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揭示地球历史和确定地球内部矿物和物质资源分布的关键。美国西南部落基山脉南部的大陆地壳形成于18亿至16亿年前。这一地壳的形成导致了北美大陆最大的岩石物质增加，并产生了一个规模与现代喜马拉雅山相似的大型山脉带。现有的使用放射性同位素的岩石年龄测定方法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，可以诊断造山过程中的变质条件。因此，确定石榴石的年龄提供了机会，以更好地了解造山运动的时间，从而在落基山脉南部的地壳形成过程中，这些科学家将试图确定角闪岩相变质作用在科罗拉多南部和北方新墨西哥州的时间，通过直接测定石榴石的Lu/Hf和Sm/Nd的方法。年龄应该提供强大的制约石榴石生长的时间，因此，变质作用的时间，因为大部分的造山带经历了峰值变质条件之间的500和650摄氏度，这是远低于封闭温度的石榴石中的Lu/Hf系统。在温度超过600摄氏度的地区，Lu/Hf和Sm/Nd年龄之间的差异可能指示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同位素系统测定辅助相，包括独居石，磷钇矿和锆石。测年将与详细研究成斑基质关系联系起来，以便将变形与变质直接联系起来。各种地质温压计将与等化学相图、成岩网格和变质结构相结合，以确定定年样品的埋藏和剥露历史。这些数据对于充分理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.