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

作者：Redlinska Marczynska Aleksandra^*; Zelazniewicz Andrzej; Fanning C Mark

来源：Geological Quarterly, 2016, 60(3): 714-736.

DOI：10.7306/gq.1287

• Cambrian
• migmatites
• Variscan orogeny
• SHRIMP
• Sudetes

The Orlica-Snieznik 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 Gieraltow gneisses include migmatites and have a longer structural history than the Snieznik augen orthogneisses. U-Pb SHRIMP analyses yielded U-Pb ages of similar to 500 Ma for cores and similar to 498 Ma for wide outer parts of zircon grains in the twice-folded Gieraltow gneisses, and an age of similar to 500 Ma for a discordant neosome vein. Neoproterozoic metasediments were among precursors of the lithologically diversified Gieraltow 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 Snieznik Gneiss Formation. The metagranite (= Snieznik 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 similar to 343 Ma to similar to 325 Ma (mean similar to 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.

• 出版日期2016