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: 2008
• 资助国家: 加拿大
• 起止时间: 2008-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=389511
• 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.

加拿大大西洋地区（和其他地方）的许多岩石单位现在并列在一起，它们的生命始于世界上遥远的地方，通常是跨越遥远的海洋。重要的是要确定这些岩石形成的环境，并确定它们是何时被吸积到北美其他地区的。当岩石被侵蚀时，“碎屑”矿物沉积在沉积物中，这些沉积物可能在烃源岩消失后很长时间内保存下来，或者在烃源岩和沉积单元或“地体”分离时留下。锆石和白云母是两种重要的碎屑矿物，因为它们各自的放射性年龄是烃源岩相当确定的“指纹”。当前应用程序中描述的项目涉及测量从选定的沉积单元中分离出来的白云母的40-Ar/39-Ar（基本上是K-Ar）年龄。因为沉积岩中的各种白云母颗粒可能来自不同的来源，所以有必要分别测定每种颗粒的年代。事实上，最近的数据表明，我们必须获得粒内年龄；也就是说，观察每个莫斯科石颗粒中存在的年龄变化。其原因是，颗粒的某些部分可能受到后来的热事件的干扰（例如，辐射“时钟”重置为零）。通过使用紫外激光烧蚀技术测量大量的“光斑”年龄，可以获得最可靠的晶粒内年龄分布模式，这是本研究选择的方法。当板块构造作用使两个地体碰撞并合二为一时，在一定的地质环境中会形成新的白云母，原有白云母的年龄可能会被由此产生的热事件完全或部分重置。在这里，紫外激光烧蚀方法也可以用来揭示晶内年龄模式，这将揭示岩层合并的时间。在目前的建议中，我们的目标是新斯科舍省西南部的Meguma地体和Avalonia地体中的晚前寒武纪至寒武纪（约6.5 -5亿年）岩石，特别是后者暴露在新不伦瑞克省南部和布雷顿角岛东部。我们寻找这些地形起源的线索，以及它们相互形成的时间，以及它们在北美其他地区形成的时间。