Brunovistulian Terrane Research Articles

Mid-Ediacaran bimodal magmatism and peri-Baltic affinity of the Brunovistulian terrane documented by the U-Pb isotope and palaeomagnetic data from the Brno Massif (Central Europe)

Palaeomagnetic and isotope (U-Pb, 40Ar-Ar39) studies were conducted in the area of the Brno Massif (Central Europe). The basaltic and granitic dykes, so far regarded as Silurian, were sampled in four localities. New U-Pb isotope ages and palaeomagnetic data indicate that the studied magmatic intrusions, most probably of bimodal type, penetrated the Brno Massif in the mid-Ediacaran (ca 575-565 Ma ago). Palaeomagnetic data support the thesis that the Brunovistulian terrane, a larger tectonostratigraphic unit containing the Brno Massif, was incorporated into the peri-Baltic part of the Cadomian orogenic belt and, at least since the mid-Ediacaran, had a common drift history with Baltica. It crossed the polar region of the southern hemisphere between 580 and 560 Ma. The Brno Massif rotated clockwise by 50 – 60o with respect to stable Europe in the Carboniferous, most probably during the oroclinal bending of the north-eastern segment of the Variscan orogenic belt.

Provenance of Precambrian basement of the Brunovistulian Terrane: New data from its Silesian part (Czech Republic, Poland), central Europe, and implications for Gondwana break-up

Brunovistulia is a complex superterrane in central Europe, the western part of which became tectonically involved in the Variscan belt and formed its Precambrian basement. The Silesian part of the Brunovistulian Terrane embraces metasediments and metavolcanic rocks whose protolith ages, provenance, geotectonic setting and pre-Variscan palaeogeography are still poorly known. Our LA-ICP-MS U-Pb zircon data, supported by geochemical studies of these rocks, show that paragneisses and quartzites in the Desná and Keprník domes stemmed from sedimentary basin(s) whose the maximum depositional age is c. 560–570 Ma. Detritus was mainly supplied from Cryogenian-Ediacaran source areas. Precambrian mica schists of the inner Velké Vrbno Dome had mainly Mesoproterozoic detrital component. The accompanied metavolcanics rocks mainly originated as subalkaline E-MORB-like basaltic effusives at c. 610–595 Ma that evolved in the within-plate environments of the retro-arc at the rear of the Ediacaran magmatic arc built of granitoids which were formed at 640–550 Ma. Because the detrital zircon age spectra indicate contributions from the sources bearing 2.2–1.1 Ga components, it is expected that the sedimentation and volcanism presumably occurred at the Amazonian part of the Gondwana mainland. A compilation of new and the existing data further indicates the complex structure of Brunovistulia. The Silesian and Moravian domains of the Brunovistulian superterrane differ in terms of lithology, age of magmatic rock units and isotopic characteristics. The two domains may have developed as different fragments of the same magmatic arc system at the Amazonian margin of peri-Gondwana which evolved for c. 100 Myr. Besides, our new data revealed an early Devonian (408 Ma) onset of volcanism in the westernmost margin of the continental Brunovistulian margin, thus c. 30–40 Ma earlier than the well-known late Devonian volcanism in the Vrbno Group further east.

Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif

The Velké Vrbno Dome crops out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19–0.82) combined with moderately positive Nd isotopic compositions (εNd (i) = +3.89 – +5.77) is used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios ( 87 Sr/ 86 Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism. U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement ( c. 535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasized by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic ( c. 2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto . The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract. Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement D) are available at https://doi.org/10.6084/m9.figshare.c.5233079

The Moldanubian Thrust Zone — A terrane boundary in the Central European Variscides refined based on lithostratigraphy and U–Pb zircon geochronology

The zircon age populations of metavolcano-sedimentary successions in the Orlica–Śnieżnik Dome (OSD), Staré Město Belt (SMB) and Velké Vrbno Dome, the Sudetes (Poland and Czech Republic), have been used to refine the location of the Moldanubian Thrust Zone (MTZ), which is a boundary between the Saxothuringian Terrane of Gondwana descent and the Brunovistulian Terrane being a promontory of Laurussia. In the northern continuation of the MTZ, a set of multiply activated, regional-scale thrusts developed and brought into contact rocks of different ages and geological histories. Metarhyolites in the Orlica–Śnieżnik Dome and the Staré Město Belt have similar geochemistry and U–Pb isotopic zircon records, which is taken in favour of their coeval formation and common Saxothuringian affinity. Felsic metavolcanic rocks from the OSD and from the thrust-bounded upper and lower units of the SMB yielded protolith ages of 500±3Ma and 493±4Ma to 498±5Ma, respectively, which indicates that metavolcano-sedimentary successions in the OSD and SMB were deposited in Late Cambrian times. Structurally below these rocks, there are the highly sheared Brousek quartzites with detrital zircons that yielded a maximum depositional age of ~530Ma. The mylonitic quartzites accommodate deformation induced by tectonic transport on the East Nýznerov Thrust, which is interpreted as the easternmost margin of Saxothuringia. This fault separates Palaeozoic rocks of the Saxothuringian Terrane from Neoproterozoic bimodal volcanogenic succession in the Velké Vrbno Dome of Brunovistulia, dated at ~558Ma. The late-Variscan thermal events in the Early Carboniferous left imprints in the form of U-rich rims around the zircons of the metavolcanic rocks from the Staré Město Belt and only a very minor overprint in the zircons from the Velké Vrbno Dome and Orlica–Śnieżnik Dome.

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.

New insights into the polyphase evolution of the Variscan suture zone: evidence from the Staré Město Belt, NE Bohemian Massif

AbstractForming a northern continuation of the Moldanubian Thrust Zone, the Staré Město Belt comprises an E-verging thrust stack of three narrow lithotectonic units that exhibit variations in their respective P–T records. The upper and lower units form the respective margins of the hanging wall and footwall of the suture zone and are dominated by amphibolite grade metasedimentary successions. The middle unit is defined by an elongated body of MORB-like amphibolites that contains inserts of migmatized mica schists. Integrating both structural studies and pseudosection modelling in the MnNCKFMASH system shows that the present-day tectonic architecture of the Staré Město Belt is the result of a polyphase Variscan evolution. During a frontal, WNW–ESE-directed (in present-day coordinates) collision between the Bohemian Massif terranes and the Brunovistulian terrane, the metasedimentary rocks of the Staré Město Belt experienced tectonic burial to depths corresponding to 7–9 kbar. The continuous indentation and underthrusting of the Brunovistulian terrane led to top-to-the-ESE folding and uplift of these rocks to depths corresponding to 5.5–6.0 kbar at peak temperature. At depths corresponding to 5.5 kbar, the Staré Město Belt underwent subsequent dextral (top-to-the-NNE) shearing that was locally associated with nearly isobaric heating, possibly related to the emplacement of a Carboniferous tonalite body in the axial part of the Staré Město Belt. Subsequent tectonic compression resulted in the Variscan WNW-dipping metamorphic foliations becoming locally (N)NE- or ESE-dipping.

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

Structural, petrological and geochronological data from marbles and mica schists combined with those from highly disputable gneisses and eclogites of the Orlica-Śnieznik Dome (OSD), gave new insights into Variscan evolution of eastern borderland of the West Sudetes. It is shown that the Variscan tectonometamorphic evolution of the OSD began with E–W oriented subhorizontal shortening (D1 stage) related to the collision of the West Sudetes terranes and the Brunovistulian terrane. The shortening led to generally upright folding, which resulted in formation of the steep N–S trending metamorphic planar fabric S1, thickening of the lithosphere and burial of the Stronie Formation under greenschist facies conditions. As a consequence of subsequent, near-coaxial gravity-controlled vertical shortening (D2), the S1 foliation was deformed in tight recumbent folds F2. The flattening strain was associated with the progression to amphibolite-facies conditions (from ca. 510 upto ca. 620°C) and uplift from depths corresponding to 9–10 kbar to depths corresponding to 7–8 kbar. On microscale, the flattening strain is documented by rotation of the mineral fabric overgrown by syn-D2 prograde garnet porphyroblasts. The Sm–Nd Grt–WR isochron age, correlated with the D2 event yields 346.5 ± 4.4 Ma. Further deformation and progressive metamorphism led to development of the subhorizontally disposed S2 axial plane schistosity, which terminated at the metamorphic temperature peak. During retrogression but still under ductile conditions of deformation, the S2 planes were reactivated during successive top-to-the-N shear movement of the OSD (D3 stage). Due to the mutual interaction of the OSD with the adjacent terranes the shear deformations were localized within marginal parts of these units. Finally, as a result of the NE–SW and the NW–SE oriented shortenings (D4 stage and D5 stage, respectively), both the structural surfaces and metamorphic isograds were regionally folded with W(NW)-ward plunges. A synthesis of new and existing data shows an overall similarity in sequence of Variscan deformations (D1–D2–D3) in the West Sudetes borderland to that observed in the eastern flank of the Moldanubian domain.

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

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.

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.

Early Cambrian location and affinities of the Brunovistulian terrane (Central Europe) in the light of palaeomagnetic data

The Brunovistulian was one of the first tectonic units of Central Europe to be defined as a ‘terrane’. In spite of extensive studies, the Early Palaeozoic palaeogeographical position and provenance of this unit remains unclear. A palaeomagnetic study of the Lower Cambrian red beds and a study of the trilobite fauna were performed to provide constraints on the palaeogeographical position of the terrane in Early Cambrian time. Good quality palaeomagnetic data obtained from the Lower Cambrian red beds suggest the mixed nature of the Early Cambrian geomagnetic field and a nearly equatorial position (a palaeolatitude of c . 7°) of the Brunovistulian terrane in the Early Cambrian. Comparison of this palaeolatitude with existing palaeogeographical models leads to the conclusion that at this time the Brunovistulian terrane was separated by a great distance from the Avalonian margin of Gondwana. The terrane was located within the Cadomian belt, occupying a position north of the present-day northern margin of Africa. It was coupled to the present-day southern margin of Baltica during the Cambrian, when Baltica moved along the Cadomian margin of Gondwana. Another possibility is that the Brunovistulian terrane could have remained near this margin of Baltica since Grenvillian time and was incorporated into the Cadomian orogen.

Collage tectonics in the northeasternmost part of the Variscan Belt: the Sudetes, Bohemian Massif

Abstract A synthesis of published and new data is used to interpret the Sudetic segment of the Variscan belt as having formed by the accretion of four major and two or three minor terranes. From west to east the major terranes are (1) Lusatia-Izera Terrane, exposing Armorican continental basement reworked by Ordovician plutonism and Late Devonian-Carboniferous collision, showing Saxothuringian affinities; (2) composite Góry Sowie-Kłodzko Terrane characterized by multistage evolution (Silurian subduction, mid- to late Devonian collision, exhumation and extension, Carboniferous deformational overprint), with analogues elsewhere in the Bohemian Massif, Massif Central and Armorica; (3) Moldanubian (Gföhl) Terrane comprising the Orlica-Śnieżnik and Kamieniec massifs, affected by Early Carboniferous high-grade metamorphism and exhumation and (4) Brunovistulian Terrane in the East Sudetes, set up on Avalonian crust and affected by Devonian to late Carboniferous sedimentation, magmatism and tectonism. The main terranes are separated by two smaller ones squeezed along their boundaries: (1) Moravian Terrane, between the Moldanubian and Brunovistulian, deformed during Early Carboniferous collision, and (2) SE Karkonosze Terrane of affinities to the Saxothuringian oceanic realm, sandwiched betwen the Lusatia-Izera and Góry Sowie-Kłodzko (together with Teplá-Barrandian) terranes, subjected to high pressure-metamorphism and tectonized during Late Devonian-Early Carboniferous convergence. The Kaczawa Terrane in the NW, of oceanic accretionary prism features, metamorphosed and deformed during latest Devonian-Early Carboniferous times, may either be a distinct unit unrelated to closure of the Saxothuringian Ocean or represent a continuation of the SE Karkonosze Terrane.