dating young zircon a case study from southeast asian megacrysts abstract u pb geochronology of quaternary zircon crystals is challenging due to their low levels of radiogenic pb the importance of accurate common pb correction and the potential need for secular disequilibrium correction u th he dating offers an independent benchmark for the quality of u pb data and their reduction including the various corrections as the closure temperature of the he system in zircon is 900 c hence u pb ages must predate u th he ages or at least be equal to them in the case of rapid cooling analyses of southeast asian mantle derived zircon megacrysts from tram poung tr tok phrom to bang kacha ba and pailin pa provide new information about their pre eruptive and eruptive history sims u pb zircon ages of 6 7 3 4 ma tr 2 45 0 23 ma to 1 44 0 13 ma ba and 0 97 0 41 ma pa correspond to u th he ages of 5 30 0 52 ma tr 2 42 0 10 ma to 1 05 0 05 ma ba and 0 91 0 02 ma pa respectively the youngest of these u pb ages pa is corroborated by la icpms mapping of a grain that yields an age of 1 017 0 067 ma the grain mapping reveals no evidence for pb diffusion and the calculated pb diffusion temperatures for these megacrysts of 1110 1250 c are significantly greater than ti in zircon apparent temperatures of 659 c these observations may imply that the u pb ages reflect magmatic crystallization and hence the close similarity of the u pb and u th he dates observed in all samples documents high zircon megacryst growth rates of 1 10 12 to to 2 10 14 cm s tr

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

late cretaceous cenozoic tectonic development of the southwest pacific region abstract a new model of the plate tectonic development of the southwest pacific integrates the continental geology of new zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations the model differs from previous ones in that the onland geology of new zealand is used to constrain the tectonic development in two important ways 1 the modern australia pacific plate boundary did not transect the new zealand sector until 23 million years ago 2 there has been a total of only c 500 km of dextral displacement on the plate boundary through new zealand the model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32 24 7 and 5 novel interpretations based on these reconstructions include the following during the late cretaceous the campbell fault was a continental transform fault between the tasman sea spreading centre and the bounty rift its 330 km of dextral displacement accompanying 25 of counterclockwise rotation of a campbell plateau block when the eastward propagating southeast indian ridge broke into the south tasman sea 57 million years ago it continued to propagate directly into southern south island and generated late eocene oligocene continental rifting through western new zealand at the same time back arc spreading in the norfolk basin caused continental rifting along a similar trend in western north island the australia pacific plate boundary originated as a transform fault between the southeast indian ridge and the pre existing kermadec trench it developed in response to the sudden cessation of spreading on the segment of the southeast indian ridge east of a fracture zone here named fracture zone z two implications arise from this model regarding the integrity of antarctica 1 1 during the cretaceous marie byrd land was probably no more than 200 km northwest of its present position 2 2 the proposition made in some earlier plate tectonic reconstructions that if the alpine fault did not form until after the late eocene there must be a late cretaceous early tertiary plate boundary within antarctica is most probably valid

timing trace and origin of basaltic migration in eastern australia eastern australian cenozoic volcanism includes a prominent southward migration of activity varied explanations have invoked hotspot or linear magma sources in the asthenosphere the movement of australia over old seafloor spreading zones with direct or delayed response in volcanism or changes in stress fields the present appraisal using new data contradicts some ideas and favours volcanism triggered directly by passage of lithosphere over initial sites of coral sea spreading the conclusions suggest that eastern australia has a major complex hotspot trail established for the past 65 myr it forms an independent and key continental reference to the hawaiian emperor oceanic trail it raises the notion that both continental and oceanic trails can mark the ghosts of former spreading sites

partial transformation of gabbro to coesite bearing eclogite from yangkou the sulu terrane eastern china an ultra high pressure uhp metamorphic slab at yangkou beach near qingdao in the sulu region of china consists of blocks of eclogite facies metagabbro metagranitoid ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss a gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra high pressures was identified in a single 30 m block metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins the metagabbro contains an igneous assemblage of pl aug opx qtz bt ilm ti mag it shows relict magmatic textures and reaction coronas fine grained garnet developed along boundaries between plagioclase and other phases primary plagioclase broke down to ab ky ms zo grt amp augite is rimmed by sodic augite or omphacite whereas orthopyroxene is rimmed by a corona of cum act and omp qtz layers or only omp qtz in transitional rocks augite and orthopyroxene are totally replaced by omphacite and the lower pressure assemblage ab ky phn zo grt coexists with domains of omp jd70 73 ky phn in pseudomorphs after plagioclase both massive and weakly deformed coesite bearing eclogites contain omp ky grt phn coe qtz rt and preserve a faint gabbroic texture coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz some intergranular coesite and quartz pseudomorphs after coesite also occur assemblages of the coronal stage transitional and uhp peak occurred at about 540 50 c at c 13 kbar 600 800 c at 15 25 kbar and 800 850 c at 30 kbar respectively garnet from the coronal through the transitional to the eclogite stage rocks show a decrease in almandine and an increase in grossular pyrope components garnet in low grade rocks contains higher mno and lower pyrope components the growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex the application of garnet zoning to estimate p t should be carried out with caution some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries these quartz aggregates as well as other primary and low p phases persisted metastably at uhp conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde p t evolution

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

some ir features of sio 4 and oh in coesite and its amorphization and dehydration at ambient pressure abstract coesite coe with grain size in the range of 30 80 m has been synthesized at 5 gpa and 1600 c for 12 h by using a cubic press its unpolarized single crystal absorption infrared ir spectra show 14 ir bands in the range of 1200 650 cm 1 five of which have high intensity at 1161 1109 1063 1028 and 994 cm 1 and are preliminarily assigned to the sio4 asymmetric stretching 3 in addition three sharp but relatively weak bands at 838 814 and 796 cm 1 are tentatively attributed to the sio4 asymmetric bending 4 the ir data also show six oh peaks in the range of 3700 3300 cm 1 with an estimated h2o content of 30 4 wt ppm following previous studies we have assigned the peaks at 3464 7 3421 8 3406 9 and 3377 cm 1 10 to the type ii hydrogarnet substitution and the peaks at 3500 6a and 3534 cm 1 6b to the b based defects with the latter aroused by possible b contamination in the synthesizing experiments annealing experiments conducted consecutively at 200 400 600 800 1000 and 1200 c with every heating step lasting for 24 h demonstrate that water diffuses quickly out of coe at t as low as 600 c the material annealed at 1200 c is completely dehydrated and amorphous a quick response of the water content in coe to the changes of p t and composition is thus possible which may be critical to the preservation of natural coe in relevant geological processes it further implies that water in coe and possibly in other nominally anhydrous minerals nams may behave distinctively different from the water located in the hydrous phases such as amphibole and mica and potentially makes significant contribution to the subduction zone related fluids

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

eduction extension and exhumation of ultrahigh pressure rocks in metamorphic core complexes due to subduction initiation the controversy over the exhumation of ultrahigh pressure uhp rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes eduction i e reverse subduction we present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together crust subducted deep enough is heated and weakened causing limited diapiric rise while crust subducted to shallower depths retains strength and is exhumed only by eduction the model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence this occurs spontaneously when subduction of buoyant crust causes a subduction zone to lock up in one place causing a new subduction zone to form in another the model is consistent with many features of the youngest region of uhp rock exhumation on earth the d entrecasteaux islands uhp exhumation and the amount of regional extension as well as the seismic structure around the islands can be explained by eduction ductile flow fabrics seen on the islands would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting reversal of motion on the north dipping continental subduction zone required by this model was likely triggered by initiation of the new britain trench as suggested previously our model implies that the crust of goodenough basin south of the islands was exhumed by eduction in the last 5 ma and this hypothesis can be tested by drilling

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

u and th zonation in fish canyon tuff zircons implications for a zircon u th he standard the inability to accurately determine the distribution of u and th in zircon crystals analysed using u th he is a major source of error and limits the confidence in subsequent data interpretation the fish canyon tuff fct zircon standard shows u th he age reproducibility in excess of 10 we have tested the extent to which this is due to u and th zonation using a combination of cathodoluminescence and ion probe analysis of a population of fct zircons primarily we find that fct zircons exhibit extreme u and th zonation and the population has large inter crystalline variability furthermore the net cathodoluminescence intensity from the fct zircons is negatively correlated with u and th concentration allowing cathodoluminescence emission to be used as a proxy for u and th zonation this correlation was exploited to constrain the u and th zonation within the crystal population and to show that the poor age reproducibility of the fct zircons is consistent with the observed zonation

precise determination of the phase boundary between coesite and stishovite in sio2 abstract the phase boundary between coesite and stishovite in sio2 was determined over the range of 1200 1700 k using a multi anvil high pressure apparatus combined with the synchrotron x ray diffraction technique the stability of each phase was determined by observing the powdered x ray diffraction data the transition boundary between the coesite and stishovite phases was found to occur at p gpa 4 7 0 0031 t k the transition pressure determined in this study was in general agreement with what was observed in previous high pressure experiments and heat capacity measurements however the slope of the transition dp dt determined in our study was higher than that estimated in a previous study utilized with the in situ x ray method in contrast the slope calculated by the heat capacity data was in good agreement with that determined in our study the phase transition determined in our study occurs at around 10 gpa at the normal mantle geotherm coinciding with the seismic discontinuity around 300 km depth known as the x discontinuity

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

volcanic eruption eye witnessed and recorded by prehistoric humans abstract human footprints in hydrovolcanic ash near cakallar volcano kula western turkey were discovered in 1968 a nearby pictograph interpreted as depicting cakallar volcano would define it as the oldest site where humans demonstrably eye witnessed a volcanic eruption and possibly artistically recorded it despite cakallar s volcanological and cultural importance its eruption age has remained controversial here two independent dating methods cosmogenic 36cl and combined u pb and u th he zircon zdd geochronology yielded the first internally consistent eruption ages controlled by detailed volcanostratigraphic mapping concordant 36cl ages of 4 7 0 6 ka errors 1 were obtained for a cone breaching lava flow zdd ages for crustal xenoliths from scoria deposits directly overlying the footprints yielded an age of 4 7 0 7 ka this firmly places the cakallar eruption and prehistoric human footprints and plausibly the rock art into the bronze age reinforcing the notion that prehistoric artwork recorded natural events

transformation kinetics of polycrystalline aragonite to calcite new experimental data modelling and implications experimental data are used to model the transformation rate of polycrystalline aragonite grain diameter 80 m to calcite optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280 to 380 c and 0 10 mpa to an expression for a grain boundary nucleated and interface controlled transformation the nucleation rate is 4 5 orders of magnitude faster than for calcite nucleated within aragonite grains and the growing in rate is slower below 300 c than for calcite growing in aragonite single crystals the activation enthalpy for growth in polycrystalline aggregate is 247kj mol compared to 163 kj mol for growth in single crystals permanent deformation of the phases limits the elastic strain energy due to the 7 volume change and reduces the coherency of the calcite aragonite interace theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures and data from 0 10 to 400 mpa are used to evaluate the effect of pressure on the grain boundary nucleation rate because of permanent deformation of the phases the effective strain energy for nucleation is 0 55 kj mol which is less than a quarter of the value for purely elastic deformation these data are used to predict the percent transformation for various p t t paths without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235 c and the kinetic data are also consistent with published p t t paths which include heating during uplift the predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite but strongly dependent on the effective strain energy

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

the effects of secular disequilibrium on u th he systematics and dating of quaternary volcanic zircon and apatite the u th he dating method applied to u rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions however in phases with crystallization ages less than 1 ma chemical fractionation within the 238 u decay series may modify the he ingrowth rate causing he ages computed from the secular equilibrium age equation to be incorrect the resulting systematic error depends on the 230 th 238 u activity ratio of the dated phase when it is erupted and on the eruption age zircons which exclude th relative to u will likely have secular equilibrium he ages that underestimate the eruption age by up to a few tens of decreasing with increasing eruption age apatites tend to accommodate u and th with little fractionation so apatite secular equilibrium he ages will be nearly concordant with eruption age if minerals are erupted immediately after crystallization the disequilibrium effect can be reasonably accounted for based on th u systematics however crystals are likely to reside for unknown but potentially long periods in a magma chamber such that the degree of secular disequilibrium will be reduced prior to the onset of he accumulation u th he analyses of co genetic phases that fractionate the u th ratio differently like apatite and zircon can be used to better constrain eruption age as well as to provide insights into magma chamber residence time we illustrate this approach with u th he analyses of zircons and apatites of the pleistocene age rangitawa tephra new zealand

seismic structure crustal architecture and tectonic evolution of the anatolian african plate boundary and the cenozoic orogenic belts in the eastern mediterranean region the modern anatolian african plate boundary is represented by a north dipping subduction zone that has been part of a broad domain of regional convergence between eurasia and afro arabia since the latest mesozoic a series of collisions between gondwana derived ribbon continents and trench roll back systems in the tethyan realm produced nearly east west trending subparallel mountain belts with high elevation and thick orogenic crust in this region ophiolite emplacement terrane stacking high p and barrovian metamorphism and crustal thickening occurred during the accretion of these microcontinents into the upper plates of tethyan subduction roll back systems during the late cretaceous early eocene continued convergence and oceanic lithospheric subduction within the tethyan realm were punctuated by slab breakoff events following the microcontinental accretion episodes slab breakoff resulted in asthenospheric upwelling and partial melting which facilitated post collisional magmatism along and across the suture zones resumed subduction and slab roll back induced upper plate extension triggered a tectonic collapse of the thermally weakened orogenic crust in anatolia in the late oligocene miocene this extensional phase resulted in exhumation of high p rocks and mediumto lower crustal material leading to the formation of metamorphic core complexes in the hinterland of the young collision zones the geochemical character of the attendant magmatism has progressed from initial shoshonitic and high k calc alkaline to calc alkaline and alkaline affinities through time as more asthenosphere derived melts found their way to the surface with insignificant degrees of crustal contamination the occurrence of discrete high velocity bodies in the mantle beneath anatolia as deduced from lithospheric seismic velocity data supports our tethyan slab breakoff interpretations pn velocity and sn attenuation tomography models indicate that the uppermost mantle is anomalously hot and thin consistent with the existence of a shallow asthenosphere beneath the collapsing anatolian orogenic belts and widespread volcanism in this region the sharp north pointing cusp isparta angle between the hellenic and cyprus trenches along the modern anatolian african plate boundary corresponds to a subductiontransform edge propagator step fault which is an artifact of a slab tear within the downgoing african lithosphere

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

an experimental investigation of hydroxyl solubility in jadeite and na rich clinopyroxenes the solubility and incorporation mechanisms of water in synthetic water saturated jadeite and na rich clinopyroxenes have been experimentally investigated infrared spectra for water saturated jadeite synthesised from 2 0 to 10 gpa show two prominent sharp peaks at 3 373 and 3 613 cm 1 together with several weaker features in the oh stretching region indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 gpa and slowly decreases with increasing pressure to about 100 ppm at 10 gpa solubility can be described by the function coh a fh2o0 5 exp p vsolid rt where coh is water solubility in ppm h2o by weight a is 7 144 ppm bar0 5 fh2o is water fugacity and vsolid 8 019 cm3 mol is the volume change of the clinopyroxene upon incorporation of oh jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions assignment of absorption bands in ir spectra verifies the importance of cation vacancies on the m2 site in providing mechanisms for hydrogen incorporation however results also suggest that substitution of lower valency cations onto the m1 site may also be important solid solution of jadeite with diopside and in particular with ca eskola component leads to a drastic increase of water solubility and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals

episodic growth and homogenization of plutonic roots in arc volcanoes from combined u th and u th he zircon dating abstract tracing the fate of unerupted magma is challenging because plutonic roots of young volcanoes are largely inaccessible here we develop the use of zircon age spectra to determine crystal provenance and source rocks for volcanic products in analogy to detrital crystals in sediments u th zircon crystallization ages for the soufriere volcanic complex saint lucia lesser antilles frequently predate their eruption as determined from combined u th and u th he zircon dating the oldest dated eruptions are 273 15 ka and 264 8 ka 1 uncertainty for morne bonin dacite and bellevue pumice deposit respectively the most recent eruptions formed morphologically pristine domes in the center of the qualibou depression belfond 13 6 0 4 ka terre blanche 15 3 0 4 ka u th u pb zircon crystallization ages determined for crystal rims and interiors range between near eruption ages to 600 ka older xenocrysts are absent zircon crystallization age distributions are complex yet systematic crystal rim ages in the most recently erupted volcanic rocks match those of co erupted plutonic inclusions whereas crystal interiors are equivalent to the cumulative distribution of zircon ages from older eruptions this is evidence that silicic lava domes and pyroclastic flows share a common source that is located underneath the qualibou depression where the intrusive roots of this long lived arc volcanic system became homogenized through thermal and mechanical reprocessing of individual batches of unerupted magma from earlier volcanic episodes within timescales of

plate boundary tectonics global sea level changes and the development of the eastern south island continental margin new zealand southwest pacific the great south basin is a mid cretaceous continental margin rift related to the break up of the new zealand sector of gondwanaland syn rift cretaceous paleogene sediment accumulated up to a thickness of 8 km followed by post rift subsidence of the northwest and southeast basin margins to form the canterbury basin and campbell plateau respectively the sedimentary evolution of the eastern south island continental margin is described within the context of four megasequences containing eight regionally recognizable seismic sequences syn rift phases 1 2 non marine graben fill post rift phases 3 6 epeiric marine transgression paraconformity phase 7 mid oligocene maximum transgression and syn orogenic phase 8 plate boundary driven terrigenoclastic regression the successive phases of this overall transgressive regressive cycle can be equated with the natural and progressive development of the sedimentary sequence within known regional factors the eastern south island margin sequence forms an appropriate southern hemisphere test of the global sea level curves of vail et al 1977 and haq et al 1986 although eustatic sea level cycles may have played some part in the differentiation of transgressive phases 3 7 the stratigraphic sequence is adequately interpreted in terms of intrinsic basin evolution within a local tectonic context

acigol rhyolite field central anatolia part 1 high resolution dating of eruption episodes and zircon growth rates protracted pre eruptive zircon residence is frequently detected in continental rhyolites and can conflict with thermal models indicating briefer magma cooling durations if scaled to erupted volumes here we present combined u th and u th he zircon ages from the acigol rhyolite field central anatolia turkey which is part of a quaternary bimodal volcanic complex unlike other geochronometers this approach dates crystallization and eruption on the same crystals allowing for internal consistency testing despite the overall longevity of acigol rhyolite volcanism and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during middle and late pleistocene times respectively for late pleistocene zircon resolvable differences exist between interior average 30 7 0 9 ka 1 error and rim 21 9 1 3 ka crystallization ages these translate into radial crystal growth rates of 10 13 to 10 14 cm s broadly consistent with those constrained by diffusion experiments rim crystallization and u th he eruption ages 24 2 0 4 ka overlap within uncertainty evidence for brief zircon residence at acigol contrasts with many other rhyolite fields suggesting that protracted zircon crystallization in or recycling from long lived crystal mushes is not ubiquitous in continental silicic magma systems instead the span of pre eruptive zircon ages is consistent with autochthonous crystallization in individual small volume magma batches that originated from basaltic precursors

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

discovery of coesite and stishovite in eucrite howardite eucrite diogenite meteorites heds probably originated from the asteroid 4 vesta we investigated one eucrite bereba to clarify a dynamic event that occurred on 4 vesta using a shock induced high pressure polymorph we discovered high pressure polymorphs of silica coesite and stishovite originating from quartz and or cristobalite in and around the shock melt veins of bereba lamellar stishovite formed in silica grains through a solid state phase transition a network like rupture was formed and melting took place along the rupture in the silica grains nanosized granular coesite grains crystallized from the silica melt based on shock induced high pressure polymorphs the estimated shock pressure condition ranged from 8 to 13 gpa considering radiometric ages and shock features the dynamic event that led to the formation of coesite and stishovite occurred ca 4 1 ga ago which corresponds to the late heavy bombardment period ca 3 8 4 1 ga deduced from the lunar cataclysm there are two giant impact basins around the south pole of 4 vesta although the origin of heds is thought to be related to dynamic events that formed the basins ca 1 0 ga ago our findings are at variance with that idea

high precision and high resolution carbonate 230th dating by mc icp ms with sem protocols abstract to facilitate the measurement of u th isotopic compositions suitable for high precision and high resolution 230th dating of coral and speleothem carbonates secondary electron multiplier sem protocol techniques for multicollector inductively coupled plasma mass spectrometry mc icp ms have been developed the instrumental sensitivities are 1 2 with a precision of 1 2 2 for abundance determination of 50 200 fg 234u 1 4 ng 238u or 230th this method features chemistry refinements improvements to procedural and instrumental blanks spectral inference reductions and careful consideration of non linear sem behavior measurement consistency of this mc icp ms combined with previous mass spectrometric results on u th standards and a variety of carbonates demonstrates the validity of the sem protocol method for fossil corals a routine u th isotopic determination at permil level precision requires only 10 50 mg of carbonate as little as 200 mg of young coral with an age of less than 20 yr can be dated with a precision of 0 3 0 8 yr about 20 200 mg speleothem samples with sub ppm to ppm u are required to earn a 5 precision on ages from 5 to 100 kyr requirement of small sample size 10 100s mg carbonate can permit high temporal resolution to date speleothems with slow growth rates i e 1 10 mm kyr this high precision 230th chronology is critical to accurately establish age models date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography the u th isotopic determination techniques described here can also be applied to different environmental samples such as waters rocks and sediments

inner forearc sequence architecture in response to climatic and tectonic forcing since 150 ka hawke s bay new zealand the influence of eustasy tectonic deformation and sediment flux as controlling parameters on basin stratigraphy and depositional sequence development are largely accepted eustasy is usually considered as the dominant mechanism of sequence generation especially for pleistocene successions in active subduction margin settings the high rates of tectonic deformation are expected to have a stronger influence on basin fill architecture while sediment flux is generally less well constrained and therefore less frequently considered the active hikurangi subduction margin in new zealand offers the opportunity to quantitatively assess the relative roles of tectonic climatic and eustatic drivers we present a quantitative source to sink like study of the late pleistocene succession from the hawke s bay sector of the inner forearc domain c 150 ka to present the interpretation of a grid of high resolution marine seismic data onland and offshore core and well descriptions and the integration of geomorphic studies enabled identification of system tracts in turn these comprise two sea level cycle depositional sequences lps1 and lps2 including one complete 100 ka sequence lps1 isopach maps of both sequences reveal changes in sediment distribution and preservation that reflect the relative roles of tectonic deformation and eustasy eustasy dominates development of sequence architecture at relatively short time scales i e 100 kyrs four long lasting depocenters are identified over the inner forearc domain and located in four subsiding basins kidnappers mahia lachlan and motu o kura basins significant shifts of the depocenter location in the basins are correlated with eustatic sea level changes estimates of sediment volumes and masses from isopach maps indicate higher mass accumulation rates during climato eustatic extremes which we correlated to the onland erosional response sediment distribution and landscape evolution are strongly influenced by the interaction of the structural deformation and sediment flux we present paleogeographic reconstructions for the inner forearc domain coincident with two paleoclimatic extremes last glacial maximum and holocene optimum these illustrate the importance of eustatic changes structural deformation and sediment flux on the pattern of sediment distribution accumulation and sequence architecture

the tertiary arc chain in the western pacific abstract the arcs bordering the pacific plate on the western and southwestern pacific rim are reconstructed since their initiation in the eocene and oligocene they occur in a zone forming an arc chain from the western pacific tropics to the eastern margin of australia they are the bonin mariana yap palau halmahera north new guinea west melanesia solomon vanuatu and tonga kermadec arcs designated here the tertiary arc chain they are associated with the formation and consumption of backarc basins reversals of arc polarity and episodic subduction has occurred in some of them the tertiary arc chain is characterized by four major stages in its evolution which can be seen characteristically in some of the arcs the first stage is the occurrence of the arc chain from the middle eocene to earliest oligocene the second stage is the formation of the backarc basins from the early to late oligocene the third stage is the occurrence of double arcs on the inner side of the arc chain in the early to middle miocene and the fourth stage is the reversal of arc polarities due to collisions since the late miocene the backarc basins associated with the arcs of the tertiary arc chain have fixed limits of duration in their evolution the backarc basins initially form 15 million years after the initiation of the volcanic arc several to 10 million years after the initial opening backarc spreading terminates approximately 20 million years after the cessation of the backarc spreading a second phase of opening occurs in the backarc region in the case of arc collision reversal of the arc polarity occurs if there is oceanic crust on the backarc side and opening of a backarc basin occurs within several million years these occurrences and durations have a variation of ca 3 5 million years

late pleistocene zircon ages for intracaldera domes at golcuk isparta turkey abstract pleistocene to quaternary volcanism in the isparta region sw anatolia turkey comprises potassic lavas and pyroclastic deposits which are largely centered around golcuk caldera trachytic intracaldera lava domes represent the latest eruptive event at golcuk and their eruption age is crucial for defining a minimum age for the preceding caldera forming explosive eruption here we present combined u th and u th he zircon geochronological data for two intracaldera lava domes constraining their crystallization and eruption ages respectively u th zircon crystallization ages peak between ca 15 and 25 ka in rare instances u th zircon crystallization ages date back to ca 59 and 136 ka u th zircon crystallization ages also permit u th he eruption ages from the same crystals to be individually corrected for uranium series decay chain disequilibrium which is mainly due to the deficit of the intermediate daughter 230 th in zircon average disequilibrium corrected u th he zircon ages are 14 1 0 5 and 12 9 0 4 ka 1 these ages are indistinguishable within analytical uncertainties suggesting that both lavas erupted quasi simultaneously this contradicts published k ar ages that suggest an extended hiatus from ca 52 to 24 ka between intracaldera dome eruptions evidence for protracted zircon crystallization over several thousands of years prior to eruption indicates the presence of a long lived magma reservoir underneath golcuk caldera implications of the revised eruptive geochronology presented here include younger ages for the latest effusive eruptions at golcuk and potentially also a more recent explosive eruption than previously assumed

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

late cenozoic rift development and intra plate volcanism in northern new zealand inferred from geochemical discrimination diagrams abstract major and trace element discrimination diagrams are used in an attempt to discover the tectonic settings in which the late cenozoic basalts and basalt andesites of northern new zealand evolved the rocks of the northland area are shown to have rift affinities whereas those of auckland and south auckland appear to have evolved in an intraplate environment it is proposed that the common link between the two groups of volcanic rocks is the hauraki rift the northland basalts are directly associated with the rift whereas the auckland and south auckland rocks either developed on the flanks of the rift or their origin is linked to the evolution of membrane stresses during the opening of this rift the basalts and basalt andesites of the waikato district also show an affinity with the rift but the geochemistry is complicated by the influence of the developing taupo volcanic zone which adds a shoshonitic character to these rocks

petrology and geochemistry of potassic rocks in the golcuk area isparta sw turkey genesis of enriched alkaline magmas abstract the lower pliocene volcanic rocks occurring in the golcuk area of sw turkey exhibit alkaline major element trends with a general potassic character the development of volcanism can be divided into 2 major stages such as trachytic ancient lavas domes and tephriphonolitic trachyandesitic to trachytic golcuk eruptions ignimbrites lava dome extrusions phreatomagmatic deposits and finally young domes volcanic rocks consist primarily of plagioclase clinopyroxene which ranges in composition from diopside to augite and are commonly zoned biotite and phlogopite amphibole phenocrysts are restricted to the pyroclastic deposits pseudoleucites are also seen only in the lava dome extrusions oxides and apatites are common accessory phenocryst phases as would be expected from their potassic alkaline nature the volcanic rocks of the golcuk area contain high amounts of lile ba sr rb and k lree and zr concentrations of compatible elements such as cr ni and v are very low possibly indicating fractionation of olivine and clinopyroxene correlation of sio2 rb sr and mgo with 87 sr 86 sr 0 703506 0 704142 exhibit an increasing trend in the direction of crustal contamination however the isotopic compositions of sr are not as high to indicate a high level of crustal contamination geochemical data are consistent with the derivation of golcuk volcanic rocks from a metasomatized and or enriched lithospheric mantle source during crustal extension in the area this metasomatism was probably occurred by fluids released from the northward subduction between african and eurasian plates during tertiary as the golcuk volcanic rocks display features of island arc magmas with having high ba nb 28 ratios and nb and ti depletions lower pliocene volcanism in the golcuk was response to extensional tectonics

subduction of continental margins and the uplift of high pressure metamorphic rocks abstract the mechanism by which high pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis we suggest that the emplacement results from partial subduction of the continental margin which because of its high flexural rigidity produces a rapid change in the trajectory of the descending slab we assume a two fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity we assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate the wedges overlying the subduction zones and the subducting slab itself experience substantial extra compressional stress at depths of around 100 km and extensional stress at shallower depths as the continental margin passes through the zone of maximum curvature the magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and or the descending plate and are experienced in a time interval of less than 5 m y for typical subduction rates the spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab which is the weakest region in the deeper parts of the subduction zone this may result in rapid upward migration of the crust of the partially subducted continental margin against the flow of subduction high pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above movement on the faults would have been coeval and would have resulted in rapid unroofing of the high pressure terranes synchronous with arrival of the continental margin at the subduction zone and therefore relatively early in the history of a collisional orogen

re anchoring the late pleistocene tephrochronology of new zealand based on concordant radiocarbon ages and combined 238u 230th disequilibrium and u th he zircon ages abstract the caldera forming rotoiti eruption from okataina volcano was one of the largest rhyolite events of the last 100 ka in the taupo volcanic zone new zealand its associated widespread tephra layer rotoehu tephra is a significant time marker in terrestrial marine paleoclimate correlation studies in the sw pacific however the accurate and precise age of this eruption and of the subsequent smaller volume earthquake flat eqf eruption has been the subject of controversy despite numerous studies we have applied combined 238u 230th disequilibrium and u th he dating of zircon from rotoiti and eqf deposits and obtained overlapping ages of 45 1 7 3 3 ka and 45 1 6 2 9 ka respectively these results are supported by new and published high precision radiocarbon data bracketing the age of the rotoiti eruption between 44 8 0 3 and 47 5 2 1 ka cal bp these age data are also in good agreement with a range of previously published estimates based on paleoclimate palynology luminescence dating of enclosing sediment and sedimentation rates in terrestrial and marine settings however these results are at variance with a commonly quoted age of 60 ka largely constrained by a single 40ar 39ar age of an overlying glassy lava flow at a distal tephra site this study demonstrates the potential for combined 238u 230th disequilibrium and u th he dating of zircon in tephra and other volcanic deposits to provide age control in the 40 100 ka time interval a period difficult to constrain using more traditional radiocarbon k ar and 40ar 39ar methods

reply to the comments of s karato on petrofabrics and seismic properties of garnet peridotites from the uhp sulu terrane china by xu et al tectonophysics 421 2006 111 127 abstract we thank karato for his interest in our recent paper in tectonophysics xu z q wang q ji s c chen j zeng l s yang j s chen f y liang f h wenk h r 2006 petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab tectonophysics 421 111 127 however karato is incorrect to take water or water related species herein referred to generically as water as an almost unique cause for any olivine fabric transition in nature no matter what geological conditions are in this reply we will first give a succinct introduction to the context of the ongoing debate and then provide some additional arguments in favour of our conclusion that the olivine c type fabric of the zhimafang garnet bearing peridotites was formed under the conditions of high pressure low temperature and low water content during the subduction of cold and dry supracrustal rocks of the yangtze plate the map of olivine slip systems as a function of differential stress and water content drawn by karato and coworkers cannot be reliably extrapolated to earth s interior until relevant scaling laws have been established the role of high water content on the fabric transitions is more likely to modify recrystallization kinetics than to switch the dominant slip systems

hydroxyl contents of accessory minerals in mantle eclogites and related rocks infrared spectra in the region 3000 to 3800 cm 1 have been measured on coesite kyanite sanidine and rutile occurring in inclusions of eclogite and on garnet and spinel in an inclusion of alkremite from south african kimberites coesite and sanidine contain virtually no h 2o or oh although one sanidine crystal was observed to contain abundant h2o apparently as fluid inclusions of molecular h2o kyanite contains significant oh consistent with previous results for similar samples corundum gives no indication of the presence of h 2o or oh rutile containing significant amounts of fe cr al and nb also contains significant amounts of structural oh garnet in alkremite contains small amounts of molecular h 2o whereas the spinel coexisting with this garnet gives no indication of h 2o or oh indicating that the spinel structure is not able to accommodate significant amounts of oh ammonium contents of all samples were less than the detection limit about 10 ppm

noble gases in mafic phenocrysts and xenoliths from new zealand we have determined the elemental and isotopic compositions of noble gases in young subduction related phenocrystic olivine and clinopyroxene samples from the taupo volcanic zone central north island new zealand and in behind arc intraplate phenocrystic and xenolithic olivine samples from the northland and auckland volcanic provinces northern north island new zealand helium isotopic ratios range from morb like 3he4he values of about 11 10 6 to lower values of about 6 10 6 consistent with previous measurements of helium isotopic ratios in subduction related samples 40ar36ar and 21ne22ne ratios range from atmosphere like compositions to maximum values of about 700 and 0 033 respectively in contrast most of the 21ne22neratios are generally indistinguishable from atmospheric values the variations in helium neon and argon isotopic ratios are interpreted as resulting from mixing of 1 a a primordial 3he rich component derived from the upper mantle and characterised by morb like 3he4he ratios of about 12 10 6 2 b a radiogenic 4he rich component derived from crustal materials and characterised by a 3he4he ratio of less than 2 10 7 and 3 c a helium poor atmosphere derived component which dominates the heavier noble gases by combining the helium and argon isotopic results it is possible to estimate the relative contribution of each of these three components to the total 40ar observed in each sample based on the present understanding of the origin and evolution of arc magmas a simple qualitative model of the mechanisms which introduce noble gases to the parent magmas of the samples is developed however it remains uncertain whether the atmospheric and crustal derived noble gas components are introduced to the mantle source regions of the magmas by subduction or alternatively are introduced by interactions between the ascending parent magmas and the overlying crust

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

occurrences of intergranular coesite in ultrahigh p rocks from the sulu region eastern china implications for lack of fluid during exhumation several coesite grains along garnet omphacite grain boundaries were discovered in eclogites from yangkou beach of the sulu region eastern china these grains exhibit variable extents of conversion of coesite to palisade through mosaic to granoblastic quartz aggregates the rare occurrence of intergranular and matrix coesite in ultrahigh pressure uhp crustal rocks is related to the fast rate of exhumation and the low amount of fluid infiltration required during retrograde recrystallization sluggish reactions due to the lack of fluid in both uhp and retrograde metamorphism resulted in the occurrence of intergranular coesite in these eclogites the preservation of gabbroic texture and relict igneous minerals and the metastable persistence of low p assemblages in the coesite stability field

elasticity and phase transformation at high pressure in coesite from experiments and first principles calculations the crystal structure and equation of state of coesite space group c 2 c and its high pressure polymorph coesite ii space group p 2 1 n under pressure have been studied using x ray powder diffraction in a diamond anvil cell dac up to 31 gpa at room temperature and first principles calculations at 0 k up to 45 gpa new diffraction peaks appear above 20 gpa indicating the formation of coesite ii structure the calculated enthalpies provide theoretical support for the pressure induced phase transformation from coesite to coesite ii at 21 4 gpa compared with coesite the coesite ii structure is characterized by a doubled b axis and the breakdown of the linear si1 o1 si1 angle in coesite into two distinct angles one is 176 close to linear whereas the other decreases by 22 to 158 coesite is very anisotropic with the a axis the shortest and twice more compressible than the b and c axis by comparison coesite ii is not so anisotropic with similar compressibilities in its a b and c axis as analyzed by a third order eulerian finite strain equation of state the bulk modulus of coesite at 21 4 gpa is 182 3 gpa and that of coesite ii is 140 8 gpa indicating that coesite ii is much more compressible than coesite the existence of coesite ii in the coldest subduction zone will change the elasticity and anisotropic properties of the subducting materials dramatically

coesite inclusions and prograde compositional zonation of garnet in whiteschist of the hp uhpm kokchetav massif kazakhstan a record of progressive uhp metamorphism coarse grained whiteschist containing the assemblage garnet kyanite phengite talc quartz coesite is an abundant constituent of the ultrahigh pressure metamorphic uhpm belt in the kulet region of the kokchetav massif of kazakhstan garnet displays prograde compositional zonation with decreasing spessartine and increasing pyrope components from core to rim cores were recrystallized at t 380 c inner to 580 c outer at p 10 kbar garnet ilmenite geothermometry margarite quartz stability and mantles at t 720 760 c and ph20 34 36 kbar coesite graphite stability phengite geobarometer kfmash system reaction equilibria textural evidence indicates that rims grew during decompression and cooling within the qtz stability field silica inclusions quartz and or coesite of various textural types within garnets display a systematic zonal distribution cores contain abundant inclusions of euhedral quartz type 1 inclusions inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite type 2 with minute dusty micro inclusions of chlorite and more rarely talc and kyanite in their cores intense radial and concentric fractures are well developed in the garnet intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims type 3 which are also surrounded by radial fractures subhedral inclusions of pure coesite without quartz overgrowths or radial fractures type 4 occur in the outer part of the mantle garnet rims are silica inclusion free type 1 inclusions in garnet cores represent the low p low t precursor stage to uhpm recrystallization and attest to the persistence of low p assemblages in the coesite stability field coesites in inclusion types 2 3 and 4 are interpreted to have sequentially crystallized by net transfer reaction kyanite talc garnet coesite h2o and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid during the retrograde evolution of the rock all three inclusion types diverged from the host garnet p t path at the coesite quartz equilibrium and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure coesite in type 2 inclusions suffered rapid intragranular h2o catalysed transformation to quartz and ruptured the host garnet at about 600 c when inclusion p 27 kbar garnet host p 9 kbar instantaneous decompression to the host garnet p t path passed through the kyanite talc chlorite quartz reaction equilibrium resulting in the dusty micro assemblage in inclusion cores type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite quartz equilibrium and finally underwent rupture and decompression when t 400 c facilitating coesite preservation type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite quartz equilibrium boundary

neotectonics of the macquarie ridge complex australia pacific plate boundary new marine geophysical data along the macquarie ridge complex the australia pacific plate boundary south of new zealand illuminate regional neotectonics we identify tectonic spreading fabric and fracture zones and precisely locate the australia pacific plate boundary along the macquarie ridge complex we interpret a 5 10 km wide macquarie fault zone between the two plates along a bathymetrie high that extends nearly the entire length of the australia pacific plate boundary south of new zealand we conclude that this is the active australia pacific strike slip plate boundary arcuate fracture zones become asymptotic as they approach the plate boundary a broad zone of less intense deformation associated with the plate boundary extends 50 km on either side of the macquarie fault zone marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike slip deformation although teleseismic evidence overwhelmingly indicates strike slip motion along the entire surveyed boundary today the mcdougall and southernmost puysegur segments show no evidence for past underthrusting whereas data from the macquarie and hjort segments strongly suggest past convergence the present day strike slip plate boundary along the macquarie ridge complex coincides with the relict spreading center responsible for australia pacific crust in the region our conceptual model for the transition from seafloor spreading to strike slip motion along the macquarie ridge complex addresses the decreasing length of spreading center segments and spacing between fracture zones as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary

reconstructions of the new zealand region for the neogene abstract the major structural elements of the present day plate boundary zone through new zealand are a subduction zone under the northern half of the country linked by a wide transform zone to the southward extension of the plate boundary through fiordland and the macquarie ridge using finite rotations of the pacific australian plate pair a sequence of reconstructions of the relative positions of the plates is presented reconstructions of the plate boundary zone based on paleomagnetic data involve essentially the same structural elements as today but with large clockwise changes in their trend with time

zircon reveals protracted magma storage and recycling beneath mount st helens current data and models for mount st helens volcano washington united states suggest relatively rapid transport from magma genesis to eruption with no evidence for protracted storage or recycling of magmas however we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust cool and crystallize beneath the volcano and are then rejuvenated and incorporated by hotter young magmas on their way to the surface estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time recording growth from hotter less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface or changes in whole rock geochemistry and petrology and providing a new time correlated record of this evolution independent of the eruption history zircon data thus reveal the history of the hidden long lived intrusive portion of the mount st helens system where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

age and origin of the new zealand orocline in relation to alpine fault movement the alpine fault and the recurved arc structures orocline in the late palaeozoic mesozoic rangitata orogen are two of the first order structures of the new zealand subcontinent a commonly held view is that the orocline originated as a gigantic drag fold with alpine fault movement in view of the post oligocene age of the alpine fault however this concept involving a single recurved arc cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the rangitata orogen and its cover rocks these data indicate at least 62 of dextral rotation during each of the mesozoic rangitata orogeny and the late cenozoic kaikoura orogeny a new model presented here postulates that there are actually two dextral recurved arcs a western arc which lies mainly outside the australia pacific plate boundary zone has a multiple origin having formed chiefly during the late triassic phase of the rangitata orogeny by the subduction accretion of the exotic rakaia terrane an eastern arc coincides with the area of the present plate boundary zone it formed as a mega brittle ductile shear zone with the late cenozoic development of the modern australia pacific plate boundary which includes the alpine fault this solves the problem of the nature and location north of the alpine fault of the relative plate movement evident as 480 km of displacement on the alpine fault a major implication of this model for plate tectonic reconstruc tions of the sw pacific is that only approximately 500 km of dextral displacement has occurred through new zealand during the late cretaceous cenozoic compared with the commonly adopted figure of 1000 km

diamond former coesite and supersilicic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established abstract we report here the first discovery of ultrahigh pressure uhp indicator minerals and textures from the rhodope metamorphic province rmp northern greece in particular we document the exsolution of quartz rutile and apatite in sodic garnet from metapelites garnet biotite kyanite gneisses which attests to the presence of a si ti na p rich precursor garnet phase similar textures in garnet have been reported in the literature only once before for eclogites from the su lu uhp metamorphic province china we also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite included in garnet from both eclogites and metapelites we then argue that these rocks had once been transported to depths exceeding 220 km well into the upper mantle thus establishing the rmp as another important uhp metamorphic belt in the world

neocene and quaternary extent and geometry of the subducted pacific plate beneath north island new zealand implications for kaikoura tectonics abstract the extent and geometry of the obliquely subduced oceanic pacific plate beneath north island new zealand for five million year intervals through the mid miocene to quaternary are presented in a series of maps and cross sections these show that the subducted plate progressively increased its extent from ne to sw beneath the north island and in the more northern regions where it was first emplaced concomitantly increased its dip from 10 to 50 the changing extent and geometry of the subducted slab has been established from the age pattern of orogenic andesites and from the geochemical k 2 o h parameter of depth of magma generation the radiometric dates show a migration of the volcanic front back towards the trench at an average rate of 20 km my the trenchward migration is explained by a model of increasing slab dip which is corroborated by the k 2 o data calibrated against the presently active arc taupo volcanic zone with the exception of northern coromandel peninsula the andesitic magmas were generated at 85 100 km depth the interpretation of the dates adopted here indicates that the subducted slab originated at the ne sw trending kermadec hikurangi trench and implies a different and much simpler evolution of the australia pacific plate boundary in the vicinity of north island than other recent models subduction geometry has been found elsewhere to be a principal influence upon the state of stress and deformational style in an over riding plate the possibility is explored that the timing nature and pattern of the neogene to quaternary kaikoura orogeny in north island is due to this influence apart from the effect of oblique subduction in eastern north island there is an accord between the onset of deformation and the emplacement sequence of the shallow slab beneath north island and between the change in subduction geometry and a progressive north to south change in northern north island from compression to extension

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

the complex facies architecture and emplacement sequence of a miocene submarine mega pillow lava flow system muriwai north island new zealand the early miocene waiatarua formation at maori bay muriwai north island new zealand consists of a complex association of basaltic andesite volcanic facies including entablature jointed thick massive facies colonnade jointed thin massive sheet facies mega pillow facies normal pillow facies and minor associated fragmental facies including vitric sandstone and breccia interpreted as hyaloclastite and peperite field observations suggest that the facies which form the waiatarua formation lava were emplaced as multiple flow lobes in a single lava flow from one sustained eruption magma discharge rate at the vent was high however magma supply rate to the more distal and lateral portions of the flow as its surface area increased varied considerably higher magma supply rates produced thick massive and thin sheet facies whereas mega pillow facies and normal pillow facies were produced contemporaneously with the thick and thin massive sheet facies but were restricted to portions of the distal flow subjected to lower magma supply rates the evolution of the waiatarua formation lava flow at maori bay has been reconstructed from the complex facies architecture this suggests that the propagation of the lava involved 7 discrete lobes that were emplaced successively

ion microprobe analysis of 231pa 235u and an appraisal of protactinium partitioning in igneous zircon ion microprobe u pa measurements in zircon crystals from a holocene rhyolite salton buttes california are utilized to assess the partitioning of pa and u between zircon and melt the relative sensitivity factor rsf for ion microprobe u pa zircon analysis was calibrated on a natural high u secular equilibrium zircon from buff peak nevada zircon crystals from salton buttes rhyolite show evidence for excess 231 pa with a weighted average 231 pa 235 u activity ratio of 1 9 from this ratio model zircon melt partition coefficient ratios d pa d u 0 9 2 2 are obtained assuming plausible limits for 231 pa 235 u melt and correcting for the age of zircon crystallization 15 ka these values roughly fit lattice strain models for tetravalent cations but are more than one order of magnitude lower than predictions for pentavalent pa based on this appraisal of pa partitioning initial 231 pa disequilibrium caused by zircon melt fractionation alone can produce minor discordance or excess 207 pb in late cenozoic zircon but is insignificant for older u pb zircon ages

a model of the cenozoic evolution of northern new zealand and adjacent areas of the southwest pacific abstract new information from the southwest pacific indicates that earlier attempts at formulating the evolution of the area assuming a single upper cenozoic magmatic arc are untenable it now appears that there were two arcs during the miocene and pliocene a western northland three kings rise arc and an eastern tonga lau kermadec colville arc both appear to have developed above west dipping subduction zones it is suggested that the norfolk and reinga basins formed as back arc basins to the western arc and that eastern north island lay adjacent to northland and formed the accretionary prism to that arc upper cenozoic evolution of the region involved the simultaneous opening of the norfolk reinga basins consumption of the western portion of the oligocene south fiji basin by subduction beneath the western arc and eastwards movement of new zealand three kings rise towards the tonga kermadec arc when the kermadec and hikurangi trenches came into line late in the pliocene the tonga kermadec arc was able to propagate rapidly southwards into north island simultaneously the western arc became extinct and the tectonic tempo and strike slip faulting accelerated markedly throughout new zealand eastern north island was moved dextrally an uncertain distance relative to the western north island and rotated 25 30 clockwise this accounts for the paradox of a 22 m y old accretionary margin lying adjacent to a 2 m y old arc taupo volcanic zone at the present day

tracking silica in earth s upper mantle using new sound velocity data for coesite to 5 8 gpa and 1073 k the compressional and shear wave velocities for coesite have been measured simultaneously up to 5 8 gpa and 1073 k by ultrasonic interferometry for the first time the shear wave velocity decreases with pressure along all isotherms the resulting contrasts between coesite and stishovite reach 34 and 45 for p and s wave velocities respectively and 64 and 75 for their impedance at mantle conditions the large velocity and impedance contrasts across coesite stishovite transition imply that to generate the velocity and impedance contrasts observed at the x discontinuity only a small amount of silica would be required the velocity jump dependences on silica d lnvp d sio2 0 38 wt 1 and d lnvs d sio2 0 52 wt 1 are utilized to place constraints on the amount of silica in the upper mantle and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs

decoupled crystallization and eruption histories of the rhyolite magmatic system at tarawera volcano revealed by zircon ages and growth rates we obtained u th disequilibrium age data on zircons from each of the four rhyolite eruptions that built tarawera volcano in the last 22 ka within the okataina volcanic center ovc caldera new zealand secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near eruption age to 100 ka prior to eruption age depth profiling of crystals reveals long periods of continuous 34 ka and discontinuous growth 90 ka growth hiatuses of up to 40 ka duration occur but do not all relate to obvious resorption surfaces age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts these features are best explained by periodic complete or partial sub solidus storage and or inclusion in larger crystal phases followed by rapid liberation prior to eruption this is supported by high abundances of u and th 500 2 000 ppm in some zircons consistent with periods of high crystallinity 70 in the magmatic system based on crystal melt partitioning contemporaneous but contrasting rim ward trends of these elements within crystals even in the same lava hand sample require synchronous growth in separate melt bodies and little connectivity within the system but also significant crystal transport and mixing prior to eruption many crystals record continuity of growth through the preceding 60 ka ovc caldera collapse and subsequent eruptions from tarawera this demonstrates a decoupling between eruption triggers such as shallow crustal extension and mafic intrusion and the crystallization state of the ovc silicic magmatic system the data highlights the need to distinguish between the time for accumulation of eruptible magma and the long term magma residence time based on the age of crystals with high closure temperatures when assessing the potential for catastrophic eruptions

pressure induced phase transitions in coesite high pressure behavior of coesite was studied on single crystals using diamond anvil cells with neon as the pressure transmitting medium by means of in situ raman spectroscopy up to pressures of 51 gpa the experimental observations were complemented with theoretical computations of the raman spectra under similar pressure conditions we find that coesite undergoes two phase transitions and does not become amorphous at least up to 51 gpa the first phase transition coesite i to coesite ii is reversible and occurs around 23 gpa the second transition coesite ii to coesite iii at about 35 gpa is also reversible but involves a large hysteresis samples recovered from the highest pressure achieved 51 gpa show raman spectra of the initial coesite the ab initio calculations gave insight into the initiation mechanism of the first phase transition implying from the analysis of unstable phonon modes that it is probably a displacive phase transition due to shearing of the four membered rings of sio4 tetrahedra upon compression the transition to the lowest symmetry phase coesite iii is possibly a first order phase transition that leads to a very distinct structure none of the metastable high pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure induced amorphization at significantly lower pressures 30 gpa the study of the high pressure behavior of coesite is important to better constrain the metastable phase diagram of silica further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms crystalline or amorphous metastable phases derived from coesite under high pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures stress reached in processes such as impacts or faulting

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

unfolding the wadati benioff zone in the kermadec new zealand region abstract as the deep seismic zones are associated with subducted oceanic crust it is important to establish the extent and continuity of each of these zones using consistent earthquake locations then the former oceanic crust can be restored to the surface of the earth by unfolding the seismic zone about the trench this unfolded crust should be used in plate reconstructions this process has been carried out for the deep seismic zone in the kermadec new zealand region where the deep seismic zone is defined by a careful selection of i s c hypocenters and the former oceanic crust is established by unfolding plate reconstructions using two sets of rotation poles show that the deepest parts of the observed seismic zone along its length were not subducted at the same time further the currently observable zone is not directly associated with earlier volcanism in northwest new zealand between 6 and 20 ma ago

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

constraints on the timing of quaternary volcanism and duration of magma residence at ciomadul volcano east central europe from combined u th he and u th zircon geochronology article i nfo high spatial resolutionzircon geochronologywas applied to constrain thetimescalesof volcaniceruptions of the youngest mostly explosive volcanic phase of ciomadul volcano carpathian pannonian region romania com bined u th and u th he zircon dating demonstrates that intermittent volcanic eruptions occurred in a time range of 56 32 ka the reliability of the eruption dates is supported by concordant ages obtained from different dating techniques such as zircon geochronology radiocarbon analysis and infrared stimulated luminescence dating for the same deposits the new geochronological data suggest that volcanism at ciomadul is much youn ger bca 200 ka than previously thought up to 600 ka a dominantly explosive volcanic phase occurred after an apparent lull in volcanism that lasted for several 10 s of ka after a period of lava dome extrusion that defines the onset of the known volcanism at ciomadul at least four major eruptive episodes can be distinguished within the 56 32 ka period among them relatively large sub plinian to plinian explosive eruptions produced distal tephra covering extended areas mostly southeast from the volcano the 38 9 ka tephra overlaps the age of the campanian ignimbrite eruption and has an overlapping dispersion axis towards the black sea region the wide range of u th model ages of the studied zircons indicates prolonged existence of a low temperature b800 c silicic crystal mush beneath ciomadul the main zircon crystallization period was between ca 100 and 200 ka coeval with the older mostly extrusive lava dome building stage of volcanism even the youngest u th model ages obtained for the outermost 4 m rim of individual zircon crystals predate the eruption by several 10 s of ka the zircon age distributions suggest re heating above zircon saturation temperatures via injection of hot mafic magmas prior to eruption intermittent intrusions of fresh magma could play a significant role in keeping the intrusive silicic magmatic reservoir in a partially melted for prolonged period the previous history of ciomadul suggests that melt bearing crystal mush resided beneath the volcano and was rapidly remobilized after a protracted several 10 s of ka lull in volcanism to trigger several eruptions in a comparatively short time window this classifies ciomadul as a volcano with potentially active magma storage pams which we propose to be common among the seemingly inactive volcanoes in volcanic arc regions the potential for reacti vation of these systems should be included into volcanic hazard assessments

basement geology from three kings ridge to west norfolk ridge southwest pacific ocean evidence from petrology geochemistry and isotopic dating of dredge samples abstract we present new petrographie x ray fluorescence electron microprobe ar ar radiometric fission track and sr nd and pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest pacific ocean the dredge samples can be divided into four groups 1 permian to cretaceous gabbroids granitoids and hornfelses from the west norfolk and wanganella ridges 2 mainly cretaceous mafic igneous rocks from the reinga ridge and vening meinesz escarpment 3 subduction related late oligocene and early miocene basalts and shoshonites from the three kings and southern norfolk ridges and 4 back arc basin and intraplate basalts of early miocene age in the south and north norfolk basins respectively and intraplate basalts of pliocene age in the reinga basin our dredge data combined with regional magnetic and gravity data provide much needed tests of earlier tectonic interpretations of the region correlatives of the permian brook street terrane and mesozoic median tectonic zone of onshore new zealand clearly extend along the west norfolk and wanganella ridges the three kings ridge was active at least in the 19 to 20 ma period and was probably generated above a west dipping subduction zone the norfolk basin was a back arc basin to this arc and had opened by 18 20 ma the oldest cenozoic volcanic rocks in the region are 26 ma subduction related volcanic breccias on the southern norfolk ridge they probably predate opening of the norfolk basin and associated eastward migration of the three kings ridge