Architecture and Evolution of an Accretionary Orogen

地球大陆地壳的大部分由前寒武纪到新生代的造山带组成，在这些造山带中，先前存在的岩石由于变质作用和变形而发生了不同程度的变化。由于碰撞构造事件和造山活动之间的联系，这一漫长的造山活动记录为重建塑造我们星球的古构造环境和事件提供了最容易获得的方法。尽管阿巴拉契亚山脉被广泛用来说明地球的构造演化和相关的造山运动，但在北纳巴拉契亚的魁北克缅因部分，驱动奥陶纪塔科尼亚造山运动的碰撞过程和事件仍然是个谜。由于对早古生代东部（当前坐标）劳伦纪边缘的性质了解甚少，该地区的变形特征不能令人满意地与特定的会聚环境或碰撞事件联系起来。关于造山运动是由蛇绿岩侵位、岛弧碰撞还是微大陆碰撞引起的，人们有不同的解释。由于塔科尼亚造山运动开始了阿巴拉契亚山脉的造山循环，了解它的结构和演变对于了解整个阿巴拉契亚山脉是至关重要的。该项目建议调查构成链湖地块和沸腾山蛇绿岩的岩石单元之间的结构、变质和时间关系，以测试阿巴拉契亚山脉北部塔科尼亚造山运动的相互竞争的模型和假设。他们的多学科方法包括制图、构造、微观构造、变质和地质年代学研究。综合野外和实验室研究，该项目将确定：(a)链湖地块内变质和变形事件的相对和绝对时间，(b)沸腾山蛇绿岩基性部分的结晶年龄，(c)蛇绿岩侵位的年龄和运动学，(d)蛇绿岩附近的基性火山岩是否代表蛇绿岩的成因部分，以及(e)链湖地块中可能与裂谷有关的基性岩脉的年龄。该项目的地质年代学部分将包括对单氮石进行原位U-Pb分析，为变质事件提供高时间分辨率；对辉长岩中的锆石或坏辉长岩进行小数量多粒U-Pb分析，以确定蛇绿岩的结晶年龄；对锆石进行U-Pb分析，以确定岩脉的年龄；对白云母进行激光步进加热40 Ar- 39 Ar分析，以确定蛇绿岩的最小侵位年龄。PI将把他的结果与现有的对魁北克南部的塞特福德矿蛇绿岩和上复单元、加斯佩半岛的Maquereau Dome和纽芬兰的dashwoods分区的解释进行比较，以评估这些潜在相关单元之间的时空关系。因为地体的范围和边界是构造模型的基本组成部分，测试地体关系将测试模型。这项工作将澄清阿巴拉契亚山脉北部塔古尼亚造山运动的性质，并将对描述塔古尼亚古地理重建和碰撞吸蚀过程的概念模型产生影响。

Much of Earth's continental crust comprises Precambrian through Cenozoic orogens,where preexisting rocks have been variably modified by metamorphism and deformation. Owingto the association between collisional tectonic events and mountain building, this long record oforogenesis provides the most accessible means of reconstructing paleotectonic settings andevents that have shaped our planet. Although the Appalachian mountains are widely used toillustrate Earth's tectonic evolution and associated orogenesis, collisional processes and eventsthat drove the Ordovician Taconian orogeny in the Quebec Maine segment of the northernAppalachians remain enigmatic. Deformational features in this region cannot be satisfactorilyrelated to a specific convergent setting or collisional event(s) because the nature of the eastern(present coordinates) Laurentian margin during the early Paleozoic is poorly understood.Interpretations are divided as to whether the orogeny was caused by ophiolite emplacement,island-arc collision, or microcontinent collision. Because the Taconian orogeny initiated theAppalachian mountain-building cycle, understanding its architecture and evolution is essentialfor understanding the Appalachians as a whole.The PI proposes to investigate structural, metamorphic, and temporal relationships among therock units that make up the Chain Lakes massif and Boil Mountain ophiolite in order to testcompeting models and hypotheses for the Taconian orogeny in the northern Appalachians. Theirmultidisciplinary approach incorporates mapping, structural, microstructural, metamorphic, andgeochronological studies. Integrating field and laboratory studies, the PI will establish: (a) therelative and absolute timing of metamorphic and deformational events within the Chain Lakesmassif, (b) the crystallization age of the mafic portion of the Boil Mountain ophiolite, (c) the ageand kinematics of ophiolite emplacement, (d) whether mafic volcanic rocks near the ophioliterepresent a genetic part of the ophiolite, and (e) the age of possibly rift-related mafic dikes in theChain Lakes massif. The geochronology portion of the project will include in-situ U-Pb analysesof monazite to provide high temporal resolution for metamorphic events, small-numbermultigrain U-Pb analyses of zircon or baddeleyite from gabbro to determine the crystallizationage of the ophiolite, U-Pb analyses of zircon to establish the age of the dikes, and laser step-heating 40 Ar- 39 Ar analysis of muscovite to determine the minimum ophiolite emplacement age.The PI will compare his results with existing interpretations of the Thetford Mines ophiolite andoverlying units in southern Quebec, the Maquereau Dome in the Gaspe Peninsula, and theDashwoods Subzone in Newfoundland in order to evaluate temporal and spatial relationshipsamong these potentially correlative units. Because the extents and boundaries of terranes arefundamental components of the tectonic models, testing the terrane relationships will test themodels. The work will clarify the nature of the Taconian orogeny in the northern Appalachians,and will bear on conceptual models describing Iapetan paleogeographic reconstruction andcollisional accretionary processes.