Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea

Abstract

Abstract The Karadag Massif in south-eastern Crimea hosts a thick sequence of volcanic rocks, which underwent significant secondary alteration. Here we deduce the effects of alteration, remove their mask and estimate the primary chemical composition and age of these volcanic rocks using their bulk chemical analyses, as well as chemical data from plagioclase, ortho- and clinopyroxene-hosted melt inclusions, and Ar isotope analyses of plagioclase and groundmass from the freshest andesite. Our data reveal two distinct magmatic series in the Karadag Massif. The first defines a subalkaline, calcic, medium-K series with high enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, which is typical of supra-subduction volcanic rocks. According to previously published trace element data, most of the Karadag volcanic rocks belong to this series. We use Ar isotope data to argue that the age of these rocks is 172.8\u202f±\u202f4.5\u202fMa, which is consistent with the biostratigraphy. No reliable major-element chemical data and no isotopic data were obtained from the second series, although its trace-element signature is similar to that of some Nb-enriched supra-subduction volcanic rocks. It is characterised by less pronounced enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, and relatively high contents of Nb and Ta. Typical supra-subduction series of the Karadag Massif is similar to other pre-Cretaceous magmatic rocks exposed in Crimea, and hence likely formed in a similar environment. Literature data on Middle Jurassic magmatic rocks of the Pontides Belt in northern Turkey also allow to distinguish two magmatic series, which closely correspond to the magmatic rocks in Crimea. This is consistent with palaeotectonic reconstructions which suggest that the Pontides Belt was joined to Crimea prior to the opening of the Black Sea, and they formed a single volcanic arc.