pre alpine variscan inheritance a key for the location of the future valaisan basin western alps the boundary between the helvetic and the penninic brianconnais zones has long been recognized as a major fault penninic front in the western alps a narrow oceanic domain has been postulated at least along part of this boundary the valaisan ocean however the information provided by the pre triassic basement has not been fully exploited and will be discussed here in detail the igneous and metamorphic history of the pre triassic basement shows significant differences between the external massifs from the helvetic zone with abundant late carboniferous granites and the basement of the brianconnais zone including the internal massifs dora maira gran paradiso monte rosa devoid of carboniferous granites a major coal bearing basin the zone houillere opened along this boundary this limnic intramontane basin has never been properly investigated the zone houillere is not comparable with the external paralic flexural basins on both sides of the variscan belt but shows similarities with the saar saale basin like the latter we interpret the zone houillere as a transtensional basin opened along a major crustal scale fault zone namely the east variscan shear zone the permian magmatism and sedimentation displays contrasting distributions being absent or very localized in the helvetic zone and widespread in the penninic zone the above data indicate that the structural inheritance from the variscan belt plays a major role in defining the future location of the valaisan basin i e the boundary between the european palaeomargin and the brianconnais microcontinent

multistage garnet in high pressure metasediments alpine overgrowths on variscan detrital grains garnet is commonly found in metamorphic and magmatic rocks where its growth records the pressure temperature history of the rock petrologists pay particular attention to multistage garnets because they document the superposition of successive metamorphic stages and or cycles in orogenic belts due to its considerable resistance to mechanical and chemical weathering garnet is also commonly present as detrital grains in modern and ancient sediments therefore some multistage garnets could potentially result from overgrowths on detrital grains we report the first discovery of this type of metamorphic overgrowth on detrital garnet grains in a high pressure metasediment money unit western alps the studied garnet crystals are found in metamorphic rocks derived from matrix supported conglomerates garnet cores display morphological characteristics of the detrital grains and testify that crystals have been corroded as a consequence of abrasion and dissolution furthermore they display a range of compositions incompatible with their growth in a single rock type implying their provenance from diverse sources garnet overgrowths developed on detrital grains during a blueschist facies event as indicated by chloritoid and rutile inclusions

from permo triassic lithospheric thinning to jurassic rifting at the adriatic margin petrological and geochronological record in valtournenche western italian alps abstract slices of polycyclic metasediments marbles and meta cherts are tectonically amalgamated with the polydeformed basement of the dent blanche tectonic system along a major alpine shear zone in the western alps becca di sale area valtournenche valley a combination of techniques structural analysis at various scales metamorphic petrology geochronology and trace element geochemistry was applied to determine the age and composition of accessory phases titanite allanite and zircon and their relation to major minerals the results are used to reconstruct the polyphase structural and metamorphic histories comprising both pre alpine and alpine cycles the pre alpine evolution is associated with low pressure high temperature metamorphism related to permo triassic lithospheric thinning in meta cherts microtextural relations indicate coeval growth of allanite and garnet during this stage at 300 ma textures of zircon also indicate crystallisation at ht conditions ages scatter from 263 to 294 ma with a major cluster of data at 276 ma in impure marble u pb analyses of titanite domains with variable al and f contents yield apparent 206pb 238u dates range from permian to jurassic chemical and isotopic data suggest that titanite formed at permian times and was then affected by extension related fluid circulation during the triassic and jurassic which redistributed major elements al and f and partially opened the u pb system the alpine cycle lead to early blueschist facies assemblages which were partly overprinted under greenschist facies conditions the strong alpine compressional overprint disrupted the pre alpine structural imprint and or reactivated earlier structures the pre alpine metamorphic record preserved in these slices of metasediments reflects the onset of the permo triassic lithospheric extension to jurassic rifting

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

combining compositional zoning and foliation intersection axes fias in garnet to quantitatively determine early p t t paths in multiply deformed and metamorphosed schists north central massachusetts usa quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite staurolite garnet grade rocks from the northeastern flank of the pelham dome north central massachusetts distinguishes the effects of acadian deformation and metamorphism from extensive overprinting alleghanian shearing the p t conditions and the metamorphic path during the acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts fias this acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an alleghanian overprint for each sample where garnet first nucleated during one of these stages in the tectonic history the pt of core growth was determined by plotting the intersection of the mn fe and ca isopleths calculated for the core composition on a p t pseudosection for that sample using thermocalc combining the pt data from all these samples indicates that the temperature and pressure increased throughout acadian orogenesis causing episodic garnet growth during the alleghanian locally intense shearing especially against the margin of the pelham dome formed the dominant schistosity which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of acadian deformation and metamorphism in the rock matrix shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700 c in those rocks lying directly adjacent to the pelham dome and resulted from the rocks of the northfield syncline being thrust a large distance southwards over the gneisses in the dome

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

superheavy s isotopes from glacier associated sediments of the neoproterozoic of south china oceanic anoxia or sulfate limitation black shales and mn carbonates interbedded with glacial deposits from the neo proterozoic of southern china exhibit extremely heavy values of pyrite s isotopes that may reflect the peculiar environment of earth at this time 34 s averages 30 at tanganshan and 44 at xiangtan compared with typical values of 0 to 5 found in younger deposits furthermore there is no distinction between the shales and the mn carbonate ores in the neoproterozoic unlike the younger deposits which show much lighter 34 s in the shales than in the mn ores the spread is 25 most other chemical parameters are very similar to both the younger mn deposits and those from the paleoproterozoic the exception is the rare earth elements ree all neoproterozoic fe ores and most neoproterozoic mn ores lack the positive eu anomaly that characterizes archean and paleoproterozoic fe mn accumulations on the other hand neoproterozoic mn deposits have positive ce anomalies on north american shale composite nasc normalized plots in contrast to other mnco 3 ores the ree is also higher than in other mn deposits but lower than in modern deep sea crusts sulfide s values in all neoproterozoic shales tend to be exceptionally variable and to often show much heavier values than can be found in marine strata from the phanerozoic therefore the anomalous 34 s values we observed reflect peculiar conditions in the world oceans at this time rather than purely local effects times of enrichment of seawater sulfate in 34 s do not correspond to periods of glaciation so the likely cause of the s isotope patterns is not worldwide glaciation but a generally low level of dissolved sulfate s in the neoproterozoic oceans that allowed modest increases in the amounts of s removed as pyrite to drive down the oceanic s reservoir enough to produce strong rayleigh reservoir effects the abundance worldwide of sturtian age mn and fe deposits indicates an increase in fe flux to the oceans that would have been sufficient to depress so 4 2 levels severely and to result in residu al dissolved s extremely enriched in 34 s ree evidence indicates that most of this enhanced fe and mn flux came from diagenetic remobilization of detrital oxides

the rheology of faults triggered by the olivine spinel transformation in mg2geo4 and its implications for the mechanism of deep focus earthquakes the relevance of the anticrack faulting mechanism to the origin of deep focus earthquakes depends critically on the physical processes operating during fault generation and movement we show here that the resistance to sliding on fault zones produced by this mechanism in mg2geo4 depends only weakly on confining pressure but strongly on sliding rate both of these characteristics are inconsistent with friction the microstructures of the fault zones and the crystal plastic rheology of the constituent phases indicate that sliding must occur primarily by grain boundary sliding previously we reported that anticrack faulting is accompanied by elastic radiation of energy acoustic emissions and that it can operate during the transformation in mg fe 2sio4 at the pressures and temperatures at which deep earthquakes occur the present results and the recent demonstration that metastable olivine is present in the subducting slab beneath japan provide additional support for the anticrack theory of deep focus earthquakes

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

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

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

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field a synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3 enstatite was deformed at high temperature and high pressure such that the olivine phase boundary was traversed both residual and new phases their grain sizes their deformation induced lattice defects and their compositions were then characterized by analytical electron microscopy owing to large heterogeneities in temperature pressure and deformation conditions contrasting microstructures were observed some areas consist essentially of untransformed olivine while the polymorph is the main constituent of other areas crystals of the polymorph are generally free of dislocations except at the grain boundaries of the larger grains where slip bands were nucleated probably induced by stress concentrations the activated glide systems are 021 100 the orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component compared to olivine crystals of the phase contain significantly higher concentrations of fe and trivalent cations all crystals are free of inclusions confirming the conclusion from previous infrared spectroscopic analysis that the high oh concentration in this specimen is dissolved in the phase coupled with these previously measured oh concentrations our new measurements suggest that incorporation of hydrogen into the phase and substitution of trivalent cations into the octahedral sites may be coupled such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

variscan relicts in alpine high p pelitic rocks from samos greece evidence from multi stage garnet and its included minerals porphyroblastic garnet schists from northern samos contain in their matrix the assemblage ca rich garnet phengite paragonite chloritoid equilibrated at 530 c and 19 kbar during early tertiary metamorphism these high pressure low temperature hp lt metapelitic rocks also exhibit mineralogical and microstructural evidence of an older higher temperature metamorphism large centimetre sized fe rich garnet showing growth zoning developed discontinuous 0 5 mm thick ca rich and mn poor overgrowths compositionally matching small 1 mm high p matrix garnet because the discontinuous garnet rims are in textural and chemical equilibrium with alpine high p minerals the central parts of the garnet porphyroblasts were found to have formed prior to the tertiary metamorphism this is supported by electron microprobe u th pb dating of monazite inclusions yielding partly reset variscan ages between 360 and 160 ma monazite xenotime and garnet muscovite thermometry applied to inclusions in the pre alpine garnet yielded temperatures of 600 625 c at 3 8 kbar prismatic al rich pseudomorphs possibly after kyanite sillimanite and inclusions in garnet composed of white k na mica quartz albite k feldspar interpreted as possible replacements of an intermediate k na feldspar further support variscan amphibolite facies conditions the samos metapelites thus experienced higher temperatures during the variscan than during alpine metamorphism diffusional relaxation was very limited between pre alpine garnet and alpine garnet both were filled with alpine garnet along overgrowths and fractures fluid mediated intergranular element transport enhanced by deformation appears crucial in transforming the variscan garnet into a grossular richer composition during alpine subduction zone metamorphism at such conditions dissolution reprecipitation appears to be a much more effective mechanism for modifying garnet compositions than diffusion amphibolite facies conditions are typical for variscan basement relics exposed in central cycladic and dodecanese islands as well as in eastern crete the samos metapelites studied comprise a north eastern extension of these basement occurrences

detrital garnet and chromian spinel chemistry of permian clastics in the renge area central japan implications for the paleogeography of the east asian continental margin abstract the japanese islands comprise a complex assembly of allochthonous continental shelf deposits and subduction related accretionary complexes the paleogeography of permian clastic strata was restored based on chemical analysis of detrital garnets and chromian spinels which are useful for provenance discrimination detrital garnet assemblages from the permian clastic rocks in japan are characterized by a grandite rich assemblage and a grandite poor assemblage most trench fill deposits in permian accretionary complexes contain the grandite poor assemblage which is composed of spessartine rich almandine grossular rich almandine and rarely pyrope rich almandine and is commonly associated with chromian spinels and volcanic rock fragments it is suggested that the sediments were derived from a volcanic arc and associated non calcareous contact metamorphic rocks along the north china block where pelitic granulites including pyrope rich almandine garnets are extensively exposed in the northern margin ophiolite and high p t schist also crop out along the continental margin most permian shelf deposits and the trench fill deposits of the central part of the accretionary complexes in japan contain the grandite rich assemblage analyses suggest provenance from a calc alkaline volcanic arc and association with calcareous contact metamorphic rocks including skarn deposits sediments lack ophiolite detritus a part of the grandite poor trench fill deposits of the accretionary complexes is overlain by grandite rich clastic rocks this indicates that the grandite rich detritus increased and the supply extended through the fore arc basin to the trench the trench fill deposits in the northernmost part of the permian accretionary complex in the renge area yield a grandite poor assemblage particularly uvarovite rich andradite garnets and chromian spinels derived from a back arc basin basalt which is very similar to the clastic composition of the permian back arc basin strata of japan hence we conclude that a back arc basin existed in the northernmost region of the permian subduction zone along the north china block

tracing two orogenic cycles in one eclogite sample by lu hf garnet chronometry the adula nappe central alps was thought to be a melange of rock fragments some of which were subducted and then exhumed from the mantle lu hf dating of two populations of garnets in one rock sample instead indicates that the nappe twice experienced subduction and exhumation as one coherent unit