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)

IF 1.9 3区 地球科学 Q1 GEOLOGY Depositional Record Pub Date : 2023-06-06 DOI:10.1002/dep2.240
Vladislav Rapprich, Pavel Čáp, Yulia V. Erban Kochergina, Eva Kadlecová, Zsolt Benkó, Jakub Sakala, Zuzana Rodovská, Jan Matějů, Daniel A. Petrash
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Abstract

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.

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基性碱性火山山麓远端碎屑沉积与生物化学沉积的相互作用:以捷克渐新世doupovskrev - Hory火山杂岩为例
晚渐新世(约25 Ma)的火山-沉积序列暴露在doupovsk Hory火山杂岩的西边缘,揭示了一个复杂的沉积历史,在不同方面受到碱玄武岩火山建筑的衰变的影响。火山岩的风化作用提供了丰富的反应物,促进了外围古湖泊的碳酸盐沉积,87Sr/86Sr值(0.7038 ~ 0.7041)是非放射性成因的。另一方面,最初浅湖的沉积物由于碎片雪崩的推平作用而变形。泥石流和雪崩沉积充填了原有的凹陷,改变了盆地的形态,使周边的湖相环境向远离火山的方向移动。在这一阶段,来自周围花岗岩的地表水流入使钙砾岩矿床具有更强的放射性成因特征(87Sr/86Sr值为0.7046 ~ 0.7049)。化石组合和石灰石结构表明了显著的季节性水位波动,可能反映了普遍潮湿的中欧晚渐新世气候的雨季和旱季交替。池塘的季节性干涸导致18O显著富集。虽然约0‰的δ13C值可能表明碳酸盐降水过程中大气CO2和火山CO2的混合作用,但在邻近地区未发现相应年龄的活火山导管。较低的δ13C值可能是地幔通过裂谷断层脱气的结果,这种现象在岩浆灭绝的Ohře裂谷中一直存在。本文认为,风化碱性玄武岩灰岩具有高度非放射性成因的Sr同位素比值(87Sr/86Sr ca 0.704),表明这些碳酸盐岩中ca的岩浆成因。这些结果表明,在碱性玄武岩环境中形成的碳酸盐岩中的Sr同位素只能揭示Sr(和Ca)离子的来源,而不一定是碳酸盐岩的存在,与发现高度非放射性成因的Sr同位素时碳酸盐岩存在的概念相反。
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CiteScore
4.10
自引率
16.70%
发文量
42
审稿时长
16 weeks
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