Collaborative Research: Explaining the Eruption of Huge Volumes of Magma-Sediment Mixtures Preceding Flood Basalt Effusion, Karoo Lip, South Africa

合作研究：解释洪水玄武岩喷发之前大量岩浆-沉积物混合物的喷发，南非卡鲁唇

• 批准号: 0332115
• 负责人: Ian Skilling
• 金额: $ 12.74万
• 依托单位: University of Pittsburgh
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-09-30 至 2004-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0332115&HistoricalAwards=false
• 关键词: Collaborative; Research; Explaining; Eruption; Huge

SkillingEAR-0106103The onset of flood basaltic volcanism is frequently explosive but this explosive volcanism is often overlooked with respect to the voluminous flood basalts that follow. We document here unusual voluminous basaltic breccias that predate and are interbedded with the earliest flood basalts of the Karoo Large Igneous Province (LIP), South Africa. No detailed studies or facies models for these deposits exist, nor can existing models for explosive volcanism explain them. The Karoo breccias have an outcrop area of 1000km2, a total volume of at least 500km3, which can be divided into flow units with volumes which may be as high as 27km3. The breccias consist of juvenile vitric clasts in a matrix dominated by sedimentary sand grains. These sediment-magma mixtures infill large caldera-like collapse structures from which they were probably erupted, and also form outflow sheets interbedded with the earliest Karoo lavas. Juvenile clasts in the breccias are very similar to those of associated fluidal peperites suggesting a continuum of mixing processes from the subsurface to surface. The collapse structures contain bodies of dolerite juxtaposed with large blocks of fused sandstone, suggesting prolonged intervals of efficient heat transfer.This study will look at pre-eruptive, eruptive and emplacement mechanisms that led to the production of the Karoo lavas, by studying and comparing infill and outflow facies, and by looking at proximal to distal variations within a representative outflow sheet. We will develop and test the hypothesis that mixing was preceded by underplating of large areas of wet unconsolidated sand by sustained and constantly replenished high-level sills of basalt. Mixing and eruption may have been accompanied by upwelling of gravitationally unstable slurries, or jetting of magma-sediment mixtures issuing from close-spaced, but ephemeral vents. We will use detailed geological mapping and facies analysis to infer emplacement processes and SEM and image analysis of juvenile clasts to constrain pre-eruptive degassing and fragmentation mechanisms. In year 1, we will study the infill sequence of the Sterkspruit Complex, selected because, in addition to excellent exposure, it contains an unusual near-vent deposit on its margin. In year 2, we will extend the study beyond the Complex to characterize outflow deposition and place the complex in a regional context. We will use chemical stratigraphy to construct the interplay between effusive and explosive phase, and will constrain this work by Ar/Ar age determinations.The study has implications for the onset of eruption in all flood basalt provinces. Similar deposits are currently under study, by White and students, for the stratigraphically equivalent Ferrar LIP of Antarctica. Little studied voluminous breccias also occur at the base of the flood basalts in the East Greenland area of the North Atlantic Tertiary LIP, and in the Siberian Traps LIP. Models for the global impact of flood basalts are predicated only on the effusive component. This study will add data that will enable refinement of these models.

溢流玄武岩火山作用的开始通常是爆炸性的，但这种爆炸性火山作用往往被忽视了大量的溢流玄武岩。 我们在这里记录不寻常的大量玄武岩角砾岩，早于最早的洪水玄武岩的卡鲁大火成岩省（LIP），南非。 没有详细的研究或相模型，这些矿床存在，也不能现有的模型爆炸火山解释他们。 卡鲁角砾岩的露头面积为1000 km 2，总体积至少为500 km 3，可划分为体积高达27 km 3的流动单元。 角砾岩由沉积砂为主的基质中的幼年玻璃质碎屑组成。 这些沉积物-岩浆混合物填充了大的类似火山口的塌陷结构，它们可能是从那里喷发出来的，也形成了与最早的卡鲁熔岩互层的流出岩片。 角砾岩中的幼年碎屑与相关的流体碎屑非常相似，表明从地下到地表存在连续的混合过程。 塌陷构造包含与大块熔融砂岩并列的辉绿岩体，表明有效传热的间隔时间较长。本研究将通过研究和比较填充相和流出相，并通过观察代表性流出岩片内近端到远端的变化，来研究导致卡鲁熔岩产生的喷发前、喷发和就位机制。 我们将发展和检验这样一个假设，即在混合之前，大面积的湿松散砂被持续不断地补充的高水平玄武岩岩床底托作用。 混合和喷发可能伴随着重力不稳定泥浆的上涌，或从近距离但短暂的喷口喷出的岩浆-沉积物混合物。 我们将使用详细的地质绘图和相分析来推断侵位过程，并使用年轻碎屑的SEM和图像分析来约束喷发前的脱气和破碎机制。 在第一年，我们将研究Sterkspruit复合体的填充序列，选择它是因为除了极好的暴露外，它的边缘还含有不寻常的近喷口存款。 在第二年，我们将把研究扩展到复杂的范围之外，以描述外流沉积的特征，并将复杂的区域背景下。 我们将使用化学地层学来构建喷溢和爆炸阶段之间的相互作用，并将限制这项工作的Ar/Ar年龄determines.The研究的爆发在所有的洪水玄武岩省的影响。 目前，白色和学生们正在研究南极洲与费拉LIP地层相当的类似矿床。 很少研究大量的角砾岩也发生在北大西洋第三纪LIP的东格陵兰地区的洪水玄武岩的基础上，并在西伯利亚圈闭LIP。 洪水玄武岩的全球影响的模型预测只有在溢流组件。 这项研究将增加数据，使这些模型的完善。