constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

response to comment by t scambos and c shuman 2016 on mass gains of the antarctic ice sheet exceed losses by h j zwally and others 2015

copper mineralization prevented by arc root delamination during alpine himalayan collision in central iran contrasting geochemical signatures of copper ore hosting eocene and some barren undeformed miocene diorites to granites in central iran temporally overlap with the alpine himalayan collision and provide key implications on the existence and lack of cu mineralization during collisional magmatism high sr and low y and yb contents of eocene arc rocks in the natanz arc segment reflect thickened andean type orogenic arc crust 45 km whereas barren miocene natanz arc rocks 21 19 ma indicate thin arc crust similar to collisional volcanism in anatolia geochemical modeling indicates a change in the mineralogy of the melt residual from precollisional eocene basaltic garnet bearing 5 30 amphibolite to syn or postcollisional miocene metasomatized mantle peridotite which can be explained by collision induced delamination of the arc lithospheric root subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet bearing arc crustal keel explain the low sr and high y and yb contents the relatively enriched initial sr isotope ratios of postcollisional miocene natanz rocks and the lack of copper mineralization in postcollisional miocene natanz arc rocks arc root delamination removes the copper and sulfur enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits lack of the dense melt residues also provides an alternative explanation for the elevated thin crustal iranian back arc plateau 38 km as a result of uplift by isostatic rebound rather than uplift by anomalous shortening miocene arc root delamination implies a minimum age of 21 ma for the alpine himalayan collision in central iran

balancing crustal thickening in arcs by tectonic vs magmatic means igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the late cretaceous to late eocene magmatic arc in north chile 21 45 22 30 s temporally changing major and trace elements of magmatic rocks from this paleo arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening balanced structural cross sections indicate a minimum of 9 km of arc normal shortening that occurred simultaneously with dextral arc parallel movements accounting for orogen parallel lengthening of 10 km this shortening produced 5 4 km of tectonic crustal thickening and resulted in a minimum of 42 km late eocene andean crustal thickness temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder 2 6 km was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression the ratio of tectonic to magmatic crustal thickening is 2 1 whole crustal magmatic addition rates during the 12 m y duration of arc transpression are 35 km 3 per kilometer of model arc length per million years mafic underplating may have thickened the andean crust considerably but most pre neogene crustal thickening was due to discrete episodes of tectonic shortening

magmatic anhydrite in plutonic rocks at the el teniente cu mo deposit chile and the role of sulfur and copper rich magmas in its formation anhydrite occurs along with biotite feldspars quartz fe oxides and sulfides in fine grained igneous rocks that form the small porphyry a stock at el teniente these minerals crystallized together in the space generated by the intrusion of this stock and they do not replace preexisting phases planar crystal boundaries between anhydrite and other phases as well as the fact that plagioclase and biotite are fresh and unaltered also suggest that anhydrite is an igneous phase and not the product of secondary hydrothermal alteration plagioclase is unusually na rich ranging from oligoclase in mafic varieties of these rocks with 20 percent biotite to albite in more felsic samples with 20 percent these unusual anhydrite bearing plutonic rocks which contain up to 3 wt percent s and 0 5 wt percent cu are isotopically similar to all other igneous rocks related to the deposit these rocks formed during a late miocene period of regional compressive deformation with no coeval volcanic activity in the area of the deposit which prevented devolitalization of the oxidized parent magma in the large productive magma chamber inferred to have underlain el teniente at 4 km below the paleosurface this allowed igneous fractionation involving the recharge of mantle derived mafic magmas into the base of the chamber as well as volatile transfer and concentration near its roof to produce the cu and s rich magmas that formed the anhydrite bearing intrusive rocks

earthquake magnitude potential of the intermountain seismic belt usa from surface parameter scaling of late quaternary faults two of the largest historical earthquakes documented for the united states occurred within the past four decades in a zone of anomalous seismicity in the western interior of the country known as the intermountain seismic belt isb paleoseismology has revealed the presence of major active faults throughout this region some of them extending through densely populated area this study presents mean value estimates of the maximum earthquake magnitudes that can be expected from 65 of the most prominent late quaternary fault segments of the isb using surface rupture parameters linear least squares regressions of magnitude on surface rupture length l maximum surface displacement d and dl respectively were computed for an updated worldwide set of historical normal slip earthquakes these were compared with similar regressions from other publications and it was found that a dl based formula restricted to normal faulting mechanisms has a relatively high correlation low standard deviation and tends to yield magnitudes between those given by d and l alone this last property is useful for applications to the isb prehistoric data where the record of historical ground rupturing earthquakes is too sparse to estimate the proportions in which d and l are likely to contribute to seismic energy release of a fault application of regressions to published paleoseismic displacements and estimated earthquake segment lengths for the isb faults shows that d consistently scales a larger magnitude than the corresponding single segment l by an average of 0 2 to 0 3 magnitude unit after accounting for bias from the regressions considerations of uncertainties in paleoseismic displacements suggest that they cannot fully explain the discrepancy since it is known that large earthquakes often encompass multiple segments of a fault these observations indicate that longer rupture lengths should be used to estimate maximum magnitudes for isb faults a dual segment rupture scenario produces better agreement between d and l based magnitudes and is supported by historical and paleoseismic earthquake data from the region magnitudes scaled by the dl based regression for dual segment rupture range from ms 6 8 to 7 5 m 0 20 and the largest are associated with the central segments of the wasatch fault and the swan grand valley teton madison and east bear lake faults

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

the fate of magmatic sulfides during intrusion or eruption bingham and tintic districts utah magmatic sulfides in 97 samples of volcanic and intrusive rocks from the tertiary bingham cu au mo and tintic ag pb zn cu au districts utah were examined to help better understand the fate of magmatic sulfides during intrusion and eruption our findings show that shallowly emplaced dikes and sills have erratic but locally high concentrations of sulfides volcanic rocks and large porphyry intrusions from these districts typically have at least two orders of magnitude fewer sulfides than the dikes sulfide concentrations vary dramatically across these dikes and sills for example in one sill in castro gulch bingham district sulfide abundance increases from 9 ppm by volume in the center to more than 2 000 ppm near the margin chalcophile metals show corresponding changes in abundance for example the whole rock copper content of the sill ranges from 23 ppm in the center to 35 ppm along the margins the textures of sulfide grains interpreted to reflect recrystallization resorption and degassing even in the most sulfide rich samples commonly have been modified suggesting that no sample preserves all of its original magmatic sulfide content immiscible liquids of monosulfide solid solution crystallized as pyrrhotite pyrrhotite and chalcopyrite or pyrite and chalcopyrite with declining temperature and pressure these locally recrystallized to pyrite and chalcopyrite or to pyrite and an fe oxide as they are oxidized the alteration and preservation textures change from subspherical sulfide blebs near the margins of dikes and sills to partially altered sulfides farther in to complete absence of sulfides in the vast majority of intrusions except where small sulfides are completely enclosed by phenocrysts sulfide concentrations appear to vary according to cooling rate and inferred pressure at the time of quenching or crystallization of the matrix most of the sulfides along the quenched margins of these dikes and sills are in the matrix slower cooling coupled with removal of magmatic volatiles including sulfurous gases e g h2s so2 allows the resorption or oxidation of magmatic sulfides to occur during final crystallization of a magma together these processes remove greater than 90 percent of the original endowment of magmatic sulfides this probably explains the low magmatic sulfide abundances of slowly cooled large porphyritic intrusions and most importantly allows metals and sulfur to participate in the formation of porphyry deposits the relative abundances of base metals lost from the center of the sill are similar to the relative abundances of the metals in the bingham deposit production and reserves suggesting that these processes also may have operated at a larger scale

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

evidence for tibetan plateau uplift before 14 myr ago from a new minimum age for east west extension important changes in south asian climate occurred in the late miocene epoch 8 myr ago 1 2 and these have been attributed by some researchers to uplift of the tibetan plateau at about the same time3 5 unfortunately this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence one way to determine the minimum age of uplift is to establish the initiation age of the north striking normal fault systems in southern tibet that are widely regarded6 10 as being related to gravitational collapse of the tibetan plateau here we report an40ar 39ar age of 14 myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north central nepal this age implies that east west extension began before 14 myr ago in at least some parts of the tibetan plateau suggesting that the plateau attained its high mean elevation well before late miocene time

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

the role of magmatic sulfides and mafic alkaline magmas in the bingham and tintic mining districts utah this paper reports the existence of abundant magmatic sulfides in introduction latite lava vitrophyres and vent facies dikes from the bingham and the eruption of sulfur rich magmas from el chichon tintic districts in central utah these dikes and flows appear to luhr et al 1984 and mt pinatubo westrich gerlach be comagmatic with the ore related monzonite intrusions in addition 1992 in recent years has attracted attention to the limited analytical data suggest that the magmatic sulfides contain most of solubility of sulfur in intermediate magmas and the factors the cu and ag present in the latites magmatic sulfides are preserved potentially responsible for the formation of magmatic only in the least oxidized and least degassed lavas and vitrophyres sulfides and anhydrite however the occurrence of magpetrography of comagmatic dikes and intrusions suggests that matic sulfides in intermediate magmas is not universal resorption and oxidation s degassing of the globules may make many volcanic suites appear to be sulfide unsaturated the metal content available to a magmatic ore fluid minette and inasmuch as magmatic sulfides may sequester chalcophile shoshonite lavas occur in subordinate amounts in the latitic volcanic metals what would their presence signify in magmatic fields associated with latite monzonite intrusions and hydrothermal systems that formed porphyry cu and mesothermal vein ore deposits of the tintic ag pb zn cu au and bingham deposits do they sequester a significant portion of the porphyry cu au mo districts utah the ages chemical and magmatic cu pb zn ag and au thereby restricting modal compositions and field relations of the mafic alkaline lavas metal availability of subsequent ore forming fluids suggest that such magmas mixed and fractionated at shallow crustal we have examined the volcanic rocks from the tintic levels to form the ore related latites and monzonites the unaltered and bingham mining districts fig 1 utah and found mafic alkaline lavas in these districts are some of the most cu rich that almost all of the volcanic units contain magmatic lavas in the great basin province sulfides some quenched dikes and intrusion margins also have preserved magmatic sulfides comparable with those in the volcanic rocks several lines of evidence indicate that the volcanic and intrusive rocks are comagmatic keith et al 1989 1991 waite 1996 the tintic district and east tintic sub district in utah have recorded over

contributions from mafic alkaline magmas to the bingham porphyry cu au mo deposit utah usa the bingham porphyry cu au mo deposit utah may only be world class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc alkaline system volcanic mafic alkaline rocks in the district are enriched in cr ni and ba as well as cu au platinum group elements pge and s the bulk of the volcanic section that is co magmatic with ore related porphyries is dacitic to trachytic in composition but has inherited the geochemical signature of high cr ni and ba from magma mixing with the mafic alkaline rocks the volcanic section that most closely correlates in time with ore related porphyries is very heterogeneous containing clasts of scoriaceous latite latitic and minette and flows of melanephelinite shoshonite and olivine latite in addition to volumetrically dominant dacite trachyte bingham ore related porphyries show ample evidence of prior mixing with mafic alkaline magmas intrusive porphyries that have not been previously well studied have several chemical and mineralogical indications of magma mixing these mixed lithologies include the hybrid quartz monzonite porphyry biotite porphyry and minette dikes even some of the more silicic latite and monzonite porphyries retain high cr and ba contents indicative of mixing and contain trace amounts of sapphire 1 mm the heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical mineralogical and isotopic data magma mixing calculations suggest about 10 of the monzonitic latitic ore related magma may have been derived from mafic alkaline magma similar to the melanephelinite if the original s content of the mafic magma was about 2 000 4 000 ppm comparable with similar magmas then the mafic magma may have been responsible for contributing more than half of the s and a significant portion of the cu au and pge in the bingham deposit

porphyry cu mo au deposits related to melting of thickened mafic lower crust examples from the eastern tethyan metallogenic domain abstract most porphyry cu deposits in the world occur in magmatic arc settings and are formed in association with calc alkaline arc magmas related to subduction of oceanic lithosphere this contribution reviews a number of significant porphyry cu deposits in the eastern tethyan metallogenic domain they widely occur in a variety of non arc settings varying from post late collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes their spatial temporal localization is controlled by strike slip faults orogen transverse normal faults lineaments and their intersections in these non arc settings these deposits are dominated by porphyry cu mo deposits with minor porphyry cu au and epithermal au deposits and exhibit a broad similarity with those in magmatic arcs the associated magmas are generally hydrous relatively high fo2 high k calc alkaline and shoshonitic and show geochemical affinity with adakites they are distinguished from arc magmas and or oceanic slab derived adakites by their occurrence as isolated complexes high k2o contents 1 2 8 5 and much wider range of end t values 10 to 3 and positive ehf t values 4 6 to 6 9 these potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower crust or delaminated lower crust but also involving various amounts of asthenospheric mantle components key factors that generate hydrous fertile magmas are most likely crust mantle interaction processes at the base of thickened lower crust in non arc settings rather than oceanic slab dehydration as in arc settings breakdown of amphibole in thickened lower crust e g amphibole eclogite and garnet amphibolite during melting is considered to release fluids into the fertile magmas leading to an elevated oxidation state and higher h2o content necessary for development of porphyry cu mo au systems copper and au in hydrous magmas are likely derived from mantle derived components and or melts which either previously underplated and infiltrated at the base of the thickened lower crust or were input into the primitive magmas by melt mantle interaction in contrast mo and part of the s in the fertile magmas are probably supplied by old crust during melting and subsequent ascent

fluid density control on vapor liquid partitioning of metals in hydrothermal systems hot aqueous fluids both vapor and saline liquid are primary transporting media for metals in hydrothermal magmatic systems despite the growing geological evidence that the vapor phase formed through boiling of magmatic ore bearing fluids can selectively concentrate and transport metals the physical chemical mechanisms that control the metal vapor liquid fractionation remain poorly understood we performed systematic experiments to investigate the metal vapor liquid partitioning in model water salt gas systems h2o nacl kcl hcl at hydrothermal conditions measurements show that equilibrium vapor liquid fractionation patterns of many metals are directly related to the densities of the coexisting vapor and liquid phases despite differences in the vapor phase chemistry of various metals that form hydroxide chloride or sulfide gaseous molecules of contrasting volatile properties water solute interaction is a key factor that controls the metal transfer by vapor like fluids in earth s crust these findings allow quantitative prediction of the vapor liquid distribution patterns and vapor phase metal transport in a wide range of conditions our density model accounts well for the vapor brine distribution patterns of na si fe zn as sb and ag observed in fluid inclusions from magmatic hydrothermal deposits for au and cu the partitioning in favor of the liquid phase predicted in a sulfur free system contrasts with the copper and gold enrichment observed in natural vapor like inclusions the formation of stable complexes of cu and au with reduced sulfur may allow for their enhanced transport by sulfur enriched magmatic hydrothermal vapors

paleoseismicity and neotectonics of the cordillera blanca fault zone northern peruvian andes the cordillera bianca fault zone is a major west dipping normal fault that bounds the west side of a 120 to 170 km wide zone of active extension along the crest of the northern peruvian andes the fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late pleistocene and holocene displacements but has not been the source of historical or teleseismically recorded earthquakes trenching and mapping of fault scarps provide new information on earthquake recurrence slip rate timing of the most recent events and andean neotectonics at quebrada queroccocha 55 km from the south end of the zone an 11 000 to 14 000 year old moraine is displaced 12 15 m and younger valley fill lacustrine and fluvial deposits are displaced 7 5 8 m scarp profiles tectonic terraces and trench exposures indicate five to seven scarp forming earthquakes of 2 3 m per event during the past 11 000 14 000 years at this location considering uncertainties in the number of events and in the age of the displaced moraine the average recurrence interval is estimated to be 2440 1060 years detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480 65 and 750 80 14c years b p respectively and stratigraphic relationships suggest that the event may be closer to the older date the late quaternary slip rate at quebrada queroccocha is 0 86 1 36 mm yr and appears to be representative of much of the fault zone at pachma bajo located 30 km from the north end of the zone on a separate segment displacement during each of the past two events was 2 to 3 m the morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000 3000 years relationships between pre inca walls and faulted deposits suggest that 1500 2000 years have elapsed since the most recent event fault behavior parameters particularly displacement per event and fault segment length show that the cordillera bianca fault zone is similar to seismogenic normal faults that nucleate m 7 7 earthquakes at depths of approximately 15 km the quebrada queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late quaternary extensional strain rates of about 1 4 mm yr across the complete zone of andean crestal extension

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

porphyry gold copper mineralisation in the cadia district eastern lachlan fold belt new south wales and its relationship to shoshonitic magmatism the cadia porphyry gold copper district is the largest hydrothermal intrusion related gold deposit in eastern australia discovered in 1992 by newcrest mining limited pre mine resources in the district were in excess of 585 t au and 2 35 mt cu the cadia district lies within shoshonitic subaqueous volcanic rocks of the late ordovician molong volcanic belt in the eastern lachlan fold belt lfb of new south wales an island arc tectonic setting is envisaged for the formation of these volcanic rocks but the nature of the arc basement and geometry of the subduction zone s is debated mineralisation occurs in four principal porphyry deposits cadia hill cadia ridgeway cadia east cadia far east and cadia quarry as sheeted and stockwork quartz sulfide veins and locally as broadly stratabound disseminated mineralisation cadia east and skarn big cadia and little cadia all of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the cadia intrusive complex cic gold copper mineralisation is hosted by these intrusions and also by the enclosing forest reefs volcanics frv wall rocks hydrothermal alteration associated with mineralisation is potassic which is overprinted by selectively pervasive propylitic and silica albite assemblages petrological studies and major and trace element analysis of rocks of the cic and frv as well as of other intrusions in the cadia district that appear unrelated to mineralisation have been conducted unaltered samples from the cic are characterised by high k2o contents up to 6 5 wt and molecular k na ratios consistently 1 confirming the alkalic and shoshonitic nature of the complex the cic samples also fall just within the shoshonite field on ce yb ta yb and th yb ta yb plots although immediately adjacent to the calc alkaline field the frv are geochemically similar to the cic although lower in p2o5 and ce lree dacitic and hornblende porphyry intrusions from within the district but spatially unrelated to mineralisation have a distinctive geochemical signature and are not shoshonitic in composition the close spatial association between gold copper mineralisation and the shoshonitic monzonitic cic argues strongly for a genetic link between the two a similar association occurs at the economic goonumbla porphyry gold copper deposits in the eastern lfb while sub economic gold copper mineralisation at cargo and copper hill is associated with calc alkaline quartz diorite and dacitic intrusions the cadia district is considered to be an excellent example of the association of alkaline potassic magmas and gold copper porphyry style mineralisation

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

geochronology and geochemistry of the bangpu mo cu porphyry ore deposit tibet abstract we have determined the ages of the ore bearing bangpu porphyries and the age of formation of an associated mo cu ore deposit using the la icpms zircon u pb and the molybdenite re os methods zircons from two adamellite porphyries give ages of 14 07 0 08 ma and 14 96 0 16 ma and zircons from a diorite porphyry give an age of 15 30 0 25 ma the ages of three molybdenite samples are 14 96 0 23 ma 15 08 0 23 ma and 16 61 0 23 ma light rare earth elements lree are enriched in the ore bearing adamellite which is shoshonitic and peraluminous while heavy rare earth elements hree and y are strongly depleted indicating an adakitic affinity in contrast to other deposits formed in the post collisional gangdese belt the rb sr ratio in bangpu is high demonstrating more crustal contamination the geochemistry of the ore bearing diorite porphyry differs significantly from that of the ore bearing adamellite the diorite porphyry displays a reverse slope in its ree rare earth element pattern and is shoshonitic and metaluminous the geochronology and geochemistry of both ore bearing rocks indicate that the bangpu ore deposit is related to two different source regions in a post collisional environment the metallic minerals formed mainly during the older diorite porphyry stage but they were recycled and reactivated during adamellite magmatism the sources of the mineralising fluids and the evolution of the northern gpcb gangdese porphyry copper belt where the bangpu ore deposit is located are more complex than in the southern gpcb

geology and origin of the post collisional narigongma porphyry cu mo deposit southern qinghai tibet narigongma is a poorly studied mo rich similar to 0 06 wt post collisional porphyry cu deposit located in southern qinghai province tibet 400 km northwest of the yulong porphyry cu mo au belt the narigongma deposit has a similar age 43 40 ma to porphyry deposits in the yulong belt but different ore assemblages the narigongma deposit is associated with eocene granodiorite and granite intrusions that were emplaced into a permian volcanic sedimentary rock sequence an similar to 43 3 ma biotite granite stock p1 porphyry is the earliest eocene intrusion and this was itself intruded by a number of smaller similar to 43 6 ma fine grained granite porphyry stocks p2 porphyry and several post ore quartz diorite porphyry dikes similar to 41 7 ma the main cu mo mineralization at narigongma is associated with the p1 porphyry hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones and an outer propylitic zone hypogene mineralization at narigongrna was characterized by early stage precipitation of molybdenite during potassic alteration and late stage deposition of chalcopyrite during phyllic alteration deposition of both the mo and cu mineralization stages was caused by decreasing temperature a high degree of crystallization of the p1 porphyry occurred prior to fluid saturation that produced mo enrichment in the residual melt due to the incompatible behavior of mo however compatible cu was sequestered by the crystallizing phases and resulted in the generation of a high mo cu magmatic hydrothermal fluid and the final mo cu mineralization assemblage zircon epsilon hf t values of 4 1 to 7 9 are indicative of magma derivation from a depleted source these isotopic data coupled with other geochemical characteristics of the narigongma porphyry such as high sio2 and k2o contents low mgo contents and compatible element abundances and highly fractionated rare earth element patterns indicate a mixing model for the origin of the porphyry bodies generation of the post collisional ore forming porphyries occurred in two stages 1 partial melting of metasomatized phlogopite bearing lithospheric mantle that generated potassic to ultra potassic mafic melts and 2 underplating of such melts beneath thickened juvenile lower crust which triggered partial melting of the lower crust to produce the ore forming high k adakitic magma c 2013 international association for gondwana research published by elsevier b v all rights reserved

relative oxidation states of magmas inferred from ce iv ce iii in zircon application to porphyry copper deposits of northern chile major and trace element compositions of zircons and whole rocks from 14 barren and seven ore bearing calc alkaline intrusions from the chuquicamata el abra porphyry copper belt of northern chile have been measured in situ by excimer laser ablation ela icp ms these data permit the ce iv ce iii ratio within zircon to be calculated using a lattice strain model for mineral melt partitioning of ce iv and ce iii zircon ce iv ce iii and eun eun ratios and by inference magmatic oxidation states generally increase from older mafic to younger felsic units within this sequence porphyry copper mineralization is directly associated only with intrusions with zircon ce iv ce iii 300 and eun eun 0 4 such trends can be understood in terms of interdependent relations between oxygen fugacity sulfur speciation and solubility and chalcophile element partitioning in silicate magmas because zircon occurs in most calc alkaline intrusions and is resistant to subsolidus alteration zircon ce iv ce iii ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic hydrothermal cu au mineralization the approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

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

high sr y magmas reflect arc maturity high magmatic water content and porphyry cu mo au potential examples from the tethyan arcs of central and eastern iran and western pakistan the temporal and geochemical evolution of arc magmatism that culminated in porphyry cu mo au deposit formation has been studied in three separate neo tethyan arc systems in central and eastern iran and western pakistan porphyry cu au deposits in the lut block of eastern iran formed in the middle eocene at the end of a period of extensive paleocene eocene volcanism porphyry cu mo deposits in the kerman belt of central iran formed in the middle miocene at the end of a period of voluminous eocene oligocene volcanism and porphyry cu au deposits in the chagai belt of western pakistan formed in four pulses during the eocene early miocene middle late miocene and late miocene pliocene after a prolonged period of arc magmatism that began in the late cretaceous and is still active in each region the late porphyry related magmas are more geochemically evolved and more hydrous as evidenced by the presence of hornblende phenocrysts than the preceding volcanic rocks we suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years leading to the generation of more evolved volatile rich magmas at later stages of the arc s history high magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore forming subvolcanic magmatic hydrothermal systems it is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites later suites are more prospective noting the common presence of hornblende or biotite phenocrysts indicating high magmatic water contents and through lithogeochemical fingerprinting of magmatic fractionation processes relatively high sr y and la yb ratios and eun eu ratios 1 indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

geochemical variations in miocene adakitic rocks from the western and eastern lhasa terrane implications for lower crustal flow beneath the southern tibetan plateau abstract it is generally believed that miocene adakitic rocks in the lhasa terrane block of southern tibet were derived from a thickened lower crust and that their compositions will therefore reflect the geochemical characteristics of their lower crustal source s adakitic rocks in the western part of the lhasa terrane possess geochemical and sr nd isotopic characteristics that are clearly different from those in the eastern part of the lhasa terrane the former display a more enriched nd sr isotopic signature together with an older model age relative to the latter suggesting that the compositions of the lower crust under the western and eastern parts of the lhasa terrane also differ in addition the adakitic rocks in the himalayas on the southern side of the indus yalu suture iys have similar geochemical characteristics and sr nd isotopic compositions to the adakitic rocks of the western lhasa terrane but differ from rocks in the eastern lhasa terrane and from igneous rocks derived from crust beneath the himalayas we suggest that the lower and or middle crustal materials beneath the western lhasa terrane probably extend to the southern side of the iys due to channel flow if mid lower crustal flow has occurred below southern tibet the southeastward moving anatectic mid lower crustal material was probably derived from below the western part of the lhasa terrane rather than the eastern moreover the emplacement ages of adakitic rocks from mayum on the southern side of the iys imply that southeastward ductile flow of mid lower crust beneath western lhasa may have occurred as early as 17 ma

some metallogenic features of gold and copper deposits related to alkaline rocks and consequences for exploration porphyry copper gold skarn copper gold sediment hosted carlin style gold breccia pipe low sulphidation epithermal gold pluton related mesothermal or orogenic gold vein and volcanogenic massive sulphide deposits associated with alkaline rocks are commonly broadly similar to those hosted by their calc alkaline counterparts in contrast porphyry molybdenum gold deposits are confined to alkaline igneous centres alkaline suites are notably deficient in high sulphidation epithermal gold deposits and even in the advanced argillic lithocaps which host them this is surprising given that the required oxidised sulphur species are seemingly more abundant than in calc alkaline igneous centres highly efficient buffering of acidic fluid by metasomatised alkaline rocks may offer a viable explanation all types of intrusion related zinc deposits also appear to be poorly developed in alkaline provinces the characteristics of several gold and copper deposits associated with alkaline rocks including the giant porgera cripple creek ladolam olympic dam and phalaborwa examples are judged to diverge appreciably from their most closely related deposit types rendering them arguably unique most of these aberrant mineralisation styles may be due to variations in magmatic fluid compositional and liberation characteristics consequent upon the extreme diversity of ore related alkaline magmas most types of gold and copper deposits developed in calc alkaline provinces also constitute exploration targets in and around alkaline igneous centres although porphyry copper gold and low sulphidation epithermal gold deposits are considered to possess the greatest potential perhaps of even greater interest however is the possibility of encountering unconventional giant gold and copper deposits which lack closely analogous examples if already defined aberrant deposits are truly unique then exploration designed specifically to detect additional examples is pointless furthermore exploration for new unique deposits is difficult because their defining geological parameters are unknown alkaline rocks within or behind calc alkaline arcs at convergent plate boundaries probably offer the greatest exploration potential although anorogenic intracontinental extensional settings should not be ignored

origin of late oligocene adakitic intrusives in the southeastern lhasa terrane evidence from in situ zircon u pb dating hf o isotopes and whole rock geochemistry abstract we describe the whole rock geochemistry biotite ar ar and zircon u pb geochronology and in situ hf o isotopic compositions of the linzhi two mica granitoid in the southeastern lhasa terrane the granitoids are calc alkaline to high k calc alkaline in composition and they have geochemical affinities with adakites the whole rock geochemistry with high sio 2 and al 2 o 3 contents low mgo cr and ni contents low mg values negative e nd t 6 2 to 2 9 and low 87 sr 86 sr i 0 7059 0 7074 together with the in situ zircon hf o isotopic data e hf t 2 7 to 2 7 18 o 7 2 8 8 indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the lhasa terrane this lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs 40 ar 39 ar ages for biotites and the presence of magmatic epidote suggest the granitoid was intruded at mid crustal depths indicating that the eastern lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22 ma the geochemical features of the linzhi adakitic rocks are significantly different from those of eocene adakitic rocks from the north himalayan belt and this suggests that we can rule out the possibility of crustal thickening in the eastern part of lhasa terrane caused by impingement of the indian lower crust post collisional 26 10 ma adakitic rocks occur within an e w trending magmatic belt that runs parallel to the yarlung tsangpo suture in the lhasa terrane these rocks have distinctive variations in geochemistry along the strike of the lhasa terrane indicating that the lower crust is isotopically heterogeneous from west to east along the terrane these geochemical variations probably result from variable degrees of input along strike of juvenile mantle derived melts into an old lower crust the middle part of the lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle derived melt underplated during neo tethyan oceanic crust subduction in contrast the lower crust below the western and eastern parts of the terrane is relatively ancient therefore crustal thickening caused by underplating of mantle derived magmas could play an important role in the middle segment of the terrane whereas crustal compression and shortening as a result of the collision of the indian and asian plates could be the major cause for crustal thickening in the western and eastern segments

minette lavas and associated leucitites from the western front of the mexican volcanic belt petrology chemistry and origin during the late pliocene k rich minette and leucitite lavas erupted in the western mexican volcanic belt near the town of los volcanes a region which is located much closer to the middle america trench than the main line of currently active andesite stratovolcanoes during this period the tectonic regime in western mexico was highly complex due to the simultaneous occurrence of active subduction of the young rivera plate and rifting caused by crustal extension most of these basic lavas contain phenocrysts of phlogopite augite and apatite along with microphenocrysts of leucite and fe ti oxide olivine is absent from all but two of the flows one an olivine leucitite and the other a felsic minette the phlogopite phenocrysts and high whole rock fe2o3 feo ratios which are characteristic of this suite record evidence of high magmatic water contents and oxygen fugacities all of these rock types are highly enriched in the large ion lithophile and light rare earth elements with sr 5100 ppm ba 4800 ppm and ce 330 ppm they are also mildly enriched in the high field strength elements e g zr 260 700 ppm and display the strong relative enrichment of the lile over the hfse that is characteristic of magmas erupted in convergent margins consideration of high pressure phase equilibria in the mg2sio4 camgsi2o6 kalsio4 sio2 system shows that the minettes from this region are not related through fractional crystallization to the more mgo rich olivinebearing minettes which have erupted in other parts of the western mexican volcanic belt during the quaternary this conclusion is consistent with both the trace element geochemistry of these lavas and with the results of fractional crystallization models instead the data suggests that these high k magmas were derived from a source region which consists predominantly of phlogopite clinopyroxene and apatite and which has formed through hydrous enrichment of the subarc mantle in response to subduction

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

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

lateral erosion toe cutting of alluvial fans by axial rivers implications for basin analysis and architecture abstract we document the neglected phenomenon of lateral erosion toe cutting of alluvial fans by non incising axial river channels field examples from the holocene of the big lost river basin idaho and the plio pleistocene of the rio grande rift new mexico help to establish architectural models with more general application to basin analysis the process of toe cutting can lead to complete fan destruction and may be a response to climate change tectonic tilting fault propagation or a combination of these variables it gives rise to near horizontal erosion surfaces cut in fan sediment steep fan margin scarps progressive up fan incision from the scarp by a network of channels soil formation up fan away from the incised channel network a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp once avulsion occurs to take the axial channel away from the bajada margin distinctive healing wedges of fan alluvium prograde across abandoned axial river channel and floodplain deposits gradually onlapping the eroded scarp and its upstream network of incised channels toe cutting has important stratigraphic basin analysis and economic consequences bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments

dynamic inland propagation of thinning due to ice loss at the margins of the greenland ice sheet cryospheric sciences branch nasa goddard space flight center greenbelt md usaabstract mass balance analysis of the greenland ice sheet based on surface elevation changesobserved by the european remote sensing satellite ers 1992 2002 and ice cloud and land elevationsatellite icesat 2003 07 indicates that the strongly increased mass loss at lower elevations 2000m of the ice sheet as observed during 2003 07 appears to induce interior ice thinning athigher elevations in this paper we perform a perturbation experiment with a three dimensionalanisotropic ice flow model aif model to investigate this upstream propagation observed thinningrates in the regions below 2000m elevation are used as perturbation inputs the model runs withperturbation for 10 years show that the extensive mass loss at the ice sheet margins does in fact causeinterior thinning on short timescales i e decadal the modeled pattern of thinning over the ice sheetagrees with the observations which implies that the strong mass loss since the early 2000s at lowelevations has had a dynamic impact on the entire ice sheet the modeling results also suggest that evenif the large mass loss at the margins stopped the interior ice sheet would continue thinning for 300 yearsand would take thousands of years for full dynamic recovery

porphyry gold deposits of the refugio district maricunga belt northern chile the porphyry gold deposits of the refugio district and similar deposits in the maricunga belt contain the lowest known copper to gold ratios cu ppm au 0 03 of any porphyry type deposit the gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks both the verde and pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite magnetite albite and or pyrite albite clay alteration to a shallow zone of pyrite albite clay and local quartz alunite ledges pancho contains an additional deepest porphyry copperlike zone with quartz veinlets a veinlets and potassic alteration relative to verde pancho is telescoped with all three zones present within a 400 m vertical interval the porphyry copperlike zone at pancho is characterized by a veinlets and pervasive potassic alteration both restricted to intrusive rocks a veinlets range from hairline streaks of magnetite biotite with minor quartz and chalcopyrite and k feldspar alteration envelopes to sugary quartz veinlets 1 cm in width with magnetite and chalcopyrite and no alteration envelopes hypersaline liquid inclusions coexisting with vapor rich inclusions indicate temperatures above 600 c and salinities as high as 84 wt percent nacl equiv a pressure estimate of 250 bars indicates a depth of 1 000 m assuming lithostatic pressure potassic alteration consists of a central zone of magnetite k feldspar oligoclase that changes outward to a biotite rich zone total sulfide content predominately as chalcopyrite is generally 1 vol percent whereas magnetite content is 2 to 5 percent where a veinlets and potassic alteration predominate grades are typically 0 1 wt percent hypogene copper and 0 5 to 1 ppm gold banded quartz veinlets are present at both verde and pancho where they occur mostly above a veinlets and cut a veinlets where they overlap they are less than 2 cm in thickness and lack alteration envelopes dark gray bands whose color is due to abundant vapor rich fluid inclusions and micrometer sized grains of magnetite commonly occur as symmetric pairs near the vein walls the bands are commonly botryoidal and are continuous through quartz grains suggesting that the quartz recrystallized from a silica gel rare liquid rich fluid inclusions in quartz indicate temperatures 350 c and salinities 35 wt percent nacl equiv estimated pressures are 200 bars suggesting depths of 190 to 1 500 m under hydrostatic pressure gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite chlorite illite and k feldspar banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures zones of abundant banded veinlets without early a veinlets generally contain 0 5 to 2 ppm gold and 0 05 wt percent hypogene copper most of the differences between porphyry gold deposits at refugio and porphyry copper deposits can be attributed to shallower depths of formation less than 1 km compared to 1 5 to 4 km that is typical for porphyry copper deposits shallower depths resulted in lower sulfide concentrations local garnet veinlets widespread albite bearing alteration and most importantly banded quartz veinlets which are unique to porphyry gold deposits banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high temperature magmatic fluids to hydrostatic pressures episodic rupturing of a brittle ductile boundary surrounding the intrusive centers at verde and pancho led to flashing of magmatic fluids loss of sulfur to vapor and low sulfide gold ratios in ore

giant ore deposits formed by optimal alignments and combinations of geological processes giant ore deposits are priority targets for mining companies a review of the characteristics of several giant porphyry and epithermal deposits worldwide suggests that they formed from ordinary processes that were fortuitously operating at maximum efficiency

grouping and migration of surface faulting and variations in slip rates on faults in the great basin province abstract nonuniformity of the occurrence of large slip events producing surface ruptures on seismogenic faults and variations in slip rate probably characterize seismogenic faulting in the great basin province western united states examples include the grouping of faulting events along the lost river fault idaho changes in tilt rates of the east range and cortez mountains nevada extension of slip along a fault on the northwest flank of the humboldt range nevada and migration or shifting of slip back and forth from one fault to another along subparallel range front faults in dixie valley nevada

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

antarctic elevation change from 1992 to 1996 satellite radar altimeter measurements show that the average elevation of the antarctic ice sheet interior fell by 0 9 0 5 centimeters per year from 1992 to 1996 if the variability of snowfall observed in antarctic ice cores is allowed for the mass imbalance of the interior this century is only 0 06 0 08 of the mean mass accumulation rate

indicators of faulting events and periods of quiescence in desert alluvial fans the main objective of the present paper is to present an integrated approach to the study of faulting stages in coarse alluvial deposits the study site is the alluvial fan of nahal shehoret presently under an extremely arid regime the exposure of fault scarps on the fan surfaces enables the identification of several geomorphic indicators employed in analysing and separating faulting events and periods of quiescence a the nature of fault scarp topography b soil catenary development on fault scarps c buried paleosols in alluvial deposits d colluvial bodies e rotated clasts along fault planes these indicators enable us to determine at all sites studied the total amount of displacement to separate faulting events and the amount of their displacement per event and to evaluate scarp stability and the duration of periods of tectonic quiescence

discrimination of productive and nonproductive porphyritic intrusions in the chilean andes a geochemical comparison between productive intrusions those associated with mineralization and nonproductive intrusions those unrelated to mineralization has been made on the basis of over 100 fresh granitoid rocks from the chilean porphyry copper belt the study was carried out on the scale of a mining district el salvador potrerillos then on the scale of a single traverse el salvador chilean coast and finally on the scale of the andes itself the productive intrusions are best characterized by large negative anomalies in y mn th and the heavy rare earth elements a discriminant diagram of y against mno is recommended for predicting whether an apparently fresh intrusion has associated peripheral mineralization the low values of y and mn in productive intrusions can be partly explained by the early crystallization of y and mn rich hydrous phases such as hornblende and by the loss of mn rich fluids from the magma the same elements make useful discriminants in other porphyry copper provinces although the exact position of the discriminant boundary varies from island arcs through continental margin to continental settings

cenozoic continental arc magmatism and associated mineralization in ecuador most of the economic ore deposits of ecuador are porphyry cu and epithermal style gold deposits associated with tertiary continental arc magmatism this study presents major and trace element geochemistry as well as radiogenic isotope pb sr signatures of continental arc magmatic rocks of ecuador of eocene to late miocene 50 9 ma elm and late miocene to recent 8 0 ma lmr ages the most primitive elm and lmr rocks analyzed consistently display similar trace element and isotopic signatures suggesting a common origin most likely an enriched morb type mantle in contrast major and trace element geochemistry as well as radiogenic isotope systematics of the whole sets of elm and lmr samples indicate strikingly different evolutions between elm and lmr rocks the elm rocks have consistently low sr y increasing rb sr and decreasing eu gd with sio2 suggesting an evolution through plagioclase dominated fractional crystallization at shallow crustal levels 20 km and most likely at sub crustal levels 40 50 km the change in geochemical signatures of tertiary magmatic rocks of ecuador from the elm to the lmr type coincides chronologically with the transition from a transpressional to a compressional regime that occurred at 9 ma and has been attributed by other investigations to the onset of subduction of the aseismic carnegie ridge

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

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

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

geochemical differences between subduction and collision related copper bearing porphyries and implications for metallogenesis abstract porphyry cu mo au deposits occur not only in continental margin arc settings subduction related porphyry cu deposits such as those along the eastern pacific rim eprim but also in continent continent collisional orogenic belts collision related porphyry cu deposits such as those in southern tibet these cu mineralized porphyries which develop in contrasting tectonic settings are characterized by some different trace element e g th and y concentrations and their ratios e g sr y and la yb suggesting that their source magmas probably developed by different processes subduction related porphyry cu mineralization on the eprim is associated with intermediate to felsic calc alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting assimilation storage and homogenization mash whereas k enriched collision related porphyry cu mineralization was associated with underplating of subduction modified basaltic materials beneath the lower crust with subsequent transformation into amphibolites and eclogite amphibolites and resulted from partial melting of the newly formed thickened lower crust these different processes led to the collision related porphyry cu deposits associated with adakitic magmas enriched by the addition of melts and the subduction related porphyry cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks all of which were associated with fluid dominated enrichment process in subduction related cu porphyry magmas the oxidation state fo2 the concentrations of chalcophile metals and other volatiles e g s and cl and the abundance of water were directly controlled by the composition of the primary arc basaltic magma in contrast the high cu concentrations and fo2 values of collision related cu porphyry magmas were indirectly derived from subduction modified magmas and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust and in part by the contribution from the rising potassic and ultrapotassic magmas both subduction and collision related porphyries are enriched in potassium and were associated with crustal thickening their high k2o contents were primarily as a result of the inheritance of enriched mantle components and or mixing with contemporaneous ultrapotassic magmas

mass balance of the antarctic ice sheet the antarctic contribution to sea level rise has long been uncertain while regional variability in ice dynamics has been revealed a picture of mass changes throughout the continental ice sheet is lacking here we use satellite radar altimetry to measure the elevation change of 72 of the grounded ice sheet during the period 1992 2003 depending on the density of the snow giving rise to the observed elevation fluctuations the ice sheet mass trend falls in the range 5 85gt yr 1 we find that data from climate model reanalyses are not able to characterise the contemporary snowfall fluctuation with useful accuracy and our best estimate of the overall mass trend growth of 27 29 gt yr 1 is based on an assessment of the expected snowfall variability mass gains from accumulating snow particularly on the antarctic peninsula and within east antarctica exceed the ice dynamic mass loss from west antarctica the result exacerbates the difficulty of explaining twentieth century sea level rise

extent of underthrusting of the indian plate beneath tibet controlled the distribution of miocene porphyry cu mo au deposits miocene igneous rocks in the 1 600 km long e w gangdese belt of southern tibet form two groups separated at longitude 89 e the eastern group is characterized by mainly intermediate felsic calc alkaline plutons with relatively high sr y ratios 23 to 342 low 87sr 86sr i ratios 0 705 to 0 708 and high endi values 5 5 to 6 1 in contrast the western group is characterized by mainly potassic to ultrapotassic volcanic rocks with relatively high th and k2o contents low sr y ratios 11 to 163 high 87sr 86sr i ratios 0 707 to 0 740 and low endi values 4 1 to 17 5 the eastern plutonic group is associated with several large porphyry cu mo au deposits whereas the western group is largely barren we propose that the sharp longitudinal distinction between magmatism and metallogenic potential in the miocene gangdese belt reflects the breakoff of the greater india slab and the extent of underthrusting by the indian continental lithosphere at that time magmas to the east of 89 e were derived by partial melting of subduction modified tibetan lithosphere mostly lower crust triggered by heating of hot asthenospheric melt following slab breakoff these magmas remobilized metals and volatile residual in the crustal roots from prior arc magmatism and generated porphyry cu mo au deposits upon emplacement in the upper crust in contrast magmas to the west of 89 e were formed by smaller volume partial melting of tibetan lithospheric mantle metasomatized by fluids and melts released from the underthrust indian plate they are less hydrous and oxidized and did not have the capacity to transport significant amounts of metals into the upper crust

contemporary seismicity in the eastern snake river plain idaho based on microearthquake monitoring microearthquake monitoring by a permanent network operated since 1971 and two recent temporary surveys performed by the idaho national engineering laboratory suggest that infrequent small magnitude earthquakes are characteristic of eastern snake river plain esrp seismicity although a total of only 19 earthquakes have been observed to date and not all have well constrained focal depths their relatively shallow occurrence at depths of 8 km is consistent with the hypothesis that elevated crustal temperatures in the esrp confine the brittle portion of the crust and hence seismogenesis to the upper 6 to 10 km a composite focal mechanism of two microearthquakes located near the axis of the esrp indicates normal faulting with a minor component of strike slip motion the northeast southwest trending t axis indicated by this composite mechanism is consistent with the direction of extension indicated by holocene volcanic rift zones within the esrp and normal faults in the adjacent basin and range province to the northwest based on this low level of small magnitude seismicity deviatoric stresses appear to be small in the esrp

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

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

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

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

the role of sulfur in the formation of magmatic hydrothermal copper gold deposits abstract essential resources of many rare metals including copper zinc molybdenum silver and gold occur in natural sulfide mineral deposits understanding the origin of these metal resources has been limited by a lack of data about the geochemistry of sulfur the most important and abundant element of ore deposits we report the first directly measured sulfur concentrations in high temperature fluids together with their ore metal contents using a new method for sulfur quantification in fluid inclusions by laser ablation inductively coupled plasma mass spectrometry la icp ms co genetic brine and vapor inclusions from magmatic hydrothermal ore deposits and granitic intrusions show an excess of sulfur over ore metals as required for efficient ore mineral precipitation the results demonstrate that s cu and au are highly enriched in vapor like magmatic fluids implying that such low salinity fluids are the key agent for the formation of porphyry copper and epithermal gold deposits

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

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

adakite like signature of late miocene intrusions at the los pelambres giant porphyry copper deposit in the andes of central chile metallogenic implications adakite like features are recognized in the late miocene 10 ma porphyritic intrusions of the los pelambres giant porphyry copper deposit central chile 32 s located within the southern portion of the flat slab segment 28 33 s of the chilean andes the al and na rich porphyries of los pelambres display distinctly higher sr y 100 300 and lan ybn 25 60 ratios than contemporaneous and barren magmatic units e g la gloria pluton cerro aconcagua volcanic rocks of the same andean magmatic belt strong fractionation of heavy rare earth elements hree absence of eu anomalies high sr y and zr sm and low nb ta ratios suggest melt extraction from a garnet amphibolite source the late miocene adakite like porphyritic intrusions at los pelambres formed closely related in time and space to the subduction of the juan fernandez ridge jfr hotspot chain along the chilean margin current tectonic reconstructions reveal that at the time of formation of the los pelambres rocks a w e segment of the jfr started to subduct beneath them producing a slow down of a previously rapid southward migration of a ne ridge trench collision these particular tectonic conditions are favorable for the origin of the los pelambres porphyry suite by melting of subducting young hotspot rocks under flat slab conditions the incorporation of crustal components into the oceanic lithopheric magma source by subduction erosion is evidenced by the sr nd isotope composition of the los pelambres rocks different from the morb signatures of true adakites a close relationship apparently exists between the origin of this adakite like magmatism and the source of the mineralization in the los pelambres porphyry copper deposit

petrology of an unusual orthopyroxene bearing minette suite from southeastern peru eastern andean cordillera al rich lamproites contaminated by peraluminous granites abstract the puno minettes eastern andean cordillera southern peru are phlogopite phyric rocks with a groundmass consisting mainly of phlogopite alkali feldspar and apatite microphenocrysts and or microlites they display the strong large ion lithophile element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan rb ba sr up to 103 primordial mantle ree up to 6 102 ci chondrites in addition to their close association with granitoids the rocks of this series show many features of minettes occurring in arc related collisional zones i e absence of mantle xenoliths up to 30 vol felsic xenocrysts and strong negative nb and ti anomalies however they are distinguished by unusual phenocryst phases whole rock compositions and groundmass mineralogy some minettes contain only a few per cent of biotite quartz plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite biotite quartz plagioclase alkali feldspar cordierite zircon pyrite in a high k latitic to trachydacitic groundmass field relations radiometric ages whole rock data and felsic xenocrysts all indicate that the puno minettes result from a mixing process involving al rich lamproites and peraluminous monzogranites the most common phenocryst assemblage highly magnesian phlogopite olivine titanian cr spinel ca rich enstatite magnesian ilmenite al and ti poor diopside is similar to that reported in al rich phlogopite lamproites from southeastern spain likewise the host rocks have diagnostic features of spanish phlogopite lamproites e g k2o al2o3 ratios 9 cao contents

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

recycling of orogenic arc crust triggers porphyry cu mineralization in kerman cenozoic arc rocks southeastern iran pre collisional eocene oligocene arc diorites quartzdiorites granodiorites and volcanic equivalents in the kerman arc segment in central iran lack porphyry cu mineralization and ore deposits whereas collisional middle late miocene adakite like porphyritic granodiorites without volcanic equivalents host some of the world s largest cu ore deposits petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment ore hosting kuh panj porphyry intrusions exhibit high sr 400 ppm low y 20 no negative eu anomalies eu eu 1 and relatively non radiogenic sr isotope signatures 87sr 86sr 0 7042 0 7047 relative to eocene oligocene granitoids mainly sr 12 la yb 15 eu eu 1 87sr 86sr 0 7053 0 7068 trace element modeling indicates peridotite melting for the barren eocene oligocene intrusions and a hydrous garnet bearing amphibolite source for middle late miocene ore hosting intrusions the presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30 35 to 40 45 km or 12 kbar the changes in residual mineralogy in the source of eocene to miocene rocks in the kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel underplating of cu and sulfur rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust mantle boundary and dehydration melting under oxidizing conditions produced syn and post collisional ore hosting intrusions while the lack of post collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur rich and oxidized adakitic magmas

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

commingling and mixing of s type peraluminous ultrapotassic and basaltic magmas in the cayconi volcanic field cordillera de carabaya se peru abstract the cayconi district of the cordillera de carabaya se peru exposes a remnant of an upper oligocene lower miocene 22 2 24 4 ma volcanic field comprising a diverse assemblage of s type silicic and calc alkaline basaltic to andesitic flows members of the picotani group of the central andean inner arc basaltic flows containing olivine plagioclase clinopyroxene ilmenite and glass and glassy rhyolitic agglutinates with phenocrystic quartz cordierite plagioclase sanidine ilmenite and apatite respectively exhibit mineralogical and geochemical features characteristic of medium k mafic and lachlan s type silicic lavas cordierite bearing dacitic agglomerates and lavas however are characterized by dispersed melanocratic micro enclaves and phenocrysts set in a fine grained quartzo feldspathic matrix they contain a bimodal mica population comprising phlogopite and biotite as well as complexly zoned sieve textured plagioclase grains sector zoned cordierite sanidine quartz irregular patches of replaced olivine clinopyroxene and orthopyroxene and accessory phases including zircon monazite ilmenite and chromite the coexistence of minerals not in mutual equilibrium and the growth dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing trachytic textured andesite flows interlayered with olivine plagioclase glomerophyric calc alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite ca rich sanidine ilmenite and aluminous spinel the mineralogical and mineral chemical relationships in both the dacites and the trachytic textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts not represented by exposed rock types and both the s type silicic and calc alkaline mafic magmas such mixing relationships are commonly observed in the oligo miocene rocks of the cordillera de carabaya suggesting that the s type rocks in this area and by extension elsewhere derive their unusually high k 2 o ba sr cr and ni concentrations from commingling and mixing with diverse mantle derived potassic mafic magmas

mass gains of the antarctic ice sheet exceed losses during 2003 to 2008 the mass gain of the antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 gt yr 2 5 of input as derived from icesat laser measurements of elevation change the net gain 86 gt yr over the west antarctic wa and east antarctic ice sheets wa and ea is essentially unchanged from revised results for 1992 to 2001 from ers radar altimetry imbalances in individual drainage systems ds are large 68 to 103 of input as are temporal changes 39 to 44 the recent 90 gt yr loss from three ds pine island thwaites smith and marie bryd coast of wa exceeds the earlier 61 gt yr loss consistent with reports of accelerating ice flow and dynamic thinning similarly the recent 24 gt yr loss from three ds in the antarctic peninsula ap is consistent with glacier accelerations following breakup of the larsen b and other ice shelves in contrast net increases in the five other ds of wa and ap and three of the 16 ds in east antarctica ea exceed the increased losses alternate interpretations of the mass changes driven by accumulation variations are given using results from atmospheric model re analysis and a parameterization based on 5 change in accumulation per degree of observed surface temperature change a slow increase in snowfall with climate warming consistent with model predictions may be offsetting increased dynamic losses

quaternary faulting near arco and howe idaho abstract well defined high angle faults were exposed in 1969 by trenching across scarps in alluvial fans along mountain ranges north of arco and howe idaho faulting along both the arco and howe scarps disrupts well developed calcic soils that are considered to be about 30 000 yr old on the other hand both scarps are interrupted by gaps occupied by unfaulted alluvial fans that probably are younger than 15 000 yr old the arco scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the lost river range from arco the scarp extends northward about 16 km 10 miles where it merges into an unstudied scarp in bedrock at a trench across the scarp 10 km 6 miles north of arco multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 12 m 40 ft measured offsets of stratigraphic units exposed in the trench indicate at least two episodes of vertical movement on individual faults one episode of movement caused a minimum offset of 4 6 to 6 1 m 15 to 20 ft and another offset was more than 3 m 10 ft the howe scarp coincides with a zone of closely spaced faults in alluvial fans along the western foot of the lemhi range a southern part of the scarp trends northwest along the southern part of the range a distance of 15 km 9 miles and a northern part trends north at least another 6 km 4 miles these parts of the scarp are separated by a bedrock ridge 3 km 2 miles wide in which the scarp is indistinct at a trench aross the scarp about 11 km 7 miles north of howe multiple movements in the fault zone have resulted in aggregate vertical displacement of at least 15 m 50 ft measured offsets of stratigraphic units exposed in the trench indicate five or more episodes of vertical movement on individual faults ranging from 0 3 m 1 ft to more than 3 m 10 ft

magmatism in the huarina belt bolivia and its geotectonic implications abstract the huarina belt is a tectono stratigraphic unit extending along the western side of the eastern cordillera and southern altiplano it includes the well known bolivian tin belt both geophysical and isotopic evidence suggest that the huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the proterozoic most of the phanerozoic backarc magmatism in the bolivian andes occurred along this belt local ordovician basalts and andesites and clasts of devonian granitoids occurring in late devonian early carboniferous sedimentary units are testimonies of a scarce palaeozoic igneous activity during the mesozoic rift related magmatism resulted in peraluminous to metaluminous high k calcalkaline triassic granitoids as well as discrete cretaceous centers of alkaline and carbonatitic rocks in the cenozoic huge high k generally peraluminous ignimbrite sheets along with numerous volcanic and intrusive centers were generated along the belt shoshonitic alkaline and ultrapotassic rocks have been locally described on the basis of both geochemical and geophysical evidence a general evolution model is proposed for the huarina belt during paleozoic and mesozoic times the predominant extensional regime of the continental crust led to lithospheric thinning basin generation and production of within plate magmas during the cenozoic prevailing compressional conditions led to a gradual thickening of the lithosphere inducing delamination of its denser and less viscous root the mantle generated magmas interplayed widely with crustal melts giving rise to the dominantly peraluminous magmas