Developing rutile chrono-thermometry in polymetamorphic assemblages for deciphering stages of mountain belt evolution

随着大陆的碰撞和山带的上升，气候变得温和，地壳内的岩石被埋在地下，承受着高温和高压，然后以一种改变的形式被运送回地表。这些岩石中的矿物质记录了它们所经历的压力和温度，再加上它们生长的时间，使得从每块岩石中构建压力-温度-时间历史成为可能。将许多这样的轨迹结合起来，就可以逐渐形成山带的整体演化。某些矿物可以告诉我们岩石埋藏的深度和温度，而其他矿物则告诉我们所涉及的总体时间尺度。目前的困难在于将温度-压力估计与这些事件发生的时间联系起来。石榴石是记录压力和温度最有效的矿物之一，但很难精确测定年代。相反，金红石、锆石和独居石等最适合测定年龄的矿物，直到最近才能够提供它们形成时的压力和温度的信息。它们的生长条件严重影响着对这些矿物可能产生的年龄或年龄范围的解释，因此，了解它们是如何形成的，巩固了它们对定义部分压力-温度-时间历史的有用性。最近的实验表明，金红石中锆的浓度与温度有关。因此，通过测量金红石晶体的锆浓度，现在有可能推断出它生长的温度，如果金红石的年龄也通过同位素方法确定，则可以直接将时间与温度联系起来。这种新方法特别有用，因为它利用了一种矿物，这种矿物在许多变质岩类型中都很常见，一旦形成，就不容易受到后续高温事件的影响。金红石因此保留了岩石早期历史的信息。金红石结晶温度和生长年龄与压力的独立信息相结合，创造了一个非常强大的工具，能够首次揭示山带深处的热条件以及岩石通过造山系统运输所需的时间。这个项目将把这种新技术应用到喜马拉雅不同类型和大块成分的岩石上。我们检查了从东喜马拉雅不丹侦察收集的少量样本。这些岩石在喜马拉雅地区是不寻常的，因为它们显示了在高压条件下形成的证据（表明埋藏深度约为50公里）。随后的高温使这一现象消失殆尽，只留下了一小部分痕迹，这使得使用传统技术来确定它们形成的压力-温度环境变得非常困难。然而，它们的进化史，提供了关于大陆地壳在大陆碰撞过程中的行为方式的重要信息，特别是在喜马拉雅地区。我们提出的研究金红石及其共存组合的地球化学和结构关系的建议可能为这一变质事件提供重要的缺失约束。通过将矿物化学数据与矿物组合的结构观察相结合，我们将推断金红石形成的反应，并确定这些反应受大块岩石组成和变质条件影响的程度。该项目的首要目标是开发合适的方法来追踪在地壳深处盛行的早期热条件，从而为喜马拉雅演化的关键部分提供见解，从而进一步了解山带上升与长期气候变化之间的联系。

As continents collide and mountain belts rise, climates are moderated, and rocks within the Earth's crust are buried and subjected to intense heat and pressure before being transported back to the surface in an altered form. Minerals in these rocks record the pressures and temperatures they experienced, which, together with the time at which they grew, enable pressure-temperature-time histories to be constructed from each rock. The overall evolution of the mountain belt can be built up piece by piece by combining many such trajectories. Certain minerals can tell us how deeply the rocks were buried and the temperatures they reached, while other minerals tell us about the overall timescales involved. The current difficulty lies in linking the temperature-pressure estimates with the timing of these events. Garnet is one of the most effective minerals for recording pressures and temperatures, yet is extremely difficult to date precisely. Conversely, the best minerals for determining age, such as rutile, zircon and monazite, have not, until recently, been able to yield information about the pressures and temperatures at which they formed. Their growth conditions critically influence the interpretation of the age, or age range, that these minerals may yield and hence understanding how they form underpins their usefulness for defining part of the pressure-temperature-time history. Recent experiments have shown that the concentration of zirconium in rutile is dependent on temperature. By measuring the zirconium concentration of a rutile crystal, it is therefore now possible to infer the temperature at which it grew and, if the age of the rutile has also been determined by isotopic methods, to directly link time to temperature. This novel approach is particularly useful because it exploits a mineral which is common to many metamorphic rock types and, once formed, is not easily affected by subsequent high-temperature events. Rutile therefore retains information about the early history of the rock. The combination of rutile crystallisation temperatures and growth ages with independent information about pressures creates a very powerful tool capable of revealing, for the first time, the thermal conditions deep under mountain belts and the amount of time it takes for rocks to be transported through the mountain-building system. This project will apply this novel technique to Himalayan rocks of different types and bulk compositions. We have examined a small number of samples from a reconnaissance collection from Bhutan in the eastern Himalaya. These rocks are unusual for the Himalaya in that they show evidence for having been formed under high-pressure conditions (indicating burial depths of about 50 km). Subsequent high temperatures have eradicated all but a small glimpse of this episode, making it very difficult to use conventional techniques for determining the pressure-temperature environment in which they formed. Their evolutionary history, however, provides vital information on the way(s) in which continental crust behaves during continental collision processes in general, and in the Himalaya in particular. Our proposal to study the geochemistry and textural relationships of rutile and its coexisting assemblages may provide the vital missing constraints on this metamorphic episode. By combining mineral chemistry data with textural observations of the mineral assemblages we shall infer the reactions responsible for rutile formation as well as determine the extent to which the reactions are influenced by bulk-rock composition and by metamorphic conditions. The overarching aim of this project is to develop the appropriate methodology for tracing the early thermal conditions that prevailed in the deep crust and so provide insight into a crucial part of Himalayan evolution, which in turn will allow further insight into the link between mountain belt rise and long term climate change.