amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

late jurassic bimodal magmatism in the northern sea floor remnant of the rocas verdes basin southern patagonian andes magmatic and detrital zircon ages from the rocas verdes basin a tectonically juxtaposed remnant of sea floor in the magallanes fold and thrust belt southern patagonia south america indicates that a rifting phase of the rocas verdes basin occurred between 152 and 142 ma and was accompanied by bimodal magmatism a dacite dyke cross cutting pillow basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal sarmiento ophiolite complex contain 150 ma zircon crystals indicating that mafic submarine volcanism had started prior to or during the late jurassic 10 15 ma earlier than previously thought the silicic pyroclastic rocks of the tobifera formation with two samples dated at 148 and 142 ma were heralded by synrift sedimentation along fault bounded grabens within palaeozoic metasediments no evidence for an active volcanic arc during the early formation of the rocas verdes basin was detected in detrital zircon grains of the lower sedimentary member of the tobifera formation a minimum of 25 ma of continuous sedimentation in the rocas verdes basin is suggested by detrital zircon grains in the upper member of the zapata formation the rocas verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution

spatial and temporal characterization of progressive deformation during orogenic growth example from the fuegian andes southern argentina abstract superposed structural fabrics in the easternmost fuegian andes reveal two distinct non coaxial deformation phases across the transition from the orogenic core to the thrust fold belt each phase is characterized by different metamorphic conditions and consistently different orientations which allow the structural correlation between the orogenic core and the internal thrust fold belt the first deformation phase was coeval with regional metamorphism reaching upper greenschist facies and featured simple shear deformation of the basement paleozoic and jurassic and cover cretaceous in the top of the underthrusting south american plate the second phase developed during collision of the orogenic wedge with the rio chico arch a promontory in the underthrusting plate this phase was characterized by thrust sheet emplacement and formation of a crustal duplex with rock uplift and consequent low to very low grade metamorphism buttressing against the rio chico arch is responsible for the change in shortening orientation that distinguishes both phases

amphibole as an archivist of magmatic crystallization conditions problems potential and implications for inferring magma storage prior to the paroxysmal 2010 eruption of mount merapi indonesia amphibole is widely employed to calculate crystallization temperature and pressure although its potential as a geobarometer has always been debated recently ridolfi et al contrib mineral petrol 160 45 66 2010 and ridolfi and renzulli contrib mineral petrol 163 877 895 2012 have presented calibrations for calculating temperature pressure fo2 melt h2o and melt major and minor oxide composition from amphibole with a large compositional range using their calibrations we have i calculated crystallization conditions for amphibole from eleven published experimental studies to examine the problems and the potential of the new calibrations and ii calculated crystallization conditions for amphibole from basaltic andesitic pyroclasts erupted during the paroxysmal 2010 eruption of mount merapi in java indonesia to infer pre eruptive conditions our comparison of experimental and calculated values shows that calculated crystallization temperatures are reasonable estimates calculated fo2 and melt sio2 content yields potentially useful estimates at moderately reduced to moderately oxidized conditions and intermediate to felsic melt compositions however calculated crystallization pressure and melt h2o content are untenable estimates that largely reflect compositional variation in the crystallizing magmas and crystallization temperature and not the calculated parameters amphibole from merapi s pyroclasts yields calculated conditions of 200 800 mpa 900 1 050 c nno 0 3 nno 1 1 3 7 7 2 wt melt h2o and 58 71 wt melt sio2 we interpret the variations in calculated temperature fo2 and melt sio2 content as reasonable estimates but conclude that the large calculated pressure variation for amphibole from merapi and many other arc volcanoes is evidence for thorough mixing of mafic to felsic magmas and not necessarily evidence for crystallization over a large depth range in contrast bimodal pressure estimates obtained for other arc magmas reflect amphibole crystallization from mafic and more evolved magmas respectively and should not necessarily be taken as evidence for crystallization in two reservoirs at variable depth

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

sources of metals in the porgera gold deposit papua new guinea evidence from alteration isotope and noble metal geochemistry the porgera gold deposit is spatially and temporally associated with the late miocene mafic alkalic epizonal porgera intrusive complex pic located in the highlands of papua new guinea png the highlands region marks the site of a tertiary age continent island arc collision zone located on the northeastern edge of the australasian craton the pic was emplaced within continental crust near the lagaip fault zone which represents an oligocene suture between the craton and volcano sedimentary rocks of the sepik terrane magmatism at porgera probably occurred in response to the late miocene elimination of an oceanic microplate and subsequent early pliocene collision between the craton margin and an arc system located on the bismarck sea plate gold mineralization occurred within 1 ma of the time of magmatism metasomatism accompanying early disseminated au mineralization in igneous host rocks resulted in additions of k rb mn s and co2 and depletions of fe mg ca na ba and sr rare earth and high field strength elements remained largely immobile pervasive development of illite k feldspar quartz carbonate alteration assemblages suggests alteration by mildly acidic 200 to 350 c fluids at high water rock ratios strontium and lead isotopic compositions of minerals from early base metal sulphide veins associated with k metasomatism and later quartz roscoelite veins carrying abundant free gold and tellurides are remarkably uniform e g 87sr86sr 0 70745 0 00044 n 10 207pb204pb 15 603 0 004 n 15 these compositions fall between those of unaltered igneous and sedimentary host rocks and specifically sedimentary rocks from the jurassic om formation which underlies the deposit igneous rocks 87sr86sr 0 7035 207pb204pb 15 560 om formation 87sr86sr t 0 7153 207pb204pb 15 636 it is therefore suggested that the hydrothermal fluids acquired their sr and pb isotopic signatures by interaction with or direct derivation from a plutonic root of the pic and host sedimentary rocks of the om formation it is likely that au was also derived from one or both of these two sources concentrations of au in unaltered igneous and sedimentary rocks from porgera 10 ppb au do not indicate that either lithology represents a significantly enriched protore although au and platinum group element pge abundances in the igneous rocks suggest a mild primary magmatic enrichment of au relative to the pge average au pt pd mantlenormalized 14 0 6 5 n 8 evidence that the porgera magmas were rich in volatiles permits speculation that au may have been concentrated in a magmatic fluid phase but alternative possibilities such as derivation of au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

high precision pb isotope measurements reveal magma recharge as a mechanism for ore deposit formation examples from lihir island and conical seamount papua new guinea abstract lihir island part of the tabar lihir tanga feni island chain in the sw pacific hosts the giant ladolam gold deposit polymetallic gold mineralization also was discovered offshore at conical seamount in 1998 during rv sonne cruise so 133 the seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites although some monzonites are found on lihir sr isotopic analyses of mineralized samples from the island and the seamount suggest that most of the sr is derived from the local alkaline lavas elevated 87sr 86sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system part of the sr was contributed either from seawater or the thick sequence of marine sediments underlying the island high precision pb isotopic analyses conducted with mc icp ms show that the ores and volcanic rocks share similar pb isotopic compositions suggesting that the pb in the mineralized zones was ultimately derived from local magmatic sources the pb isotopic data however reveal small but significant differences between the mineralized zones and the associated host lavas mineralized samples from lihir have slightly less radiogenic lead isotopic ratios than their host lavas these lead isotopic compositions are similar however to some of the fresh lavas recovered from conical seamount and to a monzonite intrusion underlying the ladolam deposit lead isotopic ratios in mineralized samples from conical seamount are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby tubaf and edison seamounts petrographic observations and microprobe elemental analyses reveal a complex magmatic history for the magma chamber inferred beneath conical seamount based on zoning patterns in the conical seamount clinopyroxenes it appears that a sub seamount magma chamber was recharged with a mafic magma similar to the most primitive volatile rich and xenolith bearing lavas recovered from tubaf and edison seamounts rapid cooling of this mafic magma accompanied by exsolution of metal bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately led to the formation of the gold mineralization similarity of the lead isotopic ratios between the ores and the mafic magmas in the area indicate that the ore metals were primarily derived from the recharging magma not by hydrothermal leaching of the host lavas suggesting that ore deposit genesis can be related to a single volatile rich magmatic pulse within otherwise barren polyphased volcano magmatic complex

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

experimentally derived high pressure cumulates from hydrous arc magmas and consequences for the seismic velocity structure of lower arc crust 1 this contribution explores the consequences of experimentally derived ultramafic plutonic rocks for estimates of continental crust composition the formation of high pressure cumulates from primitive to evolved hydrous arc magmas was investigated in the range 0 8 1 5 gpa and 1350 800 c to provide constraints on solid assemblages that form the roots of island arc crust at pressures exceeding 0 8 gpa 25km 40 60 of ultramafic cumulates are produced to obtain andesite to dacite compositions that are typical for evolved island arc volcanic and plutonic rocks calculated cumulate densities are comparable or higher than upper mantle values while seismic velocities vp vary continuously between 7 3 and 8 km s resulting in a broad crust mantle transition the moho represents a plagioclase saturation boundary our results are consistent with seismic imaging from continental margins and island arcs revealing that beneath an active volcanic front a sharp geophysical moho signature is sometimes absent

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

trace element composition of amphibole and petrogenesis of hornblendites and plutonic suites of cretaceous magmatic arcs developed in the fuegian andes southernmost south america abstract the evolution of continental crust in convergent margins can be explored in southernmost south america 54 56 s plutonic rocks of the fuegian batholith and the rear arc satellite ushuaia pluton were emplaced within the magmatic arc and the fuegian fold and thrust belt respectively they record subduction zone processes in two distinct tectonic settings during the evolution of the rocas verdes basin we report new u pb zircon geochronology bulk rock chemistry sr nd isotope data and epma and in situ la icp ms analyses of amphibole from hornblendites and gabbroic granitoid suites in order to evaluate the origin and evolution of the magmatic plumbing systems in the upper plate of the subduction zone textural relationships and amphibole compositions in hornblendite indicate crystallization at lower crustal depths with pressures of 7 8 kbar in the fuegian batholith and of 5 6 kbar in the ushuaia pluton lower cretaceous suites of hornblendite and calc alkaline hornblende gabbro diorite and tonalite in the fuegian batholith have endt values ranging between 2 and 4 they were emplaced within an island arc coeval with mid oceanic type spreading in the rocas verdes back arc basin isotope ratios and amphibole compositions in hornblendite indicate crystallization from primitive and hydrous sub alkaline basaltic melts with relatively low lree hree and low alkali contents the late cretaceous plutons in the fold and thrust belt were emplaced after the tectonic juxtaposition of rocas verdes ophiolitic complexes the ushuaia pluton consisting of clinopyroxene hornblende cumulates hornblende gabbro diorite and monzodiorite was emplaced during the waning stage of late cretaceous magmatism in this case hornblendite amphiboles show high contents of alkalis lree and incompatible elements with a strong crustal affinity th ba rb the enriched incompatible trace element patterns indicate their derivation from k rich transitional magmas formed in supra subduction settings chemical variations in amphibole from hornblendites and spatially related plutonic rocks are evaluated in terms of fluid flux from the subducted slab and partial melting of the sub arc mantle ultimately controlled by the thermal state of the subducted slab and convergence rates

new isotopic ages and the timing of orogenic events in the cordillera darwin southernmost chilean andes abstract the cordillera darwin a structural culmination in the andes of tierra del fuego exposes an orogenic core zone that has undergone polyphase deformation and metamorphism some of the classic problems of orogenic zones have remained unanswered in the cordillera darwin the age of deformed plutonic rocks the distinction of structurally reactivated basement and metamorphosed cover rocks and the timing of orogenic events this study addresses and partially answers these questions a well constrained rb sr isochron age of157 8m y and an initial 87 sr 86 sr ratio of 0 7087 obtained from a pre tectonic granitic suite suggest a genetic relation between this suite and upper jurassic silicic volcanic rocks in the cover sequence tobifera formation and also suggest involvement of continental crust in formation of these magmas a poorly constrained rb sr isochron age of240 40m y obtained from supposed basement schists is consistent with field relations in the area which suggest a late paleozoic early mesozoic metamorphism for these pre late jurassic rocks however because of scatter in the data and the uncertainties involved in dating metasedimentary rocks the significance of the isotopic age is dubious compilation of previously published ages in the area 9 with new mineral ages reported here indicate that early andean orogenic events occurred between 100 and 84 m y ago and that subduction related magmatism has contributed probably discontinuously to the crustal evolution of the region throughout the mesozoic

the patagonian batholith s of tierra del fuego chile timing and tectonic implications reconnaissance field studies on the patagonian batholith exposed s of tierra del fuego together with 49 new k ar and rb sr mineral dates render new insights into two main aspects of the late mesozoic geology the geochronology and composition of the batholith and time relations between the emplacement of the plutons and deformation the envelope of the batholith in this area is mainly composed of late jurassic early cretaceous sedimentary and volcanic rocks formed in an island arc marginal basin system three main plutonic groups have been recognized which have yielded mineral dates interpreted as crystallization ages ranging from 141 to 34 ma the earliest is a gabbroic assemblage including gabbros diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 ma the canal beagle plutonic group is composed of granitoids characterized by a penetrative syn magmatic foliation and with dates of 113 to 81 ma the seno ano nuevo plutonic group is formed of granitoids with dates of 60 to 34 ma and is mainly exposed in the outer islands indicating a local oceanwards shift of plutonism with time lower cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage and in part at least of the canal beagle plutonic group folding of the marginal basin infill which has been related to the closure of the marginal basin occurred from about 100 to 93 ma ago the gabbroic assemblage was apparently mainly pre tectonic whereas the seno ano nuevo plutonic group was clearly intruded after this deformational event a post tectonic or possibly late tectonic emplacement for the youngest plutons of the canal beagle plutonic group and a pre or syn tectonic intrusion for the oldest is inferred

strontium and oxygen isotopic constraints on fluid mixing alteration and mineralization in the tag hydrothermal deposit abstract strontium and oxygen isotopic measurements of samples recovered from the trans atlantic geotraverse tag hydrothermal mound during leg 158 of the ocean drilling program provide important constraints on the nature of fluid rock interactions during basalt alteration and mineralization within an active hydrothermal deposit fresh mid ocean ridge basalt morb with a 87 sr 86 sr of 0 7026 from the basement beneath the tag mound was altered at both low and high temperatures by seawater and altered at high temperature by near end member black smoker fluids pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater 200 c the development of a silicified sulfide mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz pyrite paragonite stockwork cut by quartz pyrite veins initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a mg bearing mixture of upwelling high temperature 300 c black smoker type fluid with a minor 300 c interaction between the wallrock and these mg bearing fluids results in the complete recrystallization of the wallrock to chlorite quartz pyrite the quartz pyrite paragonite assemblage replaces the chloritized basalts and developed by reaction at 250 360 c with end member hydrothermal fluids having 87 sr 86 sr 0 7038 similar to present day vent fluids the uniformity of the 87 sr 86 sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87 sr 86 sr ratio of end member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork precipitation of anhydrite in breccias and as late stage veins throughout most of the mound and stockwork down to at least 125 mbsf records extensive entrainment of seawater into the hydrothermal deposit 87 sr 86 sr ratios indicate that most of the anhydrite formed from 2 1 mixture of seawater and black smoker fluids 65 15 seawater oxygen isotopic compositions imply that anhydrite precipitated at temperatures between 147 c and 270 c and require that seawater was conductively heated to between 100 c and 180 c before mixing and precipitation occurred anhydrite from the tag mound has a sr ca partition coefficient kd 0 60 0 28 2 this value is in agreement with the range of experimentally determined partition coefficients kd 0 27 0 73 and is similar to those calculated for anhydrite from active black smoker chimneys from 21 n on the east pacific rise the 18 o so 4 of tag anhydrite brackets the value of seawater sulfate oxygen 9 5 dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid ocean ridges

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

fractional crystallization of primitive hydrous arc magmas an experimental study at 0 7 gpa differentiation of mantle derived hydrous basaltic magmas is a fundamental process to produce evolved intermediate to sio2 rich magmas that form the bulk of the middle to shallow continental and island arc crust this study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0 7 gpa for hydrous calc alkaline to arc tholeiitic magmas fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous higher temperature experiment temperatures ranged from near liquidus at 1 170 c to near solidus conditions at 700 c h2o contents varied from 3 0 to more than 10 wt the liquid line of descent covers the entire compositional range from olivine tholeiite 1 170 c to high silica rhyolite 700 c and evolves from metaluminous to peraluminous compositions the following crystallization sequence has been established olivine clinopyroxene plagioclase spinel orthopyroxene amphibole titanomagnetite apatite quartz biotite anorthite rich plagioclase and spinel are responsible for a marked increase in sio2 content from 51 to 53 wt at 1 040 c at lower temperatures fractionation of amphibole plagioclase and fe ti oxide over a temperature interval of 280 c drives the sio2 content continuously from 53 to 78 wt largest crystallization steps were recorded around 1 040 c and at 700 c about 40 of ultramafic plutonic rocks have to crystallize to generate basaltic andesitic liquids and an additional 40 of amphibole gabbroic cumulate to produce granitic melts andesitic liquids with a liquidus temperature of 1 010 c only crystallize 50 over an 280 c wide range to 730 c implying that such liquids form mobile crystal mushes 50 crystals in long lived magmatic systems in the middle crust allowing for extensive fractionation assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

sources of lead in the san cristobal pulacayo and potosi mining districts bolivia and a reevaluation of regional ore lead isotope provinces new lead isotope data on ores crustal rocks and leachates of crustal rocks combined with data in the literature provide important new constraints on the sources of ore metals in southwest to south central bolivia including the very large recently discovered silver zinc deposit at san cristobal the pulacayo polymetallic district and the giant potosi silver tin base metal deposit lead isotope ratios of ores and igneous rocks from the san cristobal deposit and from paleozoic and cretaceous sedimentary rocks are compared with published data on high grade middle proterozoic metamorphic basement rocks these data constrain the major source of lead and by inference of other ore metals at san cristobal to be the metamorphic basement rocks leaching experiments on samples of paleozoic and cretaceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than the whole rock compositions however lead isotope ratios of both whole rocks and leachates of these upper crustal rocks are too radiogenic for them to be major sources of ore lead at san cristobal lead isotope ratios of ores from pulacayo and potosi are similar to each other and lie within the range of paleozoic and cretaceous sedimentary whole rock compositions leaching of pb from the sedimentary rocks cannot explain the isotopic compositions of the pulacayo and potosi ores and the isotopic homogeneity of the potosi ores also argues against mixing of lead from diverse sources in the hydrothermal system lead from the sedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metals from the resulting magma lead isotope ratios of san cristobal ores are different from those of pulacayo potosi and other deposits to the east but resemble the compositions of ores and volcanic rocks in western bolivia on this basis we identify a new ore lead isotope province extending from san cristobal northward across the eastern altiplano and into southern peru this province is coincident with but smaller than the extent of the proposed arequipa antofalla metamorphic basement craton the degree of incorporation of ore metals from the metamorphic basement appears to depend on the timing and or location of the mineralizing event ore deposits in the northern part of province iv formed before the thickening of andean crust beginning around 20 ma and incorporated minor amounts of metals from the metamorphic basement younger deposits farther to the south contain major to dominant components of basement lead

continental underthrusting and obduction during the cretaceous closure of the rocas verdes rift basin cordillera darwin patagonian andes 1 the patagonian andes record a period of cretaceous neogene orogenesis that began with the compressional inversion of a late jurassic rift called the rocas verdes basin detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of 148 ma suggesting that the basin opened approximately simultaneously along its length during the late jurassic structural data and u pb isotopic ages on zircon from granite plutons near the beagle channel 55 s show that basin inversion involved two stages of shortening separated by tens of millions of years an initial stage created a small 60 km wide thrust wedge that placed the basaltic floor of the rocas verdes basin on top of adjacent continental crust prior to 86 ma structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in cordillera darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of 35 km beneath a coeval volcanic arc a subsequent stage of out of sequence thrusting culminating in the paleogene shortened basement and upper jurassic igneous rock in the internal part of the belt by at least 50 km forming a bivergent thrust wedge this latter period coincided with the exhumation of rocks in cordillera darwin and expansion of the fold thrust belt into the magallanes foreland basin this orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi oceanic rift basin

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

microfacies and depositional environment of tertiary tanjung enim low rank coal south sumatra basin indonesia abstract the south sumatra basin is among the most important coal producing basins in indonesia results of an organic petrography study on coals from tanjung enim south sumatra basin are reported the studied low rank coals have a mean random huminite reflectance between 0 35 and 0 46 and are dominated by huminite 34 6 94 6 vol less abundant are liptinite 4 0 61 4 vol and inertinite 0 2 43 9 vol minerals are found only in small amounts 0 2 vol mostly as iron sulfide based on maceral assemblages the coals can be grouped into five classes 1 humotelinite rich group 2 humodetrinite rich group 3 humocollinite rich group 4 inertinite rich group and 5 humodetrinite liptinite rich group comparing the distribution of maceral assemblages to the maceral or pre maceral assemblages in modern tropical domed peat in indonesia reveals many similarities the basal section of the studied coal seams is represented typically by the humodetrinite liptinite rich group this section might be derived from sapric or fine hemic peat often occurring at the base of modern peats the middle section of the seams is characterized by humotelinite rich and humocollinite rich groups the precursors of these groups were hemic and fine hemic peats the top section of the coal seams is typically represented by the humodetrinite rich or inertinite rich group these groups are the counterparts of fibric peat at the top of the modern peats the sequence of maceral assemblages thus represents the change of topogenous to ombrogenous peat and the development of a raised peat bog a comparison between the result of detailed maceral assemblage analysis and the paleodepositional environment as established from coal maceral ratio calculation indicates that the use of coal maceral ratio diagrams developed for other coal deposits fails to deduce paleo peat development for these young tropical coals in particular mineral distribution and composition should not be neglected in coal facies interpretations

kinematic evolution of the patagonian retroarc fold and thrust belt and magallanes foreland basin chile and argentina 51 30 s the kinematic evolution of the patagonian fold and thrust belt and cogenetic magallanes retroarc foreland basin is reconstructed using new geologic mapping two dimensional 2 d seismic reflection data and zircon u pb geochronology results span an 160 km wide transect of the thrust belt and magallanes basin near 51 30 s and highlight the influence of inherited extensional structures on basin paleogeography syntectonic sedimentation and late cretaceous neogene foreland shortening south of 50 s the patagonian fold and thrust belt developed on oceanic and attenuated crust of the predecessor late jurassic rocas verdes rift basin resulting in a collisional nature to early fold and thrust belt development and foreland sedimentation atop rifted south american crust we identify six principal stages of development between late cretaceous and neogene time a palinspastic restoration indicates 32 40 km 19 23 of retroarc shortening following closure of the rocas verdes basin and incipient growth of the thrust belt more than half of the estimated crustal shortening occurred synchronously with the deep water phase of late cretaceous foreland basin sedimentation subsequent deformation migrated into the foreland accounting for 12 km of shortening across the cretaceous early miocene basin fill thick skinned thrust faulting along multiple detachment levels in paleozoic metamorphic basement resulted in 5 km of foreland uplift and exposure of preforeland basin deposits the final phase of early miocene deformation ca 21 18 ma may reflect enhanced coupling between the continental and oceanic lithospheres causing foreland basement uplifts as the chile ridge spreading ridge approached the trench we speculate that neogene foreland shortening was accommodated by reactivation of mesozoic normal faults zones and accounts for broad uplift of the patagonian fold and thrust belt

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

late cretaceous early eocene counterclockwise rotation of the fueguian andes and evolution of the patagonia antarctic peninsula system abstract the southernmost andes of patagonia and tierra del fuego present a prominent arc shaped structure the patagonian bend whether the bending is a primary curvature or an orocline is still matter of controversy new paleomagnetic data have been obtained south of the beagle channel in 39 out of 61 sites they have been drilled in late jurassic and early cretaceous sediments and interbedded volcanics and in mid cretaceous to eocene intrusives of the fuegian batholith the anisotropy of magnetic susceptibility was measured at each site and the influence of magnetic fabric on the characteristic remanent magnetizations chrm in plutonic rocks was corrected using inverse tensors of anisotropy of remanent magnetizations normal polarity secondary magnetizations with west directed declination were obtained in the sediments and they did not pass the fold test these characteristic directions are similar to those recorded by mid cretaceous intrusives suggesting a remagnetization event during the normal cretaceous superchron and describe a large 90 counterclockwise rotation late cretaceous to eocene rocks of the fueguian batholith record decreasing counterclockwise rotations of 45 to 30 these paleomagnetic results are interpreted as evidence of a large counterclockwise rotation of the fueguian andes related to the closure of the rocas verdes basin and the formation of the darwin cordillera during the late cretaceous and paleocene the tectonic evolution of the patagonian bend can thus be described as the formation of a progressive arc from an oroclinal stage during the closure of the rocas verdes basin to a mainly primary arc during the final stages of deformation of the magallanes fold and thrust belt plate reconstructions show that the antarctic peninsula would have formed a continuous margin with patagonia between the early cretaceous and the eocene and acted as a non rotational rigid block facilitating the development of the patagonian bend

seawater metasomatism in hydrothermal sediments escanaba trough northeast pacific abstract hydrothermal sediments from the nesca vent field escanaba trough are enriched in a number of typical vent fluid metals for which fluid data for the same sediment hosted site exhibit anomalous depletion these results are consistent with cooling of rising fluids and precipitation of various metal sulphide sulphate and oxide phases in situ within the sediment column prior to expulsion of the fluids from the seabed significant enrichments of mg and u elements which are quantitatively removed from high temperature vent fluids are also observed in the escanaba trough sediments however enrichments of these elements cannot derive from simple conductive cooling of the rising vent fluids but instead are believed to result from extensive seawater metasomatism as relatively fresh unreached seawater is heated from below by hot sediments and or percolating vent fluids in the upper levels of the circulation system calculated seawater sediment interaction ratios range from 5 1 to 2500 l of seawater per gram of sediment pb sr and nd isotope distributions for the escanaba sediments are consistent with combined sediment fluid and seawater sediment interactions pb isotope ratios for hydrothermal and background sediments coincide with local sulphide deposits but are much more radiogenic than local morb this indicates not only that complete isotopic re equilibration must occur between the hot rising fluids and the host sediments but in addition that sediment composition appears to dominate the pb isotopic composition of these fluids sr and nd isotope ratios in the hydrothermally altered sediments which are distinct from background ratios coincide closely with modern seawater values providing confirmatory evidence that extensive interactions must also have occurred between these hydrothermal sediments and relatively fresh unreacted seawater

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

geophysical modeling and structure of ushuaia pluton fuegian andes argentina abstract within the area of ushuaia bay tierra del fuego southernmost south america the deformed lower cretaceous sedimentary rocks of yahgan formation host the ushuaia pluton the intrusive body is oval in map view it is compositionally varied with rocks ranging from the ultrabasic to the mesosiliceous realm the emplacement time is constrained within the albian cenomanian span by new amphibole k ar data meso and microstructures of ushuaia pluton and its host indicate a synkinematic emplacement with a dominant extensional component a set of transcurrent and normal faults related to the sinistral strike slip beagle channel fault system affects the pluton and its host on the basis of aeromagnetic data combined with field information a new model is presented for the ushuaia pluton modeling results fit well with a laccolithic body with an estimated volume of around 111 km 3 the model pluton cross section displays a central zone with an average thickness of 2000 m which progressively thins toward the margins 500 m and a southern root which reaches 5000 m deep the combined structural and geophysical model supports a transtensive scenario for the ushuaia pluton emplacement at early late cretaceous boundary

composition and isotopic constraints on the petrogenesis of alkaline arc lavas lihir island papua new guinea the sio2 undersaturated lavas from lihir island papua new guinea like most arc lavas are highly enriched in sr ba k rb and cs and depleted in hf ta nb and ti relative to ocean floor basalts and oceanic island basalts these alkali rich lavas have arc trace element signatures and nd sr and pb isotopic systematics however they are not a product of present day subduction as this volcanism has tapped mantle which was enriched by prior subduction episodes the narrow range of pb 206pb 204pb 18 74 18 76 and nd 143nd 144nd 0 51297 0 51304 isotopic compositions suggest a cogenetic origin for these lavas during the fractionation of the primitive lihir lavas elements normally considered incompatible i e the light rare earth elements lree rb th and p have high bulk solid melt partition coefficients 0 15 1 5 relatively higher partition coefficients during forma tion of the evolved lavas produced crossing rare earth element ree patterns and primitive lavas have higher incompatible elements abundances than evolved lavas this results from 1 changes in the amount of apatite in the fractionating assemblage and 2 the abnormal partitioning of trace elements into apatite microphenocrysts which nucleate at the crystal liquid interface of rapidly growing clinopyroxene phenocrysts the lihir lavas have lower alkali sr ba k rb cs and lree abundances than other tabar feni lavas they are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands the similarity of sm nd ratios of these undersaturated arc lavas to those of tholeiitic and calc alkaline arc lavas and the moderate chondrite normalized la yb la ybcn 3 7 indicates that there has been limited enrichment of the lree relative to the heavy ree during generation of the arc modified source mantle the alkaline nature of these lavas reflects their generation in a tensional tectonic environment from a fossil arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes

chemical differentiation cold storage and remobilization of magma in the earth s crust the formation storage and chemical differentiation of magma in the earth s crust is of fundamental importance in igneous geology and volcanology recent data are challenging the high melt fraction magma chamber paradigm that has underpinned models of crustal magmatism for over a century suggesting instead that magma is normally stored in low melt fraction mush reservoirs 1 9 a mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1 10 however many common features of crustal magmatism have not yet been explained by either the chamber or mush reservoir concepts1 11 here we show that reactive melt flow is a critical but hitherto neglected process in crustal mush reservoirs caused by buoyant melt percolating upwards through and reacting with the crystals10 reactive melt flow in mush reservoirs produces the low crystallinity chemically differentiated silicic magmas that ascend to form shallower intrusions or erupt to the surface11 13 these magmas can host much older crystals stored at low and even sub solidus temperatures consistent with crystal chemistry data6 9 changes in local bulk composition caused by reactive melt flow rather than large increases in temperature produce the rapid increase in melt fraction that remobilizes these cool or cold stored crystals reactive flow can also produce bimodality in magma compositions sourced from mid to lower crustal reservoirs14 15 trace element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface16 we propose that magma storage and differentiation primarily occurs by reactive melt flow in long lived mush reservoirs rather than by the commonly invoked process of fractional crystallization in magma chambers14 magma storage and differentiation in the earth s crust mainly occurs by reactive melt flow in long lived mush reservoirs rather than by fractional crystallization in magma chambers as previously thought

experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts abstract the primary goal of this investigation was to derive a set of expressions that can be used to calculate the amphibole melt partitioning behavior of the rare earth elements ree and the high field strength elements hfse in natural systems to supplement the existing data set on basaltic systems we conducted experiments on systems where amphibole was in equilibrium with dacitic tonalitic and low si rhyolitic melts these experiments doped with la sm gd lu ta nb y zr and hf were run at pressures of 2 and 5 kbar temperatures between 900 c and 945 c and oxidation conditions ranging from qfm 1 to ninio 1 the partitioning data obtained in this study were combined with published data to calculate two sets of expressions describing trace element partitioning the first set models the partitioning of trace elements into amphibole using temperature pressure and several compositional parameters including the compositionally compensated partition coefficients of ti al caand sio2 and the exchange of fe and mg between the crystal and the melt dmg dfe the second set of expressions are slightly less precise but require no specific knowledge of p t or fo2 and for application to natural systems can be constructed solely on the basis of information available from standard electron microprobe analyses these expressions predict amphibole melt partition coefficients for ree and hfse within an internal precision of 14 40 relative for alkali basalt to low si rhyolite from 850 c to 1100 c 2 20 kbar and oxygen fugacity from qfm 1 to ninio 1 partition coefficients calculated from the expressions derived in this study were used to model the partial melting and fractional crystallization of a hypothetical amphibolite and hydrous melt respectively fractionation and or melting in amphibole bearing systems produces a magma with a convex upward ree pattern a characteristic common to many hornblende bearing dacites however the removal or addition of an amphibole component cannot produce the strong hfse depletion relative to the ree observed in many arc magmas