Collaborative Research: Are Plutons Assembled Over Long Periods of Time Through Amalgamation of Small Ephemeral Magma Chambers?

岩体是大陆地壳的基本组成部分。它们被广泛认为反映了古老的岩浆房，但它们与岩浆房的真正关系是模糊和有争议的。解决这一关系涉及到诸如岩浆在地壳中的运移机制、岩体“房间”问题、岩体作用与区域变形之间的关系、岩浆系统的寿命、为什么有些岩浆喷发而有些岩浆被捕获形成岩体以及岩浆平衡、补给和岩浆系统喷发之间的关系等问题。这些问题的答案取决于精确测量深成系统的寿命和侵入历史。有两种端元概念来描述岩体所代表的东西。一方面是长期持有的观点，即深成岩体代表了在10，000 - 1，000，000年的时间尺度上在单一事件中侵位的大型岩浆体（数百或数千立方公里）。或者，深圳湾岩体也可能是在超过1000万年的时间里，通过小的幕式侵入体的合并而逐渐增长的。在后一种观点中，大体积的岩浆从来没有出现在岩体的生长过程中，岩体可能是高度的，也许是神秘的，复合的，从根本上穿时。这个项目探讨了两个嵌套的深成复合体在塞拉利昂内华达州，图奥勒米和惠特尼侵入套房。每一个晚白垩世的系统都被冰川作用暴露出来，相对来说没有变形。来自图奥勒米的新数据表明，它的侵位时间大约为1000万年，至少有一个组成侵入体（半圆顶岩体）的侵位时间超过300万年。这些结果有利于增量侵位和神秘的复合侵入体。然而，结果的普遍适用性是不确定的，类似的（但更大的）惠特尼系统的历史知之甚少。使用U/Pb地质年代学，每个岩浆系统的寿命，以及是否个别岩体侵位在单批次或在许多小增量正在确定。在需要神秘复合侵位机制的半圆顶岩体中，非常规测绘技术（例如，磁化率），试图区分复合岩体和非复合岩体，从而在野外绘制神秘复合岩体的内部结构图。这一建议的智力价值在于检验流行的关于岩体侵位和地壳生长的假设。如果我们的假设是正确的，那么，岩体侵位过程与目前的范式有明显的不同。评价岩体侵位的速率和机制涉及到构造地质学、岩石学、构造学和地质灾害等学科的基础研究问题。此外，将改进地质年代学实验室的分析技术，并正在建立识别岩体中隐蔽侵入接触的新技术。我们工作的结果将对约塞米蒂、红杉和国王峡谷国家公园的科学家和博物学家有用，因为他们致力于提高公众对科学的理解。

Plutons are basic building blocks of the continental crust. They are widely regarded to reflect ancient magma chambers, yet their true relationship to magma chambers is obscure and controversial. Resolving the relationship bears on questions such as mechanisms of magma transport in the crust, the pluton "room" problem, relations between plutonism and regional deformation, the longevity of magmatic systems, why some magma erupts and some is trapped to form plutons, and relationships between magma budget, recharge, and eruption in igneous systems. Answers to these questions depend on precisely measuring the longevity and intrusive histories of plutonic systems. There are two end-member concepts of what plutons represent. At one end is the long-held notion that plutons represent large magma bodies (hundreds or thousands of cubic kilometers) emplaced in single events on a time scale of 10,000-1,000,000 years. Alternatively, plutons may grow incrementally by amalgamation of small episodic intrusions over durations in excess of 10 million years. In the latter view, large volumes of magma never were present during growth of the pluton and the pluton may be highly, perhaps cryptically, composite, and fundamentally diachronous.This project examines two nested plutonic complexes in the Sierra Nevada, the Tuolumne and Whitney Intrusive Suites. Each of these Late Cretaceous systems has been magnificently exposed by glaciation and is relatively undeformed. New data from the Tuolumne indicate that it was emplaced over roughly 10 million years, and at least one constituent intrusion (the Half Dome pluton) was emplaced over greater than 3 m.y. These results favor incremental emplacement and cryptically composite intrusions. However, the general applicability of the results is uncertain, and the history of the similar (but larger) Whitney system is poorly known. Using U/Pb geochronology, the lifetimes of each magmatic system, and whether individual plutons were emplaced in single batches or in many small increments is being determined. In the Half Dome pluton that requires a cryptically composite emplacement mechanism, unconventional mapping techniques (e.g., magnetic susceptibility) are being explored in an attempt to distinguish composite from noncomposite plutons, and thus to map in the field the internal structures of cryptically composite plutons.The intellectual merit of this proposal lies in testing prevailing hypotheses for pluton emplacement and crustal growth. If our hypothesis is correct then pluton emplacement processes are markedly different from the current paradigm. Assessing the rates and mechanisms of pluton emplacement bears on basic research questions spanning the disciplines of structural geology, petrology, tectonics, and geologic hazards. Additionally, analytical techniques at the UNC geochronology lab will be improved and new techniques for recognizing cryptic intrusive contacts in plutons are being established. Results from our work will be useful to scientists and naturalists at Yosemite, Sequoia, and Kings Canyon National Parks as they work to increase public understanding of science.