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.

知识价值。太古宙克拉通位于岩石圈地幔之下，与克拉通外区不同：岩石圈地幔通常更冷，而且明显更难熔。后一种特征赋予了本质上较低的密度，并且可能是这些地幔根长期对流稳定性的一个促成因素（如它们的古同位素特征所证明的）。了解太古代岩石圈演化和壳幔再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。如果后者可以证明，那么，根据机制，它们的移除也可能与最下层地壳的丧失或地壳完全不稳定同时发生，从而成为地壳再循环的重要手段。华北克拉通可能是太古宙克拉通中最好的例子，它稳定了超过10亿年，但经历了一次重大转变，导致了它现在的高热流，地震活动和广泛的大陆岩浆活动（始于中生代）。古生代，含金刚石的金伯利岩通过克拉通喷发，携带矿物和包体，表明存在200公里厚的难熔岩石圈，与凉爽的地热平衡。晚白垩世高镁闪长岩中，难熔橄榄岩包体与少熔橄榄岩并存。这些橄榄岩可能表明此时存在克拉通根。相比之下，第三纪板内玄武岩携带着肥沃的地幔捕虏体，记录了较高的平衡温度，样本深度仅为80公里。这些观测结果被解释为反映了奥陶纪之后华北克拉通地下克拉通根的消失。悬而未决的问题包括：1)是否所有的克拉通地幔都被移走了；2)何时被移走了；3)被移走的机制。本文概述的项目将通过对中生代和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技术是确定橄榄岩熔体萃取时间的最可靠方法，因此也就是岩石圈形成的时间。利用电子探针、XRF和ICP-MS对地幔捕虏体进行系统表征，为Re-Os工作提供目标样品。通过详细的岩石学和硫化物相分析，以及整个岩石的PGE分析，将量化硫化物交代作用的套印程度。从晚白垩世到晚第三纪岩石圈样品的o年代将使我们能够确定目前在华北克拉通下保存的太古代岩石圈的范围，定义中生代（从中生代岩浆岩中夹带的捕虏体）发生的变化，并测试目前用来解释深层岩石圈损失的各种假设。更广泛的影响。这项工作将成为一名研究生的博士论文，并将促进与中国同事的持续、高效的合作。