Sm-Nd Thermochronology of Garnets in Metamorphic Rocks: A New Method and Tectonic Applications

Ducea/GangulyEAR-0087424我们将用一种新的热年代学方法定量研究造山环境中地壳深部岩石的冷却和折返速率。该应用程序的基本科学目标是评估正常断裂作用、韧性减薄和侵蚀对岩石折返的相对贡献。这一理解是增加我们对大陆构造知识的一个基本概念。这种方法可以通过单一的放射性衰变系统，根据矿物的年龄分布或岩心和散体年龄之间的差异来确定冷却速度。我们将用同位素稀释法和热电离质谱法测定石榴石晶体中心段和边缘段的Sm-ND年龄。年龄将通过从定义尺寸的岩芯中钻取的少量样品来确定。这些数据将与基于离子交换和矿物反应的传统温压测量相结合，以限制500oC以上岩石的折返历史。将新的和传统的热年代学数据与岩石学观察相结合，将能够构建比迄今可能的更全面的高级变质岩折返历史图景。这些样品来自碰撞造山带喜马拉雅山脉(尼泊尔)、内华达山脉弧根组合(加利福尼亚州)和俯冲杂岩卡塔琳娜岛(加利福尼亚州)。

Ducea/GangulyEAR-0087424We will quantitatively investigate cooling and exhumation rates of deep crustal rocks from orogenic environments using a new thermochronological method. The basic scientific goal of the application is to asses the relative contribution of normal faulting, ductile thinning and erosion to rock exhumation. This understanding is a fundamental concept to add to our knowledge of continental tectonics. This method permits determination of cooling rates from the age profile or the difference between the core and bulk ages of a mineral by a single radiogenic decay system. We will determine the Sm-Nd ages of central and rim segments of garnet crystals by isotope dilution and thermal ionization mass spectrometry. The ages will be determined from small quantities of samples drilled from the cores of defined dimensions. These data will be combined with conventional thermobarometry based on ion exchange and mineral reactions to constrain the exhumation history of the rocks above 500 oC. Integration of the data derived from the new and conventional thermochronology with petrological observations would permit construction of a more comprehensive picture of the exhumation history of high-grade metamorphic rocks than has been possible so far. The suites of samples are from a collisional orogen, the Himalaya Mountains (Nepal), arc root assemblages from the Sierra Nevada (California), and a subduction complex, the Catalina Island (California).