85 Ma volcanism on the Chatham Islands – A window into the geodynamic and geochemical evolution of Zealandia

85 Ma 查塔姆群岛火山活动——了解西兰大陆地球动力学和地球化学演化的窗口

• 批准号: 439265336
• 负责人: Dr. Stephan Homrighausen
• 金额: --
• 依托单位: Forschungseinheit Magmatische und Hydrothermale Systeme
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2020
• 资助国家: 德国
• 起止时间: 2019-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/439265336?language=en
• 关键词: 85; Ma; volcanism; Chatham; Islands

The Earth’s mantle is compositionally heterogeneous and exhibits geochemical heterogeneities at a variety of length scales, which were formed by continuous upper mantle depletion through melt extraction and re-enrichment by subducted and delaminated continental and oceanic materials. These ongoing processes form geochemical domains, but the distribution in the Earth is still poorly constrained. The Zealandian microcontinent, for example, is characterized by short-term intermittent HIMU-type volcanism over the last 100 Ma. HIMU (high time-integrated μ=238U/204Pb) is one of the four proposed mantle end members, which can describe the entire geochemical spectrum in multiple isotope space. There is a great debate as to whether the HIMU-type signature in Zealandia derives from the subcontinental lithospheric mantle (SCLM), from a shallow asthenospheric or deep (plume) source and consequently several models have been proposed, including subduction-zone-related metasomatism of the SCLM, decompression melting of the upper upwelling HIMU-type asthenosphere, or a mantle plume, which could have caused the break-up of Zealandia from West Antarctica. Recent studies of Zealandian intraplate lavas, reveal a more complex geochemical evolution. Using multiple isotope ratios (Sr-Nd-Pb-Hf) we could demonstrate a temporal shift from Cretaceous St. Helena HIMU end member composition to more “depleted HIMU-like” Cenozoic volcanism, which are most likely derived from distinct sources. To date, the temporal geochemical evolution of Zealandia has been reconstructed using numerous spatially-isolated localities, which are affected by distinct geochemical imprints from the highly heterogeneous lithosphere. The poorly sampled Chatham Islands represent the only known locality within Zealandia where volcanic activity has taken place over ≥80 Ma and additionally record the geochemical transition from Cretaceous St. Helena HIMU to Cenozoic HIMU-like volcanism. The Chatham Islands volcanism represents, therefore, an excellent opportunity to characterize the Zealandian volcanism from the Late Cretaceous Gondwana break-up and throughout the ~4000 km northwestward journey of the Zealandian microcontinent.The aim of the propose project is the generation of a geochemical (mineral chemistry, oxygen isotope and whole rock major/trace element and radiogenic Sr-Nd-Pb-Hf isotope) and geochronological (Ar-Ar ages) data set to determine the temporal and geochemical evolution of the previously poorly sampled Chatham Islands volcanism. In a large-scale geodynamic context, these results will provide crucial information about long-lived intraplate volcanism, the formation of both the HIMU mantle end member and HIMU-like compositions, the possible involvement of a mantle plume in the final break-up of Zealandia from Antarctica and the origin of post-breakup volcanism occurring on the Chatham Islands and Zealandia throughout the Cenozoic.

地幔是由上地幔熔体抽采和俯冲、拆离的大陆和大洋物质再富集而形成的，在不同的长度尺度上表现出不同的地球化学特征。这些正在进行的过程形成了地球化学域，但在地球上的分布仍然没有得到很好的限制。例如，在过去的100 Ma中，新西兰微大陆的特征是短期间歇性的HIMU型火山活动。HIMU（高时间积分μ= 238 U/204 Pb）是四个地幔端元之一，可以在多同位素空间描述整个地球化学谱。关于Zealandia的HIMU型特征是来自于陆下岩石圈地幔（SCLM），还是来自于浅源软流圈还是深源（羽流），存在很大的争论，因此提出了几种模型，包括SCLM的俯冲带相关交代作用，上部上涌HIMU型软流圈的减压熔融，或地幔羽流，这可能导致西兰迪亚从南极洲西部分裂。最近对新西兰板内熔岩的研究揭示了一个更为复杂的地球化学演化过程。使用多个同位素比值（Sr-Nd-Pb-Hf），我们可以证明从白垩纪圣赫勒拿HIMU端员组成的时间转移到更多的“耗尽HIMU样”新生代火山作用，这是最有可能来自不同的来源。到目前为止，时间的地球化学演化西兰迪亚已重建使用许多空间上孤立的地方，这是由不同的地球化学印记的高度异质性岩石圈的影响。采样不佳的查塔姆群岛是西兰迪亚境内唯一已知的火山活动超过80 Ma的地方，另外还记录了从白垩纪圣赫勒拿HIMU到新生代HIMU样火山作用的地球化学转变。因此，查塔姆群岛火山活动是一个很好的机会，以表征从晚白垩世冈瓦纳大陆裂解和整个4000公里的新西兰微大陆西北行程的新西兰火山活动。（矿物化学，氧同位素、全岩主/微量元素和放射成因Sr-Nd-Pb-Hf同位素）和年代学（Ar-Ar年龄）数据集，以确定以前采样不足的查塔姆群岛火山作用的时间和地球化学演化。在一个大规模的地球动力学背景下，这些结果将提供重要的信息，长寿板内火山活动，形成的HIMU地幔端成员和HIMU样的组合物，可能参与的地幔柱在最后的分裂从南极洲和起源的西兰迪亚发生在整个新生代的查塔姆群岛和西兰迪亚的解体后的火山活动。