Collaborative Research: Focused Study of Aleutian Plutons and their Host Rocks: Understanding the building blocks of continental crust

弧岩浆作用是产生现今大陆地壳的最重要的过程，并且很可能贯穿整个地球历史。然而，大陆地壳的平均成分是安山岩和钙碱性，而弧熔岩的平均成分是玄武岩和拉斑玄武岩。另一方面，弧的大部分未暴露和未取样的深成部分可能更类似于大陆地壳。因此，了解深成岩的成因是了解大陆地壳通过弧岩浆作用形成和演化的关键，这是GeoPRISMS倡议的一个关键科学目标。阿留申岛弧非常适合进行这样的研究，因为它有大量的深成岩暴露，这是任何其他洋内岛弧都无法比拟的。在阿留申岛弧，大多数长英质深成岩的成分与大块大陆地壳的成分估计重叠。我们的初步研究发现，始新世-中新世深成岩和全新世火山岩显示出明显不同的元素和同位素特征，这表明它们来自不同的母岩浆。这种差异可能反映了该区域下地幔的时间变化，或形成深成岩和熔岩的根本不同机制-可能是具有高H2O含量的强钙碱性岩浆倾向于在中地壳脱气，导致粘度和结晶度迅速增加，因此它们倾向于停止并形成深成岩;而更热、更干燥的拉斑玄武岩粘度较低，容易喷发形成熔岩。由于了解阿留申中部熔岩和深圳湾岩体在时间和空间上成分差异的程度和起源至关重要，该项目将绘制阿留申群岛中部暴露的深成岩及其同时代火山寄主岩的地图并取样。将测量一部分最有希望的样品的主要元素、微量元素和同位素组成（Sr、Nd、Hf、Pb）。岩体和/或熔岩的年龄将限制使用U-Pb锆石年代学和40 Ar/39 Ar年代学分析的斜长石斑晶和基质。这些初步结果和所获得的样品将有助于回答陆壳形成的基本问题，并有助于揭示弧内深成岩和火山岩的形成机制。

Arc magmatism is the most important process that generates the continental crust today and likely throughout Earth's history. However, average continental crust composition is andesitic and calc-alkaline, while average arc lava composition is basaltic and tholeiitic. The largely unexposed and unsampled plutonic part of the arcs, on the other hand, may be more similar to the continental crust. Therefore, understanding the genesis of plutonic rocks is a key to understanding continental crust formation and evolution via arc magmatism, a key science goal for the GeoPRISMS initiative. The Aleutian arc is uniquely well-suited for such a study, because of the extensive exposures of plutonic rocks, unmatched in any other intra-oceanic arc. In the Aleutian arc, most felsic plutonic rocks have compositions that overlap estimates for the bulk continental crust. Our pilot study found that Eocene-Miocene plutonic rocks and Holocene volcanic rocks show distinctly different elemental and isotopic signatures, which indicate that they were derived from distinct parental magmas. This difference could reflect temporal variation of the mantle under the region, or fundamentally different mechanisms that form plutons and lavas - perhaps strongly calc-alkaline magmas, with high H2O contents, tend to degas in the mid-crust, causing a rapid increase in viscosity and crystallinity, therefore they tent to stall and form plutons; while hotter, drier, tholeiitic basalts have lower viscosity and readily erupt to form lavas.As it is crucial to understand the extent and origin of the compositional difference between central Aleutian lavas and plutons through time and space, this project will map and sample plutonic rocks exposed on the central Aleutians and their coeval volcanic host rocks. A subset of the most promising samples will be measured for major element, trace element and isotopic compositions (Sr, Nd, Hf, Pb). The ages of the plutons and/or the lavas will be constrained using U-Pb zircon geochronology and 40Ar/39Ar geochronology analyses of plagioclase phenocrysts and the groundmass. These preliminary results and the samples acquired in this study will help to answer fundamental questions of continental crust formation, and shed light on the formation mechanisms of plutons and volcanics in arcs.