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一段新发现的约10公里厚的中元古代岩石组成的。这是北美地质史上一个几乎没有具体信息的时期。麦克道尔山脉剖面的详细地质、同位素年龄测定和构造研究将极大地增加目前对北美在这一时期发生的地质事件的认识和理解。这一结果也可能为评价中元古界板块构造重构提供依据。这项研究的独特之处在于，田野区域位于麦克道尔·索诺兰保护区内，这是美国最大的市政保护区之一(超过122平方公里)，毗邻大凤凰城大都市区。这项研究的科学成果将通过McDowell Sonoran保护区向公众传播，方法是准备可在网上下载的宣传材料(地质跟踪指南)和项目组的公开讲座。研究产品和样本将被移交给麦克道尔·索诺兰保护协会，这是保护区的志愿者分支机构，用于存档，并用于向保护区的游客公开宣传。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(STEM)学科中代表性不足的群体。麦克道尔山脉岩石部分由变质的沉积岩和火山岩组成，这些岩石被几个花岗岩岩体侵入。该剖面的年龄被最古老的火山凝灰岩中的早元古代铀铅年龄(1.63亿年)和侵入该剖面的最年轻花岗岩的中元古代铀铅年龄(1.423亿年)所包围。在过去的四年里，在亚利桑那州和新墨西哥州，至少有两个小得多的剖面被确定为中元古代。观察到这些岩石被大型逆冲断裂褶皱和/或切割，然后被深埋和变质，从而识别出一种新的区域范围的山脉建造事件：皮乌里斯造山。麦克道尔山脉剖面的厚度和性质为更好地了解这一造山作用的时间、性质和区域范围提供了一个独特的机会。麦克道尔山脉最近完成了地质和构造测绘以及岩石取样。对3个样品进行的先导性锆石铀铅测年研究得出了这三个样品的中元古代年龄，但最重要的年龄是该剖面中最年轻的火山岩的年龄为1.546亿年，证实该剖面为中元古代。这些年代使它成为目前已知的北美中元古代唯一包含厚厚火山岩沉积的剖面。在此提出的研究旨在建立在初步工作的基础上，以解决麦克道尔山脉剖面的以下目标：(1)进一步限制其年龄范围，(2)确定变质事件的时间和性质，(3)表征火成岩的年龄、化学、变形和地幔模式年龄，(4)约束变形的年龄和性质，以及(5)表征沉积岩的碎屑锆石-铀-铅群。这项研究将使用精心挑选的一组样品：23个用于锆石和独居石铀铅定年，其中一个子集将用于分析Lu-Hf同位素，15个样品用于火山岩和花岗岩的常量和微量元素地球化学研究，90个样品用于该剖面岩石的岩相学和微构造组构薄片研究。这五个研究目标的结果将提供麦克道尔山脉序列的全面的地质、年代学和构造特征。此外，该剖面代表了北美“岩浆缺口”(大约16亿至14.5亿年前)内唯一重要的火山岩和侵入岩，并可能代表了板块重建模型的“缺失环节”，该模型迄今一直需要澳大利亚、欧洲西北部或相关大陆碎片的存在，以解释北美沉积序列中存在15亿至16亿年的锆石，但当地来源尚不清楚。

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.