Neoproterozoic-Early Paleozoic History of the Scottish Promontory of Laurentia

[87650]康纳利，关于晚前寒武纪地球陆块分布的认识对板块构造、古气候和生物演化具有广泛的意义。前寒武纪赫布里底盾是祖先北美洲（劳伦西亚）的一小部分，在泛大陆分裂期间分离出来成为欧洲的一部分。在泛大陆形成之前，它形成了劳伦西亚一个主要海角的尖端（在拉布拉多和格陵兰边缘之间），因此可以作为东北劳伦西亚和其他大陆之间相互作用的敏感探针。尽管“苏格兰海岬”的历史对了解新元古代和早古生代全球古地理具有重要意义，但其构造史仍存在争议。早古生代变形变质作用叠加的新元古代造山带（诺达尔期）的同位素年龄和构造复杂性一直被认为是苏格兰变质变质变质变质变质的反映。然而，新元古代造山运动在东北劳伦西亚的其他地方是未知的，而是冈瓦纳兰的一个特征。这一异常新元古代事件的存在主要基于两种独立的地质年代学方法，但两者都不牢靠。此外，我们认识到一组区域性的基性岩脉可以作为可靠的时间标记，表明变质作用发生的时间比目前推测的要晚。如果正确的话，苏格兰的新元古代造山作用似乎令人怀疑，这些岩石的变形更可能与早古生代劳伦西亚和西北欧的组合有关（与劳伦西亚边缘的其他地区一致）。关键岩性的野外填图形成了详细的地质年代学和地温压学框架，将约束该地区岩浆活动、变形和变质作用的时间。事件的绝对年龄将被用来测试沿劳伦边缘的构造事件的相关性。对苏格兰新元古代造山运动进行折现，可以简化苏格兰劳伦提亚海岬的前加里东期历史，其中新元古代岩浆与里芬期（~900-800 Ma）和文甸期（~600 Ma）两个独立的延长裂陷期有关。反过来，这将使得沿着整个边缘的共同事件相互关联，从而支持了Laurentia在新元古代作为一个独立大陆的两个阶段的“分裂”，假设分别来自罗丁和潘诺蒂亚超大陆。另外，确认苏格兰新元古代造山运动的存在可能会将苏格兰与东北劳伦斯边缘的其余部分分离开来，从而推断冈瓦尼德大陆与劳伦斯大陆在前寒武纪晚期之间的地壳交换。这两种结果都将解决东北劳伦斯边缘构造事件的时间和性质，从而有助于我们理解地球历史上一个重要时期的全球古地理，其中包括显著的生物多样性和异常气候事件。

0087650ConnellyKnowledge of the distribution of land masses on Earth during the Late Precambrian has wide ranging implications for plate tectonics, paleoclimate and biological evolution. The Precambrian Hebridean Shield is a small fragment of ancestral North America (Laurentia) that detached during break up of Pangaea to become part of Europe. Prior to assembly of Pangaea, it formed the tip of a major promontory of Laurentia (between the Labrador and Greenland margins) and may thus serve as a sensitive probe of interactions between NE Laurentia and other continents. Although the history of this "Scottish promontory" is important to understanding global paleogeography in Neoproterozoic and Early Paleozoic times, its tectonic history remains controversial. Isotopic ages and structural complexities in the metasedimentary Moine and Dalradian supergroups of Scotland have long been interpreted to reflect Neoproterozoic orogenic (Knoydartian) episodes overprinted by Early Paleozoic deformation and metamorphism. Yet Neoproterozoic orogeny is unknown elsewhere in NE Laurentia and is instead a characteristic of Gondwanaland. The existence of this anomalous Neoproterozoic event is based primarily on two independent methods of geochronology, but neither appear water tight. Furthermore, we recognize a regional suite of mafic dikes that may serve as a reliable temporal marker that requires metamorphism to have occurred later than is currently presumed. If correct, Neoproterozoic orogeny in Scotland seems doubtful and deformation in these rocks is more likely related to the assembly of Laurentia and northwestern Europe during the Early Paleozoic (consistent with other regions along the Laurentian margin).Field mapping of key lithologies form a framework for detailed geochronology and geothermobarometry that will constrain the timing of magmatism, deformation and metamorphism in this region. Absolute ages of events will be used to test correlations of tectonic events along the Laurentian margin. Discounting Neoproterozoic orogeny in Scotland would simplify the pre-Caledonian history of the Scottish promontory of Laurentia in which Neoproterozoic magmas would relate to two separate phases of protracted rifting during the Riphean (~900-800 Ma) and Vendian (~600 Ma) periods. In turn, this would permit correlation of common events along this entire margin, and thus support a proposed two-stage 'breakout' of Laurentia as a discrete continent during the Neoproterozoic, hypothetically from the Rodinian and Pannotian supercontinents, respectively. Alternatively, confirming the existence of Neoproterozoic orogeny in Scotland may dissociate Scotland from the rest of the NE Laurentian margin, thereby inferring crustal exchange between a Gondwanide continent and Laurentia during the latest Precambrian. Either outcome will resolve the timing and nature of tectonic events along the margin of NE Laurentia, thereby contributing to our understanding of global paleogeography during an important segment of Earth history that includes remarkable biodiversity and anomalous climate events.