Abstract The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga ( 207 Pb– 206 Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3 Ma (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.
Erosional remnants of the Miocene Strzelin Volcanic Field in SW Poland were studied in terms of volcanology, petrology and Sr-Nd-Pb isotope geochemistry with the aim to identify the reasons for compositional variability of monogenetic volcanoes.The obtained data suggest that a heterogeneous mantle peridotite (with mixed DM/HIMU signature) was the dominant source of magmas.Partial melting and segregation of magmas in diapirically rising asthenosphere occurred within the garnet stability field.The source heterogeneity was the basic cause that controlled the compositional variability of the primary magmas, and also influenced the subsequent differentiation processes and eruptive styles.On the surface, additional role was played by variable environments (i.e.phreatomagmatic eruptions in water-saturated environments).More fertile mantle domains, with prevailing HIMU component, released melts deeper, at lower degrees of partial melting and small magma batches were formed.These nephelinitic magmas underwent only limited fractional crystallization en route to the surface and erupted with low explosivity as lava flows.In contrast, less fertile mantle domains, dominated by the DM component, released melts at higher degrees of partial melting at a shallower depth.This resulted in a more sustained magma supply that further enhanced the development of shallow-level magmatic systems, with more advanced and complex differentiation: larger degrees of fractional crystallization as well as replenishment by new batches of primitive magma.The resulting basaltic and trachybasaltic volcanoes showed a greater diversity of eruptive styles, including effusive and variably explosive eruptions.
Rhyodacite sheets (the Sady Gorne 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 ±2.8 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
Gneisses of the Góry Sowie Block, of probable Late Precambrian–Early Cambrian age, contain numerous small metabasite bodies which may have originated as dykes or sills. Many metabasites have phase assemblages and textures which show that they suVered early granulite-facies and later amphibolite- facies metamorphism, although some exhibit no evidence for the earlier event. Trace element geochemistry enabled four metabasite groups to be distinguished: (a) a dominant meta-tholeiite group (Sowie group) probably incorporating more than one set of intrusive rocks; (b) meta-tholeiites characterized by higher Nb/Y (high Nb/Y group); (c) Ti-poor metabasites (Myslêcin group) that exhibit strongly depleted HFSE; (d) alkali metabasalts (Wlóki group). Sowie group meta-tholeiites were also divided according to their dominant host lithology: granitoid gneiss, migmatitic paragneiss or gneiss containing relict granulite-facies rocks. Those in granitoid gneiss are compositionally restricted (although they fall within the same compositional range as other Sowie meta-tholeiites), and may comprise both early metabasites and a later intrusive group, postdating granite emplacement. Both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism: this suggests that both were emplaced after the granulite-facies metamorphic event, but before or during amphibolite-facies metamorphism. An Early Ordovician emplacement age for syn-metamorphic granitoid gneiss shows that the Góry Sowie Block underwent an Ordovician (early Caledonian) metamorphic event, following metabasite emplacement, or was at this grade during the Ordovician. Nonetheless, the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin, or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event.
The basaltic rocks of Sooenica Hill near Targowica (Fore-Sudetic Block) belong to the Cenozoic Central European Volcanic Province. The volcanic succession at Sooenica 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 the main 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 90-180 m high. The preserved SW sector 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.
The most important indices of volcanic eruption size include: erupted pyroclastic material volume (tephra volume); magma volume - the volume of euptive products excluding their porosity (Dense Rock Equivalent, DRE); and the magnitude, M - defined as the common logarithm of erupted magma mass minus 7. The tephra volume is the basis of the Volcanic Explosivity Index (VEI) scale. The weakest eruptions (VEI=0) produce 103km3 of tephra. Eruption of Tambora in 1815 (tephra volume = 160km3, VEI=7, DRE=50km3, M=7.3) was the strongest explosive eruption in historical times. The largest historical lava effusions occurred on Iceland, e.g. from the Laki fissure in 1783 (DRE=15km3, M=6.5). These almost recent eruptions were only modest samples of nature's powers. Mankind has not yet witnessed the largest possible eruptive events, which devastate continent-sized terrains and result in global climatic changes. Supereruptions of La Garita caldera, Colorado, USA, at 28 Ma (DRE=4500 km3, M=9.2) and Toba, Sumatra, at 74 ka (DRE=2700km3, M=8.8) were 90 and 50 times, respectively, stronger than Tambora. Products of even more powerful eruptions were recently recognized in areas of so called Large Igneous Provinces (LIPs). Largest lava effusions (DRE=9300km3, M=9.4) dated at 64.8 Ma were recognized at Deccan, and largest ignimbrites (deposits of giant explosive eruptions), dated at 132 Ma, were identified at the Parana-Etendeka province. Eruptions of that size approach the limit of largest eruptions possible on our planet, which is probably determined by the ability of formation of crustal magma reservoirs large enough.
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.
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