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

adakite like and normal arc magmas distinct fractionation paths in the east serbian segment of the balkan carpathian arc are used to assess petrogenetic and regional geodynamic processes associated with late cretaceous subvolcanic intrusions within the sparsely studied timok magmatic complex tmc and ridanj krepoljin zone rkz of eastern serbia the tmc and rkz form part of the apuseni banat timok srednogorie abts magmatic belt a cu au mineralized calc alkaline magmatic arc related to closure of thetethys ocean that extends through romania serbia and bulgaria in se europe zircon ages based on u pb laser ablation inductively coupled plasma mass spectrometry supplemented by existing isotope dilution thermal ionization mass spectrometry data respectively range from 89 to 79 ma and from 76 to 71 ma for the tmc and rkz this age pattern corresponds to cross arc younging away from the european continent adakite like trace element signatures y 18 ppm are linked with samples that extend across the arc these overlap in space and time with samples that conform to a normal arc differentiation trend we performed energy constrained assimilation fractional crystallization ec afc modeling of sr la nd yb concentrations and sr and nd isotopic data results suggest that the two distinct fractionation trends may be explained in terms of a common mantle derived parental magma but distinct fractionation and assimilation paths in the lower and upper crust petrogenesis of the adakite like magmas is consistent with extensive high pressure amphibole fractionation in the lower crust followed by ascent and plagioclase dominant fractionation and assimilation in the upper crust in contrast normal arc signatures appear to have evolved exclusively via an upper crustal differentiation process overall our interpretation supports mantle wedge melting related to weak extension during progressive rollback of a subducting slab

recognition of late ordovician yudai porphyry cu au mo mineralization in the kalatag district eastern tianshan terrane nw china constraints from geology geochronology and petrology abstract although the eastern tianshan terrane in xinjiang nw china occupies a critical position in the central asian orogenic belt caob the early paleozoic tectonic evolution and mineralization history of this terrane remain ambiguous the yudai deposit is a newly discovered porphyry cu mo au deposit in the kalatag district of the dananhu island arc a key area containing early paleozoic rocks in the eastern tianshan this paper reports the geologic characteristics molybdenite re os ages zircon la icp ms u pb ages and geochemical and lu hf isotopic findings regarding the ore hosting porphyries in the yudai deposit the yudai ore hosting porphyries include an early diorite porphyry and a late quartz diorite porphyry alteration and mineralization have formed distinct zones around the quartz diorite porphyry with disseminated vein or veinlet cu mo au mineralization types mainly related to potassic alteration e g biotite magnetite and quartz typical of a porphyry cu deposit two molybdenite samples have an re os weighted mean age of 449 5 4 2 ma and exhibit high re contents 2624 to 4704 ppm indicating a mantle component the observed mineralization was probably related to the quartz diorite porphyry which yielded a mean zircon 206pb 238u age of 452 7 2 8 ma the rock samples from the two porphyries mainly plot in the diorite field show metaluminous to weak peraluminous features and belong to tholeiitic to calc alkaline series with k2o na2o ratios of 0 16 0 34 and mg values of 44 2 50 3 the samples also exhibit steep chondrite normalized rare earth element and relatively flat heavy rare earth element patterns indicating residual amphibole with positive eu anomalies eu eu 1 09 1 45 the samples are enriched in large ion lithophile elements relative to high field strength elements typical of arc magma in addition the rocks show adakitic features sr y 47 8 73 5 and present high ba th and low th nb ratios indicating slab fluids the zircons from the quartz diorite porphyry mainly yield high ehf t values 13 5 14 7 and very young two stage hf model ages tdm2 494 556 ma consistent with a depleted mantle origin combined with the arc tectonic setting we propose that the ore metals and ore hosting porphyries at yudai were predominately derived from the partial melting of the amphibole bearing and fluid rich kangguer ocean slab that was subducted northward beneath the dananhu island arc in the late ordovician therefore the evolutionary history of the dananhu island arc can be traced back to the late ordovician the yudai deposit is the first age constrained early paleozoic porphyry cu deposit in the eastern tianshan terrane and the early paleozoic was an important epoch of mineralization on this terrane

destruction of the north china craton induced by ridge subductions abstractthe destruction of the north china craton ncc mainly occurred in the cretaceous and has been attributed to a top down rapid delamination bottom up long term thermal chemical erosions or hydration by subduction released fluids on the basis of the distribution of one jurassic and two early cretaceous adakite belts and the drifting history of the paleo pacific plate we propose that three ridge subduction events dominated the large scale decratonization in the ncc both physical erosion and magmatism induced by ridge subduction contributed to the destruction of the ncc the last ridge subduction at ma was the key driving force in the final destruction we present mineralogical geochemical and isotopic data in support of the ridge subduction model flat subduction of a spreading ridge resulted in stronger physical erosion on the thick lithosphere mantle of the ncc consequently slab melting occurred during ridge subduction forming adakites with mantle mg isotope compositions followed b

a re evaluation of the stratigraphy and evolution of the kohistan arc sequence pakistan himalaya implications for magmatic and tectonic arc building processes new field mapping and structural data combined with published geochemical data from the kohistan arc in the nw himalaya enable a re evaluation of the arc stratigraphy key lithological units and their relationships are more clearly defined permitting the construction of a revised magmatic tectonic history for the arc the oldest units are transitional oceanic type basalts which form the basement to the subduction related sequence arc type gabbroic sheets and plutons intrude the oceanic basalts together these form the kamila amphibolite belt metasediments and basaltic lavas were deposited within an extensional basin onto the kamila amphibolite belt basement this sequence exposed across the arc forms a distinct stratigraphic unit which is formally defined here as the jaglot group sediment charged turbidity currents transported material into the basin whilst submarine eruptions contributed the basaltic component this period of extension culminated in the eruption of high mg boninites of the chalt volcanic group which overlie the rocks of the jaglot group the earliest granitoids of the kohistan batholith predate suturing and intrude the jaglot and chalt sequences at c 100 ma kohistan sutured to asia suturing being accompanied by thickening of the arc with the development of major intra arc shear zones and a penetrative regionally developed steep cleavage at c 85 ma intra arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the chilas complex which clearly intrudes the kamila amphibolite belt to the south and the jaglot group to the north the chilas complex has been regarded as part of the pre suturing juvenile arc sequence field evidence summarized here show this to be not so heat advection associated with emplacement of the complex caused amphibolite facies regional metamorphism melting of the lower arc and plutonism some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 ma before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 ma at least three phases of extension and rifting each separated by short lived phases of compression charac terized arc evolution much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone

initiation of subduction and the generation of slab melts in western and eastern mindanao philippines adakite found in both the eastern and western parts of mindanao island philippines is a rare rock type characterized by low heavy rare earth elements and y contents together with high sr y ratios and is considered to be the result of the melting of young subducted oceanic crust which leaves an eclogite residue pliocene quaternary adakites from western mindanao zamboanga peninsula are probably derived from the melting of the young miocene sulu sea crust which is currently subducting beneath zamboanga associated nb enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts in eastern mindanao pliocene quaternary cones and plugs of typical adakitic composition mark the trace of the philippine fault in surigao and north davao the underlying philippine sea crust is of eocene age and therefore cannot melt under normal subduction thermal conditions thermal models indicate that melting at the start of subduction can occur subduction of the philippine sea plate began 3 to 4 ma beneath eastern mindanao and probably accounts for the presence of adakites along the philippine fault

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

plate kinematics origin and tectonic emplacement of supra subduction ophiolites in se asia abstract a unique feature of the circum pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust in se asia a majority of the ophiolites appear to have originated at convergent margins and specifically in backarc or island arc settings which evolved either along the edge of the sunda eurasia and australian cratons or within the philippine sea plate these ophiolites were later accreted to continental margins during the tertiary because of fast relative plate velocities tectonic regimes at the active margins of these three plates also changed rapidly strain partitioning associated with oblique convergence caused arc trench systems to move further away from the locus of their accretion we distinguish relatively autochthonous ophiolites resulting from the shortening of marginal basins such as the present day south china sea or the coral sea and highly displaced ophiolites developed in oblique convergent margins where they were dismantled transported and locally severely sheared during final docking in peri cratonic mobile belts i e the philippine mobile belt we find a series of oceanic basins which have been slightly deformed and uplifted varying lithologies and geochemical compositions of tectonic units in these basins as well as their age discrepancies suggest important displacements along major wrench faults we have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro geochemical data to their original location and their former tectonic settings some of the ophiolites occurring in front of the sunda plate represent supra subduction zone basins formed along the australian craton margin during the mesozoic the philippine sea basin the huatung basin south of taiwan and composite ophiolitic basements of the philippines and halmahera may represent remnants of such marginal basins the portion of the philippine sea plate carrying the taiwan philippine arc and its composite ophiolitic continental crustal basement might have actually originated in a different setting closer to that of the papua new guinea ophiolite and then have been displaced rapidly as a result of shearing associated with fast oblique convergence

evidence for an 18o depleted mantle plume from contrasting 18o 16o ratios of back arc lavas from the manus basin and mariana trough abstract back arc basin glasses from the mariana trough and manus basin display contrasting oxygen isotope characteristics that require differences in their mantle sources in both basins the lavas that are most depleted in high field strength elements possess 18o values of around 6 0 that are elevated with respect to mid ocean ridge basalt morb this characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere the nature of the more fertile mantle component differs between the two basins the lowest 18o values in the mariana trough are similar to morb and suggest that the ambient upper mantle interacts with a subduction modified mantle to produce mariana trough back arc basin basalts oxygen isotope ratios of basaltic glasses from the manus basin display a negative correlation with helium isotope ratios the subduction modified component is associated with 3he 4he ratios typical of the upper mantle glasses with 3he 4he ratios greater than average morb characteristic of a deep mantle plume have 18o values that are lower than expected for upper mantle melts this suggests that the manus basin plume taps a reservoir that is 18o depleted relative to the source of morb two mechanisms are identified that might generate this reservoir deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large 18o depleted domains in the deep mantle alternatively we propose that interactions between silicate and iron alloy during the segregation of the earth s core may have the potential to generate such reservoirs resolution between these mechanisms requires further experimental investigation of oxygen partitioning between silicates and iron alloy each of these mechanisms has distinct implications for the origins and dynamics of the manus basin plume

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

strong seismic reflections and melts in the mantle of a continental back arc basin 1 a program of explosion seismology in central north island new zealand discovered a strong reflector within the upper mantle reflections from this pmp2 are spatially confined to come from an interface 35 km deep and directly beneath a 40 km wide back arc extension zone with active volcanism high heat flow low pn wave speeds and thinned crust on the basis of relative reflection amplitudes the mantle reflections are most readily explained by an interface with a negative seismic impedance contrast a satisfactory fit is obtained for a layer with a 40 90 drop in s wave speed vs compared to the surrounding mantle we interpret this layer to be a 40 km wide reservoir of partial melt pooled at a thermal boundary layer within the upper mantle

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

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

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

reply to discussion on giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism by oyarzun r marquez a lillo j lopez i rivera s mineralium deposita 36 794 798 2001

magmatism and metallogenic mechanism of the ga erqiong and galale cu au deposits in the west central lhasa subterrane tibet constraints from geochronology geochemistry and sr nd pb hf isotopes abstract the ga erqiong galale ore district in the bangong nujiang metallogenic belt hosts two large skarn cu au deposits associated with voluminous late cretaceous volcanism and plutonism in this study we present new zircon u pb age and hf isotopic data whole rock geochemical and sr nd pb isotopic data to constrain the geodynamic mechanisms of late cretaceous magmatism and metallogenesis within the ga erqiong galale ore district the intrusive rocks in the district include diorite quartz diorite granodiorite granite and dioritic porphyrite zircon u pb analyses suggest those rocks mainly emplaced at early late cretaceous 84 95 ma which coincide with the ore forming age of ga erqiong and galale deposits 87 and 88 ma respectively geochemically the granodiorite and dioritic porphyrite are metaluminous with depletion of nb ta and ti and characterized by high mgo 2 68 4 16 wt mg 55 87 sr 414 616 ppm sr y 30 56 cr 22 5 105 0 ppm and ni 26 6 59 7 ppm low y 10 6 14 0 ppm and heavy rare earth elements hree showing geochemical signatures of high mg adakitic rocks four dioritic porphyrite and three granodiorite samples exhibit homologous sr nd pb isotope compositions and have positive whole rock end t values of 1 1 to 3 0 87sr 86sr i values of 0 7045 0 7048 206pb 204pb t ratios of 18 42 18 73 207pb 204pb t ratios of 15 58 15 63 and 208pb 204pb t ratios of 38 54 38 88 besides 116 analyses from the ga erqiong galale adakitic rocks give positive zircon ehf t values 4 3 to 12 1 the geochemical and isotopic data indicate that the early late cretaceous high mg adakitic rocks in the ga erqiong galale ore district originated from the delaminated juvenile lower mafic crust that has been modified by the previously subduction partial melting of previously subduction modified delaminated juvenile lower crust not only formed the hydrous adakitic melts in highly oxidation state but also provided the ore metals e g cu au mo for the subsequent mineralization such melts reacted with surrounding mantle peridotite to elevate the contents of metallic elements again then the melts experienced varying degrees of fractional crystallization process in the shallow magma chamber to form the dioritic and granodioritic magmas those magmas emplaced into the limestone of duo ai and jiega formation to form the ga erqiong and galale skarn cu au deposits respectively field observation together with geochronological and geochemical data indicate that the early late cretaceous mineralization events in the northern and central lhasa subterranes took place in a transition tectonic setting from collisional orogeny to extension and the input of subduction modified juvenile crust play an important role in the late cretaceous ore forming events

petrogenesis of the middle jurassic yinshan volcanic intrusive complex se china implications for tectonic evolution and cu au mineralization abstract volcanic subvolcanic rocks and associated cu au deposits in yinshan are located in the eastern part of the neoproterozoic jiangnan orogenic belt between the yangtze and cathaysia blocks constituting a large metallogenic district in southeastern china this paper presents new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating element and sr nd hf isotopic data for yinshan igneous rocks in the dexing region our results show that the yinshan igneous rocks were produced during the middle jurassic ca 176 166 ma geochemically the yinshan igneous rocks are characterized by calc alkaline and show some adakitic affinities with high sr contents up to 903 ppm sr y up to 93 2 and la n yb n ratios 12 4 to 65 1 and low y 2 53 to 15 8 ppm and yb contents 0 22 to 1 39 ppm the absence of a negative eu anomaly extreme depletion in y and yb relatively low mgo contents 0 2 to 2 9 wt and relatively high 207 pb 204 pb ratio 15 555 15 575 indicate that yinshan igneous rocks were likely derived from the partial melting of thickened lower continental crust in addition the yinshan igneous rocks show similar element characteristics to arc derived igneous rocks with bulk earth like e nd t values 1 1 to 1 8 and initial 87 sr 86 sr 0 7036 0 7090 relatively depleted e hf t values 1 5 to 5 8 and hf model ages of 0 8 1 2 ga however there is no arc related evidence in the study area in the middle jurassic which indicates that these arc related elemental and isotopic compositions are possibly inherited from meso to neo proterozoic juvenile continental crust formed by the neoproterozoic oceanic crust subduction along the jiangnan orogen the geodynamic environment responsible for the development of the middle jurassic yinshan igneous rocks is probably a localized intra continental extensional environment along the ne jiangxi deep fault as a tectonic response to far field stress at the margins of south china plate during the early stage of the paleo pacific plate subduction regarding the cu au mineralization we infer that cu au were released from subducted oceanic slab and reserved in the juvenile crust during neoproterozoic subduction along the eastern jiangnan orogen partial melting of cu au rich neoproterozoic juvenile crust during the middle jurassic time in the yinshan area caused the formation of adakitic rocks and large scale cu au deposits our model provides a new insight into genesis of adakitic rocks and related cu au deposits in a non arc setting

late cretaceous volcanic rocks in the sangri area southern lhasa terrane tibet evidence for oceanic ridge subduction abstract late cretaceous magmatism in the southern lhasa subterrane tibet provides critical constraints on the subduction processes of the neo tethyan oceanic plate here we provide data on the whole rock geochemistry zircon u pb ages trace element contents and hf isotopes of early late cretaceous volcanic rocks from sangri county to sangye temple ss along the southern lhasa subterrane the ss volcanic rocks include basalts andesites and dacites sims and la icp ms zircon u pb data indicate that the ss dacites were erupted at ca 95 ma coeval with the early late cretaceous flare up event in the southern lhasa subterrane the ss volcanic rocks belong to the calc alkaline to high k calc alkaline series and have the geochemical features of subduction related volcanic rocks with negative nb ta and ti anomalies the basalts have similar rare earth element ree patterns to e morb and are characterized by high zr abundances 65 190 ppm and zr y ratios 3 3 6 5 they were possibly derived by high degree partial melting of mantle peridotite that had been metasomatized by subduction related fluids with an input of asthenospheric components positive whole rock end t 2 9 to 3 2 values for the andesites and their variable mg values 35 61 and mgo 2 29 7 32 wt cr 12 232 ppm and ni 10 127 ppm contents can be attributed to medium degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine the calc alkaline ss dacites are similar to adakites with high sr 663 1188 ppm and low heavy rare earth element hree and y 10 7 11 5 ppm contents these adakitic dacites have positive whole rock end t 2 9 to 3 4 and zircon ehf t 8 4 to 14 9 values with relatively high values of mg 36 57 and high contents of compatible elements indicating that they were derived from the partial melting of subducted oceanic slab and with the ascending magma interacting with mantle wedge peridotite our new data indicate a distinct rock association of coeval asthenosphere component involved basalts and slab derived adakitic dacites such an association together with the coeval high temperature charnockites and high temperature granulite facies metamorphism of the charnockitic country rocks in the southern lhasa subterrane indicates an anomalous input of materials and heat during the early late cretaceous flare up event such an event can probably be attributed to the northwards subduction of a neo tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal angle subduction or low angle flat subduction and subsequent slab rollback

pedogenesis in mine tails affects macroporosity hydrological properties and pollutant flow mining activities take a direct impact on the environment particularly when this activity is focused on metal ore exploitation abandoned metal mine areas contain different types of residues from ore processing operations that are typically characterized by high concentrations of heavy metals one consequence is the generation of acid mine drainage caused by the oxidation and hydrolysis of metal sulphides mine tail soils have been studied under several points of view but not on how crack formation affects their properties and functioning before irreversible hardening the cartagena la union mining district is one of the more ancient spanish mining regions this region is placed at the south eastern part of spain and contains one of the largest lead and zinc ore deposits in south europe which was exploited for au ag pb zn fe and cu from the phoenician and carthaginian times the aim of this research work was to characterize the pedogenesis that occurred in mine tailings of different age and properties to highlight the physical and hydrological changes that occurred as a consequence of pedogenesis and their consequences on the environmental pollution risk soil profile morphology was studied in the field likewise the surficial pattern of cracks shear strength bulk density and erosion laboratory analyses were conducted to characterize physical chemical elemental mineralogical and hydrological differences between the soil mass in the macro aggregates and in the fissures macroporosity was studied through image analysis of thin sections of undisturbed soil samples x ray diffraction xrd was performed on the clay fraction studied soils resulted strongly polluted especially as for zn pb cd cl mn and ni and were classified as different kinds of spolic technosol crack formation significantly affected shear strength bulk density macroporosity saturated hydraulic conductivity and element distribution gully and tunnel erosion was also driven by crack formation the mineralogy of the clay fraction revealed that smectites are present favouring crack opening crack formation deeply affected soil features and behaviour namely hydrological properties and flow of pollutants which in turn condition environmental health and possible soil stabilization and reclamation strategies

hercynian post collisional magmatism in the context of paleozoic magmatic evolution of the tien shan orogenic belt abstract the hercynian tien shan tianshan orogen formed during late palaeozoic collision between the karakum tarim and the kazakhstan paleo continents in order to constrain timing of hercynian post collisional magmatism 27 intrusions were sampled for u pb zircon dating along a ca 2000 km long profile in uzbekistan and kyrgyzstan the samples were dated utilizing sensitive high resolution ion microprobe shrimp ii the obtained ages together with previously published age data allowed the timing of hercynian post collisional magmatism to be constrained and interpreted in the context of the paleozoic magmatic evolution of the region apart from hercynian post collisional magmatism two older magmatic episodes have been recognized and the following sequence of events has been established 1 approximately 10 ma after cessation of continuous caledonian magmatism a number of late silurian early devonian intrusions were emplaced in the middle and northern tien shan terranes between 420 and 390 ma the intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of caledonian paleo kazakhstan continent 2 the next relatively short late carboniferous episode of subduction under paleo kazakhstan was registered in the kurama range of the middle tien shan calc alkaline volcanics and granitoids with ages 315 300 ma have distinct metallogenic affinities typical for subduction related rocks and are not found anywhere outside the middle tien shan terrane west of the talas farghona fault 3 the early permian hercynian post collisional magmatism culminated after the closure of the paleo turkestan ocean and affected the whole region across terrane boundaries the post collisional intrusions formed within a relatively short time span between 295 and 280 ma the model for hercynian post collisional evolution suggests that after collision the tien shan was affected by trans crustal strike slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust this produced both granitoid magmas and hydrothermal fluid flow as a result post collisional intrusions and orogenic au deposits known in the region formed coevally and were tectonically controlled 4 between 240 and 220 ma a triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale probably the influx of heat into the crust during the triassic was tectonically focused and varied significantly in different terranes in the region under investigation the triassic thermal event was not accompanied by any significant magmatic activity thus after cessation of hercynian post collisional magmatism ca 280 ma ago there was a long magmatically quiet period in the tien shan

relations between basalts and adakitic felsic intrusive bodies in a soft substrate environment the south ouessant visean basin in the variscan belt armorican massif france the metasedimentary and magmatic terranes in the southern part of the ouessant island western brittany france are the offshore prolongation of the leon variscan metamorphic domain they mainly c

age and origin of high ba sr appinite granites at the northwestern margin of the tibet plateau implications for early paleozoic tectonic evolution of the western kunlun orogenic belt abstract the buya appinite granite is a typical high ba sr granite emplaced at the northern west kunlun orogenic belt along the northwestern margin of the tibetan plateau the granite is dated at ca 430 ma using the shrimp u pb zircon method it consists of alkaline feldspar granites with coeval appinite enclaves the granite possesses high sio2 69 77 72 69 k2o 4 44 5 10 and total alkalinity k2o na2o 8 80 9 92 sr 655 1100 ppm ba 1036 1433 ppm and lree and low hree and hfse contents and insignificant negative eu anomalies consequently the samples have very high sr y 74 141 and la yb n 37 96 ratios on the other hand they have low mgo or mg cr and ni contents and low radiogenic nd isotopes nd t 8 4 to 10 4 the high ba sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher nd t values 5 7 to 6 7 elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab the granite was likely derived from partial melting of the mafic lower crust with residual garnet associated with involvement of minor lile enriched appinitic magma followed by crystal fractionation of hornblende biotite apatite and allanite in combination with previous investigations on the evolution of the western kunlun we suggest that the buya high ba sr plutons represent the end of an early paleozoic crust thickening event after a terrane accretion on southern tarim craton and the beginning of a post orogenic collapse phase in the paleozoic west kunlun orogenic belt

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

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

the cosmos greenstone succession agnew wiluna greenstone belt yilgarn craton western australia geochemistry of an enriched neoarchaean volcanic arc succession abstract the geodynamic setting of the neoarchaean eastern goldfields superterrane egs of the yilgarn craton is the subject of debate some authors propose plume models while others advocate variants on a subduction accretion model for the origin of mineralised greenstone belt sequences felsic volcanism in the kalgoorlie terrane the westernmost terrane of the egs is considered to have a tonalite trondhjemite granodiorite dacite ttg d geochemical affinity the cosmos greenstone succession which lies in the agnew wiluna greenstone belt awb of the kalgoorlie terrane contains several komatiite hosted nickel sulphide deposits the volcanic footwall to which consists of an intercalated succession of fragmental and coherent rocks ranging in composition from basaltic andesite to rhyolite light rare earth elements lrees and large ion lithophile elements liles are strongly enriched relative to high field strength elements hfses across all volcanic units and the rocks display strong positive pb and negative nb anomalies these geochemical characteristics resemble closely those of modern high k calc alkaline to shoshonite continental arc successions contrasting ree lile and hfse concentrations coupled with assimilation fractional crystallisation afc modelling shows that the intercalated dacitic and andesitic volcanic rocks within the footwall succession are not co genetic xenocrystic zircons within the felsic volcanic lithologies indicate that some assimilation of older continental crust contributed to the generation of the footwall volcanic sequence the geochemical characteristics of the cosmos volcanic succession indicate that parental melts were derived via partial melting of enriched peridotite that had been contaminated by subducted crustal material within the mantle wedge of a subduction zone in contrast two younger felsic porphyry intrusions which cross cut the volcanic succession have a distinct ttg d affinity therefore these intrusions are considered to be generated via partial melting of a subducting slab and are related to local high ca granitoid intrusions the cosmos volcanic succession represents the first extrusive high k calc alkaline to shoshonitic volcanic arc sequence described in the kalgoorlie terrane and coupled with age dating of the stratigraphy is indicative of formation in a long lived volcanic arc setting active from 2736 ma to later than 2724 ma the composition and geochemical affinity of the volcanic footwall succession to the cosmos komatiite hosted nickel sulphide deposits contrasts with the majority of felsic volcanic rocks within the awb and also the wider kalgoorlie terrane suggesting that the overall architecture of this region is more complex than is currently thought our conclusions not only have consequences for recent models of the tectonic evolution of the egs but also contribute to the debate on the operation of plate tectonics during the late archaean in general the arc affinity of the cosmos volcanic succession containing abundant high k calc alkaline andesite lavas provides further support for the operation of plate tectonics in the neoarchaean

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

the geochemical evolution of syncollisional magmatism and the implications for significant magmatic hydrothermal lead zinc mineralization gangdese tibet abstract in addition to well known subduction processes the collision of two continents also generates abundant ore deposits as in the case of the tibetan plateau which is the youngest and most spectacular collisional belt on earth during the building history of the gangdese magmatic belt several magmatic flare up events developed however significant magmatic hydrothermal lead zinc mineralization dominantly accompanied the magmatism during the syncollisional period 65 41 ma based on integrated geochemical and isotopic data we provide insights into the genesis and evolution of syncollisional magmas and their implications for significant magmatic hydrothermal lead zinc mineralization the sr nd isotopic compositions of most syncollisional igneous rocks 87sr 86sr 0 7034 0 7123 end t 9 0 to 1 8 indicate a mixing origin between mantle derived basaltic magmas and ancient crustal melts and fractional crystallization is a fundamental mechanism by which syncollisional magmas evolve towards intermediate to silicic compositions most lead zinc mineralization related plutons are high silica 76 14 wt sio2 on average high oxygen fugacity average fmq 2 5 granites with highly evolved chemical signatures average eun eun 0 33 high rb sr average 3 9 and they represent the final products from primary magmas due to the contribution of ancient crustal melts to the genesis of mineralization related parent magmas the spatial distribution of pb zn deposits within the northern gangdese magmatic belt is controlled by the lithospheric architecture in compressional environments magmas have low evacuation efficiency and long magma chamber lifespan which is favorable for basaltic parents evolved to high silica granites through sufficient fractional crystallization this scenario contributes to our understanding of the significant magmatic hydrothermal lead zinc mineralization that occurred in the syncollisional period

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

trace element and isotopic evidence for two types of crustal melting beneath a high cascade volcanic center mt jefferson oregon mt jefferson is an andesite dacite composite volcano in the cascade range the locus of andesite and dacite dominated volcanism for at least 1 million years a large trace element data set for mt jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model fc or afc for the generation of mt jefferson andesites several incompatible element zr nb y concentrations decrease in the range from basalt to andesite and then increase in the range from andesite to rhyodacite others ba rb la th remain constant or show a slight increase in the basalt to andesite range with modest increases from andesite to rhyodacite systematic variations in highly incompatible element ratios such as ba la and rb th suggest magma mixing dominates the trace element signatures rhyodacites are isotopically uniform 87sr 86sr 0 70325 0 70343 206pb 204pb 18 75 18 85 18o 6 3 0 3 whereas andesite and dacite are more variable 87sr 86sr 0 70291 0 70353 206pb 204pb 18 59 18 86 18o 6 0 0 6 typical basaltic andesite has 87sr 86sr 0 70326 0 70358 206pb 204pb 18 78 18 85 and 18o 5 9 0 4 sr rich 1 000 ppm basaltic andesite is more variable 87sr 86sr 0 70300 0 70360 206pb 204pb 18 70 18 89 18o 5 9 0 4 the data define mixing arrays with one end member at 87sr 86sr 0 7029 206pb 204pb 18 59 another at rhyodacite and a third at 87sr 86sr 0 7036 206pb 204pb 18 89 the first end member is defined by sr rich 800 1 200 ppm andesite with high al2o3 and low k2o ba and rb th the third one by k2o and very sr rich 2 000 ppm shoshonite isotopic data for basalts in northern oregon preclude any fractionation relationship between basalt and either rhyodacite or sr rich andesite e g the minimum 206pb 204pb ratio in basalt is 18 83 considered in light of geophysical models for the cascades these data suggest two types of crustal melting beneath the arc rhyodacite may be generated at 25 30 km depth by partial melting of arc basalt like amphibolite at 850 900 c sr rich andesite may be formed by partial melting of depleted morb like mafic granulite at 35 45 km depth at 1 000 1 100 c experimental and ree evidence supports these interpretations as does the restriction of sr rich andesite in the cascades to the area south of the 100 mw m2 heat flow contour between mt jefferson and mt hood thick crust and high heat flow are necessary to produce such andesite

tectonic setting of eocene boninite magmatism in the izu bonin mariana forearc abstract middle eocene boninites were simultaneously generated over a large region during the early history of the izu bonin mariana ibm arc however widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions this suggests that an additional tectonic or thermal factor influenced the generation of the ibm boninite suite regional uplift ocean island basalt style magmatism and high heat flow also characterised the northern philippine sea plate psp at the start of the middle eocene these features are similar to those observed in large volume basaltic volcanic provinces such as the early tertiary north atlantic and suggest that the ibm boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the psp the reconstructed middle eocene location of the ibm arc and west philippine basin lies close to the present day manus basin where petrological and geochemical evidence indicate the presence of a mantle plume a calculated plume track linking these locations through time also passes close to the eauripik rise an aseismic ridge on the caroline plate during the oligocene and miocene therefore we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western pacific from the middle eocene until the present day

adakites without a slab remelting of hydrous basalt in the crust and shallow mantle of borneo to produce the miocene sintang suite and bau suite magmatism of west sarawak abstract we present new geochronological and geochemical data for neogene magmatism from west sarawak zircon u pb geochronology divides neogene magmatic rocks of west sarawak into a lower miocene west sarawak sintang suite with ages of c 19 to 21 ma and a middle miocene bau suite with ages of c 12 to 14 ma magmatism occurred in multiple short lived pulses from approximately 24 ma and was coeval with magmatic activity in nw kalimantan and east kalimantan the majority of but not all bau suite samples display adakitic chemistry while the west sarawak sintang suite is predominantly non adakitic there was no active subduction zone or subducted slab associated with this adakitic magmatism instead the geochemical diversity is consistent with the bau and west sarawak sintang suites representing mixtures of mafic mantle derived magma with felsic magma derived from remelting of hydrous mafic rock that had been emplaced into the lithosphere of borneo as arc basalt tens or hundreds of millions of years previously this origin is most evident in the main sintang suite of central borneo kalimantan which has preserved less contaminated examples of the mafic endmember this endmember resembles basaltic rocks from several locations across borneo suggesting that intraplate mantle derived magmatism was responsible for remelting older hydrated basaltic rocks in the crust

discussion on giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism by oyarzun et al mineralium deposita 36 794 798 2001 in a recent thought provoking article oyarzun et al 2001 argue that late eocene early oligocene fast oblique convergence between the south america and nazca farallon plates led to flat subduction and direct melting of the subducting plate the result of this process was the formation of adakite magmas which generated giant porphyry copper deposits related to the late eocene early oligocene structural belt of northern chile known as the west fissure fault system unfortunately no additional information was presented by oyarzun and co workers to distinguish whether this adakitic signature or ttg type magmatism as described by cornejo and matthews 2000 at the el salvador deposit reflects melting of the base of an overthickened crust high pressure crystallization of mantle derived hydrous magmas or slab melting followed by crustal contamination during ascent through the andean crust furthermore current contrasting views on flat slab subduction settings introduce additional uncertainties to the model proposed by oyarzun and co workers recently published high precision ar ar dating ballard et al 2001 richards et al 1999 2001 in conjunction with previous age information maksaev et al 1988 cornejo et al 1997 dilles et al 1997 marsh et al 1997 clark et al 1998 on giant porphyry copper deposits from the paleogene of northern chile allows to constrain the age of formation of copper ores and associated magmas between 42 and 33 ma this fact poses a strict timing constraint to the petrotectonic processes responsible for their genesis

triassic granitoids in the eastern songpan ganzi fold belt sw china magmatic response to geodynamics of the deep lithosphere the songpan ganzi fold belt sgfb sw china was developed from a passive continental margin into an orogenic belt with the consumption of the paleo tethys during the evolution of the sgfb numerous late triassic granitic plutons formed and exhibited a progressive development from adakite i type granite high ba sr granite a type granite and monzonite representative late triassic plutons were studied to unravel the bewildering evolution of the eastern sgfb the menggu pluton 224 3 ma consists of granites with high alkali k2o na2o 7 85 10 4 wt and adakitic characteristics sr y 19 38 the endt values 2 77 to 5 03 initial 87sr 86sr ratios 0 7050 0 7063 and low nb ta ratios 8 10 are indicative of an origin by partial melting of amphibolitic lower crust rocks from the niuxingou pluton 215 3 ma are richer in k than na k2o na2o 1 1 1 5 and contain high sr 1006 1662 ppm and ba 1277 2009 ppm typical of shoshonite and high ba sr granite they have less enriched endt values 0 08 to 2 04 and less radiogenic 87sr 86sri ratios 0 7047 0 7048 and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle the taiyanghe pluton 205 3 ma consists of monzonites with high al2o3 20 wt but low mgo 0 94 1 39 wt the rocks are richer in na than k k2o na2o 0 7 contain high large ion lithophile element lile 681 834 ppm sr and 2142 9453 ppm ba and display strongly fractionated ree patterns la yb n 35 63 these features together with their enriched nd sr isotopic compositions endt 4 78 to 6 50 87sr 86sri 0 7074 0 7090 suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle although a continuous compressional regime during the mid and late triassic has been invoked for the sgfb the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the late triassic the 205 ma taiyanghe pluton was emplaced simultaneously with a rapid uplift of the lithosphere when surface deposits changed from deep water turbidite to tidal flat sediments it was therefore generated during decompression probably related to the rapid removal of the overthickened lithospheric mantle the triassic magmatism in the eastern sgfb is therefore important for probing geodynamic processes in the deep lithosphere

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones the thickness of earth s mechanical lithosphere is poorly defined to investigate whether rheology controls the thickness of the overriding plate s mechanical lithosphere in subduction zones the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity it was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner leading to unrealistically high heat flow since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere we suggest that it is instead controlled by buoyancy the source of buoyancy we assume is compositional e g buoyant crust two end member models with crustal thickness of 10 and 70 km respectively were then undertaken these had lower heat flow this work supports the assumption of some earlier workers e g plank and langmuir 1988 who equated the mechanical lithosphere with the crust of the overriding plate

discussion on giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism by oyarzun et al mineralium deposita 36 794 798 2001

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

the onset of movement on the philippine fault in eastern mindanao a transition from a collision to a strike slip environment the spatial and temporal evolution of eastern mindanao from a zone of active arc arc collision to one of pure strike slip environment is addressed this is made possible by tracing the philippine fault confined to a well defined and narrow zone of deformation and by differentiating from it earlier collision related compressional features whose deformation is distributed over a broad zone a diachronous unconformity marking the end of collision related deformation reflects the southward propagating nature of the collision and provides limits on the age of initiation of the philippine fault this diachronism suggests that the initiation of the philippine fault is related to the completion of the collision we show the rapidity at which formation and propagation of the fault follows the locking of the collision zone we likewise demonstrate that the philippine fault reactivates pre existing collisional features in cutting across the southern pacific cordillera

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

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

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

age tectonic setting and metallogenic implication of phanerozoic granitic magmatism at the eastern margin of the xing an mongolian orogenic belt ne china abstract the eastern margin of the xing an mongolian orogenic belt is characterised by widespread phanerozoic granitic magmatism some of which is closely related to significant ore mineralisation this paper presents new geochronological petrogenetic and tectonic data for selected intrusions zircon u pb geochronology for five granitoid plutons indicates they were emplaced during the middle late permian 264 255 ma and cretaceous 106 94 ma and thus granitic magmatism occurred throughout the phanerozoic permian 268 252 ma early middle triassic 248 240 ma early jurassic 183 ma and cretaceous 112 94 ma the permian granitoids consist of monzogranite granodiorite tonalite and quartz diorite characterised by enrichment in na2o 3 60 4 72 wt depletion in k2o 0 97 2 66 wt and a negative correlation between p2o5 and sio2 together with the presence of hornblende these geochemical features are indicative of an i type affinity the permian granitic magmatism is associated with quartz vein type tungsten deposits 252 ma unpublished sm nd isochron age which formed in an active continental margin setting related to subduction of the palaeo asian ocean the cretaceous quartz diorites have an adakitic affinity having relatively high sr 374 502 ppm low yb 0 51 0 67 ppm and y 8 7 10 7 ppm and high sr y 39 4 46 8 and la yb n values 16 2 34 7 suggesting that they were related to the partial melting of subducted oceanic crust in addition they are associated with porphyry au cu deposits we conclude that the cretaceous granitic rocks and associated porphyry au cu mineralisation occurred in an extensional tectonic setting related to the subduction of the palaeo pacific plate beneath the eurasian plate in addition the large scale early middle triassic syn collisional granite belt at the eastern margin of the xing an mongolian orogenic belt extends from the middle of jilin province to the wangqing hunchun region constraining the timing of the final collision between the north china craton and the jiamusi khanka massif and suggesting that the xra moron river changchun suture likely extends eastward into the eastern hunchun region this collision caused the middle triassic mesothermal lode gold mineralisation

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

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

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

significance of pre 1860 ma granitoid magmatism for crustal evolution and exploration targeting in the flin flon assemblage trans hudson orogen canada granitoid plutonism a significant contributor to orogenic evolution is a commonly overlooked vector in exploration targeting using the flin flon glennie complex of the trans hudson orogen as a template we show that the geochemistry of granitoid rocks can be a useful parameter in differentiating prospective and nonprospective areas for volcanic hosted massive sulfide vhms deposits the flin flon glennie complex includes 1910 to 1860 ma plutonic rocks that can be subdivided into three suites 1 juvenile arc related rocks of ferroan and magnesian subtypes 2 contaminated arc related rocks and 3 successor arc related rocks of both low and high sr y ratio petrogenesis of ferroan juvenile and low sr y successor arc plutons indicates they formed in regions of thinner crust and hence would have been associated with higher heat flow and arc rifting conditions that favor vhms endowment regions of the orogen with both ferroan and low sr y plutonic subtypes were long lived zones of thin crust and preserve vhms mineralization related to both juvenile and successor arc magmatism in contrast volcanic belts dominated by contaminated arc or high sr y successor arc plutons formed on thicker substrates with contaminated arc rocks built on archean paleoproterozoic pericratonic slivers and high sr y successor arc rocks associated with submarine to subaerial arc systems these regions of thicker crust demonstrably lack vhms deposits and were also the locus for late unroofing our results suggest that pluton geochemistry regionally correlates with mineral occurrence and could represent a useful regional exploration tool in buried or poorly exposed parts of the flin flon glennie complex and other paleoproterozoic orogens