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

洪水玄武岩火山作用的开始常常是爆炸性的，但是这种爆炸性的火山作用相对于随后大量的洪水玄武岩常常被忽视。我们在这里记录了不寻常的体积玄武岩角砾岩，这些角砾岩早于南非卡鲁大火成岩省（LIP）最早的洪水玄武岩并与之互层。对这些矿床没有详细的研究或相模型，现有的爆发火山作用模型也不能解释它们。卡鲁角砾岩露头面积达1000km2，总积至少500km3，可划分为流单元，体积可达27km3。角砾岩由幼年玻璃质碎屑组成，基质以沉积砂粒为主。这些沉积物-岩浆混合物充满了大型的火山口状崩塌结构，它们可能就是从这些结构中喷发出来的，并且还形成了与最早的卡鲁熔岩互层的流出层。角砾岩中的幼年碎屑与伴生的流体碎屑非常相似，表明从地下到地表的混合过程是连续的。崩塌的构造包含白云岩体与大块熔融砂岩并置，表明有效的热传递间隔较长。本研究将通过研究和比较充填相和流出相，并通过观察代表性流出片的近端和远端变化，研究导致卡鲁熔岩形成的喷发前、喷发和侵位机制。我们将发展并验证这样的假设，即在混合之前，大面积的湿的未固结的沙子被持续不断地补充的玄武岩的高级技能所覆盖。混合和喷发可能伴随着重力不稳定的浆液的上涌，或者岩浆-沉积物混合物的喷射，这些混合物来自间隔很近但短暂的喷口。我们将使用详细的地质填图和相分析来推断侵位过程，并使用幼年碎屑的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杂岩的充填序列，选择这个杂岩的原因是，除了极好的暴露外，它的边缘还含有一个不寻常的近喷口矿床。在第二年，我们将把研究扩展到综合体之外，以表征流出沉积，并将综合体置于区域背景下。我们将使用化学地层学来构建涌出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。该研究对所有洪水玄武岩省的火山爆发具有指导意义。怀特和他的学生们目前正在研究类似的沉积物，在地层学上与南极费拉LIP相当。大量角砾岩也出现在北大西洋第三系东格陵兰地区的洪水玄武岩底部和西伯利亚圈闭构造中，研究很少。洪水玄武岩的全球影响的模型仅基于溢流成分进行预测。这项研究将增加数据，使这些模型的细化。

HoughtonEAR-0106237The 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.