the origin of rapakivi texture by sub isothermal decompression abstract the rapakivi texture plagioclase mantled k feldspar ovoids rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine grained granitic matrix has been studied in five proterozoic 1 64 1 55 ga anorogenic rapakivi granite batholiths in the fennoscandian shield with emphasis on mineral stability and inherited cores k feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone when the k feldspar ovoids are mantled by plagioclase the outermost part of the mantle is homogeneous but towards the k feldspar a skeletal texture develops intensive parameters obtained from different textural positions show that mineral inclusions in cores of the k feldspar ovoids and quartz megacrysts were formed at low t 680 720 c and high p 5 6 kbar conditions while inclusions in the periphery of ovoids and the plagioclase mantles display high t 780 c and intermediate to low p 3 5 1 kbar the lowest p is comparable to that during solidification of the matrix the total water equivalent of the volatile content in the magma has been calculated as 2 5 the amount of solids in the magma at 1 kbar has been estimated as 40 theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal saturated kfsp qtz pl 60 and volatile undersaturated a type granite magma was transported under approximately constant temperature 760 780 c from the lower middle crust 5 6 kbar to upper crustal levels according to phase stabilities in the eutectoid granite system quartz and k feldspar were resorbed but plagioclase remained stable and precipitated during this sub isothermal rise of magma textural presence of disequilibrium textures and mineralogical presence of different mineral assemblages including relics evidence of a sub isothermal rise of the rapakivi magmas is better preserved in subvolcanic and contact varieties of rapakivi granites than in the more deep seated rapakivi granites formed by slow cooling

rapid generation of both high and low δ18o large volume silicic magmas at the timber mountain oasis valley caldera complex nevada we present an oxygen isotope and pet rologic study of four voluminous zoned ash flow sheets of the southwestern nevada volcanic field swnvf topopah spring ts 1200 km 3 12 8 ma tiva canyon tc 1000 km in the basin and range province and it fa vors nearly total evacuation of the magma chamber during a single eruption each si licic magma unit was generated by a shal low influx of new mafic magma that melted 18 o 16 o depleted as in the case of at or 18 o 16 o enriched rm ts rocks the at tuff and associated pre and post caldera lavas are 2 5 lower in d 18 o than in the rm tuff and represent the largest known low d 18 o magma we find that all units of the at cycle contain isotopically zoned zir cons that have up to 2 core to rim zon ing and correspondingly smaller out of equilibrium quartz zircon and melt zircon fractionations air abraded cores of quartz and sphene do not preserve any d 18 o zon ing the higher d 18 o zircon cores in low d

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

endogenous growth of dacite dome at unzen volcano japan 1993 1994 a dacite dome at unzen volcano grew mainly exogenously when it was small and the effusion rate was high but endogenously when the dome became large and the effusion rate declined the endogenous dome that has grown since late 1993 shows a shape classified as pelean whereas the earlier stage had a low lava dome shape the carapace of the endogenous dome moved like the crust of a basaltic lava pillow although the dome is several hundred times larger than such pillows the surface carapace was carried from the inside of the dome where it had been produced and it thickened as it cooled the crater floor was strongly deformed by the advancing endogenous dome the movement and crater floor deformation can be compared to that of a tractor tread moving on unconsolidated ground

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

provenance constraints from detrital zircon u pb ages in the nw iberian massif implications for palaeozoic plate configuration and variscan evolution detrital zircons from pre orogenic upper ordovician to devonian low grade metasedimentary rocks have been dated by laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 constrain the variations of zircon age populations within the autochthonous sequences of nw iberia 2 compare u pb detrital zircon ages with previous data 3 test the hypothesis of the armorica microplate as a peri gondwanan terrane separated from gondwana the similarity in the ages of detrital zircons found in the palaeozoic samples studied here to those published for the upper proterozoic lower cambrian and lower ordovician sediments of nw iberia argues against the separation of armorica and points to a common source area for all of the palaeozoic detrital formations of nw iberia the west african craton and the surrounding pan african belts la icp ms u pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as early namurian this age is also a maximum age limit for thrusting and refolding in this part of the iberian massif correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of nw iberia and synorogenic sedimentation

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

the transition from pangea amalgamation to fragmentation constraints from detrital zircon geochronology on west iberia paleogeography and sediment sources abstract detrital zircon u pb geochronology data from late carboniferous to triassic clastic sedimentary rocks in sw iberia were used to investigate the regional paleogeography during the transition from pangea amalgamation to break up the major u pb zircon age peaks are middle devonian to carboniferous 390 300 ma cambrian ordovician 530 440 ma cryogenian ediacaran 750 540 ma stenian tonian 1 2 0 9 ga and paleoproterozoic 2 3 1 8 ga rapid exhumation of variscan crystalline rocks at the contact between the south portuguese zone and ossa morena zone explains the abundance of late paleozoic ages in the upper carboniferous lower permian continental successions the u pb zircon data constrain the maximum depositional age of the santa susana basin to c 304 ma and the viar basin to c 297 ma the triassic sequences despite being c 100 ma younger than the variscan tectonothermal events contain low proportions of late paleozoic zircon the major peaks in all zircon spectra closely resemble those found in the adjacent basement rocks indicating small source areas mainly located near the rift shoulders longer travelled fluvial systems are postulated for the eastern portions of the algarve basin which was closer to the westward advancing tethys ocean than the rift basins of west iberia sequences that contain significant proportions of 1 2 0 9 ga zircon are probably recycled from post collisional carboniferous permian continental deposits that were more extensive than those found today

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

rapakivi granites and related rocks an introduction abstract this article is an introductory chapter for the proceedings of the last seventh meeting of the international geological correlation programme igcp project 315 correlation of rapakivi granites and related rocks on a global scale 1991 1996 based on current knowledge progress made during the six years of the project and the seven papers published in this special issue a synthesis of the age distribution geotectonic environment petrogenesis geochemistry and metallogeny of the rapakivi granites and related rocks is presented

granites built by sheeting in a fault stepover the sanabria massifs variscan orogen nw spain the sanabria region central iberian zone variscan belt of spain shows an asymmetric thermal dome marked by migmatites accompanied by the ribadelago and the sotillo plutons these small plutons display pronounced mineralogical variations biotite granodiorites and tonalites prevail and granites and gabbros are common both plutons are confined in the releasing stepover of a transpressional shear zone that strikes 120 e and dips 70 sw most of the igneous rocks form sheets parallel to the shear zone magnetic foliations and lineations in the igneous rocks are parallel respectively to the shear bands and stretching lineations observed in the shear zone the formation of these igneous sheets at high angle to the main axis of the regional field stress is explained by a combination of the fault valve behaviour of the shear zone with the power of melt overpressure to open and ascend through previously formed planar structures like s or c planes

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

role of the ollo de sapo massive felsic volcanism of nw iberia in the early ordovician dynamics of northern gondwana abstract the ollo de sapo fm in the sanabria region of nw iberia consists of subvolcanic volcanic and volcano sedimentary rocks underlying and partly intercalated with early ordovician platform facies sediments granitic plutons of the same age presently transformed into orthogneisses are genetically related the metavolcanic rocks are dacites and rhyolites with potassic and peraluminous character and volcanic arc signature possibly inherited from the melting of calc alkaline crustal material in the study area the ollo de sapo fm is made up of two large superposed volcanic domes separated by a thin screen of contemporaneous metapelites and sandstones the domes underwent endogenous growth with subvolcanic phenocryst rich magma intruding and cooling under a volcanic and volcaniclastic shield the western dome tapers to the east and underlies the younger eastern dome suggesting that the whole formation consists of several volcanic edifices replacing each other along the 570 km outcrop belt of the ollo de sapo fm two new u pb protolith ages of 488 6 ma and 472 12 ma match within error the known age span for the ollo de sapo fm in iberia 495 483 ma the younger age dates an augen gneiss at the core of the eastern volcanic dome whereas the older date comes from the overlying volcanic sequence suggesting intrusion of the augen gneiss protolith into the overlying volcanic rocks an incipient rift involving necking of crustal scale boudinage a few hundred km landward of the edge but still within the wide passive margin of northern gondwana may explain the linear distribution of the ollo de sapo fm and the limited time span of massive volcanism the heat for crustal melting was probably supplied by mafic magmas intruding or underplatting the lower crust the early ordovician felsic magmatism of nw and central iberia seems to form part of a large siliceous province linked to break up and terrane dispersion of the northern gondwana realm

precambrian protoliths and early paleozoic magmatism in the french massif central u pb data and the north gondwana connection in the west european variscan belt abstract u pb geochronological data were collected on zircon by la mc icp ms on orthogneiss and paragneiss from the limousin area in the french massif central fmc in order to investigate the connection between the west european variscan belt and the northern margin of gondwana fifteen samples were collected in the four main tectonostratigraphic units of the fmc namely the para autochthonous unit the lower gneiss unit lgu the upper gneiss unit ugu and the thiviers payzac unit orthogneiss yield intrusion age between 521 7 and 446 6 ma considering all the results from both magmatic and metasedimentary samples two peaks at 531 and 473 ma are recognized rifting processes taking place along the north gondwana margin during the lower paleozoic went on until the end of the ordovician as suggested by the magmatic event recorded around 450 ma several maximum depositional ages were ascertained in the metasedimentary formations of the fmc as determined by the youngest detected detrital zircon crystal ranging from 604 16 ma for metasediments of the para autochthonous unit 573 12 ma for the lgu 564 9 ma for the thiviers payzac unit and 523 4 ma in the ugu minimum depositional ages are given by magmatic emplacement ages obtained in the crosscutting orthogneiss there is some evidence for a decrease of this maximum age upwards in the tectonostratigraphy detrital zircon in metasedimentary formations and inherited zircon in orthogneiss display a wide spectrum of ages with significant peaks at around 590 ma and 560 ma archean paleoproterozoic and neoproterozoic detrital zircons suggest a west african craton source of the sedimentation the large amount of neoproterozoic and lower paleozoic ages obtained in this study suggests that these periods played a significant role in the continental crustal growth history of western europe

early ordovician orogenic event in galicia nw spain evidence from u pb ages in the uppermost unit of the ordenes complex new u pb data zircon monazite and rutile obtained from rocks of the uppermost allochthon in the variscan belt of nw spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre variscan tectonothermal evolution this evolution is characterised by an early ordovician 498 500 ma bimodal magmatism followed almost immediately 493 498 ma by a barrovian style metamorphism up to the granulite facies the metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the barrovian facies pattern shown by the lithologies of the uppermost allochthon the almost coeval magmatism and metamorphism and the chemistry of the metabasites suggest an accretionary complex probably related to a volcanic arc as the most probable setting for the origin of the uppermost unit the implications of the existence of early ordovician convergent plate boundaries are discussed in the context of gondwana laurentia avalon interactions

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

what processes control the net currents through shallow straits a review with application to the bohai strait china we review the key processes controlling the net water circulation in shallow straits the circulation is mainly barotropic driven by the difference in mean sea level and tides on either side of the strait the wind the meandering of the currents in the adjoining seas the bathymetry including the shape and width of the mouths and the presence of narrows shoals sills islands and lateral embayments as the oceanography forcing can be non synchronised on either side of the strait the resulting net currents through the strait may appear chaotic we apply these findings to the net circulation through the bohai strait in china using local wind remote sensing data of currents and sea level and a hydrodynamic model the net currents through the bohai strait were episodic they were largest during winter storms when the wind generated a cyclonic coastal current in the northern yellow sea that flowed across the width of the bohai strait like the luzon strait this current meandered and a branch of the current intruded into the northern bohai strait along the deeper channel the wind set up in the bohai sea and the wind driven longshore current in the southern bohai sea generated a seaward flow through the southern bohai strait somewhat like in irbe strait as with the straits of the seto inland sea stagnation occurred in some embayments as in the torres strait the islands in the southern bohai strait retarded the net flow and increased the width of the outflow zone these flows were also modulated at very long periods 60 days by long waves in the yellow sea possibly generated by meandering of the kuroshio current that created an additional sea level slope through the strait the transit time in the bohai strait was 60 and 10 days respectively during calm weather and winter storms the residence time in the bohai sea was estimated to be about 1 68 years by the loicz model and 1 56 years by the hydrodynamic model and the influence of the river runoff including the yellow river was negligible

lower paleozoic extensional tectonics in the limit between the west asturian leonese and central iberian zones of the variscan fold belt in nw spain the limit between the west asturian leonese and central iberian zones in the nw of the iberian peninsula is described in the area of caurel truchas provinces of orense and leon from the distribution of the sedimentary formations and the variscan structures it is inferred that the transition between the two zones was a fault system which was active during at least the ordovician and lower silurian the faults were supposedly normal though they had probably some strike slip component and gave rise to a half graben in which a syn rift sequence was deposited the latter underwent inversion at the onset of the variscan tectonism producing a ramp anticline syncline pair that forms the more important folds in the area the varying geometry of these folds is interpreted partly as being due to the existence of previous en echelon folds related to the strike slip component of the faults and partly as a result of the variable intensity of the shear strain undergone during the first variscan phase

u pb zircon and monazite geochronology of variscan magmatism related to syn convergence extension in central northern portugal abstract the viseu area is located in the central iberian zone of the iberian variscan belt and hosts numerous post thickening collision related granitoids intruded into upper and middle crustal levels the present paper reports high precision u pb zircon and monazite ages for four plutons of the viseu area the syn kinematic granitoids of maceira 314 5 ma casal vasco 311 1 ma and junqueira 307 8 0 7 ma and the late kinematic biotite monzogranites of cota 306 9 ma this points to a synchronous emplacement of the different syn kinematic plutons shortly followed by the intrusion of the late kinematic granites and shows that the upper carboniferous plutonism occurred within a short time span of ca 10 million years the ascent of granite magmas took place after an extensional tectonic event d 2 and is coeval with dextral and sinistral crustal scale transcurrent shearing d 3 field and petrographical evidence suggest a narrow time span between peak t metamorphic conditions and the intrusion of granitic melts which implies very fast uplift rates accommodated through active tectonic exhumation magma compositions evolve through time reflecting an increasing involvement of mid crustal sources and the underplating effect of an upwelling asthenospheric mantle at the base of a thinning and stretching continental crust

outline of upper precambrian and lower paleozoic evolution of the iberian peninsula according to u pb dating of zircons in the northwestern part of the iberian peninsula recent geological studies have defined the following stratigraphic sequence from bottom to top basal orthogneiss of miranda do douro is overlain by a complex of schist and greywacke which have been assigned to the cambrian or late precambrian these are followed by the ollo de sapo gneisses of volcanic sedimentary origin which in turn are overlain by the fossiliferous layers of the lower ordovician the schist and greywacke complex and the lower part of the ollo de sapo formation are transected by granitic dikes viana del bollo orthogneiss all these rocks were folded faulted metamorphosed and intruded by granite plutons during variscan times the emplacement of the miranda do douro and viana del bollo orthogneisses have been respectively dated at 618 9 ma and 465 10 ma by the u pb method on zircons these data yield lower and upper limits for the deposition of the ollo de sapo formation and demonstrate the occurrence of a precambrian basement pan african or cadomian orogeny in the variscan belt in the northwestern part of the iberian peninsula the emplacement of viana del bollo orthogneiss could be genetically related to the main phase of crustal thinning 470 520 ma characteristic of the prevariscan evolution of western europe typology and u pb data of the ollo de sapo zircons indicate the occurrence of inherited old precambrian zircons and yield an age of 325 3 ma for the variscan main thermal event in the area considered four possible evolutionary models for the u pb system of the ollo de sapo zircons are discussed

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

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

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

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

multiple early paleozoic volcanic events at the northern gondwana margin u pb age evidence from the southern variscan branch sardinia italy abstract in sardinia one of the southernmost remains of the european variscan belt a crustal section through different gondwanan paleodomains is largely preserved laser ablation icp ms u pb ages on undoubtedly igneous sites of the zircons were determined on the lower palaeozoic volcanic rocks constrained by defined field relationships thus evidencing three subsequent volcanic events i intermediate and felsic 491 7 3 5 ma 479 9 2 1 concordia ages transitional volcanic rocks embedded within a cambro ordovician terrigenous succession that occurs with continuity in external and inner nappes bounded to the top by the sardic unconformity ii this cambrian lower ordovician succession is cut by calc alkalic rhyodacites which yielded a concordia age of 465 4 1 4 ma confirming their pertinence to the huge bimodal mid ordovician arc volcanism commonly interpreted as the widespread marker of the rheic ocean subduction iii alkalic metaepiclastites in the external nappe within the post caradocian transgressive sequence dated at 440 1 7 ma likely related to rifting and collapse of the mid ordovician volcanic arc in the reshaped lower palaeozoic stratigraphy of sardinia the timing of the early steps of the variscan wilson cycle can be inferred

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

grain size variations in the murray formation stratigraphic evidence for changing depositional environments in gale crater mars the lowermost exposure of the murray formation in gale crater mars was interpreted as sediment deposited in an ancient lake based on data collected by the curiosity rover constraining the stratigraphic extent and duration of this environment has important implications for the paleohydrology of gale insights into early martian environments and paleofluid flow velocity can be obtained from grain size in rocks visual inspection of grain size is not always available for rocks investigated at field sites on mars due to limited image coverage but grain sizes can also be estimated from the gini index mean score a grain size proxy that uses point to point chemical variations in chemcam laser induced breakdown spectroscopy data new gini index mean score results indicate that the murray formation is dominated by mudstones with grains smaller than the spatial resolution of all rover cameras intervals of fine to coarse sandstone also are present some of which are verified using observations of grain size and sedimentary structures in associated images overall results demonstrate that most of the murray consists of mudstone suggesting settling of grains from suspension in low energy depositional environments such as lakes some of the mudstones contain desiccation cracks indicating periods of drying with a lowering of lake water level however beds and lenses of cross bedded sandstones are common at specific intervals suggesting episodes of fluvial and possibly eolian deposition the persistence of lacustrine deposits interspersed with fluvial deposits suggests that liquid water was sustained on the martian surface for tens of thousands to millions of years

the vila de cruces ophiolite a remnant of the early rheic ocean in the variscan suture of galicia northwest iberian massif abstract the vila de cruces ophiolite is one of the ophiolitic units involved in the variscan suture of the northwest iberian massif this ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks common alternations of metasediments of pelitic or siliceous character and scarce orthogneisses metagabbros and serpentinites the protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 497 pm 4 end document ma u pb in

tectonic evolution of the upper allochthon of the ordenes complex northwestern iberian massif structural constraints to a polyorogenic peri gondwanan terrane the upper allochthon of northwest iberia represents the most exotic terrane of this part of the european variscan belt recent advances in the metamorphic petrology structural geology and geochronology of the upper allochthon in the ordenes complex are integrated into a synthesis of its tectonic evolution constraining the main tectonothermal events important aspects of this synthesis are 1 the interpretation of cambro ordovician magmatism and earliest metamorphic event as the result of drifting of a peri gondwanan terrane 2 the subsequent shortening and crustal thickening of the terrane related to its subduction and accretion to laurussia 3 a younger cycle of shortening and extension resulting from convergence between lau russia and gondwana and 4 the emplacement of this exotic terrane as the upper allochthon together with underlying ophiolitic and basal allochthons during the laurussia gondwana collision implications derived from the well established tectonothermal sequence are discussed in the context of paleozoic paleogeography and geodynamics the evolution of this part of the belt is related first to the closure of the tornquist ocean and later to that of the eastern branch of the rheic ocean furthermore the relative paleopo sitions of the upper allochthon and the iberian autochthon in northern gondwana are discussed 2007 geological society of america all rights reserved

granite emplacement during contemporary shortening and normal faulting structural and magnetic study of the veiga massif nw spain abstract the veiga massif belongs to the calc alkaline series of hercynian granitic rocks of the ibero armorican arc the veiga granodiorite intruded during the upper carboniferous into the core of the wnw ese n verging ollo de sapo antiform formed by precambrian and palaeozoic metasediments internal fabrics show that magma intrusion was contemporary with shortening measurements of feldspars orientations and anisotropy of magnetic susceptibility ams throughout the granite are consistent and indicate a foliation striking wnw ese parallel to folding with a constant dip of 75 85 n the zonation of bulk low field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders the difference in elevation between outcrops more than 600 m allows us to infer the three dimensional attitude of granite fabrics throughout the massif syn magmatic fabric folds are preserved in the inner part of the igneous body the highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion at the scale of the massif foliation is convergent toward the bottom of the intrusion along a line located at its northern border where the magma source is interpreted to be located in the western border of the massif the presence of c and s structures indicates that magma cooling was coeval with movement of the chandoiro fault a n s striking normal fault with a n290e hanging wall displacement direction these results indicate that emplacement of the veiga granite is coeval with nne ssw shortening and with an wnw ese extension direction parallel to the trend of the late folds

a new miniaturized extraction chromatography method for precise u pb zircon geochronology abstract in this paper we describe a new method for the concomitant separation of u and pb in zircons for radiometric dating purposes the proposed separation scheme is based on a blend of two extraction chromatographic materials referred to as tru spec and sr spec respectively these afford a straightforward separation of u and pb with high yield good purity and satisfactory blank levels on micro columns using very small volume 5 l of mixed resins owing to the selectivity and high capacity of the specific extraction chromatographic materials for u and pb this new technique could be further scaled down the validity of the method is illustrated by measurements of u pb isotopic ratios and determination of corresponding ages in 0 2 0 8 mg fragments of the zr91500 zircon standard