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Geologia Sudetica

Vol. 38 (2006) Abstracts

Geologia Sudetica, 38: 1-38.

The acid metavolcanic rocks of the Orlica-Śnieżnik Dome (Sudetes): their origin and tectono-metamorphic evolution

Mentor Murtezi

Institute of Geological Sciences of the Polish Academy of Sciences, Department of Geology of the Sudetes, Podwale 75, 50–449 Wrocław, Poland, e-mail: mmurtezi@poczta.fm

Key words:  sacid metavolcanic rocks, geochemistry, petrogenesis, P-T-d path, Stronie Formation, Orlica-Śnieżnik Dome

The Orlica-Śnieżnik Dome (OSD) belongs to the mosaic of geological units forming the Sudetic part (NE margin of the Bohemian Massif) of the Variscan belt. Acid metavolcanic rocks referred to as leptites appear in the OSD as intercalations in the mica schists of the Stronie Formation - the supracrustal cover of the gneiss core of the unit. Two types of acid meta-volcanogenic rocks have been distinguished: massive leptites (ML) and biotite-bearing leptites (BBL); together they are referred the leptite series. The ML originate from acid volcanic rocks that could have contained only small amounts of sedimentary material. The BBL are metamorphosed tuffites with a variable share of sedimentary material. All the analysed leptites from the entire OSD area reveal similar geochemical characteristics. The high concentration of strongly incompatible elements such as REE, as well as the enrichment in large-ion-lithophile elements indicates the continental crust affinities of the leptites' protolith. On the other hand, ascertained the high Th/Ta ratios, negative Nb and Ta anomalies, and low Ti, Zr and Hf content can be interpreted as indicators of magmatism connected with a suprasubduction environment. Such ambiguous characteristics may indicate that an ensialic rift environment, considered as the place of origin for the magmatic rocks dated at c. 500 Ma and occurring in the OSD, developed due to extension behind a collisional zone of Lower Palaeozoic age. An extensional environment created during this event could be the location for the postulated early deformation and metamorphism of the Stronie Formation. The Śnieżnik metagranites would have been emplaced into such, already deformed, supracrustal cover. The signature of an active margin found in these continental crust rocks that were stretched and melted due to the increased heat transfer resulted from an earlier (Neoproterozoic?) tectono-thermal episode.
The reconstructed path of the tectono-metamorphic evolution of the leptites entirely results from a Variscan collisional episode occurring in the borderland of the West and the East Sudetes. The sequence of the deformations established for the studied rocks is interpreted as an effect of the continuous convergence of crustal units in an E-W direction. Structural and phase-equilibria lines of evidence show that the tectono-metamorphic evolution of the Stronie Formation was that of early burial (D1) to a depth not exceeding upper amphibolite facies conditions, and subsequent uplift connected with subvertical shortening and flattening strain (D2). As a result of the continuous convergence, thrusting took place along the edges of the colliding units. The final adaptation of the colliding crustal slices led to zonal lateral oblique displacements (D3). This latter event is treated as the cause for disturbance of the isograd and isotherm pattern observed within the rocks of the Stronie Formation. It is also responsible for zonal shearing, generally taking place in a N-S direction. During the collision that occurred along the eastern margin of the OSD, the rocks of the Stronie Formation reached the highest P-T conditions of metamorphism at c. 9 kbar and 650°C. The presented data shows a considerable decrease in the peak-metamorphic conditions from the east to the west of the OSD.

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Geologia Sudetica, 38: 39-61.

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)

Marek Awdankiewicz

* Wrocław University, Institute of Geological Sciences, Department of Mineralogy and Petrology, ul. Cybulskiego 30, 50–205 Wrocław, Poland; e-mail: mawdan@ing.uni.wroc.pl

Key words:  petrology, basaltic andesites, post-collisional volcanism, Sudetes

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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Geologia Sudetica, 38: 63-80.

Syntectonic Lower Ordovician migmatite and post-tectonic Upper Viséan syenite in the western limb of the Orlica-Śnieżnik Dome, West Sudetes: U-Pb SHRIMP data from zircons

Andrzej Żelaźniewicz*, Izabella Nowak**, Alexander N. Larionov*** & Sergey Presnyakov***

* Instytut Nauk Geologicznych PAN, Podwale 75, 50-449 Wrocław, Poland; pansudet@pwr.wroc.pl
  Instytut Geologii, Uniwersytet Adama Mickiewicza, ul. Maków Polnych 16, 61-616 Poznań

** Instytut Nauk Geologicznych PAN, Podwale 75, 50-449 Wrocław, Poland
*** Centre of Isotopic Research, All Russian Geological Research Institute, St. Petersburg, Russia

Key words:  Variscan orogen, Bohemian Massif, Gierałtów gneiss, mesosome, Ca-Fe garnet, Śnieżnik gneiss, partial melting, mantled dome

In the Orlica-Śnieżnik Dome, the West Sudetes, metagranites of 515-480 Ma age occur as coarse-grained augen gneisses (~ Śnieżnik type) in the middle of the dome, whereas fine-grained, often migmatitic gneisses (~ Gierałtów type) are located more externally. Both the origin and genetic relationships of the gneisses have been disputed for many years. In a quarry near Zdobnice, in the western part of the dome, migmatitic gneisses and a post-tectonic dyke of unfoliated biotite-hornblende high-K syenite occur. The migmatititc gneiss has mesosome with relic minerals, notably Ca-Fe garnet and pseudomorphs after Al2SiO5 polymorph (?), indicative of an early granulitic metamorphism at considerably high pressure and temperature. Retrogression at still high temperature of ~720-750°C under the upper amphibolite facies conditions was accompanied by migmatization which among others produced cross-cutting neosome veins of graphic granite. Zircons from the melt derived neosome and from the syenite dyke were analysed with SHRIMP II. The former yielded a concordia age of 485±12 Ma which is taken to constrain the waning stage of the Late Cambrian-Early Ordovician migmatization. Migmatitic gneisses may have represented a metasedimentary-metaigneous Neoproterozoic crust that underwent multistage metamorphism, granulite facies inclusive, and then yielded to extensive partial melting between 515 Ma and 480 Ma. Our new data shows that the migmatization in the Orlica-Śnieżnik Dome was concurrent with the intrusion of a granitic precursor of the augen gneisses and does not support the views that the migmatitic gneisses can be a derivative of the ~500 Ma granite. In the Late Cambrian-Early Ordovician, the porphyritic granite intruded in migmatitic country rocks which mantled the granitic core. Both lithologies were later ductilely sheared and deformed under lower conditions of the amphibolite and greenschist facies during the Variscan orogeny. Four zircon grains from the post-tectonic syenite dyke yielded a concordia age of 326±3 Ma, which is interpreted as the time of its intrusion. This constrains the ductile Variscan events in the studied region.

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