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开始在世界上遥远的地方，通常是在遥远的海洋中生活。重要的是要确定这些岩石形成的环境，并确定它们何时被增生到北美其他地区。当岩石被侵蚀时，“碎屑”矿物沉积在沉积物中，这些沉积物可能在源岩消失后很久才被保存下来，或者在源岩和沉积单元或“碎屑岩”漂离时被留下。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”。本申请中所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。由于沉积岩中的各种白云母颗粒可能来自不同的来源，因此有必要分别测定每种颗粒的年代。事实上，最近的数据表明，人们必须获得颗粒内的年龄;也就是说，观察每个白云母颗粒内存在的年龄变化。其原因是，谷物的某些部分可能已经被后来的热事件干扰（例如，辐射“时钟”重置为零）。最可靠的晶粒内的年龄分布模式是通过测量大量的“点”的年龄使用紫外激光烧蚀技术，本研究的方法的选择。当板块构造过程导致两个白云母碰撞在一起，成为一个，新的白云母形成在某些地质环境，和年龄的预先存在的白云母可能会完全或部分重置所产生的热事件。在这里，紫外线激光烧蚀方法也可以用来揭示晶粒内的年龄模式，将揭示的时间的银汞齐化。在目前的建议中，我们的目标是晚前寒武纪至寒武纪（约。6.5 - 5亿年）岩石，位于西南部新斯科舍省的Meguma岩带和Avalonia岩带，特别是后者在新玩法南部和东开普省布雷东岛出露的地方。我们寻求线索的起源，这些化石和他们的时代，彼此和北美的其他地方。