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。 花岗质岩浆通常被认为起源于地壳中或浅部，那里有大量的水和适当的烃源岩，这些烃源岩部分熔融产生花岗质岩浆。 然而，萨斯喀彻温省北方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，大量的花岗岩岩浆形成于最低地壳（地球表面以下约30公里）。 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。 这个位置提供了机会，记录岩浆生产过程，现在发生在地壳地幔边界的板块碰撞区。 阿萨巴斯卡麻粒岩地体的岩石被镁铁质岩浆（2800公里长的奇普曼岩脉群）侵入，导致下地壳岩石部分熔融。 部分熔融混合和混合的基性岩脉岩浆，表明杂交过程通常在最浅的岩浆房是常见的最低地壳。 在这个项目中，将追踪下地壳花岗质岩浆的谱系，从它们部分熔融的母岩的起源，到它们从下地壳起源地出发途中遇到的污染岩浆。 该项目的目标是检验这样一种假设，即最低地壳是花岗岩岩浆产生和污染的常见场所，也是开始于大陆板下或板间的玄武岩污染的场所。 通过对全岩样品和原位锆石、石榴石、斜长石晶体的主量、微量、稀土元素地球化学和Nd、Hf同位素分析，确定了：1）近莫霍面环境下长英质和镁铁质端员岩浆的源区物质特征; 2）2.6-Ga Fehr花岗岩中奇普曼脉岩与两种不同熔融反应产生的熔体混合物的成分范围; 3）奇普曼岩脉迁移到中地壳后保持其近莫霍面成分特征的程度，太古代和太古代地幔部分熔融底侵和间侵下地壳的同位素组成特征（2.6 Ga Fon du Lac玄武岩岩床）和元古宙（1.9 Ga奇普曼岩脉）。 这项研究将定量评估中地壳和浅地壳的花岗岩和玄武岩中的不均匀性指纹是热的、部分液态的地幔/地壳过渡带的岩性多样的岩浆环境的残余的程度。

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.