the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

the p t record of synchronous magmatism metamorphism and deformation at petrel cove southern adelaide fold belt porphyroblastic schists in the thermal aureole of the victor harbor granite at petrel cove in the southern adelaide fold belt south australia preserve a record of sequential cordierite andalusite staurolite fibrolite chlorite and muscovite growth along with biotite plagioclase quartz ilmenite during progressive deformation a p t pseudo section appropriate to biotite saturated assemblages in kfmash shows that the sequence of mineral reactions records increasing pressure of at least 1 kbar from c 3 to c 4 kbar during cooling from around 580 c heating at pressures below c 3 kbar is inferred for growth of early formed cordierite porphyroblasts and is attributed in part to the thermal effects of granite emplacement while the pressure increase is attributed to tectonic burial accruing from ongoing deformation the anticlockwise p t path is consistent with convergent deformation being focussed as a consequence of heating as to be expected for a lithospheric rheology that is strongly temperature dependent

evidence of early cretaceous collisional style orogenesis in northern fiordland new zealand and its effects on the evolution of the lower crust structural metamorphic and kinematic data from a well exposed section of lower crustal rocks in northern fiordland new zealand reveal a history of intense contractional deformation and high p metamorphism at the roots of a convergent orogen high p 14 kbars granulite facies garnet clinopyroxene bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss these veins and reaction zones were variably deformed by two phases of high p granulite facies deformation quantitative kinematic analyses conducted using systems of rotated veins and reaction zones indicate that the first phase produced steeply dipping shear zones within a sinistral pure shear dominated flow regime wk 0 69 this deformation occurred at conditions of p 14 0 1 3 kbars and t 676 34 c and resulted in subhorizontal arc parallel ne sw stretching and up to 60 subhorizontal shortening in high strain zones of the lower crust at depths 45 km the second phase of deformation occurred at p 14 1 1 2 kbars and t 674 36 c and produced vertically stacked gently dipping ductile thrust faults that accommodated arc normal nw directed displacement these features reflect major tectonic thickening of the crust oblique convergence and high p metamorphism during the collision of the roots of a convergent orogen represented by plutons of the median tectonic zone in eastern fiordland with the paleo pacific margin of gondwana represented by western fiordland distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc parallel ne sw and arc normal nw se components of oblique convergence onto sinistral strike slip and ductile thrust faults respectively at lower crustal levels

cretaceous tertiary structures and kinematics of the serbomacedonian metamorphic rocks and their relation to the exhumation of the hellenic hinterland macedonia greece the kinematic pattern and associated metamorphism of the predominant ductile deformation and the subsequent deformational stages of the serbomacedonian metamorphic rocks and granitoids are presented in terms of peri tethyan tectonics a systematic record of structural and metamorphic data gives evidence of a main top to ene to ese ductile flow of cretaceous age 120 90 ma associated with a crustal stretching and unroofing a subordinate wsw to wnw antithetic sense of movement of the tectonic top is observed in places the associated metamorphic conditions are estimated at 4 5 7 5 kbar and 510 580 c during eocene to miocene times these fabrics were successively deformed by low angle extensional de ductile shear zones with top to ne and sw sense of movement and brittle shear zones of similar kinematic pattern suggesting a transition from ductile to brittle deformation de deformation was accompanied during its later stages by nw se directed shortening we also discuss the relation of this cretaceous tertiary deformation of the serbomacedonian metamorphic rocks with the eocene to miocene ductile top to southwestward crustal shear of the adjacent rhodope crystalline rocks we regard the serbomacedonian and the rhodope metamorphic rocks to represent related metamorphic provinces the most recent exhumation and cooling history of which is bracketed between the eocene and neogene

alpine extensional detachment tectonics in the grande kabylie metamorphic core complex of the maghrebides northern algeria abstract the maghrebides are part of the peri mediterranean alpine orogen they expose in their inner zone inliers of high grade crystalline rocks surrounded by oligo miocene and younger miocene cover detailed mapping coupled with structural and petrological investigations in the grande kabylie massif and the reinterpretation of the available geochronological data allow us to refute the traditional concept of rigid behaviour of this massif during alpine events we show that the dome geometry the kinematic and metamorphic evolutions and the age pattern are typical of metamorphic core complexes exhumed by extension a major low angle detachment fault defined by mylonites and by younger cataclasites has been traced in the massif the upper unit encompasses pre permian phyllites with variscan 40 ar 39 ar cooling ages capped by unconformable mesozoic to tertiary cover of the calcareous range both mainly affected by extensive tertiary brittle deformation and normal faulting the lower unit exposes in two half domes a continuous tectonic pile 6 8 km thick of amphibolite facies rocks and orthogneisses affected by syndashmetamorphic ductile deformation devoid of retrogression the regular increase of paleotemperature downward and the 40 ar 39 ar plateau ages around 80 ma suggest that the high temperature foliation and associated wnw directed shear under a high geothermal gradient relate to extensional tectonics developed during mesozoic lithospheric thinning of the variscan south european margin to the north the sidi alli bou nab massif exposes another crustal section affected throughout by wnw directed extensional shear during h p h t syndashmetamorphic thinning and with overall 40 ar 39 ar plateau ages of 25 ma the eocene oblique collisional event responsible for crustal thickening was totally overprinted by this new extensional regime synchronous with the beginning of the opening of the western mediterranean oceanic basin this was also coeval with south directed thrusting of foreland nappes to the south post miocene tectonic events cause significant overprinting

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

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

pt paths from modal proportions application to the koralm complex eastern alps thermodynamic pseudosections portray those parts of a petrogenetic grid that are relevant to a given bulk composition and the reactions appearing on them can therefore be used directly to infer the pt path that the rock followed however for many normal bulk compositions the use of pseudosections is hampered by the fact that they display only few large fields of high thermodynamic variance in the pt range of interest here it is discussed how modal information on reaction progress within these fields can be used to determine pt path information for thermodynamically high variant metamorphic assemblages we use this information on reaction progress to contour pseudosections for modal proportions of minerals using the software package thermocalc the approach is applied to di tri and quadrivariant assemblages from the koralm complex in the eastern alps a pt path for these rocks is derived from modal considerations and compared with interpretations of mineral composition contours on the same pseudosection and with conventional thermobarometry it is shown that at least part of the complex must have cooled initially near isobarically from prevalent peak conditions around 700 c and 14 kbar before the rocks commenced a barroviantype decompression path

pressure temperature deformation history for a part of the mesoproterozoic fold belt in north singhbhum eastern india abstract the supracrustal rocks in the easternmost part of the proterozoic fold belt of north singhbhum eastern india are folded into a series of large upright folds with variable plunges the regional schistosity is axial planar to the folds the folds were produced by a second phase of deformation d2 and were preceded by d1 deformation which gave rise to isoclinal folds mapped outside the study area and the locally preserved bedding parallel schistosity a shearing deformation during d2 was responsible for the sheath like geometry of a major fold the axial planes were curved by d3 warping the first metamorphic episode m1 of low pressure type produced andalusite porphyroblasts prior to or in the early stage of d1 deformation the main metamorphism m2 responsible for the formation of chloritoid kyanite garnet and staurolite porphyroblasts was late to post d2 in occurrence the staurolite isograd separates two zonal assemblages recorded in the high alumina and the low alumina pelitic schists geothermobarometric calculations indicate the peak metamorphic temperature to be 550 c at 5 5 kb fluid composition in the rocks before and during m2 metamorphism was buffered and fluid influx if any was not extensive enough to overcome the buffering capacity of the rocks from m1 to m2 the p t path is found to have a clockwise trajectory that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting followed by compressional deformation leading to loading and heating

the p t deformation path for a mid proterozoic low pressure terrane the reynolds range central australia the proterozoic low pressure high temperature lpht terrane of the reynolds range occurs in a 130 km long nw trending belt in the central part of the arunta block central australia the reynolds range has been affected by two mid proterozoic tectonic cycles di and dii associated with two metamorphic events mi and mii di mi effects are restricted to the older of two sedimentary successions the lander rock beds which are separated from the younger reynolds range group by an angular unconformity the dominant structural metamorphic features formed during dii mii affected both sedimentary successions and the various granites that intruded them and reworked most di mi effects the dii deformation history can be subdivided into one prograde two peak and one retrograde stage average p t calculations in the southeastern half of the range indicate a peak metamorphic pressure of 4 1 0 3 kbar because the calculated values are derived from the same stratigraphic level corresponding to the base of the reynolds range group which is exposed throughout the area it is likely that pressures were similar in the entire range in fact however the peak metamorphic temperature shows a dramatic increase from greenschist facies c 400 c in the northwest to granulite facies 740 60 c in the southeast indicating that mii was associated with anomalously high heat flows the p t path is anticlockwise with isobaric cooling from the metamorphic peak indicated by corona textures however the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures peak metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation suggesting they formed in response to the same thermal event if this is true the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time 110 ma based on zircon dating as it is not clear whether proterozoic asthenosphere active thermal perturbations operated for this long the alternative interpretation must be considered namely that the peak metamorphic events are separate from the shear zone event associated with reheating of the area

phase relationships in buchan facies series pelitic assemblages calculations with application to andalusite staurolite parageneses in the mount lofty ranges south australia low pressure medium to high temperature buchan type regional metamorphism of pelitic rocks in the mount lofty ranges south australia is defined by the development of biotite staurolite andalusite fibrolite prismatic sillimanite and migmatite zones k feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite concentrating particularly on well developed andalusitestaurolite parageneses in general the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and p t and t xfe pseudo sections constructed from the internally consistent data set of holland and powell 1990 in the system kfmash assuming a h2o 1 although analysed white mica compositions are systematically more aluminous than predicted importantly the stability ranges of most critical assemblages predicted by these grids and pseudo sections coincide closely with p t estimates calculated using the data set of holland and powell 1990 from the mount lofty ranges assemblages with the exception of mn in garnet and zn in one staurolite cordierite muscovite assemblage non kfmash components do not significantly appear to have affected the stability ranges of the observed assemblages an apparent local reversal in isograd zonation in which andalusite first appears down grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and together with evidence for essentially isobaric heating of individual rocks is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels

using equilibrium thermodynamics in the study of metasomatic alteration illustrated by an application to serpentinites abstract metasomatic rocks in which mineralogical change is driven by infiltration of an externally derived fluid are challenging for the application of equilibrium thermodynamics because fluid driven changes in rock bulk composition can be faster than diffusive within grain re equilibration nevertheless careful definition of systems in terms of appropriate length scales an informed choice regarding the controlling variables and detailed petrological analysis can provide useful results thermodynamic calculations using methods that solve for equilibrium between a set of independent end member reactions produce valid results regardless of the identity of the controlling variables but determination of the relationship between cause and effect in metasomatic rocks is best performed with some knowledge of the parameters that drive the evolution of the system the correspondence between observations of serpentinised harzburgites from new caledonia and the results of the application of thermodynamic techniques suggest that equilibrium thermodynamics can be applied to these rocks on appropriate small length scales varying from micron to cm the primary drivers for mineralogical change are proposed to be 1 infiltration of a h 2 o fluid which is buffered to lower a o 2 by the rock with resultant production of h 2 and the resulting change in the redox budget of the rocks and 2 gradients in a sio 2 caused by mm to cm scale variation in si mg fe inherited from the harzburgite protolith

calculated mineral equilibria in the pelite system kfmash k 2 o feo mgo al 2 o 3 sio 2 h 2 o

retrograde evolution in the central vosges mountains northeastern france implications for the metamorphic history of high grade rocks during the variscan orogeny abstract metamorphism in the vosges massif is characterized by rocks formed under a wide range of pressures and temperatures high pressure rocks garnet peridotites and eclogites facies rocks are found together with low pressure rocks garnetcordierite stability field hitherto metamorphism has been interpreted within the framework of an identical p t evolution for all rock types corresponding to either two separate geotectonic events or a unique p t evolution from high pressure to low pressure conditions we studied gneisses from the vosges massif in order to constrain the p t t path they followed p t estimation indicates that gneisses equilibrated between 5 and 7 kbar during peak metamorphism time rocks later experienced cooling from temperatures above 650 c down to 550 c it is possible that the entire p t evolution of the gneisses took place within the lower part of the amphibolite facies which is inconsistent with the p t evolution of the high pressure rocks the noteworthy absence of muscovite staurolite assemblages is attributed to conditions of low h2o and low o2 activity the consequences for the geodynamics in the vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high pressure rocks with low pressure amphibolite facies rocks

tectono metamorphic evolution of a convergent back arc the famatinian orogen sierra de quilmes sierras pampeanas nw argentina

low p metamorphism following a barrovian type evolution complex tectonic controls for a common transition as deduced in the mondonedo thrust sheet nw iberian massif the mondonedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low and medium p metamorphic domains this unit underwent an initial medium p event during the initial stages of variscan convergence related to crustal thickening subsequently the thrust sheet evolved to a low p baric type of metamorphism related to syn convergence thinning and exhumation its footwall cropping out in two tectonic windows registered a different evolution with a low p history that evolved from low to high t under a high geothermal gradient several different p t paths of the mondon edo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area following the initial crustal thickening two main syn convergence extensional shear zones developed one of them occurs in the hangingwall whereas the other affects the footwall unit both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet and their activity is viewed as a consequence of the need for gravitational re equilibration within the orogenic wedge the most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low p metamorphism is essentially a late phenomenon and is linked to late orogenic tectonic activity in the mondon edo thrust sheet our conclusions indicate that low p metamorphism may also develop during convergence and that this may occur in at least two cases one is tectonic denudation of an allochthonous unit during its emplacement and the other thinning and extension at the footwall unit of an advancing thrust sheet as a consequence the low p evolution may show different characteristics in different units of an orogenic nappe pile

late devonian carboniferous detachment faulting and extensional tectonics in western cape breton island nova scotia canada abstract shallow dipping detachment faults have played a key role in the evolution of the late devonian to carboniferous maritimes basin of western cape breton island the faults are responsible for large scale post orogenic crustal thinning and denudation of mid crustal levels tournaisian and westphalian clastic sedimentation can be related to detachment faulting along the margaree shear zone msz and the ainslie detachment ad respectively the msz consists of thick shallow dipping retrogressive mylonite overprinted by cataclastic horizons brittle detachment faults and chloritic breccia the shear zone crosscuts and transports low grade late devonian bimodal volcanic rocks and redbeds of the fisset brook formation in its hanging wall towards the west and juxtaposes them against exhumed medium to high grade metamorphic rocks and basement in its footwall locally the mylonite is 200 700 m thick and outcrops along strike from north to south for approximately 60 km tournaisian coarse clastic units and debris flow deposits of the horton group unconformably overlie ductile mylonite of the msz due to incision of brittle faults into the msz regional transgression and marine flooding of the extensional complex and exhumed units are marked by the thick visean carbonate and evaporite deposits of the windsor group the carbonates reflect a relatively quiescent tectonic regime and widespread subsidence was likely due to the cessation of extension related thermally induced buoyancy in a tectonically thinned crust the ainslie detachment nucleated as a regional flat lying fault along the carbonate evaporite contact at the top of the macumber formation forming calc mylonite near the base of the windsor group significant stratigraphic omissions in the windsor group above the detachment provide clear evidence of an extensional regime during faulting clastic deposits of the westphalian cumberland group are bounded by the detachment and sedimentation is related to listric normal faults which cut down section towards the west southwest the documentation of detachment faults has widespread implications for the interpretation of the post orogenic evolution of the northern appalachians an extensional regime can now be recognized deep reflection seismic data are interpreted in light of the new findings and can be related to late devonian carboniferous basin formation

extensional flow during gravitational collapse a tool for setting plate convergence padron migmatitic dome variscan belt nw iberia abstractplate convergence analysis in collisional orogens is usually based on the study of major contractional structures and strike slip shear zones here we show how the structural analysis of extensional structures may report the regional or far stress field during relatively local gravity driven extensional collapse of a thickened continental crust and how this information may be used to constrain the broad vectors of plate convergence at that time the padron migmatitic dome is a synconvergent extensional system developed in the axial zone of the variscan belt exposed in the nw part of the iberian massif of spain this system affected the allochthonous and autochthonous sequences involved in pangaea s assembly in southern europe it includes three major extensional shear zones which have been analyzed in detail to provide a wide ground data set for the discussion of the proposed model the tectonic flow in the padron migmatitic dome and in other coeval structures is characterized by vectors ranging

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

natural metastable reactions involving garnet staurolite and cordierite implications for petrogenetic grids and the extensional collapse of the betic rif belt medium grade metapelites of the torrox unit betic rif belt s spain contain mineral assemblages consisting of garnet grt staurolite st cordierite crd biotite kyanite sillimanite andalusite muscovite ms and quartz qtz and record complex reaction processes of cordierite growth through garnet and staurolite decomposition the reaction textures the chemical composition of the reactant and product phases including fe mg mn partitioning and the results of equilibrium thermodynamic calculations indicate that these cordierite bearing assemblages are largely deviated from equilibrium furthermore the actual cordierite forming reactions as estimated from the assemblage and associated textures conflict with the predictions of thermodynamically based petrogenetic grids for the model pelite system kfmash either those that predict the stable coexistence of cordierite muscovite plus garnet or staurolite or those that do not foresee a field of stability for these types of assemblages this conflict is explained in terms of cordierite growth at ca 575 c and 2 5 kbar through metastable reactions whose operation was conditioned by the relict persistence of higher pressure phases garnet and staurolite and phase compositions e g muscovite and biotite after fast decompression this interpretation militates against the existence of a wide p t range of stable coexistence at low p of crd ms qtz grt st in medium grade metapelites of normal composition i e poor in zn and or mn the triggering of metastable cordierite forming reactions and the preservation of even subtle disequilibrium features associated to them indicate that the rocks underwent fast near isothermal decompression from ca 12 kbar down to 2 3 kbar then rapid cooling these inferences agree with independent evidence indicating that termination of alpine metamorphism in the western betic rif belt was related to the extensional collapse of thickened crust and that the latter had consisted of a single continuous event

terrane accretion and upward extrusion of high pressure granulites in the neoproterozoic nappes of southeast brazil petrologic and structural constraints the high grade crystalline nappes exposed southeast of the sao francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of neoproterozoic deep continental crust the socorro guaxupe nappe above derived from an arc terrane and the tres pontas varginha nappe below metamorphism in the tres pontas varginha nappe is characterized by the exceptional preservation of kyanite granulites 700 750 c 15 kbar and followed by limited retrogression maximum temperatures around 900 950 c were reached toward the base of the overlying socorro guaxupe nappe during the intrusion of charnockitic mangeritic magmas lower pressure metamorphism accompanied by anatexis prevailed at shallower crustal levels our petrological results document an inverted thermal structure with isobaric heating of the top of the high pressure granulite nappe both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units both metamorphosed to high pressure but lower temperature conditions significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes the proposed geodynamic scenario considers that continental subduction took place westward underneath neoproterozoic oceanic lithosphere the two granulite units crystallised at 45 km depths under distinct paleogeotherms within this subduction zone around 630 ma the kyanite granulites were rapidely exhumed through the mechanism of low angle forced extrusion whereas syncollisional collapse affected the soft anatectic middle crust of the overlying arc terrane the final emplacement of the thinned nappe pile onto the cold sao francisco craton and its platform cover with at most anchizonal to greenschist facies metamorphism occurred around 600 ma

the neoproterozoic brasiliano orogeny in northeast brazil 40ar39ar and petrostructural data from ceara abstract the borborema province in ne brazil comprises paleoproterozoic basement and its proterozoic cover that experienced a complex tectonometamorphic history during the brasiliano pan african orogeny in ceara four major domains separated by continental scale steeply dipping dextral shear zones have been identified 40 ar 39 ar step heating of mineral separates and laser probe fusion of single grains have been used in conjunction with petro structural observations to delineate the cooling tectonic history of these domains to the north of the senador pompeu shear zone spsz amphibole and biotite pairs from low pressure granulite belts exposed along the granja shear zone and spsz record plateau ages in the range 575 560 ma corresponding to cooling rates close to 20 c m y 40 ar 39 ar laser probe muscovite dates from the chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts a muscovite biotite pair from the central ceara high pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534 525 ma related to the decompression and uplift of previously deelly buried rocks to the south of the spsz only moderate crustal thickening occurred within the various schist belts a distinct cooling pattern is indicated for these belts and the western patos shear zone attesting to a different crustal development 40 ar 39 ar dates of amphibole phlogopite muscovite and biotite range from 550 to 522 ma recording a postmetamorphic cooling rate of 6 9 c m y slower cooling rates of 3 4 c m y between amphibole and biotite closure ages in the range 540 500 ma are reported for the eastern part of the palos shear zone system the diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high temperature mylonitic belts earlier more rapid cooling occurred where the belts are narrow in the west late slower cooling occurred in wider zones to the east these thermochronological data reveal an age and cooling rate distribution at the scale of the borborema province that provides new constraints on the development of the brasiliano orogeny between 580 and 500 ma

contrasting p t paths in the eastern himalaya nepal inverted isograds in a paired metamorphic mountain belt petrology and phase equilibria of rocks from two profiles in eastern d f matrix assemblages are overprinted by decompression reaction textures and in zones f and g progress into the sillimanite field nepal from the lesser himalayan sequences across the main central thrust zone and into the greater himalayan sequences the two terranes were brought into juxtaposition during formation of sillimanite biotite gedrite foliation seams s3 formed at reveal a paired metamorphic mountain belt pmmb composed of two thrust bound metamorphic terranes of contrasting metamorphic conditions of 674 33 c and 5 7 1 1 kbar the contrasting average geothermal gradients and p t paths of these two metastyle at the higher structural level the greater himalayan sequences experienced high t moderate p metamorphism with an anmorphic terranes suggest they make up a pmmb the upperplate position of the greater himalayan sequences produced an ticlockwise p t path low p inclusion assemblages of quartz hercynitic spinel sillimanite have been overgrown by peak anticlockwise p t path with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane metamorphic garnet cordierite sillimanite assemblages that equilibrated at 837 59 c and 6 7 1 0 kbar matrix in contrast the lower plate lesser himalayan sequences were deeply buried metamorphosed in a clockwise p t path and display minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re equilibrated samples preserve inverted isograds as a result of progressive ductile overthrusting of the hot greater himalayan sequences during prograde metamorphism evidence of cooling to 600 45 c at 5 7 1 1 kbar below the main central thrust the lesser himalayan sequences are a continuous though inverted barrovian sequence of high p moderate t metamorphic rocks metamorphic zones upwards from the

stepping stones and pitfalls in the determination of an anticlockwise p t t deformation path the low p high t cooma complex australia low pressure high temperature low p high t metamorphic rocks of the cooma complex southeastern australia show evidence of an anticlockwise pressure temperature time deformation p t t d path similar to those of some other low p high t metamorphic areas of australia prograde paths are reasonably well constrained in cordierite andalusite schists cordierite k feldspar gneisses and andalusite k feldspar gneisses these paths are inferred to be convex to the temperature axis involving increase in pressure with increase in temperature evidence of the retrograde path is inconclusive but is consistent with approximately isobaric cooling as are available isotopic data on the cooma granodiorite which indicate initially rapid cooling following attainment of peak temperatures the retrograde path is inconsistent with either a clockwise p t t d path involving rapid or even moderate decompression immediately post dating the peak of metamorphism or a path in which the retrograde component simply reverses the prograde component because both these paths should cross reactions forming cordierite from aluminosilicate for which no evidence has been observed determination of the deformational metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples evidence necessary for successful elucidation of the prograde and part of the retrograde deformational metamorphic history in the cooma complex includes 1 sequentially grown porphyroblasts that can be timed relative to surrounding foliations 2 partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development 3 a tectonic marker foliation s4 at cooma that allows correlation of foliations from one location to another and 4 single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone the latest two or three foliations involve low strain accumulation allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined sequential porphyroblast growth and foliation development in the cordierite andalusite schists is examined for situations involving rotation and non rotation of porphyroblasts relative to geographically fixed coordinates although the number of foliations developed varies in the rotational situation depending on the deformation history proposed the sequential order of porphyroblast growths does not differ from the non rotational situation thus whether or not porphyroblasts rotated in the cooma rocks the sequence of reactions and therefore p t t paths inferred from the relative timing of porphyroblast growths remain the same for the deformational histories evaluated

multiple generations of extensional detachments in the rhodope mountains northern greece evidence of episodic exhumation of high pressure rocks abstract an integrated structural and petrological study shows that exhumation of high rocks of the rhodope occurred in several pulses the structurally uppermost kimi complex recording an early cretaceous high p metamorphism was exhumed between about 65 and 42 ma the sidironero kardamos and kechros complexes that record early tertiary high metamorphism at least 19 kbar at 700 c were exhumed at 42 30 ma exhumation occurred with isothermal decompression strain episodes depict thrusting of medium high grade above lower grade high p rocks syn thrusting extension and post thrusting extension polyphase extension created several generations of detachment zones which in sum excise about 20 km of material within the crustal profile this reduction in crustal thickness is consistent with a reduction of the present crustal thickness from more than 40 km to less than 30 km in the eastern rhodope we mapped the post thrusting xanthi low angle detachment system over 100km from its break up zone above the sidironero complex central rhodope into the eastern rhodope this detachment shows an overall ese dip with a ramp and flat geometry cutting across the earlier thrust structures the kimi complex is the hanging wall of all synand post thrusting extensional systems on top of the kimi complex marine basins were formed from the lutetian c 48 43 ma through the oligocene during extension successively at c 26 to 8 ma the thasos pangeon metamorphic core complexes were exhumed in these times representing the early stages of aegean back arc extension the strymon and thasos detachment systems caused crustal thinning in the western rhodope renewed heating of the lithosphere associated with magmatism and exhumation of hot middle crust from beneath the sidironero complex occurred we focusoxn the geometry timing and kinematics of extension and contraction structures related to the 42 30 ma interval and how these exhumed high p rocks we interpret high rocks exhumed in this interval as a window of the apulian plate beneath the earlier in the cretaceous accreted kimi complex

neoarchaean metamorphic evolution of the yilgarn craton a record of subduction accretion extension and lithospheric delamination abstract the greater part of the metamorphic record of the yilgarn craton formed during a protracted middle neoarchaean orogenic cycle spanning from about 2750 to 2620 ma at least six distinct metamorphic events are defined by parageneses in different tectono stratigraphic settings this review characterizes the middle neoarchaean metamorphic response of the entire craton using large databases rock descriptions from 60 000 sites in government databases and 1040 pt determinations from all sources are used to construct metamorphic map patterns for each of these metamorphic events p t paths and timing of each metamorphic event are characterized using petrology and the well constrained pt determinations from this study n 287 combined with metamorphic age determinations from all sources n 114 the spatial patterns and metamorphic conditions of each event provides critical constraints on tectonic settings and changes in tectonics over 130 million years the record of metamorphism in conjunction with stratigraphic kinematic and magmatic constraints shows that the neoarchaean craton evolved from an accretionary crustal growth phase ma and m1 to thermal reworking of the crust during massive influx of granitic intrusions m2 and lithospheric extension after the termination of subduction m3 these events were followed by lithospheric delamination resulting in a craton wide diffusion delayed thermal pulse that coincided with renewed contraction m4 m1 metamorphism at 2748 19 2727 8 and 2706 10 ma experienced high p moderate t hairpin clockwise p t paths during burial of magmatic arc margins during terrane accretion events m2 regional contact type metamorphism at 2671 6 ma tracked low p moderate t clockwise p t paths in back arc settings during voluminous felsic magmatism related to shallow subduction m3 metamorphism at 2656 5 ma experienced low p moderate t anticlockwise p t paths in post volcanic rift basins formed during lithospheric extension m4 metamorphic peaks at 2644 4 and 2629 7 ma experienced low p high t clockwise p t paths during regional scale thermal pulses resulting from lower crust and mantle lithosphere delamination at 2665 2668 ma

pseudo and real inverted metamorphism caused by the superposition and extrusion of a stack of nappes a case study of the southern brasilia orogen brazil the southern brasilia orogen is a neoproterozoic belt that occurs along the southernmost border of the sao francisco craton where the andrel ndia nappe system represents the subducted sedimentary domain and is divided into three allochthonous groups of which the ages and p t conditions of metamorphism are studied here the basal unit the andrel ndia nappe exhibits an inverted metamorphic pattern the base of the structure composed of staurolite garnet biotite kyanite quartz and muscovite marks the metamorphic peak whereas at the top the association of the metamorphic peak does not contain staurolite the liberdade nappe the middle unit presents a normal metamorphic pattern its base close to the andrel ndia nappe shows paragneiss with evidence of in situ partial melting and towards the top coarse grained staurolite schist is found the staurolite out and melt in isograds are coincident and parallel to the main foliation thus the shear zone that limits the nappes is syn metamorphic reheating the underlying andrel ndia nappe and influencing the establishment of metamorphic inversion this suggestion is supported by the monazite chemical ages which indicates that the andrel ndia nappe metamorphic peak 586 15 ma is younger than that of the liberdade nappe 622 3 7 6 ma the upper unit the serra da natureza klippe bears a typical high pressure granulite mineral assemblage that is composed of kyanite garnet k feldspar rutile and leucosome as well as a metamorphic peak at 604 5 6 1 ma this tectonic assembly with inverted and non inverted metamorphic patterns and generation of klippen structures is consistent with exhumation models and a strong indentor located in the lower continental crust

contact metamorphism in pelitic rocks on the island of kos greece eastern aegean sea a test for the na in cordierite thermometer into cordierite by a coupled substitution involving tetrahedral and most of the applicable thermometers involving cordierite require octahedral sites whereas na may not be charge balanced by additional fe mg phases and knowledge of pressure and fluid phase lattice oxygen the results obtained suggest that the experimentally composition these conditions are not necessarily met with natural established relationship between na contents of cordierite and cordierite bearing metamorphic rocks a potential pressure intemperature is a useful thermometer for cordieriteand plagioclasedependent geothermometer for cordieriteand plagioclase bearing bearing metapelitic rocks the nature of na incorporation in cordierite rocks is the experimentally determined inverse linear relationship and its relation to fluid composition however must be constrained between temperature and na contents of mg cordierite coexisting by further experiments with naoh or albite and naoh in this paper the na incordierite thermometer is empirically tested on cordierite bearing metapelites from the island of kos contact metamorphosed by a quartz monzonite intrusion the variety of phase assemblages in the metapelites allows for tight constraints to be placed on temperatures

isobaric cooling and anti clockwise p t paths in the variscan odenwald crystalline complex germany mineral assemblages in al2o3 rich sio2 and k2o poor metapelitic rocks from the western odenwald crystalline complex variscan mid german crystalline rise southern germany include corundum spinel cordierite sillimanite garnet and staurolite quartz is absent from almost all samples therefore the applicability of conventional geothermobarometry is very limited or even impossible detailed petrographic investigation on selected samples permits inference of the sequence of appearance and disappearance of several mineral assemblages the recognition of such partial re equilibration stages and their associated mineral assemblages together with mineral stabilities predicted from kfmash pseudosections enables the determination of the pressure temperature p t trajectories experienced by these rocks during the variscan metamorphism the rocks were metamorphosed under low p high t conditions and underwent an anti clockwise p t evolution a pressure increase from about 2 kbar to 4 0 5 kbar was accompanied by heating peak metamorphic conditions were reached at pressures of 4 0 5 kbar and temperatures of at least 640 c probably even higher the retrograde evolution is characterised by near isobaric cooling from 640 c to approximately 550 c the rocks underwent the anti clockwise evolution in a subduction related magmatic arc setting the close spatial association of the low p high t rocks with recently discovered metabasic eclogites in the eastern part of the odenwald crystalline complex may indicate a fossil paired metamorphic belt in the central european variscides

natural annealing of dynamically recrystallised quartzite fabrics example from the cevennes se french massif central abstract quartzite samples from the sw cevennes french massif central have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric the studied quartzite is interlayered with turbiditic micaschist series and after regional deformation the whole sequence was intruded by the st guiral granodiorite at 1 3 kbar and up to 650 700 c recorded crystallographic preferred orientation cpo patterns of the dynamically recrystallised quartzite are classified as type i with a monoclinic symmetry and type ii with an orthorhombic symmetry type i is related to simple shear dominated deformation and type ii to apparent constriction these two fabric types are consistent with structural observation that a constrictive and coaxial deformation regime controlled the formation of non cylindrical and refolded folds in the metamorphic aureole which produced essentially static annealing cpo patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low to high quartz is triggered this inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content this study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems reactivate slip systems and amplify older cpo fabrics this conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occured

kinematic vorticity and tectonic significance of superposed mylonites in a major lower crustal shear zone northern fiordland new zealand abstract new structural metamorphic finite strain and kinematic vorticity data from a 4 km wide subvertical shear zone in fiordland new zealand reveal a history of deformation reflecting different tectonic regimes an analysis of ductile fabrics within the shear zone and its wall rocks shows two distinctive stages of amphibolite facies mylonitic deformation dasz2 and dasz3 that are superimposed on older paleozoic or early mesozoic structures variations in strain intensity and well defined shear zone boundaries have allowed us to examine the progressive development of lasz2 sasz2 and lasz3 sasz3 fabrics and compare the types kinematics and conditions of deformation that produced them mineral assemblages defining lasz2 sasz2 provided calculated peak conditions of 11 9 1 1 kbar and 581 34 c indicative of lower crustal depths mineral assemblages defining lasz3 sasz3 provided calculated peak conditions of 8 7 1 2 kbar and 587 42 c finite strain and kinematic vorticity studies show that dasz2 involved ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks at some time during the cretaceous rifting of ancestral new zealand from gondwana dasz3 represents an episode of mid crustal dextral transpression that may have resulted from latest mesozoic or cenozoic oblique convergence reactivation of the subvertical sasz3 foliation by cataclastic shear zones and brittle faults dasz4 was accompanied by limited recrystallization at greenschist facies conditions dasz4 shear zones record upper crustal dextral strike slip faulting that resembles late tertiary deformation patterns associated with the australian pacific transform plate boundary

low pressure metamorphism in the sierra albarrana area variscan belt iberian massif a low pressure metamorphic zonation ranging from biotite to migorogenic belts including the variscan belt of southwest matite zones occurs in the sierra albarrana area variscan belt of europe de yoreo et al 1991 presented a review of some southwestern iberian peninsula in uppermost precambrian to lower possible tectonic settings of low pressure metamorphism palaeozoic metasedimentary rocks the principal deformation in those workers considered low pressure metamorphism this area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500 c is localized immediately to the southwest of the sierra albarrana and pressure does not exceed that of the aluminium quartzites the metamorphism is synchronous with respect to this silicate als triple point the following four tectonic deformation the metamorphic zones are symmetrically distributed situations were envisaged for the low pressure metawith respect to the sierra albarrana quartzites pressure temperature morphism de yoreo et al 1991 1 magmatic arcs p t conditions are 3 5 4 kbar and range from 400 c 2 regions of crustal extension 3 continent continent biotite zone to 500 c staurolite garnet zone up to 650 700 c collision zones where thermal relaxation occurs after migmatite zone we have not detected pressure variations along crustal thickening 4 regions with significant fluxes of the different metamorphic zones relic kyanite is observed in the aqueous fluids especially those located above subduction form of inclusions in andalusite within veins in the lower grade part zones of the staurolite andalusite zone the low pressure metamorphism of this paper is concerned with the description of a highthe sierra albarrana area arises from a two stage history including temperature low pressure facies series terrane the sierra moderate crustal thickening followed by subsequent localization of albarrana area variscan belt southwestern iberian masdeformation in a transcurrent shear zone during peak p t conditions sif figs 1 and 2 in this area a metamorphic zonation channelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developed have contributed to the heat budget of the low pressure metamorphism at low pressure garrote 1976 gonzalez del tanago peinado 1990 as will be shown here the metamorphic zonation is not related to post thickening thermal relaxation but rather exhibits a roughly symmetric pattern

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

microtextures of opaque inclusions their use as indicators for hiatuses during garnet porphyroblast growth within an analysed garnet porphyroblast opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphyroblast three different zones can been distinguished zone 1 contains a ti rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite zone 2 shows a ti rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite within zone 3 ti rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim similar microtextures within ulvospinel rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion during metamorphism of metapelites such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast because the microtextures are homogeneous within the three different zones it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate as a consequence hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next a comparison between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coincidence between these and the sites where inclusion trails become strongly deflected and truncated this correlation confirms that in the studied case sharp microstructural boundaries as truncation zones or deflection zones coincide with growth hiatuses the study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses

deformation partitioning and foliation reactivation during transpressional orogenesis an example from the central longmen shan china progressive localization of strain within the songpan garze fold belt at the eastern margin of the tibetan plateau has resulted in the formation of the wenchuan maowen shear zone and preservation of excellent overprinting relationships between three major phases of ductile and semi ductile deformation these relationships show that deformation in the eastern songpan garze fold belt evolved from crustal thickening during sw directed compression d1 to localized sinistral transcurrent shear d2 and finally to increasingly localized high temperature d3p and retrograde d3r reverse shear in response to se directed compression both d2 and d3 deformations were co planar and reactivation of the s2 foliation led to the formation of a composite s2 3 fabric however inclusion trails in garnet porphyroblasts preserve evidence of a transition between these strike slip and reverse shear end members indicating that d2 and d3 were part of an episode of progressive transpressional shear metamorphic grade increases from chlorite to kyanite zone within the wenchuan maowen shear zone reaching syn d3p peak conditions of 600 c and 10 kbar the d3p and d3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde d3r high strain zones along the margins of the xuelongbao granite and adjacent to the wenchuan maowen fault

constraining timing and p t conditions of continental collision and late overprinting in the southern brasilia orogen se brazil u pb zircon ages and geothermobarometry of the andrelândia nappe system abstract the andrel ndia nappe system ans is an important component of the neoproterozoic southern brasilia orogen sbo in se brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the sao francisco paleocontinent and paranapanema block during the brasiliano orogeny the ans is mainly composed of a pile of deformed meta sedimentary rocks that record an inverted metamorphic pattern ranging respectively from amphibolite facies at the bottom to high p granulite conditions at the top the ans is here simplified into the lower middle and upper nappes four samples from different tectonic settings within the ans were collected for zircon isotopic investigation in addition geothermobarometry modelling were performed on seven samples from the middle nappe retro eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high p conditions of 12 16 kbar with temperatures of 700 800 c during the continental subduction dated here by u pb shrimp in zircon at 625 6 ma a period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet muscovite staurolite kyanite schist with leucosome parallel to the main foliation dated by u pb shrimp in zircon at 618 5 ma re equilibrium at pressures of 4 7 kbar with temperatures of 500 700 c is ascribed to a second heat pulse under lower pressure conditions the period of melting associated to this event in the middle nappe occurred at 586 9 ma and is marked by an anatectic tourmaline garnet muscovite bearing s type granite this granite body is part of a ne sw belt of granites with ages between 605 and 563 ma and covering both the central ribeira orogen cro and the sbo the age and the alignment of this granite belt justify the interpretation that it is genetically related to the cro thus the new data provide evidence that tectonothermal events related to the collisional front of the cro extended up to 200 km overlapping the consolidated sbo at ca 580 ma

full paleostress tensor reconstruction using quartz veins of panasqueira mine central portugal part i paleopressure determination abstract paleostress tensor restoration methods are traditionally limited to reconstructing geometrical parameters and are unable to resolve stress magnitudes based on previous studies we further developed a methodology to restore full paleostress tensors we concentrated on inversion of mode i fractures and acquired data in panasqueira mine portugal where optimal exposures of mineralized quartz veins can be found to carry out full paleostress restoration we needed to determine 1 pore paleo pressure and 2 vein attitudes the present contribution focuses specifically on the determination of pore pressure to these aims we conducted an extensive fluid inclusion study to derive fluid isochores from the quartz of the studied veins to constrain p t conditions we combined these isochores with crystallisation temperatures derived from geochemical analyses of coeval arsenopyrite we also applied the sphalerite geobarometer and considered two other independent pressure indicators our results point to pore pressures of 300 mpa and formation depths of 10 km such formation depths are in good agreement with the regional geological evolution the obtained pore pressure will be merged with vein inversion results in order to achieve full paleostress tensor restoration in a forthcoming companion paper

singularities in nckfmash na2o cao k2o feo mgo al2o3 sio2 h2o a new petrogenetic grid is presented for the system nckfmash na2o cao k2o feo mgo al2o3 sio2 h2o in which ca is incorporated in garnet and caalna 1si 1 solid solution is considered for both the plagioclase and white mica structures a compatibility diagram for plagioclase bearing metapelitic assemblages within the nckfmash system also has been derived a dominant feature of the nckfmash grid is the singularities and associated singular reactions which occur along plagioclase margarite and plagioclase paragonite bearing univariant equilibria the singularities represent compositional coplanarities which occur in response to the caalna 1si 1 substitution occurring at different rates in plagioclase and white mica this is controlled by a fundamental difference in the mixing within the two mineral structures the singularities give rise to a number of intriguing phase diagram features including azeotropes from the results presented here it is predicted that the occurrence of margarite and paragonite in pelitic rocks is controlled by equilibria related to the singularities the presence of these white micas is strongly dependent upon bulk composition and plagioclase bearing margarite paragonite free assemblages typical of barrovian type terranes are predicted for bulk compositions of mg mg fe 0 4 and ca ca na 0 4 at for example 5 5 kbar

tectonometamorphic evolution of the chewore inliers partial re equilibration of high grade basement during the pan african orogeny only minor structural reworking during m2 granulite terrane the chewore inliers are isolated outcrops of the zambezi mobile samples within 2 km of the basal thrust margin preserve m1 belt within the mesozoic lower zambezi rift valley in zimbabwe mineral assemblages but these minerals were chemically re equidetailed mapping has recognized four terranes the zambezi librated without recrystallization during m2 at conditions of 5 6 quartzite granulite and ophiolite terranes apart from the 1 5 kbar and 631 100 c granulite terrane samples were ophiolite terrane all are dominated by supracrustal gneisses with totally recrystallized in shear zones at the margin of this terrane concordant granitic orthogneiss units of 1071 8 and 1083 these samples equilibrated at conditions identical to the peak of 8 ma age these terranes experienced low p high t metamorphism m2 at 7 7 1 9 kbar and 590 110 c the re equilibrated m1 terminated by isobaric cooling at 945 34 ma m1 and recrystallized sample sets define two points on the clockwise assemblages of sillimanite spinel garnet garnet orthopyroxene and p t path experienced by the granulite terrane during further two pyroxene mafics are recorded in the granulite terrane and burial and reworking in the pan african orogeny and are consistent conditions of formation were 4 4 1 7 kbar and 800 c m1 with the m2 p t path documented for the other terranes mineral parageneses and associated ductile deformation structures dominate the granulite terrane but m1 mineral parageneses are only preserved as sillimanite spinel inclusions in garnet cores in the other terranes the zambezi quartzite and ophiolite terranes were almost totally recrystallized during reworking in the m2

phase relations in andalusite sillimanite type fe rich metapelites tono contact metamorphic aureole northeast japan mineral assemblages in metapelites of the contact aureole of the tono granodiorite mass northeast japan change systematically during progressive metamorphism along an isobaric path at 2 3 kbar the bulk rock compositions of metapelites are aluminous with a values on an afm projection larger than that of the chlorite join the metapelites commonly contain paragonite in the low grade zone with increasing temperatures andalusite is formed by the breakdown of paragonite the importance of pyrophyllite as a source of al2sio5 polymorphs is limited in typical pelitic rocks the most common type of metapelite in the study area has feo feo mgo 0 5 0 6 and develops assemblages involving chlorite andalusite biotite cordierite k feldspar sillimanite and almandine with paragenetic changes similar to other andalusite sillimanite type aureoles rocks with feo feo mgo 0 8 progressively develop chloritoid bearing assemblages from bt chl cld and bt cld to and bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole the paragenetic relations are explained by a kfmash univariant reaction of chl cld and bt located to the low temperature side of the formation of cordierite by the terminal equilibrium of chlorite a p t model depicting the relative stability of chloritoid and staurolite at low and medium pressure conditions respectively is proposed based on the derived location of the chl cld and bt reaction combined with the theoretical phase relations among biotite chlorite chloritoid garnet and staurolite

metamorphic events in the eastern arunta inlier part 1 metamorphic petrology abstract in the entia dome of the eastern arunta inlier kyanite facies metamorphism is recorded over a substantial area at least 160 km 2 and thus contrasts with the regionally extensive belts of high temperature low pressure andalusite sillimanite facies metamorphism documented elsewhere in the arunta inlier this amphibolite facies event appears to postdate regional granulite facies metamorphism evidence for which is almost totally obliterated in the entia dome prograde kyanite occurs in meta basites in comparatively rare associations involving hornblende staurolite and anorthite and in other rocks in association with gedrite and hornblende or garnet and biotite these kyanite bearing assemblages equilibrated at intermediate temperatures 570 710 c at peak pressures of 7 kbar and are overprinted by lower pressure assemblages including hornblende gedrite cordierite and sillimanite while this sequence of reactions may be simply interpreted in terms of a single clockwise orogenic cycle textural evidence and the available mineral isotopic data suggest a more complex history involving multiple heating and cooling episodes during the mesoproterozoic and possibly the palaeozoic the occurrence of regional retrogressive kyanite facies assemblages in the eastern arunta inlier provides evidence that the widely held perception that proterozoic crustal evolution in australia involved a distinct ensialic orogenic style characterised by high geothermal gradients etheridge et al 1987 loosveld and etheridge 1990 sandiford and powell 1991 is not universally true rather petrological evidence presented here with isotopic constraints described by foden et al 1995 suggests that the harts range region has experienced substantial crustal reworking at elevated temperatures during both the proterozoic and palaeozoic under thermal regimes more familiar to modern continental collision zones

the influence of crystallography and kinetics on phengite breakdown reactions in a low pressure metamorphic aureole a natural example of phengite that had undergone partial thermal decomposition at a pressure of about 0 5 kbar and a temperature of about 680 c in a contact aureole was exmined in the transmission electron microscope tem partially pseudomorphed phengites were found to consist of combinations of phengite biotite k feldspar mullite sillimanite spinel and cordierite different areas within individual partially pseudomorphed phengite grains show various degrees of reaction and different reaction products the cores are the least reacted and the margins have reacted most in the cores the assemblage al mg enriched phengite biotite k feldspar mullite spinel has formed whereas the assemblage k feldspar mullite sillimanite spinel biotite cordierite has formed at the edges according to our thermodynamic calculations the breakdown of phengite should have produced cordierite spinel corundum k feldspar in regions isolated from the influx of sio2 and cordierite andalusite quartz k feldspar in regions near the edge of the grains that were essentially saturated with sio2 chemical equilibrium was not achieved in any part of the partially pseudomorphed phengites on a micron scale or larger breakdown theoretically should have been complete by about 550 c the reaction temperature was overstepped by at least 130 c for 20 25 years the variations in the degree and type of reaction are probably due partly to the availability of suitable nucleation sites in different regions partly to the need to remove h2o from reaction sites and partly to the influence of sio2 which diffused into the grains during metamorphism the presence of sio2 lowers the equilibrium temperatures thus there is a higher driving force for breakdown near the grain boundaries than in the cores most of the products show an orientation relationship with the parent phengite and have consistent habit planes they have their closest packed planes and closest packed directions parallel to one another and to those of phengite such relationships minimize the strain and surface energies at nucleation and favour most rapid nucleation and growth of the reaction products the great structural similarity of biotite to phengite resulted in its having the highest rate of nucleation and growth of any product and it occurred in all areas of the phengite pseudomorphs studied mullite and sillimanite were produced metastably mullite has more rapid nucleation kinetics than other aluminosilicates because it is structurally disordered sillimanite formed rather than andalusite in regions of the partially pseudomorphed phengites where the reaction reached an advanced stage because the reaction from phengite to andalusite requires an energetically unfavourable change in aluminium co ordination state

an experimental determination of the effect of bulk composition on phase relationships in metasediments at near solidus conditions sediments of terrigenous origin represent a conspicuouspart of the cover of the oceanic and continental crust taylor mclennan 1985 wedepohl 1991 pelitesand greywackes are involved in a variety of processes from deep mass transfer in subduction environments e g chopin 1984 smith 1984 sobolev shatsky 1990 reinecke 1991 responsible for dehydration and fluidrelease during dehydration schmidt etal 2004 to partialmelting in continental collision zones which is at theorigin of intracrustal differentiation clemens vielzeuf 1987 the evolution of metapelites and metagreywackescan be reconstructed if the dependence of phase relation ships on pressure temperature and bulk composition iswell known and under h

metamorphic history of the hemlo gold deposit from al2sio5 mineral assemblages with implications for the timing of mineralization textural relations between al2sio5 phases and deformation fabrics provide constraints on the metamorphic history of the hemlo gold deposit kyanite in the deposit is most common within and on the margins of boudinaged quartz realgar veins and less commonly as rotated porphyroblasts within the matrix of schistose rocks kyanite predates the main d2 schistosity sillimanite postdates kyanite occurring irregularly as discrete knots and foliae that run parallel to but sometimes cut across the principal d2 foliation indicating that sillimanite postdates the d2 foliation we regard kyanite to be part of the peak metamorphic assemblage with sillimanite representing a partial later overprint most likely related to fluid infiltration rare andalusite occurs in two associations as late stage clean idioblastic crystals and as large fractured grains locally overprinted by sillimanite in boudinaged quartz realgar veins we suggest two possible origins for this second form of andalusite one involving generally late growth the second involving early growth prior to the development of peak metamorphic kyanite although not unambiguous we prefer the second scenario p t conditions from petrogenetic grid constraints and new geothermobarometric estimates indicate 6 7 gpa 600 650 c for the peak kyanite grade metamorphism ca 2677 ma and 4 5 gpa 600 c for the later sillimanite overprint ca 2672 ma we see these two events as part of an evolving p t path in a single metamorphic event in our early andalusite scenario the andalusite may have formed from pyrophyllite breakdown at 2 4 gpa 450 c possibly associated with emplacement of the regional suite of granodiorite plutons ca 2686 ma late andalusite formed sporadically on the retrograde path the occurrence of deformed andalusite and aligned kyanite in and on the margins of boudinaged auriferous realgar stibnite quartz veins provides evidence in support of a premetamorphic mineralization event

calculated mineral equilibria in the greenschist blueschist eclogite facies in na 2 o feo mgo al 2 o 3 sio 2 h 2 o the new greatly expanded internally consistent dataset of holland and powell includes thermodynamic data for a wide range of mineral end members in common rock forming minerals in particular including femg 1 substitutions in glaucophane garnet chloritoid and carpholite and femg 1 and mgsial 1al 1 substitutions in talc and chlorite moreover we have the uncertainties and correlations for these data with the data we have calculated the full pressure temperature phase diagram for the system na2o feo mgo al2o3 sio2 h2o nfmash for quartz or coesite and h2o in excess in the range 300 800 c and 5 50 kbars by solving the set of non linear equations formed by the equilibrium relationships for an independent set of equations between the end members in an assemblage in nfmash the compositions of the minerals and pt can be calculated thus the changes in mgsial 1al 1 along nmash reactions and fesial 1al 1 along nfash reactions are calculated and the changes in femg 1 and mgsial 1al 1 along nfmash reactions are calculated from this information it is straightforward to generate pt diagrams for specific rock compositions mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist blueschist and eclogite facies assemblages in metapelitic rocks it is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good when semiquantitative extensions of the phase diagram to include ca mgfe 1 nasica 1al 1 fe3 al 1 and kna 1 substitutions are taken into account the agreement is essentially complete

40ar 39ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of nw iberia the tectonothermal evolution of a polyorogenic terrane in the variscan belt of nw spain has been constrained by 40 ar 39 ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures transitional levels between hp ht and ip upper units in the ordenes complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating two groups of ages have been defined 1 silurian early devonian obtained from mylonites of the fornas extensional detachment here considered as the minimum age for the start of tectonic exhumation of the hp ht units and an upper age limit for the hp ht event itself 2 early to mid devonian from structures related to the variscan convergence in the area which include top to the east thrusts and extensional detachments a single younger carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes our data indicate a polyorogenic character for a part of the iberian allochthonous complexes including variscan sensu stricto and early variscan convergence as well as an older early palaeozoic cycle

the chemical composition of ree y th u rich accessory minerals in peraluminous granites of the erzgebirge fichtelgebirge region germany part i the monazite ce brabantite solid solution series peraluminous granites of the erzgebirge fichtelgebirge germany are hosts of various members of the monazite group of minerals that display an unprecedented compositional diversity the eibenstock s type granite constitutes the third reported occurrence worldwide of brabantite and the first occurrence of this mineral in a granite many new occurrences of cheralite ce as well as a monazite group mineral intermediate between monazite ce and huttonite for which the term huttonitic monazite is proposed were discovered even common monazite ce may show extreme ranges of actinide and lanthanide element concentrations the granites that host brabantite and cheralite ce are highly differentiated strongly peraluminous low temperature residual melts of s type affinity which are rich in fluorine and other volatile constituents but depleted in thorium and the light rare earth elements such highly evolved volatile rich compositions resemble rare element pegmatites and appear favorable for the precipitation of cheralite ce and brabantite but not of monazite with large amounts of huttonitic substitution instead these minerals occur preferentially in f poor biotite and f rich li mica granites of a type affinity irrespective of the level of uranium in silicate melts which may exceed that of thorium the substitution of uranium in monazite remains limited the compositional data reported here are consistent with complete miscibility in the monazite ce brabantite solid solution series under magmatic conditions these granites contain monazites that span almost the entire compositional range reported for monazite group minerals worldwide and therefore granites appear to be ideal rocks in which to study the crystal chemistry of this mineral group in general

thermal history of the pan african brasiliano borborema province of northeast brazil deduced from 40ar 39ar analysis abstract detailed step heating 40 ar 39 ar analyses were performed on different minerals from magmatic and metamorphic rocks of the borborema province this region located in northeast brazil belongs to the pan african brasiliano belt which is about 1000 km wide and 600 km long twenty six single grains of amphibole muscovite and biotite were extracted from eighteen samples selected in an area of about 17 500 km 2 250 km by 70 km in the patos region this region has been affected by a continental scale shear zone system well defined 40 ar 39 ar plateau ages and an u pb analysis on zircon leterrier j jardim de sa e f bertrand j m and pin c 1994 ages u pb sur zircon de granitoides brasilianos de la ceinture du serido province borborema ne bresil c r acad sci paris 318 1505 1511 allow the definition of a homogeneous and unusual slow cooling history 3 4 c ma and suggest a rather slow uplift rate between 580 and 500 ma followed by a fast cooling of the whole studied area around 500 ma rapid cooling is suggested by concordant plateau ages on muscovite and biotite the existence of such an event at the end of the belt history is in agreement with data obtained on other major belts such as the alpine himalayan system or the hercynian belt

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

ice shelf basal melting in a global finite element sea ice ice shelf ocean model the finite element sea ice ocean model fesom has been augmented by an ice shelf component with a three equation system for diagnostic computation of boundary layer temperature and salinity ice shelf geometry and global ocean bathymetry have been derived from the rtopo 1 dataset a global domain with a triangular mesh and a hybrid vertical coordinate is used to evaluate sub ice shelf circulation and melt rates for present day climate the model is forced with ncep reanalysis data basal mass fluxes are mostly realistic with maximum melt rates in the deepest parts near the grounding lines and marine ice formation in the northern sectors of ross ice shelf and filchner ronne ice shelf total basal mass loss for the ten largest ice shelves reflects the importance of the amundsen sea ice shelves getz ice shelf is shown to be a major melt water contributor to the southern ocean despite their modest melt rates the cold water ice shelves in the weddell sea are still substantial sinks of continental ice in antarctica discrepancies between the model and observations can partly be attributed to deficiencies in the forcing data or to sometimes unavoidable smoothing of ice shelf and bottom topographies

metamorphic response and crustal architecture in a classic collisional orogen the damara belt namibia abstract the damara belt is a well exposed mid crustal section through a collisional orogen of cambrian age that closed the khomas ocean basin between passive margins on the congo and kalahari cratons collision resulted in a bi vergent orogen with a distinct paired metamorphic pattern of foreland vergent high p low t orogenic margins and a broad high grade low p high t orogen core spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using a large dataset n 240 of internally consistent quantitative pt determinations evolution of mineral parageneses and semi quantitative p t paths metamorphic mapping and quantitative metamorphic field gradients integration with deformation history structural profiles metamorphic chronology magmatic history and stratigraphy constrains a dynamic model of crustal architecture during peak metamorphic events the pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities md with steep pressure gradients inferring crustal scale structures that accommodated lateral exhumation of crustal wedges discontinuities are confirmed by deformation features in the field and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures crustal wedges along the orogenic margins experienced steep clockwise p t paths with peak pt conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial metamorphic chronology and over printing metamorphic fabrics constrain a sequence of foreland propagating out wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins peak metamorphism at a geothermal gradient of 20 25 c km and 8 9 kb in the southern zone wedge i was attained between 517 530 ma followed by south directed out wedging at the uis pass suture accommodated by relative extension md1 at high structural levels near the boundary with the high grade orogen core out wedging of the southern zone further buried the southern margin zone wedge ii in the footwall below the uis pass suture peak metamorphism at 17 c km and 9 5 11 5 kb in the southern margin zone was attained at 517 4 ma and followed by out wedging on basal thrusts accommodated by vertical flattening and extensional reactivation of the uis pass suture md2 peak metamorphism at 17 c km and 10 5 kb in a high p low t crustal wedge in the northern margin wedge iii occurred at 510 4 ma peak metamorphism in this wedge was terminated by isothermal decompression during north directed thrusting accommodated by extension at higher structural levels near the high grade orogen core md3 granulite facies metamorphism in the high grade orogen core which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures 4 3 6 0 kb higher t depth ratios 38 47 c km and low p t clockwise p t paths indicating only moderate burial and protracted high grade conditions metamorphic chronology confirms high grade conditions 540 505 ma persisted beyond isostatic adjustment of the high p low t orogenic margins high heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision in contrast to the high p low t margins that experienced lateral exhumation in a convergent system the high grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen peak metamorphic conditions in the central part of the orogen core record higher pressures than marginal lower grade zones to both the north and south indicating vertical exhumation as a broad massif this post peak vertical exhumation of the central core occurred after 505 ma was accommodated by brittle extensional detachments at high stratigraphic levels with km scale omission of crustal section

structural and kinematic analysis of the corredoiras detachment evidence for early variscan synconvergent extension in the ordenes complex nw spain an important detachment is described in the allochthonous ordenes complex in the nw iberian massif and its meaning is related to the kinematics of contemporaneous convergent structures the corredoiras detachment cd separates a hangingwall unit characterised by a medium pressure metamorphic gradient from a footwall high pressure and high temperature unit and an underlying ophiolitic unit an associated ductile shear zone nearly 2000 m thick developed in the lower part of the hangingwall unit where the corredoiras orthogneiss a lower ordovician metagranite was progressively transformed into augengneisses mylonitic and ultramylonitic gneisses the attitude of the stretching and mineral lineation in the mylonites varies due to late refolding at map scale but the sense of movement can be estimated being roughly top to the se according to crosscutting relationships the cd developed subsequent to the thrusting of the high pressure high temperature unit onto the ophiolitic unit and prior to younger extensional detachments upright folding and strike slip tectonics the geometric relationships of the cd with the previous structures in the footwall unit the subtractive character of the metamorphic gap between its hangingwall and footwall and the available isotopic data suggest that the cd is an early variscan ductile extensional detachment the movement of which was roughly simultaneous with the onset of thrusting of the allochthonous complexes over their relative autochthon

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

phase equilibria and oxygen isotopes in the evolution of metapelitic migmatites a case study from the pre alpine basement of northern switzerland metapelites that have undergone variable degrees of migmatization were encountered in two deep boreholes in northern switzerland microtextures and calculated phase diagrams yield three successive stages of metamorphism consistent with one p t loop 1 amphibolite grade metamorphism biotite sillimanite garnetoligoclase quartz ilmenite p t conditions are estimated at 4 5 6 5 kbars 550 650 c 2 anatectic migmatization melting reactions leucosomes do not represent pure crystallized melt but rather a mixture of melt cordierite and k feldspar grown at the expense of biotite and sillimanite 3 retrograde hydration recorded by the growth of large amounts of muscovite at the expense of cordierite and k feldspar p t conditions are estimated at 2 5 kbars 600 c oxygen isotope measurements were obtained for whole rocks and eight mineral species isotopic equilibrium among three or even four minerals grown during stage 1 can be demonstrated calculated isotopic temperatures are consistent with phase petrology migmatization did not re equilibrate the isotopic composition of the pre existing quartz grown in stage 1 even over distances as little as 10 cm high 18o in migmatitic quartz is best explained by disequilibrium melting in coexistence with an infiltrating isotopically heavy fluid lack of equilibration of oxygen isotopes between different quartz generations suggests that mineralogical and geochemical changes were rapid relative to diffusion rates a meteoric hydrothermal alteration at 300 400 c probably genetically linked to the intrusion of variscan granites strongly affected the rocks quartz did not exchange isotopes with hydrothermal fluids except in segregations where it is recrystallized the 18o values of micas feldspars and cordierite are often very low and fractionations with quartz very large which reflects significant hydrothermal effects that were previously detected in the black forest by other workers

syntectonic emplacement of the maladeta granite pyrenees deduced from relationships between hercynian deformation and contact metamorphism the relationships between hercynian deformation and contact metamorphism in the aureole of the aneto unit a constituent pluton of the maladeta granite complex spanish pyrenees provide new data relevant to the controversy between the syn and post tectonic models of emplacement for the pyrenean granites the deformation in the aneto aureole is related to two superimposed hercynian episodes the first was a top to the sw thrusting which induced sw verging recumbent folds the second which was essentially compressive in the aneto area is represented by e w trending subvertical cleavage and folds with subhorizontal axes this second episode can be assigned to the d2 main hercynian tectonic event of the pyrenees the contact metamorphic assemblages grew during the d2 event and were deformed by it close to the contact the conclusion that the pluton was emplaced during d2 is strengthened by the structural and kinematic similarities between the magmatic deformation in the southern rim of the pluton and the d2 deformation in the country rocks this syntectonic timing is supported by new geochronological data that conflict with an existing rb sr post tectonic permian whole rock age for the maladeta granite more generally in accordance with some very recent structural and u pb zircon studies our results contradict the geodynamical models of the pyrenees which relate the granite intrusions to a late extensional event

neoproterozoic high pressure metamorphism and tectonic constraint from the nappe system south of the sao francisco craton southeast brazil abstract the neoproterozoic giant nappe system exposed south of the sao francisco craton underwent a minimum of 150 km of near horizontal northeastward displacement this nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high pressure high temperature intermediate to mafic granulites at its base socorro guaxupe nappe underlain in sequence by a high pressure kyanite granulite nappe an amphibolite facies metapelite nappe and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton the kyanite granulites crystallized ca 13 kbar 750 c and have only at their top sillimanite related to t increase overprinting t 890 c a consequence of downward heat advection from the overlying socorro guaxupe granulites equilibrated at 900 c pressures of 13 14 kbar are documented in the underlying metapelite nappe with temperatures of 640 670 c in which lenses of eclogite indicate p max of ca 17 5 kbar phyllites of the parautochthonous unit are directly overlain by hp granulites in the north and record p 7 kbar and t 500 c the distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction related metamorphism and kyanite granulites from early extrusion of subducted continental crust the metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile the overall picture suggests that high pressure units recrystallized in a subduction zone located to the wsw the frontal thrusts of the nappe system show a late north northeastward transported thin skinned pattern

influences of reaction history and chemical diffusion on p t calculations for staurolite schists from the littleton formation northwestern new hampshire geothermometry from staurolite grade pelitic assemblages may fail to reflect peak temperatures of metamorphism because of 1 prograde gamet consuming reactions and 2 diffusional reequilibration during cooling in response to retrograde reactions gamet dissolution during staurolite formation leads to sharp compositional gradients within a few micrometers of garnet rims these gradients may then be modified by diffusion on cooling and lead to anomalous temperature estimates calculated from thermo barometers involving garnet numerical simulations of mineral growth and diffusion were used to examine these effects for a prograde metamorphic sequence from the salmon hole brook syncline northwestern new hampshire in higher grade assemblages mineral textures indicate that staurolite grew by the consumption of gamet and gamet biotite temperature estimates fall in the range 515 t 30 c interbedded layers containing garnet and biotite but not staurolite yield temperature estimates of 565 t 25 c in the eastern portion of the syncline rocks containing kyanite staurolite garnet biotite assemblages indicate temperatures of 550 t 25 c hypothetical growth and reaction histories that adhere to the modal mineralogical and known geochronological constraints were developed using a thermodynamically rigorous treatment of mineral modes and compositions incorporating calculations for multicomponent diffusion in garnet these models demonstrate that temperature estimates from staurolite schists may underestimate actual thermal maxima by more than 50 c gamet growth accompanying the reaction staurolite kyanite biotite garnet can lead to higher absolute temperature estimates but these may still be at least 50 c lower than actual peak temperatures simulations also demonstrate that fe enrichment in biotite accompanying retrograde production of chlorite may lead to calculated temperatures that are 10 50 c higher than would occur where only diffusional exchange operated on cooling evidence from additional metamorphic terranes suggests that this reaction model has wide applicability

metamorphic evolution partial melting and miocene exhumation of lower crust in the edough metamorphic core complex west mediterranean orogen eastern algeria abstract the edough massif is the easternmost crystalline massif of the maghrebides which represent the southeastern part of the west mediterranean orogen it emerges from below the greenschist facies tellian units that represent the thrusted mesozoic to eocene passive paleomargin of northern africa deposited on the thinned continental crust the top of the crystalline pile of the edough massif is characterised by metasedimentary rocks affected by medium to high pressure 12 14 kbar and medium temperature metamorphic conditions of ca 600 c the northern part of the massif as well as the overlying allochthonous amphibole peridotite unit in the north is characterised by high pressure granulite facies t 800 c and p 12 kbar nearly isothermal decompression is documented in metapelites by late to post kinematic crystallisation of sillimanite and andalusite overprinting kyanite the core of the dome comprises essentially diatexite and granite gneisses a late low pressure evolution and overprinting 2 3 kbar took place in paleozoic rocks in proximity to granite gneisses the predominant igneous mineralogy of granite gneisses suggests that they represent synmetamorphic and syn kinematic sheeted intrusions generated through a high degree melting of metasediments geochronology of the metamorphic rocks u pb on monazite sm nd on garnet wr pairs 40 ar 39 ar on amphiboles and micas yields ages bracketed between 24 and 16 ma suggesting that regional metamorphism of the edough massif took place during the late tertiary synchronous with the greenschist facies metamorphism recorded in the overlying mesozoic tellian units the edough massif represents a metamorphic core complex exhumed through tectonic denudation assisted by syn to late metamorphic low angle n dipping normal ductile shear zones an exhumation rate of 6 8 km ma and associated cooling rate of 200 300 c ma is implied we conclude that the edough massif represents part of the former thinned continental crust of africa involved in a late eocene northward subduction beneath the suboceanic domain that separated paleo europe and africa in mesozoic time the upward syn metamorphic tectonic emplacement at 40 45 km depth of slices of spinel peridotites detached from the lower mantle may be responsible for the local granulite facies occurrences their emplacement is a consequence of the long lived transtensional sinistral then dextral kinematics having taken place between the african and european plates

extrusional tectonics in the core of a transpressional orogen the kaoko belt namibia the pan african kaoko belt of nw namibia provides a wellexposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis the kaoko belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time 575 550ma it is divided into a barrovian style escape zone with inclined nappes and thrusts that verge outward towards the foreland a steep high grade orogen core that experienced intense wrench style deformation and in the hinterland an older 650ma exotic coastal terrane that was reworked at moderate t low p conditions during transpressional orogenesis the kinematic lineation array defined by stretching lineations and shear sense indicators shows variation in degree and polarity of lineation obliquity to the grain of the belt lineation plunge and different polarity of vergence along the plunge with respect to the overall sinistral shear sense the gross regional lineation pattern defines an arcuate array from near orogen parallel in the orogen core to higher angle obliquity across the escape zone reflecting lateral escape towards the orogen margin two highgrade lobes within the orogen core are coincident with obliqueinclined upward flow trajectories based on the kinematic lineation array and represent oblique extrusion of lower crustal material into middle crustal levels an intervening low grade metamorphic trough within the orogen core and the coastal terrane coincides with acute oblique downward and outboard directed apparent flow trajectories these kinematic patterns are consistent with barometric constraints for peak metamorphism of 5 2kbar and 4 5kbar in the low grade trough and coastal terrane and 8 0kbar in the two high grade lobes integration of the kinematic and metamorphic datasets shows that oblique shallowly inclined extrusion trajectories occurred within the orogen core of this classic transpressional orogen and that stretching lineations approximately reflect the particle paths experienced across orogen metamorphic gradients indicate a marked contrast between the escape zone and orogen core the escape zone experienced an inverted barrovian style high p moderate t metamorphism and low thermal gradients of 20 c km the orogen core experienced moderate p high t metamorphism and high thermal gradients of 30 40 c km clockwise p t paths were experienced in all parts of the kaoko belt tight and shallow conduction dominated p t paths were experienced in all orogen core domains involving heating dominated prograde paths followed by decompression consistent with extrusional tectonics steeper and more open advection dominated p t paths were experienced in the crustal over thrust escape zone which experienced low thermal gradients