Armorican Terrane Assemblage Research Articles

A palynological investigation of the Early-Middle Devonian transition and associated Choteč Event in Northern Spain

In Northern Spain there is a near complete sequence of Devonian rocks that accumulated around a relatively isolated land mass, the Armorican Terrane Assemblage, which was separated from Gondwana to the south and Laurussia to the north. The Lower to Middle Devonian transition records a dramatic facies change from carbonate to siliciclastic sedimentation. The Choteč Event, a global anoxic pulse that occurs just above the Lower-Middle Devonian boundary (392.5–393.1 Ma), has been identified within this transition. These strata are included in the Moniello and Naranco fms in Asturias and the equivalent Santa Lucía and Huergas fms in Leon. They are independently age-contrained based on conodont and invertebrate biotas. For the first time we describe palynomorph assemblages recovered from the Lower to Middle Devonian transition in Northern Spain. The assemblages contain terrestrial (spores) and marine (acritarchs, prasinophycean cysts, chitinozoans) elements, thus recording primary productivity on both the land and in the oceans. The older assemblages belong to the annulatus–sextantii Spore Zone and the younger assemblages the douglastownensis–eurypterota Spore Zone, which is concordant with the independent age constraints. Our analysis of the distribution and diversity of marine and terrestrial palynomorphs supports interpretations of the facies change associated with the Choteč Event as being related to a change to increased rainfall and terrestrial runoff. However, this change is associated with only limited taxon turnover in both land plant spores and marine phytoplankton and chitinozoans.

Middle Devonian, late Carboniferous, and Triassic magmatic flare-ups in eastern Armorica (Sakarya Zone, Turkey) as revealed by detrital zircon U-Pb-Hf isotopic data

Abstract The Sakarya Zone of northern Turkey contains a well-preserved Early–Middle Jurassic and Late Cretaceous submarine magmatic arc constructed over pre-Jurassic bedrocks that are considered to be the eastward extension of the Armorican Terrane Assemblage in Europe. In this study, we present U-Pb-Hf isotopic data from the detrital zircons of middle Permian and Lower Jurassic sandstones to reveal episodes of Paleozoic–early Mesozoic magmatic flare-ups. Detrital zircon ages, together with data from the literature, define three major age groups at 400–380 Ma, 326–310 Ma, and 250–230 Ma, which indicates three distinct magmatic flare-ups. In addition, there are minor age clusters at 460–430 Ma and 215–195 Ma. Initial εHf values of the detrital zircons indicate significant juvenile input during the Triassic flare-up, the involvement of significantly reworked crustal material during the late Carboniferous magmatic flare-up, and both juvenile and reworked crustal material during the Middle Devonian magmatic flare-up. Within the pre-Jurassic continental basement rocks of the Sakarya Zone, the late Carboniferous igneous rocks are well documented and most voluminous, and the Middle Devonian rocks are known locally, while the Triassic igneous rocks—apart from those in Triassic accretionary complexes—are hardly known. Because the Sakarya Zone is a Gondwana-derived continental block that was later involved in the Variscan and Alpine orogenies, these magmatic flare-ups cannot be explained by subduction-related processes along a single subduction zone. We propose that the Sakarya Zone rifted from the northern margin of Gondwana during the Late Ordovician–Silurian, the Devonian magmatic flare-up (400–380 Ma) was related to the southward subduction of the Rheic Ocean beneath the Sakarya Zone during its northward drift, the late Carboniferous magmatic flare-up (326–310 Ma) occurred following the collision of the Sakarya Zone with Laurussia, and the Triassic flare-up (250–230 Ma) resulted from northward subduction of the Tethys Ocean beneath the Sakarya Zone. Comparison with data from the literature shows that the Triassic and late Carboniferous magmatic flare-ups are also characteristic features of neighboring Armorican domains, such as the Balkans and the Caucasus; however, the Middle Devonian flare-up appears to be restricted to the Sakarya Zone. Along with the late Carboniferous flare-up, the Late Ordovician–Silurian flare-up, which is locally recorded in the Sakarya Zone, is typical of the Armorican Terrane Assemblage as a whole.

Zircon U-Pb-Hf isotope systematics of southern Black Forest gneiss units (Germany) – Implications for the pre-Variscan evolution of Central Europe

Presently, little is known about depositional ages, geological setting, and provenance of (meta)sedimentary rocks constituting the southern part of the Black Forest Gneiss Complex, preventing detailed reconstructions of the Cadomian to Variscan geodynamic evolution in Europe. Here, we present the first systematic U-Pb-Hf isotope data of detrital zircon grains combined with geochemical data of metasedimentary rocks from the Murgtal, Todtmoos, and Wiese-Wehra gneiss units, preserving different stages of the pre-, syn- and post-Cadomian evolution. Protoliths of Murgtal metagreywackes were deposited during the Ediacaran at <550 Ma and sourced from the Avalonian-Cadomian Belt (550–700 Ma; ∼70 %) and Africa (760–1045 Ma, 1850–2250 Ma, 2720–3230 Ma; ∼30 %). In contrast, metasedimentary rocks of the Wiese-Wehra and Todtmoos gneiss units show late Devonian depositional ages at <375 Ma, but in different geotectonic settings. Data from Wiese-Wehra metagreywackes provide evidence for the existence of a pre-Cadomian oceanic arc formed at 610 Ma (εHft = +5 to +8), accreted to the Avalonian-Cadomian Belt at ca. 540 Ma, and successively reworked until 380 Ma (εHft = +1 to +6). Finally, these rocks became part of an early Variscan arc-back arc system with juvenile input at ca. 375 Ma (εHft = 0 to +10). Todtmoos metaarkoses mainly reflect subduction-related magmatism at 420–490 Ma (∼88 %), and at 380 Ma (∼10 %) in a continental arc setting (εHft = −2 to −8), and rare Cadomian arc input (560–700 Ma; εHft = −10 to +3; ∼2 %). Zircon grains of some samples reveal a Variscan metamorphic overprint at 340–335 Ma. In combination, new and existing data from the Black Forest and other basement units throughout Europe provide evidence for the amalgamation of pre-Cadomian juvenile terranes along the northern margin of Gondwana until 540 Ma, followed by a complex rift history, accompanied by subduction between 500 and 400 Ma. Furthermore, they indicate the existence of an oceanic arc-back arc system, which was located south of the Armorican Terrane Assemblage at 380–350 Ma, immediately prior to Variscan subduction-accretion at <345 Ma.

Dispersed spore assemblages from the Lower Devonian Rañeces - La Vid groups of Northern Spain: Palaeogeographical implications

Dispersed spore assemblages are described from the Early Devonian (late Lochkovian-late Emsian) Rañeces Group (Asturias) and equivalent La Vid Group (Leon) of Northern Spain. The lower part of both groups consists predominantly of limestone and yields only impoverished palynological assemblages. However, the upper parts contain more clastic sediments and yield rich assemblages of well preserved palynomorphs. These are dominated by marine forms but also include abundant spores. Sixty-two spore taxa are reported from the Valporquero Formation (Leon). These all belong to a single assemblage that can be equated with the lindlarensis-sextantii Assemblage Zone (mid-early late Emsian age) erected in northern Gondwana. The sequences are independently age constrained by conodont and invertebrate macrofossil evidence and the ages derived from these studies are concordant with the assigned spore zones. The Spanish spore assemblages are from deposits that accumulated around the Armorican Terrane Assemblage (ATA), a group of islands that lay between the continents of Laurussia and Gondwana. Jacaard Index analysis indicates that the Valporquero Formation spore assemblage is endemic, and not closely related to those previously described from Laurussia or Gondwana, although it has more in common with the latter. This suggests that the Armorican Terrane Assemblage was widely separated from both Laurussia and Gondwana, but was located closer to and had more connectivity with Northern Gondwana.

Mercury evidence of intense submarine volcanism and hydrothermal activity during a mid-Tournaisian anoxic event in the Carnic Alps

The mid-Tournaisian Event, or Lower Alum Shale Event (LASE), is linked to severe facies changes including pelagic carbonates and widespread black siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian event in the Carnic Alps. The predominance of tiny pyrite framboids, high Corg/P, U/Th, and V/Cr ratios, generally higher enrichments of redox-sensitive metals such as Mo, U, V, and depletion in Mn suggest anoxic/euxinic conditions at the sea bottom. Moreover, we found very large anomalous Hg spikes (with maximum values reaching 3650 ppb) for the first time in the mid-Tournaisian deep-water marine succession of the Carnic Alps. Our results suggest intense volcanism during the LASE, which triggered the mid-Tournaisian anoxic event. The most likely candidate for the observed Hg anomalous contents is submarine arc volcanism and hydrothermal activity connected with the collision of the peri-Gondwanan terranes Paleo-Adria and probably Armorican Terrane Assemblage (ATA) and/or Noric terrane during the first phase of the Variscan orogeny.

Provenance of lower Palaeozoic metasediments of the East Odenwald (Mid-German-Crystalline Zone, Variscides)\u2014a correlation with the East European Platform (Poland)

U–Pb age spectra of detrital zircons related to the East European Platform could be traced in paragneiss through the whole Mid-German-Crystalline Zone (Variscides, Central Europe) from the Odenwald via the Spessart to the Ruhla crystalline forming an exotic unit between Armorica and Laurussia. The depositional ages of the paragneiss are defined by the youngest age of the detrital zircons and the oldest intrusion ages as Ordovician to Silurian. The Ediacaran dominated age spectrum of detrital zircons from the paragneiss of the East Odenwald suggests the latter to be derived from the shelf of the East European Platform (Baltica), which was influenced by the 1.5 Ga old detritus delivered from a giant intrusion (Mazury granitoid, Poland). The detrital zircon age spectrum of the lower Palaeozoic paragneiss of the East Odenwald and sandstone of the northern Holy Cross Mountains are identical. The pure Sveconorwegian spectrum of the lower Palaeozoic quartzite from the Spessart, (Kirchner and Albert Int J Earth Sci 2020) and the Ruhla (Zeh and Gerdes Gondwana Res 17:254–263, 2010) could be sourced from Bornholm and southern Sweden. A U–Pb age spectrum with 88% Palaeozoic detrital zircons from a volcano-sedimentary rock of the East Odenwald is interpreted to be derived from a Silurian magmatic arc (46%), which was probably generated during the drift of the Mid-German-Crystalline Zone micro-continent to the south. A tentative plate tectonic model of Mid-German-Crystalline Zone is presented taking into account (a) the East European Platform related age spectra of the detrital zircons (b) the Ordovician to Silurian depositional age of the metasediments (c) the Silurian and Early Devonian intrusion age of the plutonic and volcanic rocks and (d) the U–Pb ages of the Middle Devonian high-grade metamorphism. The East European Platform-related part of the Mid-German-Crystalline Zone is interpreted as a micro-continent, which drifted through the Rheic Ocean to the south and collided with the Saxothuringian (Armorican Terrane Assemblage) during the Early Devonian. Such large-scale tectonic transport from the northern continent to the southern continent is also known from the SW Iberia, where Laurussia-related metasediments of the Rheic suture zone are explained by a large scale tectonic escape (Braid et al. J Geol Soc Lond 168:383–392, 2011).

Reconstructing the terrestrial flora and marine plankton of the Middle Devonian of Spain: implications for biotic interchange and palaeogeography

A rich and well-preserved palynomorph assemblage from the Middle Devonian of northern Spain is analysed with regard to palaeobiogeography and palaeocontinental reconstruction. The communities of terrestrial plants (dispersed spores), marine zooplankton (chitinozoans) and marine phytoplankton (acritarchs and prasinophytes) all show significant endemism. They are depauperate in some respects, missing common species found elsewhere, but also containing many endemic taxa. Measures of similarity and cluster analysis reveal little relatedness to other contemporary assemblages, with both the spore and phytoplankton communities being sister groups to all other communities except one. Attempted correlation with local, regional and global sea-level and transgression–regression schemes is complicated by the absence of key index palynomorphs in this unusual assemblage. The distinctive sedimentary sequence of northern Spain may reflect an increased clastic input to the marine shelf resulting from an increasingly monsoonal climate, possibly connected to the Kačák extinction event. The assemblage's unusual, endemic character requires restrictions on dispersal. The terrestrial spore assemblage suggests that large tracts of ocean existed, without appreciable land bridges, between the Armorican Terrane Assemblage and Laurussia to the north and Gondwana to the south. The phytoplankton communities support existing evidence for significant east to west ocean currents through the Rheic Ocean. Supplementary material: Supplementary Tables 1–3 contain presence–absence data for spores, chitinozoans and phytoplankton respectively, in various assemblages, used to construct the dendrograms in Figures 4–6. The tables are available at https://doi.org/10.6084/m9.figshare.c.4726862

A new Middle Devonian chitinozoan assemblage from northern Iberia

Chitinozoans were recovered from the Naranco, Huergas and Gustalapiedra formations of northern Spain, which yielded a diverse assemblage including 29 taxa in 9 named genera. The deposits are of early Givetian age, a time when Iberia was isolated from large continents as part of the Armorican Terrane Assemblage. The formations studied here consist of a large clastic unit interrupting carbonate deposition, with large limestone formations positioned above and below the formations analyzed here. This clastic unit includes the Kačák Event, an important global extinction event associated with anoxia in the marine realm. In this paper, the relatively well-preserved chitinozoan assemblage is described and considered from a biogeographical and stratigraphical perspective. The chitinozoan community was deposited in a short space of time and includes various taxa not previously known from the Middle Devonian, though the assemblage as a whole is attributable to the period. Only moderate similarity is seen with assemblages reported from Laurussia and Gondwana, with a slight bias towards the latter. This report adds to our knowledge of chitinozoan paleobiogeography and to other recent studies of Middle Devonian palynology in northern Iberia.

An endemic microphytoplankton assemblage from Middle Devonian Iberia and its paleogeographical significance

Diverse microphytoplankton assemblages, including 72 taxa belonging to 27 genera of acritarchs and prasinophyte phycomata, have been recovered from Middle Devonian rocks in northern Spain, revealing an endemic flora dissimilar to coeval assemblages. These deposits are of early Givetian age and consist of the laterally equivalent Naranco, Huergas and Gustalapiedra formations of Asturias, León and Palencia provinces. At the time, Iberia was part of the Armorican Terrane Assemblage, a comparatively isolated island chain positioned between Laurussia and Gondwana. The studied formations represent a marine transect across a nearshore–offshore gradient and consist of a large clastic unit sandwiched between extensive carbonate deposits. This clastic unit incorporates the Kačák Event, an important global extinction event associated with marine anoxia. Herein, the suite of generally well-preserved microphytoplankton assemblages is described and their stratigraphical and biogeographical importance are considered. The microphytoplankton represents a single assemblage deposited in a short interval and is interpreted as being endemic. The assemblage is only moderately similar to contemporary assemblages from Laurussia and Gondwana and, although certain characteristic Middle Devonian taxa are present, other common species such as Arkonites bilixus and Tyligmasoma alargada are absent. While no unique taxa are found here, the taxa which are present represent a particular combination of species not seen elsewhere. Certain taxa appear which may have discordant temporal ranges, though no major inferences can be made from them as only two taxa both occur in significant numbers and have a confident identification. This assemblage adds to our knowledge of phytoplankton palaeobiogeography, representing a significantly endemic assemblage within the generally cosmopolitan microphytoplankton flora of the Middle Devonian.

An endemic flora of dispersed spores from the Middle Devonian of Iberia

AbstractDiverse assemblages of dispersed spores have been recovered from Middle Devonian rocks in northern Spain, revealing a significant endemism in the flora. Middle Devonian Iberia was part of a relatively isolated island complex (Armorican Terrane Assemblage), separated by considerable tracts of ocean from Laurussia to the north‐west and Gondwana to the south‐east. The Middle Devonian deposits of the Cantabrian Zone of northern Spain are entirely marine and comprise a thick clastic unit sandwiched between extensive carbonate units. The clastic unit, the laterally equivalent Naranco, Huergas and Gustalapiedra formations of Asturias, León and Palencia provinces, represents a nearshore‐offshore transect across a marine shelf. This unit is also believed to encompass the Kačák Event, an important global extinction event. The recovered palynomorphs include marine (phytoplankton, chitinozoans, scolecodonts) and terrestrial (spores) assemblages. These are abundant and well preserved, although of variable thermal maturity. Here, we describe the dispersed spores and consider their significance as regards biostratigraphy, palaeophytogeography and Kačák Event interpretation. The dispersed spores represent a single assemblage assignable to the lemurata–langii Assemblage Zone (lemurata Subzone) indicating a probable early (but not earliest) Givetian age. Signs of endemism include various taxa known only from this region, some taxa appearing to have discordant ranges compared with elsewhere, and the absence from Iberia of certain prominent taxa characteristic of coeval assemblages elsewhere, such as those with grapnel‐tipped processes. The abrupt interruption of carbonate deposition, with a change to rapid deposition of thick clastic deposits, provides support for a monsoonal cause of the Kačák Event.

The eoorthid brachiopod Apheoorthina in the Lower Ordovician of NW Argentina and the dispersal pathways between the Andean and Mediterranean regions of Gondwana

The eoorthid brachiopod Apheoorthina is reported for the first time from the Lower Ordovician of NW Argentina. It is represented by a species similar to A. ferrigena from the Tremadocian of the Prague Basin, increasing the faunal affinities between the Central Andean Basin and the South European microcontinents, in particular the Bohemian region (Perunica). Nine out of the fourteen brachiopod genera reported from the Tremadocian of the Central Andean Basin (~64%) are shared with the Mediterranean region, four of which (~28%) have been recorded in the Prague Basin, and two (Kvania and Apheoorthina) are restricted to the Central Andes and Perunica. Dispersal pathways around Gondwana are analyzed in the light of major factors affecting large-scale distribution of brachiopods (environment, larval capacity for dispersal, oceanic currents). The presence in Apheoorthina aff. ferrigena of a well-preserved larval protegulum measuring 420 μm in width and 210 μm in length strongly suggests that this species had planktotrophic larvae capable of long-distance dispersal. According to recent ocean-atmosphere general circulation models for the Ordovician Period, the Central Andean margin was dominated by the cold-water Antarctica Current. Despite the complex non-zonal pattern produced by current deflections around the peri-Gondwanan microcontinents, the general westward circulation sense favoured larval dispersal from the Andean region to North Africa, Avalonia, the Armorican Terrane Assemblage, and Perunica. On the other hand, the eastwards flowing Gondwana Current connected the North Gondwana waters with the South American epicontinental seas, which could explain the reversed migration of some brachiopods.

Migmatization and large-scale folding in the Orlica–Śnieżnik Dome, NE Bohemian Massif: Pressure–temperature–time–deformation constraints on Variscan terrane assembly

New research indicates that there is stratigraphic and structural continuity between the Młynowiec and Stronie Formations of the Orlica–Śnieżnik Dome (OSD), Bohemian Massif, and reveals the primary importance of the collision of Armorican Terrane Assemblage and Brunovistulian terrane in Variscan development of the OSD. The Młynowiec Formation is a locally migmatized metasedimentary unit and understanding its tectonometamorphic history is crucial for making regional-scale Variscan reconstructions. Structural studies, pseudosection modelling in MnNCKFMASH system and microprobe monazite dating revealed that this Variscan collision led to crustal thickening of the OSD, tectonic burial of the Młynowiec Formation to depths corresponding to 10–11kbar and progression of regional metamorphism at 361±5Ma. Continuous underthrusting of the Brunovistulian terrane led to exhumation of the OSD metasediments and the formation of subhorizontal, N–S-trending tight folds and a peak-metamorphic axial planar foliation. This episode took place at 650°C and 6–7kbar in the non-migmatized rocks of structurally upper parts of the Młynowiec Formation, and at 700°C and 6–7kbar in its structurally lower parts, where metamorphism was accompanied by localised in-situ migmatisation. During final consolidation of a mosaic of terranes, top-to-the-NE tectonic movements reactivated former fabrics, produced further uplift and cooled the rocks to greenschist facies. This pervasive shearing could be also responsible for the metasomatic event at ~330Ma. The continuation of the NE-transpression of the OSD involved its interactions with adjacent crustal units in the east and in the north, which led to large-scale folding represented now by synforms and associated antiforms. The metamorphic structure of the rocks subjected to the Barrovian-type metamorphism was rebuilt so that the migmatitic rocks outcrop in the cores of large-scale antiforms, and the lithological contacts, penetrative foliation and metamorphic isograds dip locally at different angles (shallow, moderate to high) and at different azimuths (E-, NE-, N, NW and W).

Constraints on the Devonian–Carboniferous closure of the Rheic Ocean from a multi-method geochronology study of the Staré Město Belt in the Sudetes (Poland and the Czech Republic)

This paper attempts to clarify the complex nature of how and when the Rheic Ocean closed in what is now Central Europe and, with respect to the various terranes that were involved, offer a regional chronology for the associated structural, metamorphic and igneous processes that accompanied and followed this closure. The Variscan orogen in Europe originated from the multiple collisions of Gondwana-derived terranes (the Armorican Terrane Assemblage) with Laurussia: however, many important structural details on the timing of these collision-related events remain obscure. In the Sudetes, the Staré Město Belt represents a WNW-dipping part of the Rheic suture that developed from the continental collision of the eastern terranes of the Armorican Terrane Assemblage (now in the Bohemian Massif) with the Brunovistulian Terrane (a part of Laurussia/Old Red Continent). In this study, the results of monazite Th–U–total Pb, garnet Lu–Hf and zircon U–Pb geochronology were integrated into a newly established D1–D3 tectonometamorphic sequence. A Th–U–total Pb age of ~368Ma from a monazite that grew concurrently with D2 metasedimentary garnet, as well as Lu–Hf ages of ~361Ma and ~355Ma obtained from D2 metasedimentary garnets, implies that the regional contractional deformation and progressive metamorphism of D2 took place mainly during the Famennian (Late Devonian) and extended into the Visean (Middle Mississippian of the Early Carboniferous). The ion probe U–Pb zircon ages of ~355Ma and ~359Ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during D1–D2. Metamorphic monazites dated at ~340Ma provide a time for the subsequent D3 dextral transpressional regime. The closure of the Staré Město Belt segment of the Rheic Ocean probably resulted from a head-on hard collision between the westerly subducting Brunovistulian promontory of Laurussia and the eastern members of the Armorican Terrane Assemblage. Thus, the Rheic Ocean closed during the Late Devonian at ~370–360Ma and preceded the collision of the Armorican Terrane Assemblage with East Avalonia at the western margin of what is now the Bohemian Massif. Following ocean closure, the Rheic slab may have broken off, resulting in the suture zone becoming dominated by lateral “tectonic escape” movements of the colliding terranes at ~340–330Ma (Visean). Syntectonic D3 intrusions of granodiorite/tonalite magma acted as a hot lubricant and stitched the suture zone together.

Chapter 2 New global palaeogeographical reconstructions for the Early Palaeozoic and their generation

Abstract New palaeogeographical reconstructions are presented at 10 myr intervals from the Lower Cambrian at 540 Ma to the Lower Devonian at 400 Ma, showing continental crustal fragments and oceans (not lands and seas), with appropriate kinematic continuity between successive maps. The maps were chiefly generated by revised and selected palaeomagnetic data and revised Apparent Polar Wandering paths linked to present-day polygons from the main continents. These have been reinforced by analysis of the distributions of some fossils and sediments. Gondwana was the dominating supercontinent from its final assembly in the Latest Neoproterozoic at about 550 Ma until the Carboniferous, and covered much of the Southern Hemisphere. The Northern Hemisphere was largely occupied by the vast Panthalassic Ocean. The relative positions of the major continents and the latitudes and rotation histories of Gondwana, Baltica, Siberia and Laurentia (Laurussia from the mid-Silurian) are now well known. Although Laurentia was oriented in a similar direction to the present, Siberia was inverted throughout the Lower Palaeozoic, and Baltica too was initially inverted, but rotated through 120° between the Late Cambrian and Late Ordovician before collision with Laurentia in the mid-Silurian Caledonide Orogeny. Through reconstructions of the Caledonide and some other orogenies, the progressive history of the Iapetus Ocean between Laurentia and Baltica/Gondwana is well constrained. Less major continents whose positions are also well known include Avalonia (initially peri-Gondwanan but migrating in the Early Ordovician to join Baltica by the end of the Ordovician), Sibumasu (now considered an integral part of Gondwana) and Mongolia (adjacent to Siberia). A large number of other terranes are reviewed and plotted on the reconstructions with varying degrees of certainty. However, significant continents with less well constrained or controversial positions are South China, North China (Sinokorea), Annamia (Indochina) and Arctic Alaska–Chukotka. The European areas of France, Iberia and southern Italy, previously considered by some as a separate Armorican Terrane Assemblage, remained parts of core Gondwana until the opening of the Palaeotethys Ocean near the end of the Silurian, but it is uncertain whether Perunica (Bohemia) was one of that group or whether it left Gondwana during the Middle Ordovician.

Plate tectonic significance of Middle Cambrian and Ordovician siliciclastic rocks of the Bavarian Facies, Armorican Terrane Assemblage, Germany — U–Pb and Hf isotope evidence from detrital zircons

We have examined the provenance and tectonic setting of the Middle Cambrian and Ordovician siliciclastic deposits and associated volcanic rocks of the Bavarian Facies, Franconia, Germany, in the Saxothuringian Zone of the Variscan orogen. The units were deposited on the North African rifted margin of Gondwana and represent deep-water equivalents of the shallow marine mature sandstone successions of the areally extensive Thuringian Facies. U–Pb ages of detrital zircons of the Middle Cambrian Wildensteiner Formation, the Middle Ordovician Plattensandstone of the Randschiefer Series, and the Upper Ordovician Döbra sandstone fall into four distinct age groups: 2.4 to 1.8 Ga (15%), 0.75 to 0.54 Ga (55%) and 0.54 to 0.44 Ga (14%); minor abundances (4%) occur around 1 Ga. This age distribution is consistent with a northern Gondwanan derivation, mainly from the Cadomian continental magmatic arc in northern Africa. The c. 2 Ga ages indicate a provenance from Eburnean or equivalent sources on the West Africa craton and in northeast Africa and Arabia. The scarcity of grains of Kibaran age (c. 1 Ga) is characteristic of a derivation from metamorphic and magmatic sources on the Arabian–Nubian Shield, rather than from distant major Kibaran age terrains. The youngest group mainly reflects Late Cambrian to Ordovician rift magmatism widespread in the northern Gondwanan Cadomian terranes. The Hf isotopic compositions of selected dated zircons at the time of their crystallization ( ε Hf( t) ; T = 3.5–0.47 Ga) vary between − 30 and + 7. Zircons with positive ε Hf( t ) values are almost exclusively restricted to the age group between 0.5 and 0.9 Ga. The Hf isotope data suggest that magmatism associated with the Cadomian continental magmatic arc and post-Cadomian (late Cambrian-Ordovician) marginal rifts in northern Gondwana mainly involved mixing of juvenile magmas with Paleoproterozoic crust.

Transition from island-arc to passive setting on the continental margin of Gondwana: U–Pb zircon dating of Neoproterozoic metaconglomerates from the SE margin of the Teplá–Barrandian Unit, Bohemian Massif

Sedimentological, petrological and geochemical data combined with SHRIMP and LA ICP-MS U–Pb ages of pebbles of granitic rocks and detrital zircons from Neoproterozoic Vletice metaconglomerates of the SE margin of the Teplá–Barrandian Unit were used to characterize the regional geotectonic setting at the Proterozoic/Paleozoic boundary. The metaconglomerates form several-metres thick intercalations in metasediments of the Svrchnice Formation (roof pendants of Central Bohemian Plutonic Complex), which is considered to have been deposited in Neoproterozoic/Cambrian times. A sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans. Geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the Teplá–Barrandian Unit Proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common. The abundance of quartz, quartzite, granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the Vletice metaconglomerates, and shallowing of the Neoproterozoic basin to the SE (in the present geographic co-ordinate system). The Neoproterozoic to lowermost Cambrian age of the Svrchnice Formation, suggested by its comparison with similar rocks in the central part of the Teplá–Barrandian Unit, was confirmed by U–Pb dating. Ages of detrital zircons from the metaconglomerate matrix cluster around two values: Archaean to Paleoproterozoic U/Pb ages between ca. 2580 and 1780 Ma and Neoproterozoic to early Cambrian ages between 660 and 520 Ma with a significant peaks at 600 and 544 Ma suggesting local volcanic activity extending shortly beyond the Proterozoic/Paleozoic boundary. Zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean U–Pb age of 610 ± 17 Ma arguing for active arc magmatism during the latest Neoproterozoic. The overall age distribution of detrital zircons and the absence of Grenvillian zircon ages favour a location of the Teplá–Barrandian Unit close to the West African craton at the Neoproterozoic/Paleozoic boundary. The rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying Cambrian shallow-water clastics alternating with predominantly felsic calc-alkaline to alkaline volcanics. During the Cambrian oblique plate convergence, the active continental margin was transformed into belts of horsts and narrow pull-apart or small rift basins. After the Ordovician break-off of Armorican Terrane Assemblage blocks from Gondwana, the extensional regime in the Teplá–Barrandian Unit continued until the beginning of Variscan orogeny in the Middle Devonian.

Defining the southern margin of Avalonia in the Pontides: Geochronological data from the Late Proterozoic and Ordovician granitoids from NW Turkey

We provide new isotopic data from the Pontides, which substantiate the presence of a Cadomian basement with latest Proterozoic granitoids but also show the existence of Ordovician intrusives. Before the opening of the Black Sea, the Pontides formed a Palaeozoic orogenic belt at the margin of the East European Platform (Baltica) in an analogous position to the Variscan terranes in the central Europe. The Pontides consist of three terranes: The Strandja terrane has a Variscan basement with Carboniferous and Permian granitoids and an epicontinental Triassic to Jurassic sedimentary cover. The I.stanbul terrane has a Cadomian basement overlain by a sedimentary sequence of Ordovician to Carboniferous age. It underwent contractional deformation during the Late Carboniferous. The Sakarya terrane is characterized by Carboniferous high temperature metamorphism and plutonism. The boundary between the I.stanbul and Sakarya terranes forms a complex tectonic zone several tens of kilometres wide. The new age data come from this boundary zone in the Armutlu peninsula in northwest Turkey. Metagranitoids, which intrude amphibolites in the Armutlu peninsula, give latest Proterozoic and Ordovician U–Pb zircon laser ablation MC–ICP–MS and Pb–Pb evaporation ages. The latest Proterozoic (ca. 570 Ma) granitoids are similar in age to those reported previously from the basement of the I.stanbul terrane; they all form part of the widespread Pan-African granitoid plutonism on the margins of Gondwana. The Mid to Late Ordovician granitoids (460 Ma), on the other hand, probably have formed during the rifting of the I.stanbul terrane away from Gondwana during the opening of the Rheic ocean. In terms of the tectonic position, stratigraphy and geological evolution, the I.stanbul and Sakarya terranes are comparable to the Avalonia and the Armorican terrane assemblage in central Europe, respectively.

Brunovistulian terrane (Bohemian Massif, Central Europe) from late Proterozoic to late Paleozoic: a review

The Brunovistulian terrane represents a microcontinent of enigmatic Proterozoic provenance that was located at the southern margin of Baltica in the early Paleozoic. During the Variscan orogeny, it represented the lower plate at the southern margin of Laurussia, involved in the collision with the Armorican terrane assemblage. In this respect, it resembles the Avalonian terrane in the west and the Istanbul Zone in the east. There is a growing evidence about the presence of a Devonian back-arc at the margin of the Brunovistulian terrane. The early Variscan phase was characterized by the formation of Devonian extensional basins with the within-plate volcanic activity and formation of narrow segments of oceanic crust. The oldest Visean flysch of the Rheic/Rhenohercynian remnant basin (Protivanov, Andelska Hora and Horni Benesov formations) forms the highest allochthonous units and contains, together with slices of Silurian Bohemian facies, clastic micas from early Paleozoic crystalline rocks that are presumably derived from terranes of Armorican affinity although provenance from an active Brunovistulian margin cannot be fully excluded either. The development of the Moravo–Silesian late Paleozoic basin was terminated by coal-bearing paralic and limnic sediments. The progressive Carboniferous stacking of nappes and their impingement on the Laurussian foreland led to crustal thickening and shortening and a number of distinct deformational and folding events. The postorogenic extension led to the formation of the terminal Carboniferous-early Permian Boskovice Graben located in the eastern part of the Brunovistulian terrane, in front of the crystalline nappes. The highest, allochthonous westernmost flysch units, locally with the basal slices of the Devonian and Silurian rocks thrusted over the Silesicum in the NW part of the Brunovistulian terrane, may share a similar tectonic position with the Giessen–Harz nappes. The Silesicum represents the outermost margin of the Brunovistulian terrane with many features in common with the Northern Phyllite Zone at the Avalonia–Armorica interface in Germany.

Combined U–Pb and Hf isotope LA-(MC-)ICP-MS analyses of detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in Central Germany

Uranium–lead ages obtained by LA-ICP-MS analyses of zircon cores from a high-grade Armorican metasediment from the Mid-German Crystalline Rise, Central Germany, yield results which are identical to, but more precise than those previously obtained by SHRIMP dating. This is mainly due to the fact that SHRIMP analyses are more sensitive than LA-ICP-MS analyses to common Pb contamination on the surface of the grain mount. The new U–Pb ages, in combination with in-situ Hf isotope analyses of zircon, provide the first evidence that detrital zircons within Armorican sediments crystallized in both juvenile and evolved magmatic rocks during the Archaean at 2.7–2.9 Ga, the Palaeoproterozoic at 1.8–2.1 Ga, and the Neoproterozoic/Early Palaeozoic at 500–720 Ma. In addition, zircons were formed at ca. 1.0 Ga by remelting of Palaeoproterozoic crust during the Grenville orogeny. The U–Pb dataset shows an age gap between 1.8 and 1.0 Ga, which is characteristic of Armorican sediments, and indicates that the metasediment protolith is younger than Late Cambrian. In addition, the data support previous conclusions that sediments constituting the Armorican terrane assemblage were derived from three crustal sources. Dominant sources were the Avalonian–Cadomian belt (ca. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West-African and/or eastern Amazonian cratons (ca. 50%). The Grenville belt was a minor source (< 5%). Variation of εHf( t) values of the Neoproterozoic/Early Paleozoic zircons indicates two periods of increased juvenile magma formation, one at 595–575 Ma and a second at 515–500 Ma. The older event is coeval with the formation of the Avalonian–Cadomian magmatic arc, whereas the younger event can be related to the break-up of the northern Gondwana margin in Cambrian/Ordovician times. In between, at around 545 Ma, only recycling of older crustal material took place.

Palaeozoic accretion of Gondwana-derived terranes to the East European Craton: recognition of detached terrane fragments dispersed after collision with promontories

Abstract Recent work in Central Europe, combined with emerging information about basement massifs in SE Europe and NW Turkey, permits a new look at the relationships between crustal blocks abutting the East European Craton (EEC) along the Trans-European Suture Zone (TESZ). The simplest model indicates that the end-Cambrian establishment of the Bruno-Silesian, Lysogory and Malopolska terranes close to their present location on the SW margin of the EEC formed a major promontory on this margin of the continent. Moesia may also have formed part of this block. Both late Ordovician accretion of Avalonia and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA) attached new continental material around the Bruno-Silesian Promontory (BSP). Palaeozoic faunal affinities and inherited isotopic signatures similar to those of Avalonia seen in the Istanbul block of NW Turkey, and in minor thrust slices in Moravia and Romania, suggest that easternmost Avalonia was severed, on collision with the BSP, and migrated east along the southern margin of the EEC. Likewise, the similarities to the ATA of the Balkan, Istranca, Sakarya and eastern Pontides blocks suggests that more easterly components of the ATA were detached at the BSP and migrated east. All the newly accreted blocks contain similar Neoproterozoic basement indicating a peri-Gondwanan origin; Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif may explain their progressive separation from Gondwana before their accretion to the EEC. Inherited ages from Avalonia contain a 1.5 Ga 'Rondonian' component arguing for proximity to the Amazonian Craton at the end of the Neoproterozoic; Armorican terranes lack such a component, suggesting that they have closer affinities with the West African Craton. Models showing the former locations of these terranes and the larger continents from which they rifted, or later became attached to, must conform to both these constraints and those provided by palaeomagnetic data. In the late Neoproterozoic and Palaeozoic, these smaller terranes, some containing Neoproterozoic ophiolitic marginal basin and magmatic arc remnants, probably fringed the end-Proterozoic supercontinent as part of a 'Pacific-type' margin. When this margin fragmented, most resulting fragments accreted to the EEC.