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

当地壳因构造板块碰撞而收缩时，山脉就形成了。这种收缩在地壳中产生花岗岩岩体，并通过剪切带（局部剧烈变形区域）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种“织物”，一种记录岩石变形的矿物排列。矿物钛矿在地壳中花岗岩的形成和岩石结构的形成过程中结晶，这两个事件都可以用钛矿晶体中铀变成铅的放射性衰变来确定年代。这种测定剪切带年代的新方法将用于研究造山过程和岩石结构的发育，从而进一步控制岩浆侵位、断裂、流体流动和金属矿化。本研究将重点研究矿物钛矿在变形过程中的物理和化学变化。构造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再结晶。钛矿的化学变化将通过x射线和质谱技术来测量，单个晶体将通过铀-铅同位素比率的测量来确定年代。这些技术将应用于两个研究区域，爱达荷州西部和加利福尼亚州中东部的剪切带。这些地区的岩石显示出广泛的组构发育，从未变形到普遍剪切。通过上述方法测量的各种变形程度将用于测试岩石组构在剪切带运动期间的强度发展情况。这两个研究领域的成果将包括建立亚百万年分辨率的变形时间，剪切带内应变积累的时空格局，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议的期刊、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.