Subduction related processes in the Borka Nappe (Inner Western Carpathians): a geochemical and petrological approach

In the Inner Western Carphatians, the rocks of the Bôrka Nappe record clear effects of subduction-related Alpine petrogenetic processes. They are metabasalts and associated metapelites. Notwithstanding the mobility of some chemicals (alkalis and light elements) during metamorphism, the geochemistry of metabasalts show features which suggest either an oceanic or a back-arc scenario for their emplacement. Specifically, trace and RE elements indicate that these rocks have a sub-alkaline to moderately alkaline affinity, and their chemical features are consistent with those of OFB and BABB. Most of the normalised REE patterns show a slight LREE enrichment, a relatively high La/Sm ratio and a clear depletion of Lu, suggesting an E-MORB affinity for these rocks. However, some samples better fit a typical N-MORB chemistry. As regards the metamorphic evolution of these metabasalts, Gln - Chl - Ep - Ab - Ttn +/- Phn and Na-Px - Gln - Chl - Ep - Ttn +/- Phn mineral assemblages indicate that these rocks underwent a blueschist facies metamorphism. This event was followed by a stage of isothermal decompression, recorded by the overprinted Act - Chl - Ep - Ab - Ttn mineral assemblage. These mineral phases nucleated at the rims of former Gln crystals, in veinlets and in small, irregularly shaped, coarser-grained domains within the matrix. The associated metapelites are characterised by the Cld - Chl - Ab - Phn +/- Pg and Gln - Grt - Ab - Phn +/- Pg mineral assemblages, referable to the earlier HP metamorphic phase. Gln was destabilised during a later metamorphic stage and locally replaced by Chl - Qtz +/- Ab. The Grt - Chl - Ms - Bt - Ab and Cld(II) - Chl - Ms - Ab mineral assemblages therefore crystallised. Both whole rock geochemistry and polyphase metamorphic evolution of metabasites and associated metapelites are consistent with a scenario involving subduction of an oceanic crust of a narrow ocean, followed by a rapid exhumation along an isothermal path.