The Proterozoic Missing Link? Deposition, Volcanism, and Deformation between 1.6 and 1.45 Ga in the McDowell Mountains, south-central Arizona.

元古代缺失的一环？

• 批准号: 1650411
• 负责人: Matthijs Van Soest
• 金额: $ 5.26万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1650411&HistoricalAwards=false
• 关键词: Proterozoic; Missing; Link; Deposition; Volcanism

The McDowell Mountains in south-central Arizona are composed of a newly identified approximately 10 kilometer-thick section of rocks of Middle Proterozoic age. This is a time period in North American geologic history for which very little concrete information exists. Detailed geologic, isotopic age determinations, and structural study of the McDowell Mountains section will greatly increase current knowledge and understanding of geologic events occurring in North America during this period. The results may also provide constraints to evaluate proposed plate tectonic reconstructions during the Middle Proterozoic. The research is unique in that it the field area lies within the McDowell Sonoran Preserve, one of the largest municipal preserves in the nation (greater than 122 square kilometers), located adjacent to the Greater Phoenix metropolitan area. The scientific results of this study will be communicated to the public via the McDowell Sonoran Preserve by the preparation of outreach materials (geologic trail guides) available for download on the web and public lectures by the project team. Research products and samples will be turned over to the McDowell Sonoran Conservancy, the volunteer branch of the Preserve, for archiving and their use in public outreach to visitors of the Preserve. The project will also contribute to the training of a graduate student and will contribute to the broadening of underrepresented groups in an important science, technology, engineering and math (STEM) discipline.The McDowell Mountains rock section is comprised of metamorphosed sedimentary and volcanic rocks that are intruded by several granite bodies. The age of the section is bracketed by an early Proterozoic uranium-lead age (1630 million years) in the oldest volcanic tuff and a Middle Proterozoic uranium-lead age (1423 million years) for the youngest granite body intruding the section. In the last four years at least two, much smaller, sections in Arizona and New Mexico, were identified as Middle Proterozoic in age. The observations that these rocks are folded and/or are cut by large thrust faults, which were then deeply buried and metamorphosed, led to the identification of a new mountain range building event of regional extent: the Picuris orogeny. The thickness and nature of the McDowell Mountains section provides a unique opportunity to gain a better understanding of the timing, nature, and regional extent of this orogeny. Geologic and structural mapping and rock sampling was recently completed in the McDowell Mountains. A pilot zircon uranium-lead dating study on 3 samples yielded Middle Proterozoic ages for all three, but the most significant age is 1546 million years for the youngest volcanic rocks in the section, confirming the age of the section as Middle Proterozoic. These ages make it the only currently known North American Middle Proterozoic section that contains thick deposits of volcanic rocks. The research proposed here intends to build on the initial work to address the following objectives for the McDowell Mountains section: (1) further constrain its age range, (2) determine the timing and nature of metamorphic events, (3) characterize the age, chemistry, deformation, and mantle model ages of the igneous rocks, (4) constrain the age and nature of deformation, and (5) characterize the detrital zircon uranium-lead populations of the sedimentary rocks. A carefully selected set of samples will be used for this study: 23 for zircon and monazite uranium-lead dating, a subset of which will be analyzed for lutetium-hafnium isotopes, 15 for major and trace element geochemistry of the volcanic and granitic rocks, and 90 for petrographic and micro-structural fabric thin section study of the rocks in the section. The results of the five research objectives will provide a comprehensive geologic, geochronologic and structural characterization of the McDowell Mountains sequence. In addition, the section represents the only significant volcanic and intrusive rocks within the North American "magmatic gap" (between about 1.6 and 1.45 billion years ago) and may represent the "missing link" for plate reconstruction models that have, heretofore, always required the presence of an exotic source terrain e.g. Australia, northwestern Europe, or associated continental fragments to explain the occurrence of 1.5 to 1.6 billion year old zircons in North American sedimentary sequences for which no local source was known.

位于亚利桑那州中南部的麦克道尔山脉由一个新发现的大约10毫米厚的中元古代岩石组成。这是北美地质历史上的一个时期，几乎没有具体的信息。详细的地质、同位素年龄测定和麦克道尔山脉部分的结构研究将大大增加目前对这一时期北美地质事件的认识和理解。这些结果也可能为评估中元古代板块构造重建提供约束。该研究的独特之处在于，该区域位于麦克道尔索诺兰保护区内，该保护区是美国最大的市政保护区之一（面积超过122平方公里），毗邻大凤凰城都会区。这项研究的科学成果将通过麦克道尔索诺兰保护区向公众传播，方法是编写可在网上下载的宣传材料（地质线索指南）和项目小组的公开讲座。研究产品和样本将移交给保护区的志愿者分支麦克道尔索诺兰保护协会，用于存档和向保护区的游客进行公众宣传。该项目还将有助于培养一名研究生，并将有助于扩大一个重要的科学、技术、工程和数学学科中代表性不足的群体。麦克道尔山脉岩石部分由变质沉积岩和火山岩组成，被几个花岗岩体侵入。该部分的年龄被最古老的火山凝灰岩的早元古代铀铅年龄（16.3亿年）和侵入该部分的最年轻的花岗岩体的中元古代铀铅年龄（14.23亿年）所包围。在过去的四年中，至少有两个，小得多，部分在亚利桑那州和新墨西哥州，被确定为中元古代的年龄。观察到这些岩石被折叠和/或被大型逆冲断层切割，然后被深埋和变质，导致确定了一个新的区域范围的山脉建设事件：Picuris orthodox。麦克道尔山脉部分的厚度和性质提供了一个独特的机会，以获得更好地了解这个orthopathy的时间，性质和区域范围。最近在麦克道尔山脉完成了地质和结构绘图以及岩石取样。对3个样品进行的锆石铀-铅测年试验研究得出了所有三个样品的中元古代年龄，但最重要的年龄是剖面中最年轻的火山岩的年龄为15.46亿年，确认了剖面的年龄为中元古代。这些年龄使它成为目前已知的唯一一个包含厚厚的火山岩沉积物的北美中元古代剖面。这里提议的研究打算在初步工作的基础上，为麦克道尔山脉部分实现以下目标：（1）进一步限定其年龄范围;（2）确定变质事件的时间和性质;（3）描述火成岩的年龄、化学、变形和地幔模式年龄;（4）限定变形的年龄和性质;（5）沉积岩碎屑锆石铀铅同位素组成特征。一组精心挑选的样品将用于这项研究：23个用于锆石和独居石铀-铅测年，其中一个子集将进行镥-铪同位素分析，15个用于火山岩和花岗岩的主要和微量元素地球化学，90个用于剖面岩石的岩相学和显微结构组构薄片研究。五个研究目标的结果将提供一个全面的地质，地质年代学和麦克道尔山脉序列的结构特征。此外，该剖面代表了北美“岩浆空白”内唯一重要的火山岩和侵入岩（大约16亿至14.5亿年前），可能代表了板块重建模型的“缺失环节”，迄今为止，这些模型总是需要存在一个外来的源地形，例如澳大利亚，欧洲西北部，或相关的大陆碎片，以解释在北美沉积序列中出现的15亿至16亿年的锆石，而这些锆石没有当地来源。