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state-of-the-art knowledge on the Cadomian basement of the TBU to redefine its principal component units, to revise an outdated stratigraphic scheme, and to interpret this scheme in terms of a recent plate-tectonic model for the Cadomian orogeny in the Bohemian Massif.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.     

Deciphering the Variscan tectonothermal overprint and deformation partitioning in the Cadomian basement of the Teplá–Barrandian unit, Bohemian Massif

The Teplá–Barrandian unit (TBU) has long been considered as a simply bivergent supracrustal ‘median massif’ above the Saxothuringian subduction zone in the Variscan orogenic belt. This contribution reveals a much more complex style of the Variscan tectonometamorphic overprint and resulting architecture of the Neoproterozoic basement of the TBU. For the first time, we describe the crustal-scale NE–SW-trending dextral transpressional Krakovec shear zone (KSZ) that intersects the TBU and thrusts its higher grade northwestern portion severely reworked by Variscan deformation over a southeastern very low grade portion with well-preserved Cadomian structures and only brittle Variscan deformation. The age of movements along the KSZ is inferred as Late Devonian (~380–370?Ma). On the basis of structural, microstructural, and anisotropy of magnetic susceptibility data from the KSZ, we propose a new synthetic model for the deformation partitioning in the Teplá–Barrandian upper crust in response to the Late Devonian to early Carboniferous subduction and underthrusting of the Saxothuringan lithosphere. We conclude that the Saxothuringian/Teplá–Barrandian convergence was nearly frontal during ~380–346?Ma and was partitioned into pure shear dominated domains that accommodated orogen-perpendicular shortening alternating with orogen-parallel high-strain domains that accommodated dextral transpression or bilateral extrusion. The synconvergent shortening of the TBU was terminated by a rapid gravity-driven collapse of the thickened lithosphere at ~346–337?Ma followed by, or partly simultaneous with, dextral strike-slip along the Baltica margin-parallel zones, driven by the westward movement of Gondwana from approximately 345?Ma onwards.     

Structure,emplacement, and tectonic setting of Late Devonian granitoid plutons in the Teplá–Barrandian unit,Bohemian Massif

The Štěnovice and Čistá granodiorite–tonalite plutons are small (~27 and ~38 km², respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Teplá–Barrandian unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated above a subducted slab in a continental margin arc setting. The U–Pb zircon age of the Štěnovice pluton, newly determined at 375 ± 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb–Pb age of 373 ± 1 Ma for the Čistá pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons (CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the Štěnovice and Čistá plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic activity from the ~NW to the ~SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath the Teplá–Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif.     

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.     

Carbon isotope composition of Upper Cambrian to Lower Ordovician carbonate in Korea,and its bearing on the Cambrian–Ordovician boundary and Lower Ordovician paleoceanography

This study presents an example of locating Cambrian–Ordovician boundary in the lower Paleozoic carbonate succession in Korea using carbon isotope stratigraphy. The Yeongweol Unit of the lower Paleozoic Joseon Supergroup comprises the Upper Cambrian Wagok Formation and the Lower Ordovician Mungok Formation in the Cambrian–Ordovician transition interval. Conventionally, the boundary was placed at the lithostratigraphic boundary between the two formations. This study reveals that the boundary is positioned in the basal part of the Mungok Formation based on the carbon isotope stratigraphy coupled with biostratigraphic information of conodont and trilobite faunas. The δ¹³C curve of the Lower Ordovician Mungok Formation shows a similar trend to that of the coeval stratigraphic interval of Argentine Precordillera (Buggisch et al., 2003), suggesting that the δ¹³C curve of the Mungok Formation reflects the Early Ordovician global carbon cycle.     

Sedimentary provenance of Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (Bohemian Massif): U–Pb and Pb–Pb geochronology of detrital zircons by laser ablation ICP-MS

Summary The provenance of the Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (TBU) of the Bohemian Massif was investigated by laser ablation ICP-MS U–Pb zircon dating, bulk sediment geochemistry and mineralogical study of the heavy mineral fraction. In contrast to the island arc provenance of the TBU Neoproterozoic sediments, the Early Palaeozoic sediments contain significant amounts of differentiated crustal material. The detrital zircon populations in the Barrandian Mid-Devonian siltstones and sandstones show ages ranging from Archaean (3.0Ga) to Early Palaeozoic (0.39Ga). Major age maxima are at 2.6Ga, 2.0–2.25Ga, 0.62 and 0.51Ga. The youngest identified zircons so far correspond to Lower and Mid-Devonian ages. The extensive mechanical abrasion of zircons having Archaean (3.0, 2.8 and 2.6Ga) to Paleoproterozoic ages (2.25–2.0Ga) suggest their provenance from recycled old sedimentary sequences. The relatively large number of zircons with ages between 2.0 and 3.0Ga may indicate the presence of relicts of the Archaean/Paleoproterozoic crust in the source areas of the studied Mid-Devonian sediments. The absence of detrital zircon ages between 0.9 and 1.2Ga and the presence of zircon ages of 2.0–2.25 and 0.5–0.8Ga correspond to the zircon age pattern from the Gondwana-related North African, rather than Gondwana-related South American and Baltic terranes. The material was entering the basin predominantly from the west and consisted primarily of detrital material of Cambrian granitoids and recycled material of Neoproterozoic meta-sedimentary sequences.     

Re–Os geochemistry and geochronology of the Ransko gabbro–peridotite massif, Bohemian Massif

The Ransko gabbro–peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex, which hosts low-grade Ni–Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites, and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase-rich peridotites. Gabbro, troctolite, peridotite and Ni–Cu ores from the Jezírka Ni–Cu (PGE) deposit, considered to be a typical example of the liquid segregation style of mineralization, were analyzed for Re–Os concentrations and isotopic ratios. Seven barren and mineralized samples from the Jezírka deposit yielded a Re–Os regression of 341.5?±?7.9 Ma (MSWD?=?69). Strongly mineralized peridotite with mantle-like initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.125 suggests that Os as well as other PGE present in the Ni–Cu mineralization are predominantly of mantle origin. On the other hand, barren and low-mineralized samples have radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios of 0.14–0.16 suggesting some import of Re and/or radiogenic ¹⁸⁷Os most likely through contamination by continental crust during magma emplacement. The Re–Os age of the Ransko Massif is significantly younger than the previously suggested Lower Cambrian age, but it is similar to and/or younger than the age of metamorphism of the adjacent Kutná Hora crystalline complex and the Moldanubian unit. Therefore, it is likely that the emplacement of the Ransko massif and its Ni–Cu mineralization was closely connected with the late-stage evolution of the Kutná Hora crystalline complex.     

Geochemistry and Sm–Nd isotopic systematics of Ediacaran–Ordovician,sedimentary and bimodal igneous rocks in the western Acatlán Complex,southern Mexico: Evidence for rifting on the southern margin of the Rheic Ocean

Ordovician igneous rocks in the western Acatlán Complex (Olinalá area) of southern Mexico include a bimodal igneous suite that intrudes quartzites and gneisses of the Zacango Unit, and all these rocks were polydeformed and metamorphosed in the amphibolite facies during the Devono-Carboniferous. The Ordovician igneous rocks consist of the penecontemporaneous amphibolites, megacrystic granitoids and leucogranite, the latter dated at ca. 464 Ma. Geochemical and Sm–Nd data indicate that the amphibolites have a differentiated tholeiitic signature, and that its mafic protoliths formed in an extensional setting transitional between within-plate and ocean floor. The amphibolites are variably contaminated by a Mesoproterozoic crustal source, inferred to be the Oaxacan basement exposed in the adjacent terrane. The most primitive samples have εNdt (t = 465 Ma) values significantly below that of the contemporary depleted mantle and were probably derived from the sub-continental lithospheric mantle. The megacrystic granites were most probably derived by partial melting of an arc crustal source (similar to the Oaxacan Complex) and triggered by the ascent of mafic magma from the lithospheric mantle. Sm–Nd isotopic signatures suggest that metasedimentary rocks from Zacango Unit were derived from adjacent Oaxacan Complex. Trace elements relationships (e.g. La/Th vs. Hf) and REE patterns suggest provenance in felsic-intermediate igneous rocks with a calc-alkaline signature. The Ordovician bimodal magmatism is inferred to have resulted from rifting on the southern flank of the Rheic Ocean and is an expression of a major rifting event that occurred along much of the northern Gondwanan margin in the Ordovician.     

Geochemistry and mineralogy of Platinum-group elements in the Ransko gabbro–peridotite massif,Bohemian Massif (Czech Republic)

The Ransko gabbro-peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex of Lower Cambrian age. The complex hosts low grade Ni-Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites and, to a much lesser extent, in both pyroxene and olivine gabbros and plagioclase-rich peridotites. The ore zone is characterized by strong serpentinization and uralitization. The total Ni + Cu locally reaches up to 4 wt%. Anomalous concentrations of platinum-group elements (PGE's) (maximum 532 ppb Pd, 182 ppb Pt, 53 ppb Rh, 15 ppb Ru, 41 ppb Ir) were detected in samples of Cu-Ni and Ni-Cu ores (maximum 2.63 wt% Ni and 2.31 wt% Cu) from the Jezírka orebody. The main ore paragenesis includes pyrrhotite, pentlandite, chalcopyrite, cubanite, pyrite, magnetite, mackinawite, valleriite, ilmenite and sphalerite. During this work, michenerite, froodite, sperrylite, gold, native bismuth, altaite, tsumoite, hessite, an unnamed Bi-Ni telluride, cobaltite-gersdorffite and galena were newly identified. The host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks.     

Ordovician Proto–basin in South China and its Tectonic Implications: Evidence from the Detrital Zircon U–Pb Ages of the Ordovician in Central Hunan,China

Caledonian orogeny is another important tectonic event in South China Block after the breakup of the Rodinia supercontinent. With a view to constrain the tectonic evolution and proto–basin in South China, this paper reports the geochemical and zircon U-Pb dating data of the Ordovician strata in central Hunan, South China. Geochemical features and paleocurrent directions suggest that the lower Ordovician deposited in a passive continental margin basin with a provenance of quartzose components and...     

Ocean floor basalt, not continental gabbro: a reinterpretation of the Hoher Bogen amphibolites, Teplá-Barrandian, Bohemian massif

At its southern margin along the Hoher Bogen mountain, the Teplá-Barrandian (Bohemian massif, Central Europe) is made up of a 1- to 4-km wide belt of amphibolites. An upper amphibolite/lower granulite facies Variscan metamorphism has brought forth coarse-grained, weakly foliated rocks with hbl+pl±cpx±opx±grt parageneses. Since the beginning of this century, these rocks, together with fine-grained or mylonitized amphibolites, have been regarded as metamorphic gabbros (gabbro amphibolites) of the Neukirchen-Kdyne igneous complex. Relics of magmatic textures, however, cannot be found anywhere. The amphibolites are therefore reinterpreted as metamorphic basalts. The Hoher Bogen amphibolites (HBA) derive from N-type MORB. The most primitive samples have Mg#s between 60 and 65. Locally occurring (garnet-)hornblendites and leucodioritic mobilisates are the products of partial melting of amphibolites during the Variscan metamorphism and do not belong to the primary magmatic rock association. Ultramafic rocks are tectonically emplaced between the HBA belt and the metapelitic rocks of the Moldanubian. At the very least, the metapyroxenites among them seem to have a cumulus origin. Together with the ultramafic rocks, the HBA belt may be regarded as a metaophiolite, comparable to the Mariánské Lazne complex. The reinterpretation of the former "gabbro amphibolites" as a metaophiolite has consequences for the geology of the Teplá-Barrandian: the size of the Neukirchen-Kdyne igneous complex is reduced. The HBA belt is a piece of oceanic crust which is possibly younger than the Precambrian metasedimentary/metavolcanic country rock of the Neukirchen-Kdyne igneous complex.     

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.

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

Mineralogical and geochemical characteristics of hydrothermal alteration and episyenitization in the Königshain granites, northern Bohemian Massif, Germany

The late-Hercynian granites of Königshain underwent multistage hydrothermal processes. Extensive high-temperature late-magmatic alteration is, for example, indicated by low Zr/Hf and an REE pattern displaying the tetrad effect. Intensive post-magmatic alteration of the granite occurred along brittle structures. At least two main stages of post-magmatic hydrothermal alteration are involved. The first high-temperature stage, which is characterized by albitization and/or quartz leaching (episyenitization), resulted from fluid–rock interaction with late-magmatic fluids that very probably mixed with external low-salinity fluids. Quartz dissolution was triggered by vapour condensation and/or the cooling of these fluids (below 450??°C) along brittle structures. The high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low-temperature fluids at stage 2; the latter is characterized by the chloritization and illitization of episyenites. Almost all major and trace elements were enriched or depleted during one of the main alteration stages. However, Zr, Hf, Th, and Ti were immobile during post-magmatic alteration. The significant depletion of LREE and the enrichment of HREE in albitized samples is controlled by the dissolution of monazite and the new formation of HREE-rich polycrase-(Y) or aeschynite-(Y) during post-magmatic stage 1. Negative Ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2.     

Provenance of Neoproterozoic and early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit (Bohemian Massif): Evidence from U–Pb detrital zircon ages

New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ～ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.     

Geochronology and geochemistry of a dyke–host rock association and implications for the formation of the Bavarian Pfahl shear zone, Bohemian Massif

To place constraints on the formation and deformation history of the major Variscan shear zone in the Bavarian Forest, Bavarian Pfahl zone, SW Bohemian Massif, granitic dykes and their feldspar-phyric massive host rock (so-called palite), zircons were dated by the U–Pb isotope dilution and Pb-evaporation methods. The dated samples comprise two host rocks and four dykes from a K-rich calc-alkaline complex adjoining the SW part of the Bavarian Pfahl shear zone. The palites, which appear to be the oldest magmatic rocks emplaced in the shear zone, yield ages of 334±3, 334.5±1.1 Ma (average ²⁰⁷Pb/²⁰⁶Pb-evaporation zircon ages) and 327–342 Ma (range of U/Pb zircon ages) suggesting a Lower Carboniferous age for the initiation of the Pfahl zone. Absence of inherited older cores in all investigated zircons indicates that incorporation of crustal zircon material has played virtually no role or that the melting temperature was very high. Determination of the dyke emplacement age is complicated by partial Pb-loss in most of the fractions analysed. This Pb-loss can be ascribed to higher U content of the dyke zircons compared to those from host rock. Upper discordia intercept ages of the different dykes range from 322±5 to 331±9 Ma. The dykes are pre- to synkinematic with respect to penetrative regional mylonitisation along the Pfahl zone, and the upper intercept ages provide a maximum age for this tectonic event.     

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.     

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy,and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K₂O/Na₂O, K₂O/TiO₂ and intermediate CaO/Al₂O₃ that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K₂O/Na₂O, K₂O/TiO₂ and CaO/Al₂O₃ that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K₂O/TiO₂ similar to those from VF, and intermediate CaO/Al₂O₃ as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO₂–eclogites) whereas the less important EM component to recycled marine or continental sediments.     

Geochemistry of sulfur and hazardous trace elements in coals and its bearing on human health

Hazardous air pollutants, including compounds of sulfur and toxic trace elements, are emitted during coal combustion. Geochemical studies of these constituents in coals provide information about their species, regional distribution and origins. The data are useful in understanding the cause and scope of human health problems related to these hazardous elements and in designing preventive or remedial measures. Sulfur in coal is a problem because sulfur dioxide emitted during coal combustion is a main source of acid rain. The sulfur isotopic evidence shows that sulfur in low-sulfur coal is derived primarily from parent plant materials. Sulfur enrichment in medium- and high-sulfur coals is caused by the sulfate in seawater that flooded the peat swamp during coal formation. The sulfur content of a coal is controlled primarily by the depositional environment of coal seams. Only pyritic sulfur can be removed by physical coal cleaning processes （coal preparation）. Sulfur dioxide emission can be reduced using clean coal technologies, such as flue-gas desulfurization, fluidized-bed combustion, and integrated gasification and combined cycle.