Emplacement depths and radiometric ages of Paleozoic plutons of the Neukirchen–Kdyn massif: differential uplift and exhumation of Cadomian basement due to Carboniferous orogenic collapse (Bohemian Massif)

The igneous complex of Neukirchen–Kdyn is located in the southwestern part of the Teplá–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the V epadly granodiorite and the Smr ovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smr ovice diorite, suggesting that this pluton has remained at shallow crustal levels (T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the V epadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyn massif (V eruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg ( ert v kámen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.     

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

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

Structure and stratigraphy of the Teplá–Barrandian Neoproterozoic,Bohemian Massif: A new plate-tectonic reinterpretation

The Teplá–Barrandian unit (TBU) of the Bohemian Massif exposes a section across the once extensive Avalonian–Cadomian belt, which bordered the northern active margin of Gondwana during late Neoproterozoic. This paper synthesizes the state-of-the-art knowledge on the Cadomian basement of the TBU to redefine its principal component units, to revise an outdated stratigraphic scheme, and to interpret this scheme in terms of a recent plate-tectonic model for the Cadomian orogeny in the Bohemian Massif. The main emphasis of this paper is on an area between two newly defined fronts of the Variscan pervasive deformation to the NW and SE of the Barrandian Lower Paleozoic overlap successions. This area has escaped the pervasive Variscan (late Devonian to early Carboniferous) ductile reworking and a section through the Cadomian orogen is here superbly preserved.The NW segment of the TBU consists of three juxtaposed allochthonous belts of unknown stratigraphic relation (the Kralovice–Rakovník, Radnice–Kralupy, and Zbiroh–?árka belts), differing in lithology, complex internal strain patterns, and containing sedimentary and tectonic mélanges with blocks of diverse ocean floor (meta-)basalts. We summarize these three belts under a new term the Blovice complex, which we believe represents a part of an accretionary wedge of the Cadomian orogen.The SE segment of the TBU exposes the narrow Pi?ín belt, which is probably a continuation of the Blovice complex from beneath the Barrandian Lower Paleozoic, and a volcanic arc sequence (the Davle Group). Their stratigraphic relation is unknown. Flysch units (the ?těchovice Group and Svrchnice Formation) overlay the arc volcanics, and both units contain material derived from volcanic arc. The former was also sourced from the NW segment, whereas the latter contains an increased amount of passive margin continental material. In contrast to the Blovice complex, the flysch experienced only weak Cadomian deformation.The new lithotectonic zonation fits the following tectonic scenario for the Cadomian evolution of the TBU well. The S- to SE-directed Cadomian subduction beneath the TBU led to the involvement of turbidites, chaotic deposits, and 605 ± 39 Ma ocean floor in the accretionary wedge represented by the Blovice complex. The accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc (the Davle Group) at ~ 620–560 Ma. Upon cessation of the arc igneous activity, the rear of the wedge and some elevated portions of the arc were eroded to supply the deep-water flysch sequences of the ?těchovice Group, whereas the comparable Svrchnice Formation (~ 560 to < 544 Ma) was deposited in a southeasterly remnant basin close to the continental margin. The Cadomian orogeny in the TBU was terminated at ~ 550–540 Ma by slab breakoff, by final attachment of the most outboard ~ 540 Ma oceanic crust, and by intrusion of ~ 544–524 Ma boninite dikes marking the transition from the destructive to transform margin during the early/middle Cambrian.     

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

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

Pre-Variscan geological events in the Austrian part of the Bohemian Massif deduced from U–Pb zircon ages

In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U–Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma (Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567±5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gföhl gneiss was recognised as a 488±6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456±3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440–450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian meta-granitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65–1.8 Ga, indicating a derivation from the South American part of Gondwana.     

New data on the Neoproterozoic – Cambrian geotectonic setting of the Teplá-Barrandian volcano-sedimentary successions: geochemistry,U-Pb zircon ages,and provenance (Bohemian Massif,Czech Republic)

The Teplá-Barrandian unit (TBU) of the Bohemian Massif was a part of the Avalonian-Cadomian belt at the northern margin of Gondwana during Neoproterozoic and Early Cambrian times. New detrital zircon ages and geochemical compositions of Late Neoproterozoic siliciclastic sediments confirm a deposition of the volcano-sedimentary successions of the TBU in a back-arc basin. A change in the geotectonic regime from convergence to transtension was completed by the time of the Precambrian-Cambrian boundary. The accumulation of around 2,500 m Lower Cambrian continental siliciclastics in a Basin-and-Range-type setting was accompanied by magmatism, which shows within-plate features in a few cases, but is predominantly derived from anatectic melts displaying the inherited island arc signature of their Cadomian source rocks. The geochemistry of clastic sediments suggests a deposition in a rift or strike-slip-related basin, respectively. A marine transgression during Middle Cambrian times indicates markedly thinned crust after the Cadomian orogeny. Upper Cambrian magmatism is represented by 1,500 m of subaerial andesites and rhyolites demonstrating several geochemical characteristics of an intra-plate setting. Zircons from a rhyolite give a U-Pb-SHRIMP age of 499±4 Ma. The Cambrian sedimentary and magmatic succession of the TBU records the beginning of an important rifting event at the northern margin of Gondwana.

How far did the Cadomian ʽterranesʼ travel from Gondwana during early Palaeozoic? A critical reappraisal based on detrital zircon geochronology

In this paper, laser ablation ICP-MS U–Pb detrital zircon ages are used to discuss provenance and early Palaeozoic palaeogeography of continental fragments that originated in the Cadomian–Avalonian active margin of Gondwana at the end of Precambrian, were subsequently extended during late Cambrian to Early Ordovician opening of the Rheic Ocean, and finally were incorporated into and reworked within the European Variscan belt. The U–Pb detrital zircon age spectra in the analysed samples, taken across a late Neproterozoic (Ediacaran) to Early/Middle Devonian metasedimentary succession of the southeastern Teplá–Barrandian unit, Bohemian Massif, are almost identical and exhibit a bimodal age distribution with significant peaks at about 2.1–1.9 Ga and 650–550 Ma. We interpret the source area as an active margin comprising a cratonic (Eburnean) hinterland rimmed by Cadomian volcanic arcs and we suggest that this source was available at all times during deposition. The new detrital zircon ages also corroborate the West African provenance of the Teplá–Barrandian and correlative Saxothuringian and Moldanubian units, questioned in some palaeogeographic reconstructions. Finally, at variance with the still popular concept of the Cadomian basement units as far-travelled terranes, we propose that early Palaeozoic basins, developed upon the Cadomian active margin, were always part of a wide Gondwana shelf and drifted northwards together before involvement in the Variscan collisional belt.     

Large P–T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite mélange

The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite mélange that contains blocks of mafic blueschist and high-pressure, granulite facies, metapyroxenite (Sm–Nd metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite facies assemblages corresponding to the high-pressure intermediate-type metamorphic facies series.Adjacent rocks of the serpentinite mélange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite facies), dolerite (actinolite–oligoclase sub-facies) and gabbro (amphibolite facies), all with assemblages that are diagnostic of the low-pressure metamorphic facies series.The difference in metamorphic facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite mélange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite mélange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.     

SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China: Geochronological constraints for the evolution of Paleo-Tethys

SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic mélange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic mélange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of the Jinshajiang belt, a granitoid batholith (268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic mélange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the mélange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.     

A ~565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

International Journal of Earth Sciences - In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565&nbsp;Ma has been detected. Such...     

U–Pb and Sm–Nd dating of high-pressure granulites from Tcholliré and Banyo regions: Evidence for a Pan-African granulite facies metamorphism in north-central Cameroon

U–Pb (TIMS–ID and SIMS) and Sm–Nd analyses of zircons and garnet-whole rock pairs were applied on high-pressure granulite facies metapelites and metagranodiorite from Tcholliré and Banyo regions, respectively in the Adamawa–Yadé and Western Domains of the Central-African Fold Belt (CAFB) of Cameroon. Cathodoluminescence (CL) images of zircons reveal that they are made up of ubiquitous magmatitic xenocrystic cores, surrounded and/or overprinted by light unzoned recrystallized domains. U–Pb data on cores yield ages ranging from Paleoproterozoic to Neoproterozoic, which we consider as dating inheritances. Data on overgrowths and recrystallized domains give ages ranging between 594 and 604 Ma, interpreted as the time of HP granulite-facies metamorphism in the Tcholliré and Banyo regions. This is also supported by ages derived from Sm–Nd garnet-whole rock pairs. Sediments of the Tcholliré region were deposited after ca. 620 Ma from Paleoproterozoic, Mesoproteroszoic and Neoproterozoic protoliths, while those from the Banyo region were deposited after 617.6 ± 7.1 Ma essentially from Neoproterozoic protoliths.     

U–Pb SHRIMP monazite ages of the giant Morro Velho and Cuiabá gold deposits, Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Minas Gerais, Brazil

U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.     

Neoproterozoic (800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc–continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt. Its Neoproterozoic amalgamation history involves early (800 Ma) and late Baikalian (600–550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage — the Gargan microcontinent and the Dunzhugur oceanic arc. They are represented by the Gargan Glyba (Block) and the island-arc ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur arc ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-arc ophiolite succession. The Dunzhugur arc faced the microcontinent, as shown by the occurrence of forearc complexes. The arc–continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the arc, and the forearc ophiolite overrode it. Continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental arc are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb–Sr mineral isochron age is 812±18 Ma, which is similar to a U–Pb zircon age of 785±11 Ma. A period of tonalite magmatism in Meso–Cenozoic orogenic belts is recognized some 1–10 m.y. after the collision. Accordingly, the Dunzhugur island arc–Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern (in modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean.     

Crustal architecture above the high-pressure belt of the Grenville Province in the Manicouagan area: new structural, petrologic and U–Pb age constraints

Recent U–Pb age determinations and P–T estimates allow us to characterize the different levels of a formerly thickened crust, and provide further constraints on the make up and tectono-thermal evolution of the Grenville Province in the Manicouagan area. An important tectonic element, the Manicouagan Imbricate zone (MIZ), consists of mainly 1.65, 1.48 and 1.17 Ga igneous rocks metamorphosed under 1400–1800 MPa and 800–900 °C at 1.05–1.03 Ga, during the Ottawan episode of the Grenvillian orogenic cycle, coevally with intrusion of gabbro dykes in shear zones. The MIZ has been interpreted as representing thermally weakened deep levels of thickened crust extruded towards the NW over a parautochthonous crustal-scale ramp. Mantle-derived melts are considered as in part responsible for the high metamorphic temperatures that were registered.New data show that mid-crustal levels structurally above the MIZ are represented by the Gabriel Complex of the Berthé terrane, that consists of migmatite with boudins of 1136±15 Ma gabbro and rafts of anatectic metapelite with an inherited monazite age at 1478±30 Ma. These rocks were metamorphosed at about the same time as the MIZ (metamorphic zircon in gabbro: 1046±2 Ma; single grains of monazite in anatectic metapelite: 1053±2 Ma) and under the same T range (800–900 °C) but at lower P conditions (1000–1100 MPa). They are mainly exposed in an antiformal culmination above a high-strain zone, which has tectonic lenses of high P–T rocks from the MIZ and is intruded by synmetamorphic gabbroic rocks. This zone is interpreted as part of the hangingwall of the MIZ during extrusion. A gap of 400 MPa in metamorphic pressures between the tectonic lenses and the country rocks, together with the broad similarity in metamorphic ages, are consistent with rapid tectonic transport of the high P–T rocks over a ramp prior to the incorporation of the mafic lenses in the hangingwall.Between the antiformal culmination of the Gabriel Complex and the MIZ 1.48 Ga old granulites of the Hart Jaune terrane are exposed. They are intruded by unmetamorphosed 1228±3 Ma gabbro sills and 1166±1 Ma anorthosite. Hart Jaune Terrane represents relatively high crustal levels that truncate the MIZ-Gabriel Complex contact and are preserved in a synformal structure.Farther south, the Gabriel Complex is overlain by the Banded Complex, a composite unit including 1403+32/−25 Ma granodiorite and 1238+16/−13−1202+40/−25 Ma granite. This unit has been metamorphosed under relatively low-P (800 MPa) granulite-facies conditions. Metamorphic U–Pb data, limited to zircon lower intercept ages (971±38 Ma and 996±27 Ma) and a titanite (990±5 Ma) age, are interpreted to postdate the metamorphic peak.The general configuration of units along the section is consistent with extrusion of the MIZ during shortening and, finally, normal displacement along discrete shear zones.     

The eastern continuation of the Cadomian orogen: U–Pb zircon evidence from Saxo-Thuringian granitoids in south-western Poland and the northern Czech Republic

We report U–Pb single zircon ages from three pre-Variscan granitoids in the NE part of the Bohemian Massif. The Platerówka granodiorite from the Lausitz-Izera Unit, the Polish Sudetes, has been dated at 533±9 Ma. The Bitouchov granite form the SW part of the South Krkonoe Unit, the Czech Sudetes, gave an age of 540+11/–10 Ma, and the Wdroe granodiorite in the Fore-Sudetic Block yielded 548±9 Ma. All these latest Vendian/Early Cambrian granitoids represent the post-tectonic expression of a late Proterozoic Cadomian orogenic cycle and demonstrate the eastward extent of the Cadomian basement into the Variscan orogen. Granodiorites of similar age have so far been reported from Brittany and especially from the Saxo-Thuringian Terrane to the NE and SW of the Elbe Fault Zone. We conclude that the Saxo-Thuringian Terrane extends across the Elbe and Sudetic Marginal Fault Zones into the Fore-Sudetic Block.     

The Cadomian Orogeny in Saxo-Thuringia, Germany: geochemical and Nd–Sr–Pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance

The Cadomian basement and the Cambro-Ordovician overstep sequence in Saxo-Thuringia is characterized by clastic sedimentation from the Late Neoproterozoic to the Ordovician. Magmatism in the Avalonian–Cadomian Arc preserved in Saxo-Thuringia occurred between ca. 570 and 540 Ma. Peri-Gondwanan basin remnants with Cadomian to Early Palaeozoic rocks are exposed as very low-grade metamorphosed rocks in six areas (Schwarzburg Anticline, Berga Anticline, Doberlug Syncline, North Saxon Anticline, Lausitz Anticline, and Elbe Zone). A hiatus in sedimentation between 540 and 530 Ma (Cadomian unconformity) is related to the Cadomian Orogeny. A second gap in sedimentation occurred during the Upper Cambrian (500 to 490 Ma) and is documented by a disconformity between Lower to Middle Cambrian rocks and overlying Tremadocian sediments. Major and trace-element signatures of the Cadomian sediments reflect an active margin (“continental arc”), those of the Ordovician sediments a passive margin. The Cambrian sediments have inherited the arc signature through the input of relatively unaltered Cadomian detritus. Initial Nd and Pb isotope data from the six Saxo-Thuringian areas demonstrate that there is no change in source area with time for each location, but that there are minor contrasts among the locations. (1) Cadomian sediments from the Lausitz Anticline, the Doberlug Syncline and the Elbe Zone have lower ²⁰⁷Pb/²⁰⁴Pb than all other areas. (2) The core of the Schwarzburg Anticline, which is overprinted by greenschist facies conditions and detached, is isotopically heterogeneous. One part of its metasedimentary units has less radiogenic Nd than sediments from other low-grade units of similar age in the same area. (3) Cadomian sediments from the Schwarzburg Anticline show an input of younger felsic crust. (4) The Rothstein Group shows distinct input of young volcanic material. Also, (5) Cadomian sediments from the Lausitz Anticline, the Elbe Zone and parts of the North Saxon Anticline are characterized by input from an old mafic crust. Nd isotope data of the remaining areas yield average crustal residence ages of the sediment source of 1.5–1.9 Ga, which suggests derivation from an old craton as found for other parts of the Iberian–Armorican Terrane Collage. Similarly, the Pb isotope data of all areas indicate sediment provenance from an old craton.The rapid change of lithologies from greywacke to quartzite from the Late Neoproterozoic (Cadomian basement) to the Ordovician does not reflect changes in sediment provenance, but is essentially due to increased reworking of older sediments and old weathering crusts that formed during various hiatus of sedimentation. This change in sediment maturity takes its chemical expression in lower overall trace-element contents in the quartzite (dilution effect by quartz) and relative enrichment of some trace-elements (Zr, MREE, HREE due to detrital zircon and garnet). The Rb–Sr systematics of the quartzites and one Ordovician tuffite was disturbed (most likely during the Variscan Orogeny), which suggests a lithology-controlled mobility of alkali and calc-alkali elements. By comparison with available data, it seems unlikely that only Nd T_DM model ages are useful to distinguish between West African and Amazonian provenance. Nd T_DM model ages of 1.5 to 1.9 Ga in combination with paleobiogeographic aspects, age data from detrital zircon, and palaeogeographic constraints, especially through tillites of the Saharan glaciation in the Hirnantian, strongly indicate a provenance of Saxo-Thuringia from the West African Craton.     

Geochemistry and geochronology of the Neoproterozoic Pan-African Transcaucasian Massif (Republic of Georgia) and implications for island arc evolution of the late Precambrian Arabian–Nubian Shield

The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNd_int = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between  750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia.     

Provenance implications of Th–U–Pb electron microprobe ages from detrital monazite in the Carboniferous Upper Silesia Coal Basin, Poland

This paper reports the results of CHIME (chemical Th–U–Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300–320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments.     

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.     

Neoproterozoic tectonic evolution of the Hongseong area, southwestern Gyeonggi Massif, South Korea; implication for the tectonic evolution of Northeast Asia

Two types of Neoproterozoic metabasites occur together with regionally intruded arc-related Neoproterozoic granitoids (ca. 850–830 Ma) in the Hongseong area, southwestern Gyeonggi Massif, South Korea, which is the extension of the Dabie–Sulu collision belt in China. The first type of metabasite (the Bibong and Baekdong metabasites) is a MORB-like back-arc basin basalt or gabbro formed at ca. 890–860 Ma. The Bibong and Baekdong metabasites may have formed during back-arc opening by diapiric upwelling of deep asthenospheric mantle which was metasomatized by large ion lithophile element (LILE) enriched melt or fluid derived from the subducted slab and/or subducted sediment beneath the arc axis. The second type of metabasite (the Gwangcheon metabasite) formed in a plume-related intra-continental rift setting at 763.5 ± 18.3 Ma and is geochemically similar to oceanic island basalt (OIB). These data indicate a transition in tectonic setting in the Hongseong area from arc to intra-continental rift between ca. 830 and 760 Ma. This transition is well correlated to the Neoproterozoic transition from arc to intra-continental rift tectonic setting at the margin of the Yangtze Craton and corresponds to the amalgamation and breakup of Rodinia Supercontinent.