Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex,Greenland  

作　　者：Hikaru Sawada Tomoaki Morishita Adrien Vezinet Richard Stern Kenichiro Tani Ikuya Nishio Kanta Takahashi D.Graham Pearson Kristoffer Szilas 

机构地区：[1]Department of Earth System Science,University of Toyama,Toyama,Japan [2]X-Star,Japan Agency for Marine-Earth Science and Technology(JAMSTEC),Kanagawa,Japan [3]Faculty of Geosciences and Civil Engineering,Kanazawa University,Kanazawa,Japan [4]Lamont-Doherty Earth Observatory,Columbia University,Palisades,USA [5]Volcanoes and Earth’s Interior Research Center,JAMSTEC,Kanagawa,Japan [6]Department of Earth and Atmospheric Sciences,University of Alberta,Edmonton,Canada [7]Canadian Centre for Isotopic Microanalysis,University of Alberta,Edmonton,Canada [8]National Museum of Nature and Science,Tsukuba,Japan [9]Department of Geosciences and Natural Resource Management,University of Copenhagen,Copenhagen,Denmark

出　　处：《Geoscience Frontiers》2023年第6期273-285,共13页地学前缘（英文版）

基　　金：supported by JSPS KAKENHI Grant Numbers 16H05741,19KK0092;Kana-zawa SAKIGAKE 2018 to T.M.,and 20 K14571 to H.S..K.S.thanks the Carlsberg Foundation for support via grant CF18-0090.

摘　　要：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 orthogneiss.This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit.In-situ U-Pb-Hf-O isotope measurements were made on the zircons.Zircons from both the chromitite and the intrusive granitoids show concordant U-Pb ages of ca.2.97-2.95 Ga.In contrast,Hf and 0 isotopic anal-yses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids.Zircons 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 crust.The 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 Ga.These zircons also haveδ^(18)O values of 4.2‰6.1‰which correspond to typical mantle values.The 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 zir-cons.These 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 metasomatism.Furthermore,the present results suggest that zircons in the chromitites originally had depleted Hf iso-topic compositions at ca.3.2-3.0 Ga.This can be explained by two different models of the evolution of the UNL unit.One 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 Ga during metamorphism or metaso-matism

关 键 词：ARCHEAN Itsaq gneiss complex Ultramafic rock METASOMATISM 

分 类 号：P548[天文地球—构造地质学] P534.2[天文地球—地质学]