The Timing and Geometry of Lithosphere Removal Beneath the North China Craton

华北克拉通岩石圈去除的时间和几何形状

• 批准号: 0635671
• 负责人: Roberta Rudnick
• 金额: $ 20.1万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-06-01 至 2010-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635671&HistoricalAwards=false
• 关键词: Timing; Geometry; Lithosphere; Removal; Beneath

Intellectual Merit. Archean cratons are underlain by lithospheric mantle that is distinct from off-craton regions: it is generally colder and significantly more refractory. The latter characteristic imparts an intrinsically lower density, and may be a contributing factor to the long-term convective stability of these mantle roots (as evidenced by their ancient isotopic characteristics). A fundamental question in understanding Archean lithospheric evolution and crust-mantle recycling is whether these roots, once formed, are preserved indefinitely, or whether they can be removed. If the latter can be demonstrated, then, depending on the mechanism, their removal may also coincide with loss of the lowermost crust or complete crustal destabilization and thereby be a significant means of crustal recycling. The North China Craton is perhaps the best example of an Archean craton that was stable for over a billion years, but underwent a significant transformation that resulted in its present state of high heat flow, seismic activity and extensive continental magmatism (which started in the Mesozoic). In the Paleozoic, diamondiferous kimberlites erupted through the craton carrying minerals and xenoliths indicative of the presence of a 200 km thick refractory lithosphere, equilibrated to a cool geotherm. In the Late Cretaceous, refractory peridotitic xenoliths occur side-by-side with less refractory peridotites in high Mg diorites. These peridotites may indicate the presence of the cratonic root at this time. In contrast, Tertiary intraplate basalts carry fertile mantle xenoliths that record high equilibration temperatures and sample to depths of only 80 km. These observations have been interpreted to reflect the loss of the cratonic root beneath the North China Craton, sometime after the Ordovician. Outstanding questions include: 1) whether all of the cratonic mantle was removed, 2) when it was removed and 3) the mechanism by which it was removed.The project outlined herein will address these questions through a detailed petrographic, geochemical and Re-Os isotopic study of mantle xenoliths carried to the surface in the Mesozoic and Cenozoic igneous rocks. The Re-Os technique is the most reliable way to date melt extraction from peridotites, hence lithosphere formation. The mantle xenoliths will be systematically characterized by electron microprobe, XRF and ICP-MS in order to target samples for Re-Os work. The degree of overprinting via sulfide metasomatism will be quantified by detailed petrography and analyses of sulfides phases, coupled with whole rock PGE analyses. The Os dates for samples from the Late Cretaceous through late Tertiary lithosphere will allow us to determine the extent of Archean lithosphere currently preserved beneath the North China Craton, to define the changes that occurred during the Mesozoic (from xenoliths entrained in Mesozoic magmatic rocks) and to test the diverse hypotheses that are currently invoked to explain the loss of the deep lithosphere.Broader Impacts. This work will form the PhD thesis for a graduate student and will foster continued, highly productive collaborations with our Chinese colleagues.

智力优势。太古宙地幔的下面是岩石圈地幔，不同于克拉通以外的地区：它通常更冷，更难熔。后者的特点赋予一个内在的低密度，并可能是一个促成因素，这些地幔根的长期对流稳定性（证明了他们的古老的同位素特征）。理解太古代岩石圈演化和壳幔循环的一个基本问题是，这些根一旦形成，是否会无限期地保存下来，或者它们是否可以被移除。如果后者能够得到证实，那么，根据其机制，它们的移动也可能与最低地壳的丧失或地壳的完全不稳定同时发生，从而成为地壳再循环的一个重要手段。华北克拉通也许是太古代克拉通的最好例子，它稳定了10亿多年，但经历了一次重大的转变，导致其目前的高热流、地震活动和广泛的大陆岩浆活动（始于中生代）。在古生代，含金刚石金伯利岩爆发通过克拉通携带矿物和捕虏体的存在下，200公里厚的耐火岩石圈，平衡到一个凉爽的地热。晚白垩世，高镁闪长岩中难熔橄榄岩捕虏体与不太难熔的橄榄岩并存。这些橄榄岩可能表明在这个时候存在的岩石根。相比之下，第三纪板内玄武岩携带肥沃的地幔捕虏体，记录高平衡温度和样品的深度只有80公里。这些观测结果被解释为反映了奥陶纪之后某个时候华北盆地下的古根的丧失。悬而未决的问题包括：本文通过对中、新生代火成岩中携带到地表的地幔捕虏体进行详细的岩石学、地球化学和Re-Os同位素研究，探讨了这些问题。Re-Os技术是确定橄榄岩熔体提取和岩石圈形成最可靠的方法。本文将利用电子探针、X射线荧光光谱和电感耦合等离子体质谱对地幔包体进行系统的表征，为铼-锇同位素研究提供样品。通过硫化物交代作用的叠加程度将通过详细的岩相学和硫化物相分析以及全岩铂族元素分析来量化。从晚白垩世到晚第三纪岩石圈样品的Os年龄将使我们能够确定目前保存在华北盆地之下的太古代岩石圈的范围，确定中生代期间发生的变化（来自中生代岩浆岩中夹带的捕虏体），并测试目前被援引来解释深部岩石圈损失的各种假设。这项工作将形成一个研究生的博士论文，并将促进与我们的中国同事持续，高效的合作。