Origins of layers in layered mafic intrusions

大型火成岩省代表了数百万立方公里的极热岩浆穿过地壳并作为侵入体集合体就位或作为大量熔岩流喷发的事件。 大型火成岩省内的侵入体以富镁和富铁为主（即，基性岩），并显示分层，表面上类似于一些沉积岩中的分层，因此统称为“层状基性岩侵入体”（LMI）。 LMI含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，钒和铌的重要来源。 在LMI矿化层的起源仍然是一个激烈的研究活动和争议的问题。 LMI研究团体倾向于支持深达10公里、宽达数百公里的岩浆房的概念，在岩浆房内，晶体沿着基底沿着，形成一系列向上加积的层。 然而，对花岗质侵入体中的层状岩石和大洋中脊背景下的层状镁铁质侵入体的研究表明，在许多情况下，分层是由于单独的水平薄岩浆片反复侵入而产生层状岩床的结果。 这种差异对于指导新发现的存款群（如安大略西北部的火圈矿床）的矿产勘探战略至关重要。 我的研究小组和其他一些人最近提出，在许多例子中，对于非常大的岩槛形LMI的岩浆房假说应该被岩槛堆叠顺序就位的想法所取代。 拟议的研究的目的是制定严格的测试的门槛假设LMI和一些竞争的假设。 需要回答的一些关键问题包括：“我们如何识别侵入到旧门槛的门槛边缘，这些门槛本身仍然非常热？我们能发现在LMI内靠近岩床的岩石在短时间内强烈受热的证据吗？“;“如果岩浆凝固而没有岩床就位，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现场实验室;标志性的南非布什维尔德复杂和蒙大拿州的斯蒂尔沃特复杂，这是岩床假说的候选人，安大略的萨德伯里火成岩复杂，这是已知的经典岩浆房，以及魁北克的基亚克湾侵入体，其地质鲜为人知，但异常暴露。 这项工作将涉及对岩石和矿物的成分和结构进行绘图、取样和测量。 现场观测结果将与预测进行比较，可以使用热力学和数值模型的火成岩过程，如化学扩散，晶体成核和生长，成熟的火成岩结构，岩浆流和侵位，以及热流内的岩浆及其固体寄主岩石。

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.