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。人们普遍认为它们反映了古代岩浆房，但它们与岩浆房的真正关系是模糊的和有争议的。这一关系的解决涉及到岩浆在地壳中的运移机制、岩体“空间”问题、岩体作用与区域变形的关系、岩浆系统的寿命、岩浆喷发和被困形成岩体的原因以及火成岩系统中岩浆的平衡、补给和喷发的关系等问题。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。对于钚所代表的东西，有两种端元概念。一方面，长期以来人们一直认为，岩体是在1万至100万年的时间尺度上，在单个事件中形成的大型岩浆体（数百或数千立方公里）。另一种可能是，在超过1000万年的时间里，通过小的偶发侵入的合并而逐渐形成的岩体。在后一种观点中，大量岩浆在岩体的生长过程中从未出现过，而岩体可能是高度（也许是神秘的）复合的，基本上是历时的。该项目研究了内华达山脉的两个嵌套的深成复合体，Tuolumne和Whitney侵入套房。每一个晚白垩纪的体系都在冰川作用下暴露无遗，相对来说没有变形。来自Tuolumne的新数据表明，它的就位时间大约超过1000万年，至少有一个组成侵入（半圆顶岩体）的就位时间超过3万年。这些结果有利于增量侵入和神秘的复合侵入。然而，结果的普遍适用性是不确定的，并且类似（但更大）惠特尼系统的历史是鲜为人知的。利用U/Pb年代学，确定了每个岩浆系统的寿命，以及单个岩体是单批植入还是以许多小增量植入。在需要隐蔽复合侵位机制的半丘岩体中，正在探索非常规的测绘技术（例如磁化率），试图区分复合与非复合岩体，从而在野外绘制隐蔽复合岩体的内部结构图。这一建议的理论价值在于检验了流行的关于岩体就位和地壳生长的假说。如果我们的假设是正确的，那么冥王星的就位过程与目前的范例明显不同。评估岩体侵位的速率和机制涉及构造地质学、岩石学、构造学和地质灾害等学科的基础研究问题。此外，将改进联合国军大学地质年代学实验室的分析技术，并正在建立新的技术，以识别岩体中隐蔽的侵入接触。我们的工作结果将对约塞米蒂、红杉和国王峡谷国家公园的科学家和自然学家有用，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.