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首页> 外文期刊>Journal of Petrology >The Petrogenesis of Volcaniclastic Komatiites in the Barberton Greenstone Belt, South Africa: a Textural and Geochemical Study
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The Petrogenesis of Volcaniclastic Komatiites in the Barberton Greenstone Belt, South Africa: a Textural and Geochemical Study

机译：南非巴伯顿绿岩带火山碎屑科马提岩的岩石成因：结构和地球化学研究

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The Onverwacht Group of the 3·5–3·2 Ga Barberton greenstone belt, South Africa contains multiple stratigraphic units that include laterally extensive beds of komatiitic ash, accretionary lapilli, and lapilli. These units have been affected by pervasive silicification, serpentinization, or, less commonly, carbonate metasomatism. Silicification resulted in SiO2 + K2O > 85 wt % and depletion of most other major and trace elements. Most of these tuffs have prominent high Hf/Hf* and Zr/Zr* (0·5–12), which cannot result from normal magmatic processes but are due to the typically immobile rare earth elements migrating during post-silicification fluid–rock interaction. Similarly, their low Ce/Ce* values do not reflect Archean surface redox conditions but the circulation of later oxidizing fluids. Despite this intense alteration, ratios of Al2O3 and TiO2 remain uniform and coherent within single volcanic units. These ratios indicate that most silicified tuffs are not petrogenetically related to the underlying or overlying komatiitic flow rocks and that each originated from either separate mantle sources or different partial melting conditions. Serpentinized tuffs retain komatiitic element abundances but Al2O3 fails to define a tight linear array with the demonstrably immobile elements Ti and Zr. We speculate that this is due to post-depositional mixing of Al-depleted and Al-undepleted tuff layers by aqueous currents. Excellent textural preservation of the silicified tuffs shows they are characterized by a dearth of phenocrysts, low particle vesicularity and abundance of fine vitric ash, suggesting the eruption and rapid quenching of superheated or near-liquidus anhydrous magmas. Minor assimilation of hydrated basaltic or ultramafic crust within the dry magma may have enhanced the surface phreatomagmatic explosivity while still allowing the magma to rise close to an adiabatic ascent path. However, textural and geochemical evidence for such a process is scarce. Temporal and compositional constraints show that the diversity in the types of komatiites throughout the Onverwacht Group can be accounted for by variations in plume–mantle dynamics and that komatiitic tuffs were deposited during intervals of volcanism characterized by low effusive eruptive volumes and/or low emplacement rates.
机译：南非3·5–3·2 Ga巴伯顿绿岩带的Onverwacht组包含多个地层单元，包括侧向广泛的科莫特灰，增生性lapilli和lapilli床。这些单元受到普遍的硅化作用，蛇形化作用或碳酸盐交代作用的影响。硅化作用导致SiO 2 + K 2 O> 85 wt％，大多数其他主要和痕量元素耗尽。这些凝灰岩中的大多数具有显着的高Hf / Hf *和Zr / Zr *（0·5-12），这不是正常的岩浆作用造成的，而是由于硅化后流体-岩石相互作用过程中迁移的稀土元素通常不可移动。同样，它们的低Ce / Ce *值也不能反映出太古宙表面的氧化还原条件，而不能反映后来的氧化流体的循环。尽管发生了剧烈的变化，但单个火山单元中Al 2 O 3 和TiO 2 的比例仍保持一致和连贯。这些比率表明，大多数硅化凝灰岩与成岩作用不与下伏或上覆的科马替质流动岩相关，并且每个都来自不同的地幔源或不同的部分熔融条件。蛇纹凝灰岩保留了丰富的komatiitic元素，但Al 2 O 3 未能与元素Ti和Zr固定在一起形成紧密的线性阵列。我们推测这是由于水流在沉积后混合贫铝和未脱铝凝灰岩层所致。硅化凝灰岩的优异质地保存表明，它们的特点是缺乏晶状体，低颗粒状泡状结构和大量细粉状灰烬，表明过热或接近液相的无水岩浆的喷发和快速淬灭。干燥岩浆中水合玄武岩或超镁铁矿壳的少量同化可能增强了地表岩浆的爆炸性，同时仍使岩浆上升到接近绝热的上升路径。但是，这种过程的质地和地球化学证据很少。时间和成分上的限制表明，整个Onverwacht组中的考玛铁矿类型的多样性可以通过羽幔动力的变化来解释，并且在火山喷发期间沉积了胶结凝灰岩，其特征是喷发量低和/或落位率低。 。

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