Thermal Structure of the Central New England Acadian-to-Alleghenian Metamorphic High, Through Regional Correlation of Accessory-Phase Paragenesis

新英格兰中部阿卡阶到阿勒根阶变质高压的热结构，通过副相共生的区域关联

• 批准号: 0440131
• 负责人: Joseph Pyle
• 金额: --
• 依托单位: Rensselaer Polytechnic Institute
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-01-15 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0440131&HistoricalAwards=false
• 关键词: Thermal; Structure; Central; New; England

One of the puzzling questions in studying the evolution of Earth's crust, and the metamorphism of rocks produced during mountain-building episodes, is "what is the heat source for very high temperature metamorphism?", especially for metamorphic rocks that locally show evidence for melting, and attainment of significantly higher temperatures than the surrounding metamorphic rocks. In such high-temperature metamorphic rocks, The PI and his co-workers have discovered that the radioactive phosphate mineral monazite undergoes 3 or 4 growth reactions in a single metamorphic event. Using thermodynamic, textural, and compositional studies, the PI and co-workers link each generation of monazite with a reaction, a temperature, and a date of formation. For multiple episodes of monazite growth, the PI and co-workers extract pressure (P)-Temperature (T)-time (t)-paths. And these P-T-t paths are compared to P-T-t paths generated from numerical models for a variety of heat sources, such as pluton intrusion, asthenospheric underplating, or exothermic reactions.The work of the PI, to date, shows that high-grade metamorphism in central New England is not linked, in many cases, to adjacent granitic plutons, as the metamorphism post-dates the pluton emplacement by 40-50 million years. P-T-t paths derived from study of monazite closely match numeric P-T-t paths modeled assuming lithospheric delamination, and asthenospheric underplating, beneath New England at about 400 million years before the present. This interpretation agrees with independent geophysical evidence for lithospheric delamination beneath present-day central New England.The PI and co-workers are extending this methodology to high-grade, partially melted metamorphic rocks along the length of New England Appalachians, and are currently examining samples from SW Maine, Central New Hampshire, S New Hampshire, and Central Massachusetts to provided a more complete picture of high-grade Paleozoic (400-300 million years before present) metamorphism, and the heat sources responsible, in the Northern U.S. Appalachians. This project represents one of the first attempts to correlate accessory phase growth on an orogen-wide scale. Correlation of monazite growth events from location to location are used in a manner analogous to stratigraphic correlation. Under the assumption that monazite rims are grown at the peak of metamorphism (during melting or melt crystallization), precise monazite rim ages provide a means to track the peak of metamorphism down the length of the orogen. Lateral (E-W) variation in monazite rim ages may record collisional or extensional vergence along the orogen. A positive correlation between pressure and peak monazite age (e.g., deeper, older peak monazite) may indicate a strong mantle heating component. Integration of growth histories, thermometry, and dating of monazite on a regional scale provides a remarkably robust tool to aid in reconstructing the thermal evolution of large-scale orogens.

在研究地壳演化以及造山过程中产生的岩石变质作用时，令人困惑的问题之一是“甚高温变质作用的热源是什么？“，特别是对于局部显示出熔融证据的变质岩，以及比周围的变质岩达到明显更高的温度。在这种高温变质岩中，PI和他的同事发现放射性磷酸盐矿物独居石在一次变质事件中经历了3或4次生长反应。利用热力学、结构和成分研究，PI和同事将每一代独居石与反应、温度和形成日期联系起来。对于独居石生长的多个事件，PI和同事提取压力（P）-温度（T）-时间（t）-路径。这些P-T-t轨迹与各种热源的数值模型产生的P-T-t轨迹进行了比较，这些热源包括侵入体、软流圈底侵作用或放热反应。迄今为止，PI的工作表明，新英格兰中部的高级变质作用在许多情况下与邻近的花岗岩侵入体无关，因为变质作用发生在侵入体就位之后4000 - 5000万年。从独居石的研究中得出的P-T-t轨迹与假设新英格兰在距今约4亿年前发生岩石圈拆沉和软流圈底侵作用而模拟的数值P-T-t轨迹非常吻合。这种解释与现今新英格兰中部岩石圈分层的独立地球物理证据相一致。PI及其同事正在将这种方法扩展到沿着新英格兰阿巴拉契亚山脉的高品位、部分熔融的变质岩，目前正在检查来自西南缅因州、中部新罕布什尔州、南部新罕布什尔州、和中马萨诸塞州提供了一个更完整的图片高级古生代（4亿至3亿年前的现在）变质作用，热源负责，在北方美国阿巴拉契亚山脉。该项目是首次尝试在造山带范围内关联副相生长的项目之一。独居石生长事件从位置到位置的相关性以类似于地层相关性的方式使用。在独居石边缘生长在变质作用高峰期（熔融或熔融结晶过程中）的假设下，精确的独居石边缘年龄提供了一种手段来跟踪变质作用高峰期的造山带的长度。独居石边缘年龄的横向（E-W）变化可能记录了造山带沿着的碰撞或伸展聚散。压力和独居石年龄峰值之间的正相关（例如，更深，更老的峰独居石）可能表明一个强大的地幔加热组件。在区域尺度上独居石的生长历史、测温和定年的综合为重建大规模造山带的热演化提供了一个非常有力的工具。