Julian Alfing , Tim E. Johnson , Jonas Kaempf , Michael Brown , Kristoffer Szilas , Kai Rankenburg , Chris Clark
{"title":"Eoarchean granulite-facies metamorphism in the Itsaq Gneiss Complex, southwest Greenland","authors":"Julian Alfing , Tim E. Johnson , Jonas Kaempf , Michael Brown , Kristoffer Szilas , Kai Rankenburg , Chris Clark","doi":"10.1016/j.epsl.2024.118977","DOIUrl":null,"url":null,"abstract":"<div><p>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, <em>n</em> = 45/67; all age uncertainties at 2σ level) and 3652 ± 69 Ma (MSWD = 1.8, <em>n</em> = 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 <sup>207</sup>Pb/<sup>206</sup>Pb ages of 3620 ± 8 Ma (MSWD = 1.2, <em>n</em> = 45) and 3630 ± 8 Ma (MSWD = 0.6, <em>n</em> = 44), respectively. Phase equilibrium modelling constrains peak <em>P–T</em> conditions of Eoarchean (3640–3630 Ma) metamorphism to 8.3–9.0 kbar and 730–820 °C. The thermobaric ratios (<em>T/P</em>) 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 <em>P–T</em> data globally, with no evidence for the bimodality in <em>T/P</em> 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.</p></div>","PeriodicalId":11481,"journal":{"name":"Earth and Planetary Science Letters","volume":"646 ","pages":"Article 118977"},"PeriodicalIF":4.8000,"publicationDate":"2024-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.sciencedirect.com/science/article/pii/S0012821X24003984/pdfft?md5=0907211de5766a1ab6e44266a689e9fa&pid=1-s2.0-S0012821X24003984-main.pdf","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Earth and Planetary Science Letters","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0012821X24003984","RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0
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.
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.