Carbon, oxygen, and strontium isotopic fingerprint in Neoproterozoic to early Cambrian limestones in an active margin setting: A record of local environment or global changes?

Abstract

The Ediacaran to early Cambrian limestones preserved within the Blovice accretionary wedge, Bohemian Massif, are part of Ocean Plate Stratigraphy (OPS) and provide an intriguing example of how paleooceanographic and palaeoenvironmental conditions at active plate margins may be recorded by isotopic composition. Field relations, petrography and geochemical data elucidate the complex interplay of oceanic, diagenetic and volcanic processes. Rare calcite-dominated primary oolitic/pisolitic limestones were deposited on the slopes of seamount volcanoes in shallow-water lagoons and their formation was simultaneous with volcanic activity. In contrast, volumetrically more abundant re-deposited limestones occurring as layers within siliciclastic, graywacke-dominated successions consist of clastic fragments derived from previously formed carbonate ramps, which were disrupted during subduction and subsequently incorporated in deep-water turbidity currents. In both limestone types, the major/trace element as well as C, O and Sr isotope signatures are well preserved and provide a fingerprint of various limestone depositional conditions that can be directly linked with paleooceanic and paleogeographic interpretations. The observed spatial distribution of carbonate-bearing silicate rocks with elemental and isotopic compositions that differ from primary limestones suggest their common origin at active margins and indicate that some of these rocks have been formed by hydrothermal activity. Finally, we demonstrate that the carbonate member of OPS may provide important insights into the polarity and dynamics of subduction zones.