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A Permian andesitic tuff ring at Rožmitál (the Intra-Sudetic Basin, Czech Republic) : evolution from explosive to effusive and high-level intrusive activity

Awdankiewicz M., Awdankiewicz H., Rapprich V., Stárková M.

Geological Quarterly

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2014

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Vol. 58, No. 4

759–-778

EN

Mafic, monogenetic volcanism is increasingly recognized as a common manifestation of post-collisional volcanism in late Variscan, Permo-Carboniferous intramontane basins of Central Europe. Although identification of individual eruptive centres is not easy in these ancient successions, the Permian Rožmitál andesites in the Intra-Sudetic Basin (NE Bohemian Massif) provide an exceptionally detailed record of explosive, effusive and high-level intrusive activity. Based on field study and petrographic and geochemical data on pyroclastic and coherent rocks, the Rožmitál succession is interpreted as the proximal part of a tuff ring several hundred metres in diameter. Initial accumulation of pyroclastic fall and surge deposits occurred during phreatomagmatic eruptions, with transitions towards Strombolian eruptions. Gullies filled with reworked tephra document periods of erosion and redeposition. Andesitic blocky lavas capped the volcaniclastic succession. Invasion of lavas into unconsolidated sediments and emplacement of shallow-level intrusions in near-vent sections resulted in the formation of jigsaw- and randomly-textured peperites. Most geochemical differences between coherent andesites and pyroclastic rocks can be linked to incorporation of quartz-rich sediments during the explosive eruptive processes and to later cementation of the volcaniclastic deposits by dolomite. The Rožmitál tuff ring could have been one of several phreatomagmatic centres in a monogenetic volcanic field located on an alluvial plain.

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Ambiguous geological position of Carboniferous rhyodacites in the Intra-Sudetic Basin (SW Poland) clarified by SHRIMP zircon ages

Kryza R., Awdankiewicz M.

Geological Quarterly

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2012

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Vol. 56, No 1

55-66

EN

Rhyodacite sheets (the Sady Górne Rhyodacites) in the lowermost part of the Permo-Carboniferous Intra-Sudetic Basin molasse fill have been mapped as intrusives but, later on, based on ambiguous field and petrographic evidence, reinterpreted as lower Carboniferous lavas and tuffs; if so, they would mark the earliest episode of late-orogenic volcanism in the Intra-Sudetic Basin and in the whole Sudetes region in SW Poland. However, re-examination of field relationships and new observations are consistent with an intrusive emplacement of the rhyodacites as conformable to semiconformable, simple to composite sheets. SHRIMP zircon study indicates that the rhyodacites contain rare inherited zircons of ca. 560 Ma, and ca. 470 Ma (or slightly older), and a main population of zircons with an average concordia age of 306.1 š2.8 Ma. This latter age documents the emplacement of the rhyodacites during a mid/late late Carboniferous (Westphalian) stage of volcanism in the Intra-Sudetic Basin in the Central European Variscides. This post-orogenic volcanism was possibly initiated several million years later than previously assumed, and could have comprised a few pulses over a relatively prolonged time span of millions of years.

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Late Palaeozoic lamprophyres and associated mafic subvolcanic rocks of the Sudetes (SW Poland): petrology, geochemistry and petrogenesis

Awdankiewicz M.

Geologia Sudetica

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2007

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Vol. 39

11-97

EN

Lamprophyric magmatism in the Sudetes, in the eastern part of the European Variscides, occurred during a period of post-collisional extension in the Carboniferous. The lamprophyres (minettes, vogesites, spessartites, kersantites) and associated mafic rocks (monzonites, micromonzodiorites) were emplaced as dyke swarms and as scattered veins that cut the crystalline basement and, locally, the overlying molasse deposits. The dyke swarms are adjacent to major regional dislocations, represent distinctive magmatic centres that are related to separate magmatic systems and each are characterized by specific parental melts that have undergone individualised shallow-level differentiation processes. The two largest dyke swarms are associated with the Karkonosze and Kłodzko-Złoty Stok granitoid massifs: these show the widest geochemical and petrographic variation, due to more advanced differentiation in long-lived magmatic systems. In contrast, a small dyke swarm emplaced in the SW part of the Orlica-Śnieżnik Dome, unrelated to granites, is strongly dominated by minettes only. Geochemical characteristics of the mafic rocks studied herein vary from (ultra)potassic in the minettes to calc-alkaline in the micromonzodiorites and from primitive (Mg# = 80-60 in many lamprophyres) to evolved (Mg# down to 30 in some micromonzodiorites). Some richterite minettes show Nb-enriched trace element patterns, but negative Nb anomalies are more typical. Richterite minettes posesess ?Nd300 and 87Sr/86Sr300 values that range from +1.9 to -8.3, and from 0.7037 to 0.715, respectively. The other rocks in this study show negatively correlated Nd and Sr isotopic ratios, between these extremes. The geochemical data suggest three types of mantle source for the lamprophyres and associated mafic rocks: (1) An asthenospheric, depleted and later re-enriched source; (2) A lithospheric source contaminated by subducted crustal rocks; (3) A lithospheric source metasomatized by subduction-related fluids. The richterite minette magmas originated from low degrees of partial melting, under high H2O/CO2 conditions, of garnet-phlogopite-peridotites. The Nb-enriched and Nb-depleted minettes are derived from sources (1) and (2), respectively. Kersantite magmas originated from source (3). The factors of source mixing, variable depths and degrees of melting, and aggregation of melts all influenced the compositions of other primitive minette and vogesite magmas. The other rocks studied (spessartites, monzonites, micromonzodiorites) are variably differentiated. Zoning and other disequilibrium textures in phlogopite, biotite, amphibole and clinopyroxene phenocrysts, together with the presence of xenocrysts, xenoliths and enclaves (cognate, restitic, migmatitic) constrain several processes that were involved in the shallow-level evolution of magmas: mixing, fractional crystallization, assimilation of crustal rocks. However, post-magmatic replacement of the igneous phases by albite, chlorite, epidotes, actinolite, blue amphiboles, titanite, carbonates, prehnite, pumpellyite and grossularite-andradite partly obscures the magmatic assemblages and textures. There are four more general results of this study. First, there is evidence for a strong heterogeneity of the upper mantle and of the presence of subduction-modified mantle beneath the Sudetes during the Late Palaeozoic. Second, the lamprophyre magmas originated and evolved in spatially and petrologically distinct, vertically extensive magmatic systems that spanned the asthenospheric and lithospheric mantle and the lower/middle crust. Third, a broad spectrum of source-related and shallow-level magmatic processes gave rise to the emplacement of primitive, mantle-derived magmas and of variably evolved magmas. Fourth, close links existed between late Variscan tectonics, the location of lamprophyric magmatism, and the shallow-level emplacement processes of mafic dykes.

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Fractional crystallization, mafic replenishment and assimilation in crustal magma chambers: geochemical constraints from the Permian post-collisional intermediate-composition volcanic suite of the North-Sudetic Basin (SW Poland)

Awdankiewicz M.

Geologia Sudetica

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2006

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Vol. 38

39-61

EN

The Permian intermediate-composition lavas of the North-Sudetic Basin represent a high-K calc-alkaline suite emplaced in an extensional, intracontinental, post-collisional setting in the eastern part of the European Variscan belt. The lavas, in a total volume of over 100 km3, erupted from fissure vents or small shield-type volcanoes in several episodes separated by repose and sedimentation periods. An idealised eruptive episode comprised basaltic trachyandesites (plagioclase- phyric, clinopyroxene lavas) followed by predominant, main-series basaltic andesites (weakly porphyrytic, two-pyroxene microcrystalline lavas) and evolved basaltic andesites (weakly porphyrytic, two-pyroxene fine-grained lavas). This volcanic suite originated in magmatic systems where differentiation processes evolved with time from (I) fractional crystallization, producing the basaltic trachyandesites, through (II) fractional crystallization coupled with mafic replenishment, resulting in the main series basaltic andesite lavas, to (III) fractional crystallization, mafic replenishment and minor crustal contamination, producing the evolved basaltic andesites. The fractionating mineral assemblages changed during the successive stages and, apart from plagioclase, olivine, clinopyroxene and Fe-Ti oxides, included ortho- pyroxene (stage II and III) and apatite with zircon (stage III). The general trace element characteristics of the volcanic rocks (enrichment in Th, LILE, Nb and Zr, but with high Th/Nb and La/Nb ratios) are transitional between those of extension-related within-plate lavas and active continental margin lavas. These characteristics are inherited from enriched lithospheric mantle sources carrying a crustal signature related to subduction processes during the earlier stages of the Variscan orogeny.

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Reconstructing an eroded scoria cone : the Miocene Soonica Hill volcano (Lower Silesia, SW Poland)

Awdankiewicz M.

Geological Quarterly

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2005

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Vol. 49, No. 4

439--448

EN

The basaltic rocks of Sośnica Hill near Targowica (Fore-Sudetic Block) belong to the Cenozoic Central European Volcanic Province. The volcanic succession at Sośnica is over 40 m thick and comprises pyroclastic fall deposits (mainly tuff breccias), subvolcanic intrusions (plug, dykes and other intrusive sheets) and aa-type lavas. Field relationships and structural data enable a detailed reconstruction of the vent location, morphology and eruptive history of the original volcano. Initial Hawaiian to Strombolian-type explosive eruptions produced a pyroclastic cone. Subsequently subvolcanic intrusions and lavas were emplaced. The lavas were fed from the central vent of the volcano, breached the cone and flowed southwards. Later eruptions resumed at a new vent on the western slopes of themain cone. The final volcanic edifice-a breached Strombolian scoria cone with a lava flow and a parasitic cone-was 500-1000 m in diameter at the base and 90-180mhigh. The preserved SWsector of this volcano, where the pyroclastic deposits were protected from erosion by the surrounding plugs and lavas, corresponds to ca. 1/2 of the height and 1/8 of the volume of the original volcano. Compared with many other remnants of Cenozoic volcanic centres in Lower Silesia, this volcano is exceptionally well preserved and exposed.

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Volcanism in a late Variscan intramontane trough: the petrology and geochemistry of the Carboniferous and Permian volcanic rocks of the Intra-Sudetic Basin, SW Poland

Awdankiewicz M.

Geologia Sudetica

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1999

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Vol. 32, nr 2

83-111

EN

The Carbonifeorus-Permian volcanic rocks of the Intra-Sudetic Basin represent products of late- to post-collisional volcanism associated with extension within the eastern part of the Variscan belt of Europe. The volcanic succession is subdivided into the older, calc-alkaline suite (the early and late Carboniferous) and the younger, mildly alkaline suite (the late Carboniferous and early Permian). The rhyodacites with subordinate basaltic andesites and andesites of the older suite show convergent plate margin affinities. The rhyolitic tuffs, rhyolites with less widespread trachyandesites and basaltic trachyandesites of the younger suite are largely characterised by within-plate affinities, with some gradations towards convergent plate margin affinities. This geochemical variation compares well with that found in some Tertiary-Recent extensional settings adjacent to former active continental margins (e.g. the Basin and Range province of the SW USA). The parental magmas for each suite of the Intra-Sudetic Basin possibly originated from similar, garnet free mantle sources at relatively shallow depths (within the subcontinental mantle?), but at variable degrees of partial melting (lower for the mildly alkaline rocks). The convergent plate margin-like geochemical signatures of the volcanic rocks may either have been inherited from their mantle sources, or be related to the assimilation of crustal rocks by the ascending and fractionating primary magmas. The intermediate-acidic rocks within each suite mainly originated due to fractional crystallisation of variable mineral assemblages equivalent to the observed phenocrysts (mainly plagioclase and pyroxenes, with hornblende and biotite in the calc-alkaline suite, and K-feldspar in the mildly alkaline suite). The trace element patterns of the volcanic rocks were also strongly influenced by fractionation of accessory minerals, such as spinels, ilmenite, zircon, apatite and others. The petrographic evidence (e.g. quartz phenocrysts with reaction rims, complexly zoned or sieve-textured feldspar phenocrysts) suggests that assimilation and/or magma mixing processes might also have taken place during the evolution of the magmas.

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Volcanism in a late Variscan intramontane trough: Carboniferous and Permian volcanic centres of the Intra-Sudetic Basin, SW Poland

Awdankiewicz M.

Geologia Sudetica

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1999

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Vol. 32, nr 1

13-47

EN

The Intra-Sudetic Basin represents a late Variscan intramontane trough situated near the NE margin of the Bohemian Massif. The Carboniferous-Permian molasse succession in the northern part of the basin provides evidence of three stages of volcanic activity during: 1) the latest Visean/earliest Tournaisian, 2) the late Westphalian-Stephanian, and 3) the early Permian, the latter corresponding to the climax of volcanism. Rhyodacites, andesites and basaltic andesites were characteristic of the earlier stages (1 and 2), while basaltic trachyandesites, trachyandesites and rhyolites erupted during the later stages (2 and 3). The earliest volcanism occurred near the northern margin of the Intra-Sudetic Basin and the successive Carboniferous and Permian volcanoes shifted SE-wards with time, consistently with the intrabasinal depositional centres. The location of the volcanoes was controlled by NNW-SSE to NW-SE aligned fault zones. The magmas intruded thicker accumulations of sedimentary rocks within intrabasinal troughs, and erupted through thinner sequences outside the troughs. Effusive to extrusive activity created lava-dominated, composite volcanic centres to the north and west. In the eastern part of the basin the most evolved acidic magmas erupted explosively, with the formation of: 1) a maar belt (late Carboniferous) and 2) a major caldera (early Permian), with subsequent emplacement of subvolcanic intrusions in both cases. The volcanic edifices represented intrabasinal elevations subjected to substantial erosion, with the largest supply of volcanogenic debris into the basin following the most voluminous rhyolitc eruptions in Permian times. The caldera was a centre of lacustrine sedimentation.