retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

a deep crustal shear zone exposed in western fiordland new zealand an approximately 180 km long deep crustal shear zone in western fiordland new zealand has been shown to have formed during continental extension between 116 and 100 ma the western fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories a detailed study of the northern part of the shear zone has revealed that the shear zone formed at depths of 40 km approximately 12 kbar and 680 c and was originally very gently dipping or subhorizontal prior to subsequent upright folding the current thickness postfolding of the shear zone is 3 4 km abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a top to the ne sense of shear unlike ductile shear zones in metamorphic core complexes from elsewhere in the world there is no evidence that decompression of the shear zone rocks or foot wall rocks occurred during the active life of the shear zone suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal this shear zone provides important evidence for the nature of deep crustal deformation occurring during continental extension

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

analogies between mineral sediment and vegetative particle dynamics in fluvial systems abstract riparian vegetation fragments and propagules form part of the organic sediment load of rivers this review paper draws analogies between the transfer of these vegetative particles and that of mineral sediment within river systems whole plants vegetation fragments and propagules are delivered to rivers by physical processes but the timing of seed release and the morphological characteristics of the plants moderate delivery the transport and deposition of plant material is partly controlled by flow patterns but is also influenced by the buoyancy and morphology of the plant material once deposited the vegetative fragments propagules and the plants that develop from them can have significant local effects on flow hydraulics and sediment erodibility producing complex assemblages of physical features aggregate vegetative particles are identified as being particularly important in accelerating vegetation colonisation of exposed river sediments driving landform development and supporting the rapid development of a diverse cover of plants the geomorphological significance of interactions between vegetative particles and fluvial processes is illustrated at different spatial scales at the catchment scale ribbons of mineral and vegetative particles are transferred downstream with inputs continuously supplied throughout the river s course individual particles experience phases of mobilisation transport and deposition as a result along larger rivers down river changes are typically seen in the size and type of mineral sediment and vegetative particles and also the species composition of the vegetative particles coupled with downstream changes in hydroclimatological conditions within a river s active zone marked downstream changes in interactions between fluvial processes vegetation propagules and landforms can result at the patch scale individual vegetative particles particularly aggregate particles deposited on exposed riverine sediments create local hydraulic complexity that induces the scour and deposition of sediment and further plant propagules resulting in a suite of distinctive landforms which may support many plant species within individual reaches patch scale interactions between mineral sediment and vegetative particles are moderated or sieved by the form of the reach and its flow patterns so that the integration of patch scale effects can support the development of different landforms in different locations

major and trace element zoning in garnets from calcareous pelites in the nw shelburne falls quadrangle massachusetts garnet growth histories in retrograded rocks

ductile fabrics in the zone of active oblique convergence near the alpine fault new zealand identifying the neotectonic overprint the mid crustal alpine schist in central southern alps new zealand has been exhumed during the past similar to3 m y on the hanging wall of the oblique slip alpine fault these rocks underwent ductile deformation during their passage through the similar to 150 km wide pacific australia plate boundary zone likely to be cretaceous in age peak metamorphism predates the largely pliocene and younger oblique convergence that continues to uplift the southern alps today late cenozoic ductile deformation constructively reinforced a pre existing fabric that was well oriented to accommodate a dextral transpressive overprint quartz microstructures below a recently exhumed brittle ductile transition zone reflect a late cenozoic increment of ductile strain that was distributed across deeper levels of the pacific plate deformation was transpressive including a dextral normal shear component that bends and rotates a delaminated panel of pacific plate crust onto the oblique footwall ramp of the alpine fault progressive ductile shear in mylonites at the base of the pacific plate overprints earlier fabrics in a dextral reverse sense a deformation that accompanies translation of the schists up the alpine fault ductile shear along that structure affects not only the 12 km thick section of alpine mylonites but is distributed across several kilometres of overlying nonmylonitic rocks c 2001 elsevier science ltd all rights reserved

equilibria within the mineral assemblage quartz muscovite biotite garnet plagioclase and implications for the mixing properties of octahedrally coordinated cations in muscovite and biotite interaction parameters derived using empirical calibration methods indicate strong non ideality in the mixing of octahedrally coordinated cations in muscovite and biotite the data set used for calibration comprises mineral compositions from 49 samples containing quartz muscovite biotite garnet plagioclase and al2sio5 kyanite or sillimanite pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet biotite femg1 exchange equilibrium and geobarometry based on the anorthite breakdown equilibrium two equilibria yielded simple expressions from which binary interaction parameters for octahedrally coordinated cations in biotite could be directly determined a four component fe2 mg al ti regular symmetric mixing model was assumed for biotite one equilibrium yielded a simple expression from which an interaction parameter for the mixing of the mgal celadonite component in muscovite could be directly determined two sets of calculations were performed utilizing different calibrations of the garnet biotite geothermometer and the anothite breakdown geobarometer and different garnet activity models both placed samples within or near the stability field of the al2sio5 phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties indicating that the values are insensitive to differences in the underlying methods using the derived interaction parameters activity models were formulated for the annite phlogopite eastonite and siderophyllite components of biotite and for the mgal celadonite component of muscovite these were utilized for the empirical calibration of 45 fluid independent equilibria involving unique combinations of phase components from the mineral assemblage garnet plagioclase biotite muscovite quartz forty three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz muscovite biotite garnet plagioclase when care is taken to insure that applications are restricted to valid compositional ranges for these the calibrations yielded multiple correlation coefficients ranging from 0 953 to 0 998 and standard deviations of the residuals ranging from 597 to 118 bars

the early metamorphic history of the haast schists and related rocks of new zealand rare but widespread relics of sodic amphibole occur in metabasites and metacherts of the haast schists and related caples terrane rocks present main stage metamorphic assemblages are frequently chemically equivalent to earlier sodic amphibole bearing assemblages indicating that these rocks underwent an earlier higher p tmetamorphism prior to formation of the present pumpellyite actinolite and greenschist facies assemblages the earlier assemblages were stable during and after early isoclinal folding but were replaced by the present moderate p t assemblages prior to the last major fabric forming deformation the change in conditions was due to thermal relaxation probably accompanied by uplift and erosion and peak metamorphic temperatures were about 350 370 c in the pumpellyite actinolite zone of the caples terrane and near 390 c in the greenschist facies chlorite zone near queenstown according to henley 1975 these greenschist facies rocks attained a pressure of at least 6 4 0 4 kb during their history but a pressure of 4 6 0 6 kb has been estimated for a chlorite zone rock from middlemarch and so the 6 4 kb estimate probably refers to the maximum pressure attained during the earlier higher p t metamorphism similar changes in metamorphic facies series with time occur in some older and more complex metamorphic belts such as the caledonides and this study suggests that it may be possible to interpret particular elements in the metamorphic development of such belts in terms of specific circum pacific analogues

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

cretaceous demise of the moa plate and strike slip motion at the gondwana margin radiolaria and paleomagnetic data from deep sea drilling project sites 595 and 596 indicate that early cretaceous seafloor was formed at moderate to high latitudes in the pacific this inference requires a cretaceous spreading ridge south of the pacific phoenix ridge and substantial dextral strike slip motion at the gondwana margin the previously unrecognized southern oceanic plate called here the moa plate was obliquely subducted beneath the gondwana margin and material accreted from it is now part of the eastern province of new zealand

spectrum response of lj well to various stresses during non seismic and seismic periods the well water level in a deep well has significant frequency dependence on various crustal stresses which can provide an effective pathway for determining the well aquifer properties and investigating the response mechanism of groundwater however due to the limitations and complexity of the field conditions there is a lack of on site stress strain data for rocks therefore it is difficult to directly and quantitatively interpret the transfer response between the strain stress and the well water level the observation item data for the lj well in china are relatively complete including rainfall barometric pressure volumetric strain and deep water level data by calculating the spectral period spectral density coherence function and transfer function of various physical variables the spectrum response characteristics of the well aquifer system to various stresses can be effectively obtained when the crust is in a relatively stable state the semidiurnal and diurnal waves are dominant moreover the transfer functions of the well water level s response to the theoretical semidiurnal and diurnal earth tides and the barometric pressure are basically consistent with the correlation analysis results in addition when earthquakes occur the signals of the volumetric strain and the well water level can more effectively highlight the relatively intermediate and high frequency information the transfer function between the well water level and the volumetric strain is nearly an order of magnitude smaller than that in the stable state indicating that the hydrogeological conditions of the lj well aquifer change instantaneously when subjected to seismic waves all the above conclusions show that the correlation and transfer function based on spectral analysis is a feasible and advantageous method of determining the correlation of various physical variables these results provide a foundation for further research on the hydrogeological characteristics of aquifers

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

igneous and metamorphic enclaves in the s type deddick granodiorite lachlan fold belt se australia petrographic geochemical and nd sr isotopic evidence for crustal melting and magma mixing the deddick granodiorite a mafic s type pluton in the lachlan introduction fold belt contains abundant enclaves most are derived from enclaves in granites and volcanic rocks can provide high grade metasediments of pelitic psammitic composition and petrogenetic information not readily available from their migmatites are common among these discrete fragments of host rocks didier 1973 didier barbarin 1991 for melanosomes rich in cordierite and garnet are restites from partial example metasedimentary enclaves can place constraints melting however compositional and nd sr isotopic data indicate on the age composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust fleming host magma but may have formed part of a spectrum of diverse 1991 steele et al 1991 enclaves formed by acmagma source lithologies alternatively they may be accidental cumulation of early phenocrysts autoliths document xenoliths regardless of their origin data for these enclaves suggest the early evolution of the host magma whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous ordovician turbidites of the lachlan fold belt three possible role of hybridization didier 1987 types of microgranular enclaves with igneous textures are disenclave suites in the relatively mafic s type granitoids tinguished the most common type are small and rounded mafic of se australia are often diverse including abundant enclaves of tonalitic composition they carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma and some contain high mg pyroxene derived from enclaves phillips et al 1981 price 1983 chen et al a mafic magma isotopic data form an array with end from 6 to 1989 wyborn et al 1991 the metasedimentary en 12 and sr sr from 0 7130 to 0 7167 host rock 10 and claves are lithologically diverse including gneissic 0 715 respectively these enclaves formed as globules of hybrid migmatitic types and biotite rich surmicaceous types mafic magma commingled with and contaminated by the more apart from their potential as indicators of crustal structure felsic host magma other enclave types were derived from disrupted and composition they are of interest as possible fragments syn plutonic dykes having a distinct isotopic composition and from of source material and or refractory residue from which a cogenetic marginal facies of the host peraluminous melts were generated and extracted by

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

thermochronological analysis of the dynamics of the southern alps new zealand the isotopic ages of material exposed close to the alpine fault in the southern alps new zealand vary in a consistent fashion along the orogen for the k ar system muscovite and biotite ages display two distinct spatial trends that meet in the area of fox glacier where a strong relationship is observed between k ar age and altitude excess argon is present in samples between mount kinnaird and haast pass in the southwest of the southern alps giving unrealistically old inconsistent k ar ages across this region incorporation of excess argon appears to be highly variable however and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area such treatment indicates that excess argon contamination does not play an important role in the systematic k ar age variation identified along the southern alps the observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the southern alps beginning ca 5 ma k ar mica ages older than this south of the copland valley indicate that substantial late cenozoic exhumation occurred along the southern alps prior to the development of the modern tectonic regime but at rates lower than at present total exhumation during the current phase of tectonism in the southern alps decreases from the whataroa river southward north of fox glacier the southern alps have attained a dynamical steady state such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions

early history of earth s crust mantle system inferred from hafnium isotopes in chondrites the 176lu to 176hf decay series has been widely used to understand the nature of earth s early crust mantle system1 2 3 4 5 6 the interpretation however of lu hf isotope data requires accurate knowledge of the radioactive decay constant of 176lu as well as bulk earth reference parameters a recent calibration of the value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3 9 gyr implying widespread continental crust extraction from an isotopically enriched mantle source more than 4 3 gyr ago7 but does not provide evidence for a complementary depleted mantle reservoir here we report lu hf isotope measurements of different solar system objects including chondrites and basaltic eucrites the chondrites define a lu hf isochron with an initial 176hf 177hf ratio of 0 279628 0 000047 corresponding to 1 983 0 033 10 11 yr 1 using an age of 4 56 gyr for the chondrite forming event this value indicates that earth s oldest minerals were derived from melts of a mantle source with a time integrated history of depletion rather than enrichment7 the depletion event must have occurred no later than 320 myr after planetary accretion consistent with timing inferred from extinct radionuclides8 9 10

potassium argon age studies of a low grade progressively metamorphosed greywacke sequence dansey pass south island new zealand 39 potassium argon age determinations from a progressively metamorphosed greywacke schist sequence in southern new zealand prehnite pumpellyite to greenschist facies range from 199 to 154 ma early to late jurassic they show an inverse correlation of age with metamorphic grade concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium bearing minerals was completed during metamorphism the lowest grade metamorphic rocks which yield the oldest ages about 200 ma have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest jurassic thus it is shown that the jurassic cretaceous rangitata orogeny in new zealand commenced earlier is more complex and of longer duration than previously supposed the remaining ages from pumpellyite actinolite and lower greenschist rocks range uniformly through the jurassic and are interpreted as cooling ages from these data a long term uplift rate of 0 23 mm yr is calculated for the jurassic cretaceous

reprint of extensional shear band development on the outer margin of the alpine mylonite zone tatare stream southern alps new zealand abstract schistose mylonitic rocks in the central part of the alpine fault af at tatare stream new zealand are cut by pervasive extensional c shear bands in a well understood and young natural ductile shear zone the c shears cross cut the pre existing mesozoic aged foliation displacing it ductilely synthetic to late cenozoic motion on the af using a transect approach we evaluated changes in geometrical properties of the mm cm spaced c shear bands across a conspicuous finite strain gradient that intensifies towards the af precise c attitudes c foliation dihedral angles and c s intersections were calculated from multiple sectional observations at both outcrop and thin section scales based on these data the direction of ductile shearing in the alpine mylonite zone during shear band activity is inferred to have trended 20 clockwise down dip of the coeval pacific australia plate motion indicating some partitioning of oblique slip motion to yield an excess of dip slip relative to plate motion azimuth or some up dip ductile extrusion of the shear zone as a result of transpression or both constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary szb across all examined parts of the shear zone the mean dihedral angle between the c shears and the mylonitic foliation s remains a constant 30 1 1 the aggregated slip accommodated on the c shear bands contributed only a small bulk shear strain across the shear zone 0 6 0 8 uniformity of per shear slip on c shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain hardening rheology such that they locked up at a finite shear strain inside c bands of 12 15 shear band boudins and foliation boudins both record extension parallel to the szb as do the occurrence of extensional shear band sets that have conjugate senses of slip we infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with w k

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

a phenomenological numerical approach for investigating grain size evolution in ductiley deforming rocks abstract the sizes of recrystallised grains in exhumed ductile shear zones are often used to infer conditions of deformation i e stress strain rate and temperature here we present a simple numerical method of calculating the dynamic evolution of grain size during ductile deformation our phenomenological method is based on the fact that the dynamic competition between grain growth and recrystallisation will drive grains towards a steady state size at each time increment grain growth and reduction contributions are calculated with magnitudes which depend on the difference between the current grain size and a desired steady state grain size in our models we use a recrystallised grain size piezometer to calculate the steady state grain size for a given stress our numerical routine is incorporated into the sulec finite element package allowing us to explore spatial and temporal changes in grain size as a test we compare model results to measured grain sizes in quartz layers thinned and recrystallised around rigid garnet porphyroclasts under simple shear dominated deformation in the alpine fault zone of new zealand numerical models are able to replicate observed grain size variations with boundary conditions consistent with those constrained for the central alpine fault zone

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

tracking crustal processes by ft thermochronology in a forearc high hikurangi margin new zealand involving cretaceous subduction termination and mid cenozoic subduction initiation abstract raukumara peninsula is a forearc high on the pacific margin of new zealand its basement comprises two sedimentary terranes that formed in subduction prisms during the late jurassic mid cretaceous marking the end of subduction along the pacific margin of eastern gondwanaland the cover includes allochthonous late cretaceous oligocene passive margin slope to basin facies emplaced into the forearc during the early miocene initiation of the current subduction regime as well as autochthonous neogene sediments apatite and zircon fission track ft data are reported and interpreted for the basement and cover sediments the detrital zircon ft ages constrain the maximum depositional ages of the low grade zeolite and pumpellyite facies basement pahau terrane is no older than 152 13 ma this zircon ft mode and older ones 198 8 ma 232 58 ma recording cooling denudation in the source areas probably including rakaia terrane waioeka terrane located oceanward of pahau terrane is no older than 125 2 ma being the age of volcanic derived zircons and supports early cretaceous dinoflagellate ages reported previously omaio petrofacies comprises the youngest part of the waioeka terrane and has a zircon ft age of 108 6 3 ma this indicates that basement sequences accumulated in the northeast contemporaneously with cover sequences koranga fm in the southwestern part of the peninsula these zircon ft ages together with the results of thermal history modelling of certain waioeka terrane samples are consistent with migration of a thrust system in a subduction prism environment during the late jurassic cretaceous until about 85 ma as proposed earlier when mesozoic subduction terminated apatite ft data indicate for all basement samples a phase of neogene heating and cooling associated with basin formation sediment accumulation and subsequent uplift and erosion resulting in almost complete removal of the neogene stratigraphic record over western parts of the peninsula forward modelling of the data imply rapid earliest miocene downwarping of the crust in bay of plenty coinciding with initiation of subduction of pacific plate followed by emplacement of a 1 5 2 0 km thick wedge of east coast allochthon from the northeast early miocene sediments accumulated in a foredeep along the front sw margin of the allochthon probably sourced from a block of pahau basement west of whakatane fault which was exhumed and denuded by 4 km contemporaneously during the late early to middle miocene the basin depocentre migrated further to the southwest while areas previously downwarped northeast were uplifted and eroded the depocentre migration and subsequent inversion is a pattern that affected most of north island and is related to mechanical or dynamical interaction of the two plates as the slab of pacific oceanic lithosphere was emplaced beneath north island this may be a general feature of young subduction zones

trace element zoning in a metamorphic garnet trace element zoning has been measured in an amphibolite garnet from the tauern window austria by using an ion microprobe humps in the zoning profiles of na sc v y and the heavy rare earth elements mark a period of open system behavior these humps correspond to a part of the major element zoning profile that is interpreted as a p t reversal the source of the mass excesses of these elements remains ambiguous they were derived either on a thin section scale by the breakdown of trace element enriched refractory minerals or externally from unusual trace element enriched fluids p t paths determined from garnet zoning may require modification if open system behavior is important during garnet growth

mid cretaceous tectonic evolution of the tongareva triple junction in the southwestern pacific basin the trace of the ridge ridge ridge triple junction that connected the pacific farallon and phoenix plates during mid cretaceous time originates at the northeast corner of the manihiki plateau near the tongareva atoll for which the structure is named the triple junction trace extends 3250 km south southeast to and beyond a magnetic anomaly 34 bight it is identified by the intersection of nearly orthogonal abyssal hill fabrics which mark the former intersections of the pacific phoenix and pacific farallon ridges a distinct trough is commonly present at the intersection a volcanic episode from 125 to 120 ma created the manihiki plateau with at least twice its present volume and displaced the triple junction southeast from the nova canton trough to the newly formed manihiki plateau almost simultaneously the plateau was rifted by the new triple junction system and large fragments of the plateau were rafted away to the south and east the tongareva triple junction originated ca 119 ma when carbonate sedimentation began atop the manihiki plateau subsequent spreading rates on the pacific phoenix and pacific farallon ridges averaged 18 20 cm yr until 84 ma

timing of prograde metamorphism in the zanskar himalaya the timing of the prograde metamorphism of continental crust buried during the collision of india and eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism our knowledge of the himalaya indicates that continental collision at 55 50 ma culminated in crustal melting and leucogranitic magmatism at 24 18 ma however the timing of events in the intervening 30 m y is poorly understood here we present sm nd isotope analyses of garnets from the zanskar himalaya that also preserve information on crustal and orogenic evolution during that period the prograde p t t paths indicate that heating and burial from 6 to 10 kbar continued until at least 25 30 ma and imply peak temperatures 700 c close to those necessary to cause the crustal melting in the himalaya at ca 20 ma without a requirement for additional heat sources such as shear heating on major crustal shear zones the persistence of high temperatures as late as 20 ma in the himalaya may be related to postcollisional thrusting south of the suture zone and within the indian plate at 40 35 ma

origin of small scale segmentation and transpressional thrusting along the alpine fault new zealand the alpine fault is the major structural feature of the australian pacific plate boundary in the south island of new zealand geologic evidence suggests that half to three quarters of the plate boundary displacement is accommodated by movement on the fault detailed investigation of the central section of the alpine fault has revealed that it consists of oblique thrust sections striking 020 050 that are linked by subvertical right lateral faults striking between 065 and 090 the segmentation is on a scale of 1 10 km similarly oriented right lateral faults are abundant southeast of the alpine fault and are consistent with stresses induced in an elastic layer by an oblique slip ductile fault zone below propagation of the fault to the surface is predicted to result in an en echelon arrangement of strike slip and thrust segments it is suggested that the spatial distribution of segments is affected by the existence of deeply incised valleys in the hanging wall that disturb the stress field to depths of 1 4 km the segmentation is near surface and does not appear to act as a barrier to the propagation of large earthquakes

metapelite phase equilibria modeling in mnnckfmash the effect of variable al2o3 and mgo mgo feo on mineral stability a series of mno na2o cao k2o feo mgo al2o3 sio2 h2o mnnckfmash metapelite pseudosections highlights the dependence of predicted mineral assemblages on bulk rock al2o3 and mg mgo mgo feo t xal pseudosections portray the dependence of staurolite biotite and aluminum silicate on al2o3 content allowing the distinction between high al and low al pelite as commonly portrayed with kfmash modeling the mnnckfmash system also shows the effect of al2o3 on plagioclase and zoisite stability which cannot be done in the kfmash system comparison of mnnckfmash to kfmash pseudosections highlights the consequence of ignoring the important rock constituents mno na2o and cao when constructing pseudosections kfmash cannot model important phases such as plagioclase and zoisite and there are significant differences in predicted garnet biotite and chloritoid stability in the two different systems in particular kfmash does not model garnet stability appropriately at low pressures and temperatures because it cannot account for the stabilizing effect of mn the comparisons also show that the method of calculating a kfmash bulk rock composition equivalent to a real rock composition is problematic and has significant implications for the predicted pseudosection assemblage stability comparison of the mnnckfmash pseudosections to natural assemblages observed in the waterville fm maine indicates that the mnnckfmash system comes very close to modeling naturally developed mineral assemblages successfully the only major discrepancy between predicted and observed assemblages is the inability to predict the paragenesis staurolite andalusite using an average or natural waterville fm composition garnet thermobarometric results from the waterville fm are in poor agreement with pseudosection topology for an average waterville fm composition this suggests that if quantitative p t path information is to be derived through a combination of pseudosections and thermobarometry samples will have to be investigated on an individual basis in more detail than was done in this study

disequilibrium for ca during growth of pelitic garnet compositional zoning in hundreds of almandine rich garnets in amphibolite by facies micaceous quartzites from the picuris range north central new mexico usa indicates that although mn mg and fe achieve chemical equilibrium at hand sample scale during garnet growth ca does not instead ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment yielding disequilibrium for ca at scales larger than the region immediately surrounding an individual porphyroblast detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite and core analyses were made of 97 additional crystals each analysis was made on a section that passed precisely through the morphological centre of the crystal located by means of 3 d imagery from computed x ray tomography the data reveal strong correlations between crystal size and concentrations of mn mg and fe but not ca in garnet cores a relationship between crystal size and isolation rigorous cross correlations among concentrations of mn mg and fe but not ca and systematic variations in ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations we interpret these observations as the result of thermally accelerated diffusion controlled garnet growth in circumstances that promoted rapid intergranular diffusion and thus rock wide equilibration of mn mg and fe but that prevented equilibration at similar scale for ca because of its more sluggish intergranular diffusion the anomalous behaviour of ca is made evident in these garnets by the presence of sharp spikes in ca concentration which are demonstrably not a consequence of any simultaneous rock wide event such as a change in pressure temperature or some other intensive parameter instead ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast to the degree that such kinetic factors introduce departures from chemical equilibrium for ca thermobarometric estimates that involve grossular contents of pelitic garnet will be in error

new garnets for old cautionary tales from young mountain belts major and trace elements in garnet may be used to characterize p t t paths in orogenic belts but they are also often used without age constraints to obtain p t estimates alone during investigations into the prograde phase of the alpine himalayan tertiary orogeny garnets with anomalously old sm nd ages were discovered in three different areas they occurred in identical parageneses to garnets that grew during tertiary prograde metamorphism ages 500 ma were determined for homogenized garnets from the garhwal himalaya while garnets with distinctive growth zonation from the betic cordillera s spain gave an age of 235 1 1 7 ma although the garhwal date coincides with a recognized magmatic and metamorphic event the 235 1 ma age from the betics at first sight appears one of the more unlikely times for garnet growth duplicate analyses precisely date growth of the garnet cores which x ray maps reveal to be entirely distinct from the rims implying a hiatus before resorption and rim development this initial garnet growth may be linked to early crustal extension during pangaea break up these results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts at the same time the data show the potential for unravelling the history of these complex zones and strike a cautionary note for thermobarometry

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

plate boundary deformation in south island new zealand is related to inherited lithospheric structure abstract the alpine fault is the main active structure in the oblique continental collision zone of south island new zealand it is continuous at the surface for 800 km and accommodates 70 of current plate motion a 460 km offset of basement rocks suggests it has accommodated 50 of plate displacement since 45 ma geophysical geological and contemporary kinematic data are successfully modeled by slip on a narrow alpine fault zone extending into the lower crust the data are consistent with plate boundary deformation occurring on either northwest or southeast dipping shear zones in the upper mantle or with widely distributed strain in the upper mantle we propose that the position of the alpine fault and any associated lithospheric discontinuity is controlled by an eocene passive margin that separated palaeozoic continental lithosphere of the challenger plateau from much younger oceanic lithosphere strike slip motion since 25 ma has translated the passive margin into the continental collision zone resulting in subduction of oceanic lithosphere beneath south island and progressive localisation of shear strain near the alpine fault the eocene rift boundary formed by exploiting cretaceous oceanic transform faults and an older discontinuity within the new zealand continent we propose that a precursor to the alpine fault could be as old as palaeozoic inherited structure has clearly controlled the first order deformation pattern through new zealand since at least 100 ma and demonstrates that ancient faults can play an important role in determining lithospheric scale patterns of deformation at continental plate boundaries

brittle and ductile deformation in a zone of rapid uplift central southern alps new zealand schists exposed in the central southern alps new zealand 1 5 km east of the alpine fault zone have been rapidly uplifted during the late cenozoic a regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle ductile transitional behaviour to ductile deformational features kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction indicating their relation to late cenozoic uplift of the southern alps analysis of deformed veins suggests a minimum shortening strain of about 50 perpendicular to the foliation the veins crosscut upright mesoscopic to macroscopic folds which commonly develop high strain zones on their limbs within such zones a strong stretching lineation plunges gently sw approximately perpendicular to the stretching lineation in the mylonites along the alpine fault the folds and high strain zones do not appear to be related to late cenozoic uplift but may have originated during an earlier phase of dominantly strike slip motion fluid inclusion studies give depth temperature estimates of around 3 km and 285 c for the development of the brittle structures and 6 8 km at 310 350 c for the brittle ductile transition the deformed veins show evidence of extensive fluid infiltration during the later phases data from fluid inclusions and metamorphic assemblages give an estimate of 15 20 km and 400 450 c for their deformation these data combined with uplift rates determined by other studies allow the construction of a depth temperature time path which indicates nearly isothermal decompression associated with rapid uplift a shallow brittle ductile transition 6 8 km is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area because of the small temperature change over a large amount of decompression pressure may be an important factor in controlling the depth of the brittle ductile transition the marked temperature drop above the brittle ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism

new insight into the dynamic development of the southern alps new zealand from detailed thermochronological investigation of the mataketake range pegmatites

tectonic development of the alpine fault zone new zealand a fission track study fission track dating of zircons from the alpine fault zone in new zealand and associated granites and gneisses reveals at least three stages in the development of the present fault zone mylonites northwest of the fraser fault give ages of ca 80 ma zircons from the fraser formation bounded by the fraser and alpine faults give ages of ca 9 ma samples east of the alpine fault give ages below 5 ma these youngest ages result from the currently active uplift of the alpine schists against the alpine fault the 9 ma ages record a rapid uplift of the fraser formation owing to the onset of compression across the plate boundary mylonites to the west of the mapped fraser fault that give zircon fission track ages of ca 80 ma indicate a third much earlier phase of fault movement structural and other geologic evidence supports these conclusions the observation that the present plate boundary was located along a preexisting zone of weakness suggests that mylonite zones may merit greater attention as possible sites for initiation of rifting and or continental transform faults 26 references

trace element abundance patterns of garnet inclusions in peridotite suite diamonds abstract individual sub calcic chrome pyrope crystals from finsch and kimberley diamonds finsch and bultfontein kimberlite heavy mineral concentrate and from diamondiferous harzburgite dunite xenoliths from the udachnaya kimberlite pipe were analyzed for rare earth elements ree sc ti and zr with the ion microprobe the abundances and abundance ratios of these trace elements including lree enrichment and low ti together with high and variable cr contents are inconsistent with a simple equilibrium relationship between peridotite suite garnet and silicate and carbonate liquids it is suggested that the trace element abundance patterns represent a signature of ancient mantle metasomatism which preceded the formation of peridotite suite garnet and diamond

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

the influence of high u th inclusions on the u th pb systematics of almandine pyrope garnet results of a combined bulk dissolution stepwise leaching and sem study abstract the isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts this approach to geochronology has the distinct advantage that time information is directly related to the pressure temperature deformation history recorded by the same mineral one potential problem with the technique however is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself inclusions of old refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth establishing pt histories and constraining tectonic mechanisms in this contribution we present u th pb data obtained on bulk dissolutions of garnet from the himalaya as well as the results of a detailed sem and u th pb stepwise leaching study in order to further clarify these issues our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted the u th pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 ma to 30 ma the stepwise leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the sem the bulk of the radiogenic 206pb in these samples was released only by treatment at high pressure in a bomb in addition the 208pb 206pb ratio of the radiogenic pb released in earlier leach steps is inconsistent with the available constraints on the th u ratio of garnet lattices as measured by sims in this case the u pb ages of these garnets is dependent purely on the age of the included high u th phases and the degree to which they have isotopically re equilibrated with the whole rock during the subsequent metamorphism that grew garnet simple mass balance arguments show that in this particular instance very small amounts of these inclusions are required amounts that escape detection during conventional sample preparation these data also have implications for current thinking on the closure temperature for the sm nd and u pb systems in garnet

kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks central southern alps new zealand the hanging wall of the alpine fault near franz josef glacier has been exhumed during the past similar to2 3 m y providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen presently exposed rocks of the pacific plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid crustal detachment initially they were transpressed across the outboard part of the orogen resulting in oblate fabrics with a down dip stretch later they encountered the alpine fault experiencing an oblique slip backshearing on vertical planes this escalator like deformation tilted and thinned the incoming crust onto that crustal scale oblique ramp this style of hanging wall deformation may affect only the most rapidly uplifting central part of the southern alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp angle at depth a 3d transpressive flow affected mylonites locally near the fault but their shear direction remained parallel to plate motion ruling out ductile extrusion as an important process in this orogen outside the mylonite zone late cenozoic shortening is inferred to be modest 30 40 as measured from deformation of younger biotite grains oblique collision is dominated by translation on the alpine fault and rocks migrate rapidly through the deforming zone preventing the accumulation of large finite strains transpression may play a minor role in oblique collision c 2001 elsevier science ltd all rights reserved

retrogressive dissolution of garnet effect on garnet biotite geothermometry cation exchange geothermometry is predicated on the basis of chemical equilibration between phases involved in ion exchange reactions if chemical equilibrium is attained between the phases at any temperature and subsequently locked in temperature determinations should be consistent throughout a given sample the attainment of chemical equilibrium is commonly inferred where phases used for thermobarometry are in mutual contact or where sharp straight grain boundaries exist between grains thermometric determinations on garnet biotite pairs in a metapelite from the cheyenne belt wyoming illustrate the problem of using these criteria to infer chemical equilibrium in one example garnet biotite pairs from different parts of a single zoned garnet grain yield apparent temperatures that range over 150 c and are anomalously higher than documented metamorphic conditions almandine garnets from low to intermediate grade pelitic rocks commonly show strong compositional zoning retrograde modification coupled with strain induced dissolution of garnet may result in meaningless temperatures because of nonequilibration of phases used for geothermometry apparent textural equilibrium or mutual contact of grains may not be indicative of chemical equilibrium dissolution of garnet under conditions of biotite stability can produce garnet biotite pairs with sharp straight grain boundaries suggestive of textural equilibrium in the absence of visible truncation of compositional zones compositional maps or microprobe traverses both parallel and normal to schistosity are necessary to evaluate the possibility of dissolution failure to do so may result in the incorrect interpretation of t p determinations

uplifted marine terraces along the alpine fault new zealand three types of evidence indicate that marine terraces are widespread in the southern alps of new zealand i remnants of shore platforms occur as distinct levels of notched ridge crests and flat summits degraded sea cliffs are common ii scattered quartz beach pebbles occur on 16 of 18 levels of exhumed shore platforms in the fox franz josef type area to altitudes as high as 1700 meters iii altitudinal spacings of new zealand terrace flights allow correlation with 18 dated global marine terraces at new guinea which were formed during glacio eustatic highstands of sea level within the last 336 x 10 3 years inferred uplift rates at fox franz josef increased from 3 2 to 7 8 meters per 10 3 years since about 135 x 10 3 to 140 x 10 3 years ago presumably because of increased convergence between the pacific and australian plates

ion microprobe determinations of trace element concentrations in garnets from anatectic assemblages abstract trace element data for garnet cordierite plagioclase alkali feldspar and tourmaline have been obtained by ion microprobe analysis of assemblages from a pelite derived leucogranite a sillimanite grade anatectic gneiss and a kyanite grade sub solidus schist from the high himalaya results confirm that for equilibrium melting residual plagioclase will cause depletion in sr and eu in the melt and residual garnet will result in strong depletion of y and hree consistent with their published partition coefficients all trace elements of interest are strongly incompatible in cordierite implying that for equilibrium fractional melting of pelites at shallow depths hree concentrations in garnets from metasediments are independent of metamorphic grade but the metamorphic garnets are significantly less enriched in hree than is the leucogranite garnet this suggests either that the leucogranite is derived from a source with lower modal abundances of garnet than the analysed metasediments or that garnet from the anatectic gneiss has not reached chemical equilibrium with a granitic melt the latter possibility is consistent with sluggish self diffusion rates in garnets for ree at temperatures of 5 kbar

metamorphism and uplift of alpine schist in the franz josef fox glacier area of the southern alps new zealand geothermometry and geobarometry of 10 garnet oligoclase zone schists in the franz josef fox glacier area southern alps new zealand give temperatures ranging from 415 to 625 c and pressures from 5 2 to 9 2 kbar indicating a t p array of about 50 c kbar and inferred peak temperature conditions over a c 15 km thick section at depths between c 20 and 34 km the present day distribution of the schist samples implies that only about one third of the original crustal section is now exposed the garnet oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late palaeozoic early mesozoic subduction along the gondwana continental margin partial uplift c 0 2 m ma and erosion of the complex during jurassic cretaceous times rangitata uplift was synchronous with d2 deformation and recrystallization as recorded by the p t array cenozoic kaikoura uplift and exhumation of the schist since c 30 ma to form the southern alps was associated with oblique slip movement on the alpine fault the present day position and steep eastward dip of isograds and d2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two thirds of the original schist sequence largely due to simple shear along schistosity planes as the schist generally shows only incipient greenschist facies retrograde recrystallization an apparently complete although contracted prograde mineral sequence has been preserved by rapid uplift 5 km ma of hot rock and the effects of limited shear heating near the alpine fault

middle to late miocene age for the end of amphibolite facies mylonitization of the alpine schist new zealand implications for onset of transpression across the alpine fault

trace element zoning in garnet from the kwoiek area british columbia disequilibrium partitioning during garnet growth trace element zoning in garnets from two contact metamorphosed rocks from the kwoiek area british columbia hollister 1969a was measured with an ion microprobe zoning profiles have three distinct parts with chemical breaks defined by co variation of major and trace elements important features of the trace element zoning profiles are 1 roughly bell shaped zoning profiles for y and the hrees 2 an abrupt increase in ti at a point midway through each garnet with inflections in the zoning profiles of other elements li na cr v y zr and the hree and 3 irregular cr and v profiles unlike mn zoning the zoning profiles of most other trace elements cannot be easily modeled using simple rayleigh fractionation models ti activity in the two samples is buffered by phase relations with ilmenite garnets from a continuously heated contact metamorphic environment should display continuous ti zoning profiles if equilibrium was maintained and provided the ti buffering assemblage did not change during garnet growth the irregular ti profiles suggest disequilibrium behavior several elements cr v may indicate breakdown of a phase enriched in trace elements during metamorphism the source for the mass excess of these elements is probably the refractory cores of ilmenite grains either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point the preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth

the effect of whole rock mno content on the stability of garnet in pelitic schists during metamorphism garnet bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions modelling of thermodynamic data for minerals in the system na2o k2o feo mgo al2o3 sio2 h2o however predicts that garnet should be observed only in rocks of a narrow range of very high fe mg bulk compositions traditionally the nearly ubiquitous presence of garnet in medium to high grade pelitic schists is attributed qualitatively to the stabilizing effect of mno based on the observed strong partitioning of mno into garnet relative to other minerals in order to quantify the dependence of garnet stability on whole rock mno content we have calculated mineral stabilities for pelitic rocks in the system mno na2o k2o feo mgo al2o3 sio2 h2o for a moderate range of mno contents from a set of non linear equations that specify mass balance and chemical equilibrium among minerals and fluid the model pelitic system includes quartz muscovite albite pyrophyllite chlorite chloritoid biotite garnet staurolite cordierite andalusite kyanite sillimanite k feldspar and h2o fluid in the mno free system garnet is restricted to high fe mg bulk compositions and commonly observed mineral assemblages such as garnet chlorite and garnet kyanite are not predicted at any pressure and temperature in bulk compositions with xmn mn fe mg mn 0 01 however the predicted garnet bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole rock mno content the small mno contents of normal pelitic schists xmn 0 01 0 04 are both sufficient and necessary to account for the observed stability of garnet

thermal regime in the central alpine fault zone new zealand constraints from microstructures biotite chemistry and fluid inclusion data abstract the thermal state within plate boundary fault zones is likely to influence the dimensions of large co seismic fault ruptures and strain localisation in underlying mylonitic shear zones vein microstructures biotite chemistry and fluid inclusion data offer new insights into the thermal structure of the mylonitic shear zone down dip of the alpine fault the main pacific australian plate boundary structure through southern new zealand quartz veins within the fault rocks were formed and deformed at temperatures ranging from 500 c to immediately above that of the brittle viscous transition 325 c illustrating that fluids are present throughout the fault zone microthermometric data from primary mixed co 2 h 2 o fluid inclusions in boudin neck quartz carbonate veins indicate that these inclusions were trapped at temperatures of 325 15 c and pressures of c 40 mpa vein microstructures indicate that fluid temperatures were equal to rock temperatures assuming hydrostatic fluid pressure the veins were trapped at depths of 2 o co 2 fluids in the shallower crust of 500 kg m 3 biotite grains in the mylonites are a mixture of porphyroclastic grains with high r ti mg inherited from the parent alpine schist and smaller low r grains that recrystallised during mylonitisation particularly those that are found within syn mylonitic shear bands assuming a simple step function model approximating diffusive equilibration the changes in biotite chemistry indicate t during mylonitic deformation of 340 c according to the ti in biotite thermometer of henry et al 2005 a best fit uplift p t path to existing data and these new results indicate that the uplift path p t gradient of fault zone rocks above the brittle viscous transition is of the order of 40 c km 1 below the brittle viscous transition the uplift path p t gradient is 10 c km 1 the p t profile within the fault rocks is comparable to that in the immediate hanging wall alpine schist indicating there is no peak in the crustal isotherms in the fault hanging wall as was indicated by previous numerical models and analyses that did not consider data from the fault rocks themselves

metamorphic discontinuities in orogenic belts example of the garnet biotite albite zone in the otago schist new zealand a regional petrographic reconnaissance of psammitic and pelitic rocks in the otago schist new zealand has revealed the presence of garnet grossalspite with typical rim composition almandine41 spessartine25 grossular33 pyrope1 and biotite in 37 new samples more than doubling the previously known number a new garnet biotite albite zone can now be defined in the greenschist facies otago schist that is distinct from the better known biotite garnet and oligoclase zones in the along strike alpine schist the garnet biotite albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple continuous progressive jurassic regional metamorphism in otago the structurally higher lower grade boundary of the zone coincides in at least three places with previously mapped regional shear zones the structurally lower expected higher grade boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to alpine schist recrystallisation of cretaceous age the otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts

sub critical fracture and associated retrogression of garnet during mylonitic deformation garnets from the alpine fault mylonites are fractured most fractures have a retrogressive fracture fill a small number have garnet fill a detailed sem study has been carried out to elucidate the mechanisms of fracture and associated retrogression wide fracture zones contain free floating islands of garnet in a retrogressive matrix electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations where rotations are observed they are small and systematic some large fractures show clearly that fracture and retrogression occurs by a replacement process fracture surface morphologies are indicative of surface dissolution a model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet