T4[Topic] Metamorphic rocks and tectonics

3:30 PM - 3:45 PM JST(6:30 AM - 6:45 AM UTC)

[T4-O-12] Muscovite forming in an environment with high ambient argon concentrations during Abukuma low P/T type of metamorphism, NE Japan

*Tetsumaru Itaya¹ (1. Hiruzen Institute for Geology & Chronology)

Keywords:

Abukuma metamorphic belt,NE Japan,Low P/T type of metamorphism,K-Ar ages of biotite & muscovite,high non-radiogenic Ar content

The Abukuma Mountains constitute one of the important geological units in NE Japan. In the southern region, metamorphic rocks are extensively distributed alongside the granitic bodies. This Abukuma metamorphic belt is well-known as a type locality for andalusite–sillimanite-series progressive metamorphism (Miyashiro, 1961: Journal of Petrology). Based on the lithology, the Abukuma metamorphic rocks can be divided into the Gosaisyo series on the east side and the Takanuki series on the west side. The Gosaisyo series are mainly composed of metabasites and metacherts with subordinate amount of meta-ultrabasites, metapelites and limestones. The geochemical analysis of the metabasites and the finding of radioralian fossils in siliceous metamorphic rocks revealed that the Gosaisyo series formed by the metamorphism of the upper part of the Jurassic oceanic crust (Hiroi and Kishi, 1989: Geological Society, London, Special Publications). On the other hand, the Takanuki series consists dominantly of pelitic-psammitic gneisses with small amounts of coarse-grained metabasites and marbles containing lenses of metalaterites. The host rocks of the Takanuki series are thought to be essentially of terrigenous origin (Hiroi and Kishi, 1989: Geological Society, London, Special Publications). The K-Ar analyses of muscovite and biotite from the two types of series were carried out, resulting in the two different types of series were the same in age of ca. 100Ma (Itaya, 2025: Engineering Geology of Japan). This indicates that both series underwent the low P/T type of metamorphism together in middle Cretaceous after the different geological bodies juxtaposed tectonically with each other. Itaya (2025: Engineering Geology of Japan) showed that the non-radiogenic Ar in the mucovites is extremely high and variable (ranging from 1 to 73%), in stark contrast to the low value less than 2% reported for phengitic mica from the Sanbagawa eclogites in SW Japan (Itaya and Tsujimori, 2015: International Geology Review) despite both the Abukuma and the Sanbagawa having similar ages (ca. 100 Ma). The Higo metamorphic rocks in SW Japan also have muscovites with low non-radiogenic Ar content less than 2% (Nagakawa et al., 1997: Journal of the Geological Society of Japan). It is also noteworthy that, in the same samples, muscovite yields younger apparent ages and significantly higher non-radiogenic Ar contents than biotite. This may be attributed to that muscovite forming in an environment with high ambient argon concentrations during Abukuma metamorphism, but without incorporation of excess radiogenic argon, as there is no positive correlation between age and non-radiogenic Ar content. Biotite likely formed under similar condition; however, due to its lower closure temperature compared to that of muscovite, some of the trapped argon may have been lost during post-metamorphic cooling, resulting in a reduced non-radiogenic Ar content. A similar relationship between muscovite and biotite is observed in the granitic sample, suggesting that this phenomenon may be more widespread. This would be due to the low P/T type of metamorphism of the upper part of the Jurassic oceanic crust that is close to the earth surface. To our knowledge, this is the first report of such behavior, and further investigation is required to clarify its underlying mechanism.