low to medium pressure variscan metamorphism in galicia nw spain evolution of a kyanite bearing synform and associated bounding antiformal domains

recrystallization of shell carbonate in soil 14c labeling modeling and relevance for dating and paleo reconstructions abstract mollusk shells are commonly present in a broad array of geological and archaeological contexts the shell carbonate can serve for numerical age determination 14 c and as a paleoenvironmental indicator 18 o 13 c shell carbonate recrystallization in soils however may re equilibrate the carbon c isotopic signature with soil co 2 the equilibration dynamics remain poorly understood because of the absence of suitable experimental approaches here we used the artificial 14 c labeling technique to study the process of shell carbonate recrystallization as a function of time organic free and organic containing shell particles of protothaca staminea were mixed with loess or a carbonate free loamy soil the mixtures were placed in air tight bottles where the bottle air containing 14 co 2 p co 2 2 the 14 c activity of shells was measured over time and related to the recrystallization of shell carbonate recrystallization of shell carbonate already began after one day the recrystallization rates were 10 3 day 1 in organic containing shell embedded in soil and 1 6 10 2 day 1 in organic free shells in loess removal of organic compounds increased shell porosity and so increased the contact surface for exchange with soil solution organic free shells recrystallized much faster in loess 0 56 in 56 days than in other treatments recrystallization was 2 to 7 times higher in loess in the presence and absence of organic compounds respectively than in carbonate free soil loess carbonate itself can recrystallize and accumulate on shells leading to overestimation of shell carbonate recrystallization a model for shell carbonate recrystallization as a function of time was developed the model considers the presence or absence of organic compounds in shell structure and geogenic carbonates in the embedding matrix the model enabled all results to be fitted with r 2 0 98 the modelled time necessary for nearly full recrystallization 95 of shell carbonate was 88 years for organic free shells in loess and up to 770 years for organic containing shells in carbonate free soil after this period the original isotopic signature will vanish completely and will be replaced by a new 13 c and 14 c signature in the shell structure thus shell carbonate recrystallization may proceed relatively rapidly in terms of geologic time this is necessary to consider in the interpretation of dating results and paleoenvironmental reconstructions

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

low thermal gradient hercynian metamorphism in the eastern pyrenees fluids with compositions in the system co2 h2o nacl were trapped in quartz veins enclosed in low grade metamorphic rocks chlorite zone on the southern flank of the canigo massif eastern pyrenees the veins which also contain arsenopyrite crystals were formed contemporaneously with the main hercynian foliation and metamorphism volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest p t trapping conditions of 4 6 6 kbar and 450 530 c this implies that an episode of metamorphism with an average geothermal gradient of 25 c km 1 occurred during the main deformation event this episode preceded the low p high t metamorphism described around domes and to date considered as characteristic of the hercynian orogeny in the pyrenees

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

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

tracing crustal evolution in the nw iberian peninsula through the rbsr and upb systematics of palaeozoic granitoids a review abstract the abundant granitoids in the palaeozoic basement of the northwestern iberian peninsula include 470 440 ma old suites early middle ordovician now present as orthogneisses and several variscan suites the age data presently available suggest that the latter have been generated in three separate time intervals 330 320 ma ago late visean early namurian 310 300 ma ago westphalian and 290 280 ma ago late stephanian autunian rb sr whole rock and u pb zircon systematics along with field and chemical evidence testify to the generation of the early and late palaeozoic granitoids from lower palaeozoic and precambrian maximum age 1 7 1 6 ga sedimentary sequences respectively the source rocks contained zircons derived from a late archaean early proterozoic continental crust that probably underlies the whole of western europe although for the greater part modified beyond recognition by younger crustal reworking

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

probable anticlockwise p t evolution in extending crust hlinsko region bohemian massif in the hlinsko region variscan bohemian massif czech republic a major extensional shear zone separates low grade metasedimentary series hlinsko schists and high grade rocks of the moldanubian terrane svratka crystalline unit during late variscan extension a tonalite intruded syntectonically into the normal ductile shear zone and caused contact metamorphism of the overlying schists concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists in order to decipher the detailed petrological evolution of the hlinsko unit situated in the hangingwall of this tectonic contact a phase diagram approach and pctrogenetic grids calculated with the thermocalc computer program were used the crystallization deformation relationships and the paragenetic analysis of the hlinsko schists define a p t path with an initial minor increase in pressure followed by cooling calculated pseudosections constrain this anticlockwise p t evolution to the upper part of the andalusite field between 0 36 and 0 40 gpa for temperatures ranging from 570 to 530 c a low uhz0 is required to explain the presence of andalusite biotite bearing assemblages and could be related to the presence of abundant graphite in contrast the footwall rocks of the svratka crystalline unit record decompression from around 0 8 gpa at a relatively constant temperature followed by cooling thus the footwall and the hangingwall units display opposite but convergent p thistories decompression in the footwall rocks is related to a rapid exhumation we propose that the inverse anticlockwise p t path recorded in the hangingwall pelites is related to the rapid extension controlled sedimentation of the overlying flysch series

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

a cavitation seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi brittle field vivero fault nw spain abstract bands of multi phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions indeed they are one of the most typical manifestations of strain localization in high strained polymineralic rocks in the lithosphere especially in the semi brittle field yet many questions remain on their origin and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove particularly for the inter grain mixing of phases here we present a case study of ultramylonite bands developed in a coarse grained granite deformed at conditions near the base of the seismogenic zone i e showing coseismic deformation microstructures overprinted by thermally activated ones we found that in the mylonite to ultramylonite transition two deformation mechanisms coexist promoting inter grain phase mixing through kfs precipitation 1 dislocation accommodated grain boundary sliding in quartz and 2 diffusion accommodated grain boundary sliding in feldspar the model proposed to explain grain mixing is based on a cavitation seal mechanism which is strongly dependent on grain boundary sliding for the opening of transient micro cavities and on the diffusivity of potassium feldspar for their progressive sealing

growth and concentration of fibrous sillimanite related to heterogeneous deformation in k feldspar sillimanite metapelites in foliated k feldspar sillimanite metapelites fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser grained aggregates rich in quartz feldspar and biotite with or without garnet cordierite and residual andalusite many of the folia appear to be limbs of crenulation microfolds the sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non coaxial deformation by grain boundary sliding fibre sliding without appreciable build up of dislocations whereas other minerals are unstable in these zones and so concentrate in lower strain interfolial zones initiation and especially concentration of the sillimanite in folia may be assisted by fluid flow and local base cation leaching whereby minerals unstable in zones of strong non coaxial strain are dissolved and removed from these zones

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

a multi proxy approach to decode the end cretaceous mass extinction mass extinctions generally involve a complex array of interrelated causes and are best evaluated by a multi proxy approach as applied here for the end cretaceous mass extinction this study documents and compares the planktic foraminiferal records carbonate dissolution effects stable isotopes and magnetic susceptibility in france bidart austria gamsbach and tunisia elles in order to explore the environmental conditions during the uppermost maastrichtian plummerita hantkeninoides zone cf1 leading up to the mass extinction planktic foraminiferal assemblages at bidart and gamsbach appear to be more diverse than those at elles with unusually high abundance 20 30 and diversity 15 species of globotruncanids in the two deep water sections but lower abundance 10 and diversity 10 species at the middle shelf elles section oxygen isotopes in zone cf1 of elles record rapid climate warming followed by cooling and a possible return to rapid warming prior to the mass extinction the onset of high stress conditions for planktic foraminifera is observed 50 60 cm below the ktb at bidart and gamsbach and 4 5 m below the ktb at elles due to much higher sediment accumulation rates these intervals at bidart and gamsbach record low magnetic susceptibility and high planktic foraminiferal fragmentation index fi at elles bidart and gamsbach an increased abundance of species with dissolution resistant morphologies is also observed at gamsbach the correlative interval in india records significantly stronger carbonate dissolution effects in intertrappean sediments between the longest lava flows ending with the mass extinction based on current evidence this widespread dissolution event stratigraphically coincides with the climate cooling that follows the late maatrichtian global warming and may be linked to ocean acidification due to deccan volcanism the estimated 12 000 28 000 gigatons gt of co2 and 5200 13 600 gt of so2 introduced into the atmosphere likely triggered the carbonate crisis in the oceans resulting in severe stress for marine calcifiers leading to mass extinction

outline of upper precambrian and lower paleozoic evolution of the iberian peninsula according to u pb dating of zircons in the northwestern part of the iberian peninsula recent geological studies have defined the following stratigraphic sequence from bottom to top basal orthogneiss of miranda do douro is overlain by a complex of schist and greywacke which have been assigned to the cambrian or late precambrian these are followed by the ollo de sapo gneisses of volcanic sedimentary origin which in turn are overlain by the fossiliferous layers of the lower ordovician the schist and greywacke complex and the lower part of the ollo de sapo formation are transected by granitic dikes viana del bollo orthogneiss all these rocks were folded faulted metamorphosed and intruded by granite plutons during variscan times the emplacement of the miranda do douro and viana del bollo orthogneisses have been respectively dated at 618 9 ma and 465 10 ma by the u pb method on zircons these data yield lower and upper limits for the deposition of the ollo de sapo formation and demonstrate the occurrence of a precambrian basement pan african or cadomian orogeny in the variscan belt in the northwestern part of the iberian peninsula the emplacement of viana del bollo orthogneiss could be genetically related to the main phase of crustal thinning 470 520 ma characteristic of the prevariscan evolution of western europe typology and u pb data of the ollo de sapo zircons indicate the occurrence of inherited old precambrian zircons and yield an age of 325 3 ma for the variscan main thermal event in the area considered four possible evolutionary models for the u pb system of the ollo de sapo zircons are discussed

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

structural and metamorphic evidence of local extension along the vivero fault coeval with bulk crustal shortening in the variscan chain nw spain abstract the vivero fault is a w dipping n s striking ductile shear zone separating the ollo de sapo antiform in its western hangingwall and the lugo dome in its eastern footwall two stages of deformation f 1 and f 2 produced nearly coaxial folds with sub horizontal axes a crenulation cleavage s 2 transposes an older s 1 three sets of shear bands in the hangingwall define a pervasive fabric consistent with an e w bulk shortening perpendicular to a composite s 1 2 foliation and nne stretching parallel to l 2 the vivero fault zone is marked by a mylonitic foliation with a steeply nw plunging stretching lineation and extensional crenulation cleavage ecc indicating normal slip in the vicinity of the fault sub horizontal nne trending f 3 folds with a crenulation cleavage s 3 deform earlier formed fabrics including a mylonitic foliation pressure temperature conditions obtained from mineral assemblages on both sides of the vivero fault yield a minimum throw of 5 5 km andalusite bearing pelite in the hangingwall was infolded by an f 2 synform into the kyanite field at 450 500 c the eastern edge of these rocks was later accreted to the footwall and heated to andalusite staurolite conditions at 600 c slip on the vivero and valdovino faults is kinematically related east west shortening during f 2 involved folding and sinistral strike slip on the valdovino fault which induced local extension along the newly generated vivero fault synkinematic emplacement of granitoids along the vivero fault is favoured by extension coeval slip on both faults took place during the later stages of f 2 folding geometrical constraints caused northwards escape of the crustal block bounded by the valdovino and vivero faults recorded by nne stretching defined by l 2

a dynamic model of the structural evolution of the hercynian pyrenees abstract the structural history of the hercynian pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes the migmatization and the intrusions of plutonic and basic and ultrabasic rocks the metamorphism can be shown to have progressed upwards with time attaining the presently exposed levels just before during and a short time after the major penetrative regional deformation the intrusion of major plutonic anatectic and gneissic massifs which has determined the metamorphic domes in country rocks was roughly synchronous with the progression of the metamorphism but seems to have continued later in the uppermost levels the basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation thus showing that their intrusion predated this deformation in the lowermost levels these results together with the presently available sedimentological data lead us to propose a dynamic model of the tectonic evolution of the hercynian pyrenees the first step was a crustal extension and thinning attaining its maximum during frasnian time marked in the upper levels by a horst and graben controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low pressure metamorphism and anatexis characteristic of the hercynian pyrenees the second step was a crustal shortening marked by large scale crustal thrusting followed by widespread regional penetrative deformation the flysch troughs formed in front of the progressing and emerging thrusts in lower levels the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels

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

setting new constrains on the age of crustal scale extensional shear zone vivero fault implications for the evolution of variscan orogeny in the iberian massif the vivero fault is crustal scale extensional shear zone parallel to the variscan orogen in the iberian massif belt with an associated dip slip movement toward the hinterland to constrain the timing of the extension accommodated by this structure we performed zircon u pb la icp ms geochronology in several deformed plutons some of them emplaced syntectonically the different crystallization ages obtained indicate that the fault was active at least between 303 2 and 287 3 ma implying a minimum tectonic activity of 16 5 ma along the fault the onset of the faulting is established to have occurred later than 314 2 ma the geochronological data confirm that the vivero fault postdates the main variscan deformation events in the nw of the iberian massif and that the extension direction of the late carboniferous early permian crustal scale extensional shear zones along the ibero armorican arc was consistently perpendicular to the general arcuate trend of the belt in sw europe

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

thermobarometric constraints on the conditions of metamorphism in the canigou massif pyrenees implications for hercynian geothermal gradients quantitative thermobarometry of garnet biotite sillimanite f cordierite pelitic assemblages from the canigou massif and constraints based on consideration of high grade metapelitic reaction equilibria indicate that peak temperatures during hercynian metamorphism reached 725 25 c at depths of 16 2 km 4 5 0 5 kbar this corresponds to an average peak geothermal gradient of 45 50 c km 1 which is in broad agreement with results from studies of granulite facies assemblages in the pyrenees but is significantly lower than gradients proposed for some other amphibolite facies areas in the terrane it is suggested that the claims by numerous authors for peak hereynian geothermal gradients in the pyrenees in excess of 70 100 c km 1 fail to consider the effects of post metamorphic attenuation of metamorphic sequences and the unreliable nature of p estimates made in the absence of suitable assemblages for quantitative thermobarometry

structural metamorphic and magmatic history of the mondonedo nappe hercynian belt nw spain abstract the mondonedo nappe is one of the main units in the internal zone of the hercynian belt in the iberian peninsula it consists of low to medium grade metasediments late precambrian and palaeozoic in age folded by large d 1 e verging recumbent structures and thrusted several tens of km towards the east during a d 2 deformation episode at the base of the thrust sheet a shear zone of up to 3 km thick was developed granitoid bodies which were intruded after d 1 and before or during d 2 were deformed in this basal zone the geometry of d 1 and d 2 structures and that of the isograds are described as well as the relationships between metamorphism and deformation a structural evolution is inferred which shows an episode of generalized ductile deformation d 1 followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone d 2 and culminated with the thrusting along an important fracture the shear zone originated in a thermally softened level which according to metamorphic data was at temperatures in the range 550 600 c the structural metamorphic and magmatic features of the mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs this could be a consequence of the collision against another continental mass to the west

c o h s fluid composition and oxygen fugacity in graphitic metapelites c 0 h fluid produced by the equilibration of hzo and excess graphite must maintain the atomic h o ratio of water 2 1 this constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric isothermal conditions the o h20 and co fugacities j fhto and fc of such fluids have been estimated from equations of state and fit as a function of pressure and temperature these fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration e g pelitic lithologies because there are no compositional degrees of freedom for graphite saturated fluids produced entirely by dehydration the variance of the dehydration process is not increased in comparison with that in non graphitic systems thus compositional buffering of c 0 h fluids by dehydration equilibria a common petrological model requires that redox reactions decarbonation reactions or external h o 2 fluid sources perturb the evolution of the metamorphic system such perturbations are not likely to be significant in metapelitic environments but their tendency will be to increase the fo of the fluid phase at high metamorphic grades pyrite desulphidation reactions may cause a substantial reduction of fhz0 and slight increases info and fcs relative to sulphur free fluid at low metamorphic grade sulphur solubility in h o s 2 fluids is so low that pyrite decomposition must occur by sulphurconserving reactions that cause iron depletion in silicates a common feature of sulphidic pelites with increasing temperature and sulphur solubility pyrite desulphidation may be driven by dehydration reactions or infiltration of h o rich fluids the absence of magnetite and the assemblages carbonate aluminosilicate or pyrite pyrrhotite ilmenite from most graphitic metapelites is consistent with an h o 2 model for gcoh s fluid for graphitic rocks in which such a model is inapplicable a phase diagram variable that defines the h o ratio of gcoh s fluid is more useful than the conventional

structural controls on metamorphism and syn tectonic magmatism the portuguese hercynian collision belt upper palaeozoic collisional movements along the ibero armorican arc were accommodated on oblique strike slip ductile shear zone systems in north central portugal these shear zones are coincident with long linear belts of high temperature low pressure metamorphism and s type granitic plutons a model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12 15 km this was achieved by a combination of i increased heat flow ii lowering of melting temperatures caused by fluxing of hydrous fluids and iii a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts shear zones are proposed as the most important influence in controlling the site of development of such belts anatexis of metasedimentary crust and emplacement of the resulting s type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source