neogene tectonic evolution and exhumation of the southern ecuadorian andes a combined stratigraphy and fission track approach abstract coastal marine and continental sedimentary facies of middle to late miocene age are exposed in the andes of southern ecuador cuenca giron santa isabel loja malacatos vilcabamba and catamayo gonzanama basins the chronostratigraphy of the basin series was established by zircon fission track dating on a total of 120 tephra layers subsequently the timing of tectonic events was estimated through the well dated stratigraphic sequences and intervening unconformities sedimentation from 15 to 9 ma termed pacific coastal stage was dominantly of coastal marine type extending over an area far greater than the present basin perimeters it ended when a period of east west oriented compression at 9 5 8 ma exhumed the region and sedimentation was then restricted to smaller basins termed intermontane stage these late miocene continental sediments were for the first time sourced from the west in the rising western cordillera apatite fission track analysis was applied to some of the tephras in the cuenca basin and also to the older eocene 42 35 ma quingeo basin series in order to quantify the basin histories with respect to timing and amount of burial and later exhumation in the quingeo basin burial of the oldest sediments reached temperatures of 100 c at 18 ma when they started to cool down during a period of exhumation this process preceded the pacific coastal stage development of the other basins in the cuenca basin the oldest sediments were buried to temperatures of ca 120 c by 9 ma when a period of inversion began and a phase of erosion was dominant this timing correlates well with that estimated from structural evidence at ca 6 ma the cooling rate slowed down and maybe even reverted to a small increase in temperature until 3 ma when the final stages of exhumation took place assuming a geothermal gradient of 35 c km total uplift for this part for ecuador is about 6100 m over the last 9 million years assuming a steady state continuous movement this means a mean rock uplift rate of 0 7 mm yr and a surface uplift of 0 3 mm yr to the present

the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

an accreted continental terrane in northwestern peru abstract a paleomagnetic study of over 250 cores from 26 sites sampled in early to late cretaceous and paleogene volcanic plutonic and sedimentary formations of the lancones basin in the piura province of northern peru indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable south america a clockwise rotation ranging from 90 for the lowermost units to 35 for the uppermost ones has been documented although the lack of precise chronology has not allowed a detailed temporal description four sites from late carboniferous pennsylvanian formations in the amotape tahuin range also show a 110 clockwise rotation and yield evidence for a northward displacement when considered together with previous geological studies these data are consistent with the hypothesis of the accretion of an amotape tahuin continental terrane to the peruvian margin in neocomian times the accretion was followed by in situ rotation suggesting a dextral shear regime these results indicate that the geodynamical evolution of northern peru is more closely related to the processes observed in ecuador than to those classically assumed for the central andes of peru

the development of antarctic glaciation and the neogene paleoenvironment of the maurice ewing bank abstract a micropaleontologic magnetostratigraphic and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late miocene to holocene depositional and erosional history of the intermediate depth maurice ewing bank meb located at the eastern extremity of the falkland malvinas plateau southwest atlantic ocean sedimentation on the meb is controlled presently by the position of the polar front and strong bottom currents beneath the axis of the antarctic circumpolar current acc fluctuations in the position of the polar front and in the intensity of the acc are inferred to have largely controlled deposition on the meb since the initiation of acc flow over the bank in the miocene the sedimentary record of the meb is used to infer the late miocene to holocene oceanographic and climatic conditions in the south atlantic sector of the southern ocean the meb suffered major erosion by intense acc flow during late miocene time which exposed cretaceous miocene sediment near the surface and shaped the present configuration of the bank this erosional event is inferred to have occurred during late miocene to early pliocene time 7 2 4 7 m y b p with the major phase of erosion occurring between middle magnetostratigraphic chron 7 and late chron 6 7 2 6 2 m y b p we suggest that the miocene sedimentary record of the meb and other paleoenvironmental evidence from the circum antarctic region precludes the existence of a west antarctic ice sheet or extensive ice shelves along the antarctic margin prior to the late miocene during the late miocene a west antarctic ice shelf formed in the former west antarctic sea and rapidly thickened until it grounded below sea level to form the west antarctic ice sheet formation of this ice sheet with floating and partially grounded extensions ice shelves in the ross and weddell embayments led to the first major production of true antarctic bottom water formed in a manner similar to today which permanently altered global abyssal circulation late miocene changes in oceanic ccd levels and a permanent shift in the oceanic 13 c 12 c composition may be attributed to this major change in abyssal circulation a decrease in acc velocity during an early pliocene amelioration of climate led to resumption of deposition on the bank from 4 7 to 3 9 m y b p intensification of the acc during the late gilbert and early gauss chrons again resulted in limited deposition and widespread erosion and or non deposition over most of the meb from 4 0 to 3 2 m y b p acc velocity apparently began to wane during the middle of the gauss chron and allowed widespread deposition on the bank during late gauss time 2 9 2 48 m y b p and over more limited areas during earliest matuyama time 2 48 to about 2 2 m y b p during middle to late matuyama time bottom current intensity reached its greatest post miocene level forming a regional disconformity between sediments of about 2 0 and 1 0 m y in age this increased circumpolar circulation and associated erosion is inferred to have peaked during the late matuyama chron 1 2 1 0 m y b p and is approximately the same age as the tentatively dated greatest patagonian glaciation in nearby southern argentina between 1 0 and 0 7 m y b p the bank was blanketed with a coarse erosion resistant layer of ice rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion sedimentation on the meb during the bruhnes chron 720 000 yr b p present was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest pleistocene time

lithosphere erosion and crustal growth in subduction zones insights from initiation of the nascent east philippine arc the philippine trench marks a nascent plate margin where subduction initiation is propagating from north to south magma compositions in the east philippine arc record thinning of arc lithosphere as it is eroded from below lithosphere is thicker beneath the younger southern part of the arc causing basaltic magma to stall and fractionate garnet at high pressure in the mature northern section basaltic magma differentiates at shallower levels at pressures where garnet is not stable local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust varying fractionation depths of hydrous basalt may help to explain the andesitic composition of bulk continental crust

petrogenetic evolution of arc magmatism associated with late oligocene to late miocene porphyry related ore deposits in ecuador we present a data set on the geochemical and isotopic composition of mineralized porphyry intrusions se riate intrusions and arc volcanic formations spatially associated with late oligocene to late miocene porphyry related ore deposits in ecuador our aim is to constrain the petrogenetic evolution of these arc magmas to evaluate geodynamic and tectonic impact factors on arc magmatism and to explore the metallogenic signifi cance of these parameters we distinguish four petrogenetic groups associated with the investigated magmatic centers indicating dif ferent paths of low and high pressure crustal magma evolution tertiary ecuadorian arc magmas which dif ferentiated at higher crustal pressures are commonly characterized by adakite like compositions related to fractionation and or restite equilibration effects of amphibole garnet titanite particularly in hydrous silicic melts without concomitant or subsequent major plagioclase fractionation trace element distribution patterns of arc magmas through time are broadly systematic and indicate that crustal magma differentiation progres sively extends to higher pressures from the late oligocene to the late miocene possibly related to crustal thick ening locally adakite like features possibly related to high melt h2o contents or accessory phase fractiona tion are already observed in the early miocene in a regional setting otherwise dominated by low pressure magma evolution compositional differences between mineralized porphyry intrusions and spatially associated seriate precur sor intrusions in ecuador mostly reflect regional scale changes in magma composition through time and are not observed if the relative emplacement age difference is small where porphyry intrusions are associated with shallow crustal batholith systems in northern central ecuador their emplacement is commonly late with re spect to batholith magmatism intrusive compositions indicate that porphyry parental melts mostly evolved at higher crustal pressures where metallogenically favorable melt preconditioning might possibly occur

ecuadorian andes volcanism a review of late pliocene to present activity abstract this article provides background information about ecuador s general volcanic setting as an introduction to the numerous articles in this special issue of the journal dealing with the many facets of ecuadorian volcanism it also provides a brief description of ecuador s physiography as well as its geodynamic setting a comprehensive bibliography is included as a guide to further reading the ecuadorian andes contain at least 20 volcanoes active during the holocene several reach altitudes of 5000 to 6000 m and are glacier clad eleven have erupted in pre columbian and historic times and have greatly affected local populations and caused fatalities

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

simple bouguer gravity anomaly field and the inferred crustal structure of continental ecuador the simple bouguer gravity anomaly field of continental ecuador corresponds closely to the physiographic provinces of the country the sierra which includes the andes and their foothills is characterized by a pronounced low with values to 292 mgal which reflects the deep andean root bouguer anomalies over the oriente become less negative away from the sierra chiefly in response to progressive thinning of continental crust eastward the costa between the sierra and the pacific shore in the north has the most positive on land bouguer anomalies 162 mgal so far known in the western hemisphere this part of the costa is underlain by an ancient oceanic plate now welded to the northwestern corner of the otherwise continental south american plate

cool la nina during the warmth of the pliocene the role of el nino southern oscillation enso in greenhouse warming and climate change remains controversial during the warmth of the early mid pliocene we find evidence for enhanced thermocline tilt and cold upwelling in the equatorial pacific consistent with the prevalence of a la nina like state rather than the proposed persistent warm el nino like conditions our pliocene paleothermometer supports the idea of a dynamic ocean thermostat in which heating of the tropical pacific leads to a cooling of the east equatorial pacific and a la nina like state analogous to observations of a transient increasing east west sea surface temperature gradient in the 20th century tropical pacific

syn collisional adakitic granodiorites formed by fractional crystallization insights from their enclosed mafic magmatic enclaves mmes in the qumushan pluton north qilian orogen at the northern margin of the tibetan plateau the qumushan qms syn collisional granodiorite which is located in the eastern section of the north qilian orogen at the northern margin of the greater tibetan plateau has typical adakitic characteristics and also contains abundant mafic magmatic enclaves mmes this recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed mmes the mmes and their host granodiorites share many characteristics in common including identical crystallization age similar to 430 ma same mineralogy similar mineral chemistry and whole rock isotopic compositions indicating their genetic link the mmes are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the qms adakitic granodiorite subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as mmes dispersed in the adakitic granodiorite host during emplacement the geochemical data and petrogenetic modeling of trace elements suggest that the qms adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the mmes the parental magma for the qms granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision which may have also experienced interaction with mantle peridotite during ascent c 2016 elsevier b v all rights reserved

contourite porosity grain size and reservoir characteristics abstract contourites are now recognised as having a significant potential as hydrocarbon reservoirs in the subsurface and several fields have been interpreted as comprising bottom current reworked turbidite sands however very little has been published on the porosity characteristics of contourites this study documents porosity data from iodp expedition 339 sites in the gulf of cadiz we use grain size analyses porosity depth plots and exponential models to yield a better understanding of grain size characteristics and facies porosity characteristics and the reservoir potential of contourites in the subsurface new grain size data for over 350 samples from the cadiz contourites is presented building on earlier work these data confirm the distinctive trends in textural properties linked to depositional processes under the action of bottom currents the finest muddy contourites porosity depth relationships from four cadiz sites show a moderately high initial porosity for both sand and mud facies 50 60 and a systematic decrease with depth to around 35 40 near 500 m burial depth according to the exponential models of porosity with depth contourite porosity should exceed 10 at 2500 m burial depth we compare the data from the gulf of cadiz contourite depositional system with those of the eirik drift newfoundland drift gardar drift and canterbury slope drifts similar depth trends are observed and all show anomalies linked to interbedded sandy and muddy facies composition carbonate vs siliciclastic and the presence of hiatuses in the sediment record these data provide good insight into the likely reservoir characteristics of contourites for both conventional and unconventional reservoirs they are comparable with those of existing contourite fields most of which are mixed turbidite contourite systems

rapid transition to long lived deep crustal magmatic maturation and the formation of giant porphyry related mineralization yanacocha peru abstract the yanacocha magmatic field northern peru hosts the largest high sulfidation gold deposit on earth mineralization is associated with porphyritic intrusions distributed along a ne trending magmatic structural corridor eight of these intrusions investigated in this study range in age from 12 4 to 8 4 ma and show systematic chemical and isotopic changes through time they are interpreted to derive from hydrous mafic magmas evolving through amphibole clinopyroxene garnet fractionation and lower crust melting leaving a garnet residue at deeper levels which led to variably strong adakite like signatures and through plagioclase amphibole fractionation at shallower levels both accompanied by crustal assimilation and recharge recharge assimilation fractional crystallization rafc processes systematic geochemical and isotopic changes with intrusion ages coupled with plagioclase zoning and amphibole geobarometry suggest that the evolution of the magmatic system occurred through interaction of mantle derived melts with an increasing length of the crustal column and propagation from deep towards shallower crustal levels through time this was probably the result of a steadily increasing compression that has progressively slowed down magma ascent forcing magmas to evolve at a series of intermediate level chambers between the lower and upper crust increased compression might have been related to the onset of subduction of the buoyant inca oceanic plateau estimated to occur at 12 ma i e the same time of the onset of the rapid transition from normal to adakite like signatures the giant yanacocha ore system developed in coincidence with the 3 6 4 0 ma long intrusion of the adakite like magmas 12 4 12 0 8 4 ma formed by the above processes into a small upper crustal volume and peaked during the last 2 4 ma 10 8 8 4 ma of magmatic activity after a 1 4 ma long 12 4 11 0 ma maturation of magmas at deep crustal levels further investigation is needed to understand whether the association of adakite like magmas with ore which is typical of other giant porphyry systems is the result of the build up of incompatible volatiles and metals in oxidized magmas that evolve under high pressure conditions of recycling of lower crustal sulfide rich cumulates and or of a long lived focused transfer of magmas from deep to shallow crustal levels

transition from calc alkalic to adakitic magmatism at cayambe volcano ecuador insights into slab melts and mantle wedge interactions in volcanic arcs two main types of magmatism are recognized 1 widespread calc alkalic magmatism generated by hydrous partial melting of a metasomatized mantle wedge and 2 less common adakitic volcanism produced by subducted slab melting the cayambe volcanic complex in ecuador shows a progressive temporal change from an older calc alkalic volcano viejo cayambe to a younger adakitic edifice nevado cayambe this evolution may be related to the unusual geodynamic setting of the ecuadorian andes controlled mainly by the subduction of the nazca plate including the carnegie ridge thickened oceanic crust the viejo cayambe magmas appear to be generated from a mantle wedge source slightly metasomatized by slab derived melts conversely the nevado cayambe magmas imply either stronger and more advanced interactions between slab melts and mantle peridotite or that adakitic magma could reach the surface due to higher degrees of slab melting fractional crystallization assimilation and mixing processes also contributed to lava diversity we propose that the magmatic evolution of the cayambe volcanic complex is due to increasing efficiency of interaction between slab melts and the mantle wedge because of higher degrees of slab melting in response to the subduction of the carnegie ridge

middle pliocene sea surface temperatures a global reconstruction identification and analyses of pliocene marine microfossils from 64 globally distributed stratigraphic sequences have been used to produce a middle pliocene sea surface temperature reconstruction of the earth this reconstruction shows little or no change from current conditions in low latitude regions and significant warming of the ocean surface at mid and higher latitudes of both hemispheres this pattern of warming is consistent with terrestrial records and suggests a combination of enhanced meridional ocean heat transport and enhanced greenhouse effect were responsible for the middle pliocene warmth

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

modelling pliocene warmth contribution of atmosphere oceans and cryosphere the relative role of the atmosphere oceans and cryosphere in contributing towards middle pliocene warmth ca 3 ma bp is investigated using the hadcm3 coupled ocean atmosphere general circulation model the model was initialised with boundary conditions from the usgs prism2 data set and a pliocene atmospheric co2 level of 400 ppmv and run for 300 simulated years the simulation resulted in a global surface temperature warming of 3degreesc compared to present day in contrast to earlier modelling experiments for the pliocene surface temperatures warmed in most areas including the tropics 1 5degreesc compared with present day the model predicts a general pattern of ocean warming 1 5degreesc in both hemispheres to a depth of 2000 m below which no significant differences are noted sea ice coverage is massively reduced up to 90 the flow of the gulf stream north atlantic drift is up to 100 mm s 1 greater in the pliocene case analysis of the model predicted meridional streamfunction suggests a global pattern of reduced outflow of antarctic bottom water aabw up to 5 sv a shallower depth for north atlantic deep water formation and weaker thermohaline circulation 3 sv the decrease in aabw occurs mainly in the pacific rather than atlantic ocean model diagnostics for heat transports indicate that neither the oceans nor the atmosphere are transporting significantly more heat in the pliocene scenario rather these results indicate that the major contributing mechanism to global pliocene warmth was the reduced extent of high latitude terrestrial ice sheets 50 reduction on greenland 33 reduction on antarctica and sea ice cover resulting in a strong ice albedo feedback these results highlight the need for further studies designed to improve our knowledge regarding pliocene terrestrial ice configurations

warmer tropics during the mid pliocene evidence from alkenone paleothermometry and a fully coupled ocean atmosphere gcm 1 traditional reconstructions of sea surface temperatures ssts produced by the pliocene research interpretations and synoptic mapping prism group indicate that mid pliocene surface ocean temperatures were unchanged or slightly cooler than modern at the tropics and low latitudes and significantly warmer at higher latitudes particularly in the north atlantic this change in the latitudinal pattern of ssts has been attributed to enhanced meridional ocean heat transport generated by more vigorous surface ocean gyres and or thermohaline circulation thc this study derives new sst estimates using the alkenone paleothermometer in the pacific ocean these estimates combined with published alkenone sst data from two atlantic sites are located in tropical and subtropical regions where the distribution of prism sites is sparse a fully coupled ocean atmosphere general circulation model oagcm running for the mid pliocene was also used to predict sst significant differences are noted between absolute prism ssts interpolated to our core locations and alkenone derived ssts these differences may be related to errors in the prism and alkenone paleothermometry estimates or to regional differences between localities where data was collected however the available alkenone and model based sst estimates are consistent in the sign of temperature change they predict and provide the first indication of warmer ssts during the mid pliocene in the tropics and subtropics this contrasts with prism s estimates of unchanged or slightly cooler ssts for the same geographical regions the pattern of ssts produced by alkenone estimates and modeling is not characteristic of that produced by enhanced meridional ocean heat transport or thc instead the pattern is similar to that which might be expected as a result of higher concentrations of atmospheric co2 which would act to warm the oceans at the tropics and other latitudes furthermore model diagnostics indicate that reduced sea ice and terrestrial ice sheet extents and a strong ice albedo feedback played a major role in forcing mid pliocene warmth further work aimed at validating these conclusions should concentrate on expanding the mid pliocene sst data set particularly at low latitudes as well as undertaking model model comparisons to assess model dependency of the results

enhanced circulation during a warm period the early pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today to characterize thermohaline circulation during this warm period 4 4 3 1 ma we combined new benthic foraminiferal isotopic data from the southeast atlantic with published data and reconstructed pacific and atlantic ocean nutrient distributions the data indicate enhanced ventilation of the deep atlantic and intermediate pacific during the early pliocene enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of north pacific and atlantic subtropical gyres was probably also enhanced future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

cretaceous to early paleogene tectonic evolution of the northern central andes 0 18 s and its relations to geodynamics the tectonic phases subsidence patterns and location of the magmatic arc along the peruvian margin during late cretaceous and paleogene times are compared with the main geodynamic parameters absolute plate motion convergence velocity and direction age of the subducted slab and accretional events for the same periods during the late cretaceous long termed compression seems to be controlled by the absolute trenchward motion of the overriding plate and to a minor extent by the young age of the subducted lithosphere short lived contractional or extensional phases are mainly linked to the acceleration or deceleration of the convergence between the oceanic and continental plates and probably to changes in the convergence direction periods of high and low convergence rates coincide with increased and decreased subsidence rates along the margin respectively and seem to be independent of compressional tensional regimes during the paleogene in addition to the mentioned processes the eastward shift of the magmatic belt and subsidence in the forearc zone are interpreted as the result of the subduction erosion of the margin this process seems to be related to the decrease of the dip of the benioff zone expressed by the drastic widening of the magmatic arc

simulations of warm tropical conditions with application to middle pliocene atmospheres during the early and mid pliocene the period from 5 to 3 million years ago approximately the earth is believed to have been significantly warmer than it is today but the reasons for the higher temperatures are unclear this paper explores the impact of recent findings that suggest that at that time cold surface waters were absent from the tropical and subtropical oceanic upwelling zones el nino was in effect a perennial rather than intermittent phenomenon and sea surface temperatures in low latitudes were essentially independent of longitude when these conditions are specified as the lower boundary condition for an atmospheric gcm we find that the trade winds along the equator and hence the walker circulation collapse the low level stratus clouds in low latitudes diminish greatly thus reducing the albedo of the earth the atmospheric concentration of water vapor increases and enhanced latent heat release due to stronger evaporation warms up the tropical atmosphere particularly between 40 s and 20 n moreover teleconnection patterns from the pacific induce a warming over north america that is enhanced by surface albedo feedback a process that may have helped to maintain this region ice free before 3 ma the results presented here indicate that the suggested absence of cold surface waters from the tropical and subtropical oceanic upwelling zones could have contributed significantly to the pliocene warmth

metallogenic features of miocene porphyry cu and porphyry related mineral deposits in ecuador revealed by re os 40ar 39ar and u pb geochronology mineralization and alteration events at ten miocene porphyry cu and porphyry related epithermal mineral deposits in southern central and northern ecuador were dated by means of molybdenite re os biotite and alunite 40ar 39ar and titanite u pb geochronology most of these hydrothermal events show a spatio temporal correlation with porphyry intrusion emplacement as constrained by zircon u pb ages the total age range for these events spans the 23 5 6 1 ma period without displaying systematic along or across arc age distribution trends while epithermal deposits tend to be spatially associated with volcanic rocks of a similar age porphyry cu deposits in ecuador are frequently spatially associated with deeper seated basement units and batholith scale precursor intrusive systems assembled over 5 m y time periods in most cases formation of the porphyry cu deposits is related to the youngest magmatic hydrothermal event in a given area postdating batholith construction at a regional scale the majority of miocene deposits occurs in southern ecuador where areally extensive post mineralization late miocene to recent volcanic sequences with the potential to conceal mineralization at depth are lacking only few miocene deposits occur in northern central ecuador where they mainly crop out in the western cordillera west of the productive present day volcanic arc the surface distribution of post mineralization arc volcanism reflects along arc variations in subducting slab geometry porphyry cu and epithermal deposits in ecuador define a miocene metallogenic belt broadly continuous with its coeval counterpart in northern central peru although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting their respective metal endowments differ significantly

allochthonous terranes in northwestern ecuador abstract formations and lithostratigraphic units of the ecuadorian northwestern cordillera can be divided into three major groups the first group comprises lower cretaceous e morb basalts conformably overlain by tholeiitic to boninitic components of an ensimatic island arc the second group consists mainly of tholeiitic to calc alkaline volcanic and volcano sedimentary island arc deposits unconformably on top of the first group and of eocene to early oligocene age the third group is represented by calc alkaline continental volcanics ranging from late oligocene to recent two main tectonic events occur during early eocene and middle oligocene times the first event can be interpreted as an accretion of a slab of oceanic basalt with its overlying arc material the suture zone is marked by slices of morb type material delineating the southward extension of the colombian cauca patia fault zone the second event is related to a major rearrangement of plate tectonic regimes in the eastern pacific during oligocene times its nature is most probably transpressional accompanied by thrusting

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

el nino s tropical climate and teleconnections as a blueprint for pre ice age climates 1 at 2 7 million years ago the warm equable climates of early and middle pliocene time used here to mean from 5 to 2 7 ma were replaced by recurring ice ages most attempts to explain the change appeal either to changes in co2 in the atmosphere or reduced heat transport by the atlantic ocean the sources of the strongest teleconnections in the current climate however lie in the tropics and such connections occur by transport of heat and moisture by the atmosphere the most prominent of these teleconnections link aberrations in sea surface temperatures in the equatorial pacific el nino southern oscillation enso variations with warm and dry or cool and wet anomalies in extratropical climates we show that in most cases early and middle pliocene climate both in equatorial regions and in the extratropics differ from present day climates with the same spatial pattern as that associated with enso for instance not only was canada warmer during early pliocene time than at present as it is during el nino but the region surrounding the gulf of mexico appears to have been cooler and a bit wetter as it commonly is during el nino a virtually permanent el nino like state appears to have characterized pre ice age climates suggesting that transport of heat by the atmosphere was the principal mechanism that maintained extratropical warmth accordingly cooling and the growth of recurring ice sheets in the northern hemisphere resulted from the development of a strong walker circulation and a weakening of the hadley circulation

the generation of high sr y plutons following late jurassic arc arc collision blue mountains province ne oregon abstract high sr y plutons sr y 40 occupy large areas in ancient and modern orogenic belts yet considerable controversy exists regarding mechanisms of their generation the tectonic settings in which they form and their relationship to contractional deformation through time in the blue mountains province ne oregon a suite of late jurassic 148 145 ma high sr y plutons intrude middle jurassic 162 157 ma low sr y 600 ppm and sr y values 40 and positive initial epsilon hf values 10 5 to 7 8 2 weighted average these geochemical and isotopic results are consistent with geochemical models suggesting derivation from partial melting of island arc crust in the presence of a plagioclase poor to absent clinopyroxene hornblende garnet bearing source depths 35 40 km we propose that the transition from low sr y to high sr y magmatism resulted from orogenic thickening of island arc crust in the dixie butte area during late jurassic arc arc collision between the olds ferry and wallowa island arcs at 159 154 ma this fundamental change in crustal structure influenced post orogenic magmatism and resulted in a relatively brief 3 myr 148 145 ma episode of high sr y magmatism other high sr y plutons occur throughout the us sector of the western north american cordillera e g salmon river suture zone klamath mountains peninsular ranges and closely follow major contractional events involving arc arc and arc continent collisions

timing of bottom water scour recorded by sedimentological parameters in the south australian basin abstract the south australian basin has been the site of extensive antarctic bottom water aabw scour during the late cenozoic the combination of detailed stratigraphy and sedimentological analyses of three piston cores allowed the identification of the major episodes of high bottom current paleovelocity the chronology of the cores was determined by magnetostratigraphy and biostratigraphy five parameters mean mode skewness coarsest 1 and sorting of the carbonate free silt particle size distribution were used to infer periods of high paleovelocity sorting was found to be an insensitive parameter within the inferred paleovelocity regimes the two major high velocity episodes during the last 3 3 m y occurred during the gauss chron and the upper matuyama chron several short episodes of high bottom water velocity occurred during the matuyama chron only one minor pulse was identified during the brunhes chron 0 0 73 m y b p the high velocity episode during the gauss chron produced a disconformity separating miocene and middle pliocene sediments while the upper matuyama episode may be responsible for the widespread disconformity at or near the seafloor which has been identified in many cores in the region

allochthonous terranes in the andes of ecuador and northwestern peru in ecuador and northwestern peru the andes and adjacent country particularly on the pacific side are composed of at least five distinctive geologic terranes the terranes are distinguished from one another and from cratonic south america to the east by dissimilar basements cover rocks intrusive rocks and bouguer gravity anomaly fields the pinon terrane occupying most of coastal ecuador has a basaltic basement characterized by the largest known on land positive bouguer anomalies in the western hemisphere the tahuin terrane occupies most of northwestern peru and the southwestern corner of ecuador the terrane has an especially complex basement and is the site of generally positive bouguer anomalies the small biron terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent late cretaceous k ar mineral ages the wedge shaped chaucha terrane lies in part on the western andean slope between the oceanic pinon terrane on the north and the continental bir

the middle permian to triassic tectono magmatic system in the southern korean peninsula abstract the permo triassic adakitic magmatism in the southern korean peninsula preserves important tectonomagmatic records along the east asian continental margin in this study we present shrimp la mc icpms u pb zircon ages and hf isotope signature with whole rock geochemical compositions of the middle permian to middle triassic granitoid gneiss granitoid suites and the middle to late triassic plutonic rocks in the southeastern korean peninsula the former yields three age groups of ca 272 263 ma ca 258 251 ma and ca 243 240 ma respectively the latter group yields ca 237 216 ma ages indicating a long term magmatism that lasted from the middle permian to late triassic the granitoid gneiss granitoid suites have chemical compositions similar to high silica adakitic rocks showing high sio2 59 55 73 97 wt na2o 3 55 5 30 wt sr y 30 401 and la ybn 11 173 and low mgo 0 18 1 82 wt y 2 14 ppm and ybn 2 7 these rocks are peraluminous with high ba and sr and show relatively high k2o contents and k2o na2o ratios 0 7 with positive correlation between la ybn vs sr y ratios and high initial sr ratios these features reflect different contribution of the slab derived adakite melts that interacted with peridotitic mantle in the metasomatized mantle wedge and the middle paleoproterozoic lower crustal basement of the yeongnam massif on the contrary the triassic alkaline magmatism in the yeongnam massif is alternatively interpreted either by a tectonic switch to the extension dominated arc system or by a delamination of an overthickened arc the results from this study offer important evidence for a common subduction accretion system existed between the north and south china cratons and microcontinents between them along margins of the east asian continental blocks with different plate interaction in different locations generating distinct magmatic suites

bottom current erosion along a traverse in the south atlantic sector of the southern ocean abstract the magnetostratigraphy and biostratigraphy of twenty five piston cores on a profile from antarctica to the cape basin south of africa reveal disconformities in the sedimentary record which are attributed to bottom current erosion increased antarctic bottom water production removed all sediment in the weddell and cape basins younger than middle pliocene 2 5 m y with miocene sediment exposed at or near the seafloor at some locations increases in the velocity of the circumpolar deep water eroded lower pliocene to upper pliocene 4 0 2 0 m y sediment on the crest and south flank of the atlantic indian ridge and maintains an active scour zone today with middle pleistocene 1 0 m y sediment exposed at the seafloor

mid pliocene warmth stronger greenhouse and stronger conveyor abstract three million years ago prior to the onset of northern hemisphere glaciation global mean temperatures may have been as much as 3 5 c warmer than at present we present evidence based on the carbon isotopic composition of marine organic matter that atmospheric co2 levels at this time were on average only about 35 higher than the preindustrial value of 280 ppm we also present carbon isotopic evidence for stronger thermohaline circulation in the atlantic ocean during the warmest intervals and propose that the north atlantic conveyor belt may act as a positive feedback to global warming by enhancing sea ice retreat and decreasing high latitude albedo based on our results it seems unlikely that the mid pliocene warm period was a doubled co2 world

accreted oceanic terranes in ecuador southern edge of the caribbean plate the western part of ecuador is made of several oceanic terranes which comprise two oceanic plateaus of early 120 ma and late cretaceous age 90 ma respectively the older oceanic plateau was accreted to the andean margin in the late campanian 75 ma fragments of the turonian coniacian plateau were accreted to the ecuadorian margin in the late maastrichtian 68 ma guaranda terrane and late paleocene 58 ma pinon naranjal terrane the guaranda terrane received either fine grained oceanic sediments of coniacian maastrichtian age or island arc back arc volcanic suites of middle campanian middle maastrichtian age the pinon naranjal terrane recorded a comparable history completed in the maastrichtian paleocene either by pelagic cherts or by island arc products macuchi arc the late cretaceous plateau of ecuador is interpreted as part of the caribbean oceanic plateau cop because their evolutions are comparable if so the cop was not formed by the galapagos hotspot but on the farallon oceanic plate south of ecuador and close to the south american margin the cop belonged to the farallon plate until a subduction zone separated both plates in the middle campanian giving way to a well developed mid campanian mid maastrichtian island arc accretion in the late maastrichtian triggered a change in the subduction system and the development of a new arc system of late maastrichtian late paleocene age which crosscut the south america cop plate boundary the last accretion occurred in the late paleocene

separate lead isotope analyses of leachate and residue rock fractions implications for metal source tracing in ore deposit studies separate lead isotope analyses of leachate and residue fractions are applied to a broad spectrum of rocks commonly investigated in metallogenic studies resulting data highlight a systematic behavior of leachate and residue fractions with respect to lead isotope compositions which essentially depends on the mineralogical composition of the rock granitoid and high grade metamorphic rocks have residue compositions virtually identical to common lead in contrast low grade metasedimentary rocks may have residue compositions swamped by radiogenic lead of leach resistant zircons mafic magmatic rocks have residues that are often more radiogenic than leachates depending on the ratio of leach refractory zircons to common lead in the residual fraction of these rocks separate leachate and residue analyses of source rocks provide two lead isotope end members whose mixture may represent lead with the appropriate ore fluid composition our leaching experiments indicate that hot acid solutions and by inference hydrothermal fluids preferentially leach radiogenic lead from medium to high grade metamorphic and granitoid rocks whereas they preferentially leach common lead from low grade metasedimentary and mafic magmatic rocks the method presented in this study provides a reliable alternative to other methods i e age correction of bulk rock compositions for the determination of the common lead signature of felsic to intermediate magmatic rocks this may be preferable to age corrected bulk rock analyses where ages to apply for corrections of bulk rock data are not known or where moderately to highly altered rocks must be used case studies of orogenic gold and mvt districts of peru pataz and san vicente respectively show that separate leachate and residue lead isotope analyses carried out systematically on whole rocks allow a more thorough evaluation of metal source reservoirs than does the standard method of age corrected or uncorrected bulk rock analyses

role of tropics in changing the response to milankovich forcing some three million years ago appeared in regions that today have intense oceanic upwelling the eastern equatorial pacific and the coastal zones of southwestern africa and california there was furthermore a significant change in the earth s response to milankovich forcing obliquity signals became large but those associated with precession and eccentricity remained the same the latter change in the earth s response can be explained by hypothesizing that the global cooling during the cenozoic affected the thermal structure of the ocean it caused a gradual shoaling of the thermocline around 3 ma the thermocline was sufficiently shallow for the winds to bring cold water from below the thermocline to the surface in certain upwelling regions this brought into play feedbacks involving

bottom current speed in the vema channel recorded by particle size of sediment fine fraction abstract the particle size distribution of the carbonate free silt fraction was determined in forty two core top samples from the east flank of the vema channel in order to allow comparison of the mean particle size with near bottom current speed based on nearby ctd and current meter observations the silt mean particle size fluctuates insignificantly between 1450 3950 m but coarsens markedly below 4000 m under antarctic source bottom currents the top of the zone of coarse sizes approximates the top of lower circumpolar water lcpw at 4000 m the steep gradient to coarsest sizes in the axis of the channel marks the transition to antarctic bottom water aabw the vertical profile of mean particle size on the east flank correlates in general respects with the vertical velocity gradient in the channel axis although the core sites on the east flank are as far as 200 miles from the current meter arrays nevertheless this relationship is used to infer that the non carbonate particle sizes deposited on the east flank of the channel may be an indication of bottom current speed and therefore the mean size is used to reconstruct profiles of mean current speed for three time slices 18 120 and 140 ka b p during glacial isotopic stages 2 and 6 the particle sizes deposited on the east flank of the channel are characterized by finer sizes at nearly all depths and the zone of coarse sizes corresponding to lcpw and aabw deepens during interglacial isotopic stage 5e the particlesize profile is very similar to the modern profile however lcpw and aabw appear to be deeper than at present the finer particle sizes deposited at nearly all depths during glacial periods indicates that the velocity of deep circulation in the atlantic was reduced in response to the ice ages deep circulation during isotopic stage 5e closely resembled modern circulation however aabw production rates may have been less that at present

petrogenesis of andean andesites from combined o sr isotope relationships the descent of the oceanic nazca plate below the western margin of south america is responsible for the seismicity tectonism and magmatism characteristic of andean type continental margins within this geological setting there is much debate concerning the petrogenesis of the calc alkaline volcanic association a thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust the asthenospheric mantle wedge and the lower crust might occur1 and hence calc alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources in the andes for example pichler and zeil2 have invoked partial melting of lower continental crust while others3 5 envisage petrogenesis of andesites from sub crustal mantle sources possibly in conjunction with crustal contamination6 9 we present here new o isotope data for volcanic rocks from three contrasted volcanic settings in ecuador north chile and north west argentina we then consider the significance of observed o sr isotope relationships and attempt to evaluate the important factors in the petrogenesis of andean andesitic lavas

physiographic and tectonic settings of high sulfidation epithermal gold silver deposits of the andes and their controls on mineralizing processes abstract gold and silver ores in the vast majority of andean high sulfidation epithermal au ag deposits occur at high present day elevations and typically 200 500 m below low relief landforms situated at 3500 to 5200 m a s l most deposits are middle miocene and younger and include el indio tambo pascua lama veladero el indio belt chile argentina cerro de pasco central peru pierina lagunas norte yanacocha northern peru quimsacocha ecuador and the california vetas mining district santander colombia jointly accounting for 130 moz au resources slightly older examples are only preserved in the atacama desert and include the middle eocene el guanaco and el hueso and the late oligocene early miocene la coipa deposits the absence of paleocene and older high sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift these conditions prevailed predominantly in segments of shallow angle subduction of the nazca or caribbean plate below the south american continent a tectonic setting also common for porphyry style cu au mo deposits stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent recording the terminal stages of local arc magmatism however dacitic domes are important at e g yanacocha and la coipa at lagunas norte a small stratovolcano largely pre dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high elevation low relief surfaces in the california vetas mining district and el indio belt hydrothermal alunite ages become generally younger upstream along the incising valleys indicating that hydrothermal activity and by inference ore deposition were facilitated by erosion the lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids ultimately enhancing ore deposition the host rock composition permeability and location of the water table control the distribution of alteration zones and ore intermediate volcanic rocks are the most common ore hosts but they typically pre date mineralization by several ma however high sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks california vetas mining district significantly older plutonic rocks pascua lama or quartzites lagunas norte large vuggy quartz alteration zones and commonly oxidized low grade large tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies whereas coherent volcanic plutonic or metamorphic rocks may host fault and breccia controlled ores the near surface steam heated zone can attain a thickness of several hundred meters in dry climates e g veladero pascua lama tambo but is typically poorly developed and less than 20 m thick in humid climatic zones the physiographic and tectonic settings of high sulfidation epithermal deposits are distinct from low sulfidation epithermal districts such as those of patagonia el penon chile or fruta del norte ecuador the latter range to significantly older ages jurassic to early eocene occur at mainly lower elevations and were emplaced in extensional settings a temporal coincidence between uplift erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low sulfidation districts

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

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

quaternary sanukitoid like andesites generated by intracrustal processes chacana caldera complex ecuador implications for archean sanukitoids high mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences where they are associated with trondhjemite tonalite granodiorite ttg suites archean rocks with these geochemical features are known as sanukitoids and despite their limited abundance are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction related high mg andesites and diorites understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes the accepted petrogenetic models for modern enriched high mg andesites and their archean equivalents the sanukitoids consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge we present new data on the petrography mineral chemistry and whole rock geochemistry major and trace elements and sr nd pb isotopes of andesitic rocks from an 30 ka lava flow yuyos flow from the chacana caldera complex eastern cordillera of ecuador these rocks show a remarkable geochemical affinity with archean sanukitoids including high magnesium numbers 0 58 0 63 accompanied by high contents of incompatible elements e g th 17 23 ppm u 6 7 5 ppm ba 1600 1800 ppm sr 1430 1565 ppm la 74 94 ppm additionally the sanukitoid like andesites of yuyos are associated with predominant silica rich adakite like andesites which are widespread throughout the quaternary arc of ecuador this makes the quaternary ecuadorian magmatic province a close equivalent of the archean ttg sanukitoid association the bulk rock geochemistry petrography and mineral chemistry data indicate that the sanukitoid like features of the andesites of the yuyos flow derive from intracrustal recycling of the felsic intermediate to mafic ultramafic roots of the quaternary volcanic arc of ecuador by normal mantle derived basaltic magmas with the geochemical characteristics of continental arc basalts or high alumina basalts in view of the similarities between the yuyos andesites and archean sanukitoids in terms of geochemistry and lithological association we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both archean sanukitoids and modern enriched high mg andesites

post miocene fluctuations of antarctic bottom water paleospeed in the southwest atlantic ocean abstract a gap in the falkland escarpment serves as the primary channel through which antarctic bottom water aabw flows into the argentine basin from the georgia basin the particle size distributions of the biogenic free silt fraction in four cores near the falkland gap and the hiatus frequency at water depths greater than 4000 m throughout the southern ocean are used to infer post miocene aabw paleospeed analysis of aabw paleocirculation indicates episodes of increased bottom current speeds at 5 3 4 5 4 0 3 5 3 3 3 1 2 5 2 3 2 0 1 5 and 1 0 ma these fluctuations in inferred paleospeed are compared to both northern and southern hemisphere paleoceanographic and paleoclimatic events in order to examine the response of bottom water circulation to post miocene climates during the early gilbert middle matuyama 5 3 1 5 ma southern hemisphere climatic events provided the primary forcing mechanisms on aabw activity the model invoked involves increased salinity and overturn of weddell shelf water in response to increased sea ice formation during cold episodes from the late matuyama after 1 5 ma to present fluctuations in warm deep water resulting from northern hemisphere climatic evolution may have been the predominant factor influencing aabw paleospeed these episodes of increased bottom current activity may correlate to periods of mudwave migration in the argentine basin during the past 5 ma the mudwave fields consist of silty siliceous clays deposited in response to enhanced sediment supply and fast moving currents in the interior basin we speculate that the periods of enhanced bottom water activity may have been responsible for major episodes of mudwave growth that began since the late oligocene and have persisted into the holocene

middle pliocene vegetation reconstructions paleoclimatic inferences and boundary conditions for climate modeling abstract the general characteristics of global vegetation during the middle pliocene warm period can be reconstructed from fossil pollen and plant megafossil data the largest differences between pliocene vegetation and that of today occurred at high latitudes in both hemispheres where warming was pronounced relative to today in the northern hemisphere coniferous forests lived in the modern tundra and polar desert regions whereas in the southern hemisphere southern beech apparently grew in coastal areas of antarctica pliocene middle latitude vegetation differed less although moister than modern conditions supported forest and woodland growth in some regions now covered by steppe or grassland pliocene tropical vegetation reflects essentially modem conditions in some regions and slightly cooler than or warmer than modern climates in other areas changes in topography induced by tectonics may be responsible for many of the climatic changes since the pliocene in both middle and lower latitudes however the overall latitudinal progression of climatic conditions on land parallels that seen in the reconstruction of middle pliocene sea surface temperatures pliocene paleovegetational data was employed to construct a 2 2 global grid of estimated mid pliocene vegetational cover for use as boundary conditions for numerical general circulation model simulations of middle pliocene climates continental outlines and topography were first modified to represent the pliocene landscape on the 2 2 grid a modern 1 1 vegetation grid was simplified and mapped on this pliocene grid and then modified following general geographic trends evident in the pliocene paleovegetation data set

concept of continuum as opposed to periodic tectonism in the andes abstract results of a study of various andean sedimentary basins that are distinct in terms of age and geodynamic position allow us to postulate the idea of a compressional tectonic continuum as opposed to the short tectonic events which have so far been considered as one of the main features of the dynamic evolution of the andean belt in these basins the development of kilometre scale synsedimentary folded structures implies the existence of compressional stress over a period of several million years the existence of tectonic events of long duration allows us to modify the interpretation of the classic andean tectonic model the timing of the continuum tectonism is perhaps dependent on the variation in the convergence rate between the farallon nazca and south american plates

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

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

neogene stratigraphy and andean geodynamics of southern ecuador abstract the present paper reviews tertiary volcanic and sedimentary formations in the inter andean region of southern ecuador between 2 s and 4 20 s in order to develop a geodynamic model of the region the formations occur in the southern shallow prolongation of the inter andean valley between the cordillera real to the east and the cordillera occidental and amotape tahuin provinces to the west one hundred fifty zircon fission track analyses has established a detailed chronostratigraphy for the sedimentary and volcanic formations and several small intrusions the paleogene to early miocene formations are dominated by intermediate and acidic volcanic and pyroclastic rocks in addition relics of eocene continental sedimentary series have been identified the neogene sedimentary series lie unconformably on deformed and eroded metamorphic sedimentary and volcanic formations they were deposited in two stages which are separated by a major unconformity dated at 10 9 ma 1 during the middle and early late miocene 15 10 ma marginal marine deltaic lagoonal lacustrine and fluvial environments prevailed which we group under the heading pacific coastal sequences they presumably covered a greater surface area in southern ecuador than their present occurrence in small topographic depressions we suggest that they were deposited in the shallow marine cuenca and loja embayments deposition in a marginal marine environment is also supported by the occurrence of brackish water ostracods and other fauna 2 above the regional angular unconformity the coastal facies are overlain by late miocene 9 5 ma continental alluvial fan and fluvial facies which are in turn covered by mainly airborne volcanic material they represent the intermontane sequences of the basins of cuenca giron santa isabel nabon loja and malacatos vilcabamba sedimentologic and stratigraphic results are used to discuss the tectonic setting of neogene sedimentation in the forearc and arc domain of the ecuadorian subduction system during the pacific coastal stage northward displacement of the coastal forearc block along the calacali pallatanga fault zone has driven crustal collapse in the inter andean region as a result extensional subsidence drove the eastward ingression of shallow seas into the cuenca and loja embayments from the manabi and progreso basins to the west tectonic inversion in the forearc area during the early late miocene at 9 5 ma reflects the initiation of w e oriented compression and uplift in the inter andean region and the establishment of smaller intermontane stage basins which host the continental sequences coeval topographic rise of the cordillera occidental is indicated by the onset of clastic input from the west the small intermontane basin of nabon 8 5 7 9 ma formed during the period of maximum compression the present data prove that the neogene andean forearc and arc area in southern ecuador was a site of important but variable tectonic activity which was presumably driven by the collision and coupling of the carnegie ridge with the ecuadorian margin since 15 9 ma

contemporaneous paleogene arc magmatism within continental and accreted oceanic arc complexes in the northwestern andes and panama abstract the basement rocks of the northwestern andes are composed of accreted fault bound slivers of the cretaceous caribbean large igneous province clip previous geologic mapping geochronological and geochemical data show that paleogene arc like units make up part of the accreted clip in sw colombia timbiqui complex nw colombia santa cecilila la equis complex and at the boundary between colombia and panama acandi region similar paleogene igneous units have been reported in ecuador silante and macuchi arc and panama chagres bayano arc these arc like rocks are commonly interpreted to have formed as part of an intra oceanic arc before accretion to the continental margin alternative explanations suggest that the paleogene igneous rocks in ecuador were formed within a continental magmatic arc we compare paleogene igneous rocks in the san blas complex chagres bayano arc and acandi region santa cecilia la equis complex and the sw complex timbiqui complex silante and macuchi arcs using geochemical major and trace elements geochronological zircon u pb and isotopic zircon hf and whole rock pb sr nd isotopes data zircon u pb analyses yield ages of 46 42 ma for the acandi region 59 51 ma for the santa cecilia la equis complex and 44 42 ma for the timbiqui complex major and trace element data show low k to calc alkaline compositions with negative nb anomaly and lree hree enrichment typically attributed to arc like signatures for the three complexes pb sr and nd whole rock and zircon lu hf isotopic data for the santa cecilia la equis and the san blas complexes are consistent with a depleted mantle source mixed with pacific sediments however the presence of inherited proterozoic zircons elevated 207pb 204pb ratios in some evolved igneous rocks and crystallization under low pressure conditions suggest assimilation of continental derived sediments these data indicate that the santa cecilia la equis and the san blas complexes likely originated in an intra oceanic island arc setting in a close proximity to the south american margin before accretion to the andean margin in the case of the sw complex the absence of eu anomaly slightly depleted hree as well as increased sr y values with fractionation indicate that these magmas formed in a moderately mature arc within a mid shallow crust level moreover radiogenic pb and sr isotopic values for the less evolved igneous rocks of the sw complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source and assimilation of the mafic basement crust this indicates that the paleogene magmatic rocks of the sw complex originated in a continental arc like setting within previously accreted slivers of the clip

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

geochemical window into subduction and accretion processes raspas metamorphic complex ecuador the high pressure low temperature p 1 3 2 gpa t is 600 c raspas metamorphic complex is an exhumed fragment of the partially accreted partially subducted amotape chaucha terrane in southwest ecuador comparative analysis of major and trace elements plus sr nd and pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments subduction and exhumation of oceanic lithosphere resulted in selective bulk trace element geochemical changes rb ba and sr have been lost in amounts from approximately 85 50 from the high p low t metamorphosed pelites and basalts whereas pb is enriched in mafic rocks during formation of the eclogite u pb and rare earth elements rees were immobile high p low t metamorphosed terranes form the basement of active ecuadorian arc volcanoes partial melting of this basement by mantle wedge derived basalt is a likely source of adakitic components

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

correlations between nazca farallon plate kinematics and forearc basin evolution in ecuador the tertiary evolution of the forearc basins of ecuador shows a close correlation between the changing convergence rate of the farallon and later nazca oceanic plates and continental south america the correlation occurs during the subduction of a relatively young slab and in the late miocene onset of the subduction of the carnegie aseismic ridge the ecuador forearc basins lie on a basement of oceanic crust known as the pinon terrane the accretion of this terrance occurred in the paleocene as the leading edge of the farallon plate the macuchi island arc collided with south america in the middle eocene this forearc terrane was the site of major pull apart basin formation and turbiditic sedimentation coincident with a phase of very rapid convergence between chron 21 and chron 13 48 37 ma this deformation was bounded by the trench and a major dextral strike slip fault zone and resulted in the northward translation of the forearc with respect to continental south america during the oligocene a phase of extension normal to the trend of the active margin occurred coincident with a phase of relatively slow convergence chron 13 to chron 6 37 20 ma this extension was followed in the middle miocene by inversion of most of the forearc basins coincident with a return to relatively fast convergence from chron 6 20 ma to the present day subduction of the carnegie aseismic ridge occurred during this period circa 8 ma to present and may have enhanced the compressive event further northward translation of the forearc silver accompanied this later deformation the relationships outlined for the forearc may be modeled in terms of a dynamic orogenic wedge which responds directly to changes in convergence rate at the subduction zone the convergence rate appears to be an important control on the coupling between the downgoing slab and overriding continental plate

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

metal sources in mineral deposits and crustal rocks of ecuador 1 n 4 s a lead isotope synthesis ecuador consists of terranes having both continental chaucha tahuin loja terranes and oceanic macuchi alao salado terranes affinity which were accreted to the amazon craton from late jurassic to eocene four main magmatic arcs were formed by the subduction of the farallon nazca plate since the juras sic a jurassic continental arc on the western margin of the amazon craton a jurassic island arc alao terrane an early tertiary island arc macuchi terrane and a middle late tertiary continental arc encompassing the ter ranes of macuchi chaucha tahuin loja and alao after complete assembly of the ecuadorian crust mineral deposits formed during these magmatic arc activities include porphyry cu and gold skarn deposits in associa tion with the jurassic continental arc polymetallic volcanic hosted massive sulfide deposits vhms in associ ation with the jurassic island arc of alao au cu zn vhms deposits in association with the early tertiary is land arc of macuchi and porphyry cu and precious metal epithermal deposits in association with the middle late tertiary continental arc magmatism on the newly assembled crust of ecuador macuchi chaucha tahuin loja and alao terranes in this study we have compiled 148 new and 125 previously published lead isotope analyses on paleozoic to miocene metamorphic intrusive volcanic and volcanosedimentary rocks as well as on jurassic to miocene magmatic related ore deposits of ecuador lead isotope compositions of the magmatic rocks of the four main arc events derive from mixing of various sources including mantle variably enriched by pelagic sediments and or by a high 238u 204pb component and heterogeneous continental crust rocks lead isotope compositions of the ecuadorian ore deposits display a broad range of values 206pb 204pb 18 3 19 3 207pb 204pb 15 54 15 74 208pb 204pb 38 2 39 2 which is as large as the range previously re ported for all magmatic related ore deposits of the central andean provinces i and ii combined ore deposits formed before complete assembly of the ecuadorian crust through complete accretion of the several terranes i e pre eocene have lead isotope compositions overlapping those of the associated magmatic rocks sug gesting a largely magmatic origin for their lead in contrast post assembly ore deposits i e post eocene have lead isotope compositions that only partly overlap those of the coeval magmatic rocks of the continental arc in fact several ore deposits have lead isotope compositions shifted toward those of the basement rocks that host them suggesting that lead derives from a mixture of magmatic lead and basement rock lead leached by hy drothermal fluids most ecuadorian ores have high 207pb 204pb values 15 55 suggesting a dominant continental crust or pelagic sediment origin of the lead however we caution against concluding that chalcophile metals for ex ample cu and au also have a continental crust origin ore deposits of the different terranes of ecuador irrespective of their age plot in distinct isotopic fields which are internally homogeneous this suggests that lithologic factors had an important control on the lead isotope compositions ultimately lead isotope compositions of the ore deposits of ecuador mirror the isotopic compositions of the rocks of the host terranes and are consistent with the multiterrane nature of the ecuado rian crust

portovelo a volcanic hosted epithermal vein system in ecuador south america the portovelo epithermal vein system in southwestern ecuador has produced more than 120 tons of gold and about 250 tons of silver the veins result from hydrothermal processes close to a miocene volcano which produced an andesitic to dacitic sequence followed by collapse and post collapse rhyolitic activity which generated most of the alteration and mineralization three main structural segments are defined by nw trending strike slip faults which show later stages of vertical movement these faults are responsible for development of an extensive n s dilatational jog within andesitic rocks which acted as the main host to ore deposition a large scale propylitic aureole surrounds a quartz chlorite sericite adularia core centered on the rhyolites within a system of collapse related ring structures a quartz chlorite sericite adularia calcite assemblage is the most common wall rock alteration close to the veins the size 4 x 15 km and vertical range 1400 m of the vein system is exceptional alteration textures and mineral assemblage including a quartz calcite gangue sulfides abundant sulfosalts and free gold electrum are quite typical of an adularia sericite epithermal deposit spatially the mineralization is arranged in three zones in addition three successive stages can be distinguished the bulk of economic mineralization was deposited during the second stage in association with a clear quartz and calcite gangue tm ice and tm clath data of fluid inclusions in the clear quartz indicate a high salinity similar to 10 5 eq wt nacl the homogenization temperatures of fluid inclusions in the gangue minerals and in the altered host rocks vary between 180 degrees and 310 degrees c quartz delta 18 o values from hydrothermally altered wall rocks reflect the original isotopic values of the latter these values show a narrower range in vein quartz delta o 18 between 7 7 parts per thousand and 11 5 parts per thousand smow in addition the delta o 18 values of the vein quartz increase systematically with decreasing homogenization temperature this suggests that quartz was in equilibrium with a large reservoir of water of constant o 18 o 16 composition at decreasing equilibrium temperatures the estimated isotopic composition of the fluids from which milky quartz and calcite of the main mineralization stage precipitated lies around 1 smow this value indicates a meteoric rather than a magmatic origin of the ambient fluid clear quartz of the second stage seems to have formed from a fluid with a delta 18 o of 3 parts per thousand smow this higher value can be due to a more intense water rock interaction or to mixing of meteoric with magmatic water

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

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

lead isotopic evidence for evolutionary changes in magma crust interaction central andes southern peru abstract lead isotopic measurements were made on andean igneous rocks of jurassic to recent age in moquegua and tacna departments southernmost peru to clarify the petrogenesis of the rocks and in particular to investigate the effect of crustal thickness on rock composition this location in the cordillera occidental is ideal for such a study because the ca 2 ga precambrian basement rocks arequipa massif have a distinct pb isotopic signature which is an excellent tracer of crustal interaction and because geomorphological research has shown that the continental crust was here thickened drastically in the later tertiary seven samples of quartz diorites and granodiorites from the ilo and toquepala intrusive complexes and seven samples of toquepala group subaerial volcanics were analyzed for pb isotopic compositions the plutonic rocks range in age from jurassic to eocene the volcanic rocks are all late cretaceous to eocene with one exception the pb isotopic ratios are in the ranges 206 pb 204 pb 18 52 18 75 207 pb 204 pb 15 58 15 65 and 208 pb 204 pb 38 53 38 74 the data reflect very little or no interaction with old continental material of the arequipa massif type lead from four miocene huaylillas formation ash flow tuffs two pliocene capillune formation andesites and five quaternary barroso group andesites has lower 206 pb 204 pb than that in the pre miocene rocks but relatively high 207 pb 204 pb and 208 pb 204 pb 206 pb 204 pb 18 16 18 30 207 pb 204 pb 15 55 15 63 208 pb 204 pb 38 45 38 90 tilton and barreiro 9 have shown that contamination by arequipa massif granulites can explain the isotopic composition of the barosso group lavas and the new data demonstrate that this effect is evident to varying degrees in all the analysed neogene volcanic rocks the initial incorporation of such basement material into the magma coincided with the early miocene uplift of this segment of the cordillera occidental 32 and thus with the creation of a thick crustal root the data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area but a rigorous relationship is not yet established

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538

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

geochemical evolution of the quaternary chachimbiro volcanic complex frontal volcanic arc of ecuador abstract the chachimbiro volcanic complex cvc is a composite volcano located in the frontal part of the ecuadorian quaternary arc four periods of activity can be differentiated in chachimbiro 400 ka long volcanic history named ch1 405 7 20 0 298 6 32 9 ka ch2 121 75 23 2 36 08 2 8 ka ch3 36080 280 22730 120 ybp and ch4 5 760 30 ybp respectively the magmatic suite ranges in composition from andesites to rhyodacites and displays systematic increases of sio2 sr y la yb incompatible elements e g lree mree lile and depletion of hree with time these geochemical features coupled with radiogenic isotope data and textural observations e g several resorption and overgrowth cycles in plagioclase phenocrysts suggest an open system evolution for cvc including typical crustal magmatic processes such as recharge assimilation and fractional crystallisation to better constrain the magmatic crustal processes of the cvc we use a multi model approach including three open system models rfc recharge fractional crystallisation afc assimilation fractional crystallisation and ec rafc energy constrained recharge assimilation fractional crystallisation different simulations have been developed to best fit cvc s geochemistry including trace elements sr y ratios and sr isotopes which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of cvc this is consistent with an overall thermal decline of the magmatic system through time the genetic model here presented involves the input of a magmatic recharge component matured in the low mid crust through dominant fractionation of amphibole garnet into intermediate crustal reservoir s where magma undergoes extensive fc

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

accreted fragments of the late cretaceous caribbean colombian plateau in ecuador the eastern part of the western cordillera of ecuador includes fragments of an early cretaceous c123 ma oceanic plateau accreted around 85 80 ma san juan unit west of this unit and in fault contact with it another oceanic plateau sequence guaranda unit is marked by the occurrence of picrites ankaramites basalts dolerites and shallow level gabbros a comparable unit is also exposed in northwestern coastal ecuador pedernales unit picrites have lree depleted patterns high endi and very low pb isotopic ratios suggesting that they were derived from an extremely depleted source in contrast the ankaramites and mg rich basalts are lree enriched and have radiogenic pb isotopic

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