Eoarchean granulite-facies metamorphism in the Itsaq Gneiss Complex, southwest Greenland

IF 4.8 1区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Earth and Planetary Science Letters Pub Date : 2024-09-14 DOI:10.1016/j.epsl.2024.118977
Julian Alfing , Tim E. Johnson , Jonas Kaempf , Michael Brown , Kristoffer Szilas , Kai Rankenburg , Chris Clark
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Abstract

Using petrography, in situ garnet Lu–Hf geochronology, garnet rare-earth element (REE) analysis, zircon U–Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu–Hf geochronology from two samples of variably migmatitic metabasic rocks least affected by subsequent (Neoarchean) reworking yield Lu–Hf isochron ages of 3641 ± 62 Ma (MSWD = 1.7, n = 45/67; all age uncertainties at 2σ level) and 3652 ± 69 Ma (MSWD = 1.8, n = 83/84) from garnet with REE patterns typical of single-stage prograde growth. From the same two samples, zircon grains with textures consistent with metamorphic growth give weighted-mean 207Pb/206Pb ages of 3620 ± 8 Ma (MSWD = 1.2, n = 45) and 3630 ± 8 Ma (MSWD = 0.6, n = 44), respectively. Phase equilibrium modelling constrains peak P–T conditions of Eoarchean (3640–3630 Ma) metamorphism to 8.3–9.0 kbar and 730–820 °C. The thermobaric ratios (T/P) of 800–1000 °C/GPa recorded by the investigated samples are considerably higher (warmer) than previously proposed for granulite-facies metamorphism in the northern IGC, and broadly similar to Archean metamorphic P–T data globally, with no evidence for the bimodality in T/P that characterizes younger metamorphism. Either subduction-driven metamorphism (and plate tectonics) did not operate in the Eoarchean, or the Eoarchean lithosphere had a rheology that prohibited exhumation of subducted rocks.

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格陵兰西南部伊萨格片麻岩群的始新世花岗岩层变质作用
利用岩相学、原位石榴石lu-hf地质年代学、石榴石稀土元素(REE)分析、锆石U-Pb地质年代学和相平衡建模,我们为格陵兰西南部Itsaq片麻岩群(IGC)北部的新元古代花岗岩变质作用提供了明确的证据。从两个受后续(新元古代)再加工影响最小的变质玄武岩样品中提取的原位石榴石lu-hf地质年代为3641 ± 62 Ma(MSWD = 1.7,n = 45/67;所有年龄不确定性均为2σ级)和3652 ± 69 Ma(MSWD = 1.8,n = 83/84),石榴石的lu-hf等时线年龄具有典型的单级顺行生长的REE模式。在同样的两个样品中,质地与变质生长一致的锆石颗粒的加权平均 207Pb/206Pb 年龄分别为 3620 ± 8 Ma(MSWD = 1.2,n = 45)和 3630 ± 8 Ma(MSWD = 0.6,n = 44)。相平衡模型将新元古代(3640-3630 Ma)变质作用的峰值P-T条件约束为8.3-9.0 kbar和730-820 °C。调查样本记录的热压比(T/P)为800-1000 °C/GPa,比之前提出的北IGC地区花岗岩成因变质作用的热压比要高(更热)得多,与全球范围内的阿新世变质作用P-T数据大体相似,没有证据表明年轻变质作用的T/P具有双峰性。要么是俯冲驱动的变质作用(和板块构造作用)在始新世并不发生,要么是始新世岩石圈的流变性禁止了俯冲岩石的吐出。
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来源期刊
Earth and Planetary Science Letters
Earth and Planetary Science Letters 地学-地球化学与地球物理
CiteScore
10.30
自引率
5.70%
发文量
475
审稿时长
2.8 months
期刊介绍: Earth and Planetary Science Letters (EPSL) is a leading journal for researchers across the entire Earth and planetary sciences community. It publishes concise, exciting, high-impact articles ("Letters") of broad interest. Its focus is on physical and chemical processes, the evolution and general properties of the Earth and planets - from their deep interiors to their atmospheres. EPSL also includes a Frontiers section, featuring invited high-profile synthesis articles by leading experts on timely topics to bring cutting-edge research to the wider community.
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