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Late Cretaceous high-potassium volcanism in eastern Srednogorie, Bulgaria

¹ Istituto di Geologia e Paleontologia dell'Università, Via Lamarmora, 4 Firenze, Italy
² Istituto di Mineralogia, Petrografia e Geochimica dell'Università, Via Lamarmora, 4 Firenze, Italy
³ Geological Institute of Bulgarian Academy of Sciences, Sofia, Bulgaria

The Srednogorie zone of Bulgaria was the site of extensive Late Cretaceous calc-alkalic and shoshonitic magmatism. The calc-alkalic volcanism developed mainly in the central and western parts, whereas shoshonitic volcanism was the predominant magmatic feature of the eastern part. Leucitic basanites, limburgites, and picrites occur in minor amounts in eastern Srednogorie. The shoshonitic volcanic rocks are intermediate to silicic in composition and are characterized by high K₂O, K₂O/Na₂O around unity, high Rb, Sr, and Ba, and low TiO₂ contents. They are associated with calc-alkalic rocks less strongly enriched in K and incompatible elements. Petrographically, the calc-alkalic rocks are characterized by the presence of hydrous mineral phenocrysts that are not present in the shoshonitic rocks. Shoshonitic and calc-alkalic rocks have some common chemical characteristics such as low TiO₂, lack of absolute iron enrichment, and large overlap in many trace-element abundances. These are considered evidence for a genetic relationship between their primary magmas, which may have undergone different degrees of enrichment in incompatible elements.

The eastward increase of K-rich volcanic rocks relative to calc-alkalic rocks in the Srednogorie zone is believed to be related to distension tectonics connected with the opening of the Black Sea.

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