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

深成岩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室，但它们与岩浆室的真正关系是模糊的，也是有争议的。如何解决这些关系，关系到地壳岩浆运移机制、深成岩体“房间”问题、深成作用与区域变形的关系、岩浆系统的寿命、岩浆为什么有的喷发、有的被困成深成，以及岩浆收支、补给和喷发的关系等问题。这些问题的答案取决于准确测量深成系统的寿命和侵入性历史。关于钚的含义，有两个终端成员的概念。一端是长期持有的观念，即深成岩浆体(数百或数千立方千米)在10,000-1,000,000年的时间尺度上以单一事件的形式存在。或者，在超过1000万年的持续时间内，通过合并小插曲的侵入岩，深成岩可能会逐渐增长。在后一种观点中，在岩体的生长过程中从未出现过大量的岩浆，而岩体可能是高度的，也许是神秘的，复合的，并且基本上是历时的。这个项目研究了内华达山脉的两个嵌套的深成杂岩，Tuolumne和Whitney侵入岩套。这些晚白垩世系统中的每一个都被冰川作用壮观地暴露出来，相对来说没有变形。来自Tuolumne的新数据表明，它大约侵位超过1000万年，至少有一个成分侵入体(半穹顶岩体)侵位超过3英里。这些结果有利于增量侵位和隐蔽的复合入侵。然而，结果的普遍适用性是不确定的，类似的(但更大的)惠特尼系统的历史也鲜为人知。利用U/Pb年代学，正在确定每个岩浆系统的寿命，以及单个深成岩浆是以单批次还是以许多小增量侵位的。在需要隐蔽的复合侵位机制的半穹顶岩体中，人们正在探索非常规的映射技术(例如磁化率)，试图区分复合和非复合深成岩体，从而在野外绘制隐蔽复合深成岩体的内部结构图。这一提议的智力优势在于检验了目前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当前的范式有明显的不同。评估岩体侵位的速率和机制涉及到跨越构造地质学、岩石学、构造学和地质灾害等学科的基础研究问题。此外，北卡罗来纳大学地质年代学实验室的分析技术将得到改进，正在建立识别深成岩中隐秘侵入接触的新技术。我们的工作成果将对约塞米蒂国家公园、红杉国家公园和国王峡谷国家公园的科学家和博物学家有用，因为他们致力于增加公众对科学的理解。

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.