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

大陆形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，以及是什么使它们保持稳定，对人类来说非常重要。大陆最古老的部分，即那些在25亿年前形成的区域，被称为“克拉通”，通常是地球上最稳定的表面特征。它们没有经历过活火山活动或地震，大陆地壳下厚而坚硬的岩层一直延伸到200-250公里深处，这被普遍认为是这些古老地区稳定的关键。38亿年前的华北克拉通是个例外。人类历史上一些最具破坏性的地震，以及相对较新的火山活动和矿床形成都发生在这个克拉通。有强有力的证据表明，几亿年前华北克拉通地壳下原本存在的厚岩层被剥离，代之以更薄、更弱的岩层，但这种转变的时间和过程尚不确定。该提案概述的项目旨在通过研究过去4亿年间爆发的火山所采集的深层岩石样本，确定华北克拉通下厚岩层何时以及如何被替换，这段时间跨越了替换事件。本文拟利用岩石学、Re-Os和Lu-Hf同位素地球化学以及地幔捕虏体中铂族元素（PGE）丰度的测定，揭示华北克拉通下中生代和新生代地幔岩石圈的历史。为了从对流上地幔观测到的Re-Os光谱中区分太古宙后岩石圈年龄，将在目标位置收集具有统计意义的大型Re-Os数据集。将对选定捕虏体中的斜辉石进行Lu和Hf同位素测量，以比较两个系统的年龄系统，并评估Lu-Hf系统在确定岩石圈形成年龄方面的效用。PGE数据将用于评估单个样品的Os模型年龄的准确性，并且还将提供可能对地幔捕虏体起作用的过程的信息，例如再作用和熔融岩石相互作用。研究结果有望阐明华北克拉通不同部位岩石圈地幔的现今年龄分布，将中生代岩石圈与更早和现在的岩石圈地幔进行比较，并将这些信息与现有的地震图像联系起来，以推断克拉通的深部结构，确定岩石圈地幔的横向和纵向替换程度。反过来，这将为一个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.