calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

southern ocean bottom water of the australian new zealand sector the bottom water characteristics of the southeast indian ocean and southwest pacific ocean reveal northward transfer of cold water across the mid ocean ridge at the discordance zone near 120 e and at the balleny fracture zone near 155 e a principal northward flow which transfers cold continental margin waters of antarctic origin to lower latitudes within the south indian basin occurs to the east of kerguelen plateau within the south indian basin near the 0 c isotherm the s o2 and si relations display significant slope changes the more variable water below 0 c is of local origin while the water above 0 c represents larger scale regional effects

epidote in calcalkaline magmas an experimental study of stability phase relationships and the role of epidote in magmatic evolution experiments on tonalite and granodiorite were performed at conditions ranging from 2 1 to 18 kbar and 550 to 850 c to establish the magmatic stability field of epidote as a function of p t and f02 at water saturated conditions and f02 buffered by nno epidote has a wide magmatic stability field in tonalite at 13 kbar this field extends from the wet solidus at 630 to 790 0c the low pressure intersection of the magmatic epidote crystallization reaction and the h20 saturated tonalite solidus and hence the minimum pressure for magmatic epidote occurs at about 5 kbar the clapeyron slopes of epidote melting reactions are moderately positive in p t space at pressures below the intersection of the epidote melting and the gamet in reactions at 13 kbar 790 c at this intersection epidote reaches its maximal thermal stability 790 c in tonalite h20 in the presence of garnet that is above 14 kbar epidote melting reactions have steep negative clapeyron slopes in p t space at p t conditions near the low pressure intersection of the epidote melting reaction with the water saturated solidus experiments were also performed with f02 buffered by hm at these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar in granodiorite the stability of epidote has been investigated at pressures from 7 5 to 18 kbar with respect to the tonalite the epidote out reaction is located almost at the same p t conditions at 10 kbar but the maximum thermal stability is about 50 c lower toward higher pressures although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions modal abundances of epidote are low 05 vol in the presence of major 40 vol melt fractions thus epidote in calc alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization

a transitional alkalic dolerite dike suite of mesozoic age in southeastern new england dike rocks from the new england platform of rhode island and adjacent massachusetts consist of premetamorphic and post metamorphic suites the older group includes metamorphosed dolerite minette and schistose dioritic rocks post metamorphic dikes consist of dolerite and sparse monchiquite the post metamorphic dolerites are of comparable age to the eastern north american dolerite suite associated with the mesozoic basins along the eastern seaboard of north america however the southeastern new england dolerites exhibit mineralogy and chemistry more typical of a transitional alkalic suite compared to the more subalkalic tholeiitic dolerites of the eastern north american suite both suites are compatible with a rift tectonic setting but the more alkalic dolerites may represent a deeper source of small volume melts compared to the eastern north american dolerites these more alkaline melts may have concentrated at local centers or they may be typical of flank dolerites as opposed to the less alkalic varieties that occur within the central axial rift

chronology and dynamics of a large silicic magmatic system central taupo volcanic zone new zealand the central taupo volcanic zone in new zealand is a region of intense quaternary silicic volcanism accompanying rapid extension of continental crust at least 34 caldera forming ignimbrite eruptions have produced a complex sequence of relatively short lived nested and or overlapping volcanic centers over 1 6 m y silicic volcanism at taupo is similar to the yellowstone system in size longevity thermal flux and magma output rate however taupo contrasts with yellowstone in the exceptionally high frequency but small size of caldera forming eruptions this contrast reflects the thin rifted nature of the crust which precludes the development of long term magmatic cycles at taupo 11 refs 4 figs 1 tab

magma extraction pressures and the architecture of volcanic plumbing systems abstract the deposits of large volcanic eruptions provide the sole record of the architecture of magmatic plumbing systems in the moments when large pools of crystal poor eruptible magma are present in the crust it is widely accepted that silicic magmas form by segregation of melt rich crystal poor magma from a crystal rich source however the depths at which segregation takes place and the distribution of the magma within the crust are not well constrained we present a new approach to calculate pressures at which crystal poor eruptible magma is extracted from a crystal rich source i e mush we apply the approach to a sequence of large 50 km3 eruptions from the taupo volcanic zone tvz new zealand which were part of a volcanic flare up we compare the calculated extraction pressures with pre eruptive storage pressures for the same units our results show that storage and extraction pressures do not always coincide instead eruptible magma can be completely segregated from the crystal rich source and stored at shallower levels in the crust prior to eruption in the case of the tvz flare up repeated input of material and heat probably coupled with tectonic extension gradually conditioned the crust and allowed extraction of eruptible magma over a growing range of pressures with time our approach has the potential to reveal important information on the structure and distribution of magmatic systems within the shallow crust

the circulation south of australia derived from an inverse model an inverse model is used to determine circulation and volume transports in the australian sector of the southern indian ocean from observations during woce water movement in the upper 1000 m is westward along the continental slope and eastward in the subantarctic front saf the subtropical front is not associated with continuous eastward flow however such flow appears to build up between 120 e and 132 e where it dominates the region between 37 s and 42 s antarctic intermediate water is found to move from the pacific into the indian ocean except in the region of the saf where it moves from the indian to the pacific ocean circumpolar deep water exhibits weak flow which can only be reliably resolved in the upper part where it is predominantly westward antarctic bottom water enters the australian basin through two depressions in the australian antarctic discordance at 125 37 e and at 126 30 e

pre eruptive crystallization conditions of mafic and silicic magmas at the plat pays volcanic complex dominica lesser antilles andesitic to dacitic magmas of the plat pays volcanic complex dominica lesser antilles that erupted during the last not vert similar100 ka equilibrated at temperatures of 800 880 c and total pressures of 1 1 2 3 kbar magmatic conditions were moderately oxidizing fo2 not vert similar1 5 0 3 log units above fmq and estimated melt water contents vary from 5 2 to 6 5 wt the crystal rich nature of the samples the relatively narrow bulk rock compositional range and the similarity of the intensive crystallization parameters throughout the sample suite are consistent with the presence of a long lived highly crystallized magma reservoir further crystallization during ascent produced highly evolved rhyolitic glass compositions the silicic magma reservoir was occasionally invaded by more mafic melts of basaltic andesitic to basaltic composition these were rarely erupted but they mixed with the silicic magmas and were frequently incorporated as mafic enclaves the mafic magmas record temperatures of not vert similar1060 c at fo2 1 6 2 0 log units above fmq the similarities in mineralogy petrology and inferred magma storage conditions of the plat pays volcanic complex with the soufriere hills volcano montserrat and montagne pelee martinique can serve as a useful guide for forecasting and evaluation of possible future eruptions

construction of a pluton evidence from an exposed cross section of the searchlight pluton eldorado mountains nevada a top to bottom cross section of the searchlight pluton is exposed in a large tilt block in the colorado river extensional corridor of southern nevada hornblende barometry and geologic relations indicate that the pluton was about 10 km thick extending from approximately 3 to 13 km depth the pluton is stratified with about 2 km of fine grained quartz monzonite below the roof 2 km of granite in the center and 6 km of coarser more mafic quartz monzonite at the bottom the lower unit has a pronounced magmatic foliation that was subhorizontal prior to tilting contacts between the units are gradational over a few centimeters to about 20 m geometry field relations and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber the upper unit formed as a solidification front that migrated downward from the roof and the middle unit granite and the lower unit represent complementary segregated melt and crystal rich cumulate thus the dominant part of the pluton appears to have solidified in monotonic fashion from more or less uniform magma that was compositionally similar to the upper unit the basal quartz monzonite and the lower part of the granite enclose widely scattered synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale these rocks are commonly fine grained and formed as quenched mafic melts they are isotopically distinct from the main sequence and represent discrete injections into the magma chamber the mafic magmas were contaminated by but did not strongly affect the main sequence magma the nd and sr isotopic compositions of the main sequence magma are intermediate between those of regional crust and enriched mantle derived basalts like those represented by the mafic pods this suggests that the principal magma was a hybrid with about 60 mantle component and thus the pluton represents both addition to and internal reorganization of the crust

evidence of early cretaceous transpression in the sulu orogenic belt eastern china abstract recent studies have documented marine turbidites with syn sedimentary deformation features in the central sulu orogenic belt of eastern china these units preserve essential information on the late mesozoic evolution of the sulu orogenic belt referred to as the baxiandun formation the turbidites exhibit similar lithologic characteristics to nearby units such as the lingshandao formation that have been well studied even though precise geochronologic constraints are lacking for a more precise correlation this study reports detrital zircon u pb age data that correlate the baxiandun formation turbidites of the central sulu orogenic belt to the early cretaceous lingshandao formation we also report al in hornblende emplacement depth estimates for granitic intrusions of the sulu orogenic belt s laoshan mountain a sharp contact between the laoshan granites and the marine baxiandun formation indicates that the baxiandun basin rapidly subsided to the emplacement depth of the laoshan plutons lateral correlation among the marine turbidites the lingshandao and baxiandun formations combined with information established by previous studies indicates initiation of transpressional tectonics at 122 121 ma transpression ceased with the emplacement of the laoshan granites whose a1 type composition indicates a return to extensional tectonics at ca 111 ma

differentiation conditions of a basaltic magma from santorini and its bearing on the production of andesite in arc settings santorini volcano in the aegean region greece is characterized by andesitic to silicic dominated explosive activity and caldera forming eruptions sourced from magmatic reservoirs located at various structural levels beneath the volcano there is a good understanding of the silica rich magmatism of the island whereas the andesite dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood to fill this gap we have performed crystallization experiments on a representative basalt from santorini with the aim of determining the conditions of differentiation pressure temperature volatile fugacities and the parental magma relationship with the andesitic eruptive rocks experiments were carried out between 975 and 1040 c in the pressure range 100 400 mpa fo2 from qfm to nno 3 5 where qfm is quartz fayalite magnetite and nno is nickel nickel oxide with h2omelt contents varying from saturation to nominally dry conditions the results show that basalt phenocrysts within the basalt crystallized at around 1040 c in a magma storage reservoir located at a depth equivalent to 200 400 mpa pressure with 3 5 wt dissolved h2o and fo2 around qfm comparison with the xenocryst and phenocryst assemblages of the upper scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 c and 400 mpa following 60 80 wt crystallization of an ol cpx plag ti mag opx 6 pig ilm assemblage with melt water contents around 4 6 wt at santorini the andesitic low viscosity and water rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs eventually erupting upon mixing with resident magma changes in prevailing oxygen fugacity may control the tholeiitic calc alkaline character of santorini magmas explaining the compositional and mineralogical differences observed between the recent thyra and old eruptive products from akrotiri

late proterozoic and devonian plutonic terrane within the avalon zone of rhode island the u th pb radiometric age of zircons demonstrates that much of rhode island consists of a late proterozoic plutonic complex that subsequently was intruded by a large devonian alkalic to subalkalic igneous complex two groups of late proterozoic rocks can be recognized 1 the esmond granite and related plutonic rocks in northern rhode island ranging in composition from gabbro to granite and yielding an upper concordia intercept of 621 8 m y and 2 quartz rich recrystallized gneissic rocks ten rod granite gneiss and hope valley alaskite gneiss in southern rhode island giving an upper concordia intercept age of 601 5 m y the lower intercept of the zircon discordia for the gneissic rocks indicates the isotopic systems were disturbed during the late paleozoic alleghanian orogeny whereas zircons from the esmond granite and related plutonic rocks have been subjected mainly to loss of lead attributable to recent dilatancy rocks determined to be of devonian age include the scituate granite and plutonic rocks of the east greenwich plutonic suite the quincy granite of quinn 1971 in northeastern rhode island may be of a similar devonian age or it may be slightly younger zircon data from these rocks fall upon a chord that gives an upper concordia intercept of 370 7 m y and a lower intercept close to the origin collectively these rocks form a composite pluton with an area 700 km 2 the devonian rocks exhibit petrologic characteristics distinct from slightly older acadian orogenic rocks that occur in lithostratigraphic zones to the west and are more comparable to anorogenic ordovician to devonian alkalic granitoids that trend north northeast across massachusetts and the gulf of maine the new age determinations confirm that much of rhode island contains plutonic rocks intruded during the avalonian orogeny but that a hertofore unrecognized major devonian plutonic episode also occurred the distinctly different ages probably had gone unrecognized because of generally similar lithologic and textural features exhibited by the late proterozoic and devonian rocks we present new petrologic and geochemical criteria that should assist in distinguishing the rocks of different ages as additional mapping and geologic study proceeds

age determination of mixed water masses using cfc and oxygen data we present a new method based on a combination of optimum multiparameter analysis and cfc oxygen mixing analysis to determine the ages of water masses in regions of mixing it enables us to follow water mass movements in greater detail than with other methods which give only the combined pseudoage of a water mass mixture we define the age of a water mass as the time a water parcel needs to spread from its source region where it received its individual tracer characteristics to the point of observation the age distribution allows us to determine pathways of water masses which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes we apply the method to hydrographic data along world ocean circulation experiment section i5 in the south east indian ocean in the thermocline indian central water icw and subantarctic mode water samw meet and mix these distinct water masses have different formation mechanisms but similar temperature salinity characteristics it is shown that the convective formation of samw is a major ventilation mechanism for the lower indian thermocline in the eastern part of the south indian ocean samw dominates the oceanic thermocline and is found to be about 5 years old pure icw is present only in the thermocline of the region 48 degrees 55 degrees e with increasing age with depth confirming the subduction theory while most samw joins the equatorward gyre movement of the southeastern indian ocean some of it propagates westward through turbulent diffusive mixing reaching 55 degrees e after 15 20 years it takes icw some 25 30 years to reach 110 degrees e

evolution of the caldera forming eruption at crater lake oregon indicated by component analysis of lithic fragments crater lake caldera 8 10 km formed 6845 years b p 14c age during the climactic eruption of the volcanic edifice known as mount mazama is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of mount mazama initial activity was from a single vent that produced a rhyodacite pumice fall from a plinian column altered preexisting volcanic rocks are the predominant lithic type in the plinian deposit and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin the wineglass welded tuff lies atop the plinian deposit and contains a higher proportion of fresh volcanic rocks suggesting enlargement of the single vent by slumping of its walls this same vent enlargement caused the plinian eruption column to collapse and feed valley hugging pyroclastic flows that deposited the wineglass welded tuff when enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported mount mazama collapsed to form the caldera while highly energetic pyroclastic flows produced the climactic ignimbrite a lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the wineglass welded tuff and contains predominantly altered volcanic rocks of deeper origin accompanied by minor granitoids from the magma chamber walls azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence such as crater lake nevertheless may form by syneruptive subsidence along ring fractures

a northern boundary current along australia s southern shelves the flinders current 1 south of australia the monthly mean wind stress curl is positive during both summer and winter and leads to ekman pumping and downwelling throughout the region sverdrup dynamics indicates that this downwelling should lead to a northward transport of around 5 10 sv 1 sv 10 m3 s 1 classical arguments for western boundary currents are adapted to show that this transport should be deflected into an upwelling favorable boundary current that flows from east to west along australia s southern shelves the flinders current support for this proposition is obtained from results of the ocean circulation and climate advanced modelling project occam sverdrup transports and limited observations for the region in addition the occam results show that the northward transport leads to upwelling at depths below 400 m and that the flinders current 1 intensifies from victoria to western australia transport 8 sv with speeds up to 15 cm s 1 2 can extend from the surface to depths of 800 m and 3 is found during both summer and winter during winter the winds are downwelling favorable and lead to a coastal current that flows from west to east and opposes the flinders current further support for the origin and nature of the flinders current is obtained from simple numerical experiments made using a rectangular domain and idealized representations of the summer and winter wind stress

crystallisation ages in coeval silicic magma bodies 238u 230th disequilibrium evidence from the rotoiti and earthquake flat eruption deposits taupo volcanic zone new zealand abstract the timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent successive eruption deposits in the taupo volcanic zone new zealand the earlier caldera forming rotoiti eruption 100 km3 magma at okataina volcano was followed within months at most by the earthquake flat eruption 10 km3 magma from nearby kapenga volcano both generated non welded ignimbrite and coeval widespread fall deposits the rotoiti and earthquake flat deposits are both crystal rich high silica rhyolites with sparse glass bearing granitoid fragments also occurring in rotoiti lag breccias generated during caldera collapse here we report 238u 230th disequilibrium data on whole rocks and mineral separates from representative rotoiti and earthquake flat pumices and the co eruptive rotoiti granitoid fragments using tims and in situ zircon analyses by sims multiple grain zircon controlled crystallisation ages measured by tims from the rotoiti pumice range from 69 3 ka 350 ka with a pronounced peak at 70 90 ka the weighted mean of isochrons is 83 14 ka in accord with the tims data one glass bearing rotoiti granitoid clast yielded an age of 57 8 ka by tims controlled by th rich phases that however are not apparently present in the juvenile pumices another glass bearing rotoiti granitoid yielded sims zircon model ages peaking at 60 90 ka having a similar age distribution to the pumice age data from pumices are consistent with a published 64 4 ka eruptive age now modified to 62 2 ka but chemical and or mineralogical data imply that the granitoid lithics are not largely crystalline rotoiti rhyolite but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies similarly although the earthquake flat eruption immediately followed and probably was triggered by the rotoiti event age data from juvenile material are significantly different a multiple grain zircon controlled crystallisation age measured by tims from a representative pumice is 173 5 ka while sims model ages range from 70 26 34 ka to 350 ka with a peak at 105 ka these age data coupled with previously published geochemical and isotopic data show that the rotoiti and earthquake flat deposits were erupted from independent unconnected magma bodies

structural control of volcanism and caldera development in the transtensional taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand comprises several segments with variable components of extension and is characterised by longitudinal segmentation into a central zone dominated by rhyolitic calderas and extremities with only andesitic composite volcanoes modern

hornblende cummingtonite and biotite hydrogen isotopes direct evidence of slab derived fluid flux in silicic magmas of the taupo volcanic zone new zealand abstract hornblende cummingtonite and biotite d values from rhyolitic and mingled mixed mafic rocks from the central taupo volcanic zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone hand samples were obtained from pyroclastic flow airfall and lava flow deposits as well as co eruptive cognate plutonic blocks mineral d values range from 88 to 35 smow standard mean ocean water cummingtonite phenocryst d overlaps with hornblende from the same rocks while biotite values are consistently 7 more negative than hornblende phenocrysts from the same hand sample calculated volcanic steam d values ca 46 to 25 are consistent with an island arc crustal source ca 40 to 15 with a negligible to absent mantle ca 100 to 60 component earlier researchers suggested that the taupo volcanic zone fluid flux included dilute concentrations of slab derived phases i e co 2 cl similar to back arc volcanic systems at other convergent margins however the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyroclastic deposits and volcanic fumaroles from the kamchatka and japanese arcs the silicic rocks from the 2 0 1 0 ma mangakino caldera the most inland caldera system we studied have the lowest d hornblende and biotite suggesting that silicic volcanism distal to the subduction trench was derived from a back arc like i e mixture of mantle and slab fluid however we found that the currently active more proximal locus of silicic volcanism reflects typical arc like fluid compositions our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters ca 45 to 23

plutonic lithics in ignimbrites of taupo volcanic zone new zealand sources and conditions of crystallisation abstract plutonic rock fragments occur as rare accessory lithics within some of the ignimbrites erupted from calderas and caldera complexes in central taupo volcanic zone tvz new zealand the two main suites of plutonic lithics are 1 medium to fine grained granitoids and 2 fine grained dolerites and microdiorites the granitoids are mostly medium grained rocks with both equigranular and granophyric intergrowth textures the latter indicating rapid crystallisation at shallow crustal levels biotite is the predominant mafic phase in most granitoid samples although rare hornblende and orthopyroxene bearing granitoids also occur glass occurs in some granitoid samples from rotoiti ignimbrite indicating that these samples were incompletely crystallised prior to eruption dolerite and microdiorite fragments typically contain abundant hornblende and up to 10 vesicular interstitial glass sio2 abundances in the granitoids range between 70 3 and 77 9 they are weakly metaluminous to weakly peraluminous and show an enrichment in large ion lithophile lil elements rb k ba and a depletion in nb relative to morb similar to recent tvz rhyolitic volcanic rocks chemically and petrographically the granitoids fall into two distinct groups high sr granodiorites 120 205 ppm sr and chemically more evolved low sr monzogranites 26 65 ppm sr with an apparent compositional gap between the two eu eu anomalies and strong correlations between al2o3 cao and sr indicate that compositional variation between the two granitoid groups is dominantly controlled by plagioclase fractionation the high and low sr groups coincide with the highest and lowest sr concentrations observed in tvz rhyolites ree data and sr nd pb and o isotope ratios for granitoids are comparable to published data for tvz rhyolites and consequently they are considered to represent crystallised portions of zoned silicic magma chambers associated with quaternary tvz volcanic activity and not fragments of older pre tvz basement most granitoids are regarded as comagmatic with their host ignimbrites but those from rotoiti ignimbrite have different mineral assemblages and whole rock chemistry to the host ignimbrite these fragments appear to be incompletely crystallised portions of an earlier magma system possibly the same system that generated the matahina ignimbrite dolerites and microdiorites are geochemically similar to evolved high alumina basalts erupted from the central tvz and are considered to be intrusive equivalents

radiocarbon evidence for mid late holocene changes in southwest pacific ocean circulation variability in the southwest sw pacific ocean circulation is influenced by the changes in the south pacific subtropical gyre and its western boundary current the east australian current eac the eac plays a significant role in transporting warm well ventilated nutrient poor waters to more temperate higher latitudes recent climate changes associated with eac intensification have led to anomalous warming in the south tasman with implications for marine ecosystems and environment a clear understanding of the significance of these changes requires knowledge of past natural variability here we have reconstructed a 4500 year record of regional sea surface radiocarbon reservoir ages r and local reservoir effects r our results reveal the centennial scale variability over the last 4500 years with r ranges as large as 390 14c yr older r 410 14c yr between 1610 to 1860 a d in our record corresponding to the little ice age suggests a weaker influence of the eac in the south tasman between 4000 and 1900 cal years b p r and r were significantly younger than the modern with values of 170 and 130 14c yr respectively indicating increased eac transport of tropical waters into the south tasman we propose that the large r variability was influenced by strong and abrupt el nino events which punctuated the muted el nino southern oscillation enso period in the mid late holocene and enabled increased westward flow of gyre waters into the sw pacific the strengthening of the eac extension appears to have been a response to the precession modulated enso southern annular mode interactions

40ar 39ar ages from the narragansett basin and southern rhode island basement terrane their bearing on the extent and timing of alleghenian tectonothermal events in new england pennsylvanian sedimentary rocks record the effects of progressive barrovian metamorphism and polydeformation in the narragansett basin of rhode island whole rock phyllite samples from low grade portions of this terrane display internally concordant 40 ar 39 ar age spectra with plateau ages of 248 to 258 m y four samples biotite from higher grade rocks records plateau ages of 240 to 248 m y eight samples similar 40 ar 39 ar biotite ages are displayed westward across the southern rhode island basement terrane 239 to 250 m y nine samples although coexisting hornblende records slightly older plateau ages of 255 m y two samples the 40 ar 39 ar plateau ages are interpreted to date times of post metamorphic cooling through temperatures required for argon retention in the various mineral phases examined the ages suggest that a distinct late paleozoic alleghenian thermal event affected much of southern rhode island together with the depositional ages of the variably metamorphosed pennsylvanian rocks westphalian b stephanian a or b the 40 ar 39 ar data permit calibration of the sequence of alleghenian tectonothermal events recorded in the narragansett basin the controls suggest a permian event lasting about 40 to 50 m y between initial barrovian regional metamorphism and synchronous d 1 d 2 ductile deformation between 295 and 275 m y and post d 3 cooling below argon retention temperatures between 250 and 240 m y

mineralogy geochemistry and petrology of neoarchean ferroan to magnesian granites of carajas province amazonian craton the origin of hydrated granites associated with charnockites abstract 2 75 to 2 73 ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the carajas province the vila jussara suite was recognized recently and its geologic mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the estrela complex serra do rabo and igarape gelado granites and planalto suite monzogranites and syenogranites are dominant in most units but in the igarape gelado granite and vila jussara suite granodiorites and tonalites are also relatively abundant the main mafic phases in all these granitoids are amphibole potassian hastingsite with subordinate magnesiohastingsite 0 95 fe fe mg 0 47 and biotite 0 88 fe fe mg 0 52 with rare occurrence of clinopyroxene relict crystals magmatic epidote of pistacite component from 26 to 31 occurs only in the vila jussara suite the rocks of estrela serra do rabo and planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced ferroan granites whole rock feot feot mgo 0 89 that evolved at low f o 2 fmq 0 5 the igarape gelado granite needs additional studies but it is formed apparently by reduced ferroan and oxidized ferroan granites the vila jussara suite is also composed of reduced ferroan granites that are similar to those of the other granite units however it has additionally oxidized ferroan and magnesian granitoids which contain magnetite ilmenite as the main fe ti oxide phase and evolved respectively at moderate f o 2 nno 0 5 and at comparatively higher f o 2 nno to nno 1 the magmas of these granites were formed at temperatures 900 c and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc alkaline in composition plagioclase orthopyroxene clinopyroxene and fe ti oxides were the main phases in the melt residues and the liquids were enriched in ba zr y and ree except for eu but not in sr compared to their sources the magmas should have relatively high water content 4 wt the magmatic evolution was strongly influenced by partial melting processes the different varieties of the vila jussara suite are not linked by fractional crystallization and derived from distinct magmas the neoarchean granites of the carajas province were emplaced in a collisional tectonic setting they are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling the magmas were generated in the crust at pressures of 800 to 1000 mpa and emplaced at pressures of 200 to 500 mpa the studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites the magnesian variety of the vila jussara suite is similar to the pyroxene free granitoids of the neoarchean matok pluton of the collisional limpopo belt the neoarchean granites of carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the matok pluton

reconstructing the geological and structural history of an active geothermal field a case study from new zealand abstract the utilisation of geothermal systems benefits from an understanding of the host rock geology locations and controls of permeability pathways and the nature and timing of magmatic sources providing thermal energy kawerau geothermal field in the central taupo volcanic zone tvz of new zealand is currently developed for electricity generation and direct uses of high temperature steam to 200 mw electrical output the kawerau geothermal system is hosted in a sequence of volcanic lithologies tuffs lavas and intrusive bodies and sediments that overlie faulted mesozoic metasedimentary greywacke basement identification of lithologies in the volcanic sedimentary sequence is challenging due to the levels of hydrothermal alteration and lithological similarities a combination of detailed petrological investigations consideration of the emplacement processes and greater certainty of crystallisation or eruption ages through u pb age determinations on zircons is used to reconstruct the depositional and faulting evolution of the rocks hosting the currently active hydrothermal system the oldest event inferred is faulting of the greywacke along northwest southeast orientated dominantly strike slip structures to generate half grabens that were filled with sediments incorporating two dated ignimbrites 2 38 0 05 and 2 17 0 05 ma a 1 46 0 01 ma ignimbrite was deposited relatively evenly across the field implying that any topographic relief was subdued at that time subsequent deposition of ignimbrites occurred in episodes around 1 0 0 55 0 6 and 0 32 ma interspersed with thin sedimentary sequences that accumulated at average rates of 0 06 mm yr 1 andesite lavas from a buried composite cone occur as a conformable package between units dated at 1 0 and 0 6 ma bodies of coherent rhyolite occur at multiple stratigraphic levels two magma types with associated tuffs were emplaced as domes and sills at 0 36 0 03 ma and a third type at 0 138 0 007 ma as dikes and domes that are exposed at surface the andesitic putauaki composite cone southwest of the field first erupted around 8 ka but earlier hydrothermal eruption breccias imply that magma was intruded to shallow depths as early as 16 ka age data and associated correlations show that post 1 5 ma normal faulting has accompanied episodic subsidence of the kawerau area with fault movement focused between northeast southwest structures associated with the geometry of the modern tvz and the reactivated northwest southeast structures associated with most displacement in the area prior to 1 5 ma contrasts between emplacement of coherent rhyolite as sills at 0 36 ma and a dike at 0 138 ma reflect a shift in orientation of the principal stress axes in response to initiation of the modern tvz rifting regime most volcanic rocks at kawerau are distally sourced from elsewhere in the tvz but form local marker horizons that delineate topographic relief within the field and additionally constrain past subsidence rates current rates of subsidence and thermal output at kawerau are geologically recent features associated with latest quaternary rifting processes

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 2 application to taupo volcanic zone rhyolites constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems however it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems in this study we use the recently developed rhyolite melts geobarometer to constrain the crystallisation pressures of rhyolites from the taupo volcanic zone tvz as sanidine is absent from the studied deposits we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass the quartz 1 feldspar constraint of gualda and ghiorso contrib mineral petrol 168 1033 doi 10 1007 s00410 014 1033 3 2014 we use glass compositions matrix glass and melt inclusions from seven eruptive deposits dated between 320 and 0 7 ka from four distinct calderas in the central tvz and we discuss advantages and limitations of the rhyolite melts geobarometer in comparison with other geobarometers applied to the same eruptive deposits overall there is good agreement with other pressure estimates from the literature amphibole geobarometry and h2o co2 solubility models one of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions regardless of volatile saturation the examples presented also emphasise the utility of this method to filter out spurious glass compositions pressure estimates obtained with the new rhyolite melts geobarometer range between 250 to 50 mpa with a large majority at 100 mpa these results confirm that the tvz hosts some of the shallowest rhyolitic magma bodies on the planet resulting from the extensional tectonic regime and thinning of the crust distinct populations with different equilibration pressures are also recognised which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events

vent evolution and lag breccia formation during the cape riva eruption of santorini greece the 18 500 yr b p cape riva cr eruption of santorini vented several km3 or more of magma generating four eruption units a basal plinian fall deposit cr a and three pyroclastic flow deposits cr b to cr d upwards cr b and cr d are welded ignimbrites cr c consists predominantly of up to 25 m thick coarse lithic rich co ignimbrite lag breccias resulting from a climactic phase of the eruption the initial plinian phase occurred from a localized vent in n santorini and subsequent column collapse resulted in emplacement of cr b towards the end of cr b new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias cr c cr d probably records a return to a localized vent as the eruption waned the eruption sampled a zoned magma chamber containing rhyodacite overlying andesite and leaks of these magmas were manifested as the skaros therasia lavas preceding the cr eruption plinian and initial ignimbrite stages occurred while the magma chamber was ove

mixing and mingling in the evolution of andesite dacite magmas evidence from co magmatic plutonic enclaves taupo volcanic zone new zealand abstract the southeastern side of the taupo volcanic zone new zealand is marked by a line of andesite dacite low silica rhyolite complexes co magmatic plutonic enclaves occur within the lavas of the four youngest complexes white island motuhora whale island edgecumbe and tauhara the enclaves range from coarse grained gabbros diorites granodiorites and a syenite to finer grained dolerites and microdiorites the more mafic types are generally porphyritic with large phenocrysts of plagioclase usually with extensive sieve textures in the cores and corroded margins most of these enclaves including the coarser grained plutonic examples contain glass and many are miarolitic diorites and microdiorites dolerites predominate at white island motuhora and edgecumbe many are porphyritic enclaves at tauhara are more variable those collected from hipaua dome include a range from microdiorites to quartz microdiorites and those from rubbish tip dome include microdiorites a granodiorite and a syenite most enclaves show textural evidence for disequilibrium with multiple populations of plagioclase and pyroxene they also show considerable textural variation even within a thin section with coarse grained gabbros diorites intimately mixed with finer grained dolerites microdiorites geochemically and isotopically most enclaves have a similar composition with their host lavas although some have lower silica contents enclaves at motuhora and tauhara are isotopically more variable indicating multiple sources and a more complex petrogenesis most diorite microdiorite enclaves are interpreted to represent parts of a crystal mush formed during fractionation of andesite dacite magma and entrained during later rise of magma to the surface the granodiorite from rubbish tip dome tauhara probably represents part of a silicic magma chamber within the crust that fed the host low silica rhyolite lava dome variability within the enclaves indicates the complexity likely to occur within the magma chambers beneath a typical volcanic arc

two feldspar geothermometry a review and revision for slowly cooled rocks recent improvements in the experimental and thermodynamic basis of two feldspar geothermometry allow one to recover temperatures of coexistence more reliably some problems however persist 1 the experimental solvi by seck 1971a and elkins and grove 1990 differ from each other 2 it is not known to what extent na k ca exchange equilibrium is approached 3 both solvi are probably metastable with regard to al si order 4 it is difficult to judge how closely high temperature natural feldspars compare to this situation 5 the thermodynamic treatment neglects phase transformations 6 the temperature dependence of the margules parameters used to model non ideal mixing behaviour may not be linear 7 it is not clear which expressions should be used to describe ideal activities with these caveats in mind we treat the problem of retrograde resetting in high grade metamorphic rocks that were slowly cooled under essentially dry conditions coexisting feldspars from such rocks commonly do not plot on a common isotherm thus temperatures derived from such pairs using any of the proposed two feldspar geothermometers will necessarily be in error we suggest that the non equilibrium compositions result from retrograde intercrystalline k na exchange this exchange continues after the plagioclase and alkali feldspar have already become essentially closed systems with respect to al si exchange which is a prerequisite for na k ca exchange we use a modified version of the fuhrman and lindsley 1988 programme to reverse the k na exchange and derive concordant temperatures

a variscan pressure temperature time path for the n e mont blanc massif the northeastern portion of the mont blanc massif in western switzerland is predominantly comprised of the granitic rocks of the mont blanc intrusive suite and the mont blanc basement gneisses within these metamorphic rocks are a variety of sub economic fe skarns the mineral assemblages and fluid inclusions from these rocks have been used to derive age pressure temperature and fluid composition constraints for two variscan events metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40ar 39ar and indicate that these metamorphic events have a minimum age of approximately 334 ma garnet hornblende plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 c and pressures ranging from 5 to 8 kbar garnet hornblende magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures ranging from 400 to 500 c in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites late paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of ladinien ichnofossils are on the order of 0 32 mm year and 20 c km respectively these uplift rates and geothermal gradients differ from those obtained from the neighbouring aiguilles rouges massif and indicate that these two massifs experienced different metamorphic conditions during the carboniferous and permian periods during the early to late carboniferous period the relative depths of the two massifs were reversed with the aiguilles rouges being initially unroofed at a much greater rate than the mont blanc but experiencing relatively slower uplift rates near the termination of the variscan orogeny

oblique back arc rifting of taupo volcanic zone new zealand taupo volcanic zone tvz is a back arc rift innorth island new zealand its geometry andkinematics are investigated using shaded reliefimages field examination of faults and offset streamchannels the results show that tvz trends nne is 250 km long by hikurangi margin in the north island of new zealand 20 km wide and consists of fivesegments extension is principally manifest as steeplydipping 60 90 normal faults parallel to tvz these in the last 300 kyr have experienced acomponent of dextral shear tvz is therefore anoblique back arc rift the dextral shear is 37 of thetotal displacement which for previously estimatedspreadingrates 7mm yr correspondsto 2 6mm yr thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in thenorthisland distributeddextralshearacrosstvzthussuggests that strain partitioning across the plateboundary at latitudes of tvz is less significant thanpreviously thought

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

evidence from zircon u pb age spectra for crustal structure and felsic magma genesis at taupo volcano new zealand u pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at taupo volcano as well as unexpected crustal structure beneath the taupo volcanic zone ages from the xenolith in a 28 ka rhyolite dome match those from early cretaceous pahau terrane graywackes that crop out 75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process boundaries between graywacke terranes in the north island are thus more complex than implied from map views in contrast ages from zircons in the 20 ka omega dacite match the nearby surficial graywacke of the jurassic kaweka terrane 55 of grains quaternary plutonic sources 39 and pahau terrane graywacke 6 five of 72 grains analyzed contain graywacke cores and pleistocene rims survival of euhedral xenocrystic zircons through the generation processes for omega dacite magma reflects wholesale melting of crustal protoliths including varieties not previously considered for petrogenetic modeling open system behavior and crustal melts being generated and erupted within 1 10 yr

millennial timescale resolution of rhyolite magma recharge at tarawera volcano insights from quartz chemistry and melt inclusions most rhyolite eruption episodes of tarawera volcano have emitted several physiochemically distinct magma batches 1 10 km3 these episodes were separated on a millennial timescale the magma batches were relatively homogeneous in temperature and composition at pumice scale 4 cm but experienced isolated crystallisation histories at the sub cm scale matrix glasses have trace element compositions sr ba rb that vary by factors up to 2 5 indicating incomplete mixing of separate melts some quartz hosted melt inclusions are depleted in compatible trace elements sr ti ba compared to enclosing matrix glasses this could reflect re melting of felsic crystals deeper in the crystal pile individual quartz crystals display a variety of cathodoluminescence brightness and ti zoning patterns including rapid changes in melt chemistry and or temperature 50 100 c and point to multi cycle crystallisation histories the tarawera magma system consisted of a crystal rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

frontal structure water masses and circulation in the crozet basin due to topographic steering by the crozet and kerguelen plateaus the antarctic circumpolar current acc core in the crozet basin area is shifted to its northernmost position in the southern ocean along the southern limb of the south indian subtropical gyre here the jet like current is embedded within a narrow band 26 5 kg m3 in the western half of the basin and heavier mode 26 7 kg m3 in its eastern half these different varieties of crozet basin mode water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone the area north of the kerguelen plateau appears to be an important source of water mass modification antarctic intermediate water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher oxygen richer colder subantarctic water energetic eddy activities observed along the frontal zone likely enhance such cross frontal exchange of water masses equatorward spreading of the modified thermocline water masses mode water antarctic intermediate water occurs as part of the anticyclonic circulation in the south indian subtropical gyre which is centered within the basin strong injection of north indian deep water into the acc south of amsterdam island is clearly evidenced for the first time this deep oxygen minimum water centered at about 3000 m can be traced to the east of madagascar from where it spreads southward through the eastern part of the crozet basin due to the influence of this water mass north atlantic deep water characteristics of the circumpolar deep water are significantly altered in kerguelen amsterdam passage our results indicate a cyclonic deep level circulation pattern in the crozet basin with a northward flow 9 sv of circumpolar deep water antarctic bottom water as a deep western boundary current and a partially compensating southward flow 3 sv of north indian deep water through the eastern part of the basin it is concluded that the crozet basin constitutes an important passage for the exchange of water masses between the southern ocean and the indian ocean

chapter 3 mid crustal emplacement ofmesozoic plutons san gabriel mountains california and implicationsfor the geologic history of the san gabriel terrane

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

zircon crystallization and recycling in the magma chamber of the rhyolitic kos plateau tuff aegean arc in contrast to most large volume silicic magmas in continental arcs which are thought to evolve as open systems with significant assimilation of preexisting crust the kos plateau tuff magma formed dominantly by crystal fractionation of mafic parents deposits from this 60 km3 pyroclastic eruption the largest known in the aegean arc lack xenocrystic zircons secondary ion mass spectrometry sims u pb ages on zircon cores never older than 500 ka and display sr nd whole rock isotopic ratios within the range of european mantle in an area with exposed paleozoic and tertiary continental crust this evidence implies a nearly closed system chemical differentiation consequently the age range provided by zircon sims u th pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus the age distribution from 160 ka age of eruption by sanidine 40ar 39ar dating smith et al 1996 1 to ca 500 ka combined with textural characteristics high crystal content corrosion of most anhydrous phenocrysts but stability of hydrous phases suggest 1 a protracted residence in the crust as a crystal mush and 2 rejuvenation reduced crystallization and even partial resorption of minerals prior to eruption probably induced by new influx of heat and volatiles this extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned aegean microplate acts as a refractory container for magmas in the dying aegean subduction zone continent continent subduction 1 ref 32

structural implications of some radiometric ages of igneous rocks in southeastern new england u th pb zircon rb sr whole rock and k ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern new england one in late proterozoic z time 600 650 m y ago and the other in devonian to ordovician time 375 450 m y ago the dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late paleozoic assembly these provinces generally are bounded by major faults and or abrupt changes in metamorphic grade and from east to west include i the milford dedham zone ii the putnam nashoba zone iii the merrimack synclinorium and iv the bronson hill zone from the present and other recently published studies reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks other units remain less well constrained but probably may be placed into one or the other of the general age groupings case by case examination of the geochronologic data makes us cautious in accepting some apparent fine structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated with the above mentioned qualifications in mind igneous rock ages summarized according to structural province are as follows i milford dedham zone a late proterozoic z dedham granite and related rocks 630 15 m y 595 15 m y 612 646 m y milford granite 630 15 m y 591 50 m y hope valley alaskite gneiss and scituate granite gneiss ages uncertain unnamed granite near assonet 600 650 m y b devonian to ordovician alkalic rocks including quincy cape ann and peabody granites wenham monzonite and nahant gabbro ii putnam nashoba zone a late proterozoic z no plutonic rocks identified but may include some metavolcanic rocks b devonian to ordovician sharpners pond diorite 430 5 m y andover granite 408 22 m y 450 22 m y muscovite granite near west berlin age uncertain preston gabbro 424 5 m y ill merrimack synclinorium a late proterozoic z granitic orthogneiss unit of the massabesic gneiss complex 600 620 m y metavolcanic unit of the massabesic gneiss complex 646 m y or older b devonian to ordovician fitchburg complex at maiden hill 402 11 m y fitchburg complex at rollstone quarry 390 15 m y newburyport complex 450 15 m y ayer granite 433 5 m y chelmsford granite age uncertain muscovite biotite granite at millstone hill 372 7 m y canterbury gneisses 395 10 m y 392 9 m y iv bronson hill zone a late proterozoic z dry hill gneiss of the pelham dome 600 m y b devonian to ordovician monson gneiss 440 10 m y these distinctive age patterns together with contrasts in petrography and texture of the igneous rocks in stratigraphy of the country lock and in metamorphic grade accentuate the separate geologic development of each structural province no correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province

discussion of blundy and holland s 1990 calcic amphibole equilibria and a new amphibole plagioclase geothermometer

ophiolitic melanges in crustal scale fault zones implications for the late palaeozoic tectonic evolution in west junggar china the baijiantan and darbut ophiolites in west junggar are exposed in steep fault zones 70 containing serpentinite melange in contact on either side with regionally distributed upper devonian lower carboniferous ocean floor peperitic basalts and overlying sedimentary successions the ophiolitic melanges show classic structural features created by strike slip faulting and consistent shear sense indicators of left slip kinematics sandstone blocks within the melanges resemble the surrounding sediments in lithology and age indicating that the ophiolitic melanges consist of locally derived rocks the ophiolitic melanges therefore originated from left slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones sandstone is the youngest lithology involved in the melange and provides a maximum age for the melange of 322 ma whereas stitching plutons are younger than 302 ma multiple clusters in zircon ages from single gabbro blocks in the melange at 375 360 354 and 340 ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers late paleozoic tectonic evolution of west junggar involved late devonian to carboniferous relative motion between the junggar block and west junggar ocean basin which triggered the left slip fault zones within a remnant ocean basin along which the oceanic crust was disrupted to form linear ophiolitic melanges final filling of this remnant ocean basin and its dismemberment by strike slip faulting occurred in the late carboniferous followed by crustal thickening by juvenile granites at the carboniferous permian boundary

age and tectonic setting of the camaguey volcanic intrusive arc cuba late cretaceous extension and uplift in the western greater antilles abstract ar ar ages determined on rocks and minerals from the camaguey area in central cuba provide age constraints on events that accompanied the northward migration of cuba into the caribbean region and its subsequent collision with the bahamas bank whole rock samples from the camujiro and piragua formations part of the camaguey volcanic sequence yielded ar ar ages of 74 72 ma distinctly younger than the 100 80 ma ages indicated by fossils in interlayered sedimentary rocks syenite and granodiorite in the camaguey batholith which cut these volcanic rocks yield generally similar ar ar ages of 75 72 ma for hornblende biotite and feldspar these ages are interpreted to reflect relatively rapid uplift and cooling of most of the volcanic intrusive arc additional constraints on the timing of this uplift are provided by ar ar ages of 71 75 ma for rhyolite rhyodacite domes of the la sierra formation and 52 ma for andesitic basalt of la mulata formation which appear to have been emplaced onto erosion s

deciphering magmatic processes in calc alkaline plutons using trace element zoning in hornblende hornblende in the kuna crest lobe kcl of the tuolumne intrusive complex tic and the upper zone of the wooley creek batholith wcb precipitated over a temperature range of 835 to 700 c and thus has the potential to record magmatic processes we measured trace element concentrations in hornblende from the wcb from the kcl of the tic and from one sample from an adjacent interior unit of the tic to compare and contrast magmatic processes in these two mid crustal intrusions in both systems the magmatic amphibole is magnesiohornblende in which ti zr hf nb sr ba and rare earth elements ree typically decrease from crystal interiors to rims an indication of compatible behavior of these elements and the size of the negative eu anomaly decreases in the kuna crest lobe hornblende from individual mapped units differs in trace element abundances and zoning trends some samples contain at least two distinct hornblende populations which is particularly evident in the shapes of ree patterns in contrast compositions of hornblende from all structural levels of the upper wcb and related dacitic roof zone dikes form a single broad array and the ree patterns are essentially indistinguishable regardless of rock type from quartz diorite to granite in the wcb zr hf ratios in hornblende are consistent with crystallization from a melt with chondritic zr hf values in contrast most hornblende in the kcl has zr hf values lower than expected from crystallization from a melt with chondritic values suggesting that zircon fractionation occurred before and during crystallization of the hornblende simple fractional crystallization models indicate that ree high field strength elements sr and ba were compatible in kcl and wcb magmas as hornblende grew these trends require removal of hornblende plagioclase zircon ilmenite biotite the uniform variations of trace element concentrations and patterns in the upper wcb and roof zone dikes indicates crystallization from a large magma body that was compositionally uniform probably stirred by convection caused by influx of mafic magmas at the base of the zone coint et al 2013a 2013b cf burgisser and bergantz 2011 in contrast in the kcl each analyzed sample displays distinct hornblende compositions and zoning patterns some of which are bimodal these features indicate that each analyzed sample represents a distinct magma and that individual magmas were variably modified by fractionation and mixing hornblende trace element contents and zoning patterns prove to be powerful tools for identification of magma batches and for assessing magmatic processes and thereby relating plutonic rocks to hypabyssal and volcanic equivalents

geochemistry and petrogenesis of proterozoic diabase in the southern death valley region of california diabase sills and dikes of proterozoic age intrude crystalline basement and the overlying crystal spring formation in the southern death valley region of california despite pervasive deuteric alteration analyses of relict plagioclase an66 45 titaniferous augite and ilmenite permit the calculation of initial crystallization temperatures of 1 165 25 c for plagioclase and 1 110 25 c for augite with an oxygen fugacity of 10 11 atm the early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase

ventilation processes and water mass ages in the thermocline of the southeast indian ocean indian central water icw and subantarctic mode water samw are the two major sources for the ventilation of the permanent thermocline in the indian ocean icw is formed by subduction in the region of negative wind stress curl while samw is formed by convective overturning at the subantarctic front samw contributes to the depth range of icw but is not easily identified because most hydrographic properties temperature salinity and nutrients of samw do not differ much from those of icw this study identifies icw and samw in a zonal section near 32 s and evaluates the relative importance of convection vs subduction for the ventilation process oxygen and nutrient data from the eastern part 50 114 e of woce section 15 are used with temperature and salinity to determine water mass fractions of subducted icw and of samw from water mass mixing analysis the individual age fields of the two water mass components are then derived from a combination of the fractions obtained with a linear oxygen cfc mixing model unlike earlier studies which derive an uncalibrated apparent age our results express water mass age in true units of time years the core of the samw near 114 e is about 5 years old while the core of the subducted icw at 60 80 e shows an increase of age with depth in agreement with the subduction process icw moves eastward with the south indian current reaching an age of 35 years at 114 e samw spreads westward against the mean flow through turbulent diffusion reaching an age of 25 years at 50 e

crust mantle interaction and craton destruction evidence from late mesozoic plutons in the north china craton late mesozoic granitoids widely distributed in the yanshan belt of the north china craton ncc are important markers of cratonic destruction and lithospheric thinning we report new petrological geochemical zircon u pb and lu hf isotopic data on two late mesozoic granitic plutons from the central ncc near the trans north china orogen we report zircon u pb emplacement ages of 162 156 ma and 132 ma from the siganding and fangshan plutons respectively indicating late jurassic early cretaceous magmatism the presence of a dusty zone and discontinuous zoning in plagioclase phenocrysts and the occurrence of mafic microgranular enclaves in both granitoid plutons suggest mingling and mixing of felsic and basic magmas the rocks investigated in this study show high na 2 o k 2 o and high mg mg mg fe mg and are metaluminous with enrichment in light rare earth elements and large ion lithophile elements rb ba u and sr and depletion in high strength field elements th u nb ta p and ti they also display negative ehf t values and high sr y ratios comparable with adakitic rocks suggesting that the felsic magmas of these intrusions were probably derived from the partial melting of the palaeoproterozoic two stage zircon hf depleted model ages between 2 5 and 1 9 ga thickened lower crust of the ncc whereas the mafic magma was probably derived from subcontinental lithospheric mantle we correlate the magmatism with lithospheric thinning beneath the ncc and associated crust mantle interaction it is inferred that lithospheric thinning was of limited extent in late jurassic 162 156 ma times the magma flare up at around 130 ma was produced by large scale melting of the enriched subcontinental lithospheric mantle with development of extensive intracontinental adakitic magmatic rocks derived from thickened ancient lower continental crust with variable involvement of subcontinental lithospheric mantle supplementary material mineral assemblages and abundance representative electron microprobe analyses la icp ms zircon u pb age data and la mc icp ms lu hf isotopic data are available at https doi org 10 6084 m9 figshare c 3815308

pre eruptive physical conditions of el reventador volcano ecuador inferred from the petrology of the 2002 and 2004 05 eruptions abstract a petrological study of the eruptive products of el reventador allowed us to infer the magmatic processes related to the 2002 and 2004 05 eruptions of this andesitic stratovolcano on november 3 2002 el reventador experienced a highly explosive event which was followed by emplacement of two lava flows in november december 2002 silica contents range from 62 to 58 wt sio2 for the november 3 pyroclastic deposits to 58 56 and 54 53 wt sio2 for the successive lava flows in november 2004 eruptive activity resumed supplying four new lava flows 56 54 wt sio2 between november 2004 and august 2005 volatile contents in matrix glasses and glass inclusions from the november 3 pyroclastic deposits allow us to estimate the total amount of so2 and hcl released into the atmosphere during the paroxysmal phase i e 80 kt of so2 and 280 kt of hcl pre eruptive pressure temperature conditions of the magmas range from 300 to 150 mpa and 1000 c with high water contents 5 wt we propose the existence of an andesitic magma body located at 7 12 km depth that is frequently intruded by more primitive hydrous magmas from a deeper source the initial crystallization of amphibole from the hydrous primitive magma seems typical of el reventador as well as the historically recurrent and regular periods of eruptive activity lasting several years this eruptive behaviour coupled with the fractionation and mixing processes inferred from the 2002 and 2004 05 petrologic data suggest that deep magmatic recharge at el reventador is frequent and is probably responsible for the high frequency of eruptions

the tectono metamorphic evolution of gneiss complexes in the middle urals russia a reappraisal abstract the middle urals are characterized by a major virgation in the linear trend of the urals orogen and represent the most highly contracted part of the late palaeozoic collisional belt this part of the orogen is dominated by metamorphic complexes and major fault and shear zones the main uralian fault zone muf the east dipping suture of the orogen containing low grade metamorphic rocks separates the sysert complex in the east from the ufaley complex in the west the sysert complex in the hanging wall of the muf consists of intensely deformed gneisses granitic intrusions and a metamorphosed melange zone tectonic and isotopic investigations suggest the following stages for the evolution of the sysert complex a pre orogenic rifting and magmatism during ordovician and silurian times b oceanic closure island arc formation related to convergence and subduction during devonian times c major ductile deformation under amphibolite facies conditions related to nw directed thrusting associated with crustal stacking during collision in carboniferous times d exhumation and contractional intracontinental tectonics during permian times and e closing of isotope systems related to cooling and the end of orogenic shortening through triassic times the ufaley complex in the footwall of the muf is interpreted as an east dipping crustal stack that records an amphibolite facies uralian metamorphism lithologically the complex can be divided into pre orogenic european basement west ufaley and intensely deformed palaeozoic metasediments and amphibolites east ufaley high pressure relics in the east ufaley complex are interpreted to be the result of subduction whereas intense ductile deformation is related to overthrusting onto west ufaley the west ufaley complex is composed of gneisses amphibolites migmatites and granitic intrusions and has been thrust onto devonian limestones along a major shear zone in both sysert and ufaley complexes nw trending stretching lineations and top to the nw kinematic indicators suggest an oblique plate convergence with a significant sinistral component the muf is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall

petrological constraints on crystallization conditions of mesoarchean sanukitoid rocks southeastern amazonian craton brazil we report petrological and geochemical data for the 2 87 ga rio maria sanukitoid granodiorite and associated rocks from mesoarchean granite greenstone terranes of the eastern amazonian craton brazil the dominant rocks have granodiorite to subordinate monzogranitic compositions with minor proportions of intermediate quartz diorites or quartz monzodiorites in addition to mafic endmembers occurring as layered rocks or as enclaves the mineral assemblage is dominated by amphibole plagioclase biotite and epidote minerals all of inferred magmatic origin pyroxenes being notably absent textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the rio maria rocks crystallization conditions have been derived from a comparison between natural phase assemblages proportions and compositions and experimental studies carried out on similar magma compositions the comparison shows that the parental magmas were water rich with more than 7 wt dissolved h2o with crystallization temperatures in the range 950 680 c the mg mg fe ratios of both amphibole and biotite indicate fo2 conditions in the range nno 0 5 to nno 2 5 where nno is nickel nickel oxide buffer therefore pointing to both water rich and oxidizing conditions for sanukitoid magmas amphibole compositions indicate emplacement at around 200 mpa and record a high pressure stage of magma crystallization around 600 900 mpa sanukitoid magmas share two of the principal characteristics of modern arc magmas elevated redox state and volatile contents which suggest that they may have formed in a geodynamic environment broadly similar to present day subduction zones

timing of arrival of the avalon zone in the northeastern appalachians a new look at the straddling granite the straddling granite was previously thought to straddle the hermitage bay dover fault system which marks the boundary between the avalon and gander zones of eastern newfoundland and hence to fix the earliest juxtaposition of these two zones before 504 12 ma more detailed investigation shows that these rocks are divisible into two distinct geochemical suites the indian point granite to the west and the hardy9s cove intrusive suite to the east of the samples that provided the 504 ma isochron five were obtained from the hardy9s cove suite and one from within the fault zone hence the isochron must be taken as representing only a typical date for plutonic rocks within the avalon zone the indian point granite intrudes the gaultois granite dated at 350 18 ma and is thus considerably younger than the hardy9s cove suite these observations now allow for juxtaposition of the avalon and gander zones to have been as late as early carboniferous

provenance analysis using conglomerate clast lithologies a case study from the pahau terrane of new zealand abstract the field of provenance analysis has undergone a revolution with the development of single crystal isotope dating techniques using mainly silt to sand sized single minerals this study focuses on coarser grained rocks notably conglomerates which involve much shorter transport distances and therefore may be used to trace proximal sources the torlesse terranes part of the new zealand eastern province are accretionary complexes that comprise an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages two terranes are recognised in the south island the permian to late triassic rakaia terrane and the late jurassic to early cretaceous pahau terrane geochronology geochemistry and sr nd isotopes of igneous clasts from pahau terrane conglomerates identify the median tectonic zone s l as a major contributor of detritus to the pahau depositional basin shrimp u pb zircon ages of igneous clasts range from 128 to 123 ma and from 147 to 135 ma calc alkaline clasts of the latter range are indistinguishable in age chemical composition and petrogenesis from the calc alkaline granitoids of the darran suite whereas alkaline clasts correlate with the electric granite both parts of the median tectonic zone an early jurassic calc alkaline rhyolite clast from kekerengu 188 3 ma correlates with the bounty island granite campbell plateau the petrography and geochemistry of pahau terrane sandstone clasts indicate the recycling of permian to early late triassic older rakaia terrane rocks this is further supported by the detrital zircon age data igneous conglomerate clasts are capable of placing exceptionally tight constraints on pahau provenance and on its mesozoic tectonic setting within the southwest pacific margin of gondwana they provide the best evidence yet that the pahau terrane is locally derived

andean type active margin formation in the eastern taurides geochemical and geochronogical evidence from the baskil granitoid elazig se turkey abstract the southeast anatolian orogen resulted from collision of the afro arabian and the eurasian plates following the cretaceous to miocene closure of the southern neotethyan oceanic basin in this orogenic belt there are number of tectonomagmatic stratigraphic units in the kahramanmaras malatya elazig region that are important to understand the geological evolution of southeast anatolia during the late cretaceous these are a metamorphic massifs i e malatya keban platform b ophiolites i e goksun ispendere komurhan c ophiolite related metamorphics i e berit metaophiolite and d granitoids i e goksun dogansehir and baskil the baskil granitoid crops out to the northwest of elazig and is a large magmatic body 170 km2 that intruded all of the above mentioned units during late cretaceous time the baskil granitoid comprises both mafic and felsic plutonic sub plutonic rock associations the felsic plutonic phase includes granite granodiorite tonalite and quartz monzonite whereas the felsic sub plutonic phase is characterized by aplite granophyre granite porphyry and granodiorite porphyry the granite and granodiorite contain mafic microgranular enclaves mme the mafic plutonic phase comprises gabbro diorite quartz diorite whereas the mafic sub plutonic phase is represented by diabase microdiorite quartz microdiorite diorite porphyry quartz diorite porphyry and dykes of orbicular gabbro dyke geochemically the baskil granitoid rocks have i type metaluminous peraluminous calc alkaline characteristics the ree and ocean ridge granite normalized multi element patterns and tectonomagmatic discrimination diagrams together with biotite geochemistry suggest that the granitoids were formed in a volcanic arc setting the40ar 39ar geochronology of the granitoid rocks yielded biotite ages of 81 9 0 7 ma and 81 5 0 8 ma 95 confidence level coexisting hornblende and hornblende from additional samples yielded ages ranging from 84 0 0 7 ma to 81 5 1 1 ma the ophiolites are believed to have formed in a suprasubduction zone tectonic setting whereas the ophiolite related metamorphic rocks formed either during the initiation of intraoceanic subduction or later stage thrusting 90 ma these units were then thrust beneath the malatya keban platform during the progressive closure of the southern neotethys this was followed by intrusion of the granitoids 85 82 ma along the tauride active continental margin of the southern neotethys

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

deep sections through the south australian basin and across the australian antarctic discordance fronts and water masses in the australian basin are described based on three sections made by r v franklin during 1994 as part of the woce one time hydrographic survey the subtropical front is found closer to the australian continent than previously thought convection of subantarctic mode water is seen to reach 700 m depth a locally formed variety of antarctic intermediate water with a temperature near 6 c is found near 120 e antarctic bottom water enters the indian ocean through the discordance at 125 37 e and 127 e but not at 120 e where the deep passage seen in the topography may be closed for bottom water inflow

the origin of lavas and ignimbrites of the taupo volcanic zone new zealand in the light of oxygen isotope data abstract the taupo volcanic zone consists predominantly of andesites at each end and about a 20times larger volume of rhyolites incl ignimbrites and pumice in the center in a 18o vs sio2 diagram the rhyolites do not continue the increasingly 18o contaminated andesite basalt to dacite trend but have lower 18o values than the most differentiated andesites comparison with 18o of greywackes and conjectural granitoids seems to rule out a partial fusion origin in retrospect the sr isotope data of ewart and stipp 1968 are seen to confirm this conclusion isotopic adjustment of source rocks before partial fusion by exchange with circulating subsurface waters is highly unlikely because of the combined demands for o and sr isotope shifts and the constraint offered by the normal igneous to somewhat enriched 18o values of the rhyolites oxygen isotope results confirm separate origins or at least different modes of ascent of basalt and andesites on one hand and rhyolites on the other both magma types appear to have originated below crustal formations and both have become contaminated by greywacke through which they passed to reach the surface

observations of seismic volcanic signals at stromboli volcano italy abstract digital recordings of ground motion during a seismic operation organized at stromboli volcano italy from may 31 1986 to june 26 1986 have been studied the amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies 2 0 and 6 0 hz with subdominant frequencies at 4 1 4 8 and 8 hz according to their waveform and their spectral content three types of discrete volcanic signals are distinguishable the first type is characterized by a dominant frequency at 2 hz and a subdominant frequency at 4 1 hz the second type also has a dominant peak frequency at 2 hz with two important subdominant frequencies at 5 and 8 hz the type 3 has a sharp dominant frequency at 6 hz and subdominant frequency at 3 2 hz the frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin the discrete signals are linked to explosions at the active vents within the craters

the mechanism for antarctic intermediate water renewal in a world ocean model abstract realistic representation of the low salinity tongue of antarctic intermediate water aaiw has been achieved in a coarse resolution ocean general circulation model the authors find that this water mass is not generated by direct subduction of surface water near the polar front instead the renewal process is concentrated in the southeast pacific ocean off southern chile the outflow of the east australian current progressively cools by heat loss to the atmosphere and freshens by assimilation of polar water carried north by the surface ekman drift during its slow movement across the south pacific toward the aaiw formation zone further deep warm advection near chile enables more convective overturn resulting in very deep mixed layers from which aaiw is fed into the south pacific and also into the malvinas current along with this isolated region of aaiw renewal the model relies on alongisopycnal mixing of fresh surface water from the polar front to capture a realistic circumpolar tongue

the record of middle jurassic volcanism in the carmel and temple cap formations of southwestern utah altered volcanic ash beds in the middle jurassic temple cap and carmel formations in southwestern utah record a pulse of active arc related volcanism between 166 and 171 ma a second pulse between 148 and 155 ma has previously been documented in the upper jurassic morrison formation volcanic and volcaniclastic rocks of these same ages have also been identified closer to or within the arc in california in the inyo mountains the cowhole mountains the palen mountains and the central mojave desert the upper part of the volcaniclastic mount wrightson formation and the strata of cobre ridge in southern arizona are ca 170 ma in age and appear to be time correlative with the middle jurassic formations in southwestern utah the altered ash beds found in the temple cap and carmel formations typically contain phenocrysts of sanidine quartz biotite apatite zircon and titanite plagioclase was likely present originally in all of the ashes but was removed by alteration and is now found only in the temple cap red beds quartz and sanidine are absent in two crystal poor ash beds that contain two pyroxenes hornblende and biotite although major and trace element concentrations in the ash beds have been substantially modified compositions of relict phenocrysts reveal that the magmas were calc alkaline rhyolites to andesites two pyroxene two feldspar biotite and biotite apatite thermometers suggest that crystallization occurred at temperatures ranging from 740 to 910 8c hornblende geobarometry yields pressures of 1 2 kilobars for the two ash beds that contain the appropriate buffer assemblage the mafic silicates all have moderately high mg fe ratios this fact combined with the presence of hornblende biotite and titanite suggests that the phenocrysts crystallized at high oxygen fugacities similar to those of the granites of the batholiths of california the ash probably erupted from a low lying arc cut by strikeslip faults in what is now southern california and western nevada major jurassic unconformities occur near or within the ash bearing formations in southwestern utah laser fusion singlecrystal 40 ar 39 ar measurements have defined the ages of the unconformities and the associated volcanism the age of the j 1 unconformity found at the base of the temple cap formation in southwestern utah is older than ca 170 5 ma the j 2 unconformity which lies between the temple cap and carmel formations formed between ca 169 and 168 ma the origin of these unconformities is still unclear but may be related to the middle jurassic pulse of magmatism and the oblique plate convergence along the western margin of north america the age range of ash beds in the carmel formation between 166 3 and 168 0 6 0 5 ma is consistent with a bajocian bathonian boundary of ca 166 ma

subsurface andesite lavas and plutonic rocks in the rotokawa and ngatamariki geothermal systems taupo volcanic zone new zealand abstract deep 2 km drillholes into the rotokawa and ngatamariki geothermal systems taupo volcanic zone tvz new zealand penetrate a sequence of silicic volcanic rocks and andesite lavas the latter being locally more than 1100 m thick one drillhole at ngatamariki bottoms in diorite 2200 m the first plutonic rocks reached by any well drilled into the taupo volcanic zone the rotokawa andesite lavas which occur below 1400 m depth are older than 330 ka and locally rest upon mesozoic metasedimentary basement rocks of the torlesse terrane they are typically dense jointed and where fresh contain phenocrysts of andesine labradorite augite hypersthene titanomagnetite and in places hornblende geochemically these rocks are medium k calc alkaline orogenic andesites their major and trace element contents and sr isotopic ratios show them to be chemically coherent with little lateral or vertical variation hydrothermal alteration has typically added volatiles but has been largely isochemical on a hand specimen scale mineralogical geochemical and stratigraphic evidence indicates that the rotokawa andesites are not genetically or temporally related to andesite lavas exposed at rolles peak 5 km east of rotokawa nor to andesite or diorite penetrated at ngatamariki but they have closer chemical affinities with the plagioclase pyroxene phyric lavas of the tongariro volcanic centre at the south end of the tvz the ngatamariki andesites have distinctive trace element compositions particularly ti and zr and are chemically and petrographically different from most other tvz lavas the occurrence of thick subsurface andesite sequences in the rotokawa ngatamariki area suggests that early eruptives here were of intermediate composition rather than the more silicic volcanics now so widespread both at the surface and in the shallow levels of several nearby geothermal systems

comment on blundy and holland s 1990 calcic amphibole equilibria and a new amphibole plagioclase geothermometer

petrology and geochemistry of the early mesozoic caraquet dyke new brunswick canada the caraquet dyke of new brunswick is one of three early mesozoic dykes recognized in atlantic canada it has a quartz normative tholeiitic composition intermediate in character between low ti and high ti appalachian diabase major element e g mgo sio2 and trace element concentrations e g ba zr suggest that the dyke is less evolved than the shelburne dyke but slightly more evolved than the avalon dyke although related to continental rifting and opening of the atlantic ocean the dykes of the atlantic provinces show little in common with continental rift volcanic rocks they resemble morb except that their k rb and ba concentrations are much higher semi quantitative modelling suggests that the elevated large ion lithophile element concentrations reflect assimilation of continental crust differences in composition among the three dykes may be due to variations in the quantity and composition of assimilated material as well as the degree of differentiation as with other early mesozoic dyk

juxtaposed mesozoic diabase dike sets from the carolinas a preliminary assessment aeromagnetic anomaly maps of the north carolina piedmont provide confirming evidence of juxtaposed mesozoic diabase dike sets dikes in north carolina may be divided into four domains on the basis of dike orientations most dikes are olivine normative but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages two orientations dominate northwest and north south the northwest trending set dominates in the south carolina piedmont and the north south set extends through much of the central virginia piedmont and under the coastal plain in south carolina the north south dikes are part of a set that converges near charleston and georgetown south carolina and diverges northward into virginia preliminary information suggests that the north south dikes are younger than the more common northwest trending dikes current models proposed for stress regimes associated with the breakup of pangea can accommodate the northwest set but not the north south convergent set this convergent set may be evidence for crustal deformation above a mantle plume centered near charleston and georgetown in middle jurassic time it is possible that presumed mafic plutons under the coastal plain near charleston regional seismicity patterns and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid mesozoic time the western margin of northwest trending dikes may be related to tectonic features inherited from pre mesozoic time

lithic types in ignimbrites as a guide to the evolution of a caldera complex taupo volcanic centre new zealand taupo volcanic centre is one of two active rhyolite centres in the taupo volcanic zone tvz and has been sporadically active over the past ca 300 ka at least four large scale ignimbrites have erupted from the centre including the well documented 26 5 ka oruanui ignimbrite and 1 8 ka taupo ignimbrite because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism disrupted by regional tectonics and obscured by poor exposure indirect methods must be applied in order to determine their source regions in this paper detailed componentry density and petrology of lithic fragments from three ignimbrites rangatira point oruanui taupo are used to reveal aspects of the sub taupo caldera geology including the evolution of the taupo volcanic centre to assist in ignimbrite correlation and to evaluate structures within the taupo caldera complex lithic fragments identify a complex subsurface geology the rangatira point ignimbrite sampled dominantly rhyolite lavas plus a variety of welded ignimbrites rare high silica dacites and a single dolerite most lithic fragments in the oruanui ignimbrite are andesite with minor rhyolite welded ignimbrite dacite and rounded greywacke while in the taupo ignimbrite rhyolite is again the dominant lithic component with subordinate welded ignimbrites andesite and greywacke the densities of lithic fragments indicate similar ranges of values for all lava types and thus density is a poor indicator of lithology care must therefore be taken before interpreting subcrustal stratigraphy using density as the sole criterion the petrography and geochemistry of lithic types are more specific and the variation can be used to identify sources for the ignimbrites both pumice chemistry and rhyolite lithic fragments from the rangatira point ignimbrite are comparable to domes exposed at the southern end of the western dome complex and combined with limited outcrop information suggest the most likely source for this unit is in the northern part of the taupo caldera complex the dominance of andesite lithic fragments in the oruanui ignimbrite suggests a major andesite cone existed beneath the source area and the different lithic suites between oruanui and taupo ignimbrites suggest these ignimbrites came at least in part from mutually exclusive collapse structures we believe that the oruanui caldera is sited principally in the northwestern part of present day lake taupo and the taupo caldera in the northeastern part identification of abundant ignimbrite lithics in the taupo ignimbrite which are considered to represent an intracaldera facies of an earlier ignimbrite that is not exposed at the surface suggest there was a further pre oruanui ignimbrite caldera in the taupo ignimbrite eruptive vent region

timing and conditions of formation of granitoid clasts erupted in recent pyroclastic deposits from tarawera volcano new zealand abstract granitoid clasts in pyroclastic deposits of the 0 7 ka kaharoa eruption from the intra caldera tarawera volcano of the okataina volcanic centre ovc new zealand provide an insight to the plutonic processes beneath one of the most productive quaternary rhyolite centers on earth sims u th data for 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 208 4 ka mswd 1 2 n 72 the remaining crystals are at secular equilibrium and u pb analyses indicate that a few of these outliers date back to 750 ka a period significantly longer than the known volcanic record at ovc probably 550 ka in contrast zircon crystallization in co erupted pumice and lava of the 0 7 ka kaharoa event and that of the three preceding rhyolite eruptions occurred mostly during 0 50 ka reflecting a separate magmatic history brittle deformation features incipient alteration and low 18o whole rock compositions 3 are consistent with a shallow solid intrusion that has interacted with meteoric hydrothermal fluids however 18o sims analyses of zircons 5 4 0 2 n 11 are consistent with magmatic compositions and thus meteoric interaction occurred post emplacement the kaharoa granitoids contrast with those ejected in the 40 60 ka caldera forming rotoiti event that were partly molten and display zircon age spectra indistinguishable from those for co erupted pumices suggesting the latter were derived from contemporaneous crystal mush the 0 7 ka kaharoa case shows that zones of the magmatic system remained solid despite frequent and voluminous magma production that included caldera forming events such long lived sub solidus zones in magma systems would act as barriers to melt connectivity and interaction with country rock but are also a potential source for antecrysts xenocrysts in subsequent eruptions this exemplifies how plutonic and volcanic evolution can diverge even in close proximity

caldera resurgence during magma replenishment and rejuvenation at valles and lake city calderas a key question in volcanology is the driving mechanisms of resurgence at active recently active and ancient calderas valles caldera in new mexico and lake city caldera in colorado are well studied resurgent structures which provide three crucial clues for understanding the resurgence process 1 within the limits of 40ar 39ar dating techniques resurgence and hydrothermal alteration at both calderas occurred very quickly after the caldera forming eruptions tens of thousands of years or less 2 immediately before and during resurgence dacite magma was intruded and or erupted into each system this magma is chemically distinct from rhyolite magma which was resident in each system 3 at least 1 km of structural uplift occurred along regional and subsidence faults which were closely associated with shallow intrusions or lava domes of dacite magma these observations demonstrate that resurgence at these two volcanoes is temporally linked to caldera subsidence with the upward migration of dacite magma as the driver of resurgence recharge of dacite magma occurs as a response to loss of lithostatic load during the caldera forming eruption flow of dacite into the shallow magmatic system is facilitated by regional fault systems which provide pathways for magma ascent once the dacite enters the system it is able to heat remobilize and mingle with residual crystal rich rhyolite remaining in the shallow magma chamber dacite and remobilized rhyolite rise buoyantly to form laccoliths by lifting the chamber roof and producing surface resurgent uplift the resurgent deformation caused by magma ascent fractures the chamber roof increasing its structural permeability and allowing both rhyolite and dacite magmas to intrude and or erupt together this sequence of events also promotes the development of magmatic hydrothermal systems and ore deposits injection of dacite magma into the shallow rhyolite magma chamber provides a source of heat and magmatic volatiles while resurgent deformation and fracturing increase the permeability of the system these changes allow magmatic volatiles to rise and meteoric fluids to percolate downward favouring the development of hydrothermal convection cells which are driven by hot magma the end result is a vigorous hydrothermal system which is driven by magma recharge

a review of the shelf slope circulation along australia s southern shelves cape leeuwin to portland abstract a review is presented of the ocean circulation along australia s southern shelves and slope uniquely the long zonal shelf is subject to an equatorward sverdrup transport that gives rise to the flinders current a small sister to the world s major western boundary currents the flinders current is strongest near the 600 m isobath where the current speeds can reach 20 cm s and the bottom boundary layer is upwelling favourable it is larger in the west but likely intermittent in both space and time due to possibly opposing winds thermohaline circulation and mesoscale eddies the flinders current may be important to deep upwelling within the ubiquitous canyons of the region during winter the leeuwin current and local winds act to drive eastward currents that average up to 20 30 cm s the currents associated with the intense coastal trapped wave field 6 12 day band are of order 25 30 cm s and can peak at 80 90 cm s wintertime winds and cooling also lead to downwelling to depths of 200 m or more and the formation of dense coastal water within the great australian bight and the south australian sea within the great australian bight the thermohaline circulation associated with this dense water is unknown but may enhance the eastward shelf edge south australian current the dense salty water formed within spencer gulf is known to cascade as a gravity current to depths of 200 m off kangaroo island this dense water outflow and meanders in the shelf circulation also fix the locations of a sequence of quasi permanent mesoscale eddies between the eyre peninsula and portland during summer the average coastal winds reverse and surface heating leads to the formation of warm water in the western great australian bight and the south australian sea no significant exchange of shelf water and gulf water appears to occur due to the presence of a dense nutrient rich sub surface pool that is upwelled off kangaroo island the winds lead to weak average coastal currents increasing evidence suggests that el nino events 4 7 year period can have a major impact on the winter and summer circulation these events propagate from the pacific ocean and around the shelf slope wave guide of west australia and into the great australian bight during winter el nino events the average shelf currents may be largely shut down during summer the thermocline may be raised by up to 150 m the nature and role of tides and surface waves is also discussed along with uncertainties in the general circulation and future research

thermal history of the giant qulong cu mo deposit gangdese metallogenic belt tibet constraints on magmatic hydrothermal evolution and exhumation abstract a complete thermal history for the qulong porphyry cu mo deposit tibet is presented zircon u pb geochronology indicates that the mineralization at qulong resulted from brecciation veining events associated with the emplacement of a series of intermediate felsic intrusions combined with previously published ages our results reveal a whole intrusive history of the qulong composite pluton causative porphyries were emplaced at 16 0 ma as revealed by 40ar 39ar dating of hydrothermal biotite 15 7 0 2 ma and sericite 15 7 0 2 ma zircon and apatite u th he zhe and ahe dating of qulong revealed that both followed similar monotonic thermal trajectories from 900 c u pb ages 17 5 15 9 ma to 200 c zhe 15 7 14 0 ma and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200 c myr the qulong deposit was exhumed between 13 6 ma and 12 4 ma ahe at an estimated rate of 0 16 0 24 mm y which is consistent with previous estimates for other gangdese miocene porphyry deposits our ahe thermochronology results suggest that neither the gangdese thrust system nor the yadong gulu graben affected or accelerated exhumation at the qulong deposit

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract recent drilling at the ngatamariki geothermal field taupo volcanic zone new zealand has provided new constraints on the stratigraphy and volcanic evolution of the region over 2800 m thickness of volcanic products are present at ngatamariki mainly comprised of rhyolitic ignimbrites linked to large caldera forming events at sources outside the field area but locally sourced andesite and rhyolite lavas and domes are also encountered most of the rocks are allocated to the pre 0 35 ma tahorakuri formation crystallisation age spectra and consequent best estimates of eruption age have been obtained by u pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by secondary ion mass spectrometry techniques using a shrimp rg instrument the oldest rock dated is an ignimbrite which yields an eruption age estimate of 1 85 0 06 ma this ignimbrite plus comparable aged units dated at the adjacent rotokawa and ohaaki geothermal fields are interpreted to represent the oldest silicic volcanic deposits in the area and onlap the basal andesite lava pile that is best developed at rotokawa to the south other pyroclastic units and associated volcaniclastic sediments with another intercalated andesite lava unit return age estimates between 1 85 0 06 and 0 701 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma the area was buried by two ignimbrite packages of the whakamaru group erupted from sources just west and well north respectively of the field ignimbrites of the waiora formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0 282 0 011 and 0 257 0 011 ma coeval with more extensive volcanic activity in the maroa dome complex west of the field sediments of the huka falls formation and deposits of the 25 4 0 2 ka oruanui eruption then cap and seal the system the new u pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the ngatamariki geothermal field the field despite 2 8 km of subsidence does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of quaternary age two chemically and temporally distinct hydrothermal events are located at the ngatamariki field with no evidence of continuity between the two

the south indian ocean current in this paper the historical hydrographic database for the south indian ocean is used to investigate i the hydrographic boundary between the subtropical gyre and the antarctic circumpolar current acc the subtropical front stf and especially ii the southern current band of the gyre a current band of increased zonal speeds in the upper 1000 m is found just north of the stf in the west near south africa and at the surface stf in the open indian ocean until the waters off the coast of australia are reached as neither any other investigation of this current nor a name for it are known the flow has been called the south indian ocean current sioc this name is anologous to the same current band in the south atlantic ocean the south atlantic current the stf is located in the entire south indian ocean near 40 degrees s the associated current band of increased zonal speeds is the sioc which is found at or north of the stf east of 100 degrees e the sioc separates from the stf and continues to the northeast the zonal flow south of the stf is normally weak and serves to separate the south indian ocean and circumpolar currents near africa the sioc has a typical volume transport of 60 sv 1 sv 10 6 m3 s 1 in the upper 1000 m relative to deep potential density surfaces of sigma 4 45 87 kg m 3 2800 3500 m or sigma 2 36 94 kg m 3 1500 2500 m near western australia the sioc is reduced to about 10 sv as it turns to the northeast