[S-CG51] ハイブリッド年代学−ジルコン年代学の次へ−

[SCG51-P02] Zircon in Archean chromitite records juvenile Hf isotopic contribution at ca. 3.2–3.0 Ga in the Ujaragssuit Nunât layered unit, southwest Greenland

*沢田 輝¹、森下 知晃⁴、谷 健一郎⁵、Vezinet Adrien²、Stern Richard ²、西尾 郁也^3,4、 Pearson Graham²、Szilas Kristoffer³ (1.海洋研究開発機構、2.アルバータ大学、3.コペンハーゲン大学、4.金沢大学、5.国立科学博物館)

キーワード：太古代、超苦鉄質岩、クロミタイト、ジルコン、ハフニウム同位体

Zircon is an accessary mineral found mainly in felsic rocks, while it also occurs rarely in ultramafic rocks such as peridotite and chromitite, and its origin is controversial. The origin of the zircons in ultramafic rocks can be classified as (1) igneous zircons retaining their primary composition, (2) igneous zircons that have been metamorphosed and altered, and (3) metamorphic or hydrothermal zircons precipitated from fluid or through decomposition of other minerals. We obtained zircons from Chromitites in the Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, as well as granitoids that were intruded into the ultramafic unit, and carried out in–situ U–Pb–Hf isotopic analyses by using LA-ICPMS. The UNL unit chromitite have been considered as one of the oldest chromitites based on ~ca. 4.1 Ga Hadean whole rock Pt–Os model ages, although what the age means are in debate due to metamorphism and metasomatism on the UNL unit. Zircons from both the chromitite and granitic rocks show concordant U–Pb ages of 2.97–2.95 Ga. Cathodoluminescence (CL) images and Th/U ratio indicate that zircons from the intrusive granitoids are igneous in origin. The zircons from granitoids show initial Hf isotopic ratio between 0.2805 and 0.2807 indicating the granitoid were generated from melting of typical Archean continental crustal rocks. On the other hand, most of the zircons from chromitite have dark CL images without zoning and Th/U ratio lower than 0.1, which means that those zircons have been highly affected by metamorphism or metasomatism. Some minor zircons from a chromitite show relatively bright CL images and Th/U ratio of ~0.4, indicating least altered, and exhibited initial Hf isotopic ratios of 0.28078~0.28084 (i.e., initial εHf value of -1.1~-0.4) which are close to that of the chondritic value at ca. 3.0 Ga and the depleted mantle at ca. 3.2 Ga. The other highly altered chromitite zircons show Hf isotope ratio between the least altered chromitite zircons and those from the intrusive granitoid, suggesting isotopic mixing during metasomatism. We conclude that the UNL ultramafic unit is younger than previously thought, formed at ca.3.2–3.1 Ga with igneous zircon, and was intruded by the granitoid at 2.97–2.95 Ga. Furthermore, the present result indicates that U-Pb ages of the chromitite zircons are totally reset by the granitoid intrusion, whereas resets of Hf isotopic composition of them are the overwriting of Hf isotope ratios occurred incompletely. Research in metamorphic and metasomatic zircons in ultramafic rocks will require understanding of the U–Pb–Hf isotopic systematics in zircon.