dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

metallogenic evolution of convergent margins selected ore deposit models abstract metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust in general tin mercury antimony and fluorine are most abundant in areas of thickest crust although the composition of ore deposits is strongly controlled by deep crustal and mantle features the actual form and type of many ore deposits is controlled by upper crustal features particularly the relative abundance of carbonate and silicate country rock for instance epithermal precious metal deposits are commonly associated with subaerial silicic volcanism which generally forms in the later stages of arc evolution deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc similarly chimney manto limestone replacement lead zinc silver deposits are common only in terranes with extensive thick limestone units which are commonly formed in shallow shelf environments several important types of convergent margin deposits remain poorly understood acid sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc alkaline porphyry copper deposits are closely associated with calc alkaline porphyry copper deposits commonly in the same volcanic arc although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution in addition to tectonic and petrologic factors metallogenic patterns probably reflect global forcing mechanisms the most important of which are global seawater anoxia and plate reorganizations global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs

the deep layers of a paleozoic arc geochemistry of the copley balaklala series northern california abstract ree zr nb concentrations and sr nd isotope compositions have been measured in copley basalts and andesites balaklala rhyolites and mule mountain trondhjemites northern california which represent the deep layers of a well preserved intra oceanic island arc of siluro devonian age 87sr 86sr is shifted towards high values up to 0 707 whereas ce is preferentially removed from rhyolites a large proportion of the analyzed samples including some acidic rocks shows a pronounced depletion in light ree the end t values of most copley balaklala and mule mountain rocks fall in the range 6 to 8 which suggests that they originated from a normal morb type source end t 9 contaminated with either sediments or an oib type component in modern island arcs only the shallow levels are accessible comparison with the copley balaklala mule mountain series suggests that at depth an immature island arc is likely to comprise thick layers of lile depleted tholeiites and rhyolites intensely altered by pervasive circulation of seawater least square solutions of trace element models suggest that rhyolites and trondhjemites represent remelting of mafic volcanics from the arc basement rather than residual melts of basalt andesite differentiation

the guerrero suspect terrane western mexico and coeval arc terranes the greater antilles and the western cordillera of colombia a late mesozoic intra oceanic arc accreted to cratonal america during the cretaceous the guerrero suspect terrane composed of late jurassic early cretaceous sequences extends from baja california to acapulco and is considered to be coeval with the late mesozoic igneous and sedimentary arc sequences of the greater antilles the west indies venezuela and the western cordillera of colombia these sequences represent the remnants of an arc which accreted to the north american and northern south american cratons at the end of the cretaceous in western mexico the arc sequences built on continental crust consist of high k calc alkaline basalts andesites and rhyolites enriched in lree with abundant siliceous pyroclastic rocks interbedded either with aptian albian reefal limestones or red beds they do not show magmatic changes during the arc development in contrast the arc sequences built on oceanic crust show an evolution with time arc activity began with the development of depleted low k tholeiitic mafic suite guanajuato igneous sequence followed first by mature tholeiitic basalts and then by calc alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of early cretaceous age at the end of the arc growth during aptian albian times calc alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc the tholeiitic and shoshonitic mafic rocks as well as the calc alkaline lavas are mildly enriched in lree y and nb and show high nd ratios typical of oceanic arcs in contrast the calc alkaline mafic suite enriched in lree y and nb exhibits lower nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by paleozoic subducted sediments or old source enrichments oib the cretaceous arc rocks of the greater antilles interbedded with and or capped by aptian albian limestones the cretaceous andesites of northern colombia the cretaceous tholeiitic and calc alkaline volcanic rocks of venezuela and the cretaceous volcano plutonic arc assemblage of tobago share a similar magmatic evolution with the western mexican oceanic arc the tholeiitic plutono volcanic assemblage of tobago depleted in lree and characterized by high nd values is similar to the guanajuato volcano plutonic sequence of mexico considered to represent the pristine stage of the arc the mature tholeiitic sequences exposed in the proto caribbean arc show flat to moderately enriched lree patterns like those of the guerrero terrane however felsic plutonic and volcanic rocks prevail in the caribbean calc alkaline suites accompanied locally by shoshonitic lavas characterize the end of arc magmatic activity in both places thus the geochemical features of the late jurassic cretaceous arc series of the guerrero terrane and the proto caribbean are consistent with the following plate tectonic model the guerrero terrane and the proto caribbean probably belonged to the same intra paleo pacific arc system the development of which was related to the subduction of oceanic basins fringing the north and northern south american cratons this subduction zone was wsw dipping while subduction was going on these magmatic arcs drifted moved closer to the north and south american cratons and finally collided with the american borderlands at different periods during the cretaceous the late mesozoic guerrero and proto caribbean arc sequences show striking similarities with the miocene calc alkaline lavas dredged from the banda ridges the north marianas seamount province and the halmahera and philippine arcs we suggest that the diverse but mostly submarine segments of this late mesozoic intra pacific arc rimmed the north and south american cratons as much as these tertiary arcs rim southeast asia

the influence of crustal structure on compositions of subduction related magmas abstract a survey of sr isotopic ratios and other compositional features of subduction related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust i e thickness composition and age these observations lead to the conclusion that crust and or mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas typically basalt or basaltic andesite it is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks the extent to which such processes manifest themselves will depend upon details of the local crustal structure in particular the composition and age of the crust will strongly influence such parameters as sr nd and pb isotopic compositions such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust e g central andes depending upon the level of magma stagnation and evolution within the crust different trends in isotopic composition may result these isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas and locally they may reflect subduction of continental sediments interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust cascades e eleutians and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes consequently most island arc magmas and especially those more evolved than basalt are probably not primary in the sense that they do not represent direct melts of the upper mantle studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered it appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle derived magmatic components are more mafic in composition than average continental crust

a lithologic tectonic framework for the metallogenic provinces of california the lithologic tectonic framework of california developed principally during mesozoic time when various terranes of oceanic crust and island arc crust were accreted to older sialic crust resulting in westward growth of the continent emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the mesozoic period the discrete tectonostratigraphic terranes that resulted from these events and subsequent tertiary and quaternary volcanic events are characterized by specific types of metallic mineral deposits or in some terranes by the virtual absence of deposits lead silver zinc replacement type deposits are common in the paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island arc terranes the vast majority of contact metasomatic tungsten deposits including all the large ones are in pendants of miogeoclinal rocks in the sierra nevada batholith but the important atolia deposits reside in granitoid rocks that invade oceanic terrane molybdenum distribution closely follows that of tungsten all the large contact metasomatic iron deposits in california are in craton and miogeoclinal terranes but sparse small deposits of this type also occur in island arc terranes of the northern sierra nevada and eastern klamath mountains lode gold deposits although widely scattered show a marked preference for oceanic and island arc terranes that have been invaded by granitoid plutons all the major deposits including late tertiary bonanza deposits such as bodie are in such terranes it appears that magmatic processes were responsible for mobilizing and transporting the gold but the metal was perhaps derived from the eugeosynclinal rocks notably the mafic volcanics most mercury deposits are found in the coast ranges where they commonly occur in silica carbonate rock an alteration product of serpentinite the deposits appear to be spatially related to the coast range thrust and the source of the mercury may have been sedimentary rocks of the underlying franciscan assemblage epigenetic mineralization occurred at several different times during the mesozoic and again during miocene and pliocene time the timing of mineralization events and the distribution of various deposit types indicate that no broad scale zoning of epigenetic deposits exists around the sierra nevada batholith syngenetic deposits are represented mainly by massive sulfides chert associated manganese and chromite the massive sulfide deposits with one exception are restricted to island arc terranes and nearly all of these deposits are in silicic volcanic rocks they are interpreted to be syngenetic with the enclosing rocks although some redistribution of metals may have occurred after the original deposition the deposits occur in volcanic sequences of at least five different ages ranging from early devonian to late jurassic or early cretaceous and along with their enclosing rocks were probably formed at some distance from their present sites chert associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments chromite is confined to ultramafic rock much of which occupies suture zones separating various accreted terranes

a jurassic cretaceous island arc in the ladakh himalayas abstract the ladakh mesozoic ophiolite belt western himalaya contains a pile of volcanic thrust sheets dras unit which differ significantly in structure and composition from the ophiolitic melange zones the dras unit is composed of pillow lavas doleritic sills very irregular basaltic basaltic andesites and dacitic flows intercalated with pyroclastics volcanoclastic sediments and radiolarian cherts according to fossil evidence this volcanism must have been active between upper jurassic and upper cretaceous the presence of relict primary minerals such as magnesiochromite clinopyroxene hastingsitic hornblende and ti magnetite as well as distinctive bulk chemistries suggests that the volcanics belong to island arc tholeiite and to calc alkaline rock series typical of present island arcs in the caribbean and pacific model calculations incorporating probed phenocryst phases indicate that in addition to olivine clinopyroxene and plagioclase amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt the latter process must have taken place at about 1000 c and at moderate depth of 5 15 km within or underneath the island arc today hornblende bearing mafic cumulates appear in the vicinity of kargil within and close to the dras volcanics in a sr evolution diagram the dras volcanics have yielded a pseudo isochron with a low initial 87 sr 86 sr ratio of 0 7035 0 0003 which is in the same range as the mean of modern island arc volcanics however a geologically unrealistic age of 263 m y is obtained from the slope of this isochron the upper mantle is regarded as the source material for the island arc tholeiitic magmas enrichment in k ba sr and lree supports the involvement of components derived from dehydration or incipient melting of subducted tethyan oceanic crust in the mantle

the nicola group late triassic and early jurassic subduction related volcanism in british columbia mafic lavas of the nicola group are divided into three distinct petrographic and geochemical types type 1 lavas are strongly augite porphyritic picrites basalts and andesites that belong to a high potassium to shoshonitic rock series type 2 lavas are augite and plagioclase porphyritic basalts and andesites that belong to a low potassium calc alkaline series and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites low concentrations of ti zr y and nb and moderate to high concentrations of k rb ba and sr in type 1 and 2 lavas clearly indicate a subduction related tectonic setting of eruption type 3 lavas show chemical affinities intermediate between modern day island arc and intraplate volcanics type 1 shoshonitic lavas generally lie east of and are younger than type 2 calc alkaline lavas a relationship that implies an east dipping early mesozoic subduction zone beneath the nicola arc these interpretations resolve previous uncertainties regarding th

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

an island arc origin for the canyon mountain ophiolite complex eastern oregon u s a abstract the canyon mountain ophiolite oregon is exceptional in lacking sheeted dikes basaltic pillow lavas and sediments that are characteristic of many other ophiolites instead the uppermost portion of the complex consists of a significant volume of plagiogranites which in addition to minor basalts intrude a large section of keratophyres believed to be of volcanic origin the trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite it is suggested that the plagiogranites basalts and keratophyres comprise a sill complex both the plagiogranites and the keratophyres are similar respectively to low k2o plutonic and extrusive rocks of island arcs the mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation related to magmatism and diapirism above a benioff zone radiometric age dates of plagiogranites confine the minimum age of the complex to the early permian the canyon mountain ophiolite may thus be correlative with other fragments of a lower permian arc terrane throughout northeastern oregon which were chaotically mixed during renewed subduction in middle to late triassic time

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

endeavour 26 north a porphyry copper gold deposit in the late ordovician shoshonitic goonumbla volcanic complex new south wales australia endeavour 26 north is the largest of 12 pipelike porphyry cu au deposits found near parkes in central west new south wales within the late ordovician goonumbla volcanic complex the dominant volcanic rocks of the complex the goonumbla volcanics are latitic flows and volcanic breccias which are overlain by the trachytic wombin volcanics the volcanic rocks are characterized by high k 2 o and high k 2 o na 2 o ratios which are typical of the shoshonite association as well as an enrichment in large ion lithophile elements and a low concentration of high field strength elements the nelungaloo volcanics form an early ordovician nonshoshonitic volcanic substrate to the complex which initially developed in a submarine and subsequently subaerial setting the 87 sr 66 sr epsilon nd and pb isotope data indicate the magmas were derived from mantle sources the shoshonitic magmas may be products of high pressure fractionation in the subcontinental lithosphere the porphyry deposits are associated with a gravity low which is taken to reflect a subvolcanic chamber beneath the center of the complex all deposits are located within a pronounced circular feature in the aeromagnetic data that is interpreted as the margin of a collapsed caldera dioritic to monzonitic stocks which in part define the caldera rim represent a resurgent phase of magmatism the endeavour 31 stock which was part of this phase is ringed by six porphyry cu au deposits including endeavour 26 north the deposits are centered on fingerlike quartz monzonite porphyries which cut the endeavour 31 stock at depth geochemical anomalies of cu and zn in the bedrock define the size of the alteration system associated with the stock in part the distribution of the cu au deposits is controlled by the endeavour lineament in comparison to most other volcanic and intrusive rocks of the goonumbla volcanic complex the endeavour

late triassic arc related potassic igneous rocks in the north american cordillera igneous rocks of late triassic age are widespread in the cordillera of western north america and except in wrangellia consist of subduction related plutonic and volcanic suites many of these including those in the stikinia quesnellia rattlesnake creek and jackson terrenes and in southern california are clinopyroxene rich and belong to high potassium and shoshonitic rock series features that are generally absent from older and younger igneous rocks in the same terranes the late triassic subduction related rocks are exposed in two discontinuous belts that lie east and west of the cache creek terrane in canada and correlative melange terranes farther south stratigraphic and structural data suggest that these belts were spatially separate magmatic arcs in late triassic time tectonic implications of this analysis include an explanation of middle jurassic cordilleran deformation as the result of collision of the western with the eastern belt absence of late triassic links between stikinia and quesnellia disassociation of stikinia with terranes in northwestern nevada and tentative correlation of the wallowa seven devils terrane with stikinia rather than wrangellia

metallogenic episodes of the tasman fold belt system eastern australia precise dating of alteration minerals and host rocks to au and base metal deposits in the tasman fold belt system eastern australia has established a number of distinct metallogenic epochs which correspond to episodes of magmatism or to deformation events base and precious metal mineralization was emplaced in the mount read volcanics tasmania at about 500 ma and other hydrothermal or deformational events may have occurred in the hellyer deposit between approximately 480 and 440 ma copper and au mineralization was emplaced in the lachlan fold belt new south wales at about 440 ma and appears to be related to the end stages of ordovician magmatism that spans a period from approximately 480 through to 440 ma gold deposits or base metals which formed in new south wales and tasmania at about 420 to 380 ma are essentially coeval with deformation and or are broadly related to siluro devonian magmatism that largely occurred from about 420 to 390 ma gold mineralization in the anakie inlier northeast drummond basin north queensland was essentially coeval with the emplacement of the host drummond sequence at about 350 ma

origin of rare earth element enriched hematite breccias at the olympic dam cu u au ag deposit roxby downs south australia hematite breccias the host rocks to cu u au ag ore at the olympic dam deposit occur as steeply dipping northwest striking dikelike bodies within fractured granite the breccia complex has a strike length of over 5 km and extends to depths greater than 1 km both the deposit as a whole and the individual breccia bodies are zoned from weakly brecciated sericitized and hematized granite on the margins through heterolithic breccias to hematite quartz microbreccia at the center hematite occurs as euhedral laths and fine grained aggregates in the matrices of all breccia types in veins and fragments as pseudomorphs after feldspar and rarely as pseudomorphs after siderite and magnetite relict magnetite is rare and most iron oxide was deposited as hematite heterolithic breccias include fragments of earlier breccia and veins minor sedimentary rocks including bedded hematite and laminated barite also are present as clast components in the upper portion of the deposit volcaniclastic conglomerate and siltstone occur in a fault bounded block approximately 500 x 500 m in plan view copper iron sulfides occur as interstitial grains most common microveinlets and rare clasts they are most abundant in heterolithic breccias and are zoned laterally from chalcocitebornitc to chalcopyrite pyrite zonal boundaries are subparallel to steep breccia contacts copper iron sulfides commonly are intergrown with fluorite and locally rim corroded quartz grains the breccias are highly enriched in light lree and heavy rare earth elements hree avg approximately 5 000 ppm ree in hematite rich rocks total lree content and la lu ratios are correlated with hematite abundance maximum la values 10 000 x chondrite occur in hematite quartz microbreccias from the geographic center of the deposit five hydrothermal ree phases have been identified bastnaesite florencite monazite xenotime and britholite they occur in all breccia types and in a variety of habits disseminated in quartz sericite matrices of granite breccia in quartz sericite veins interstitial to hematite laths intergrown with barite in laminated sedimentary rocks and as inclusions in hematite and in sulfide grains the hematite breccias formed by progressive hydrothermal brecciation and iron metasomatism of the granite host minor sedimentary components preserved as blocks and fragments within the upper portion of the deposit point to a near surface environment for the breccia complex textural relations indicate that most of the copper was introduced late relative to hematization and brecciation based on spatial relations between barren zones oxidized sulfide grains and high grade chalcocite ore some supergene redistribution of copper probably occurred the abundance of ree in hydrothermal phases ree enrichment of altered relative to unaltered wall rock concentration of ree in the center of the system and variable slopes of chondrite normalized patterns combined with the lack of evidence of intrusion of unusual magmas at the present level of exposure attest to extensive transport and deposition of ree by hydrothermal fluids responsible for hematization and breccia formation this study demonstrates hydrothermal ree mobility on a scale previously undocumented and suggests that ree may be particularly mobile in f rich hydrothermal systems

subduction arc reversal and the origin of porphyry copper gold deposits in island arcs the tertiary quaternary porphyry copper gold deposits of the southwest pacific rim particularly in luzon mainland papua new guinea bougainville guadalcanal and fiji mostly formed after a reversal of arc polarity where this reversal has not occurred e g new zealand and japan porphyry copper gold deposits are absent or scarce hence older porphyry copper gold deposits may be useful clues to tectonic history enrichment of magmas in gold may have resulted from two stage melting the tectonic setting of porphyry copper gold deposits differs from that inferred for volcanic hosted massive sulfide deposits

analysis of supergene ore forming processes and ground water solute transport using mass balance principles analysis of metal distribution in weathering profiles developed above ore deposits provides a simple and direct method to determine the geologic controls on surficial leaching and enrichment of metals by supergene descending fluids this approach offers an understanding of the interrelationships of metal concentration zone thicknesses and bulk rock density of the various zones within a weathered rock column analytical solutions to mass balance equations provide expressions describing the chemical evolution paths of enrichment blanket metal grade b in terms of original protore metal grade p leached zone metal grade 1 and the ratio of total leached zone column height to blanket thickness l o t b neglecting density terms for the sake of simplicity this function is expressed as b p l o t b p 1 linear data arrays in the coordinate system b versus p 1 for groups of drill holes in individual geologic domains e g wall rock types strongly suggest that enrichment blanket metal grades evolve from a protore metal grade p with a fairly constant slope of l o t b to a final value b determined by the extent of metal leaching in the zone of oxidation the constancy of the ratio l o t b within geologic domains indicates a simultaneous deepening of the zone of oxidation and thickening of the enrichment blanket under conditions of a descending ground water table analysis of metal mass balance in drilling profiles also provides a means of reconstructing paleosurficial topography at the time of active oxidation even in highly eroded terrains application of these methods using rock density and assay data from the butte district of montana shows the paleo oxidation surface to have been eroded by as much as 450 ft down to the present unconformity between weathered bedrock and overlying tertiary gravels geomorphic stability and slow erosion rates often thought necessary for effective chemical weathering are therefore questioned we propose instead a general balance of rates of erosion and descent of the ground water table for optimal enrichment preservation of the fossil weathering profiles occurs by submergence of the zone of oxidation below the paleoground water table resulting in cessation of supergene oxidation and preservation of the blanket the mass balance methods derived in this study also relate overall leaching efficiency to protore characteristics showing a strong hypogene alteration control at butte as described by mcclave 1973 lateral copper transport is evaluated by identifying local anomalies in the computed oxidation surface zones of copper introduction near faults and zones of high permeability appear as apparent topographic highs whereas zones of depletion appear as topographic depressions preliminary analysis of this type at the porphyry copper deposit at la escondida chile indicates deeper oxidation and leaching effects than at butte the assumption of vertically homogeneous protore copper grades leads to the conclusion that erosion of at least 200 m of leached capping has occurred throughout much of the la escondida district geologic domains defined by computed values of l o t b correlate well with hypogene alteration zoning computation of the magnitude and direction of overall subsurface lateral metal fluxes indicates transport distances of at least 1 km in directions consistent with present surface topography and fracture patterns there is a strong correlation of the thickest and highest grade enrichment zones with inferred positive lateral fluxes whereas the most thoroughly leached cap rocks make up zones of inferred negative lateral flux which are likely source regions alternative explanations to the lateral flux hypothesis are evaluated including assessment of variations in protore metal grade projections both in vertical and horizontal directions despite the approximations involved in the present calculations the lateral flux pattern appears to be the most likely interpretation of the present metal distribution in the supergene system at la escondida

late miocene to present exhumation in the ligurian alps southwest alps with evidence for accelerated denudation during the messinian salinity crisis detrital apatite u th he and fission track thermochronometry are applied to constrain the exhumation history of the ligurian alps southwest alps exhumation of the ligurian alps is recorded from ca 11 ma and continues until present more than 4 km of exhumation is recorded at an average rate of 0 4 0 2 mm yr exhumation is accommodated along a north dipping thrust at the foot of the ligurian margin combined u th he and fission track thermal modeling show a phase of accelerated denudation between 7 and 5 ma associated with the base level drop during the messinian salinity crisis

paleozoic and lower mesozoic magmas from the eastern klamath mountains north california and the geodynamic evolution of northwestern america abstract the paleozoic to early mesozoic geology of the eastern klamath mountains n california is characterized by three major magmatic events of ordovician late ordovician to early devonian and permo triassic ages the ordovician event is represented by a calc alkalic island arc sequence lovers leap butte sequence developed in the vicinity of a continental margin the late ordovician to early devonian event consists of the 430 480 ma old trinity ophiolite formed during the early development of a marginal basin and a series of low k tholeiitic volcanic suites lovers leap basalt keratophyre unit copley and balaklala formations belonging to intraoceanic island arcs finally the permo triassic event gave rise to three successives phases of volcanic activity nosoni dekkas and bully hill represented by the highly differentiated basalt to rhyolite low k tholeiitic series of mature island arcs the permo triassic sediments are indicative of shallow to moderate depth in an open warm sea the geodynamic evolution of the eastern klamath mountains during paleozoic to early mesozoic times is therefore constrained by the geological petrological and geochemical features of its island arcs and related marginal basin a consistent plate tectonic model is proposed for the area consisting of six main stages 1 1 development during ordovician times of a calc alkalic island arc in the vicinity of a continental margin 2 2 extrusion during late ordovician to silurian times of a primitive basalt andesite intraoceanic island arc suite which terminated with boninites the latter suggest rifting in the fore arc followed by the breakup of the arc 3 3 opening and development of the trinity back arc basin around 430 480 ma ago 4 4 eruption of the balaklala rhyolite either in the arc or in the fore arc ending in early devonian time with intrusion of the 400 ma mule mountain stock 5 5 break in volcanic activity from the early devonian to the early permian and 6 6 development of a mature island arc from the early permian to the late triassic the eastern klamath mountains island arc formations and ophiolitic suite are part of the cordilleran suspect terranes considered to be gondwana margin fragments that have undergone large northward translations before final collision with the north american craton during late mesozoic or cenozoic times these eastern klamath mountains island arcs could be associated with the paleo pacific oceanic plate that led to accretion of these allochthonous terranes to the american margin

partitioning of gold in young calc alkalic volcanic rocks from guatemala gold analyses of whole rock mineral and groundmass of seven calc alkalic volcanic rocks basalt to rhyolite show that gold is less abundant in the dominant phenocrystic phases than in the whole rock groundmass concentrations are also lower than whole rock crystal liquid partition coefficients are 1 for plagioclase and hornblende and about 1 for olivine pyroxene magnetite and biotite if only the principal phases are considered the bulk gold partition coefficient is typically 1 yet gold seems to be depleted rather than enriched in many evolved magmas a significant proportion of gold in these rocks is in neither the groundmass nor the dominant phenocrysts but instead is contained in tiny sulflde blebs that are found in these rocks during crystallization the continual separation of a sulflde phase that concentrates gold is consistent with compatible gold behavior and petrography further data is needed to evaluate this hypothesis

is the lower duarte igneous complex hispaniola a remnant of the caribbean plume generated oceanic plateau eleven samples of metapicrites metaankaramites diabases and cumulates of the lower duarte complex in central hispaniola were analyzed for major trace element and nd sr isotopic compositions t

the tectonic significance of ordovician magmatism in the eastern lachlan fold belt abstract the shoshonitic rock series is a suite of igneous rocks derived from the mantle and containing high potassium plus low abundances of high field strength elements it is commonly associated with calcalkaline igneous rocks but in most modern examples a contemporary active subduction zone is not present mantle derived shoshonitic magmatism in the lachlan fold belt commenced in the early ordovician and terminated at the end of the ordovician when it was replaced by tholeiitic magmatism and the onset of wholesale melting of the lower to middle crust associated calcalkaline igneous rocks are insignificant the shoshonites are widely distributed across the eastern part of the fold belt in a series of discontinuous belts and volcanic centres the largest of which is the orange wellington belt extending from east of cowra to dubbo the broad geographical distribution is not typical of the curvilinear belts of volcanics associated with subduction zones and would be even greater if the intervening basins or troughs were unfolded all rocks analyzed to date are either clearly shoshonitic or show chemical features that indicate shoshonitic affinities prior to alteration and metamorphism in some areas lavas rich in olivine and clinopyroxene potassic trachybasalts dominate in others the more evolved plagioclase rich shoshonites are most abundant it is suggested that the ordovician magmatism was not accompanied by coeval subduction as has been generally assumed more likely the lithospheric mantle beneath the lachlan fold belt was modified during an earlier subduction event associated with the assembling of the basement blocks recognized from granite studies the cause of melting in the ordovician may be related to lithospheric heating or overturning prior to the upwelling of unmodified asthenosphere the hot 1200 c asthenosphere was the source for the silurian tholeiites and provided conductive heat for the melting of the lower to middle crust

nd sr isotope systematics of nicola group volcanic rocks quesnel terrane volcanic rocks of the nicola group belong to an arc built on the western margin of the quesnel terrane in the late triassic to early jurassic low k high k compositional types define a rb sr isochron of 222 15 ma with initial 87sr 86sr 0 70367 2 the corresponding nd isotopic compositions of these samples end 222 ma 5 1 to 7 8 fall within the range for early mesozoic island arcs a comparable range of end 222 ma 5 0 to 7 9 in picrite shoshonite samples precludes generation of increasingly potassic magmas by progressive metasomatism of the mantle wedge alone source region heterogeneity possibly imparted by changes in the composition of subducted slab components or interaction with amphibole or phlogopite in the source remnant of an earlier permian arc on the quesnel terrane is required to account for geochemical differences between these rock suites crustal contamination is severely limited from the high end values such that continental basement now underlying the quesnel terrane

major episodes of geologic change correlations time structure and possible causes abstract published data sets of major geologic events of the past 250myr extinction events sea level lows continental flood basalt eruptions mountain building events abrupt changes in sea floor spreading ocean anoxic and blackshale events and the largest evaporite deposits have been synthesized with estimated errors these events show evidence for a statistically significant periodic component with an underlying periodicity formally equal to 26 6 myr and a recent maximum close to the present time the cycle may not be strictly periodic but a periodicity of 30myr is robust to probable errors in dating of the geologic events the intervals of geologic change seem to involve jumps in sea floor spreading associated with episodic continental rifting volcanism enhanced orogeny global sea level changes and fluctuations in climate the period may represent a purely internal earth pulsation but evidence of planetesimal impacts at several extinction boundaries and a possible underlying cycle of 28 36 myr in crater ages suggests that highly energetic impacts may be affecting global tectonics a cyclic increase in the flux of planetesimals might result from the passage of the solar system through the central plane of the milky way galaxy an event with a periodicity and mean phasing similar to that detected in the geologic changes

petrology and geochemistry of plagiogranite in the canyon mountain ophiolite oregon plagiogranites in the canyon mountain ophiolite oregon include a wide range of rock types ranging from diorite to trondhjemite the plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates the plagiogranites are typically low in k2o and high in na2o and are enriched 10 20 times chondrites in ree and overlap with abundances in basic rocks from canyon mountain all samples of plagiogranite are relatively depleted in lree with more silicic samples characterized by a slightly lesser degree of lree depletion total ree content is not consistently correlated with contents of major and other trace elements

origin and ore forming consequences of the advanced argillic alteration process in hypogene environments by magmatic gas contamination of meteoric fluids metasomatic effects have been thermodynamically computed for magma derived sulfur gases which may contaminate convecting meteoric fluids in the monzonite k silicate protore and are compared with natural assemblages in the butte district of montana the advanced argillic assemblage with covellite is produced in hypogene environments at 300 degrees c by sulfur metasomatism of protore only for a limited range of net sulfur valences essentially 1 to 2 placing limiting constraints on the possible ratios of so 2 and h 2 s gases a second important factor is the relative reaction rates of protore and contaminating sulfur gases contamination by sulfur gases is a third factor of importance a final factor is that a minimum negative net volume of reaction causes by wall rock dissolution and fracture enlargement the high permeability necessary for circulation of enormous quantities of hydrothermal fluids and vein growth characteristic of this extreme alteration facies modified journal abstract

porphyry deposits of the northwestern cordillera of north america edited by t g schroeter special volume 46 canadian institute of mining metallurgy and petroleum montreal 1995 888 pp canadian 95 plus 15 handling isbn 0 919086 56 x

gold deposition by sulfidation of ferrous fe in the lacustrine sediments of the pueblo viejo district dominican republic the effect of fe c s diagenesis on later hydrothermal mineralization in a maar diatreme complex abstract the pueblo viejo district located in the cordillera central of the dominican republic contains large au ag deposits associated with acid sulfate alteration within spilites conglomerates and carbonaceous sedimentary rocks that were deposited in a maar diatreme complex much of the au mineralization occurs in pyritic carbonaceous siltstones of the pueblo viejo maar diatreme member of the cretaceous los ranchos formation pyrite is the only fe bearing phase in mineralized rock whereas siderite is the dominant fe bearing phase in siltstones distal to mineralization disseminated pyrite occurs as framboids cubes pyritohedra concretions and cement early framboids occur throughout the district au occurs as inclusions in later non framboid disseminated pyrite nfdp an occurrence that is interpreted to be indicative of contemporaneous deposition pyrite framboids exhibit a wide range of 34s cdt values 17 5 to 4 8 and are interpreted to have formed during biogenic reduction of pore water sulfate the nfdp yield restricted 34s cdt values x 5 2 s 2 4 n 43 similar to those obtained from later vein pyrite x 6 4 s 1 5 n 12 alunite and barite have 34s values ranging from 18 8 to 21 6 the interpretation that the nfdp vein pyrite alunite and barite and possibly even the framboidal pyrite share a common source of igneous sulfur is supported by the 34s data siderite occurs as concretions and cement contains abundant mg fe0 75mg0 19mn0 03ca0 02co3 and has 13c pdb and 18o smow values ranging from 2 5 to 1 1 and 14 6 to 19 5 respectively these data are consistent with the interpretation that the siderite formed in lacustrine sediments and that the carbonate in the siderite is probably methanogenic although contributions from oxidation of organic matter during biogenic sulfate reduction thermal decarboxylation of organic matter or magmatic vapor cannot be ruled out disseminated au mineralization in the sedimentary rocks formed when a hydrothermal fluid encountered reactive fe2 in diagenetic siderite the ensuing pyrite deposition consumed h2s and destabilized the au hs 2 complex leading to precipitation of au the capacity of the sedimentary rocks to consume h2s and precipitate au was controlled by the amount of non pyrite fe present as siderite the abundance of siderite was controlled by the extent of pyrite formation during diagenesis

evaporitic source model for igneous related fe oxide ree cu au u mineralization we propose that many igneous related fe oxide rich ree cu au u bearing deposits form by hydrothermal processes involving evaporitic ligand sources either coeval salars or older evaporites these deposits are abundant in both phanerozoic and proterozoic extensional continental and continent margin settings they commonly form in global arid zones but they also occur where magmatism is superimposed upon older evaporites magmatic compositions exert only second order control mainly on alteration mineralogy and on element abundances hot s poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments siderophile lithophile and hydrothermal alteration sodic locally alkaline found in these systems this model contrasts with immiscible oxide melt and magmatic hydrothermal origins commonly proposed for these deposits although all three mechanisms can occur 31 refs 3 figs 1 tab

shoshonitic volcanism in the northern mariana arc 1 mineralogic and major and trace element characteristics petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern mariana and southern volcano arcs define two distinct rock series the central and northern mariana arc to 23 n and the northern volcano arc north of iwo jima are characterized by plagioclase clinopyroxene orthopyroxene titantomagnetite bearing subalkaline rocks including both low k and medium k series the northern mariana arc and southern volcano arc from 23 n to iwo jima are erupting rocks of a shoshonitic series with phenocrysts of plagioclase clinopyroxene olivine and biotite these rocks are less saturated than those of the subalkaline provinces and are substantially enriched in ba 400 900 ppm sr 600 1000 ppm k2o 1 4 5 and k2o na2o 0 4 1 2 relative to the subalkaline lavas ba y and ba zr increase by a factor of 3 to 4 in the shoshonitic rocks but k rb ba sr and k ba are relatively constant throughout the arc there is no relationship between degree of enrichment and volcano volume or degree of fractionation seamounts and islands within each province have the same range of compositions much of the intra edifice variation in lava composition can be modeled by 10 70 crystallization of plagioclase and clinopyroxene with smaller amounts of olivine orthopyroxene and titanomagnetite or by accumulation of 10 40 phenocrysts dominantly plagioclase the differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources the occurrence of this enriched mantle in the northern marianas may be a consequence of the propagation of the mariana trough spreading center into the volcano arc

u pb and k ar dates related to the timing of magmatism and deformation in the cache creek terrane and quesnellia southern british columbia u pb dating of zircon from the guichon creek batholith indicates an emplacement age of 210 3 ma comparison with previously published k ar 211 188 ma and rb sr 205 and 196 ma dates reveals that intrusion mineralization cooling and uplift of the batholith took some 20 ma spanning the triassic jurassic boundary on the decade of north american geology dnag time scale the mount martley pluton and tiffin creek stock yield late jurassic dates of 155 2 ma u pb zircon and 152 5 ma k ar hornblende respectively and provide a reliable minimum age kimmeridgian for penetrative deformation in the cache creek terrane k ar whole rock dates from cache creek terrane and ashcroft formation argillites range from early permian 266 8 ma and early jurassic 194 6 ma to late jurassic kimmeridgian 154 5 ma we interpret the younger dates as recording middle late jurassic tectonism and the older ones as possible relics from earlier deformation episodes an early cretaceous k ar date 129 5 ma for a lamprophyre dike that cuts the nicola group suggests that the early cretaceous magmatic arc of the coast plutonic complex had an eastern alkalic fringe in the intermontane belt

early devonian volcanism in the eastern klamath mountains california evidence for an immature island arc the early devonian volcanic sequence of the eastern klamath mountains northern california consists of the copley greenstone a basalt andesite series overlain by the balaklala rhyolite all the rocks are metamorphosed to the greenschist facies the copley greenstone consists of massive flows and pyroclastic deposits in the lower part that are overlain by pillow lavas no sediments are interlayered in the volcanic pile except in the easternmost outcrop area where sandstones with granitic debris and shaly tuffs are interbedded in the uppermost flows high mg andesites occur only near the top of the basic volcanic succession the balaklala rhyolite is formed of massive flows breccias and tuffs the copley volcanic rocks poor in titanium belong to a low k tholeiitic suite formed in an island arc geodynamic environment the occurrence of olivine with chromium spinels in almost all the lava types the enrichment in magnesium chromium and nickel and the depletion in hygromagmaphile elements suggest that

early magnetite amphibole plagioclase alteration mineralization in the island copper porphyry copper gold molybdenum deposit british columbia the initial hydrothermal event in the evolution of the ca 377 mt 0 41 percent cu island copper porphyry cu au mo deposit northern vancouver island was the formation of an extensive alteration mineralization facies dominated by magnetite calcic amphibole and intermediate to sodic plagioclase early stage quasipervasive magnetite rich alteration and associated magnetite rich veinlets in this middle jurassic island arc hosted hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata the magnetite rich alteration zone 500 to 700 m wide and originally extending over at least 450 m vertically exhibits an outward zonation from quartz magnetite albite an sub or 6 amphibole trace apatite trace through quartz magnetite amphibole albite an sub 6 10 or oligoclase andesine an sub 15 36 apatite trace or scapolite trace to amphibole magnetite oligoclase or sodic andesinc an sub 15 39 or quartz or apatite trace assemblages definition of early stage mineral assemblages impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration was facilitated by the use of novel petrographic techniques including incident light nomarski differential interference contrast microscopy early stage mineralization comprises several well defined vein veinlet types which were demonstrably emplaced prior to the main stage chalcopyrite pyrite stockwork and associated k silicate alteration minor chalco pyrite and pyrite occur in some early stage veins and magnetite is a constituent of many later main stage sulfide rich veins but the distinction between these two major vein systems is clear mass exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks with an fe enrichment approaching 450 percent 20 g 100 cc in the dacite dike and lesser and more variable na enrichment with both ti and al behaving as mobile constituents paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite rich alteration mineralization and imply that this event was initiated above 650 degrees c and at pressures of at least 1 kbar and was generated by fecl 2 rich brines at first single phase and moderately saline avg 15 wt nacl equiv but latterly boiling at ca 560 degrees to 645 degrees c and or ca 0 55 kbars the main stage potassic alteration zone has the configuration of an annulus ca 100 to 150 m in width which was entirely superimposed on the more extensive early alteration mineralization zone at ca 430 degrees to 575 degrees c and below 450 bars gold although correlated overall with cu and potassic alteration was probably extensively introduced in the early stage it is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids in which so super 2 exceeded both h 2 s and so super 2 4 the deposition of abundant magnetite and the widespread and intense fe metasomatism which define the early stage record the high solubility of fe as fecl 2 in high temperature brines in equilibrium with magnetite bearing quartzofeldspathic rocks as well as the retrograde solubility of magnetite between ca 550 degrees and 750 degrees c whereas the high so 2 h 2 o so 4 ratios of the initial hydrothermal fluids at island copper and in other similar deposits may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating s rich mafic melts the mineral assemblages of the early stage veins and associated quasi pervasive alteration cannot be assigned to the accepted alteration mineralization facies of porphyry systems such as a veins and potassic alteration and record specific and distinct conditions of fluid rock interaction