[S-CG57] Hybrid Geochronology from Date to Age

[SCG57-P04] Complexity in zircon geochronology of serpentinite bodies

*Hikaru Sawada^1,6, Ryosuke Oyanagi^2,6, Mitsuhiro Nagata³, Sota Niki⁴, Takafumi Hirata⁵ (1.University of Toyama, 2.Kokushikan University, 3.Nihon University, 4.Nagoya University, 5.The University of Tokyo, 6.JAMSTEC)

Keywords:rodingite, subduction zone, metasomatism, metamorphism

Over the past two decades, hydrothermal zircon is frequently discovered in serpentinized ultramafic rocks, including serpentinite and associated alteration rocks such as rodingites. The U–Pb dating of them is a potential tool for constraining the timing of serpentinization and reconstructing the tectonic evolution of geological units. However, recent studies have increasingly revealed that the origins of zircon in serpentinite are diverse, making it difficult to classify them solely based on petrological, mineralogical, or geochemical proxies. In this presentation, we illustrate the complexity of serpentinite geochronology using rodingites from the Kanesaki serpentinite body in the Mikabu Unit of the Sanbagawa belt, Saitama, Japan, as a case study. This serpentinite body contains two distinct types of rodingite: Type-1, composed of clinozoisite, and Type-2, composed of diopside and grossular, occurring as lenticular bodies ranging from tens of centimeters to several meters in size and separated by a distance of just over 10 meters. Zircon U-Pb dating of these rodingites revealed distinct characteristics: Type-1 yielded a concentrated age of 129 Ma with Th/U being below 0.1, whereas Type-2 exhibited a wide range of ages from 95 Ma to 3 Ga with Th/U exceeding 0.1. The results suggest that Type-1 originated from volcanic or similar rocks that initially lacked zircon, where hydrothermal zircon formed during serpentinization. In contrast, Type-2 is interpreted as an altered sandstone containing detrital zircon that retained its original ages. Furthermore, a rodingite dyke with a mineral assemblage similar to Type-2 has been reported in the Nomozaki serpentinite body, Nagasaki, but in this case, it contains hydrothermal zircon and is considered to have an igneous origin. Since rodingites are highly altered rocks that have lost nearly all textural and compositional information of their protoliths, zircon dating provides crucial insights into their origins. Zircon ages obtained from serpentinite bodies do not necessarily reflect the timing of serpentinization. It is essential to interpret their significance by integrating multiple analyses, including dating of multiple zircon grains, petrological observations, and geological context.