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

地幔在不同长度尺度上均表现出地球化学非均质性和成分的不均匀性，这是由于上地幔在熔融萃取作用下的持续衰竭和俯冲、脱层的陆、大洋物质的再富集而形成的。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很少受到限制。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（高时积分μ=238U/204Pb）是提出的四个地幔端元之一，可以描述多同位素空间的整个地球化学谱。关于西兰洲的himu型特征是来自次大陆岩石圈地幔（SCLM）、浅软流圈还是深（羽流）源，存在很大的争论，因此提出了几种模式，包括与俯冲带相关的SCLM交代作用、上层上升流的himu型软流圈减压融化或地幔柱，这些模式可能导致西兰洲从西南极洲分裂。最近对新西兰板内熔岩的研究揭示了更为复杂的地球化学演化。利用多种同位素比值（Sr-Nd-Pb-Hf），我们可以证明从白垩纪圣赫勒拿岛的HIMU末端成员组成到更“枯竭的HIMU样”的新生代火山活动的时间转变，这很可能来自不同的来源。迄今为止，西兰迪亚的时间地球化学演化已经利用许多空间孤立的地点进行了重建，这些地点受到来自高度非均质岩石圈的不同地球化学印记的影响。查塔姆群岛（Chatham Islands）是西兰洲（Zealandia）内唯一已知的火山活动超过80 Ma的地区，此外还记录了从白垩纪圣赫勒拿火山活动到新生代类似火山活动的地球化学转变。因此，查塔姆群岛的火山活动是刻画晚白垩世冈瓦纳大陆分裂后的西兰火山活动以及西兰微大陆向西北移动约4000公里的火山活动的绝佳机会。该项目的目的是生成地球化学（矿物化学，氧同位素和整个岩石主要/微量元素和放射性成因Sr-Nd-Pb-Hf同位素）和地质年代学（Ar-Ar年龄）数据集，以确定以前采样不足的查塔姆群岛火山活动的时间和地球化学演化。在大尺度地球动力学背景下，这些结果将提供关于长期存在的板内火山作用、HIMU地幔端段和类HIMU成分的形成、地幔柱在西兰迪亚从南极洲最终分裂过程中的可能参与，以及整个新生代发生在查塔姆群岛和西兰迪亚的分裂后火山作用的起源等重要信息。

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.