PGE, Re-Os and Lu-Hf Isotopic Signature of Lithosphere Removal Beneath the North China Craton

大陆构成了我们物种进化的平台，我们几乎所有的自然资源都来自大陆。因此，了解大陆是如何形成和演化的，以及是什么让它们保持稳定，对人类来说非常重要。大陆最古老的部分，也就是那些形成于25亿年前、被称为克拉通的区域，通常是地球上最稳定的地表特征。它们没有经历活火山活动或地震，大陆地壳下厚而硬的岩层的存在，延伸到200-250公里深处，通常被认为是这些古老地区稳定的关键。距今约38亿年的北中国克拉通是这一总体情况的一个例外。一些人类历史上最具破坏性的地震，以及相对较新的火山作用和矿床形成都发生在这个克拉通。有强有力的证据表明，最初存在于北中国克拉通地壳下的厚层岩石在数亿年前被移除，取而代之的是更薄、更弱的岩层，但这种转变的时间和过程尚不确定。这项提案中概述的项目试图通过研究过去4亿年火山喷发的深部岩层的碎片，来确定北中国克拉通之下厚厚的岩层何时以及如何被置换。过去4亿年里，火山喷发的时间跨越了置换事件。利用岩相学、Re-Os和Lu-Hf同位素地球化学以及测定地幔包体中铂族元素的丰度来揭示北中国克拉通之下中、新生代地幔岩石圈的历史。将在目标位置收集大量具有统计意义的Re-Os数据集，以区分太古代以后的岩石圈年龄与在对流上地幔中观察到的Re-Os谱。将对部分包体中的单斜辉石进行Lu和Hf同位素测量，以比较两个系统之间的年龄系统学，并评估Lu-Hf系统在确定岩石圈形成年龄方面的实用性。PGE数据将用于评估个别样品Os模型年龄的准确性，还将提供有关可能作用于地幔包体的过程的信息，如参考和熔岩相互作用。这些结果有望阐明北中国克拉通不同部分之下的岩石圈地幔的现今年龄分布，中生代年龄的岩石圈与更古老的和现代的岩石圈地幔的对比，以及这些信息如何与现有的地震学图像联系起来，以推断克拉通的深部结构，并确定岩石圈地幔交代的横向和垂直程度。反过来，这将为稳定的克拉通转变为当今世界上构造最活跃的地区之一所涉及的过程提供模型。

Continents form the platform on which our species has evolved, and nearly all of our natural resources derive from the continents. Understanding how continents form and evolve and what makes them stable is, therefore, of importance for humans. The oldest portions of continents, those regions that formed before 2.5 billion years ago and termed 'cratons', are typically the most stable surface features on Earth. They experience no active volcanism or earthquakes, and the presence of a thick and stiff rock layer beneath the continental crust, extending down to depths of 200-250 km, is generally considered key for the stability of these ancient regions. The ~3.8 billion year old North China Craton is an exception to this general picture. Some of the most devastating earthquakes in human history, as well as relatively recent volcanism and ore deposit formation have occurred in this craton. There is strong evidence that the thick rock layer, which was originally present beneath the crust of the North China Craton, was removed and replaced with a thinner and weaker rock layer several hundred million years ago, but the timing and process of this transformation is uncertain. The project outlined in this proposal seeks to determine when and how the thick rock layer was replaced beneath the North China Craton by studying pieces of this deep layer sampled by volcanoes that erupted over the last 400 million years, a time period that spans the replacement event. The research proposed involves the use of petrography, Re-Os and Lu-Hf isotope geochemistry, as well as measuring the abundances of the platinum group elements (PGE) in mantle xenoliths to unravel the history of the mantle lithosphere beneath the North China Craton during the Mesozoic and Cenozoic. Large, statistically significant data sets for Re-Os will be collected at targeted locations in order to distinguish post-Archean lithospheric ages from the Re-Os spectrum observed in the convecting upper mantle. Clinopyroxene from select xenoliths will be measured for Lu and Hf isotopes in order to compare age systematics between the two systems and evaluate the utility of the Lu-Hf system for determining lithosphere formation ages. PGE data will be used to evaluate the veracity of Os model ages for individual samples, and will also provide information on processes, such as refertilization and melt-rock interaction, that may have acted on the mantle xenoliths. The results are expected to clarify the present-day age distribution of lithospheric mantle beneath different portions of the North China Craton, how the Mesozoic aged lithosphere compares with older and present-day lithospheric mantle, and how this information can be linked with available seismological images in order to deduce the deep architecture of the craton and determine the lateral and vertical extent of lithospheric mantle replacement. This, in turn, will inform models for the processes involved in the transformation of a stable craton into one of the most tectonically active regions of the world today.