the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

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

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

estudio sismico de la corteza iberica norte 3 3 a seismic image of the variscan crust in the hinterland of the nw iberian massif an offshore vertical incidence reflection seismic study with simultaneous on land wide angle recording has been conducted as part of the estudio sismico de la corteza iberica norte escin project

episodic and short lived granitic pulses in a post collisional setting evidence from precise u pb zircon dating through a crustal cross section in corsica abstract in western corsica a large granitoid batholith was emplaced during carboniferous and permian times these rocks display an evolution through time from high mg k calc alkaline to calc alkaline then alkaline affinities moreover the magmatic processes can be observed at different crustal levels new and precise isotope dilution and thermo ionisation mass spectrometry id tims u pb dating suggests that granite magmatism in corsica did not simply reflect the continuous thermal evolution of a thickened crust a precise timing for the post collisional magmatism is documented with four distinct short lived episodes respectively at 345 338 305 and 280 ma separated by periods of inactivity these four magma pulses display variable nd isotopic signatures related to mantle crust interactions granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere an increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event this is interpreted to reflect the emplacement of depleted mantle materials in the lower crust which enhanced heat flow and triggered partial melting of previously underplated mafic rocks this increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime preluding to the opening of the alpine orogenic tract

kaneohe acoustic thermometer further validated with rays over 3700 km and the demise of the idea of axially trapped energy the kaneohe acoustic source transmitted 133 hz 60 ms resolution signals over 3709 km from oahu at 183 m depth to a naval receiver at 1433 m depth near northern california ray theory successfully models the acoustic multipaths whose travel times are unambiguously tracked between 1983 89 despite the fact that the sound bounces one or more times from the oahu slope before becoming trapped in the sound channel the eigenrays are inclined at about 15 at the axis of the sound channel the upper turning depths of the eigenrays are insensitive to realistic perturbations along the section this supports the finding that the changes in delay of 0 2 s between 1983 89 are due to temperature and not due to changes in the multipaths compared with transmission through a smoothed representation of the ocean s acoustic waveguide the mesoscale and submesoscale features vertically scatter axially trapped energy about 200 and 800 m respectively the submesoscale structure may be associated with internal waves scatt

where the ocean influences the impulse response and its effect on synchronous changes of acoustic travel time in 1983 sounds at 133 hz 0 06 s resolution were transmitted in the pacific for five days at 2 min intervals over 3709 km between bottom mounted instruments maintained with atomic clocks in 1989 a technique was developed to measure changes in acoustic travel time with an accuracy of 135 microseconds at 2 min intervals for selected windows of travel time within the impulse response the data have short lived 1 to 10 ms oscillations of travel time with periods less than a few days excluding tidal effects different windows exhibited significant synchronized changes in travel time for periods shorter than 10 h in the 1980s this phenomenon was not understood because internal waves have correlation lengths of a few kilometers which are smaller than the way sound was thought to sample the ocean along well separated and distinct rays corresponding to different windows the paradox s resolution comes from modern theories that replace the ray picture with finite wavelength representations that predict sound

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

cortlandtitic enclaves associated with calc alkaline granites from tapia asturias hercynian belt northwestern spain abstract petrographic and mineralogical characteristics of amphibole olivine and pyroxene bearing ultramafic rocks from asturias nw spain are dealt with in this paper these rocks are of cortlandtitic type and occur as small rare enclaves in basic rocks related to hercynian calc alkaline post tectonic epizonal granites in the northwest of the iberian peninsula these particular ultramafic enclaves are characterized by poikilitic cumulate microtexture olivine fo 77 81 spinel from chromite to pleonaste composition enstatite subordinated diopside and sulphides are included in large brown calcic amphibole crystals displaying an irregular zonation phlogopite and plagioclase are also found in a much lower proportion between the large amphibole crystals mineral assemblage and chemical composition of minerals indicate formation conditions of 1150 c 7 8 kbar of p total and p h 2 o p total these rocks could represent the earlier products of fractional crystallization from a hydrated high alumina basalt involved in the genesis of the calc alkaline granites this basic magma would start crystallizing at a relatively deep level carrying up the first products of its crystallization during its ascent

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

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

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

magma pulses in the central variscan belt episodic melt generation and emplacement during lithospheric thinning magmatism in the central part of the variscan orogenic belt in europe has occurred in several distinct episodes each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting i between 340 and 330 ma visean high k diorites and granitoids within a zone extending parallel the orogen axis possibly tracing a palaeosuture ii at 310 307 ma westphalian calc alkaline granitoids in the intra alpine domain iii at 304 295 ma stephanian subalkaline granitoids in intra alpine and pennine domains and finally iv at 280 270 ma permian granitoids in pennine austroalpine and south alpine domains the generation and emplacement of granitoid melts in the variscan crust are suggested to be short term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle and decompressional melting

u pb baddeleyite ages of the kerns and triangle mountain intrusions nipissing diabase ontario u pb ages have been determined for two diabase intrusions from kerns and hudson townships that are part of the nipissing diabase suite from the proterozoic southern province canada the ages of the kerns and triangle mountain intrusions are 2217 2 4 and 2209 6 3 5 ma respectively based on analysis of primary baddeleyite from both intrusions together with a published u pb age for the castle mine intrusion these data indicate that the nipissing sills were emplaced separately over a time span of 2 8 1 6 ma given by the maximum and minimum overlap of errors on the u pb ages

u pb ages for late magmatism and regional deformation in the shebandowan belt superior province canada five precise u pb zircon and titanite ages from different lithologic units of the shebandowan greenstone belt in the western wawa subprovince of the superior province put tight constraints on the time of late archean magmatism and of two major deformation events a porphyry sill from the older supracrustal sequence has an age of 2733 3 ma another porphyritic rock a trondhjemite occurring as a clast in a conglomerate of the unconformably overlying timiskaming type supracrustal sequence formed 2704 2 ma ago and defines a maximum age for the deposition of the timiskaming type sequence an alkalic volcanic rock from this sequence has been directly dated at in accord with the above constraint and with another probable maximum age of deposition given by the date of 2696 2 ma for the shebandowan lake pluton a first deformation event d1 was related to a predominantly vertical tectonic regime and occurred during or before intrusion of the shebandowan lake pluton at 2696 2 ma the second deformatio

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

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

magnetic fabric studies of the nipissing sill province and senneterre dykes canadian shield and implications for emplacement the 2 21 ga nipissing diabase sills intrude the huronian supergroup of the southern province coeval senneterre dykes in the adjacent portion of the superior province are potential feeders for the sills anisotropy of magnetic susceptibility ams data were obtained from both of these units to study flow patterns and test emplacement models the sills exhibit three distinct paleomagnetic directions that are likely primary and represent a time correlation tool within the magmatic event two of the three directions are also found in senneterre dykes sites in the northern cobalt embayment carry an n1 reverse paleomagnetic signature they possess maximum ams axes that trend north northwest south southeast interpreted as the direction of magma emplacement in one area the data rule out a cone sheet emplacement mechanism the n3 paleomagnetic signature is restricted to a sill and or dyke like bodies near lake timiskaming where the ams axes are dominantly north northwest south southeast similar to that of the nearby n1 reverse sites sites in a geographically restricted area farther south carry an n1 normal paleomagnetic direction the ams patterns in these sites exhibit very poor within and between site consistency compatible with the magma having crystallized statically in this volumetrically minor magmatic pulse in the sudbury elliot lake area the nipissing diabase is deformed and outcrops exhibit east west elongation parallel to the fold axes of the enclosing huronian sedimentary rocks ams fabrics are interpreted as reflecting this deformation rather than primary magmatic flow three of eight senneterre dyke sites show horizontal flow patterns whereas the remainder have more complicated ams patterns

further evidence that the sound speed algorithm of del grosso is more accurate than that of chen and millero estimates of the bottom depth of the ocean at 11 sites were determined by combining round trip acoustic travel time measurements made by inverted echo sounders on the ocean bottom with sound speed profiles determined from simultaneously measured temperature and salinity profiles these depths were converted into pressures and were compared to independently measured bottom pressures to determine the accuracy of the algorithms used to calculate the sound speed profile the sound speed algorithms tested were those derived by del grosso j acoust soc am 56 1084 1091 1974 and by chen and millero j acoust soc am 62 1129 1135 1977 as well as the corrected version of chen and millero s algorithm recently published by millero and li j acoust soc am 95 2757 2759 1994 the results of this study agree with the results from recent acoustic tomography experiments which indicate that the algorithm of del grosso for the speed of sound in seawater is more accurate than the currently accepted standard algorithm of chen and millero del grosso s algorithm also produces more accurate results than those from the millero and li correction to the chen and millero algorithm

tracing crustal evolution in the nw iberian peninsula through the rbsr and upb systematics of palaeozoic granitoids a review abstract the abundant granitoids in the palaeozoic basement of the northwestern iberian peninsula include 470 440 ma old suites early middle ordovician now present as orthogneisses and several variscan suites the age data presently available suggest that the latter have been generated in three separate time intervals 330 320 ma ago late visean early namurian 310 300 ma ago westphalian and 290 280 ma ago late stephanian autunian rb sr whole rock and u pb zircon systematics along with field and chemical evidence testify to the generation of the early and late palaeozoic granitoids from lower palaeozoic and precambrian maximum age 1 7 1 6 ga sedimentary sequences respectively the source rocks contained zircons derived from a late archaean early proterozoic continental crust that probably underlies the whole of western europe although for the greater part modified beyond recognition by younger crustal reworking

time constraints for the tectono thermal evolution of the cantabrian zone in nw spain by illite k ar dating abstract contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to low grade metamorphic activity form a complex pattern in the northwestern spanish cantabrian area of the variscan orogen the distribution of these metamorphic features is based on field observations clay mineralogy mineral crystallinity indices vitrinite reflectance data and observation of rock microtextures k ar dating of fine grained illite rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern cantabrian area the usual k ar isochron plotting was combined with the bench type age distribution to unravel the successive illite crystallization episodes several tectono thermal episodes occurred in the southeastern pisuerga carrion province initially during the early permian at 293 4 ma in the ruesga line valsurvio dome and sierra del brezo area a further mid permian event was detected at 268 6 ma in the palentine nappes and during the mid triassic at 243 5 ma again in the valsurvio and sierra del brezo areas and along the ruesga line a later reactivation of the ruesga line was detected during mid jurassic time at 175 6 ma

relationships between syn orogenic sedimentation and nappe emplacement in the hinterland of the variscan belt in nw iberia deduced from detrital zircons flysch type syn orogenic deposits of carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the variscan belt of nw iberia new u pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry la icp ms are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting the age populations of four syn orogenic formations are compared with those of the pre orogenic sequence in the autochthon and parautochthon representing the gondwanan passive margin and in the allochthon formed by peri gondwanan and oceanic terranes in addition a new structural study has been carried out to understand the relationships between the syn orogenic deposits and the development of variscan structures the aims are to identify the sources of sediments and to establish the relationship between variscan structural evolution and syn orogenic sedimentation development of a forebulge outwards from the allochthonous front deduced from the structural study suggests the existence of depocentres that hosted the syn orogenic sediments together with the trend shown by the more recent zircons in each formation that are younger towards the external zones the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement the zircon age populations point to the allochthon as the main source of detritus for the syn orogenic basins with perhaps a limited participation of the parautochthon and autochthon in the younger formations

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

an updated perspective on basin scale tomography there seems to be little doubt that pulse like acoustic transmissions at 1000 to 5000 km can be used to measure small changes in average temperature in many oceanic regions given the numerous results published by different groups the early experimental results in this field which led to these conclusions have been confirmed by recent experiments our purpose in this letter is to summarize the work in this field in the context of addressing a few reservations expressed about the early experiments

mesozoic exhumation history and palaeolandscape of the iberian massif in eastern galicia from apatite fission track and u th he data apatite fission track aft and u th he ahe data combined with time temperature inverse modelling reveal the cooling and exhumation history of the iberian massif in eastern galicia since the mesozoic the continuous cooling at various rates correlates with variation of tectonic boundary conditions in the adjacent continental margins the data provide constraints on the 107 timescale longevity of a relict paleolandscape aft ages range from 68 to 174 ma with mean track lengths of 10 7 2 6 to 12 6 1 8 m and ahe ages range from 73 to 147 ma fastest exhumation 0 25 km ma occurred during the late jurassic to early cretaceous main episode of rifting in the adjacent western and northern margins exhumation rates have decreased since then and have been approximately one order of magnitude lower across inland galicia the aft data are consistent with early cretaceous movement on post variscan ne trending faults this is coeval with an extensional episode offshore the ahe data in this region indicate less than 1 7 km of denudation in the last 100 ma this low exhumation suggests the attainment of a mature landscape during late cretaceous post rift tectonic stability whose remains are still preserved the low and steady rate of denudation prevailed across inland galicia despite minor n s shortening in the northern margin since 45 ma ago in north galicia rock uplift in response to nw strike slip faulting since early oligocene to early miocene has caused insufficient exhumation 3 km to remove the mesozoic cooling signal recorded by the aft data

on equations for the speed of sound in seawater long range acoustic transmissions made in conjunction with extensive environmental measurements and accurate mooring position determinations have been used to test the accuracy of equations used to calculate sound speed from pressure temperature and salinity the sound speed field computed using the del grosso equation v a del grosso j acoust soc am 56 1084 1091 1974 give predictions of acoustic arrival patterns which agree significantly better with the long range measurements than those computed using the chen and millero equation c chen and f j millero j acoust soc am 62 1129 1135 1977 the predicted ray travel times and travel time error have been calculated using objectively mapped sound speed fields computed from ctd and xbt data using the measured and predicted ray travel times a negligible correction to del grosso s equation of 0 05 0 05 m s at 4000 m depth is calculated small errors of about 50 m in the gps determination of mooring positions lends a depth independent error of 0 1 m s to the sound speed equation correction work supported by nsf and onr

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

genesis and rank distribution of upper carboniferous coal basins in the cantabrian mountains northern spain abstract the cantabrian mountains located in the nw of the iberian peninsula constitute the most important coal mining district of spain anthracitic and bituminous coals high and medium rank coals have been mined in the area since the end of the nineteenth century and they currently account for about 70 of the total coal resources of the country the region forms part of the cantabrian and west asturian leonese zones of the iberian variscan fold belt and is strongly deformed by a set of imbricate thrusts coeval folds and high angle faults coal bearing successions are westphalian and stephanian pennsylvanian in age and are exposed in numerous coalfields of variable size arranged roughly parallel to the tectonic structures coal rank varies from medium rank bituminous d coals rr 0 5 to high rank anthracites a coals rr

mantle evolution in the variscides of sw england geochemical and isotopic constraints from mafic rocks abstract the geology of sw england has long been interpreted to reflect variscan collisional processes associated with the closure of the rhenohercynian ocean and the formation of pangea the cornish peninsula is composed largely of early devonian to late carboniferous volcanosedimentary successions that were deposited in pre and syn collisional basins and were subsequently metamorphosed and deformed during the variscan orogeny voluminous early permian granitic magmatism cornubian batholith is broadly coeval with the emplacement of ca 280 295 ma lamprophyric dykes and flows although these lamprophyres are well mapped and documented the processes responsible for their genesis and their relationship with regional variscan tectonic events are less understood pre to syn collisional basalts have intra continental alkalic affinities and have ree profiles consistent with derivation from the spinel garnet lherzolite boundary end values for the basalts range from 0 37 to 5 2 and tdm ages from 595 ma to 705 ma the lamprophyres are extremely enriched in light rare earth elements large iron lithophile elements and are depleted in heavy rare earth elements suggesting a deep garnet lherzolite source that was previously metasomatised they display end values ranging from 1 4 to 1 4 initial sr values of ca 0 706 and tdm ages from 671 ma to 1031 ma suggesting that metasomatism occurred in the neoproterozoic lamprophyres and coeval granite batholiths of similar chemistry to those in cornwall occur in other regions of the variscan orogen including iberia and bohemia by using new geochemical and isotopic data to constrain the evolution of the mantle beneath sw england and the processes associated with the formation of these post collisional rocks we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation

a review of the arcuate structures in the iberian variscides constraints and genetic models abstract in the iberian variscides several first order arcuate structures have been considered in spite of being highly studied their characterization formation mechanisms and even existence is still debatable the main ibero armorican arc iaa is essentially defined by a predominant nw se trend in the iberian branch and an e w trend in the brittany one however in northern spain it presents a 180 rotation sometimes known as the cantabrian arc ca the relation between both arcs is controversial being considered either as a single arc due to one tectonic event or as the result of a polyphasic process according to the last assumption there is a later arcuate structure ca overlapping a previous major one iaa whatever the models they must be able to explain the presence of a variscan sinistral transpression in iberia and a dextral one in armorica and a deformation spanning from the devonian to the upper carboniferous another arcuate structure in continuity with the ca the central iberian arc cia was recently proposed mainly based upon on magnetic anomalies geometry of major folds and ordovician paleocurrents the critical review of the structural stratigraphic and geophysical data supports both the iaa and the ca but as independent structures however the presence of a cia is highly questionable and could not be supported the complex strain pattern of the iaa and the ca could be explained by a devonian carboniferous polyphasic indentation of a gondwana promontory in this model the ca is essentially a thin skinned arc while the iaa has a more complex and longer evolution that has led to a thick skinned first order structure nevertheless both arcs are essentially the result of a lithospheric bending process during the iberian variscides

evolution of a kyanite bearing belt within a ht lp orogen the case of nw variscan iberia kyanite replaces andalusite in a belt of ordovician and silurian pelitic rocks that form a narrow synform pinched between high grade antiforms in nw variscan iberia kyanite occurs across the belt in al rich black pelites in assemblages i kyanite chloritoid chlorite muscovite and ii kyanite staurolite chlorite muscovite in i kyanite occurs in the matrix and in kyanite muscovite aggregates that pseudomorph earlier andalusite porphyroblasts the aggregates are found across the belt and can still be recognized in assemblage ii and even in iii andalusite staurolite biotite muscovite this latter being a hornfelsic silurian schist where kyanite is relic and staurolite occurs in the matrix and is resorbed inside new massive pleochroic andalusite kfmash and mnkfmash pseudosections have been constructed using thermocalc for al rich and al poorer compositions from the belt chloritoid zoning in al rich rocks containing assemblage i plus chloritoid chlorite thermometry complemented with garnet chlorite thermometry in al poorer lithologies mean that the path is one of increasing pressure and temperature conditions prior to assemblage i with earlier andalusite stable are those of the andalusite chloritoid chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims the passage from assemblage i to ii implies a prograde path within the kyanite field assemblage iii represents peak conditions indicating a prograde staurolite consuming reaction across a kfmash field leading eventually to a locally found andalusite biotite muscovite hornfels the lowest pressure stages are recorded by cordierite biotite in al poor pelites garnet bearing mnkfmash assemblages in al poorer pelites record conditions similar to assemblages ii and iii the replacement of andalusite by kyanite in assemblage i is attributed to downdragging of andalusite bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the vivero fault

u pb zircon and monazite geochronology of post collisional hercynian granitoids from the central iberian zone northern portugal abstract in the central iberian zone ciz of the iberian massif large volumes of granitoids were emplaced during the post collisional stage of the hercynian orogeny syn to post d3 the last ductile deformation phase twelve granitic units and a quartz monzodiorite were selected for a u pb zircon and monazite geochronological study they represent successive stages of the d3 event the ucanha vilar lamego sameiro and refoios do lima plutons are coeval 313 2 ma 319 4 ma 316 2 ma and 314 2 ma respectively and belong to the earliest stage later on the braga massif was emplaced its different units yielding the same age 309 3 ma for the braga granite 309 1 ma for the gonca granite and 311 5 ma for a related quartz monzodiorite the braga massif is subcontemporaneous with the agrela and celeiros plutons 307 3 5 ma and 306 2 ma respectively in agreement with field data the briteiros granite is younger 300 1 ma followed by the emplacement of the peneda geres massif geres paufito illa and carris granites the geres granite emplaced at 296 2 ma seems to represent a first magmatic pulse immediately followed by the intrusion of the paufito granite at 290 2 5 ma for the carris granite a minimum emplacement age of 280 5 ma was obtained based on these results the following chronology is proposed 1 syn d3 biotite granitoids 313 319 ma 2 late d3 biotite dominant granitoids 306 311 ma 3 late to post d3 granitoids ca 300 ma 4 post d3 granitoids 290 296 ma these chronological data indicate that successive granitic intrusions were emplaced in the ciz during a short time span of about 30 ma that corresponds to the latest stages of the hercynian orogeny a rapid and drastic change occurred at about 300 ma between a compressive ductile tectonic regime d3 ca 300 320 ma associated to calc alkaline monzonitic and aluminopotassic plutonism and a fragile phase of deformation d4 which controlled the emplacement of the subalkaline ferro potassic plutonism at 290 296 ma

devonian mississippian i type granitoids in the western carpathians a subduction related hybrid magmatism abstract sims zircon u pb dating of i type granitoids from four variscan crystalline basement outcrops in the western carpathians tribec nizke tatry cierna hora and slovenske rudohorie mts implies that they originated between 367 and 353 ma therefore they belong to the earliest variscan i type granitoids the oldest devonian age at ca 367 364 ma is obtained from an enclave bearing tonalite and associated dikes of the tribec mts several granodiorites from the nizke tatry vepor and cierna hora mts 353 357 ma show younger early carboniferous ages these data require a re assessment of older models that explained the genesis of i type granitoids in the western carpathians based on the assumption of younger middle to upper carboniferous ages the i type granite massifs of the western carpathians most probably originated in an arc related environment within the galatian superterrane an assemblage of gondwana derived fragments the early age of i type magmatism in the western carpathians marks the beginning of a north dipping subduction of the paleotethys ocean under ibero ligerian and intra alpine terranes we suggest a term proto tatricum for that part of the galatian superterrane where devonian mississippian i type granitoids were emplaced now the granitoids are incorporated as a part of crystalline basement into the alpine tatric and veporic units within present west carpathian mountain chain

upper and middle crustal response to delamination an example from the lachlan fold belt eastern australia a delamination model applied to the lachlan fold belt in eastern australia shows that lithospheric delamination began in the early silurian 430 ma after crustal thickening in the wagga omeo zone the delamination induced isostatic uplift farther east to produce localized transient extensional basins and coeval silicic magmas which formed by partial melting of the lower crust during asthenospheric upwelling however the voluminous granitoid intrusions were emplaced during and after compression reflecting reestablishment of horizontal compression associated with plate convergence continued sinking of the slab produced an east ward widening orogenic belt where the granite batholiths and associated thermally softened zones of deformation tracked the direction of delamination once the slab was completely detached at 400 ma the compressive stresses were transmitted across the belt and delamination recommenced about 1000 km westward against the cratonic margin the process was repeated at 400 370 ma within 60 m y the two sinking slabs had converted a 1700 2000 km wide continental margin into a 700 km wide orogenic belt important effects of delamination on the upper and middle crust are 1 production of regional scale low pressure metamorphic belts of limited crustal thickness 2 widespread voluminous silicic magmatism 3 lack of deep crustal exposures even with high degrees of crustal shortening and 4 partial replacement of the lower crust by a mafic underplate that cooled completely after deformation ceased

self subduction of the pangaean global plate one of the most striking and rare occurrences in the earth s history is the amalgamation of most of the continental lithosphere into one supercontinent the most recent supercontinent pangaea lasted from 320 to 200 million years ago here we show that after the continental collisions that led to the formation of pangaea plate convergence continued in a large wedge shaped oceanic tract we suggest that plate strain at the periphery of the supercontinent eventually resulted in self subduction of the pangaean global plate when the ocean margin of the continent subducted beneath the continental edge at the other end of the same plate our scenario results in a stress regime within pangaea that explains the development of a large fold structure near the apex of the palaeotethys ocean extensive lower crustal heating and continental magmatism at the core of the continent as well as the development of radially arranged continental rifts in more peripheral regions of the plate

the architecture of the european mediterranean lithosphere a synthesis of the re os evidence rhenium depletion model ages t rd of sulfides in peridotite xenoliths from the subcontinental mantle beneath central spain the calatrava volcanic field reveal that episodes of mantle magmatism and or metasomatism in the iberia microplate were linked to crustal growth events mainly during supercontinent assembly and or breakup at ca 1 8 1 1 0 9 0 6 and 0 3 ga a synthesis of available in situ and whole rock os isotope data on mantle derived peridotites shows that this type of mantle maximum t rd of ca 1 8 ga is widespread in the subcontinental mantle of europe and africa outboard from the betics maghrebides appenines front in contrast the mantle enclosed within the alpine domain records t rd as old as 2 6 ga revealing a previously unrecognized archean domain or domains in the central and western mediterranean our observations indicate that ancient fragments of subcontinental lithospheric mantle have played an important role in the development of the present architecture of the mediterranean lithosphere

ediacaran palaeozoic tectonic evolution of the ossa morena and central iberian zones sw iberia as revealed by sm nd isotope systematics abstract sm nd isotopic analyses of palaeozoic sedimentary and igneous rocks in the southwest iberian massif western end of the european variscan belt are presented in order to unravel its complex poly orogenic evolution during the closure of the rheic ocean and the amalgamation of pangea the gondwanan margin in southwest iberia sw iberia is subdivided into the ossa morena and central iberian zones separated by the badajoz cordoba shear zone which represents a cryptic suture zone between these terranes the relationships between these terranes and between units preserved within the suture zone e g the sierra albarrana group during the palaeozoic and neoproterozoic are controversial sm nd isotopic studies of representative sedimentary sequences covering the entire pre variscan record of the ossa morena and central iberian zones show very similar characteristics from the uppermost ediacaran onwards these data indicate that their accretion to one another must have been completed by the late neoproterozoic ediacarin that time an event assigned to cadomian orogeny and that they never separated substantially from each other since that time the sm nd isotopic composition of the sierra albarrana group metasedimentary rocks is similar to that of the pre cadomian sequences of the ossa morena zone serie negra suggesting derivation from a common source the common provenance of the palaeozoic sequences in the two zones is identical to that of the pre cadomian serie negra of the ossa morena zone which in accordance with the data presented herein and published u pb zircon data indicates a west african affinity

the vilalba gold district a new discovery in the variscan terranes of the nw of spain a geologic fluid inclusion and stable isotope study abstract this work defines the vilalba gold district a new gold region in the nw of spain comprising three mineralized areas castro de rei valina azumara and arcos and two different types of mineralization 1 w au skarn in castro de rei and 2 au as ag pb zn cu sb vein disseminated mineralization in valina azumara and arcos mineralization is mainly hosted by impure limestone and black slates lower cambrian and spatially related to variscan and late variscan structures rhyolite dikes occur in arcos 299 6 ma and castro de rei both consisting of peraluminous high potassium and calc alkaline rocks these dikes represent the fractionated expression of unexposed post tectonic granitoids located at depth the castro de rei area exhibits a contact aureole with biotite cordierite and calc silicate hornfels prograde skarn consists of garnet pyroxene quartz wollastonite and scheelite the retrograde alteration took place in two stages and mainly consists of amphibole epidote zoisite quartz calcite chlorite and sulfides gold mineralization in the skarn occurs as invisible gold in sulfoarsenides and as electrum related to bi te s minerals the mineralization of valina azumara and arcos comprises two stages the first consists of as rich pyrite and arsenopyrite with invisible gold the second stage mainly consists of base metal sulfides and ag pb zn cu sb sulfosalts hydrothermal fluids involved in the skarn formation are low salinity up to 6 6 wt nacl eq aqueous carbonic fluids h2o co2 ch4 n2 evolving into aqueous fluids during the last stage of gold mineralization stable isotope geochemistry indicates the importance of a magmatic fluid in the early stages 18osmow from 7 5 to 11 34scdt from 2 7 to 5 9 that evolved through interaction with host rocks and mixing with metamorphic and or meteoric fluids the 34scdt values 7 8 10 9 of second retrograde stage sulfides suggest the incorporation of 34s enriched from host rocks in valina azumara and arcos the fluid inclusion and isotope studies indicate similar hydrothermal fluids to those associated with the second retrograde skarn stage taking into account the fluid inclusion study and mineral geothermometry an attempt was made to determine the p t conditions of ore formation the prograde skarn of castro de rei formed at temperatures and pressures ranging from 520 to 560 c and from 1 7 to 2 5 kb the mineralization of valina azumara formed at minimum p t conditions of 1 7 kb and 355 c the estimated temperatures for the arcos mineralization are similar to those of valina azumara up to 383 c but at lower pressures we propose the model of intrusion related gold systems irgss to explain the mineralizations of the vilalba gold district in this model the castro de rei skarn and the valina azumara mineralization represent a proximal and a distal deposit respectively relative to an unexposed granitoid situated at depth we also consider arcos as a distal deposit in relation to another unexposed granitoid located further south in the district

a test of basin scale acoustic thermometry using a large aperture vertical array at 3250 km range in the eastern north pacific ocean broadband acoustic signals were transmitted during november 1994 from a 75 hz source suspended near the depth of the sound channel axis to a 700 m long vertical receiving array approximately 3250 km distant in the eastern north pacific ocean the early part of the arrival pattern consists of raylike wave fronts that are resolvable identifiable and stable the later part of the arrival pattern does not contain identifiable raylike arrivals due to scattering from internal wave induced sound speed fluctuations the observed ray travel times differ from ray predictions based on the sound speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates suggesting that the equation used to compute sound speed requires refinement the range averaged ocean sound speed can be determined with an uncertainty of about 0 05 m s from the observed ray travel times together with the time at which the near axial acoustic reception ends used as a surroga

are crustal thickness variations in old mountain belts like the appalachians a consequence of lithospheric delamination comparison of moho geometry and reflection character of active collision zones like the alps with those of fossil collision zones like the appalachians lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny in particular the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature acceptance of the delamination hypothesis in turn fundamentally alters one s view of the tectonic evolution of old collisional orogens in the case of the appalachians delamination has direct implications for the interpretation of alleghanian granites the direction of subduction leading to final closure between laurentia and gondwana and the thermal state of the lithosphere at the onset of atlantic rifting a corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens conversely the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions

the sphene centered ocellar texture an effect of grain supported flow and melt migration in a hyperdense magma mush abstractthe sphene centered ocellar texture consists of leucocratic ocelli with sphene titanite crystals at the center enclosed in a biotite rich matrix this texture has been recognized worldwide in hybrid intermediate rocks on the basis of structural petrological and geochronological data from selected outcrops of the variscan ribadelago pluton nw iberian massif we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase and biotite dominated crystalline framework at the late stage of crystallization the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field entering the domain of grain supported flow microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of reynolds di

early neoproterozoic assembly of the yangtze block decoded from metasedimentary rocks of the miaowan complex abstract the present yangtze block was proposed to be formed as a unified block in the mesoproterozoic neoproterozoic transition but the detailed timing and process are poorly constrained here we report integrated in situ u pb and hf o isotopic analyses of detrital zircons from the xiaoxikou metamorphosed sedimentary rocks of the miaowan complex in the yangtze block as well as u th pb and nd o isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks u pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 ma the 970 ma zircons are dominantly characterized by mantle like 18ozir values 4 5 5 8 and highly depleted ehf t values 5 to 14 the 900 ma zircons mostly have o and hf isotopic features similar to the 970 ma zircons except relative small proportion of grains with high 18o values 6 0 9 2 and variable ehf t values 33 to 7 monazites from the vein are dated at ca 850 ma and have uniform 18o values 5 4 6 6 but significantly enriched end t values 14 5 to 12 6 based on the new isotopic results and immature lithology the xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 ma and 850 ma in a fore arc basin and clastic materials derived from 970 900 ma arc rocks were related to the northward subduction present day coordinates of the miaowan oceanic plate the 850 ma leucocratic vein shows geological and isotopic characteristics consistent with local 860 840 ma post tectonic granitic plutons and may represent a fractionated product of local coeval granitic magmas we propose that the closure of the miaowan oceanic basin and formation of the current unified yangtze block occurred between 900 ma and 860 ma i e later than the deposition of the fore arc metasedimentary rocks while earlier than the emplacement of the post tectonic plutons

structural and metamorphic evidence of local extension along the vivero fault coeval with bulk crustal shortening in the variscan chain nw spain abstract the vivero fault is a w dipping n s striking ductile shear zone separating the ollo de sapo antiform in its western hangingwall and the lugo dome in its eastern footwall two stages of deformation f 1 and f 2 produced nearly coaxial folds with sub horizontal axes a crenulation cleavage s 2 transposes an older s 1 three sets of shear bands in the hangingwall define a pervasive fabric consistent with an e w bulk shortening perpendicular to a composite s 1 2 foliation and nne stretching parallel to l 2 the vivero fault zone is marked by a mylonitic foliation with a steeply nw plunging stretching lineation and extensional crenulation cleavage ecc indicating normal slip in the vicinity of the fault sub horizontal nne trending f 3 folds with a crenulation cleavage s 3 deform earlier formed fabrics including a mylonitic foliation pressure temperature conditions obtained from mineral assemblages on both sides of the vivero fault yield a minimum throw of 5 5 km andalusite bearing pelite in the hangingwall was infolded by an f 2 synform into the kyanite field at 450 500 c the eastern edge of these rocks was later accreted to the footwall and heated to andalusite staurolite conditions at 600 c slip on the vivero and valdovino faults is kinematically related east west shortening during f 2 involved folding and sinistral strike slip on the valdovino fault which induced local extension along the newly generated vivero fault synkinematic emplacement of granitoids along the vivero fault is favoured by extension coeval slip on both faults took place during the later stages of f 2 folding geometrical constraints caused northwards escape of the crustal block bounded by the valdovino and vivero faults recorded by nne stretching defined by l 2

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

successful ray modeling of acoustic multipaths over a 3000 km section in the pacific a new ray model is compared with tomographic data from basin scale transmissions in the northeast pacific 250 hz 0 016 ms resolution 3000 km model calculations resemble the arrival pattern of the acoustic multipaths quite well to date this is the longest distance in the ocean over which ray theory has been shown to accurately predict the acoustic impulse response of the ocean the new ray model confirms previous analysis that basin scale tomography is feasible and that the international algorithm for the speed of sound in seawater is too fast however the new ray model looks much more like the data than the previous ray model because the new model has a more realistic bottom and a more realistic sound speed field at 3000 km there is no evidence that ray theory suffers from any lack of predictability due to chaotic behavior in this particular experiment the new model is suitable for automating the identification of acoustic multipaths over basin scales

comparison of basin scale acoustic transmissions with rays and further evidence for a structured thermal field in the northeast pacific from may to september of 1987 250 hz 16 ms resolution acoustic signals were transmitted between four sources and nine receivers in the northeast pacific this paper examines the acoustic transmissions across nine of the sections within this group with path lengths ranging from approximately 1700 to 3300 km acoustic multipaths are tracked in the data and ray theory is successfully used to identify the multipaths where the spring and summer levitus climatological databases are used to determine the sound speeds the observed multipaths arrive on the order of 1 s later than the predicted rays travel time differences greater than 0 15 s are due to temperature errors in levitus climatology within the ocean s upper 1 km the resulting corrections to levitus spring and summer oceans are 0 2 and 0 3 c respectively the upper turning depths for all rays are found to vary by less than 50 m from spring to summer variations in the measured travel times over the four month period are about 0 5 s some s

petrogenetic and tectonic significance of young leucogranites from the northwestern himalaya pakistan in the himalaya of northwestern pakistan small discordant bodies of leucogranite intrude metamorphosed basement of the indian crust as the first step in assessing the tectonic significance of these granites we have used the shrimp ion microprobe to obtain u pb dates on zircon zircons of igneous appearance are scarce in the leucogranites and with one exception are associated with xenocrystic components which occur as discrete grains and as cores rimmed by young zircon zircons of igneous appearance are high in u 5000 to 40 000 ppm low in th u 0 01 to 0 05 and frequently colored a distinct blue xenocrystic components give a range of proterozoic ages consistent with ages previously reported for indian crust in sharp contrast zircons of igneous appearance yield clusters of concordant tertiary ages two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 ma swat and about 50 ma naran together with previously determined 40ar 39ar and fission track cooling ages these ages document immediate and rapid metamorphism and denudation following eocene collision three leucogranite dikes from northern localities within the nanga parbat haramosh massif are very young having emplacement ages of about 2 3 ma 5 ma and 7 ma the formation and emplacement of these leucogranites during the rapid late tertiary denudation of the nanga parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis

some geodynamic and compositional constraints on postorogenic magmatism postorogenic magmatic suites are common to many orogens in many cases apparently just postdating the cessation of deformation they differ from preceding orogenic suites in that they have higher temperatures more primitive isotopic signatures and bimodal natures and thus are compositionally similar to suites found in extensional regimes we propose that thinning of the lithospheric mantle which may be an automatic response to orogenic lithospheric thickening is responsible for these magmatic suites mantle lithospheric thinning moves the asthenospheric lithospheric thermal boundary higher in the lithospheric column thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism this removal of much of the dense lithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenic buoyancy forces that will oppose and potentially terminate deformation extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional lithospheric stress regime to produce accompanying felsic a type magmas this model explains why the cessation of deformation is coincident with high temperature bimodal magmatism and inferred uplift as well as the long noted similarity and confusion between postorogenic and extensional magmatic suites this arises because both reflect thinning of the lithospheric mantle not because both reflect extensional tectonics we suggest that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust

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

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

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

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

tidal signals in basin scale acoustic transmissions travel times of acoustic signals were measured between a bottom mounted source near oahu and five bottom mounted receivers located near washington oregon and california in 1988 and 1989 at three out of five receivers observed travel times at m2 and s2 periods agree with predictions from a barotropic tide model to within 30 deg in phase and a factor of 1 6 in amplitude the discrepancies at the fourth and fifth receivers can be removed to first order by including predicted effects of phase locked baroclinic tides generated by guyots in the moonless mountains a simple model is used to estimate the conversion of energy from barotropic to baroclinic tides by the world s seamounts at m2 the conversion amounts to about 4 of the total dissipation at m2 although this estimate is very approximate it is similar to other published values

structural metamorphic and magmatic history of the mondonedo nappe hercynian belt nw spain abstract the mondonedo nappe is one of the main units in the internal zone of the hercynian belt in the iberian peninsula it consists of low to medium grade metasediments late precambrian and palaeozoic in age folded by large d 1 e verging recumbent structures and thrusted several tens of km towards the east during a d 2 deformation episode at the base of the thrust sheet a shear zone of up to 3 km thick was developed granitoid bodies which were intruded after d 1 and before or during d 2 were deformed in this basal zone the geometry of d 1 and d 2 structures and that of the isograds are described as well as the relationships between metamorphism and deformation a structural evolution is inferred which shows an episode of generalized ductile deformation d 1 followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone d 2 and culminated with the thrusting along an important fracture the shear zone originated in a thermally softened level which according to metamorphic data was at temperatures in the range 550 600 c the structural metamorphic and magmatic features of the mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs this could be a consequence of the collision against another continental mass to the west

listening for climatic temperature change in the northeast pacific 1983 1989 data are presented from an acoustic experiment designed to detect climatic trends of temperature in the ocean with basin scale resolution these data are presented as an intriguing new way to recognize changes in spatially averaged temperature in 1983 travel times of acoustic signals 133 hz 60 ms resolution were measured over 4000 km between a source and receiver mounted near oahu and northern california respectively in 1987 measurement was begun on the travel times along six additional sections in the northeast pacific each at a distance of 3000 to 4000 km travel times changed by about 0 2 s at each receiver at interannual periods changes in acoustic travel time exceeding about 0 03 s are due to changes in the spatially averaged temperature along each section a change of 0 03 s is equivalent to a change in spatially averaged temperature of only about 0 02 c in the upper kilometer of the ocean the dynamical processes responsible for the temperature variability along the acoustic sections are not yet identified no evidence is seen for a long term change in climate from intermittent data between 1983 and 1989 along a 4000 km section

the genesis of felsic mafic plutonic associations a sr and nd isotopic study of the hercynian braga granitoid massif northern portugal abstract the late tectonic braga plutonic complex in the centro iberian zone north portugal was emplaced during the hercynian orogeny within a silurian sedimentary sequence and displays an acid basic association which consists of three well defined intrusions the biotite dominant braga granite minor bodies of gabbro to granodiorite composition and the mildly peraluminous gonca leucogranite these three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a rb sr age of 310 10 ma is obtained the distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related however the rb sr and sm nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process the gabbros have chemical characters high k ba sr and light rare earth elements and low nb ti and zr contents and isotopic compositions sri 0 70497 and nd 2 5 which suggest an alkaline magmatic affinity of shoshonitic type they are probably derived from an enriched mantle source in contrast the peraluminous gonca leucogranite sri 0 70933 and nd 6 8 is more likely the result of crustal anatexis the braga granite and the evolved members of the basic series have an intermediate isotopic composition sri 0 70532 to 0 70733 and nd 3 0 to 6 2 which can be interpreted in terms of an hybridization process between the two previous end members the chemical and sr nd isotopic compositions of the braga plutonic series can be explained by an assimilation fractional crystallization afc model between a mantle derived magma equivalent to the gabbros and a crust derived magma the gonca granite

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

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

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 eastern iranian orocline abstract the eastern iranian range is seen as a large scale india eurasia collisional event and the subsequent amalgamation of the afghan and lut micro continents including the neo tethyan accretionary complexes and associated island arcs revised examination of the nature of tectono stratigraphic units distributed in both sides of the n s elongated eastern iranian orogen reveals two remarkable facts first from a general perspective it is structurally a giant parallel antiform in profile view second lithologically the most time equivalent comparable tectonic units from both northern and southern domains comprising the eastern iranian orogen are connectable follow the strike of hinge and limbs zones of the antiform and a m shaped disharmonic pattern in its core these characters are thought to be formed in response to two principal deformational phases resulting from two perpendicular stress directions the first was n s associated with the north dipping subduction of the tethyan oceanic lithosphere beneath the roughly ribbon shaped cimmerian supercontinent including the afghan and lut blocks before late eocene afterward the second was e w shortening originated by the westward escape of the afghan block as a consequence of the indian plate indentation into eurasia this orogen parallel stress was correspondingly accommodated by tens of degrees counter clockwise rotation of the lut block around a vertical axis the result of such shortening was oroclinal buckling and concomitant retreating of the subduction zone and closure of the sistan ocean a remaining wedge shaped part of the neotethys ocean formed by the ribbon curvature the development of the eastern iranian orogen already included the sistan suture zone an enormous mass of folded eocene flysch deposits with a general n s axial surface and some cretaceous ophiolitic remnants the plate scale buckling which was formed by this late stage orogenic event is hereby considered as an orocline call eastern iranian orocline lithosphere buckling around a vertical axis caused a large scale crustal folding coeval with the bending of its mantle part increasing the lithospheric thickness of the orocline compared to adjacent regions the syn orocline deformation associated delamination and resulting magmatic events took place in eastern iran during a short period of about 10 to 20 ma in the late paleogene the orocline hinge zone is occupied by the khorasan outer arc and the birjand core while its limbs are composed of the baluch inner arc and the southwestern and southeastern terminations of the lut and afghan blocks the eastern iranian orocline seems to be the middle segment of a triple bending belt with n s two opposite facing bend including afghan lut and sanandaj sirjan blocks this large curvature is possibly an example of secondary oroclines which was formed by collision related escape tectonics although the limited available kinematic data show a kind of progressive orocline

geochemical and isotopic constraints on petrogenesis the serra da freita pluton a typical granite body from the portuguese hercynian collision belt the hercynian serra da freita pluton in north central portugal is emplaced within a sillimanite bearing flychoid sequence of graywackes and pelitic schists here tested as a possible protolith for the granite there is a marked overlap in most major element values between the schists and the granite however high k values preclude anatexis of pelite and the 3 fold na enrichment shown by the granites could be achieved by 25 30 partial melting of immature graywacke depletions of ba nb ce zr ti and y in the granite reflect retention of these elements in the source restite separation may have occurred to some limited extent however enrichment in rb may be due to late stage fractional crystallization overlapping 18 o values for both schists and granites at 10 to 14 combined with an initial 87 sr 86 sr of 0 7136 for the pluton strongly support the local graywacke rich beira schists as the most likely protolith for this strongly peraluminous granite magma

potential field constraints on the deep structure of the lugo gneiss dome nw spain abstract the lugo gneiss dome in the nw iberian massif spain is a variscan structure developed during late stages of orogenic collapse crustal extension was mainly accomplished by two kilometre scale conjugate extensional shear zones and by the late development of the dome and a huge normal fault these structures overprint previous contractional recumbent folds and a thrust fault the lugo dome and its southward continuation the sanabria dome are the site of the conspicuous eastern galicia magnetic anomaly egma a n s band 50 km wide and 190 km long with a maximum amplitude of 190 nt integrated potential field modelling of the egma and its corresponding gravity signature have been carried out aided by constraints provided by the measurement of c 900 magnetic susceptibilities and by previous geophysical data mainly seismic refraction and reflection profiles results suggest that a large volume of low density migmatites and associated inhomogeneous granites are the main source of the magnetic anomaly small massifs of basic and ultrabasic rocks inside the migmatites and high susceptibility iron ore bodies sparsely distributed in low grade middle ordovician slates are also thought to contribute to the anomaly but to a minor extent although otherwise similar to other gneiss domes the lugo dome is accompanied by a striking magnetic anomaly whose origin is discussed in terms of the tectonic evolution of this structure and the provenance of the magnetite bearing migmatites and inhomogeneous granites that core it

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma

measurements of gulf stream meandering and evidence of seasonal thermocline development using long range acoustic transmissions abstract phase coded signals with 60 rms resolution were transmitted twice weekly for several months from acoustic sources at 2000 m depth in the sargasso sea to three bottom mounted receives designed as west east and north stations at ranges approximately between 1000 and 2000 km the transmission paths to west and east stations were entirely in the sargasso sea the path to north station crossed the gulf stream and so traversed one of the most time and range dependent environments found anywhere in the ocean arrivals at all three stations were stable and could be identified from range dependent ray traces travel times at west station clearly change is response to the warming of the seasonal thermocline from spring to summer the travel time change with predictions travel time changes at north station primarily respond to the north south meandering of the gulf stream