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

亚利桑那州中南部的麦克道尔山脉是由新近发现的大约10公里厚的中元古代岩石组成的。这是北美地质历史上的一段时期，关于这段时期的具体信息很少。详细的地质、同位素年龄测定和麦克道尔山脉剖面的构造研究将大大增加当前对这一时期发生在北美的地质事件的认识和理解。这些结果也为评价中元古代的板块构造重建提供了约束条件。这项研究的独特之处在于，它的研究区域位于麦克道尔索诺兰保护区内，这是美国最大的市政保护区之一（面积超过122平方公里），毗邻大凤凰城大都市区。这项研究的科学结果将通过麦克道尔索诺兰保护区向公众传播，其方式是准备外联材料（地质路径指南），可在网上下载，并由项目团队进行公开讲座。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（McDowell Sonoran Conservancy）存档，并用于向保护区的游客进行公共宣传。该项目还将有助于培养一名研究生，并将有助于扩大在重要的科学、技术、工程和数学（STEM）学科中代表性不足的群体。麦克道尔山岩石剖面由变质沉积岩和火山岩组成，并被若干花岗岩岩体侵入。该剖面的年龄由最古老的火山凝灰岩的早元古代铀铅年龄（1630亿年）和侵入该剖面的最年轻的花岗岩岩体的中元古代铀铅年龄（1423亿年）所包围。在过去的四年里，亚利桑那州和新墨西哥州至少有两个更小的部分被确定为中元古代。观察到这些岩石被大型逆冲断层折叠或切割，然后被深埋和变质，从而确定了一个新的区域性山脉形成事件：Picuris造山运动。麦克道尔山脉剖面的厚度和性质为更好地了解该造山运动的时间、性质和区域范围提供了独特的机会。最近在麦克道尔山脉完成了地质和构造测绘以及岩石取样。对3个样品进行了锆石铀铅定年试验，得到了3个样品的中元古代年龄，但最重要的是该剖面最年轻的火山岩年龄为1.546亿年，确定了该剖面的中元古代年龄。这些年代使它成为目前已知的北美中元古代唯一含有厚厚的火山岩沉积物的剖面。本文拟在前期工作的基础上，对麦克道尔山剖面进行以下研究：(1)进一步确定其年龄范围；(2)确定变质事件的时间和性质；(3)确定火成岩的年龄、化学、变形和地幔模式年龄；(4)确定变形的年龄和性质；(5)确定沉积岩的碎屑锆石铀铅种群特征。这项研究将使用一组精心挑选的样品：23个用于锆石和独居石铀铅测年，其中一个子集将用于分析镥铪同位素，15个用于火山和花岗岩的主要和微量元素地球化学，90个用于岩石的岩石学和微观结构织物薄片研究。五个研究目标的结果将为麦克道尔山层序提供一个全面的地质、地质年代学和构造特征。此外，该剖面代表了北美“岩浆裂谷”（约16亿至14.5亿年前）内唯一重要的火山岩和侵入岩，可能代表了板块重建模型的“缺失环节”，在此之前，这些模型总是需要存在异国情调的源地，如澳大利亚、欧洲西北部、或相关的大陆碎片，以解释北美沉积序列中15至16亿年前的锆石的出现，而这些锆石在当地的来源是未知的。

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.