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

在研究地壳的演化和造山过程中产生的岩石的变质作用时，一个令人困惑的问题是“高温变质作用的热源是什么？”，特别是对于那些局部显示出熔化证据的变质岩石，它们的温度明显高于周围的变质岩石。在这种高温变质岩中，PI和他的同事们发现，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。通过热力学、结构和成分研究，PI及其同事将每一代的独居石与反应、温度和形成日期联系起来。对于多期的独居石生长，PI和同事提取了压力(P)-温度(T)-时间(T)-路径。并将这些P-T-t路径与各种热源（如岩体侵入、软流层底板或放热反应）的数值模型生成的P-T-t路径进行了比较。迄今为止，PI的工作表明，在许多情况下，新英格兰中部的高变质作用与邻近的花岗岩岩体没有联系，因为变质作用比岩体的就位时间晚了4000万到5000万年。从独居石研究中得出的P-T-t路径与假设距今约4亿年前新英格兰岩石圈剥离和软流圈底化所模拟的数值P-T-t路径非常吻合。这一解释与现代新英格兰中部岩石圈剥离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新英格兰阿巴拉契亚山脉的高级别、部分融化的变质岩，目前正在检查缅因州西南部、新罕布什尔州中部、新罕布什尔州南部和马萨诸塞州中部的样品，以提供美国北部阿巴拉契亚山脉高级别古生代（距今4 -3亿年前）变质作用的更完整图景，以及相关的热源。该项目代表了在整个造山带范围内将副相生长联系起来的首次尝试之一。从一个地点到另一个地点的独居石生长事件的对比与地层对比类似。假设单独居石的边缘生长在变质作用的高峰（在熔融或熔融结晶期间），精确的单独居石边缘年龄提供了沿造山带长度追踪变质作用高峰的方法。独居石边缘年龄的横向（东西向）变化可能记录了沿造山带的碰撞或伸展辐合。压力与独居石峰值年龄（如更深、更老的独居石峰值）之间的正相关关系可能表明有强烈的地幔加热成分。结合区域尺度上的独居石的生长历史、测温和定年，为重建大型造山带的热演化提供了一个非常有力的工具。

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.