lithochemistry of the lower paleozoic goldenville and halifax groups southwestern nova scotia canada implications for stratigraphy provenance and tectonic setting of the meguma terrane

ordovician paleotopography as evidenced from original dips and differential compaction of dolostone and shale unconformably overlying precambrian basement on manitoulin island canada abstract the middle to basal upper ordovician lindsay formation unconformably overlies the paleoproterozoic bar river and lorrain formations on manitoulin island ontario canada representing a depositional hiatus of at least 1 7 billion years the paleoproterozoic inliers currently exposed as basement highs in the sheguiandah region comprised a 14 km2 area of quartz arenite archipelago during the middle to late ordovician inland sea transgression of north america the quartz arenite outcrop collectively covers an area of approximately 1 8 km2 high energy transport and reworking during wave and tropical storm activity along a high relief rocky shoreline is indicated by dolostone containing 2 5 m size rounded to subrounded quartz arenite boulders with percussion marks up to 4 5 m size subangular quartz arenite blocks consistent with mechanical breakage along joints fractures or bedding planes lenticular conglomerate beds and cracks in quartz arenite basement containing mixtures of dolostone quartz granules and fossiliferous debris with continued transgression organic rich carbonate muds were deposited under low energy conditions either on carbonate strata or directly on quartz arenite basement in restricted regions protected from the open sea clast size distribution in the dolostone clast rounding and the location of the shale quartz arenite unconformity relative to basement highs are all consistent with a longshore current running from the present northeast to southwest with the leeward sides of rocky islands protected from high wave and storm energy processes subsequent deposition of upper ordovician and silurian strata resulted in burial and differential compaction of the lindsay formation units the compaction may have been associated with fractures through which mg rich fluids migrated locally altering the lowermost lindsay formation to dolostone evidence for differential compaction is provided by bed thickening down dip of basement highs and boulder flanks and higher regional dips in shale beds proximal to basement highs compared with dolostone strata the relatively high angle strata of the lindsay formation are a result of the combined effects of paleotopography a high relief rocky shoreline and differential compaction

minas fault zone late paleozoic history of an intra continental orogenic transform fault in the canadian appalachians abstract the minas fault zone mfz defines the boundary between the avalon and meguma terranes in the canadian appalachians and is exposed in mainland nova scotia and southern new brunswick these terranes originated along the gondwanan margin but had accreted to laurentia by the middle devonian the surface trace of the mfz is adjacent to the southern margin of the latedevonian permian maritimes basin the late devonian late carboniferous evolution of the mfz involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism regional fluid flow and mineralization in the late devonian early carboniferous asymmetric rifting accompanied by dextral shear produced two coeval sequences the horton group which is dominated by continental clastic strata and the fountain lake group which consists predominantly of bimodal volcanic rocks that overlie high level plutons emplaced along active shear zones the overall tectonic environment may have been dominated by dextral transtension along the southern margin of laurentia which corresponded with the northern flank of the rheic ocean a major change in the evolution of the minas fault zone occurred in the late mississippian early pennsylvanian and produced the e w chedabucto fault clockwise rotation of pre existing structures local zones of transtension and transpression as well as regional fluid flow and extensive mineralization this major change may reflect the onset of laurentia gondwana oblique collision the effects of which continued into the latest carboniferous with coeval development of flower structures and pull apart basins in zones of local transpression and transtension

orogenesis and basin development u pb detrital zircon age constraints on evolution of the late paleozoic st marys basin central mainland nova scotia abstract the st marys basin along the southern flank of the composite late paleozoic magdalen basin in the canadian appalachians and along the avalon meguma terrane boundary contains late devonian early carboniferous continental clastic rocks of the horton group that were deposited in fluvial and lacustrine environments after the peak of the acadian orogeny shrimp ii geological survey of canada data on approximately 100 detrital zircons from three samples of horton group rocks from the st marys basin show that most of the zircons have been involved in a multistage history recycled from clastic rocks in the adjacent meguma and avalonian terranes although there is a minor contribution from early silurian 411 ma and late devonian suites ca 380 370 ma neoproterozoic ca 700 550 ma and paleoproterozoic ca 2 0 2 2 ga zircon populations predominate with a minor contribution from ca 1 0 1 2 and 1 8 ga zircons published u pb single zircon analyses on clastic sedimentary rocks indicate th

geochemistry and provenance of the carboniferous baixo alentejo flysch group south portuguese zone abstract this work is focused on the turbiditic sediments from the carboniferous baixo alentejo flysch group bafg in the south portuguese zone an external zone of the iberian variscides the aim of this work is to constrain the provenance and tectonic setting of these sediments in a context of a complex evolution of sw iberian variscides for this purpose we performed a systematic study of petrographical and geochemical signatures of greywackes and shales from the three bafg formations mertola mira and brejeira major and trace element composition and ratios suggest heterogeneous source area composition for bafg shales and greywackes for the oldest mertola formation greywackes source area is dominated by granitoid rocks with minor mafic input the latter becomes residual in the mira formation the youngest brejeira formation greywackes show clear felsic affiliation associated with an increase in recycled components the shales of all three bafg formations denote a granodioritic affiliation chemical index of alteration cia and plagioclase index of alteration pia values suggest moderate weathering in the source areas of mertola and mira formations these indices together with a cn k relations point out to steady state weathering conditions in the source areas of both formations in contrast both cia and pia values for the brejeira formation indicate variable conditions of palaeoweathering from moderate to intense as a consequence of non steady state conditions probably triggered by tectonic instability in the provenance area compared to the greywackes the shales of all three bafg formations exhibit higher cia and pia values as well as low k2o al2o3 0 2 and index of compositional variability

the missing rheic ocean magmatic arcs provenance analysis of late paleozoic sedimentary clastic rocks of sw iberia abstract early carboniferous turbiditic sedimentary rocks in synorogenic basins located on both sides of the rheic suture in sw iberia were studied for provenance analysis an enigmatic feature of this suture which resulted from closure of the rheic ocean with the amalgamation of pangea in the late carboniferous is that there are no recognizable mid to late devonian subduction related magmatic rocks which should have been generated during the process of subduction on either side of it u pb la icp ms geochronology of detrital zircons from early carboniferous turbidites in the vicinity of the rheic suture in sw iberia where it separates the ossa morena zone with gondwana continental basement to the north from the south portuguese zone with unknown meguma continental basement to the south reveals the abundance of mid to late devonian 51 81 and early carboniferous 13 25 ages the cabrela and mertola turbidites of the ossa morena and south portuguese zones respectively are largely devoid of older zircons differing from the age spectra of detrital zircons in the oldest late devonian strata in the underlying south portuguese zone which contain abundant cambrian and neoproterozoic ages mid to late devonian zircons in the cabrela formation age cluster at c 391 ma eifelian givetian transition and mertola formation age clusters at c 369 ma and at c 387 ma famennian and givetian respectively are attributable to a source terrane made up of magmatic rocks with a simple geological history lacking both multiple tectonic events and older continental basement the terrane capable of sourcing sediments dispersed on both sides of the suture is interpreted to have been completely removed by erosion in sw iberia given that closure of the rheic ocean required subduction of its oceanic lithosphere and the absence of significant arc magmatism on either side of the rheic suture we suggest 1 the source of the zircons in the sw iberia basins was a short lived rheic ocean magmatic arc and 2 given the lack of older zircons in the sw iberia basins this short lived arc was probably developed in an intra oceanic environment

acadian deformation in the shallow crust an example from the siluro devonian arisaig group avalon terrane mainland nova scotia the silurian early devonian arisaig group of the avalon terrane in northern mainland nova scotia consists mainly of thinly bedded sandstones siltstones and shales deposited in a near shore environment these strata were deformed in the middle devonian to form regional northeast to nne trending folds and record deformation processes in the shallow crust during the acadian orogeny one of the most regionally extensive orogenic events in the canadian appalachians structural features in the arisaig group are consistent with fold propagation associated with thrust fault geometry and coeval local extension recorded by a set of conjugate normal faults many outcrop scale folds have sheared limbs and show evidence of a complex progressive deformation folding was predominantly accomplished by bulk rotation and flattening above thrust fault tips early structures d1 d2 produced regional cylindrical folds whereas later d3a d3b d3c structures produced conical folds d1 d3 fold orientations show high var

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

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

variscan ophiolite belts in the ossa morena zone southwest iberia geological characterization and geodynamic significance abstract the boundary between the iberian and south portuguese terranes in sw iberia is traced by a suture recording the closure of rheic and related oceans through nne directed subduction the western segment of the suture comprises imbrications of the iberian terrane relative autochthon neoproterozoic lower palaeozoic and allochthonous complexes the iberian terrane relative autochthon is overlain by lower middle devonian metasediments and an unconformable cover of lower carboniferous calc alkaline volcano sedimentary complexes both preserving evidence of very low grade metamorphism the allochthonous complexes include disrupted slivers of eclogite ca 370 ma klippen and slices of an internal ophiolite sequence iomzos a basal tectonic melange moura phyllonitic complex and an external ophiolite beja acebuches complex baoc a large layered gabbroic sequence lgs whereas part of the beja igneous complex intruded ca 350 340 ma the sw suture domains the iomzos represents obducted remnants of the rheic ocean for which available shrimp u pb data indicate an age of 479 5 ma this ocean which separated iberia from a promontory of gondwana and avalonia now represented by the south portuguese terrane opened near the cambrian ordovician boundary through a rift jump from an intra cratonic setting within the iberian terrane to oceanic rifting the baoc displays geochemical features compatible with derivation from a short lived back arc basin lower middle devonian t dm sm nd 380 440 ma the variscan ophiolite belts preserved along the sw variscides were emplaced by antithetic obduction in two stages both with shearing sense towards the nne the first stage which involved iomzos cold obduction exhumed hp rocks ca 370 ma and created a foreland bulge towards the iberian terrane s sw flank this mechanism controlled the development of the volcanic related carbonate shelves and the contemporaneous terena flysch trough to the ne the second stage corresponds to baoc hot obduction carrying the iomzos klippen in piggy back style and affecting the southern margin of the lgs at ca 350 340 ma

contrasting isotopic sources sm nd of late ediacaran series in the iberian massif implications for the central iberian ossa morena boundary abstract in the iberian massif westernmost sector of the variscan orogen the nature of the boundary between the central iberian zone and the ossa morena complex has been largely discussed peri gondwanan ediacaran series similar in age c 565 ma but different in composition occur at both sides of that boundary the lower alcudian series central iberian zone is located to the north while the serie negra ossa morena complex can be found to the south of the boundary major and trace element compositions and sm nd isotopic systematics of siliciclastic rocks from both series suggest a common active margin setting for their deposition the serie negra likely occupying a more external fore arc position within the arc system relative to the lower alcudian series which would have been deposited within a large back arc domain tdm ages for these two series are notably different yielding values of 1421 2040 ma and 1256 1334 ma for the serie negra and lower alcudian respectively these values suggest higher input from old cratonic sources in the ossa morena complex with probable participation of the west african craton which would be less prominent in the central iberia zone the whole data set indicate deposition of the ediacaran series in separated basins probably far away from each other along across the margin of gondwana current juxtaposition of these basins is explained by variscan and probably also pre variscan tectonics similar conclusions could be also be extended to other similar boundaries in the variscan orogen

baltic provenance of top famennian siliciclastic material of the northern rhenish massif rhenohercynian zone of the variscan orogen the provenance of top famennian sedimentary rocks linked to the hangenberg crisis from the northern rhenish massif germany was investigated by the means of detrital zircon u pb geochronology based on the obtained age spectra three main tectonothermal domains are recognized as possible sources paleo and mesoproterozoic 2000 1000 ma units of baltica and early paleozoic caledonian orogen 500 400 ma our interpretation of the detritus having been derived from northern source areas i e baltica and the scandinavian caledonides with a minor input of german polish rugen pomeranian caledonides contradicts the traditional view that during the upper devonian the northern rhenish massif was supplied by detritus from the south complementary mineralogical textural and geochemical analyses point to a derivation of the detritus of drewer and hangenberg sandstones mainly from felsic recycled continental crust the elevated concentrations of pb and zn in the studied sections are a feature attributed to hydrothermal alteration related to the terminal devonian synsedimentary volcanism or post depositional variscan deformation

early carboniferous deformation and mineralization in the cobequid shear zone nova scotia an 40ar 39ar geochronology study latest devonian to early carboniferous igneous rocks along the cobequid shear zone of central nova scotia record a complex history of intrusion volcanism deformation and hydrothermal alteration twenty new 40ar 39ar age determinations have been made on biotite and hornblende separates from rocks that constrain these events the mafic plutons of the cobequid shear zone are synchronous with the previously dated granitoid plutons 363 355 4 ma with the exception of the younger folly lake gabbro 355 350 4 ma high temperature shear deformation continued along the cobequid shear zone during this entire period of pluton emplacement several samples from mylonitic and hydrothermally altered rocks with dates ca 340 ma indicate renewed movement along the cobequid shear zone at this time accompanied by minor magmatism younger 330 320 ma ages reflect final movement along the rockland brook fault resulting in mid namurian uplift of the cobequid highlands following this regional alleghenian event ther

shallow depth caco3 dissolution evidence from excess calcium in the south china sea and its export to the pacific ocean national basic research program of china 973 program 2009cb421201 2009cb421206 national science foundation of china nsfc 40821063 90711005 south china sea institute of oceanology the chinese academy of sciences

geomorphology and coastal erosion of a quartzite island hongdo in the yellow sea off the sw korean peninsula abstractas a dense metamorphic rock with silica grains that are strongly fused quartzite exposed on rocky shores is generally resistant to marine erosion the longevity of quartzite coasts compared to those formed by other rock types is underscored by the preservation of many former islands that retain notable topographic relief as inliers of archean and proterozoic quartzite surrounded by paleozoic marine strata in north america and western australia among the few places where modern rocky shores composed principally of quartzite may be studied with regard to marine erosion hongdo in the yellow sea off the southwestern korean peninsula is a natural laboratory where intersection of structural geology coastal geomorphology oceanography sedimentology and climate converge to demonstrate how a quartzite island evolves through time although small in size 6 87 km2 hongdo is open to the ocean on all sides the island s structure as an asymmetrical anticline permits a wide range of attack angles for se

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

u pb geochronology and petrology of the late paleozoic gil marquez pluton magmatism in the variscan suture zone southern iberia during continental collision and the amalgamation of pangea the origin of plutonic complexes that stitch suture zones developed during collision is not well understood in southern iberia the pulo du lobo suture zone pdlz is intruded by the syn to postcollisional gil marquez pluton gmp thought to be part of the sierra norte batholith u pb la icpms zircon data on various phases of the gmp yield from oldest to youngest 1 a 354 4 7 6 ma unfoliated gabbro 2 a 345 6 2 5 ma foliated intermediate phase 3 a 346 5 5 4 ma unfoliated porphyritic granite 4 a 335 1 2 8 ma unfoliated biotite granite this sequence is consistent with cross cutting relationships observed in the field the range in ages is consistent with interpretations that the gmp is part of the composite ca 350 308 ma snb inherited ages preserved in the gmp intermediate and felsic phases indicate that its magmas traversed through south portuguese zone and pdlz crust during emplacement the ca 345 ma emplacement of the late kinematic foliated intermediate phase constrains the age of late stage strike slip deformation within the pdlz and the lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement nor during its subsequent history the unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision while the ca 335 ma biotite granite intrudes the santa ira flysch thereby providing a tight constraint for the latest stage of deformation in the pdlz

440 ma igneous activity in the meguma terrane nova scotia canada part of the appalachian overstep sequence abraded zircons from the basal rhyolitic tuff member of the white rock formation which disconformably overlies the cambrian early ordovician meguma group have yielded a nearly concordant u pb age of 442 or 4 ma interpreted as the age of extrusion this age straddles the approximately 443 ma ordovician silurian boundary abraded zircons from the brenton granite lie on a chord with an upper intercept age of 439 4 3 ma which is interpreted to be the age of intrusion thus supporting its inferred subvolcanic nature on the other hand monazite from the brenton pluton yielded a nearly concordant analysis with a 207 pb 206 pb age of 380 or 3 ma which is interpreted to be the time of the low pressure high temperature metamorphism using these new data the following observations suggest that the meguma and avalon terranes were neighbors during the silurian early devonian 1 both have latest ordovician earliest silurian bimodal subaerial alkalic tholeiitic rift related volcanic rocks with nd signatures indicating a similar continental basement source 2 both show a similar progression of depositional environments subaerial in the earliest silurian progressively deeper water marine strata in the llandovery and wenlock switching to gradually shallowing marine environments in the ludlow and reverting to subaerial in the pragian and 3 both contain rhenish bohemian early devonian fauna furthermore the southeast to northwest transition from an offshore sandbar to a beach sand in the silurian white rock formation suggests the presence of land to the north now recognized as avalonia these conclusions support published suggestions that the siluro devonian successions in the meguma and avalon terranes form part of the overstep sequence that extends across most of the northern appalachians published data indicate that avalonia was adjacent to gondwana in the neoproterozoic that it separated from gondwana in the early ordovician and was accreted to eastern laurentia in the late ordovician early silurian on the basis of the data and correlations presented here we suggest that the meguma terrane travelled with avalonia this is consistent with the absence of a phase of deformation between the cambro ordovician meguma group and the silurian white rock formation

superposed hercynian and cadomian orogenic cycles in the ossa morena zone and related areas of the iberian massif polyphase ductile deformation and metamorphism in the ossa morena zone of the iberian massif are related to a complex evolution including two major tectonothermal episodes of cadomian late proterozoic to early cambrian and hercynian age middle to late paleozoic petrological structural and geochronological data attest to a number of tectonic episodes in the 620 480 ma interval the model presented for this orogeny comprises stages of volcanic arc generation backarc extension tectonic inversion crustal thickening and cratonization in an andean type continental margin the hercynian cycle began in the early paleozoic with episodes of continental rifting orogenic events occurred in the 390 300 ma interval and affected both the cadomian basement which was heterogeneously reworked and overprinted and the paleozoic cover hercynian regional metamorphism was generally of low grade high pressure assemblages and thermal domes developed locally and were related to subduction and late extensional events the geodynamic model proposed for the cadomian and hercynian orogenies and the correlation with comparable areas from pre mesozoic massifs elsewhere in western europe account for the geodynamic scenarios that drove the fate of cadomian terranes

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

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

late neoproterozoic to carboniferous genesis of a type magmas in avalonia of northern nova scotia repeated partial melting of anhydrous lower crust in contrasting tectonic environments avalonian rocks in northern mainland nova scotia are characterized by voluminous 640 600 ma calc alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc however after the cessation of arc activity repeated episodes of felsic magmatism between ca 580 ma and 350 ma are dominated by a type geochemical characteristics sm nd isotopic data combined with zircon saturation temperature estimates indicate that these magmas were formed by high temperature 800 1050 c melting of the same anhydrous crustal source regional tectonic considerations indicate that a type felsic magmatism was produced 1 at 580 ma in a san andreas type strike slip setting 2 at 495 ma as avalonia rifted off gondwana 3 at 465 and 455 in an ensialic island arc environment and 4 at 360 350 ma during post collisional intra continental strike slip activity as avalonia was translated dextrally along the laurentian margin these results attest to the importance of crustal source rather than tectonic setting in the generation of these a type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane they also suggest that the shallow crustal rocks in northern mainland nova scotia were not significantly detached from their lower crustal source between ca 620 ma and 350 ma a time interval that includes the separation of avalonia from gondwana its drift and accretion to laurentia as well as post accretionary strike slip displacement

tectonics of the beja acebuches ophiolite a major suture in the iberian variscan foldbelt the beja acebuches ophiolitic complex baoc south portugal spain corresponds to a high grade metamorphic belt along the boundary between ossa morena and the south portuguese zones and comprises a lithostratigraphic sequence including from top to bottom metabasalts metamorphosed multiple dyke intrusions in gabbro flasergabbros and metaserpentinites it is affected by three deformation phases d1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north north east this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the serpa antiform affecting precambrian basement and cambrian cover rocks the obduction polarity ist north eastwards similar to the subduction polarity that generates the beja gabbroic complex bgc implying a flake geometry the second deformation phase d2 is represented by an intense wnw ese sinistral shear event which is responsible for the shattered appearance of the suture d2 is reactivated later by a more brittle d3 event involving thrusting to the south west again with a sinistral component 40ar 39ar isotopic ages were obtained for metamorphosed multiple dyke intrusions in the baoc s gabbro 342 6 1 4 ma for metagabbroic cumulates 340 7 1 9 ma and for the undeformed unmetamorphosed bgc 341 1 1 3 ma occurring to the north of the ophiolitic suture these ages reflect a last regional cooling event in the area which post dates the ophiolite emplacement and the intrusion of the bgc through this oceanic sequence

two stage collision exploring the birth of pangea in the variscan terranes abstract the variscan suture exposed in nw iberia contains a stack of terranes including two allochthonous units with continental affinity and gondwanan provenance upper and basal units separated by an ophiolite belt where the most common units show protolith ages at c 395 ma recent lu hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust hence the ophiolites could not have originated in a deep ocean basin associated with a mature mid ocean ridge or intraoceanic subduction the tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction the upper units record an initial high p ultra high p metamorphic event that occurred before 400 390 ma while the basal units were affected by a second high p low to intermediate t metamorphic event dated at c 370 ma continental subduction affected the most external margin of gondwana and developed in a setting of dextral convergence with laurussia development of the two high p events alternated with the opening of an ephemeral oceanic basin probably of pull apart type in early devonian times this ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the variscan suture current ideas for the assembly of pangea advocate a single collisional event between gondwana and laurussia in the carboniferous however the new evidence from the allochthonous terranes of the variscan belt suggests a more complex scenario for the assembly of the supercontinent with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered based on modern analogs of continental interaction the development of complex collisions as here suggested for gondwana and laurussia during the assembly of pangea could have been the norm rather than the exception throughout earth history

uniformitarianism as a guide to rocky shore ecosystems in the geological record literature on pleistocene and neogene rocky shores is reviewed from a uniformitarian perspective to assess the fidelity of physical and biological information entrained in the geological record through present day processes coverage by latitude spans tropical to subarctic shores variations in paleotopography are illustrated by unconformities between sedimentary accumulations and rocks of igneous or non igneous origin former rocky shores occur across a range of geographic scales that include i spot localities ii embayments with continuous transition from exposed outer to protected inner shores and iii entire islands with windward and leeward shores differences in rate and mag nitude of sea level change affect rocky shore physiography terrace deposits characterize the pleistocene but ramp de posits typify older neogene shores on average species with hard parts account for about 50 of the modern rocky intertidal fauna on a regional basis regardless of latitude biodiversity is highest in mid latitudes fixed organisms with encrusting wedging or boring habits corals barnacles oysters other bivalves and vermetid gastropods show high potential for fossilization in growth position mobile organisms that cling to rocks gastropods crabs and echinoids suffer post mortem transport but may be immured within neptunian dikes or interstices among cobbles and boulders at best localized fossil deposits from the pleistocene reflect 30 of the biodiversity of skeletonized organisms tabulated from modern rocky shores on a regional basis uniformitarianism provides little insight into the diversity of soft bodied organisms excluding stromatolites that joined the ecosystem in precambrian times and the extent to which soft bodied organisms dominated shore life through later times

tectonic significance of late ordovician silicic magmatism avalon terrane northern antigonish highlands nova scotia12 1 this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2 contribution to international geological correlation programme igcp project 497 avalonia was a microcontinent during most of the ordovician separating the iapetus ocean to the north from the rheic ocean to the south in the northern antigonish highlands nova scotia volcanic rocks dunn point and mcgillivray brook formations underlie early silurian early devonian strata arisaig group and were thought to represent extension that heralded the development of the basin into which arisaig group strata were deposited however recent u pb zircon thermal ionization mass spectrometry data from rhyolite in the dunn point formation dpf yielded an age of 460 0 3 4 ma and here we report a concordant age of 454 5 0 7 ma for an ignimbrite in the overlying mcgillivray brook formation mbf these data confirm a ca 10 million year gap between volcanism and onset of arisaig group deposition which occurred after accretion of avalonia to baltica the dpf and mbf both resemble a type sio2 rich magmas the mbf has very high concentrations of zr 745 1965 ppm y 65 213 ppm nb 57

chemical and isotopic composition of the lower crust beneath the meguma lithotectonic zone nova scotia evidence from granulite facies xenoliths granulite facies xenoliths from an upper devonian lamprophyre dyke near tangier nova scotia provide new information about the lower crust in the meguma zone two mineralogically and chemically distinet groups of xenoliths occur both groups contain quartz feldspar biotite fe ti oxides sulfides in addition group a contains garnet graphite aluminosilicates spinel sapphirine hight al2o3 subgroups a1 and a2 or clinopyroxene sphene apatite high cao sub group a3 group b has highly variable proportions of orthopyroxene b1 clinopyroxene b2 and amphibole b3 trace element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite facies rocks lowp t sedimentary assemblages of quartz chlorite clay minerals calcite albite or paragonite can account for the compositions of group a xenoliths within group b a high mgo mgo 13 wt subgroup with high transition metal contents and low mgo mgo 9 wt sub groups with higher lil large ion lithophile element contents exist although the rare earth and high field strength elements indicate a tholeiitic or low k calc alkaline chemistry the lil elements are as high as those from high k calc alkaline volcanic are rocks isotopically group a ranges from endt 2 56 to 0 80 and87sr 86srt 0 7046 to 0 7182 fort 370 ma for group b these values are 1 45 to 5 33 and 0 7028 to 0 7048 respectively model ages tchur are correspondingly low and are tightly constrained 544 52 ma these young ages contrast with the middle proterozoic nd model ages of the overlying meguma zone low grade flysch this time inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous tangier lower crust avalon and an older allochthonous meguma group upper crust

neoproterozoic early devonian magmatism in the antigonish highlands avalon terrane nova scotia tracking the evolution of the mantle and crustal sources during the evolution of the rheic ocean abstract the antigonish highlands lies in the avalon terrane avalonia in the canadian appalachians and records four episodes of continental rift related bimodal magmatism ranging in age from late neoproterozoic to middle devonian the oldest episode ca 615 ma clydesdale formation was emplaced in a local rift setting in an andean style arc when avalonia resided along the northern gondwanan margin early cambrian magmatism arbuckle brook formation occurred in a local transtensional setting after arc magmatism had given way to a san andreas type transform environment middle ordovician dunn point and bears brook formations magmatism occurred in a local rift in an ensialic island arc setting analogous to the modern taupo volcanic zone in northern new zealand after avalonia had separated from gondwana middle devonian mcarras brook formation and correlatives occurred after avalonia had collided with laurentia in each episode mafic rocks are characterized by high feot feot mgo tio2 zr y and ti y typical of differentiated within plate mafic magmas multi element spider diagrams indicate that all four episodes were generated in an enriched sub continental lithospheric mantle lil hfs and ree patterns indicate that the clydesdale dunn point bears brook and mcarras brook formations were sourced in the shallow spinel lherzolite mantle and that the arbuckle brook formation was generated in the deeper garnet lherzolite mantle in each episode coeval felsic rocks were generated by crustal anatexis taken together the sm nd isotopic data for the mafic rocks form an envelope that defines the evolution of the mantle source beneath the antigonish highlands that was enriched between 0 8 and 1 1 ga and has an average sm nd ratio of ca 0 24 a value that is typical of an enriched mantle source sm nd isotopic data for the felsic rocks are characterized by a recurrence of tdm ages between 0 95 and 1 0 ga interpreted to reflect repeated melting of the lower crust that was itself derived from a depleted mantle source between 0 95 and 1 1 ga the remarkable geochemical and isotopic similarity of the neoproterozoic to middle devonian mafic and felsic magmas indicate that the crust and sub continental lithospheric mantle beneath the antigonish highlands were coupled during the rift and drift of avalonia from the gondwanan margin during the formation of the rheic ocean in the ordovician its accretion to baltica and to laurentia in the silurian and post accretionary strike slip movement of avalon along the laurentian margin although the sm nd isotopic data suggest that avalonian basement and lithospheric mantle formed over the same time interval 0 8 to 1 1 ga the divergence of their respective envelopes with time reflects their differing end values and sm nd ratios we suggest that avalonian crustal basement and its enriched mantle were both formed between 0 8 and 1 1 ga in a panthalassa type ocean that surrounded the supercontinent rodinia and were accreted to the northern gondwanan margin at about 650 ma prior to the oldest magmatic event in the antigonish highlands

variability over time in the sources of south portuguese zone turbidites evidence of denudation of different crustal blocks during the assembly of pangaea this study combines geochemical and geochronological data in order to decipher the provenance of carboniferous turbidites from the south portuguese zone sw iberia major and trace elements of 25 samples of graywackes and mudstones from the mertola visean mira serpukhovian and brejeira moscovian formations were analyzed and 363 u pb ages were obtained on detrital zircons from five samples of graywackes from the mira and brejeira formations using la icpms the results indicate that turbiditic sedimentation during the carboniferous was marked by variability in the sources involving the denudation of different crustal blocks and a break in synorogenic volcanism the visean is characterized by the accumulation of immature turbidites mertola formation and the base of the mira formation inherited from a terrane with intermediate to mafic source rocks these source rocks were probably formed in relation to devonian magmatic arcs poorly influenced by sedimentary recycling as indicated by the almost total absence of pre devonian zircons typical of the gondwana and or laurussia basements the presence of carboniferous grains in visean turbidites indicates that volcanism was active at this time later serpukhovian to moscovian turbiditic sedimentation mira and brejeira formations included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism the abundance of precambrian and paleozoic zircons reveals strong recycling of the gondwana and or laurussia basements a peri gondwanan provenance is indicated by zircon populations with neoproterozoic cadomian avalonian and pan african zircon forming events paleoproterozoic and archean ages the presence of late ordovician and silurian detrital zircons in brejeira turbidites which have no correspondence in the gondwana basement of sw iberia indicates laurussia as their most probable source

early cambrian granitoids of north gondwana margin in the transition from a convergent setting to intra continental rifting ossa morena zone sw iberia two distinct cambrian magmatic pulses are recognized in the ossa morena zone sw iberia an early rift er and a main rift related event this cambrian magmatism is related to intra continental rifting of north gondwana that is thought to have culminated in the opening of the rheic ocean in lower ordovician times new data of whole rock geochemistry 19 samples sm nd sr isotopes 4 samples and id tims u pb zircon geochronology 1 sample of the early cambrian er plutonic rocks of the ossa morena zone are presented in this contribution the er granitoids barreiros barquete calera salvatierra de los barros and tablada granitoid massifs are mostly peraluminous granites the sm nd isotopic data show moderate negative endt values ranging from 3 5 to 0 1 and tdm ages greatly in excess of emplacement ages most er granitoids are crustal melts however a subset of samples shows a transitional anorogenic alkaline tendency together with more primitive isotopic signatures documenting the participation of lower crust or mantle derived sources and suggesting a local transient advanced stage of rifting the barreiros granitoid is intrusive into the ediacaran basement of the ossa morena zone serie negra succession and has yielded a crystallization age of 524 7 0 8 ma consistent with other ages of er magmatic pulse this age 1 constrains the age of the metamorphism developed in the ediacaran back arc basins before the intrusion of granites and 2 defines the time of the transition from the ediacaran convergent setting to the lower cambrian intra continental rifting in north gondwana

zircon u pb geochronology of paragneisses and biotite granites from the sw iberian massif portugal evidence for a palaeogeographical link between the ossa morena ediacaran basins and the west african craton

petrology age and tectonic setting of the seal island pluton offshore southwestern nova scotia the seal island pluton outcrops only on small islands located on the continental shelf 45 km south of nova scotia although geophysical data indicate that the pluton is part of large granitoid units that cover thousands of square kilometres farther offshore based on the island outcrops the seal island pluton consists of biotite monzogranite and muscovite biotite monzogranite of uncertain relative age metasedimentary xenoliths combined with characteristic magnetic patterns indicate that the pluton intruded the cambrian ordovician meguma group compared with the biotite monzogranite the muscovite biotite monzogranite is higher in sio2 more peraluminous and more depleted in heavy rare earth elements and also has lower end 1 39 versus 0 82 possibly the result of more contamination by meguma group sedimentary rocks the biotite monzogranite yielded a late devonian u pb zircon age of 362 8 0 7 ma although the relatively minor petrological differences between the two units do not preclude a co m

the multistage crystallization of zircon in calc alkaline granitoids u pb age constraints on the timing of variscan tectonic activity in sw iberia cl imaging and u th pb data for a population of zircons from two of the evora massif granitoids ossa morena zone sw iberia show that both calc alkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in th u ratio and having ages in the range ca 350 335 ma early carboniferous multistage crystallization of zircon is revealed in two main growth stages ca 344 342 ma and ca 336 335 ma well represented by morphologically complex zircons with cores and rims with different ages and different th u ratios that can be explained by 1 crystallization from melts with different compositions felsic peraluminous to felsic intermediate metaluminous 0 001 837 c granulite facies and or because they were derived from inheritance poor felsic and mafic rocks from a previous cycle as suggested by the internal structures of zircon cores these variscan magmatic rocks with crystallization ages estimated at ca 336 335 ma are spatially and temporally related to high temperature metamorphism anatexis processes of interaction between crustal and mantle derived magmas and intra orogenic extension that acted in sw iberia during the early carboniferous

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

u pb zircon ages and provenance of the white rock formation of the rockville notch group meguma terrane nova scotia canada evidence for the sardian gap and west african origin the white rock formation is the lowermost formation of the rockville notch group an assemblage of silurian devonian rocks preserved in five areas along the northwestern margin of the meguma terrane of nova scotia the formation consists mainly of mafic and felsic metavolcanic rocks interlayered with and overlain by marine metasedimentary rocks felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range with errors of about 446 434 ma these dates confirm a 30 ma hiatus after deposition of the early ordovician hellgate formation in the underlying halifax group this hiatus is coeval with the sardian gap in the lower palaeozoic of peri gondwanan europe the metavolcanic metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses some of the latter were previously interpreted to be ordovician tillite an interpretation no longer viable new detrital zircon data from metas

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

the elusive nature of the rheic ocean suture in sw iberia the rheic ocean suture resulted from pre carboniferous oceanic subduction followed by late devonian carboniferous variscan collision in sw iberia this suture has been classically located along the boundary between the ossa morena and south portuguese zones based on the presence of three units i a conspicuous metamafic unit beja acebuches that crops out along this boundary and has been interpreted as a pre carboniferous rheic ocean ophiolite ii a low grade metasedimentary unit with minor mid ocean ridge basalt like lithologies pulo do lobo unit thought to represent a rheic ocean subduction related accretionary prism and iii the allochthonous cubito moura unit that contains high pressure and ophiolitic like rocks we report new structural and geochronological data that allow us to reinterpret the origin and internal structure of the beja acebuches and the pulo do lobo units thus both the beja acebuches protoliths and the pulo do lobo metabasalts would have been formed in the context of an intracollisional extensional stage that interrupted the variscan collision at early carboniferous time after the rheic ocean consumption and the first continental collision later on collision was resumed in an oblique left lateral regime that gave way to coeval frontal folds and thrusts and lateral shear zones and strike slip faults structures with variable pressure temperature conditions and space distribution along time as a consequence of the superposition of transtension and complex transpression the rheic suture in sw iberia has an obscure nearly cryptic appearance

probing the composition of unexposed basement south portuguese zone southern iberia implications for the connections between the appalachian and variscan orogens geochemistry and sm nd and u pb magmatic zircon isotope data from a postcollisional batholith that crosscuts the allochthonous south portuguese zone spz of southern iberia suggest that the base

exceptionally preserved late ordovician biotas from manitoba canada there are few body fossil biotas known from early paleozoic accretionary shorelines and very few examples of ordovician soft bodied assemblages this study documents two recently discovered biotas from separate sedimentary basins in manitoba canada that provide unique information about tropical shoreline communities shortly before the late ordovician extinction event each site represents a distinct depositional environment but they share biotic elements including eurypterids xiphosurids and large problematic tubes the william lake biota representing more restricted conditions includes jellyfish that are among the best hydromedusan body fossils known rocks at the airport cove site deposited under more open circulation contain scolecodonts and noncalcified algae these biotas have some parallels with the recently described middle ordovician winneshiek lagerstatte but are also similar to some late silurian assemblages considered together early paleozoic marginal marine deposits are a rich but as yet poorly known source of paleobiodiversity data

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

a study of inherited zircons in granitoid rocks from the south portuguese and ossa morena zones iberian massif support for the exotic origin of the south portuguese zone abstract trace element and u pb isotopic analyses of inherited zircon cores from a sample of gil marquez granodiorite south portuguese zone spz and almonaster nebulite ossa morena zone omz in the aracena metamorphic belt have been obtained using laser ablation inductively coupled plasma mass spectrometry these data reveal differences in the age of deep continental crust in these two zones inherited zircon cores from the ossa morena zone range at 600 100 ma 1 7 2 ga and 2 65 2 95 ga while those from the south portuguese zone range at 400 500 and 700 800 ma these data support the exotic origin of the south portuguese zone basement relative to the rest of iberian massif the young ages of inherited zircon cores and nd model ages of magmatic rocks of the south portuguese zone are comparable to results from granulite facies xenoliths and granitic rocks from the meguma terrane and avalonia and support a correlation between the basement of the southernmost part of the iberian massif and the northern appalachians

regional significance of new u pb age data for neoproterozoic igneous units in avalonian rocks of northern mainland nova scotia canada gondwanan neoproterozoic tectonothermal events pan african and brasiliano are represented in northern mainland of nova scotia by volcanic and sedimentary rocks assigned to the jeffers and georgeville groups and by gabbroic to granitoid plutons these rocks comprise part of avalonia an exotic terrane in the appalachian orogen that was deposited in an arc related environment along the periphery of gondwana prior to accretion to laurentia lavas sampled in the basal units of the jeffers and georgeville groups yielded slightly discordant u pb zircon and monazite data that fall on chords with upper intercept ages of 628 ma and 617 7 1 6 ma respectively syntectonic to late syntectonic plutons intruded into these groups yielded u pb zircon ages of 606 6 1 6 ma and 603 9 5 ma the former intrusion also yielded a concordant titanite age of 607 3 ma when combined with previously published ages these data indicate that the back arc deposition recorded in these groups lasted 10 15 million years 628 613 ma and was closely followed by c 613 595 ma metamorphism intrusion and heterogeneous strike slip related deformation assuming no significant shuffling of fault blocks the relative locations of the cobequid antigonish back arc basin and the southern cape breton island volcanic arc are consistent with their genesis above a north west dipping subduction zone the age range of arc related magmatism in nova scotia is similar to that of avalonian rocks in southeastern newfoundland and britain lending support to hypotheses of neoproterozoic linkages

inherited arc signature in ediacaran and early cambrian basins of the ossa morena zone iberian massif portugal paleogeographic link with european and north african cadomian correlatives abstract geochemical data from clastic rocks of the ossa morena zone iberian massif show that the main source for the ediacaran and the early cambrian sediments was a recycled cadomian magmatic arc along the northern gondwana margin the geodynamic scenario for this segment of the avalonian cadomian active margin is considered in terms of three main stages 1 the 570 540 ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust a continental magmatic arc and the development of related marginal basins 2 the ediacaran early cambrian transition 540 520 ma coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc and 3 gondwana fragmentation with the formation of early cambrian 520 510 ma shallow water platforms in transtensional grabens accompanied by rift related magmatism these processes are comparable to similar cadomian successions in other regions of gondwanan europe and northwest africa ediacaran and early cambrian basins preserved in the ossa morena zone portugal and spain the north armorican cadomian belt france the saxo thuringian zone germany the western meseta and the western high atlas morocco share a similar geotectonic evolution probably situated in the same paleogeographic west african peri gondwanan region of the avalonian cadomian active margin

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

rift related volcanism predating the birth of the rheic ocean ossa morena zone sw iberia abstract two very different periods of magma emplacement in the crust of the ossa morena zone early and main events in sw iberia have been previously interpreted to record a cambrian early ordovician rifting event that is thought to have culminated in the opening of the rheic ocean during the early ordovician new stratigraphic petrographic geochemical and sm nd isotope data from cambrian volcanic rocks included in six key low grade sections in both portugal and spain considerably improve our understanding of these events these data 1 confirm the existence of two rift related magmatic events in the cambrian of the ossa morena zone 2 demonstrate that the early rift related event was associated with migmatite and core complex formation in the mid upper crust and is represented by felsic peraluminous rocks the parent magmas of which were derived mainly from crustal sources and 3 show the main rift related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks some of the mafic rocks show n morb affinity whereas others have oib or e morb affinities suggesting different heterogeneous mantle sources depleted and enriched asthenospheric and lithospheric plume like and non plume like the acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle derived magmas this new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge trench collision we interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower crust flow in a high heat flow setting to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates

rheic ocean ophiolitic remnants in southern iberia questioned by shrimp u pb zircon ages on the beja acebuches amphibolites 1 the rheic ocean was a major oceanic domain between avalonia and gondwana in ordovician silurian times most of the paleozoic plate reconstructions assume that the rheic ocean suture lies within southern iberia coinciding with the contact between the south portuguese zone and the ossa morena zone this paper reports four sensitive high resolution ion micro probe shrimp u pb zircon ages from mid ocean ridge basalt morb featured rocks of the beja acebuches amphibolite unit which crops out along the boundary between the ossa morena and the south portuguese zone and is considered its most conspicuous suture unit the obtained ages range from 332 3 to 340 4 ma corresponding to the crystallization of the mafic protoliths these early carboniferous ages for the beja acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the rheic ocean suture since this oceanic domain was presumably closed in devonian times tectonic reconstructions joining in a single suture line the beja acebuches amphibolite unit in southern iberia to either the devonian lizard ophiolite in southern england or the root zone of the devonian ordovician ophiolitic units in northwest iberia must be therefore reconsidered because of the age difference we interpret the beja acebuches amphibolite unit to represent a narrow and very ephemeral realm of oceanic like crust that opened in early carboniferous times after total consumption of the rheic ocean we suggest that a mantle plume underneath southern iberia in early carboniferous times is the most plausible large scale geodynamic scenario for the formation of these morb featured rocks

geochemistry of the ediacaran early cambrian transition in central iberia tectonic setting and isotopic sources abstract a complete ediacaran early cambrian stratigraphic transition can be observed in the southern part of the central iberian zone iberian massif two different stratigraphic units underlying ordovician series display geochemical and sm nd isotopic features in agreement with an evolving geodynamic setting pusa shales early cambrian rest unconformably on greywackes of the lower alcudian formation late ediacaran both sequences present minor compositional variations for major and trace element contents and similar ree patterns close to those of paas post archean australian shale trace element contents and element ratios suggest mixed sources with intermediate to felsic igneous contributions for both units tectonic setting discrimination diagrams for the ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin volcanic arc however the compositions of the cambrian shales fit better with a more stable context a back arc or retro arc setting end t and tdm ages are compatible with dominance of a similar cratonic source for both sequences probably the west africa craton end 565 values for the ediacaran greywackes 3 0 to 1 4 along with tdm ages 1256 1334 ma imply a significant contribution of juvenile material probably derived from the erosion of the volcanic arc however end 530 values in the cambrian shales 5 2 to 4 0 together with older tdm ages 1444 1657 ma suggest a higher contribution of cratonic isotopic sources probably derived from erosion of the adjacent mainland coeval with the progressive cessation of arc volcanism along the peri gondwanan realm during the cambrian there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas the geochemistry of the ediacaran cambrian transition in central iberia documents a tectonic switch in the periphery of gondwana from an active margin to a more stable context related to the onset of a passive margin

contiguous rather than discrete paleozoic histories for the avalon and meguma terranes based on detrital zircon data upper ordovician lower devonian strata of the meguma terrane in the canadian appalachians contain zircon populations including an important mesoproterozoic zircon population 1 0 1 4 ga similar to those in coeval strata of avalonia and strongly suggest contiguous rather than discrete histories for these terranes throughout the paleozoic that these terranes were juxtaposed throughout the early paleozoic is indicated by the absence of a cambrian ordovician accretionary event the lack of intervening suture zone ophio litic units and the similarity of avalonian and meguma basement nd isotope signatures in early paleozoic igneous suites as avalonia had accreted to laurentia baltica by the early silurian these data suggest that the meguma terrane like avalonia resided along the same northern margin of the rheic ocean at that time these conclusions have implications for reconstructions of the northern gondwanan margin in the early paleozoic and imply that the silurian devonian acadian orogeny in maritime canada occurred in an andean type setting and was not related to collision of the meguma terrane with the laurentian margin

tectonic influence on sedimentation along the southern flank of the late paleozoic magdalen basin in the canadian appalachians geochemical and isotopic constraints on the horton group in the st marys basin nova scotia the late devonian early carboniferous st marys basin in mainland nova scotia canada consists of horton group intracontinental clastic rocks that were deposited along the avalon meguma terrane boundary during the waning stages of the acadian orogeny and prior to terminal collision in the appalachian orogen these rocks represent the earliest stages of deposition along the southern flank of the composite magdalen basin late devonian to permian that extends over much of maritime canada and oversteps terrane boundaries in the canadian appalachians as such the horton group represents deposition in an episutural basin the horton group in the st marys basin predominantly consists of 3000 4000 m of clastic sedimentary rocks deposited in fluviatile and lacustrine nearshore environments geochemical analyses of clastic rocks reveal a range of sio2 content 60 94 wt high total rare earth elements and initial 87sr 86sr and strongly negative nd t t 360 ma the signature of the passive margin to active continental margin implied by standard discrimination diagrams probably is inherited largely from rocks in the source region and is not an indication of the setting in which the basin formed although the data indicate that some lithologies contain a minor avalonian basement component the geochemical and isotopic signatures of the sedimentary rocks reveal mixed sources that can be attributed largely to uplift and erosion of meguma terrane metasedimentary and granitoid rocks immediately to the south of the st marys basin regional syntheses indicate that this uplift occurred before and during horton group deposition and was coeval with dextral ramping of the meguma terrane over the avalon terrane along the southern flank of the magdalen basin

jens munk archipelago ordovician silurian islands in the churchill area of the hudson bay lowlands northern manitoba abstract the name jens munk archipelago is proposed for a cluster of small monadnocks that rise a maximum of 45 m above present sea level on the shores of hudson bay in the churchill area of northern manitoba resistant precambrian quartzite formed shoals and islands during the late ordovician and early silurian periods outcrop control in the present tundra environment of the hudson bay lowlands is spotty at best but the quartzitic monadnocks are fringed by upper ordovician and lower silurian ramp deposits mainly carbonate in composition five isolated outliers of ordovician silurian strata are described and the location of four other potential outliers is indicated ramp dip at any one site generally ranges from 10 to 15 and the surviving ramps on opposite flanks of paleoislands suggest preservation of near synsedimentary conditions extensive conglomerates of reworked quartzite are most abundant on a northern exposure within the islands ripple marks and oriented cephalopod debris are related t

outer rocky shores of the mowanbini archipelago devonian reef complex canning basin western australia abstract the oscar range in western australia s canning basin features inliers of folded paleoproterozoic quartzite quartzitic conglomerate and phyllite that formed islands during the late devonian undisturbed strata of the pillara limestone upper devonian frasnian stage surround individual paleoislands that rise above the former seabed with a maximum topographic relief of 90 m on average the mowanbini archipelago aboriginal name for the oscar range lies 15 km off the granitic and metamorphic mainland represented by the kimberley block to the north devonian facies on outer rocky shores were studied at three localities near the southeast end of the oscar range at the west end of the study area a devonian reef margin sits unconformably on paleoproterozoic phyllite but to the east the reef is separated from rocky shores by a wide lagoon massive clast supported conglomerate and sandstone beds interfinger with backreef carbonates where the reef is closest to land across the lagoon 2 5 km eastw

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

tectonic significance of a llanvirn age for the dunn point volcanic rocks avalon terrane nova scotia canada implications for the evolution of the iapetus and rheic oceans abstract the paleozoic evolution of avalonia is crucial to the understanding of the development of the appalachian orogen and the iapetus and rheic oceans paleomagnetic faunal structural and isotopic data indicate that avalonia had accreted to laurentia by the early silurian in the avalon terrane of nova scotia ordovician early devonian rocks consist of bimodal volcanic rocks at the base dunn point formation disconformably overlain by a thick sequence of fossiliferous siliciclastics arisaig group which contain llandoverian to lochkovian fossils until now the only published age data for the volcanic rocks yielded an imprecise rb sr whole rock isochron age of 421 15 ma and the volcanism has been interpreted to reflect local extension and basin development related to the oblique collision between avalonia and laurentia after closure of the iapetus ocean we present u pb zircon data from a rhyolite which indicate an age of 460 0 3 4 ma for the dunn point formation an age that requires a re interpretation of its tectonic setting the dunn point volcanism probably developed on the avalonian microcontinent outboard from both laurentia and gondwana possibly in a rifted arc setting analogous to the modern taupo zone in northern new zealand a llanvirn age for these felsic rocks also reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for avalonia in the ordovician silurian and implies 10 northward movement of avalonia between 460 and 440 ma and about 5 5 cm year for the latitudinal component of the convergence between avalonia and laurentia i e the destruction of the iapetus ocean and divergence between gondwana and avalonia development of the rheic ocean during this interval

apparent enhancement of 234th based particle export associated with anticyclonic eddies national basic research program of china 973 program 2009cb421200 national natural science foundation of china nsfc 41121091 41130857

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source