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

在研究地壳演化和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“极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并达到比周围变质岩高得多的温度的变质岩。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到4次生长反应。利用热力学、质构和成分研究，PI和同事们将独居石的每一代与反应、温度和形成日期联系起来。对于独居石生长的多个阶段，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、软流圈底侵或放热反应)的数值模型生成的P-T-t路径进行了比较。到目前为止，PI的工作表明，在许多情况下，新英格兰中部的高级变质作用与邻近的花岗岩深成岩没有联系，因为变质作用比岩体侵位晚4,000万-5,000万年。从独居石研究得出的P-T-t轨迹与假设岩石圈拆沉和软流圈底侵作用的数值P-T-t轨迹非常吻合，大约在此之前的4亿年。这一解释与现今新英格兰中部之下岩石圈拆沉的独立地球物理证据一致。Pi和他的同事正在将这一方法扩展到沿着新英格兰阿巴拉契亚山脉的高级、部分熔融的变质岩，目前正在研究来自缅因州西南部、新罕布夏州中部、新罕布夏州S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古生代(距今4-3亿年)变质作用和热源的更完整的图景。该项目是首次尝试在造山带范围内关联附着期生长。独居石生长事件从一个位置到另一个位置的对比使用的方式类似于地层对比。在独居石边缘生长于变质作用峰期(熔融或熔融结晶期间)的假设下，精确的独居石边缘年龄提供了沿造山带追踪变质峰期的手段。独居石边缘年龄的横向(E-W)变化可能记录了沿造山带的碰撞或伸展收敛。压力与独居岩峰龄(例如，更深、更老的峰期独居岩)之间的正相关可能指示了强烈的地幔加热成分。区域范围内独居石的生长历史、测温和测年的综合为重建大规模造山带的热演化提供了一个非常强大的工具。

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.