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

洪水玄武岩火山活动的爆发往往是爆炸性的，但相对于随之而来的大量洪水玄武岩来说，这种爆炸性火山活动往往被忽视了。我们在这里记录了在南非卡鲁大火成岩省(LIP)最早的泛滥玄武岩(LIP)之前并与之互层的异常巨大的玄武岩角砾岩。目前还没有关于这些矿床的详细研究或相模型，现有的火山爆发模型也无法解释它们。卡鲁角砾岩露头面积1000平方公里，总体积至少500平方公里，可划分为体积高达27平方公里的流动单元。角砾岩由以沉积砂粒为主的基质中的幼年玻璃碎屑组成。这些沉积物-岩浆混合物充填了巨大的火山口状坍塌结构，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床层。角砾岩中的幼年碎屑与伴生的流体珍珠岩中的幼年碎屑非常相似，表明了从地下到地表的连续混合过程。坍塌构造含有与大块熔融砂岩并列的榴辉岩体，表明有效的热传递间隔很长。这项研究将通过研究和比较充溢和流出相，以及通过观察典型流出表中近端和远端的变化，来研究导致Karoo熔岩产生的喷发前、喷发和侵位机制。我们将开发和测试这一假设，即在混合之前，通过持续和不断补充的高水平玄武岩底板，将大片湿润的松散沙子衬底。混合和喷发可能伴随着重力不稳定的泥浆的上涌，或者从距离较近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填图和相分析来推断侵位过程，并对年轻碎屑进行扫描电子显微镜和图像分析，以约束喷发前的脱气和破碎机制。在第一年，我们将研究Sterkspruit杂岩的充填序列，之所以选择它，是因为除了良好的曝光率外，它的边缘还包含一个不同寻常的近喷口沉积。在第二年，我们会将研究范围扩展至该综合设施以外的地方，以确定外流沉积的特征，并把该综合设施放在一个区域的范围内。我们将使用化学地层学来构建喷流相和爆炸相之间的相互作用，并将通过Ar/Ar年龄测定来限制这项工作。这项研究对所有溢流玄武岩省的喷发开始都有意义。怀特和学生们目前正在研究类似的沉积物，以寻找与南极洲地层学上相当的菲拉尔唇。在北大西洋第三系东格陵兰地区的泛滥玄武岩底部和西伯利亚圈闭唇部也发现了鲜为人知的大量角砾岩。洪泛玄武岩对全球影响的模型仅以喷流成分为基础。这项研究将增加数据，使这些模型能够得到完善。

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.