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.

在几乎每个现代地质环境中，包括海洋和大陆火山区域，火山岩（高sio2）火山主义在几乎所有现代地质环境中都是常见的。 长英质火山也被广泛分布在古代绿岩带中，并且通常寄主火山质巨大的硫化物（VM）沉积物（Cu，Zn，PB）。 因此，长英质岩浆是加拿大经济的主要贡献者。 英质火山岩的起源仍然是一个基本的地质问题。 一些长英质火山岩与大量的地幔衍生的镁铁质（低sio2）岩浆（海洋，热点设置）相关，其中可能主要是通过从镁铁质岩浆中结晶而衍生的。 大陆环境中硅质岩浆的起源不太清楚，在这里，热地幔衍生的熔体可以驱动地壳融化，从而导致地幔和地壳熔体混合，然后再进一步的地壳同化和岩浆结晶。与地球化学的直接反映的镁铁质岩浆不同，英质岩浆的起源被外来矿物的结晶所掩盖，这些矿物质实质上改变了不断发展的岩浆的地球化学特征。 同位素系统学提供了长信火山岩起源的关键证据。放射同位素比是熔体来源的特征，并且稳定的同位素在熔融来源和后续结晶中产生信息。挥发性含量和喷发温度还有助于用不同的来源区分长毛岩浆。在伟大的盆地中，一个明显的长英质火山事件被称为Ignimbrite耀斑，在40到1500万年前，整个犹他州和内华达州西部的火山活动向西南扫荡。在某些情况下，由于岩石的断层和倾斜，爆发的熔岩/ignimbrites非常暴露。 尽管早期的研究表明，巨大的盆地长英质岩石主要是地壳融化，但最近的研究允许地幔衍生的岩浆对墨岩岩的贡献。尽管这些长英质岩浆系统通常构成许多活跃地雷的大型水热改变系统，但迄今为止仅发现了一个主要的黄金沉积物，我们的工作将确定其他勘探区域。 在奴隶省，已经确定了两种类型的2.6至2.7 Ga Felsic火山络合物，一种直接与Coeval Mafic Magmatism（KAM-Type）直接相关，而第二种可以通过大大融化的镁质下层壳（Banting-type）进行建模。 这两种岩浆类型的起源对于在这段时间内对奴隶克拉顿的构造历史的模型至关重要，因为它们的一代中需要非常不同的熔融和熔融后过程。 像伟大的盆地长英质岩浆系统一样，水热改变在奴隶长英质络合物中也很常见，因此进一步研究了从奴隶长英质复合物中的矿物潜力。 矿藏是否优先出现在一种墨岩型中，而在夏威夷火山的另一层？**，英质岩石极为罕见。 它们可以通过两个过程，通过玄武岩的结晶或形成大部分火山的旧火山岩的熔化形成。 在夏威夷，长英质岩浆仅发生在某些岛屿的活动的晚期，而长英质岩浆的起源告诉我们有关地表以下岩浆储层的冷却速度以及火山中水循环的重要性。