Collaborative Research: Using Titanite as a Petrochronometer for Direct Fabric Dating of High Temperature Systems

山脉的形成是由于地壳板块的碰撞而导致的地壳收缩。这种收缩在地壳中产生花岗岩岩体，并通过剪切带（局部强烈变形区域）使这些花岗岩岩体变形。这两个过程都在花岗岩上留下了一个“织物”，一种记录岩石变形的矿物排列。在地壳中花岗岩的形成和岩石组构的形成过程中，钛石矿物结晶，这两个事件都可以通过钛石晶体中铀的放射性衰变为铅来确定年代。 这种测定剪切带年龄的新方法将用于研究造山过程和岩石组构的发展，这些岩石组构随后可以控制进一步的岩浆侵位、断层、流体流动和金属矿化。本研究将集中在变形过程中的矿物钛的物理和化学变化。结构地质学技术将通过岩石的露头和微观结构来研究物理变化，电子显微镜将量化单个矿物颗粒的重结晶。将通过X射线和质谱技术测量钛铁矿的化学变化，并使用铀-铅同位素比率测量单个晶体的年代。这些技术将应用于两个研究领域，在剪切带在西部爱达荷州和中东部加州。从这些地区的岩石显示了广泛的组构发展，从非变形到普遍剪切。通过上述方法测量的变形程度的变化将用于测试岩石组构的强度在剪切带运动期间如何发展。这两个研究领域的成果将包括建立变形时间的百万分之一年分辨率的方法，剪切带内应变积累的时空模式，以及岩浆活动和变形之间的反馈。该项目将更广泛地支持研究生研究培训、在线科学学习模块和地理信息学数据库开发。这项研究的结果将通过同行评审期刊、专业会议、数据库和网站广泛传播。

Mountain ranges form when the Earth's crust contracts due to the collision of tectonic plates. This contraction generates granite plutons in the crust and deforms those granite plutons through shear zones (localized regions of intense deformation). Both processes imprint a "fabric" on the granite, an arrangement of minerals that records the deformation of the rock. The mineral titanite crystallizes during the formation of granite in the crust and during the formation of rock fabric, and both events can be dated using the radioactive decay of uranium into lead within titanite crystals. This new method for dating shear zones will be used to examine mountain building processes and the development of rock fabrics that subsequently can control further magma emplacement, faulting, fluid flow, and metal ore mineralization. This study will focus on the physical and chemical changes in the mineral titanite during deformation. Structural geology techniques will investigate the physical changes through the outcrop and microscale structures of the rock, and electron microscopy will quantify the recrystallization of individual mineral grains. Chemical changes in titanite will be measured by X-ray and mass spectrometry techniques, and individual crystals will be dated using measurements of uranium-lead isotope ratios. These techniques will be applied in two study areas, in shear zones in western Idaho and east-central California. Rocks from these areas show a wide range of fabric development, from non-deformed to pervasively sheared. The variety of degrees of deformation measured through the methods described above will be used to test how the intensity of a rock fabric develops during shear zone movement. Outcomes in both study areas will include methodologies to establish sub-million year resolution of the timing of deformation, the spatial-temporal pattern of strain accumulation within shear zones, and the feedbacks between magmatism and deformation. This project will more broadly support graduate research training, on-line science learning modules, and geoinformatics database development. Results of this research will be widely disseminated through peer-reviewed journals, professional meetings, data repositories, and websites.