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Zircon within chromitite requires revision of the tectonic history of the Eoarchean itsaq gneiss complex,Greenland

Geoscience Frontiers 2023年第6期14卷 273-285页

作者： Hikaru Sawada Tomoaki Morishita Adrien Vezinet Richard Stern Kenichiro Tani Ikuya Nishio Kanta Takahashi D.Graham Pearson Kristoffer Szilas Department of Earth System Science University of ToyamaToyamaJapan X-Star Japan Agency for Marine-Earth Science and Technology(JAMSTEC)KanagawaJapan Faculty of Geosciences and Civil Engineering Kanazawa UniversityKanazawaJapan Lamont-Doherty Earth Observatory Columbia UniversityPalisadesUSA Volcanoes and Earth’s Interior Research Center JAMSTECKanagawaJapan Department of Earth and Atmospheric Sciences University of AlbertaEdmontonCanada Canadian Centre for Isotopic Microanalysis University of AlbertaEdmontonCanada National Museum of Nature and Science TsukubaJapan Department of Geosciences and Natural Resource Management University of CopenhagenCopenhagenDenmark

The Ujaragssuit Nunat layered(UNL)unit in the itsaq gneiss complex,west Greenland,has been consid-ered to contain one of the oldest chromitites on Earth based on~ca.4.1 Ga Hadean whole rock Pt-Os model ages and ca.3.81 Ga zircon U-Pb age of the surrounding *** study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL ***-situ U-Pb-Hf-O isotope measurements were made on the *** from both the chromitite and the intrusive granitoids show concordant U-Pb ages of ca.2.97-2.95 *** contrast,Hf and 0 isotopic anal-yses indicate that zircons in the chromitites have a different origin from those in the intrusive *** from granitoids yielded Th/U ratios higher than 0.2,initial Hf isotope ratios of 0.2805-0.2807(i.e,initial:Hf value of-11 to-5),andδ^(18)O values of mostly 6.0‰-7.0‰,which are typical for felsic igneous rocks in Archean continental *** least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078-0.28084(i.e.,initial:Hf value of-1.1 to-0.4),close the chondritic value at ca.3.0 Ga and the depleted mantle at ca.3.2 *** zircons also haveδ^(18)O values of 4.2‰6.1‰which correspond to typical mantle *** other chromitite zircons yielded Th/U ratios lower than 0.1,and Hf and 0 isotopic compositions ranging between the least altered zircons and the intrusive granitoid *** results indicate that the zircons in the chromitites crystallized before or during the 2.97-2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent ***,the present results suggest that zircons in the chromitites originally had depleted Hf iso-topic compositions at ca.3.2-3.0 *** can be explained by two different models of the evolution of the UNL *** is that if the UNL unit was formed at>3.81 Ga as previously thought,with the zircons in the chromitites subsequently being precipitated by ca.3.2-2.95 G

关键词： Archean itsaq gneiss complex Ultramafic rock Metasomatism

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