rutile u pb age depth profiling a continuous record of lithospheric thermal evolution abstract understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history using laser ablation inductively coupled plasma mass spectrometry we outline here a method to simultaneously acquire 206 pb 238 u age and trace element profiles from u bearing accessory phases inversion of 206 pb 238 u age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships thermally activated volume diffusion of pb and zr in rutile is sensitive to the thermal evolution of the mid to lower lithosphere we document the ability of laser ablation depth profiling to simultaneously resolve 206 pb 238 u age and zr diffusion profiles in the outer 35 m of lower crustal rutile euhedra from the ivrea zone southern alps with 1 2 m depth resolution inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from 600 c at 180 ma followed by a period of slower cooling from 525 c to 450 c combined with the topology of zr diffusion profiles these data indicate that the ivrea zone underwent a brief thermal pulse in the early jurassic plausibly associated with hyperextension of the adriatic margin inversion of near surface 206 pb 238 u age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere

trace elements in titanite a potential tool to constrain polygenetic growth processes and timing abstract the petrological information preserved in the trace element signature of titanite is a valuable complement to in situ u pb geochronology and can be used to refine interpretations on the growth of this mineral we present trace element and u pb isotopic compositions of titanite grains in five amphibolite to granulite facies samples from the east albany fraser orogen of western australia chondrite normalised rare earth element ree abundance patterns discriminate between titanite populations and correlate with backscatter electron bse zonation in two samples titanite ree composition correlates with proximity to garnet and in one sample titanite composition correlates with gneissic compositional banding the dy yb ratio can be used as a geochemical indicator to link titanite growth to garnet growth titanite that crystallised in a garnet bearing assemblage is hree depleted with dy yb 2 whereas titanite that crystallised in a garnet free assemblage typically has flat hree patterns with dy yb by integrating the ree signatures with u pb data the five new titanite u pb ages can be linked to a range of processes magmatic titanite crystallisation metamorphic titanite growth garnet growth and or cooling through the closure temperature for pb diffusion in titanite

u pb dating of prograde and retrograde titanite growth during the scandian orogeny u pb ages of metamorphic titanite from high grade gneisses of the nasafjallet window in the north central scandinavian caledonides record growth over an 20 m y interval during the late silurian to early devonian biotite amphibolite gneisses located near the basement cover boundary contain metamorphic titanite in two microtextural settings one as lenticular grains distributed throughout a recrystallized matrix of metamorphic phases and one as plate like grains enclosed within biotite grains these distinct morphological populations exhibit core overgrowth relationships with compositionally similar cores and overgrowths but the plate like titanite grains have a much higher proportion of overgrowth and give slightly younger ages cores and overgrowths in both types are inferred to have formed along the prograde ca 413 ma and retrograde ca 395 ma paths respectively of a single metamorphic cycle associated with the burial and exhumation of a slice of baltic basement during the caledonide orogeny baltica laurentia collision the existence of polygenic u pb systematics in metamorphic titanite demonstrates the potential usefulness of titanite to reveal multiple metamorphic and deformational episodes and to constrain the duration of metamorphic events

the thermal structure of collisional orogens as a response to accretion erosion and radiogenic heating thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies here we demonstrate that high temperature low pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat producing elements a new two dimensional finite difference model described here incorporates the subduction of lithosphere with heat producing material in the upper crust accretion of crustal material from the subducting plate to the upper plate and surface erosion of the upper plate these processes result in the development of a wedge of heat producing material within the upper plate the rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures our model yields inverted upper plate geotherms when heat production rates exceed 0 75 w m3 and the heat producing wedge extends to a depth greater than 35 km temperatures in excess of 500 c at depths of 20 30 km are computed when heat production rates are greater than 1 75 w m3 and the wedge extends to a depth 50 km other processes such as shear heating fluid flow or mantle delamination need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems

a p t t d discontinuity in east central nepal implications for the evolution of the himalayan mid crust abstract rock specimens collected from lower middle and upper portions of the exhumed himalayan mid crust in the upper tama kosi region of east central nepal yield pressure temperature time deformation p t t d paths that demonstrate the presence of a structural metamorphic and geochronologic discontinuity a p t t d path from below the discontinuity defines a loading path with deformation at staurolite grade metamorphic conditions occurring at 10 8 ma p t t d paths from above the discontinuity in contrast record an earlier and protracted metamorphic history that includes decompressional heating monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be 21 ma with decompression related garnet breakdown initiating at c 19 ma and likely continuing until c 15 ma these contrasting p t t d histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen these results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen

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

timescales of collisional metamorphism from sm nd lu hf and u pb thermochronology a case from the proterozoic putumayo orogen of amazonia abstract the construction of mountain belts by continent continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction driven plate tectonics on earth yet quantitative estimation of the rate and duration of prograde orogenic metamorphism in high grade collisional belts has proven elusive to constrain differences in the chemical partitioning and diffusive behavior of elements associated with the 147sm 143nd and 176lu 176hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes here we present new sm nd and lu hf isotopic results from multiple garnet and whole rock aliquots from a granulite grade metasedimentary rock from the proterozoic putumayo orogen of south america using a combination of internal isochron dating by these two decay systems combined with apatite u pb dating and pressure temperature time p t t constraints from mineral equilibria and chemical diffusion geospeedometry we provide first order insights into the time scales of a proterozoic continent continent collisional metamorphic event in amazonia associated with its incorporation to the supercontinent rodinia our results indicate that these metasediments attained peak granulite grade metamorphic conditions of nearly 0 62 gpa and 680 c at approximately 1035 ma and started to cool slowly presumably during exhumation at a rate of 5 k myr simple diffusion scaling relationships and numerical modeling of sm nd and lu hf in garnet indicate that while sm and nd were both fully re equilibrated at peak temperatures and therefore date cooling lu and hf underwent diffusive decoupling due to incomplete hf re equilibration by modeling the impact of this differential re equilibration in the development of the sm nd and lu hf isochrons we constraint the duration of the prograde metamorphic path and argue that garnet growth in this sample took place over millions of years approximately 25 myr according to our preferred interpretation the determined apatite u pb date vs grain size relationship cannot be reconciled with the thermal history described above when modeled using the existing experimental diffusion data and instead suggest that much slower pb diffusivities in this mineral compared to those predicted by the existing experimental data may have inhibited its resetting we note that the inferred temperature time history for the metamorphic loop of this segment of the putumayo basement is comparable to that proposed for the metamorphism and exhumation of metasediments in the india asia collision zone suggesting that similar tectonic processes and rates likely operated during the mesoproterozoic putumayo orogeny

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

geochronology and trace element geochemistry of titanite in the machangqing cu mo dominated polymetallic deposit yunnan province southwest china abstract the machangqing cu mo dominated polymetallic deposit is a porphyry skarn epithermal cu mo au metallogenic system located in the middle part of the jinshajiang ailaoshan alkaline porphyry metallogenic belt the skarn mineralization of the machangqing deposit mainly occurs along the contacts between the alkalic porphyry intrusions and the surrounding lower ordovician xiangyang formation rocks we present la icp ms u pb ages and trace element data for titanite from the machangqing deposit in order to investigate the origin of this deposit based on mineral textures and assemblages two types of titanite are recognized in machangqing magmatic titanite type i from the granite porphyry and hydrothermal titanite from the mineralized skarn the coarse grained magmatic titanite is euhedral and occurs as discrete grains in the interstices of feldspar quartz and biotite whereas fine to medium grained hydrothermal titanite crystals type ii are euhedral to subhedral and occur in association with skarn minerals such as garnet pyroxene and magnetite magmatic titanite has lower feo al2o3 f and nb ta but higher tio2 th u hfses and lu hf than hydrothermal titanite the magmatic titanite has higher lree hree ratios and total ree contents with stronger negative eu anomalies than its mineralized skarn counterpart trace elemental characteristics of hydrothermal titanite in machangqing are consistent with relatively low f contents and oxygen fugacities when compared to the neighboring beiya gold dominated polymetallic deposit in the same metallogenic belt the weighted average 206pb 238u age of 34 3 1 2 ma of hydrothermal titanite is within error but slightly younger than the age of magmatic titanite 37 5 4 1 ma indicating that the skarn mineralization followed the emplacement of the granite porphyry and was broadly coeval with the porphyry mineralization the porphyry and skarn types of mineralization at machangqing were formed from the same metallogenic system different ore forming conditions such as oxygen fugacities and f contents might be responsible for different metal enrichments at the machangqing and beiya deposits

continental collision and slab break off a comparison of 3 d numerical models with observations abstract conditions and dynamics of subduction collision and subsequent 3 d slab break off and slab tear propagation are quantified for the first time using fully dynamic numerical models model results indicate that collision after the subduction of old strong subducting oceanic slab leads to slab break off at 20 25 myr after the onset of continental collision and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100 150 mm yr in contrast young weak oceanic slabs show the first break off already 10 myr after continental collision and can experience tear migration rates up to 800 mm yr slab strength plays a more important role in the timing of slab break off and the speed of a propagating slab tear than negative slab buoyancy does slab break off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle the density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break off a 40 km thick continental crust can be buried to depths greater than 200 km although this maximum depth is significantly less for younger or very weak slabs or thicker continental crust slab break off typically starts at a depth of 300 km mostly independent of mantle rheology but like continental crustal burial can be shallower for young or buoyant plates our 3 d models illustrate how due to the difference in necking in 2 d and 3 d break off has an intrinsic small preference to start as a slab window within the slab s interior rather than as a slab tear at the slab edge however any significant asymmetry in the collision setting e g earlier collision at one end of the subduction zone would override this and leads to slab tearing starting near one edge of the slab these results put important new constraints on the dynamics of the collision and subsequent slab break off for modern collision belts for a proposed timing of the initial arabia eurasia collision at 35 ma break off of the 200 myr old neo tethys slab is unlikely to have occurred before 15 10 ma furthermore our results illustrate that shallow early break off of weak slabs provides a viable explanation for the absence of blueschists and ultra high pressure metamorphism in the precambrian geological record

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

protracted thrusting followed by late rapid cooling of the greater himalayan sequence annapurna himalaya central nepal insights from titanite petrochronology the greater himalayan sequence ghs has commonly been treated as a large coherently deforming high grade tectonic package exhumed primarily by simultaneous thrust and normal sense shearing on its bounding structures and erosion along its frontal exposure a new paradigm developed over the past decade suggests that the ghs is not a single high grade lithotectonic unit but consists of in sequence thrust sheets in this study we examine this concept in central nepal by integrating temperature time t t paths based on coupled zr in titanite thermometry and u pb geochronology for upper ghs calcsilicates with traditional thermobarometry textural relationships and field mapping peak zr in titanite temperatures are 760 850 c at 10 14 kbar and u pb ages of titanite range from c 30 to c 15 ma sector zoning of zr and distribution of u pb ages within titanite suggest that diffusion rates of zr and pb are slower than experimentally determined rates and these systems remain unaffected into the lower granulite facies two types of t t paths occur across the chame shear zone csz between c 25 and 17 16 ma hanging wall rocks cool at rates of 1 10 c ma while foot wall rocks heat at rates of 1 10 c ma over the same interval temperatures increase structurally upwards through the hanging wall but by 16 17 ma temperatures converge in contrast temperatures decrease upwards in foot wall rocks at all times while the foot wall is interpreted as an intact structurally upright section the thermometric inversion within the hanging wall suggests thrusting of hotter rocks over colder from c 25 to c 17 16 ma retrograde hydration that is restricted to the hanging wall and a lithological repetition of orthogneiss are consistent with thrust sense shear on the csz the csz is structurally higher than previously identified intra ghs thrusts in central nepal and thrusting duration was 3 6 ma longer than proposed for other intra ghs thrusts in this region cooling rates for both the hanging wall and foot wall of the csz are comparable to or faster than rates for other intra ghs thrust sheets in nepal the overlap in high temperature titanite u pb ages and previously published muscovite 40ar 39ar cooling ages imply cooling rates for the hanging wall of 200 c ma after thrusting causes of rapid cooling include passive exhumation driven by a combination of duplexing in the lesser himalayan sequence and juxtaposition of cooler rocks on top of the ghs by the stds normal sense displacement does not appear to affect t t paths for rocks immediately below the stds prior to 17 16 ma this article is protected by copyright all rights reserved

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

petrology and geochronology of rutile rutile tio2 is an important accessory mineral that when present offers a rich source of information about the rock units in which it is incorporated it occurs in a variety of specific microstructural settings contains significant amounts of several trace elements and is one of the classical minerals used for u pb age determination here we focus on information obtainable from rutile in its original textural context we do not present an exhaustive review on detrital rutile in clastic sediments but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile for further information on the important role of rutile in provenance studies the reader is referred to previous reviews e g zack et al 2004b meinhold 2010 triebold et al 2012 coarse rutile is the only stable tio2 polymorph under all crustal and upper mantle conditions with the exception of certain hydrothermal environments smith et al 2009 as such we will focus on rutile rather than the polymorphs brookite anatase and ultrahigh pressure modifications in this chapter we first review rutile occurrences trace element geochemistry and u pb geochronology individually to illustrate the insights that can be gained from microstructures chemistry and ages then in the spirit of petrochronology we show the interpretational power of combining these approaches using the ivrea zone italy as a case study finally we suggest some areas of future research that would improve petrochronologic research using rutile rutile is a characteristic mineral in moderate to high pressure metapelitic rocks in high pressure metamorphosed mafic rocks and in sedimentary rocks e g force 1980 frost 1991 zack et al 2004b triebold et al 2012 rutile also occurs rarely in magmatic rocks e g anorthosites as well as in some hydrothermal systems coarse grained

channel flow extrusion model to constrain dynamic viscosity and prandtl number of the higher himalayan shear zone constraining magnitudes of mechanical and thermo mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics talbot in j struct geol 21 949 957 1999 such parameters aid dynamic scaling of analogue tectonic models ramberg in gravity deformation and the earth s crust in theory experiments and geological applications 2nd edn academic press london 1981 which are useful to unravel tectonics in further details schultz ela and walsh in j struct geol 24 247 275 2002 the channel flow extrusion of the higher himalayan shear zone hhsz higher himalaya can be explained by a top to s sw simple shear i e the d2 deformation in combination with a pressure gradient induced flow against gravity presuming its newtonian incompressible rheology with parallel inclined boundaries the viscosity of this shear zone along a part of the himalayan chain through india nepal and bhutan is estimated to vary widely between 1016 and 1023 pa s and its prandtl number pr within 1021 1028 the estimates utilized ranges of known thickness 6 58 km of the hhsz that of its top subzone of ductile shear of normal shear sense stdsu 0 35 9 4 km total rate of slip of its two boundaries 0 7 131 mm year 1 pressure gradient 0 02 6 kb km 1 density 2 2 3 1 g cm 3 and thermal diffusivity 0 5 10 6 2 1 10 6 m s 2 along the orogenic trend considering most of the parameters specifically for the sutlej section india the calculated viscosity and the prandtl number pr of the hhsz are deduced to be 1017 1023 pa s and pr 1022 1028 the upper limits of the estimated viscosity ranges are broadly in conformity with a strong tibetan mid crust from where a part of the hhsz rocks extruded on the other hand their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure the estimated mechanical and thermo mechanical parameters of the hhsz will help to build dynamically scaled analogue models for the himalayan deformation of the d2 phase

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

thermometry and microstructural analysis imply protracted extensional exhumation of the tso morari uhp nappe northwestern himalaya implications for models of uhp exhumation

applications and limitations of u pb thermochronology to middle and lower crustal thermal histories abstract volume diffusion of pb occurs over micron length scales in apatite and rutile at temperatures relevant to the evolution of the middle and lower crust continuous thermal history information can be resolved from inversion of intracrystalline u pb date profiles preserved within individual grains recent developments in microbeam analysis permit rapid measurement of these age profiles at sub micron spatial resolution thus heralding a new era for u pb thermochronology here we review the theoretical experimental and empirical basis for u pb thermochronology and show that rutile in particular presents an exceptional opportunity to obtain high resolution thermal history information from the deep crust we present a bayesian procedure that is well suited to the inversion of u pb date profile datasets and balances computational efficiency with a full search of thermal history coordinate space complications relevant to accurate application of u pb thermochronology are discussed i theoretically and ii empirically using a rutile u pb dataset from the lower crust of the grenville orogeny purely diffusive date profiles are shown to be the exception to uniform or step like young profiles suggesting that processes other than thermally activated volume diffusion may control u pb systematics in rutile residing in the lower crust however the data obtained from apparent diffusive profiles systematically match cooling histories inferred from other thermochronometers this result emphasises the importance of integrating microtextural observations and trace element concentrations with u pb age data in order to discriminate between diffusive and non diffusive pb transport mechanisms in accessory phases and thus minimize the risk of generating spurious thermal histories

campaign style titanite u pb dating by laser ablation icp implications for crustal flow phase transformations and titanite closure abstract u pb dates of titanite from 150 samples of chiefly quartzofeldspathic gneiss and leucosomes were measured across the western gneiss region of norway to understand deformation and metamorphism of typical crustal rocks during ultrahigh pressure uhp subduction and exhumation titanite is unstable at these high temperatures and pressures and indeed most of the titanite yielded post uhp dates a modest number of titanites sampled across large areas however have pre uhp u pb dates indicating that they survived their excursion to and return from mantle depths metastably this has three important implications 1 titanite grains can remain closed to complete pb loss during regional metamorphism at temperatures as high as 750 c and pressures as high as 3 gpa 2 phase transformations in quartzofeldspathic rocks can be inhibited at the same conditions 3 quartz bearing rocks can remain undeformed even at high temperature and pressure both of the latter were previously recognized the present study simply presents a new method for evaluating both using titanite u pb dates

thermo kinematic evolution of the annapurna dhaulagiri himalaya central nepal the composite orogenic system the himalayan orogen represents a composite orogenic system in which channel flow wedge extrusion and thrust stacking operate in separate orogenic domains with distinct rheologies and crustal positions we analyze 104 samples from the metamorphic core greater himalayan sequence ghs and bounding units of the annapurna dhaulagiri himalaya central nepal optical microscopy and electron backscatter diffraction ebsd analyses provide a record of deformation microstructures and an indication of active crystal slip systems strain geometries and deformation temperatures these data combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the ghs the profiles define a three stage thermokinematic evolution from midcrustal channel flow stage 1 7008c to 550 6508c to rigid wedge extrusion stage 2 400 6008c and duplexing stage 3 280 4008c these tectonic processes are not mutually exclusive but are confined to separate rheologically distinct orogenic domains that form the modular components of a composite orogenic system these orogenic domains may be active at the same time at different depths positions within the orogen the thermokinematic evolution of the annapurna dhaulagiri himalaya describes the migration of the ghs through these orogenic domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems

implications of near rim compositional zoning in rutile for geothermometry geospeedometry and trace element equilibration in principle compositional profiling of the near rim region of minerals can provide insight into cooling rates but presumes that loss or gain of material from the crystal rim is not kinetically restricted trace element depth profiles collected for zr hf ta nb and u in amphibolite facies rutile grains of the catalina schist southern california show significant variability within a single rock profiles of the same element among different grains can have significantly different slopes grains with indistinguishable zr profiles show vastly different nb profiles and grains with indistinguishable nb profiles show different zr profiles textural and kinetic idiosyncrasies within the matrix apparently affect the ability of specific crystals to accept or release trace elements and impugn the common assumption that mineral surfaces maintain equilibrium at amphibolite facies conditions a new model that limits the flux of zr from rutile grains helps explain commonly observed compositional profiles and implies that inversion of compositional profiles assuming equilibrium among grain surfaces will invariably overestimate cooling rates few grains may record the low closure temperatures that experimentally determined diffusivities imply rather higher temperatures will be retained depending on the proximity of reactants and products in the matrix silicon diffusion does not control zr reequilibration in rutile and relative diffusion coefficients d s of trace elements in rutile are dzr dhf 10dnb 20dta 40du

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

aluminous titanite sphene from the eclogite zone south central tauern window austria abstract highly aluminous titanite with al 2 o 3 contents up to 14 wt 50 of the caalsio 4 f oh end member occurs in siliceous dolomites from the eclogite zone south central tauern window austria 140 chemical analyses of titanites from 13 samples from the eclogite zone and adjacent tectonic units reveal that al is incorporated into titanite primarily by the mechanism ti o al f oh there is also a strong positive correlation between al and f content of these titanites indicating that the combined substitution is enhanced by the presence of f al contents of titanite vary between tectonic units with the most aluminous varieties occurring in the high pressure eclogite zone variations also occur from sample to sample within a single tectonic unit and both between and within individual grains in a single sample complex zoning with sharp chemical and optical discontinuities is observed with al 2 o 3 variations of up to 6 wt within a single sample individual grains generally have low al cores mantled by high al zones and rimmed by low al edges it is suggested that the incorporation of al into titanite is favoured by high p low t and high f activity

laser ablation split stream analysis of the sm nd and u pb isotope compositions of monazite titanite and apatite improvements potential reference materials and application to the archean saglek block gneisses abstract continued improvements in both icpms inductively coupled plasma mass spectrometry and laser ablation technologies are fueling advancements in accessory mineral investigations and their related isotope systems of interest and are now being applied to a wide range of geological applications in this contribution we present an updated methodology for laser ablation split stream lass analysis of the light rare earth element lree enriched minerals monazite titanite and apatite for simultaneous analysis of the u th pb age or trace element content and the sm nd isotope system the data were collected with the current generation of high sensitivity icpms and laser systems thermofinnigan neptuneplus mc icpms and element xr sf icpms coupled to a resolution 193 nm arf excimer laser system the increased sensitivity of these icpms instruments allows for improved spatial resolution and the ability to target minerals which previously contained insufficient concentrations of elements of interest e g sm nd in apatite making their analysis difficult if not impossible using less sensitive instruments furthermore the higher sensitivity allows less aggressive ablation parameters that facilitate thin section sampling and reduces inter element and isotopic fractionation to assess and improve the accuracy precision and efficiency of the technique three new potential lass reference materials rms are evaluated for dual u pb and sm nd analysis tory hill apatite tory hill titanite and steenskralkamp monazite the homogeneity of these materials was first assessed using reconnaissance laser ablation analyses with final characterization of u pb age and sm nd isotope composition using isotope dilution thermal ionization mass spectrometry id tims the precision and accuracy of the lass method is explored using secondary mineral reference materials of known age and sm nd isotope composition the utility of the technique is evaluated with a case study of monazite and apatite from the eoarchean uivak gneiss complex of labrador canada finally we suggest that there is a wide variety of geological applications for this methodology such as multi mineral detrital provenance crustal growth and petrogenic studies

combined oxygen isotope and u pb zoning studies of titanite new criteria for age preservation this material is based upon work supported by the u s national science foundation award ear 0838058 the u s department of energy office of basic energy sciences under award number de fg02 93er14389 and geological society of america student research grant 9224 10 wiscsims is partially supported by nsf ear 1053466 and 1355590

early ordovician u pb metamorphic ages of the eclogite bearing seve nappes northern scandinavian caledonides eclogite grade metamorphism of the seve nappe complex snc in norrbotten sweden records the attempted subduction of the baltic continental margin during the early palaeozoic evolution of the iapetus ocean metamorphic titanite sampled from several calcsilicate gneisses of the snc in norrbotten occurs as part of a prograde eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant u pb ages of 500 475 ma later structural disruption of these rocks occurred during the siluro devonian scandian phase of the caledonide orogeny but the u pb systematics show no evidence of a second generation metamorphic or recrystallized of titanite or of post early ordovician disturbance through pb loss hence the u pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the snc these results support earlier sm nd and 40ar 39ar studies indicating an early ordovician metamorphic age for the eclogitic norrbotten snc and confirm the early ordovician destruction of at least this segment of the palaeozoic passive margin of baltica these results indicate that the snc in the northern scandinavian caledonides was subducted and metamorphosed to high grade some 50 70 myr prior to the high grade metamorphism of the snc in the central scandinavian caledonides this result requires significantly different early palaeozoic tectonic histories for rocks mapped as snc in the northern caledonides and those in the central caledonides despite a seemingly similar tectonostratigraphic position and broadly similar high grade metamorphism

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

u pb systematics and trace element characteristics in titanite from a high pressure mafic granulite abstract simultaneous acquisition of u pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic high pressure granulite using la icp ms split stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high temperature melt present environments although the titanite studied here have a comparatively low initial pb pb 0 component pb 0 pb the pb 0 is highly radiogenic relative to model crustal values indicating inheritance from u bearing accessory minerals consumed in the melt titanite forming reactions additionally titanite crystals typically exhibit core rim decreases in pb 0 pb as defined by 204 pb 206 pb calculated 206 pb 0 206 pb t and uncorrected 206 pb 238 u spot date profiles near the margins this is clearly dominated by local u enrichment but in the uniformly low u interiors outwardly decreasing pb 0 pb appears to reflect decreasing pb 0 concentrations during growth the positive correlation among pb 0 and sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation given the high crystallization temperatures 800 c and likely slow cooling rates 5 c our data support slow pb diffusivity in titanite even at high temperature conditions as has been proposed in a number of recent studies along the outer 50 100 m u th zr and ree concentrations are variably elevated relative to the crystal interiors with profiles taking one of two forms 1 sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge or 2 gradually increasing to highest concentrations nearest the crystal edge high contrast bse imaging shows that the former profiles are associated with 1 2 m wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is or formerly was in contact with matrix feldspar i e crystallized melt and are inferred to represent trace element enriched dissolution precipitation reaction fronts the latter profiles are associated with diffuse locally wispy brightness gradients adjacent to resorbed crystal boundaries subgrain boundaries or thicker bright bands formed in contact with matrix hornblende crystals and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro porosity pathways at some point in the recrystallization process

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

thermal evolution of the scandian hinterland naver nappe northern scotland the northern highlands terrane of scotland hosts several thrust nappes that were deformed and metamorphosed during the silurian scandian orogeny quantitative petrological analysis of metamorphic assemblages indicates that the hinterland positioned naver nappe experienced decompression heating from 8 9 kbar and 600 c to 6 7 kbar and 700 c monazite xenotime thermometry and geochronology delineate a detailed temperature time history for the naver nappe monazite often exhibits compositional zoning which is used to establish multiple temperature time points in several samples these data indicate that the naver nappe experienced relatively fast heating c 50 c myr 1 and relatively slow cooling 15 20 c myr 1 with peak temperatures occurring at c 425 ma this temperature time evolution is compatible with the early emsian 407 403 ma deposition of unmetamorphosed conglomerates that rest on high grade metamorphic rocks in the naver nappe but requires an acceleration in the cooling rate to 40 50 c myr 1 at 420 410 ma geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the scandian orogen in northern mainland scotland are younger than the c 430 ma deformation in the foreland positioned moine thrust zone we postulate that heat from pervasive granitic intrusions in the naver nappe weakened the crust allowing deformation to retreat to the hinterland of the orogen supplementary material a description of our analytical methods all u pb trace element data additional figures explaining our petrological analysis and other relevant data are available at https doi org 10 6084 m9 figshare c 4458041

fluid driven resetting of titanite following ultrahigh temperature metamorphism in southern madagascar abstract la icp ms u pb dates and epma trace element maps of titanite were collected from calc silicate gneisses of the ikalamavony peak t 750 800 c and anosyen peak t 900 950 c domains in southern madagascar to evaluate how titanite responds to high temperature metamorphism cooling and retrogression fluid mediated replacement of precursor titanite by titanite domains of different composition interface coupled dissolution precipitation icdr was the primary mechanism by which titanite was reset following high grade metamorphism comparison of titanite u pb dates 530 490 ma with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300 500 c apparent zr temperatures temperatures calculated assuming titanite quartz zircon equilibrium in altered titanite are less than or equal to the metamorphic peak but higher than the inferred alteration temperature implying that zr was removed but that titanite quartz zircon equilibrium was not achieved during alteration although evidence for icdr was observed in titanite samples separated by 10 s to 100 km differences in u pb dates among samples and even among titanite grains in the same thin section suggest that alteration at any given time was localized

preserved zr temperatures and u pb ages in high grade metamorphic titanite evidence for a static hot channel in the himalayan orogen abstract titanite grains from 800 c gneisses of the c 6 km thick greater himalayan sequence of central nepal were analyzed for zr in titanite temperatures and u pb ages to investigate the formation and evolution of a former weak mid crustal channel in the himalaya laser ablation icp ms depth profiles and spot analyses of titanite zr and u pb isotopic compositions are inconsistent with diffusional reequilibration on scales greater than 1 2 m and instead data appear to faithfully record both temperature and age titanite records protracted heating from 700 750 c at 37 ma to 775 c at 24 ma and possibly slight cooling to 765 c at 20 ma such temperatures exceed partial melting reactions in associated metapelites and imply profoundly weak rheologies i e a hot channel yet predate initiation of the bounding main central thrust and south tibetan detachment systems by as much as 15 myr evidently thick but weak crustal channels may remain stationary with respect to the convergence direction for 10 myr even during one of earth s biggest continent continent collisions

flattening the bhutan himalaya a detailed thermobarometric transect of 35 samples across the greater and tethyan himalayan sequences in central bhutan demonstrates a tectonostratigraphically intact section with uniform apparent thermal and baric field gradients of 2072 1c km and 0 5770 08 kbar km pressure temperature paths determined from chemically zoned garnets in 6 samples demonstrate that these p t conditions correspond with maximum pressures the super lithostatic baric gradient cannot be explained by pre to syn metamorphic tectonic processes or by extension within an inclined slab instead the data imply 50 post peakmetamorphic flattening of the himalayan metamorphic core accommodated by distributed top to thenorth shear consistent with microstructural analysis orogenic flattening best explains the development of the south tibetan detachment system as a strain incompatibility feature rather than a structure bounding the top of a tectonically inserted wedge and helps reconcile debate attributing first order himalayan structural features to either wedge failure critical taper or pipe flow channel flow

an evaluation of the inverted metamorphic gradient at langtang national park central nepal himalaya the crystalline core of the himalayan orogen in the langtang area of nepal located between the annapurna manaslu region and the everest region contains middle to upper amphibolite grade pelitic gneisses and schists these rocks are intimately associated with the main central thrust mct one of the major compressional structures in the northern indian plate which forms a 3 7 km wide zone containing rocks of both footwall and hangingwall affinity an inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks where metamorphic conditions increase from garnet grade near the mct zone to sillimanite k feldspar grade in the upper hangingwall petrographic data distinguish two metamorphic episodes that have affected the area a high pressure moderate temperature episode m1 and a moderate pressure high temperature episode m2 comparison with appropriate reaction boundaries suggests that conditions for m1 in the hangingwall were approximately 900 1200 mpa and 425 525 c thermobarometric results for 24 samples from the footwall mct zone and hangingwall reflect p t conditions during the m2 phase of 400 1200 mpa and 490 660 c the decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 mpa km 1 with slight fluctuations in the mct zone reflecting structural discontinuities in contrast to the palaeopressures palaeotemperatures are indistinguishable across the entire area sampled although field evidence suggests the presence of the inverted palaeothermal gradient well known in the himalaya quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section at langtang change in pressure is responsible for the presence of the sequence of index minerals through the section i interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence

shocked titanite records chicxulub hydrothermal alteration and impact age abstract hydrothermal activity is a common phenomenon in the wake of impact events yet identifying and dating impact hydrothermal systems can be challenging this study provides the first detailed assessment of the effects of shock microstructures and impact related alteration on the u pb systematics and trace elements of titanite catisio5 focusing on shocked granite target rocks from the peak ring of the chicxulub impact structure mexico a 1 mm long shock twinned titanite grain preserves a dense network of irregular microcracks some of which exploit shock twin interfaces secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks in situ laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms analysis reveals a mixture of three end member pb components the pb components are 1 common pb consistent with the pb isotopic signature of adjacent alkali feldspar 2 radiogenic pb accumulated since magmatic crystallization and 3 a secondary younger pb signature due to impact related complete radiogenic pb loss the youngest derived ages define a regression from common pb that intersects concordia at 67 4 ma in agreement with the established age of 66 04 0 05 ma for the chicxulub impact event contour maps of la mc icpms data reveal that the young ages are spatially restricted to microstructurally complex domains that correlate with significant depletion in trace elements ree y zr nb mo sn th and reduction in magnitude of the eu eu anomaly mapping by time of flight secondary ion mass spectrometry tof sims show that patterns of localised element depletion in titanite are spatially related to microcracks which are enriched in al the spatial correlation of ages and trace element abundance is consistent with localised removal of pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid mediated element transport process associated with the impact event here we interpret the 67 4 ma u pb age to represent hydrothermal pb loss in the chicxulub peak ring in the wake of the impact event these results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post impact hydrothermal activity in impact structures elsewhere

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

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

interpreting titanite u pb dates and zr thermobarometry in high grade rocks empirical constraints on elemental diffusivities of pb al fe zr nb and ce length scales of compositional heterogeneity in titanite from 750 to 1000 c metamorphic rocks from southern madagascar were measured to provide empirical constraints on elemental diffusivities the calculated pb diffusivity is comparable to experimental estimates of sr diffusivity because of this u pb dates from rocks that reached peak temperatures 900 c thus zr in titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks al and nb diffuse at similar rates to zr ce and fe diffuse slower than pb but faster than zr differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near end member titanite used in experiments

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

titanite petrochronology linked to phase equilibrium modelling constrains tectono thermal events in the akia terrane west greenland abstract the mesoarchean akia terrane in west greenland contains a detailed magmatic and metamorphic mineral growth record from 3 2 ga to at least c 2 5 ga this time span makes this region an important case study in the quest to track secular changes in geodynamic style which may ultimately inform on the development of plate tectonics as a globally linked system of lateral rigid plate motions the common accessory mineral titanite has recently become recognised as a powerful high temperature geochronometer whose chemistry may chart the thermal conditions of its growth furthermore titanite offers the potential to record the time temperature history of mafic lithologies which may lack zircon although titanite suffers from higher levels of common pb than many other u pb chronometers we show how measurement of 207pb 206pb in texturally coeval biotite may assist in the characterization of the appropriate common pb composition in titanite titanite extracted from two samples of mafic gneisses from the akia terrane both yield u pb ages of c 2 54 ga although coeval their chemistry implies growth under two distinctly different processes in one case the titanite has elevated total ree high th u and grew from an in situ partial melt consistent with an identical date to granite dyke zircon in contrast the second titanite sample contains greater common pb lower total ree lower th u and grew from dominantly hydrothermal fluids zr in titanite thermometry for partial melt derived titanite with activities constrained by phase equilibrium modelling indicates maxima of c 690 c elsewhere in the akia terrane coeval metamorphism linked to growth of hydrothermal titanite is estimated at temperatures of c 670 c these new results when coupled with existing findings indicate punctuated repeated metamorphic events in the akia terrane in which high temperature conditions re occurred at least three times between 3 0 and 2 5 ga but crucially changed in style across a c 3 0 ga change point we interpret this change in metamorphism as reflecting a fundamental shift in geodynamic style in west greenland at 3 0 ga consistent with other estimates for the onset of widespread plate tectonic type processes

age and tectonic setting of the udokan sediment hosted copper silver deposit transbaikalia russia abstract the udokan cu ag deposit in transbaikalian russia is one of the largest individual examples of the sediment hosted stratiform type mineralization is hosted by the udokan complex a 12 000 m thick metasedimentary sequence deposited between 2 2 and 2 06 ga in a large intra to peri cratonic basin constructed over archean basement of the siberian craton the basin was inverted and metamorphosed to dominantly greenschist facies at 1 9 ga as a consequence of the collision between the aldan and stanovoi domains titanite is a common constituent of the greenschist metamorphic assemblages of the udokan complex as well as being an integral part of the stratiform disseminated and associated veinlet cu sulfide mineralization at udokan titanite crystals were separated from a sample collected in the medny area of the deposit where typical high grade disseminated and veinlet chalcocite bornite mineralization is well exposed titanite crystals from the disseminated and veinlet fractions were separately dated by the id tims u pb method with the disseminated fraction returning a concordia age of 1895 3 9 7 ma and the veinlet fraction a concordia age of 1896 7 7 8 ma combination of both titanite fractions produced a concordia age of 1896 2 6 2 ma the titanite age effectively dates the associated cu sulfide mineralization since there is no evidence that titanite growth is appreciably younger than either the mineralization in the medny area or the udokan deposit as a whole a 1 9 ga age for the stratiform cu mineralization is considered likely this age is undistinguishable from that of regional metamorphism during the collisional orogeny that led to final amalgamation of the siberian craton and its incorporation into columbia the age also confirms udokan as the world s oldest sediment hosted cu deposit and that its formation coincided with a continental scale orogeny rather than with diagenesis and extensional basin development 150 m y earlier

sediment underthrusting within a continental magmatic arc coast mountains batholith british columbia nsf ear 0309885 ear 1338583 idaho state university romanian national science funding agency uefiscdi grant pn iii p4 id pce 2016 0127

earthquake source time function from coherent and incoherent rupture abstract theoretical study has been made investigating the seismic source spectrum generated from a coherent and incoherent rupture an earthquake is modeled by a finite propagating rupture on a fault plane where fault heterogeneities fault patches are randomly distributed the dislocation velocity of such a fracture is assumed to be approximated by a stochastic process of random impacts of particles obeying brownian motion the parameters of the present stochastic source are seismic moment fault dimension fault patch intensity and patch fracture time the model predicts two corner frequencies one originates from the fault finiteness and the other from the fracturing of fault patches the seismic source spectrum from the model consequently shows distinct frequency dependence of 0 2 2 with increasing angular frequency where is about 1 0 the seismic moment is controlled by an average dislocation on the fault and by the fault dimension the short period spectrum which is much more abundant than that of the ordinary deterministic models is controlled by the product of the fault patch intensity and the square root of their total number the 2 high frequency asymptote of the theoretical spectrum is in conformity with the white acceleration spectrum usually found in the literature and it guarantees the finite total energy of the rupture process

prolonged partial melting of garnet amphibolite from the eastern himalayan syntaxis implications for the tectonic evolution of large hot orogens

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

miocene faulting at plate tectonic velocity in the himalaya of central nepal abstract combination of geochemical zoning in metamorphic garnet and monazite plus in situ th pb isotopic dating of monazite yields p t conditions ages and convergence rates for the main central thrust mct and affiliated faults in central nepal inferred rates were 1 5 0 9 cm yr langtang thrust 19 ma 2 2 0 7 cm yr main central thrust 15 ma and 7 3 cm yr ramgarh thrust 10 ma the lower values are similar to modern convergence rates across the himalaya but the ramgarh thrust may have briefly absorbed all indo asian convergence at 10 ma when foreland and marine sedimentation rates markedly increased and at least one major strike slip fault in tibet experienced a hiatus in movement variable rates of convergence across the himalaya on myr timescales imply myr variations in strain rates throughout all components of the indo asian orogen

synthrusting metamorphism cooling and erosion of the himalayan kathmandu complex nepal in this paper we tackle some of the outstanding problems of the himalaya in particular the external zone in the kathmandu complex using an integrated approach involving field mapping microstructure thermobarometry and geochronology the result is a new model showing the evolution of one major main central thrust therefore we refute suggestions that the kathmandu complex is a klippe or separate thrust sheet compared to the main central thrust sheet in the high himalaya the kathmandu complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the high himalaya fairly near the leading edge unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the main central thrust wedge was 40 km thick on the northern side of the kathmandu complex and 20 km thick on the southern margin initiation of the main central thrust occurred at 22 ma possibly during the closing stages of peak amphibolite facies metamorphism slip at elevated temperature 500 600 c continued until 14 ma this is a little longer than has previously been proposed in marked contrast to the famous inverted metamorphism on the main central thrust in the high himalaya the metamorphic zonal scheme in the kathmandu complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base these mylonites postdate the high grade regional amphibolite metamorphism and give an illusion of inverted metamorphism a likely reason for the contrast is that the main central thrust cut up section toward the foreland and therefore at kathmandu carries high levels in the metamorphic structure our model involves reactivation of the main central thrust at 7 8 ma as inferred from published monazite and mica ages but because the kathmandu rocks show no evidence for high temperature reactivation at this time we presume that the late reactivation involved only the internal high himalaya zone while the main central thrust was inactive in the external kathmandu zone we attempt to quantify rates of cooling exhumation and thrusting during time period 22 ma to the present

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

structural expressions and tectonic implications of general noncoaxial flow in the midcrust of a collisional orogen the northern scandinavian caledonides mapping and strain analysis in the northern scandinavian caledonides indicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kinematically compatible manner is through the ductile flow of rocks in the midcrust of these mountain belts in the ofoten efjorden region of north norway amphibolite facies metamorphism and strong penetrative strain occurred at deep levels of the caledonian nappe stack as it was thrust eastward onto the western margin of baltica kinematic analysis of deformation in the lower portion of the caledonian nappe stack indicates that the rock mass underwent heterogeneous noncoaxial flow with components of foreland directed simple shear and subvertically shortened pure shear general noncoaxial flow simultaneous simple shear pure shear can accomplish foreland directed tectonic transport subvertical thinning and transport parallel elongation at deep levels of a nappe stack this pattern of flow is significant technically for two reasons first it suggests spreading of the caledonian allochthon during thrust emplacement and second it can form a kinematic link between normal faulting at shallower structural levels and thrusting at the base of the nappes by analogy with the caledonides general noncoaxial flow of midcrustal rocks may represent an important process in other collisional and contractional orogens particularly in those that have undergone upper crustal extension while thrusting continued in the foreland

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

single shot laser ablation split stream ss lass petrochronology deciphers multiple short duration metamorphic events single shot laser ablation split stream ss lass petrochronology offers spatial resolution of 1 m per surface analysis this enables the technique to interrogate metamorphic zircon overgrowths that are very thin or that preserve a composite age structure to demonstrate the advantages of ss lass the technique was applied to metamorphic zircon overgrowths in five rocks from the cordillera de la costa venezuela in these rocks ss lass was able to decipher discrete short duration 106 yr zircon growth events at c 33 0 c 28 3 c 23 0 and c 18 2 ma comparison with existing geo thermochronology suggests that the ss lass dates represent hydro thermal events that mark distinct episodes of tectonism affecting the northern margin of south america with an external error of 5 incorporated into isotope ratios as required to achieve satisfactory analytical precision ss lass dates for reference materials were accurate to within 1 5 of published values the ss lass technique is thus in favorable cases capable of providing 106 yr resolution for cenozoic rocks methods in geospeedometry which employ forward models to reproduce observed diffusion length scales have obtained anomalously short time scales of 104 106 yr for metamorphism the high spatial and temporal resolution of ss lass offers tremendous promise for investigating the veracity of these claims

initiation of crustal shortening in the himalaya new monazite u th pb petrochronological data from the annapurna region of central nepal outline a protracted thermal history spanning 30 ma from the early eocene c 48 ma to the early miocene c 18 ma rare earth element data collected concomitant with the isotopic analyses are consistent with prograde metamorphism and crustal thickening between 48 and 30 ma and anatexis between 28 and 18 ma the timing of metamorphism recorded in these rocks is consistent with records of crustal shortening derived from ultrahigh pressure rocks in the western himalaya and exhumed metamorphic rocks in southern tibet although previous records of early shortening metamorphism related to the initial collision of india with asia are spatially associated with the northern margin of the indian plate the ages presented in this study extend that early record south into the main himalayan range these new data provide important geological constraints on this early poorly documented history

diffusion obstacles and opportunities in petrochronology many of the approaches in petrochronology are rooted in the assumption of equilibrium diffusion is an expression of disequilibrium the movement of mass in response to chemical potential gradients and isotopes in response to isotopic gradients it is extremely important that we be aware of how the effects of diffusion can place obstacles across our path towards petrochronologic enlightenment conversely the effects of diffusion also provide opportunities for understanding rates processes and conditions experienced by rocks the enormity of the field does not permit us to provide a comprehensive review of either the mathematics of diffusion or quantitative data that have been obtained relevant to the interpretation of diffusive processes in rocks and minerals many resources cover these topics including rimg volume 72 diffusion in minerals and melts zhang and cherniak 2010 and several textbooks crank 1975 glicksman 2000 particularly relevant to the discussion of petrochronology are summaries of the theory and controls on diffusion brady and cherniak 2010 zhang 2010 as well as diffusion rates in feldspar cherniak 2010a accessory minerals cherniak 2010b garnet ganguly 2010 mica pyroxene and amphibole cherniak and dimanov 2010 and melts zhang and ni 2010 zhang et al 2010 rather than duplicate that material our goal is to explore the obstacles and opportunities presented by the effects of diffusion as they inform the rates of petrologic processes to achieve this goal we emphasize key principles and illustrative examples quantitative interpretation of the effects of diffusion assumes predictability of numerous factors that may affect chemical or isotopic transport including temperature initial and boundary conditions water and oxygen fugacities activities of other components multiple mechanisms of diffusion and crystal chemistry coupling of the substitution of elements into different crystallographic sites additionally the extraction of meaningful ages durations of events and temperatures requires

in situ la icp ms u pb geochronology and trace elements analysis of polygenetic titanite from the giant beiya gold polymetallic deposit in yunnan province southwest china abstract the beiya gold polymetallic deposit located in the middle of the jinshajiang ailaoshan alkaline porphyry metallogenic belt is one of the largest gold deposits in china the mineralization mainly occurs in skarn along the intrusive contacts between the alkaline porphyries and middle triassic limestone in this paper we present u pb age as well as major and trace element geochemistry of titanite from the beiya deposit and distinguish the titanite into a magmatic and a hydrothermal suite our study indicates that the titanite from the ore related porphyry and from the mineralized skarn is texturally and geochemically very different the euhedral envelope shaped titanite from the ore related porphyry has lower feo f hfses nb ta and lu hf together with higher tio2 and th u than the subhedral titanite from the mineralized skarn the titanite from the porphyry also displays higher lree hree and more subtle negative eu anomaly than its mineralized skarn counterpart this suggests a magmatic and a hydrothermal origin for respectively the titanite from the ore related porphyry and from the mineralized skarn in situ magmatic titanite u pb dating has yielded an eocene age of 36 0 5 9 ma consistent with the porphyry zircon u pb age 36 07 0 43 ma obtained in previous studies hydrothermal titanite has yielded a weighted average 206pb 238u age of 33 1 1 0 ma mswd 2 0 which represents the age of the retrograde skarn alteration and the maximum age for the gold mineralization together with the previous molybdenite re os age we have further constrained the beiya gold polymetallic metallogeny to 33 1 34 1 ma the mineralization age is slightly younger than the porphyry emplacement indicating that the beiya metallogeny was likely to be a post magmatic hydrothermal product of the himalayan orogenic event the ree characteristics of hydrothermal titanite also reveal that the ore forming fluids may have been derived from a highly oxidized magma

metamorphic history of the central himalaya annapurna region nepal and implications for tectonic models pressure temperature time p t t conditions of metamorphism have been determined in the annapurna region of central nepal that place new constraints on the structural and tectonic evolution of the himalayan orogenic wedge peak p t conditions increase structurally upward 525 c and 8 kbar in the lesser himalayan sequence 650 c and 12 kbar at the base of the greater himalayan sequence across the main central thrust 750 c and 12 kbar in the middle of the greater himalayan sequence and 775 c and 13 kbar near the top of the greater himalayan sequence metamorphic monazite ages in the greater himalayan sequence also increase structurally upward 16 21 ma for subsolidus growth at the base of the greater himalayan sequence to 25 ma for peak t metamorphism and anatexis near the top of the greater himalayan sequence these ages are several million years older than at equivalent structural levels at langtang 200 km to the east the p t t data recommend reinterpretation of the bhanuwa fault within the greater himalayan sequence as a thrust and the presence of a different thrust structurally above the bhanuwa thrust here named the sinuwa thrust the new data are consistent with progressive stacking of tectonic slices with calculated overthrust rates that are consistent with some but not all models that presume 2 cm yr convergence across the himalaya since 25 ma despite differences in absolute ages similarities among the chemical systematics of monazite peak p t conditions and overthrust rates calculated for annapurna when compared to langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers although the timing and duration of movement on discrete thrust surfaces may differ

titanite petrochronology of the southern brasilia orogen basement effects of retrograde net transfer reactions on titanite trace element compositions abstract combining u pb geochronology and trace element characteristics of accessory minerals more reactive during metamorphism and deformation than zircon such as titanite catisio5 is a powerful tool for elucidating pressure temperature time p t t evolution of orogenic basement complexes this study reports in situ titanite u pb and trace element data from upper amphibolite facies metatexitic orthogneisses of the pouso alegre complex in the basement of the neoproterozoic southern brasilia orogen se brazil the titanite data is combined with thermodynamic modeling major phase compositions and u pb zircon dating and used to constrain the p t t path of the complex during the neoproterozoic orogenesis the investigated garnet amphibole gneisses record peak metamorphic conditions of 750 775 c and 10 5 12 5 kbar associated with partial melting and dated at ca 630 620 ma based on u pb analyses of hree y depleted titanite cores inferred to have crystallized in the presence of garnet near the peak p t conditions extensive melt crystallization at ca 616 ma is constrained by u pb dating of leucosome zircons the exhumation of the complex and re equilibration to ca 550 625 c at 6 9 kbar led to widespread generation of biotite epidote clinozoisite partially replacing amphibole garnet along the main foliation trace element signatures of these retrograde net transfer reactions are recorded in the titanite rims which are hree y enriched strontium depleted and were dated at 605 7 6 2 ma the average zirconium contents of the titanite rims are higher than cores suggesting disequilibrium partitioning of zirconium into titanite during the retrograde path resulting in overestimated zr in titanite temperatures for the rims the exhumation of the complex was concomitant with the intrusion of folded and recrystallized leucogranite dykes dated at 607 0 5 7 ma attesting the persistence of deformation after their intrusion assuming that the pouso alegre complex is part of the sao francisco paleocontinent reworked during the brasilia orogeny the data constrain the minimum age for the collision between the sao francisco paleocontinent and the paranapanema block at ca 620 630 ma and agree with the idea of long lived metamorphism and deformation in the southern brasilia orogen