Ar-Ar geochronology: provenance and thermal history from laser ablation analysis of muscovite

Ar-Ar 地质年代学：白云母激光烧蚀分析的来源和热历史

• 批准号: 7375-2007
• 负责人: Reynolds, Peter
• 金额: $ 1.06万
• 依托单位: Dalhousie University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2007
• 资助国家: 加拿大
• 起止时间: 2007-01-01 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=363132
• 关键词: Ar; geochronology; provenance; thermal; history

Many of the rock units in Atlantic Canada (and elsewhere) now juxtaposed began their lives in distant parts of the world typically across distant seas. It is important to identify the environment in which these rocks formed, and also to establish when they were accreted to the rest of North America. When rocks erode, 'detrital' minerals are deposited in sediments that may be preserved long after the source rocks have disappeared, or otherwise have been left behind as the source and sedimentary units or 'terranes' drifted apart. Zircon and muscovite are two important detrital minerals because their respective radiometric ages are together fairly definitive 'fingerprints' of the source rocks. Projects described in the current application involve the measurement of 40-Ar/39-Ar (essentially K-Ar) ages of muscovites separated from selected sedimentary units. Because the various muscovite grains in a sedimentary rock may have come from different sources, it is necessary to date each grain separately. In fact, recent data suggest that one must obtain intra-grain ages; that is, observe what age variation exists within each muscovite grain. The reason for this is that some parts of the grain may have been disturbed (e.g. radiometric 'clock' reset to zero) by later thermal events. Intra-grain age distribution patterns are most reliably made by measuring a large number of 'spot' ages using the UV laser ablation technique, the method of choice for the present research. When plate tectonic processes cause two terranes to collide together and become one, new muscovites are formed in certain geologic environments, and the ages of pre-existing muscovites may be completely or partially reset by the resulting thermal event. Here too, the UV laser ablation method can be used to reveal intra-grain age patterns that will reveal the timing of terrane amalgamation. In the current proposal, we target Late Precambrian to Cambrian (ca. 650-500 million year old) rocks in the Meguma terrane in SW Nova Scotia and in the Avalonia terrane, specifically where the latter is exposed in southern New Brunswick and in eastern Cape Breton Island. We seek clues to the origin of these terranes and to the times of their accretion to each other and to the rest of North America.

现在，加拿大大西洋（和其他地方）的许多岩石单元现在并列在世界遥远的地区，通常在遥远的海洋中。重要的是要确定这些岩石形成的环境，也必须确定何时被吸收到北美其他地区。当岩石侵蚀时，“碎屑”矿物会沉积在源岩石消失后很长时间后可能保存的沉积物中，或者以其他方式被抛在后面，因为源头和沉积单元或“地形”散落。锆石和白云母是两个重要的碎屑矿物质，因为它们各自的辐射年龄是源岩的相当确定的“指纹”。当前应用中描述的项目涉及与所选沉积单元分离的麝香木的40-AR/39-AR（基本上是K-AR）年龄的测量。由于沉积岩石中的各种白云母颗粒可能来自不同的来源，因此有必要分别与每种谷物分开日期。实际上，最近的数据表明必须获得粒内年龄。也就是说，观察每种白云母谷物中存在什么年龄变化。这样做的原因是，谷物的某些部分可能会因以后的热事件而受到干扰（例如辐射“时钟”重置为零）。晶粒内年龄分布模式是通过使用紫外线激光消融技术测量大量“斑点”年龄的最可靠的，这是本研究的首选方法。当板块构造过程导致两个地层碰撞并成为一个地形时，在某些地质环境中形成了新的棉花箱，并且预先存在的墨藤的年龄可能会被结果的热事件完全或部分重置。在这里，紫外线激光消融方法可用于揭示晶粒内年龄模式，以揭示地层融合的时机。在目前的提案中，我们将晚期前寒武纪瞄准了新星新斯科舍省Meguma Terrane和Avalonia Terrane的Cambrian（大约650-5亿年的岩石），尤其是在新布鲁伦斯特南部和东部披风布雷顿岛。我们寻求有关这些地形的起源以及它们彼此和北美其他地区积聚时期的线索。