Contrasting provenance signals in Riphean and Vendian sandstones in the SW Urals (Russia): constraints for a change from passive to active continental margin conditions in the Neoproterozoic

Abstract

Two contrasting provenance areas for Neoproterozoic sandstones can be distinguished in the southwestern Urals by light and heavy mineral analyses as well as by mineral chemistry. These reflect a dramatic change of geotectonic conditions at the eastern border of the Baltica protocontinent at around 610–620 Ma. Detritus from Riphean and Lower Vendian sandstones representing about 1 Ga of continuous sedimentation is characterised by a ‘continental platform provenance’ reflecting its derivation from the Proterozoic basement of the East European platform. Source rocks include mainly minerals and few lithic clasts from granitoids and high-grade metamorphic rocks as well as reworked clastic sediments and silicic volcanic rocks. The provenance signal changes abruptly with Upper Vendian detritus representing a ‘recycled orogenic provenance’. This includes mineral and lithic clasts of low-grade siliciclastic metasediments and mylonites containing phengites with a high-pressure signature as well as clasts of bimodal volcanic rocks and reworked siliciclastic sediments. The composition of phengites contrasts strongly with those of detrital white mica in the Riphean rocks. Also, the composition of tourmalines derived from a metapsammopelitic source indicates a mainly Al-poor metasediment provenance, whereas Riphean tourmalines were derived from mainly Al-rich metapsammopelites. Zircon morphology, tourmaline zoning and a reduced heavy mineral spectrum provide evidence for polycyclic sedimentation during Riphean and Vendian. Upper Vendian sedimentary rocks were deposited in a foreland basin derived from a proximal uplift to the east. Provenance characteristics of the Upper Vendian detritus are consistent with areas affected by a high-pressure/low-temperature metamorphism during a pre-Uralian orogenic event, most likely from the metamorphic complex of Beloretsk, which was emplaced and exhumed during the Upper Vendian. The change of geotectonic conditions in the Upper Vendian reflects a change from a passive continental margin in the South Urals throughout the entire Riphean since at least 1350 Ma to a convergent continental margin within a presumed transpressional setting.