Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)
Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)
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DOI：
10.1007/s00445-012-0619-8
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发表时间：
2012-09-01
影响因子：
3.5
通讯作者：
Stuart, F. M.
中科院分区：
文献类型：
--
作者：
Brown, Richard J.;Manya, S.;Stuart, F. M.
坦桑尼亚的Igwisi火山（IHV）在保存火山口外的熔岩和火山碎屑建筑物方面是独特和重要的。它们为金伯利岩岩浆的喷发行为提供了重要的见解，而这在其他已知的金伯利岩火山中是无法获得的。IHV熔岩中橄榄石晶体的宇宙成因He-3定年和古地磁分析表明，它们的时代为晚更新世至全新世。这使它们成为地球上已知最年轻的金伯利岩体，年龄超过30 Ma，可能表明坦桑尼亚克拉通金伯利岩火山作用的新阶段。地质测绘、全球定位系统测量和实地调查显示，每座火山都由部分侵蚀的火山碎屑岩、火山口和熔岩组成。这些火山位于周围地面以上40米以下，其大小与小型单成因玄武岩火山相当。火山碎屑锥由散层火山碎屑落沉积物组成，包括火山渣、球粒和致密的幼年火山碎屑。火山碎屑与许多古代金伯利岩管道中记录的火山碎屑相似，表明Igwisi火山喷发和其他金伯利岩喷发之间的岩浆破碎动力学存在重叠。火山碎屑锥沉积物的特征，包括弹道碎屑和主要是不良泡状火山渣火山灰岩和火山砾凝灰岩的情况下，表明相对较弱的爆炸活动。熔岩流特征表明金伯利岩具有出乎意料的高粘度（估计> 10（2）至10（6）Pa s），这归因于脱气和火山口冷却。每一座火山都被推断为是一次小规模的、短暂的（几天到几周）单成因喷发的结果。每座伊格维西火山的喷发过程大致相似，并经历了三个阶段：（1）在爆炸挖掘火山口和管道时，含岩屑的火山碎屑岩沉降;（2）不稳定喷发柱的幼年火山砾沉降，并在喷口周围建造火山碎屑大厦;（3）熔岩流时，脱气的粘性岩浆渗出。这些过程类似于其他小体积的单成喷发（如玄武岩浆）。
The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic He-3 dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by > 30 Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand < 40 m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at > 10(2) to 10(6) Pa s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).