the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

new insights into the mechanics of sill emplacement provided by field observations of the njardvik sill northeast iceland sills are concordant sheet like bodies of magma their mechanics of emplacement is an important but still not fully understood topic the well exposed basaltic njardvik sill in the extinct tertiary dyrfjoll volcano in northeast iceland offers exceptionally clear insights into the mechanism of sill emplacement the sill is multiple and consists of at least 7 units sills all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows each sill unit was supplied with magma from an inclined sheet the contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets since the njardvik sill consists of separate units it obviously did not evolve into a homogeneous magma body nevertheless the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections the local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills the njardvik sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill using these field observations as a basis a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile along which the sill emplacement takes place the results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties there the orientation of the maximum principal compressive stress 1 is horizontal hence such contacts can represent interfaces along which sill emplacement is encouraged once a sill has been emplaced it extends the stress field with a horizontal orientation of 1 consequently inclined sheets and dykes injected near the sill will be deflected into sills the injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma in case of the njardvik sill the injection frequency was low so subsequently emplaced sill units can be distinguished

basaltic magma reservoirs factors controlling their rupture characteristics and evolution the manner in which magma enters and leaves a magma reservoir is a fundamental aspect of magmatic activity a quantitative model is developed to evaluate the variation of stress acting on the wall of a magma reservoir as a function of depth factors assessed include the size of the reservoir its depth of burial the strength of the country rocks and the variation with depth of the densities of the magma and country rocks it is shown that small magma reservoirs with halfheights less than 1 km centered at levels of neutral buoyancy can grow in all directions with nearly equal ease by injecting dikes into their surroundings at the sites of wall ruptures in larger reservoirs with half heights greater than 2 km however the stress required to cause wall failure is much less at the depth corresponding to the center of the reservoir than it is near the top or bottom thus larger reservoirs are likely to grow predominantly sideways by lateral dike injection in these cases vertical growth will not be important unless significant accumulation of low density material can occur in the upper part of the reservoir the formation of a low density layer due to gas exsolution or chemical differentiation of the magma can facilitate failure of the roof and upward dike migration from a mature reservoir as the reservoir grows and evolves through time the increasing magma pressure head at the depth corresponding to the center of the reservoir means that the amount of gas exsolution or chemical differentiation required to allow the occurrence of vertical dike emplacement also increases ultimately a vertical reservoir size is reached beyond which the excess stress required to cause lateral dike injection is so much less than that required to cause vertical injection that essentially no further vertical growth of the reservoir will occur the reservoir will thus develop a more laterally elongate shape with time the final aspect ratio will then depend mainly on the supply rate of magma to the reservoir from the mantle

deciphering the sardic ordovician and variscan deformations in the eastern pyrenees sw europe detailed geological mapping of the la cerdanya area canigo unit eastern pyrenees provides new data characterizing the different structural styles exhibited by cambrian lower ordovician jujols group and upper ordovician successions their unconformable contact related to the sardic phase ranges from 0 paraconformity to 90 angular discordance the jujols group rocks topped by the unconformity are affected by sardic foliation free open folds the pre sardic succession the sardic unconformity and the lower part of the post sardic succession rabassa conglomerate and cava formations are cut and offset by several late ordovician nne ssw trending synsedimentary extensional faults associated with hydrothermal activity which dramatically affected the thickness of the lower part of the upper ordovician succession we relate 1 the mid ordovician sardic uplift and erosion and 2 a mid to late ordovician upward propagating extensional fault system bounding the outline of half grabens subsequently infilled by alluvial deposits to a thermal doming event about 475 450 ma that led to the uplift and stretching of the ordovician lithosphere thermal doming may be caused by mafic magma underplating and responsible for the coeval calc alkaline magmatic activity broadly developed in the eastern pyrenees we discuss the similarities between the mid ordovician sardic unconformity and other early paleozoic unconformities described in neighbouring areas finally we suggest a geodynamic scenario in which a regional scale thermal event was related to the opening of the rheic ocean that led to the drift of avalonia from the sw european margin of gondwana

mechanism of crystal redistribution in a sheet like magma body constraints from the nosappumisaki and other shoshonite intrusions in the nemuro peninsula northern japan processes of crystal separation in a magma heavily laden with crystals without phase change are investigated from observations on frozen magma systems nosappumisaki and other shoshonite intrusions in the nemuro peninsula japan for which the origin of the crystals and the initial conditions are well constrained the nosappumisaki intrusion is 120 m in thickness and extends for more than 1 5 km it exhibits a wide range of lithological variation principally as a result of crystal redistribution after intrusion crystals in each lithology can be clearly divided into two kinds according to their composition and texture those present before the intrusion of the magma phenocrysts and those that crystallized in situ after intrusion from the vertical change in mode and size of phenocrysts it is shown that 1 augite phenocrysts were rapidly deposited with little overgrowth after intrusion by significant coagulation or clustering on a time scale of more than a few years and 2 plagioclase phenocrysts definitely denser than the melt but concentrated in the upper level floated by counter flow of massive deposition of augite phenocrysts these results indicate that in a magma heavily laden with crystals of a few millimeters in size 20 vol crystal crystal and crystal melt interaction play an important role in the separation of crystals from the host melt

post granulite facies monazite growth and rejuvenation during permian to lower jurassic thermal and fluid events in the ivrea zone southern alps u pb analyses of single monazite grains from two granulite facies metapelites in the ivrea zone southern alps reveal the presence in both samples of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted one group of monazite data defines a subconcordant discordia line with an upper intercept age of 293 4 5 8 ma and a lower intercept age of 210 14 ma the upper intercept is interpreted as the real cooling age of the monazites the lower intercept is interpreted as an episode of fluid driven pb loss indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 12 ma another group of monazite data lies above the concordia the presence of excess 206pb indicates that these crystals have grown below the monazite blocking temperature thus after the granulite facies metamorphism the age of growth of the new monazite crystals is approached by their 207pb 235u ages that range between 273 and 244 ma the two groups of post cooling age post 293 4 5 8 ma monazite data correspond to two distinct late and post variscan geotectonic regimes that affected the southern alps 1 permian transtension with decompression and anatectic melting 2 upper triassic to lower jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism

structure and u pb dating of the saint arnac pluton and the ansignan charnockite agly massif a cross section from the upper to the middle crust of the variscan eastern pyrenees in the agly massif pyrenees two variscan plutons the saint arnac pluton and the ansignan charnockite intrude different levels of a c 10 km thick crustal section the saint arnac pluton intrudes through upper crustal rocks and the ansignan charnockite cuts mid crustal country rocks a structural study of these plutons based on the anisotropy of magnetic susceptibility technique combined with a kinematic study of the country rocks shows that the structures and emplacement modes of the plutons are compatible with those of the other plutons of the pyrenees emplaced during the d 2 transpressive phase u pb dating on zircons from the saint arnac pluton yields a 308 3 1 2 ma age for a diorite and a 303 6 4 7 ma age for a granodiorite the charnockite was previously dated at 315 ma the emplacement ages of these two intrusions are thus separated by at least 5 ma first numerous sills and laccoliths such as the ansignan laccolith were injected in the middle crust and this induced heating and thickening subsequently the large saint arnac pluton was injected in the upper crust at the beginning of formation of a gneissic dome

durbachites vaugnerites a geodynamic marker in the central european variscan orogen durbachites vaugnerites are k mg rich magmatic rocks derived from an enriched mantle source observed throughout the european variscan basement their present day geographical distribution does not reveal any obvious plate tectonic context published geochronological data show that most durbachites vaugnerites formed around 335 340 ma plotted in a visean plate tectonic reconstruction the occurrences of durbachites vaugnerites are concentrated in a hotspot like cluster in the galatian superterrane featuring a distinctive regional magmatic province reviewing the existing local studies on variscan durbachite vaugnerite rocks we interpret their extensive appearance in the visean in terms of two factors i long term mantle enrichment above early variscan subduction systems and ii melting of this enriched subcontinental mantle source during the variscan collision stage due to thermal anomalies below the galatian superterrane possibly created by slab windows and and or the sinking of the subducted rheic slab into the mantle the tectonic reorganization of europe in the late palaeozoic and during the alpine orogeny has torn apart and blurred this marked domain of durbachites vaugnerites

african provenance for the metasediments and metaigneous rocks of the cyclades aegean sea greece u pb geochronology on detrital and inherited zircons from metasediments as well as gneissic and metaigneous basement rocks of the cyclades aegean sea greece defines several periods of crustal growth prior to the carboniferous these ages are consistent with cycladic rocks being derived from the northern margin of gondwana there is a distinct mesoproterozoic age gap evident in zircons from the 25 samples considered this is characteristic of the west african craton and we suggest it will also prove characteristic of the aegean region this mesoproterozoic age gap may be used as a tool in terrane reconstruction to distinguish crustal material derived from different parts of gondwana

paleomagnetism and 40ar 39ar geochronology of gabbro sills at mariscal mountain anticline southern big bend national park texas implications for the timing of laramide tectonism and vertical axis rotations in the southern cordilleran orogenic belt mariscal mountain anticline is a major laramide asymmetric fold located along the frontal margin of the cordilleran orogenic belt in the trans pecos region of southwest texas exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement based largely on their arcuate outcrop geometry has been interpreted to predate folding paleomagnetic fold tests on samples from large 30 m thick and thin l m thick gabbro sills are negative at the 99 confidence level baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro whereas sample directions from a site well removed from thermal effects of the gabbro are scattered results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization the negative fold test indicates that sill intrusion at mariscal mountain anticline postdated laramide folding two 40ar 39ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37 0 1 3 ma 2 this date is similar to those from rocks of similar composition elsewhere in trans pecos texas but significantly younger than whole rock and pyroxene k ar dates of late cretaceous and early eocene age commonly cited for the mariscal mountain gabbro the paleomagnetic and geochronologic data indicate that laramide deformation had clearly ceased by late eocene early oligocene time consistent with absence of contractional deformation in middle eocene to lower oligocene volcanic and sedimentary rocks elsewhere in the region although paleomagnetic data from the gabbro sills may not adequately average secular variation the in situ group mean direction d 348 6 i 50 9 k 65 1 95 4 3 n 18 sites is indistinguishable from expected mid tertiary reference directions this indicates no evidence of significant vertical axis rotation in the mariscal mountain area analysis of paleomagnetic data from studies elsewhere in the big bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with basin and range extension is equivocal

geochronology and geochemistry of the pola de allande granitoids northern spain their bearing on the cadomian avalonian evolution of northwest iberia the pola de allande pre variscan tonalite granodiorite plutons are located in the narcea antiform at the boundary zone between the cantabrian and west asturian leonese zones of the iberian variscan belt these granitoids were intruded into a neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by variscan thrust related shear deformation u pb laser ablation inductively coupled plasma mass spectrometry icp ms dating of zircons from two plutons yielded concordant ages of intrusion of 605 10 and 580 15 ma to the authors knowledge this is the first reported u pb cadomian avalonian age for igneous rocks in this section of the iberian variscan belt these intrusions are coeval with the main episode of dominantly calc alkaline magmatic activity related to cadomian avalonian subduction major and trace element composition of the granitoids is characteristic of i type high k calc alkaline granitoids generated in continental arc se

emplacement of a xenolith rich sill lajitas texas abstract a 2 15 m thick hawaiite sill in the cretaceous boquillas formation west of the big bend national park texas locally contains a zone 1 12 m thick containing 25 vol xenoliths the inclusions ranging in size from 2 cm to 1 m are dominated by cumulate gabbros and white quartzite the mafic inclusions broke from the sidewall cumulates in a reservoir that fed the hawaiite sill many xenoliths show contacts between quartzite and igneous rock marked by clinopyroxene crystals perpendicular to the contacts the quartzite xenoliths are permeated variably by quenched mafic liquid now devitrified glass with plagioclase clinopyroxene smectite and some quartz grains are fringed by inverted tridymite stratigraphic reconstruction indicates that the sill was emplaced at a depth shallower than 0 5 km but the source of the paleozoic quartzite and sidewall cumulate xenoliths was at least 1 km below the sill the high concentration of xenoliths their lack of imbrication orientation or sorting the preservation of unbroken thin slabs of limestone and the absence of xenoliths at the top and bottom of the sill all support emplacement by plug flow of xenolith charged magma in the center lubricated by less viscous xenolith free magma at the margins crystal size distribution of plagioclase indicates that mean crystal residence time for cumulates was about 75 years and crystallization time at the aphyric upper margin of the sill was about 2 years

u pb age and lead isotopic characterization of au bearing skarn related to the andorra granite central pyrenees spain auriferous skarns are associated with post and late kinematic hercynian granites that intruded into cambro ordovician to devonian sediments of the central pyrenees we determined the age of the andorra granite and the associated skarn at 305 3 ma by u pb dating titanite from the endo skarn the sulfur isotopic composition from sulfides in the skarn cardellach et al 1992 shows a significant variation with isotopically light sulfur 34s 3 in the barren skarns and heavy sulfur 34s 11 in the gold bearing skarns outwards it increasingly resembles sulfur from arsenopyrite disseminations in the cambro ordovician sediments the lead isotopic composition from sulfides of the skarns is very homogeneous 206pb 204pb 18 410 207pb 204pb 15 699 208pb 204pb 38 574 in contrast to the one in gold bearing arsenopyrite veins and in arsenopyrite disseminations in the sediments e g 206pb 204pb varies from 18 54 to 30 36 combined sulfur and lead isotope data indicate that the lead in the skarn is dominantly derived from the granite whereas the sulfur is derived at variable portions from both the granite and the sediments

polyphase wrench tectonics in the southern french massif central kinematic inferences from pre and syntectonic granitoids in the variscan french massif central the south limousin area consists of low to medium grade metamorphic rocks intruded by two granitic bodies the structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area this study shows that the south limousin area experienced successivelly two strike slip events along two geographically distinct shear zones from north to south the left lateral estivaux and the right lateral south limousin strike slip faults respectively these ductile faults subdivide the south limousin into three structural units from north to south they are the upper gneiss unit thiviers payzac unit and genis unit the two granitic bodies intrude the thiviers payzac unit only the younger estivaux granite is a syntectonic pluton which emplaced during left lateral wrenching 40ar 39ar dates from biotites indicate an early carboniferous age 346 3 ma the older granite is a pretectonic body it is the ordovician saut du saumon augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid state deformation our microtectonic data indicate that the right lateral motions overprint the left lateral ones and produce apparently symmetrical fabrics

igneous evolution of a complex laccolith caldera the solitario trans pecos texas implications for calderas and subjacent plutons the solitario is a large combination laccolith and caldera herein termed laccocaldera with a 16 km diameter dome over which developed a 6 2 km caldera this laccocaldera underwent a complex sequence of predoming sill laccolith and dike intrusion and concurrent volcanism doming with emplacement of a main laccolith ash flow eruption and caldera collapse intracaldera sedimentation and volcanism and late intrusion detailed geologic mapping and 40ar 39ar dating reveal that the solitario evolved over an interval of approximately 1 m y in three distinct pulses at 36 0 35 4 and 35 0 ma the size duration and episodicity of solitario magmatism are more typical of large ash flow calderas than of most previously described laccoliths small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36 0 ma emplacement of the main laccolith doming ash flow eruption and caldera collapse occurred at 35 4 ma during the most voluminous pulse a complex sequence of debris flow and debris avalanche deposits megabreccia trachyte lava and minor ash flow tuff subsequently filled the caldera the final magmatic pulse at 35 0 ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture solitario rocks appear to be part of a broadly cogenetic metaluminous suite peralkaline rhyolite lava domes were emplaced north and west of the solitario at approximately 35 4 ma contemporaneous with laccolith emplacement and the main pulse in the solitario the spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic thermal flux represented by the main pulse of solitario magmatism current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith although the solitario laccocaldera followed this sequence of events our field and 40ar 39ar data demonstrate that it developed through repeated episodic magma injections separated by 0 4 to 0 6 m y intervals of little or no activity this evolution requires a deep long lived magma source well below the main laccolith laccoliths are commonly thought to be small shallow features that are not representative of major silicic magmatic systems such as calderas and batholiths in contrast we suggest that magma chambers beneath many ash flow calderas are tabular floored intrusions including laccoliths evidence for this conclusion includes the following 1 many large plutons are recognized to be laccoliths or at least tabular 2 the solitario and several larger calderas are known to have developed over laccoliths and 3 magma chambers beneath calderas which are as much as 80 km in diameter cannot be as deep as they are wide or some would extend into the upper mantle the solitario formed during a tectonically neutral period following laramide deformation and preceding basin and range extension therefore space for the main laccolith was made by uplift of its roof and possibly subsidence of the floor not by concurrent faulting laccolith type injection is probably a common way that space is made for magma bodies of appreciable areal extent in the upper crust

partially melted lithic megablocks in the yardea dacite gawler range volcanics australia implications for eruption and emplacement mechanisms lithic megablocks ranging from 200 m felsic volcanic unit in the mesoproterozoic gawler range volcanic province grv of south australia throughout its vast extent the yardea dacite shows typical lava like features in that it is massive columnar jointed and evenly porphyritic with 30 40 crystals in a spherulitic and granophyric groundmass in addition flow banding is present at many locations the megablocks are abundant at two sites 50 km apart but isolated megablocks and smaller 6 cm lithic clasts are also scattered throughout the unit at both sites the megablocks are matrix supported non graded randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite the most abundant and largest megablocks are granitoids derived from older basement and from early crystallised plutons of the hiltaba suite which is broadly coeval and comagmatic with the grv the granitoid megablocks have been partially melted most likely prior to eruption when resident in the thermal aureole of the yardea dacite magma chamber the lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas consistent with the lava like character of the host dacite using reasonable estimates of megablock density magma density and magma viscosity we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks implying that they could have been entrained and erupted effusively all but the largest and least melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow the rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber dislodging megablocks that were later entrained and effusively erupted

mechanisms of differentiation in shallow mafic alkaline intrusions as illustrated in the big bend area western texas abstract syenitic bodies are a common feature in alkaline sills and laccoliths that range in composition from syenogabbroic to syenodioritic the syenitic bodies are generally accepted to be the result of in situ differentiation such bodies are usually called segregations but relatively little discussion is given to the actual ways they may form subhorizontal sheets or layers are most common although rounded ocelli and vertically elongated cylindroidal forms are also common these are all systematically arranged in the intrusions such features are found in at least ten sills in the big bend area of texas and are reported as well in montana utah australia sakhalin scotland and new zealand detailed chemical and mineralogical analyses demonstrate crystal fractionation very convincingly for the generation of syenitic liquids that form the bodies the analyses also allow the calculation of viscosities cooling time and mineral settling rates in the sill studied most thoroughly these factors indicate or are compatible with proposed mechanisms for the formation of syenite bodies in many intrusions there is a bimodal distribution of syenite compositions one type is relatively plagioclase rich and the other type is plagioclase poor mass balance calculations on several intrusions show that a residual liquid of plagioclase rich composition will form after about 30 of the parent magma has crystallized whereas a residual liquid of plagioclase poor composition will form only after about 50 of the original magma has crystallized the several mechanisms by which plagioclase rich residual liquids are aggregated into the different forms of syenite bodies include localized crystal settling and sagging of a crystaline framework on a scale of tens of centimeters to a meter within an upper solidification front formation of cylindroidal columns of syenitic differentiate in a crystal mush within a lower solidification front and diapiric rise of such masses after the amount of magma crystallized reaches 50 or more rupture of a rigid crystalline framework occurs through the processes of contraction upon cooling shearing caused by deformation and expansion of bubbles in the interstitial residual liquid while plagioclase poor differentiate flows into spaces created in these processes important for the effectiveness of these processes is the buildup of volatiles in the residual liquids which results from crystallization of anhydrous minerals that causes second boiling and lowering of the viscosity of the liquids

structure age and mode of emplacement of the hercynian borderes louron pluton central pyrenees france the hercynian borderes louron pluton 20 km2 in the central pyrenees intrudes devonian and carboniferous metasediments it shows a concentric zoning and consists of a significant proportion of quartz gabbros in its periphery and of granodiorites biotite monzogranites and biotite muscovite monzogranites in its core ams study shows that the pluton corresponds to an elongated dome with a n100 e trending axis the anisotropy intensity ppara is high in the south and in the core of the pluton whereas it is low in the north the shape parameter t indicates that the fabric is strongly planar in a large central band oriented nw se whereas it is strongly linear in the western and eastern tips of the pluton these characteristics suggest that the borderes louron pluton emplaced in two episodes 1 intrusion of mafic magmas along a n100 e sill parallel to the regional foliation of the host metasediments and 2 injection of three successive silicic batches granodiorite biotite monzogranite two micas monzogranite which pushed aside the early mafic injections in situ u pb dating of zircon grains indicates that the emplacement age of the biotite monzogranite is 309 4 ma synchronous with the d 2 dextral transpressive event and close to the ages of the eastern pyrenean plutons may be slightly older

early variscan visean granites in the core of central pyrenean gneiss domes implications from laser ablation u pb and th pb studies abstract laser ablation u pb and th pb single zircon and monazite geochronology was applied to seven samples from two variscan gneiss domes in the central pyrenean axial zone detrital zircon ages obtained from a paragneiss in the western aston dome range from neoarchean ca 2 57 ga to late neoproterozoic ca 560 ma suggesting that late ediacaran sediments were deposited along the northern margin of gondwana cadomian magmatism is recorded for the first time in the central pyrenees from a biotite metagranite ca 545 ma in the central aston dome a second sample of an orthogneiss from the aston dome yields a middle ordovician protolith age ca 467 ma similar in age to other dated orthogneisses of the axial zone three samples from granite stocks situated in the core of the aston and bossost domes have crystallization ages of ca 339 337 ma providing the first evidence of visean igneous activity in the axial zone in contrast to the larger and significantly younger ca 312 305 ma granite plutons along the margins of the gneiss domes visean magmatism caused lp ht contact metamorphism m 2 migmatization throughout the aston dome though less well constrained by overgrowth ages on zircon ca 331 ma and monazite ages ca 340 320 ma is likely to be related to visean intrusions the main s 2 schistosity in the aston and bossost domes is coeval with the visean intrusions inclusion trails in porphyroblasts nucleating in the contact aureoles document the continuous development of s 2 related to local extension crystal plastic deformation of the granites indicates that the d 2 deformation event continued after their crystallization visean magmatism played a crucial role in early stages of dome formation by thermally weakening the metasedimentary mantle and to a certain degree the orthogneiss as well enhancing the development of orogen parallel shear zones in an overall n s compressive setting

in situ u pb zircon ages for early ordovician magmatism in the eastern pyrenees france the canigou orthogneisses u pb age determinations with an ion probe on zircons within orthogneisses from the canigou massif eastern pyrenees give an emplacement age for their granitic protolith of 475 10 ma arenigian this invalidates the previous interpretation of basement cover relationships between the orthogneisses and the overlying lower cambrian canaveilles metasedimentary series and corroborates the intrusive character of the orthogneisses which is further supported by field evidence the data also define two groups of inherited zircons paleo meso proterozoic and cadomian in addition to partial lead loss during the hercynian late carboniferous high temperature low pressure metamorphic event these results are in agreement with ages reported from other orthogneissic bodies across the entire variscan chain of western europe and indicate the existence of a widespread cambro ordovician magmatic event

arc related ediacaran magmatism along the northern margin of gondwana geochronology and isotopic geochemistry from northern iberia abstract field petrographic geochemical and geochronological studies carried out in northwest iberia constrain the nature and origin of the ediacaran magmatic rocks cropping out in the narcea antiform the results indicate the presence of a long lived subduction related magmatic arc between ca 580 and ca 560 ma in two tectonic units magmatic rocks in the allande group are major intrusive calc alkaline bodies and limited volcanic rocks in the eastern navelgas group there are no intrusive bodies and volcanic rocks are widespread and represent large edifices exposed over tens of square km sm nd and rb sr geochemical isotopic data indicate derivation of the magmatic rocks of both units from a ca 1 1 ga lithospheric mantle as well as varying degrees of crustal contamination geocronological u pb la icp ms and id tims data constrain the magmatic activity to 575 565 ma in the allande group and to ca 560 ma in the navelgas group further constraints on the evolution of the magmatic arc have been obtained from the study of pebbles in early cambrian conglomerates that were derived from the magmatic rocks of these ediacaran units we interpret these ediacaran units to have been formed in a back arc environment in the narcea antiform and to record a multi stage evolution along this segment of the northern gondwana magmatic arc as revealed by their isotopic and geochronological differences

unfolding the arc the use of pre orogenic constraints to assess the evolution of the variscan belt in western europe abstract we present a pre orogenic early paleozoic palinspastic reconstruction of the northern gondwana margin that was subsequently involved in the late paleozoic variscan orogeny in central and western europe our reconstruction is based on two pre orogenic data sets the age and distribution of cambrian ordovician magmatism and the detrital zircon age signature of late neoproterozoic early paleozoic clastic rocks we obtain this reconstruction by unfolding the ibero armorican arc and by restoring the movement of the large scale dextral strike slip faults that transect the different tectono stratigraphic units our results favour an irregular shape for this part of the northern gondwana margin with a n s central segment linking two e w oriented segments the proposed reconstruction and the structural restoration of the main features of variscan deformation is in accordance with some aspects of previously proposed structural models such as the curved geometry of the gondwanan margin required by the indentor model for continental collision the role played by the large strike slip faults in dispersing formerly juxtaposed units and the regional scale oroclinal folding of part of this margin during late carboniferous early permian times the combined use of the pre orogenic geological constraints and palinspastic restoration is a useful approach that may provide a foundation for continual refinement of reconstructions as more data become available

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

u pb age constraints on variscan magmatism and ni cu pge metallogeny in the ossa morena zone sw iberia new u pb zircon ages from the santa olalla igneous complex have been obtained which improve the knowledge of the precise timing of variscan magmatism in the ossa morena zone sw iberia this complex has a special relevance as it hosts the most important ni cu platinum group element pge mineralization in europe the aguablanca deposit u pb zircon ages have been obtained for seven samples belonging to the santa olalla igneous complex and spatially related granites with the exception of the cala granite 352 4 ma which represents an older intrusion the bulk of samples yield ages that cluster around 340 3 ma the santa olalla tonalite 341 5 3 ma the sultana hornblende tonalite 341 3 ma a mingling area at the contact between the aguablanca and santa olalla stocks 341 1 5 ma the garrote granite 339 3 ma the teuler granite 338 2 ma and dioritic dykes from the aguablanca stock 338 6 0 8 ma the bodonal cala porphyry which has also been dated 530 3 ma comprises a group of sub volcanic rhyolitic intrusions belonging to the bodonal cala volcano sedimentary complex which hosts the igneous rocks the knowledge that emplacement of the aguablanca deposit was related to episodic transtensional tectonic stages during the variscan orogeny will be fundamental in future mineral exploration in the ossa morena zone

geochemical characterization and isotopic age of caradocian magmatism in the northeastern iberian peninsula insights into the late ordovician evolution of the northern gondwana margin this paper presents new geochronological geochemical and isotopic data for late ordovician magmatism 455 2 ma from the variscan massifs in the northeastern iberian peninsula this magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc alkaline ignimbrites and epiclastic volcanic rocks which were deformed and metamorphosed during the variscan orogeny the metaplutonic rocks are located in the lower part of a neoproterozoic early cambrian metasedimentary sequence in the canigo massif eastern pyrenees aluminous orthogneisses have isotopic signatures that indicate a crustal origin whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts the metavolcanic rocks define a calc alkaline ignimbritic sequence of crustal origin located in the lower part of the late ordovician sequence in the les gavarres massif catalan coastal ranges tdm ages for all the studied late ordovician rocks range from mesoproterozoic to paleoproterozoic 1 3 ga 1 5 ga 1 7 ga and 2 2 ga suggesting a neoproterozoic crust with paleoproterozoic components and an arc signature as the most probable source for the crustal melts the isotopic similarities between the studied samples and other early ordovician magmatic rocks of the central and western iberian massif suggest repeated melt extractions from a common basement as the source for the early and late ordovician magmatism in the iberian realm however the studied massifs are more closely linked to other massifs from the mediterranean realm the most probable tectonic setting for the emplacement of the late ordovician volcanic and plutonic bodies is that of an extensional regime which postdates a previous ordovician contractional event and the opening of the rheic ocean

tectonic setting of the north gondwana margin during the early ordovician a comparison of the ollo de sapo and famatina magmatic events abstract this paper presents a comparison of compiled geochronological and geochemical data from the ollo de sapo and famatina magmatic events the ollo de sapo magmatic sequence is located in northwest iberia and was emplaced during the early ordovician from 495 to 474 ma the famatina complex is a magmatic sequence located in northern argentina that was emplaced during the early to mid ordovician from 483 to 463 ma these magmatic events are currently interpreted to have been emplaced in different tectonic settings despite both having occurred along the north gondwana margin geochronological data indicates that these magmatic events occurred contemporaneously over at least 9 m y and therefore can provide a snapshot of the northern gondwana margin during the mid ordovician major element data indicates that both magmatic suites are calc alkaline to alkali calcic and trace element and ree data show magmatic signatures that are indistinguishable this study highlights the similarity between the ollo de sapo and the famatina magmatic suites and discusses alternative models for their emplacement based on paleomagnetic and paleobiogeographical data these data indicate that the ollo de sapo was likely emplaced in a subduction zone setting while the famatina magmatic suite may be of parautochthonous origin to gondwana implying that the pampeanan margin may not have been active during the early to mid ordovician

age determination of ordovician magmatism in ne sardinia and its bearing on variscan basement evolution a newly discovered pre variscan cover basement relationship in ne sardinia is defined by differences in structural and metamorphic histories the oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover cover and basement are further distinguished by contrasting metapelite compositions superimposed metamorphism is represented by pre variscan garnet and variscan plagioclase porphyroblasts that appear in the field as a mixed garnet albite zone in situ u pb zircon dating of meta igneous rocks from cover and basement by laser ablation inductively coupled plasma mass spectrometry gives ages of 458 7 ma for the tanaunella orthogneiss 456 14 ma for the lode orthogneiss and 474 13 ma for the lula porphyroid the internal sardinian variscides thus incorporate a mid ordovician magmatic belt developed upon an older basement correlation is proposed with a tectonostratigraphic domain that is at present exposed in the alps within the mid ordovician peri gondwanan realm the internal sardinian variscides appear to have developed along an active margin whereas the foreland correlates with the ibero aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of avalonia to form the rheic ocean

decollements in slate belts examples from the european variscides and the qin ling belt of central china decollements are conspicuous features of collision belts as a result of shortening and shearing of rocks of different competencies in response to intracontinental subduction and crustal stacking when these decollements occur at upper levels the classical thin skin tectonics of the foredeeps without important internal strain results at mid crustal levels slaty cleavage and foliation development occurs in the more internal parts of the chains in the lower crust near the crust mantle boundary the more severe conditions granulitic metamorphism result in ductile deformation in the deepest parts of the mountain belts examples of middle and lower crust are well documented in central china and the variscan belt of europe from field evidence and deep seismic profiling from these examples it appears more and more likely that most of the mid crustal slate belts showing vertical slaty cleavage at the erosion surface overly flat decollements at greater depth large decollements must occur as well in the lower continental crust and at crust mantle boundary typical features of the deep seismic vertical profiles such as the strongly layered lower crust may represent the image of such deep decollements

relationship between plagioclase crystallization and cooling rate in basaltic melts rock textures commonly preserve a record of the near surface crystallization history of volcanic rocks under conditions of simple cooling without convection or mixing textures will reflect sample cooling rate the temperature at which crystallization was initiated and the distribution of mineral phase precipitation across the crystallization interval compilation of plagioclase size and number density data on natural dike sill and lava lake and experimental samples suggests that 1 growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate and 2 the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization evaluation of textural data in the context of recent numerical models of crystallization in simple one and two component systems provides new insight into regularities in the crystallization behavior of basaltic magmas for example the rate of change in crystal size and number density as dictated by mass balance has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems in natural samples plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width solidification time comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes distributions broaden and the grain size at peak frequency increases with increasing crystallization times decreasing cooling rates in contrast partially crystallized makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size measured size distributions in dikes can be explained by late stage modification of makaopuhi type distributions through loss of small crystals possibly the consequence of growth without nucleation finally this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near surface basaltic systems predicted growth rates will be slow and relatively constant 10 10 10 11 cm s for crystallization times expected in most shallow volcanic systems 1000 years

provenance of quartz rich metamorphic tectonite pebbles from the black flysch w pyrenees n spain an ebsd and detrital zircon la icp ms study abstract metasandstone and quartz tectonite conglomerate pebbles from the albian black flysch encapsulate evidence of pre mesozoic igneous and metamorphic events quartz deformation microstructures and crystallographic fabrics characterize plastic flow under sub greenschists 250 300 c to high grade metamorphic conditions above 550 600 c detrital zircon grains from the pebbles provide concordant 206pb 238u radiometric ages ranging from 473 26 ma to 2261 59 ma with four major populations clustered in the intervals 1 96 1 80 ga paleoproterozoic 1 11 0 93 ga stenian tonian 765 571 ma neoproterozoic and 500 475 ma cambro ordovician the youngest zircons and the probability distribution of the studied population permit us to establish an early ordovician maximum depositional age for tectonite pebble protoliths and relate them with the armorican quartzite detrital zircon signature we identify the currently submerged landes massif of the armorican platform as the most likely candidate to being the source of black flysch metamorphic conglomerate pebbles

nature of a flood basalt magma reservoir based on the compositional variation in a single flood basalt flow and its feeder dike in the mesozoic hartford basin connecticut the erosional remains of the mesozoic holyoke basalt in the hartford pomperaug and deerfield basins of connecticut and massachusetts indicate an original flow volume of 1200 km3 its feeder dike which is about 50 m wide and 160 km long can be traced down through 2 km of mesozoic sediments and as a result of faulting associated with basin formation through an additional 6 km of paleozoic metamorphic rocks chemical profiles through the distal and proximal parts of the flow and through the dike at depths of 2 4 and 8 km provide sequential samples of the magma that rose during this one eruptive event the flow and dike have restricted compositions that indicate saturation with olivine augite and plagioclase at depth the flow consisted largely of a liquid at the pigeonite reaction point dike compositions can be modeled as mixtures of this liquid with up to 24 crystals of plagioclase augite and olivine the dike compositions indicate equilibration with these minerals at 3 8 kbar this pressure corresponds to a depth of 12 2 km which is believed to have been the depth of the brittle ductile transition in the crust at the time this transition appears to be the only reasonable barrier that could have caused ponding of the magma at the mid crustal level the holyoke liquid is interpreted to have segregated from a compacting crystal mush following 30 crystallization of the magma in this mid crustal reservoir eruption of the basalt exhausted the supply of segregated liquid and when the remaining crystal mush began to rise in the dike the average density of the magma column increased until it matched the average density of the intruded crust and the eruption ended by analogy with the differentiation that took place in the solidifying holyoke flow on the surface the mid crustal magma reservoir is estimated to have had a volume of at least 12 000 km3 the magma in this chamber must have come from a still deeper chamber because it was too fractionated to have come directly from a mantle source

rb sr whole rock investigations in the aston massif central pyrenees rb sr whole rock data are presented from four magmatic units in the aston massif central pyrenees the data are interpreted to reflect the following variscan events 1 an almost complete homogenization of the sr isotopic composition through the aston orthogneiss about 315 ma ago obscuring the original age of gneiss 2 intrusion of the ax granite 301 15 ma ago and of the soulcem pegmatite muscovite granite 292 13 ma ago both from crustal sources the variscan bassies granodiorite partly suffered later rb sr whole rock resetting presumably related to the development of alpine shear zones

polyphase magmatic pulses along the northern gondwana margin u pb zircon geochronology from gneiss domes of the pyrenees abstract alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the aston and hospitalet domes of the axial zone of the pyrenees are subjects of a laser ablation icp ms u pb zircon geochronology study the age spectra recorded by detrital magmatic xenocrystic and inherited zircons reveal a more complex nearly continuous paleozoic magmatic history of the variscan basement of the pyrenees than previously known inherited and detrital zircons of mesoarchean paleoproterozoic to ediacaran ages attest to the peri gondwana location of the cambrian sediments that later form the metamorphic core of the variscan pyrenees the youngest magmatic zircon ages fall into the late carboniferous and earliest permian ranging from ca 306 297 ma and represent the emplacement ages of the dikes and small granite intrusions the age spectra of magmatic xenocrystic zircons contain several maxima middle 475 465 ma and late ordovician 455 445 ma early 415 402 ma and late devonian 385 383 ma early 356 351 ma and middle carboniferous ca 328 ma middle ordovician and middle carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded the aston and hospitalet orthogneisses and the soulcem granite the presence of early mid carboniferous magmatic zircons in several samples lends further support to a wide spread early variscan magmatic activity in the central pyrenees the other age peaks do not have equivalent igneous or metaigneous rocks in the central axial zone but are thought to be present in the pyrenean crust not exposed and yet to be identified the diversity of ordovician devonian and carboniferous up to permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre variscan and variscan orogenies the source of the heat for the devonian to early mid carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone lithospheric scale shearing or a mantle plume tuzo

relationship between amazon and high andes rainfall 1 a regional climate model is used to study the connections between amazon and high andes rainfall the modeling effort focuses on understanding the rainfall response during the last glacial maximum lgm 21 000 years ago as paleoproxy evidence generally suggests that the high andes were wet during this period while conditions over the amazon were drier although there is more uncertainty about the latter analysis of the yearlong present day and full lgm integrations reveals that the delayed onset of amazon convection in the lgm during austral spring is associated with an enhancement of rainfall over the high andes in the absence of convection over the amazon in the austral spring zonal geopotential height gradients over the amazon strengthen resulting in anomalous easterly flow between the amazon and the high andes instead of the low level flow paralleling the andes e g northwesterly the flow impinges more directly onto the eastern slopes of the andes during the lgm where it rises and transports moisture from the amazon up into the mountains enhancing convection over the andes this circulation mechanism explains how the moisture and rainfall over the high andes of peru and bolivia can increase such as the case for the lgm despite large scale drying over south america present day observations are then explored for 1984 2002 and 2003 to identify whether mechanisms identified in the paleoclimate modeling are observable in the present day climate comparison of these particular springs reveals a response analogous to the lgm model response suggesting that this mechanism that connects tropical atlantic sea surface temperatures amazon convection and high andes rainfall is operating in the present climate

chronological link between deep seated processes in magma chambers and eruptions permo carboniferous magmatism in the core of pangaea southern pyrenees abstract in the southern pyrenees there are upper carboniferous lower permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions rhyolitic ignimbrites and andesitic flows these basins are spatially associated with granodiorites and dacitic dykes emplaced in variscan basement rocks u pb shrimp dating of zircons extracted from three granodiorites an andesitic flow a dacitic dyke and six ignimbrites revealed that magmatism occurred over an extended period of thirty eight million years from ca 304 ma to ca 266 ma upper carboniferous middle permian a scattering of zircon ages in each sample shows that the history of melt crystallization was complex with more than one zircon forming event in each magma chamber the prolonged crystallization history was transferred to the product of the eruptions a chronological link between the deep seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at ca 309 307 ma upper carboniferous ca 304 296 ma upper carboniferous lower permian ca 294 282 ma lower permian and ca 276 ma lower permian the variation of zircon u th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the permian zircons probably crystallized from melt phases related to both a felsic intermediate metaluminous source from ca 310 293 ma mostly 0 1

variscan extension in the pyrenees evidence for variscan extension in the pyrenees is summarized and used with other geological and geochronological data to reconstruct the variscan extensional geometry and kinematics at a crustal scale a regionally consistent pattern of ese wnw trending synmetamorphic stretching directions indicates variscan extension roughly parallel to the present alpine trend of the belt the spatial distribution of variscan low pressure high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping crustal scale shear zones allowing heterogeneous ductile extension of the variscan middle and lower crust extension occurred synchronous with uplift and emergence of large parts of the crust and deposition during the upper carboniferous and permian of up to 2500 m of continental sediments and intercalated basaltic andesitic volcanics in fault bounded extensional halfgrabens the extensional structures and related metamorphism postdate a namurian to early westphalian stage of crustal thickening characterized by thrusting and steep folding it is suggested that variscan extension in the pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere enhanced by detachment of a cold and gravitationally unstable lithospheric root which caused progressive uplift tectonic denudation and a highly anomalous thermal structure of the variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the early permian

a hybrid composite dike suite from the northern arabian nubian shield southwest jordan implications for magma mixing and partial melting of granite by mafic magma abstract the arabian nubian shield is an exemplary juvenile continental crust of neoproterozoic age 1000 542 ma the post collisional rift related stage 610 to 542 ma of its formation is characterized among others by the intrusion of several generations of simple and composite dikes this study documents a suite of hybrid composite dikes and a natural example of partial melting of granite by a mafic magma from the northernmost extremity of arabian nubian shield in southwest jordan the petrogenesis of this suite is discussed on the basis of field petrographic geochemical and rb sr isotopic data these dikes give spectacular examples of the interaction between basaltic magma and the granitic basement this interaction ranges from brecciation partial melting of the host alkali feldspar granite to complete assimilation of the granitic material field structures range from intrusive breccia angular partially melted granitic fragments in a mafic groundmass to the formation of hybrid composite dikes that are up to 14 m in thickness the rims of these dikes are trachyandesite latite with alkali feldspar ovoids up to 1 cm in diameter while the central cores are trachydacite to dacite and again with alkali feldspar ovoids and xenoliths from the dike rims the granitic xenoliths in the intrusive breccia have been subjected to at least 33 partial melting a seven point rb sr isochron from one of these composite dikes yields an age of 561 33 ma and an initial 87 sr 86 sr ratio of 0 70326 0 0003 2 and mswd of 0 62 geochemical modeling using major trace rare earth elements and isotopes suggests the generation of the hybrid composite dike suite through the assimilation of 30 to 60 granitic crustal material by a basaltic magma while the latter was undergoing fractional crystallization at different levels in the continental crust

new late middle to early late ordovician u pb zircon ages of extension related felsic volcanic rocks in the eastern pyrenees ne iberia tectonic implications pre variscan basement rocks from the pyrenees provide evidence of several magmatic episodes with complex geodynamic histories from late neoproterozoic to palaeozoic times one of the most significant episodes consisting of several granitic and granodioritic bodies and volcanic rocks mostly pyroclastic in nature dates from the late ordovician period in the eastern pyrenees this magmatism is well represented in the ribes de freser and nuria areas here the nuria orthogneiss and the ribes granophyre both dated at c 457 460 ma seem to form a calc alkaline plutonic suite emplaced at different crustal levels the presence of numerous pyroclastic deposits and lavas interbedded with upper ordovician sandbian lower katian formerly caradoc sediments intruded by the ribes granophyre suggests that this magmatic episode also generated significant volcanism moreover the area hosts an important volume of rhyolitic ignimbrites and andesitic lavas affected by alpine deformation these volcanic rocks were previously attributed to late variscan volcanism extensively represented in other areas of the pyrenees here we present the first five laser ablation u pb zircon dates for this ignimbritic succession and two new ages for the ribes granophyre the ages of the ignimbrites overlapping within error are all 460 ma suggesting a genetic relationship between the plutonic and volcanic rocks and indicating that the sandbian katian magmatism is much more voluminous than reported in previous studies and possibly includes mega eruptions linked to the formation of collapse calderas

anatomy of an extensional shear zone leading to the exhumation of the middle crust within the canigou dome eastern pyrenees axial zone abstract to investigate the mechanism of exhumation of deep crustal rocks in hot orogens we focus on the southwestern part of the variscan canigou gneiss dome pyrenees where the transition between the middle and upper crust can be observed on the basis of new structural data geological mapping cross sections microstructural analyses and petrologic observations we propose that flat lying foliation within the gneiss dome and steep cleavage developed above it formed coevally we note the presence of regional scale shear zones at the boundary between the middle crust nuria canigou unit and upper crust puigmal unit at this boundary the e w south canigou shear zone is an extensional shear zone showing top to the southwest kinematics deformation which is localized within the andalusite biotite transition zone occurred under retrogressive conditions from high grade ductile to brittle conditions during the main deformation stage d2 the tectonic contact was folded during a later stage d3 we propose that the shape of the canigou dome resulted from the activation of extensional shear zones at the top of the orthogneiss leading to partial exhumation of high grade rocks the canigou gneiss dome can be considered an extensional dome that formed during late variscan ca 310 290ma transpression

multi stage magma ascent beneath the canary islands evidence from fluid inclusions gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from gran canaria la palma el hierro lanzarote and la gomera canary islands contain abundant co2 dominated fluid inclusions inclusion densities are strikingly similar on a regional scale histogram maxima correspond to one or more of the following pressures 1 minimum 0 55 to 1 0 gpa within the upper mantle 2 between 0 2 and 0 4 gpa the moho or the lower crust 3 at about 0 1 gpa upper crust fluid inclusions in several rocks show a bimodal density distribution the lower density maximum comprising both texturally early and late inclusions this is taken as evidence for an incomplete resetting of inclusion densities and simultaneous formation of young inclusions at well defined magma stagnation levels for gran canaria pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0 9 to 1 0 gpa indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas magma chamber pressures within the mantle inferred to represent levels of mantle xenolith entrainment are 0 65 0 95 gpa for el hierro 0 60 0 68 gpa for la palma and 0 55 0 75 gpa for lanzarote the highest density fluid inclusions in many canary island mantle xenoliths have probably survived in situ near isobaric heating at the depth of xenolith entrainment inclusion data from all islands indicate ponding of basaltic magmas at moho or lower crustal depths and possibly at an additional higher level strongly suggestive of two main crustal accumulation levels beneath each island we emphasize that repeated magmatic underplating of primitive magmas and therefore intrusive accretion are important growth mechanisms for the canary islands and by analogy for other ocean islands comparable fluid inclusion data from primitive rocks in other tectonic settings including iceland etna and continental rift systems hungary south norway indicate that magma accumulation close to moho depths shortly before eruption is not however restricted to oceanic intraplate volcanoes lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception independent of the tectonic setting

entrainment of country rock during basaltic eruptions of the lucero volcanic field new mexico as magma rises through the lithosphere it may entrain wall rock debris the entrainment process depends on the local hydrodynamic regime of the magma e g velocity temperature bulk density the extent of interaction of magma with groundwater and the mechanical properties of the wall rocks wall rock entrainment results in local flaring of dikes and conduits which in turn affects the hydrodynamics of magma ascent and eruption we studied upper crustal xenoliths erupted from small volume basaltic volcanoes of the lucero volcanic field west central new mexico in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles including strongly hydrovolcanic strombolian and effusive processes total xenolith volume fractions ranged from 0 3 0 9 in hydrovolcanic facies to 10 4 10 2 in strombolian facies the volcanoes erupted through a thick well characterized sequence of paleozoic and lower mesozoic sedimentary rocks so that erupted xenolith

highly heterogeneous precambrian basement under the central deccan traps india direct evidence from xenoliths in dykes abstract crustal or mantle xenoliths are not common in evolved tholeiitic flood basalts that cover huge areas of the precambrian shields yet the occasional occurrences provide the most direct and unequivocal evidence on basement composition few xenolith occurrences are known from the deccan traps india and inferences about the deccan basement have necessarily depended on geophysical studies and geochemistry of deccan lavas and intrusions here we report two basalt dykes rajmane and talwade dykes from the central deccan traps that are extremely rich in crustal xenoliths of great lithological variety gneisses quartzites granite mylonite felsic granulite carbonate rock tuff because the dykes are parallel and only 4 km apart and only a few kilometres long the xenoliths provide clear evidence for high small scale lithological heterogeneity and strong tectonic deformation in the precambrian indian crust beneath measured 87sr 86sr ratios in the xenoliths range from 0 70935 carbonate to 0 78479 granite mylonite the rajmane dyke sampled away from any of the xenoliths shows a present day 87sr 86sr ratio of 0 70465 and initial at 66 ma ratio of 0 70445 the dyke is subalkalic and fairly evolved mg no 44 1 and broadly similar in its sr isotopic and elemental composition to some of the lavas of the mahabaleshwar formation the xenoliths are comparable lithologically and geochemically to basement rocks from the archaean dharwar craton forming much of southern india as several lines of evidence suggest the dharwar craton may extend at least 350 400 km north under the deccan lava cover this is significant for precambrian crustal evolution of india besides continental reconstructions

polymetamorphism and ductile deformation of staurolite cordierite schist of the bossost dome indication for variscan extension in the axial zone of the central pyrenees the bossost dome is an e w trending elongated structural and metamorphic dome developed in cambro ordovician metasedimentary rocks in the variscan axial zone of the central pyrenees a steep fault separates a northern half dome cored by massif granite from an e w trending doubly plunging antiform with granitic sills and dykes in the core to the south the main foliation is a flat lying s 1 2 schistosity that grades into a steeper dipping slaty cleavage at the dome margins three major deformational and two metamorphic phases can be differentiated s 1 2 schistosity is an axial planar cleavage to w vergent recumbent folding that probably occurred in mid westphalian time peak regional metamorphism m 1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust strong non coaxial deformation recording uniform top to the se extension during d 2 a is preserved in staurolite garnet schists in a 1 5 km thick shallowly se dipping zone in the southeastern dome a 500 m thick contact aureole m 2 was imprinted on the regionally metamorphosed rocks following the intrusion the bossost granite during d 2 b more coaxial deformation prevailed during synkinematic growth of m 2 phases in the inner part of the contact aureole around the northern part of the dome where it obliterated d 2 a fabrics progressive non coaxial deformation continued in the southeastern antiform and is recorded by late synkinematic growth of cordierite successive overprinting of the m 1 staurolite garnet assemblage by andalusite and cordierite of m 2 is preserved in the southern part the assemblage muscovite cordierite staurolite biotite is considered metastable given the low mn and zn contents of staurolite and cordierite and interpreted as the result of prograde metamorphism during decompression p t conditions during m 2 were approximately 3 kbar and 600 c pervasive crenulations and mesoscopic to regional southerly verging folds are the result of d 3 nne ssw compression post dating ductile deformation and contact metamorphism polymetamorphic assemblages of the bossost dome preserve a regionally confined zone of ese directed extensional shearing within an overall n s compressional setting exact timing of extensional shearing is not known but can be constrained by recumbent folding during the mid westphalian and granitic intrusions which confine it to late carboniferous time c 305 ma crustal scale flat lying extensional shear zones with similar orientation and time frame are observed in the hospitalet massif of the eastern axial zone this suggests that crustal extension though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting was not uncommon in late carboniferous time in the axial zone of the pyrenees

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

field and model constraints on silicic melt segregation by compaction hindered settling the role of water and its effect on latent heat release to investigate how large volumes of silicic melts segregate to form granitic plutons we conducted a case study of a zoned pluton in which sio 2 increases from intermediate 69 wt to highly silicic compositions 74 wt toward the contact with metasedimentary wallrock in the outer 25 m of the pluton all other major minor and trace elements vary systematically with sio 2 and indicate that outward increasing sio 2 is due to a decrease in mafic elements and minerals whole rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the si rich boundary layer instead mafic enclaves which are common in the pluton also decrease in abundance in the outer 25 m of the pluton suggesting a mechanical origin for the si rich boundary layer the coupling of mechanical and geochemical boundary layers combined with geochemical modeling indicate that the silica rich enclave poor boundary layer formed by hindered settling or compaction of a crystal rich crystal fractions 60 magmatic mush segregation of melts at high crystal fraction is known to be a slow process however petrography and zr based thermometry indicate that the residual si rich liquids were water saturated water decreases melt viscosity which helps expulsion but equally importantly water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma we show that the higher the water content the longer the time interval over which a magma chamber resides at the stage when water saturated high silica liquids form allowing sufficient time for exfiltration of silicic liquids before the magma body freezes

ordovician magmatism in the eastern pyrenees implications for the geodynamic evolution of northern gondwana new data on the geochemistry and geochronology of different felsic gneisses and metabasites from the variscan massifs of eastern pyrenees have allowed us to shed some light on the ordovician magmat

evolution of the middle crust of the pyrenees during the paleozoic new data on the plutonic rocks from the north pyrenean agly massif the variscan middle crust of the eastern pyrenees is represented by the belesta and caramany gneissic units of the north pyrenean agly massif in this study we date and reinterpret the orthogneisses and granites composing these units we characterize a recurrent plutonic activity spanning all the paleozoic in this part of the pyrenean crust the belesta unit amphibolite facies is mainly composed of sills of latest ediacaran earliest cambrian 542 4 and 540 4 ma orthogneisses 70 of the series almost unknown in the pyrenees until now and latest carboniferous earliest permian 307 3 298 3 ma time span granites 15 of the series these plutonic rocks were emplaced in probably ediacaran aluminous pelites later transformed into paragneisses 15 of the series the caramany unit granulite facies is mainly composed of precambrian aluminous paragneisses migmatized during the latest carboniferous earliest permian 299 4 ma sills of earliest cambrian 529 5 ma orthogneisses and sills about 30 of the series of late carboniferous 308 3 299 3 ma opxbearing and opx free granodiorites grt bearing leucogranites and norites among these last rocks the ansignan charnockite forming a 800 m thick laccolith near the base of the caramany unit and previously dated at 314 6 7 ma respaut lancelot neues jahrbuch miner abh 147 1 21 34 1983 is refined at 307 3 ma our results confirm the importance of the late carboniferous plutonism in deep levels of the pyrenean crust which largely resulted from recycling of older para and orthoderived rocks of this crust the emplacement of hundreds of metres in thickness of granites and other plutonic rocks in the variscan pyrenees occurred in a rather short time both as sills in these deep levels and as large plutons in shallower levels a fact which necessarily had consequences on the ht lp carboniferous metamorphism

relationships between the occurrence of accessory ge minerals and sphalerite in variscan pb zn deposits of the bossost anticlinorium french pyrenean axial zone chemistry microstructures and ore deposit setting abstract the presence of unique accessory ge minerals containing up to 70 wt ge is a widespread phenomenon in pb zn deposits of the variscan pyrenean axial zone paz such a mode of occurrence is however rare worldwide with germanium more typically occurring as a trace component of sulfides notably sphalerite

was there a super eruption on the gondwanan coast 477 ma ago precise zircon and monazite id tims u pb dating of three lower ordovician altered ash fall tuff beds k bentonites in the cantabrian zone of nw iberia yielded coeval ages together with an equivalent previously studied sample 477 5 1 gutierrez alonso et al 2007 of 477 1 3 ma 477 2 1 1 ma and 477 3 1 ma with a pooled concordia age all analyses in the four samples of 477 2 0 74 ma a conservative estimation of the volume and mass of the studied k bentonite beds using exclusively the cz data yields a volume for the preserved deposits of ca 37 5 km3 volcanic explosivity index vei 6 colossal when considering other putative equivalent beds in iberia and neighboring realms i e armorica sardinia the volume of ejecta associated to this event would make it reach the supervolcanic apocalyptic status vei 8 1000 km3 at variance with most known cases of this kind of gigantic eruption events geological observations indicate that the studied magmatic event was related to continental margin extension and thinning and not to plate convergence we speculate that a geochronologically equivalent large caldera event recognized in the geological record of nw iberia could be ground zero of this super eruption

steady state volcanism paleoeffusion rates and magma system volume inferred from plagioclase crystal size distributions in mafic lavas dome mountain nevada abstract the dome mountain dm lavas located in the moat of the timber mountain caldera of the sw nevada volcanic field cap a thick sequence of welded tuffs massive and bedded tuffs and mafic and rhyolitic lava flows these 10 9 m y old flows comprise a pile of some twenty flows with a total thickness of about 300 m individual flows range in thickness from 3 to 20 m and are characterized by a basal vesicular zone a massive central region and an upper vesicular region containing in most cases cooling fractures the flows dip gently away from the summit of dm a prominent topographic feature of the region with an elevation of 2065 m silica contents range from 48 8 wt at the base of the pile to 59 4 wt at the top although most of the flows are andesitic crystal size distributions csds for plagioclase in these comagmatic lavas have been determined to provide information on magma storage times these times when coupled with phase equilibria can be used to infer cooling rates which are indicative of the relative dimensions of magmatic bodies and the vigor of various magmatic cooling processes most of the csds show no sign of crystal fractionation or accumulation assuming a growth rate of 10 10 cm s calculated mean crystal residence times for most lavas cluster in the range of 1 5 to 4 years two of the lower lavas have residence times in the range of 4 to 9 years the residence times correlate inversely with nucleation density and broadly correlate inversely with silica content high residence time low silica lavas occur at the base of the pile whereas low residence time high silica lavas occur at the top of the pile one interpretation of the residence time data is that the lavas are from an open system of more or less constant residence time but varying spatially in system locations e g a magma chamber in which steady state recharge and eruption may have been reached alternatively these lavas may represent small samples closely spaced in time of a slowly cooling large batch or reservoir of magma the greater residence times associated with the lower flows may indicate the increased amount of time necessary to form the conduit of the magma volcano system by the oldest magma or simply that part of the system with the highest liquidus temperature a minimum volume of 1 4 to 2 5 km 3 for the magma chamber is estimated for the dm system using the estimated residence times in a simplified cooling model of a sheet of aspect ratio r the actual system volume depends on r which is strictly unknown and thus the actual system volume may be much larger e g 200 km 3 a mean paleoeffusion rate of 0 05 km 3 yr is obtained for the lavas by assuming that the residence time of each lava corresponds to the amount of time elapsed since the eruption of the underlying flow

the palaeogeographic position of central iberia in gondwana during the ordovician evidence from zircon chronology and nd isotopes terra nova 22 341 346 2010 abstract the abnormally elevated abundance of inherited zircon locally up to 80 90 of zircon grains contain pre magmatic cores in the cambro ordovician magmatic rocks of central iberia made possible an estimation of the distribution of ages of their source rocks the comparison of inherited u pb ages and whole rock nd model ages with those of three main north african neoproterozoic terranes reveals that the iberian magmas can only have been generated from a crust similar to that of the east african orogen west of the arabian nubian shield such crust is currently found in the three precambrian inliers of the western desert of egypt east of the archaean terranes of gebel kamel palaeontological evidence also indicates that the ordovician fauna of central iberia is similar to that of the region between eastern algeria and arabia therefore we conclude that during the ordovician iberia was not attached to north west africa near morocco as is usually assumed but instead to the east african orogen in northern egypt

thermotectonic evolution of the north pyrenean agly massif during early cretaceous hyperextension using multi mineral u pb thermochronometry