development of petrogenetic models

petrogenetic and metallogenetic relationships in the eastern cordillera occidental of central peru abstract ore deposits spatially and temporally related to high k calc alkaline intrusions or domes are widespread in the central peruvian cordillera occidental geochronology and petrochemistry of intrusive rocks associated with mineralization reveal that most ore deposits 1 were emplaced in the late middle to early late miocene and 2 are related to calc alkaline igneous rocks of granodioritic composition with documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm sm mathrm yb 3 end document documentclass aas

estimation of uniaxial compressive strength of rock materials by index tests using regression analysis and fuzzy inference system abstract this paper presents a comparative evaluation of efficacies of different index tests and analysis techniques i e regression analyses and fuzzy inference system in predicting uniaxial compressive strength ucs of granite schist and sandstone ucs and indices such as block punch index point load strength schmidt rebound hardness ultrasonic p wave velocity and physical properties effective porosity and density were determined for the concerned rocks from simple regression analyses it was apparent that for granite and sandstone performances of all six indices are reasonably good in predicting ucs in case of granite block punch index and point load strength are the best indices whereas effective porosity point load strength and schmidt rebound hardness are the most efficient indices for sandstone in case of schist however ultrasonic p wave velocity does not seem to be a competent index unlike other indices where point load strength proves to be the best one from the critical analysis of the tests results it was demonstrated and subsequently concluded that index test results of different rock types with different geology should not be clubbed together for statistical correlation with any rock mechanical parameter like ucs both multiple regression analyses and the fuzzy inference system exhibited better predictive performances for ucs than simple regression analyses in addition to the coefficient of correlation the variance account for vaf and the root mean square error rmse were also calculated to check the predictive performances of these two models and it was found that the predictive performances of both models are comparable however one should be cautious while employing multiple regression analysis in predicting ucs as there is always a chance of cumulating plausible errors that might have remained within individual index test results on the other hand fuzzy inference system seems to be an efficient tool in predicting ucs of rock materials from indices because of its efficacy in handling uncertainties in the test results with transparency

tectonomagmatic development of the eocene pasevh pluton nw iran implications for the arabia eurasia collision abstract the pasveh pluton is one of a number of intrusive complexes in the northwestern part of the sanandaj sirjan zone ssz nw iran the major part of this complex is composed of high k calc alkaline granitic rocks the geochemical data indicate that the granitic rocks although compositionally similar are not cogenetic and can be divided into three groups the first group mesocratic granitoids is represented by the largest outcrop in the pasveh pluton it is mainly composed of metaluminous magnesian monzodiorite to granodiorite with abundant mme and gabbroic enclaves zircon la icp ms u pb dating yielded eocene crystallization ages ca 40 ma for this group the monzodiorite endt 2 0 to 2 5 granodiorite endt 1 5 to 1 6 mme and gabbroic enclave endt 3 3 to 4 7 have distinct nd isotopic compositions the second group is composed of eocene 39 7 0 6 ma weakly peraluminous monzogranites that have carboniferous and cretaceous inherited zircons leucocratic granitic dikes of adakitic composition comprise the third group of pasveh silicic rocks the dikes are slightly younger 37 5 0 3 ma than host rocks and cross cut the mesocratic granitoids our results indicate that the underplating of basaltic magmas related to extensional stress in the overlying iranian plate during the eocene induced partial melting of the lower crust differences in the amount of partial melting of the source rocks are likely responsible for the volume and heterogeneity of their melt compositions subsequent to melt generation it is possible that the silicic magmas mixed with mafic magmas in variable proportions to produce the hybridized rocks and enclaves the intrusion of leucogranite dikes after the emplacement of the pasveh pluton suggests that during or slightly after neo tethys subduction there was another period of magmatism that was likely related to the melting of the subducted slab

mineral compositional and textural evidence for magma mingling in the saraykent volcanics abstract upper cretaceous paleocene calc alkaline dacitic and rhyodacitic lava flows and domes are exposed in the saraykent yozgat region within central anatolian crystalline complex cacc saraykent volcanics are generally plagioclase hornblende biotite titanomagnetite quartz augite phyric subrounded vesicular magmatic enclaves ranging from a few millimeters to a few centimeters in size are present in the saraykent dacites the enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites almost all phases in saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling plagioclase phenocrysts display sieved and normal type populations a wide range in rim compositions and oscillatory zoning maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol increase in an some plagioclase core compositions are unusually ca rich they range from an27 to an75 for the saraykent dacites and from an30 to an66 for the saraykent rhyodacites hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma both hornblende and biotite crystals show reverse zoning in terms of decreasing mg mg fe quartz phenocrysts are corroded and embayed presence of normally and reversely zoned augites in the same sample and high ti high al augites more comparable with basaltic compositions are other lines of evidence for disequilibrium the enclave hornblendes are both edenitic and pargasitic the latter being more typical of mafic compositions the enclave biotite crystals have higher mg mg fe2 ratios than the host dacite biotite crystals and display normal reverse and oscillatory zoning plagioclase core compositions vary between an44 and an55 in the enclaves although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression this cannot explain the observed compositional change and normal type of plagioclase in the same sample similarly a cognate i e fractionation origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite however other criteria such as oscillatory zoning and unusually calcic cores of plagioclases heterogeneity of plagioclase rims resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the saraykent volcanics

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

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

silicic magma chambers as traps for basaltic magmas the cadillac mountain intrusive complex mount desert island maine the cadillac mountain intrusive complex located on mount desert island maine provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma the complex consists of three units the cadillac mountain granite cmg the somesville granite sg and gabbro diorite g d basaltic magmas were emplaced before during and after the emplacement and crystallization of the granitic plutons and the chambers of silicic magma acted as traps for basaltic magma basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma the g d unit about 1 5 km thick consists of interlayered gabbroic dioritic and granitic rocks although thin 1 m thick gabbroic layers in the g d unit are typically chilled on all margins thicker layers of gabbro commonly form the lower parts of macrorhythmic units

constraints on the sources of ore metals in mississippi valley type deposits in central and east tennessee usa using pb isotopes abstract the mississippi valley type mvt central and east tennessee tn districts contain lead zinc deposits that occur in the early ordovician carbonates of the knox group although both districts share similar host rock have similar temperatures of formation and typically fill open spaces of collapse breccias or replace their host carbonates previous studies may suggest that these ores did not form from the same mineralizing fluids and may have different pb sources nu plasma mc icp ms pb isotopic analyses on sulfides from central and east tn were conducted and the metal sources evaluated pb isotope ratios from the young mine in the east tn district plot within ranges defined by other east tn district samples from previous studies generally pb isotope ratios or ores from the elmwood mine in the central tn district are more radiogenic and plot between samples from the southern appalachians and illinois kentucky districts it appears that the thick black metal rich shales of the appalachian basin have been the source of the pb zn and ore fluids of mvt deposits that formed the east tn deposits when east tn pb data from this study are compared to central tn pb data central tn pb is commonly more radiogenic suggestive of a mineralizing fluid with different compositions lead isotopic signature of central tn ores may imply potential mixing between end member sources originating from the southern appalachians and illinois basins lead isotopes in mvt and carbonate hosted base metal deposits in us south america and europe display two basic types of pb isotopic signatures the classic mvt deposits in the mid continent us se and central missouri illinois kentucky and upper mississippi valley districts have highly radiogenic pb isotopic compositions and they generally display linear pb isotopic arrays in contrast deposits from europe irish type silesian type alpine type and cevennes region the appalachians and andean type deposits have lower pb isotopic values the european deposits except for the irish type have a restricted range of isotopic ratios which suggest a single mineralizing event of relatively short duration it appears that basement rocks or sedimentary rocks derived from the basement contributed pb to many mvt and carbonate hosted base metal deposits worldwide hydrothermal leaching of radiogenic pb from the crystalline basement rocks may have occurred during formation of these ore deposits high 238u 204pb values for several districts also suggest a supracrustal origin of pb

lead isotope variations across the central peruvian andes lead isotope values of ore and host rock samples from 18 deposits of nine mining districts in an andean transect between latitudes 9 degrees and 14 degrees s are reported the ore deposits discussed in this paper comprise a relatively narrow age span of about 200 m y upper triassic to early pliocene the lead isotope ratios from different ore deposits correlate closely with their geotectonic settings there is a systematic increase of radiogenic lead isotope ratios from west to east i e from ore deposits located near the coast in the lower cretaceous marginal basin of central peru to ore deposits in the carbonate platform at the margin of the brazilian shield for the coastal deposits ral condestable and los icas the lead may be mantle derived and was supplied via the casma volcanics the more radiogenic lead of the leonila graciela santa cecilia ores may be related to lateral facies variations within the marginal basin it is assumed that here a mantle derived component was mixed with a crustal component the lead isotope ratios from ore deposits in the western cordillera mainly indicate a contribution from the orogen reservoir leaching of the paleozoic basement is probably the main source igneous activity may have facilitated remobilization processes during younger tectonic and magmatic cycles mixing between upper crust and orogen reservoirs with participation of magmatic lead can explain the lead isotope ratios of the ore deposits in the atacocha district the lead isotope signature of the shalipayco ores is interpreted in relation to the permo triassic mitu volcanism leaching of permian rift related volcanics could be responsible for the less radiogenic lead in this deposit the ores of the san vicente deposit are characterized by highly radiogenic values and negative model ages typical of upper crustal leads in cratonic regions the ore lead of san vicente could have been derived by leaching clastic sediments derived from precambrian upper crust brazilian shield

use of the block punch test to predict the compressive and tensile strengths of rocks this study aimed to explore the applicability of the block punch test in estimating uniaxial compressive and brazilian tensile strengths of granite schist and sandstone from india it was experimentally shown that when predicting uniaxial compressive strength of rocks the block punch test is as useful as the point load test efficacy of the existing empirical equations for predicting uniaxial compressive strength by the block punch index was also evaluated prediction of brazilian tensile strength by block punch index appears to be more precise than by the point load strength performances of the existing strength anisotropy transformation factors in the literature in order to normalize the block punch index in the direction perpendicular to foliations were assessed based on the results of the tests performed on the schistose rocks

pb isotopic compositions of some zn pb deposits and occurrences from urumieh dokhtar and sanandaj sirjan zones in iran abstract this study reports pb isotope ratios of galena pbs sampled from some zn pb deposits and occurrences n 18 within two major structural provinces of iran the sanandaj sirjan zone ssz and urumieh dokhtar zone udz regardless of the nature of the host rock and ore type the pb isotopic data plot in distinct isotopic fields according to their geographic situation which suggest that the structural terranes had an important control on the lead isotope compositions the zn pb deposits in the udz record distinctly higher 206pb 204pb and 208pb 204pb ratios compared to those within the ssz both zones however have radiogenic 207pb 204pb ratios 15 55 suggesting a dominant input of pb from crustal sources in thorogenic 208pb 204pb vs 206pb 204pb and uranogenic 207pb 204pb vs 206pb 204pb plots the pb isotope data define two linear arrays which may be attributed to mixing between mantle like end members and at least two distinct crustal sources characterized by high u pb and th u ratios in plumbotectonic discrimination diagrams the samples plot proximal to the orogenic curve which again supports interaction between crustal and mantle reservoirs however both neo tethys subduction and in particular mesozoic and tertiary orogenic activities seem to have played important roles in remobilizing pb and forming zn pb deposits in ssz and udz respectively the similarity of pb isotopic compositions between udz deposits and those for previously reported eocene volcanics demonstrates that long lived and widespread hydrothermal activity promoted remobilization of lead from the crust the latter occurred subsequent to neo tethys lithosphere subduction in the tertiary and continued subsequent the collision of the iranian and arabian plates in the miocene

mixing basaltic and dacitic magmas by forced convection although magma mixing is an old concept which has been applied to many magma types recent petrographic studies have revived interest in the theory by suggesting that it may account for the origin of calk alkaline magma1 6 and mid ocean ridge basalt7 9 there has been no previous experimental investigation into how two magmas with different properties can mix to form a homogeneous magma of andesitic composition or to produce textures such as observed in banded dacite we present here the results of experiments demonstrating that basaltic and dacitic magmas can be easily mixed by forced convection to form both banded dacite and homogeneous andesite in less than a few hours

field evidence of magma mixing from microgranular enclaves hosted in palaeoproterozoic malanjkhand granitoids central india abstract the malanjkhand granitoids mg pluton about 1500 sq km occurs in the balaghat district of madhya pradesh the mg 2400 ma represent an episode of palaeoproterozoic felsic magmatism in central india and hosts potential cu mo au deposits the enclaves hosted in mg can be broadly classified into two categories microgranular enclaves dark coloured fine grained magmatic and xenoliths of country rocks the microgranular enclaves me may be rounded ellipsoidal discoid elongated lenticular or tabular and their size commonly reaches up to 2 metres across the me have sharp and in places diffuse contacts with their host granitoids the shape and size of me indicate contemporaneous flow and mingling of partly crystalline felsic mafic magmas some me exhibit dark crenulated margins giving them a pillow like form that has been attributed to undercooling of a me magma as globules intruded into a granitoid magma the presence of corroded felsic and mafic minerals xenocrysts in me is interpreted as the result of mechanical transfer during the mafic felsic magma interaction and mixing event mafic minerals biotite rim the quartz xenocrysts giving rise to ocellar texture which exhibit signatures of resorption under hybrid enclave magma conditions all these features suggest an origin for the calc alkaline intermediate granitoid magma in malanjkhand involving a magma mixing process

isotopic record of lead pollution in lake sediments from the northeastern united states abstract although it is common knowledge that pb concentrations have increased in lake sediments in the northeastern united states over the last 150 years the processes responsible have been the subject of debate in this study differences in lead isotopic compositions and concentrations in sediment from large lakes lake erie ontario and michigan and small ones deep lake and lake andrus are used to infer temporal changes in the source s of anthropogenic pb in the great lakes region a natural background component of pb is present in sediment deposited prior to 1860 in lake erie and the other lakes as indicated from low pb concentrations and uniform lead isotopic compositions changes in isotopic ratios of lake sediment reflect differing sources of anthropogenic pb superimposed on the natural component such as regional deforestation from 1860 1890 followed by coal combustion and ore smelting through 1930 combustion of leaded gasoline was the dominant anthropogenic pb source to the atmosphere and by inference to lake sediment from 1930 1980 temporal changes in lead isotopic compositions in lake sediment suggest that the source of the pb used in gasoline additives gradually changed from 1930 to present the best example is a distinct shift in lead isotopic ratios in lake sediment deposited after 1970 which corresponds to increased pb production from the viburnum trend deposits in missouri present in all lakes except ontario however the changes in lead isotopic compositions are much less variable than and do not parallel those calculated on the basis of annual united states mine production and imports rather anthropogenic recycling of pb as well as natural mixing processes during emission transport and deposition of pb in lake sediment control most of the variation in lead isotope ratios differences in lead isotopic ratios in lake michigan erie and deep lake sediment preserve regional differences in lead isotopic ratios from u s and canadian sources first noted in aerosols by sturges and barrie 1987 more localized sources of pb such as point discharges are needed to explain the results from lake ontario and andrus

metal sources in mineral deposits and crustal rocks of ecuador 1 n 4 s a lead isotope synthesis ecuador consists of terranes having both continental chaucha tahuin loja terranes and oceanic macuchi alao salado terranes affinity which were accreted to the amazon craton from late jurassic to eocene four main magmatic arcs were formed by the subduction of the farallon nazca plate since the juras sic a jurassic continental arc on the western margin of the amazon craton a jurassic island arc alao terrane an early tertiary island arc macuchi terrane and a middle late tertiary continental arc encompassing the ter ranes of macuchi chaucha tahuin loja and alao after complete assembly of the ecuadorian crust mineral deposits formed during these magmatic arc activities include porphyry cu and gold skarn deposits in associa tion with the jurassic continental arc polymetallic volcanic hosted massive sulfide deposits vhms in associ ation with the jurassic island arc of alao au cu zn vhms deposits in association with the early tertiary is land arc of macuchi and porphyry cu and precious metal epithermal deposits in association with the middle late tertiary continental arc magmatism on the newly assembled crust of ecuador macuchi chaucha tahuin loja and alao terranes in this study we have compiled 148 new and 125 previously published lead isotope analyses on paleozoic to miocene metamorphic intrusive volcanic and volcanosedimentary rocks as well as on jurassic to miocene magmatic related ore deposits of ecuador lead isotope compositions of the magmatic rocks of the four main arc events derive from mixing of various sources including mantle variably enriched by pelagic sediments and or by a high 238u 204pb component and heterogeneous continental crust rocks lead isotope compositions of the ecuadorian ore deposits display a broad range of values 206pb 204pb 18 3 19 3 207pb 204pb 15 54 15 74 208pb 204pb 38 2 39 2 which is as large as the range previously re ported for all magmatic related ore deposits of the central andean provinces i and ii combined ore deposits formed before complete assembly of the ecuadorian crust through complete accretion of the several terranes i e pre eocene have lead isotope compositions overlapping those of the associated magmatic rocks sug gesting a largely magmatic origin for their lead in contrast post assembly ore deposits i e post eocene have lead isotope compositions that only partly overlap those of the coeval magmatic rocks of the continental arc in fact several ore deposits have lead isotope compositions shifted toward those of the basement rocks that host them suggesting that lead derives from a mixture of magmatic lead and basement rock lead leached by hy drothermal fluids most ecuadorian ores have high 207pb 204pb values 15 55 suggesting a dominant continental crust or pelagic sediment origin of the lead however we caution against concluding that chalcophile metals for ex ample cu and au also have a continental crust origin ore deposits of the different terranes of ecuador irrespective of their age plot in distinct isotopic fields which are internally homogeneous this suggests that lithologic factors had an important control on the lead isotope compositions ultimately lead isotope compositions of the ore deposits of ecuador mirror the isotopic compositions of the rocks of the host terranes and are consistent with the multiterrane nature of the ecuado rian crust

mineralogical constraints on the petrogenesis of trachytic inclusions carpenter ridge tuff central san juan volcanic field colorado although bulk rock normative analyses of the trachytic inclusions from the carpenter ridge tuff yield abundant quartz and minor corundum a portion of the phenocryst assemblage is indicative of an alkaline parentage sanidine and biotite contain up to 8 and 5 wt bao respectively in addition both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica undersaturated magma amphibole is magnesiohastingsite with 3 wt tio2 and less than 0 3 mole fraction vacancies in the a site clinopyroxene compositions straddle the calcic augite salite boundary chrondite normalized ree patterns are similar for both inclusions and rhyolites the inclusions are slightly poorer in ree and have a positive eu anomaly versus the negative anomaly of the rhyolites the similarity in ree patterns would seem to indicate that the two rock types are genetically related with the positive eu anomalies resulting from feldspar accumulation however this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types we suggest that the best explanation for these discrepancies is that a mildly silica undersaturated magma was the parent for the phenocrysts this magma intruded the carpenter ridge chamber and because the crystallization temperatures of both magma overlapped the alkaline magma mixed with the ambient rhyolite to form a hybrid this hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk rock chemistry depended upon the proportions of the endmember liquids the abundance of normative quartz the minor normative corundum and the similarity of ree patterns indicates that the inclusions are mixtures dominated by the rhyolitic component additional processes such as liquid state diffusion crystal accumulation and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process

field chemical and physical constraints on mafic felsic magma interaction in the lamarck granodiorite sierra nevada california evidence of magma interactions resulting in both hybridization and mingling are preserved in a diverse suite of gabbroic to intermediate rocks associated with the compositionally zoned hornblende biotite lamarck granodiorite of the eastern sierra nevada california ellipsoidal mafic enclaves were formed by quenching of small amounts of high alumina basaltic magma upon injection into and dispersal through granodiorite magma early in its crystallization synplutonic intrusions of hornblende gabbro through hybridized mafic granodiorite represent injection of mafic magma at a later stage of crystallization of the granodiorite as they crosscut regional trends in foliation and compositional zoning in the host pluton where compositional contrasts between intrusion and host granodiorite are large contacts are sharp and abundant enclaves derived from the mafic intrusion are present in the granodiorite where the host is relatively mafic or where the local scale proportion of mafic magma is large contacts are zones of extensive hybridization that contain both enclaves and hybrid schlieren uncontaminated mafic intrusions have high alumina basaltic compositions whereas hybridized intrusions have silica contents as high as 63 5 mafic intrusions locally contain coarse grained cumulus gabbro inclusions mafic schlieren in granodiorite far from mafic intrusions represent localized accumulations of hornblende fe ti oxides and biotite from the granodiorite intrusion of late mafic dikes mobilized and entrained granitic residue from the granodiorite and formed composite dikes of aplite and pillowed diorite whether interacting magmas mix or mingle is a function of the heat contents and mass fractions of the end members calculations that account for compositions heats of fusion heat capacities of liquids and crystals and a range of initial temperatures crystallinities crystal sizes and magma water contents indicate that in most circumstances the basalt end member quenches the resulting large viscosity contrast between the end members prevents hybridization homogenization is likely only if the compositional difference between host and injected mafic magma is less than 10 sio2 or if the mass fraction of mafic magma is greater than 0 5 resulting mixtures have the composition of tonalite or mafic granodiorite thus the more silicic rocks of the granodiorite pluton must represent differentiation products rather than direct hybrids of mafic or intermediate magma and felsic magma

biogeography of circum mediterranean miocene pliocene rodents a revision using factor analysis and parsimony analysis of endemicity abstract the biogeographic relationships of north african miocene pliocene rodent faunas are assessed using two different methods factor analysis and parsimony analysis of endemicity the mediterranean basin is a true biogeographic province within it north africa makes up a sub province of its own only slightly closer to the ibero occitanian sub province than to the eastern mediterranean one these three sub provinces retain their permanency over time localities within them being clustered according to their geological age rather than by geographic location the miocene pliocene is a period of strong but transitory similarity between the north and south while there is a steady trend toward homogenisation of the northern mediterranean most of the exchanges occur through france between the eastern mediterranean and eastern spain and between the latter area and the rest of iberia and north africa while there are very few trans mediterranean and southern longitudinal exchanges

local equilibrium of mafic enclaves and granitoids of the turtle pluton southeast california mineral chemical and isotopic evidence free to read on publisher s website major element and trace element compositions of whole rocks mineral compositions and rb sr isotopic compositions of enclaveand host granitoid pairs from the early cretaceous calc alkalineturtle pluton of southeasterncalifornia suggest that the local environment profoundly affects some enclave types in the turtle pluton where the source of fine grained mafic enclaves can be deduced to be magmatic by the presence of partially disaggregated basaltic dikes mineral chemistry suggests partial or complete local equilibrium among mineral species in the enclaveand its host granitoid because of local rb sr isotopic equilibration betweenfine grained enclaves and host granitoid one cannot use sr isotopes to distinguish an enclave source independent of its host rocks from an enclave sourcerelated to the enclosing pluton however preliminary nd isotopic data suggestan independent mantle source for enclaves

mineralization and deformation of the malanjkhand terrane 2 490 2 440 ma along the southern margin of the central indian tectonic zone the malanjkhand cu mo au deposit located near the northwest margin of the malanjkhand batholith terrane is a strategic and significant porphyry style deposit that experienced a protracted 50 m y deformational history shortly after its formation at 2 490 8 ma stein et al 2004 in a recent study panigrahi et al 2004 averaged u pb shrimp zircon data from a pooled set of samples from the malanjkhand batholith to advocate a meaningless intermediate age of 2 476 ma for the malanjkhand granitoid and its cu mo au deposit in the northwest part of the malanjkhand batholith re os dating of occurrence specific molybdenite captures not only the age of porphyry style mineralization and associated magmatism but also elucidates a complex deformational history that extends to 2 450 ma in the central part of the malanjkhand batholith re os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the devgaon mo prospect unequivocally shows that deformation ceased at this location no later than 2 470 2 465 ma the deformational history recorded at the malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the malanjkhand terrane with units in the poorly understood proto central indian tectonic zone citz along its southern margin the central indian shear zone the timing for this convergence is late archean early paleoproterozoic