High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps)

Coarse-grained gabbros from two different localities in the Gets nappe (Upper Prealps) have been dated by U-Pb and Ar-40/Ar-39 isotopic analyses. Zircons from both gabbros gave identical concordant U-Pb ages of 166 +/- 1 Ma (Fig. 4). Amphibole from one of them gave an Ar-40/Ar-39 plateau age of 165.9 +/- 2.2 Ma (Fig. 5). This concordance implies that 166 +/- 1 Ma is the age of magmatic crystallization of these gabbros. The Gets wildflysch with its mafic and ultramafic lenses is an ophiolitic melange, that we infer to come from a proximal part of the accretionary prism at the foot of the active SE margin of the Piemont ocean. In this position we can expect to find remnants of the oldest parts of the Piemont oceanic crust. These are the first high-precision dates using modern techniques from an Alpine ophiolite and are in excellent agreement with the following: 1) The few, somewhat younger, reliable ages on ophiolites from the probable continuation of the Piemont basin into the Apennines and Corsica; 2) Recent data on the age of the first supra-ophiolitic sediments (Late Bathonian to Early Callovian radiolarites); 3) The structural and stratigraphic evolution of the Brianconnais (s.s.) domain, the future NW margin of the Piemont ocean. We note a remarkable coincidence, in Late Bajocian time, between: (A) the end of tensile fracturing in the Brianconnais continental crust; (B) the beginning of its subsidence; (C) the age of the Gets ophiolites. This coincidence is consistent with an ocean opening mechanism based on a combination of subhorizontal extension and thermally driven vertical movements of the lithosphere.

Geochemistry and 40Ar/39Ar geochronology of pseudotachylyte associated with UHP whiteschists from the Dora Maira massif, Italy

In situ UV-Iaser ablation Ar-40/(39) Ar geochronological and geochemical data, together with rock and mineral compositional data, have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the UHP whiteschists of the Dora Maira Massif, Italy. Several generations of fresh pseudotachylyte occur as irregular veins up to a few cur thick both parallel and at high angles to the foliation. Whole rock XRF data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith. Minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss. The pseudotachylyte contains newly crystallized grains of biotite and K-feldspar in a matrix of glass with partially fused grains of quartz, zircon, apatite, and titanite. Electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer-scale changes in mineralogy. UV-Iaser ablation ICP-MS traverses across the mylonitic gneiss-pseudotachylyte contact are consistent with cataclastic communition of REE carriers such as epidote, monazite, allanite, zircon, and apatite before melting as an efficient mechanism of REE homogenization in the pseudotachylyte. The 40Ar/39Ar data from one band of pseudotachylyte indicate formation at 20.1 +/- 0.5 Ma, when the mylonitic gneisses were already in a near surface position. The variable effects of top-to-the-west shear deformation within outcrops of the coesite-bearing unit are reflected in localized zones of protomylonite, cataclasite, ultracataclasite, and pseudotachylyte. Preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the UHP rocks. It is speculated that deeper crustal seismic events potentially played a role in the unroofing of the UHP rocks at earlier stages in their exhumation history. (c) 2005 Elsevier B.V. All rights reserved.

Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez-Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez-Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents Ar-40/Ar-39 ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported Ar-40/Ar-39, white mica ages from cover units of the central and southern Siviez-Mischabel Nappe. In cover units from the central and southern Siviez-Mischabel regions of the study area, Ar-40/Ar-39 ages appear to date synkinematic white mica growth. Results suggest that the Siviez-Mischabel :Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez-Mischabel, however, Ar-40/Ar-39 white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez-Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high-pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years. Copyright 1998 by the American Geophysical Union.

U-Pb and Rb-Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland

New geochronological data which clarify the timing of syn-orogenic magmatism and regional metamorphism in the Connemara Dalradian are presented. U-Pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 Ma whereas the earliest, basic magmatism has been dated previously at 490 Ma; a fine-grained, fabric-cutting granite contains discordant zircons which also appear to be 465 Ma old. Are magmatism in Connemara therefore spanned a period of at least 25 Ma. Recent U-Pb data on titanite from central Connemara which gave a peak metamorphic age of 478 Ma are supplemented by U-Pb data on titanite and monazite from metamorphic veins in the east of Connemara which indicate that low-P, high-T regional metamorphism ism continued there to 465 Ma, i.e. at least 10 Ma later than in the central region dated previously. New Rb-Sr data on muscovites from coarse-grained segregations in different structural settings range from 475 to 435 Ma; in part this range probably also reflects differences in age from west to east, with three ages close to 455 Ma from the eastern area, which is also the site of the lowest pressure metamorphism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1-2 Ma. The new dates emphasize the complexity in the spatial and temporal distribution of high-level regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the southern part of Connemara. The later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism, and were localized in the eastern part of the region.

Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

We have selected and dated three contrasting rock-types representative of the magmatic activity within the Permian layered mafic complex of Mont Collon, Austroalpine Dent Blanche nappe, Western Alps. A pegmatitic gabbro associated to the main cumulus sequence yields a concordant U/Pb zircon age of 284.2 +/- 0.6 Ma, whereas a pegmatitic granite dike crosscutting the latter yields a concordant age of 282.9 +/- 0.6 Ma. A Fe-Ti-rich ultrabasic lamprophyre, crosscutting all other lithologies of the complex, yields an 40Ar/39Ar plateau age of 260.2 +/- 0.7 Ma on a kaersutite concentrate. All ages are interpreted as magmatic. Sub-contemporaneous felsic dikes within the Mont Collon complex are ascribed to anatectic back-veining from the country-rock, related to the emplacement of the main gabbroic body in the continental crust, which is in accordance with new isotopic data. The lamprophyres have isotopic compositions typical of a depleted mantle, in contrast to those of the cumulate gabbros, close to values of the Bulk Silicate Earth. This indicates either contrasting sources for the two magma pulses - the subcontinental lithospheric mantle for the gabbros and the underlying asthenosphere for the lamprophyres - or a single depleted lithospheric source with variable degrees of crustal contamination of the gabbroic melts during their emplacement in the continental crust. The Mont Collon complex belongs to a series of Early Permian mafic massifs, which emplaced in a short time span about 285-280 Ma ago, in a limited sector of the post-Variscan continental crust now corresponding to the Austroalpine/ Southern Alpine domains and Corsica. This magmatic activity was controlled in space and time by crustal-scale transtensional shear zones.

Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary Islands

High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 +/- 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. Ar-40/Ar-39 amphibole dating yielded ages from 21.9 +/- 0.6 to 21.8 +/- 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between Ar-40/Ar-39 and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i. e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (T-c) of zircon (699-988 degrees C) to amphibole (500-600 degrees C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the Ar-40/Ar-39 and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole T-c is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 +/- 0.07/0.08/0.15 Ma and 21.58 +/- 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope delta O-18 values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Termination of major ductile strike-slip shear and differential cooling along the Insubric line (Central Alps): U-Pb, Rb-Sr and 40Ar/39Ar ages of cross-cutting pegmatites

To constrain the age of strike-slip shear, related granitic magmatism, and cooling along the Insubric line, 29 size fractions of monazite and xenotime were dated by the U-Pb method, and a series of 25 Rb-Sr and Ar-40/Ar-39 ages were measured on different size fractions of muscovite and biotite. The three pegmatitic intrusions analyzed truncate high-grade metamorphic mylonite gneisses of the Simplon shear zone, a major Alpine structure produced in association with dextral strike-slip movements along the southern edge of the European plate, after collision with its Adriatic indenter. Pegmatites and aplites were produced between 29 and 25 Ma in direct relation to right-lateral shear along the Insubric line, by melting of continental crust having Sr-87/Sr-86 between 0.7199 and 0.7244 at the time of melting. High-temperature dextral strike-slip shear was active at 29.2 +/- 0.2 (2 sigma) Ma, and it terminated before 26.4 +/- 0.1 Ma. During dike injection, temperatures in the country rocks of the Isorno-Orselina and Monte Rosa structural units did not exceed approximate to 500 degrees C, leading to fast initial cooling, followed by slower cooling to approximate to 350 degrees C within several million years. In one case, initial cooling to approximate to 500 degrees C was significantly delayed by about 4 m.y., with final cooling to approximate to 300 degrees C at 20-19 Ma in all units. For the period between 29 and 19 Ma, cooling of the three sample localities was non-uniform in space and time, with significant variations on the kilometre scale. These differences are most likely due to strongly varying heat flow, and/or heterogeneous distribution of unroofing rates within the continuously deforming Insubric line. If entirely ascribed to differences in unroofing, corresponding rates would vary between 0.5 and 2.5 mm/y, for a thermal gradient of 30 degrees/km.

A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps) - Evidence for fluid-flow and constraints on nappe kinematics

Stable isotope and Ar-40/Ar-39 measurements,were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the delta(18)O values (up to 6 parts per thousand) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the delta(13)C values of calcite is observed only in the deepest sections (up to 3 parts per thousand). The delta D-SMOW (SMOW = standard mean ocean water) values of white mica are -54 parts per thousand +/- 8 parts per thousand (n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated delta(18)O and delta D values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12 parts per thousand to 16 parts per thousand and -30 parts per thousand to +5 parts per thousand, respectively, for assumed temperatures of 250 to 450 degrees C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies, It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface, This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica Ar-49/Ar-39 ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 degrees C. The;plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma Ar-40/Ar-39 age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.

U-Pb and Ar-Ar geochronological data from the Pelagonian basement in Evia (Greece): geodynamic implications for the evolution of Paleotethys

High precision U-Pb zircon and Ar-40/Ar-39 mica geochronological data on metagranodiorites, metagranites and mica schists from north and central Evia island (Greece) are presented in this study. U-Pb zircon ages range from 308 to 1912 Ma, and indicate a prolonged magmatic activity in Late Carboniferous. Proterozoic ages represent inherited cores within younger crystals. Muscovite Ar-40/Ar-39 plateau ages of 288 to 297 Ma are interpreted as cooling ages of the magmatic bodies and metamorphic host rocks in upper greenschist to epidote-amphibolite metamorphic conditions. The multistage magmatism had a duration between 308 and 319 hla but some older intrusions, as well as metamorphic events, cannot be excluded. Geochemical analyses and zircon typology indicate calc-alkaline affinities for the granites of central Evia and alkaline to calc-alkaline characteristics for the metagranodiorites from the northern part of the island. The new data point towards the SE continuation, in Evia and the Cyclades, of a Variscan continental crust already recognised in northern Greece (Pelagonian basement). The Late Carboniferous magmatism is viewed as a result of northward subduction of the Paleotethys under the Eurasian margin.

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) - Tso Monad transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55 Ma and metamorphosed by ca. 48-40 Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10 km) by 40-30 Ma in the Tso Monad dome (northern section of the transect) and by 30-20 Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.

Alpine tectonic evolution of a Jurassic subduction-accretionary complex: Deformation, kinematics and 40Ar/39Ar age constraints on the Mesozoic low-grade schists of the Circum-Rhodope Belt in the eastern Rhodope-Thrace region, Bulgaria-Greece

Deformation of the Circum-Rhodope Belt Mesozoic (Middle Triassic to earliest Lower Cretaceous) low-grade schists underneath an arc-related ophiolitic magmatic suite and associated sedimentary successions in the eastern Rhodope-Thrace region occurred as a two-episode tectonic process: (i) Late Jurassic deformation of arc to margin units resulting from the eastern Rhodope-Evros arc-Rhodope terrane continental margin collision and accretion to that margin, and (ii) Middle Eocene deformation related to the Tertiary crustal extension and final collision resulting in the closure of the Vardar ocean south of the Rhodope terrane. The first deformational event D-1 is expressed by Late Jurassic NW-N vergent fold generations and the main and subsidiary planar-linear structures. Although overprinting, these structural elements depict uniform bulk north-directed thrust kinematics and are geometrically compatible with the increments of progressive deformation that develops in same greenschist-facies metamorphic grade. It followed the Early-Middle Jurassic magmatic evolution of the eastern Rhodope-Evros arc established on the upper plate of the southward subducting Maliac-Meliata oceanic lithosphere that established the Vardar Ocean in a supra-subduction back-arc setting. This first event resulted in the thrust-related tectonic emplacement of the Mesozoic schists in a supra-crustal level onto the Rhodope continental margin. This Late Jurassic-Early Cretaceous tectonic event related to N-vergent Balkan orogeny is well-constrained by geochronological data and traced at a regional-scale within distinct units of the Carpatho-Balkan Belt. Following subduction reversal towards the north whereby the Vardar Ocean was subducted beneath the Rhodope margin by latest Cretaceous times, the low-grade schists aquired a new position in the upper plate, and hence, the Mesozoic schists are lacking the Cretaceous S-directed tectono-metamorphic episode whose effects are widespread in the underlying high-grade basement. The subduction of the remnant Vardar Ocean located behind the colliding arc since the middle Cretaceous was responsible for its ultimate closure, Early Tertiary collision with the Pelagonian block and extension in the region caused the extensional collapse related to the second deformational event D-2. This extensional episode was experienced passively by the Mesozoic schists located in the hanging wall of the extensional detachments in Eocene times. It resulted in NE-SW oriented open folds representing corrugation antiforms of the extensional detachment surfaces, brittle faulting and burial history beneath thick Eocene sediments as indicated by 42.1-39.7 Ma Ar-40/Ar-39 mica plateau ages obtained in the study. The results provide structural constraints for the involvement components of Jurassic paleo-subduction zone in a Late Jurassic arc-continental margin collisional history that contributed to accretion-related crustal growth of the Rhodope terrane. (C) 2011 Elsevier Ltd. All rights reserved.

Calcareous plankton biostratigraphy and 40Ar /39Ar dating of miocene volcaniclastic layers from Monferrato (NW Italy)

The results of Ar-40/Ar-39 dating integrated with calcareous plankton biostratigraphical data performed on two volcaniclastic layers (VLs) interbedded in Burdigalian to Lower Langhian outer shelf carbonate sediments cropping out in Monferrato (NW Italy) are presented. The investigated VLs, named Villadeati and Varengo, are thick sedimentary bodies with scarce lateral continuity. They are composed of prevalent volcanogenic material (about 87 up to 90% by volume) consisting of glass shards and volcanic phenocrysts (plagioclase, biotite, quartz, amphibole, sanidine and magnetite) and minor extrabasinal and intrabasinal components. On the basis of their composition and sedimentological features, the VLs have been interpreted as distal shelf turbidites deposited below storm wave base. However, compositional characteristics evidence the rapid resedimentation of the volcanic detritus after its primary deposition and hence the VL sediments can be considered penecontemporaneous to the encasing deposits. Biostratigraphical analyses were carried out on the basis of a quantitative study of calcareous nannofossil and planktonic foraminifer associations, whilst Ar-40/Ar-39 dating were performed on biotite at Villadeati and on homeblende at Varengo. The data resulting from the Villadeati section have permitted to estimate an age of 18.7 +/- 0.1 Ma for the last common occurrence (LCO) of Sphenolithus belemnos whereas those from Varengo allowed to extrapolate an age of 16.4 Ma +/-0.1 Ma for the first occurrence (FO) of Praeorbulina sicana. This latter biovent is commonly used to approximate the base of the Langhian stage, that corresponds to the Early-Middle Miocene boundary.

Prograde mica 40Ar/39Ar growth ages recorded in high pressure rocks (Syros, Cyclades, Greece)

Phengites from the eclogite and blueschist-facies sequences of the Cycladic island of Syros (Greece) have been dated by the in situ UV-laser ablation Ar-40/Ar-39 method. A massive, phengite-rich eclogite and an omphacite-rich metagabbro were investigated. The phengites are eubedral and coarse-grained (several 100 mum), strain-free and exhibit no evidence for late brittle deformation or recrystallization. Apparent ages in these samples range from 43 to 50 Ma for the phengite-rich eclogite and 42 to 52 Ma for the ompbacitic metagabbro. This large spread of ages is visible at all scales-within individual grains as well as in domains of several 100 mum and across the entire sample (ca. 2 cm). Such variations have been traditionally attributed to metamorphic cooling or the incorporation of excess argon. However, the textural equilibrium between the phengites and other high pressure phases and the subtle compositional variations within the phengites, especially the preservation of growth textures, alternatively suggest that the observed range in ages may reflect variations of radiogenic argon acquired during phengite formation and subsequent growth, thus dating a discrete event on the prograde path. This implies that the oldest phengite 40Ar/39Ar ages provide the best estimate of a minimum crystallization age, which is in agreement with recently reported U-Pb and Lu-Hf geochronological data. Our results are consistent with available stable isotope data and further suggest that, under fluid-restricted conditions, both stable and radiogenic isotopic systems can survive without significant isotopic exchange during subduction and exhumation from eclogite-facies P-T conditions. (C) 2004 Elsevier B.V. All rights reserved.

Late Labradorian metamorphism and anorthosite-granitoid intrusion, Cape-Caribou River allochthon, Grenville Province, Labrador - Evidence from U-Pb geochronology

In the Cape Caribou River allochthon (CCRA), metaigneous and gneissic units occur as a shallowly plunging synform in the hanging wall of the Grand Lake thrust system (GLTS), a Grenvillian structure that forms the boundary between the Mealy Mountains and Groswater Bay terranes. The layered rocks of the CCRA are cut by a stockwork of monzonite dykes related to the Dome Mountain suite and by metadiabase-amphibolite dykes that probably form part of the ca. 1380 Ma Mealy swarm. The mafic dykes appear to postdate much of the development of subhorizontal metamorphic layering within the lower parts of the CCRA. The uppermost (least metamorphosed) units of the CCRA, the North West River anorthosite-metagabbro and the Dome Mountain monzonite suite, have been dated at 1625 +/- 6 and 1626 +/- 2 Ma, respectively. An amphibolite unit that concordantly underlies the anorthosite-metagabbro and is intruded discordantly by monzonite dykes has given metamorphic ages of 1660 +/- 3 and 1631 +/- 2 Ma. Granitoid gneisses that form the lowest level of the CCRA have given a migmatization age of 1622 +/- 6 Ma. The effects of Grenvillian metamorphism become apparent in the lower levels of the allochthon where gneisses, amphibolite, and mafic dykes have given new generation zircon ages of 1008 +/- 2, 1012 +/- 3, and 1011 +/- 3 Ma, respectively. A posttectonic pegmatite has also given zircon and monazite ages of 1016(-3)(+7) and 1013 +/- 3 Ma, respectively. Although these results indicate new growth of Grenvillian zircon, this process was generally not accompanied by penetrative deformation or melting. Thus, the formation of gneissic fabrics and the overall layered nature of the lower CCRA are a result primarily of Labradorian (1660-1620 Ma) tectonism and intrusion, and probably reflect early movement on an ancestral GLTS. Grenvillian heating and metamorphism (up to granulite facies) was strongly concentrated towards the base of the CCRA and probably occurred during northwestward thrusting of the allochthon over the Groswater Bay terrane.

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): Geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 +/- A 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 +/- A 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.