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: $ 5.25万
• 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: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=751167
• 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，这些岩浆要么是侵入物的集合，要么是大量熔岩流的喷发。大型火成岩省内的侵入岩以富镁和富铁（即基性）岩石为主，其层状结构与某些沉积岩的层状结构相似，因此俗称“层状基性侵入岩”（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的研究界倾向于岩浆库的概念，岩浆库深10公里，宽数百公里，其中晶体沿着底部积聚，形成向上堆积的一系列层。然而，对花岗质侵入岩中的层状岩石和洋中脊背景下的层状基性侵入岩的研究表明，在许多情况下，分层是由分开的水平薄岩浆片的反复侵入造成的，以产生层状的岩基堆。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）的矿产勘探策略至关重要。我的团队和其他一些人最近提出，在许多例子中，超大型岩台形LMI的岩浆室假说应该被岩台堆叠顺序放置的想法所取代。拟议研究的目的是对LMI的静止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：“我们如何识别侵入旧技能的边缘，这些技能本身仍然很热门？”“我们能否探测到LMI内岩石附近的岩石在短时间内剧烈加热的证据？”“如果岩浆凝固而没有在岩浆中嵌入岩浆岩，预计会产生什么样的岩石序列？”我们将使用四个LMI作为自然实地实验室的例子；南非标志性的布什维尔德杂岩和蒙大拿州的斯蒂尔沃特杂岩，它们都是岩床假说的候选者；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房；魁北克省的恰克湾侵入岩，它的地质鲜为人知，但却非常充分地暴露出来。这项工作将包括绘制、取样和测量岩石和矿物组成和质地。野外观测结果将与火成岩过程的热力学和数值模型的预测结果进行比较，这些预测结果包括化学扩散、晶体成核和生长、火成岩结构的成熟、岩浆流动和就位、岩浆及其固体寄主岩石内的热流。