Petrogenesis of Felsic Volcanic Rocks in Multiple Geologic Environments and Controls on Mineralization

多种地质环境下长英质火山岩的岩石成因及成矿控制

• 批准号: RGPIN-2014-06063
• 负责人: Cousens, Brian
• 金额: $ 2.19万
• 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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=648208
• 关键词: Petrogenesis; Felsic; Volcanic; Rocks; Multiple

Felsic (high-SiO2) volcanism is common in virtually every modern geologic setting, including oceanic and continental volcanic regions. Felsic volcanism is also widely distributed in ancient greenstone belts and commonly hosts volcanogenic massive sulphide (VMS) deposits (Cu, Zn, Pb). Thus, felsic magmatism is a major contributor to the Canadian economy. The origin of felsic volcanic rocks remains a fundamental geological problem. Some felsic volcanic rocks are associated with abundant mantle-derived mafic (low-SiO2) magmatism (oceanic, hotspot settings) where derivation primarily by crystallization from mafic magma is likely. The origin of silicic magmas in continental settings is less clear, where hot mantle-derived melts can drive crustal melting, leading to mixing of mantle and crustal melts, followed by further crustal assimilation and magma crystallization. Unlike mafic magmas whose geochemistry is a direct reflection of the mantle source, the origin of felsic magmas is muddled by crystallization of exotic minerals that substantially modify the geochemical character of the evolving magma. Isotope systematics provides key evidence of the origin of felsic volcanic rocks. Radiogenic isotope ratios are characteristic of the source of melts, and stable isotopes yield information both on melt sources and subsequent crystallization of the felsic magma. The volatile content and eruption temperature also help distinguish felsic magmas with different sources.* *The goal of this proposal is to better understand the origin of high-silica volcanic rocks in different tectonic settings, primarily in the Great Basin of the western United States, greenstone belts in the Canadian Shield, and in Hawaii. In the Great Basin, a period of pronounced felsic volcanic events, termed the ignimbrite flare-up, punctuate a southwestward sweep of volcanic activity across western Utah and Nevada between 40 and 15 million years ago. In some cases, the erupted lavas/ignimbrites are exceptionally exposed due to later faulting and tilting of the rocks. Although early studies suggested that Great Basin felsic rocks are primarily crustal melts, recent studies allow for greater contribution of mantle-derived magmas to felsic rocks. Although these felsic magmatic systems commonly host large hydrothermal alteration systems typical of many active mines, to date only one major gold deposit has been discovered, and our work will identify other areas for exploration.**In the Canadian Shield, Archean greenstone belts typically include small proportions of felsic volcanic rocks. In the Slave province, two types of 2.6 to 2.7 Ga felsic volcanic complexes have been identified, one related directly to coeval mafic magmatism (Kam-type) and a second that can be modeled largely by melting of mafic lower crust (Banting-type). The origin of these two magma types is critical to models of the tectonic history of the Slave craton during this time period because very different melting and post-melting processes are required in their generation. Like Great Basin felsic magma systems, hydrothermal alteration is common in Slave felsic complexes, thus further investigation of the mineral potential in Slave felsic complexes is part of this project. Do mineral deposits appear preferentially in one felsic magma type vs. the other?**At Hawaiian volcanoes, felsic rocks are extremely rare. They may form by two processes, either by crystallization of basalt or by melting of older volcanic rocks that form the bulk of the volcano. At Hawaii, felsic magmas occur only in the late stages of activity on some islands, and the origin of the felsic magmas tells us about the rate of cooling of magma reservoirs beneath the surface and the importance of water circulation within the volcano.

长英质(高二氧化硅)火山作用在几乎所有现代地质环境中都很常见，包括大洋和大陆火山区。长英质火山作用也广泛分布在古绿岩带中，通常赋存火山成因块状硫化物(VMS)矿床(铜、锌、铅)。因此，长英质岩浆作用是加拿大经济的主要贡献者。长英质火山岩的成因仍然是一个基本的地质问题。一些长英质火山岩与丰富的地幔起源的镁铁质(低SiO_2)岩浆作用(大洋、热点环境)有关，其中可能主要由镁铁质岩浆结晶而产生。大陆环境中硅质岩浆的起源不太清楚，那里来自地幔的热熔体可以驱动地壳熔融，导致地幔和地壳熔体混合，随后进一步的地壳同化和岩浆结晶。与镁铁质岩浆的地球化学直接反映地幔来源不同，长英质岩浆的起源被外来矿物的结晶所混杂，这些矿物实质上改变了不断演化的岩浆的地球化学性质。同位素系统学为长英质火山岩的成因提供了关键证据。放射性同位素比率是熔体来源的特征，稳定的同位素提供有关熔体来源和随后的长英质岩浆结晶的信息。挥发分含量和喷发温度也有助于区分不同来源的长英质岩浆。**这项提议的目标是更好地了解不同构造环境下高硅质火山岩的起源，主要是在美国西部的大盆地、加拿大地盾的绿岩带和夏威夷。在大盆地，一段显著的长英质火山事件，被称为褐沸石爆发，标志着4000万至1500万年前横跨犹他州西部和内华达州的西南火山活动。在某些情况下，由于后来的断层作用和岩石的倾斜，喷发的熔岩/火成岩异常暴露。虽然早期的研究表明大盆地长英质岩石主要是地壳熔岩，但最近的研究允许地幔岩浆对长英质岩石做出更大的贡献。尽管这些长英质岩浆系统通常拥有许多活跃矿山所特有的大型热液蚀变系统，但到目前为止只发现了一个主要金矿床，我们的工作将确定其他勘探领域。**在加拿大盾中，太古宙绿岩带通常包括少量长英质火山岩。在Slave省，已发现两种2.6~2.7Ga的长英质火山岩杂岩，一种直接与同时代的镁铁质岩浆作用有关(KAM型)，另一种主要可通过镁铁质下地壳的熔融来模拟(板条型)。这两种岩浆类型的起源对于这一时期奴隶克拉通的构造史模型至关重要，因为它们的生成需要非常不同的熔融和后熔融过程。与大盆地长英质岩浆系统一样，热液蚀变在次长英质杂岩中普遍存在，因此，进一步研究次长英质杂岩中的矿物潜力是本项目的一部分。矿藏是否优先出现在一种长英质岩浆类型中，而不是另一种类型？**在夏威夷火山，长英质岩石极其罕见。它们可以通过两个过程形成，要么是玄武岩结晶，要么是形成火山主体的较老的火山岩熔融。在夏威夷，长英质岩浆只出现在一些岛屿活动的后期，长英质岩浆的来源告诉我们地表下岩浆储藏的冷却速度和火山内部水循环的重要性。