white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

provenance of upper precambrian lower cambrian shales in the central iberian zone spain evidence from a chemical and isotopic study geochemical and isotopic sr nd o data are presented for upper precambrian and lower cambrian shale sequences from the central iberian zone spain in order to define geochemical discriminants between them and to assess the nature of the source regions supplying these basins elemental data especially th sc ratios coupled with endt values of 2 5 suggest that the upper precambrian rocks contain a greater proportion of a much younger more mafic component than the lower cambrian sequence which has high th sc and endt of 7 3 coherent trends between elemental and o isotope data imply a homogeneous recycled source as the dominant provenance for the upper precambrian shales whereas a lack of such coherence for the lower cambrian sediments suggests a more heterogeneous provenance the sedimentological geochemical and isotopic data are consistent with the upper precambrian succession being derived ultimately from a polycyclic mixture of young pre cadomian mafic sources and older 2 ga crust in contrast the lower cambrian sequence was mainly derived from old more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion

siliciclastic carbonate slope apron in an immature tensional margin upper precambrian lower cambrian central iberian zone salamanca spain abstract the middle upper vendian and upper series lower cambrian of the central part of the central iberian zone of the iberian massif comprise for the most part siliciclastic sediments in the upper part of the middle series and at the base of the upper series a siliciclastic carbonate succession is predominant in the middle series the siliciclastic carbonate succession consists of limestone breccias sandstone limestone couplets and sandy limestone facies these were resedimented by debris flows low and high concentration turbidity currents and occasionally tractive flows the facies associations and the lack of well defined sequences in them suggest sedimentation on a mixed slope apron in the upper series the siliciclastic carbonate succession is composed in the east of chaotic facies conglomerates sandstones and shales corresponding to sedimentation by rockfalls slides debris flows and high concentration turbidity currents whereas in the west it is composed of conglomerates sandstones and shales that correspond to sedimentation by traction storms tides and deposition from suspension the facies associations and their sequential arrangement in the west suggest that sedimentation occurred on a prograding shelf with the development of bars and channels whereas in the east sedimentation occurred on a slope the relationship between the proposed depositional model and the previously established tectonic model ortega et al 1988 of a tensional margin suggests that the slopes were formed by faults that led to the development of horsts and grabens the faults were active during deposition of the middle and upper series the general tendency towards rising sea level observed in other parts of the world during late vendian early cambrian times shanmugam and moiola 1982 has been recorded in the study area

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

sm nd isotope characteristics of late cadomian granite magmatism in northern france and the channel islands sm nd isotopic studies of granites within the late precambrian cadomian orogenic belt of the north armorican massif northwestern france and channel islands reveal differences between arc related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes late cadomian c 570 ma arc related granitoids exhibit a range of e nd 2 to 6 and nd model ages t dm 1 0 1 3 ga reflecting variable contamination between late precambrian mantle derived magmas and ancient c 2 0 ga continental crust the contamination did not involve exposed granitic icartian basement to anygreat degree a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition or early cadomian plutons which were themselves contaminated by lower crust voluminous granites of the mancellian region c 550 540 ma share common isotopic characteristics e nd 4 to 7 t dm 1 5 1 7 ga with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the st malo region consistent with a common source

contrasting geochemical features of the central iberian zone shales iberian massif spain implications for the evolution of neoproterozoic lower cambrian sediments and their sources in other peri gondwanan areas abstract more than 90 samples of shales have been collected for geochemical purposes from neoproterozoic and lower cambrian sedimentary series in an extensive area of the central iberian zone ciz for similar sio2 and al2o3 abundances the neoproterozoic shales ns are characterized by relatively higher tio2 and zr abundances and by higher cr th and ti nb ratios by contrast the lower cambrian shales lcs have higher al2o3 tio2 rb zr la yb cn and th yb ratios diagrams such as al2o3 tio2 al2o3 tio2 ti nb tio2 zr tio2 zr nb and ti nb rb zr are useful to define two distinct fields that discriminate the neoproterozoic from the lower cambrian compositions these and other geochemical features suggest that these rocks were derived from different source compositions and that the lower cambrian shales lcs were chemically more fractionated than those of the neoproterozoic these distinctive chemical features are also recorded by neoproterozoic and cambrian fine grained rocks in other areas in the central iberian zone and in other zones within iberia published neodymium isotopic results consistently distinguish both compositions and together with the geochemical data support a relatively fractionated nature for the lower cambrian sediments compared to the neoproterozoic ones it is proposed that continental deposits derived from the interior orogens in north gondwana would have been the sources for neoproterozoic sediments in the iberian zones whereas the lower cambrian sediments in these zones would have been derived from older crust beneath those continental deposits thus the neoproterozoic lower cambrian stratigraphic succession in iberia reflects an inverse stratigraphy of the source area and contrasts with zones of central and western europe where equivalent sedimentary series record important coeval igneous contributions and show less fractionated geochemical characteristics finally the possible connection between the central iberian zone and the meguma zone mz in canada is discussed on the basis of certain sedimentological and geochemical features it is proposed that the sedimentary series of the central iberian zone and of the meguma zone represent respectively proximal and distal north gondwana derived sediments

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

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

seismic source mechanism of degassing bursts at kilauea volcano hawaii results from waveform inversion in the 10 50 s band the current march 2008 to february 2009 summit eruptive activity at kilauea volcano is characterized by explosive degassing bursts accompanied by very long period vlp seismic signals we model the source mechanisms of vlp signals in the 10 50 s band using data recorded for 15 bursts with a 10 station broadband network deployed in the summit caldera to determine the source centroid location and source mechanism we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account the vlp signals associated with the bursts originate in a source region 1 km below the eastern perimeter of halemaumau pit crater the observed waveforms are well explained by the combination of a volumetric component and a vertical single force component for the volumetric component several source geometries are obtained which equally explain the observed waveforms these geometries include 1 a pipe dipping 64 to the northeast 2 two intersecting cracks including an east striking crack dike dipping 80 to the north intersecting a north striking crack another dike dipping 65 to the east 3 a pipe dipping 58 to the northeast intersecting a crack dipping 48 to the west southwest and 4 a pipe dipping 57 to the northeast intersecting a pipe dipping 58 to the west southwest using the dual crack model as reference the largest volume change obtained among the 15 bursts is 24 400 m3 and the maximum amplitude peak to peak of the force is 20 gn each burst is marked by a similar sequence of deflation and inflation trailed by decaying oscillations of the volumetric source the vertical force is initially upward synchronous with source deflation then downward synchronous with source reinflation followed by oscillations with polarity opposite to the volumetric oscillations this combination of force and volume change is attributed to pressure and momentum changes induced during a fluid dynamic source mechanism involving the ascent expansion and burst of a large slug of gas within the upper 150 m of the magma conduit as the slug expands upon approach to the surface and more liquid becomes wall supported by viscous shear forces the pressure below the slug decreases inducing conduit deflation and an upward force on the earth the final rapid slug expansion and burst stimulate vlp and lp oscillations of the conduit system which slowly decay due to viscous dissipation and elastic radiation consideration of the fluid dynamic arguments leads us to prefer the dual crack vlp source model as it is the only candidate model capable of producing plausible values of length scales and pressure changes the magnitudes of the vertical forces observed in the 15 bursts appear consistent with slug masses of 104 to 106 kg

upper neoproterozoic lower cambrian sedimentary successions in the central iberian zone spain sequence stratigraphy petrology and chemostratigraphy implications for other european zones o3 tio2 zr and nb abundances in shales from all the units strongly suggest a gradual compositional change within this sedimentary succession together with the petrological data the chemical results do not reveal any obvious coeval volcanic contribution to the sediments on the basis of the chemical data a comparison is made with other european zones containing detrital sediments composed of reworked crustal components

brioverian volcanism and cadomian tectonics baie de st brieuc brittany stages in the evolution of a late precambrian ensialic basin

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

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

the late neoproterozoic magmatism in the ediacaran series of the eastern pyrenees new ages and isotope geochemistry geochronological u pb la icp ms geochemical and isotopic data from metavolcanic felsic rocks of the canigo and cap de creus massifs in the eastern pyrenees provide evidence of an ediacaran magmatic event lasting 30 ma in ne iberia these data also constrain the age of the late neoproterozoic succession in the cap de creus massif where depositional ages range from 577 to 558 ma and in the canigo massif where the data 575 568 ma represent minimum ages the geochemistry of the felsic rocks indicates that they were formed in a back arc environment and they record a fragment of a long lived subduction related magmatic arc 620 520 ma in the active northern gondwana margin the homogeneity shown by all these crustal fragments along this margin suggests that the individualization of the pyrenean basement from the iberian massif started later probably during its transition from an active to a passive margin in cambro ordovician times

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

the upper neoproterozoic lower cambrian of the central iberian zone spain chemical and isotopic sm nd evidence that the sedimentary succession records an inverted stratigraphy of its source the upper neoproterozoic lower cambrian sedimentary succession in the central iberian zone has recently been divided into 12 sedimentary units i to xii units i to iv are of late neoproterozoic age and units v to xii are early cambrian throughout the stratigraphic section shales with similar ranges of sio2 and al2o3 have tio2 and zr contents that are coupled and vary gradually from the bottom unit i tio2 1 0 1 1 zr 245 287 ppm to the top unit xii tio2 0 74 0 86 zr 141 192 ppm these two parameters clearly distinguish upper neoproterozoic from lower cambrian compositions as do certain elemental ratios al2o3 tio2 rb zr nb ti which vary from low values at the bottom of the section to high at the top however other elements the rare earth elements rees y do not fit this general trend with heavy rees and y showing extreme concentrations in some lower cambrian units which also have negative ce anomalies units i to iii and units xi to xii have uniform tdm neodymium model ages 1 1 and 1 7 ga respectively but the units between these two groups have erratic values between 1 4 ga unit vii and 3 4 ga unit viii thus the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions a composition that records a juvenile contribution younger than 1 1 ga and a composition mainly derived from an old basement some samples from lower cambrian units have disturbed ree abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in ree and y redistribution this alteration probably affected the material now preserved as unit iv during a period of relative sea level fall at the end of the late neoproterozoic both elemental and isotopic results support the suggestion that the upper neoproterozoic lower cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement through time the deeper crust gradually increased its contribution to the sediments in the basin from late neoproterozoic to lower cambrian times these results are compatible with data from many other european zones

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

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

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

neoproterozoic cambrian synsedimentary magmatism in the central iberian zone spain geology petrology and geodynamic significance petrographic geochemical and field studies in low grade metamorphic areas ciudad rodrigo hurdes sierra de gata domain crhsg central western spain show that neoproterozoic lowermost cambrian series in the central iberian zone ciz record two kinds of provenance sources including 1 detrital material derived from recycled orogens and 2 a cadomian coeval juvenile contribution that governs their isotopic signature evidence of magmatism contemporaneous with neoproterozoic cambrian sedimentation is provided by the presence of coherent massive volcanic rocks metabasalts metaandesites and metarhyolites volcaniclastic shales sandstones conglomerates and breccias the appearance of volcanogenic lithic fragments and crystals mixed in different proportions with siliciclastic constituents and also present within calcareous components in the sedimentary succession reinforces this evidence although most of the selected volcanic and volcaniclastic samples appear to show tholeiitic affinity some of them display calc alkaline affinity different trace element ratios such as sm nd nb yb and ta yb suggest a magmatic evolution in the same tectonic setting the geochemical results reported here support the existence of an active geodynamic setting as a direct contributor to the synsedimentary and magmatic content of the neoproterozoic lowermost cambrian successions in the ciz in particular the relatively high end t values and the high range of f sm nd ratios are consistent with an active margin during the neoproterozoic early cambrian the existence of tectonic activity is also confirmed by the presence of synsedimentary deformation and volcanic rocks all of these traits favour a geodynamic model in which the iberian cadomian segment represented in the ciz would have been part of an active northern margin of gondwana with an associated magmatic arc and related basins during neoproterozoic lower cambrian times a proposed link between the ossa morena and the central iberian zones might account for late cadomian pull apart basins developed on both sides of the magmatic arc sharing the same scenario and involving similar magmatic activity during the neoproterozoic cambrian transition

upper neoproterozoic series in the central iberian cantabrian and west asturian leonese zones spain geochemical data and statistical results as evidence for a shared homogenised source area abstract analyses 100 of six groups of upper neoproterozoic shales from different areas in central and northern spain covering an extension of over 60 000 km2 reveal relatively uniform major and trace element abundances and values of relevant ratios such as al2o3 tio2 k2o al2o3 mol ti zr ti nb rb zr rb th sm nd eu eu la yb n la nb zr y zr nb th nb th yb th u and cr zr the six groups of shales define indistinguishable chondrite normalised and upper continental crust normalised patterns for rare earth and trace elements respectively and show relatively low fractionated geochemical features compared with those of the continental upper crust statistical calculations demonstrate a normal distribution of the abundances of major and trace elements and element ratios they suggest a remarkable geochemical homogeneity for the upper neoproterozoic series studied and they separate this series from the lower cambrian one in the central iberian zone on the basis of 15 element ratios the extension of the upper neoproterozoic detrital series and the similar geochemical features of shales in all the studied areas suggest an extensive and homogenised source region thus the mean of the 100 analyses neoproterozoic iberian average shale nibas is proposed as a standard of comparison to identify the specific recycled and homogenised source for detrital upper neoproterozoic sediments in central and northern iberia and probably also in other european peri gondwanan areas the ranges of statistically distinctive geochemical parameters separating the upper neoproterozoic from the lower cambrian series are given as possible criteria to help in the studies of areas where no other data provide conclusive information about the series ages

geochemistry of the upper neoproterozoic and lower cambrian siliciclastic rocks and u pb dating on detrital zircons in the central iberian zone spain sandstones and shales from the upper neoproterozoic un succession in the central iberian zone ciz show parallel ree patterns and relatively restricted and similar ranges and average values of some element ratios such as al2o3 tio2 ti nb eu eu la yb n gd yb n and th u this remarkable geochemical homogeneity for related medium and fine grained rocks is unusual and strongly suggests a recycled source area however the lower cambrian lc equivalent rocks are in general terms geochemically less mature more heterogeneous and more fractionated their average ree patterns are practically coincident probably as a consequence of ree redistribution related to the reworking of sediments during a stage of sea level fall in lower cambrian times ti and zr abundances chemical index of alteration cia values and element ratios such as al2o3 tio2 k rb ti nb and rb zr can be used for discriminating purposes between the un and lc siliciclastic rocks detrital zircons from a un and a lc sandstone display morphological differences however u pb data are discordant and cannot be interpreted in a straightforward manner nevertheless age data are compatible with a model in which the continent of gondwana would have supplied zircons showing a bimodal age distribution in variable proportions to the respective sandstone units

investigating the kinematics of local thrust sheet rotation in the limb of an orocline a paleomagnetic and structural analysis of the esla tectonic unit cantabrian asturian arc nw iberia the esla tectonic unit lies along the southern boundary of the cantabrian asturian arc a highly curved foreland fold thrust belt that was deformed during the final amalgamation of the pangea supercontinent previous structural and paleomagnetic analyses of the cantabrian asturian arc suggest a two stage tectonic history in which an originally linear belt was bent into its present configuration creating an orocline the esla tectonic unit is a particularly complex region due to the interaction of rotating thrust sheets from the southern limb of the arc and the southward directed thrusts of the picos de europa tectonic domain during late stage north south shortening and oroclinal bending these structural interactions resulted in intense modification of early phase thin skinned tectonic structures that were previously affected by a deeper out of sequence antiformal stack that passively deformed the early thrust stack a total of 75 paleomagnetic sites were collected from the portilla and santa lucia formations two carbonate passive margin reef platform units from the middle devonian similar to other regions of the cantabrian asturian arc esla unit samples carry a secondary remanent magnetization that was acquired after initial thrusting and folding of variscan deformation in the late carboniferous protracted deformation during late stage oroclinal bending caused reactivation of existing thrust sheets that include the esla and younger corniero and valbuena thrusts when combined with existing structural data and interpretations these data indicate that the present day sinuosity of the esla unit is the consequence of both secondary rotation of originally linear features in the western esla exposures e g frontal thrusts and secondary modification and tightening of originally curvilinear features in the eastern esla exposures e g hanging wall lateral oblique ramps differences in structural style between the esla and other tectonic units of the arc highlight the complex kinematics of oroclinal bending which at the orogen scale buckled an originally linear north south in present day coordinates trending cantabrian asturian thrust belt during the final stages of pangea amalgamation

oxygen carbon and strontium isotope records of ediacaran carbonates in central iberia spain carbon oxygen and strontium isotope ratios were determined in the uppermost neoproterozoic carbonate rocks at pastores sw salamanca central iberia the low 18o v smow values 15 19 indicate an alteration of the whole stratigraphic series however the alteration process does not seem to have affected most of the 13c and 87sr 86sr data these data reveal that only one particular level of the carbonate section shows simultaneous relative increases in mn sr and 87sr 86sr ratios decreases in the 18o and 13c values and covariations between 87sr 86sr and the other parameters despite this most of the carbonate section lacks these covariations and its 13c 4 6 to 0 5 and 87sr 86sr 0 70845 0 70853 ratios probably reflect the seawater isotopic features at the time of sedimentation the data for c and sr isotopes at the pastores carbonate section and the presence of cloudina in platform carbonates together with an overlying major sub cambrian unconformity following a relative sea level fall are features comparable to those frequently shown by proterozoic cambrian boundary series across the world thus our results consistently support the notion that the carbonates at pastores are equivalent to other carbonates deposited close to the ediacaran cambrian boundary

diachronous post orogenic magmatism within a developing orocline in iberia european variscides 1 u pb zircon crystallization ages of 52 late variscan granitoid intrusions from nw iberia 19 from new data 33 from previous studies constrain the lithospheric evolution of this realm of the variscan belt of western europe and allow assessment of the relationship between oroclinal development and magmatism in late carboniferous early permian times the u pb ages in conjunction with a range of geological observations are consistent with the following sequence of events i oroclinal bending starts at 310 305 ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts ii between 305 and 300 ma melting continues under the outer arc of the orocline central iberian zone of the iberian variscan belt and mid crustal melting is initiated coevally the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure iii between 300 and 292 ma foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle iv cooling of the lithosphere between 292 and 286 ma resulting in a drastic attenuation of lower crustal high temperature melting by 285 ma the thermal engine generated by orocline driven lithospheric thinning delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts the model proposed explains the genesis of voluminous amounts of granitoid magmas in post orogenic conditions and suggests that oroclines and similar post orogenic granitoids common constituents of numerous orogenic belts may be similarly related elsewhere

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

geochemistry and petrology of recycled orogen derived sediments a case study from upper precambrian siliciclastic rocks of the central iberian zone iberian massif spain abstract two areas of upper precambrian siliciclastic rocks separated from each other by 150 km were selected in the central iberian zone ciz for a petrological and geochemical study the petrology of the coarser grained rocks indicates the absence or scarcity of igneous clasts the presence of extrabasinal and intrabasinal clasts and homogeneous features of sandstones ranging from very fine to fine grained the modal data of these rocks plot close to the qt l and qm lt sides of qtfl and qmflt triangular diagrams suggesting provenance from a recycled orogen the geochemical data on the shales and sandstones from the two areas studied are very uniform both groups of rock show similar ranges of relevant major element ratios such as al 2 o 3 tio 2 17 46 21 51 17 46 21 34 for shale and sandstone respectively al 2 o 3 feo t 2 40 3 79 2 18 3 33 and feo t feo t mgo 0 71 0 77 0 68 0 76 together with parallel average rare earth element ree patterns and similar values for some key trace element ratios la th th yb th u zr hf sm nd and cr sc other trace element ratios la sc th sc cr th and cr zr are uniform in the shales group and also in the sandstones group although minor differences become apparent when both groups are compared these differences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and indirectly the sc and cr contents given their covariations with al2o3 the zr yb zr y and zr th ratios of the shales are uniform the sandstones show higher and more variable values and there is no correlation between zr abundance and any of these ratios in the shales or sandstones together with the parallel ree patterns shown by both groups of rock these results suggest the absence of heavy mineral fractionation during the transport of detrital material and or the relative importance of other minerals probably phyllosilicates in the control of yb y and th abundances apparently all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling areas thus the petrological and chemical data are consistent and point to a very extensive homogeneous and stable source region as the most plausible contributor to the upper precambrian siliciclastic rocks in the ciz the possible relationship between this zone and n gondwana is also discussed

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

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

three dimensional velocity structure and earthquake locations beneath the northern tien shan of kyrgyzstan central asia we used the arrival times of local earthquakes and quarry blasts recorded by the kyrgyzstan broadband network knet to obtain three dimensional 3 d p and s wave velocity models of the upper crust beneath an actively deforming mountain front and its associated foreland in the kyrgyz tien shan the continuous velocity models described by cubic b spline interpolation of the squared slowness over a regular 3 d grid were computed by simultaneous inversion of hypocenter and medium parameters exact ray tracing was done in the smooth 3 d medium by shooting rays from the sources to the stations by an analytical perturbation method based on the paraxial ray theory the deduced large sparse linear system was solved using the damped iterative least squares algorithm lsqr the stability and resolution of the result was qualitatively tested by two synthetic tests the spike test and the checkerboard resolution test we found that the models are well resolved up to a depth of 27 km for most parts of our image domain the p and s wave velocity models are consistent with each other and provide evidence for marked heterogeneity in the upper crustal structure beneath the northern tien shan at shallower depths 7 km the sediment filled foreland is imaged as a relatively lower velocity feature compared to the mountains which are cored by crystalline basement rocks in contrast at midcrustal depths the mountains are underlain by relatively lower velocity materials compared to the foreland a distinct contrast in velocity structure is also observed between the eastern and western parts of the kyrgyz range at midcrustal depths with the presence of relatively higher velocities toward the east the seismicity is concentrated near the traces of major active faults and extends deeper beneath the foreland compared to the mountains the regional compression in the tien shan is accommodated along a series of high angle reverse faults distributed throughout the orogenic system that extend from the surface down to midcrustal depths the range bounding fault zone can be identified by a sharp lateral gradient in seismic velocities with a pronounced southward dip combined with a zone of seismicity that also deepens to the south and reverse fault source mechanisms from moderate sized events a pronounced low velocity zone lvz is imaged in the p wave field at midcrustal depths beneath the western part of the kyrgyz range this lvz is presumably correlatable with reported high conductivity zones in this region that have been proposed to mark active fault zones along which fluid migration occurs the location of the lvz which is closely coincident with the depth of maximum earthquake generation might imply that it is a crustal decoupling zone at the brittle ductile transition

strain partitioning in the footwall of the somiedo nappe structural evolution of the narcea tectonic window nw spain abstract the somiedo nappe is a major thrust unit in the cantabrian zone the external foreland fold and thrust belt of the north iberian variscan orogen exposed at the narcea tectonic window are precambrian rocks below the basal decollement of the somiedo nappe which exhibit a different deformation style than the overlying paleozoic rocks above the basal decollement during variscan deformation folding and widespread subhorizontal bedding parallel decollements were produced in the hanging wall within the paleozoic rocks vertical folding with related axial planar cleavage at a high angle to the decollement planes developed simultaneously in the upper proterozoic narcea slates of the footwall below the detachment the relative magnitude of finite strain measured in the footwall rocks diminishes towards the foreland these observations indicate that 1 significant deformation may occur in the footwall of foreland fold and thrust belts 2 the shortening mechanism in the footwall may be different from that of the hanging wall and 3 in this particular case the partitioning of the deformation implies the existence of a deeper blind decollement surface contemporaneous with the first stages of the foreland development that does not crop out in the region this implies a significant shortening in the footwall which must be taken into account when restoration and balancing of cross sections is attempted a sequential diagram of the evolution of the narcea tectonic window with a minimum shortening of 85 km is proposed explaining the complete variscan evolution of the foreland to hinterland transition in the north iberian variscan orogen

factors affecting finite strain estimation in low grade low strain clastic rocks the computer strain analysis methods sape mrl and dtnnm have permitted the characterization of finite strain in two different regions with contrasting geodynamic scenarios 1 the talas ala tau tien shan kyrgyzs republic and 2 the somiedo nappe and narcea antiform cantabrian to west asturian leonese zone boundary variscan belt nw of iberia the performed analyses have revealed low strain values and the regional strain trend in both studied areas this study also investigates the relationship between lithology grain size and percentage of matrix and strain estimates the two methodologies used the results show that these methods are comparable and the absence of significant finite strain lithological control in rocks deformed under low metamorphic and low strain conditions

probing crustal and mantle lithosphere origin through ordovician volcanic rocks along the iberian passive margin of gondwana abstract northwestern iberia preserves one of the most complete paleozoic sequences that document the origin and development of a passive margin along the southern gondwanan flank of the rheic ocean in addition to a well preserved sedimentary record there is widespread ordovician volcanic activity that can be used to probe the nature of the lower crust and mantle lithosphere that sourced the volcanic rocks during the rheic ocean opening the ordovician rift related volcanic sequences provide first order constraints on the early evolution of the rheic ocean in addition to published and new lithogeochemical data we provide sm nd isotopic data which together constrain the mantle or crustal source and allow an assessment on the role of the basement in rheic ocean magmatism the data imply that the mafic rocks are derived from a variety of sources including juvenile mantle that was contaminated by subduction coeval with early ordovician magmatism suggesting the importance of arc activity in northwest iberia during the opening of the rheic ocean other basalts were derived from a subcontinental lithospheric mantle that was enriched at about 1 0 ga basalts derived from a mantle enriched at ca 1 0 ga occur along other parts of the gondwanan margin avalonia oaxaquia and so the iberian basalts may be a local representation of a regionally significant enriched mantle the sm nd isotopic characteristics permit a genetic connection between this mantle source and the basement rocks recently identified in northwest iberia felsic magmas are predominantly intra crustal magmas derived from melting a mesoproterozoic crust lending support to other lines of data that the gondwanan margin of northwest iberia was predominantly underlain by a south american rio negro source

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

amazonian mesoproterozoic basement in the core of the ibero armorican arc 40ar 39ar detrital mica ages complement the zircon s tale the 40 ar 39 ar age data on single detrital muscovite grains complement u pb zircon ages in provenance studies as micas are mostly derived from proximal sources and record low temperature processes ediacaran and cambrian sedimentary rocks from northwest iberia contain unmetamorphosed detrital micas whose 40 ar 39 ar age spectra suggest an amazonian middle american provenance the ediacaran sample contained only neoproterozoic micas 590 783 ma whereas the cambrian sample contained three age groups neoproterozoic 550 640 ma avalonian cadomian pan african mesoproterozoicneoproterozoic boundary ca 920 1060 ma grenvillian sunsas and late paleoproterozoic ca 1580 1780 ma rio negro comparison of 40 ar 39 ar muscovite ages with published detrital zircon age data from the same formations supports the hypothesis that the neoproterozoic basins of northwest iberia were located in a peri amazonian realm where the sedimentary input was dominated by local periarc sources tectonic slivering and strike slip transport along the northern gondwanan margin affected both the basins and fragments of basement that were transferred from amazonian to northern african realms during the latest neoproterozoic earliest cambrian exhumation and erosion of these basement sources caused shedding of detritus to the cambrian basins in addition to detritus sourced in the continental mainland the apparent dominance of rio negro aged micas in the cambrian sandstone suggests the presence of unexposed basement of that age beneath the core of the ibero armorican arc

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