Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages

Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. ⁴⁰Ar/³⁹Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record ⁴⁰Ar/³⁹Ar plateau ages of c. 527 and 521 Ma. An internally discordant ⁴⁰Ar/³⁹Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant ⁴⁰Ar/³⁹Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a ⁴⁰Ar/³⁹Ar plateau age of c. 307 Ma. The ⁴⁰Ar/³⁹Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.     

Fault rocks as indicators of progressive shear deformation in the Guingamp region, Brittany

The North Armorican Shear Zone is a major structural feature running from the island of Moiene in the west to Moncontour in the east of the Armorican Massif. In the region of Guingamp it cuts through a Precambrian migmatite complex and granitoid rocks of both Precambrian and Hercynian age. A variety of fault rocks are present in this part of the shear zone, and are thought to represent a time sequence in which deep level, ductile deformation gave way to higher level brittle displacements. Mylonite and cataclasite series rocks, and pseudotachylites are described and their conditions of formation considered. The Hercynian Quintin granite post-dates the main movement of the shear zone but is itself dextrally displaced during the late stages of shear movement.     

Gondwana break-up and the northern margin of the Saxothuringian belt (Variscides of Central Europe)

At the northwestern edge of the Hercynian Bohemian Massif (Saxothuringian belt) new U-Pb zircon age data from rift-related magmatic rocks indicate that the initiation of Gondwana break-up in this area started during the Middle to Upper Cambrian. Magmatic rocks from a bimodal, MORB- to within-plate volcanic sequence in the Vesser area are dated between ca. 517 and 501 Ma. The volcaniclastic sequences analysed exhibit basal layers of conglomerates and mature sandstones, which can be correlated with a widespread Gondwana-derived onlap horizon of an uppermost Cambrian/Tremadocian age that links the Vesser area with the Saxothuringian continental basin. The association of the Vesser rocks with the Saxothuringian terrane as part of the Armorican terrane assemblage is further demonstrated by a coeval magmatic development and by identical detrital components which are derived from a common Cadomian basement (white mica with a ca. 539 Ma K-Ar minimum age and inherited zircon signatures). The Vesser unit, situated between the NW margin of the Saxothuringian zone and the Mid-German Crystalline Zone, probably represents a N-facing remnant of an ocean-continent transition of the, or within the, Armorican terrane assemblage and involves sections of the early break-up process at the peri-Gondwanan shelf south of the Rheic ocean.     

Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)

The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.     

The development of Brioverian structures and Brioverian/Palaeozoic relationships in west Finistère (France)

Following an outline of Brioverian stratigraphy (see part 1 for explanation of the term) in western Finistère (France), a detailed account is given of a polyphase Cadomian orogeny involving two periods of major folding and a regional metamorphism. A brief comment on Palaeozoic sedimentation and the influence of the Brioverian basement at this time leads on to an analysis of the different responses of Palaeozoic and Brioverian rocks to the Variscan deformation.     

U/Pb and Pb/Pb zircon ages for arc-related intrusions of the Bolu Massif (W Pontides, NW Turkey): evidence for Late Precambrian (Cadomian) age

We present new U/Pb and Pb/Pb radiometric age data from two tectono-stratigraphic units of the regionally extensive Bolu Massif, in the W Pontides (İstanbul Fragment), N Turkey. A structurally lower unit (Sünnice Group) is cut by small meta-granitic intrusions, whereas the structurally higher unit comprises meta-volcanic rocks (Çaşurtepe Fm) cut by meta-granitic plutons (Tüllükiriş and Kapıkaya plutons). U/Pb single-crystal dating of zircons from the Kapıkaya Pluton yielded a concordant cluster, with a mean ²³⁸U/²⁰⁶Pb age of 565.3 ± 1.9 Ma. Zircons from the Tüllükiriş Pluton (affected by Pb loss) gave a ²⁰⁷Pb/²⁰⁶Pb age of 576 ± 6 Ma age (Late Precambrian). Small meta-granitic intrusions cutting the Sünnice Group yielded a less precise ²⁰⁷Pb/²⁰⁶Pb age of 262 ± 19 Ma (Early Permian). The older ages from the Bolu Massif confirm the existence of latest Precambrian arc magmatism related to subduction of a Cadomian ocean. We infer that the Bolu Massif represents a fragment of a Cadomian active margin. Cadomian orogenic units were dispersed as exotic terranes throughout the Variscan and Tethyan orogens, and the Bolu Massif probably reached its present position prior to latest Palaeozoic time. Our dating results also confirm that NW Turkey was affected by Hercynian magmatism related to subduction of Palaeotethys, as inferred for other areas of the Pontides.     

Geodynamic evolution of a Cadomian arc region: the northern Ossa-Morena zone, Iberian massif

Ductile deformation and polyphase metamorphism in the Ossa-Morena zone of the Iberian massif are related to two major tectonothermal episodes of Cadomian (late Neoproterozoic to early Cambrian) and Variscan age (middle to late Paleozoic). The available petrological, structural and geochronological data suggest that a number of tectono-metamorphic and magmatic episodes occurred during the 620–480 Ma interval that would comprise a complete Cadomian Wilson cycle. The geodynamic scenario was that of an Andean-type continental margin. An evolutionary model is presented for this orogeny comprising stages of volcanic arc generation, crustal thickening, back-arc extension, tectonic inversion and cratonization. A correlation with comparable areas from pre-Mesozoic massifs elsewhere in Europe is proposed, in particular with the Armorican massif of northern France.     

Hydrothermalisme anté-Hercynien en Sud-Ibérie : apport de la géochimie isotopique du plombPre-Hercynian hydrothermalism in South Iberia: lead isotope geochemistry constraints

Lead isotope study has been performed on massive sulphide deposits of Ossa-Morena and Aracena Belt (South Iberia). Results suggest the existence of at least two ancient hydrothermal events. The first one, Upper Brioverian in age (～600–570 Ma), gave birth to Maria-Luisa and Puebla de la Reina massive sulphide deposits; it thus confirms the existence of a Cadomian orogen in South Iberia. Isotopic compositions indicate a local contribution of mantle-derived material (Maria Luisa mine), confirming the presence of ancient oceanic crust in Aracena Belt. This mineralising event seems to extend till the Armorican Massif. The second episode, Eo-Hercynian in age (～400–350 Ma) has allowed genesis of massive sulphide deposits of la Nava Paredon and Aguas Blancas, and could be coeval with the emplacement of South-Iberian massive sulphide ore deposits in the neighbouring South-Portuguese Zone. A more continental crustal source for later ore deposits could explain the much more important metal accumulation in this zone. To cite this article: É. Marcoux et al., C. R. Geoscience 334 (2002) 259–265.     

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

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.     

中国东北麻山杂岩晚泛非期变质的锆石SHRIMP年龄证据及全球大陆再造意义

中国东北地区佳木斯地块南部麻山杂岩正、副片麻岩 7个样品的锆石 SHRIMP年龄数据首次明确地表明 ,东北地区存在 500 Ma的晚泛非期高级变质作用事件。峰期麻粒岩相变质导致柳毛地区 (502± 10)Ma (2σ )深熔花岗岩的形成。正、副片麻岩变质年龄的一致性表明它们已在变质前发生了构造叠置。西麻山副片麻岩中含有在后期麻粒岩相变质过程中未重结晶的碎屑锆石，由此形成从协和一致的 550 Ma到弱不一致 1 900 Ma的较大 207Pb/206Pb年龄变化范围，表明其原岩具有从新元古代到中元古代-古元古代的年龄。柳毛地区变质的片麻状闪长岩中所含的古老锆石的 207Pb/206Pb年龄为 546～ 1 460 Ma表明，该闪长岩大约在 1 400 Ma就位，并受到 500 Ma变质事件的影响，从而说明柳毛地区存在中元古代基底。然而，与以前的认识相反，麻山杂岩不存在具有太古宙基底的同位素证据。晚泛非期变质事件年龄的确定对重塑晚前寒武纪-显生宙早期麻山杂岩和佳木斯地块的古地理位置具有重要意义。根据目前获得的有关证据，认为佳木斯地块可能曾经位于冈瓦纳大陆北缘的华北克拉通附近。     

Precadomian relicts in the Armorican Massif: Their age and role in the evolution of the western and central European Cadomian-Hercynian belt

Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses).There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial ⁸⁷Sr/⁸⁶Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial ⁸⁷Sr/⁸⁶Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.     

Peri-Gondwanan Ordovician crustal fragments in the high-grade basement of the Eastern Rhodope Massif,Bulgaria: evidence from U-Pb LA-ICP-MS zircon geochronology and geochemistry

Field, geochemical, and geochronologic data of high-grade basement metamafic and evolved rocks are used to identify the nature and timing of pre-Alpine crustal growth of the Rhodope Massif. These rocks occur intrusive into clastic-carbonate metasedimentary succession. Petrography and mineral chemistry show compositions consistent with Alpine amphibolite-facies metamorphism that obliterated the original igneous textures of the protoliths. Bulk-rock geochemistry identifies low-Ti tholeiitic to calc-alkaline gabbroic-basaltic and plagiogranite precursors, with MORB-IAT supra-subduction zone signature and trace elements comparable to modern back-arc basalts. The U-Pb zircon dating revealed a mean age of 455 Ma for the magmatic crystallization of the protoliths that contain inherited Cambrian (528–534 Ma) zircons. Carboniferous, Jurassic, and Eocene metamorphic events overprinted the Ordovician protoliths. The radiometric results of the metamorphic rocks demonstrate that Ordovician oceanic crust was involved in the build-up of the Rhodope high-grade basement. Dating of Eocene-Oligocene volcanic rocks overlying or cross-cutting the metamorphic rocks supplied Neoproterozoic, Ordovician and Permo-Carboniferous xenocrystic zircons that were sampled en route to the surface from the basement. The volcanic rocks thus confirm sub-regionally present Neoproterozoic and Paleozoic igneous and metamorphic basement. We interpret the origin of the Middle-Late Ordovician oceanic magmatism in a back-arc rift-spreading center propagating along peri-Gondwanan Cadomian basement terrane related to the Rheic Ocean widening. The results highlight the presence of elements of Cadomian northern Gondwana margin in the high-grade basement and record of Rheic Ocean evolution. The eastern Rhodope Massif high-grade basement compared to adjacent terranes with Neoproterozoic and Cambro-Ordovician evolution shares analogous tectono-magmatic record providing a linkage among basement terranes incorporated in the Alpine belt of the north Aegean region.     

Petrology and Ar-Ar dating of granulites from the Galicia Bank (Spain): African craton relics in Western Europe

Intermediate orthogranulites were collected on the western flank of the Galicia bank during the Galinaute II cruise in 1995. The petrography of these rocks reveals two types of granulites. The first type is hydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + hornblende + garnet + biotite + opaque + zircon + apatite assemblage. Both hornblende and orthopyroxene define a weak foliation plane. A late deformation event is expressed by some fractures cross-cutting the foliation. The second is anhydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + clinopyroxene + opaque + zircon + apatite assemblage. The rocks display a granoblastic texture and are affected by brittle deformation as testified by the development of numerous microfractures. The P–T conditions (7 ± 1 Kbar, 750 ± 50 °C) calculated from two representative samples demonstrate that the rocks equilibrated under granulite facies conditions. Ar-Ar dating gives Precambrian ages ranging between ca. 2500–2000 Ma for the amphibole from the hydrous granulite and 1600–1500 Ma for the core of the K-feldspar from the anhydrous and hydrous granulites. A younger age of 900 Ma is obtained from the recrystallized rims of the K-feldspar from the two samples. These data indicate that the granulitic rocks in the Galicia Bank had already been exhumed and cooled below ca. 140–400 °C (blocking T° for K-feldspar) in Precambrian times (900 Ma). Given the very well preserved granulitic minerals assemblage of the rocks, the granulites behaved as competent and metastable boudins during their exhumation. The granulitic samples were previously interpreted as fragments of the lower continental crust sampled by the main detachment fault during Cretaceous rifting, but they were part of an upper continental crust from the Precambrian. Geochronological data and petrological assemblages suggest that the granulite blocks in the Galicia Bank probably were derived from the North Armorican Domain (northern part of France) where a Precambrian terrain outcrops. The opening of the Bay of Biscay could be responsible for the scattering of the Precambrian terrain and may explain the presence of the granulitic blocks on both sides of the Bay of Biscay. During the subduction of Europe below the Iberian peninsula the granulite blocks were transported southward and incorporated into a Cretaceous conglomerate forming the accrecionary prism on the Northern Iberia Margin. The granulite facies blocks found on the Galicia Bank represent another example of Gondwanian relics supporting the idea that the West European plate belonged to the West African craton during the Proterozoic.     

Petrology and Ar-Ar dating of granulites from the Galicia Bank (Spain): African craton relics in Western Europe

Abstract

Intermediate orthogranulites were collected on the western flank of the Galicia bank during the Galinaute II cruise in 1995. The petrography of these rocks reveals two types of granulites. The first type is hydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + hornblende + garnet + biotite + opaque + zircon + apatite assemblage. Both hornblende and orthopyroxene define a weak foliation plane. A late deformation event is expressed by some fractures cross-cutting the foliation. The second is anhydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + clinopyroxene + opaque + zircon + apatite assemblage. The rocks display a granoblastic texture and are affected by brittle deformation as testified by the development of numerous microfractures. The P-T conditions (7 ± 1 Kbar, 750 ± 50 °C) calculated from two representative samples demonstrate that the rocks equilibrated under granulite facies conditions. Ar-Ar dating gives Precambrian ages ranging between ca. 2500–2000 Ma for the amphibole from the hydrous granulite and 1600–1500 Ma for the core of the K-feldspar from the anhydrous and hydrous granulites. A younger age of 900 Ma is obtained from the recrystallized rims of the K-feldspar from the two samples. These data indicate that the granulitic rocks in the Galicia Bank had already been exhumed and cooled below ca. 140–400 °C (blocking T° for K-feldspar) in Precambrian times (900 Ma). Given the very well preserved granulitic minerals assemblage of the rocks, the granulites behaved as competent and metastable boudins during their exhumation. The granulitic samples were previously interpreted as fragments of the lower continental crust sampled by the main detachment fault during Cretaceous rifting, but they were part of an upper continental crust from the Precambrian. Geochronological data and petrological assemblages suggest that the granulite blocks in the Galicia Bank probably were derived from the North Armorican Domain (northern part of France) where a Precambrian terrain outcrops. The opening of the Bay of Biscay could be responsible for the scattering of the Precambrian terrain and may explain the presence of the granulitic blocks on both sides of the Bay of Biscay. During the subduction of Europe below the Iberian peninsula the granulite blocks were transported southward and incorporated into a Cretaceous conglomerate forming the accrecionary prism on the Northern Iberia Margin. The granulite facies blocks found on the Galicia Bank represent another example of Gondwanian relics supporting the idea that the West European plate belonged to the West African craton during the Proterozoic. © 2000 Éditions scientifiques et médicales Elsevier SAS     

Armorican provenance for the mélange deposits below the Lizard ophiolite (Cornwall,UK): evidence for Devonian obduction of Cadomian and Lower Palaeozoic crust onto the southern margin of Avalonia

Devonian sedimentary rocks of the Meneage Formation within the footwall of the Lizard ophiolite complex in SW England are thought to have been derived from erosion of the over-riding Armorican microplate during collision with Avalonia and the closure of the Rheic Ocean. We further test this hypothesis by comparison of their detrital zircon suites with those of autochthonous Armorican strata. Five samples analysed from SW England (Avalonia) and NW France (Armorica) have a bimodal U–Pb zircon age distribution dominated by late Neoproterozoic to middle Cambrian (c. 710–518 Ma) and Palaeoproterozoic (c. 1,800–2,200 Ma) groupings. Both can be linked with lithologies exposed within the Cadomian belt as well as the West African craton, which is characterized by major tectonothermal events at 2.0–2.4 Ga. The detrital zircon signature of Avalonia is distinct from that of Armorica in that there is a much larger proportion of Mesoproterozoic detritus. The common provenance of the samples is therefore consistent with: (a) derivation of the Meneage Formation mélange deposits from the Armorican plate during Rheic Ocean closure and obduction of the Lizard Complex and (b) previous correlation of quartzite blocks within the Meneage Formation with the Ordovician Grès Armoricain Formation of NW France.     

Evolution of geochemistry and depositional settings of Early Palaeozoic siliciclastics of the Barrandian (Teplá-Barrandian Unit,Bohemian Massif,Czech Republic)

Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Teplá-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Píbram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Teplá-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Teplá-Barrandian unit within the Bohemian Massif terrane mosaic.Dr. Patoka deceased in July 2004.     

Time scales and mechanisms of growth of active margins of Gondwana: A model based on detrital zircon ages from the Neoproterozoic to Cambrian Blovice accretionary complex,Bohemian Massif

New U–Pb detrital zircon ages from (meta-)graywackes of the Blovice accretionary complex, Bohemian Massif, provide an intriguing record of expansion of the northern active margin of Gondwana during late Neoproterozoic and Cambrian. The late Neoproterozoic (meta-)graywackes typically contain a smaller proportion of Archean and Paleoproterozoic zircons and show a 1.6–1.0 Ga age gap and a prominent late Cryogenian to early Ediacaran age peak. The respective zircon age spectra match those described from other correlative Cadomian terranes with a West African provenance. On the other hand, some samples were dominated by Cambrian zircons with concordia ages as young as 499 Ma. The age spectra obtained from these samples mostly reflect input from juvenile volcanic arcs whereas the late Cambrian samples are interpreted as representing relics of forearc basins that overlay the accretionary wedge.The new U–Pb zircon ages suggest that the Cadomian orogeny, at least in the Bohemian Massif, was not restricted to the Neoproterozoic but should be rather viewed as a continuum of multiple accretion, deformation, magmatic and basin development events governed by oceanic subduction until late Cambrian times. Our new U–Pb ages also indicate that the Cadomian margin was largely non-accretionary since its initiation at ~ 650–635 Ma and that most of the material accreted during a short time span at around 527 Ma, closely followed by a major pulse of pluton emplacement. Based on the new detrital zircon ages, we argue for an unsteady, cyclic evolution of the Cadomian active margin which had much in common with modern Andean and Cordilleran continental-margin arc systems. The newly recognized episodic magmatic arc activity is interpreted as linked to increased erosion–deposition–accretion events, perhaps driven by feedbacks among the changing subducted slab angle, overriding plate deformation, surface erosion, and gravitational foundering of arc roots. These Cadomian active-margin processes were terminated by slab break-off and/or slab rollback and by a switch from convergent to divergent plate motions related to opening of the Rheic Ocean at around 490–480 Ma.The proposed tectonic evolution of the Teplá–Barrandian unit is rather similar to that of the Ossa Morena Zone in Iberia but shows significant differences to that of the North Armorican Massif and Saxothuringian unit in Western and Central Europe. This suggests that the Cadomian orogenic zoning was complexly disrupted during early Ordovician opening of the Rheic Ocean and Late Paleozoic Variscan orogeny so that the originally outboard tectonic elements are now in the Variscan orogen's interior and vice versa.     

From Cadomian arc to Ordovician passive margin: geochemical records preserved in metasedimentary successions of the Orlica-Śnieżnik Dome in SW Poland

The chemical composition of metamorphosed siliciclastic rocks in the Orlica-?nie?nik Dome (Bohemian Massif) identifies the main sources for the Neoproterozoic [the M?ynowiec Formation (MF)], Early Cambrian [the Stronie Formation (SF)] and Late Cambrian/Early Ordovician [the Goszów quartzites (GQ)] sediments. The MF developed from erosion of a Cadomian magmatic arc along the northern Gondwana margin. The variegated SF, with supra-subduction affinities, shows chemical characteristics pointing to erosion of the freshly exhumed Cadomian orogen and detritus deposition in the back-arc basin. The very different chemical features of the GQ indicate deposition in a basin sited on a passive continental margin. The explanation proposed for the observed changes in chemical composition involves three main stages: (1) The pre ~540 Ma evolution of an active continental margin and related back-arc basin ceased with the collision and accretion of the magmatic arc to the Gondwana margin; (2) Early Cambrian rift to drift transition (540–500 Ma) and development of a depositional basin filled with detritus derived from remnants of the magmatic arc; (3) Peri-Gondwana break-up leading to the formation of shallow-water passive margin depositional basins filled with quartz-rich detritus resembling Early Ordovician Armorican quartzites known from other parts of the Variscan Belt.     

Pre-Caledonian basement in Ireland and its cover relationships

Pre-Caledonian basement is exposed in three areas within the Irish orthotectonic Caledonides: 1. In northwest County Mayo the Erris Complex comprises the Annagh Division, which is largely Grenvillian but includes older gneisses, and the Inishkea Division, which is probably equivalent to the older Moines of Scotland. 2. In the northeast Ox Mountains, Rosses Point and Lough Derg inliers there is a granulite fades, dominantly metasedimentary basement which is probably late Grenville in age. 3. Laxfordian and probably older gneisses are seen in the Inishtrahull Platform northeast of Malin Head, County Donegal. In addition, some gneisses within the belt of ‘Connemara migmatites’ in south Connemara may be fragments of allochthonous basement and some high grade metamorphic rocks in the Tyrone Central inlier have been referred to as pre-Caledonian basement, but there is at present no conclusive support for these suggestions. The Ox Mountains succession seen in the Clew Bay region and the southwest and central Ox Mountains is probably not pre-Caledonian basement. Basement in the paratectonic Caledonides is confined to the Cadomian and possibly pre-Cadomian Rosslare Complex (older than 625 Ma) and the nearby late Precambrian (to early Cambrian ?) Cuilenstown Formation in southeast County Wexford. In addition, an unseen block of basement defined by magnetics immediately to the north of the Dingle peninsula of County Kerry is probably of Rosslare Complex affinity. Granulite facies xenoiiths recovered from volcanic rocks of Carboniferous age about 70 km to the west of Dublin, indicate the character of basement there, though its age is not proven. All contacts between basement and Lower Palaeozoic Caledonian cover in the orthotectonic Caledonides are synmetamorphic slide zones and faults and in the paratectonic Caledonides the contacts are either faults, which are often rooted in older mylonites, or are unexposed and presumed to be major structural breaks.     

黑龙江省东部马家街群碎屑锆石年代学及其大地构造意义

马家街群分布在黑龙江省东部佳木斯地块桦南隆起的西南缘,主要由一套经历了接触变质作用的富铝、富碳沉积碎屑岩所组成。区域上,这套接触变质岩系具有变质矿物分带特征,由西向东依次出现十字石、红柱石、石榴石和黑云母。红柱石碳质板岩和石榴云母石英片岩2件样品获得的LA-ICP-MS U-Pb碎屑锆石年龄谱均显示有272~310Ma、479~533Ma和>800Ma三组年龄。根据两件样品显示的最小年龄均未小于272Ma,而且二者的最小年龄组（272~310Ma）具有类似的峰值年龄,分别为276Ma和279Ma,这限定了马家街群主体岩石沉积年龄的下限应在中二叠世之前。侵入马家街群的花岗岩的锆石年龄为259Ma,说明其接触变质作用时代为晚二叠世早期,限定了马家街群形成时代的上限。479~533Ma年龄组中,2件样品的峰值年龄分别为499Ma和522Ma,这是佳木斯地块麻山群中最为重要的高级变质和花岗质岩浆作用年龄。>800Ma的年龄组具有多个峰值年龄,说明源区（佳木斯地块）具有前寒武纪-早前寒武纪地壳。上述证据表明,马家街群是晚二叠世早期形成的一套接触变质岩系,而非前寒武纪区域变质岩系。鉴于479~533Ma的麻山群在佳木斯地块中普遍存在,说明以麻山群为代表的早古生代变质结晶岩系既是马家街群沉积的基底,也是重要的物源区;而276~279Ma的早二叠世火山岩在佳木斯地块东缘分布广泛,表明其对马家街群的沉积也具有一定的贡献。