crystallisation and zircon saturation of calc alkaline tonalite from the adamello batholith at upper crustal conditions an experimental study the understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent here we present experimental results for a tonalitic system at a pressure of 200 mpa and under water saturated conditions melting experiments were performed at temperatures between 700 and 1000 c in externally heated hcm pressure vessels with oxygen fugacity controlled close to the ni nio buffer equilibrium nno employing an argon methane mixture as pressure medium and co pd redox sensors to verify fo2 conditions natural rock powder of a medium k tonalite from the adamello batholith in northern italy served as experimental starting material based on compositional data of stable phases in the run products and images of entire run charges mass balance calculations as well as image processing were performed to investigate the evolution of the crystal melt ratio with respect to temperature furthermore compositional trends of minerals as well as the liquid line of descent of residual melts were obtained orthopyroxene clinopyroxene and plagioclase were identified as near liquidus phases below 990 c at 900 c amphibole joins the solid phase assemblage at the expense of clinopyroxene indicating the existence of a peritectic relationship after an initial near linear decrease with temperature residual melt fractions exhibit a plateau of 45 55 vol between 750 and 850 c followed by a rapid decrease coinciding with quartz saturation at 725 c compositions of residual liquids evolve along a typical calc alkaline differentiation trend with decreasing temperature increasing sio2 and decreasing tio2 al2o3 cao mgo and feo contents and become peraluminous below 900 c intermediate to acidic rocks from the adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low pressure magma differentiation and liquid extraction experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used zr saturation models but fully consistent with observed and modelled zr evolution trends from the natural rock record of the southern adamello batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800 830 c corresponding to a melt fraction of about 50 vol

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

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

a quantitative approach to trace element and sr isotope evolution in the adamello batholith northern italy a development of de paolo s mathematical procedure 1981 for magmatic afc assimilation fractional crystallization processes is discussed with respect to both trace element and sr isotopic ratio behaviours during the genesis and evolution of adamello batholith northern italy resolution of a two equation system one relative to 87sr 86sr ratio variation in a magma generated by an afc process the other to its trace element content variations gives the f mass of magma at time t mass of initial magma and d bulk partition coefficient values by which one can deduce the r rate of assimilation rate of crystallization value during each step of magmatic evolution this quantitative approach suggests that 1 there was a common precursor magma for all the adamello granitoids with a mg rich tholeiitic composition 2 each intrusive unit appears to have been generated by different extents of afc 3 the trace element distribution in the magma seems essentially influenced by mineral fractionation rather than by the composition of the assimilated crustal material

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

geodynamic controls on tertiary arc magmatism in ecuador constraints from u pb zircon geochronology of oligocene miocene intrusions and regional age distribution trends abstract we obtained u pb zircon ages of late tertiary intrusions in the northern andes to provide robust time calibration points for the intrusive geochronologic framework of ecuador which is mostly based on k ar data magmatism associated with intrusion emplacement ranges from 31 to 7 ma with ages mainly pooling in the late oligocene early miocene where both k ar and u pb data exist for a given intrusive system ages obtained by the different methods are usually concordant within 1 4 m y suggesting that k ar ages are accurate indicators for the timing of tertiary arc magmatism on a regional multi m y scale except for a single sample the investigated intrusions completely lack externally inherited zircons in agreement with dominantly zircon poor oceanic crustal basement domains spatio temporal distribution trends of tertiary arc magmatism inferred from screened u pb k ar and zircon fission track geochronologic data allow tracking of the progressive along arc broadening of a flat slab segment below southernmost ecuador in the mid to late miocene and of moderate slab shallowing in northern central ecuador in the late miocene these regional arc migration patterns correlate in time with the subduction of the buoyant inca plateau and the carnegie ridge seamount chain the temporal distribution of tertiary ecuadorian arc magmatism indicates a late oligocene early miocene arc magmatic flare up event comprising widespread ignimbrite eruption and batholith construction initiation of the flare up event coincides in time with accelerating less oblique farallon nazca south america plate convergence suggesting a positive feedback between convergence rates asthenospheric melt production mantle crust melt flux and upper crustal arc magmatic productivity in ecuador

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

dissecting complex magmatic processes an in depth u pb study of the pavia pluton ossa morena zone portugal the build up of large magmatic complexes can proceed piecemeal over periods of several million years through sequences of complex processes of magma production differentiation assimilation final crystallization and subsequent metasomatic modification all these stages can produce or modify minerals used as geochronometers such as zircon monazite and titanite the present study exemplifies such complex relationships also demonstrating how a systematic approach with comprehensive sampling and careful high resolution u pb analyses can yield a coherent picture of the entire magmatic process the study was conducted on the pavia pluton an elongated variscan intrusion in the ossa morena zone of portugal the geochronological data show that the pavia pluton was emplaced by the amalgamation of multiple magma pulses into the crust over a period of c 11 myr an early event at 340 ma revealed by xenocrystic zircon preceded the magmatic activity at the exposed level of the pluton but is recognized as the main magmatic event elsewhere in the ossa morena zone a second event at 328 ma formed tonalite trondhjemite and granodiorite and subordinate differentiates in the central domains of the pluton units i and ii a third event at c 324 ma emplaced granodiorite in the flanking domains iii v and the contemporaneous and widespread two mica granite in domainvi together with late rhyodacite porphyries microgranodiorites aplite pegmatite and pegmatite dikes a fourth event at 319 317 ma was characterized by the emplacement of some microgranites and pegmatite dikes these two last magmatic events also had an effect on the previously emplaced rocks causing local overgrowths and isotopic resetting of minerals the occurrence of a fifth magmatic event at depth at 313 ma is the inferred cause of the hydrothermal activity responsible for local zircon monazite and titanite resorption and or recrystallization and for some of the textures exhibited by the main rock forming minerals the magmatic episodes were interspersed with periods of quiescence this cyclicity presumably reflects an external control by the transtensional tectonic regime of the ossa morena zone

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

brittle ductile brittle deformation during cooling of tonalite adamello southern italian alps abstract late to post magmatic deformation in slightly diachronous contiguous intrusions of the north western adamello batholith southern alps italy is recorded as from oldest to youngest i joints ii solid state ductile shear zones iii faults associated with epidote k feldspar veins and iv zeolite veins and faults structures ii to iv are localized on the pervasive precursory network of joints i which developed during the earliest stages of pluton cooling high temperature 500 c ductile overprinting of joints produced lineations defined by aligned biotite and hornblende on the joint surfaces and highly localized mylonites the main phase of faulting producing cataclasites and pseudotachylytes occurred at 250 c and was associated with extensive fluid infiltration cataclasites and pseudotachylytes are clustered along different e w striking dextral strike slip fault zones correlated with the activity of the tonale fault a major tectonic structure that bounds the adamello batholith to the north ductile deformation and cataclastic veining episodes occurred at p 0 25 0 3 gpa during rapid cooling of the batholith to the ambient temperatures 250 c that preceded the exhumation of the batholith timing of the sequence of deformation can be constrained by 39 ar 40 ar ages of 30 ma on pseudotachylytes and various existing mineral ages in the whole composite adamello batholith multiple magma pulses were intruded over the time span 42 30 ma and each intrusive body shows the same ductile to brittle structural sequence localized on the early joint sets this deformation sequence of the adamello might be typical of intrusions undergoing cooling at depths close to the brittle ductile transition

zircon and titanite recording 1 5 million years of magma accretion crystallization and initial cooling in a composite pluton southern adamello batholith northern italy abstract the southern part of the adamello batholith the so called re di castello unit is an example of a composite pluton ranging from gabbro to granodiorite in composition u pb dating of single zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies showing apparent durations of growth between 90 and 700 ka the youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite the formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization in contrast to antecrystic zircons present in the same sample which had formed earlier in the magmatic column or were derived from re mobilized earlier magma the autocryst derived emplacement ages range from 42 43 0 09 ma to 40 90 0 05 ma recording 1 5 ma of intrusion and crystallization history we anticipate that extended periods of zircon crystallization may be common in silicic rocks whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0 1 0 2 in 206 pb 238 u age decreasing th u ratios of dated zircons within one melt batch document the depletion of the residual melt portion in th due to the contemporaneous crystallization of titanite preliminary hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern re di castello unit represent hybrid melts with an important crustal component e hf between 2 8 and 3 0 subsequently emplaced melts are more juvenile at e hf values at 6 4 to 8 9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

earthquake mechanics and deformation in the tonga kermadec subduction zone from fault plane orientations of intermediate and deep focus earthquakes 1 we make use of rupture directivity to analyze 82 deep earthquakes 100 km depth in the tonga kermadec subduction zone identifying the fault planes for 25 of them we are able to place new constraints on both the physical mechanism of intermediate and deep focus earthquakes and deformation within the subducting slab we find that half of deep earthquakes with mw 6 have detectable directivity we compare the obtained fault orientations with those expected for the reactivation of outer rise normal faults and with those expected for the creation of new faults in response to the ambient stress field earthquakes 300 km depth match the patterns expected for the creation of a new system of faults we observe both subhorizontal and subvertical fault planes consistent with a downdip compressional stress field slip along these faults causes the slab to thicken rupture propagation shows no systematic directional pattern in contrast at intermediate depths 100 300 km all ruptures propagate subhorizontally and all identified fault planes whether in the upper or lower region of the double seismic zone are subhorizontal rupture propagation tends to be directed away from the top surface of the slab after accounting for the angle of subduction the subhorizontal fault plane orientation is inconsistent with the orientation of outer rise normal faults allowing us to rule out mechanisms that require the reactivation of these large surface faults subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation if all deformation takes place on these subhorizontal faults it would cause the slab to thin assuming the slab is incompressible this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths

some thermal constraints on crustal assimilation during fractionation of hydrous mantle derived magmas with examples from central alpine batholiths following assimilation at different levels in the crust we test by heat and mass balance the extent of assimilation of crustal country rocks in different local plutonic

rates of magma differentiation and emplacement in a ballooning pluton recorded by u pb tims tea adamello batholith italy abstract geochemical structural field and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths models for pulsed intrusion of small magma batches over long timescales 1 ma versus transfer of larger magma bodies on shorter timescales predict a different thermal metamorphic and rheological state of the crust highlighting the importance of robust time constraints this study focuses on a well characterized upper crustal intrusion the 15 km2 lago della vacca complex lvc adamello batholith n italy previous studies used structural and petrologic data to argue that the lvc was emplaced through pulsed magma injection and in situ expansion or ballooning of a short lived 105 yr magma chamber we test this model using a dense sampling strategy and high precision id tims u pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral u pb tims tea these data show that the marginal mafic pulses of magma crystallized zircon with primitive hf isotopes and negligible eu anomalies during fractional crystallization and ascent through the crust on 10 30 ka timescales subsequent more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the lvc to

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

syn tectonic emplacement of the late mesozoic laojunshan granite pluton in the eastern qinling central china an integrated fabric and geochronologic study abstract the porphyritic monzogranite of laojunshan eastern qinling in central china forms a w e elongated pluton characterized by a varying abundance of cm scale potassium feldspar kfs megacrysts with or without plagioclase mantles mafic microgranular enclaves mmes and associated mafic dykes the measurements of the shape preferred orientation of kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton s margin possibly resulting from eastward non coaxial magmatic flow felsic mafic magma mixing and mingling were important during magma emplacement as indicated by abundant mmes coeval mafic dykes and disequilibrium textures within kfs megacrysts quantitative analyses of the crystal size distribution and the in situ chemistry of kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of kfs megacrysts the syn tectonic and two main stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic submagmatic solid state fabrics in the pluton and wall rock deformations as well as zircon u pb dating results which produce eastward younging emplacement ages spanning from 122 ma to 119 ma combined with other studies our work supports the idea that the qinling dabie orogen was in a sinistral transtensional regime during the early cretaceous which may correspond to the eastward extrusion tectonics in eastern china

insights into the mechanisms and timescales of pluton assembly from deformation patterns of mafic enclaves 1 we propose a novel method to constrain both timescales and assembly styles of intrusive bodies using the strain recorded by mafic enclaves a common component of granitic rocks petrology thermal modeling and magma rheology are combined to investigate the evolution of strain recorded by enclaves during the piecemeal assembly of a pluton growing at various rates of magma input the different compositions and hence phase relations of host magma and enclaves limits homogeneous deformation of these two materials to restricted temperature ranges which we term windows of mutual deformability outside these windows only the less viscous host granite records any appreciable deformation and enclaves are mainly transported as rigid objects the temporal and spatial development of the windows of mutual deformability reflects the emplacement rate of magmas into the pluton consequently the radial distribution of strain recorded by enclaves can provide a picture of the thermal and rheological evolution of a magmatic body during its construction our approach is applied to deformation patterns of mafic enclaves in the lago della vacca complex lvc of the adamello massif italy to estimate the timescales of pluton emplacement our calculations suggest total emplacement timescales of the order 50 to 150 ky in excellent agreement with recent high precision radiometric dating of zircons from the lvc

coastal circulation caused by an isolated storm abstract the strength of storm generated coastal current and the intensity of coastal upwelling depend on the scales of the storm and the position of the storm with respect to the position or measurement it is known that long coastal trapped waves travel poleward along eastern ocean boundaries and that a storm may generate these waves at its equatorward and poleward edges if a storm lasts longer than coastal trapped waves take to propagate from the equatorward edge of the storm to the position of measurement then the strongest current and upwelling occur poleward of the storm if the storm is brief in this sense then the strongest current and upwelling appear within the storm region after the storm decays a single storm with equatorward winds will generate a poleward traveling upwelling event followed by a more slowly traveling undercurrent event the alongshore scale and time scale of the upwelling event bear a strong correspondence to the scales of the storm only if the storm starts and stops abrup

magma evolution leading to porphyry au cu mineralization at the ok tedi deposit papua new guinea trace element geochemistry and high precision geochronology of igneous zircon

evaluation of the astronomical time scale for the paleocene and earliest eocene abstract the astronomical tuned time scale is rapidly extended into the paleogene but due to the existence of an eocene gap different tuning options had to be presented for the paleocene these options differ both in number and tuning of 405 kyr eccentricity related cycles and are only partially consistent with recalculated 40 ar 39 ar constraints for the cretaceous paleogene k pg and paleocene eocene p e boundaries in this paper we evaluate the cyclostratigraphic interpretation of records from odp leg 198 and 208 sites and the zumaia section to solve the problem of the different tuning options we found that the interval between the k pg boundary and the early late paleocene biotic event elpe comprises 17 instead of 16 405 kyr eccentricity related cycles as previously proposed while the entire paleocene contains 25 405 kyr cycles starting from 40 ar 39 ar age constraints for the k pg boundary a new tuning to 405 kyr eccentricity is presented for the paleocene and earliest eocene which results in ages of 66 0 and 56 0 ma for the k pg and p e boundaries respectively this tuning introduces considerable differences in age for a number of nannofossil events at odp sites 1209 and 1262 in the interval between 61 and 63 ma but eliminates large and abrupt changes in the seafloor spreading rate the tuning seems further consistent with recalculated 40 ar 39 ar ages for ash layer 17 of early eocene age however despite this apparent consistency with existing radio isotopic constraints an alternative 405 kyr younger or less likely older tuning cannot be excluded at this stage

emplacement related deformation of granitoid magmas southern adamello massif italy the lago della vacca suite in the tertiary adamello massif italian alps is a small 10 km 2 crudely zoned granitoid pluton emplaced at a pressure of 3 5 kbar into folded mesozoic sediments and precursor intrusive rocks field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early passive stoping by the mafic to intermediate composition marginal units to later forceful emplacement of the more silicic core marginal units of the pluton have highly discordant stoped outer contacts against folded triassic carbonates and precursor mafic ultramafic rocks in contrast the core units have steeply inward dipping contacts against these marginal units magmatic foliations defined by the planar alignment of plagioclase amphibole and biotite and by mafic enclaves are well developed in all units they describe steeply inward dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin lineations are rarely present the most intensely foliated rocks occur in the marginal units as anastomosing sinuous bands that wrap around lenses of less intensely foliated granitoid three dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain strain ellipsoids are weakly prolate within the core of the pluton becoming increasingly oblate as strain increases toward the pluton margin the short axis z of the strain ellipsoid is uniformly subnormal to the foliation suggestive of radial flattening textures of the marginal rocks which record high strain coupled with amphibole plagioclase thermometry indicate that deformation occurred in the magmatic or submagmatic state 650 780 c and was accompanied by significant migration of interstitial melt the documented transition from passive to forceful emplacement and final geometry of the suite resulted from temporal changes in the rheology of both the magma and adjacent country rocks pulses of low density hydrous magma were initially emplaced by stoping denser country rocks on cooling and solidification these magmas contracted by 15 20 creating an unpressured volume which allowed subsequent magma pulses to be emplaced into the pluton core these pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas emplacement of the lago della vacca suite was therefore the product of several internal or magma related processes an interaction between stoping radial expansion and volume contraction on solidification of the hydrous magmas the heterogeneous distribution of strain in the marginal units reflects local variations in temperature and most importantly in the distribution of interstitial melt during emplacement

poleward along shore current pulses on the inner shelf of the bay of biscay abstract we analyzed strong events of coastal poleward along shore currents above 10 cm s 1 and up to more than 50 cm s 1 on the inner shelf 50 80 m depth of the bay of biscay bob from the spanish coast to the brittany coast we used data from four acoustic doppler current profilers adcps deployed from july 2009 to august 2011 the goal of this study was to analyze current variability at meso and subinertial scales and its generation mechanism these currents occurred all year long and were classified into three types events occurring principally in the southern part of the bob were classified as southern events bay scale events were defined when strong poleward currents were detected over all the shelf typically stronger on the spanish and the southern brittany shelves strong events were characterized by depth averaged current velocities over 40 cm s 1 in the southern part of the bob at short time lags the along shore currents were clearly related to along shore wind stress at upstream locations an explanation is provided for longer time lags in terms of coastal trapped wave ctw dynamics the first ctw mode phase speeds were in agreement with the propagation speeds of the fastest events 5 m s 1 while inner shelf modes could explain the slowest events 1 3 m s 1 the cross shelf density gradient and the extension of the ipc were also associated with strong coastal poleward along shore currents the duration of the events the vertical structure of the currents and the associated coastal trapped waves were studied in relation with the stratification

inner shelf circulation patterns and nearshore flow reversal under downwelling and stratified conditions off a curved coastline 1 the role of a curved coastline and associated bathymetry in the development of downwelling circulation in a stratified inner shelf is examined through a number of numerical experiments different scenarios include constant versus variable wind forcing and variations in bottom friction the three dimensional response of the shelf within the domain embayment enclosed by capes is associated with the generation of a velocity pycnocline disturbance at the upstream cape and its subsequent downstream advection this disturbance is more pronounced under variable wind conditions its downstream advection through the bay exhibits different patterns depending on the competition between inertia and bottom friction near the cape when inertia dominates the disturbance separates from the cape and travels downwind with an enhanced downstream flow offshore and a countercurrent inshore the separation occurs at a low rossby number ro 0 15 which is attributed to the positive curvature of the coastline forming the cape when friction dominates the advection path is constrained along the coastline resulting in an alongshore temperature gradient and a transient thermal front running almost perpendicular to the coast isobaths simulations with spatially variable bottom friction with higher friction toward the coast result in the generation of eddy like features the numerical results are in agreement with both observations and surface temperature imagery from long bay south carolina an embayment enclosed by two capes and emphasize the role that coastline and associated shelf morphology can play in enhancing cross shelf transport and exchange

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

resonance of long waves generated by storms obliquely crossing shelf topography in a rotating ocean the oceanic forced wave beneath a moving atmospheric disturbance is amplified by proudman resonance when modified by the earth s rotation this classical resonance only occurs if the disturbance time scale is smaller than the inertial period with or without coriolis effects free transients generated by storm forced waves obliquely crossing step changes in water depth at particular angles are shown to resonate by exciting a range of barotropic free waves rotationally influenced slow atmospherically forced waves crossing a vertical coast at a critical angle lead to a form of sub critical resonance which occurs only when the component of the disturbances phase velocities along the coast matches that of a free kelvin wave kw in a rotating ocean transients generated by disturbances crossing a step at a particular angle are shown to excite a free double kelvin wave dkw this new type of resonance only occurs for sufficiently large steps and disturbances with time scale greater than the inertial period a storm crossing a step shelf can result in the excitation of an infinite set of edge waves a single kw a unique dkw and a first mode continental shelf wave depending on the topography and the disturbance time scale translation speed and incident angle the study of resonances and wave mode excitations generated by storms crossing a coast or a continental shelf may contribute to understanding how a particular combination of the storm characteristics can result in destructive coastal events with time scales encompassing the typical meteotsunami period band tens of minutes and storm surges with periods of several hours or days

u pb re os and 40ar 39ar geochronology of the nambija au skarn and pangui porphyry cu deposits ecuador implications for the jurassic metallogenic belt of the northern andes new u pb re os and 40ar 39ar dates are presented for magmatic and hydrothermal mineral phases in skarn and porphyry related ores from the nambija and pangui districts of the subandean zone southeastern ecuador nambija has been one of the main gold producing centers of ecuador since the 1980s due to exceptionally high grade ores average 15 g t but frequently up to 300 g t au pangui is a recently discovered porphyry cu mo district the geology of the subandean zone in southeastern ecuador is dominated by the i type subduction related jurassic zamora batholith which intrudes triassic volcanosedimentary rocks the zamora batholith is in turn cut by porphyritic stocks which are commonly associated with skarn formation and or porphyry style mineralization high precision u pb and re os ages for porphyritic stocks u pb zircon associated prograde skarn u pb hydrothermal titanite and retrograde stage skarn re os molybdenite from veins postdating gold deposition of the nambija district are all indistinguishable from each other within error 145 ma and indicate a late jurassic age for the gold mineralization previously gold mineralization at nambija was considered to be early tertiary based on k ar ages obtained on various hydrothermal minerals the new jurassic age for the nambija district is slightly younger than the 40ar 39ar and re os ages for magmatic hydrothermal minerals from the pangui district which range between 157 and 152 ma mineralization at nambija and pangui is associated with porphyritic stocks that represent the last known episodes of a long lived jurassic arc magmatism 190 to 145 ma a jurassic age for mineralization at nambija and pangui suggests that the northern andean jurassic metallogenic belt which starts in colombia at 3 n extends down to 5 s in ecuador it also adds a new mineralization style au skarn to the metal endowment of this belt

geochronological constraints on post extinction recovery of the ammonoids and carbon cycle perturbations during the early jurassic abstract this paper presents the first quantitative study of the early jurassic recovery of ammonoids after the end triassic mass extinction based on detailed u pb id tims isotope dilution thermal ionization mass spectrometry geochronology from ash bed zircons placed within a clear phylogenetical and biochronological framework at the subzonal and species level this study was triggered by the discovery of a rich peruvian succession of ammonites deposited concomitantly with an unusually large number of ash beds two major phases of rediversification are observed during the psiloceras spelae and angulaticeras zones that correspond to positive peaks in the 13corg curve providing a possible link between biodiversity and the global carbon cycle in the case of the post extinction recovery the development of the earliest hettangian ammonites occurs within the genus psiloceras which begins with the occurrence of p spelae and then explodes into worldwide development of smooth psiloceratids of the psiloceras planorbis group s l this rapid biodiversification likely occurred less than 100 ka after the end triassic crisis the genus psiloceras occupied all the possible ecological niches worldwide from the pacific deep waters to the nw european shallow deposits and also in some rare tethyan occurrences like at germig in tibet this global dispersion allowed the differentiation of the group in several major phyla the schlotheimiidae discamphiceratinae arietitidae and lytocerataceae which were the roots of all other jurassic and cretaceous ammonites

shrimp u pb dating of the preeruption growth history of zircons from the 340 ka whakamaru ignimbrite new zealand evidence for 250 k y magma residence times the whakamaru group ignimbrites are products of one of the largest caldera forming eruptions from the central taupo volcanic zone new zealand ca 340 ka imaging of zircons separated from low silica rhyolite pumice from whakamaru group ignimbrite reveals a complex magmatic crystallization history some grains contain resorbed cores cores and rims of individual zircons were dated using the sensitive high resolution ion microprobe due to the high u contents of the zircons 206 pb count rates were sufficiently high to obtain 206 pb 238 u ages with precisions of 5 10 1 s using longer than usual 206 pb count times and this has allowed zircon core and rim ages to be distinguished zircon rim and outer growth zone ages range from within 1 s error of the eruption age to 449 20 ka whereas zircon cores produced ages to 608 20 ka indicating magmatic residence times exceeding 250 k y for some zircons the data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years involving magma replenishment and mixing with residual phases of older silicic magma chambers in addition to crystal fractionation processes prior to eruption the study has shown that 206 pb 238 u dating using high resolution ion microprobe can be used to determine zircon preeruptive growth histories in quaternary lavas and pyroclastic rocks

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

a modeling study of the coastal eastern boundary current system off iberia and morocco to investigate the northern canary current system nccs results from four numerical experiments of increasing complexity are examined experiment 1 which uses seasonal wind forcing only shows that as expected wind forcing is the key generative mechanism for the current upwelling meander eddy and filament structures experiments 2 and 3 which have the additional effects of irregular coastline geometry show the following capes are areas for enhanced upwelling extensive filaments maximum current velocities and enhanced growth of cyclonic meanders and eddies an embayment like the gulf of cadiz is a primary region for anticyclonic meander and eddy development the results from the complex flow regime of experiment 4 which has the additional effects of thermohaline gradients and mediterranean outflow highlights the major characteristics and unique features such as the generation of meddies of the nccs with relatively close similarities to field observations

large scale edge waves generated by hurricane landfall 1 direct observations of the storm surge induced by hurricane wilma s landfall revealed a formation of a wave pulse propagating alongshore the height of the wave pulse exceeded 1 5 m in the detided sea level the duration of this wave pulse was 6 h and the propagation speed was of o 10 m s 1 this wave has been identified as an edge wave of large spatial and temporal scales a set of numerical experiments has been conducted to delineate a generation of edge waves with large spatial and temporal scales by a fast moving storm system the model of the coastal ocean was set in a two dimensional configuration with the continental shelf and slope topography reminiscent of the west florida shelf a moving cyclonic system in the gradient wind balance was prescribed analytically in order to identify a long wave response in the model a linear boundary problem was solved yielding dispersion characteristics and a structure of the edge wave modes a fast moving storm system crossing the shelf at a right angle produces a nearly symmetrical response of two edge wave trains propagating both downstream in the direction of a kelvin wave and upstream typically zero mode edge waves dominate the response as the translation speed of the storm becomes lower its eulerian timescale becomes longer and the waves are more affected by the earth s rotation in that case the wave energy propagates predominantly downstream when the storm trajectory deviates from the normal approach the edge wave response is not symmetric most of the energy propagates in the direction of the alongshore component of the storm translation velocity

anticyclonic eddies trapped on the continental shelf by topographic irregularities nonlinear effects produced during the scattering of a barotropic shelf wave bsw by a spatially varying mean current are studied using a primitive equation numerical model both the bsw phase and the mean current propagate in the same positive direction along shelf slope topography which is uniform everywhere except for a localized topographic irregularity e g a submarine canyon the mean current is specified at the upstream boundary and adjusts to the topography closely following isobaths through the model domain the incident bsw signal is then introduced at the upstream boundary either as a harmonic wave or as a pulse of finite duration the bsw signal scatters its energy into other available wave modes when it encounters the topographic irregularity the scattered wave field is dominated by evanescent modes which are trapped at the topographic irregularity and appear as intense mesoscale flows between the coast and the mean current nonlinear dynamics transform these large amplitude evanescent modes into persistent eddy like features on the shelf the nonlinear interaction is much stronger when the current on the shelf associated with the bsw is opposite to the mean current direction i e negative so anticyclonic eddies are preferentially generated at the topographic irregularity for a harmonic bsw an anticyclonic eddy periodically appears when the negative current phase passes and disappears when the positive current phase passes a bsw pulse with negative velocity at the coast produces a strong anticyclonic eddy which persists after the pulse has passed for a time period substantially longer than the pulse duration a pulse with positive velocity at the coast does not generate any persistent features on the shelf the anticyclonic eddies produce mass exchange between the shelf and the mean current and could contribute significantly to cross shelf exchange on continental shelves

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

incremental growth of the patagonian torres del paine laccolith over 90 k y the miocene paine granite in the torres del paine intrusive complex southern chile is an extraordinary example of an upper crustal mafic and granitic intrusion the granite intruded as a series of three sheets each one underplating the previous sheet along the top of the basal paine mafic complex high precision u pb geochronology on single zircons using isotope dilution thermal ionization mass spectrometry yields distinct ages of 12 59 0 02 ma and 12 50 0 02 ma respectively for the first and last sheet of the laccolith this age relationship is consistent with field observations the zircon ages define a time frame of 90 40 k y for the emplacement of a 2000 m thick granite laccolith these precise u pb zircon ages permit identification of the pulses in a 20 k y range the data obtained for the paine granite fill the gap between 100 k y and 100 1000 yr pulses described in the literature for crustal magma chambers

evolution of large silicic magma systems new u pb zircon data on the nw permian athesian volcanic group southern alps italy abstract during the permian the southern alps were affected by prominent igneous activity that produced voluminous basic to acidic volcanic and plutonic rocks the magmatism originated during a period in which postorogenic extension and transtensional faulting controlled magma ascent and emplacement within this scenario the athesian volcanic group ag with lithologies ranging from basaltic andesites to rhyolites constitutes a major part of the permian magmatism in the central and eastern southern alps we present an up to date stratigraphic reconstruction of the northwestern sector of the ag volcanic sequence combining detailed field mapping and new geochronological data single zircon u pb age data suggest that the volcanism in the ag lasted ca 10 m yr between 285 and 274 ma with a deposition rate that increased throughout the eruptive cycle the processes of magma deposition and basin formation were strongly influenced by tectonic activity over the entire history of the ag at least two major

zircon inheritance in an igneous rock suite from the southern adamello batholith italian alps zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern adamello batholith s ab southern alps n italy display complex u pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous inherited radiogenic pb and to minor postmagmatic loss of pb inherited pb is mainly composed of 1 a 1100 ma pb component located in zircons devoid of visible cores and 2 a 450 ma component associated with conspicuous bubble rich turbid cores in concordia representation the data points conform to lower intercept ages of 40 ma a linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of 39 3 ma u pb systematics of zircon in particular u content and crystal morphology are clearly related zircons of type g1 which form relatively late in the zircon crystallization sequence consistently show the highest u contents in each zircon population these late zircons however are not devoid of inherited radiogenic lead in a population from a granodiorite cores are randomly distributed throughout the morphological spectrum the presence of old inherited zircon components in all investigated samples furnishes proof for involvement of crustal material in the genesis of the s ab rocks samples characterized by crustal nd and sr values usually show enhanced zircon inheritance inheritance varies with differentiation and reaches a maximum for intermediate to acidic members these compositions show the highest zr saturation temperatures calculated for the rock spectrum studied textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with zr at an early stage of crystallization on the other hand zr solubilities and textural relations consistently show that melts of basic to intermediate rocks were not saturated with zr extension of the zr solubility model to mineral melt mixtures of tonalitic bulk composition demonstrates that zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole survival of xenocrystic zircons in the mafic to intermediate rocks of the s ab can best be explained in terms of dissolution kinetics since temperature and h2o content of these zr undersaturated melts were favourable for relatively rapid zircon dissolution inherited zircons in particular trace element rich unstable cores cannot have been exposed to such conditions over extended time periods therefore the tonalitic or more basic magmas of the s ab cannot have been derived from crustal sources by slow processes such as burial metamorphism

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

episodic and long lasting paleozoic felsic magmatism in the pre alpine basement of the suretta nappe eastern swiss alps the suretta nappe of eastern switzerland contains a series of meta igneous rocks with the rofna porphyry complex rpc being the most prominent member we present la icp ms u pb zircon data from 12 samples representing a broad spectrum of meta igneous rocks within the suretta nappe in order to unravel the pre alpine magmatic history of this basement unit fine grained porphyries and coarse grained augengneisses from the rpc give crystallization ages between 284 and 271 ma which either represent distinct magma pulses or long lasting magmatic activity in a complex magma chamber there is also evidence for an earlier variscan magmatic event at 320 310 ma mylonites at the base of the suretta nappe are probably derived from either the rpc augengneisses or another unknown carboniferous permian magmatic protolith with a crystallization age between 320 and 290 ma two polymetamorphic orthogneisses from the southern suretta nappe yield crystallization ages of 490 ma inherited zircon cores are mainly of late neoproterozoic age with minor neo to paleoproterozoic sources we interpret the suretta nappe as mainly representing a gondwana derived crustal unit which was subsequently intruded by minor cambrian ordovician and major carboniferous permian magmatic rocks finally the suretta nappe was thrust into its present position during the alpine orogeny which hardly affected the u pb system in zircon

time scales of pluton construction at differing crustal levels examples from the mount stuart and tenpeak intrusions north cascades washington deciphering the magmatic history of continental magmatic arcs in general and the growth history of individual intrusions in particular is key to understanding the complex history of magma generation segregation and transport that define the dynamics of crustal growth we utilize high precision u pb geochronology to resolve a detailed magmatic history from two composite intrusions the 2 4 kbar mount stuart batholith and the 7 10 kbar tenpeak pluton emplaced in the cretaceous north cascades arc this temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts u pb zircon crystallization ages were obtained from 12 samples of the mount stuart batholith and 8 samples of the tenpeak intrusion representing the range of compositional diversity and geographical extent these dates indicate that the mount stuart batholith was constructed over a 5 5 m y time period that was punctuated by four intervals of high magma flux the durations of the high flux periods are short a few hundred thousand years relative to the duration of the batholith the consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time transgressive magma fabrics in contrast the tenpeak intrusion was constructed over a 2 6 m y time period with magma influx distributed throughout the intrusive history and texturally distinct magma bodies the tenpeak intrusion lacks distinct age domains which suggests that any magma reservoir was smaller in size and potentially more ephemeral although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally neither pluton bears resemblance to a purely end member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little if any interaction in particular ages from the youngest domain of the mount stuart batholith indicate that a melt rich magma reservoir of 520 km 3 existed over a 170 90 k y time span

geochemistry of the adamello massif northern italy the tertiary adamello massif outcropping over an area of more than 550 km2 in the southern alps northern italy is composed mainly of granitoid rocks granodiorite tonalite quartz diorite with minor amounts of diorite and gabbro the major and trace element composition of these rocks is comparable to calc alkaline volcanic rocks of continental margins the granitoid rocks display spatial and temporal variations in their composition particularly in na p sr la nb and y contents and 87sr 86sr ratios the variations were probably produced by concurrent contamination wall rock assimilation and fractional crystallization of high alumina basaltic magma

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

mo rich source and protracted crystallization of late mesozoic granites in the east qinling porphyry mo belt central china constraints from zircon u pb ages and hf o isotopes abstract in the east qinling orogenic belt central china there are numerous mo deposits with over 6 mt mo metal the giant nannihu shangfanggou and yuchiling porphyry mo deposits are hosted in porphyry stocks associated with the heyu batholith whereas the giant donggou porphyry mo deposit is hosted in a stock associated with the taishanmiao batholith zircon grains from the heyu batholith have concordant 238u 206pb ages scattered from 150 3 to 130 2 ma with an age interval of nearly 20 m y whereas ore hosting granitic porphyry stocks from the nannihu shangfanggou and yuchiling deposits have 206pb 238u ages ranging from 151 1 to 135 1 ma 143 1 to 132 1 ma and 143 5 to 131 5 ma respectively it is likely that the heyu batholith has a prolonged history of incremental assembly and the ore bearing granitic porphyry stocks may have originated from the same magma reservoir and emplaced in different stages likewise zircon grains from the taishanmiao batholith have concordant 238u 206pb ages spanning in a period from 130 2 to 111 3 ma whereas those from the donggou granitic porphyry stock have 238u 206pb ages ranging from 125 2 to 110 3 ma nearly coeval with the taishanmiao batholith the molybdenite re os model ages for each deposit are coincident with relatively young zircon 238u 206pb ages of the host stock indicating that mo mineralization is likely related to late stage magmatism zircon grains from the batholiths and granitic porphyry stocks have ehf t ranging from 10 to 30 tdm2 ages from 1 6 to 2 5 ga and 18o from 5 0 to 8 7 which are taken to suggest the involvement of the subducted continental crust of the yangtze block in the source we thus propose that the extensive mo mineralization in the east qinling porphyry mo belt is genetically related to the prolonged granitic magmatism in the late mesozoic and a mo rich source that was related to the subducted continental crust of the yangtze block stagnating beneath the southern margin of the north china craton

paleozoic evolution of southern alpine crust northern italy as indicated by contrasting granitoid suites abstract the western southalpine crust of northern italy and southern switzerland consists of high grade ivrea verbano and medium grade rocks serie dei laghi the latter contains ordovician og and permian granitoids pg the og which range in composition from diorite to leucogranite show a calcalkaline metaluminous character the felsic lithologies yield a rb sr w r isochron of 466 5 ma with i r of 0 7087 the most mafic lithologies do not plot on the isochron but arrange on a mixing line indicating mantle crust interaction 87 sr 86 sr i 0 704 0 709 their initial pb isotope ratios 206 pb 204 pb i 17 47 18 00 207 pb 204 pb i 15 55 15 63 208 pb 204 pb i 37 66 38 12 fall within typical values of the lower crust linear trends in the pb pb diagrams are in agreement with a mantle crust interaction the pg consist of mafic to intermediate dykes and granitic plutons with calcalkaline and weakly peraluminous character two felsic plutons give two whole rock isochrons at 276 7 and 277 8 ma with the same i r of 0 710 the mafic dykes show sr isotopic variations indicating mantle crust interaction 87 sr 86 sr i 0 705 0 710 the pb isotope ratios of pg 206 pb 204 pb i 18 12 18 53 207 pb 204 pb i 15 63 15 73 208 pb 204 pb i 38 08 38 73 are distinctly higher than those of og and similar to those of the ivrea verbano metasediments og are depleted in both lile and hfse show less pronounced negative anomalies of sr p and ti and have lower sr and pb isotope ratios than pg for both og and pg we suggest that the most mafic representatives were generated through interaction of mantle derived magma and crustal material while the granitoids derived from the resulting hybrid magma mainly through crystal fractionation the ultimate sources of the ordovician and permian magmatisms were different both geochemically and isotopically reflecting two different geodynamic environments a convergent plate boundary in an accretionary terrane in the ordovician a post orogenic extensional regime in cratonized continental crust in the permian geological data are compatible with such environments

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation