Architecture and Evolution of an Accretionary Orogen

地球的大部分陆壳由前寒武纪到新生代造山带组成，在那里原有的岩石经过变质和变形而发生了不同程度的变化。由于碰撞构造事件与造山之间的联系，这种长期的造山作用记录为重建塑造了我们星球的古构造环境和事件提供了最容易获得的手段。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作用，但驱动阿巴拉契亚北部魁北克缅因州段奥陶纪塔科尼亚造山的碰撞过程和事件仍然是个谜。由于对早古生代劳伦斯东缘(现今坐标)性质的认识较差，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有关(S)。关于造山作用是由蛇绿岩侵位、岛弧碰撞还是微陆碰撞引起的，有不同的解释。由于塔克期造山启动了阿巴拉契亚造山旋回，了解其结构和演化对于整体理解阿巴拉契亚山脉至关重要。PI建议调查组成链湖地块和沸腾山蛇绿岩的岩石单元之间的构造、变质和时间关系，以检验阿巴拉契亚北部塔克期造山作用的相互竞争的模型和假说。他们的多学科方法结合了测绘、构造、微构造、变质和地质年代学研究。结合野外和实验室研究，PI将确定：(A)湖体链内变质和变形事件的相对和绝对时间，(B)沸山蛇绿岩镁铁质部分的结晶年龄，(C)蛇绿岩侵位的年龄和运动学，(D)蛇绿岩附近的镁铁质火山岩是否存在蛇绿岩的成因部分，以及(E)Chain Lake地块中可能与裂谷有关的镁铁质岩脉的年龄。该项目的年代学部分将包括独居石的现场U-Pb分析以提供高时间分辨率的变质事件，来自辉长岩的少量多颗粒U-Pb分析以确定蛇绿岩的结晶，锆石的U-Pb分析以确定岩脉的年龄，以及白云母的激光阶梯加热40Ar-39Ar分析以确定最小蛇绿岩侵位。PI将把他的结果与现有的对魁北克南部的Thetford Mines蛇绿岩和俯冲单元、Gaspe半岛的Maquereau穹顶和纽芬兰的达什伍兹次带的解释进行比较，以评估这些潜在相关单元之间的时空关系。由于地体的范围和边界是构造模型的基本组成部分，因此测试地体关系将测试模型。这项工作将阐明阿巴拉契亚山脉北部塔科尼亚造山作用的性质，并将涉及描述伊阿佩坦古地理重建和碰撞增生过程的概念模型。

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.