The Origin and Contamination of Magmas in the Lowermost Crust

花岗岩是地球大陆地壳所特有的，花岗岩岩浆是地球上最大的火山爆发的来源。一般认为，花岗质岩浆起源于地壳的中浅层，那里有丰富的水和适当的源岩，这些源岩部分熔融生成花岗质岩浆。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地之一，它提供的证据表明，在地球表面下约30公里的最下层地壳中形成了大量的花岗岩岩浆。目前在阿萨巴斯卡麻粒岩地体表面的岩石是大约26亿年前下地壳的一部分。这个位置提供了记录岩浆产生过程的机会，现在发生在地壳-地幔边界的构造板块碰撞带。阿萨巴斯卡麻粒岩地体中的岩石被基性岩浆（2800公里长的奇普曼岩群）侵入，导致下地壳岩石部分融化。部分熔融体与基性岩脉岩浆相互混合，表明在最浅层岩浆房中常见的混合作用在最下层地壳中也很常见。在这个项目中，下地壳的花岗质岩浆的谱系将被追踪，从它们部分融化的母岩的起源，到它们从起源的下地壳地点在旅途中遇到的污染岩浆。这个项目的目的是验证这样一个假设，即最下面的地壳是花岗岩岩浆产生和污染的共同场所，也是玄武岩污染的共同场所，玄武岩最初是作为大陆底板或板块间的。利用全岩样品和原位锆石、石榴石、斜长石晶体的主要元素、微量元素、稀土元素地球化学和Nd、Hf同位素分析，确定：1)近莫霍环境中长英质和镁质端元岩浆的物质来源特征；2) 2.6 ga Fehr花岗岩中两种不同熔融反应产生的Chipman岩脉与熔体杂合物的组成范围；3)Chipman岩脉迁移至中地壳后，其近莫霍（moho）成分特征在多大程度上保持不变；4)太古宙（2.6 Ga Fon du Lac玄武岩岩）和元古代（1.9 Ga Chipman岩脉）下地壳下覆和间覆地幔部分熔体的同位素和组成特征。本研究将定量评价中、浅层地壳花岗岩和玄武岩的非均质指纹在多大程度上是热的、部分液态的地幔/地壳过渡带岩性多样化岩浆环境的残余。

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.