U-Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton
U-Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton
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皮尔巴拉克拉通东部北极穹窿二长岩 U-Pb 锆石年代学
DOI：
10.1111/iar.12248
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发表时间：
2018
期刊：
影响因子：
1.5
通讯作者：
Maruyama Shigenori
中科院分区：
文献类型：
--
作者：
Asanuma Hisashi;Sawaki Yusuke;Sakata Shuhei;Obayashi Hideyuki;Suzuki Kazue;Kitajima Kouki;Hirata Takafumi;Maruyama Shigenori
西澳大利亚北极穹丘地区的表壳岩为地壳过程、地表环境和生物圈演化的早期记录提供了有价值的信息。由于发现了世界上最古老的微体化石，北极穹丘地区的地层演替引起了许多研究者的兴趣。古太古代地层（Warrawoona群）主要由镁铁质-超镁铁质绿岩组成，其间夹有硅质岩和长英质熔岩。对沉积物的年龄限制主要基于锆石U-Pb年代学。然而，许多锆石颗粒遭受了变质作用，并含有异常高的普通Pb含量，这使得U-Pb数据的解释变得复杂。为了提供更有说服力的年代学约束，U-Pb Concordia年龄被广泛接受为最佳估计。从两个二长花岗岩中分离出来的大多数锆石也遭受了严重的变质作用。在我们的分析中，使用预烧蚀技术结合元素映射选择较少变质的域，然后用激光烧蚀电感耦合等离子体质谱法测定它们的U-Pb同位素组成。大多数分析域含有一定量的常见Pb（204 Pb/206 Pb> 0.000 1），而获得了3个和5个U-Pb数据点，其中较少常见Pb（204 Pb/206 Pb < 0.000 1）。这两个U-Pb数据集分别获得了约3445 Ma和3454 Ma的U-Pb Concordia年龄。这些年龄代表了二长花岗岩侵入的时间，并限制了Warrawoona群的最小沉积年龄。此外，单粒捕虏晶锆石的207 Pb/206 Pb年龄为3545 Ma，支持皮尔巴拉火山的硅铝质基底存在于3500 Ma之前的观点。原位U-Pb锆石定年结合预烧蚀技术有可能识别非变质部分，甚至从部分变质锆石中获得精确和准确的地质年代学数据。
Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages ofca3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a207Pb/206Pb age ofca3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. Thein situU–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.