Collaborative Research: Testing the Timescale and Geometry of Incremental Pluton Assembly Through 3-D Modeling and Thermochronology

本研究旨在验证岩体记录增量组合的地质年代学、热年代学和野外证据的假设。尽管从理论上考虑，支持增量增长的地质年代学和野外数据（特别是数百万年至数千万年）仍存在激烈的争论。内华达山脉岩基岩石的角闪石、黑云母和钾长石氩热年代学被用来解决几个广泛的问题。首先，从铀-铅锆石中推断出缓慢增加的钚组装，来自相同样品的氩-氩日期为铀-铅结果的可靠性提供了强有力的测试。其次，如果大型“均质”岩体是逐渐聚集起来的，那么增量之间的接触在野外就被忽略了，而利用大块岩石磁化率的新测绘正在被用来识别隐藏的接触。最后，对岩体和围岩温度历史的模拟表明，冷却路径对增量岩体组合的速率和几何形状都很敏感。因此，基于锆石和钛矿铀-铅系统的冷却历史，以及角闪石、黑云母和钾-长石氩系统的冷却历史，正在与模拟的冷却历史进行比较，以解决侵入增量的空间安排和它们的添加速率。本研究主要探讨岩浆房的物理化学演化，包括岩浆侵入机制、岩体组构发育、岩浆分异过程等。模拟结果表明，岩体及其围岩的复杂热史有助于解释岩体角闪石氩-氩年代学；接触变质作用的最高温度和持续时间；早期侵入体在成为后期侵入体围岩过程中的热、构造和岩石学演化；岩体古地磁资料的解释；而大体积可喷发岩浆体的形成程度是由岩体记录的。这项研究的重点是为地球表面的火山提供岩浆的岩浆库的组装速度。它采用了各种各样的年代测定技术，这些技术相互制衡，努力了解岩浆体的形成方式和速度，以及它们发生灾难性喷发的可能性。增量组合假说预测，“超级火山”岩浆房的生长将与其他火山岩浆房的构造和热历史截然不同。我们应该能够利用这一认识来评估像黄石公园这样的大型火山复合体发生灾难性喷发的可能性。这项研究支持了北卡罗来纳大学一名学生的博士研究，以及北卡罗来纳大学和犹他大学其他硕士和本科生的教育活动。该项目包括洛斯阿拉莫斯国家实验室、新墨西哥理工大学、北卡罗来纳大学和犹他大学之间新的合作关系的发展。这项研究也是我们对国家公园的持续支持的一部分。它有助于我们更新约塞米蒂国家公园的地质展览，并在公园的护林员培训中发挥作用。

This research is designed to test the hypothesis that plutons record geochronologic, thermochronologic and field evidence for incremental assembly. Although suggested by theoretical considerations, geochronologic and field data in support of incremental growth (particularly over millions to tens of millions of years) are hotly debated. Hornblende, biotite and potassium-feldspar argon thermochronology of rocks in the Sierra Nevada batholith are being used to address several broad questions. First, slow incremental pluton assembly has largely been inferred from uranium-lead zircon ages, and argon-argon dates from the same samples are providing a robust test of the reliability of the uranium-lead results. Second, if large "homogeneous" plutons were assembled incrementally, then contacts between increments have been overlooked in the field, and new mapping using bulk rock magnetic susceptibility is being used to identify cryptic contacts. Finally, modeling pluton and wall rock temperature histories indicates that cooling paths are sensitive to both the rate and the geometry of incremental pluton assembly. Therefore, cooling histories based on the zircon and titanite uranium-lead systems, and hornblende, biotite and potassium-feldspar argon systems, are being compared to modeled cooling histories in order to resolve spatial arrangements of intrusive increments and the rates at which they are added. This research is addressing the physical and chemical evolution of magma chambers, including mechanisms of intrusion, development of pluton fabrics, and permissible magmatic differentiation processes. The complex thermal histories of plutons and their wall rocks suggested by our modeling bear on interpretation of hornblende argon-argon dates from plutons; the maximum temperatures and durations of contact metamorphism; the thermal, structural and petrologic evolution of early intrusions as they become the wall rocks for later intrusions; the interpretation of paleomagnetic data collected from plutons; and the extent to which formation of large-volume eruptible magma bodies is recorded by plutons. This research focuses on the rates of assembly of magma chambers that feed volcanoes at the surface of the Earth. It employs a variety of dating techniques that act as checks and balances on each other in an effort to understand how, and how fast, the magma bodies are constructed, and their potential for catastrophic eruption. The incremental assembly hypothesis predicts that growth of "super volcano" magma chambers will be profoundly different than the construction and thermal histories of magma chambers that feed other volcanoes. We should be able to use this understanding to assess the potential for catastrophic eruption of huge volcanic complexes such as Yellowstone. The research is supporting the Ph.D. research of a student from the University of North Carolina, as well as the educational activities of other M.Sc. and undergraduate students at the University of North Carolina and the University of Utah. The project includes the development of new collaborative ties between Los Alamos National Laboratory, New Mexico Tech, University of North Carolina, and the University of Utah. The research is also an ongoing part of our support of the National Parks. It is contributing to our efforts to update the geology exhibits in Yosemite National Park, and play a role in Ranger training for the Park.