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

地球行星演化中的一个基本问题是，在致密的岩石圈板块下沉的驱动下，现代风格的板块构造过程何时成为热对流和地壳变形的主要模式。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晚太古代至早元古代由北美西部边缘之下的大洋板块俯冲形成的增生杂岩。这些岩石是南部法明顿峡谷杂岩的一部分，在约1700 Ma的后来的碰撞事件中发生了变化，现在以角闪岩级别的片麻岩为代表。法明顿峡谷南部杂岩包括镁铁质变火山岩(角闪岩、辉石角闪岩、石榴石角闪岩)、超镁铁岩和石英岩--其中一些可能代表与镁铁质变火山岩有关的变钙长岩--在一种化学成分类似于灰瓦克的石英长石片麻岩中。放射性同位素研究给出了古老的钕模式年龄和太古代继承的锆石成分，这表明这种增生杂岩形成于怀俄明州克拉通的西南边缘，与同时代的大陆边缘一起形成，部分由法明顿峡谷杂岩(正片麻岩、混合岩、伟晶岩)的北部代表。这一大陆边缘弧在中元古代明显地与火山桑塔金弧碰撞。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盐湖城以北的法明顿峡谷综合体的部分区域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(变质角闪岩)和超镁铁质岩石(地幔构造岩？)。为了确定变质岩的原岩及其形成的构造环境，正在进行常量元素和微量元素浓度的全岩地球化学研究。对石榴石-单斜辉石角闪岩进行了详细的古压力-温度研究，以确定在任何镁铁质地块中是否保存了早期的高压变质作用。这项研究提供了有关古北美大陆西缘太古宙/中原生动物构造事件性质的重要信息。

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.