strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

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

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

the maladeta granite polydiapir spanish pyrenees a detailed magnetostructural study abstract the maladeta granitic complex spanish central pyrenees has been subjected to a detailed magnetostructural study through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites due to the dominant paramagnetic property of the granites a first order correlation can be derived between the susceptibility magnitude and the bulk rock iron content hence the petrographic nature of the rock the maladeta complex is shown to be composed of four principal juxtaposed plutons each one being normally zoned i e becoming more basic towards the periphery the largest pluton namely the colomers unit is itself made of a number of sub plutons due to distinct but more or less coeval batches of magmas the various magnetic data complemented by field and microstructural observations allow us to propose that the maladeta polydiapir probably exploited ne and se trending openings of the brittle crust for its emplacement that accompanied or just preceded a sw verging compressive event a later n s compression marked by inverse dextral and e w trending shears is preferably attributed to late variscan rather than to alpine tectonics

clay minerals as geo thermometer a comparative study based on high spatial resolution analyses of illite and chlorite in gulf coast sandstones texas u s a phyllosilicates are among the most important stable minerals within the earth s crust their use as geo thermometer bears great potential for application to the thermal history of rocks within the stability range of layered silicates and was tested here a high resolution analytical technique combining focused ion beam fib milling and analytical electron microscopy aem analysis has been applied to a series of sandstone core samples from the gulf coast texas u s a the nanoscale compositional variations of k deficient mica and chlorite flakes show that rim compositions are the most likely to approach equilibrium compositions whereas core compositions may be relict especially for illite like phases these rim analyses were used to test existing empirical or thermodynamically formulated thermo baro meters against maximum temperatures which are perfectly constrained for the selected samples as they were measured in situ during drilling 100 230 c and 300 1200 bars the results show that most of the empirical models overestimate the temperature while thermodynamic models yields reasonable estimates for diagenetic to anchizonal conditions especially if the fe3 content is taken into account this study clearly shows that phyllosilicates thermometry is reliable when combined with an analytical technique giving access to the fine scale compositional variations that may represent local equilibration whereas using micrometric compositional analysis precludes trustworthy application of such thermometers

the main variscan deformation event in the pyrenees new data from the structural study of the bielsa granite abstract a structural and magnetic fabric study of the bielsa granite axial zone of the pyrenees provides new data indicating that this pluton was emplaced during the main variscan event recognized in the pyrenees we argue that the later post triassic deformation was localized along mylonitic bands reactivating earlier variscan shear bands the bielsa granite intrudes cambro ordovician metasediments at its northern border and the pluton is unconformably overlain by sediments triassic in age to the south and east the main structures in the igneous body are magmatic foliations and lineations defined by feldspar phenocrysts and biotite and dextral shear bands with dominant wnw ese trends the wnw ese striking magmatic foliations have variable dips and the magmatic lineation a subhorizontal wnw ese trend the zonation of low field magnetic susceptibility coarsely related to mineral content indicates a layer arrangement of rock types with more basic compositions at the northern border the magnetic ellipsoids are prolate along a wnw ese central band and oblate at the pluton borders where flattening is more important all these data allow us to interpret the bielsa intrusion as being caused by coeval nne shortening and dextral shear the presence of shear bands indicates that strain localization under dextral transpression continued during late variscan times

pre orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central pyrenees pre kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts but rarely dated and accounted for in orogenic cycle reconstructions studying pre kinematic alteration is challenging because of its usual obliteration by subsequent syn kinematic metamorphism often occurring at conditions typical of the brittle ductile transition it is however to be expected that pre kinematic alteration has major implications for the rheology of the upper crust in the 305 ma old variscan basement of the bielsa massif located in the axial zone of the pyrenees successive fluid rock interaction events are recorded in granites below 350 c combined microstructural and petrographic analysis low t thermobarometry and in situ u th pb dating of anatase titanite and monazite show extensive pre orogenic pre alpine and pre kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270 350 c at 230 300 ma this event is followed by a second fluid rock interaction stage marked by new crystallization of phyllosilicates at 200 280 c and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes u pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40 70 ma consistent with independent regional geology constraints the variscan basement was therefore softened at late to post variscan time at least 150 200 ma before the main alpine shortening while alpine age compression c 35 50 ma leads to the formation of a dense net of mylonites the associated deformation both distributed at the scale of the bielsa massif and localized at decametric scale in mylonitic corridors precedes the strain localization along the major thrusts of the axial zone the bielsa massif is a good example where inherited pre orogenic upper crustal softening controls the deformation patterns in granitic basement units through low grade metamorphic reactions

source characteristics of the sinaia romania sequence of may june 1993 a particular sequence of crustal earthquakes h 10 km ml 5 1 occurred in the southwestern extremity of the eastern carpathian arc bend sinaia region starting on may 4 1993 the main shock ml 5 1 occurred on may 23 after significant 20 day foreshock activity the first locations delineate an epicentral area of approximately 225 km2 elongated in a nw se direction which agrees with the fault plane solution for the main shock the digital recordings of the romanian telemetered network are analyzed in order to estimate source parameters and to investigate scaling relations an empirical deconvolution technique and the spectral ratio method are used for this purpose the main results such as 1 the departure from a m13o scaling of the source radius as a function of the seismic moment 2 the scaling of the peak velocity parameter as a function of local magnitude and 3 the scaling of the dynamic stress drop with earthquake size are interpreted in terms of an inhomogeneous source model

collision kinematics in the western external alps the kinematics of the collision in western alps are investigated through five balanced cross sections of the whole external domain from the oisans to the mont blanc massif these cross sections were built using published data for the jura and subalpine fold and thrust belts and new structural and field analysis for the external crystalline massifs five units are defined the sedimentary nappes from innermost parts of the external zone e g ultra dauphinois helvetic the crystalline units with their dysharmonically folded cover e g morcles nappe sedimentary nappes over the frontal parts of the crystalline massifs the aravis granier unit the subalpine belts e g vercors chartreuse bauges and bornes and the jura except for the ultra dauphinois nappes the shortening including the cover shortening always corresponds to basement shortening the total amount of shortening increases from south 28 km 20 to north 66 km 27 moreover the shortening is slightly older in the south than in the north deepwater turbidites flysch and shallow marine to freshwater clastics molasse basins are more developed in the north pressure and temperature conditions are higher in the north the average uplift rates are about 3 times higher in the north and more localized in space we propose that these differences are due to along strike variations in the structure of the european continental margin inherited from mesozoic times we then build five palinspastic maps one at cretaceous times showing the inherited european mesozoic margin structure and four from priabonian to upper miocene times showing the collision kinematics and the related rotation of adria

thermochronology constraints for the propagation sequence of the south pyrenean basement thrust system france spain 1 in this work we combined apatite fission track and biotite k feldspar 40ar 39ar ages with tectonic data in the west central part of the axial zone of the pyrenees we discuss the exhumation ages and rates of the neouvielle bordere louron and bielsa variscan granites and their relationships with the timing and sequence of south vergent basement thrusting within the pyrenean orogenic prism the 40ar 39ar ages on k feldspars from the neouvielle massif sample nv7 seem to indicate tectonic movements on the eaux chaudes thrust during the early middle eocene fission track results suggest that the exhumation of the neouvielle massif occurred around 35 ma and exhumation of the bordere louron massif around 32 ma in relation to thrusting on the gavarnie thrust the bielsa massif was exhumed from around 19 ma by out of sequence movements on the bielsa thrust we thus show that whereas most of the pyrenean basement thrust faults here the eaux chaudes gavarnie and guarga thrusts were active in sequence toward the southern foreland from the early eocene to the earliest miocene some of them here the bielsa thrust were activated out of sequence in the hinterland later than the generally accepted aquitanian age for the end of the pyrenean compression finally the apatite fission track modeling indicate a last cooling episode starting around 5 ma which is most certainly related to the pliocene reexcavation of the southern and northern flanks of the pyrenees

permian granites in the pyrenees the aya pluton basque country we show that the permian volcanism in the pyrenees was accompanied with emplacement of granite plutons with transitional chemistry reassessment of age structure and petrological characteristics of the aya pluton intrusive in the cinco villas massif basque country indicates that it was emplaced at 267 1 1 1 ma in a dextral transtensive context and that it marks a transition between the high k calcalkaline and the alkaline emissions of the first and second permian volcanic cycles respectively this age is close to that of c 267 ma reported for lamprophyres dykes of the pyrenean axial zone these data characterize a period of transition between the variscan orogeny and the subsequent alpine cycle in the pyrenees with an earlier extensive regime that became widespread in the pyrenees during the mesozoic times

succession of permian and mesozoic metasomatic events in the eastern pyrenees with emphasis on the trimouns talc chlorite deposit recent studies proposing pre orogenic mantle exhumation models have helped renew the interest of the geosciences community in the pyrenees which should be now interpreted as a hyper extended passive margin before the convergence between iberia and eurasia occurred unresolved questions of the pyrenean geology as well as the understanding of the formation of hyper extended passive margins are how the crust was thinned and when where and how the crustal breakoff occurred the study of the variscan and pre variscan pyrenean basement is thus critical to document and understand this cretaceous crustal thinning in order to specify the timing of mesozoic metasomatism and the associated deformation in the pre mesozoic basement of the pyrenees we carried out a u th pb laser ablation icp ms study on a large panel of ree and titanium rich minerals titanite and rutile from talc chlorite ores from the eastern pyrenees with a special emphasis on the trimouns deposit the world s largest talc quarry our results suggest that the trimouns talc formation was restricted to the upper aptian cenomanian time while the talc and chlorite formation in the eastern pyrenees occurred during several distinct permian jurassic and cretaceous episodes these results give strong constraints on the tectonic setting of the pyrenean domain during the transition between the variscan and alpine orogenic cycles and particularly on when and how the upper crust was thinned before the crustal breakoff and the final mantle exhumation

titaniferous accessory minerals in very low grade metamorphic rocks keweenaw peninsula michigan usa abstract titanite tio 2 and pseudorutile are associated with primary igneous fe oxide grains in basalt and rhyolite clasts from the keweenaw peninsula michigan usa which were metamorphosed to the prehnite pumpellyite facies pseudorutile occurs with titanite tio 2 in broad lamellae within titaniferous magnetites it also occurs as intergrowths with titanite and fe oxide in embayments within primary oxide grains and in composite fe oxide titanite grains that appear to represent nearly complete replacement of original ti bearing fe oxides thin 111 lamellae in titaniferous magnetite grains contain titanite titanite tio 2 and titanite tio 2 fe oxide tio 2 also occurs by itself in networks of closely spaced small elongated lenses in fe ti oxides concentrations of caalsio 4 f caalsio 4 oh in titanite range from 0 to 30 the large variation in titanite compositions suggests that equilibrium was not achieved except on a very local scale a conclusion also supported by local variations in the assemblages of ti bearing secondary minerals textures and mineral assemblages indicate that ulvospinel lamellae were altered to titanite and or tio 2 polymorphs while ilmenite was altered to pseudorutile and titanite tio 2 the relative proportions of tio 2 and titanite appear to reflect local variations in the composition of the metamorphic fluid phase that may be linked to the degree of interaction with a hydrothermal fluid titanite rich regions may indicate a greater degree of fluid flushing than tio 2 rich regions textures and mineral assemblages cannot distinguish between models in which pseudorutile and a tio 2 polymorph formed during weathering or diagenesis followed by conversion of tio 2 to titanite during metamorphism and models in which pseudorutile formed together with titanite and tio 2 during metamorphism in either case pseudorutile was able to persist through prehnite pumpellyite metamorphism suggesting that it can be a significant ti rich accessory mineral in very low grade metamorphic rocks

isotopic systematics o h c sr nd of superimposed barren and u bearing hydrothermal systems in a hercynian granite massif central france abstract the hercynian 325 ma granitic complex of saint sylvestre experienced two hydrothermal episodes the first one leading to the formation of a typical alteration facies called episyenite mainly characterized by the dissolution of granitic quartz the second one leading to the deposition at 280 ma of a major u deposit the present study documents the isotopic characteristics of these hydrothermal events 18 o 16 o d h and 13 c 12 c systematics demonstrate that barren episyenites were generated during hydrothermal interaction of large amounts of mainly meteoric water with the granite the ore depositing fluid equilibrated in a sedimentary formation before entering the granite episyenitization was dated at 301 2 ma which is 25 ma later than the granite emplacement the rb sr isochron relationship implies isotopic equilibrium within the fluid phase and between the fluid and the host rocks which confirms the high water rock ratio inferred from o h isotope systematics a coupled nd and sr isotope investigation demonstrates that at least nd was reworked from the granite in the episyenite genesis but sr and possibly nd had an exotic origin consistent with the inferred sedimentary origin during the ore forming stage the present study emphasizes the fact that spatially superimposed hydrothermal events in a granitic environment can be 1 disconnected from the magmatic history of the granite itself and 2 mutually disconnected from chronological and chemical points of view

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

nucleation of ductile shear zones in a granodiorite under greenschist facies conditions neouvielle massif pyrenees france abstract the problem of ductile shear zone nucleation under greenschist facies conditions is approached from the example of small scale shear zones developed in the neouvielle granodioritic pluton pyrenees france by means of field and microstructural observations and chemical analyses these shear zones are not related to pre existing fractures and exhibit networks of numerous conjugate fairly parallel and regularly spaced centimetre scale brittle ductile shear zones involving diffuse localization mechanisms although the mode of deformation depends on the minerals present deformation in the shear zones is basically controlled by hydration processes hydration and consequent fluid controlled alteration and deformation are related to fluid migration towards developing cracks all these cracks are extensional and formed on the scale of at most a few grains and frequently single grains a comparison of chemical compositions of undeformed and sheared granodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems we propose a sequence of mechanisms by which the nucleation of a small scale brittle ductile shear zones spreading out within a granodioritic rock may occur in these mechanisms shear zone nucleation occurs independently of pre existing fractures and results from the heterogeneous character of the polymineralic rock in the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain scale hydraulic fracturing fluid focusing generates instabilities by local softening and subsequently shear zone nucleation

physico chemical control on the ree minerals in chloritoid grade metasediments from a single outcrop central alps switzerland this study assesses the effects of fluid whole rock composition and oxygen fugacity on the texture and composition of monazite allanite and xenotime for this purpose these were investigated in 13 mono metamorphic metasediments from a single locality of the central alps switzerland which record greenschist facies conditions t 400 450 c two of the samples contain hydrothermal veins dominated by quartz and calcite quartz respectively in metasediments devoid of veins the light rare earth elements lree are concentrated in allanite in all samples except for one metamarl ga06 allanite formation is texturally coeval with apatite chloritoid and xenotime during the main tectono metamorphic stage allanite formation implies significant mass transfer of ca and p via a fluid phase which is not clearly related to advective transport in ga06 elongate monazite grains have a detrital core rimmed by newly formed monazite significant arsenic contents are found in newly formed monazite xenotime and apatite monazite texture and composition suggest re crystallization by pressure solution at an oxygen fugacity sufficient to partly oxidize as s u and fe whether or not monazite is preserved appears to be but weakly dependent on the ca al ratio and thus whole rock composition at greenschist facies conditions samples with veins show peculiar features along carbonate bearing veins allanite occurs as porphyroblasts overgrowing the main foliation of the host rock as similar allanite porphyroblasts occur in calcite bearing metamarl their formation is attributed to environments rich in co2 bearing fluid a new generation of monazite is found in and along retrograde quartz veins this study demonstrates that at least in metasediments up to chloritoid grade ree minerals record fluid rock interaction that occurred at different deformation stages arsenic concentrations in ree phosphates appear to reflect conditions of elevated oxygen fugacity in cases where such conditions are not inherited from the sedimentary protolith the oxidation reflects a hydrothermal event the age of which may be directly datable by u th pb of as rich monazite

fluid mixing and recycling during pyrenean thrusting evidence from fluid inclusion halogen ratios abstract syntectonic fluids have been sampled through fluid inclusion microthermometry and crush leach analyses cations and halogens from a 50 km n s transect through the central southern pyrenees the fluid inclusions are contained in syntectonic quartz veins in triassic redbeds cretaceous carbonates and hercynian basement rocks with some calcite and dolomite data from limestones and evaporites in more external parts of the belt the main datasets come from 1 alpine shear zones cutting the neouvielle granodiorite in the hercynian axial zone at the north end of the transect 2 an imbricate zone beneath the alpine gavarnie thrust at the pic de port vieux 3 several localities in the footwall and hangingwall of the gavarnie thrust on the southern margin of the axial zone the inclusion fluids generally decrease in salinity from 27 35 at the northern end of the transect to 7 22 on the southern margin of the axial zone the majority of the inclusions have cl br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper trias this interpretation is supported by cl br na systematics which are consistent with a change from halite to halite sylvite precipitation with progressive evaporation fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations although it is possible that final concentration of brines in the neouvielle massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals the fluids are also very similar in salinity and halogen chemistry to those found in veins associated with mesozoic pb zn f deposits which predate the thrusting the lower salinities seen at the southern margin of the axial zone are interpreted to reflect mixing of the brines with a higher level fluid connate or meteoric water circulating within the mesozoic carbonates of the higher thrust sheets at one locality where triassic evaporites are still present high cl br ratios at relatively low salinities are present in inclusions within the underlying triassic redbeds but low cl br ratios at higher salinities are seen lower in the sequence this is consistent with dissolution of halite by a dilute fluid but with limited penetration downwards we suggest that the fluid history of the pyrenees evolved through a series of stages 1 upper triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks 2 circulation of brines with formation of pb zn deposits along faults at some time between the triassic and the upper cretaceous 3 renewed extension with erosion of triassic rocks in many areas and further drawing down of triassic brines into the basement 4 deposition of u cretaceous and palaeocene carbonates containing connate waters of marine origin 5 formation of the pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults 6 creation of topography with a high level circulation system in the mesozoic thrust sheets driven largely by topography at the southern margin of the axial zone there was limited mixing of the deeper overpressured brines with these more dilute hydrostatically pressured fluids an important point is that because of their density hypersaline brines are difficult to expel from the upper crust and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years

metastability mechanical strength and the support of mountain belts exhumed high pressure rocks from the caledonian root zone in norway provide analogues for processes occurring today under southern tibet allowing large scale geophysical observations from gravity and earthquakes to be linked with the mechanical properties of metastable rocks under high mountains metastability is essential for the survival of thick mountain roots and hence of high mountains and is in turn controlled by water if water is absent dry granulite formed from earlier melting episodes is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years but if hydrous fluid is introduced the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength in norway this transformation was initiated by water infiltration along fractures formed by earthquakes the same process as marked by deep earthquakes may be occurring today beneath southern tibet

p t deformation fe3 fe2 mapping at the thin section scale and comparison with xanes mapping application to a garnet bearing metapelite from the sambagawa metamorphic belt japan quantitative x ray maps of composition from a chlorite k white mica albite quartz and garnet bearing thin section from a sambagawa blueschist facies metapelite were combined with a multiequilibrium calculation method to calculate a p t fe3 fe2 deformation map at the millimetre scale the studied sample was chosen because elongated chlorite crystallization tails pressure shadows rimmed by phengite are present which is an appropriate assemblage for the quantification of the p t evolution chlorite temperature and fe3 content maps were calculated by successive iterations for each pixel analysis of fe3 until convergence of the four chlorite quartz h2o equilibria that can be written using the fe and mg amesite clinchlore daphnite and sudoite chlorite end members the calculated map of fe2 fe3 in chlorite is in good qualitative agreement with the in situ mapping of this ratio using xanes x ray absorption near edge structure techniques the temperature map indicates that high temperature chlorite zones with low fe3 contents alternate with lower temperature zones and higher fe3 contents in the crystallization tail late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite 250 c showing fe3 fetotal up to 0 4 groups of chlorite and mica pixels were then identified based on compositional and structural criteria and a p tdeformation map was calculated using representative analyses of these groups the calculated p tdeformation map suggests that in contrast to chlorite the composition of most mica did not change significantly during exhumation mica reequilibrated in late ew shear bands only ew shearing was already active at 0 1 gpa 500 c which corresponds to the peak temperature and probably pressure conditions at reduced redox conditions the intensity of deformation probably decreased with decrease in temperature to 350 400 c at this temperature a second main deformation event corresponding to a further ew stretching occurred and was still active below 250 c and more oxidizing conditions these results indicate that the scale at which p t data can be obtained is now close to the scale of observation of structural geologists a close link between deformation and mineral reaction is therefore possible at the microscopic scale which provides information about the relationship between deformation and mineral reactivity the modalities of deformation with time and the p t conditions at which it occurred

300 million years of episodic hydrothermal activity stable isotope evidence from hydrothermal rocks of the eastern iberian central system the eastern iberian central system has abundant ore showings hosted by a wide variety of hydrothermal rocks they include sn w fe and zn w calcic and magnesian skarns shear zone and episyenite hosted cu zn sn w orebodies cu w sn greisens and w sn base metal and fluorite barite veins systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning variscan orogeny the first event was at about 295 ma followed by younger pulses associated with early alpine rifting and extension and dated near 277 150 and 100 to 20 ma respectively events ii iv the 18o d and 34s studies of hydrothermal rocks have elucidated the hydrological evolution of these systems the event i fluids are of mixed origin they are metamorphic fluids h2o co2 ch4 nacl 18o 4 7 to 9 3 d ab 34 related to w sn veins and modified meteoric waters in the deep magnesian sn w skarns h2o nacl 4 5 6 4 wt nacl eq 18o 7 3 7 8 d 77 to 74 and epizonal shallow calcic zn w and fe skarns h2o nacl 14 wt nacl eq from evaporites hosted in the post variscan sequence the d values are very similar to most of those recorded by kelly and rye in panasqueira and confirm that the upper paleozoic meteoric waters in central iberia had very negative d values 52 whereas those of early mesozoic age ranged between 65 and 36

recurrent hydrothermal activity induced by successive extensional episodes the case of the berta f pb zn vein system ne spain abstract a mineralogical and geochemical fluid inclusion stable and radiogenic isotopes study of the berta f pb zn vein system has identified the source and temperature of the fluid reservoirs involved and proved the existence of two separate hydrothermal events at the mine scale which reflect distinct periods of regional fluid circulation main stage minerals fluorite i sulphides calcite i and barite i precipitated by mixing between a polysaline h 2 s bearing 34 s 11 brine up to 23 nacl eq salinity and a more dilute fluid 18 o from 3 2 to 0 at temperatures between 80 and 150 c the progressive increase in 87 sr 86 sr ratio from the early precipitated minerals 0 71242 in calcite i to the late ones 0 71894 in fluorite ii is mainly but not exclusively due to a difference in age separating the two hydrothermal events the assumed genetic model for the main stage fluorite i is based on a convective circulation of surficial waters leaching the crystalline basement rocks acquiring a high salinity high 87 sr 86 sr ratios and a high temperature these fluids then mixed with low salinity low temperature waters having a low 87 sr 86 sr ratio an at least jurassic age is suggested for the main period of vein filling contemporaneous with the extensional regime during the mesozoic when fluid circulation was probably enhanced by crustal thinning during the early burdigalian lower miocene a new period of important extension in this area took place hydrothermal activity related to this new and younger extensional regime is geochemically different and produced a distinctive mineralogical record developing a set of veinlets filled with green octahedral fluorite fluorite ii calcite ii and barite ii the sr isotope compositions of these late stage vein minerals are compatible with leaching the granodiorite host rocks during recent times the existence of successive hydrothermal events in the same area is not surprising as geothermal systems like la garriga samalus are still active and currently precipitating fluorite

analysis of continental midcrustal strain localization induced by microfracturing and reaction softening 1 low angle extensional shear zones which often characterize the brittle ductile transition of the continental crust are seen here to result from strain localization the potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the east tenda shear zone alpine corsica the coupling between microfracturing and feldspar to mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km this reaction is proposed as the main destabilizing factor responsible for the onset of localization with feldspar having a stationary dislocation creep flow stress larger than mica to test this hypothesis a rheological model is constructed based on the field observations for a mixture of three phases mica quartz and feldspar deforming at a common strain rate the phase concentrations change with time according to the feldspar to mica reaction which takes place only if feldspar grains are fractured a condition detected with the mohr coulomb criterion the tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust which sustains an overall simple shear the onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust the upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur the time lapse for this velocity change determines the depth of the shear zone 15 17 km the kinetics of the metamorphic reaction and the final amount of white mica control its width 1 4 km the time of the shear zone formation is less than half a million years

quantifying orthoclase and albite muscovitisation sequences in fault zones abstract a new graphical method is described for assessing the degrees of alteration of orthoclase and albite feldspars to muscovite the method uses major element data from whole rock samples and given a range of samples of varying degrees of alteration pathways of the alteration sequences may be determined the method may be used quantitatively if likely starting compositions are known and aluminium immobility is assumed molar proportions of k na and al are plotted in the form of an na na k versus na k al graph modelled plots of muscovitisation of an orthoclase albite system are presented for comparison with actual data so that actual compositions may be assessed in terms of feldspar muscovitisation an example of the application of this technique to muscovitised granitic fault rocks in the external western alps is presented illustrating how the method is best used in conjunction with microstructural and field constraints on the fault rock evolution this yields an evolution pathway for muscovitisation of the feldspars during fault zone deformation and links specific alteration steps with particular microstructural changes within the fault zone we also apply this technique to a previously published data set on basement shear zone geochemical changes from the pyrenees where orthoclase albitisation provides an additional interest given that these reactions are fluid induced this work provides further information on the effects of fluid flow in such fault zones in terms of basement fault zone rheologies