East Avalonia Research Articles

U–Pb zircon–rutile dating of the Llangynog Inlier, Wales: constraints on an Ediacaran shallow-marine fossil assemblage from East Avalonia

The Llangynog Inlier of south Wales contains an assemblage of Ediacaran macrofossils from a shallow-marine environment, including discoidal morphs of Aspidella and rare examples of Hiemalora , Palaeopascichnus and Yelovichnus . These are taxa found at other sites in the Avalonian microcontinent (e.g. Charnwood Forest and eastern Newfoundland) and in the younger White Sea Ediacaran assemblages. As the Charnwood fossils reflect a deep-water environment, and no macrofossils have been found in the Ediacaran rocks of the Long Mynd, the fossils of the Llangynog Inlier represent a unique glimpse of shallow-marine life in southern Britain (East Avalonia). However, the lack of absolute age constraints has hampered direct comparison with other assemblages. Here, we report in situ zircon and rutile U–Pb dates from a rhyolitic ash-flow layer of the Coed Cochion Volcaniclastic Member, Llangynog Inlier, which constrains the age of the fossiliferous strata. A weighted mean single grain zircon isotope dilution thermal ionization mass spectrometry U–Pb age of 564.09 ± 0.70 Ma is interpreted as the rhyolite's crystallization age. This age is consistent with in situ laser ablation inductively coupled plasma mass spectrometry zircon and rutile U–Pb dating. The Llangynog age temporally correlates these fossils to dated horizons within East Avalonia at the Beacon Hill Formation, Charnwood (565.22 ± 0.89 Ma) and the Stretton Shale Formation, Long Mynd (566.6 ± 2.9 Ma). Correlations to West Avalonia include the time-equivalent Fermeuse Formation, St John's Group, eastern Newfoundland (564.13 ± 0.65 Ma). The data presented here establish the biota of the Llangynog Inlier as a lateral equivalent to the similarly shallow-marine, tidally influenced ecosystem of the upper Fermeuse Formation. Intra-terrane depositional environmental variability also affects what is preserved in Avalonian fossil sites. Further, time-constrained geochemical data reinforce the Llangynog Inlier's classification within the Wrekin Terrane. Supplementary material : U–Pb data, reference material and figures showing sedimentary structures are available at https://doi.org/10.6084/m9.figshare.c.6965642

The Harz Mountains (Germany) – Cadomia meets Avalonia and Baltica: U–Pb ages of detrital and magmatic zircon as a key for the decoding of Pangaea's central suture

Abstract U–Pb ages of detrital ( n = 2391) and magmatic ( n = 170) zircon grains from the Harz Mountains were obtained by LA-ICP-MS for provenance studies and absolute age dating. Results point to a complete closure of the Rheic Ocean at c. 419 Ma. A narrow Rhenish Seaway then re-opened in Emsian to mid-Devonian time ( c. 390–400 Ma). Devonian sedimentary rocks of the Harz Mountains were deposited on the northwestern (Rheno-Hercynian) and on the southeastern (Saxo-Thuringian) margins of the Rhenish Seaway. A new U–Pb zircon age from a plagiogranite (329 ± 2 Ma) within a harzburgite makes the existence of oceanic lithosphere in the Rhenish Seaway probable. The Rhenish Seaway was completely closed by Serpukhovian time ( c. 328 Ma). Existence of a terrane in the seaway is not supported by the new data. Provenance studies and spatial arrangement allow reconstruction of the thin- to thick-skinned obduction style of the Harz Mountains onto the southeastern margin of East Avalonia (Rheno-Hercynian Zone) during the Variscan orogeny. Detrital zircon populations define Rheno-Hercynian and Saxo-Thuringian nappes. Intrusion of the granitoid plutons of the Harz Mountains occurred in a time window of c. 300 to 295 Myr and constrained the termination of Variscan deformation.

Geological and geochemical characteristics of Late Ordovician volcanic levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for global events during the Ordovician

The İstanbul-Zonguldak Tectonic Unit is regarded as the easternmost fragment of Avalonia-Carolina and stands for Far East Avalonia. Its stratigraphy also is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as the İstanbul Terrane, the Gözdağ Formation is represented by lagoonal sedimentary rocks consisting of shale-sandstone with limestone of Middle Ordovician-Lower Silurian age. Here, I report on stratigraphic positions and petrographical and geochemical data of fine- and coarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grained tuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are Sandbian and Katian ages, and the lavas are Hirnantian ages, according to the stratigraphic positions of the Late Ordovician volcanic rocks. Geochemically, the-fine grained tuffs have High-K calc-alkaline character and the coarse-grained tuffs and lavas have calc-alkaline affinity. They are devoid of noticeable with-in plate components, as deduced by the presence of obvious negative Nb anomalies, and they have subduction signatures. In conjunction with the data from literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul-Zonguldak Tectonic Unit during the Taconic orogeny formed in the earliest late Ordovician between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were deposited in a shallow marine in it during the last phase of the consumption of the Teisseyre-Tornquist Ocean from volcanoes occurred in volcanic arc-related settings. Besides, I the Hirnantian lavas flowed in a lagoonal depocenter in it during the soft-docking of Avalonia and Baltica, known as the pre-Caledonian orogeny.

A detective duo of apatite and zircon geochronology for East Avalonia, Johnston Complex, Wales

The Johnston Complex represents a rare inlier of the Neoproterozoic basement of southern Britain and offers a window into the tectonomagmatic regime of East Avalonia during the assembly of Gondwana. This work presents in situ zircon (U–Pb, Lu–Hf), apatite (U–Pb), and trace element chemistry for both minerals from the complex. Zircon and apatite yield a coeval crystallization age of 570 ± 3 Ma, and a minor antecrystic zircon core component is identified at 615 ± 11 Ma. Zircon Hf data imply a broadly chondritic source, comparable to Nd data from East Avalonia, and T DM 2 model ages of c. 1.5 Ga indicate source extraction during the Mesoproterozoic. Zircon trace element chemistry is consistent with an ensialic calc-alkaline continental arc setting and demonstrates that magmatism was ongoing prior to terrane dispersal at 570 Ma. Apatite trace element chemistry implies a sedimentary component within the melt consistent with voluminous S-type granite production during the formation of Gondwana. The similarity of the ɛHf and geochemistry between both zircon age populations suggest derivation from a uniform source that did not undergo significant modification between 615–570 Ma. Time-constrained apatite–zircon chemistry addresses complexities in dating S-type granitoids (zircon inheritance) and permits inferences on post-magmatic thermal histories. Supplementary material: Zircon U–Pb, Lu–Hf and trace element data, and apatite U–Pb and trace element data are available at https://doi.org/10.6084/m9.figshare.c.6484464

Ordovician arc and syncollisional magmatism in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for the consumption of the Teisseyre-Tornquist Ocean in Far East Avalonia

Ordovician arc and syncollisional magmatism in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for the consumption of the Teisseyre-Tornquist Ocean in Far East Avalonia

Age and implication of Late Ediacaran dykes in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for the rifting of the Rheic Ocean

ABSTRACT The İstanbul-Zonguldak Tectonic Unit is a part of Avalonia-Carolina and represents Far East Avalonia. It includes Ordovician to Carboniferous deposits which unconformably overlie Late Neoproterozoic metamorphic rocks. Timing of its detachment from West Gondwana-land, thus rifting of the Rheic Ocean, is poorly known. Here, I report on dacite, diabase and andesite dykes of Late Ediacaran igneous crystallization ages from the basement rocks and rift-deposits in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They are folded about a consistent axis and have porphyritic to intersertal textures. U-Pb dating on igneous zircons from folded dykes yielded Late Ediacaran ages of ca. 556.2 ± 2.2 Ma (2σ) and 552.1 ± 2.8 to 549.2 ± 2.3 Ma (2σ). Late Ediacaran dykes show calc-alkaline and alkaline affinities, and contain with-in plate components. Rift-related Late Ediacaran magmatism shows that the Yellice arc changed from an arc to a back-arc basin and the rift-deposits formed during the late Ediacaran due to the arc rifting. In conjunction with the data from literature, I suggest that the rift-related magmatism is related to the rifting event during the late Ediacaran, leading to the detachment of this continental block from the West Gondwana-land, thus to the opening of the Rheic Ocean, and Early Cambrian deformation event correspond to the adding of the Kraishte terrane with Far East Avalonia. The docking of the Kraishte terrane led to the deformation of Late Ediacaran dykes. Overall, these data indicate that the depositional time of sedimentary rocks in the continental fragment should have started during the late Ediacaran instead of Ordovician.

U–Pb zircon geochronology and geochemistry of mafic and intermediate dykes in the İstanbul–Zonguldak Tectonic Unit (NW Turkey): evidence for Devonian and Carboniferous magmatism and the north-subducting Rheic Ocean in Far East Avalonia

U–Pb zircon geochronology and geochemistry of mafic and intermediate dykes in the İstanbul–Zonguldak Tectonic Unit (NW Turkey): evidence for Devonian and Carboniferous magmatism and the north-subducting Rheic Ocean in Far East Avalonia

Tonian–Ediacaran tectonomagmatic evolution of West Avalonia and its Ediacaran–early Cambrian interactions with Ganderia: an example of complex terrane transfer due to arc–arc collision?

Abstract The Neoproterozoic tectonomagmatic evolution of West Avalonia comprises four major events. Tectonism started with the formation of a Tonian passive margin on a Baltica-derived ribbon dispersed into the Mirovoi Ocean. Obduction of an oceanic terrane onto the ribbon produced olistostromes, deformation and metamorphism before 750 Ma. Obduction was followed by a Tonian (750–730 Ma) arc on the created composite crust. A pause in magmatism between 730 and 700 Ma is the next event. Subsequently, a Cyrogenian (700–670 Ma) arc was formed, which may have collided with Baltica or another buoyant element nearby. Thereafter, a long-lasting (640–565 Ma) continental arc was erected which, combined with the late Ediacaran–Early Paleozoic sedimentary cover, represents the hallmark of West Avalonia. A Caribbean-style incursion of the Ediacaran arc into the widening Tornquist gap between Amazonia and Baltica led to a diachronous collision with the Ganderian arc. Strike-slip slivering produced a complex transfer of terranes to both: Carolinia and smaller terranes to Ganderia, and East Avalonia to West Avalonia. The Rheic Ocean opened diachronously at c. 500 Ma, following a plate reorganization and re-establishment of an oblique subduction zone beneath Amazonia. As a result, Avalonia and Ganderia became progressively separated and dispersed into the Iapetus Ocean.

Baltica and the Cadomian orogen in the Ediacaran\u2013Cambrian: a perspective from SE Poland

In the supercontinent of Rodinia, Baltica occurred next to Amazonia, then the two drifted away when Rodinia broke up. By the end of the Neoproterozoic, Baltica became an independent continent. At that time, Timanide orogen developed at its modern northeastern margin. In most paleogeographical reconstructions, the opposite (SW, Tornquist) edge faced the Tornquist Ocean and remained just a passive margin till the arrival of the Gondwana-born East Avalonia in the late Ordovician. However, preliminary isotopic studies of detrital zircons from the Tornquist passive margin succession hinted that rock components of Gondwana derivation reached Baltica already in the early Cambrian. In this paper, we examine 18 drill-cores of Ediacaran-Cambrian and Ordovician siliciclastic rocks from the tectonostratigraphic units along the SW–NE transect from Upper Silesia (USB) via Małopolska (MB) and the Holy Cross Mts (HCM) to the East European Platform (EEP), SE Poland, in terms of the provenance data gained from the LA-ICP-MS and SHRIMP analyses of 32 zircon samples. Rocks from all the units revealed abundant Cadomian 0.7–0.55 Ga detrital zircons (15–50% of the total analyzed grains) and other grains that yielded peaks at 0.9–1.2, 1.4–1.6, 1.8–2.2, 2.7–3.0 Ga assignable to Baltica rather than Amazonia. Such age spectra in the USB, HCM and EEP prove the proximity of peripheral (peri-Gondwanan) fragments of the Cadomian orogen to Baltica. These fragments formed the Teissyere-Tornquist Terrane Assemblage (TTA) that obliquely docked and overrode the thinned southwestern edge of Baltica which earlier accumulated Neoproterozoic rift and passive margin deposits. Our data show that in the late Ediacaran-early Cambrian, parts of the Cadomian orogenic belt became accreted to Baltica.

Detrital zircon characterization of early Cambrian sandstones from East Avalonia and SE Ireland: implications for terrane affinities in the peri-Gondwanan Caledonides

AbstractThe Caledonides of Britain and Ireland include terranes attributed to both Laurentian and Gondwanan sources, separated along the Solway line. Gondwanan elements to the south have been variably assigned to the domains Ganderia and East Avalonia. The Midland Platform forms the core of East Avalonia but its provenance is poorly known. Laser ablation split-stream analysis yields information about detrital zircon provenance by providing simultaneous U–Pb and Lu–Hf data from the same ablated volume. A sample of Red Callavia Sandstone from uppermost Cambrian Stage 3 of the Midland Platform yields a U–Pb age spectrum dominated by Neoproterozoic and Palaeoproterozoic sources, resembling those in the Welsh Basin, the Meguma Terrane of Nova Scotia and NW Africa. Initial εHf values suggest that the Neoproterozoic zircon component was derived mainly from crustal sources < 2 Ga, and imply that the more evolved Palaeoproterozoic grains were transported into the basin from an older source terrane, probably the Eburnean Orogen of West Africa. A sample from Cambrian Stage 4 in the Bray Group of the Leinster–Lakesman Terrane shows, in contrast, a distribution of both U–Pb ages and εHf values closely similar to those of the Gander Terrane in Newfoundland and other terranes attributed to Ganderia, interpreted to be derived from the margin of Amazonia. East Avalonia is clearly distinct from Ganderia, but shows evidence for older crustal components not present in West Avalonia of Newfoundland. These three components of the Appalachian–Caledonide Orogen came from distinct sources on the margin of Cambrian Gondwana.

Early Carboniferous extension in East Avalonia: 350 My record of lithospheric memory

Abstract Despite reactivations during Variscan and Alpine orogenies and the opening of the Northern Atlantic, Avalonia is one of the few regions where the initial Mid-Paleozoic basin structure is still recognisable, largely intact, and well studied. Its kinematics and dynamics remain, however, largely unknown, in particular in view of the successive late Paleozoic to recent evolution of the basin. Consequently, the importance of the mid-Palaeozoic tectonic evolution and structural controls are often overlooked and poorly understood in basin studies. In this paper, we reassess the importance of Mid-Palaeozoic tectonics on subsequent sedimentary basin evolution of north-western Europe. To this end, we analyse the dynamics of early Variscan extension in Avalonia based on the integration and re-evaluation of available geophysical and geological data from lithosphere to basin scales. Based on a revised crustal map of the Thor suture zone, we present a new paleo-tectonic reconstruction and tectonic scenario for the Devonian-Carboniferous rifting. These findings are key for a better understanding of long-lived tectonic segmentation and post-rifting deformation phases. Our findings indicate that the structural grain of many crustal fault dominated sedimentary basin structures such as the North Sea Central Graben were created in the early Carboniferous. Consequently, the main basement structuration of northwest Europe was completed before the Variscan orogeny and successive post-Variscan extension and inversion phases reactivated the existing basement structures without creating major new fault groups. Incorporation of the Paleozoic structural grain allows for a consistent tectonic framework for the Mesozoic, contributing to fundamental understanding of basin evolution. From our tectonic framework analysis, Avalonia therefore stands out as a fine example of long lived lithosphere memory, spanning over 350 My of structural control in geodynamic evolution.

First U–Pb geochronology on detrital zircons from Early-Middle Cambrian strata of the Torgau-Doberlug Syncline (eastern Germany) and palaeogeographic implications

LA-ICP-MS U–Pb data from detrital zircons of the Ediacaran to Cambrian siliciclastic sequence of the Torgau-Doberlug Syncline (TDS, Saxo-Thuringia, Germany) are reported for the first time. The majority of 203 analysed zircon grains is Proterozoic with minor amount of Archean and Palaeozoic grains. The U–Pb ages fall into three groups: 2.8–2.4 Ga (3%), Neoarchean to earliest Palaeoproterozoic; 2.3–1.6 Ga (46%), early to late Palaeoproterozoic; 1.0–0.5 Ga (47%), Neoproterozoic to Cambrian. This age distribution is typical for the West African Craton as the source area and for Cadomian orogenic events in northwestern Gondwana. The samples show an age gap between 1.6 and 1.0 Ga, which is characteristic for West African provenance and diagnostic in distinguishing this unit from East Avalonia and Baltica. The dataset shows clusters of Palaeoproterozoic ages at 2.2–1.7 Ga, that is typical for western Gondwana, which was affected by abundant magmatic intrusions (ca. 2.2–1.8 Ga) during the Eburnean orogeny (West African craton). Neoarchean zircon ages (3%) point to recycling of magmatic rocks formed during the Liberian and Leonian orogenies. Ediacaran to earliest Cambrian rocks of the TDS originated in an active margin regime of the Gondwanan shelf. The following early Palaeozoic overstep sequence was deposited within rift settings that reflects instability of the West-Gondwanan shelf and the separation of terranes from Ordovician onward. The results of this study demonstrate distinct northwestern African provenance of the Cambrian siliciclastics of the TDS. Due to Th–U ratios from concordant zircon analysis, igneous origin from felsic melts is concluded as the source of these grains.

The lithosphere–asthenosphere system beneath Ireland from integrated geophysical–petrological modeling II: 3D thermal and compositional structure

The lithosphere–asthenosphere boundary (LAB) depth represents a fundamental parameter in any quantitative lithospheric model, controlling to a large extent the temperature distribution within the crust and the uppermost mantle. The tectonic history of Ireland includes early Paleozoic closure of the Iapetus Ocean across the Iapetus Suture Zone (ISZ), and in northeastern Ireland late Paleozoic to early Mesozoic crustal extension, during which thick Permo-Triassic sedimentary successions were deposited, followed by early Cenozoic extrusion of large scale flood basalts. Although the crustal structure in Ireland and neighboring offshore areas is fairly well constrained, with the notable exception of the crust beneath Northern Ireland, the Irish uppermost mantle remains to date relatively unknown. In particular, the nature and extent of a hypothetical interaction between a putative proto Icelandic mantle plume and the Irish and Scottish lithosphere during the Tertiary opening of the North Atlantic has long been discussed in the literature with diverging conclusions. In this work, the present-day thermal and compositional structure of the lithosphere in Ireland is modeled based on a geophysical–petrological approach (LitMod3D) that combines comprehensively a large variety of data (namely elevation, surface heat flow, potential fields, xenoliths and seismic tomography models), reducing the inherent uncertainties and trade-offs associated with classical modeling of those individual data sets. The preferred 3D lithospheric models show moderate lateral density variations in Ireland characterized by a slightly thickened lithosphere along the SW-NE trending ISZ, and a progressive lithospheric thinning from southern Ireland towards the north. The mantle composition in the southern half of Ireland (East Avalonia) is relatively and uniformly fertile (i.e., typical Phanerozoic mantle), whereas the lithospheric composition in the northern half of Ireland (Laurentia) seems to vary from moderately depleted to fertile, in agreement with mantle xenoliths erupted in northwestern Ireland.

Origin and evolution of Avalonia: evidence from U–Pb and Lu–Hf isotopes in zircon from the Mira terrane, Canada, and the Stavelot–Venn Massif, Belgium

Laser ablation inductively coupled plasma mass spectrometry U–Pb and Lu–Hf isotope analyses of detrital zircon from Neoproterozoic–Cambrian clastic sedimentary rocks in the Mira terrane (Cape Breton Island, Nova Scotia, Canada; West Avalonia) and the Stavelot–Venn Massif (East Belgium; East Avalonia) support deposition on an originally coherent microcontinent. Crustal evolution trends defined by ϵHf (t) values varying with age reflect juvenile magma production in the source continent at 1.2–2.2 and 2.4–3.1 Ga. Mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0.5–0.72, 1.4–1.7, 1.8–2.2 and 2.4–2.7 Ga. These results concur with the crustal evolution in Amazonia, the likely parent craton. Crustal evolution in Avalonia is recorded in detrital and magmatic zircon from Neoproterozoic arcs (680–550 Ma). Positive ϵHf (t) values suggest juvenile input and mixing with recycled crust. Most negative ϵHf (t) values represent recycling of predominantly Mesoproterozoic underlying crust. Avalonian arc magmatism was followed by late Neoproterozoic–early Cambrian sedimentation in various belts in West Avalonia. These belts were juxtaposed by strike-slip during late early Cambrian deposition in a rift basin. The youngest detrital zircon population ( c . 517 Ma) probably represents synrift magmatism before break-up of Avalonia. Migmatization at 406 ± 2 Ma in a xenolith from the East Avalonian crust reflects post-collisional heating. Supplementary materials: Details of sample locations and analytical results are available at www.geolsoc.org.uk/SUP18641 .

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.

Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections

Abstract Within the Appalachian–Variscan orogen of North America and southern Europe lie a collection of terranes that were distributed along the northern margin of West Gondwana in the late Neoproterozoic and early Palaeozoic. These peri-Gondwanan terranes are characterized by voluminous late Neoproterozoic ( c . 640–570 Ma) arc magmatism and cogenetic basins, and their tectonothermal histories provide fundamental constraints on the palaeogeography of this margin and on palaeocontinental reconstructions for this important period in Earth history. Field and geochemical studies indicate that arc magmatism generally terminated diachronously with the formation of a transform margin, leading by the Early–Middle Cambrian to the development of a shallow-marine platform–passive margin characterized by Gondwanan fauna. However, important differences exist between these terranes that constrain their relative palaeogeography in the late Neoproterozoic and permit changes in the geometry of the margin from the late Neoproterozoic to the Early Cambrian to be reconstructed. On the basis of basement isotopic composition, the terranes can be subdivided into: (1) Avalonian-type (e.g. West Avalonia, East Avalonia, Meguma, Carolinia, Moravia–Silesia), which developed on juvenile, c . 1.3–1.0 Ga crust originating within the Panthalassa-like Mirovoi Ocean surrounding Rodinia, and which were accreted to the northern Gondwanan margin by c . 650 Ma; (2) Cadomian-type (e.g. North Armorican Massif, Ossa–Morena, Saxo-Thuringia, Moldanubia), which formed along the West African margin by recycling ancient ( c . 2.0–2.2 Ga) West African crust; (3) Ganderian-type (e.g. Ganderia, Florida, the Maya terrane and possible the NW Iberian domain and South Armorican Massif), which formed along the Amazonian margin of Gondwana by recycling Avalonian and older Amazonian basement; and (4) cratonic terranes (e.g. Oaxaquia and the Chortis block), which represent displaced Amazonian portions of cratonic Gondwana. These contrasts imply the existence of fundamental sutures between these terranes prior to c . 650 Ma. Derivation of the Cadomian-type terranes from the West African craton is further supported by detrital zircon data from their Neoproterozoic–Ediacaran clastic rocks, which contrast with such data from the Avalonian- and Ganderian-type terranes that suggest derivation from the Amazonian craton. Differences in Neoproterozoic and Ediacaran palaeogeography are also matched in some terranes by contrasts in Cambrian faunal and sedimentary provenance data. Platformal assemblages in certain Avalonian-type terranes (e.g. West Avalonia and East Avalonia) have cool-water, high-latitude fauna and detrital zircon signatures consistent with proximity to the Amazonian craton. Conversely, platformal assemblages in certain Cadomian-type terranes (e.g. North Armorican Massif, Ossa–Morena) show a transition from tropical to temperate waters and detrital zircon signatures that suggest continuing proximity to the West African craton. Other terranes (e.g. NW Iberian domain, Meguma) show Avalonian-type basement and/or detrital zircon signatures in the Neoproterozoic, but develop Cadomian-type signatures in the Cambrian. This change suggests tectonic slivering and lateral transport of terranes along the northern margin of West Gondwana consistent with the transform termination of arc magmatism. In the early Palaeozoic, several peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Ganderia, Meguma) separated from West Gondwana, either separately or together, and had accreted to Laurentia by the Silurian–Devonian. Others (e.g. Cadomian-type terranes, Florida, Maya terrane, Oaxaquia, Chortis block) remained attached to Gondwana and were transferred to Laurussia only with the closure of the Rheic Ocean in the late Palaeozoic.

Development of Trans-European Suture Zone in Poland: from Ediacaran rifting to Early Palaeozoic accretion

This contribution summarizes selected results of the Accretion of Poland Project. Emphasis is placed on geochronological, geochemical and palaeomagnetic constraints on the Late Neoproterozoic to Early Palaeozoic development of the Trans-European Suture Zone (TESZ). During the Late Neoproterozoic break-up of Rodinia, a major rift developed in the area of the future TESZ along which Baltica was separated from peri-Gondwana and Laurentia, resulting in opening of the Tornquist Ocean and development of the southwestern Baltica passive margin. This was paralleled by the development of the Cadomian orogenic system along the margin of Gondwana and the eastern and southern margins of Baltica. Some tectonic units involved in the TESZ, such as the Brunovistulian Terrane and the Malopolska Massif characterized by Cadomian basement, were derived from the internal and external parts of the Cadomian Orogen, presumably somewhere at the SE or SW corners of Baltica. Determination of areas where these terrains were originally located depends strongly on the Ediacaran plate model that is adopted for Baltica. The Malopolska Massif was re-accreted to Baltica, presumably due to latest Ediacaran strike-slip tectonics, during the late Middle to Late Cambrian, causing at that time an interruption of its passive margin subsidence pattern and minor erosion. During Late Ordovician to Silurian times, the Caledonian collision of Gondwana-derived East Avalonia Terrane with Baltica gave rise to the development of a foredeep basin along the southwestern margin of Baltica. The proximal part of this foredeep corresponds to the Pomeranian region to the Koszalin-Chojnice Zone, and its distal parts to the Baltic Basin, both of which developed on Baltica basement. During Ordovician and Silurian times clastics were shed into the Koszalin-Chojnice Zone and the Baltic Basin from the evolving Caledonian orogenic wedge, consisting of a subduction-related volcanic arc, obducted ophiolites and accretionary prism, as well as crustal units that were detached from basement of Baltica and Avalonia. The Brunovistulian Terrane was accreted to the Malopolska Massif at the turn from the Silurian to the Devonian. Proximal terranes, such as the Pomerania and Łysogory units remained after Late Neoproterozoic rifting in a position close to the relatively mobile SW margins of Baltica.

The oldest rocks of Greece: first evidence for a Precambrian terrane within the Pelagonian Zone

The Pelagonian Zone in Greece represents the westernmost belt of the Hellenide hinterland (Internal Hellenides). Previous geochronological studies of basement rocks from the Pelagonian Zone have systematically yielded Permo-Carboniferous ages. In this study we demonstrate, for the first time, the existence of a Precambrian crustal unit within the crystalline basement of the Pelagonian Zone. The U–Pb single-zircon and SHRIMP ages of these orthogneisses vary from 699 ± 7 Ma to 713 ± 18 Ma, which identify them as the oldest rocks in Greece. These Late Proterozoic rocks, which today occupy an area of c. 20 × 100 km, are significantly different from the neighbouring rocks of the Pelagonian Zone. They are therefore interpreted as delineating a terrane, named here the Florina Terrane. During the Permo-Carboniferous, Florina was incorporated into an active continental margin, where it formed part of the basement for the Pelagonian magmatic arc. The activity of this arc was dated in this study by single-zircon Pb/Pb ages as having taken place at 292 ± 5 Ma and 298 ± 7 Ma. During the Alpine orogeny, Florina, together with the Pelagonian Zone, eventually became a constituent of the Hellenides. Geochemically, the Florina orthogneisses represent granites formed at an active continental margin. Because of the Late Proterozoic ages, this Late Proterozoic active continental margin can be correlated to a Pan-African or Cadomian arc. As the gneisses contain inherited zircons of Late to Middle Proterozoic age, the original location of Florina was probably at the northwestern margin of Gondwana. Similar to other Gondwana-derived terranes, such as East Avalonia, Florina approached the southern margin of Eurasia during Palaeozoic times, where it became part of an active continental margin above the subducting Palaeotethys. These interpretations further indicate that terrane accretion was already playing an important role in the early pre-alpine evolution of the Hellenides.

Neoproterozoic?Early Paleozoic evolution of peri-Gondwanan terranes: implications for Laurentia-Gondwana connections

Neoproterozoic tectonics is dominated by the amalgamation of the supercontinent Rodinia at ca. 1.0 Ga, its breakup at ca. 0.75 Ga, and the collision between East and West Gondwana between 0.6 and 0.5 Ga. The principal stages in this evolution are recorded by terranes along the northern margin of West Gondwana (Amazonia and West Africa), which continuously faced open oceans during the Neoproterozoic. Two types of these so-called peri-Gondwanan terranes were distributed along this margin in the late Neoproterozoic: (1) Avalonian-type terranes (e.g. West Avalonia, East Avalonia, Carolina, Moravia-Silesia, Oaxaquia, Chortis block that originated from ca. 1.3 to 1.0 Ga juvenile crust within the Panthalassa-type ocean surrounding Rodinia and were accreted to the northern Gondwanan margin by 650 Ma, and (2) Cadomian-type terranes (North Armorica, Saxo-Thuringia, Moldanubia, and fringing terranes South Armorica, Ossa Morena and Tepla-Barrandian) formed along the West African margin by recycling ancient (2–3 Ga) West African crust. Subsequently detached from Gondwana, these terranes are now located within the Appalachian, Caledonide and Variscan orogens of North America and western Europe. Inferred relationships between these peri-Gondwanan terranes and the northern Gondwanan margin can be compared with paleomagnetically constrained movements interpreted for the Amazonian and West African cratons for the interval ca. 800–500 Ma. Since Amazonia is paleomagnetically unconstrained during this interval, in most tectonic syntheses its location is inferred from an interpreted connection with Laurentia. Hence, such an analysis has implications for Laurentia-Gondwana connections and for high latitude versus low latitude models for Laurentia in the interval ca. 615–570 Ma. In the high latitude model, Laurentia-Amazonia would have drifted rapidly south during this interval, and subduction along its leading edge would provide a geodynamic explanation for the voluminous magmatism evident in Neoproterozoic terranes, in a manner analogous to the Mesozoic-Cenozoic westward drift of North America and South America and subduction-related magmatism along the eastern margin of the Pacific ocean. On the other hand, if Laurentia-Amazonia remained at low latitudes during this interval, the most likely explanation for late Neoproterozoic peri-Gondwanan magmatism is the re-establishment of subduction zones following terrane accretion at ca. 650 Ma. Available paleomagnetic data for both West and East Avalonia show systematically lower paleolatitudes than predicted by these analyses, implying that more paleomagnetic data are required to document the movement histories of Laurentia, West Gondwana and the peri-Gondwanan terranes, and test the connections between them.

Tethyan, Mediterranean, and Pacific analogues for the Neoproterozoic–Paleozoic birth and development of peri-Gondwanan terranes and their transfer to Laurentia and Laurussia

Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) ∼1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or ∼2 Ga (Cadomia) basement; (2) 750–600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic–Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician–Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an “accordion” model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a “bulldozer” model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a “Baja” model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge–trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a “Baja” model to a “bulldozer” model. By analogy with the eastern Pacific, we name the oceanic plates off northern Gondwana: Merlin (≡Farallon), Morgana (≡Pacific), and Mordred (≡Kula). If Neoproterozoic subduction was towards Gondwana, application of this combined model requires a total rotation of East Avalonia and Carolina through 180° either during separation (using a western Transverse Ranges model), during accretion (using a Baja British Columbia “train wreck” model), or during dispersion (using an Australia “bulldozer” model). On the other hand, Siluro-Devonian orthogonal transfer (“accordion” model) from northern Africa to southern Laurussia followed by a Carboniferous “Baja” model appears to best fit the existing data for Cadomia. Finally, Oaxaquia, Chortis, Maya, and Suwannee appear to have been transported along the margin of Gondwana until it collided with southern Laurentia on whose margin they were stranded following the breakup of Pangea. Forward modeling of a closing Mediterranean followed by breakup on the African margin may provide a modern analogue. These actualistic models differ in their dictates on the initial distribution of the peri-Gondwanan terranes and can be tested by comparing features of the modern analogues with their ancient tectonic counterparts.