Collaborative Research: Constraining the Formation and Evolution of the Proterozoic Orogenic Belt of the SW USA Based on Geochronology of Garnet and Accessory Minerals

了解大陆地壳的形成是解开地球历史和确定地球内部矿产和物质资源分布的关键。美国西南部落基山脉南部地区的陆壳形成于18亿至16亿年前。地壳的形成导致了北美大陆最大规模的岩石物质增加之一，并产生了一条规模类似于现代喜马拉雅山脉的大山带。现有的用放射性同位素测定岩石的年龄并不能唯一地确定造山和地壳形成的时间。这些研究人员将使用最近开发的技术来确定矿物石榴石的年龄，石榴石是一种重要的造岩矿物，可以在造山过程中诊断变质条件。因此，确定石榴石的年龄为更好地了解落基山脉南部造山的时间，从而了解地壳形成过程提供了机会。这些科学家将尝试通过Lu/Hf和Sm/ND方法直接测定石榴石的年龄，来确定科罗拉多州南部和新墨西哥州北部角闪岩相变质的时间。由于造山带的大部分经历了500-650摄氏度的高峰变质条件，因此石榴石的生长时间和变质作用的时间应该提供了强有力的制约；这远远低于石榴石中Lu/Hf系统的闭合温度。在温度超过600摄氏度的地区，Lu/Hf和Sm/ND年龄的差异可能指示了高温变质事件的持续时间。对于关键样品，我们还将使用U/Pb同位素系统测定副相的年龄，包括独居石、辉石和锆石。为了直接将变形与变质作用联系起来，测年将与成斑细胞基质关系的详细研究联系在一起。各种地质温压计将与等化学相图、岩石成因网格和变质结构相结合，以确定年代样品的埋藏和折返历史。这些数据对于全面了解导致北美大陆西南部形成的构造和地球动力学演化至关重要。

Understanding the formation of the continental crust is key to unraveling earth history and determining the distribution of mineral and material resources within the earth. The continental crust of the southern Rocky Mountain region of the southwestern United States was formed between 1.8 billion and 1.6 billion years ago. The formation of this crust resulted in one of the largest additions of rock material to the North American continent, and produced a large mountain belt similar in scale to the modern Himalaya. Existing age determinations of rocks using radiogenic isotopes have not uniquely determined the time of mountain building and crustal formation. These researchers will use recently developed techniques to determine the age of the mineral garnet, which is an important rock-forming mineral that is diagnostic of metamorphic conditions during mountain building. Determining the ages of garnet therefore provides the opportunity to better understand the timing of mountain building, and therefore processes of crustal formation in the southern Rocky Mountains.These scientists will attempt to determine the timing of amphibolite facies metamorphism in southern Colorado and northern New Mexico by directly dating garnets by the Lu/Hf and Sm/Nd methods. The ages should provide robust constraints on the timing of garnet growth and therefore the timing of metamorphism because much of the orogenic belt experienced peak metamorphic conditions between 500 and 650 degrees Celcius; this is well below the closure temperature for the Lu/Hf system in garnet. In regions where temperatures exceeded 600 degrees Celcius, the difference between the Lu/Hf and Sm/Nd ages may indicate the duration of the high temperature metamorphic event. For key samples, we will also date accessory phases using the U/Pb isotopic system, including monazite, xenotime, and zircon. The dating will be tied to detailed studies of porphyroblast matrix relationships in order to directly link deformation to metamorphism. A variety of geothermobarometers will be combined with isochemical phase diagrams, petrogenetic grids, and metamorphic textures to determine the burial and exhumation histories of the dated samples. These data are essential to fully understand the tectonic and geodynamic evolution that led to the formation of the southwestern portion of the North American continent.