Volcaniclastic turbidites of the Coşuştea Nappe: a record of Late Cretaceous arc volcanism in the South Carpathians (Romania)

Abstract

Sedimentological and mineralogical composition of Upper Cretaceous terrigenous and volcaniclastic sandstones are presented for the Coşuştea Nappe of the South Carpathians, Romania, in order to constrain the provenance and tectonic setting of deposition. Existing geochemical data on volcaniclastic rocks were interpreted using discrimination diagrams in order to get additional information. The Coşuştea Nappe includes terrigenous turbidites, overlain by upward coarsening sequences of volcaniclastic turbidites, both associated with a strongly dismembered mélange complex. Facies association and vertical facies distribution suggest that terrigenous successions are midfan turbidites, dominated by deposition in suprafan channels. Their sandstone mineralogy indicates that a major sediment source, located on the upper plate, provided detritus of Getic type metamorphic basement and withinplate volcanic rocks, with minor input from the accretionary wedge. Volcaniclastic sedimentation took place as dominantly sandstone deposition in supracone lobes, followed by coarse sedimentation as channelized debris flows. Vertical facies distribution suggests evolution in time from midfan to proximal fan turbidites. Mineralogical composition of volcaniclastic sandstones indicates provenance from a major volcanic source, with minor contributions from the accretionary wedge and from an upper continental plate supplying terrigenous siliciclastic detritus, and suggests that volcaniclastic turbidites accumulated in a trench or a slope forearc basin. Geochemical data indicate resedimented volcanic arc material, with intermediate to basic composition and calc-alkaline geochemistry. The volcanic source was very likely represented by the Maastrichtian volcanism related to the Banatitic Magmatic and Metallogenetic Belt from the western South Carpathians.