The Cook-Austral volcanic lineament is thought to be the product of three overlapping South Pacific hotspots tracks: Macdonald, Arago, and Rarotonga. However, the entire Rarotonga hotspot track consists of volcanism on just two islands, Rarotonga and Aitutaki, making it difficult to evaluate a hotspot origin for the young volcanism on these islands. We present new Sr-Nd-Hf-Pb-He isotopic and major and trace element data on Aitutaki—which has two periods of volcanism, older (9.39 Ma) and younger (1.382 to 1.941 Ma)—and Atiu Islands in the Cook Islands to evaluate a hotspot origin. Like Atiu, the “older volcanic series” at Aitutaki was tentatively linked with the Arago hotspot based on its age, but this link has never been tested with geochemistry. New geochemical data on the older Aitutaki volcanic series show clear affinities to the Arago hotspot volcano Atiu. In contrast, the younger Aitutaki volcanic series, erupted on a near atoll, exhibits evidence for a rejuvenated, non-plume volcanic origin. If Rarotonga Island (1.157 to 1.697 Ma) and the young series at Aitutaki share a common heritage and mechanism for formation as previously proposed—they erupted only 260 km apart over an overlapping time period and exhibit overlapping radiogenic isotopic compositions—then Rarotonga Island lavas may also represent a subaerial rejuvenated volcanic stage capping an older volcano related to a different hotspot, consistent with no direct involvement from a mantle plume in the origin for subaerial lavas at Rarotonga.Rarotonga, Aitutaki, and Atiu islands exhibit some of the most extreme enriched mantle (EM) signatures in the Cook-Austral volcanic lineament. There are three geographically separated geochemical domains along the Cook-Austral volcanic lineament—a northern geochemical domain with EM geochemical signatures is separated from a southern EM domain by a ∼1000-km-long central HIMU (“high μ”, or high ²³⁸U/²⁰⁴Pb) domain—and Rarotonga, Aitutaki, and Atiu islands are part of the northernmost geochemical domain. There is a possible role for lithospheric thickness in controlling geochemical compositions of melt extracted from the mantle along Cook-Austral volcanic lineament, particularly across the Austral Fracture Zone. However, this is complicated by the lineament’s juxtaposition with the South Pacific Superswell, which may modify lithospheric behavior and complicate a simple lithospheric thickness model describing variable hotspot volcano compositions in the region.

中文翻译：

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库克群岛艾图塔基的古老和年轻的火山形成对比：对库克-澳大利亚火山链起源的启示

库克-澳大利亚火山岩系被认为是南太平洋三个重叠的热点轨道的产物：麦克唐纳，阿拉戈和拉罗汤加。但是，整个拉罗汤加岛热点地区仅由两个岛（拉罗汤加岛和艾图塔基）上的火山活动组成，因此很难评估这些岛上年轻火山岩的热点成因。我们提供了新的Sr-Nd-Hf-Pb-He同位素以及主要分布在艾图塔基（Aitutaki）的数据，该图有两个时期的火山活动，年龄较大（9.39 Ma），年龄较小（1.382至1.941 Ma），并且位于库克群岛的阿蒂乌群岛评估热点起源。像阿蒂乌（Atiu）一样，艾图塔基（Aitutaki）的“较古老的火山序列”也根据其年龄暂时与Arago热点联系在一起，但是这种联系从未经过地球化学的检验。关于较旧的艾图塔基火山系列的新地球化学数据表明，它与阿拉戈热点火山阿蒂乌（Atiu）有明显的相似性。相反，年轻的艾图塔基火山系列在环礁附近爆发，显示出复兴的非蓬松火山起源的证据。如果拉罗汤加岛（1.157至1.697 Ma）和艾图塔基的年轻系列如前所述具有共同的传承和形成机制-它们在重叠的时间段内仅相隔260公里爆发，并显示出重叠的放射性同位素组成，那么拉罗汤加岛的熔岩可能还代表了一个空中复苏的火山阶段，该阶段覆盖了与另一个热点相关的一个较老的火山，这与拉罗汤加岛地下熔岩的起源中没有地幔柱直接相关。拉罗通加，艾图塔基，阿蒂乌岛（Atiu）和库克群岛（Cook-Austral）火山岩系中表现出一些最极端的地幔特征（EM）。库克-澳大利亚火山沿线沿线有三个地理上分离的地球化学域，一个具有EM地球化学特征的北部地球化学域与一个南部EM域之间相距约1000公里长的中央HIMU（“高μ”或高²³⁸ U / ²⁰⁴ Pb）域-拉罗汤加岛，艾图塔基岛和阿蒂乌岛是最北端地球化学域的一部分。岩石圈厚度可能在控制沿库克-澳大利亚火山岩系（特别是整个澳大利亚断裂带）从地幔中提取的熔体的地球化学成分方面发挥着作用。但是，由于线状体与南太平洋超高涌浪并置而使情况复杂化，这可能会改变岩石圈的行为，并使描述该地区可变热点火山成分的简单岩石圈厚度模型复杂化。