Is the Southern Farmington Canyon Complex a Late Archean/Early Proterozoic Accretionary Complex?

地球行星演化的一个基本问题是，在致密岩石圈板块下沉的驱动下，现代板块构造过程何时成为热对流和地壳变形的主要方式。该项目研究了犹他州北部瓦萨奇山脉的岩石组合，这些岩石组合可能代表了太古代晚期到元古代早期的增生复合体，该复合体是由北美西部边缘下的海洋板块俯冲形成的。这些岩石是法明顿峡谷南部建筑群的一部分，后来在1700万年左右的碰撞事件中被修改，现在以角闪岩级片麻岩为代表。南法明顿峡谷杂岩包含基性变质火山岩块体（角闪岩、辉石角闪岩、石榴石角闪岩）、超基性岩和石英岩，其中一些可能代表与基性变质火山岩相关的变质岩，它们位于化学成分与灰岩相似的石英长石片麻岩基质中。放射性成因同位素研究得出了古老的钕模式年龄和太古宙继承的锆石成分，表明该增生杂岩形成于怀俄明克拉通的西南边缘，与同时期的大陆边缘弧（部分由法明顿峡谷杂岩北部组成）（正长岩、混辉岩、辉晶岩）相结合。该大陆边缘弧在中元古代明显与火山圣塔坎弧碰撞。为了更好地了解该地区的地质演化，我们正在对法明顿南部和盐湖城北部的法明顿峡谷的部分地区进行地质测绘和地球化学分析。取样主要集中在角闪岩（变质玄武岩）、石英岩（变质玄武岩）和超镁铁质岩（地幔构造岩）。为了确定变质岩的原岩及其形成的构造环境，正在进行主要元素和微量元素浓度的全岩地球化学研究。对石榴石斜辉长角闪岩进行了详细的古压力-温度研究，以确定在任何基性块体中是否保存了早期的高压变质作用。这项研究为古北美大陆西缘太古代/中元古代构造事件的性质提供了重要信息。

One of the fundamental questions in the planetary evolution of Earth centers on when modern style plate tectonic processes, driven by the sinking of dense lithospheric plates, became the dominant mode of thermal convection and crustal deformation. This project examines rock assemblages in the Wasatch Mountains of northern Utah that may represent a late Archean to early Proterozoic accretionary complex that formed by subduction of oceanic plates beneath the western margin of North America. These rocks, part of the southern Farmington Canyon complex, were modified by a later collisional event circa 1700 Ma and are now represented by amphibolite grade gneisses. The southern Farmington Canyon complex contains blocks of mafic metavolcanic rock (amphibolite, pyroxene amphibolite, garnet amphibolite), ultramafic rock, and quartzite - some of which may represent metachert associated with mafic metavolcanic rocks - in a matrix of quartzo-feldspathic gneiss with a chemical composition similar to greywacke. Radiogenic isotopic studies yield old Neodymium model ages and Archean inherited zircon components which suggests that this accretionary complex formed on the southwestern margin of the Wyoming craton, in conjunction with a coeval continental margin arc represented in part by the northern portion of the Farmington Canyon complex (orthogneiss, migmatite, pegmatite). This contintental margin arc apparently collided with the volcanic Santaquin arc during Middle Proterozoic time. To better understand the geologic evolution of this area, we are conducting geologic mapping and geochemical analysis of parts of the Farmington Canyon complex south of Farmington and north of Salt Lake City. Sampling is focused on the amphibolites (metabasalts), quartzites (metacherts), and ultramafic rocks (mantle tectonites?). Whole rock geochemical studies for major and trace element concentrations are being carried out in order to establish the protoliths of the metamorphic rocks and the tectonic setting in which they originated. Detailed paleo pressure-temperature investigations of garnet-clinopyroxene amphibolites are being used in order to determine if an earlier episode of high-pressure metamorphism is preserved in any of the mafic blocks. This study is providing critical information bearing on the nature of Archean/Mesoprotoerozoic tectonic events along the western margin of the ancient North American continent.