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Refining the granite, gneiss and schist interrelationships within the Lusatian-Izera Massif, West Sudetes, using SHRIMP U-Pb zircon analyses and new geologic data

Żelaźniewicz A., Fanning C., M., Achramowicz S.

Geologia Sudetica

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2009

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Vol. 41

67-84

EN

The eastern part of the Lusatian-Izera Massif, West Sudetes, comprises different types of gneissose rocks, collectively known as the Izera gneisses, with a subordinate component of petrographically varied mica schists. Coarse-grained gneisses and their protoliths have been dated at 515-480 Ma, but the lack of age data for other rocks has impeded accounts of their mutual relationships and, thus, the region's geological evolution. This paper reports new sensitive high-mass resolution ion microprobe (SHRIMP) U-Pb zircon data, and some new field and petrographic observations, for three representative rock types: 1) the Złotniki schist (a fine-grained quartz-albite-chlorite-sericite-biotite schist); 2) a fine-grained gneiss that grades to 'porphyroblastic' granite and which occurs on the slopes of Mt. Stóg Izerski; 3) a leucogranite found just the south of the village of Kotlina. A volcanogenic intercalation in the Złotniki Lubańskie schists developed at 560 Ma and contained xenocrystic zircons that grew in the source at 620 Ma and 600-580 Ma. The schists are interpreted as the metamorphosed equivalent of the Lusatian greywackes, which were derived from a dissected arc and deposited in a convergent-margin basin along northern peri-Gondwana. The zircons from the fine-grained gneisses yielded four age groups: 515 +- 7 Ma, 500 +- 12 Ma, 487 +- 13 Ma and 471 +- 8 Ma. Similar age groups of zircons can also be found in the coarse-grained metagranites. Rifting of Gondwana during the mid-Cambrian-early Ordovician was a protracted thermal event lasting ~30-45 m.y., with episodic attenuation of the mainland crust every ~5-10 m.y. before continental fragments finally became separated. Each episode successively promoted an increased heat flux from the mantle that facilitated melting of the crust, causing metamorphism and fusion of the Precambrian Lusatian-Izera basement and a final phase of S-type felsic magmatism. The leucogranite sample yielded zircons in two age groups, 508 +-5 Ma and 483.1 +- 3.6 Ma, with low Th/U ratios, which is interpreted as a product of an anatectic melting at deeper crustal levels. These leucogranites are in close spatial relation with belts of mica schist, which could mean that these granites used some rheologically weak zones that were introduced into the Izera pluton where large fragments of country rocks were trapped within the ~500 Ma granites.

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Famennian, thermally altered chert exotic clasts from the Jędrzychowice/Ludwigsdorf wildflysch (Sudetes)-stratigraphic and tectonic implications

Wajsprych B., Haydukiewicz J., Achramowicz S.

Geological Quarterly

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2006

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Vol. 50, No. 2

281-288

EN

A wildflysch sequence recently recognized in the Görlitzer Schiefergebirge/western Kaczawskie Mts. boundary zone permits the characterization of the westernmost Kaczawa Unit as a chaotic complex. The cherts,many of which contain numerous radiolarians, occur as exotic clasts within the olistostrome deposits of the wildflysch. They are associated with allochthonous blocks of blueschist, andesite pillow lavas and pyroclastic rocks, flysch facies clastic rocks, mudstones (often of black shale lithology) and carbonates. The conodonts extracted from the grey-greenish radiolarite cherts indicate a mid-Famennian age (Early rhomboidea Zone to Early marginifera Zone). The conodont-bearing radiolarite clasts reveal no trace of a tectonometamorphic fabric. Their 4-4.5 conodont colour alteration index (CAI) indicates that the rock underwent low-temperature (250-285C) thermal alteration. The lack of thermal overprint in the olistostrome matrix allows this alteration to be interpreted as a part of the tectonothermal, post-mid-Famennian evolution of the source area. A number of the exotic blocks of unmetamorphosed volcanic rocks associated with the chert exotics seems to indicate igneous (volcanic) activity as the reason for the chert alteration. The fact that exotic blocks of the Devonian chert have been found as clasts within deposits earlier believed to represent Upper Proterozoic (Cadomian) flysch suggests the need for reassessement of the extent of Cadomian rocks in the westernmost Sudetes.

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On the new wildflysch-to-flysch, blueschist-rich Lower Visean succession of the Western Sudetes (SW Poland)

Wajsprych B., Achramowicz S.

Annales Societatis Geologorum Poloniae

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2003

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Vol. 73, No 2

123-137

EN

Field and laboratory works realized in last years allowed us to redefine one of the lithotectonic units of the western Sudetes, till now considered to be composed of Cambrian to Lower Carboniferous, stratigraphically coherent, volcano-sedimentary succession, as the Lower Carboniferous, probably lower Visean, wildflysch-to- flysch succession. This unit forms the westernmost part of the Kaczawa Complex and is situated at the boundary zone between the Kaczawa Mts. (Polish Sudetes) and the Görlitzer Schiefergebirge (Germany) or - at a larger scale - between the Sudetes and Lugian tectonostratigraphic zones of the Variscan orogen. Petrographical studies of rocks sampled from the larger allochthonous bodies (olistolithes and slide-sheets) of the wildflysch sequence have revealed some peculiar features of the lithic composition of this succession. They consist in the presence of: (1) - unmetamorphosed and metamorphosed volcanites with a distinct HP-overprint, characteristic of supra- subduction zones, gabbro-type plutonites, and relatively numerous detrital chromite grains indicating the occur- rence of ophiolite ultramafics in the source area; and (2) - large block(s) of rock composed of quartz (almost 100% SiO 2 ), previously interpreted as Palaeozoic quartz vein, now documented by the authors to be totally silicified primary evaporites, composed of gypsum (selenite), anhydrite and salt. The last finding would be of special significance as the first strong evidence of evaporites within the Variscan orogenic complex in Europe, if further studies confirm proposed here tectonic position of the silicified evaporites. General lithic composition of the Jędrzychowice/Ludwigsdorf wildflysch detrital material is characterised by the presence of such litholog ies, as: black and gray-to-green cherts, black shale mudstones and cherts, (turbi- dite-)siliciclastics, carbonates (both bioclastic and diagenetic), basic and acid, unmetamorphosed and epi-to- HP-metamorphosed volcanites, and gabbros and ultrabasites (the latter noted only by detrital chromites). Moreover, the siliciclastic material of the olistosthrome matrix discloses the presence of acid magmatic (granites) and high-grade metamorphics of gneiss-to- mica-schist type in the source areas. Such a compositio n of detrital material clearly reflects a typical tectonic mélange as the source terrane for the wildflysch deposit. It would mean that the Jędrzychowice/Ludwigsdorf wildflysch should be considered as the next, strong and unequivocal signal of large-scale tectonic mélange stage in a tectonic/geodynamic evolution of the Central European Palaeozoic orogenic system.