strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

a quaternary solution model for white micas based on natural coexisting phengite paragonite pairs a thermodynamic model for the quaternary white mica solid solution with end members muscovite mg celadonite paragonite feceladonite ms mgcel pg fecel is presented the interaction energies for the mgcel pg join the fecel pg join and the ternary interactions were obtained from natural coexisting phengite paragonite pairs phengite paragonite pairs were selected based on the criteria that their chemical compositions may be represented as a linear combination of the model end member compositions and that the respective formation conditions 350 650 c 4 21kbar are accurately known previously published excess free energy expressions were used for the ms pg ms mgcel and ms fecel binaries the suggested mixing model was tested by calculating multicomponent equilibrium phase diagrams this proved to be particularly well suited to reproduce compositional variations of white micas from amphibolite facies metapelites

the western ailaoshan volcanic belts and their se asia connection a new tectonic model for the eastern indochina block the western ailaoshan wal volcanic belts mark the western boundary of the south china indochina suture zone along the ailaoshan fold belt geochronological and geochemical evidence suggests that the wal volcanic belts vary in ages and origins and includes 1 l carboniferous earliest m permian ca 300 270 ma incipient backarc basin magmatism 2 m permian ca 265 ma island arc mature backarc basin magmatism 3 early l permian ca 260 255 ma wal indochina arc continent collision and the associated syn collisional granitic magmatism as well as subsequent 4 e triassic ca 250 245 ma post collisional magmatism regional geological comparison indicates that the post l carboniferous wal tectonic evolution was closely related to the eastern indochina block we present a new g plate geodynamic evolution model for the ailaoshan fold belt and its surrounding south china indochina region based on new and published u pb zircon ages together with whole rock and pb isotope geochemical data

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

young ages from old garnet in the polyphase metamorphic terrane of the limpopo belt south africa abstract the p t t path of high grade metamorphic rocks is significant for understanding vertical motions and heating of the crust in general garnet is used to constrain p t paths and metamorphic zircon is used to obtain t for different metamorphic stages however this approach may be complicated by polyphase tectonothermal events the central zone of the limpopo belt in south africa is a complex precambrian metamorphic terrane that has experienced three distinct high grade metamorphic events at 3 22 ga m1 2 62 ga m2 and 2 02 ga m3 based on zircon dating a previous investigation of a metapelite enclave in the bulai pluton in the central zone showed that metamorphic zircon in the enclave recorded mainly m2 granulite facies metamorphism and was protected by the bulai pluton however titanite and rutile u pb ages of the same sample record the m3 event here we investigated garnet sm nd isotopic systematics of four metapelite samples from the evelyn enclave in the bulai pluton we constructed single mineral sm nd isochrons for garnet using micro samples in order to avoid the risk of isotopic disequilibrium between different minerals or whole rocks metamorphic zircon with sillimanite inclusions has yielded a granulite facies metamorphic age of ca 2 6 ga however the sm nd isotopic data for the garnet record much younger ages of ca 2 03 ga these ages are consistent with the m3 event in this area but were rarely identified in previous zircon dating studies this suggests that the m3 event reached sufficiently high temperatures to totally reset the rutile and titanite u pb and garnet sm nd systems moreover the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing p t t paths and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes

chemical and textural equilibration of garnet during amphibolite facies metamorphism the influence of coupled dissolution reprecipitation metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low temperature amphibolite facies metamorphic reactions garnet is widely assumed to be a reactant in staurolite isograd reactions and the evidence for this has been assessed in the late proterozoic dalradian pelitic schists of the scottish highlands the 3d imaging of garnet porphyroblasts in staurolite bearing schists reveals a good crystal shape and little evidence of marginal dissolution however there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction staurolite forms directly adjacent to the garnet and its nucleation is strongly associated with deformation of the muscovite rich fabrics around the porphyroblasts cloudy fluid inclusion rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet such cloudy garnet typically has a mg rich mn poor composition and is interpreted to have formed during a coupled dissolution reprecipitation process triggered by a local influx of fluid all garnet in the muscovite bearing schists present in this area is potentially reactive irrespective of the garnet composition but very few of the schists contain staurolite the staurolite producing reaction appears to be substantially overstepped during the relatively high pressure barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history the remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning such zoning in metamorphic garnet created during coupled dissolution reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re equilibration fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists in addition the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history however the partial re equilibration of the porphyroblasts during coupled dissolution reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions

early paleozoic orogenesis along gondwana s northern margin constrained by provenance data from south china abstract the cambrian ordovician boundary unconformity in the southern part of the south china craton is spatially and temporally related to coeval orogenic activity along the indian margin of east gondwana detrital zircon age spectra from strata above and below the unconformity range in age from 3580 450 ma with peaks in the late mesoproterozoic and neoproterozoic the patterns are similar to time equivalent sequences elsewhere in south china and together with regional facies relationships and paleocurrent data indicate derivation from a gondwana source the disconformity at the base of the ordovician succession forms part of a regional break that has also been documented in the himalaya qiangtang lhasa sibumasu and western australia all these successions have similar detrital zircon age spectra suggesting derivation from common source s in south china the effects of this tectonic event are relatively mild and are represented by a local disconformity at the base of the ordovician succession but elsewhere in north gondwana this event is marked by an angular unconformity with metamorphism of older units and relatively widespread magmatic activity south china was likely located in a distal location to the northeast of the pulse of tectonic activity which was focused in the himalaya region and was close to the continent ocean boundary between northern gondwana and the proto tethys

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

late permian to middle triassic sedimentary records in shiwandashan basin implication for the indosinian yunkai orogenic belt south china abstract the shiwandashan basin is an elongate region of the late permian middle triassic potential sediment accommodation that formed on continental crust between the yunkai orogen and the adjacent yangtze craton block south china several terms such as remanent marine trough structural window rift and pull apart basin have been proposed studies of the late permian middle triassic sedimentary sequences and the tectonics lithofacies and paleogeography of the shiwandashan basin indicate that the late permian middle triassic succession is composed of thick coarse molasse deposits and fine clastics organized into coarsening upward and fining upward stratigraphic sequences the shiwandashan basin went through the process of growth and disappearance from molasse to turbidite deposition and finally to volcanic eruption during late permian middle triassic to some extent the facies evolution reflects the tectonic controls on the sedimentation of the foreland basin the coarsening upward sequences hundreds to thousands of metres scale occurring immediately adjacent to the thrust front demonstrate that important episodic thrusting orogenesis and piedmont flexuring subsidence with low velocity took place in the northwestern part of the yunkai orogen whereas fining upward sequences are the indicator of continued deformation with high thrust velocity the following four facts have been demonstrated 1 a foreland basin existed in late permian middle triassic 2 the development of the foreland basin was controlled by the evolution the yunkai orogen 3 the basin and mountains continuously changed and migrated 4 the shiwandashan basin was a key demarcation between an active region in southeastern south china cathaysian block and a relatively stable region in northwestern yangtze block it was a convergent boundary in late permian middle triassic and its tectonic nature changed in the wake of the tectonic evolution of the surrounding area

origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone detailed electron and ion microprobe analyses were carried out on atoll shaped and normal garnets in ultrahigh pressure uhp metamorphic eclogite from dabie east central china compositional profiles of both normal garnets and rings of atoll garnets show well preserved growth zoning with a decrease in mn ca and heavy rare earth elements hrees and an increase in mg toward rims manganese and middle rare earth element mree enrichments are observed near garnet rims island and peninsula shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims whereas the hree concentrations are similar to those of the normal garnet cores electron back scatter diffraction ebsd analyses show that the island and peninsula shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings these observations suggest that the atoll garnets were formed by the consumption of earlier formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals nams and lawsonite decomposition at the onset of exhumation i e garnet breakdown was from the inside and re growth from outside to inside though somewhat restricted this study reveals that because garnet and zircon act as sinks for hrees and zr respectively the majority of released hrees and zr were likely re incorporated into newly grown garnet and zircon during the dabie uhp metamorphic slab exhumation

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

dynamic interactions between erosion deposition and three dimensional deformation in compressional fold belt settings 1 this study investigates whether the three dimensionality of erosion and deposition can influence three dimensional folding even when compression is purely convergent this problem is studied by employing a three dimensional mathematical model incorporating dynamic thin plate deformation coupled with fluvial and hillslope sediment transport the approach taken is to impose several fundamentally different sediment routing patterns by changing the initial regional topographic slope and the ratio of the characteristic timescales for deformation to fluvial surface processes r and to investigate how deformation responds numerical results indicate that when and or r are small i e initial topography is regionally flat and or rates of surface processes are slow compared to rate of imposed deformation folding is two dimensional and unaffected by surface processes since loads created by surface processes are imposed after deformation this situation favors development of small scale drainage networks transporting sediment from anticlines to adjacent basins when and or r are relatively large the development of a large scale transverse drainage network which can be maintained during folding is able to strongly amplify and localize folding this leads to interesting features where the largest transverse rivers coincide with the greatest structural and topographic amplitude of doubly plunging folds which is caused by erosion enhanced deformation for intermediate parameter values folding and surface processes interact in a complex manner giving rise to three dimensional en echelon structures that migrate in a transient manner this study demonstrates that under some circumstances one can anticipate major two way interaction between surface processes and three dimensional deformation

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

application of k feldspar jadeite quartz barometry to eclogite facies metagranites and metapelites in the sesia lanzo zone western alps italy the eclogite facies assemblage k feldspar jadeite quartz in metagranites and metapelites from the sesia lanzo zone western alps italy records the equilibration pressure by dilution of the reaction jadeite quartz albite the metapelites show partial transformation from a pre alpine assemblage of garnet alm 63 prp 26 grs 10 k feldspar plagioclase biotite sillimanite to the eo alpine high pressure assemblage garnet alm 50 prp 14 grs 35 jadeite jd 80 97 di 0 4 hd 0 8 acm 0 7 zoisite phengite plagioclase is replaced by jadeite zoisite kyanite k feldspar quartz and biotite is replaced by garnet phengite or omphacite kyanite phengite equilibrium was attained only in local domains in the metapelites and therefore the k feldspar jadeite quartz kjq barometer was applied only to the plagioclase pseudo morphs and k feldspar domains the albite content of k feldspar ranges from 4 to 11 mol in less equilibrated assemblages from val savenca and from 4 to 7 mol in the partially equilibrated samples from monte mucrone and the equilibrated samples from montestrutto and tavagnasco thermodynamic calculations on the stability of the assemblage k feldspar jadeite quartz using available mixing data for k feldspar and pyroxene indicate pressures of 15 21 kbar 1 6 1 9 kbar at 550 50 c this barometer yields direct pressure estimates in high pressure rocks where pressures are seldom otherwise fixed although it is sensitive to analytical precision and the choice of thermodynamic mixing model for k feldspar moreover the kjq barometer is independent of the ratio p h 2 o p t the inferred limiting a h 2 o for the assemblage jadeite kyanite in the metapelites from val savenca is low and varies from 0 2 to 0 6

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part i methods and techniques macroscopic textures resulting from different atomic scale mechanisms for metamorphic crystallization display different degrees of order clustering intergrowth and relative isolation of porphyroblasts data on the sizes and locations of thousands of crystals in a three dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features these data can now be acquired by means of high resolution computed x ray tomography numerical models that simulate porphyroblast formation governed by either interface controlled or diffusion controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities these numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples from which inferences can be drawn concerning the relative importance of these mechanisms in nature an independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius rate relations

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

thermobarometry of garnet clinopyroxene hornblende granulites from the kapuskasing structural zone amphibole mixing properties and standard state properties for the end members tremolite tschermakite pargasite ferrotremolite ferrotschermakite and ferropargasite have been refined through analysis of relevant phase equilibrium experiments coexisting cummingtonite hornblende pairs and natural mineral parageneses when merged with the mineral properties given by r g berman these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades the method was applied to texturally equilibrated garnet pyroxene hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet biotite gneisses from the kapuskasing structural zone thermometer resetting is apparent only in garnet biotite gneisses whereas touching garnet pyroxene pairs in mafic gneiss show little evidence of retrograde fe mg exchange computed metamorphic temperatures garnet clinopyrox

a ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the middle adula nappe central alps switzerland a ternary solid solution model for omphacite with the end members jadeite naalsi2o6 diopside camgsi2o6 and hedenbergite cafesi2o6 was derived from experimental data from the literature the subregular solution model fitted by linear programming is best suited to omphacites with very little aegirine component in common eclogites applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the adula nappe central alps switzerland results in large stability fields for common eclogite assemblages garnet omphacite quartz h2o kyanite within this field the compositions of garnet and omphacite show very little variation a precise determination of the peak pressure and temperature is not possible the occurrence of amphibole overgrowing the peak pressure assemblage in fresh eclogite suggests retrograde re equilibration still under eclogite facies conditions the computation of isopleths for garnet and pyroxene end members allows the estimation of the pressure and temperature conditions of this re equilibration event 19 21 kbar c 700 c

metamorphic evolution and 40ar 39ar geochronology of the wuguan complex eastern qinling area china implications for the late paleozoic tectonic evolution of the qinling orogen abstract the late paleozoic tectonic evolution of the qinling orogen is poorly constrained the wuguan complex which lies between the north qinling and south qinling tectonic belts represents a late paleozoic metamorphic belt of the qinling orogen in this contribution we report new metamorphic pressure temperature p t constraints and 40ar 39ar ages of garnet amphibolites and garnet bearing metapelites from the wuguan complex textural observations and mineral compositions especially garnets with two stages of growth indicate the wuguan complex experienced two distinct metamorphic events conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak p t conditions of 660 725 c and 8 0 12 0 kbar with a clockwise p t path whereas the later metamorphic overprint records p t conditions of 560 610 c and 7 5 9 0 kbar hornblendes from amphibolites yield 40ar 39ar ages of 306 299 ma whereas muscovites from garnet bearing metapelites yield 40ar 39ar ages of 252 251 ma the two groups of 40ar 39ar ages correspond to the cooling time of the two distinct metamorphic events respectively combining with previously published zircon u pb ages from the wuguan complex and two groups of metamorphic ages from the guishan complex and the xiongdian eclogite belt we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during carboniferous subduction of oceanic crust the later metamorphic overprint might have been related to the initial collision between the sino korean and yangtze cratons during the late permain

thermal models of the middle america trench at the nicoya peninsula costa rica 1 we present thermal models of the 24 ma cocos plate subducting under the caribbean plate at the nicoya peninsula costa rica our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism initial geotherms of the incoming plate representing different effective cooling depths of hydrothermal circulation affect the thermal structure and temperatures along the main subduction thrust excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required conductive plate cooling over predicts heat flow across the frontal forearc with no or very low frictional heating models with hydrothermal cooling to depths of 2 km under predicts the heat flow comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100 150 c but there are large along strike variations in both the seismicity and thermal regime

permian high temperature metamorphism in the western alps nw italy during the late palaeozoic lithospheric thinning in part of the alpine realm caused high temperature low to medium pressure metamorphism and partial melting in the lower crust permian metamorphism and magmatism has extensively been recorded and dated in the central eastern and southern alps however permian metamorphic ages in the western alps so far are constrained by very few and sparsely distributed data the present study fills this gap we present u pb ages of metamorphic zircon from several adria derived continental units now situated in the western alps defining a range between 286 and 266 ma trace element thermometry yields temperatures of 580 890 c from ti in zircon and 630 850 c from zr in rutile for permian metamorphic rims these temperature estimates together with preserved mineral assemblages garnet prismatic sillimanite biotite plagioclase quartz k feldspar rutile define pervasive upper amphibolite to granulite facies conditions for permian metamorphism u pb ages from this study are similar to permian ages reported for the ivrea zone in the southern alps and austroalpine units in the central and eastern alps regional comparison across the former adriatic and european margin reveals a complex pattern of ages reported from late palaeozoic magmatic and metamorphic rocks and relics thereof two late variscan age groups 330 and 300 ma are followed seamlessly by a broad range of permian ages 300 250 ma the former are associated with late orogenic collapse in samples from this study these are weakly represented clearly dominant is the permian group which is related to crustal thinning hinting to a possible initiation of continental rifting along a passive margin

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

drivers for late paleozoic to early mesozoic orogenesis in south china constraints from the sedimentary record abstract unconformity bound permo triassic successions in the qinfang basin south china craton reflect tectonic processes associated with tectonothermal activity along the margins of the craton the provenance record of these successions based on modal analysis geochemical composition detrital zircon u pb ages and paleocurrent data indicate derivation from the adjoining precambrian yunkai massif and surrounding paleozoic sedimentary units the quartz dominated nature of the siliciclastic units along with their high th sc and zr sc ratios indicate a recycled orogen source detrital zircons from late permian to early triassic samples display major age groups at 1100 800 ma 650 500 ma 480 420 ma with subordinate age groups at 2800 2400 ma 1900 1600 ma and 1400 1200 ma late triassic sandstones show a similar detrital zircon age pattern as well as a 300 250 ma age group integration of this provenance data with regional geological information suggests that the unconformity at the base of the permian succession is related to subduction of the paleo tethys ocean to the south of the craton whereas the major driver for triassic tectonic activity corresponds with terrane accretion and the termination of paleo tethys ocean subduction along the southwest margin of the craton

ne sw trending hepu hetai dextral shear zone in southern china penetration of the yunkai promontory of south china into indochina abstract the ne sw trending hepu hetai shear zone extends for about 480 km along the boundary between guangdong and guangxi provinces in the southern part of south china field mapping and kinematic analyses indicate that the zone is characterized by dextral ductile strike slip deformation with an estimated displacement along the fault zone in excess of 500 km studies of synkinematic phengite and gas liquid inclusions in quartz within mylonite suggest that the ductile shear deformation occurred under medium temperature pressure conditions of the greenschist facies and 40ar 39ar analyses of muscovite yield ages of 213 195 ma based on these data and late paleozoic mesozoic paleogeographic data for southern china we propose that the hepu hetai shear zone originated via the penetration of the yunkai promontory of south china into indochina during the triassic

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

high pressure metamorphism in the nappes of the valle dell orco traverse western alps collisional belt a detailed field and petrological study of rocks from nappes cut by the valle dell orco italian western alps in particular the sesia lanzo composite unit has revealed geological and metamorphic histories which started in pre alpine times and lasted up to the alpine subduction collisional processes during these processes the nappes sustained an early high p low t stage and a later low p greenschist facies stage but followed partly distinctive p t time trajectories this paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction underthrust to the later exhumation stage the metamorphic crystallization is often governed by incomplete and or local equilibrium and the pervasive syn metamorphic deformation and the composition of the syn metamorphic fluid phase if present have exerted an effective local control on reaction kinetics

origin and mechanical significance of honeycomb garnet in high pressure metasedimentary rocks from the tauern window eastern alps high pressure schists 2 2 5 gpa from the eclogite zone in the tauern window contain honeycomb garnet in which fine webs of garnet surround strain free quartz carbonate grains high resolution x ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions and that the quartz carbonate inclusions have random orientations in contrast matrix quartz exhibits a prominent crystallographic preferred orientation cpo high resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz most honeycomb garnet is chemically homogeneous but some displays asymmetric core rim zoning taken together these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz quartz grain boundaries or possibly via wholesale precipitation of garnet quartz carbonate from a fluid in either case a silicate rich aqueous fluid must have been present the likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry rather than via dislocation creep mechanisms at high pressures the absence of a cpo in early quartz may thus point to involvement of a pervasive grain boundary fluid rather than requiring low differential stresses during subduction

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

stratigraphy of volcanic rocks in the changning menglian belt in southwestern yunnan china abstract the changning menglian belt is regarded as an important geosuture of the paleotethyan ocean in southwest china volcanic rocks occur extensively in this belt and are critical for understanding its tectonic evolution however the lithostratigraphic division age and correlation of these volcanic rocks remain contentious in this paper the sequences in two lithostratigraphic units in the manxin area which include volcanic rocks the pingzhang and huoju formations have been determined this is a key area for interpreting the changning menglian belt using systematic biostratigraphy and information from the recently published 1 50 000 geological map the huoju formation is assigned to the middle permian five sections in the pingzhang formation from different parts of the changning menglian belt are correlated on the basis of their biostratigraphy this correlation suggests that the top of the pingzhang formation becomes progressively younger from visean to early late carboniferous from north to south recent progress on the late permian laochang formation is also reviewed it is concluded that most of the volcanic rocks were formed in seamount oceanic island settings and that the changning menglian belt represents a late devonian to late permian paleotethyan ocean basin

a thermodynamic model of mineral segregations in the lower sillimanite zone near rangeley maine a local equilibrium irreversible thermodynamic approach is used to model metamorphic segregations in pelites from the lower sillimanite zone near rangeley maine relative phenomenological coefficients for the model are derived from a reaction hvolving sillimanite and another reaction involving staurolite using a least squares technique to give the best fit to the observed data the mean values of the relative coefficients in an sio2 fixed 1a t1 marker reference frame from five rocks in the middle of the lower sillimanite zone are alci 5 5 9 feo 2 6 mgo 1 8 koos 1 0 naoos 0 9 tior 0 4 and cao 0 2 the phenomenological coefficients are used with gibbs duhem relations and conservation equations to give the sequence of mantles mineral modes in each mantle chemical potential profiles and size of segregations that are expected to form around reacting sillimanite staurolite and garnet the model predicts that sillimanite growth will produce a segregation with a muscovite free biotite rich mantle surrounding a sillimanite and quartz rich core staurolite will be replaced by a muscovite rich pseudomorph and matrix garnets should be rimmed by thir biotite free mantles comparison of the computed textures with those observed in the rocks shows a good match except for the case of garnet where new garnet growth was not sufficient to produce well defned mantles in the relatively coarse grained matrix

aulacogen formation in response to opening the ailaoshan ocean origin of the qin fang trough south china abstractthe qin fang trough south china trends northeast southwest orthogonal to the adjoining southern margin of the craton the devonian strata within the trough are unconformable on late neoproterozoic units of the yunkai massif indicating that strata within the trough are autochthonous u pb ages and hf isotope compositions of detrital zircons from the silurian to devonian succession are consistent with derivation from the massif in comparisons of our data with those from equivalent units in the ailaoshan belt and hainan island detrital zircons from the silurian strata show similar age distributions and hf isotope compositions indicating that the three areas shared a common source and were adjacent to each other during the silurian in contrast the age distributions of detrital zircons preserved in devonian strata in the qin fang area are different from those of equivalent units in the ailaoshan belt this along with the absence of devonian strata on hainan island suggests that the qin fang

massive fluid influx beneath the colorado plateau usa related to slab removal and diatreme emplacement evidence from oxygen isotope zoning in eclogite xenoliths

crustal reworking and hydration insights from element zoning and oxygen isotopes of garnet in high pressure rocks sesia zone western alps subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere a better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial in this study major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the sesia zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin garnet shows a variety of textures requiring dissolution precipitation processes in presence of fluids in polycyclic metasediments garnet preserves a partly resorbed core related to pre alpine high temperature low pressure metamorphism and one or multiple rim generations associated with alpine subduction metamorphism in eclogites garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim in metasediments pre alpine garnet relics show 18o values up to 5 3 higher than the alpine rims while no significant variation is observed among different alpine garnet generations within each sample this suggests that an extensive re equilibration with an externally derived fluid of distinct lower 18o occurred before or in correspondence to the first alpine garnet growth while subsequent influxes of fluid had 18o close to equilibrium the observed shift in garnet 18o is attributed to a possible combination of 1 interaction with sea water derived fluids during pre alpine crustal extension and 2 fluids from dehydration reactions occurring during subduction of previously hydrated rocks such as the serpentinised lithospheric mantle or hydrated portions of the basement

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

geochronology and petrogenesis of the early paleozoic i type granite in the taishan area south china middle lower crustal melting during orogenic collapse abstract the early paleozoic wuyi yunkai orogeny represents the first extensive tectonothermal event in south china since the neoproterozoic break up of the rodinia supercontinent the early paleozoic igneous rocks in the south china block scb are important for understanding the processes responsible for the orogeny the taishan batholith in the inner cathaysia block consists dominantly of granodiorite and hornblende bearing granite zircon u pb dating of four samples gives a consistent crystallization age of ca 436 ma synchronous with widespread late orogenic s type granitic intrusions in the wuyi yunkai orogen the taishan granitoid rocks are metaluminous or peraluminous a cnk 0 94 1 15 and have variable sio 2 61 5 74 9 wt mgo 0 4 2 4 wt fe 2 o 3 1 8 7 4 wt and cao 1 1 5 5 wt with gradually decreasing p 2 o 5 from mafic to felsic components thus the taishan batholith displays the petrological and geochemical characteristics of i type granites additionally all samples have strongly negative e nd t values 11 to 9 suggesting an overall ancient source the early paleozoic i type granitoid rocks in the taishan area were derived from partial melting of ancient middle to lower crust at a high thermal gradient 35 c km and underwent extensive fractional crystallization of hornblende plagioclase and biotite the wuyi yunkai orogenic collapse may have been responsible for middle to lower crustal melting of the cathaysia block the early paleozoic i and s type granitoid magmatism lack significant input of a juvenile component indicating that the processes accompanying orogenic collapse such as asthenosphere upwelling and basaltic underplating contributed heat to trigger widespread early paleozoic felsic magmatism in the scb an intraplate orogenic event instead of continental or arc collision may be the preferred model for the wuyi yunkai orogeny

temperature micro mapping in oscillatory zoned chlorite application to study of a green schist facies fault zone in the pyrenean axial zone spain oscillatory compositional zoning in minerals has been observed in hydrothermal magmatic and metamorphic environments and is commonly attributed to chemical or physical cyclical changes during crystal growth chemical zoning is a common feature of solid solutions which has been rarely reported in phyllosilicates in this study oscillatory zoning in chlorite is described in samples from the pic de port vieux thrust a minor thrust fault associated to the major gavarnie thrust fault zone central pyrenees spain the pic de port vieux thrust sheet comprises a 1 20 m thick layer of triassic red pelite and sandstone thrust over mylonitized cretaceous dolomitic limestone the thrust fault zone deformation comprises secondary faults and cleavage affecting the triassic pelite and sandstone an important feature responsible to this deformation is a set of veins filled by quartz and chlorite chlorite is present in crack seal extension veins and in shear veins both structures opened under the same stress conditions in some shear veins chlorite occurs as pseudo uniaxial plates arranged in rosette shaped aggregates these aggregates appear to have developed as a result of radial growth of the chlorite platelets oscillatory zoning has been imaged by backscattered scanning electron microscopy and by x ray quantitative micro mapping these oscillations correspond to chemical zoning with alternating iron rich and magnesium rich bands the chlorite composition ranges from a fe rich pole to a mg rich pole fe 3 fe values were measured in chlorite using xanes spot analyses and vary from 0 23 to 0 44 the highest values are in the fe rich area temperature maps built from standardized microprobe x ray images and redox state using the program xmaptools indicate oscillatory variations from about 310 to 400 50 c during chlorite crystallization these temperature variations are correlated with a fe 3 fe variation by al 3 fe 3 1 and di trioctahedral substitutions highlighted by mg and fe tot contents fe mg zoning chemical variations could be then explained by alternation of cooling times and cyclical pulses of a fluid hotter than the host rock it is however not excluded that kinetic effects influence the incorporation of mg or fe during chlorite crystallization

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

detrital zircon u pb ages and hf isotopes of lower middle devonian to middle jurassic sandstones in the qinfang basin southern south china block constraints on provenance and tectonic setting abstract the affinity and tectonic setting of the qinfang basin have long been an enigma here we conduct detrital zircon u pb geochronology and hf isotope analysis of lower middle devonian to middle jurassic samples from the basin along with published age data and paleontology from the basin and its east and west sides this study constrains the sediment provenance and tectonic setting of the basin sandstones of devonian jurassic exhibit an age distribution with major age groups at 2700 2300 ma absence for jurassic 1200 800 ma 600 400 ma and 334 246 ma for jurassic alone zircons peak 2500 ma were sourced from the yangtze block while 1200 800 ma grains may originate from the yunkai massif and jiangnan orogen 600 400 ma zircons came from yunkai massif and or cambrian to silurian sediments in the basin carboniferous early triassic 334 246 ma zircons were probably supplied by an older sequence in the great youjiang basin and the darongshan shiwandashan igneous rocks the qinfang basin and its two sides are considered to share a common east gondwana source in silurian the similarity in age spectra between silurian devonian samples in the basin and ailaoshan belt suggests that the basin is a remnant of the paleo tethyan ailaoshan ocean however the distinct age distribution between the basin and indochina block shows that the basin is not a part of the indochina block by middle jurassic following the final closure of the ailaoshan ocean in the late triassic the adjacent western area may be still tectonically active thereby feeding the basin with detritus of early permian

eclogitic metamorphism in carbonate rocks the example of impure marbles from the sesia lanzo zone italian western alps the impure marbles of the internal sesia lanzo zone underwent a multi stage metamorphic evolution of alpine age and retain early alpine eclogitic assemblages partially recrystallized under blueschist to greenschist facies conditions these high p assemblages consist of carbonates phengite quartz omphacite grossular rich locally spessartinic garnet zoisite and al rich titanite retrogressive stages are characterized by the growth of glaucophane paragonite phlogopite tremolite and albite halogen rich biotite and amphibole are also present p t estimates of the early alpine metamophism have been calculated from these unique high p assemblages in order to test the applicability of some calibrations to impure carbonate systems in particular some gt cpx calibrations and the phengite geobarometer give results t 575 45 c at 15 kbar for the eclogitic climax and t 500 c at ph2o 9 kbar for early alpine retrogressive stages which are within the range obtained from the surrounding lithologies phase relationships in p t xco2 space indicate that mineral assemblages in the impure marbles coexisted with h2o rich fluids xco2 0 03 during their entire alpine evolution

basaltic activity preserved in an upper permian radiolarian chert from the paleo tethys in the inthanon zone northern thailand basaltic sandstone composed mainly of clay to sand sized fragments of glass and basaltic rock has been discovered in a pelagic chert sequence in northern thailand the sandstone conformably underlies and is partly intercalated with the upper permian wuchiapingian early changhsingian radiolarian bearing chert geological age dating based on radiolarians petrographic examination of the bedded chert with the basaltic sandstone and geochemical studies were conducted to investigate the origin and sedimentary mode of the lithologic sequence the bedded chert overlying the basaltic sandstone yielded late permian radiolarians and was correlatable with the wuchiapingian follicucullus charveti albaillella yamakitai assemblage zone as well as the early changhsingian neoalbaillella ornithoformis assemblage zone lithofacies and microfacies of the bedded chert indicated deposition in a pelagic realm of the paleo tethys although the tectonic setting was difficult to estimate based on geochemical features because of element mobility and alteration the basaltic fragments likely originated as oceanic basalt such as oceanic island basalt microfacies and sedimentary structures of the chert and sandstone indicated that the studied sequence is composed of intermittent high sedimentation rates mainly gravity currents of basaltic fragments in a background setting of very slowly accumulating radiolarian chert the sedimentary mode of the sequence the rare terrigenous influx and the presence of fusulinoidean or other foraminiferal tests indicate that the basaltic rocks fragment were originally sourced from an oceanic topographic high such as a seamount or an oceanic plateau and that the volcanic activity occurred in the chert accumulated oceanic basin of the paleo tethys in the wuchiapingian

macroscale dynamics of experimental landscapes abstract we review results from laboratory scale modelling of erosion and relief dynamics under variable uplift and rainfall rates under constant values of these forcing parameters an experimental landscape typically evolves towards a steady state between uplift and erosion and we show how the geometry of the steady state landscape adjusts to the rates of uplift and rainfall the comparison between these laboratory scale landscapes and the natural ones is not straightforward because contrary to analogue modelling in tectonics natural conditions of relief evolution cannot be downscaled to the laboratory without any scale distortions laboratory scale modelling in geomorphology is therefore only experimental not analogue despite these limitations experimental models may be used to provide physical tests for numerical models and they give insights into first order behaviours and directions for future research

emplacement at granulite facies conditions of the sesia lanzo metagabbros an early record of permian rifting the evolution of the pre alpine corio and monastero metagabbros points to strong chemical and mineralogical similarities with that of other permian gabbro bodies of the alps which are concentrated in the southalpine and austroalpine domains the structural and metamorphic pre alpine evolution of these gabbros records a re equilibration following the emplacement in the deep crust p 0 6 0 9 gpa and t 850 70 c exhumation through amphibolite facies conditions p 0 5 0 35 gpa and t 570 670 c followed by a greenschist facies imprint 0 25 p 0 35 gpa and t 550 c this retrograde p t evolution suggests that the exhumation occurred in a high thermal gradient regime such as that induced by upwelling of an asthenospheric plume during continental rifting this would be consistent with the crustal thinning known to have occurred in both the southalpine and austroalpine domains during permian times the gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the lanzo massif this lithologic association and the metamorphic evolution is similar to that of the fedoz gabbro austroalpine domain of the central alps and completely different from that observed in passive margins where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites the location of permian gabbro bodies in the austroalpine and southalpine domains and their absence in the helvetic domain is evidence for asymmetric rifting

diffusion control of garnet growth harpswell neck maine usa a detailed analysis of chemical zoning in two garnet crystals from harpswell neck maine forms the basis of an interpretation of garnet nucleation and growth mechanisms garnet apparently nucleates initially on crenulations of mica and chlorite and quickly overgrows the entire crenulation giving rise to complex two dimensional zoning patterns depending on the orientation of the thin section cut contours of ca zoning cross those of mn fe and mg indicating a lack of equilibrium among these major garnet constituents zoning of fe mg and mn is interpreted to reflect equilibrium with the rock matrix whereas ca zoning is interpreted to be controlled by diffusive transport between the matrix and the growing crystal image analysis reveals that the growth of garnet is more rapid along triple grain intersections than along double grain boundaries moreover different minerals are replaced by garnet at different rates the relative rate of replacement by garnet along double grain boundaries is ordered as muscovite chlorite plagioclase quartz flux calculations reveal that replacement is limited by diffusion of si along double grain boundaries to or from the local reaction site it is concluded that multiple diffusive pathways control the bulk replacement of the rock matrix by garnet with si and al transport being rate limiting in these samples

permo triassic structural evolution of the shiwandashan and youjiang structural belts south china abstract we conducted field mapping coupled with radiometric dating across the shiwandashan and youjiang structural belts swsb and yjsb to investigate how southwest south china evolved and to better understand its links to plate boundary dynamics during the late permian to middle triassic our results reveal an episodic tectono magmatic evolutionary history of the swsb and yjsb the swsb underwent significant nw se shortening punctuated by 250 240 ma s type pluton emplacement during the late permian to middle triassic the shortening was expressed by thin skinned nw verging thrusts and folds and conjugate sets of n trending sinistral and e trending dextral faults the nw se shortening overlapped with and was succeeded by triassic ne sw shortening in the yjsb the ne sw shortening was expressed by ne verging thrusts and folds which documented a northeastward propagation of foreland deformation the ne verging folds overprinted older nw verging folds forming superimposed folds at the juncture of the yjsb and swsb in the long an area our results combined with regional considerations support a model of the nw se shortening as an andean type orogeny that developed in response to westward subduction of the paleo pacific plate and the ne sw shortening as a product of the indochina south china collision the subduction of the paleo pacific plate assisted in westward motion of the south china oceanic lithosphere which may have facilitated the closure of the paleo tethys ocean and subsequent collisions of south china with north china and indochina

unraveling the schneeberg garnet puzzle a numerical model of multiple nucleation and coalescence a quantitative kinetic model for the growth of the different garnet porphyroblast microstructures type 1 and type 2 of the western schneeberg complex wsc is presented these porphyroblasts formed by a multiple nucleation and coalescence mechanism our numerical simulation shows that at constant diffusion rates 1 low interface reaction rates result in a fully amalgamated porphyroblast type 2 2 intermediate reaction rates result in a porphyroblast where coalescence of grains closer to the margin prevented amalgamation of those in the centre similar to type 1 porphyroblasts and 3 high interface reaction rates result in a porphyroblast microstructure with an atoll form all three microstructures are characterised by distinctive cluster size distributions a 2 d cluster size distribution analysis of type 1 porphyroblasts of wsc shows that these did not form because of intermediate interface reaction rates but because the diffusion rate of nutrients was too low to keep pace with the interface reaction rate

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

u pb dating and tectonic implication of ophiolite and metabasite from the song ma suture zone northern vietnam the song ma suture zone including the classical song ma suture and the nam co antiform between indochina and south china blocks in northern vietnam is a crucial area for understanding the tectonic evolution of southeast asia to explore their natures and evolutionary history this paper reports new petrological studies and sims zircon u pb age data for the song ma ophiolites metabasites and fine grained quartzite the song ma ophiolites consist mainly of peridotite fo90 91 en92 basalt and gabbro and experienced greenschist to lower amphibolite facies ocean floor metamorphism lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite we identified two types of zircon in the altered pyroxenite i uniform metamorphic 70 vol and ii zircon having inherited magmatic cores and metamorphic rims 30 vol all u pb analyses of both zircon types yield an upper intercept age of 340 29 ma that we interpret as the protolith age type i zircon and type ii metamorphic rims together yield a mean 206pb 238u age of 280 2 ma with rodingite forming broadly coevally at 283 to 282 ma metabasalt plagioclase amphibole schist has a morb type geochemical affinity magmatic zircon from a plagioclase amphibole schist yields a protolith age of 315 4 ma high pressure eclogite garnet phengite quartz schist and garnet hornblende rock are distributed in the northwestern nam co antiform zircon crystals from both eclogite and garnet hornblende rock have low th u ratios of 0 01 to 0 07 and 0 02 to 0 06 and yield metamorphic ages of 231 8 ma and 228 3 ma respectively the ages of detrital zircon grains from fine grained banded quartzite from the nam co antiform range from paleozoic to proterozoic with a peak at 1 7 to 1 8 ga all available data indicate a three stage tectonic evolution 1 340 to 280 ma the paleotethys that separated the indochina and south china blocks was characterized by oceanic crust formation at 340 to 315 ma and ocean floor metamorphism at 283 to 280 ma 2

development of garnet porphyroblasts by multiple nucleation coalescence and boundary misorientation driven rotations two types of garnet porphyroblast occur in the schneeberg complex of the italian alps type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz decussate mica and small garnet grains and a margin containing large garnet grains orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains individual garnet grains without subdomains are small 50 m faceted and idioblastic and have simple zoning profiles with ca rich cores and ca poor rims subdomains of larger garnet grains are similar in size to the individual small garnet grains type 2 porphyroblasts comprise only ellipsoidal garnet with small subdomains in the centre and larger subdomains at the margin each subdomain has its own ca high ca dropping towards subdomain boundaries garnet grains with or without subdomains all have the same ca poor composition at rims in contact with other minerals the compositional zonation patterns are best explained by simultaneous multiple nucleation followed by growth and amalgamation of individual garnet grains the range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre the difference between type 1 and type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast electron backscatter diffraction shows that small individual garnet grains are randomly oriented large marginal garnet grains and subdomain bearing garnet grains have a strong preferred orientation clustering around a single garnet orientation misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations the only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other this process was driven by the reduction of subdomain boundary energy associated with misorientation loss rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals quartz and mica around the garnet an analytical model in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz suggests that at the conditions of metamorphism the rotation required to give a strong preferred orientation can occur on a similar time scale to that of porphyroblast growth

microstructural chemical and textural records during growth of snowball garnet the growth history of two populations of snowball garnet from the lukmanier pass area central swiss alps was examined through a detailed analysis of three dimensional geometry chemical zoning and crystallographic orientation the first population collected in the hinge of a chevron type fold shows an apparent rotation of 360 the first 270 are characterized by spiral shaped inclusion trails gradual and concentric mn zoning and a single crystallographic orientation whereas in the last 90 crenulated inclusion trails and secondary mn maxima centred on distinct crystallographic garnet domains are observed microstructural geochemical and textural data indicate a radical change in growth regime between the two growth sequences in the first 270 growth occurred under rotational non coaxial flow whereas in the last 90 garnet grew under a non rotational shortening regime the second population collected in the limb of the same chevron type fold structure is characterized by a spiral geometry that does not exceed 270 of apparent rotation these garnet microstructures do not record any evidence for a modification of the stress field during garnet growth concentric mn zoning as well as a single crystallographic orientation are observed for the entire spiral electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one 001 axis oriented sub parallel to the symmetry axis and by another 001 axis oriented sub parallel to the orientation of the internal foliation these features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis the internal foliation and the crystallographic orientation

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

rates of fe mg mn and ca diffusion in garnet extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations some of this uncertainty derives from long down temperature extrapolations but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature and reveals a systematic dependence of diffusion rates on host garnet composition a comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for fe mg mn and ca for nearly all garnet compositions across the full range of geologically relevant temperatures pressures and oxygen fugacities

matrix dependent garnet growth in polymetamorphic rocks of the sesia zone italian alps mushroom and atoll shaped garnet crystals were found in high pressure quartz rich pelitic layers from the monte mucrone area western alps italy these garnet crystals are characterized by a peninsula shaped core surrounded by a partially crystallized dodecahedral external rim textural observations and thermodynamic modelling point towards growth of the atoll garnet from the monte mucrone area during two distinct orogenic cycles the core region and the inner part of the ring forming the edge of the atoll grew under barrovian metamorphic conditions of likely hercynian age while the outer rim of the atoll structure developed under alpine high pressure conditions electron backscatter diffraction analyses indicate that the atoll shaped structure has one single crystallographic orientation despite its complex compositional zoning thermodynamic modelling reveals marked changes in equilibrium assemblage leading to changes in stoichiometry of the garnet forming reaction which in turn explain the characteristic garnet morphology small amounts of quartz are consumed during the earlier stage of garnet growth history whereas the production of garnet requires a much larger amount of quartz during the final stage of its growth this leads to a change from initial poikiloblastic to non poikiloblastic textures this change is responsible for the formation of atoll shaped garnet finally garnet in intercalated mica rich layers forms idiomorphic crystals continuous from the centre to rim this study highlights the importance of the difference between the local matrix composition and the aggregate composition of the reactants needed for the garnet forming reaction finally we show that interaction between matrix and reaction stoichiometry can lead to porphyroblast precipitation inside the already grown porphyroblast

dependence of reaction kinetics on h2o activity as inferred from rates of intergranular diffusion of aluminium quantitative constraints on the accelerative effects of h2o on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of al in natural systems that are fluid saturated hydrous but fluid undersaturated and nearly anhydrous widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the makhavinekh lake pluton northern labrador combined with time temperature histories from conductive thermal models yield intergranular diffusivities for al from 700 900 c under nearly anhydrous conditions those rates when extrapolated down temperature are approximately three orders of magnitude slower than rates derived from re analysis of garnet resorption coronas formed under hydrous but fluid undersaturated conditions near 575 c in rocks of the llano uplift of central texas which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid saturated conditions in rocks from the picuris range of north central new mexico thus even at constant temperature rates of intergranular diffusion of al and corresponding length scales and timescales of metamorphic reaction and equilibration may vary by as much as seven orders of magnitude across the range of h2o activities found in nature