collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

interpretation of porphyroblast inclusion trails limitations imposed by growth kinetics and strain rates porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock however there remains considerable controversy over whether porphyroblasts rotate during bulk non coaxial deformation with reference to an area of the scandinavian caledonides and utilizing existing data from theoretical and experimental modelling this study demonstrates that both straight and s shaped inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to newtonian simple shear at crustal strain rates of 10 14 s 1 and porphyroblast growth times of 0 1 1 0 ma it is shown that a maximum of 5d 9d angular rotation would occur during growth at faster strain rates of 10 12 s 1 e g those in a shear zone porphyroblast angular rotations of 90d are shown to occur in 0 1 0 25 ma i e times comparable with or faster than porphyroblastesis in view of this s shaped inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain whereas straight inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates

control of granite emplacement by regional deformation abstract a combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings the deepest zones revealed by gravity data provided those zones also show vertical lineations in outcrop are interpreted as magma feeder channels these structures or root zones reflect the flow pattern of the upwelling magma while being emplaced they can be interpreted within the regional stress field active during that time granites intruded during transcurrent shearing have only a few feeder channels these roots are not connected to the main shear zone all of them lie within locally extensional areas of the regional stress field this situation is also observed in granites emplaced during a compressional phase of deformation when emplaced during a regionally extensional phase the granitic plutons are very thin with many small deeper zones in which variously evolved granite types may be intrusive these regions vertically differentiated are also interpreted as feeder channels the different morphology at depth as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons the root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust if the pluton is emplaced higher in a brittle crust then the roots are aligned with the maximum compression according to the andersonian theory of fracture the orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma in all the examples the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

genesis of a variscan batholith field petrological and mineralogical evidence from the corsica sardinia batholith abstract the genesis of the corsica sardinia batholith csb can be explained in a continental collision setting during the time of construction of the batholith which is dated at about 70 ma three magmatic associations were emplaced mg k calc alkaline u1 composite mafic silicic calc alkaline u2 and post collisional alkaline u3 in addition the level of intrusion became progressively shallower from 5 6 kbar for u1 to less than ikbar for u3 in a large scale uplift associations u1 and u2 exhibit non cogenetic mafic and felsic components the processes and products of interaction between mantle and crustal derived melts are however different and may be related to the thickness of the crust which controls both the parameters t pco2 ph2o andptot and the time of mingling of the products of anatexis and basaltic melts both u1 and u2 granitoids were formed by crystallization of a magma derived by fusion of a source of greywacke composition the melting occurred successively under granulitic u1 and amphibolite facies conditions u2 in association u1 basaltic melts displaying potassic lamprophyre like composition up to 8 k2o are of uncertain affinity furthermore crustal interaction with the basic magma precludes precise definition of the source in the u2 composite association the primary tholeiitic character of the mantle derived basic rocks is constrained by both mineralogical and geochemical data dry and hydrated subunits are distinguished among the basic rocks associated with the granitoids no hybridization processes have been identified in the core of the mafic complexes of the dry subunits but they are recognized at the level of emplacement at the margins at the contact with the granitoids contamination in the mafic hydrated subunit is due to the increase in water content of crustal origin in the magma during the ascent and emplacement of the basic complexes leading to the development of abundant green amphibole extensive hybridization is restricted to the base of the crust where it led to incomplete mixing yielding granodiorites and enclaves the genesis and emplacement of the composite calc alkaline u2 association took place in an extensional setting in an uplifting basement postdating a collisional event during the devonian an eastern austro alpine block was thrust onto the western ebro balearic continent the u1 mg k granitoids resulted from anatexis under conditions of pco2 ph2o this took place at the base of the austro alpine crust which was undergoing fusion due to its adiabatic ascent the u2 calc alkaline granitoid rocks were derived in a hydrated environment by fusion of the same austro alpine crust in corsica and of the ebro balearic crust on sardinia and in the pyrenees the conditions of interaction between mafic and felsic magmas may serve as a geodynamic marker providing information on the thickness of the crust in time and space and on the genesis of continental crust the persistence during the palaeozoic of dominantly tholeiitic mafic magmatism suggests that hot spot activity occurred in this area of southwestern europe during pre variscan and variscan times up to the time of opening of the liguro piedmontese ocean birthplace of the alpine orogen

microstructural features of albite porphyroblasts as indicators of sequential barrovian metamorphic mineral growth in the caledonides of the sw scottish highlands inclusion porphyroblast and porphyroblast porphyroblast relationships show that abundant albite in mica schists in the caledonides of the sw scottish highlands are part of the barrovian metamorphic assemblage growth early in the d2 deformational phase of porphyroblast cores followed the growth of mn rich garnet but preceded the growth of porphyroblasts of the index mineral almandine two sets of inclusion trails in the albite correspond to the regionally expressed s1 and s2 straight trails of muscovite chlorite quartz epidote and the earliest growth of biotite make up s1 crenulated trails express deformation of s1 early in d2 with muscovite chlorite biotite quartz epidote and the mn rich garnet associated with the development of s2 crenulation cleavage the geometries of these trails uniquely record early stages of d2 deformational history an 0 3 growth is related to the temporal coincidence of the formation of s1 s2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component the main characteristics of the regionally expressed d2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth essentially inclusion free an 4 19 rims grew on the inclusion containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite biotite and chlorite indicate that mineral growth extended from the later stages of d2 to post d2 previous interpretations of the albite porphyroblast growth having been during d4 to post d4 contemporaneous with retrogression are inconsistent with the microstructural evidence

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

inverted metamorphic zonation and variscan thrust tectonics in the rouergue area massif central france p t t record from mineral to regional scale

inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations can it be reliably done abstract the timing of porphyroblast inclusion trails can be confidently interpreted relative to surrounding external foliations only where there is continuity between the two where this continuity is broken timing is ambiguous where single or multiple growths of two or more different porphyroblastic minerals have occurred during a relatively complex deformation history the risk of misinterpreting the relative timing of porphyroblast growth is high and can lead to wrong inferences about pressure temperature time deformation p t t d paths misinterpreting porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions the effects of porphyroblast rotation vs non rotation relative to an externally fixed reference frame on inferred p t t d paths are poorly understood however the d part of the path can differ considerably depending on whether or not porphyroblasts are inferred to have rotated the effect on the p t t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred sequence of porphyroblast growth

from thickening to extension in the variscan belt kinematic evidence from sardinia italy the variscan nappe stack of se sardinia originated as a result of several stages of nappe imbrication during the lower carboniferous phases of the variscan orogeny the crustal shortening caused regional ssw and w directed thrusting greenschist facies metamorphism and open to isoclinal polyphase folding the final stage of shortening produced large scale antiforms and synforms post collisional deformation resulted in inversion of earlier thrusts as normal faults development of low angle normal faults and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms suggesting that they cannot be regarded as parasitic folds of the latest thickening phase but instead are the consequence of vertical shortening during gravitational collapse of dome like km scale antiforms leading to denudation of antiformal culminations

mylonite development in the hercynian basement of sardinia italy abstract plastic deformation predominates over large areas in the hercynian basement of sardinia during lower carboniferous continental shortening mylonitization associated with ubiquitous top to south southwest thrusting decreases from internal central sardinia to foreland areas southern sardinia in the barbagia unit mylonites completely obliterate all the previous structures in the meana sardo unit plastic deformation strongly overprints early folds and in the gerrei unit mylonites are found only below the meana sardo thrust a thick shear zone the baccu locci mylonite zone develops between the gerrei and riu gruppa units inferences from deformation mechanisms in quartz mylonites suggest a temperature during deformation below 400 c along thrusts throughout the study area overprinting relationships between foliations deformation of annealed microstructures and porphyroblasts with inclusion patterns indicate that crystal growth occurred after early thrusting events and before the main nappe emplacement phase microstructural investigations in the baccu locci mylonites record two changes in the main deformation mechanism operating during progressive mylonitization in the early stages of deformation most of the strain is accommodated by dislocation creep in a fine grained quartz matrix at higher strain dynamic recrystallization fully affects larger quartz crystals producing pure quartz layers where plastic deformation and strain localize ongoing deformation together with syntectonic breakdown of feldspar producing mica leads to mineral changes grain size reduction and reaction softening with strain localization in the fine matrix again dynamically deformed quartz is boudinaged in the later stages of mylonitization significant fluid infiltration during deformation can account for some of the large ore bodies hosted in the baccu locci mylonites

kinematics of orogenic collapse in the variscan pyrenees deduced from microstructures in porphyroblastic rocks from the lys caillaouas massif in the pyrenees a phase of bulk vertical shortening d3 is indicated by a horizontal crenulation cleavage s3 which is developed in a pre existing schistosity s2 porphyroblasts of the lys caillaouas massif regionally preserve 1 1 the initial pre d3 orientation of s2 in the form of a vertical and e w trending preferred orientation of their inclusion trails 2 2 a vertical l2 stretching lineation defined by the elongation of the inclusions these data are consistent with n s compression and crustal thickening during d2 the angle between the inclusion trails s2 i and the external schistosity s2 e records the subsequent rotation of s2 from vertical to gently dipping orientations at higher crustal levels s2 remained steep suprastructure and little d3 deformation is evidenced with depth d3 deformation progressively intensifies and zones can be distinguished with opposite dip directions of s2 opposite apparent porphyroblast rotation senses and opposite f3 crenulation asymmetries these micro structures on a n s section through the lys caillaouas and the garonne dome massifs record a regional partitioning of bulk coaxial vertical shortening this deformation partitioning was probably controlled by a not exposed gneiss dome which behaved relatively rigid as compared to its metasedimentary cover in zones of non coaxial d3 deformation s2 was extended reactivated and forms the dominant schistosity where d3 was locally more coaxial s2 was shortened isoclinally crenulated and s3 forms the macroscopic cleavage the fact that d3 extension in the lys caillaouas massif immediately followed d2 compression and that similar extension is recorded in large parts of the variscan foldbelt suggests gravitational collapse of a thickened orogen this is consistent with the flattening type of d3 strain recognized in a number of pyrenean variscan massifs peak high t low p metamorphic conditions coincide with the onset of orogenic extension when very little crustal thinning could have been accomplished yet this suggests that heat induced strain softening within the orogen may have triggered its collapse

an outline of shear sense analysis in high grade rocks ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved however high grade shear zones are common and dominate in many precambrian terrains high grade shear zones should be analysed in a different way from low grade zones the plane on which shear sense markers should be observed the vorticity profile plane is more difficult to find than in low grade shear zones the most reliable shear sense markers in high grade shear zones are shear bands mineral fish mantled porphyroclasts sigmoids and asymmetric boudins 2006 published by elsevier b v on behalf of international association for gondwana research

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

emplacement of the arzachena pluton corsica sardinia batholith and the geodynamics of incoming pangaea abstract the assembly of the corsica sardinia batholith c sb coincides with final shaping of the variscan belt and represents a key structure to unravel the feedbacks between partial melting rheology and the evolution of collisional orogens this paper presents a model for the genesis of the arzachena pluton azn one of the major calc alkaline massifs of the c sb based on u pb zircon dating thermobarometry and structural analysis major and trace element compositions indicate that azn has hybrid characteristics between that of typical s and i type granites that could be explained in terms of incremental melting of a heterogeneous crustal source made of metatexites and ordovician calc alkaline granitoids growth of the pluton started around 320 315 ma with the emplacement at middle crustal level 0 37 0 4 gpa of granodioritic melts within narrow conjugate nw se sinistral and e w dextral shear zones the main growth stage 311 6 4 ma is marked by emplacement of large volumes of monzogranitic melts that induced a local decrease of the crustal strength expressed by horizontal channel flow driven by the gravity finally 307 6 3 5 ma leucogranites emplaced within radial and peripheral dilatant fractures developed during the cooling of the main body the transition from magmatic to sub magmatic and ht solid state fabric observed throughout azn indicates that deformation plays a non trivial role during the growth of the magmatic system restoring the position of the corsica sardinia block to early permian coordinates allow to recast the birth of the c sb in a consistent geodynamic framework that conciliates the development of conjugates strike slip structures the oroclinal bending of the chain and the thermal relaxation this study indicates that the c sb had an active role during post orogenic extension rather than being just a consequence of it

extensional tectonics and gravitational collapse in an ordovician passive margin the western argentine precordillera abstract the paper describes ubiquitous extensional structures developed in a passive margin of ordovician age in the argentine precordillera these extensional structures include normal faults and boudinaged sequences in some places the boudinage reaches very high extension values giving rise to block in matrix formations most of these extensional structures developed when sediments were not well lithified as recorded by hydroplastic fractures slump folds and pinch and swell structures the presence of slump folds coeval with the extensional deformation the variable extension directions obtained from the kinematic analysis and a weak cleavage recording layer perpendicular shortening support the interpretation that gravitational collapse related to submarine sliding was the cause for extensional deformation well consolidated rocks located at the lower part of the stratigraphic sequence also display scarce extensional faults these extensional faults predate folding because they were breached by flexural slip faults and as a result of their passive rotation in fold limbs these initial normal faults may now appear as reverse faults particularly in steep and overturned limbs the truncation of extensional faults by flexural slip faults produces typical bed thickness changes across the extensional faults giving rise to apparent synsedimentary faults these normal faults can be attributed to the crustal extension that generated the passive continental margin or may represent deep parts of faults related to gravitational collapse

metamorphism and microstructures along a high temperature metamorphic field gradient the north eastern boundary of the kralovsky hvozd unit bohemian massif czech republic a metamorphic field gradient has been investigated in the moldanubian zone of the central european variscides encompassing from base to the top a staurolite kyanite zone a muscovite sillimanite zone a k feldspar sillimanite zone and a k feldspar cordierite zone respectively the observed reaction textures in the anatectic metapsammopelites of the higher grade zones are fully compatible with experimental data and petrogenetic grids that are based on fluid absent melting reactions from structural and microstructural observations it can be concluded that the boundary between the kyanite staurolite zone and the muscovite and k feldspar sillimanite zones coincides with an important switch in deformation mechanism s besides minor syn anatectic shearing melt enhanced deformation microstructural criteria point a to a switch in deformation mechanism from rotation recrystallization climb accommodated dislocation creep to prism slip and high temperature fast grain boundary migration in quartz b to the activity of diffusion creep in quartz feldspar layers and c to accommodation of strain by intense shearing in fibrolite biotite layers it is suggested that any combination of these deformation mechanisms will profoundly affect the rheological characteristics of high grade metamorphic rocks and significantly lower rock strength hence the boundary between these zones marks a major rheological barrier in the investigated cross section and probably also in other low to medium pressure high temperature areas at still higher metamorphic grades k feldspar cordierite zone where the rheologically critical melt percentage is reached rock rheology is mainly governed by the melt and other deformation mechanisms are of minor importance in the study area the switch in deformation mechanism s is responsible for large scale strain partitioning and concentration of deformation within the higher temperature hanging wall during top to the s thrusting thus preserving a more complete petrostructural record within the rocks of the footwall including indications for a devonian high to medium pressure medium temperature metamorphic event thrusting is accompanied by diapiric ascent of diatexites of the k feldspar cordierite zone and infolding of the footwall suggesting local crustal overturn in this part of the moldanubian zone

architecture of late orogenic plutons in the aracuai ribeira fold belt southeast brazil abstract post collisional to late orogenic magmatism 580 to 480 ma in the aracuai ribeira fold belt se brazil is characterized by the predominance of high k metaluminous allanite titanite bearing granitoids small lenses of coronitic gabbro anorthosite pyroxenite and phlogopite peridotite are also common in deeper exposed areas of this fold belt in the region of southern espirito santo state the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions a tendency to funnel shaped bodies with sub vertical hemi ellipsoidal conical roots changing upwards to shallow dipping tops associated stocks sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes local ductile shear zones and parallel to fold hinges the contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e g santa angelica venda nova mimoso do sul and varzea alegre but at shallow levels e g castelo pedra azul and conceicao de muqui agmatic stoping zones occur along the borders a magmatic foliation within the granitoids is usually well marked but the schistosity in the surrounding gneisses wraps around the plutons the intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores gabbro diorite to tonalite surrounded by a mingled marble cake zone where basic and acid rocks are interfingered and an external zone of syenomonzonite and granite widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and or syenomonzonitic compositions is characteristic for all intrusive complexes we suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust the mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis the bimodal plutons result from interaction among the contrasting magmas ascent and emplacement followed older regional structures such as regional fold hinges and ductile shear zones

change of nappe transport direction during the variscan collisional evolution of central southern sardinia italy abstract the collisional variscan early carboniferous evolution of the greenschist facies metamorphic basement of sardinia is characterized by three deformation phases and two 90 reorientations of shortening direction during the first phase of deformation n s shortening occurred with south facing isoclinal folding south directed nappe emplacement and mylonitization in central and southeastern sardinia and large scale e w striking upright folds in southwestern sardinia emplacement of the barbagia meana sardo gerrei and riu gruppa units took place during this event this deformation phase was followed by e w shortening with w directed emplacement of the sarrabus and arburese units along the villasalto thrust and upright to e dipping large scale n s striking upright folds in southwestern sardinia during the third deformation event the shortening direction again rotated 90 and n s shortening produced kilometer scale antiforms and synforms the most prominent features in the metamorphic basement of sardinia the limbs of the kilometer scale antiforms and synforms are vertically shortened during gravitational collapse with contemporaneous faulting and conjugate shear zone development presence of large scale e w oriented antiforms was therefore critical for the formation of late variscan extensional features such as e w striking low angle normal faulting

the variscan post collisional volcanism in late carboniferous permian sequences of ligurian alps southern alps and sardinia italy a synthesis abstract in the considered wide sector of the west mediterranean southern europe the collisional phase of the variscan orogeny during late carboniferous and permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental alluvial to lacustrine basins originated by wrench to normal fault systems the first volcanic cycle generally late carboniferous early permian in age is represented by early calc alkaline andesites and rhyolites in variable amounts and by following large volume of rhyolites and by dacites both andesites and rhyolites show k normal and high k calc alkaline character the origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle crust interface telescoped within a thickened crust the melting is considered as the consequence of thermal re equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt the ascent of liquids occurred in a trans tensional regime the second magmatic cycle is represented by alkaline magmatism and exhibits typical anorogenic features consistent with a rifting regime this event was no more related with the collapse of the variscan belt but rather to the post variscan global re organization of plates that evolved during late triassic times to the neo tethyan rifting in both cycles important differences in timing areal distribution and outpoured volumes arise

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

thermobarometry and granite genesis the hercynian low p high t velay anatectic dome french massif central the velay dome french massif central offers a quasi continuous section across an anatectic domain comprising low to high grade schists gneisses and granites two main tectonometamorphic events and their related generation of granitic material were recognized in addition to a major barrrovian tangential event d2 attributed to intracontinental collision tectonics i a medium to low p high t event d3 which gave rise to migmatites and syntectonic monzonitic granites and granodiorites and ii a widespread melting event d4 which led to the generation of migmatities the velay granite and post anatectic granites thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms i a first associated with the d3 event characterized by p 5 kbar t 750 c and water present melting biotite remains stable which led to large scale migmatization but minor amount of granites ii a second associated with the d4 event and characterized by vapour absent melting p 4 5 kbar t 760 850 c which gave rise to the velay granites and late migmatitic granites the temperature increase during the d4 event is attributed to the intrusion of hot mafic magmas within the crust the time integrated features of the different granitic rocks in the velay dome can be directly related to ah2o in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 ma

diamictite and oolitic ironstones a sedimentary association at ordovician silurian transition in the north gondwana margin new evidence from the inner nappe of sardinia variscides italy abstract the contact between the silurian black phyllite and the cambro ordovician underlying rocks has been investigated over different tectonic units affected by green schist facies metamorphism in the inner nappe zone of the sardinia variscides in spite of strain and metamorphism the field work highlighted the occurrence of diamictic sediments in the canaglia tectonic unit the diamictite consists of dark massive metamorphic claystone bearing chamositic ooliths chamositic nodules and millimetre to centimetre sized clasts dispersed or gathered in clusters within the muddy matrix in the argentiera tectonic unit the diamictite consists of angular clasts ranging in size from few millimetres to several decimetres scattered within a finely laminated black sericitic meta argillite field data textural and compositional analyses suggest a glacio marine environment for the formation of the diamictites the canaglia diamictite deposited in a protected glacial influenced shore compositionally it can be defined as ironstone in the upper part it hosts a horizon of clast supported conglomeratic hard ironstone mostly made of magnetite which testifies for sub aerial reworking the source of the iron is to be related to local glacio eustatic driven emergence of upper ordovician alkaline mafic volcanics these are widespread in the uppermost ordovician of the canaglia unit possibly linked to the rifting stage that invested the north gondwana margin before the uppermost ordovician early silurian sea level rise the argentiera diamictite deposited beyond the iron rich diamictite in the outer euxinic shelf that was reached by rain out of rafted debris

on the emplacement of tabular granites granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement in both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally with the exception of very high level epizonal plutons structural studies suggest that space for many tabular intrusions must be provided mainly by floor depression lopolith emplacement rather than roof lifting laccolith emplacement an emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits a crustal scale balance in the rates of melt extraction magma ascent and pluton filling is required by the model and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude the process is evaluated with end member cantilever and piston sinking mechanisms the models predict that large 10 100 km wide tabular plutons 3 km thick can be emplaced quickly 100 a to 1 ma with floor depression and related wall rock strain rates similar those expected during tectonic deformation 10 10 to 10 15 s 1 bulk strains in the intervening crustal column rarely exceed a strain ratio of 1 5 which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton

late orogenic collapse and thermal doming in the northern gondwana margin incorporated in the variscan chain a case study from the ozieri metamorphic complex northern sardinia italy abstract a branch of the south european variscan chain is noticeably exposed in sardinia the early stage of collision between the northern gondwana margin and the armorica terrane assemblage ata generated syn metamorphic folding and thrusting the evidences of such deformation are well preserved in the nappe zone a structural domain characterized by stacking of different tectonic units under metamorphism of barrovian greenschist facies a late post nappe shortening under retrograde metamorphic conditions gave rise to wide upright n120 n160 trending antiforms that control the trend of the chain the structural analysis of the ozieri metamorphic complex omc shows evidence of an important phase of late variscan extensional tectonics deformation results in the formation of oppositely dipping normal shear zones which developed at upper and middle structural level along the limbs of major regional antiforms causing fabric reactivation crustal thinning and exhumation of the omc core within the omc the activity of the shear zones was coeval with ht lp metamorphism as suggests the occurrence of syn kinematic cordierite andalusite sillimanite biotite whereas syntectonic dykes and a tonalite body in the deeper part of the omc indicate that early emplacement of melt along shear zones and or in the antiform hinges possibly supplied the heat for the anomalous thermal gradient and triggered the exhumation of a core complex like structure

energy growth in laharic mass flows lahars debris flows and sediment rich floods are frequent and deadly hazards at all mountain forming volcanoes their hazard potential is traditionally assessed through mass conserving closed system models where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks this belies evidence of extremely high energy and deadly catastrophes caused by such flows at large distances from volcanoes here we use the first high resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass flow structure we show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path we also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front

the cooma metamorphic complex a low p high t lpht regional aureole beneath the murrumbidgee batholith the cooma complex of the lachlan fold belt south eastern australia is characterised by a large c 10 km wide low p high t metamorphic aureole surrounding a small 3 6 km granite pluton the aureole extends northward to envelop the eastern lobe of the murrumbidgee batholith and progressively narrows to a kilometre wide hornfelsic aureole some 50 km north of cooma at its northern extremity the batholith has intruded its own volcanic cover these regional relations suggest that the murrumbidgee batholith is gently tilted to the north with the cooma complex representing the aureole beneath the batholith two main deformation events d3 and d5 affected the aureole the inner high grade migmatitic domain contains upright f5 folds defined by a composite transposed s3 s0 fabric and s3 s0 concordant leucosomes the folded stromatic migmatites define the western limb of a f5 synform with its axis located in the batholith lenses and sheets of the murrumbidgee batholith intruded along s3 but also preserve s3 as a strong solid state foliation s3 and the granite sheets but are also folded by f5 outlining a fanning positive flower structure these relations indicate that most of the batholith was emplaced before and during d3 but intrusion persisted until early syn d5 formation of the cooma granodiorite occurred post d3 to early syn d5 after formation of the wide metamorphic aureole during early syn d3 to early syn d5 the murrumbidgee batholith was emplaced between pre d3 to early syn d5 synchronous with the formation of the cooma complex the structural and metamorphic relations indicate that the murrumbidgee batholith was the ultimate heat source responsible for the cooma metamorphic complex d3 structures and metamorphic isograds are subparallel to the batholith margin for over 50 km this concordance probably extends vertically suggesting that the isograds also fan outward from the batholith margin this implies an inverted metamorphic sequence focused on the murrumbidgee batholith although the base has been almost completely removed by erosion in the cooma complex the field evidence at cooma combined with previous thermal modelling results suggest that extensive lpht metamorphic terranes may represent regional metamorphic aureoles developed beneath high level granitic batholiths

the early palaeozoic break up of northern gondwana sedimentology physical volcanology and geochemistry of a submarine volcanic complex in the bavarian facies association saxothuringian basin germany abstract ordovician volcano sedimentary successions of the bavarian facies association in the saxothuringian basin record the continental rift phase of the separation of the saxothuringian terrane from gondwana an 80 m succession from the vogtendorf beds and randschiefer series arenig middle ordovician exposed along the northern margin of the munchberg gneiss massif in northeast bavaria were subjected to a study of their sedimentology physical volcanology and geochemistry the randschiefer series previously has been interpreted as lavas tuffs sandstones and turbidites but the studied ordovician units include four main lithological associations mature sandstones and slates pillowed alkali basalts and derivative mass flow deposits trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites eight lithofacies have been distinguished based on relict sedimentary structures and textures which indicate deposition on a continental shelf below wave base the explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills and was succeeded by the eruption of pillowed basalts debris flow deposits overlie the basalts ordovician volcanism in this region therefore alternated between effusive and explosive phases of submarine intermediate to mafic volcanism based on geochemical data the volcanic and pyroclastic rocks are classified as basalts and trachyandesites according to their geochemical characteristics especially to their variable concentrations of incompatible elements such as the high field strength elements hfse they can be divided into three groups group i which is formed by massive lavas at the base of the succession has extraordinarily high contents of hfse the magmas of this group were probably derived from a mantle source in the garnet stability field by low ca 1 degrees of partial melting and subsequent fractionation group ii which comprises the pillow lavas at the top of the sequence displays moderate enrichment of hfse this can be explained by a slightly higher degree of melting ca 1 6 for the primary magma group i and ii melts fractionated from their parental magmas in different magma chambers the eruption centres of groups i and ii therefore cannot be the same and the volcanic rocks must have originated from different vents the sills and pyroclastic flow deposits of group iii stem at least partly from the same source as group i rocks of group i most likely mixed together with group ii components during the formation of the group iii flows which became hybridised during eruption transportation and emplacement the sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism analogous to modern continental rift zones hence the rift associated volcanic activity preserved in the vogtendorf beds and randschiefer series represents an early ordovician stage of rift volcanism when the separation of the saxothuringian terrane from gondwana had just commenced

temperature time relationships across metamorphic zones evidence from porphyroblast matrix relationships in progressively deformed metapelites abstract in a rock pile undergoing prograde regional metamorphism the isotherms corresponding to certain metamorphic reactions shift continuously while the same rock volume develops progressive deformation fabrics as a result of increasing strain porphyroblasts in pelitic schists can provide crucial information on both these aspects of metamorphism not only do they reflect the pressure temperature evolution of that terrain but their inclusion trails are clues to the state of deformation at the time of porphyroblast growth microstructural relationships of inclusion trail patterns versus matrix foliations combined with petrological data on prograde reaction sequences can be used to reconstruct temperature time relationships in different zones within metamorphic terrains in two proterozoic areas in northern queensland the western mary kathleen fold belt and the robertson river subgroup a systematic variation of inclusion trail patterns has been found across metamorphic zones each porphyroblast species contains more advanced stages of s 2 foliation development towards its isograd compared with higher grade zones in which these porphyroblasts are present the combined microstructural petrographic observations allow to place constraints on the t t evolution of the various zones the regional distribution of inclusion trail patterns in the two study areas indicates that deformation affected high and low grade zones simultaneously

late orogenic carboniferous extensions in the variscan french massif central the variscan french massif central experienced two successive stages of extension from middle carboniferous to early permian in the northern massif central the first stage began in the late visean immediately after nappe stacking and is well recorded by namurian westphalian synkinematic plutonism the middle carboniferous leucogranites widespread in the nw massif central limousin and sioule area were emplaced within a crust extending along a ne sw direction at the same time the hanging wall or gueret extensional allochton moved toward the se several examples of the synextensional plutonism are also recognized in central limousin saint mathieu dome la porcherie and cornil leucogranites these examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by nw se stretching and ne sw shortening in the central and southern massif central cevennes ch taigneraie and margeride areas plutonism is dominantly granodioritic and exhibits the same structural features nw se maximum stretching and overturning to the se middle carboniferous namurian westphalian extension was parallel to the variscan belt both in the massif central and southern armorican area this extensional regime was active from the late visean in the north while compression dominated in the southernmost domains montagne noire and pyrenees the second extensional stage occurred from late carboniferous to early permian this event was responsible for the opening of intramontane coal basins brittle deformation in the upper crust and ductile normal faulting localized on the margin of cordierite granite migmatite domes data from the coal basins show that the half graben is the dominant structural style except for basins located along submeridianal left lateral faults which have pull apart geometries late carboniferous extension occurred along the ne sw direction the ne sw maximum stretching direction can be found in the whole massif central but is more developed in the eastern part the extensional direction is transverse to the general trend of the belt and top to the ne shearing is dominant correlations of these two extension directions with neighboring variscan massifs are discussed

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

hercynian tectonometamorphic evolution of the bosost dome french spanish central pyrenees metamorphic and structural studies show that the bosost dome central pyrenees is a metamorphic as well as a structural dome developed in cambro ordovician to devonian metasediments around a migmatitic core during the hercynian orogeny the structural metamorphic dome occurs in epizonal terranes and is twice as large as originally described the structural dome shows a concentric zoning of d2 and d3 local deformations developed around the migmatites and superimposed on the d1 regional hercynian deformation the structural data suggest that migmatites were emplaced diapirically into palaeozoic metasediments during d1 and are responsible for the local d2 and d3 deformations an early medium pressure high temperature metamorphism occurred during d1 leading to anatexis under water rich conditions then evolved progressively to a high temperature low pressure metamorphism during the d2 and d3 deformations the corresponding p t time path is a clockwise loop representing the evolution of the metamorphism from migmatiza tion to the diapiric emplacement of migmatites in a compressional setting pressure values obtained from geobarometry are three times as high as expected from sedimentological data this suggests that the hercynian metamorphism developed in a thickened crust during the n s shortening responsible for the large scale e w folding observed throughout the pyrenean axial zone

the velay dome french massif central melt generation and granite emplacement during orogenic evolution this paper is a synthesis of available data on the velay dome that include both small and large scale lithologic and structural mapping strain analysis isotope geochemistry geochronology and pressure temperature estimates the velay dome one of the largest granite migmatite domes of the variscan belt formed during orogenic collapse at around 300 ma its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the variscan orogenic crust a first melting stage developed in connection with south verging thrust zones during the early carboniferous leading to a crustal thickening estimated at 20 km minimum the involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water saturated melts the volume of biotite granite extracted from melt during this period was limited the second phase of melting corresponded to generalized melting of gneiss achieved by biotite dehydration melting reactions and accompanied by the generation of cordierite bearing granites at this stage crustal scale detachment faults were activeandpartiallyobliteratedtheearlierstructures thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe velay dome due to the southward and lateral ballooning of the granitic dome the reconstructed p t path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting at the base of the crust this melting event is coeval with granulite facies metamorphism associated to underplating of mantle derived magmas as suggested by the geochemical signature of late paleozoic lower crustal xenoliths sampled by cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions accordingly itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals d 2001 published by elsevier science b v

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