Neoproterozoic-Early Paleozoic History of the Scottish Promontory of Laurentia

苏格兰劳伦西亚海角的新元古代-早古生代历史

• 批准号: 0087650
• 负责人: James Connelly
• 金额: $ 11.38万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-04-01 至 2004-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0087650&HistoricalAwards=false
• 关键词: Neoproterozoic; Early; Paleozoic; History; Scottish

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.

了解地球上晚前寒武纪陆块的分布对板块构造、古气候和生物进化具有广泛的意义。前寒武纪赫布里底斯盾是祖先北美(劳伦西亚)的一小块碎片，在泛古大陆解体时分离出来，成为欧洲的一部分。在泛古大陆组装之前，它形成了劳伦蒂角(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蒂亚与其他大陆之间相互作用的灵敏探测器。尽管这个“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很重要，但它的构造史仍然存在争议。长期以来，苏格兰变质沉积的莫因和达拉迪亚超群的同位素年龄和构造复杂性一直被解释为反映了早古生代变形和变质作用叠加的新元古代造山(诺伊达特)幕。然而，新元古代造山作用在东北劳伦西亚的其他地方是未知的，相反，这是冈瓦瓦兰的一个特征。这一新元古代异常事件的存在主要基于两种独立的年代学方法，但两种方法似乎都不严密。此外，我们认识到一套区域镁铁质岩脉可能是一个可靠的时间标志，要求变质作用发生得比目前推测的晚。如果正确，苏格兰的新元古代造山作用似乎值得怀疑，这些岩石中的变形更有可能与早古生代劳伦亚和欧洲西北部的集合有关(与劳伦斯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气压学框架，将限制该地区岩浆作用、变形和变质的时间。事件的绝对年龄将被用来检验沿劳伦斯边缘的构造事件的相关性。不考虑苏格兰的新元古代造山作用，将简化苏格兰劳伦蒂海角的前加里东历史，其中新元古代岩浆与里斐时期(~900-800 Ma)和文迪亚时期(~600 Ma)两个不同的拉张裂谷阶段有关。反过来，这将允许沿着整个边缘的共同事件的相关性，从而支持劳伦西亚在新元古代作为一个离散大陆的两阶段‘突破’，假设分别来自罗迪尼亚和潘诺提亚超级大陆。或者，证实苏格兰新元古代造山作用的存在，可能使苏格兰与东北劳伦斯边缘的其余部分分离，从而推断冈德瓦尼德大陆与劳伦蒂亚大陆在最新的前寒武纪之间的地壳交换。这两种结果都将决定东北劳伦西亚边缘的构造事件的时间和性质，从而有助于我们理解地球历史的重要部分，包括显著的生物多样性和异常气候事件的全球古地理。