The Origin and Contamination of Magmas in the Lowermost Crust

地壳最底层岩浆的起源和污染

• 批准号: 1250422
• 负责人: Sheila Seaman
• 金额: $ 18.78万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-01 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1250422&HistoricalAwards=false
• 关键词: Origin; Contamination; Magmas; Lowermost; Crust

Granites are unique to Earth's continental crust, and granitic magma is the source of the planet's largest explosive volcanic eruptions. Granitic magmas are generally considered to originate in the middle or shallow crust, where water and appropriate source rocks that partially melt to produce granitic magma are abundant. However, the Athabasca Granulite Terrane of northern Saskatchewan, one of the largest exposures of lower crustal rocks on Earth, provides evidence showing that large volumes of granitic magmas form in the lowermost crust (approximately 30 km beneath the Earth's surface). The rocks presently on the surface in the Athabasca Granulite Terrane were part of the lower crust approximately 2.6 billion years ago. This location provides the opportunity to document the magma-producing processes now occurring at the crust-mantle boundary in zones of collision of tectonic plates. Rocks in the Athabasca Granulite Terrane were invaded by mafic magmas (the 2800-km-long Chipman dike swarm) that caused the lower crustal rocks to partially melt. The partial melts mingled and mixed with the mafic dike magmas, showing that hybridization processes commonly identified in the shallowest magma chambers are common in the lowermost crust as well. In this project, the lineage of granitic magmas of the lower crust will be traced, from the origin of the parent rock from which they partially melted, through the contaminating magmas that they encountered on their journey from the lower crustal site of their origin. The goal of this project is to test the hypothesis that the lowermost crust is a common site of granitic magma production and contamination, and of contamination of basalts that began as continental underplates or interplates. Major, trace, and rare earth element geochemistry and Nd and Hf isotopic analyses both on whole rock samples and on zircon, garnet, and plagioclase crystals in situ will be used to identify: 1) the source material characteristics of felsic and mafic end member magmas in the near-Moho setting; 2) the compositional range of hybrids between Chipman dikes and melts produced by two distinct melting reactions in the 2.6-Ga Fehr granite, 3) the degree to which the Chipman dikes retain their near-Moho compositional characteristics after migrating to the middle crust, and 4) the isotopic and compositional characteristics of mantle partial melt that underplated and interplated the lower crust during both the Archean (2.6 Ga Fon du Lac basaltic sills) and the Proterozoic (1.9 Ga Chipman dikes). This study will quantitatively evaluate the degree to which the fingerprint of heterogeneity in both granites and basalts of the middle and shallow crust is a remnant of the lithologically diverse magma environment of the hot, partially liquid mantle/crust transition zone.

花岗岩是地球大陆地壳独有的，花岗岩浆是地球上最大的火山爆发的来源。花岗岩浆通常被认为起源于中地壳或浅地壳，那里有大量的水和适当的源岩，这些岩石部分熔融产生花岗岩浆。然而，萨斯喀彻温省北部的阿萨巴斯卡麻粒岩地体是地球上最大的下地壳岩石暴露之一，它提供的证据表明，大量花岗岩浆形成于最下面的地壳(约在地球表面下30公里处)。目前在阿萨巴斯卡麻粒岩地体地表的岩石是大约26亿年前下地壳的一部分。这一地点提供了记录目前在构造板块碰撞带的壳幔边界发生的岩浆产生过程的机会。阿萨巴斯卡麻粒岩地体中的岩石受到镁铁质岩浆(长2800公里的奇普曼岩墙群)的侵袭，导致下地壳岩石部分熔融。部分熔体与镁铁质岩浆混合混合，表明在最浅的岩浆室中通常发现的混合过程在最下面的地壳中也很常见。在这个项目中，将追溯下地壳花岗岩浆的谱系，从它们部分熔融的母岩的起源，到它们从起源的下地壳遗址旅行时遇到的污染岩浆。该项目的目标是检验一种假设，即最下面的地壳是花岗岩浆产生和混染的常见场所，以及始于大陆底板或板间的玄武岩混染的地点。全岩样品和锆石、石榴石、斜长石晶体的主量元素、痕量元素和稀土元素的元素地球化学及NdHf同位素分析将用于鉴定：1)近莫霍面环境中长英质和镁铁质端元岩浆的源区物质特征；2)2.6-Ga Fehr花岗岩中两次不同熔融反应产生的Chipman岩脉与熔体的混合成分范围；3)Chipman岩脉迁移至中地壳后保留其近莫霍面成分特征的程度；4)太古代(2.6Ga Fon Du Lac玄武岩)和元古界(1.9Ga Chipman岩脉)下地壳底侵和夹杂的地幔部分熔体的同位素和成分特征。这项研究将定量评估中浅地壳花岗岩和玄武岩中的不均一性指纹在多大程度上是热的、部分液态的地幔/地壳过渡带岩浆环境的残留物。