TY - JOUR

T1 - Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex, Greenland

AU - Sawada, Hikaru

AU - Morishita, Tomoaki

AU - Vezinet, Adrien

AU - Stern, Richard

AU - Tani, Kenichiro

AU - Nishio, Ikuya

AU - Takahashi, Kanta

AU - Graham Pearson, D.

AU - Szilas, Kristoffer

N1 - Publisher Copyright: © 2023 China University of Geosciences (Beijing) and Peking University

PY - 2023

Y1 - 2023

N2 - The Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, west Greenland, has been considered 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 O isotopic analyses 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 εHf value of − 11 to − 5), and δ18O 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 εHf 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 have δ18O 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 O isotopic compositions ranging between the least altered zircons and the intrusive granitoid zircons. 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 isotopic 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 metasomatism. The other model is that the UNL unit itself was actually formed at ca. 3.2–3.0 Ga, with zircon in the chromitite representing the crystallisation age of the unit, which was then tectonically incorporated into the ca. 3.81 Ga orthogneiss prior to the 2.97–2.95 Ga granitoid intrusion event. In either case, our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2–2.95 Ga. Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

AB - The Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, west Greenland, has been considered 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 O isotopic analyses 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 εHf value of − 11 to − 5), and δ18O 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 εHf 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 have δ18O 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 O isotopic compositions ranging between the least altered zircons and the intrusive granitoid zircons. 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 isotopic 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 metasomatism. The other model is that the UNL unit itself was actually formed at ca. 3.2–3.0 Ga, with zircon in the chromitite representing the crystallisation age of the unit, which was then tectonically incorporated into the ca. 3.81 Ga orthogneiss prior to the 2.97–2.95 Ga granitoid intrusion event. In either case, our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2–2.95 Ga. Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

KW - Archean

KW - Itsaq gneiss complex

KW - Metasomatism

KW - Ultramafic rock

U2 - 10.1016/j.gsf.2023.101648

DO - 10.1016/j.gsf.2023.101648

M3 - Journal article

AN - SCOPUS:85163488004

VL - 14

JO - Geoscience Frontiers

JF - Geoscience Frontiers

SN - 1674-9871

IS - 6

M1 - 101648

ER -