the role of climate in the accumulation of lithium rich brine in the central andes abstract lithium rich brine within the sub surface of the salar del hombre muerto shm salt pan in the andes of northwestern argentina has a chemical and isotopic composition which is consistent with li derived from several sources the modern halite saturated lagoon li rich salts and brines formed recently and dissolution of halite which precipitated from ancient saline lakes shm lies in the closed basin that includes part of the massive cerro galan caldera which is drained by the rio los patos which is responsible for 90 of surface runoff into the salar the low li isotope composition 3 4 of this river is consistent with significant contributions of geothermal spring water as water drains through the volcaniclastic deposits which cover a large proportion of the basin li removal as indicated by decreasing li na occurs but without significant isotope fractionation this indicates a mechanism of surface sorption onto smectite or ferrihydrite rather than li incorporation into octahedral structural sites of clays these observations suggest that conditions in this high altitude desert have limited the dilution of hydrothermal spring water as well as the formation of clay minerals which jointly have allowed the li resource to accumulate rapidly changes in climate on a multi millennial time scale specifically in the hydrologic budget have resulted in solute accumulation rates that have been variable through time and decoupled li and na fluxes inflow to the salar under modern conditions has high li na 7 9 10 3 by wt with 7 li indistinguishable from basement rocks 0 3 to 6 4 while under pluvial climate conditions the li na of the saline lake was 40 times lower than the modern lagoon 0 1 0 3 10 3 compared to 10 6 13 4 10 3 with slightly higher 7 li 6 9 to 12 3 reflecting the uptake of 6 li into secondary minerals which formed under a wetter climate

high temperature emplacement of the cerro galan and toconquis group ignimbrites puna plateau nw argentina determined by trm analyses estimates of pyroclastic flow emplacement temperatures in the cerro galan ignimbrite and toconquis group ignimbrites were determined using thermal remanent magnetization of lithic clasts embedded within the deposits these ignimbrites belong to the cerro galan volcanic system one of the largest calderas in the world in the puna plateau nw argentina temperature estimates for the 2 08 ma cerro galan ignimbrite are retrieved from 40 sites in 14 localities 176 measured clasts distributed at different distances from the caldera and different stratigraphic heights additionally temperature estimates were obtained from 27 sample sites 125 measured clasts from seven ignimbrite units forming the older toconquis group 5 60 4 51 ma mainly outcropping along a type section at rio las pitas vega real grande the paleomagnetic data obtained by progressive thermal demagnetization show that the clasts of the cerro galan ignimbrite have one single magnetic component oriented close to the expected geomagnetic field at the time of emplacement results show therefore that most of the clasts acquired a new magnetization oriented parallel to the magnetic field at the moment of the ignimbrite deposition suggesting that the clasts were heated up to or above the highest blocking temperature tb of the magnetic minerals tb 580 c for magnetite tb 600 630 c for hematite we obtained similar emplacement temperature estimations for six out of the seven volcanic units belonging to the toconquis group with the exception of one unit lower merihuaca where we found two distinct magnetic components the estimation of emplacement temperatures in this latter case is constrained at 580 610 c which are lower than the other ignimbrites these estimations are also in agreement with the lowest pre eruptive magma temperatures calculated for the same unit i e 790 c hornblende plagioclase thermometer folkes et al 2011b we conclude that the cerro galan ignimbrite and toconquis group ignimbrites were emplaced at temperatures equal to or higher than 620 c except for lower merihuaca unit emplaced at lower temperatures the homogeneity of high temperatures from proximal to distal facies in the cerro galan ignimbrite provides constraints for the emplacement model marked by a relatively low eruption column low levels of turbulence air entrainment surface water interaction and a high level of topographic confinement all ensuring minimal heat loss

receiver function images from the moho and the slab beneath the altiplano and puna plateaus in the central andes summary teleseismic data recorded during one and a half years are investigated with the receiver function technique to determine the crustal and upper mantle structures underneath the highly elevated altiplano and puna plateaus in the central andes a series of converting interfaces are determined along two profiles at 21 s and 25 5 s respectively with a station spacing of approximately 10 km the data provide the highest resolution gained from a passive project in this area so far the oceanic nazca plate is detected down to 120 km beneath the altiplano whereas beneath the puna the slab can unexpectedly be traced down to 200 km depth at longer periods a shallow crustal low velocity zone is determined beneath both plateaus exhibiting segmentation in the case of the altiplano the segments present vertical offsets and are separated by inclined interfaces which coincide with major fault systems at the surface an average depth to moho of about 70 km is determined for the altiplano plateau a strong negative velocity anomaly located directly below the moho along with local crustal thinning is interpreted beneath the volcanic arc of the altiplano plateau between 67 w and 68 5 w a deep section of the puna profile reveals thinning of the mantle transition zone although poorly resolved the detected anomaly may suggest the presence of a mantle plume which may constitute the origin of the anomalous temperatures at the depth of the upper mantle discontinuities

volcanic biotite sanidine 40ar 39ar age discordances reflect ar partitioning and pre eruption closure in biotite the 40 ar 39 ar radioisotope system is widely used to date eruption and cooling of volcanic tephra marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained this chronometer bridges the gap between 14 c and longer lived isotopic systems that are too imprecise for dating young samples however 40 ar 39 ar ages obtained from coevally erupted biotite and sanidine do not always match here we use an independent chronometer 238 u 230 th disequilibrium to demonstrate that 40 ar 39 ar age disparity is not caused by differences in pre eruption crystallization times our findings indicate that the presence of extraneous 40 ar in biotite and its absence in sanidine may result from violations of two assumptions implicit in 40 ar 39 ar geochronology on volcanic samples 1 prior to eruption minerals are devoid of 40 ar due to rapid loss to an infinite reservoir such as the atmosphere and 2 closure to volume diffusion is geologically instantaneous and coincident with eruption we propose a mechanism whereby the presence of extraneous ar in certain minerals is explained by the relative sequence of four events in a magmatic system 1 crystallization 2 mineral closure with respect to ar diffusion 3 isotopic equilibration of magmatic and atmospheric ar and 4 quenching of the system by eruption these data have potentially far reaching implications for studies that depend on geochronological data necessitating re evaluation of interpretations based solely on biotite with no independent age control particularly in young samples where the effects are most pronounced

partitioning of sr ba rb y and lree between alkali feldspar and peraluminous silicic magma samples from well known rhyolitic systems in the western usa were analyzed to determine trace element partition coefficients between sanidine and coexisting melt both d sr and d eu correlate positively with an concentration in sanidine barium not surprisingly shows more correlation with or in sanidine all three show negative correlations with whole rock al2o3 partition coefficients for ree show a decreasing trend with strong positive d eu d lree correlate negatively with the whole rock al2o3 in peraluminous rocks al2o3 in magma seems to exert a major influence on partition coefficients of trace elements with increasing al2o3 concentration the melt composition becomes increasingly similar to that of feldspar incompatible trace elements rb y nb and zr in sanidine show positive trends whereas compatible trace elements sr eu and ba show negative correlations when plotted against whole rock al2o3 lrees which are expected to substitute for ca in the sanidine structure behave like sr and eu because trace element partitioning is influenced by both crystal and magma compositions multiple regression analysis has been used to predict trace element partition coefficients the equations will assist workers in the selection of partition coefficients used in petrogenetic models of silicic systems

columnar jointing in vapor phase altered non welded cerro galan ignimbrite paycuqui argentina columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits however the non welded cerro galan ignimbrite at paycuqui argentina contains well developed columnar joints that are instead due to high temperature vapor phase alteration of the deposit where devitrification and vapor phase crystallization have increased the density and cohesion of the upper half of the section thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures above 630 c but the lack of welding textures indicates temperatures below the glass transition temperature in order to remain below the glass transition at 630 c the minimum cooling rate prior to deposition was 3 0 10 3 8 5 10 2 c min depending on the experimental data used for comparison alternatively if the deposit was emplaced above the glass transition temperature conductive cooling alone was insufficient to prevent welding crack patterns average 4 5 sides to each polygon and column diameters average 75 cm are consistent with relatively rapid cooling where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer the presence of regularly spaced complex radiating joint patterns is consistent with fumarolic gas rise where volatiles originated in the valley confined drainage system below joint spacing is a proxy for cooling rates and is controlled by depositional thickness valley width we suggest that the formation of joints in high temperature non welded deposits is aided by the presence of underlying external water where vapor transfer causes crystallization in pore spaces densifies the deposit and helps prevent welding

correlation of upper cenozoic tephra layers between sediments of the western united states and eastern pacific ocean and comparison with biostratigraphic and magnetostratigraphic age data five widespread upper cenozoic tephra layers that are found within continental sediments of the western united states have been correlated with tephra layers in marine sediments in the humboldt and ventura basins of coastal california by similarities in major and trace element abundances four of these layers have also been identified in deep ocean sediments at dsdp sites 34 36 173 and 470 in the northeastern pacific ocean these layers erupted from vents in the yellowstone national park area of wyoming and idaho y the cascade range of the pacific northwest c and the long valley area california l are the huckleberry ridge ash bed 2 0 ma y rio dell ash bed ca 1 5 ma c bishop ash bed 0 74 ma l lava creek b ash bed 0 62 ma y and loleta ash bed ca 0 4 ma c the isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments for example the widespread bishop ash bed is correlated from proximal localities near bishop in east central california where it is interbedded with volcanic and glacial deposits to lacustrine beds near tecopa southeastern california to deformed on shore marine strata near ventura southwestern california to deep ocean sediments at site 470 in the eastern pacific ocean west of northern mexico the correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control relative age variations of as much as 0 5 m y exist between marine biostratigraphic datums for example highest occurrence level of discoaster brouweri and calcidiscus tropicus c macintyrei as determined from sedimentation rate curves derived from other age control available at each of several sites these discrepancies may be due to several factors among which are 1 diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds 2 upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western united states and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums 3 dissolution of microfossils or selective diagenesis of some taxa 4 lack of precision in isotopic age calibration of these datums 5 errors in isotopic ages of tephra beds and 6 large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums correlation of tephra layers between on land marine and deep ocean deposits indicates that some biostratigraphic datums diatom and calcareous nannofossil may be truly time transgressive because at some sites they are found above and at other sites below the same tephra layers

the geological and structural evolution of the long lived miocene pleistocene la hoyada volcanic complex in the geodynamic framework of the central andes argentina abstract the long lived la hoyada volcanic complex lhvc is located in the east of cordillera de san buenaventura in argentina this region is important for understanding of the geodynamic evolution of the southern puna since it is located in its southern edge where the transition between normal and low angle subduction occurs the aim of this work is to contribute to the knowledge of the volcanism of the southern puna through the analysis of the stratigraphy and geological evolution of lhvc fieldwork and image analysis have led to a new lithostratigraphic scheme that identifies lithosomes and unconformity bounded stratigraphic units defining the stratigraphic architecture of the lhvc new 40ar 39ar ages further constrain the geological evolution of this complex the resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units with the identification of at least six superimposed eruptive centers and their products during the lhvc evolution a new stratigraphic framework for the eastern cordillera de san buenaventura composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity catastrophic destructive events and changes in the magmatic plumbing system is proposed 1 san buenaventura supersynthem ca 9 7 ma including mainly explosive rhyolitic volcanism 2 lower la hoyada supersynthem ca 7 d pyroclastic and epiclastic deposits and 4 purulla supersynthem

geology geochemistry geochronology and economic potential of neogene volcanic rocks in the laguna pedernal and salar de aguas calientes segments of the archibarca lineament northwest argentina abstract this study presents new geochemical geochronological isotopic and mineralogical data combined with new geological mapping for a 2400 km 2 area of neogene volcanic rocks in northwestern argentina near the border with chile between 25 10 s and 25 45 s the area covers the zone of intersection between the main axis of the cordillera occidental and a set of nw se trending structures that form part of the transverse archibarca lineament this lineament has localized major ore deposits in chile e g the late eocene la escondida porphyry cu deposit and large volcanic centers such as the active llullaillaco and lastarria volcanoes on the border between chile and argentina and the neogene archibarca antofalla and cerro galan volcanoes in argentina neogene volcanic rocks in the laguna pedernal and salar de aguas calientes areas are mostly high k calc alkaline in composition and range from basaltic andesites through andesites and dacites to rhyolites magmatic temperatures and oxidation states estimated from mineral compositions range from 1000 c and fmq 1 0 1 5 in andesites to 850 c and fmq 1 5 2 0 in dacites and rhyolites the oldest rocks consist of early middle miocene andesite dacite plagioclase pyroxene phyric lava flows and ignimbrites with 40 ar 39 ar ages ranging from 17 14 0 10 ma to 11 76 0 27 ma their major and trace element compositions are typical of the andean central volcanic zone and show strong crustal contamination trends for highly incompatible elements such as cs rb th and u these rocks are geochemically grouped as sub suite 1 this widespread intermediate composition volcanism was followed in the middle late miocene by a period of more focused rhyodacitic flow dome complex formation these felsic rocks are characterized by less extreme enrichments in highly incompatible elements and increasing depletion of heavy rare earth elements these rocks are geochemically grouped as sub suite 2 the youngest rocks in this sub suite show the highest la yb ratios and are characterized by abundant hornblende phenocrysts not commonly seen in other rocks from the area in the pliocene pleistocene there was a return to more typical andesite dacite volcanism with geochemical characteristics similar to the early middle miocene lavas and are also grouped in sub suite 1 finally extensional tectonics in the quaternary led to localized outpouring of mafic basaltic andesitic to andesitic monogenetic lava flows and cones one particularly large flow the vega aguas calientes lava flow covers approximately 90 km 2 and samples form two groupings with affinities similar to the least evolved samples from sub suites 1 and 2 sub groups ba1 and ba2 respectively nd and sr isotopic compositions indicate moderate to strong crustal contamination especially in more felsic rocks and extend from 87 sr 86 sr 0 706 and end 2 4 values typical of central volcanic zone rocks to more evolved compositions 0 709 and 6 8 respectively typical of large volume ignimbrites of the altiplano puna volcanic complex and cerro galan the latter compositions are thought to be derived by extensive interaction between mantle derived arc magmas and paleozoic granitoid rocks that form much of the crustal column in this region the distinctive mineralogy and geochemistry of the sub suite 2 middle late miocene rhyodacitic flow dome complexes indicate that these magmas had higher water content than both the earlier and later sub suite 1 andesites dacites they were erupted during a period of tectonic quiescence following the quechua orogenic phase and geophysical evidence suggests that they were proximally derived from a large upper crustal magma chamber which partially collapsed to form a trap door caldera strong fumarolic alteration associated with the youngest of these felsic volcanoes cerro abra grande suggests the potential for the existence of epithermal type mineralization within the volcanic edifice or porphyry type mineralization at depth

geochemical isotopic and single crystal 40ar 39ar age constraints on the evolution of the cerro galan ignimbrites the giant ignimbrites that erupted from the cerro galan caldera complex in the southern puna of the high andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone major and trace element analyses of cerro galan ignimbrites 68 71 sio2 that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels afc modeling and new fractionation corrected 18o values from quartz 7 63 8 85 are consistent with the ignimbrite magmas being near 50 50 mixtures of enriched mantle 87sr 86sr 0 7055 and crustal melts 87sr 86sr near 0 715 0 735 processes at lower crustal levels are predicated on steep heavy ree patterns sm yb 4 7 high sr contents 250 ppm and very low nb ta 9 5 ratios which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet bearing residue processes at mid crustal levels create large negative eu anomalies eu eu 0 45 0 70 and variable trace element enrichment in a crystallizing mush zone with a temperature near 800 850 c the mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction crystallinity differences in the ignimbrites are attributed to biotite zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near eutectic crystallization occurs at temperatures as low as 680 c just preceding eruption 40ar 39ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the galan ignimbrite erupted in more than one batch including a 2 13 ma intracaldera flow and outflows to the west and north at near 2 09 and 2 06 ma episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas whose changing chemistry reflects their evolution over a steepening subduction zone could trigger the eruptions of the cerro galan ignimbrites

teleseismic tomography of the southern puna plateau in argentina and adjacent regions abstract we performed a teleseismic p wave tomography study using seismic events at both teleseismic and regional distances recorded by a temporary seismic array in the argentine puna plateau and adjacent regions the tomographic images show the presence of a number of positive and negative anomalies in a depth range of 20 300 km beneath the array the most prominent of these anomalies corresponds to a low velocity body located in the crust most clearly seen in the center of the array 27 s 67 w between the cerro peinado volcano the cerro blanco caldera and the farallon negro in the east this anomaly southern puna magmatic body extends from the northern most part of the array and follows the line with the highest density of stations towards the south where it becomes smaller it is flanked by high velocities on the west and the east respectively on the west the high velocities might be related to the subducted nazca plate on the northeast the high velocity block coincides with the position of the hombre muerto basin in the crust and could be indicating an area of lithospheric delamination where we detected a high velocity block at 100 km depth on the eastern border of the puna plateau north of galan this block might be related to a delamination event in an area with a thick crust of paleozoic metamorphic rocks at the border between puna and eastern cordillera in the center of the array the southern puna magmatic body is also flanked by high velocities but the most prominent region is located on the east and is interpreted as part of the sierras pampeanas lithosphere with high velocities the position of the sierras pampeanas geological province is key in this area as it appears to limit the extension of the plateau towards the south

crustal thickness in the central andes from teleseismically recorded depth phase precursors summary although crustal thickness is a critical constraint for geodynamic models of andean orogenesis relatively few measurements exist from local seismic studies in this study we use reflections from the underside of the moho as illuminated by intermediate depth earthquakes to provide new estimates of crustal thickness for the central andes 16 34 s these reflected signals pmp are identified as precursors to the depth phase pp when recorded at teleseismic distances 35 85 although relatively small in amplitude the pmp phase is often clear even on single seismograms less obvious pmp phases were enhanced by stacking traces from arrays this method was most effective for events of m w 6 and depth 100 km crustal thickness determined in this study ranges from 59 to 70 km in southern peru from 49 to 80 km across the puna altiplano and from 50 to 60 km above the pampean flat slab the lack of such phases for certain events may be evidence of heterogeneity at the moho and some precursors appear to correspond to intracrustal magma and lithospheric mantle discontinuities the results obtained using the pmp technique combined with those from other studies show the crustal thicknesses under the central andes are more variable than expected from purely isostatic considerations we infer that one or a combination of geodynamic processes involving lower crustal flow lower crustal delamination and mantle crustal lithospheric coupling can explain the highly variable topography on the moho beneath the central andes

tulelake california the last 3 million years abstract the tulelake basin formed by east west extension and faulting during the past several million years contains at least 550 m of lacustrine sediment interdisciplinary studies of a 334 m long cored section from the town of tulelake california near the center of the basin document a 3 m y record of environmental changes the core consists of a thick sequence of diatomaceous clayey silty and marly lacustrine sediments interbedded with numerous tephra layers paleomagnetic study puts the base of the core at about 3 0 ma twelve widespread silicic tephra units provide correlations with other areas and complement age control provided by magnetostratigraphy mafic and silicic tephra units erupted from local sources are also common in the core widespread tephra units include the llao rock pumice tsoyawata 7 ka the trego hot springs bed 23 ka and the rockland 0 40 ma lava creek 0 62 ma and rio dell 1 5 ma ash beds as well as several ash beds also found at summer lake oregon and an ash bed originally recognized in dsdp hole 173 in the northeastern pacific several tephra layers found in the core also occur in lacustrine beds exposed around the margins of the basin and elsewhere in the ancestral lacustrine system diatoms are present throughout the section pollen is present in most of the section but some barren zones are found in the interval between 50 and 140 m the greatest change in behavior of the pollen record takes place just above the top of the olduvai normal polarity subchronozone ostracodes are present only in high carbonate 10 caco3 intervals evolutionary changes are found in the diatom and ostracode records bulk geochemical analyses show significant changes in elemental composition of the sediment through time

delamination of southern puna lithosphere revealed by body wave attenuation tomography the southern puna plateau has been proposed to result from a major pliocene delamination event that has previously been inferred from geochemical geological and some preliminary geophysical data seventy five seismic stations were deployed across the southern puna plateau in 2007 2009 by scientists from the u s germany chile and argentina to test the delamination model for the region the puna passive seismic stations were located between 25 and 28 s using the seismic waveform data collected from the puna experiment we employ attenuation tomography methods to resolve both compressional and shear quality factors qp and qs respectively in the crust and uppermost mantle the images clearly show a high q nazca slab subducting eastward beneath the puna plateau and another high q block with a westward dip beneath the eastern cordillera we suggest that the latter is a piece of delaminated south american lithosphere a significant low q zone lies between the nazca slab and the south american lithosphere and extends southward from the northern margin of the seismic array at 25 s before vanishing around 27 5 s this low q zone extends farther west in the crust and uppermost mantle at the southern end of the seismic array the low q zone reaches 100 km depth beneath the northern part of the array but only 50 km depth in the south lateral variations of the low q zone reflect the possible mechanism conversion between mantle upwelling related to delamination and dehydration the depth of the nazca slab as defined by q images decreases from north to south beneath the plateau which is consistent with the steep flat transition of the angle of the subducting slab as defined by previous earthquake studies

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

geomorphology and spatial distribution of monogenetic volcanoes in the southern puna plateau nw argentina abstract monogenetic volcanoes are the most common volcanic landforms on earth several works have shown that the study of geomorphology and spatial distribution of such small volcanoes reveals important aspects related to the volcanic field from eruption dynamics to tectonic processes the altiplano puna plateau 15 to 27 s central andes comprises the largest non collisional plateau on earth characterized by intensive deformation uplift and volcanism this region is marked by the presence of hundreds of mafic monogenetic volcanoes despite its discrete occurrence the monogenetic volcanoes show a close relationship with large scale lithospheric foundering events in the region in this contribution we applied a geographic information system gis approach in order to build the first complete catalog of monogenetic landforms present in the southern puna plateau 24 30 to 27 00 s nw argentina exploring their evolution and relationship with geotectonic events combining spatial analysis with a geomorphological study of each volcano it was possible to identify the presence of 285 monogenetic volcanoes in the region which could be grouped in seven clusters southern puna shows a predominance of cinder cones and lava domes suggesting a small influence of water in the eruption dynamics despite its high clustering degree southern puna monogenetic vents present a low density 8 3 10 3 vents km2 when compared to other monogenetic fields around the world in addition puna plateau cones show a close relationship with monogenetic cones associated with an extensional environment considering their back arc context based on relative and available absolute ages it was possible to map the volcanism evolution through time the results indicate that the first monogenetic volcanoes appeared in the central region of the plateau sierra de calalaste mainly associated with large scale nne ssw reactivated faults the activity then expanded antofalla and antofagasta regions and finally reached the borders of the plateau pasto ventura and arizaro regions which show volcanoes associated with nw se normal faults

structure of the crust and the lithosphere beneath the southern puna plateau from teleseismic receiver functions abstract we present a teleseismic p and s receiver function study using data from a temporary passive source seismic array in the southernmost puna plateau and adjacent regions the p receiver function images show the distribution of crustal thickness and v p v s ratio for this area over much of the southern puna plateau the crustal thickness is 50 55 km whereas to the west a thicker crust 60 to 75 km is observed beneath much of the andean volcanic arc region from the puna southward there is little obvious change in the crustal thickness across the border of the plateau south of 28 s the crust is seen to progressively thin towards the east in the pampean ranges where it is 35 40 km thick the southern puna plateau is characterized overall by a low crustal v p v s ratio less than 1 70 implying a felsic crustal composition an anomalously high v p v s ratio of 1 87 is observed beneath the cerro galan volcanic center in the region where a prominent crustal low velocity zone identified below 10 km depth probably extends into the lower crust the crustal thickness determined under the cerro galan area 59 km is close to that of the rest of the southern puna the prominent high v p v s ratio and low velocity zone beneath the galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the galan caldera a widespread crustal low velocity layer observed beneath much of the southern puna correlates well with crustal low velocity anomalies observed by teleseismic tomography the lithosphere asthenosphere boundary beneath the array can be clearly observed by both p and s receiver functions at depths of 70 90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau the mantle transition zone discontinuities appear at expected depths

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

imaging a magma plumbing system from mash zone to magma reservoir the puna plateau of the central andes is a well suited location to investigate the processes associated with the tectono magmatic development of a cordilleran system these processes include long lived subduction including shallow and steep phases substantial crustal thickening the emplacement of large volumes of igneous rocks and probably delamination to elucidate the processes associated with the development of a cordilleran system we pair common conversion point derived receiver functions with rayleigh wave dispersion data from ambient noise tomography the resulting high resolution shear wave velocity model of the southern puna plateau reveals the details of a lithospheric scale magma plumbing system slow velocities near the crust mantle transition are interpreted as a mash zone a partially molten zone where mantle derived melts interact with the lithosphere and undergo density differentiation with 4 9 4 9 melt after differentiation less dense and presumably more felsic melts propagate to shallower depths within the crust 20 km below surface and comprise vertically 10 km and laterally 75 km extensive slow velocity bodies that span the frontal arc and plateau interior these large slow velocity bodies represent a partially molten mid crust up to 22 where magma can further evolve to higher silica concentrations the periodic influx of melt from the underlying mash zone into these mid crustal bodies may serve as a trigger to the eruption of the voluminous ignimbrites observed in the southern puna plateau many of the active tectonic processes operating along the southern puna plateau are thought to be analogous to the processes that formed the north american cordillera thus these results could provide insight into some of the processes associated with the development of a cordilleran margin

pulsating flow dynamics of sustained forced pyroclastic density currents insights from a facies analysis of the campo de la piedra pomez ignimbrite southern puna argentina the quaternary campo de la piedra pomez ignimbrite cppi is a superbly exposed partially indurated rhyolitic ignimbrite emplaced on the southern puna of argentina it is characterized by a variety of facies that record in unprecedented detail the flow dynamics of the parent pyroclastic density currents pdcs detailed facies analysis and internal architecture defined using a sequential stratigraphy approach reveal that cppi was formed by sustained pdcs generated from a low fountain eruptive style boiling over the pdcs had overall flow conditions characterized by high particle concentration and limited capability to surmount topographic obstacles the mobility of the pdcs was largely controlled by high pore pressure and the continuous supply at the source sustained forced convection dominated pdcs the successive forestepping backstepping stacking patterns identified in the cppi reflect a marked unsteadiness of its parent pdcs due to a pulsating discharge rate at the source the lateral facies variations in the cppi record the non uniform character of its parent pdcs as they flowed outward from the base of the collapsing fountain proximal medial areas characterized by steady aggradation rates pass through to distal braided thalwegs with highly variable aggradation rates to frontal edges characterized by the step aggradations of secondary decoupled pumice rich lobes collectively the facies variations in the cppi record the complex dynamics of sustained pdcs that include pulsating discharge rate at the source progressive modification of the original topography and lateral flow transformations

petrogenesis of early neogene magmatism in the northern puna implications for magma genesis and crustal processes in the central andean plateau of uolm 2 and mm group rocks assuming the same endnew compositional data and petrogenetic models are presented for member compositions the modelling suggests genesis of the mm pre upper miocene volcanism in the northern puna of argentina magmas at higher pressure than the uolm 2 centres 10 kbar 22 s 24 s two phases of volcanism produced small dome vs 7 kbar which may reflect the influence of crustal thickening in complexes of mainly silicic andesite to low sio2 rhyolite the the plateau region by the mid miocene the felsic dome complexes upper oligocene lower miocene phase uolm 20 17 ma of this study are compositionally similar to the large volume calderaproduced two distinct groups of rocks the uolm 1 group is sourced felsic ignimbrites that dominated volcanism in the region metaluminous and mainly andesitic with isotopic compositions like from 10 to 2 ma and our results suggest that there is no fundamental those of the recent arc sr srt 0 706 ndt 3 the difference in magma genesis between them the differences in the uolm 2 group is more silicic and peraluminous and has isotopic volumes and the mode of eruption reflect changes in the stress and compositions indicating a substantial crustal contribution sr thermal regime with time srt 0 713 ndt 8 the mid miocene phase mm 15 12 ma produced rocks similar in composition to those of the uolm 2 group sr srt 0 710 ndt 7 but with higher incompatible element contents ratios of ba nb and zr

development of the southern eastern cordillera nw argentina constrained by apatite fission track thermochronology from early cretaceous extension to middle miocene shortening 1 for the puna plateau and eastern cordillera of nw argentina the temporal and spatial pattern of deformation and surface uplift remain poorly constrained analysis of completely and partially reset apatite fission track samples collected from vertical profiles along an ese trending transect extending from the plateau interior across the southern eastern cordillera at 25 s reveals important constraints on the deformation and exhumation history of this part of the andes the data constrain the neogene andean development of the eastern cordillera as well as rift related exhumation for some of the sampled locations in the late jurassic early cretaceous an intervening eocene oligocene exhumation episode in the southern eastern cordillera was probably related to crustal shortening subsequent reburial of the area by andean foreland basin strata commenced between 30 and 25 myr magnitude and duration of sedimentation revealed by thermal modeling differ between the sample locations pointing to an eastward propagating basin system in the southern eastern cordillera andean deformation commenced at 22 5 21 myr predating both the inferred formation of significant topography by 5 7 5 myr and preservation of sediments in the adjacent cenozoic basins by 6 5 8 myr comparing the calculated structural depth of partially reset samples suggests that newly formed west dipping reverse faults along the former salta rift margin accommodated most of the neogene tectonic movement late cenozoic deformation at the southern eastern cordillera began earlier in the west and subsequently propagated eastward the lateral growth of the orogen is coupled with a foreland basin system developing in front of the range and then becomes subsequently compartmentalized by later emergent topography

revised age of the rockland tephra northern california implications for climate and stratigraphic reconstructionsin the western united states lanphere et al 1999 1 presented new data for the age of the rockland pumice tuff breccia of wilson 1961 2 using the incremental heating 40ar 39ar technique their age 610 ka is 200 ka older than zircon fission track ages obtained on this tuff by meyer et al 1980 3 1991 4

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude

multi proxy evidence for tectonic control on the expansion of c4 grasses in northwest argentina the timing of the rise of c4 grasses and the climatic conditions driving their expansion are widely debated with recent studies suggesting that the expansion was more complex than previously thought proxy reconstructions from northwestern argentina record multiple increases and decreases in c4 abundance throughout the late miocene to early pliocene as well as variations in the timing of the spread of c4 grasses into the region here we present a multi proxy paleovegetation reconstruction of the proportions of c3 and c4 vegetation using organic carbon and phytoliths preserved in paleosols at entre rios in the santa maria basin of catamarca argentina between 6 2 and 3 3 ma both corg13 and phytolith assemblages indicate a c3 dominated ecosystem with no evidence for substantial amounts of c4 vegetation from the late miocene through mid pliocene despite their rise to dominance elsewhere during this time these data suggest that by 6 2 ma the santa maria basin floor was at an elevation high enough to create locally cool conditions that allowed c3 vegetation to outcompete c4 grasses phytolith assemblages show that later uplift of the sierra calchaquies and sierra aconquija ranges through the pliocene increased aridity and drove the expansion of c3 grasses at the expense of palms and other forest indicators this multi proxy paleovegetation reconstruction demonstrates that tectonics played an important role in controlling the late miocene expansion c4 grasses by suppressing their spread into northwest argentina

central andean mantle and crustal seismicity beneath the southern puna plateau and the northern margin of the chilean pampean flat slab earthquake hypocenters recorded in the andean southern puna seismic array 25 28 s 70 65 w provide new constraints on the shape of the subducting nazca plate beneath the puna plateau the transition into the chilean pampean flat slab and the thermal state of the mantle and crust some 270 new mantle hypocenters suggest that the subducting slab under the puna shoals into the flat slab segment more abruptly and farther to the north than previously indicated the revised geometry is consistent with the central volcanic zone incapillo caldera being the southernmost center with pleistocene activity until reaching the southern side of the flat slab region evidence for the revised slab geometry includes three well defined hypocenter clusters in the pipanaco nest 27 5 29 s 68 66 w which are interpreted to reflect slab bending stresses a few low magnitude earthquakes with strongly attenuated s waves in the long recognized antofalla teleseismic gap 25 5 27 5 s support a continuous slab under the southern puna the paucity of gap earthquakes and the presence of mafic magmas are consistent with a hot mantle wedge reflecting recent lithospheric delamination evidence for a hot overlaying puna crust comes from new crustal earthquake hypocenters concentrated at depths shallower than 5 km two notable short duration swarms were recorded under the resurgent dome of the 2 ma back arc cerro galan caldera and the near arc cerro torta dome new crustal earthquake focal mechanisms from 17 events in the array along with two existing mechanisms have strike slip oblique reverse and oblique normal solutions fitting with regional e w compression and n s extension

correlation of the rockland ash bed a 400 000 year old stratigraphic marker in northern california and western nevada and implications for middle pleistocene paleogeography of central california abstract outcrops of an ash bed at several localities in northern california and western nevada belong to a single air fall ash layer the informally named rockland ash bed dated at about 400 000 yr b p the informal rockland pumice tuff breccia a thick coarse compound tephra deposit southwest of lassen peak in northeastern california is the near source equivalent of the rockland ash bed relations between initial thickness of the rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the mazama ash from crater lake oregon identification of the rockland tephra allows temporal correlation of associated middle pleistocene strata of diverse facies in separate depositional basins specifically marine littoral estuarine and fluvial strata of the hookton and type merced formations correlate with fluvial strata of the santa clara formation and unnamed alluvium of willits valley and the hollister area in northwestern and west central california and with lacustrine beds of mohawk valley fluvial deposits of the red bluff formation of the eastern sacramento valley and fluvial and glaciofluvial deposits of fales hot spring carson city and washoe valley areas in northeastern california and western nevada stratigraphic relations of the rockland ash bed and older tephra layers in the great valley and near san francisco suggest that the southern great valley emerged above sea level about 2 my ago that its southerly outlet to the ocean was closed sometime after about 2 my ago and that drainage from the great valley to the ocean was established near the present northerly outlet in the vicinity of san francisco bay about 0 6 my ago

fission track age 400 000 yr of the rockland tephra based on inclusion of zirco grains lacking fossil fission tracks abstract a zircon fission track age of about 400 000 yr b p has been determined for the rockland tephra a widespread pyroclastic layer in northern california and western nevada new ages of zircon separates from both proximal and distal exposures of this layer range from 370 000 to 460 000 yr ages of the best material provide a narrower range from 370 000 yr for unwelded ash flow tuff to 420 000 yr for distal air fall ash that appears to be uncontaminated by clastic detritus or xenocrysts detrital or xenocrystic grains in the ash flow tuff may have been annealed during emplacement and cooling of the tuff detrital and xenocrystic zircons are identified on the basis of their physical characteristics and distinctly older ages independent stratigraphic and magnetostratigraphic data constrain the age of the rockland tephra between 300 000 and 600 000 yr a range that is compatible with the fission track age zircon grains containing no spontaneous fossil tracks are regarded as part of the normal population of comagmatic grains because maximum ages calculated for these grains form a population that mimics the distribution of ages of individual zircon grains that contain fossil tracks modal ages of both groups fall between 250 000 and 500 000 yr induced fission tracks from grains that lack fossil tracks are included in the age calculations resulting in significantly younger and more coherent dates than would result if these tracks had been omitted especially those of the finer grained distal samples

the geochemistry of a dying continental arc the incapillo caldera and dome complex of the southernmost central andean volcanic zone 28 s the pleistocene incapillo caldera and dome complex 5 570 m marks the southernmost siliceous center of the andean central volcanic zone 28 s where the steeply dipping 30 segment of the subducting nazca plate transitions into the chilean flatslab to the south the eruption of the incapillo caldera and dome complex began with a 3 1 ma effusive phase characterized by 40 rhyodacitic dome eruptions this effusive phase was terminated by an explosive caldera forming event at 0 51 ma that produced the 14 km3 incapillo ignimbrite distinctive and virtually identical chemical signatures of the domes and ignimbrites sio2 67 72 wt la yb 37 56 ba la 16 28 la ta 30 50 87sr 86sr 0 70638 0 70669 e nd 4 2 to 4 6 indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor the strong hree depletion sm yb 6 8 that distinguishes incapillo magmas from most of the large ignimbrites of the altiplano puna plateau can be explained by the extent and degree of partial melting at lower crustal depths 40 km in the presence of garnet at upper crustal depths this high pressure residual geochemical signature also common to adjacent late miocene pliocene pircas negras andesites was partially overprinted by shallow level assimilation and fractional crystallization processes energy constrained afc modeling suggests that incorporation of anatectic upper crustal melts into a fractionated adakite like dacitic host best explains the petrogenesis of incapillo magmas the diminution of the sub arc asthenospheric wedge during nazca plate shallowing left the incapillo magma chamber unreplenished by both mafic mantle derived and lower crustal melts and thus stranded at shallow depths within the andean crust based on its small size and distinctive high pressure chemical signature the incapillo caldera and dome complex provides an endmember model for an andean caldera erupting within a waning magmatic arc over a shallowing subduction zone

isothermal plateau fission track age and revised distribution of the widespread mid pleistocene rockland tephra in west central united states a tephra bed in southeastern idaho is correlated to rockland tephra a widespread silicic air fall unit previously identified in northern california and western nevada correlation of proximal rockland tephra and its distal correlative is based on glass chemistry determined by electron microprobe and neutron activation analysis an age estimate of proximal rockland tephra is precisely determined by application of the isothermal plateau fission track itpft technique to its hydrated glass shards a weighted mean age of 0 47 0 04 ma compares well with age estimates based on co existing mineral phases using the external detector fission track and additive dose thermoluminescence techniques the presence of rockland tephra 800 km east of its presumed eruptive source will improve the development of detailed chronologies and correlation of pleistocene sedimentary sequences located in coalescent intermontane basins of west central united states

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

the evaluation of brine prospects and the requirement for modifications to filing standards the recent increase in demand for lithium has led to the development of new brine prospects particularly in the central andes the brines are hosted in closed basin aquifers of two types mature halite dominant and immature clastic dominant the estimate of elemental resources in these salars depends on a detailed knowledge of aquifer geometry porosity and brine grade the geometry of the aquifers can be evaluated by classical geophysical and drilling techniques but because the resource is a fluid with the attendant problems of in aquifer mixing and reorganization existing codes for filing resource and reserve estimates need modification total porosity is relatively straightforward to measure but effective porosity and specific yield which are required to estimate the resource are more difficult recovery factors are low compared with most metalliferous and industrial mineral deposits due to reliance on pumping of the brine from wells for extraction these and related issues lead us to believe that modifications to the existing standards for reporting mineral resources and reserves are required for these prospects

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

implications of quaternary volcanism at cerro tuzgle for crustal and mantle evolution of the puna plateau central andes argentina the high k tuzgle volcanic center 24 s 66 5 w along with several small shoshonitic centers developed along extensional quaternary faults of the el toro lineament on the east central puna plateau 275 km east of the main front of the andean central volcanic zone cvz these magmas formed by complex mixing processes in the mantle and thickened crust 50 km above a 200 km deep scismic zone tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate like ti group element characteristics lower incompatible element concentrations and lower 87sr 86sr ratios at a given end underlying mio pliocene volcanic rocks erupted in a compressional stress regime and have back arc like calc alkaline chemical characteristics the tuzgle rocks can be divided into two sequences with different mantle precursors a an older more voluminous rhyodacitic ignimbrite to mafic andestitic 56 to 71 sio2 sequence with la yb ratios 35 la yb ratios are controlled by the mafic components low ratios result from larger mantle melt percentages than high ratios shoshonitic series lavas 52 to 62 sio2 contain small percentage melts of more isotopically enriched arc like mantle sources some young tuzgle lavas have a shoshonitic like component variable thermal conditions and complex stress system are required to produce the tuzgle and shoshonitic series magmas in the same vicinity these conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the altiplano to the thinner mantle lithosphere that produced back are calc alkaline and intraplate type flows in the southern puna substantial upper crustal type contamination in tuzgle lavas is indicated by decreasing end 2 5 to 6 7 with increasing 87sr 86sr 0 7063 to 0 7099 ratios and sio2 concentrations and by negative eu anomalies eu eu 0 78 in lavas that lack plagioclase phenocrysts trace element arguments indicate that the bulk contaminant was more silicic than the tuzgle ignimbrite and left a residue with a high pressure mineralogy crustal shortening processes transported upper crustal contaminants to depths where melting occurred these contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity decollement the tuzgle ignimbrite erupted from this level when melting rates were highest subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas

origin of late miocene peraluminous mn rich garnet bearing rhyolitic ashes in the andean foreland northern argentina abstract the ramadas volcanic center on the eastern margin of the central andean puna plateau along the olacapato el toro lineament in argentina erupted a rare strongly peraluminous mn rich garnet bearing rhyolitic tuff in the late miocene the voluminous ashes from this eruption which are distinctive in having euhedral spessartine almandine garnets alm70 72sps22 26grs2 4prp0 5 1 as their only phenocryst are widely dispersed in the andean foreland among these tuffs are those in the guanaco formation foreland basin sediments along the xibi xibi and los alisos rivers in the rio grande de jujuy basin and the metan valley some 100 200 km east of the ramada volcanic center the co occurrence of tubular to cellular pumice fragments and blocky glass shards in an ash matrix in these tuffs is interpreted as indicating that they erupted in an initial vent opening event with pulsating pyroclastic surges at the initiation of the strong plinian eruption of the ramada volcanic center new ar ar ages from the guanaco fm glass shards agree with fossil ages in placing the eruption at 6 3 0 3 ma a number of distinctive chemical isotopic and mineralogical features including mg rich biotite and mg hastingsite xenocrysts of the guanaco formation and ramadas volcanic center tuffs are consistent with the melt having been derived by extensive crystallization of a mantle derived mafic shoshonitic series magma contaminated by assimilation dehydration melts of metapelitic sediment and the puna crust distinctive chemical features include whole rock sio2 contents of 75 76 wt a cnk indices 1 2 low ca mg ti and fe concentrations steep ree patterns with extreme negative eu anomalies low ba sr lree and high cs rb u concentrations and recalculated initial ratios of 87sr 86sr at 0 7119 and 143nd 144nd of 0 5123 at 6 3 ma the erupted magma has a transitional chemical character between those of the 11 ma mn rich garnet bearing coyaguayma ignimbrites to the north and the 6 ma cerro galan ignimbrites to the south unlike these crystal rich ignimbrites the ramadas tuff records the extraction of an extensively fractioned melt from a plagioclase k feldspar quartz and biotite bearing mush with accessory titanomagnetite and apatite in line with existing experimental studies on mn rich garnets and comparisons with the coyaguayma ignimbrite pre eruption crystallization of the rhyolite segregated from the mush likely occurred at 800 to 720 c at a depth of no

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

timing and genesis of ore formation in the qarachilar cu mo au deposit ahar arasbaran metallogenic zone nw iran evidence from geology fluid inclusions o s isotopes and re os geochronology in the qarachilar cu mo au deposit of the ahar arasbaran metallogenic zone aamz northwest iran mineralization occurs as three quartz sulfide veins that cut granodiorite quartz monzodiorite rocks of the qaradagh batholith qdb ore formation can be divided into three stages with chalcopyrite molybdenite and gold bearing pyrite appearing mainly in the first two stages the main wall rock alteration is silicification and intermediate argillic carbonate and propylitic alteration fluid inclusion microthermometry indicates trapping of medium to high salinity 9 2 55 wt nacl equiv fluids at qarachilar fluid inclusion trapping conditions are estimated to be 190 degrees c 530 degrees c and 0 1 3 kbar the variable phase ratios as well as spatial coexisting of liquid and vapor rich two phase and halite bearing multiphase fluid inclusions homogenizing over the same temperatures are consistent with fluid boiling during ore formation obtained delta o 18 h2o values of quartz from ore stage veins are 5 7 parts per thousand to 9 7 parts per thousand signifying that the ore fluid system was predominantly magmatic water the average calculated delta s 34 h2s values are 1 1 parts per thousand for pyrite chalcopyrite and molybdenite consistent with a magmatic source for sulfur combined the fluid inclusion and stable isotope data indicate that the ore forming fluids at qarachilar were magmatic in origin and were subsequently cooled and diluted by meteoric water fluid boiling and mixing facilitated hydrothermal alteration and mineralization molybdenite re os dating shows that mineralization occurred at 42 35 0 16 ma coincident with formation of porphyry cu mo mineralization at agarak deposit and hanqasar aygedzor and dastakert prospects in the lesser caucasus however qarachilar is older than all porphyry cu mo mineralization in the aamz and urumieh dokhtar magmatic arc udma which suggests that collision between arabia and eurasia were oblique and thus diachronous our data suggest that mineralization at qarachilar is related to collisional eocene magmatic hydrothermal activity related to neo tethys subduction and shares a number of similarities with the vein type cu mo au mineralization related to cu mo porphyries

magma sources and tectonic setting of central andean andesites 25 5 28 s related to crustal thickening forearc subduction erosion and delamination

age of the rockland tephra western usa abstract the age of the rockland tephra which includes an ash flow tuff south and west of lassen peak in northern california and a widespread ash fall deposit that produced a distinct stratigraphic marker in western north america is constrained to 565 000 to 610 000 yr by 40ar 39ar and u pb dating 40ar 39ar ages on plagioclase from pumice in the rockland have a weighted mean age of 609 000 7000 yr isotopic ages of spots on individual zircon crystals analyzed by the shrimp rg ion microprobe range from 500 000 to 800 000 yr a subpopulation representing crystal rims yielded a weighted mean age of 573 000 19 000 yr overall stratigraphic constraints on the age are provided by two volcanic units including the underlying tephra of the lava creek tuff erupted within yellowstone national park that has an age of 639 000 2000 yr the basaltic andesite of hootman ranch stratigraphically overlies the rockland in the lassen peak area and has 40ar 39ar ages of 565 000 29 000 and 565 000 12 000 yr for plagioclase and groundmass respectively identification of rockland tephra in odp core 1018 offshore of central california is an important stratigraphic age that also constrains the eruption age to between 580 000 and 600 000 yr

on the eruption age and provenance of the old crow tephra abstract tephrochronology is used to correlate and reconstruct geographically disparate sedimentary records of changing environment climate and landscape throughout geologic time single tephra layers represent isochronous markers across broad regions thus accurate and precise radiometric constraints on the timing of eruption are critical to their utility the old crow tephra is found throughout eastern beringia and represents the largest preserved pleistocene ashfall event in the region despite its volume and significance as a stratigraphic marker the provenance of this tephra is debated and the interpreted eruption age of marine isotope stage mis 5 at 125 ka has vacillated to investigate provenance and eruption age we develop a geochemical fingerprint for the old crow tephra via titanomagnetite geochemistry and zircon crystallization cooling age via coupled u pb u th and u th he zircon geochronology our results indicate that old crow oxides are geochemically distinct from the commonly assumed source caldera system at the emmons lake volcanic center elvc zircon crystals from the old crow tephra range in age from proterozoic to pleistocene with concordant zircon u pb u th and u th he dates on the youngest population of grains suggesting crystallization in their parent magma and in turn eruption at 202 9 9 5 ka we discuss strengths and shortcomings of our radiogenic datasets in light of this result and review the far reaching implications of a change in old crow eruption age