petrogenesis of the greenhills complex southland new zealand magmatic differentiation and cumulate formation at the roots of a permian island arc volcano a permian 265 ma intrusive complex which formed as a magmatic feeder reservoir to an immature island arc volcano is fortuitously exposed in southern new zealand known as the greenhills complex this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes cumulates which include dunite olivine clinopyroxenite olivine gabbro and hornblende gabbro norite are related products of parent magma fractionation both primary magmatic and secondary platinum group minerals occur within dunite at one locality using the composition of cumulus minerals mafic dykes and melt inclusions we have determined that the parent magmas of the complex were hydrous low k island arc tholeiites of ankaramitic affinities progressive magmatic differentiation of this parent magma generated fractionated melt of high alumina basalt composition which is now preserved only as dykes which cut the complex field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate rock formation recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface similar processes are inferred to account for volcanic rock compositions in other parts of this arc terrane and in modern island arc systems

calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

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

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

a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

igneous petrogenesis and tectonic setting of plutonic and volcanic rocks of the cordillera de talamanca costa rica panama central american arc

evolution of crystallizing interstitial liquid in an arc related cumulate determined by la icp ms mapping of a large amphibole oikocryst high resolution chemical mapping of a large single oikocrystic introduction amphibole grain from the eriksberg gabbro by laser ablation the most commonly studied cumulus rocks are those inductively coupled plasma mass spectrometry la icp ms formed by crystallization of relatively dry basaltic liquids documents the progressive evolution of interstitial liquid in a hydrous e g parsons 1986 cawthorn 1996 these include most basaltic system the amphibole has a nearly constant mg number layered intrusions ophiolites oceanic cumulates and and only minor variations in most major elements in part because most cumulates recovered from the moon and in metof the resorption of olivine buffering the liquid composition however eorite collections studies of these cumulates have innon buffered trace elements including sc v nb zr th u and cluded efforts to understand how the crystals are the rare earth elements ree show at least an order of magnitude segregated from the liquid cumulus processes as well variation for example v varies from 1000 ppm zr as how the resulting pile of crystals plus liquid evolves from 4 3 to 1000 ppm nb from 0 14 to 12 3 ppm and the during slow cooling postcumulus processes by comree patterns range from bowed up with a negative eu anomaly parison very little effort has been directed toward underto bowed down with a positive eu anomaly all within a single standing these processes in the crystallization of relatively crystal the distribution of amphibole compositions indicates that hydrous basaltic liquids such as those associated with arc the interstitial liquid was not uniformly distributed as crystallization magmatism the presence of a significant amount of proceeded rather the compositional variations reflect progressively water in the liquid is especially important for the postmore channelized flow of interstitial liquid during compaction when cumulus evolution of a crystal pile as it will stabilize the interstitial liquid evolved so that the crystallizing amphibole had hydrous minerals and cause vapor saturation while a 300 ppm v vapor saturation was reached and the behavior of the significant amount of interstitial liquid remains incompatible trace elements changed markedly the final amphibole to the importance of fractional crystallization in procrystallize is enriched in soluble incompatible elements and depleted ducing compositional variability in crystallizing silicate in zr and nb a relationship that is consistent with crystallization liquids has been well established since bowen s exfrom a fluid where compaction returns such evolved liquid or fluid perimental work bowen 1928 during perfect fractional to an overlying magma reservoir resulting fractionation trends will crystallization the solids are isolated from the liquid before the liquid evolves appreciably and thus they proshow the relative depletions in ti nb and zr seen in arc related vide an instantaneous record of the liquid evolution basalts incorporation of liquid in a crystal pile can change both the composition of the solids and the evolution of the supernatant liquid initially the parental liquid if the

rare earth element signatures of economic and sub economic porphyry copper systems in urumieh dokhtar magmatic arc udma iran abstract a wide variety of porphyry copper deposits have been reported along the cenozoic urumieh dokhtar magmatic arc in iran the formation of these deposits is attributed to closure of neo tethys and the subsequent collisional tectonic regime during the tertiary this study presents whole rock rare earth element ree data for the giant to small sarcheshmeh meiduk sungun darreh zerreshk dalli iju parkam and ali abad with sub economic daraloo and reagan porphyry copper systems the data indicate a temporal and geochemical evolution from sub economic porphyry systems to economic deposits all the economic and sub economic porphyry copper systems exhibit lree enrichment relative to hree whereas the economic deposits exhibit a relatively steep downward lree to hree profile reflecting higher lree abundance the eu anomalies vary from distinct negative in sub economic deposits eu eu 0 28 0 70 to either markedly less negative or positive anomalies eu eu 0 45 1 67 in economic deposit the economic porphyry deposits are characterized by relatively high la sm and sm yb values representing high crustal assimilation in a relatively thickened crust and provide insight into fractionation of hornblende with minor garnet in deep crustal parts mash zone compared with sub economic deposits the steep downward lree to mree and flatter to slightly upward mree to hree in economic deposits indicate hornblende involvement magma evolution toward more volatile content it seems that in an ongoing process of closure of neo tethys during compression and crustal shortening the early eocene oligocene sub economic intrusions are followed by adakite like hydrous miocene and younger economic cu bearing intrusions

petrogenesis of the early cretaceous adakite like porphyries and associated basaltic andesites in the eastern jiangnan orogen southern china abstract early cretaceous quartz porphyries are associated with mantle derived mafic rocks basaltic andesites in the eastern segment of the neoproterozoic jiangnan orogen southern china zircons from two quartz porphyries yield identical u pb ages at 142 1 ma by the la icp ms method hornblende and augite are the dominant phenocrysts in the basaltic andesites and the groundmass plagioclase has a high anorthite content 80 both of which indicate high h 2 o 6 wt contents in the magma the basaltic andesites have low tio 2 0 68 0 78 wt and nb la 0 23 0 28 nb and ta depletions high initial 87 sr 86 sr ratios 0 710 and unradiogenic nd isotopes e nd t 3 4 to 3 7 implying that they were generated from the partial melting of an isotopically enriched lithospheric mantle source that had been metasomatized by subduction related fluids the quartz porphyries sio 2 65 6 70 5 wt show adakite like geochemical features with high sr y 18 1 53 4 and la yb 18 6 39 4 ratios but low y 9 44 15 2 ppm and yb 0 84 1 43 ppm contents they have low mgo 0 72 1 46 wt ni 1 04 14 7 ppm and cr 2 34 36 7 ppm contents and lower nd isotopic ratios e nd t 6 02 to 7 64 than the basaltic andesites suggesting that they may have been generated from the partial melting of the lower part of a thickened continental crust without interaction with the underlying lithospheric mantle the mafic magmas ascended from the metasomatized and hydrated lithospheric mantle and heated the lower part of thickened continental crust triggering partial melting to form the adakite like porphyries the moderate levels of sr 204 546 ppm and negative eu anomalies eu eu 0 69 0 86 in the porphyries require the presence of minor plagioclase as residual phase in the magma source indicating the continental crust may be not as thick 50 km as previously suggested the new recognition of adakite like porphyries and associated mafic rocks in the eastern jiangnan orogen reveals the significant role of neoproterozoic orogen related crust and oceanic sediments in the formation of mesozoic adakite like rocks of central eastern china

the petrogenesis of the neoarchean kukuluma intrusive complex nw tanzania abstract the kukuluma intrusive complex kic is a late archean igneous complex dominated by monzonite and diorite with subordinated granodiorite the monzonite and the diorite suites have low silica content sio2 62 wt moderate mg mg average 49 high sr y sr yaverage 79 and high la yb la ybaverage 56 ratios and strongly fractionated lan ybn 9 69 ree patterns their moderate ni niaverage 50 ppm cr craverage 85 ppm variable cr ni ratio 0 65 3 56 and low tio2 tio2average 0 5 wt indicate little to no interaction with the peridotitic mantle for most major elements al2o3 feot na2o tio2 and p2o5 the monzonite and the diorite suites display subparallel trends for the same sio2 content indicating they represent distinct melts intrusions belonging to the diorite suite have high na2o na2oaverage 4 2 wt dy ybn dy ybn average 1 6 a positive sr anomaly and uncorrelated nb la and zr sm ratios suggesting derivation from partial melting of garnet bearing amphibolite intrusions belonging to the monzonite suite have higher na2o na2oaverage 5 61 wt dy ybn dy ybn average 2 21 a negative sr anomaly and correlated nb la and zr sm ratios consistent with derivation from partial melting of eclogite with residual rutile small variations in the th u ratio and near chondritic morb average values th umonzonite 3 65 th udiorite 2 92 are inconsistent with a subducting slab signature and it is proposed that the monzonite and the diorite suites of the kic formed by partial melting of garnet bearing lower mafic crust of an oceanic plateau the granodiorite suite has lower mg mg average 41 moderately fractionated ree low sr y sr yaverage 20 la yb la ybaverage 15 dy ybn dyybn average 1 24 and small negative eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation near morb th u th uaverage 2 92 and zr sm zr smaverage 30 21 ratios are consistent with intracrustal melting of amphibolite archean rocks with an adakitic geochemical signature have been used to argue in favour of a plate tectonics regime in the archean the results presented here suggest that tectonic models for the tanzania craton which invoke a subduction related setting for all greenstone belts may need revision

zircon u pb and hf o isotopes trace the architecture of polymetallic deposits a case study of the jurassic ore forming porphyries in the qin hang metallogenic belt china abstract the qin hang intra continental porphyry skarn cu polymetallic belt qhmb is among the economically important metallogenic belts in south china the significant differences in the size and metal assemblage of the jurassic magmatic hydrothermal ore deposits in this belt remain as an enigma here we employ zircon u pb and hf o isotopes of the tongshanling and baoshan cu pb zn deposits in the central part of the qhmb to investigate the contrasting metallogenic architecture our sims zircon u pb data indicate that the tongshanling and baoshan granodiorite formed at 160 ma these rocks show high mg values and negative zircon e hf t and high 18 o values suggesting that the magmas of the granodiorite porphyries were mainly generated through the anatexis of older crustal components triggered by the input of mantle derived magma the minor content of amphibole phenocrysts low sr y ratios negative eu anomaly and low zircon ce 4 ce 3 ratios indicate that the porphyries are relatively less oxidized with less water content compared with the ore bearing porphyries in the dexing and yuanzhuding porphyry cu deposits in the northern and southern part of the qhmb suggesting that high magmatic water content and oxidation state are important prerequisites for the formation of large size porphyry skarn copper deposits in the qhmb the positive correlation between zircon e hf t values with the cu reserves as well as zircon 18 o values with the cu cu pb zn ratios of the deposits indicate that the magmatic sources exerted a first order control on the volume and metal assemblage of deposits in the qhmb the hf and nd isotope contour maps indicate that the central part of the qhmb has high potential for pb zn dominated magmatic hydrothermal deposits whereas the northern and southern part of the qhmb are prospective for large cu deposits our results have important implications in formulating regional exploration strategies for jurassic porphyry skarn cu pb zn deposits in the qin hang belt

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

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

early silurian to early carboniferous ridge subduction in nw junggar evidence from geochronological geochemical and sr nd hf isotopic data on alkali granites and adakites abstract the central asian orogenic belt caob evolved through a long lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the paleozoic the northern part of the western junggar region nw junggar offers a window into these processes particularly to constrain the timing of closure of the paleo asian ocean here we report geochemical geochronological and isotopic data from k feldspar granites and adakitic rocks from the nw junggar region zircon u pb ages suggest that the granites were emplaced during early silurian to the early carboniferous 434 328 ma the granites show geochemical characteristics similar to those of a type granites with high sio2 71 13 76 72 wt na2o k2o 8 00 9 59 wt and al2o3 12 28 14 08 wt but depleted sr nb ta and eu they display moderate to high positive end t and ehf t values 4 26 8 21 and 7 69 14 60 respectively and young nd and hf model ages t2dm nd 489 740 ma and t2dm hf 471 845 ma suggesting magma derivation through partial melting of lower crust in the boshchekul chingiz and zharma saur arcs the adakites are characterized by high sr content 406 5 751 6 ppm and low y 13 8 16 4 ppm and yb 1 5 1 8 ppm content yielding relatively high sr y ratios 25 38 49 41 similar to those of modern adakites they have high positive end t and ehf t values 7 85 8 25 and 13 23 15 97 respectively and young nd and hf model ages t2dm nd 429 535 ma and t2dm hf 355 550 ma indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the boshchekul chingiz arc the petrogenesis and distribution of the a type granites and adakites as well as the tectonic architecture of the region suggest that a ridge subduction event might have occurred during the early silurian to early carboniferous in combination with previous studies in the chinese altai we suggest a two sided ridge subduction model for the junggar altai region

andesite and dacite genesis via contrasting processes the geology and geochemistry of el valle volcano panama the easternmost stratovolcano along the central american arc is el valle volcano panama several andesitic and dacitic lava flows which range in age 5 10 ma are termed the old group after a long period of quiescence approximately 3 4 ma volcanic activity resumed approximately 1 55 ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0 9 0 2 ma these are referred to as the young group all of the samples analyzed are calc alkaline andesites and dacites the mineralogy of the two groups is distinct two pyroxenes occur in the old group rocks but are commonly absent in the young group in contrast amphibole has been found only in the young group samples several disequilibrium features have been observed in the minerals e g oscillatory zoning within clinopyroxenes these disequilibrium textures appear to be more prevalent among the old as compared with the young group samples and are most likely the result of magma mixing assimilation and or polybaric crystallization mass balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young group rocks or between the old and young group volcanics we believe that the old group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge the young group however does not appear to be related to more primitive magmas by differentiation the young group samples cannot be related by fractionation including realistic amounts of amphibole distinctive geochemical features of the young group including la yb ratios 15 yb 1 sr y 150 and y 6 suggest that these rocks were derived from the partial melting of the subducted lithosphere these characteristics can be explained by the partial melting of a source with residual garnet and amphibole dacitic material with the geochemical characteristics of subducted lithosphere melting is generated apparently only where relatively hot crust is subducted based on recent work the young dacite genesis at el valle volcano is related to the subduction of relatively hot lithosphere

laser 40ar39ar ages of tephra from indian ocean deep sea sediments tie points for the astronomical and geomagnetic polarity time scales tephra layers within deep sea sediment cores are valuable stratigraphic markers which can be radiometrically dated with high precision by the laser 40ar39ar technique two neogene ash layers from ocean drilling program odp site 758 have been dated these layers are located within detailed oxygen isotope and paleomagnetic stratigraphies which have previously been assigned ages based on the astronomically derived geomagnetic polarity time scale adgpts 7 10 28 we use the radiometric dates from the ash layers to independently evaluate the ages assigned to oxygen isotope stage 19 1 and the paleomagnetic reversals marking the brunhes matuyama boundary and the termination of the nunivak event ash d 20 21 yields an age of 800 20 ka which is consistent with an adgpts age of 780 ka for the overlying brunhes matuyama transition and with the astronomical age for oxygen isotope stage 19 1 ash i located near the top of the nunivak subchron bears evidence of excess 40ar an error weighted mean of five plateau ages gives an apparent age of 4 519 012 ma but an errorchron through a subset of the data suggests a possible eruption age of 4 43 03 ma the plateau age is in agreement with the adgpts 7 10 28 and the errorchron age is in agreement with the time scale of baksi 29 the laser 40ar39ar ages of both ashes are significantly older than the ages from the radiometrically derived time scale rdgpts of mankinen and dalrymple 3 which is primarily based upon a large dataset of k ar dated paleomagnetic poles it is essential to reconcile the adgpts with the rdgpts because radiometric age tie points are the best hope of extending the adgpts to times before 5 ma an analysis of the expected and actual variance for the astronomical and mankinen and dalrymple 3 time scales suggests that only a handful of low k ar ages may have biased the original gpts it is not necessary to invoke general systematic errors in the ar dating system to explain the discrepancies between the rdgpts and adgpts

late miocene to pliocene crustal extension and lithospheric delamination revealed from the 5 ma palopo granodioritic intrusion in western sulawesi indonesia abstract recent studies have revealed that following collision between the western sulawesi and sula spur microcontinents during the early to middle miocene the western sulawesi microcontinent subsequently underwent extension however the geodynamic process that resulted in extension is unknown the petrogenesis of late miocene to pliocene granitoids in the region provides insights to the mechanism of extension to achieve this goal we studied the palopo granodioritic intrusion a well exposed representative pluton that crops out along the palu koro fault zircon u pb dating indicates a 4 8 4 7 0 1 ma crystallization age for the pluton compared with arc igneous rocks the pluton has high sr contents and low y and yb contents with relatively high sr y and lan ybn ratios these geochemical signatures are similar to those of typical adakites the samples exhibit high mgo 1 60 2 54 wt cr 46 65 ppm and ni 14 1 19 0 ppm contents relative to adakitic rocks derived from thickened lower crust in addition negative ehf t 9 7 to 5 5 and end t 8 5 to 11 1 values together with relatively low 143nd 144nd and high initial 87sr 86sr ratios reflect an origin through recycling of ancient crustal materials which is clearly different from adakites derived from melting of a subducting slab the most plausible mechanism for generation of the late miocene pliocene adakitic granodiorites in the western sulawesi microcontinent is partial melting of delaminated lower crust that interacted with the mantle we propose that the late miocene to pliocene crustal extension in the western sulawesi microcontinent was triggered by delamination of pre existing thickened lithosphere

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

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

magma mixing and gold mineralization in the maevatanana gold deposit madagascar abstract the maevatanana gold deposit in madagascar is hosted by archean metamorphic rocks in quartz sulfide veins that are structurally controlled by nnw sse trending shear zones fluid inclusion data show that the trapping conditions in quartz range from 0 87 to 2 58 kbar at temperatures of 269 362 c laser raman spectroscopy confirms that these inclusions consist of co 2 so 2 and h 2 o the 34 s values of the pyrites range from 1 7 to 3 6 with an average of 2 25 supporting a magmatic origin noble gases he ne ar ke xe are chemically inert thus will not be involved in chemical reactions during geological processes also due to the low concentration of he in the atmosphere and the low solubility of he in aqueous fluids the atmosphere derived he is unlikely to significantly affect he abundances and isotopic ratios of crustal fluids ensures that he production should have the typical crust 3 he 4 he ratios the 3 he 4 he ratios of fluid inclusions in pyrite from the deposit range from 0 06 to 0 12 ra while the 40 ar 36 ar ratios range from 6631 to 11441 we infer that the ore forming fluids could have been exsolved from a granitic magma the oxygen and hydrogen isotope compositions of the ore forming fluids 1 5 18 o h2o 7 8 72 d 117 indicate they were derived from a granitic magma four pyrite samples from the gold deposit yield a precise re os isochron age of 534 13 ma given that the post collisional granites in northern and central madagascar were derived by melting of sub continental lithospheric mantle and formed between 537 and 522 ma we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas these data could be accounted for the formation of the maevatanana gold deposit first the shear zones hosting the deposit formed around 2 5 ga when the madagascan micro continental blocks collided with other continental blocks triggering large scale tectono magmatic activity and forming nnw sse trending shear zones the gold mineralization at maevatanana is coeval with the crystallization age of the cambrian post collisional a type granitoid plutons in northern and central madagascar implying that this deposit is associated with extensional collapse of the east african orogen this extension in turn induced asthenospheric upwelling melting of sub continental lithospheric mantle these magmas underplated the lower crust generating voluminous granitic magmas by partial melting of the lower crust the mixing magma during tectono thermal reactivation of the east african orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced au s complexes e g au hso 3 2 the shear zones which were then placed under extensional collapse of the east african orogen in the cambrian formed favorable pathways for the magmatic ore forming fluids these fluids then precipitated gold sulfides that form the maevatanana gold deposit

a fifty year perspective of magmatic evolution on ruapehu volcano new zealand verification of open system behaviour in an arc volcano geochemical and petrological data for samples from well documented eruptions that occurred at ruapehu volcano over the period 1945 1996 can be used to illustrate the complexity of short term geochemical variation in an arc type volcano collectively data from ruapehu volcano show trends with time of increasing sio2 abundance and rising 87sr 86sr ratios consistent with broad control by assimilation and crystal fractionation processes afc however the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano these complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short term effects that are imposed during recharge on the overall afc trend we show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice probably in dikes and sills these processes are probably replicated over longer time periods hundreds to thousands of years as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches arc type volcanoes such as ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity this style of activity is difficult to predict through the usual seismicity ground deformation lake water geochemistry volcano monitoring techniques and petrology and geochemistry may provide the basis for an alternative strategy

age and geochemistry of host rocks of the cobre panama porphyry cu au deposit central panama implications for the paleogene evolution of the panamanian magmatic arc abstract the cobre panama porphyry cu au deposit located in the petaquilla district of central panama is hosted by a sequence of medium to high k calc alkaline volcanic and sub volcanic rocks new crystallisation ages obtained from a granodiorite petaquilla batholith and associated mineralised diorite to granodiorite porphyry stocks and dikes at cobre panama indicate that the batholith was emplaced as a multi phase intrusion over a period of 4 million years from 32 20 0 76 ma to 28 26 0 61 ma while the porphyritic rocks were emplaced over a 2 million year period from 28 96 0 62 ma to 27 48 0 68 ma both the volcanic to sub volcanic host rocks and intrusive rocks of the cobre panama deposit evolved via fractional crystallisation processes as demonstrated by the major elements e g al2o3 fe2o3 tio2 and mgo displaying negative trends with increasing sio2 the petaquilla intrusive rocks including the diorite granodiorite porphyries and granodiorite batholith are geochemically evolved and appear to have formed from more hydrous magmas than the preceding host volcanic rocks as evidenced by the presence of hornblende phenocrysts higher degrees of large ion lithophile element lile and light rare earth element lree enrichment and heavy rare earth element hree depletion and higher sr y and la yb values however the degree of lree enrichment hree depletion and la yb values are insufficient for the intrusive rocks to be considered as adakites collectively the volcanic and intrusive rocks have lile ree and mobile trace element concentrations similar to enriched miocene age cordilleran arc magmatism found throughout central and western panama both the petaquilla and cordilleran arc magmatic suites are geochemically more evolved than the late cretaceous to eocene chagres bayano arc magmas from northeastern panama as they display higher degrees of lile and lree enrichment the geochemical similarities between the petaquilla and cordilleran arc magmas suggest that evolved calc alkaline arc magmatism may extend to the late eocene at least 10 million years earlier than previously estimated the crystallisation ages for intrusive rocks associated with mineralisation at cobre panama imply that the deposit formed in the early oligocene between a period of late cretaceous to eocene magmatism ca 66 42 ma chagres bayano arc and cordilleran arc magmatism 22 7 ma similarities in the timing of intrusive suite emplacement and the fingerprinting of magmatic fractionation processes between the cobre panama porphyry deposit and the cerro colorado porphyry deposit in western panama ca 5 3 ma suggest that these features provide favourable geodynamic and geochemical prerequisites for the formation of porphyry deposits along the panamanian magmatic arc during the cenozoic

transition from the farallon plate subduction to the collision between south and central america geological evolution of the panama isthmus this paper presents new geological constraints on the collision of southern central america with south america and the resulting deformational episodes that have affected the panama isthmus since the late cretaceous the panama isthmus is located in southwestern central america and it represents the zone of contact between the two land masses central america and south america this collision event is still active today it has resulted in regional uplift since the late miocene pliocene and is responsible for the great american biotic interchange between south and north america depending on the methods of investigation used and due to the lack of data available the time when this collision began is still widely debated and poorly constrained to better constrain this age we have studied the rock formations and the tectonic deformations in central and eastern panama that have occurred since the late cretaceous this study presents new rock ages field work documentation and analyses and seismic line interpretations and it is complemented by spatial images for the eastern panama area during the middle eocene a number of changes suddenly appeared in the geological records that were synchronous with the break up of southern central america into two smaller blocks chorotega and choco our main results identify the prevalence of an extensional tectonic regime from the middle eocene to the middle miocene that caused the formation of horst and graben structures with thick sedimentary basin fills and a synchronous clockwise block rotation here we propose that these geologic events are associated with the initiation of the oblique collision of southern central america with south america the first contact of the southeastern extremity of central america occurred around 40 ma to 38 ma and then propagated northwestwards we describe here this long term collision episode in relation to the history of the panama isthmus

geochemical characteristics of the adakite like dodurga pluton central pontides n turkey implications for middle triassic continental arc magmatism in southern black sea region the tracing of triassic arc magmatism is much debated for a long time along the sakarya composite terrane in turkey and the southern black sea region the dodurga pluton is located in the central pontides n turkey one of the most complicated regions around the black sea region and represents the first documented middle triassic continental arc in the region this pluton comprises of dacite porphyry and granodiorite and displays calc alkaline signatures which is in agreement with arc related and i type magmas the pm normalized multi element and cn normalized ree patterns are akin to volcanic arc granites and characterized by liles and lrees enrichments relative to hfses and hrees coupled with negative nb anomalies which confirm derivation in a subduction related tectonic setting moreover the major and trace element systematics of the samples including higher sr y ratios 22 93 101 36 and lower yb contents 0 31 1 30 ppm are consistent with those from adakitic melts compared to classical arc magmas therefore this pluton also represents the first middle triassic adakite like magmatic occurrence in turkey overall petrographic and geochemical signatures reported here indicate that the dodurga pluton might have been derived from a heterogeneous source mixed by melts from subducted oceanic slab and lower continental crust in an active continental margin based on the tethyan evolution in the anatolian realm three geodynamic scenarios are possible for its formation and it could have been formed as a result of northward or southward subduction of the paleotethys ocean or northward subduction of the northern branch of neotethys ocean during the middle triassic

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

fracturing of the panamanian isthmus during initial collision with south america tectonic collision between south america and panama began at 23 25 ma the collision is significant because it ultimately led to development of the panamanian isthmus which in turn had wide ranging oceanic climatic biologic and tectonic implications within the panama canal zone volcanic activity transitioned from hydrous mantle wedge derived arc magmatism to localized extensional arc magmatism at 24 ma and overall marks a permanent change in arc evolution we interpret the arc geochemical change to result from fracturing of the panama block during initial collision with south america fracturing of the panama block led to localized crustal extension normal faulting sedimentary basin formation and extensional magmatism in the canal basin and bocas del toro synchronous with this change both panama and inboard south america experienced a broad episode of exhumation indicated by u th he and fission track thermochronology coupled with changing geographic patterns of sedimentary deposition in the colombian eastern cordillera and llanos basin such observations allow for construction of a new tectonic model of the south america panama collision northern andes uplift and panama orocline formation finally synchroneity of panama arc chemical changes and linked uplift indicates that onset of collision and isthmus formation began earlier than commonly assumed

oxygen isotope and trace element geochemistry of zircons from porphyry copper system implications for late triassic metallogenesis within the yidun terrane southeastern tibetan plateau abstract it is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore bearing ones from barren ones especially by examining the oxygen isotopic and trace element compositions of zircon the oxygen isotopic composition and trace element abundances of zircons from the mineralized pulang porphyries and the coeval disuga and lannitang andesites of the yidun terrane eastern tibetan plateau have been determined by secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms in this study respectively analyzed zircons have relatively high but invariant 18 o values 5 8 6 9 that are indicative of derivation from homogeneous mantle or mantle derived magmas together with contribution of mature crustal materials we propose a model for the late triassic magmatism in the yidun terrane involving magmas derived from partial melting of subduction metasomatized mantle peridotites that subsequently experienced melting assimilation storage homogenization mash processes the low calculated ti in zircon temperatures 644 c 863 c generally between 640 c and 690 c for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas and partly controlled by the crystallization of amphibole zircon grains from porphyry copper system display small negative europium anomalies mostly eu eu 0 5 contrasting with pronounced negative eu anomalies in zircons mostly eu eu magmato hydrothermal quartz vein cu mo w deposit magmato hydrothermal quartz vein mo w deposit this study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits

an early permian ca 280 ma silicic igneous province in the alxa block nw china a magmatic flare up triggered by a mantle plume abstract silicic igneous provinces have important implications for geodynamic processes in this study we show that the predominantly early permian ca 280 ma granitoids in the alxa block correspond to an important silicic igneous province with areal distribution in northwest nw china we present zircon u pb age and hf o isotopic and whole rock geochemical and sr nd isotopic data for the newly identified i and a type granites and high sr y granodiorites in the bayannuoergong batholith and gabbros diorites and quartz diorites in the bijiertai complex alxa block the i type monzogranites granodiorites and a type granites of the bayannuoergong batholith were probably generated by mixing between crust and mantle derived magmas partial melting of a thickened eclogitic lower crust and underplated mantle derived mafic rocks respectively the gabbros quartz diorites and diorites of the bijiertai complex were likely generated by partial melting of the lithospheric mantle differentiation from basaltic magmas and partial melting of the middle crust respectively despite pronounced geochemical and isotopic heterogeneity of these rocks new sims and la icp ms u pb zircon dating indicates that they were formed over a very short time interval at ca 280 ma thus the mantle lower crust and middle crust underwent widespread melting at this time given that these magmatic rocks have no spatial or temporal zonation and were probably formed in an extensional setting we suggest that the ca 280 ma tarim mantle plume in nw china may have triggered the magmatic flare up of the alxa silicic igneous province

crustal forensics in arc magmas abstract the geochemical characteristics of continental crust are present in nearly all arc magmas these characteristics may reflect a specific source process such as fluid fluxing common to both arc magmas and the continental crust and or may reflect the incorporation of continental crust into arc magmas either at source via subducted sediment or via contamination during differentiation resolving the relative mass contributions of juvenile mantle derived material versus that derived from pre existing crust of the upper plate and providing these estimates on an element by element basis is important because 1 we want to constrain crustal growth rates 2 we want to quantitatively track element cycling at convergent margins and 3 we want to determine the origin of economically important elements and compounds traditional geochemical approaches for determining the contributions of various components to arc magmas are particularly successful when applied on a comparative basis studies of suites from multiple magmatic systems along arcs for which differentiation effects can be individually constrained can be used to extrapolate to potential source compositions in the lesser antilles arc for example differentiation trends from individual volcanoes are consistent with open system evolution however such trends do not project back to a common primitive magma composition suggesting that differentiation modifies magmas that were derived from distinct mantle sources we propose that such approaches should now be complemented by petrographically constrained mineral scale isotope and trace element analysis to unravel the contributing components to arc magmas this innovative approach can 1 better constrain true end member compositions by returning wider ranges in geochemical compositions among constituent minerals than is found in whole rocks 2 better determine magmatic evolution processes from core rim isotopic or trace element profiles from the phases contained in magmas and 3 constrain rates of differentiation by applying diffusion controlled timescales to element profiles an example from nguaruhoe volcano new zealand underscores the importance of such a microsampling approach showing that mineral isotopic compositions encompass wide ranges that whole rock isotopic compositions are consequently simply element weighted averages of the heterogeneous crystal cargo and that open system evolution is proved by core rim variations in sr isotope ratios nguaruhoe is just one of many systems examined through microanalytical approaches the overwhelming conclusion of these studies is that crystal cargoes are not truly phenocrystic but are inherited from various sources the implication of this realization is that the interpretation of whole rock isotopic data including the currently popular u series needs careful evaluation in the context of petrographic observations

microplate model for the present day deformation of tibet 1 site velocities from 349 global positioning system gps stations are used to construct an 11 element quasi rigid block model of the tibetan plateau and its surroundings rigid rotations of five major blocks are well determined and average translation velocities of six smaller blocks can be constrained where data are well distributed the velocity field can be explained well by rigid block motion and fault slip across block boundaries residual misfits average 1 6 mm yr compared to typical one standard deviation velocity uncertainties of 1 3 mm yr any residual internal straining of the blocks is small and heterogeneous however residual substructure might well represent currently unresolved motions of smaller blocks although any smaller blocks must move at nearly the same rate as the larger blocks within which they lie undetected relative motions between them could be significant particularly where there are gaps in gps coverage predicted relative motions between major blocks agree with the observed sense of slip and along strike partitioning of motion across major faults however predicted slip rates across tibet s major strike slip faults are low only 5 12 mm yr a factor of 2 3 smaller than most rates estimated from fault offset features dated by radiometric methods as 2000 to 100 000 year old previous work has suggested that both gps data and low fault slip rates are incompatible with rigid block motions of tibet the results reported here overcome these objections

the 860 ma cordilleran type guandaoshan dioritic pluton in the yangtze block sw china implications for the origin of neoproterozoic magmatism abstract the tectonic setting of the widespread neoproterozoic igneous rocks in south china has long been a matter of debate situated in the western margin of the yangtze block of south china the guandaoshan pluton is composed of diorite in the core and quartz diorite in the margin both types contain abundant mafic enclaves and mafic dikes this pluton is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 858 pm 7 mathrm ma end document by the shrimp zircon u pb method the rocks have sio2 contents of 52 8 63 8

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

mineral inventory of continuously erupting basaltic andesites at arenal volcano costa rica implications for interpreting monotonous crystal rich mafic arc stratigraphies abstract except for the first 2 years since july 29 1968 arenal volcano has continuously erupted compositionally monotonous and phenocryst rich 35 basaltic andesites composed of plagioclase plag orthopyroxene opx clinopyroxene cpx spinel olivine detailed textural and compositional analyses of phenocrysts mineral inclusions and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments we suggest three distinct crystallization environments crystallized low mg 7 al ti and high 4 wt al2o3 titanomagnetite with considerable al2o3 10 18 wt and possibly olivine but appears to lack plagioclase a fourth crystallization environment is characterized by clinopyroxene e g mg 78 85 cr2o3 0 15 0 7 wt al cr rich spinel olivine fo80 and in some circumstances high an 80 plagioclase this assemblage seems to record mafic inputs into the arenal system and crystallization at high to low pressures single crystals cannot be completely classified as xenocrysts antecrysts cognate crystals or phenocrysts because they often contain different parts each representing a different crystallization environment and thus belong to different categories bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions on the other hand they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources the driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time may also go through compositional modification by fractionation and assimilation of country rocks thus mafic magmas become basaltic andesite through mixing fractionation and assimilation by the time they arrive at subvolcanic depths we infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing subvolcanic magma thus the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite furthermore we propose that this quasi constant supply of small increments of magma is the fundamental cause for small scale decade long continuous volcanic activity that is the current eruption of arenal is flux controlled by inputs of mantle magmas

creation of the cocos and nazca plates by fission of the farallon plate abstract throughout the early tertiary the area of the farallon oceanic plate was episodically diminished by detachment of large and small northern regions which became independently moving plates and microplates the nature and history of farallon plate fragmentation has been inferred mainly from structural patterns on the western pacific plate flank of the east pacific rise because the fragmented eastern flank has been subducted the final episode of plate fragmentation occurred at the beginning of the miocene when the cocos plate was split off leaving the much reduced farallon plate to be renamed the nazca plate and initiating cocos nazca spreading some oligocene farallon plate with rifted margins that are a direct record of this plate splitting event has survived in the eastern tropical pacific most extensively off northern peru and ecuador small remnants of the conjugate northern rifted margin are exposed off costa rica and perhaps south of panama marine geophysical profiles bathymetric magnetic and seismic reflection and multibeam sonar swaths across these rifted oceanic margins combined with surveys of 30 20 ma crust on the western rise flank indicate that i localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide fissural volcanism along some of these fractures built volcanic ridges e g alvarado and sarmiento ridges that are 1 2 km high and parallel to absolute farallon plate motion they closely resemble fissural ridges described from the young western flank of the present pacific nazca rise ii for 1 2 m y prior to final rupture of the farallon plate perhaps coinciding with the period of lithospheric stretching the entire plate changed direction to a more easterly nazca like course after the split the northern cocos part reverted to a northeasterly absolute motion iii the plate splitting fracture that became the site of initial cocos nazca spreading was a linear feature that at least through the 680 km of ruptured oligocene lithosphere known to have avoided subduction did not follow any pre existing feature on the farallon plate e g a fracture zone trail of a transform fault iv the margins of surviving parts of the plate splitting fracture have narrow shoulders raised by uplift of unloaded footwalls and partially buried by fissural volcanism v cocos nazca spreading began at 23 ma reports of older cocos nazca crust in the eastern panama basin were based on misidentified magnetic anomalies there is increased evidence that the driving force for the 23 ma fission of the farallon plate was the divergence of slab pull stresses at the middle america and south america subduction zones the timing and location of the split may have been influenced by i the increasingly divergent northeast slab pull at the middle america subduction zone which lengthened and reoriented because of motion between the north america and caribbean plates ii the slightly earlier detachment of a northern part of the plate that had been entering the california subduction zone contributing a less divergent plate driving stress and iii weakening of older parts of the plate by the galapagos hotspot which had come to underlie the equatorial region midway between the risecrest and the two subduction zones by the late oligocene

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

magmatic degassing histories from apatite volatile stratigraphy measurements of oh and cl growth zoning in apatite phenocrysts from the cerro galan ignimbrite describe the tumultuous volatile history of a magma chamber in the days to months prior to a supereruption the chemical stratigraphy in these crystals provides a unique relative time scale not otherwise available when studying pre eruptive volatiles in ancient magma chambers and it allows us to temporally correlate the data within each crystal resulting in a volatile history the five crystals studied show broadly similar volatile histories but distinctive differences between the various crystals suggest that each recorded a specific perspective on the magma chamber prior to eruption perhaps correlating to different depths or laterally separated regions within the 1000 km 3 magma chamber all crystals are consistent with the interpretation of a multistage magmatic history including a period of residence in a quiescent magma and a period of rapid growth under strongly varying conditions several apparent events during this period of growth consistent with two competing processes of h 2 o degassing and recharge are recorded by nearly all of these crystals such zoning could not have survived for more than 400 d prior to eruption suggesting that during the days to months leading up to a supereruption apatite records key aspects of the processes that directly lead to and influence these catastrophic events

zircon u pb and molybdenite re os dating and geological implications of the shimadong porphyry molybdenum deposit in eastern yanbian northeastern china the shimadong porphyry mo deposit is located in eastern yanbian in the eastern part of the north margin of the north china craton northeastern china here we present the whole rock major and tra

mineral compositions of syn collisional granitoids and their implications for the formation of juvenile continental crust and adakitic magmatism continental collision zones have been proposed as primary sites of net continental crustal growth therefore studies on syn collisional granitoids with mafic magmatic enclaves mmes are essential for testing this hypothesis the baojishan bjs and qumushan qms syn collisional plutons in the north qilian orogen nqo on the northern margin of the tibetan plateau have abundant mmes in sharp contact with host granitoids sharing similar constituent minerals but with higher modal abundances of mafic minerals in mmes the qms host granitoids have high sr y and la yb ratios showing adakitic compositions different from the bjs granitoids based on bulk rock compositions and zircon u pb age dating recent studies on these two plutons proposed that mmes represent cumulates crystallized early from the same magmatic system as their host granitoids and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies here we focus on trace element geochemistry of the constituent mineral phases of both mmes and their host granitoids of the qms and bjs plutons we show that different mineral phases preferentially host different trace elements e g most rare earth elements rees and y reside in titanite only found in the qms pluton amphibole apatite epidote and zircon mostly heavy rees and high field strength elements hfses reside in biotite titanite amphibole and zircon based on the mineral chemical data we testify that for these two plutons mmes are of similar cumulate origin crystallized from primitive andesitic melts in the early stage of granitoid magmatism the primitive andesitic melts for these syn collisional granitoids are most likely produced by partial melting of the oceanic crust supporting the hypothesis of continental crustal growth considering the syn collisional granitoids represent juvenile continental crust as evidenced by distinct mineral compositions the two plutons have different parental magma compositions e g higher tio2 content higher sr y and la yb ratios in the qms parental magmas a signature best understood as being inherited from the source the higher tio2 content of the parental magma for the qms pluton leads to the common presence of titanite in the qms pluton absent in the bjs pluton crystallization of which in turn controls the trace element ree y nb ta and others systematics in the residual melts towards an adakitic signature therefore parental magmas with high tio2 content and high sr y and la yb ratios as well as their further fractionation of titanite are important factors in the development of adakitic compositions as represented by the qms host granitoids this model offers a new perspective on the petrogenesis of adakitic rocks the present study further demonstrates that in general mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks

high water contents of magmas and extensive fluid exsolution during the formation of the yulong porphyry cu mo deposit eastern tibet abstract the yulong porphyry cu mo ore district in eastern tibet features a series of eocene felsic porphyry intrusions only one of which is intensively mineralized in this study zircon u pb ages and major and trace element compositions of whole rock samples and in situ accessory minerals zircon apatite and titanite from these various intrusions were analyzed to determine the factors controlling cu mineralization new zircon u pb dating together with published zircon u pb ages suggest that these porphyry intrusions were emplaced over a relatively short period of time 43 9 0 6 ma to 40 9 0 3 ma with the mineralized yulong intrusion emplaced at a late stage of the magmatic activity 41 1 0 3 ma to 40 9 0 3 ma these intrusions have indistinguishable whole rock major and trace element compositions but the mineralized yulong intrusion has higher apatite and titanite la yb ratios average apatite la yb 115 1 28 5 n 52 average titanite la yb 22 4 4 9 n 38 than the earlier subeconomic intrusions average apatite la yb 50 4 12 3 n 72 average titanite la yb 12 4 3 5 n 58 suggesting more fractionation of amphibole from the magmas that sourced it additionally a negative relationship between la yb and cen cen ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution indistinguishable zircon eun eun values and apatite core so3 contents from the mineralized zircon eun eun 0 69 0 06 n 25 apatite so3 0 65 0 24 wt n 21 and subeconomic intrusions zircon eun eun 0 62 0 05 n 64 apatite so3 0 61 0 27 wt n 15 suggest that the entire magmatic suite was relatively oxidized hydrous and sulfur rich and was therefore fertile for ore formation however apatite crystals from the mineralized yulong intrusion have significantly lower cl contents 0 08 0 03 wt n 72 and higher f cl ratios 47 80 21 97 n 72 than those from the subeconomic intrusions cl 0 29 0 29 wt f cl 12 87 5 16 n 78 these data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing yulong magmas sudden and voluminous release of volatiles coeval with emplacement of the yulong intrusion was triggered by an as yet unidentified process but possibly including magmatic recharge

water saturated magmas in the panama canal region a precursor to adakite like magma generation amphibole while uncommon as a phenocryst in arc lavas is increasingly recognized as a key constituent in the petrogenesis of arc magmas fractional crystallization of water saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that depending on the pressure of crystallization may also contain garnet fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature this study examines newly dated late oligocene 25 37 0 13 ma hypabyssal amphibole rich andesites from cerro patacon in the panama canal region these andesites contain nodules of amphibole cumulates that are 4 6 cm in diameter and are almost entirely composed of 5 10 mm amphibole crystals dominantly ferri tschermakite geochemical variations optical and chemical zoning of the cerro patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite amphiboles hosted within the cumulate nodules differ from those hosted in the cerro patacon andesite and contain consistently higher values of ti we suggest these nodules represent the early stages of fractionation from a water saturated magma cerro patacon andesites have ree concentrations that plot at the most depleted end of central american arc magmas and exhibit a distinctive depletion in the middle ree these geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the cerro patacon andesite consistent with the petrographic evidence fractionation of water saturated magmas is a mechanism by which adakitic compositions may be produced and the cerro patacon andesites do exhibit adakite like geochemical characteristics e g elevated sr y 28 34 however the relatively elevated concentrations of y and hree indicate garnet was not stable in the fractionating assemblage during this early stage of arc development

the paleogene arcs of the northern andes of colombia and panama insights on plate kinematic implications from new and existing geochemical geochronological and isotopic data abstract the northern andes of colombia comprise two non cogenetic paleogene arcs formed in contrasting geodynamic settings including continental and oceanic domains new whole rock geochemistry and isotopic constraints together with a review of 332 geochemical 76 isotopic and 204 geochronological data from paleocene to eocene volcanic and plutonic rocks exposed in central and western colombia and panama are used to evaluate cause effect relations between regional plate kinematics and the spatio temporal distribution of the circum caribbean magmatic arcs short lived 60 45 ma arc like magmatism in the central cordillera of colombia was emplaced in a thickened continental crust due to the oblique subduction of the caribbean oceanic plate underneath south america as suggested by the high sr y ratios conversely the panama arc and its poorly explored extension in the northwestern segment of the western cordillera of colombia document a major phase of tholeiitic to calc alkaline arc magmatism between 71 ma and 34 ma mostly derived from a hydrated mantle wedge and emplaced in an oceanic plateau crust this arc record a major phase of magmatic activity between 40 ma and 50 ma that can be associated to changes in the convergence direction and subduction rates of the farallon plate farther to the south in the pacific region of colombia eocene arc related rocks from the timbiqui complex show a geochemical signature that suggests a magmatic origin from melting of both a subduction modified mantle and lower tectonically thickened crust a possible interpretation for the pacific and western colombia panama arc systems and its analogous in ecuador recall an aleutian type convergent margin in which the continental arc laterally switches to a purely oceanic system along the trailing edge of the caribbean plate as consequence of eastward subduction of the farallon plate

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

synchronous a type and adakitic granitic magmatism at ca 2 2 ga in the jiao liao ji belt north china craton implications for rifting triggered by lithospheric delamination abstract the jiao liao ji belt jljb underwent back arc or intra continental rifting during the early palaeoproterozoic but the underlying mechanism remains unclear to address this issue this study focuses on the petrogenesis of widespread palaeoproterozoic granitoids on liaodong peninsula china zircon u pb dating and geochemical and hf isotopic data suggest that these granitoids were formed at ca 2 2 ga and they can be divided into a type and adakitic granites the a type granites have relatively high sio2 k2o zr ga nb and y contents and low cao mgo al2o3 and sr contents based on these characteristics ehf t values 0 8 to 2 8 and tdm2 ages 2797 2569 ma the a type granites are inferred to be derived from neoarchaean tonalite trondhjemite granodiorites in a rift setting in contrast the coeval adakitic granites have relatively high na2o al2o3 and sr contents and high sr y and lan ybn ratios but low mgo ni yb and y contents suggesting a magma source with garnet but no plagioclase in the residue beneath thickened lower crust their variable ehf t values 8 5 to 6 9 and tdm2 ages 3276 2305 ma indicate that the adakitic magmas were mainly generated by the partial melting of meso neoarchaean crust the co existence of contemporaneous ca 2 2 ga adakitic and a type granites provides evidence that the jljb underwent regional rifting in the palaeoproterozoic most likely due to delamination of a thickened lithosphere as follows 1 in the neoarchaean the lower crust of the longgang block underwent granulite and or eclogite facies metamorphism due to continuous thickening which resulted in increasing lower crustal density 2 the thickened lower crust became unstable and delaminated at ca 2 2 ga asthenospheric upwelling following this lithospheric delamination triggered the thinning of the overlying lithosphere the upper crust and remnant thickened lower crust were partially melted to form the a type and adakitic granites respectively the rifting likely resulted in the separation of the longgang and nangrim blocks prior to their amalgamation in the late palaeoproterozoic to form the jljb

increased magmatic water content the key to oligo miocene porphyry cu mo au formation in the eastern gangdese belt tibet the eastern gangdese magmatic belt between 88 e and 94 e in southern tibet preserves a series of cenozoic collision related igneous rocks that have formed since the start of the india asia collision at 55 to 50 ma the paleocene eocene intrusive rocks have intermediate la yb n ratios average 12 0 8 6 n 150 range 1 15 64 0 intermediate to low sr y ratios mostly 40 and weak or absent eu anomalies oligocene average eun eu 0 99 0 27 n 35 range 0 67 1 58 miocene average eun eu 0 95 0 13 n 93 range 0 44 1 46 and their mineralogy consists mainly of plagioclase quartz and amphibole as phenocrysts these geochemical and mineralogical characteristics suggest that the paleocene eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase whereas the oligo miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement oligocene and miocene magmatic rocks crop out as small volume intrusions and are associated with several large to giant sized porphyry cu mo au deposits as well as numerous smaller porphyry and skarn deposits in contrast earlier more voluminous paleocene eocene magmatism is only known to be associated with three small porphyry deposits erosional loss of subvolcanic porphyry systems from the older paleocene eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution because exhumation rates after 55 ma were relatively low and coeval linzizong volcanic rocks are extensively preserved in the gangdese belt instead we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits the paleocene eocene magmas were generated during the onset of collision between india and asia and were triggered by rollback of the neo tethyan oceanic slab this magmatism records the final stage of subduction of oceanic lithosphere beneath southern tibet and the relatively low water contents of these magmas may reflect final dehydration of the remnant neo tethyan slab after oceanic slab breakoff at 40 to 38 ma the greater india slab began to subduct beneath the gangdese belt until hard collision with the indian continent at 35 ma and breakoff of the greater india slab at 25 to 10 ma this last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction modified hydrated tibetan lithosphere the resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism giving rise to a suite of postsubduction porphyry type ore deposits in the oligo miocene

petrogenesis of a voluminous quaternary adakitic volcano the case of baru volcano the origin of adakite magmas remains controversial because initially the term adakite had petrogenetic significance implying an origin by direct melting of the eclogitized subducting oceanic crust many models have been produced for their origin and until now there has not been a straightforward method to discriminate between these models in a given adakite suite here we use detailed chronological and geochemical studies of selected adakitic edifices that allows for the determination of the magmatic output rate parameter qe which has been correlated with the rates of magma generation deep within subduction zones by providing temporal and eruption rate estimates we provide constraints on the possible petrogenetic processes involved in the generation of adakite like signatures adakite magmas derived from the melting of the subducting slab should be volumetrically insignificant when compared to the adakite like magmas produced by typical arc magma generation processes in this study we use this observation and the extraordinary stratigraphic exposure from miocene to present in an adakitic volcano in panama and to study the temporal and chemical variation in erupted magmas to estimate rates of magma generation detailed chemical and geochronological analyses of baru volcano indicate that the volcanic edifice was constructed in its entirety during the quaternary and magmas display adakite like features such as steep rare earth elements patterns pronounced depletions in the heavy rare earth elements low y high sr and high sr y the magmatic output rates qe that we have calculated show that compared to other typical adakitic volcanoes most of the volcanic edifice of baru volcano was constructed extremely rapidly 213 k a and in time frames that are similar to typical arc volcanoes the observed chemical and mineralogical variation coupled with the high magma production rates indicate that baru volcano is more representative of a typical arc volcano than a small volume melt of the subducting oceanic crust the technique we outline may have broader application in determining the petrogenetic conditions of other adakite suites

triggers for the formation of porphyry ore deposits in magmatic arcs porphyry ore deposits supply much of the copper molybdenum gold and silver used by humans a review of the main processes that trigger porphyry ore formation suggests that sulphide saturation of the magmas that supply the metals could be the overriding mechanism that helps control the temporal and spatial distribution of the ore deposits

the role of clinopyroxene in amphibole fractionation of arc magmas evidence from mafic intrusive rocks within the gangdese arc southern tibet abstract amphibole is thought to be a key control on the differentiation of arc magmas but the fractionation of this mineral remains enigmatic because it is rarely present in erupted lavas this study presents detailed petrological mineralogical and geochronological data for newly identified late triassic 217 215 ma mafic intrusive rocks from the gangdese arc of southern tibet these gabbros host an amphibole rich cumulate enclave and clinopyroxene crystal clots including two mineral assemblages in chemical disequilibrium 1 amphibole and type 1 clinopyroxene within the enclave and clots and 2 small crystals of type 2 clinopyroxene plagioclase and biotite within the host textural and geochemical evidence indicate that the enclave and clots represent captured fragments of amphibole and clinopyroxene dominated lower crustal crystal mushes respectively whereas small host crystals formed by the shallow crystallization of evolved melts extracted from these crystal mushes the compositional variations between the two types of clinopyroxene also reveal this melt segregation process amphibole with similar rare earth element patterns to associated type 1 clinopyroxene inclusions suggest that the former formed through peritectic reactions consuming clinopyroxene enhanced amphibole fractionation may also have occurred as a result of the presence of high density clinopyroxene inclusions this indicates that the fractionation of amphibole together with precursory clinopyroxene inclusions within the deep crust controls the early chemical evolution of mafic arc magmas a process that is consistent with the geochemical variations in lavas from three other active arc volcanoes the associated cumulates can efficiently delaminate back into the mantle ultimately leading to the generation of sio2 rich continental crustal material