Interactions between distal epiclastic and bio-chemogenic sedimentation at the foothills of a mafic alkaline volcano: The case of the Oligocene Doupovské Hory Volcanic Complex (Czech Republic)

Late Oligocene (ca 25 Ma) volcano-sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non-radiogenic ⁸⁷Sr/⁸⁶Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (⁸⁷Sr/⁸⁶Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water-level fluctuations, possibly reflecting the alternating rainy and dry-seasons of a prevalently humid Central-European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant ¹⁸O enrichments. Although the ca 0‰ δ¹³C values might suggest mixing of atmospheric and volcanic CO₂ during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ¹³C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non-radiogenic Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non-radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt-sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.