forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

magmatic evolution of a volcano studied by 230th 238u disequilibrium and trace elements systematics the etna case age determinations of several lava flows from etna through 230th 238u disequilibrium internal isochrons yield a precise chronology of the volcano s history for the last 200 000 years and emphasize the main episodes in the formation of this huge complex strato volcano study of 230th232th 0 initial ratios of lavas together with their trace element compositions yields a consistent model of magmatic evolution implying the existence for 200 000 years of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity these short periods of mixing appear to be related to the formation of the large calderas of etna in addition to these processes affecting the deep reservoir fractional crystallization also occurred in more superficial levels of the volcanic edifice thereby yielding several series of differentiation of relatively short duration as for its geochemistry etna s volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context at the edge of the african plate

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

formal ratification of the quaternary system period and the pleistocene series epoch with a base at 2 58 ma in june 2009 the executive committee of the international union of geological sciences iugs formally ratified a proposal by the international commission on stratigraphy to lower the base of the quaternary system period to the global stratotype section and point gssp of the gelasian stage age at monte san nicola sicily italy the gelasian until then had been the uppermost stage of the pliocene series epoch the base of the gelasian corresponds to marine isotope stage 103 and has an astronomically tuned age of 2 58 ma a proposal that the base of the pleistocene series epoch be lowered to coincide with that of the quaternary the gelasian gssp was also accepted by the iugs executive committee the gssp at vrica calabria italy which had hitherto defined the basal boundary of both the quaternary and the pleistocene remains available as the base of the calabrian stage age now the second stage of the revised pleistocene in ratifying these proposals the iugs has acknowledged the distinctive qualities of the quaternary by reaffirming it as a full system period correctly complied with the hierarchical requirements of the geological timescale by lowering the base of the pleistocene to that of the quaternary and fully respected the historical and widespread current usage of both the terms quaternary and pleistocene copyright 2009 john wiley sons ltd

trench migration and subduction zone geometry seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries we numerically investigate the effect of trench migration on the evolution of a slab with temperature dependent viscosity encountering a viscosity interface we find that subduction zone geometry is sensitive to even small rates 1 cm yr of retrograde motion increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities subduction zone geometry appears to be strongly time dependent we conclude that individual tectonic setting and time dependent slab behaviour can account for many different types of observable subduction zone anomalies our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography

crustal xenoliths from calbuco volcano andean southern volcanic zone implications for crustal composition and magma crust interaction crustal xenoliths in the 1961 andesite flow of calbuco volcano in the southern southern volcanic zone ssvz of the andes consist predominantly of pyroxene granulites and hornblende gabbronorites the granulites contain plagioclase pyroxene magnetite amphibole and have pronounced granoblastic textures small amounts of relict amphibole surrounded by pyroxene plagioclase magnetite glass symplectites are found in some specimens these and similar textures in the gabbronorites are interpreted as evidence of dehydration melting mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism a moderate pressure high temperature stage accompanied by melting and melt extraction from some samples followed by thermal metamorphism after entrainment in the calbuco andesite lavas high nd t values 4 0 to 8 6 nd isotope model ages of 1 7 2 0 ga and trace element characteristics like chondrite normalized la yb and la nb 1 indicate that the protoliths were oceanic basalts similar oceanic metabasalts of greenschist to amphibolite facies are found in the paleozoic metamorphic belt that underlies the chilean coastal ranges mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate crystallizing from the basaltic andesite magma at calbuco crystallization pressures for the gabbros based on total al contents in amphibole are 6 8 kbar these pressures point to middle to lower crustal storage of the calbuco magma neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of calbuco andesites although an ancient sedimentary contaminant is indicated by the lava compositions the presence of oceanic metabasaltic xenoliths together with the sedimentary isotopic imprint suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt which is an accretionary complex of intercalated basalts and sediments that formed along the paleozoic gondwanan margin if this is the case the geochemical composition of the lower and middle crust beneath the ssvz is significantly different from that of most recent ssvz volcanic rocks

transient shallow reservoirs beneath small eruptive centres constraints from mg fe interdiffusion in olivine abstract small eruptive centres commonly have more primitive lavas than those associated with stratovolcanoes an observation that has been taken to indicate a short magma residence in the crust relative to those reservoirs below stratovolcanoes the caburgua cones of the andean southern volcanic zone from a basaltic small eruptive centre where this can be tested here we use melts simulations and the available thermobarometry data to determine the conditions of olivine crystal rim formation and the mg fe diffusion modelling to determine the magma residence times of those rims in the crust results yield timescales varying from a few days to dozens of days and if freezing is to be avoided can only be explained by some form of storage or slow transport through at least one shallow magma body the longest durations of magma residence seen in the olivine rim zones are up to 471 days these timescales are shorter than those estimated decadal from the nearby more differentiated and well established stratovolcano villarrica which has a dominantly basaltic andesite composition for caburgua cones we propose the existence of a transient reservoir in contrast to a long lived reservoir such as that inferred beneath the adjacent villarrica stratovolcano

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

morpho structural evolution of the cordon caulle geothermal region southern volcanic zone chile insights from gravity and 40ar 39ar dating the cordon caulle geothermal region 40 5 s is a 13 km 6 km nw se elongate volcano tectonic depression hosting numerous monogenetic volcanic centers fumaroles and hot springs most of which occur spatially associated with the edges of the depression volcanism started at approximately 0 3 ma with dominantly mafic effusions and expressed ultimately in post glacial and historic dacite rhyolite eruptions 1921 1922 and 1960 three ne trending gravimetric profiles were carried out across the depression of cordon caulle yielding a residual negative anomaly with the greatest amplitude 20 mgal at the center of the depression a 2d gravity model given by a 490 m thick shallow body with a density contrast 650 kg m3 interpreted as a shallow package of dacite rhyolite lavas and tephra was found to match the residual anomaly a dense rock equivalent volume of felsic material on the order of 28 km3 25 of the total output of cordon caulle is derived from this thickness previous workers explain the origin of felsic rocks at cordon caulle in terms of the orientation of the borders of cordon caulle relative to the maximum stress direction 1 i e ne trending prevailing during the quaternary according to which nw trending domains were compressional favoring stagnation storage and differentiation of magmas the dominantly monogenetic style of volcanism the plateau like morphology and the negative bouguer anomaly on the contrary are interpreted as an indication of extensional tectonics intra arc extension is proposed to be related to coseismic or postseismic stress relaxation accompanying major thrust events such as the great 1960 chilean earthquake which is consistent at cordon caulle with the direction of propagation of dikes of the 1960 eruption i e coseismic extension and present day subsidence inferred from insar data i e postseismic extension prior to the quaternary 8 2 1 6 ma nearly east west compression prevailed at the intra arc zone a 5 44 0 04 ma old granodiorite intrusion bracketed by two sub parallel branches of the regional margin parallel liquine ofqui fault zone lofz in a prominent pop up structure 15 km to the east of cordon caulle accounts for reverse reactivation of the lofz and rapid fault related exhumation of basement rocks 1 mm year we argue that under compression reverse reactivation of the lofz was facilitated by the combined effect of cyclic increase in fluid pressure induced by magmatic hydrothermal input and removal of lithostatic load induced by glacial erosion 1 5 mm year in the vicinity of cordon caulle

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

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

multiple subduction components in the mantle wedge evidence from eruptive centers in the central southern volcanic zone chile in the andean central southern volcanic zone basalts from small eruptive centers near the large composite center volcan villarrica are poor in fluid mobile elements such as b cs rb k pb ba and u compared with concurrently erupted villarrica basalts new 1 0 be and u series isotopic data for these centers show that fluid mobile element poor small eruptive center basalts have small 1 0 be 9 be ratios 1 6 1 9 10 1 1 and 2 3 8 u 2 3 0 th activity ratios near 1 0 whereas basalts from villarrica show 2 3 8 u enrichment and larger 1 0 be 9 be ratios 4 0 6 4 10 1 1 these results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k y to 3 m y that these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting in contrast the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere the results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales

evolution of the laacher see magma chamber evidence from sims and tims measurements of uth disequilibria in minerals and glasses abstract u th disequilibrium was measured by secondary ion mass spectrometry th and thermal ionization mass spectrometry u in mineral separates from pumices and cumulate nodules from the laacher see volcano 12 5 0 5 kyr corrected 14 c age in east eifel germany phonolitic glasses all plot to the left of the equiline with higher u th observed in the most felsic glasses measured partitioning of u and th in the mineral phases is consistent with the observed increase in u th with progressive fractional crystallization apparent ages of crystallization inferred from internal isochrons in the pumices from the main part of the magma chamber are about 13 3 kyr indistinguishable from the eruption age this implies that the residence time of these phenocrysts did not exceed 1 2 kyr older less precise ages for the upper part of the magma chamber 30 kyr suggest mixing of older crystals from cumulates and or longer residence times mineral isochrons for cumulate nodules are not well defined with ages ranging from 10 to 30 kyr suggesting in situ cumulate formation 10 20 kyr prior to the eruption the time required to explain the 230 th 232 th difference between parental basanite and laacher see phonolite is inferred to be about 100 kyr together these observations favor a cooling model where differentiation of parental basanite takes place in a deep magma body over about 100 kyr followed by emplacement of the resulting mafic phonolite into an upper crustal chamber and continued differentiation toward evolved phonolite the time scale for the development of zonation in the laacher see magma chamber is estimated to be 10 20 kyr at most

u series evidence for crustal involvement and magma residence times in the petrogenesis of parinacota volcano chile in this study we present th u disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the nevados de payachata volcano which erupted through 70 km of continental crust in the central volcanic zone of the andes 18 s 69 w both lavas and mineral separates were analyzed by mass spectrometry for 238u 230th disequilibria the lavas are characterized either by 230th enrichment or depletion relative to its parent nuclide 238u mineral separates are used to derive u th isochron ages and these ages compare favorably with inferred stratigraphic ages or k ar ages although in one case the u th age is significantly older than the stratigraphic age despite relatively constant sr nd and pb isotope ratios the lavas display inverse trends in 230th 238u versus ce yb or ba hf diagrams these trends cannot be interpreted by simple two component mixing rather there must be three and perhaps four components involved in the genesis of the parinacota lavas a mantle wedge a slab fluid and a lower crustal component can be identified a sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence the existence of binary arrays can be explained by variable amounts of crustal material the process of crust mantle interaction must have been short enough to preserve u th disequilibrium 300 ka

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

volcanologic and petrologic evolution of antuco sierra velluda southern andes chile abstract the andean southern volcanic zone comprises 30 active arc front volcanoes that grew over periods of hundreds of thousands of years quantifying the rates at which these volcanoes grow is key to appreciating geological hazards clarifying petrologic evolution and exploring possible relationships between volcanism ice loading and climate the integration of precise geochronology and geologic mapping together with new lava compositions and volume estimates reveal the evolution of the antuco sierra velluda volcanic complex at 37 2 s thirty one new 40ar 39ar age determinations illuminate a punctuated eruptive history that spans at least 430 kyr sierra velluda comprises 130 km3 and began to grow prior to 426 8 ka a lacuna in the volcanic record between 343 5 and 150 4 ka coincides with glaciations associated with marine isotope stages mis 10 and 8 although shallow intrusions were emplaced at 207 0 and 190 0 ka antuco began to grow rapidly on the northeast flank of sierra velluda erupting 60 km3 of lava during three phases 1 an early phase that began at 150 4 ka 2 a post mis 2 phase between 16 3 and 6 2 ka and 3 a post sector collapse phase after 6 2 ka volcanism has been continuous during the last 100 kyr with an average rate of cone growth during this period of 0 46 km3 kyr that has accelerated by about 50 during the past 6 kyr whereas sierra velluda erupted basaltic andesitic to andesitic 53 5 to 58 7 wt sio2 lavas during the last expansion of glaciers between 130 and 17 ka early antuco erupted a wider spectrum of lavas ranging from basaltic andesite to dacite 52 0 to 64 5 wt sio2 notably eruptions following the last glacial termination at 17 ka produced basalts and basaltic andesites 50 9 53 7 sio2 and following the 6 2 ka cone collapse they have been exclusively olivine basalt 50 9 53 0 sio2 with 5 wt mgo thermodynamic and trace element modeling suggests that lavas from sierra velluda and early antuco reflect extensive fractional crystallization of parental basaltic magmas with low water content 1 wt at pressures between 0 9 and 1 5 kbar in contrast eruptions following rapid deglaciation tapped asthenospheric mantle derived basalt that has been extensively modified by assimilation of partial melts of lower crustal rocks

re examination of crystal ages in recent mount st helens lavas implications for magma reservoir processes u series data for recent mount st helens lavas suggest that crystallization preceded eruption by more than 0 5 ka but are complicated by possible evidence of crystal recycling and or addition of radium to the liquid after crystallization we report new ion and electron microprobe trace and major element data for plagioclase and pyroxene in these recent mount st helens lavas and use these data to reassess 226ra 230th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates revised 226ra 230th model crystallization ages are 2 4 ka for plagioclase with the exception of the 1982 dacite and 0 15 5 7 ka for pyroxene in contrast to previous interpretations no late stage addition of ra to the liquid after precipitation of the minerals is required the variability of ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re equilibration of xenocrysts with a new host liquid ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases ra th ages have not been significantly modified by ra diffusion into or out of the crystals high 226ra ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal liquid chemical equilibrium one possibility is that of growth entrapment of surface enrichments during rapid crystallization which could lead to less discrimination between ra and ba than predicted by calculated bulk partition coefficients 226ra 230th crystal ages for the castle creek andesite and basalt that are younger than 230th 238u ages of the same crystals could be explained by mixing of crystals into melts with different 230th 232th ratios by combinations of older and younger crystal growth within the same magma or for the basalt by diffusion of ra into crystals at high magmatic temperatures average plagioclase ages in most of the samples of 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano this observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch

origin of 226ra 230th disequilibria in arc lavas from southern chile and implications for magma transfer time abstract improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface the rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238u series young lavas from southern chile in which 238u 230th disequilibria have been measured sigmarsson et al nature 346 1990 163 165 sigmarsson et al nature 394 1998 566 569 were analyzed for 226ra abundances the disequilibrium between 226ra and 230th in these lavas is found to correlate with 238u 230th disequilibria and 10be be morris et al nature 344 1990 31 36 these correlations strongly suggest that the excess of 226ra over 230th is due to the addition of a slab derived fluid to the magma source since ra and u are fluid mobile elements and the cosmogenic 10be is most likely derived from the subducting nazca plate beneath the andes the largest slab signature is observed in the lavas of villarrica volcano which is the most active volcano in south america a model for subduction fluxing is discussed in which the u series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10be be b be correlation for different arcs morris et al nature 344 1990 31 36 the fact that excesses of 226ra and 238u over 230th are correlated indicates that linear arrays on the 230th 232th 238u 232th diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts the 226ra 230th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer or less than 8000 years this disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m year finally the correlations of 226ra 230th with 238u 232th and 10be be in andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

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

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

u th pa ra study of the kamchatka arc new constraints on the genesis of arc lavas abstract the 238u 230th 226ra and 235u 231pa disequilibria have been measured by mass spectrometry in historic lavas from the kamchatka arc the samples come from three closely located volcanoes in the central kamchatka depression ckd the most active region of subducted related volcanism in the world the large excesses of 226ra over 230th found in the ckd lavas are believed to be linked to slab dehydration moreover the samples show the uncommon feature of 230th 238u activity ratios both lower and higher than 1 the u series disequilibria are characterized by binary trends between activity ratios with 231pa 235u ratios all 1 it is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting we suggest that they are more likely to reflect mixing between two end members a high magnesia basalt hmb end member with a clear slab fluid signature and a high alumina andesite haa end member reflecting the contribution of a slab derived melt the u th ra characteristics of the hmb end member can be explained either by a two step fluid addition with a time lag of 150 ka between each event or by continuous dehydration the inferred composition for the dehydrating slab is a phengite bearing eclogite equilibrium transport or dynamic melting can both account for 231pa excess over 235u in hmb end member nevertheless dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid derived 226ra 230th a continuous flux melting model is also tested in this model 231pa 235u is quickly dominated by fluid addition and for realistic extents of melting this process cannot account for 231pa 235u ratios as high as 1 6 as observed in the hmb end member the involvement of a melt derived from the subducted oceanic crust is more likely for explaining the haa end member compositions than crustal assimilation melting of the oceanic crust is believed to occur in presence of residual phengite and rutile resulting in no 226ra 230th disequilibrium and low 231pa excess over 235u in the high alumina andesites consequently it appears that high alumina andesites and high magnesia basalts have distinct origins the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle it seems that there is no genetic link between these two magma types in contrast with what was previously believed

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

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

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

u series isotopes and destructive plate margin magma genesis in the lesser antilles high precision tims230th232th isotope ratios are presented from a suite of lavas which span the length of the lesser antilles island arc compositions vary from low k tholeiitic lavas with low87sr86sr in the north to high k calc alkaline to alkalic lavas with higher87sr86sr in the south the northern lavas are characterised by 230th238u 1 a three component model is required for magma genesis in the lesser antilles sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates releasing fluids enriched in u ba sr and k but with negligible th and low87sr86sr which promote partial melting in the wedge ta zr increases with87sr86sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment mass balance calculations are consistent with a relatively constant fluid flux except for u and little variation in the degree of partial melting 10 15 along the arc and they suggest that the sediment contribution to the arc lavas increases from 2 in the north to 15 in the south in the northern parts of the arc the addition of u by these fluids overprints any increases inth u that might have been caused by partial melting and results in lavas with 230th238u 1 m yr 1 which imply channelled flow through the mantle wedge

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

geochemical variations in andean basaltic and silicic lavas from the villarrica lanin volcanic chain 39 5 s an evaluation of source heterogeneity fractional crystallization and crustal assimilation at 39 5 s in the southern volcanic zone of the andes three pleistocene recent stratovolcanoes villarrica quetrupillan and lanin form a trend perpendicular to the strike of the andes 275 to 325 km from the peru chile trench basalts from villarrica and lanin are geochemically distinct the latter have higher incompatible element abundances and la sm but lower ba la and alkali metal la ratios these differences are consistent with our previously proposed models involving a a west to east decrease in an alkali metal rich high ba la slab derived component which causes an across strike decrease in degree of melting or b a west to east increase in the contamination of subduction related magma by enriched subcontinental lithospheric mantle silicic and mafic lavas from the stratovolcanoes have overlapping sr nd and o isotopic ratios silicic lavas also have geochemical differences that parallel those of their associated basalts e g rhyolite from villarrica has lower la sm and incompatible element contents than high sio2 andesite from lanin at each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing apparently crustal contamination was not an important process in deriving the evolved lavas basaltic flows from small scoria cones 20 35 km from villarrica volcano have high incompatible element contents and low ba la like lanin basalts but trend to higher k rb 356 855 and lower 87sr 86sr 0 70361 0 70400 than basalts from either stratovolcano however all basalts have similar nd pb and o isotope ratios the best explanation for the unique features of the cones is that the sources of svz magmas e g slab derived fluids or melts of the subcontinental lithospheric mantle have varying alkali metal and radiogenic sr contents these heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing this model is preferable to one involving crustal contamination because it can account for variable sr isotope ratios and uniform nd and pb isotope ratios among the basalts and the divergence of the cones from across strike geochemical trends defined by the stratovolcanoes

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

calbuco volcano and minor eruptive centers distributed along the liquine ofqui fault zone chile 41 42 s contrasting origin of andesitic and basaltic magma in the southern volcanic zone of the andes calbuco volcano is a late pleistocene holocene composite stratovolcano located at 41 20 s in the southern region of the southern volcanic zone of the andes ssvz 37 46 s in contrast to basalt and basaltic andesite which are the dominant lava types on the volcanic front from 37 to 42 s calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths they have sio2 contents in the 55 60 range and have comparatively low k2o rb ba th and lref abundances relative to other ssvz centers incompatible element abundance ratios are similar to those of most ssvz volcanics but 87sr 86sr and 143nd 144nd are respectively higher and lower than those of adjacent volcanic centers basalts from nearby osorno stratovolcano 25 km to the northeast are similar to other basaltic ssvz volcanoes however basalts from several minor eruptive centers mec located east of calbuco and osorno volcano along the liquine ofqui fault zone lofz are enriched in ba nb th and lree and have higher la yb and lower ba la k la and rb la 87sr 86sr and 143nd 144nd in mec basalts are respectively lower and higher than those of osorno and calbuco lavas we suggest that mec basalts were produced by lower extents of mantle melting than basalts from osorno and other ssvz stratovolcanoes probably as a result of lower water content in the source of mec basalts calbuco andesites formed from basaltic parents similar to osorno basalts by moderate pressure crystallization of a hornblende bearing assemblage accompanied by crustal assimilation hornblende stability in the calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks which are also an appropriate endmember for isotopic trends together with magma storage at mid crustal depths the unique characteristics of calbuco volcano i e the stability of hornblende at andesitic sio2 contents low 143nd 144nd and high 87sr 86sr and abundant crustal xenoliths provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the ssvz