palaeoecology and archaeology of the karlich seeufer open air site middle pleistocene in the central rhineland germany abstract the karlich seeufer archaeological site in germany s central rhineland was excavated between 1980 and 1992 the site provides evidence for hominid activity during a middle pleistocene interglaciation known up to now only from the karlich clay pit and therefore defined as the karlich interglaciation which is considered to be post cromer iv and pre holstein sensu stricto in age the site is characterized by acheulean artifacts a fauna dominated byelephas palaeoloxodon antiquus and a unique and outstanding preservation of wooden and other palaeobotanical remains assuming all finds are associated the site previously was interpreted as an elephant hunting camp with a wooden structure together with wood and bone implements preservedin situ recent analysis of the same features has shown that the site can also be interpreted as a reworked archaeological sample hominid occupation occurred in the vicinity of a small lake with prevailing meso oligotrophic conditions expanding boreal forests and fen vegetation characterized the landscape

variable parent magmas and recharge regimes of the parinacota magma system n chile revealed by fe mg and sr zoning in plagioclase abstract zoning patterns of an content and fe mg and sr concentrations in plagioclase phenocrysts in andesites from parinacota volcano n chile reflect alternating recharge events with two chemically distinct mafic magmas these magmas are characterized by low and high sr contents similar to two recent mafic flank eruptions one end member basaltic andesite shows large sr enrichment and heavy rare earth depletions and thus equilibrated with lower crustal rocks at depth where plagioclase high sr is unstable and garnet high hree y is stable a second end member magma is lower in sr ba contents and has ree patterns typical for parent magmas elsewhere in the central andes the number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone variations of an fe and mg contents and morphology of zones suggest also changes in water pressure including decompression under water under saturated and water saturated conditions evidence for decompression is more present in post collapse samples suggesting that the change in the volcano dynamics involves changes in magma chamber location this shows the importance of the cone collapse event in the volcano s magmatic evolution we propose that both end members are only seen at the surface of parinacota volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the central volcanic zone

petrological inferences on the evolution of magmas erupted in the andagua valley peru central volcanic zone abstract major and trace element and sr and nd isotope data is presented from the andagua valley scoria cone and lava field 15 32 s 72 19 w southern peru in the northernmost part of the central volcanic zone cvz the rocks are all quite evolved in composition sio2 55 64 wt and classify as benmoreites latites and few mugearites and trachytes samples are characterized by high na2o 4 2 5 2 wt sr 600 1300 ppm ba 800 1600 ppm the main difference between the benmoreites and latites is in the na2o content that reach the highest so far reported from cvz for these sio2 concentrations the rocks are generally nearly aphyric but latites and trachytes are more porphyritic amphibole microphenocrysts generally are only present in latites and trachytes the difference between benmoreite and latite samples is reflected in lower p2o5 and zr content of the latite samples documenting the existence of two compositional different parental magma types the investigated volcanic activity spans the pleistocene to recent with the historic activity concentrated in the area just south of andagua combined relative stratigraphy petrography and geochemistry define volcanic units and demonstrate that rocks from chilcayoc grande chilcayoc chico 2 jenchana sucna 1 and chilcayoc chico 1 represent the most recent volcanic activity the main trend samples each form a co magmatic group resulting in sub parallel trends in many variations diagrams it is furthermore shown that these trends point towards calculated mixing lines relating the individual units through a binary mixing process thus indicating a two stage evolution in the case of jenchana sucna 1 and chilcayoc chico 1 the samples define positive correlation trends in the sr vs rb diagram that can be extrapolated back towards origo indicating nearly perfect incompatibility of sr and rb this together with generally high sr y 50 105 and low y content

the volcanic and magmatic evolution of volcan ollague a high k late quaternary stratovolcano in the andean central volcanic zone abstract volcan ollague is a high k calc alkaline composite volcano constructed upon extremely thick crust in the andean central volcanic zone volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the vinta loma series and an overlying coalescing dome and coulee sequence of the chasca orkho series following cone construction the upper western flank of ollague collapsed toward the west leaving a collapse amphitheater about 3 5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano the deposit is similar to the debris avalanche deposit produced during the may 18 1980 eruption of mount st helens u s a and was probably formed in a similar manner it presently covers an area of 100 km2 and extends 16 km from the summit subsequent to the collapse event the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse amphitheater additional post collapse activity included construction of a dacitic dome and coulee of the la celosa series on the northwest flank field relations indicate that vents for the vinta loma and post collapse series were located at or near the summit of the cone the vinta loma series is characterized by an anhydrous two pyroxene assemblage vents for the la celosa and chasca orkho series are located on the flanks and strike n55 w radial to the volcano the pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks a possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation in addition to the lava flows at ollague magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas all eruptive activity at ollague predates the last glacial episode 11 000 a b p because post collapse lava flows are overlain by moraine and are incised by glacial valleys present activity is restricted to emission of a persistent 100 m high fumarolic steam plume from a vent located within the summit andesite dome sr and nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation in contrast narrow ranges in 143nd 144nd and 87sr 86sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination

constraints on the origin of archean trondhjemites based on phase relationships of nuk gneiss with h2o at 15 kbar we report the t x h2o phase relations for the trondhjemitic nuk gneiss which comprises the principal component of the second phase of archean 3 0 2 8 by igneous activity in the godthab region of southwestern greenland a pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths under h2o saturated conditions a melting interval of 135 c separates the solidus at 610 c from the liquidus at 745 c h2o saturation at 15 kbar occurs at approximately 15 5 wt h2o the h2o undersaturated liquidus extends along a curved path from 745 c at 15 5 wt h2o to 1100 c at 2 h2o lower h2o contents were not investigated at low h2o contents 7 mafic residues consisting of some combination of hb ga ep and bi are possible and could correspond to amphibolitic source rocks at lower melt h2o contents 5 possible residues consist of na pl qz ga and could correspond to an earlier generation of tonalitic trondhjemitic rocks however such residues would not impart the highly fractionated ree patterns characteristic of archean trondhjemites if a first generation of tonalitic trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth it would crystallize to an assemblage consisting almost entirely of na pl qz with highly fractionated ree patterns these rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated ree patterns because neigher pl nor qz is capable of significantly fractionating hree from lree

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

geochemical effects of decoupled fractional crystallization and crustal assimilation most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization afc in some magmatic systems however assimilation and crystallization are not strictly related and the mass assimilated is decoupled from and therefore varies independently of the mass crystallized fca the geochemical consequences of fca are examined and compared to those of afc the behavior of incompatible elements is identical during afc and fca and ratios of these elements do not allow discrimination between the two processes major oxide least squares mass balance models do not discriminate between afc and fca at f 0 7 f fraction of melt remaining however fca yields magmas richer in compatible elements and with higher sr isotopic ratios than afc at a given value of f repeated cycles of fca and afc combined with magma mixing fam may result in unusual geochemical trends such as the evolution of a calc alkaline basaltic parent to a tholeiitic daughter magma or the evolution of low and medium k calc alkaline basalts to high k andesites dacites trachyandesites or trachydacites lavas erupted by the volcano micro profitis ilias on santorini hellenic arc greece provide an example of magmas which evolved by combined fractionation assimilation by fca and mixing

pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central andes the late miocene 7 9 to 4 5 ma pocho volcanic field in argentina occurs 700 km east of the chile trench over the modern shallowly dipping andean wadati benioff zone near 32 s latitude in argentina the field is located in the sierra de cordoba which is the easternmost laramide style block faulted range in the sierras pampeanas pampean ranges the arrival of the shallowly dipping slab initiated both volcanism and the uplift of the sierra de cordoba pocho rocks 52 to 68 sio2 feo mgo 2 2 comprise an older 7 5 0 5 ma high k and a younger 5 3 0 7 ma shoshonitic series mineralogic data and fractionation models show that crystallization occurred under hydrous oxidizing conditions which were most extreme in the high k series an unusual pattern of successively lower ree at higher sio2 concentrations can be modeled by sphene apatite and amphibole removal an arc like trace element signature attributed to an arc component is strongest in the younger shoshonitic series an important depleted lower crustal mantle lithospheric source component in both series is indicated by non radiogenic sr and pb isotopic ratios at nd 0 to 2 low rb sr ratios and low u and th concentrations this depleted signature contrasts with the enriched one in potassic back arc central volcanic zone cvz lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone first deep crustal mash processes in the nearly normal thickness crust beneath pocho incorporated depleted proterozoic basement components and not complexly mixed structurally thickened crustal components as in the cvz second the association of pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components third miocene shallowing of the subduction zone beneath the flat slab required thinning of both the astenosphere and the subcontinental lithosphere thus an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere similar magmatic sources would have existed during laramide shallow subduction in western north america

generation of normal and adakite like calc alkaline magmas in a non subductional environment an sr o h isotopic study of the apuseni mountains neogene magmatic province romania abstract neogene magmatism in the apuseni mountains of romania was active between 15 and 7 ma with an isolated final eruption at 1 6 ma several groups of magmatic rocks can be distinguished in the province on the basis of ages trace elements and isotopic compositions most are of calc alkaline affinity despite the fact that the region is located 200 km from the postulated carpathian subduction trench however adakite like calc alkaline magmas were also erupted and the youngest eruptions were of alkaline affinity magmatic rocks older than 13 5 ma are characterised by the highest 87sr 86sr 18o and d values in their phenocryst phases this early magmatism is considered to be a result of crustal melting related to the fast rotation of small crustal blocks volcanic rocks younger than 13 5 ma have lower 87sr 86sr and 18o ratios with a narrow range for pyroxene and amphibole mineral separates indicating closed system fractional crystallization a larger scatter of 87sr 86sr and 18o values is found in plagioclase phenocrysts and groundmasses indicating that a small amount of crustal assimilation also occurred the d values for hydrogen bearing phases such as amphibole biotite and groundmass indicate fractional crystallisation in a closed system and a trend of decreasing d in the source with time the magma source of normal calc alkaline adakite like magmas was not related to contemporaneous subduction but was likely due to melting of delaminated eclogitic mafic lower crust in contrast the late stage alkaline magmas were related to asthenospheric upwelling petrological analysis of the complex succession of magmas in the apuseni mountains province demonstrates that normal calc alkaline and adakite like magmatism can be generated in a non subductional environment in this case associated with an unusual extensional setting

evolution of the late pleistocene mojanda fuya fuya volcanic complex ecuador by progressive adakitic involvement in mantle magma sources the mojanda fuya fuya volcanic complex consists of two nearby volcanoes mojanda and fuya fuya the older one mojanda volcano 0 6 to 0 2 ma was first constructed by andesites and high silica andesites forming a large stratovolcano lower mojanda this edifice was capped by a basaltic andesite and andesitic cone upper mojanda which collapsed later to form a 3 km wide summit caldera after large phreatomagmatic eruptions the lower fuya fuya edifice was constructed by the extrusion of viscous si rich andesitic lavas and dacitic domes and the emission of a thick sequence of pyroclastic flow and fallout deposits which include two voluminous rhyolitic layers an intermediate construction phase at fuya fuya is represented by a mainly effusive cone andesitic in composition san bartolo edifice the construction of which was interrupted by a major sector collapse in the late pleistocene finally a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre forming the upper fuya fuya volcanic centre this paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source although all the rocks of the complex are included in the medium k field of continental arcs the fuya fuya suite 61 75 wt sio2 shows depletion in y and hree and high sr y and la yb values compared to the less silicic mojanda suite 55 66 5 wt sio2 the mojanda calc alkaline suite was generated by partial melting of an adakite metasomatised mantle source that left a residue with 2 garnet followed by fractional crystallization of dominant plagioclase pyroxene olivine at shallow intra crustal depths for fuya fuya geochemical and mineralogical data suggest either 1 partial melting of a similar metasomatised mantle with more garnet in the residue 4 followed by fractional crystallization involving plagioclase amphibole and pyroxene or 2 mixing of mafic mantle derived magma from the mojanda suite and slab melts followed by the same fractional crystallization process

different sources involved in generation of continental arc volcanism the carboniferous permian volcanic rocks in the northern margin of the north china block abstract new zircon u pb dating results on the carboniferous permian volcanic rocks in the northern margin of the north china block ncb indicate their eruption during the early carboniferous to late permian from 347 3 ma to 258 1 ma and a slight decrease of the upper limits of the volcanic sequences from west to east they have a main rock association of basalt basaltic andesite andesite dacite rhyolite tuff and tufaceous sandstone most of them have calc alkaline compositions and exhibit variable sio2 contents from 48 2 wt to 77 1 wt there is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams indicating that they are not bimodal in composition the carboniferous permian volcanic rocks exhibit subduction related geochemical features such as negative nb and ta anomalies of mafic to intermediate rocks on primitive mantle normalized diagrams indicating they were formed in an andean type continental arc during southward subduction of the paleo asian oceanic plate beneath the northern ncb however their wide range of whole rock sr nd and zircon hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks geochemical and sr nd hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust however most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust there is an increasing input of crustal materials from the carboniferous to permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences the results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments mantle wedge and arc continental crust could be involved in their generation for the permian andesite dacite and rhyolitic volcanic rocks there is a striking correlation between their sr nd hf isotopic compositions and those of the underlying basement sr nd hf isotopic differences between the permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the north china craton suggest that mixture and homogenization of the magmas in the melting assimilation storage hybridization mash zone within this continental arc was incomplete and incomplete homogenization of magmas is likely a common feature of many mash zones within continental arcs our results on the volcanic rocks also show that the final closure of the paleo asian ocean along the solonker suture zone has not been finished prior to the middle permian and the decease of the paleo asian oceanic subduction beneath the northern ncb in its middle western parts is a little earlier than that in its eastern part

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

shallow and deep crustal control on differentiation of calc alkaline and tholeiitic magma article u 230 th isotopes 87 sr 86 sr lower crust assimilation the role of changing crustal interaction and plumbing geometry in modulating calc alkaline vs tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath volcan parinacota changes in petrologic affinity correlate with periods of cone building sector collapse and rebuilding of the volcano over the last 52 ka and are well explained by changes in magma recharge regime with increasing recharge and magma output lavas transition from low fe strongly calc alkaline phenocryst rich silicic compositions to medium fe near tholeiitic mafic and aphanitic characteristics strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously these are characterized by distinctive 87 sr 86 sr ratios which suggest an initially compartmentalized system relatively high 230 th 232 th activity ratios of 0 72 in early erupted calc alkaline lavas are consistent with interaction with high u upper crust low 230 th 232 th activity ratios of 0 55 and up to 33 th excess in younger near tholeiitic lavas correlate with steep ree patterns indicating lower crustal interaction thorium excesses at the time of eruption approach the maximum that can be generated via small degree garnet residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short on the order of b2 10 4 yr in contrast lavas with greatest calc alkalinity are also at or near secular

neogene quaternary magmatism and geodynamics in the carpathian pannonian region a synthesis in the carpathian pannonian region in neogene times westward dipping subduction in a land locked basin caused collision of two lithospheric blocks alcapa and tisia with the southeastern border of the european plate calc alkaline and alkaline magmatism was closely related to subduction rollback collision and extension from the spatial distribution of the magmatic activity four segments can be defined western segment magmatism occurring on the alcapa block central segment magmatism occurring on both alcapa and the tisia blocks south eastern segment and interior segment both on the tisia block most calc alkaline magmatism in the region resulted from melting of a heterogeneous asthenospheric mantle source modified by addition of fluids and sediment assimilation and fractionation processes at shallow crustal levels occurred in most of the segments strongly masking the deeper source processes long term subduction rollback and or delamination led to contamination of the asthenosphere beneath the western segment here large volume partial melts of the contaminated mantle caused underplating and crustal anatexis leading to mixing of mantle derived calc alkaline magmas with crustal melts in the central segment calc alkaline magmas were formed by subduction and rollback followed by back arc extension and slab breakoff avariable mantle source is indicated in the back arc setting and larger amount of fluid induced metasomatism source enrichment and assimilation nearer to the trench in the interior segment evolution from typical calc alkaline magmas to adakite like ones was related to extension due to fast rotations and transtensional tectonics here calc alkaline magmas formed by decompression melting of a heterogeneous crust mantle lithosphere while adakite like melts resulted from fliud dominated melting of the lithosphere along the south eastern segment slab breakoff was responsible for the generation of typical calcalkaline magmas but in the extreme south of the segment shallow level tearing of the slab followed breakoff strike slip tectonics allowed the rise of hot asthenosphere and generation of adakite like magmas via slab melting along the torn edge of the east european plate alkalic basaltic volcanism with an oib like asthenosphere source followed the calc alkaline stage

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

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

the eruptive chronology of the yucamane calientes compound volcano a potentially active edifice of the central andes southern peru abstract we reconstruct the eruptive chronology of the yucamane calientes compound volcano in southern peru based on extensive fieldwork and a large dataset of geochronological k ar 40ar 39ar u pb and 14c and geochemical major and trace element analyses this compound volcano is composed of two edifices that have experienced discontinuous volcanic activity from the middle pleistocene to the holocene the calientes volcano has been constructed in four successive stages calientes i is composed of andesitic lava flows dated at 500 ka subsequently the callazas ignimbrite calientes ii stage was emplaced 160 190 ka followed by the main cone building stage calientes iii at 130 100 ka finally the holocene calientes domes were emplaced and represent the last eruptive products of this edifice the yucamane volcano has been constructed in three stages yucamane i consists of a succession of andesitic lava flows exposed at the base of the volcano that are older than 40 ka yucamane ii 36 30 ka comprises a thick sequence of block and ash flow deposits that represents an episode of dome growth predating the younger yucamane cone yucamane iii built after 20 25 ka during the holocene yucamane vulcanian to sub plinian activity has emplaced tephra fall and pyroclastic density current deposits the most recent explosive eruptions occurred ca 3000 bp and emplaced a tephra fall and pumice flow deposits most samples from calientes volcano are andesites and dacites 60 1 67 7 wt sio2 while rocks from yucamane volcano are basaltic andesites to dacites 53 4 66 9 wt sio2 the rocks have a mineral assemblage of plagioclase amphibole biotite orthopyroxene clinopyroxene olivine and fe ti oxides the analyzed samples are categorized within a high k calc alkaline series calientes volcano erupted mostly andesitic magmas but its history is punctuated by rare eruptions of silica rich magmas in contrast yucamane volcano follows a different trend characterized by a gradual decrease in silica content through post glacial time from the large vei 3 sub plinian andesitic eruption of 3 ka to moderate vei 2 vulcanian eruptions of basaltic andesitic on the basis of such recurrent and recent holocene low to moderate explosive activity yucamane must be considered an active and potentially threatening volcano which may affect the province of candarave with 8000 inhabitants

the origin and evolution of magmas from the san pedro pellado volcanic complex s chile multicomponent sources and open system evolution the san pedro pellado volcanic complex is located at 36 s in the chilean andes the eruptive rocks of the complex record the development and collapse of a caldera followed by voluminous largely basaltic andesite volcanism at each stage of evolution crystal fractionation was accompanied by variable degrees of contamination and mixing large variations in incompatible element ratios cannot be produced by closed system evolution correlations between indices of differentiation and incompatible element ratios together with high 18o values indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at san pedro pellado even in the most mafic rocks however incompatible element characteristics are variable as a result of source heterogeneity and deep level processes the restricted ranges in isotope ratios of sr nd and pb among san pedro pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock three source components are recognized as contributing to parental magmas at san pedro pellado although the relative contributions of each cannot be quantified the volumetrically dominant source component is the sub arc asthenospheric mantle morb source the major source of lile is thought to be slab derived fluids which modified the sub arc mantle other incompatible elements may also have been enriched by interaction with the continental lithosphere mantle and or lower crust during ascent

intracrustal derivation of na rich andesitic and dacitic magmas an example from volcan ollague andean central volcanic zone andesitic and dacitic lavas erupted from quaternary stratovolcanoes in the central volcanic zone cvz of the andes have major and trace element compositions similar to proposed melts of subducted mid ocean ridge basalt and na rich trondjhemites tonalites and granodiorites ttg in early archean high grade gneiss terrains suites of lavas from individual volcanoes have average la yb of 25 45 sr y of 30 65 yb contents of 1 5 1 1 ppm and na 2 o contents 3 8 wt with lavas erupted on the volcanic front displaying the most affinity to the early archean suites because slab melting is not involved in the derivation of the cvz magmas these data indicate that rocks with early archean ttg compositions can also be produced through intracrustal differentiation processes isotopic major element and ree data from volcan ollague are consistent with a model incorporating contamination of mantle derived basaltic magmas in garnet bearing deep continental crust to produce basaltic andesite magma followed

spatiotemporal structure of the east india coastal current from satellite altimetry we use a newly processed altimeter data set to present a hitherto unprecedented description of the spatiotemporal structure of the east india coastal current eicc the data set resolves timescales ranging from a few months to a few years and the high along track resolution yields the first description of the cross shore structure of the current the seasonal cycle dominates the variability but the nonannual timescales have similar energy levels all along the eicc path there are short lived intense intraseasonal bursts in contrast to the seasonal cycle the interannual and intraseasonal components are decorrelated along the coast and possible mechanisms for the decorrelation discontinuity in the flow are discussed in the cross shore direction the current is highly correlated at all timescales the eicc is trapped against the shelf with the current offshore flowing in the opposite direction at most locations the eicc appears as an inherently discontinuous flow taking the form of a few recirculating loops along the eicc path with a typical cross shore spatial scale of 150 200 km the loops are highly variable in direction at all timescales from intraseasonal to interannual this discontinuity of the eicc in space and time implies that the basic pathways and advective timescales for the interbasin exchange of water masses between the bay of bengal and the arabian sea are not robust when the full spatiotemporal variability of the eicc is considered

sangay volcano ecuador structural development present activity and petrology abstract sangay 5230 m the southernmost active volcano of the andean northern volcanic zone nvz sits 130 km above a 32 ma old slab close to a major tear that separates two distinct subducting oceanic crusts southwards quaternary volcanism is absent along a 1600 km long segment of the andes three successive edifices of decreasing volume have formed the sangay volcanic complex during the last 500 ka two former cones sangay i and ii have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the amazon plain sangay iii being constructed within the last avalanche amphitheater has been active at least since 14 ka bp only the largest eruptions with unusually high plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano however given the volcano s relief and unbuttressed eastern side a future collapse must be considered that would seriously affect an area of present day colonization in the amazon plain 30 km east of the summit andesites greatly predominate at sangay there being few dacites and basalts in order to explain the unusual characteristics of the sangay suite highest content of incompatible elements except y and hree of any nvz suite low y and hree values in the andesites and dacites and high nb la of the only basalt found a preliminary five step model is proposed 1 an enriched mantle in comparison with an morb source or maybe a variably enriched mantle at the site of the sangay prior to quaternary volcanism 2 metasomatism of this mantle by important volumes of slab derived fluids enriched in soluble incompatible elements due to the subduction of major oceanic fracture zones 3 partial melting of this metasomatized mantle and generation of primitive basaltic melts with nb la values typical of the nvz which are parental to the entire sangay suite but apparently never reach the surface and subordinate production of high nb la basaltic melts maybe by lower degrees of melting at the periphery of the main site of magma formation that only infrequently reach the surface 4 afc processes at the base of a 50 km thick crust where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated producing andesites with low y and hree contents due to garnet stability at this depth 5 low pressure fractionation and mixing processes higher in the crust both an enriched mantle under sangay prior to volcanism and an important slab derived input of fluids enriched in soluble incompatible elements two parameters certainly related to the unique setting of the volcano at the southern termination of the nvz apparently account for the exceptionally high contents of incompatible elements of the sangay suite in addition the low cr ni values of the entire suite another unique characteristic of the nvz also requires unusual fractionation processes involving cr spinel and or clinopyroxene either in the upper mantle or at the base of the crust

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

the negra muerta volcanic complex southern central andes geochemical characteristics and magmatic evolution of an episodically active volcanic centre abstract this petrologic analysis of the negra muerta volcanic complex nmvc contributes to understanding the magmatic evolution of eruptive centres associated with prominent nw striking fault zones in the southern central andes specifically the geochemical characteristics and magmatic evolution of the two eruptive episodes of this complex are analysed the first one occurred as an explosive eruption at 9 ma and is represented by a strongly welded fiamme rich andesitic to dacitic ignimbrite deposit the second commenced with an eruption of a rhyolitic ignimbrite at 7 6 ma followed by effusive discharge of hybrid lavas at 7 3 ma and by emplacement of andesitic to rhyodacitic dykes and domes both explosive and effusive eruptions of the second episode occurred within a short time span but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber pressurising the magmatic system and triggering explosive eruption of rhyolitic magma chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition which could have been strong enough to inhibit the interaction between the two magmas after the initial explosive stages of the eruption at 7 6 ma the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber at 7 3 ma increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera assimilation during turbulent ascent ata is invoked to explain a conspicuous reversed isotopic signature 87sr 86sr and 143nd 144nd in the entire volcanic series therefore the 7 6 to 7 3 ma volcanic rocks of the nmvc resulted from synchronous and mutually interacting petrological processes such as recharge fractional crystallization hybridisation and assimilation during turbulent ascent ata geochemical characteristics of both volcanic episodes show diverse type and or depth in the sources and variable influence of upper crustal processes and indicate a recurrence in the magma forming conditions similarly other minor volcanic centres in the transversal volcanic belts of the central andes repeated their geochemical signatures throughout the miocene

magmatic differentiation at la poruna scoria cone central andes northern chile evidence for assimilation during turbulent ascent processes and genetic links with mafic eruptions at adjacent san pedro volcano abstract la poruna 21 53 s 68 30 w is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic andesite to andesite lava flow that is up to 8 km in length automated mineralogical analysis describes a suite of porphyritic mafic samples comprising olivine and pyroxene bearing rocks well defined major element compositional trends as well as trace and rare earth element characteristics e g sr y 96 ka from san pedro volcano exhibit similar isotopic characteristics therefore evidence of similar magmatic processes this new dataset clearly defines magma compositional changes during the la poruna eruption ca 100 ka revealing an increase in crustal contamination at shallow crustal levels for the younger san pedro lavas

geochemistry and crustal evolution of volcano sedimentary successions and orthogneisses in the sao gabriel block southernmost brazil relics of neoproterozoic magmatic arcs abstract precambrian metaplutonic rocks of the sao gabriel block in southernmost brazil comprise juvenile neoproterozoic calc alkaline gneisses cambai complex the connection with associated ultra mafic metavolcanic and metasedimentary rocks palma group is not well established the whole complex was deformed during the brasiliano orogenic cycle both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the cambai complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the sao gabriel block and its role during the assembly of west gondwana the major element compositions of the igneous rocks palma group and cambai complex indicate a subalkaline character most orthogneisses have a calc alkaline chemistry many metavolcanic rocks of the palma group show signatures of low k tholeiitic volcanic arc basalts trace element data especially ti zr y nb of most igneous samples from both the lower palma group and the cambai complex indicate origin at plate margins i e in a subduction zone environment this is corroborated by relative enrichment in lree low contents of nb and other high field strength elements and enrichment in lile like rb ba and th the data indicate the possible existence of two suites an oceanic island arc and a continental arc or active continental margin however some ultramafic samples of the lower palma group in the western sao gabriel block indicate the existence of another volcanic suite with intra plate character which possibly represents relics of oceanic island basalts oib trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks the patterns of chondrite and n morb normalized spider diagrams resemble the patterns of the igneous rocks i e lile and lree enrichment and hfs depletion the geochemical signatures of most igneous and metasedimentary samples and their low 87sr 86sr t ratios suggest only minor contribution of old continental crust a geotectonic model for the sao gabriel block comprises east ward subduction and following accretion of an intra oceanic island arc to the eastern border of the rio de la plata craton at ca 880 ma and westward subduction beneath the newly formed active continental margin between ca 750 and 700 ma the sao gabriel block represents relics of an early brasiliano oceanic basin between the rio de la plata and kalahari cratons this ocean to the east of the rio de la plata craton might be traced to the north and could possibly be linked with neoproterozoic juvenile oceanic crust in the western brasilia belt goias magmatic arc

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

the sediment record of the unicorn cave southern harz mountains germany abstract we demonstrate the utility of a multi pronged approach to geomorphological studies we show how the synthesis of results from 3 d imaging varied geotechnical methods and dating reveals hitherto unknown features of a cave and of its sediment infill and elucidates its geomorphological history the unicorn cave einhornhohle southern harz mountains germany long famous for its cave bear lion and mammoth fossils is a dry vadose trunk cave high above base level characterized by large rooms connected by shallow passages and filled with sediments up to 40 m thick the sediment is an archive for natural and anthropogenic environmental change the succession of river gravels breccia and silts and sands provides a paleo geomorphic and climatic archive while human artefacts from the paleolithic period provide information about early settlement phases here we derive a detailed description of the unicorn cave and its sediment infill by utilising several sources i we synthesise data from the literature into a comprehensive picture of the sediment infill ii we present new geophysical measurements of the sediments in the large room leibnizhalle and discuss these new measurements in view of our older findings iii we map the larger chambers with the photogrammetric structure from motion method to build three dimensional models iv we present new age estimates from speleothem samples in the cave and discuss them with other published age estimates finally we use our newly collected evidence to reconstruct the evolution of the entire unicorn cave including the newly revealed not yet accessible passages and their pristine sediment infill as yet undisturbed by fossil hunters the main period of speleogenesis ended with base level incision in the mid pleistocene the deposition of basal river gravels probably in the late cromerian was followed by a sequence of clays and loams capped by speleothem at mis 3 the dates suggest at least two bone beds mis 3 and mis 5e along with bone material older than mis 7

the dynamics of the southwest monsoon current in 2016 from high resolution in situ observations and models abstractthe strong stratification of the bay of bengal bob causes rapid variations in sea surface temperature sst that influence the development of monsoon rainfall systems this stratification

lithospheric contributions to arc magmatism isotope variations along strike in volcanoes of honshu japan major chemical exchange between the crust and mantle occurs in subduction zone environments profoundly affecting the chemical evolution of earth the relative contributions of the subducting slab mantle wedge and arc lithosphere to the generation of island arc magmas and ultimately new continental crust are controversial isotopic data for lavas from a transect of volcanoes in a single arc segment of northern honshu japan have distinct variations coincident with changes in crustal lithology these data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts

eddies and currents in the bay of bengal during summer monsoons abstract the southwest monsoon current smc in the bay of bengal bob and prominent eddies surrounding it in particular an anticyclonic eddy ae to the southeast and a cyclonic eddy sri lanka dome sld to the east of sri lanka were investigated using field observations numerical simulations satellite imagery laboratory experiments and theoretical studies the field campaigns were conducted in 2015 and 2018 respectively in conjunction with the air sea interactions in the northern indian ocean asiri and monsoon intra seasonal oscillations in bay of bengal miso bob research initiatives ship observations confirmed the presence of a annually recurring ae about 500 km southeast of sri lanka with surface velocities up to 1 m s 1 size in the meridional direction 200 km and penetration approximately to the depth of the thermocline 150 m satellite observations and coamps model simulations show that the ae and the sld are formed following the appearance of the smc in early summer and both evolve during july august and disappear in september this result is at odds with the prevailing notion that the ae is a result of the interaction between the smc and rossby waves arriving from the southeast of the bob carrying energy of a blocked wyrtki jet at the sumatra coast such waves do not arrive in the southwestern bob until august september simultaneous appearance of ae and sld is inconsistent with a recently proposed mechanism in which sld is formed due to separation of the smc from the sri lanka coast a new hypothesis is proposed wherein the ae and the sld are generated by topographically trapped rossby wave response of the smc to perturbations by the sri lankan coast observations of the size location and origins of the ae were broadly consistent with this hypothesis so were the results of a laboratory experiment designed to mimic natural flow in the bob by creating an eastward jet smc on a simulated plane sea surface temperature sst observations show that the smc carries colder upwelled water from the southern sri lankan and indian coasts and distributes it within the surface waters of the ae and beyond thus the surface waters of the ae are colder than that of the sld notwithstanding perceived local upwelling in the sld suggesting that smc may play a controlling role in the air sea interactions of the southern bob

late silurian early devonian rifting during dextral transpression in the southern gaspe peninsula quebec petrogenesis of volcanic rocks the upper silurian lower devonian volcanic rocks in the southern gaspe peninsula of the quebec appalachians crop out at the northeast end of the connecticut valley gaspe synclinorium these shallow marine and subaerial sequences reach a thickness of up to at least 2000 m and comprise two groups 1 the late silurian volcanic rocks which are mainly transitional alkalic tholeiitic basalts with steeply sloping ree patterns 2 the early devonian volcanic rocks which include a significant proportion of intermediate rocks in addition to tholeiitic basalts compared with the silurian rocks the devonian basalts have lower abundances of strongly incompatible trace elements such as ba th ta nb and light ree and relatively flat heavy ree patterns basalts of both groups display negative nb and ta anomalies relative to th and la although the basalts of both sequences were derived from lithospheric mantle the silurian basalts were generated from garnet peridotite at 80 km depth while the devonian ba

magmagenesis in a subduction related post collisional volcanic arc segment the ukrainian carpathians abstract calc alkaline magmatism in the south west ukraine occurred between 13 8 and 9 1 ma and formed an integral part of the neogene subduction related post collisional carpathian volcanic arc eruptions occurred contemporaneously in two parallel arcs here termed outer arc and inner arc in the ukrainian part of the carpathians outer arc rocks mainly andesites are characterized by lile enrichment e g k and pb nb depletion low compatible trace element abundances high 87sr 86sr high 18o and low 143nd 144nd isotopic ratios 0 7085 0 7095 7 01 8 53 0 51230 0 51245 respectively inner arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas they also show low compatible element abundances but have lower 87sr 86sr 18o and higher 143nd 144nd ratios 0 7060 0 7085 6 15 6 64 0 5125 0 5126 respectively than outer arc rocks both high nb and low nb lithologies are present in the inner arc based on the lile enrichment especially pb a higher fluid flux is suggested for the outer arc magmas compared with those of the inner arc combined trace element and sr nd o isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex compositional differences between the inner and outer arcs were produced by introduction of variable proportions of slab derived sediments and fluids into a heterogeneous mantle wedge and by different extents of upper crustal contamination degrees of magmatic fractionation also differed between the two arcs the most primitive magmas belong to the inner arc isotopic modelling shows that they can be produced by adding 3 8 subducted terrigenous flysch sediments to the local mantle wedge source up to 5 upper crustal contamination has been modelled for fractionated products of the inner arc the geochemical features of outer arc rocks suggest that they were generated from mantle wedge melts similar to the inner arc primitive magmas but were strongly affected by both source enrichment and upper crustal contamination assimilation of 10 20 bulk upper crust is required in the afc modelling assuming an inner arc parental magma we suggest that magmagenesis is closely related to the complex geotectonic evolution of the carpathian area several tectonic and kinematic factors are significant 1 hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes 2 thermal disturbance caused by ascent of hot asthenospheric mantle during the back arc opening of the pannonian basin 3 clockwise translational movements of the intracarpathian terranes which facilitated eruption of the magmas

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

foreland basin magmatism in the western moroccan meseta and geodynamic inferences 1 basaltic magmatism occurs in the hercynian foreland basin of the western moroccan meseta as pillow lavas flows interbedded with flysch deposits and sills of dolerite and gabbro a sedimentological and tectonic study shows that the deposition of the flysch and thus the intrusion of the lava flows and sills was controlled by the northwestward propagation of thrust related folds in the wedge top depozone of a syncontractional foreland basin system the pillow lavas appear as either massive sheets of stacked lava flows or thin lava flows interlayered with the syntectonic turbiditic deposits the sills are composed of several mafic units up to six in the most evolved marziqallal sill cut through by granophyric veins which can be interpreted as a result of in situ crystallization the pillows lava basalts and the dolerites gabbros are calc alkaline and cogenetic a discussion of the possible mechanisms of magma production in subduction related orogens leads us to conclude that this foreland basin magmatism was generated in a retrolithospheric setting a comparison with the other early carboniferous volcanic and plutonic rocks of the western and eastern mesetas shows that calc alkaline foreland basin magmatism was widespread in the entire mesetan domain and was probably a result of wet melting of the metasomatised mantle lithosphere of the overriding plate because the moroccan hercynides are likely to have been a result of continental subduction and the magmatism is observed at a distance of more than 500 km from the suture it is suggested that the retroforeland basin magmatism was a result of either dehydration of a shallow dipping 15 foregoing oceanic slab or underplating of the continental lithosphere ending with slab breakoff beneath the foreland basin

tectonic setting of primitive magmas in volcanic arcs an example from the antarctic peninsula abstract primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment most examples are either associated with high rates of arc parallel extension or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression in two cases the vanuatu and solomon islands arcs such arc compression is being caused by collision of seamounts in the antarctic peninsula primitive mafic dykes were emplaced perpendicular to the continental arc ar ar and k ar data suggest intrusion of the dykes at c 126 106 ma possibly during mid cretaceous regional compression of the arc the dykes form two compositional groups one group has low la n yb n ratios 0 31 0 49 lower nb yb and higher th nb than n morb age corrected nd values of 7 3 to 7 9 and are interpreted as melts of subduction modified sub arc asthenosphere the other has high la n yb n ratios 3 86 8 92 higher nb yb and th nb than n morb age corrected nd values of 2 8 to 3 4 and are interpreted as melts of sub arc lithosphere the absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers

combined sr o isotope relationships and petrogenesis of andean volcanics of south america active andesitic type volcanism in the southern andes lat 42 36 s is characterized by low sr and o isotopic compositions which fall in the range 87sr 86sr 0 7036 0 7043 18o 5 2 6 8 in contrast volcanic centres in the central andes lat 24 21 s are significantly enriched in both 87sr and 18o we present the first combined sr and o isotope study of the southern volcanic zone svz in chile crustal thickness 40 km 1 and of the southern part of the central volcanic zone cvz in the borderlands of bolivia chile and argentina crustal thickness 60 70 km 1 only one of 13 volcanic centres studied lascar exhibits a correlated increase in both 87sr and 18o parallel with the basalt andesite rhyolite compositional change indicative of contamination by upper crust through a fractional crystallization assimilation mechanism the other centres of the cvz for which there is an overall sr o correlation document the variable contamination of mantle derived magmas at depth in the lower crust which had been thickened by mesozoic and earlier plutonism

the nevados de payachata volcanic region 18 s 69 w n chile subduction related volcanism in the nevados de payachata region of the central andes at 18 s comprises two temporally and geochemically distinct phases an older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late miocene age the pleistocene to recent phase 0 3 ma includes the twin stratovolcanoes volcan pomerape and volcan parinacota the nevados de payachata volcanic group and two small centers to the west i e caquena and vilacollo both stratovolcanoes consist of an older dome and flow series capped by an andesitic cone the younger cone i e v parinacota suffered a postglacial cone collapse producing a widespread debris avalanche deposit subsequently the cone reformed during a brief second volcanic episode a number of small relatively mafic satellitic cinder cones and associated flows were produced during the most recent activity at v parinacota at the older cone i e v pomerape an early dome sequence with an overlying isolated mafic spatter cone and the cone forming andesitic dacitic phase mostly flows have been recognized the two nevados de payachata stratovolcanoes display continuous major and trace element trends from high k2o basaltic andesites through rhyolites 53 76 sio2 that are well defined and distinct from those of the older volcanic centers petrography chemical composition and eruptive styles at v parinacota differ between pre and post debris avalanche lavas precollapse flows have abundant amphibole at sio2 59 wt and lower mg numbers than postcollapse lavas which are generally less silicic and more restricted in composition compositional variations indicate that the magmas of the nevados de payachata volcanic group evolved through a combination of fractional crystallization crustal assimilation and intratrend magma mixing isotope compositions exhibit only minor variations pb isotope ratios are relatively low 206pb 204pb 17 95 18 20 and208pb 204pb 38 2 38 5 87sr 86sr ratios range 0 70612 0 70707 143nd 144nd ratios range 0 51238 0 51230 and 18osmow values range from 6 8 o to 7 6 o smow a comparison with other central volcanic zone centers shows that the nevados de payachata magmas are unusually rich in ba up to 1800 ppm and sr up to 1700 ppm and thus represent an unusual chemical signature in the andean arc these chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components primary mantle derived melts which are similar to those generated by subduction processes throughout the andean arc are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface these magmas subsequently evolve at shallower levels through assimilation crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast

late cenozoic magmatism of the bolivian altiplano small basalt to dacite volcanic centers are distributed sparsely over the bolivian altiplano behind the andean volcanic front most are pliocene to recent in age and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts true phenocrysts are rare in the more mafic samples compared with recent volcanic rocks from andean stratovolcanoes the bolivian centers overlap in major element trends incompatible element contents tend to be higher particularly in the eastern altiplano the ranges of isotopic compositions reflect ubiquitous crustal contamination pb isotope compositions are dominated by pb from isotopically heterogeneous basement resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region rocks sampled from the bolivian altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region major element trends are largely controlled by the fractionating phase assemblage olivine clinopyroxene and amphibole trace element and isotope systematics however defy realistic attempts at modeling due to the geographic scatter of samples the uniformity of compositions at a given center and the heterogeneity of the contaminant nevertheless there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone in particular life hfse exemplified by ba nb and zr nb ratios decrease from the arc front eastward into the altiplano these variations are not easily reconciled with control by crustal contamination alone a model is proposed in which the asthenospheric source is fluxed by high ba nb slab derived fluid to induce melting beneath the arc high fluid flux increases the ba nb ratio of the asthenosphre and leads to high degrees of partial melting high zr nb to the east lower or no fluid flux leads to low ba nb and low degrees of partial melting low zr nb melting in the back arc region of the altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter flowing asthenosphere there is no unequivocal evidence that requires a significant role for the lithospheric mantle

sources of lead in the san cristobal pulacayo and potosi mining districts bolivia and a reevaluation of regional ore lead isotope provinces new lead isotope data on ores crustal rocks and leachates of crustal rocks combined with data in the literature provide important new constraints on the sources of ore metals in southwest to south central bolivia including the very large recently discovered silver zinc deposit at san cristobal the pulacayo polymetallic district and the giant potosi silver tin base metal deposit lead isotope ratios of ores and igneous rocks from the san cristobal deposit and from paleozoic and cretaceous sedimentary rocks are compared with published data on high grade middle proterozoic metamorphic basement rocks these data constrain the major source of lead and by inference of other ore metals at san cristobal to be the metamorphic basement rocks leaching experiments on samples of paleozoic and cretaceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than the whole rock compositions however lead isotope ratios of both whole rocks and leachates of these upper crustal rocks are too radiogenic for them to be major sources of ore lead at san cristobal lead isotope ratios of ores from pulacayo and potosi are similar to each other and lie within the range of paleozoic and cretaceous sedimentary whole rock compositions leaching of pb from the sedimentary rocks cannot explain the isotopic compositions of the pulacayo and potosi ores and the isotopic homogeneity of the potosi ores also argues against mixing of lead from diverse sources in the hydrothermal system lead from the sedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metals from the resulting magma lead isotope ratios of san cristobal ores are different from those of pulacayo potosi and other deposits to the east but resemble the compositions of ores and volcanic rocks in western bolivia on this basis we identify a new ore lead isotope province extending from san cristobal northward across the eastern altiplano and into southern peru this province is coincident with but smaller than the extent of the proposed arequipa antofalla metamorphic basement craton the degree of incorporation of ore metals from the metamorphic basement appears to depend on the timing and or location of the mineralizing event ore deposits in the northern part of province iv formed before the thickening of andean crust beginning around 20 ma and incorporated minor amounts of metals from the metamorphic basement younger deposits farther to the south contain major to dominant components of basement lead

andean trending mobile belts in the brazilian shield much of the phanerozoic andean mountain chain is floored by precambrian basement1 3 in places as old as 2 000 myr4 5 but relations between these rocks and those of the brazilian shield have long been obscure mainly because of the intervening cenozoic sedimentary cover we describe here recent work over the bolivian sector of the brazilian shield fig 1 which shows a pattern of proterozoic mobile belts approximately parallel with the andean chain to the west when the trends of these bolivian belts are projected towards the andes under the cenozoic cover their ages broadly accord with those of the andean basement our interpretation is that the two domains are linked in the form of a chain of five andean trending mobile belts with a combined width of 1 000 km becoming progressively younger to the west with ages ranging from 1 300 myr to the 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

reply to comment on crustal contributions to arc magmatism in the andes of central chile by w hildreth and s moorbath

crust magma interactions and the evolution of arc magmas the san pedro pellado volcanic complex southern chilean andes field petrographic and trace element data indicate that volcanic rocks of the quaternary san pedro pellado complex lat 36 s chile have incorporated a silicic component that may be derived from the crust isotope ratios of sr nd and pb vary little among the volcanic rocks this is due primarily to the limited isotopic contrast between the mesozoic tertiary crust and the young magmas isotope and element compositions of san pedro pellado rocks are similar to those reported from other localities in the southern volcanic zone of the chilean andes we suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone the san pedro pellado example illustrates the importance of considering all types of evidence not just isotopic data in the evaluation of crustal contributions to arc magmas

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

crustal sources involved in continental arc magmatism a case study of volcan mocho choshuenco southern chile volcan mocho choshuenco a pleistocene holocene basaltic andesite to dacite shield volcano located at lat 40 s in the southern volcanic zone svz of the andes is built on the thinnest svz continental crust 30 km which consists largely of paleozoic sedimentary and metamorphic rocks and mesozoic plutons sr pb nd and o isotopic ratios of mocho choshuenco lavas 87 sr 86 sr 0 70399 to 0 70422 206 pb 204 pb 18 57 to 18 61 207 pb 204 pb 15 60 to 15 63 208 pb 204 pb 38 45 to 38 57 143 nd 144 nd 0 512787 to 0 512881 18 o 5 8 to 6 8 standard mean ocean water are similar to those of other svz centers and exhibit no correlation with indices of differentiation however incompatible trace element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5 to 15 rb ba k ba rb la and k la ratios increase with increasing sio 2 content as a result of assimilation of orogenic calc alkalic plutonic rocks enriched in rb and k relative to ba and la xenoliths of plutonic rocks recovered from mocho choshuenco lavas are slightly more radiogenic in 87 sr 86 sr less radiogenic in 143 nd 144 nd and comparable in pb isotopic ratios and they have variable 18 o contents 87 sr 86 sr 0 70482 to 0 70557 206 pb 204 pb 18 60 to 18 64 207 pb 204 pb 15 60 to 15 62 208 pb 204 pb 38 53 to 38 56 143 nd 144 nd 0 512686 18 o 3 3 to 5 5 smow contamination of mocho choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions these heterogeneities probably were produced during magma crust interaction at the crust mantle boundary or in the lower crust

proterozoic crustal evolution of central east antarctica age and isotopic evidence from glacial igneous clasts and links with australia and laurentia abstract rock clasts entrained in glacial deposits sourced from the continental interior of antarctica provide an innovative means to determine the age and composition of ice covered crust zircon u pb ages from a suite of granitoid clasts collected in glacial catchments draining central east antarctica through the transantarctic mountains show that crust in this region was formed by a series of magmatic events at 2 01 1 88 1 85 1 79 1 57 1 50 1 41 and 1 20 1 06 ga the dominant granitoid populations are ca 1 85 1 45 and 1 20 1 06 ga none of these igneous ages are known from limited outcrop in the region in addition to defining a previously unrecognized geologic history zircon o and hf isotopic compositions from this suite have 1 mantle like 18o signatures 4 0 4 5 and near chondritic hf isotope compositions ehf 1 5 for granitoids of 2 0 ga age 2 mostly crustal 18o 6 0 8 5 and variable hf isotope compositions ehf 6 to 5 in rocks with ages of 1 88 1 85 1 79 and 1 57 ga in which the 1 88 1 79 ga granitoids require involvement of older crust 3 mostly juvenile isotopic signatures with low mantle like 18o 4 5 and radiogenic hf isotope signatures ehf 6 to 10 in rocks of 1 50 1 41 ga age with some showing crustal sources or evidence of alteration and 4 mixed crustal and mantle 18o signatures 6 0 7 5 and radiogenic hf isotopes ehf 3 to 4 in rocks of 1 2 ga age together these age and isotopic data indicate the presence in cratonic east antarctica of a large composite igneous province that formed through a punctuated sequence of relatively juvenile proterozoic magmatic events further they provide direct support for geological correlation of crust in east antarctica with both the gawler craton of present day australia and proterozoic provinces in western laurentia prominent clast ages of 2 0 1 85 1 57 and 1 45 ga together with sediment source linkages provide evidence for the temporal and spatial association of these cratonic elements in the columbia supercontinent abundant 1 2 1 1 ga igneous and metamorphic clasts may sample crust underlying the gamburtsev subglacial mountains indicating the presence of a mesoproterozoic orogenic belt in the interior of east antarctica that formed during final assembly of rodinia

temporal evolution of magmatism in the northern volcanic zone of the andes the geology and petrology of cayambe volcanic complex ecuador in the northern volcanic zone of the andes the cayambe volcanic complex consists of 1 a basal mostly effusive volcano the viejo cayambe whose lavas andesites and subordinate dacites and rhyolites are typically calc alkaline and 2 a younger essentially dacitic composite edifice the nevado cayambe characterized by lavas with adakitic signatures and explosive eruptive styles the construction of viejo cayambe began 1 1 myr ago and ended at 1 0 ma the young and still active nevado cayambe grew after a period of quiescence of about 0 6 myr from 0 4 ma to holocene its complex history is divided into at least three large construction phases angureal cone main summit cone and secondary summit cone and comprises large pyroclastic events debris avalanches as well as periods of dome activity geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite on the contrary field observations and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex the adakitic signature of nevado cayambe magmas is related to partial melting of a basaltic source which could be the lower crust or the subducted slab however reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab thus the viejo cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab derived melts adakites whereas the nevado cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis this temporal evolution can be related to the presence of the subducted carnegie ridge modifying the geothermal gradient along the wadati benioff zone and favouring slab partial melting

separation of upslope flow over a uniform slope motivated by the importance of understanding mountain weather during periods of thermal convection a laboratory study was conducted to investigate the separation of an upslope anabatic flow on a two dimensional heated mountainous slope in the absence of a background mean flow three flow regimes were identified in the first at slope angles larger than a critical value the separated flow generated a rising plume completely fed by the anterior upslope flow for this case a simple model based on a balance between the opposing vorticities of baroclinicity and shear was proposed to predict the location of the separation point relative to the mountain base the model also predicts the velocity and length scales at separation as well as those of the rising plume after separation in the second regime the volume flow of the separated plume was not fully supplied by the upslope flow requiring entrainment of additional ambient fluid at the base of the plume source the third regime occurred when wherein the plume almost completely engulfed the slope similar to a buoyant plume emanating from a source of finite dimensions thus overshadowing the upslope flow measurements of the separation point conducted during the materhorn field research program were consistent with the results of the laboratory experiments and modelling

arabian sea and bay of bengal exchange of salt and tracers in an ocean model exchanges of mass and salt between the mixed layers in the arabian sea and the bay of bengal are examined in a model of the indian ocean using passive tracers as a tool to map the pathways inflow of high salinity water from the arabian sea into the bay of bengal is significant and occurs after the mature phase of the southwest monsoon freshwater transport out of the bay of bengal is southward throughout the year along the eastern boundary of the indian ocean low salinity water transport into the arabian sea occurs in the somali current during the southwest monsoon closing a clockwise path of water mass transport only a small fraction of low salinity water is advected into the eastern arabian sea from the bay of bengal

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

volcanological and petrological evolution of volcan tata sabaya sw bolivia tata sabaya 19 08 s 68 32 w 5430 m a s l is located on the northern shores of salar de coipasa on the altiplano of bolivia it is a 1700 m high symmetrical composite cone one of two holocene centers in a chain of edifices trending e w from the main n s trending volcanic arc of the andean central volcanic zone cvz four main stages are envisaged for the evolution of the edifice stage 1 formation of a pyroclastic shield possibly around a central dome complex stage 2 formation of an andesitic composite cone on top of the early pyroclastic sequence stage 3 collapse of a 120 sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering 300 km2 of the salar de coipasa to the south collapse was induced by inflation of the edifice by an input of fresh magma into the sub volcanic magma chamber stage 4 flows and domes which have since healed the edifice tata sabaya shows a restricted range of whole rock compositions most units fall in the range 60 62 5 sio2 with a general evolution to the more silicic stage 4 lavas through time the typical phenocryst assemblage is plagioclase hornblende hypersthene augite oxide biotite is found in the stage 4 lavas and is dominant over pyroxene hornblende and biotite are thought to be a late stage sub liquidus products by far the most mafic compositions at tata sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes 52 sio2 macroscopic microscopic and chemical evidence for magma mixing mixed phenocryst and matrix populations resorption features mixed glass compositions contraindicative temperature indicators etc are found throughout tata sabaya the whole rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system despite being located 35km to the east of the main arc front the rocks of tata sabaya are compositionally similar to those from other centers in the northern 17 19 s segment of the cvz the high k2o calc alkaline suite is characterised by lree and extreme lile enrichments up to 1800 ppm ba and sr 87sr 86sr 0 7056 0 7068 and 143nd 144nd 0 51217 0 51227 are similar to other cvz lavas this is regarded as a baseline signature derived during deep crustal contamination this is corroborated by pb isotope data which indicates influence of the low 206pb 204pb precambrian basement that is thought to underlie the northern portion of the cvz modification of the isotope ratios during open system processes at shallow level is also indicated the most extreme incompatible element enrichments are found in the quenched mafic inclusions these characteristics have been reported from other holocene mafic lavas elsewhere in the northern cvz and may represent a distinct recent magmatic episode

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

early holocene collapse of volcan parinacota central andes chile volcanological and paleohydrological consequences the catastrophic gravitational collapse of the old cone of volcan parinacota produced a 6 km 3 debris avalanche that traveled 22 km and covered more than 150 km 2 the upper lauca drainage a broad high altitude basin in the chilean altiplano was permanently altered by the collapse although the eruptive history of parinacota before and after the debris avalanche is well known from petrologic and geochronologic studies previous age limits on the debris avalanche based on multiple chronometers span ca 8 20 ka new cosmogenic surface exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ 10 be and indicate that the avalanche occurred at 8 8 0 5 ka these data demonstrate that cosmogenic 10 be surface exposure dating can be successfully applied to quartz bearing volcanic debris avalanche deposits and that this method offers a distinct advantage over 14 c chronologies that provide only minimum or maximum age limits the 8 8 ka exposure age for the debris avalanche 1 agrees with 14 c age limits of paleosol material incorporated in the debris avalanche 2 requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone building episodes at most continental arc volcanoes and 3 suggests that volcano collapse did not result in the formation of lago chungara but instead led to a major expansion of a preexisting closed basin

common pb isotopic compositions of the lima arequipa and toquepala segments in the coastal batholith peru implications for magmagenesis abstract common pb isotopic compositions for 65 feldspar samples from the 188 37 ma old gabbroic to monzogranitic plutons of the peruvian coastal batholith show changes along strike that can be related to variable contamination of mantle derived magmas by the local precambrian basement results have the following isotopic ranges 206 pb 204 pb 17 580 20 803 207 pb 204 pb 15 555 15 709 and 208 pb 204 pb 38 104 41 177 with averages of 206 pb 204 pb 18 630 207 pb 204 pb 15 610 and 208 pb 204 pb 38 500 the gabbroic dioritic and tonalitic plutons north and east of lima the lima segment have ratios similar to the isotopically homogeneous reservoir identified by tilton 1979 tilton and barreiro 1980 and barreiro and stern 1982 for rocks in central and southern chile the homogeneous reservoir has been suggested to be enriched subcontinental mantle more siliceous rocks granodiorites and monzogranites in the lima segment have considerably higher ratios 206 pb 204 pb 18 680 20 803 207 pb 204 pb 15 610 15 709 and 208 pb 204 pb 38 500 41 177 nine of ten monzogranites from the ring dike complexes in the lima segment define a linear array with a slope of 0 1143 correlation coefficient 0 9398 and a pb isochron age of 1 9 ga this isochron age has no simple chronological meaning although it may represent a component of sedimentary debris derived from 1 9 ga crustal materials feldspars from plutons in the arequipa and toquepala segments south of lima tend to have low pb isotopic ratios 206 pb 204 pb 17 580 18 603 207 pb 204 pb 15 555 15 633 and 208 pb 204 pb 38 104 38 749 the ratios plot between the isotopic signatures of local precambrian granulites and gneisses and the chile type enriched subcontinental magmatic reservoir indicating that these end members were the principal magma sources the oldest plutonic rocks in the segments early to mid jurassic are closest to the pb isotopic ratios of the precambrian basement contamination of plutons in the arequipa and toquepala segments by unradiogenic pb from the precambrian basement and lack of it in the lima segment supports crustal models by couch et al 1981 and jones 1981 which show a thick precambrian basement layer throughout southern peru and an extremely thin layer beneath the lima segment

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

the temporal record of magmatism at cerro uturuncu bolivian altiplano twenty six new 40ar 39ar plateau ages for 23 lavas and domes from the uturuncu volcano in the altiplano of sw bolivia reveal a protracted eruptive history from 1050 5 to 250 5 ka eruptions have been exclusively effusive producing some 50 km3 of high k dacites and silicic andesites bimodal mineral compositions complex mineral textures the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at uturuncu post 458 ka distinct high and low mgo cr magmas are resolved these magmas erupt during similar times suggesting that eruptions are tapping different parts of the magma system albeit from the same vent system volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath uturuncu and calculated buoyancy forces are sufficient to drive effusive eruptions eruptive activity is episodic with six eruptive periods separated by hiatuses of 50 kyr cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka the episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying altiplano puno magma body

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

subduction controls on the compositions of lavas from the ecuadorian andes three volcanoes of the ecuadorian andes atacazo antisana and sumaco lie in a transect perpendicular to the trench and the main trend of the andean arc each of the volcanoes lies on crust of substantially different age composition and thickness few compositional or isotopic features correspond in a straightforward way to the type of the crust through which the magmas have passed isotopic data limit assimilation to 15 at each of the volcanoes instead a systematic relationship exists between the compositions of the lavas and the depth to the benioff zone suggesting that subduction imparts the principal control on the compositions of the magmas atacazo s lavas have low concentrations of the incompatible trace elements and very large lil hfs ratios sumaco s lavas are strongly enriched in the incompatible trace elements and have small lil hfs ratios antisana s lavas are intermediate in almost every respect these features are consistent with devolatilization of the subducted slab controlling the extent of partial melting of a depleted mantle source a mixing and melting model suggests the volcanic front magmas are made by large extent of partial melting 15 and include a large slab input 1 1 added to the depleted mantle the magmas of the middle belt of volcanoes are made by smaller extent of partial melting 3 induced by moderate amounts of slab derived fluid 0 06 the back arc magmas result from small degrees of melting 2 and small slab input

the nevados de payachata volcanic region 18 s 69 w n chile ii evidence for widespread crustal involvement in andean magmatism volcanism extending over 11 ma is represented in the rocks of the nevados de payachata region culminating in the formation of two large composite stratocones within the last 500 000 years chemically distinct mafic magmas are erupted at a number of parasitic centers these cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones two distinct trends are defined by the intermediate and evolved rocks a high lile large ion lithophile element tio2 and ce yb lineage among the youngest rocks including the two major stratocones and a more typical calc alkaline trend among the older 1 ma rock types within individual volcanic centers differentiation involves fractionation of plagioclase pyroxene and hornblende with biotite and k feldspar in the more evolved rock types isotopic compositions sr pb nd o vary little with differentiation from basaltic andesite to rhyolite or with age contamination during differentiation from basalt to rhyolite may occur but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust the majority of data are similar to baseline compositions cenozoic parental magmas from other parts of the central andes and may reflect a relatively homogeneous magma source or source mixture throughout this central volcanic zone cvz which is distinct from the southern and northern andes and from island arc volcanic rocks

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

the magmatic to magmatic hydrothermal evolution of the el laco deposit chile and its implications for the genesis of magnetite apatite deposits

a summer monsoon pump to keep the bay of bengal salty the bay of bengal receives a large influx of freshwater from precipitation and river discharge outflow of excess freshwater and inflow of saltier water is required to prevent the bay from freshening relatively fresh water flows out of the bay along its boundaries and inflow of saltier water occurs via the summer monsoon current smc which flows eastward from the arabian sea into the bay this saltier water however slides under the lighter surface water of the bay maintaining the salt balance of the bay therefore demands upward mixing of this saltier subsurface water here we show that an efficient mechanism for this mixing is provided by upward pumping of saltier water in several bursts during the summer monsoon along the meandering path of the smc advection by currents can then take this saltier water into the rest of the basin allowing the bay to stay salty despite a large net freshwater input

southern bay of bengal currents and salinity intrusions during the northeast monsoon shipboard velocity and hydrographic profiles collected in december 2013 along with drifter observations satellite altimetry global ocean nowcast forecast products and coupled model simulations were used to examine the circulation in the southern bay of bengal as part of ongoing international research efforts in the region the observations captured the southward flowing east india coastal current eicc off southeast india and east of sri lanka the eicc was approximately 100 km wide with speeds exceeding 1 m s 1 in the upper 75 m east of the eicc a subsurface intensified 300 km wide northward current was observed with maximum speeds as high as 1 m s 1 between 50 m and 75 m the eicc moved low salinity water out of the bay and the subsurface northward flow carried high salinity water into the bay during typical northeast monsoon conditions during a time period when the central equatorial indian ocean was experiencing a westerly wind burst related to the madden julian oscillation mjo event while the northward subsurface high salinity flow has previously been observed during the southwest monsoon it was observed during the northeast monsoon the observations are consistent with northward high salinity subsurface flow in numerical model solutions the analysis suggests that direct forcing along the equator may play a significant role for high salinity intrusions east of sri lanka

geodynamical evolution of central andes at 24 s as inferred by magma composition along the calama olacapato el toro transversal volcanic belt abstract miocene to recent volcanism on the puna plateau central andes developed in three geological settings a volcanic arc in the western cordillera miocene recent b trans arc along the main nw se transverse fault systems miocene and c back arc mainly monogenic volcanic centres pliocene quaternary we have studied the evolution of the arc trans arc volcanism along one of the most extensive transverse structures of central andes the calama olacapato el toro at 24 s compositional variations from arc to trans arc volcanism provide insights into petrogenesis and magma source regions puntas negras and rincon volcanic centres are arc type and have typical calc alkaline geochemical and sr nd pb isotopic characteristics east of the arc lavas of the tul tul del medio and pocitos complexes tumepo are heavy rare earth element depleted and could be derived from 20 30 of partial melting of a lower crustal garnet bearing metabasite these liquids could be variably mixed with arc magmas at the base of the crust mash this suggests important contributions from lower crustal sources to tumepo centres products at the quevar and aguas calientes volcanic complexes to the east of tumepo show a prominent upper crustal signature high 86sr 87sr low 143nd 144nd and could represent mixtures of 20 30 tumepo type liquids with up to 70 80 of upper crustal melts we propose a geodynamic model to explain geochemical variations for the arc trans arc transverse volcanism from the upper miocene to recent in our model arc volcanism is linked to dehydration of the subducting nazca plate which produces typical calc alkaline compositions during the upper miocene 10 5 ma lithospheric evolution in the puna plateau was dominated by thickening of ductile lower crust and thinning of the lithosphere lower crustal melting was promoted by concomitant asthenospheric upwelling and water release from the amphibolite eclogite transformation yielding tumepo magmas with lower crustal signatures at the same time the eastern sector of the puna plateau experienced westward underthrusting of the brazilian shield and upper crustal brittle deformation partial melts from underthrust upper crustal wedges mixed with lower crustal magmas to produce lavas at aguas calientes and quevar during pliocene to quaternary delamination of the lithosphere and lowermost crust promoted a widespread monogenetic ocean island basalt type and shoshonitic volcanism

structure from motion photogrammetry a low cost effective tool for geoscience applications abstract high resolution topographic surveying is traditionally associated with high capital and logistical costs so that data acquisition is often passed on to specialist third party organisations the high costs of data collection are for many applications in the earth sciences exacerbated by the remoteness and inaccessibility of many field sites rendering cheaper more portable surveying platforms i e terrestrial laser scanning or gps impractical this paper outlines a revolutionary low cost user friendly photogrammetric technique for obtaining high resolution datasets at a range of scales termed structure from motion sfm traditional softcopy photogrammetric methods require the 3 d location and pose of the camera s or the 3 d location of ground control points to be known to facilitate scene triangulation and reconstruction in contrast the sfm method solves the camera pose and scene geometry simultaneously and automatically using a highly redundant bundle adjustment based on matching features in multiple overlapping offset images a comprehensive introduction to the technique is presented followed by an outline of the methods used to create high resolution digital elevation models dems from extensive photosets obtained using a consumer grade digital camera as an initial appraisal of the technique an sfm derived dem is compared directly with a similar model obtained using terrestrial laser scanning this intercomparison reveals that decimetre scale vertical accuracy can be achieved using sfm even for sites with complex topography and a range of land covers example applications of sfm are presented for three contrasting landforms across a range of scales including an exposed rocky coastal cliff a breached moraine dam complex and a glacially sculpted bedrock ridge the sfm technique represents a major advancement in the field of photogrammetry for geoscience applications our results and experiences indicate sfm is an inexpensive effective and flexible approach to capturing complex topography

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

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

the geology of damavand volcano alborz mountains northern iran damavand volcano located in northern iran is a large 400 km 3 composite cone that is currently dormant it shows fumarolic activity near the summit but no evidence of eruption in the past 1000 yr the volcano represents an isolated focus of magmatism of uncertain tectonic affinity although geophysical and geochemical constraints point toward a local hotspot plume origin possibly associated with lithospheric delamination rather than any association with subduction new u th he and 40 ar 39 ar geochronological constraints indicate that the present cone young damavand has been constructed over 600 k y on an older eroded edifice of indistinguishable composition younger than 1 8 ma damavand activity has been characterized by the eruption of radially directed trachyandesite lava flows almost exclusively from summit vents limited pyroclastic activity has yielded thin fallout pumice lapilli layers and a few pyroclastic flows only one significant pyroclastic event is recognized in the remnants of a welded ignimbrite ponded and preserved along the haraz river drainage relatively short periods of volcanic eruptive activity were interspersed with longer periods of erosion in which volcanic products were transported particularly as hyperconcentrated flows into the surrounding drainage systems to be further reworked into epiclastic deposits occasional catastrophic events punctuated this interplay between volcanism and erosion at least one sector collapse is signified by the presence of a large debris avalanche deposit and the regional drainage systems appear to have been frequently dammed by incursions of volcanic material

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

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

origin of the absarokite banakite association of the damavand volcano iran trace elements and sr nd pb isotope constraints the activity of the damavand volcano central alborz northern iran began 1 8 ma ago and continued up to 7 ka bp although the volcanic suite is clearly of shoshonitic affinity only two petrographic types can be distinguished in the studied lavas 1 weakly differentiated absarokites 49 sio2 51 scattered around the volcano but with a regional extension 2 highly differentiated banakites 59 sio2 63 which form the bulk of the 4 000 m thick volcanic pile all lavas are alkalic 3 7 k2o 5 ree and lile rich e g 85 la 148 ppm 9 th 32 ppm and show highly fractionated ree patterns 69 la yb 115 and pronounced nb ta negative anomalies the absarokites are characterised by sr 0 7045 0 7046 and nd 0 51266 0 51269 isotope compositions close to the bulk earth values and distinct from those of the banakites 0 7047 87sr 86sr 0 7049 0 51258 143nd 144nd 0 51262 the pb isotope ratios are also slightly lower in the absarokites than in the banakites 18 71 206pb 204pb 18 77 15 62 207pb 204pb 15 63 38 85 208pb 204pb 38 91 and 18 77 206pb 204pb 18 84 15 62 207pb 204pb 15 64 38 94 208pb 204pb 39 06 respectively overall there is a clear tendency towards higher sr pb and lower nd isotope ratios with increasing degree of differentiation this study suggests that the absarokites result from a low degree of partial melting 5 of a highly metasomatized mantle source which inherited its characteristics from an old subduction setting the initiation of volcanic activity 1 8 ma ago results from variations in the lithospheric thermal regime probably related to lithospheric delamination as proposed for anatolia pearce et al 1990 the banakites are mainly generated by extensive fractional crystallisation 70 of the absarokitic magma with a limited amount a few percents of assimilation of an old crustal component in the form of bulk assimilation or afc processes which both can explain the sr nd and pb isotope data

low frequency currents from deep moorings in the southern bay of bengal abstracthigh resolution currents and hydrographic fields were measured at six deep water moorings in the southern bay of bengal bob by the naval research laboratory as part of an international effort focused on the dynamics of the indian ocean currents temperature and salinity were sampled over the upper 500 m for 20 months between december 2013 and august 2015 one of the major goals is to understand the space time scales of the currents and physical processes that contribute to the exchange of water between the bob and the arabian sea the observations captured southwest and northeast monsoon currents seasonally varying large eddies including a cyclonic eddy the sri lanka dome sld and an anticyclonic eddy southeast of the sld the observations further showed intraseasonal oscillations with periods of 30 70 days near inertial currents and tides monthly averaged velocities commonly exceeded 50 cm s 1 near the surface and extreme velocities exceeded 150 cm s 1 during the southwest monsoon ti

volcanological and petrological evolution of vulcano island aeolian arc southern tyrrhenian sea petrological and geochemical data are reported for volcanic rocks from vulcano island the subaerial volcanism 120 ka to present built up a nw se elongated composite structure affected by two intersecting multistage calderas volcanics older than 20 ka consist mostly of high k calc alkaline hkca to shoshonitic sho mafic rocks these magmas interacted significantly with the continental crust which generated variable sr isotopic ratios 0 70412 0 70520 however a major role was also played by input of parental liquids into the magma chamber which prevented further evolution of the magmas hkca sho and potassic ks rocks formed from 20 to 8 ka display a much larger range of sio2 from shoshonites to rhyolites and higher concentrations of incompatible elements with respect to the previous stage sr isotopic ratios show small variations 0 70448 0 70486 mixing of silicic and mafic liquids and fractional crystallization processes fc were the main evolutionary processes during this stage volcanics younger than 8 ka consist of sho and leucite bearing ks mafic rocks with abundant intermediate and silicic products mafic and intermediate rocks display similar incompatible element abundances and sr isotopic ratios as the previous stage volcanics whereas higher 87sr 86sr 0 70494 0 70583 are observed in some rhyolites these products originated from a complex interplay of fc crustal assimilation and magma mixing processes the most mafic rocks show increasing incompatible element abundances rb sr rb ba mg al mg ca and a decrease in large ion lithophile to high field strength element ratios passing from older hkca sho to the younger sho ks volcanics these variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere fertile peridotite to a more strongly metasomatized lithospheric mantle residual peridotite time related petrological and geochemical variations have been used to develop a model for the evolution of the vulcano plumbing system