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An insight into a gneiss core of the Orlica–Śnieżnik Dome, NE Bohemian Massif : new structural and U-Pb zircon data

Redlińska-Marczyńska A., Żelaźniewicz A., Fanning C. M.

Geological Quarterly

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2016

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Vol. 60, No. 3

714--736

EN

The Orlica–Śnieżnik Dome in the Sudetes, the NE Bohemian Massif, embraces two formations of felsic gneisses of controversial origin and evolution. Our study shows that despite similar geochemical signatures, they carry systematic minor differences in mineral, isotope, zircon and geothermobarometric characteristics. Four variants of the Gierałtów gneisses include migmatites and have a longer structural history than the Śnieżnik augen orthogneisses. U-Pb SHRIMP analyses yielded U-Pb ages of ~500 Ma for cores and ~498 Ma for wide outer parts of zircon grains in the twice-folded Gierałtów gneisses, and an age of ~500 Ma for a discordant neosome vein. Neoproterozoic metasediments were among precursors of the lithologically diversified Gierałtów Gneiss Formation. First deformation, metamorphism, and migmatisation of these rocks occurred at 515–475 Ma and overlapped with the development and emplacement of a porphyritic S-type granite precursor to the Śnieżnik Gneiss Formation. The metagranite (= Śnieżnik augen orthogneiss) embraced migmatitic xenoliths. Zircon grains from such xenoliths revealed distorted and replaced cores which yielded U-Pb ages that dispersed around 507–487 Ma, whilst wide darker poorly zoned outgrowths yielded ages from ~343 Ma to ~325 Ma (mean ~340 Ma). These outgrowths were interpreted as a record of Carboniferous metamorphism assisted by rich Zr- and U-carrying fluids. The Variscan metamorphic overprint was heterogeneous, and variously affected rocks of the two gneiss formations.

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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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A Carboniferous/Permian, calc-alkaline, I-type granodiorite from the Małopolska Block, Southern Poland: implications from geo chemi cal and U-Pb zircon age data

Żelaźniewicz A., Pańczyk M., Nawrocki J., Fanning M.

Geological Quarterly

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2008

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Vol. 52, No 4

301-301

EN

A granodiorite from bore hole WB-102A in the Dol ina Będkowska, the Małopolska Block (MB), South ern Po land, yielded a mean U-Pb zir con age of 300 š3 Ma with SHRIMP II. No in her ited older com po nent was de tected. Geo chemically, it is a K-rich, I-type, calc-al ka line granodiorite with su pra-subduction char ac ter is tics (neg a tive Nb and Ti anom a lies). Silicic ig ne ous rocks are abun dant at the MB mar gin along the Kraków-Lubliniec Fault Zone (KLFZ) across which it ad joins to the Up per Silesian Block (USB) where such rocks are scarce. Both blocks be long to the Variscan fore land. Gra nitic rocks can not, how ever, gen er ate at fore land set tings. Thus, the hy poth e sis is put for ward that the par ent melt for the silicic rocks was de rived from the thick ened lower crust of the Variscan orogenic belt ow ing to extensional de com pres sion melt ing, and trans ported away to wards pul-apart open ings de vel oped along the crustal-scale fault zone (KLFZ) that un der went a com plex strike-slip his tory around the Car bon if er ous/Perm ian bound ary.