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，由致密岩石圈板块下沉驱动的现代板块构造过程何时成为热对流和地壳变形的主导模式。本项目研究犹他州北方瓦萨奇山脉的岩石组合，这些岩石组合可能代表了晚太古代至早元古代的增生杂岩，该杂岩是由北美西缘下方的大洋板块俯冲而形成的。这些岩石是南部法明顿峡谷杂岩的一部分，被后来大约1700 Ma的碰撞事件所改变，现在以角闪岩级片麻岩为代表。南部法明顿峡谷杂岩包含镁铁质变火山岩（角闪岩、辉石角闪岩、石榴石角闪岩）、超镁铁质岩和石英岩块体-其中一些可能代表与镁铁质变火山岩有关的变黑岩-在化学成分类似于杂砂岩的石英-蓝宝石片麻岩基质中。放射性同位素研究产生旧的新石器时代的模型年龄和太古代继承锆石成分，这表明，这增生复杂的形成在西南边缘的怀俄明州克拉通，与同时代的大陆边缘弧的一部分代表的北方部分的法明顿峡谷复杂（正片麻岩，混合岩，伟晶岩）。该陆缘弧在中元古代与火山桑塔昆弧碰撞。 为了更好地了解这一地区的地质演化，我们正在对法明顿以南和湖城以北的法明顿峡谷复合体的部分地区进行地质测绘和地球化学分析。取样集中在斜长角闪岩（变质玄武岩）、石英岩（变质玄武岩）和超镁铁岩（地幔构造岩？）。目前正在进行全岩主要和微量元素含量的地球化学研究，以确定变质岩的原岩及其形成的构造背景。详细的古压力-温度调查的石榴石单斜辉石斜长角闪岩正在使用，以确定是否早期的高压变质作用是保存在任何镁铁质块。 这项研究为古北美大陆西缘沿着太古宙/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.