u pb zircon and geochemical constraints on the age origin and evolution of paleozoic arc magmas in the oyu tolgoi porphyry cu au district southern mongolia abstract uranium pb zircon ages are linked with geochemical data for porphyry intrusions associated with giant porphyry cu au systems at oyu tolgoi to place those rocks within the petrochemical framework of devonian and carboniferous rocks of southern mongolia in this part of the gurvansayhan terrane within the central asian orogenic belt the transition from devonian tholeiitic marine rocks to unconformably overlying carboniferous calc alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation radiogenic nd and pb isotopic compositions e nd t range from 3 1 to 7 5 and 206 pb 204 pb values for feldspars range from 17 97 to 18 72 as well as low high field strength element hfse contents of most rocks mafic rocks typically have 2 are consistent with magma derivation from depleted mantle in an intra oceanic volcanic arc the late devonian and carboniferous felsic rocks are dominantly medium to high k calc alkaline and characterized by a decrease in sr y ratios through time with the carboniferous rocks being more felsic than those of devonian age porphyry cu au related intrusions were emplaced in the late devonian during the transition from tholeiitic to calc alkaline arc magmatism uranium pb zircon geochronology indicates that the late devonian pre to syn mineral quartz monzodiorite intrusions associated with the porphyry cu au deposits are 372 ma whereas granodiorite intrusions that post date major shortening and are associated with less well developed porphyry cu au mineralization are 366 ma trace element geochemistry of zircons in the late devonian intrusions associated with the porphyry cu au systems contain distinct th u and yb gd ratios as well as hf and y concentrations that reflect mixing of magma of distinct compositions these characteristics are missing in the unmineralized carboniferous intrusions high sr y and evidence for magma mixing in syn to late mineral intrusions distinguish the late devonian rocks associated with giant cu au deposits from younger magmatic suites in the district

determination of recent 238u 234u and 230th mobility in granitic rocks application of a natural analogue to the high level waste repository environment abstract the isotope activity ratios 234u 238u and 230th 234u have been investigated in drill core samples up to 1300 m depth in three granitoids from australia to identify the nature of possible recent 1 0 which has generated excess 230th these enriched ground waters do not appear to have penetrated below 530 m depth and 230th excess indicates bulk loss of u the observed disequilibria indicate that significant radionuclide mobilisation has occurred in these rocks during the recent past

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

re os ages for the erdenet and tsagaan suvarga porphyry cu mo deposits mongolia and tectonic implications the erdenet and tsagaan suvarga deposits are the two largest known porphyry cu mo deposits in mongolia replicate re os analyses of molybdenite from these deposits yield ages of 240 7 0 8 and 240 4 0 8 ma for erdenet and 370 1 1 2 and 370 6 1 2 ma for tsagaan suvarga 2 errors the re os age of erdenet is significantly older and that of tsagaan suvarga is slightly older than published 40ar 39ar ages for sericite associated with mineralization in these deposits 207 4 5 0 and 364 9 7 0 ma respectively these re os results suggest that in the hydrothermal environment 40ar 39ar ages may be partly to significantly reset whereas re os ages record and maintain the age of primary mineralization events these re os ages indicate that porphyry mineralization at erdenet and tsagaan suvarga are related to porphyritic intrusions representing the youngest event during regional magmatism in northern and southern mongolia respectively the re os age of erdenet marks the final stage of arc magmatism in northern mongolia followed by late permian collision between the mongolian north china and siberian blocks the re os age of tsagaan suvarga indicates that devonian arc magmatism in southern mongolia was active at the present latitude of 44 n at least until 370 ma

lascar volcano northern chile evidence for steady state disequilibrium lascar volcano located in the central volcanic zone of northern chile is a medium to high k calc alkaline stratovolcano three stages are recognized in the construction of the edifice with activity ranging from andesitic fissure eruptions to a large volume dacitic pumice flow the soncor flow the eruption products are predominantly two pyroxene andesites and dacites with some hornblende dacites occurring in the soncor flow disequilibrium textures resulting from mixing between mafic and acid end members

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

magma evolution and ascent at volcanic arcs constraining petrogenetic processes through rates and chronologies large 226 ra excesses in a number of mafic arc magmas and geophysical observations of earthquake hypocenters locations indicate that mafic melts can get transferred from source to surface within days to 1 kyr decreasing 226 ra 230 th disequilibria with increasing sio2 in co magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems or less in open systems however the rapid decrease of global 238 u 230 th disequilibria with increasing sio2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238 u excesses in the case of rapid magmatic evolution open system processes are frequently involved in the production of andesites and dacites and the data imply that the open system component is close to 238 u 230 th secular equilibrium melts modeling of magma evolution at santorini in the aegean arc provides corroborating evidence from an individual well studied arc volcano that closed system fractional crystallization fails to produce the more evolved compositions and increasing 87 sr 86 sr ratios with increasing sio2 from andesites to rhyolites suggests assimilation of old crust is involved these observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust melt production in deep crustal hot zones provides an explanation for low 238 u 230 th disequilibria of the evolved magmatic component as significant accumulation of evolved melts through partial re melting of previously intruded basalts requires n100 kyr incubation time at typical magmatic flux rates thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems further insights into magmatic evolution can be gained from dating of minerals from mafic intermediate and felsic arc magmas a many u th mineral isochrons in mafic arc magmas yield ages significantly older than those

dating recent volcanic rocks through 230th 238u disequilibrium in accessory minerals example of the puy de dome french massif central precise dating of very young 10 000 yr silicic volcanic rocks can be achieved by measurements of 230 th 238 u disequilibria in accessory minerals such as apatite and zircon it is illustrated here by the isochron age of 12 000 1000 yr obtained on a trachyte from the puy de dome french massif central the crystallization age is virtually the same as the eruption age determined by thermoluminescence and 14 c dating this indicates a short time interval a few centuries at most between crystallization and eruption in such small volume silicic magmas

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

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

evolution of lascar volcano northern chile lascar volcano in the central andes is an ese wnw elongate composite stratocone stage i 3 at 26 45 ka producing the soncor ignimbrite no caldera formed laharic and alluvial fan deposits and a debris avalanche formed from 26 45 to 19 2 ka indicating a humid climatic period the dacitic and andesitic western stratocone stage iii developed over the soncor vent stage iii culminated in the 9 2 ka tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater stage iv activity switched to the east forming an andesite lava three nested summit craters which repeatedly filled with lava and collapsed absence of significant erosion since 9 2 ka is due to the much drier holocene climate activity since 1984 displays cycles of lava dome formation in the summit crater lava dome subsidence with crater collapse vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19 20 april 1993 the total volume of magma erupted from lascar over a 43 ka period is estimated at 30 to 40 km 3

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

continuous mixing of crystal mush and replenished magma in the ongoing unzen eruption zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing the dacite was formed by mixing of relatively high and low temperature t end member magmas the magnetite phenocrysts derived from the low t magma are reversely zoned by the mixing with high t magma a diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching for the mixed dacite erupted from may 1991 to may 1993 the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr this indicates that the mixing was continuous during the effusion the invariability of the other mixing signatures such as the thickness of reaction rims around biotite phenocrysts also supports the continuous mixing model low t end member magma is estimated by mass balance calculation to be a crystal rich mush of dacitic composition these observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion the proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes

u series disequilibrium in arc magmas induced by water magma interaction abstract volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down going slab u series disequilibrium in such magmas is commonly attributed to this particular melting process a detailed study of u series isotopes in the 650 y b p eruptive sequence of mt pelee martinique shows that plinian products are in radioactive equilibrium whereas dome forming products of the same eruption are characterized by 238u 230th disequilibrium the same features apply to other plinian and dome forming products of this volcano and systematically correspond to different eruptive styles we attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes this conclusion might be generally applicable to arc magmas

the link between volcanism and tectonics in the southern volcanic zone of the chilean andes a review abstract the nature of the interplay between tectonics and volcanism is a major question in continental margin tectonics the southern andes volcanic zone svz located at the obliquely convergent nazca south america plate margin between 33 s and 46 s offers a unique opportunity to address this question because of along strike changes in crustal thickness tectonic style and well constrained long term and short term kinematic history the complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems main first order factors accounting for the along strike variations in the nature and composition of volcanism are crustal thickness and the existence of a major intra arc fault system the liquine ofqui fault zone lofz second order factors include the local nature of the volcanic arc basement two main categories of volcano tectonic associations have been identified according to the spatial distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second order active inactive basement faults kinematically coupled associations those that are directly related to the current dextral transpressional tectonic regime include ne trending volcanic alignments of stratovolcanoes and or monogenetic cones kinematically uncoupled associations in which coupling with the current regime is not required include stratovolcanoes built on top of ancient reverse and strike slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the lofz whereas mostly primitive magmas are found in the first category more evolved terms can occur genetically associated with the second category we propose a simplified three dimensional model for the magma plumbing system along and across the svz that integrates kinematically coupled and kinematically uncoupled volcano tectonic associations within the framework of first and second order geological and structural constraints

inter and intracrystalline isotopic disequilibria techniques and applications in the last 20 years we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences from constraining pre eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes these techniques have had significant impacts in a wide variety of scientific fields nowhere has the impact been greater than in identifying the sources processes and timing of processes involved in igneous magmatic systems both technique refinements and the development of new technologies have aided in advancing microsampling applications thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems in this chapter we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems including melts and minerals from the use of petrographic microscopes and the later introduction of the electron microprobe the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks even today these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context for trace elements and isotopes early studies e g cortini and van calsteren 1985 confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost later studies such as geist et al 1988 focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic andesitic and rhyolitic magmas and davidson et al 1990 used

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

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

magmatic hydrothermal evolution of the cretaceous duolong gold rich porphyry copper deposit in the bangongco metallogenic belt tibet evidence from u pb and 40ar 39ar geochronology abstract the duolong gold rich porphyry copper deposit was recently discovered and represents a giant prospect inferred resources of 4 5 mt fine cu with a grade of 0 72 cu 30 50 t fine gold with a grade of 0 23 g t au in the bangongco metallogenic belt tibet zircon shrimp and la icp ms u pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes the first at about 121 ma bolong mineralized granodiorite porphyry bmgp and barren granodiorite porphyry bgp and the second about 116 ma duobuza mineralized granodiorite porphyry dmgp moreover the basaltic andesites also have two episodes at about 118 ma and 106 ma respectively one andesite yields an u pb zircon age of 111 9 1 9 ma indicating it formed after the multiple granodiorite porphyries by contrast the 40ar 39ar age of 115 2 1 1 ma hydrothermal k feldspar vein hosted in dmgp reveals the close temporal relationship of ore bearing potassic alteration to the emplacement of the dmgp the sericite from quartz sericite vein hosted in dmgp yields a 40ar 39ar age of 115 2 1 2 ma therefore the ore forming magmatic hydrothermal evolution probably persisted for 6 m y additionally the zircon u pb ages 106 121 ma of the volcanic rocks and the porphyries suggest that the neo tethys ocean was still subducting northward during the early cretaceous

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

seismological aspects of the 1988 1989 lonquimay chile volcanic eruption abstract more than 1600 earthquakes were recorded in a four month period on portable seismographic stations installed in the lonquimay area immediately after the initiation of the volcanic eruption of december 25 1988 hypocentral parameters were calculated on a subset of 150 events showing clear p and s arrivals seismic activity was restricted to a north south elongated region that included the main crater of the lonquimay volcano the new navidad crater lies along the eastern margin of the area of seismicity most of the hypocenters are located at depths shallower than 6 km and none have depths exceeding 10 km the largest earthquakes of the sequence took place on dec 28 m 4 7 jan 12 13 m 4 8 and 5 0 and feb 24 m 5 3 the number of events per day varied it peaked on dec 27 with more than 260 events and more than 200 events were recorded on jan 13 both of which corresponded to the days of large earthquakes the relation of large to small events given by the b value of the magnitude frequency distribution log n a bm is typical of volcanic sequences b 1 7 and much larger than the equivalent for tectonic environments the rate of decay of the number of aftershocks of each large earthquake is proportional to t p with p increasing from 0 63 to 1 08 as the eruption progressed in time initial low values of p indicate a slow stress release in the region larger absolute values of p are comparable with tectonic environments and might be indicators of the last stages of the eruption process the total seismic energy release is 9 6 10 19 ergs about five orders of magnitude less than the estimated thermal energy the spatial distribution of earthquakes suggests that the sources of stress change associated with the eruption are located under and possibly to the south of the main crater of the lonquimay volcano and that the evacuation channels of magma occupied a zone of weakness in the region of the new crater

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

geomorphology and structural development of the nested summit crater of lascar volcano studied with terrestrial laser scanner data and analogue modelling abstract many volcano summits host craters that are partially overlapping the formation of such nested craters has been commonly interpreted as vent migration here we present an additional mechanism that may explain the geometry of nested craters at volcanoes lascar volcano the most active volcano of the central volcanic zone in the chilean andes hosts ene wsw trending summit craters that are partially overlapping nested details on the evolution and interaction between the different craters remain unclear to create a robust dataset terrestrial laser scanner tls data were collected at the summit of lascar in 2013 the resulting topographic data set consisting of more than 15 million data points with centimetre sampling allows visualising almost the complete eastern edifice of the volcano s summit from the tls data a digital elevation model dem and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features to further improve our understanding of the processes responsible for the formation of the craters and geomorphology we designed sandbox analogue models results suggest that one of the craters is a parasite crater formed as a consequence of ongoing activity in the adjacent crater our data suggest that the nested craters have all been modified since the last major eruption in 1993 by near surface effects associated with cooling compaction and gravitational sliding of the crater floor infill as the active crater deepens the adjacent inactive crater extends and partially slumps towards the active one understanding the structural development of these nested craters is relevant for assessing potential future eruption sites thus making lascar a dynamic target for a detailed morphology study these findings may similarly be applied to other volcanoes where nested craters have developed

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

u pb and re os geochronology of the bainaimiao cu mo au deposit on the northern margin of the north china craton central asia orogenic belt implications for ore genesis and geodynamic setting abstract the bainaimiao cu mo au deposit located at the northern margin of the north china craton ncc in central inner mongolia is a large cu dominant deposit orebodies are hosted in strongly deformed and metamorphosed granodiorite porphyry intrusions or in ew trending shear zones in greenschist and sericite felsic schist of the bainaimiao group mineralization can be divided into early middle and late stages characterized by quartz pyrite polymetallic quartz sulfides and carbonate quartz respectively hydrothermal activity led to pervasive silicification k feldspar biotite and chlorite epidote carbonate alteration eight samples from cu mo bearing quartz veins and massive sulfide ores in greenschist were collected from the viii orebody in the north ore belt and molybdenite separates were selected for re os analyses the concentrations of re and 187 os range from 216 6 to 1051 ppm and 2302 to 4913 ppb respectively all samples give re os model ages between 441 2 and 445 9 ma and yielded an isochron age of 445 0 3 4 ma mswd 0 26 with an initial 187 os of 4 17 ppb a granodiorite porphyry sample for shrimp u pb dating was collected from the underground of the xiii orebody of the north ore belt spots from 14 zircon grains have 206 pb 238 u values between 426 2 and 462 9 ma and form a tight cluster on concordia yielding a weighted mean 206 pb 238 u age of 445 6 ma this is considered to be the best estimate of crystallization age of the granodiorite porphyry the re os isochron age of the molybdenite is consistent with the shrimp u pb age of the granodiorite porphyry which indicates that the deposit is a porphyry system of paleozoic age according to geodynamic setting and geological history of the northern margin of the ncc the deposit was probably related to magmatic activity of the middle paleozoic ondor sum subduction accretion complex bainaimiao arc during the devonian the bainaimiao arc was accreted to the northern margin of the ncc collision between the bainaimiao arc and ncc led to regional metamorphism of the ore hosting terrane and ore metals were remobilized in this process gold mineralization was probably overprinted onto the earlier cu mo mineralization

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

gold deposits of the northern margin of the north china craton multiple late paleozoic mesozoic mineralizing events the northern margin of the north china craton is well endowed with lode gold deposits hosting a resource of approximately 900 tonnes t of gold the 1 500 km long region is characterized by east trending blocks of metamorphosed archean and proterozoic strata that were episodically uplifted during variscan indosinian and yanshanian deformational and magmatic events at least 12 gold deposits from the daqinshan yan liao includes the zhangjiakou yanshan and chifeng gold districts and changbaishan gold provinces contain resources of 20 100 t au each most deposits are hosted in uplifted blocks of precambrian metamorphic rocks although felsic paleozoic and mesozoic plutons are typically proximal and host 30 of the deposits the lodes are characterized by sulfide poor quartz veins in brittle structures with low base metal values and high au ag ratios although phyllic alteration is most common intensive alkali feldspar metasomatism characterizes the wulashan dongping and zhongshangou deposits but is apparently coeval with variscan alkalic magmatism only at wulashan stepwise 40ar 39ar geochronology on 16 samples from gangue and alteration phases combined with unpublished shrimp u pb dates on associated granitoids suggest that gold mineralizing events occurred during variscan indosinian and yanshanian orogenies at circa 350 250 200 180 150 and 129 ma however widespread permo triassic 250 ma and early jurassic 180 ma thermal events caused variable resetting of most of the white mica and k feldspar argon spectra as well as previously reported k ar determinations compiled and new stable isotope and fluid inclusion data show that most 18o values for ore stage veins range from 8 to 14 indicating a fluid in equilibrium with the precambrian metamorphic basement rocks d values from fluid inclusions range widely from 64 to 154 which is indicative of a local meteoric component in some veins and highly variable 34s data 7 to 17 even within individual deposits indicate various local country rock sources for sulfur fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 c and are consistently characterized by low salinity h2o co2 ch4 n2 solutions although the data are largely consistent with that from orogenic gold veins intrusion related veins and epithermal veins are also recognized the multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the north china craton electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 001 0239 2

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

recycling of crystal mush derived melts and short magma residence times revealed by u series disequilibria at stromboli volcano abstract the presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts representing the crystalline portion of old magmas recycled in the juvenile material but very little is known about the fate of interstitial liquid hosted within the crystal rich mush i e antemelt u series disequilibria measured in magmas erupted in the past 18 years at stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano despite almost constant major and trace element composition significant variations in isotope ratios are observed 230 th 232 th decreases with time whilst 238 u 232 th 226 ra 230 th and 87 sr 86 sr are different in the two types of magma erupted magma with low phenocryst content lp is erupted as pumices during paroxysm and is thought to belong to a deep reservoir highly porphyritic magma hp is erupted during the normal strombolian activity as scoria and during the effusive events as lavas and it is considered to derive from the former one within a shallow reservoir through degassing driven crystallisation mixing and incorporation of antecrysts the distinct 238 u 232 th of lp and hp magma requires the involvement of a component with high 87 sr 86 sr and 238 u 232 th deriving from older magmas erupted earlier in the volcano history up to 2 5 ka the incompatibility of u and th in major mineral phases limits the possible effect of antecrysts hence requiring the involvement of a u and th rich antemelt the decrease of 226 ra excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old antemelt the variation with time of 230 th 232 th within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to 55 yrs inferred reservoir volume 0 5 km 3 and 2 10 yrs inferred reservoir volume 0 02 0 09 km 3 respectively our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so called steady state volcano such as stromboli we show that while most of the magma is erupted within few years a portion of it made of both crystals antecrysts and residual liquid antemelt can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir the presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes hence favouring their remobilisation

comparative study of lahars generated by the 1961 and 1971 eruptions of calbuco and villarrica volcanoes southern andes of chile abstract the villarrica and calbuco volcanoes of the andean southern volcanic zone are two of the most active volcanoes in chile and have erupted several times in the xx century the 1961 eruption at calbuco volcano generated lahars on the north east and southern flanks while the 1971 eruption at villarrica volcano generated lahars in almost all the drainages towards the north west and south of the volcano the deposits from these eruptions in the voipir and chaillupen river villarrica and the tepu river calbuco are studied the 1971 lahar deposits on villarrica volcano show a great number of internal structures such as lamination lenses grading of larger clasts and a great abundance of large floating blocks on top of the deposits the granulometry can be unimodal or bimodal with less than 5 by weight of silt clay material sem images reveal a great variety of forms and compositions of clasts the 1961 lahar deposits on calbuco volcano have a scarce number of internal structures steeper margins and features of hot emplacement such as semi carbonized vegetal rests segregation pipes and a more consolidated matrix the granulometry usually is bimodal with great quantities of silt clay material 10 by weight sem images show a uniformity of composition and forms of clasts differences on deposits reveal different dynamics on both lahars the villarrica lahar was generated by sudden melt of ice and snow during the paroxysmal phase of the 1971 eruption when a high fountain of lava was formed the melted water flowed down on the flanks of the volcano and incorporated sediments to become transition flows highly energetic and were emplaced incrementally dilution of the flows occurs when the lahars reached unconfined and flatter areas in cases where the lahar flow found large water streams dilution is enhanced the calbuco lahars were generated by the dilution of block and ash pyroclastic flows by flowing over the ice or snow or by entering active rivers transforming to debris flows the differences on dynamics of both flows show the importance to understand initiation processes of lahars in order to make better hazard assessment due to laharics flows

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

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

effects of volcano loading on dike propagation in an elastic half space we use laboratory experiments and numerical models to examine the effects of volcano loading on the propagation of buoyant dikes in a two dimensional elastic half space in laboratory experiments we simulate the propagation of buoyant dikes in an isotropic regional stress field by injecting air into tanks of solidified gelatin a weight resting on the surface of the gelatin represents a volcanic load a numerical model is used to simulate these experiments both experiments and numerical simulations show that as a dike ascends it begins to curve toward the load in response to the local stress field imposed by the load the lateral distance over which dikes curve to the load increases with the ratio of average pressure at the base of the load to the dike driving pressure for realistic volcano and dike dimensions this pressure ratio is going to be large suggesting that dikes can converge to a volcano over lateral distances several times the load width numerical calculations involving an anisotropic regional stress field however predict that the lateral extent of dike attraction shrinks as the regional horizontal compressive stress decreases relative to the vertical compressive stress dike focusing will be substantial if the regional differential stresses are less than the average pressure at the base of the load if this is the case then our models predict a positive feedback between the size of volcanoes and the area of dike attraction this feedback may promote the development of large discrete volcanoes and also predicts a positive correlation between the spacing and sizes of adjacent volcanoes to test this prediction we examine nearest neighbor pairs of the 21 largest volcanoes in the cascade range the 14 pairs examined show a large range in volcano spacing 6 115 km and a statistically significant correlation between spacing and average volcano height this result is consistent with our model results and suggests that the local compressive stress induced by these volcanoes may be an important factor in controlling magma transport in the lithosphere

use and abuse of the terms calcalkaline and calcalkalic the terms calcalkaline and calcalkalic are currently defined and used in multiple and non equivalent ways generally the variation of total fe as feo mgo within evolving subalkaline rock suites is regarded as the most distinctive character of calcalkaline compared with tholeiitic suites reflecting the relative timing of phase saturation with fe ti oxides plagioclase and ferromagnesian silicates other classification schemes are widely used including a minority of adherents to the original formalized definition of calcalkalic by peacock 1931 journal of geology 39 54 67 given the prevailing contradictory and confusing usage of these terms which leads to miscommunication it is proposed that the spectrum of subalkaline rocks be divided into high medium and low fe suites complementing divisions made on the basis of k contents the terms calcalkaline and calcalkalic should be restricted to rock suites that conform to peacock s definition

geological characteristics and genesis of the tuwu porphyry copper deposit hami xinjiang central asia abstract the tuwu copper deposit located in the dananhu tousuquan island arc between the kazakhstan junggar and tarim plates is one of the largest porphyry copper deposits in western china ore hosting rocks consist of early carboniferous 333 2 ma granodiorite and plagiogranite porphyries both the porphyries are elongate the orebodies consisting of veinlet disseminated mineralization within the porphyries grade range from 0 30 to 1 50 cu mineralization can be assigned to six stages wall rock alteration associated with the mineralization include silicification chloritization epidotization sericitization and carbonatization three alteration zones have been recognized phyllic zone chlorite biotite zone and propylitic zone re os dating indicates a 322 7 3 ma age for the deposit 34 s values of the ore sulfides range from 0 9 to 1 3 with a mean of 0 11 reflecting a deep sulfur source 18 o values of quartz in the ores range from 7 70 to 9 70 and the 18 o value of chlorite in the ores is 6 62 calculated 18 o water values of quartz and chlorite range from 5 37 to 6 62 and the d values range from 48 to 63 suggesting that the ore fluids were mainly magma derived in the early stage of mineralization however values for the late mineralization stage decrease suggesting the addition of meteoric waters during the mineralization fluid inclusions within quartz grains are generally 3 to 30 m in size with gas proportions ranging from 5 to 60 mostly from 10 to 15 homogenization temperatures of fluid inclusions are relatively low bracketed between 101 and 409 c mostly between 100 and 200 c salinities exhibit a wide range from 0 35 to 14 61 wt nacl equiv with a mean of 7 72 wt nacl equiv suggesting that ore fluids have low to moderate salinity the ore bearing plagiogranite porphyries show a strong depletion in heavy rare earth elements with average values of yb and y of 0 65 and 5 90 ppm respectively this being a characteristic geochemical signature for adakite the geochemical signature of the adakites within the granitoid rocks represents a characteristic guide for further exploration for copper porphyry type ore deposit in eastern tianshan

seismicity and coda q values in the eastern pyrenees first results from the la cerdanya seismic network seismicity in the la cerdanya region of the eastern pyrenees has been accurately mapped for the first time using data from a local seismic network the majority of earthquakes lies on or near the la cerdanya fault or secondary faults to the south coda magnitudes determined for these earthquakes using magnitude relations from other regions range between 0 5 and 2 2 these are however presumed to be underdetermined values sinceq values appear to be very low in the la cerdanya region codaq values at a frequency of 1 5 hz range between 17 and 120 the lowest values being obtained for the most seismically active regions codaq values also increase with increasing distance a result which indicates decreasing seismic attenuation with increasing depth in the crust

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

magmatic residence times of zoned phenocrysts introduction and application of the binary element diffusion modelling bedm technique this paper describes a general technique binary element diffusion modelling bedm for determining single crystal residence times in magmas that relies on modelling the diffusion of two or more elements in the crystal bedm has the advantage over other diffusion based models in that it does not need a precisely defined initial compositional profile for the crystal at zero time and instead requires that the concentrations of the two elements are correlated during crystallisation any differences subsequently observed between the two elements are caused by intracrystalline diffusion during residence in hot magma these differences are removed by artificially ageing the slower diffusing of the two elements and the amount of time taken to undo the difference between the elements is simply related to the crystal residence time decoupling time at high temperatures the bedm principle is demonstrated using artificial data and is then applied to literature data for sr and ba in a zoned sanidine crystal from the bishop tuff anderson et al in j petrol 41 3 449 473 2000 for this crystal the method gives a residence time estimate of 114 ka at 800 c which is then compared with estimates from other methods in theory the method can be further expanded for use as a geothermometer as well as geochronometer however this is not easily possible with the diffusivity data currently available

contrasting late carboniferous and late permian middle triassic intrusive suites from the northern margin of the north china craton geochronology petrogenesis and tectonic implications two contrasting intrusive suites have been identified from the northern margin of the north china craton a late carboniferous dioritegranodiorite suite mainly made up of quartz diorite diorite granodiorite tonalite and hornblende gabbro and a late permian middle triassic suite of granitoid intrusions consisting of monzogranite syenogranite and quartz monzonite plutons from the late carboniferous suite exhibit variable sio2 contents and calc alkaline or high k calc alkaline metaluminous geochemical features most have low negative whole rock nd t values where t is the crystallization age of 17 1 to 11 5 and zircon hf t values of 38 3 to 11 2 indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials however an older pluton in the suite exhibits higher nd t values of 11 5 to 9 9 nd model ages of 1 82 1 64 ga lower initial 87sr 86sr ratios of 0 7046 0 7048 and it contains some zircon grains that are characterized by high negative to positive zircon hf t values of 8 7 to 1 2 indicating strong involvement of juvenile materials derived from the lithospheric mantle the late carboniferous plutons are interpreted as subduction related and to have been emplaced in an andean style continental margin arc during the southward subduction of the paleo asian oceanic plate beneath the north china craton rocks from the late permian middle triassic intrusive suite display geochemical signatures ranging from highly fractionated i type to a type they exhibit higher zircon hf t values of 14 9 to 6 7 whole rock nd t values of 10 6 to 8 8 and younger hf and nd model ages than most of the late carboniferous plutons indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle and crust derived melts they are interpreted as postcollisional postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break off and subsequent sinking after final collision and suturing of the mongolian arc terranes with the north china craton these two contrasting intrusive suites suggest that the final closure of the paleo asian ocean and collision between the mongolian arc terranes and the north china craton occurred during the late permian and these events were followed by postcollisional postorogenic extension large volume magmatism and significant continental growth no significant syncollisional crustal thickening high pressure metamorphism or s type granitoid magmatism occurred during the collision process

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

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

the effects of magma replenishment processes on 238u 230th disequilibrium short lived isotope data are increasingly used to investigate magma chamber residence times arc lavas have excellent potential as tracers of magma evolution because their magmas are often out of u series isotope equilibrium and some arc magmas have yielded surprisingly old pre eruption u series ages from mineral separates a model is presented to examine the behaviour of u th isotopes for a range of conditions between closed system and steady state magmatic systems magmas can be maintained out of secular equilibrium for long periods of time by periodic mixing events with new magma batches the latter results in oscillations in 230th 232th consistent with the relatively constant values observed in lavas of different ages from some centres the effects of periodic reinjection of magma and mixing of old and young crystals on u series mineral isochrons are also considered mixtures of crystals produced by either replenished or closed system processes can readily result in linear arrays in many cases it may be valid to interpret these as minimum average residence times for the crystals whereas the liquid residence times calculated from u series data for periodically replenished systems are maximum values

understanding crystal populations and their significance through the magma plumbing system crystals are rarely composed of a single crystal population that have grown solely from the batch of magma in which they are resident on emplacement either by eruption or shallow intrusion close investigation of the majority of crystal populations reveal that they comprise up to four main components phenocrysts crystals co genetic with their magmatic host xenocrysts crystals wholly or in part foreign to the magmatic host and magma system antecrysts crystals which are recycled one or several times before inclusion in the host magma but have an origin within the magmatic system and microlites which represent small co genetic crystals which nucleate and grow rapidly on decompression and eruption textural analysis techniques are employed to quantify key aspects of the crystal population including crystal shape crystal size distributions spatial distribution patterns and textural modification using dihedral angles santorini provides a case study of an active volcanic system where a combined textural analysis study has been developed highlighting how the crystal population is being continuously modified by a series of replenishment and mixing events developing textural and microgeochemical techniques provides the next stage in the interrogation of crystal populations linking textures to isotopic heterogeneities and providing fingerprints of where crystals are sourced and re cycled

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

carboniferous granitic plutons from the northern margin of the north china block implications for a late palaeozoic active continental margin we report four late palaeozoic zircon sensitive high resolution ion microprobe shrimp u pb ages for granitic plutons from the inner mongolia palaeo uplift on the northern margin of the north china block these cast a new light on the poorly understood tectonic history of the northern margin of the north china block and the central asian orogenic belt during the late palaeozoic the plutons have for a long time been considered to belong to the early precambrian basement of the north china block our new shrimp u pb zircon dating of four plutons at longhua daguangding boluonuo and hushiha has yielded intrusive ages of 311 2 ma 324 6 ma 302 4 ma and 310 5 ma respectively geochemical data suggest that these granitoids have a calc alkaline subduction related i type signature indicating the existence of an andean style continental arc along the northern margin of the north china block during the late palaeozoic our results also indicate that the palaeo asian ocean still existed during latest carboniferous earliest permian time and that the final collision between the southern mongolia composite terranes and the north china block occurred later than c 290 ma we suggest that the northern margin of the north china block was an active continental margin and the inner mongolia palaeo uplift is a deeply exhumed mid crustal root of a late palaeozoic andean style continental arc

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

uth fractionation by fluids in k rich magma genesis the vico volcano central italy abstract the 230th 238u disequilibrium method has been applied to the potassic volcanic series of vico latium central italy the 3 main events of the recent volcanic activity have been dated using whole rock isochrons the lava flows of the main cone were erupted between 260 and 180 ky pyroclastic flows are dated around 90 ky and the final effusive activity mte venere around 60 ky these results are in good agreement with previous k ar ages the intense th u fractionation th u varying from 3 2 to 11 is apparently not related to processes of partial melting and crystal fractionation the th u fractionation is the most intense in basic terms of the first episode and is due to the percolation of a deep hydrothermal fluid which impoverishes u in different batches of primary magmas more limited th u fractionations in highly differentiated magma are due to u accompanied by sb and alkalies enrichments in the shallow magma chambers a time integrated model is proposed for the evolution of the 230th 232th ratio in the mantle source of vico it shows that th u and th ta fractionations are probably related to the bulk k u and th enrichment of the mantle of the roman province around 300 ky ago a metasomatic enrichment by a silicate melt is proposed to explain the anomalously high th u and th ta fractionations

geodynamic settings and tectonic model of skarn gold deposits in china an overview seventy skarn type gold deposits including 1 super large 19 large and 24 medium sized are known from different geotectonic units of china they contain a total resource of approximately 1000 t of gold 625 t in south china and account for 20 of china s gold reserves these skarn deposits are sited in collisional orogenic belts fault controlled magmatic belts and reactivated cratonic margins all of the chinese skarn gold provinces were affected by phanerozoic collisional orogenesis the timing of the metallogenic events and the spatial temporal distribution of the chinese skarn gold deposits indicates that they were formed during ore forming processes linked to the transition from shortening to extension in the geodynamic evolution of a collision orogen and not to subduction systems as is commonly advocated for porphyry copper systems around the pacific rim

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

geodynamics and metallogeny of the central eurasian porphyry and related epithermal mineral systems a review abstract major porphyry cu au and cu mo deposits are distributed across almost 5000 km across central eurasia from the urals mountains in russia in the west to inner mongolia in north eastern china these deposits were formed during multiple magmatic episodes from the ordovician to the jurassic they are associated with magmatic arcs within the extensive subduction accretion complex of the altaid and transbaikal mongolian orogenic collages that developed from the late neoproterozoic through the palaeozoic to the jurassic intracratonic extension the arcs formed predominantly on the palaeo tethys ocean margin of the proto asian continent but also within two back arc basins the development of the collages commenced when slivers of an older proterozoic subduction complex were rifted from an existing cratonic mass and accreted to the palaeo tethys ocean margin of the combined eastern europe and siberian cratons subduction of the palaeo tethys ocean beneath the karakum and altai tarim microcontinents and the associated back arc basin produced the overlapping late neoproterozoic to early palaeozoic tuva mongol and kipchak magmatic arcs contemporaneous intra oceanic subduction within the back arc basin from the late ordovician produced the parallel urals zharma magmatic arc and separated the main khanty mansi back arc basin from the inboard sakmara marginal sea by the late devonian the tuva mongol and kipchak arcs had amalgamated to form the kazakh mongol arc by the mid palaeozoic the two principal cratonic elements the siberian and eastern european cratons had begun to rotate relative to each other drawing in the two sets of parallel arcs to form the kazakh orocline between the two cratons during the late devonian to early carboniferous the palaeo pacific ocean began subducting below the siberian craton to form the sayan transbaikal arc which expanded by the permian to become the selanga gobi khanka arc by the middle to late permian as the kazakh orocline continued to develop both the sakmara and khanty mansi back arc basins were closed and the collage of cratons and arcs were sutured by accretionary complexes during the permian and triassic the north china craton approached and docked with the continent closing the mongol okhotsk sea an embayment on the palaeo pacific margin to form the mongolian orocline subduction and arc building activity on the palaeo pacific ocean margin continued to the mid mesozoic as the indosinian and yanshanian orogens significant porphyry cu au mo and au cu deposits were formed during the ordovician in the kipchak arc e g bozshakol cu au in kazakhstan and taldy bulak porphyry cu au in kyrgyzstan silurian to devonian in the kazakh mongol arc e g nurkazgan cu au in kazakhstan and taldy bulak levoberezhny au in kyrgyzstan devonian in the urals zharma arc e g yubileinoe au cu in russia devonian in the kazakh mongol arc e g oyu tolgoi cu au and tsagaan suvarga cu au in mongolia carboniferous in the kazakh mongol arc e g kharmagtai au cu in mongolia tuwu yandong cu au in xinjiang china koksai cu au kounrad cu au and the aktogai group of cu au deposits in kazakhstan carboniferous in the valerianov beltau kurama arc e g kal makyr dalnee cu au in uzbekistan benqala cu au in kazakhstan late carboniferous to permian in the selanga gobi khanka arc e g duobaoshan cu au in inner mongolia china triassic in the selanga gobi khanka arc and jurassic in the selanga gobi khanka arc e g wunugetushan cu mo and jiguanshan mo in inner mongolia china in addition to the tectonic geologic and metallogenic setting and distribution of porphyry cu au mo mineralisation within central eurasia the setting geology alteration and mineralisation at each of the deposits listed above is described and summarised in table 1

paleozoic northward drift of the north tien shan central asia as revealed by ordovician and carboniferous paleomagnetism abstract three new middle late ordovician and two new early carboniferous paleomagnetic poles have been obtained from the north tien shan zone ntz of the ural mongol belt in kyrgyzstan and kazakhstan paleolatitudes for the carboniferous are unambiguously northerly and average 15 5 n whereas the ordovician paleolatitudes 6 9 and 9 are inferred to be southerly given that a very large 180 rotation of the ntz would be necessary during the middle paleozoic if the other polarity option was chosen thus the ntz drifted northward during much of the paleozoic east west drift cannot be determined as is well known from paleomagnetic data in addition detailed thermal demagnetization analysis reveals two overprints one of recent age and the other of permian age which is a time of strong deformation in the ntz the paleolatitude of the combined permian overprint is 30 5 2 n the paleolatitudes collectively track those predicted for the area by extrapolation from baltica very well but are different from those of siberia for ordovician times this finding is compatible with sengor and natal in s sengor a m c natal in b a 1996 paleotectonics of asia fragments of a synthesis in yin a harrison m eds the tectonic evolution of asia cambridge univ press cambridge pp 486 640 model of tectonic evolution of the ural mongol belt and disagrees with the models of other researchers declinations of the ordovician and early carboniferous results range from northwesterly to northeasterly and are clearly affected by local relative rotations which seem characteristic for the entire ntz because the permian overprint declinations also show such a spread apparently the important latest paleozoic triassic deformation involved shear zone related rotations as well as folding and significant granitic intrusions

redistribution of u and th in shallow plutonic environments radioactive heat production in the idaho batholith and in the alamosa river stock in the san juan mountains correlates with the degree of hydrothermal alteration of plutons by meteoric groundwater fission track mapping shows that as much as half of the u in whole rock samples of the most intensively altered zone of the alamosa river stock is associated with secondary minerals the hydrothermally altered shallow plutons in the idaho batholith contain significant amounts of u in secondary minerals whereas unaltered deeply emplaced plutons contain virtually all of their u in primary accessory minerals the relative distributions of u and th measured by gamma ray spectrometry the associations of u observed by fission track mapping and a depth dependence in thermally induced hydraulic fracturing suggest a depth dependent geochemical process that could produce an upward increase in the abundance of u and th in batholiths the process adds u and th to chemically receptive secondary minerals during alteration of plutons by convecting groundwater

u pb 39ar 40ar geochronology of the metamorphosed volcanic rocks of the bainaimiao group in central inner mongolia and its implications for ore genesis and geodynamic setting abstract the greenschist facies volcano sedimentary sequence of the bainaimiao group in central inner mongolia hosts the bainaimiao cu mo au deposit and the bainaimiao lode gold deposit but the ore classification and genetic process of the bainaimiao cu mo au deposit is still disputed in this study we have selected the interbedded metamorphosed volcanic rocks of the bainaimiao group for la icp ms zircon u pb analysis 17 analyses form a tight cluster on the concordia and yield a weighted mean 206 pb 238 u age of 465 1 ma 95 confidence level mswd 0 95 n 17 which represents the best estimate of the crystallization age of the volcanic rocks of the bainaimiao group biotite from the greenschist of the bainaimiao group yielded a well defined plateau with an age of 429 2 4 1 ma the inverse isochron age is 429 1 4 2 ma with an elevated 40 ar 36 ar i ratio of 299 9 22 7 which represents the time of regional greenschist facies metamorphism previous studies showed that the re os isochron age of the molybdenite from the bainaimiao cu mo au deposit is 445 0 3 4 ma and the mineralized granodiorite porphyry yielded a weighted mean 206 pb 238 u age of 445 6 ma these data indicate that the bainaimiao volcanic rocks were formed at about 465 ma and the porphyry cu mo au mineralization took place at about 445 ma volcanic rock emplacement is 20 ma older than the cu mo au mineralization indicates it is a porphyry deposit rather than vms deposit the dynamic setting for the porphyry mineralization is the paleo asian oceanic plate subduction along the northern margin of the north china craton in early paleozoic subsequently the collision between the bainaimiao arc and the north china craton led to regional metamorphism and deformation at about 430 ma and the old porphyry system may have been overprinted during this metamorphic process

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

magma cumulate mixing identified by u th disequilibrium dating magma mixing is believed to be important in magmagenesis1 particularly in volcanic arcs2 where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4 nevertheless isotope studies particularly of uranium series dis equilibria have concentrated on the analysis of bulk rock samples5 9 comprehensive u th mineral separate data from arc volcanics are scarce10 14 here we present the first u th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of santorini in the aegean arc the lava contains crystal populations from two sources of distinct thorium isotope composition one from a basic cumulate the other phenocrysts from a dacite magma the crystal populations have statistically indistinguishable crystallization ages of 79 14 12kyr and 93 29 22kyr and are thought to have physically intermingled just before eruption the crystals did not re equilibrate and hence retain the isotopic compositions and ages of the parental materials

some remarks on u th mineral ages from igneous rocks with prolonged crystallisation histories abstract mineral isochron dating is a frequently used geochronological tool one of its assumptions is that the minerals grow over a time period that is small compared to the half life of the radiogenic isotope system used in recent years increasing analytical precision has promoted the use of the short lived u series isotope system in order to date young crystallisation events three whole rock zircon u th isochrons from the 26 5 ka oruanui eruption in the taupo volcanic zone new zealand yield pre eruptive model ages of 5 5 0 8 ka 9 7 1 7 ka and 12 3 0 8 ka for the sub 63 m 63 125 m and 125 250 m zircon size fractions respectively this suggests that in this case the assumption of instantaneous crystal growth breaks down instead the u th data may be explained by continuous zircon growth over a period of 90 ka however cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector zoned euhedral rim and the u th data can also be modelled by mixing an older 27 ka model age population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the oruanui rhyolite this indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment it is concluded that conventional u series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude in future microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales

eruptive history and geochronology of mount mazama and the crater lake region oregon geologic mapping k ar and 40 ar 39 ar age determinations supplemented by paleomagnetic measurements and geochemical data are used to quantify the quaternary volcanic history of the crater lake region in order to define processes and conditions that led to voluminous explosive eruptions the cascade arc volcano known as mount mazama collapsed during its climactic eruption of 50 km 3 of mainly rhyodacitic magma 7700 yr ago to form crater lake caldera the mazama edifice was constructed on a pleistocene silicic lava field amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for mount mazama between 420 ka and 35 ka mazama produced medium k andesite and dacite in 2 1 proportion the edifice was built in many episodes some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region and some were possibly related to deglaciation following marine oxygen isotope stages mis 12 10 8 6 5 2 and 2 magmas as evolved as dacite erupted many times commonly associated with or following voluminous andesite effusion establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca 27 ka the silicic melt volume then grew incrementally at an average rate of 2 5 km 3 k y 1 for nearly 20 k y the climactic eruption exhausted the rhyodacitic magma and brought up crystal rich andesitic magma mafic cumulate mush and wall rock granodiorite postcaldera volcanism produced 4 km 3 of andesite during the first 200 500 yr after collapse followed at ca 4800 yr b p by 0 07 km 3 of rhyodacite the average eruption rate for all mazama products was 0 4 km 3 k y 1 but major edifice construction episodes had rates of 0 8 km 3 k y 1 the long term eruption rate for regional monogenetic and shield volcanoes was d 0 07 km 3 k y 1 but only 0 02 km 3 k y 1 when the two major shields are excluded plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath mount mazama is several times that which has been erupted the eruptive and intrusive history reflects competition between 1 crystallization driven by degassing and hydrothermal cooling and 2 thermal input from a regional magma flux focused at mazama before ca 30 ka relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal poor melt subsequently and perhaps not coincidentally during mis 2 a large volume of eruptible silicic magma accumulated in the climactic chamber probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of mazama

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods

eruptions of pavlof volcano and their possible modulation by ocean load and tectonic stresses all nine magmatic eruptions at pavlof volcano in six of the years from 1973 to 1984 have occurred between september 9 and november 20 volumes of erupted material range from 0 1 to 15 9 106 m3 dense rock equivalent at an average rate of 3 106 m3 yr 1 the volumes are estimated from eyewitness reports for two eruptions the others are estimated from a relationship that we derive between eruption volume and harmonic tremor duration and amplitude the volume of lava erupted is approximately time predictable a significant correlation exists between the eruptions and yearly nontidal variations in sea level and may result from ocean loading calculated volume changes beneath the volcano due to ocean loading are from 0 02 to 2 times eruption volumes and we postulate that the volcano acts as a long period several month volume strain meter with lava being preferentially erupted when strain beneath the volcano is compressive the volcano did not erupt during the period 1978 1980 when tilt seismic data and sea level data indicate that deep aseismic slip may have occurred models of this event predict a volume increase beneath the volcano that might have compensated strain from magma injection these observations indicate that pavlof volcano may be responsive to small slow changes in ambient stresses or strains and that these changes may modify or trigger eruptions

interaction of arc magmas with subvolcanic hydrothermal systems insights from compositions and metasomatic textures of olivine crystals in fresh basalts of daisen and mengameyama western honshu japan