Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones
Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones
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南非巴伯顿绿岩带 3.5-Ga Komati 组的泡状科马提岩：海底熔岩的膨胀和三齿稃带的起源
DOI：
10.1007/s004450100164
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发表时间：
2001
影响因子：
3.5
通讯作者：
J. Dann
中科院分区：
文献类型：
--
作者：
J. Dann
抽象。3.5 Ga科马蒂组的科马蒂岩是在海底熔岩平原环境中喷发的超镁铁质熔岩（>23% MgO）。新发现的泡状科马提岩具有被同火山结构破坏的泡状上地壳，这种结构类似于现代熔岩流的膨胀相关结构。详细的露头图显示了具有2-15 m厚的上部泡状带的流动，这些泡状带（1）因差异膨胀而旋转，（2）被来自流动内部的堤坝侵入，（3）延伸，形成淹没地堑，和/或（4）完全吞没。最大的充气结构是一个20米的表面浮雕，这是由一个复合流动单位的刺流叶覆盖的古墓。熔岩膨胀并旋转了上部的囊状地壳，但没有形成囊状，而是结晶成一个厚的刺带，里面有拳头大小的骨架橄榄石。在膨胀期间，三齿刺带在绝缘的上地壳下缓慢冷却，而不是代表快速冷却的熔岩。膨胀的熔岩的超压可能抑制了水泡形成。这项工作描述了已知的最古老的泡状科马提岩，说明了第一个现场证据膨胀结构科马提岩流，提出了一个新的因素，在发展的刺带，并得出结论，膨胀模型是有用的了解科马提岩海底流场的演变。
Abstract. Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2–15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.