Origins of layers in layered mafic intrusions

大型火成岩省代表着数百万立方千米的极热岩浆穿过地壳，要么作为侵入岩的集合体侵位，要么作为大量熔岩流喷发。大型火成岩省内的侵入岩以富镁铁(即镁铁质)岩石为主，并显示出表面上类似于某些沉积岩中的分层，因此通常被称为层状镁铁质侵入岩(LMI)。LMI拥有世界上最重要的重要金属储量，包括铬和铂；它们也是铜、镍、钯、磷、铁、钛、钒和Nb的重要来源。LMI中矿化层的成因仍然是一个激烈的研究活动和争议问题。LMI研究界倾向于支持深达10公里、宽达数百公里的岩浆室的概念，在其中晶体沿着底部堆积形成向上堆积的一系列层。然而，对花岗岩侵入体中的层状岩石和大洋中脊环境中的层状镁铁质侵入体的研究表明，在许多情况下，分层是分离的水平薄片岩浆反复侵入产生层状岩床堆叠的结果。这种差异对于指导新发现的矿床群(如安大略省西北部的火环矿床)的矿产勘探战略至关重要。我的团队和其他一些人最近建议，在许多例子中，非常大的地台形状的LMI的岩浆室假说应该被层叠的岩床顺序就位的想法所取代。这项拟议研究的目标是对LMI的窗台假设及其一些竞争假说进行严格的测试。一些需要回答的关键问题将包括：“我们如何识别侵入较老岩基的岩床边缘，这些岩床本身仍然非常热？”；“我们能探测到LMI中岩床附近岩石发生短暂而强烈的加热的证据吗？”；“如果岩浆固化而没有在其内部部署岩床，预计会产生什么样的岩石序列？”我们将使用LMI的四个例子作为自然野外实验室：南非的标志性Bushveld杂岩和蒙大拿州的Stillwater杂岩，它们是底板假说的候选者；安大略省的萨德伯里火成岩杂岩，它被认为是一个经典的岩浆室；以及魁北克的Kyak Bay侵入体，它的地质鲜为人知，但暴露得非常好。这项工作将涉及岩石和矿物成分和纹理的测绘、采样和测量。现场观测将与使用热力学和数值模型进行的预测进行比较，这些过程包括化学扩散、晶体成核和生长、火成岩结构成熟、岩浆流动和侵位以及岩浆及其固体围岩内的热流。

Large igneous provinces represent events during which millions of cubic km of extremely hot magma passed through the Earth's crust and were either emplaced as collections of intrusions or erupted as voluminous lava flows. Intrusions within large igneous provinces are dominated by magnesium- and iron-rich (i.e., mafic) rocks and display layering that superficially resembles the layering in some sedimentary rocks, hence the general term "layered mafic intrusions" (LMI). LMI contain the world's most important reserves of important metals including chromium and platinum; they are also important sources of copper, nickel, palladium, phosphorus, iron, titanium, vanadium, and niobium. The origins of mineralized layers in LMI remain a matter of intense research activity and controversy. The LMI research community tends to favor the concept of magma chambers as large as ten km deep and hundreds of km wide within which crystals accumulate along the base to form an upward-aggrading series of layers. However, the study of layered rocks in granitic intrusions and of layered mafic intrusions at mid-ocean ridge settings has shown that in many cases layering results from the repeated intrusion of separate thin horizontal sheets of magma to produce a layered stack of sills. The difference is critical for guiding mineral exploration strategies in newly discovered deposit groups like the Ring of Fire deposits of northwestern Ontario. My group and a few others have recently suggested that the magma chamber hypothesis for very large sill-shaped LMI should be replaced by the idea of sequential emplacement of stacks of sills in many examples. The objective of the proposed research is to develop rigorous tests of the sill hypothesis for LMI and some of its competing hypotheses. Some key questions to be answered will include: "How can we recognize the margins of sills intruded into older sills that remain very hot themselves?"; "Can we detect evidence for short, intense periods of heating of rocks adjacent to sills within LMI?"; "What sequence of rocks is expected to be produced if a magma solidifies without having sills emplaced within it?" We will use four examples of LMI as natural field laboratories; the iconic Bushveld Complex of South Africa and Stillwater Complex of Montana, which are candidates for the sill hypothesis, the Sudbury Igneous Complex of Ontario, which is known to have been a classical magma chamber, and the Kyak Bay Intrusion of Quebec, whose geology is poorly known but exceptionally well-exposed. The work will involve mapping, sampling and measurement of rock and mineral composition and textures. The field observations will be compared with the predictions that can be made using thermodynamic and numerical models of igneous processes such as chemical diffusion, crystal nucleation and growth, maturation of igneous textures, magma flow and emplacement, and heat flow within magmas and their solid host rocks.