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GEOCHEMISTRY OF METABASIC DYKE-LIKE BODIES IN THE SREDOGRIV LOW-GRADE METAMORPHIC ROCKS, WESTERN BALKAN ZONE, NW BULGARIA
Abstract
The Sredogriv low-grade metamorphic rocks belongs to the Montana unit of the Western Balkan Zone in northwestern Bulgaria. The low-grade rocks mainly consist of metamorphosed greenschist facies clastic-marly-tuffaceous precursors and minor basic to acidic, presumably olistostromic magmatic rock bodies. Here, we present whole-rock geochemistry of metabasic dyke-like bodies in the Sredogriv metamorphic rocks. Field observations revealed that the dyke-like bodies are intruded into the Sredogriv sedimentary succession and do not represent olistostromic bodies, which is demonstrated by consistent elongation of the dykes that strike ESE-WNW to E-W. The dyke-like bodies demonstrate a weak metamorphic overprint. Major elements were analyzed by X-ray fluorescence facility at the Sofia University St. Kliment Ohridski, and the trace elements and rare-earth elements were determined by laser-ablation inductively coupled mass-spectrometry at the Geological Institute of the Bulgarian Academy of Sciences. Our recent study on the detrital zircons in the Sredogriv metaconglomerate yielded a maximum depositional age of 523 Ma, additionally maximum age of deposition is defined at 263 Ma for a conglomerate of the Smolyanovtsi Formation from the first unconformable sedimentary cover. Mostly continental detrital material from magmatic source built the sedimentary protolith of the Sredogriv metamorphic rocks, which are recycled in the clastic cover rocks. However, at present, the bulk chemical composition of the metabasic dyke-like bodies remains poorly understood. Our study shows that the dyke-like bodies represent enriched ocean-floor magmatic products, which are more likely crystallized in a back-arc tectonic setting.
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