Geochemistry of Metabasites in the Stronie Group and Nové Město Group, the Orlica-Śnieżnik Dome, West Sudetes

Content Provider	Semantic Scholar
Author	Nowak, I.
Copyright Year	2006
Abstract	The Stronie Group (SG) supracrustal rocks, together with a variety of gneisses enclosing small bodies of granulites and eclogites, form the middle and eastern parts of the Orlica-Śnieżnik dome (OSD). The westernmost flank of the OSD is formed by the Nové Město Group (NMG). Metabasites within the Stronie Group appear as small lensoid or irregular bodies, rarely exceeding 0.5 km2, associated with mid-grade metamorphosed metapelitic schists, subordinate marbles, quarzites and acid metavolcanic rocks. They form two groups: alkali basalts of WPB type and subalkali tholeiites similar to MORB, with typical ratios of characteristic elements. No low-Ti metabasites has been identified. Alkali basalts are characterized by Nb/Y > 1.5, Ti/V > 50, Zr/Y > 4, Zr/Nb < 5. In metatholeiites the ratios are: Nb/Y < 0.7, Ti/V < 50, Zr/Y < 4, Zr/Nb > 20. Metabasites of WPB type occur as laminated biotite amphibolites which pass laterally to mica schists or to calcareous schists next to marble bodies. At least part of these amphibolites likely originate from tuffitic protolith merged with clastic rocks and represent pyroclastic products of early stage of a continental rift volcanism. MORB-like metatholeiites are more frequent, though their occurences are widely separated and differ in size. They have finegrained gabbroic or diabasic protoliths interpreted as hypabysal lava bodies or dykes feeding individual volcanoes during more advanced rifting. Marbles contain problematic fossils of early Cambrian age (e. g. Gunia 1984), which suggests similar age for the interlayered metatuffs. U-Pb SHRIMP analyses of zircons from acid metavolcanics throughout the SG yielded similar ages of ~500 Ma (Murtezi and Fanning 2005). Accordingly the same age is assumed for the SG metabasites. In the NMG, Domečka and Opletal (1980) and Opletal et al. (1990) identified subalkalic tholeiites interpreted as ocean floor basalts and co-magmatic calc-alkaline felsic volcanites, apparently different from (meta)basites of the Stronie Group at the OSD core. Our studies show that the NMG metabasites, generally similar to the main series tholeiites of Floyd et al. (1996, 2000), can be subdivided into 3 different types: within-plate tholeiites (WPT) which dominate in the region, less common MORB-like tholeiites, and scarce Ti-tholeiites, based on Zr/Y, Ti/Y, Ti/V, Zr/ Nb, (La/Yb)N and (La/Sm)N ratios and abundances of HFSE and REE. In the field, the WP-tholeiites (Zr/Y ca. 3.57 to 5.70; Ti/Y ca. 290–450) and MORB-like tholeiites (Zr/Y < 3.5; Ti/Y < 320) form roughly parallel belts concordant with the regional meridional strike of the main lithological boundaries and that type of their distribution continues E-ward to the Stronie Group. It is observed in the field that the WP tholeiites and Ti-tholeiites are accompanied by felsic volcanites which injected the former and the MORB-like tholeiites intruded other mafites. Both field evidence and geochemistry confirm earlier conclusion of Opletal et al. (1990) who concluded that the bimodal magmatic rocks were derived from the same magma source. U-Pb SHRIMP datings of acid metavolcanics in the NMG reveal an age of ~500 Ma, identical with that determined for the SG rocks. In the NMG, the WPT and MORB-like metabasites have rather uniform isotopic signature εNd(500) ~5–7 suggestive of similar mantle source and possibly weak contamination with crustal material. A MORB source transitional between N-MORB and E-MORB and an enriched mantle source are suggested. A N-MORB-like source might possibly be mixed with an enriched OIB-like source (plume). For the WPT, both the isotopic and characteristic elemental ratios may also point to mild contamination due to some crustal admixtures. The SG MORB-like metabasites have similar isotopic signature to the NMG metatholeiites εNd(500) ~5–6. They come from a common depleted mantle source. Isotopic signature εNd(500) ~2–4 of the SG alkali basalts suggest derivation of the alkali magmas from a different more enriched mantle source than the source of the metatholeiites. The mafic magmatism recorded in the NMG and SG likely developed in a relatively short period of Mid-Late Cambrian magmatic episode in a continental rift-related (probably backarc) setting which reached the stage of new oceanic crust production represented by MORB-like metabasites. The scarce alkali basalts from the eastern part of the SG area are assigned to earlier stages of the rifting. Palaeogeographic assignment of the rift basin needs dating of its sedimentary-volcanogenic infilling. Further U-Pb datings are under way.
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