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

合作研究：基于石榴石及副矿物年代学约束美国西南部元古界造山带的形成和演化

• 批准号: 1019877
• 负责人: Jeffrey Vervoort
• 金额: $ 9.74万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-08-01 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1019877&HistoricalAwards=false
• 关键词: Collaborative; Research; Constraining; Formation; Evolution

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.

了解大陆地壳的形成是解开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18亿到16亿年前。这个地壳的形成导致了北美大陆最大的岩石物质的增加，并产生了一个与现代喜马拉雅山脉规模相似的大山带。现有的用放射性成因同位素测定岩石年龄的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新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更好地了解造山时间的机会，从而更好地了解落基山脉南部的地壳形成过程。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代，来确定科罗拉多州南部和新墨西哥州北部角闪岩相变质作用的时间。年龄应该为石榴石生长的时间以及变质作用的时间提供强有力的约束，因为造山带的大部分经历了500至650摄氏度之间的变质高峰条件；这远远低于石榴石中Lu/Hf体系的闭合温度。在温度超过600℃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同位素系统确定辅助相的年代，包括独居石、xenotime和锆石。测年将与卟啉母细胞基质关系的详细研究联系起来，以便将变形与变质作用直接联系起来。各种地温计将与等化学相图、岩石成因网格和变质结构相结合，以确定年代样品的埋葬和挖掘历史。这些数据对于充分了解导致北美大陆西南部分形成的构造和地球动力学演化至关重要。