Origins of layers in layered mafic intrusions

层状镁铁质岩体中层的起源

• 批准号: RGPIN-2018-05327
• 负责人: Mungall, James
• 金额: $ 2.62万
• 依托单位: Carleton University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=712947
• 关键词: Origins; layers; layered; mafic; intrusions

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.

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的事件，它们要么作为侵入物的集合而形成，要么以大量的熔岩流的形式喷发。 大型火成岩省内的侵入岩以富含镁和铁（即镁铁质）的岩石为主，其层状结构表面上类似于某些沉积岩中的层状结构，因此通称为“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、钒和铌的重要来源。 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 研究界倾向于支持深达十公里、宽数百公里的岩浆室的概念，在岩浆室中，晶体沿着底部堆积，形成一系列向上侵蚀的岩浆层。 然而，对花岗质侵入体中的层状岩石和大洋中脊环境下的层状镁铁质侵入体的研究表明，在许多情况下，分层是由单独的薄水平岩浆片重复侵入产生层状岩床的结果。 这种差异对于指导新发现的矿床群（例如安大略省西北部的火环矿床）的矿产勘探策略至关重要。 我的小组和其他一些人最近建议，在许多例子中，非常大的基台形状的 LMI 的岩浆室假说应该被基台堆叠的顺序放置的想法所取代。 本研究的目的是对 LMI 的基台假设及其一些竞争假设进行严格的检验。 需要回答的一些关键问题包括：“我们如何识别侵入到本身仍然很热的旧窗台中的窗台边缘？”； “我们能否发现 LMI 内岩台附近岩石短暂、强烈加热的证据？”； “如果岩浆在没有放置岩台的情况下凝固，预计会产生什么序列的岩石？”我们将使用 LMI 的四个例子作为自然现场实验室；南非标志性的布什维尔德杂岩体和蒙大拿州斯蒂尔沃特杂岩体（它们是基台假说的候选者）、安大略省的萨德伯里火成岩杂岩体（已知是经典的岩浆房）以及魁北克省的奇亚克湾侵入岩体（其地质情况鲜为人知，但暴露得非常充分）。 这项工作将涉及岩石和矿物成分和纹理的绘图、采样和测量。 现场观测结果将与使用火成岩过程的热力学和数值模型进行的预测进行比较，这些模型包括化学扩散、晶体成核和生长、火成岩结构的成熟、岩浆流和侵位以及岩浆及其固体母岩内的热流。