The origin and evolution of cratons: Age and chemical constraints from the lithospheric root beneath West Greenland.

克拉通的起源和演化：西格陵兰下方岩石圈根部的年龄和化学限制。

• 批准号: NE/C519054/1
• 负责人: David Pearson
• 金额: $ 27.14万
• 依托单位: Durham University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FC519054%2F1
• 关键词: origin; evolution; cratons; Age; chemical

Like an iceberg, the main volume of a continent is hidden at depth in 'mantle roots'. Beneath ancient crustal rocks (>2.5 billion years old), known as cratons, many of these roots extend to great depth (at least 250 km and perhaps 400 km), are very ancient and are host to the worlds supply of gem diamonds. Arguably the most interesting aspect of these cratonic mantle roots is that they seem to assist in the long-term survival of continental crust. To understand the mechanisms by which cratons and their roots are formed and stabilised requires information on the bulk composition of various cratonic roots and their age of formation. This is difficult to achieve because samples of the roots are only erupted by deeply derived volcanic rocks in certain areas and many samples are too small to obtain representative estimates of their bulk compositions or obtain age information. The one craton where addressing these issues has been possible in a detailed way, the Kaapvaal craton, S. Africa, appears to have an unusual composition and hence we are in need of an alternative benchmark dataset for cratonic lithosphere, with which to improve our knowledge of how typical cratons form and evolve. The North Atlantic Craton, exposed in S.W. Greenland, together with its surrounding areas of deformed cratonic crust, is intruded by deeply derived volcanic rocks carrying an exceptional inventory of large fragments of the mantle root beneath. Using published data from the experimental melting of mantle at different pressures, together with the analysis of elements that have differing affinities for the melt compared to the solid residue during melting at different pressures, we plan to constrain the depth of melting that formed the mantle root beneath the differing parts of this region. This will enable us to test a model that suggests that cratonic roots are formed by the subduction stacking of older oceanic plates. Mineral analysis of the mantle fragments will allow us to evaluate the depths from which they were derived and hence look at the thickness of the root beneath different areas. We will also use the radiometric decay systems 187Re-1870s and 176Lu-176Hf, on both minerals and the bulk rock, to determine the age of the melt extraction event and to try to determine the age of any subsequent disturbance of the root associated with tectonic episodes responsible for the fragmentation of a much larger craton. In this way we can obtain an understanding of the age and depth of the deep mantle root beneath crustal regions of different ages. This will provide us with a picture of not only how cratons are formed, but how they are affected by major tectonic events and why they sometimes breakup. Lastly, diamonds have been found in the volcanic rocks intruding both the craton and the region surrounding the craton, that has been heavily influenced by tectonic events. Understanding how the age, depth and composition of the cratonic root beneath region changes laterally will also improve our understanding of how diamonds are distributed in the roots in and around cratons and ultimately what processes by cause their formation or destruction.

就像冰山一样，大陆的主体体积隐藏在“地幔根部”深处。在被称为克拉通的古老地壳岩石(&gt；25亿年前)之下，许多根部延伸到很深的地方(至少250公里，也许400公里)，非常古老，是世界宝石钻石供应的所在地。可以说，这些克拉通地幔根最有趣的方面是，它们似乎有助于大陆地壳的长期生存。要了解克拉通及其根部形成和稳定的机制，需要了解各种克拉通根部的整体成分及其形成年龄的信息。这很难实现，因为根部的样本只在某些地区由深得来的火山岩喷发，而且许多样本太小，无法获得对其整体成分的代表性估计或获得年龄信息。有可能以详细的方式解决这些问题的唯一克拉通是南非的卡帕瓦尔克拉通，它似乎具有不同寻常的成分，因此我们需要一个克拉通岩石圈的替代基准数据集，用它来改进我们对典型克拉通如何形成和演化的了解。暴露在格陵兰岛西南部的北大西洋克拉通，以及它周围变形的克拉通地壳，被深层次的火山岩侵入，这些火山岩携带着一大批地幔根部的特殊碎片。利用公布的不同压力下地幔熔化实验的数据，以及对不同压力下熔化过程中与固体残留物具有不同亲和力的元素的分析，我们计划限制在该区域不同部分之下形成地幔根部的熔融深度。这将使我们能够测试一个模型，该模型表明克拉通根是由较老的海洋板块俯冲堆积而成的。对地幔碎片的矿物分析将使我们能够评估它们产生的深度，从而观察不同区域下根部的厚度。我们还将使用矿物和块状岩石上的187Re-1870s和176Lu-176Hf辐射衰变系统来确定熔融提取事件的年龄，并试图确定与导致更大的克拉通碎裂的构造事件有关的任何后续根部扰动的年龄。这样，我们就可以对不同时代地壳区域下的深部地幔根的年龄和深度有一个了解。这不仅将为我们提供克拉通是如何形成的，而且还将为我们提供一幅克拉通如何受到重大构造事件的影响以及为什么它们有时会破裂的图景。最后，在侵入克拉通和克拉通周围地区的火山岩中发现了钻石，这些火山岩受到构造事件的严重影响。了解区域下克拉通根部的年龄、深度和成分如何横向变化，也将有助于我们理解钻石如何分布在克拉通内部和周围的根部，以及最终是什么过程导致它们的形成或破坏。