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.

智力上的功绩。太古宙克拉通是由岩石圈地幔覆盖的，与克拉通外地区不同：通常更冷，更难熔。后一种特征提供了本质上较低的密度，并可能是这些地幔根长期对流稳定的一个贡献因素(从它们的古老同位素特征中得到了证明)。理解太古宙岩石圈演化和壳幔循环的一个基本问题是，这些树根一旦形成，是否会被无限期地保存下来，或者它们是否可以被移除。如果可以证明后一种情况，那么根据机制，它们的清除也可能与最下部地壳的丧失或地壳的完全不稳定同时发生，因此是地壳再循环的重要手段。北中国克拉通可能是太古代克拉通最好的例子，它稳定了10亿多年，但经历了重大转变，导致了其今天的高热流、地震活动和广泛的大陆岩浆活动(始于中生代)。在古生代，含钻石的金伯利岩通过克拉通喷发，携带矿物和捕虏体，表明存在200公里厚的耐火岩石圈，与凉爽的地热相平衡。晚白垩世，在高镁闪长岩中，难熔橄榄岩包体与少量难熔橄榄岩共生。这些橄榄岩可能表明此时克拉通根的存在。相比之下，第三系板内玄武岩携带有肥沃的地幔包体，记录了高的平衡温度，样品深度仅为80公里。这些观察被解释为反映了北中国克拉通之下的克拉通根的丧失，在奥陶纪之后的某个时候。悬而未决的问题包括：1)克拉通地幔是否全部被移除，2)它何时被移除，3)它被移除的机制。本文概述的项目将通过对中、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同位素研究来解决这些问题。Re-Os技术是从橄榄岩中提取熔体，从而形成岩石圈的最可靠的方法。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了系统的表征，以便为Re-Os工作提供靶标。硫化物交代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PGE分析来量化。晚白垩世至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北中国克拉通之下的太古代岩石圈的范围，确定中生代发生的变化(来自中生代岩浆岩中的包体)，并检验目前用于解释深部岩石圈丧失的各种假说。这项工作将形成研究生的博士论文，并将促进与我们的中国同事持续的、富有成效的合作。