review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

subsalt play gulf of mexico a review exploration for deep water sandstone reservoirs beneath allochthonous salt in the gulf of mexico represents a major new frontier play in north america more than 30 wells thus far have been drilled with subsalt targets resulting in 8 discoveries at least 3 of which are commercial and 3 that have reserves of 100 million bbl oil equivalent or more reservoirs consist of miocene pleistocene sandstones deposited in submarine fan channel system environments on the paleoslope where salt deformation has a complex late cenozoic history salt sheets exist at various stratigraphic levels and have overridden sandstone fairways on the present day outer continental shelf and upper slope where water depths are moderate and where pipeline and other infrastructure facilities already exist potential reserves for the subsalt play have been estimated at 1 2 billion bbl of oil and 15 tcf gas from 25 or more significant fields recent success in the subsalt play has depended upon 1 advances in 3 d three dimensional seismic acquisition and processing in particular 3 d prestack depth imaging and 2 improved geologic modeling of salt deformation and depositional systems in addition better understanding of the drilling risks frequently encountered in penetrating salt sheets has been important progress in all these areas ismore certain to continue and will result in significant new tools and techniques for exploration as a whole less

the eastern continuation of the cadomian orogen u pb zircon evidence from saxo thuringian granitoids in south western poland and the northern czech republic we report u pb single zircon ages from three pre variscan granitoids in the ne part of the bohemian massif the platerowka granodiorite from the lausitz izera unit the polish sudetes has been dated at 533 9 ma the bitouchov granite form the sw part of the south krkonose unit the czech sudetes gave an age of 540 11 10 ma and the w droze granodiorite in the fore sudetic block yielded 548 9 ma all these latest vendian early cambrian granitoids represent the post tectonic expression of a late proterozoic cadomian orogenic cycle and demonstrate the eastward extent of the cadomian basement into the variscan orogen granodiorites of similar age have so far been reported from brittany and especially from the saxo thuringian terrane to the ne and sw of the elbe fault zone we conclude that the saxo thuringian terrane extends across the elbe and sudetic marginal fault zones into the fore sudetic block

the western mediterranean extensional basins and the alpine orogen the western mediterranean late oligocene miocene basins alboran valencia and provencal basins are a coherent system of interrelated troughs in all basins normal faults and thermal subsidence migrated toward the east progressively moving to the miocene to pleistocene algerian and tyrrhenian basins all those troughs appear elements of the back arc opening related to the eastward roll back of the w directed apennines maghrebides subduction zone similarly to western pacific back arc settings these late oligocene early miocene basins nucleated both within the betic cordillera e g alboran sea and in its foreland valencia and provencal troughs the n40 70 direction of grabens is oblique to the coexisting n60 80 trending orogen and shows its structural independence from the orogenic roots thus as the extension cross cuts the orogen and developed also well outside the thrust belt front the westernmost basins of the mediterranean had to develop independently from the alps betics orogen therefore the alboran extension considered a classic example of a basin generated by the collapse of an orogen cannot be ascribed to the detachment or annihilation of the lithospheric root in contrast with the eastward migrating extensional basins the betic balearic thrust front was migrating westward producing interference or inversion structures

records of precambrian early palaeozoic volcanic and sedimentary processes in the central european variscides a review of shrimp zircon data from the kaczawa succession sudetes sw poland abstract the early pre orogenic stages of evolution in the variscan belt i e rifting processes opening of sedimentary basins and associated igneous activities are often obscure because many successions have yielded little or no biostratigraphic data have a strong metamorphic overprint and are tectonically deformed and dislocated the increasing application of shrimp zircon dating has provided useful constraints on magmatic and metamorphic processes helped locate probable source areas for detritus within sedimentary successions and facilitated large scale palaeogeographic correlations this methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the kaczawa complex in the polish west sudetes thus recent shrimp studies in the kaczawa mountains have yielded early ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the kaczawa succession c 503 ma for metarhyodacites of crustal derivation and c 485 ma for alkaline metatrachytes of mantle signature these dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break up of the northern peripheries of gondwana new shrimp data from metavolcaniclastic and metasedimentary rocks of the kaczawa complex have yielded results that have provided significantly changed interpretations on their age and relationships for instance a siliciclastic sequence interpreted as belonging to the lower part of the kaczawa complex the gackowa sandstones and seemingly sourced using an array of geochemical and mineralogical evidence from nearby early ordovician volcanic rocks has surprisingly yielded zircon ages not younger than precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the neoproterozoic lusatian graywackes felsic metavolcaniclastic rocks embedded in the carbonate succession of the wojcieszow limestone have yielded shrimp zircon ages of c 497 ma thus supporting a late cambrian early ordovician age for this shallow water limestone sequence which had been proposed to be late ordovician silurian or younger in age based on controversial foraminifera evidence and more recently assigned to the mid cambrian based on newly found archaeocyathid fragments and age spectra of zircon populations in the metamudstone sequence of the radzimowice slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early palaeozoic rifting with source areas similar to those known from parts of gondwana e g nw africa overall the new shrimp zircon studies both in metavolcanic and metasedimentary sequences have considerably illuminated their affinities and should provide encouragement for further application of this technique in this region in general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along north gondwana during latest precambrian and early palaeozoic times

ordovician passive continental margin magmatism in the central european variscides u pb zircon data from the se part of the karkonosze izera massif sudetes sw poland approximately 500 ma old orthogneisses are widespread in the eastern part of the variscan belt and are commonly interpreted to have intruded mica schist series of assumed neoproterozoic age new shrimp zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the karkonosze izera massif sw poland indicate that they are late cambrian early ordovician rather than neoproterozoic in age based on the zircon age spectra distributed mainly between ca 500 and 660 ma with a few proterozoic inherited minimum ages of ca 970 and 1 825 ma younger zircon dates dispersed between ca 412 and 464 ma are interpreted as a result of pb loss likely caused by subsequent metamorphism consequently the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca 500 ma old orthogneiss protoliths with the pooled concordia age of 487 8 ma interpreted as the best approximation of the protolith intrusive age field relationships petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca 500 ma granite intrusions and indicate that their recent close proximity is the result of tectonic juxtaposition however both the mica schists enclosing the bimodal metavolcanic rocks and the orthogneisses are interpreted to represent a cambro ordovician passive continental margin sequence being part of the saxothuringian domain they are tectonically overlain to the east by hp t metamorphic units comprising morb type metaigneous rocks and delineating a tectonic suture separating the saxothuringian block in the west from an assumed continental block tepla barrandian to the south east

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

u pb dating of serpentinization hydrothermal zircon from a metasomatic rodingite shell sudetic ophiolite sw poland abundant zircon grains were found within a metasomatic chlorite corrensite trioctahedral regularly interstratified chlorite smectite shell that surrounds rodingite from the jordanow gogolow jg serpentinite massif sudetic ophiolite bohemian massif the zircon grains bear two types of primary fluid inclusions containing either liquid co2 or aqueous solutions the inclusions were used to assign the p t conditions of the zircon formation 270 300 c ca 1 kbar using the crossed isochore method abraded single zircon grains give a u pb age of 400 4 3 ma which is interpreted as the age of zircon crystallization this zircon formed during rodingitization i e a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process the zircon age from the jordanow gogolow massif is consistent with the crystallization age of metagabbro protoliths from the sudetic ophiolite as determined by oliver et al j geol soc lond 150 1993 355 a ce anomaly combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from sudetic ophiolite suggests that the sm nd whole rock isotopic data of pin et al lithos 21 1988 195 353 21 ma possibly characterize the age of later metamorphic event

estimating thermal conductivity in sedimentary basins using lithologic data and geophysical well logs a method for estimating in situ thermal conductivity profiles in oil and gas wells is advanced to rectify a major shortcoming in thermal analyses of sedimentary basins thermal conductivity estimates are made in a two stage procedure and are based on a model for the conductivity of mixtures and input data from lithological and geophysical logs first rock matrix conductivity for an arbitrary depth interval i e drill cuttings sample interval which is about 3 m is determined from the laboratory calibrated conductivities and volumetric representation of its individual lithologic components using a geometric mean model in situ conductivity is then estimated by a second application of the model correcting for temperature and porosity determined from geophysical logs the method is illustrated for three uinta basin utah wells that penetrate a series of tertiary sandstones shales and muddy carbonates detailed lithologic descriptions together with sonic and neutron logs were digitized and used for estimating in situ conductivity the validity of the method was tested by comparing the prediction against laboratory measurements on 565 samples from the same wells rock matrix thermal conductivity ranges from 1 6 to 6 8 w m k and is predicted within 20 of the actual measurement for 90 of the samples both in situ conductivity values and variations for a given lithologic unit are reduced at increased porosity and increased temperature thermal conductivity nomograms are presented as useful tools to predict directly the in situ thermal conductivit of a formation from a well log signal

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

zircon shrimp u pb ages and in situ hf isotopic analysis for the mesozoic intrusions in south taihang north china craton evidence for hybridization between mantle derived magmas and crustal components abstract the north china craton ncc was stabilized in paleo proterozoic times and the eastern part of it was remobilized in the mesozoic as is indicated by the presence of voluminous mesozoic magmas in eastern ncc in particular the lithospheric mantle beneath eastern ncc was significantly thinned during the intense mesozoic tectono magmatic activity which makes the ncc different from most other ancient cratons the mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the ncc from craton to a tectonically active region the south taihang magmatic complex lies in the western part of the mesozoic magmatic zone in the ncc the complex falls into three main plutonic rocks the hongshan syenite the xishu gabbroic diorite monzonite and the wu an monzonitic rocks zircon u pb dating reveals that the three plutonic rocks were emplaced contemporaneously at 132 ma in situ hf isotopic analyses for the dated zircons indicate that the hongshan syenites show homogeneous hf t values 10 to 13 which are slightly lower than those 9 0 for the enriched subcontinental lithospheric mantle sclm as revealed by zircons from mafic rocks that originated from melting of sclm this suggests that the parental magma to the syenites was likely derived from melting of the enriched sclm and contaminated slightly by old lower continental crustal material during ascent the xishu and wu an gabbroic diorites to monzonitic rocks however show significantly varied hf isotopic compositions with hf t 9 2 to 16 and 15 5 to 24 respectively the wide range of hf t values for a single sample suggests that the xishu and wu an plutons formed through a process of hybridization between enriched mantle derived mafic magma and crustal components in variable proportions this is supported by the textural and compositional disequilibrium of plagioclase phenocrysts from the two plutons the wu an monzonitic rocks show lower hf t values than the xishu gabbroic diorite monzonite suggesting more incorporation of crustal components in the source the sudden surge of magmatic activity in the south taihang orogen took place as a result of lithospheric extension that caused partial fusion of the enriched portions of sclm generating voluminous mafic magmas underplating of the mafic magmas in the lower crust consequently caused partial melting of the old basement rocks to produce granitic melts which was followed by mixing between the mafic and granitic magmas

the izera metabasites west sudetes poland geologic and isotopic u pb zircon evidence of devonian extension in the saxothuringian terrane abstract in the lausitz izera massif of the saxothuringian terrane part of the central european variscides 515 480 ma metagranites have been intruded by a swarm of basic veins that have within plate alkali basalt geochemistry or rarely normal mid ocean ridge basalt n morb geochemistry based on their spatial orientation distribution in the region and structural relationships to the host metagranites these basic veins could be grouped into three types that displayed six varieties of host vein contacts zircons extracted from eight of the basic veins revealed that there were two populations clear zircons and brown zircons each type were subjected to both single grain conventional u pb analysis and to sensitive high resolution ion microprobe shrimp u pb analysis the clear grains yielded ages 480 ma clustering at 500 ma the brown grains yielded ages between 390 and 365 ma clustering at 370 ma the colourless zircons possess an oscillatory zoned structure and rare earth element ree distribution pattern that suggests they come from the host granites and from deeper neoproterozoic granitoid basement however the basic host rocks show almost no obvious contamination by felsic material the turbid brown grains which are dark under cathodoluminescence also seem to be mainly magmatic because of their internal structures high th u ratios and ree pattern when the zircon results are integrated into the regional geology an 30 m y period of episodic intra plate predominantly alkaline basic magmatism at the passive margin of the saxothuringian terrane is suggested with maximum activity at 370 ma during an extensional regime combining the factors of the relatively high thermal gradient during early metamorphism of the metabasites 450 c 2 kbar the geology of saxothuringia and the regional structural data we propose a plume model of hot fingers type whereby the basic magmatism of the izera region was due to repeated devonian intrusions derived from garnet bearing enriched oib type and less commonly depleted asthenosphere sources the waning stages of this activity were coeval with the early deformation of the izera granite host basic vein system and matched by rifting and extensional widening of the saxothuringian basin in its izera kaczawa segment

a structural model for the western central sudetes a deformed stack of variscan thrust sheets a model based on new structural and geochemical data is presented it unifies the structural history of the izera rudawy janowickie and kaczawa complexes with the fore sudetic block despite their current separation by the intra sudetic and marginal sudetic faults above the granitoid izera kowary and w droze gneisses at the base of the structural sequence the ductile kowary shear zone marks the basal decollement of the wierzawa thrust sheet comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites above it the ductile kaczorow shear zone corresponding to the main mylonitic zone within the leszczyniec shear zone in the rudawy janowickie complex marks the base of the dobromierz thrust sheet characterized by voluminous morb like meta tholeiites and minor metasediments in the higher parts of the rudawy janowickie and kaczawa complexes and the pyszczynska hill area of the fore sudetic block in the east the sleza ophiolite and the gory sowie block override the entire nappe stack kinematic fabrics in the major and related shear zones indicate d 1 compressional transport towards the northwest followed by minor d 2 extensional movements the thrust stack was deformed during d 3 by southwest verging folds was subsequently intruded by post orogenic granites and later disrupted by the intra sudetic marginal sudetic and associated faults

the tepla saxothuringian suture in the karkonosze izera massif western sudetes central european variscides the southern and eastern karkonosze izera massif northern bohemian massif exposes blueschist facies rocks and morb type magmatic complexes during late devonian to early carboniferous times these were overthrust within a nappe pile toward the nw onto the pre variscan saxothuringian basement composed of the izera kowary metagranitoids and their envelope the lowermost nappe or parautochthonous unit of the pile is the low grade metamorphosed jest d complex comprising a devonian to early carboniferous sedimentary succession of the saxothuringian passive margin this is tectonically overlain by the south karkonosze complex which represents ordovician silurian volcano sedimentary infill of the saxothuringian basin affected by late devonian hp metamorphism the uppermost nappe is the early palaeozoic epidote amphibolite grade leszczyniec morb like complex cropping out on the eastern margin of the karkonosze izera massif it probably represents a fragment of obducted saxothuringian basin floor the nappe pile was stacked beneath the overriding upper plate margin now concealed below the intra sudetic basin and hypothesized to represent a fragment of the tepla barrandian terrane the nappe stacking triggered by buoyancy controlled upward extrusion of the subducted continental slab was the main mechanism for the exhumation of hp rocks the final stages of the nw ward nappe stacking were accompanied and followed by se directed early carboniferous extensional collapse the lower plate of the suture zone was uplifted at that time and intruded by the 330 ma old nearly undeformed karkonosze granite pluton as a result of the collapse the tepla barrandian upper plate was downthrown on shear zones and brittle faults and buried under several km thick synorogenic late tournaisian through namurian and post orogenic late carboniferous early permian succession of the intra sudetic basin the south and east karkonosze suture most probably is a fragment of the tepla saxothuringian munchberg tepla suture belt known from the western bohemian massif

geochronology and nd sr systematics of lusatian granitoids significance for the evolution of the variscan orogen in east central europe a variety of pre variscan granitoids and two variscan monzogranites occurring in the central and western parts of the lusatian granodiorite complex lgc saxonia were dated by the single zircon evaporation method complemented by whole rock nd isotopic data and rb sr whole rock and mineral ages the virtually undeformed pre variscan granitoids constitute a genetically related mostly peraluminous magmatic suite ranging in composition from two mica granodiorite muscovitebearing biotite quartz diorite tonalite and granodiorite to biotite granodiorite and monozogranite

high heat flow anomalies over salt structures on the texas continental slope gulf of mexico we made 74 closely spaced 2 km apart heat flow measurements around and over two salt structures on the texas continental slope gulf of mexico the values outlined the shape of the heat flow anomalies over both structures based on a preceding high resolution seismic survey we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall shaped feeder the heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation the values over the salt plug are nearly all greater than 70 mw m the measurements over the salt tongue have a sharp heat flow peak of 90 mw m associated with the presumed feeder and rather uniform values around 60 mw m over the remainder the variation of heat flow over both structures is smooth and shows no apparent scatter heat flow values off these features are uniformly low around 30 mw m thermal effects from bottom water temperature fluctuation slope sedimentation diapiric movement of the salt body and pore fluid migration appear unable to provide a satisfactory explanation for the observations however thickness variations of a highly conductive salt body can easily account for the heat flow anomalies we suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

geology sulfide mineralogy and petrogenesis of the angstberg ni cu pge sulfide mineralization lausitz block bohemian massif germany a potential ni cu exploration target in central europe abstract germany s largest known magmatic ni cu pge sulfide enrichment is associated with the gabbroic angstberg intrusion that is one of several small scaled ni cu sulfide bearing intrusions in the northern bohemian massif central european variscan belt the intrusions are part of a middle devonian to early carboniferous mafic ultramafic magmatic system that emplaced within cadomian granodiorites of the lausitz block the angstberg intrusion represents an exposed sub vertical dike like body 320 60 m in plan view and comprises olivine gabbronorite olivine gabbro gabbro olivine hornblende gabbro and hornblende pyroxene gabbro the hornblende bearing and hornblende free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their ti lree hree and volatile contents fractionation probably took place in deeper seated sections of the magmatic system possibly within staging magma chamber s overall enriched incompatible trace element patterns suggest that the angstberg lithologies are linked with intra plate magmatism likely associated with a transtensional setting in the course of the variscan orogeny the previously suggested devonian age of the intrusion is confirmed by an 40ar 39ar biotite age of 373 4 0 9 ma for the olivine gabbro the magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine richest parts of the olivine gabbronorite and olivine hornblende gabbro lithologies in addition some sulfide matrix breccias up to 70 modal sulfides occur directly on the footwall contact of the intrusion to granodiorite both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite and accessory pge au and ag phases nickel rich bismuthotellurides of the melonite merenskyite solid solution series and froodite are the most common platinum group minerals disseminated sulfides are characterized by high ni moderate cu and low pt and pd tenors 4 9 wt ni 2 6 wt cu 200 800 ppb pt 200 1000 ppb pd sulfide matrix breccias feature comparable ni but lower cu pt and pd tenors 4 6 wt ni 0 3 1 wt cu 10 100 ppb pt 40 200 ppb pd fractional crystallization and wall rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the angstberg intrusion sulfur isotope compositions of pyrrhotite pentlandite chalcopyrite assemblages 1 4 to 0 2 vcdt propose a mantle origin of s whereas addition of crustal s cannot be completely ruled out as the s isotope values of the sulfides overlap with those of the granodiorite 7 to 4 vcdt olivine and sulfide textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine rich crystal mush during magma rise to upper levels the sulfide matrix breccias also resulted from entrained sulfide melts or from gravity driven downward percolation of sulfide melts from stratigraphically higher now eroded levels in both cases the formation of sulfide matrix breccias most likely was amplified by in situ thermomechanical erosion of the granodioritic wall rock caused by flash boiling of thermally dehydrating biotite despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern bohemian massif the angstberg intrusion as well as other neighboring magmatic sulfide occurrences may represent a new exploration target for ni and cu in central europe

dissolution reprecipitation of igneous zircon in mid ocean ridge gabbro atlantis bank southwest indian ridge abstract zircons recovered from oceanic gabbro exposed on atlantis bank southwest indian ridge typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas in one rock of twenty investigated weak oscillatory zonation patterns are overprinted by secondary textural features characterized by mottled convoluted and wavy internal zonation patterns that are frequently associated with secondary micron to submicron scale micro porosity these zircons are hosted in a felsic vein that intruded an oxide gabbro both of which are cross cut by monomineralic amphibole and quartz rich veinlets zircons with weak oscillatory zonation patterns record a weighted average 206pb 238u age of 12 76 0 20 ma mswd 1 5 and have high trace element concentrations e g rees 0 4 2 2 wt y 0 6 2 8 wt p 0 4 0 9 wt and th u 0 1 0 5 these zircons are anomalously old 1 myr relative to the magnetic age for this portion of oceanic crust 11 75 ma in contrast zircons with non igneous secondary textures have a younger weighted average 206pb 238u age of 12 00 0 16 ma mswd 1 7 and have lower trace element concentrations e g rees 0 2 0 8 wt y 0 3 1 0 wt p 0 1 0 3 wt and slightly lower th u 0 1 0 3 the weighted average age of these zircons is similar to the magnetic anomaly age and other 206pb 238u ages of nearby rocks we do not observe a correlation between crystallographic misorientation internal texture or trace element chemistry we suggest that the decrease in trace element concentrations associated with the development of non igneous alteration textures is attributed to the purging of non essential structural constituent cations from the zircon crystal lattice at amphibolite facies conditions the mechanism of alteration re equilibration was likely an interface coupled dissolution reprecipitation processes that affected pre existing anomalously old zircons during shallow level magmatic construction of atlantis bank at 12 0 ma

geochemical evidence for progressive rift related early palaeozoic volcanism in the western sudetes the volcanigenic rocks of the kaczawa mts western sudetes poland in the eastern variscides show changing geological and geochemical evolution during early palaeozoic time the lower part of the succession cambrian ordovician has three components 1 shallow marine to subaerial metabasalts associated with limestones and volcaniclastics the lavas are dominantly of a transitional tholeiitic alkaline type and their trace element patterns typically represent a rift related environment they pass laterally and upwards into more depleted basalts resembling enriched morb with nd isotopic characteristics indicating contamination by continental crust 2 interlayered rhyodacitic lavas and volcaniclastics which show negative nd values suggesting formation of the original magma by crustal melting 3 an overlying alkaline bimodal suite of lavas and volcaniclastic rocks as well as alkaline metabasites of shallow intrusive character the geochemistry of the latter resembles oceanic island volcanics but they may well have been emplaced in the same evolving rift environment the upper part of the kaczawa succession ordovician silurian in age comprises a thick and monotonous sequence of deep marine pillowed and massive metabasalts associated with black shales and cherts these lavas exhibit morb trace element characteristics with minor evidence of crustal contamination during this stage of rifting true oceanic crust probably formed it is thus suggested that the studied part of the kaczawa succession developed in a progressively evolving rift initially within an ensialic environment and finally reaching the stage of a basin underlain by oceanic type crust together with similar cambrian ordovician volcanic sedimentary associations widely distributed in western europe from portugal through france and germany they represent a record of the early palaeozoic rifting in the northern periphery of gondwana

holocene sea level database from the atlantic coast of europe abstract high quality relative sea level rsl data reveal spatial and temporal variations in crustal movements during the holocene which are used for many applications ranging from calibrating models of earth rheology and ice sheet reconstructions to the development of coastal lowlands and human occupation here we present a holocene rsl database for the atlantic coast of europe ace and estimate rates of rsl change from the ace database using a spatio temporal empirical hierarchical model the database contains 214 index points which locate the rsl position in space and time and 126 limiting dates which constrain rsl to above or below a certain elevation at a specific point in time the temporal distribution extends from present to 11 5 ka with only 42 index points older than 7 ka the spatial distribution spans 1700 km from french flanders france to algarve portugal with more than half of the index points concentrated along the french coast the ace database shows rsl was below present during the holocene rates of rsl change were highest during the early holocene ranging between 6 8 0 5 mm yr 1 in middle portugal and 6 3 0 8 mm yr 1 in southern france from 10 to 7 ka mid to late holocene rates decreased over time with rates ranging between 0 9 0 4 mm yr 1 in middle france and 0 1 0 5 mm yr 1 in middle portugal from 4 ka to present comparison of the rsl data to output from a glacial isostatic adjustment model suggests that deglaciation of the british irish and fennoscandian ice sheets dominates the large scale variability captured by the ace database which reflects a decreasing influence of the collapsing british irish and fennoscandian peripheral forebulge that migrated from the northeast to the northwest after 4 ka

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

geochemistry and 40ar 39ar geochronology of the mafic metavolcanic rocks from the rýchory mountains complex west sudetes bohemian massif palaeotectonic significance the mafic metavolcanic rocks blueschists and greenschists of the r chory mountains crystalline complex west sudetes experienced sea floor hydrothermal alteration spilitization prior to regional metamorphism the metabasite geochemistry namely trace element and ree abundances indicates that the protolith was comparable in composition with 1 tholeiitic to transitional ocean floor basalts and 2 transitional and alkaline intra oceanic island basalts two main metamorphic events affected the r chory mountains metabasites in an earlier high pressure low temperature metamorphic event the rocks experienced blueschist facies metamorphism the results of 40 ar 39 ar geochronology studies on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 ma the greenschist metamorphic overprint followed around 340 ma the elongated bodies of mafic metavolcanic rocks are situated within the prominent nne ssw leszczyniec shear zone following the trend of the r chory mountains and the rudawy janowickie mountains both the geochemical affinities and the blueschist facies metamorphism of the metabasites suggest that this shear zone evolved from the variscan suture dividing western and central terranes of the west sudetes according to the radiometric age for the end of the high pressure low temperature metamorphism the terranes accreted during the famennian a considerable time span between the formation of the metabasite protolith and the blueschist metamorphism may indicate long lasting subduction of a large oceanic plate between gondwana and laurussia possibly accompanied by terrane accretion prior to the variscan orogeny

glaucophane bearing assemblage overprinted by greenschist facies metamorphism in the variscan kaczawa complex sudetes poland an early palaeozoic ordovician metamudstone sequence near wojcieszow kaczawa mts western sudetes poland contains numerous metabasite sills up to 50 m thick these subvolcanic rocks are of within plate alkali basalt type primary igneous phases in the metabasites clinopyroxene salite and kaersutite are veined and partly replaced by complex metamorphic mineral assemblages particularly the kaersutite is corroded and rimmed by zoned sodic sodic calcic and calcic amphiboles the matrix is composed of actinolite pycnochlorite albite an 0 5 epidote ps 27 33 titanite calcite opaques and occasionally biotite phengite and stilpnomelane the sodic amphiboles are glaucophane to crossite in composition with nab from 1 9 to 1 6 they are rimmed successively by sodic calcic and calcic amphiboles with compositions ranging from magnesioferri winchite to actinolite no compositions between nab 0 92 and nab 1 56 have been ascertained the textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist or blueschist greenschist transitional assemblage the presence of glaucophane and no traces of a jadeitic pyroxene quartz association indicate pressures between 6 and 12 kbar during the high pressure episode temperature is difficult to assess in this metamorphic event the replacement of glaucophane by actinolite chlorite albite with associated epidote allows restriction of the upper pressure limit of the greenschist recrystallization to 8 kbar between 350 and 450 c the mineral assemblage representing the greenschist episode suggests the p t conditions of the high pressure part of the chlorite or lower biotite zone the latest metamorphic recrystallization under the greenschist facies may have taken place in the visean

single zircon ages and whole rock nd isotopic systematics of early palaeozoic granitoid gneisses from the czech and polish sudetes jizerske hory krkonose mountains and orlice sněžnik complex granitoid orthogneisses make up the predominant rock type in the west sudetes from jizerske hory in the nw to the orlicke hory and sn nik mountains in the se these generally strongly foliated gneisses are calc alkaline in composition and display trace element characteristics suggesting generation in a volcanic arc setting single zircon ages reflecting the time of emplacement of the gneiss protoliths define a relatively narrow cambro ordovician range between 502 and 515 ma this is similar to previously reported zircon ages from the czech and polish west sudetes and documents an important and regionally extensive post cadomian magmatic event that we relate to continental arc magmatism on the margin of avalonia that developed during closure of the tornquist ocean an age of 492 ma for a microgranite dyke cutting deformed and metamorphosed orthogneisses in the orlicke hory shows the main deformation to be early paleozoic zircon xenocryst minimum ages range between 546 and 2070 ma and show maxima in the cadomian pan african 550 850 ma and grenvillian 1000 1300 ma time brackets the grenvillian event is also evident from nd mean crustal residence ages that vary between 1 34 and 1 87 ga from these data we suggest that the pre variscan granitoid gneisses of the czech west sudetes were largely generated by melting of a predominantly grenville age basement that was part of the northern margin of gondwana and may have been related to grenville age basement now identified in northern south america

early palaeozoic initial rift volcanism in the central european variscides the kaczawa mountains sudetes sw poland evidence from sims dating of zircons early palaeozoic volcanic suites are widespread throughout the variscan belt and have commonly been ascribed to incipient rifting along the northern periphery of gondwana in cambrian to ordovician times their distribution across europe defines the present day extent of gondwana derived terranes and constrains the timing of their separation from the gondwanan margin the kaczawa mountains in the west sudetes at the eastern termination of the variscan belt include bimodal rift related rocks but their protolith age and hence their significance have been highly uncertain we have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite yielding ages of 502 4 2 6 ma and 485 7 1 6 ma respectively this constrains the initial rift magmatism to c 500 485 ma in this part of the european variscides the rift related early palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri gondwanan terranes of europe

three dimensional inverse modeling of the refractive heat flow anomaly associated with salt diapirism this article introduces a technique for three dimensional inverse modeling of geothermal heat conduction through heterogeneous media the technique is used to determine the basal geometry of a diapiric salt structure found on the continental slope offshore texas salt is two to four times more thermally conductive than other sedimentary rocks the geothermal field is perturbed by the presence of salt and results in an anomaly in the heat flow through the seafloor the spatial variation pattern of the anomalous heat flow reflects the geometry of the salt body the inverse modeling obtains a model for the thermal conductivity structure that causes the heat flow anomaly observed on the seafloor the inversion algorithm systematically searches for an optimal thermal conductivity model by iteratively minimizing the misfit between the model predicted and the observed heat flow to reduce the problem of nonuniqueness the inversion incorporates a priori information constrained independently such as the upper surface geometry of the salt and the lateral extent of the salt body which can be delineated by a limited coverage of two dimensional seismic data in addition it is assumed that the thermal conductivity of the sedimentary strata surrounding the salt body is well constrained this inversion method is applied to a heat flow data set obtained over a salt structure on the texas continental slope the salt structure was first surveyed with the single channel seismic reflection which yielded the a priori information necessary the base of the salt was not imaged seismically then three dozen heat flow measurements were obtained on the seafloor over and off the salt feature the inverse heat flow modeling performed here shows that this structure is a salt tongue which has a diapiric root on one side according to the most optimal thermal conductivity model obtained the root seems to extend to 6 km below the seafloor refinement in the model geometry and additional constraints on thermal conductivities of the surrounding strata should yield a model that is more detailed and would allow more thorough geological interpretation of the salt structure