Neoproterozoic-Early Paleozoic History of the Scottish Promontory of Laurentia

0087650康纳利晚前寒武纪地球陆块分布的知识对板块构造、古气候和生物进化具有广泛的影响。 前寒武纪赫布里底地盾（英语：Hebridean Shield）是北美大陆（Laurentia）的一小部分，在泛大陆分裂时分离，成为欧洲的一部分。 在泛大陆形成之前，它形成了劳伦大陆（位于拉布拉多和格陵兰边缘之间）的一个主要海角的尖端，因此可以作为东北劳伦大陆和其他大陆之间相互作用的敏感探针。 虽然这个“苏格兰海角”的历史对于理解新元古代和早古生代的全球古地理学很重要，但它的构造历史仍然存在争议。苏格兰变质沉积岩Moine和Dalradian超群的同位素年龄和结构复杂性一直被解释为反映新元古代造山（Knoydartian）叠加了早古生代变形和变质作用。 然而，新元古代的造山带在东北劳伦大陆的其他地方是未知的，而是冈瓦纳大陆的一个特征。 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，但两者都没有表现出水密性。此外，我们认识到一个区域套件的基性岩脉，可能作为一个可靠的时间标志，需要变质作用发生晚于目前的假设。 如果正确的话，苏格兰的新元古代造山带似乎是可疑的，这些岩石的变形更可能与早古生代劳伦提斯和欧洲西北部的组合有关（与劳伦提斯边缘沿着的其他地区一致）。关键岩性的野外测绘形成了详细的地质年代学和地质温压学框架，这将限制该地区岩浆活动、变形和变质作用的时间。 事件的绝对年龄将被用来测试构造事件的相关性沿着劳伦阶边缘。 不考虑苏格兰新元古代造山运动，将简化苏格兰劳伦蒂亚海角的前加里东历史，其中新元古代岩浆将与里菲期（~900-800 Ma）和文迪期（~600 Ma）两个独立的长期裂谷阶段有关。 反过来，这将允许沿沿着这整个边缘的共同事件的相关性，从而支持一个建议的两个阶段的“突破”劳伦作为一个离散的大陆在新元古代，假设从Rodinian和Pannotian超大陆，分别。 或者，确认新元古代造山带在苏格兰的存在可能会将苏格兰与东北劳伦大陆边缘的其他部分分离开来，从而推断在最新的前寒武纪期间冈瓦纳大陆和劳伦大陆之间的地壳交换。 无论哪种结果都将解决东北劳伦大陆边缘沿着构造事件的时间和性质，从而有助于我们了解地球历史上一个重要阶段的全球古地理，其中包括显着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.