identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

hornblende bearing leucosome development during syn orogenic crustal extension in the tormes gneiss dome nw iberian massif spain abstract the high grade lower unit of the tormes gneissic dome tgd nw sector of the iberian massif contains tonalitic orthogneissic bodies that were migmatized during a heterogeneous ductile shearing related to a major episode of syn orogenic extensional deformation d2 both peak and retrograde p t conditions were deduced from the analysis of reaction textures related to superimposed s2 fabrics developed during exhumation analysis of mineral zoning and thermobarometric calculations in cakfmash and kfmash systems syn thermal peak partial melting of the tonalitic orthogneisses via a biotite dehydration melting reaction yielding hornblende k feldspar and melt produced stromatic migmatites with leucosomes parallel to the s2 foliation of the mesosome as well as discordant patchy pegmatitic migmatites the deduced post thermal peak p t path comprises an initial phase of decompression combined with cooling from 5 5 6 kbar at 750 c to ca 4 4 5 kbar at 675 c consistent with the sequence of initial retrograde mineral assemblages present in high t mylonitic s2 fabrics the later part of the syn d2 p t path indicates a significant cooling and is recorded by lower amphibolite to greenschist assemblages related to the later mylonitic s2 fabrics the proposed melt reaction generating hornblende bearing leucosomes may be important for crustal differentiation processes at mid crustal levels during syn orogenic extension as well as for the production of certain types of high level granites the necessary conditions are large volumes of source material of intermediate composition and the production of a sufficient melt fraction

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

s type granite generation and emplacement during a regional switch from extensional to contractional deformation central iberian zone iberian autochthonous domain variscan orogeny zircon grains extracted from s type granites of the meda escalhao penedono massif central iberian zone variscan orogen constrain the timing of emplacement and provide information about potential magma sources simple and composite zircon grains from three samples of s type granite were analyzed by la icp ms new u pb data indicate that granites crystallized in the bashkirian 318 7 4 8 ma overlapping the proposed age range of ca 321 317 ma of the nearby s type granitic rocks of the carrazeda de anciaes lamego and ucanha vilar massifs the timing of emplacement of such s type granites seems to coincide with the waning stages of activity of a d2 extensional shear zone i e pinhel shear zone developed in metamorphic conditions that reached partial melting and anatexis ca 321 317 ma dykes of two mica granites resembling diatexite migmatite are concordant and discordant to the compositional layering and s2 main foliation of the high grade metamorphic rocks of the pinhel shear zone much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid state deformation field relationships suggest contemporaneity between the ca 319 317 ma old magmatism of the study area and the switch from late d2 extensional deformation to early d3 contractional deformation inherited zircon cores are well preserved in these late d2 early d3 s type granite plutons u pb ages of inherited zircon cores range from ca 2576 to ca 421 ma the spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the ediacaran to silurian metasedimentary and metaigneous rocks of the iberian autochthonous and parautochthonous domains this is evidence of a genetic relationship between s type granites and the host metamorphic rocks there is no substantial evidence for the addition of mantle derived material in the genesis of these late d2 early d3 s type granitic rocks the end arrays of heterogeneous crustal anatectic melts may be just inherited from the source probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures the generation of the bashkirian s type granites has been dominated by continental crust recycling rather than the addition of new material from mantle sources

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

shrimp u pb zircon geochronology and thermal modeling of multilayer granitoid intrusions implications for the building and thermal evolution of the central system batholith iberian massif spain abstract this work shows the results of a u pb shrimp zircon geochronological study of the central part of the gredos massif spanish central system batholith the studied batholith is composed of several granodiorite and monzogranite tabular bodies around 1 km thick each intruded into partially molten pelitic metasediments granodiorites and monzogranites belonging to three distinct intrusive bodies and samples of anatectic leucogranites have been selected for shrimp u pb zircon geochronology distinct age groups separated by up to 20 ma have been distinguished in each sample important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies 312 6 2 8 ma for the circo de gredos bt granodiorites floor intrusive layer 306 9 1 5 ma for the barbellido plataforma granitoids top intrusive layer and 303 5 2 8 ma for las pozas crd monzogranites middle intrusive layer these age differences are interpreted in terms of sequential emplacement of the three intrusive bodies contemporary with the late paleozoic d3 deformation phase the anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites 305 4 1 6 ma for refugio del rey leucogranites and 303 2 ma for migmatitic hornfelses it is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions thermal modeling with comsol multiphysics reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts around 750 c for several million of years favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions

tracing the cambro ordovician ferrosilicic to calc alkaline magmatic association in iberia by in situ u pb shrimp zircon geochronology gredos massif spanish central system batholith abstract u pb geochronological study of zircons from nodular granites and qtz diorites comprising part of variscan high grade metamorphic complexes in gredos massif spanish central system batholith points out the significant presence of cambro ordovician protoliths among the variscan migmatitic rocks that host the late carboniferous intrusive granitoids indeed the studied zone was affected by two contrasted tectono magmatic episodes carboniferous variscan and cambro ordovician three main characteristics denote a close relation between the cambro ordovician protholiths of the prado de las pozas high grade metamorphic complex strongly reworked during the variscan orogeny and other cambro ordovician igneous domains in the central iberian zone of the iberian massif 1 geochemical features show the ferrosilicic signature of nodular granites they plot very close to the average analysis of the metavolcanic rocks of the ollo de sapo formation iberia qtz diorites present typical calc alkaline signatures and are geochemically similar to intermediate cordilleran granitoids 2 both qtz diorite and nodular granite samples yield a significant population of cambro ordovician ages ranging between 483 and 473 ma and between 487 and 457 ma respectively besides 3 the abundance of zircon inheritance observed on nodular granites matches the significant component of inheritance reported on cambro ordovician metagranites and metavolcanic rocks of central and nw iberia the spatial and temporal coincidence of both peraluminous and intermediate granitoids and specifically in nodular granites and qtz diorite enclaves of the prado de las pozas high grade complex is conducive to a common petrogenetic context for the formation of both magmatic types tectonic and geochemical characteristics describe the activity of a cambro ordovician arc back arc tectonic setting associated with the subduction of the iapetus tornquist ocean and the birth of the rheic ocean the extensional setting is favorable for the generation emplacement and fast rise of subduction related cold diapirs supported by the presence of typical calc alkaline cordilleran granitoids contemporary with ferrosilicic volcanism

continuous deformation ductile thrusting and backfolding of cover and basement in the sierra de guadarrama hercynian orogen of central spain abstract hercynian deformation has affected two different crustal ensembles a crystalline basement and a metasedimentary cover in the sierra de guadarrama the rheological basement is a medium to high grade metamorphic complex composed mostly of granitic orthogneisses with minor amounts of sediments the cover consists of a thick sequence of low to high grade metasediments with minor gneissic bodies ranging in age from late proterozoic to devonian the deformation in the cover was analysed mainly in the eastern part of the sierra de guadarrama whereas the crystalline basement was analysed in the segovia area to the west both cover and basement deformation started with a stage d 1 of continuous penetrative deformation the deformational regime has a strong component of subhorizontal simple shear with east vergence a second phase d 2 of shear deformation gave rise to a broad subhorizontal shear zone and a thrust in the cover and to narrow ductile shear zones in the basement the transition between the two first phases of deformation seems to have been gradual the third phase d 3 is a generalized backfolding episode which induced an intense and penetrative strain in broad areas of both ensembles finally large open folds with steep axial surfaces developed in two mutually perpendicular systems giving rise to dome and basin interference patterns and some subvertical ductile shear zones and faults were also created the structural evolution of the basement and cover points to a thicker skinned model of deformation giving rise to an important crustal thickening a salient feature of the model arose during the second phase when a thick detachment level developed in the lower part of the cover the generalized existence of this level may account for the scarcity of basement outcrops in the spanish hercynian belt

x ray computed micro tomography of spiral garnets a new test of how they form abstract garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with x ray computed tomography for each garnet crystal the orientation of its internal foliation intersection inflexion axes fia was measured its volume and shape orientation the latter via calculation of best fit ellipsoids additionally image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to fia our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples this supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating the fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments our study exemplifies the strength of x ray computed micro tomography compared to previous techniques based on serial thin sections for distinguishing multiple fia sets present in single samples

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

extension in compressional orogenic belts the eastern alps extension in the eastern alps encompasses the following 1 extension accompanying simple shear crustal stacking subhorizontal stretching occurs along flat lying foliations and regionally consistent stretching lineations with associated high strains on a local scale stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust sheet imbrication the overall geometric effect is crustal thickening 2 classical extensional tectonics extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks the overall geometric effect is crustal thinning extension is generally oriented subparallel to the strike of the orogen

provenance signatures recorded in transgressive sandstones of the upper cretaceous iberian seaway abstract maximum mesozoic transgressions flooded many continental areas worldwide forming a wide and shallow seaway on the iberian microplate during the late cretaceous several siliciclastic and mixed siliciclastic carbonate units developed along the iberian massif cratonic area while at the same time epeiric carbonate platforms spanned iberia these siliciclastic deposits unconformably cover the variscan basement as well as the permian and triassic deposits this study focuses on sandstone provenance in central spain in a transgressive relatively quiet tectonic setting with a wide rock variety in the potential source areas results shown in this paper can be applied in analogue systems transgressive sandstones often form important hydrocarbon reservoirs due to their high primary porosity the cretaceous siliciclastic deposits are included in four formations ranging in age from the cenomanian to the early coniacian the deposits were initially of fluvial origin followed by a coastal tidal setting quantitative petrographic analysis of sandstones from these formations reveals compositional variations related to provenance and weathering degree the oldest formation utrillas fm was deposited on a paleorelief surface sandstone composition changes from arkosic sandstones in the depressed area petrofacies p1a to younger quartzarenites when sediments were deposited on top of the paleotopographic high petrofacies p1b quartz grains in both sub petrofacies are angular to subangular and the framework is poorly sorted intense weathering conditions are deduced for petrofacies p1b manifested by grain infiltration clay coatings and the absence of plagioclase grains and alteration features on k feldspars all these observations have been related to wetter conditions during p1b deposition main source areas of the utrillas fm were ordovician metasediments and orthogneisses permian volcanic rocks and triassic buntsandstein facies petrofacies 2 encompasses the three coastal tidal formations constituted mainly by subarkoses generally showing a subangular and poorly sorted framework the deduced source areas are mainly associated with variscan granitoids pre ordovician schists and paragneisses and ordovician metasediments glauconite and carbonate grains support intrabasinal inputs the low degree of sorting of this petrofacies related to a low energy sedimentary environment produced lower values of original porosity compared to transgressive sandstone hydrocarbon reservoirs also intrabasinal inputs promoted carbonate cementation and contribute to almost occlude the porosity

intraplate deformation and basin formation during the tertiary within the northern iberian plate origin and evolution of the almazan basin the tertiary almazan basin was a topographic high during the mesozoic rifting at the northern part of the iberian plate it was bordered by two mesozoic basins 1 the castilian basin with a maximum thickness of more than 2000 m to the south and 2 the cameros basin with a maximum thickness of 9000 m to the north to the east the almazan basin is bordered by paleozoic rocks of the nw se trending aragonian range which underwent little subsidence during the mesozoic the two mesozoic basins show an e w direction during the tertiary nne ssw compression mainly late eocene early miocene both mesozoic basins were inverted and uplifted and the almazan basin with an overall synclinal geometry was filled with syn tectonic terrestrial sediments reaching a maximum thickness of 3500 m deformation during the tertiary was strongly controlled by preexisting structures both mesozoic extensional faults and reactivated variscan folds the depocenter of the tertiary basin migrated southward during the paleogene this migration is consistent with a piggyback transport of the whole basin northward due to the movement on the underlying crustal scale north verging cameros thrust the geometry and sedimentary evolution of the almazan thick skinned piggyback basin was controlled by the uplift associated with the main crustal scale thrust which was the main source area and was separated from the foreland basin since the early stages of filling late eocene this makes the main difference with thin skinned piggyback basins controlled by the foreland thrust belt and separated from the foreland basin only at their late stages

granite emplacement at the termination of a major variscan transcurrent shear zone the late collisional viseu batholith abstract a major event of plutonic activity occurred all across the central iberian zone of the iberian variscan belt at the end of late paleozoic variscan collisional tectonism the present study focuses on the western sector of the viseu late post tectonic batholith central portugal a large composite intrusion comprising three main plutonic units a small bodies of mafic to intermediate composition preferentially concentrated along the northern border b a wide ring of coarse porphyritic biotite monzogranite cota viseu granite and c a more evolved medium porphyritic biotite muscovite monzogranite occupying the central part of the intrusion alcafache granite the compositional zonation pattern of the whole batholith and the complex mixing mingling relationships between the voluminous cota viseu porphyritic granite and the mafic intermediate rocks suggest that these melts were withdrawn from a lower crustal source region undergoing partial melting invasion by mantle derived mafic magmas mixing and fractional crystallization new ca id tims u pb zircon ages indicate that pluton assembly via multipulse injection of successive magma batches took place between 299 4 0 4 ma and 296 0 0 6 ma a detailed anisotropy of magnetic susceptibility ams survey suggests that pluton emplacement occurred at the extensional termination of a regional scale ene wsw trending sinistral d3 shear zone the juzbado penalva shear zone jpsz a dilational opening model involving the development of en echelon tensional gashes at the extensional termination of the fault followed by progressive opening and widening of north south trending fractures provided the space into which the successive magma batches arriving from below were emplaced vertical inflation was accommodated by depression of the pluton floor the proposed model is consistent with the asymmetric wedge shaped geometry of the intrusion steep root zone on the northern side discordant subvertical walls and a shallowing pluton floor towards the south

tectonothermal evolution associated with variscan crustal extension in the tormes gneiss dome nw salamanca iberian massif spain abstract the tectonothermal variscan evolution of the tormes gneiss dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics that induced a quick exhumation of the deep parts of the metamorphic complex during crustal thinning the shearing affected a broad band of metamorphic rocks but as the temperature decreased became progressively concentrated in a low grade ductile detachment that separates two distinct units characterized by their lithology and different tectonothermal evolution kinematic indicators in non coaxial fabrics show a displacement of the hanging wall to the southeast i e parallel to the trend of the foldbelt the subhorizontal shearing is related to the extensional collapse of the variscan crust previously thickened during the collision tectonics

decompressional coronas and symplectites in granulites of the musgrave complex central australia abstract in granulite facies metapelitic rocks in the musgrave complex central australia reaction between s1 garnet and sillimanite involves the development in s2 of both garnet cordierite hercynitic spinel biotite and hercynitic spinel cordierite sillimanite biotite the s2 assemblages occur either in coronas and symplectites mainly around garnet or in rocks in which s2 is more strongly developed as recrystallized assemblages ignoring the presence of biotite and ilmenite the mineral textures can be accounted for qualitatively by a consideration of the model system feo mgo al2o3 sio2 fmas the textural relationships accord with decompression accompanying the change from s1 to s2 however since biotite and ilmenite are involved in the assemblages the parageneses are better accounted for in terms of equilibria in the expanded model system k2o feo mgo al2o3 sio2 h2 tio2 fe2o3 kfmashto i e afm tio2 fe2o3 the coronas reflect the tectonic unroofing of at least part of the musgrave complex from peak s1 conditions of about 8 kbar to s2 conditions of about 4 kbar

p t paths derived from garnet growth zoning in an extensional setting an example from the tormes gneiss dome iberian massif spain the tormes gneiss dome nw iberian massif variscan belt of spain comprises a metamorphic core complex lower unit bounded by a major extensional detachment despite metamorphic temperatures in the upper amphibolite facies 700 740 c metapelites from the highest levels of the lower unit contain garnet with preserved growth zoning these rocks were used for reconstruction of quantitative p t paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning the results are consistent with a two stage tectonothermal evolution under high grade conditions 1 an early compressional phase of deformation that led to upper amphibolite facies barrovian type metamorphism and to p increase and t rise to approximately 9 kbar and 700 725 c 2 a subsequent major extensional phase of deformation that led to quasi isothermal decompression from 8 9 to 3 kbar at t conditions between 700 and 740 c several lines of structural textural and petrological evidence suggest that up to 15 20 km of overburden was removed from the lower unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions a final stage of quasi isobaric cooling to greenschist facies conditions is locally recorded in late low grade detachments

assessing bulk assimilation in cordierite bearing granitoids from the central system batholith spain experimental geochemical and geochronological constraints

time space distribution of silicic plutonism in a gneiss dome of the iberian variscan belt the evora massif ossa morena zone portugal abstract in the iberian variscan belt polyphasic deformation has been recognized as comprising an early phase of crustal thickening followed by an intermediate phase of crustal extension and doming and a later phase of shortening the evora massif is a gneiss dome of the westernmost domains of the ossa morena zone sw iberia which provides a remarkable insight into the late paleozoic deep crustal structure of the variscan continental crust of northern gondwana in this study we bring new structural and geochronological u pb data for the northern hanging wall of the evora massif we describe the existence of low dipping d2 extensional shear zones associated with buchan type metamorphism m2 this enables three tectono metamorphic units to be distinguished the lower gneiss unit the intermediate schist unit and the upper slate unit d2 m2 structures experienced subhorizontal shortening d3 and were transposed by low plunging folding thrusting and strike slip faulting zircon grains extracted from pavia quartz feldspathic gneiss of the lower gneiss unit yielded a crystallization age of ca 521 ma cambrian stage 2 3 which establishes a correlation with tectono metamorphic units of the footwall and southern hanging wall of the evora massif u pb zircon dating of divor foliated quartz diorite 339 7 ma malarranha weakly foliated biotite rich granite 322 3 ma and undeformed porphyritic granite of the pavia pluton 314 4 ma constrain the timing of emplacement of granitic magmas synchronously with doming carboniferous magmatism initiated with doming me1 ca 343 335 ma continued through d2 m2 me2 ca 328 319 ma and lasted until the waning stage of crustal extension me3 ca 317 313 ma the evora massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle derived i e dioritic gabbroic magmas rocks found in the footwall of the evora massif

c32 and c34 hopanoids in recent sediments of european lakes novel intermediates in the early diagenesis of biohopanoids a study of hopanoids in the sediments of five european lakes has identified bacteriohopane 32 33 34 35 tetrol 35 methoxybacteriohopane 32 33 34 triol and two new hopanoids with shorter side chains 35 norbacteriohopane 32 33 34 triol and 33 34 35 trinorbacteriohopane 31 32 diol their structures have been revealed by mass spectrometry polyfunctionalised hopanoids with shortened side chains are not often reported in sediments mainly due to the analytical procedures used their systematic investigation could shed further light upon the early diagenesis of biohopanoids

new u pb ages for early ordovician magmatism in central spain felsic orthogneisses occur widely in the ollo de sapo domain of the central iberian zone most of which were previously considered to represent a precambrian basement to the palaeozoic sequences of this domain however new u pb dating across the berzosa riaza shear zone sierra de guadarrama central spain indicates an early ordovician age for the most representative types of orthogneisses in the eastern part of the ollo de sapo domain dated rocks include the volcaniclastic cardoso gneiss 480 2 ma from the low medium grade hanging wall the riaza gneiss 468 8 16 ma mylonitic granite in the berzosa riaza shear zone three types of leucogneiss buitrago gneiss 488 8 10 ma megacrystic granite 482 11 14 ma aplitic vein and 482 8 9 ma gneissic leucogranite and the la morcuera granitic augen gneiss 477 4 ma in the high grade footwall the new age of the cardoso gneiss brackets to the mid late arenig the so called early ordovician sardic unconformity characteristic of the central iberian zone these new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the sardic events and that these orthogneisses were part of a felsic magmatic belt which extended along the ollo de sapo domain of the central iberian zone this magmatic belt is interpreted to have been active during the early ordovician break up of the peri gondwanan margin of the iapetus ocean

sequential limb rotation and kink band migration recorded by growth strata almazan basin north spain abstract the geometry of growth strata in the almazan monocline demonstrates that limb rotation and kink band migration can coexist as folding mechanisms the almazan basin showing an overall synclinal geometry was one of the most actively subsiding areas during the tertiary in the inner part of the iberian plate with more than 3500 m of preserved tertiary non marine sediments they are arranged in five tectonosedimentary units tsu a1 to a5 bounded by unconformities at the basin borders the southern basin border is defined by the wnw ese trending almazan monocline that can be followed for 25 km along strike with a maximum structural relief of about 2500 m the maximum changes in thickness on both limbs of the monocline occur in units a2 and a3 which constrains the maximum deformation period to between the middle upper eocene headonian top of a1 and upper oligocene lower miocene agenian base of a4 the detailed geometry of the mesozoic pre growth and the tertiary growth strata of the almazan monocline was determined from the analysis of six seismic reflection sections at high angles or perpendicular with the fold axis the average dip of the monocline changes from 15 in the east to more than 45 in the western sector being replaced westwards by a north verging steeply dipping thrust in some sections growth strata in the dipping limb show rotational offlap onlap syntectonic unconformities with progressively decreasing dips from bottom to top indicating that limb rotation took place during the first stages of folding at the limits of the dipping limb anticline and syncline two kink bands appear in some of the sections the one located on the syncline being more developed within each kink band beds show a constant dip of 30 40 the axial surfaces of each kink band converge upwards in the third and fourth tecto sedimentary units being parallel in the older units the fixed axial surface occurs in a down dip position with respect to the active axial surface the geometry of growth strata indicates that the mechanism of folding followed a sequence of i a first stage of large scale limb rotation wavelength larger than 5 km with minor development of kink bands in some sections and ii a second stage of more localised kink band migration probably favoured by fault propagation in the basement

correlating deformation in variscan nw iberia using porphyroblasts implications for the ibero armorican arc abstract 3 d microstructural analysis of 30 porphyroblastic samples from the variscan orogen in nw iberia has revealed four sets of inclusion trails with distinctive geographic trends and relative timing the chronological and orientational consistency of these microstructures provides a reference frame for the correlation of macroscopic structural successions the two younger inclusion trail sets correlate with the main map scale structures of middle to upper carboniferous age whereas the two older sets correspond to mesoscopic structures preserved locally in low strain pods and dating back as far as 365 ma upper devonian inclusion trail orientations appear uninfluenced by a major orocline in the study area ibero armorican arc outlined by late stage folds and thrusts with variable trends it is proposed that the orocline developed by wnw ese trending transpression superposed on originally n s to ne sw trending structures during a late stage of the orogeny upper carboniferous domains of low or zero strain e g porphyroblasts are inferred to have maintained relatively constant orientations in the zone of transpression presumably due to deformation partitioning effects consequently they preserve a succession of pre orocline structural trends the collected microstructural data also indicate moderate preferred vertical and horizontal orientations of inclusion trails in the analysed samples as reported earlier in other variscan regions this probably witnesses a dynamic equilibrium between tectonic forces and gravity during four periods of differently oriented crustal shortening that generated the four recognised sets of inclusion trails

recycled metaigneous crustal sources for s and i type variscan granitoids from the spanish central system batholith constraints from hf isotope zircon composition abstract the spanish central system scs is one of the largest granite batholiths in the european variscan belt zircons from five granitic intrusions from the eastern scs have been separated and analysed for hf isotopes by laser ablation mc icpms in order to evaluate the nature of granitic magma source two samples are cordierite bearing s type granites the rest are amphibole bearing i type monzogranites hf isotope composition of zircons from these granites defines a narrow range of negative hf values 1 1 to 5 8 typical of crustal protoliths the within sample variation of zircon hf isotope composition is slightly above the analytical uncertainty 1 5 units suggesting the mixing of different magma batches during pluton assembly the s and i type granites display the same range of initial hf values that is in agreement with previous data showing that both scs granite types have similar sr nd o pb isotope signatures these isotopic data suggest that they derived from similar sources while the variation in peraluminosity could be related to different partial melting conditions the presence of a lower ordovician inherited zircon population 478 462 ma at least in i type granites suggests the involvement of cambro ordovician orthogneisses as components in the source of some scs granites inherited zircon population of similar age has also been identified in the scs lower crustal granulites the hf isotope composition of the analysed granite zircons is within that of zircons from the above granulites moreover some evidence of a juvenile input at 560 595 ma is recorded by hf isotope data in zircons from both the scs granites and granulite xenoliths hf isotope signature in conjunction with other chemical isotopic sr nd o pb and geochronological evidences suggests that scs granites were probably derived from metaigneous lower crustal sources the variscan magmatism in central spain is dominantly a crustal reworking event

structural relationship between the sea of marmara basin and the north anatolian fault zone the north anatolian fault naf zone is 1500 km long extending almost up to the greek mainland in the west it is a seismically active right lateral strike slip fault that accommodates the relative motion between the turkish block and black sea plate the sea of marmara lies along the western part of the naf and shows evidence of subsidence in this area pure strike slip motion of the fault zone changes into extensional strike slip movement that is responsible for the creation of the sea of marmara and the north aegean basins the northern half of the sea of marmara is interpreted as a large pull apart basin this basin is subdivided into three smaller basins separated by strike slip fault segments of uplifted blocks ne sw basinal areas are covered by horizontally layered sedimentary sequences uplifted blocks have undergone compressional stress all the blocks are subsiding and are undergoing vertical motions and rotations relative to one another the uplifted blocks exhibit positive bouguer gravity anomalies according to gravity interpretation there is relative crustal thinning under the sea of marmara the northern side of the sea of marmara is marked by a distinctive deep rooted magnetic anomaly which is dissected and shifted southward by strike slip faulting the southern shelf areas of the sea of marmara are dominated by short wavelength magnetic anomalies of shallow origin

a seismic geotraverse across the iberian variscides orogenic shortening collisional magmatism and orocline development 1 a crustal geotraverse through the iberian variscides is presented by integrating the available geological and seismic profiling data different modes of orogenic shortening are identified with varying degrees of coupling between upper and lower crust in northern and southern regions of the geotraverse a decoupling in the middle crust permits the lower crust to subduct underthrust thus compensating for strong upper crustal shortening this behavior does not result in great crustal thickening except in sectors of crustal underthrusting in southern central iberia moderate upper crustal shortening is due to a mechanically strong lower crust impeded to subduct underthrust in this region shortening is partitioned between upper and lower crust deformation being distributed in the upper crust while localized at major fault zones in the lower crust finally the central region of the geotraverse northern central iberia shows a coupled crustal deformation having given way to the largest orogenic thickening in the iberian variscides the thermal maturity of this much thickened crust originated voluminous crustal melting and concomitant normal fault detachments developed while shortening dominated in other regions theoretical models suggest that compressive stresses may prevail in the lower crust beneath the extending upper crust thus explaining the efficient syncollisional exhumation in this part of the orogen a particular feature of the iberian variscan geotraverse is the great importance of out of section mass movements mainly left lateral shear zones concentrated in two suture boundaries which displaced to the nw present coordinates central and northern iberia with respect to southern iberia

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

extensional tectonics in the central iberian peninsula during the variscan to alpine transition abstract the passage from the variscan cycle to the early alpine framework in the central part of the iberian peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes 1 1 a first variscan compressional stage vi middle devonian to early carboniferous dominated by compressional conditions leading to the building up of the orogenic edifice the stress regime was relevant to what might be called variscan type compression e w oriented this stage was characterized by major himalayan type tectonics with frontal nappes thrusts overturned folds lateral transcurrent ramps and localized anatectic magmatism minor synorogenic extension and plutonism was also recorded during this stage in the tormes granitic dome 2 2 a second variscan stage v2 early to middle carboniferous was characterized by increasing extensional conditions leading to widespread plutonism adamellites granodiorites wanning compressional conditions were restricted to the eastern and southern realms of central iberia the eastern part of the spanish central system and the toledo mountains 3 3 a third stage here defined as late variscan lv developed from middle carboniferous to early permian as a result of n s late orogenic extension this episode is relevant to detachment tectonics and the gravitational collapse of the variscan orogenic edifice under combined simple pure shear conditions plutonism granites and leucogranites was still of major importance early permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the spanish central system 4 4 a fourth stage here defined as early alpine ea early permian to triassic marks the onset of the alpine framework this stage was characterized by what might be called an early alpine type regional stress regime i e e w extension and n s compression within a simple shear model and resulted in the configuration of the iberian peninsula into two contrasted realms a western inherited variscan block and an eastern alpine block subjected to post orogenic extension elements developed during this event include n s high angle normal faults nw se and ne sw conjugate strike slip faulting and asymmetric rifting involving listric low angle detachments

monazite paragenesis and u pb systematics in rocks of the eastern mojave desert california u s a implications for thermochronometry abstract studies of the paragenesis and u pb systematics of monazite in rocks from the eastern mojave desert california corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance unmetamorphosed latham shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition monazite is absent in the shale and low grade schists but it is abundant in schists at staurolite and higher grades lower grade schists instead include minute th and ce oxides and unidentified ce poor lree phosphates that apparently are lower temperature precursors to monazite thus monazite originates when the pelite passes through lower amphibolite facies conditions monazites from three upper cretaceous granites yield ages that are strongly discordant upper intercepts of 1 6 1 7 ga are similar to those defined by u pb data for coexisting zircons and coincide with a period of copious magmatism in the mojave crust as the host upper cretaceous granitic magmas were all above 700 c effective closure of the restitic monazites to pb loss must be well in excess of this temperature u pb compositions of monazite from proterozoic granitoids and schist also indicate high pb retentivity taken together these studies support the suggestion that monazite can be an effective prograde thermochronometer at least in pelites it is not usually retained as a detrital mineral but rather forms during moderate temperature metamorphism its u pb system should not be reset by subsequent higher grade metamorphism

extensional flow produces recumbent folds in syn orogenic granitoids padron migmatitic dome nw iberian massif this contribution provides a case example on the generation of large scale recumbent folds in syn orogenic granitoids we analyze the progressive reworking of extension related structures into later ones after a period of crustal thickening the padron migmatitic dome formed after the climax of the gondwana laurussia collision in the late paleozoic petrostructural analysis carried out in the eastern flank of this dome reveals that extensional flow resulted in progressive exhumation of mainland gondwana which rested under peri gondwanan allochthonous terranes and a suture zone during maximum crustal thickening exhumation proceeded up to upper crust levels andalusite stability field along with partial melting of the middle lower crust and with the generation of granitoid laccoliths during an early extensional stage newly formed lithological and mechanical anisotropies such as the presence of variably sized sheet shaped bodies of syn orogenic granitoids provided a favorable rheological setting for fold nucleation during the intermediate stages of extension in extending orogenic crust whether recumbent folds occur after significant melt production depends on the lateral vertical flow ratio and on the orientation of deforming bodies with regard to kinematic strain axes we suggest that subhorizontal extensional flow dominated over vertical flow during the early and intermediate stages of the evolution of the padron dome a component of vertical diapiric flow caused progressive tilting of the sheet like bodies and obliquity respect to strain axes this resulted in the development of regional scale folds at the expense of syn orogenic granitoids such as in the case of the portomouro recumbent synform extensional ductile flow was oblique to the trend of the orogen during the whole process and directed to the nnw during the formation of recumbent folds non coaxial shearing favored an nnw sse elongate shape for the syn kinematic granitic massifs as well as the subsequent nucleation of recumbent folds deformation concentrated along discrete detachments during the late stages of extension

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

c o h n fluids in quartz segregations from a major ductile shear zone the berzosa fault spanish central system the berzosa fault is a major ductile shear zone the berzosa shear zone bsz which separates the ollo de sapo anticline from the inner higher grade crystalline axis of the iberian hercynian belt this shear zone is the site of abundant early kinematic quartz al silicates segregations rich in fluid inclusions host rocks are medium grade staurolite schists and sillimanite gneisses fluid inclusions in selected quartz segregations across the berzosa shear zone have been studied by microthermometric methods as well as in some instances by raman analysis the recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late peak metamorphic conditions and lasts until late in the uplift history of the zone three types of inclusions have been found which in a time sequence are co2 h2o h2o salt b type and n2 ch4 three types of b inclusion may be distinguished in turn depending on whether they were trapped during an earlier dynamic recovery phase b1 type formed later as intergranular trails b2 type or were trapped apparently along with n2 ch4 in clusions from a heterogeneous fluid b3 type considerations from isochores confirm that co2 h2o inclusions were trapped during late peak and high t retrograde metamorphic conditions in the range 650 500 c and 5 2 kbar whilst n2 ch4 inclusions along with the b3 type of inclusions formed at low pressures 1 kbar and temperatures 300 c b2 type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone inferred p t paths for the area are convex to the t axis

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

high resolution correlations between gotland and estonia near the base of the wenlock the strata of the ireviken event can be correlated between gotland sweden and the viki core on saaremaa estonia with a high degree of precision most of the ten datum planes are found with a precision of better than 0 1 m in the lower and upper visby beds on gotland and high in the mustjala member of the jaani formation on saaremaa five of the latest llandovery early wenlock units defined by these datum planes are between c 0 12 and 1 77 m thick on gotland and 0 12 and 0 67 m in estonia the duration of each was between a few thousand and c 31 000 years the stratigraphic resolution achieved in the interval of the ireviken event is 10 000 years ultimate range ends are lacking within a preceding slightly longer interval but the pattern of intermittent presence of some taxa agrees well with an unchanged relative rate of sedimentation their coeval presence in different places indicates ecological regional or more widespread changes

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

extensional orogenic collapse captured by strike slip tectonics constraints from structural geology and upb geochronology of the pinhel shear zone variscan orogen iberian massif abstract the late paleozoic collision between gondwana and laurussia resulted in the polyphase deformation and magmatism that characterizes the iberian massif of the variscan orogen in the central iberian zone initial continental thickening d1 folding and thrusting was followed by extensional orogenic collapse d2 responsible for the exhumation of high grade rocks coeval to the emplacement of granitoids this study presents a tectonometamorphic analysis of the trancoso pinhel region central iberian zone to explain the processes in place during the transition from an extension dominated state d2 to a compression dominated one d3 we reveal the existence of low dipping d2 extensional structures later affected by several pulses of subhorizontal shortening each of them typified by upright folds and strike slip shearing d3 d4 and d5 as identified by superimposition of structures the d2 pinhel extensional shear zone separates a low grade domain from an underlying high grade domain and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust crustal thinning decompression melting and magma intrusion progressive lessening of the gravitational disequilibrium carried out by this d2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the pinhel shear zone by strike slip tectonics during renewed crustal shortening high grade domains of the pinhel shear zone were folded together with low grade domains to define the current upright folded structure of the trancoso pinhel region the d3 tamames marofa satao synform new dating of syn orogenic granitoids shrimp u pb zircon dating intruding the pinhel shear zone together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca 331 311 ma with maximum tectonomagmatic activity at ca 321 317 ma the capture and apparent cessation of movement of the pinhel shear zone occurred at ca 317 311 ma

fabric evidence for granodiorite emplacement with extensional shear zones in the variscan gredos massif spanish central system abstract three granitoid bodies in the central part of the gredos massif spanish central system batholith are tabular around 1 km in thickness and intruded into a migmatitic middle crust during the d3 deformation phase of the variscan orogeny petrologically they are composed of bt granodiorite and crd monzogranite and they show varying abundance of large cm scale feldspar megacrysts a detailed study of the shape preferred orientation spo magmatic fabric defined by these megacrysts together with a kinematic analysis of the structures due to interactions between them and the measurement of quartz c axis fabrics in migmatites and granitoids suggests that granitic magma and country rocks were mechanically coupled during deformation the emplacement took place along large scale extensional shear zones active during the first stages of the d3 phase the shape of the spo ellipsoids varies from constrictional at the centre of the granitic bodies to flattening or even oblate at their external contacts with the migmatitic host rocks the favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments followed by shear zone development commonly at the top of the granitic bodies the entire structure was later folded during the last stages of the d3 phase

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

convergence in a thermally softened thick crust variscan intracontinental tectonics in iberian plate rocks the subduction phase in the development of the variscan orogen in sw europe was followed by an extended period of intracontinental tectonics the progressive temperature rise in the hinterland during plate convergence was accompanied by widespread partial melting in the lower crust and the nucleation of kilometric buckle folds and crustal scale shear zones in the stronger upper crust thermal mechanical weakening in the core of the orogen was contemporaneous with shortening and thickening in the foreland fold and thrust belt we evaluate lithospheric strength profiles in the hinterland and foreland based on the metamorphic and structural record for three tectonic stages we find that lower crustal strength varied in space as well as in time during orogenesis strength contrasts between the foreland and the hot hinterland during convergence may have led to the additional indentation of the foreland into the hinterland of the ibero armorican arc

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

crustal density structure in the spanish central system derived from gravity data analysis central spain abstract shallow and deep sources generate a gravity low in the central iberian peninsula long wavelength shallow sources are two continental sedimentary basins the duero and the tajo basins separated by a narrow mountainous chain called the spanish central system to investigate the crustal density structure a multitaper spectral analysis of gravity data was applied to minimise biases due to misleading shallow and deep anomaly sources of similar wavelength first an estimation of gravity anomaly due to cenozoic sedimentary infill was made power spectral analysis indicates two crustal discontinuities at mean depths of 31 1 3 6 and 11 6 0 2 km respectively comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the moho discontinuity a 3d depth model for the moho was obtained by inverse modelling of regional gravity anomalies in the fourier domain the moho depth varies between a mean depth of 31 km and 34 km maximum depth is located in a nw se trough gravity modelling points to lateral density variations in the upper crust the central system structure is described as a crustal block uplifted by ne sw reverse faults the formation of the system involves displacement along an intracrustal detachment in the middle crust this detachment would split into several high angle reverse faults verging both nw and se the direction of transport is northwards the detachment probably being rooted at the moho

presence of palaeoproterozoic and archean components in the granulite facies rocks of central iberia the hf isotopic evidence abstract hf isotope analyses in previously dated zircon crystals from four variscan granulite facies rocks from central iberia have been studied to examine crustal evolution processes two samples are from exposed migmatite terranes the other two are granulite xenoliths from lower crustal levels hf model ages show that lower crustal granulites display a bigger contribution of archean components than exposed high grade rocks 14 of the zircon population instead of 6 reinforcing ideas of preservation of ancient crustal material in the lower crust the distribution of age hf isotope data from zircon suggests two juvenile crust formation events in central iberia the main one during the neoproterozoic 540 620 ma and a possible second mesoproterozoic event around 1 0 1 2 ga magmatism appears in the central iberian zone exclusively related to that younger juvenile addition hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central iberia as a peri gondwanan terrane located between the west african craton and the arabian shield far from the amazonia microplate

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

residence and redistribution of ree y zr th and u during granulite facies metamorphism behaviour of accessory and major phases in peraluminous granulites of central spain accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as ree y zr th and u during crustal melting processes under high grade metamorphic conditions although this is probably the case at middle crustal levels when a comparison is made with granulite facies lower crustal levels differences are seen in trace element behaviour between accessory minerals and some major phases such a comparison can be made in central spain where two granulite facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt rich migmatites are newly crystallized this implies that an important redistribution of trace elements occurs during the early stages of granulite facies metamorphism in addition the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions in granulitic xenoliths from lower crustal levels the situation seems to be different as major minerals show high concentration of certain trace elements the distribution of which is thus controlled by reactions involving final consumption of al ti phlogopite a marked redistribution of hree y zr between garnet and xenotime where present and zircon but also of lree between feldspars k feldspar and plagioclase and monazite is suggested

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

as ag sulphide veins in the spanish central system further evidence for a hydrothermal event of permian age the spanish central system scs has been subjected to repeated deformation and fluid flow events which have produced both sulphide bearing and barren vein systems although several hydrothermal episodes took place between 300 and 100 ma fluid circulation during the permian was especially important giving rise to a range of different types of deposits this study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the as ag mineralised stockwork of mo nica mine bustaviejo madrid fluid inclusion data indicate the presence of two different fluids an initial ore stage i formed from a low to moderate salinity 3 8 wt eq nacl h2o nacl co2 ch4 fluid at minimum trapping temperature of 350 15 oc and 0 3 kbar a second h2o nacl fluid is found in three types of fluid inclusions a high temperature and low salinity type 340 20 oc 0 8 3 1 wt eq nacl also associated to ore stage i a moderate temperature and very low salinity type 160 255 oc 0 1 5 wt eq nacl represented in ore stage iii and a very low temperature and hypersaline type 60 70 oc 30 35 wt nacl unrelated to the mineralising stages and clearly postdating the previous types 40ar 39ar dating on muscovite from the early as fe stage i has provided an age of 286 4 ma synchronous with the late emplacement phases of la cabrera plutonic massif 288 5 ma and with other permian hydrothermal events like sn w skarns and w sn sulphide veins 18o of water in equilibrium with stage i quartz 5 3 7 7x d of water in equilibrium with coexisting muscovite 16 0 to 2 0 and sulphide 34s 1 5 3 6 values are compatible with waters that leached metamorphic rocks the dominant mechanism of the as ag deposition was mixing and dilution processes between aqueous carbonic and aqueous fluids for stage i as fe and nearly isobaric cooling processes for stages ii zn cu sn and iii pb ag the origin and hydrothermal evolution of as ag veins is comparable to other hydrothermal permian events in the spanish central system

variscan syncollisional extension in the iberian massif structural metamorphic and geochronological evidence from the somosierra sector of the sierra de guadarrama central iberian zone spain abstract the rocks affected by the variscan orogeny in the sierra de guadarrama central iberian zone can be separated into two domains a high grade western domain and a medium to low grade eastern domain these domains are separated by the berzosa riaza shear zone a new integrated analysis of structural metamorphic and u pb monazite age data indicates that the berzosa riaza shear zone is a major d2 extensional structure associated with oblique crustal scale extensional shearing and important decompression low p high t metamorphism associated with the d2 event overprints earlier barrovian metamorphic assemblages formed during the variscan d1 compressional event the kilometre scale berzosa riaza shear zone exhibits a s c mylonitic front at its base and a crenulation front at its top with successive fabric development under different p t conditions indicating crustal thinning textural relationships and u pb dating of monazite indicate that the main d2 extensional event took place between 337 2 and 326 3 ma early mid carboniferous a late d2 extensional episode overprinted the major d2 ductile shear zone along narrow low grade oblique normal detachment zones kinematic indicators in both d2 and late d2 fabrics indicate a displacement of the upper structural levels to the southeast field relationships microstructures and qualitatively derived p t t d paths suggest that the early mid carboniferous syncollisional extension in the sierra de guadarrama was associated with the major extensional collapse of the variscan hinterland central iberian zone and was contemporaneous with the earliest thrusting in the foreland

folds with axes parallel to the extension direction an experimental study abstract experimental studies of single and multilayer folding have generally considered shortening of layers oriented perpendicular to the maximum extension direction x i e layers parallel to yz or in a more limited number of cases oblique layers still containing the intermediate y axis few experimental studies have considered the case where the extension direction x lies within the layer itself although in nature folds with axes parallel to x are quite commonly seen these folds have often been ascribed to passive rotation of fold axes during continued shear but it has been shown both theoretically and experimentally that active buckle folds can also develop with axes parallel to x single and multilayer analogue model experiments were performed on planar layers oriented initially perpendicular to the intermediate y axis and with the extension direction x lying within the layer itself all experiments were conducted in plane strain either in pure shear or simple shear paraffin waxes of different melting ranges were used as analogues for rocks with a power law rheology stress exponent around 2 3 with a viscosity ratio of ca 30 1 no measurable fold amplification was discernible for shortening of 36 or shear strain of 3 6 neither domed initial perturbations with circular sections parallel to the layer nor cylindrical perturbations elongate parallel to the initial stretching direction were significantly amplified only at much higher viscosity ratios ca 600 1 did active buckle folding develop this folding at high viscosity ratio was associated with flow of the matrix in the x direction around the layer developing a strong linear fabric parallel to x in the matrix immediately adjacent to the layer the development of this flow discontinuity between matrix and layer may be characteristic of active buckling of layers parallel to xz with fold axes parallel to x

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

crustal scale thrusting and extension in the hercynian schwarzwald and vosges central europe the hercynian outcrop areas in the schwarzwald of sw germany and vosges of easternmost france consist from north to south of three major lithotectonic complexes very low to medium grade metamorphics of the saxothuringian belt the polymetamorphic central gneiss complex and the tectonic melange of the southern complex juxtaposition of these three complexes occurred by oblique convergent motions in early carboniferous time the saxothuringian belt is part of a major se dipping accretionary wedge that consists of lower paleozoic meta sedimentary and metaigneous rocks and superimposed upper devonian to lower carboniferous synorogenic volcanics and elastics the central gneiss complex consists of polymetamorphic paragneisses orthogneisses metabasites and meta ultramafics dipping to the nw the southern complex consists of an imbricated melange of north to nw dipping synorogenic elastics volcanics and blocks of ultramafics and gneissic basement the central gneiss complex cgc experienced pervasive hercynian low pressure high temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins this convergence 340 330 ma produced mylonitic cataclastic shear zones in the cgc the most significant shear zones are the se verging todtnau fault zone between the cgc and the southern complex and the nw verging lubine fault zone between the central gneiss complex and the rocks of the saxothuringian belt these two fault zones enclose a bivergent pop up of the polymetamorphic basement the todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km crustal thickening during early carboniferous convergence led to an extensional phase of crustal reordering between about 330 and 280 ma that was accompanied by the massive rise of posttectonic granitic plutons from middle to high crustal levels simultaneous creation of a structural lamination in the refractory lower crust neglected the main convergent tectonic boundaries it produced a basin and range type lower crust and mantle crust transition in early permian time and involved a major advective input of heat

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

syn collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics the salamanca detachment zone central iberian zone spain the area south of the town of salamanca is a zone of the spanish variscan belt characterized by a first compressional event which gave rise to steep structures followed by the development of an extensional ductile shear zone more than 4 km thick that is described as the salamanca detachment zone the strain associated to the detachment is analyzed using quartz pebbles in conglomerates comparing the measured strain values with theoretical strain paths leads to the conclusion that the deformation was approximately of plane strain type quartz c axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation though coaxial components were probably involved the extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution taken together the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust and that it was syn collisional the movement of the hangingwall unit was to the southeast parallel to the trend of the fold belt and the translation was of the order of one to a few dozens of km though important this is not exceedingly large and allowed the preservation of low grade metamorphic conditions in the hangingwall preventing it from being affected by extensive brittle tectonic processes

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

u pb ages of zircons from meta igneous and meta sedimentary rocks of the sierra de guadarrama implications for the central iberian crustal evolution u pb data of zircons from various gneisses of the eastern sierra de guadarrama central spain make it possible to reconstruct the pre hercynian and hercynian history of this area 1 upper intercept ages 2400 ma for meta igneous and 2000 ma for meta sedimentary rocks of discordias defined by data of anhedral zircon fractions document the existence of early proterozoic crust 2 lower intercept minimum ages of anhedral zircon fractions show radiogenic lead loss at about 540 ma due to a thermal event leading to volcanism and emplacement of granitoid rocks into precambrian crust growth of euhedral zircons is possibly related to this event 3 lower intercept minimum ages of about 380 ma defined by anhedral zircon fractions in meta sedimentary rocks prove an early hercynian event according to present knowledge on the metamorphism in the sierra de guadarrama this event could be explained in terms of a barrow type medium pressure metamorphism the inferred cadomian igneous event relates the geological history of central spain with that of western africa to the south and brittany to the north furthermore similarities of the crustal evolution in the area studied and other internal zones of the hercynian belt moldanubian zone french central massiv are confirmed the early hercynian event for the first time affected all the rocks of the area together the pre hercynian evolution of the two complexes is different and the present association of the basement rocks may be explained by thrusting or in terms of early hercynian nappe transport

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

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

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin