Argon/Argon, micropaleontological and Uranium/Lead geochronological data for igneous and sedimentary rocks dredged during the VESPA scientific cruise

Abstract: The 2015 VESPA voyage (Volcanic Evolution of South Pacific Arcs) was a seismic and rock dredging expedition to the Loyalty and Three Kings Ridges and South Fiji Basin. In our paper we present 33 40Ar/39Ar, 22 micropaleontological, and two U/Pb ages for igneous and sedimentary rocks from 33 dredge sites in this little-studied part of the southwest Pacific Ocean. Igneous rocks include basalts, dolerites, basaltic andesites, trachyandesites, and a granite. Successful Ar/Ar dating of altered and/or low-K basalts was achieved through careful sample selection and processing, detailed petrographic and element mapping of groundmass, and incremental heating experiments on both phenocryst and groundmass separates to interpret the complex spectra produced by samples having multiple K reservoirs. The 40Ar/39Ar ages of most of the sampled lavas, irrespective of composition, are latest Oligocene to earliest Miocene (25-22 Ma); two are Eocene (39-36 Ma). The granite has a U/Pb zircon age of 23.6±0.3 Ma. 40Ar/39Ar lava ages are corroborated by microfossil ages from associated sedimentary rocks. The VESPA lavas are part of a >3000 km long disrupted belt of Eocene to Miocene subduction-related volcanic rocks. The belt includes arc rocks in Northland New Zealand, Northland Plateau, Three Kings Ridge, and Loyalty Ridge and, speculatively, D’Entrecasteaux Ridge. This belt is the product of superimposed Eocene and Oligocene-Miocene remnant volcanic arcs that were stranded along and near the edge of Zealandia while still-active arc belts migrated east with the Pacific trench. Plain word summary: Samples of lava from the seabed between New Zealand and New Caledonia have been dated using atomic clocks and fossils. Most lavas erupted in a big pulse of volcanic activity between 25 and 22 million years ago. They are part of a belt of now-extinct undersea volcanoes that stretches for more than 3000 km between New Zealand and the Solomon Islands. These volcanoes were formed by subduction of the Pacific Plate under the Australian Plate.

摘要：2015年VESPA航次（Volcanic Evolution of South Pacific Arcs，南太平洋弧火山演化）是针对洛亚蒂海岭、三王海岭与南斐济盆地开展的地震勘探与岩石拖网采样科考航次。本研究论文报道了西南太平洋这片研究程度极低海域33个拖网站位的火成岩与沉积岩样品的33组氩-氩（40Ar/39Ar）年龄、22组微古生物年龄以及2组铀-铅（U/Pb）年龄。所涉火成岩包括玄武岩、粒玄岩、玄武安山岩、粗安岩与花岗岩。针对蚀变及（或）低钾玄武岩的氩-氩定年难题，研究团队通过严谨的样品筛选与前处理、对基质开展详细岩相学与元素制图分析，同时对斑晶及基质分离组分进行阶段加热实验，以解析含多钾储库样品产生的复杂年龄谱。多数采样熔岩的氩-氩年龄无论成分差异如何，均为渐新世晚期至中新世早期（25~22 Ma），另有两例为始新世（39~36 Ma）。该花岗岩的铀-铅锆石年龄为23.6±0.3 Ma。熔岩的氩-氩年龄与伴生沉积岩中的微古生物年龄相互印证。VESPA航次所获熔岩属于一条长逾3000 km、形成于始新世至中新世的俯冲相关火山岩扰动带。该带包含新西兰北部地区、北部高原、三王海岭与洛亚蒂海岭的弧岩，推测还包括当特尔卡斯托海岭。该带是叠加的始新世与渐新世-中新世残余火山弧的产物，这些残余弧残留在西兰大陆（Zealandia）边缘及附近区域，而活动弧带则随太平洋海沟向东迁移。通俗说明：采自新西兰与新喀里多尼亚之间海底的熔岩样品，通过原子定年技术与化石定年手段获得了年龄数据。多数熔岩形成于2500万至2200万年前的一次大规模火山活动脉冲。它们属于一条现已灭绝的海底火山带的一部分，该带在新西兰与所罗门群岛之间绵延逾3000 km。这些火山由太平洋板块向澳大利亚板块俯冲作用形成。