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ALLANITE-(Ce) AS AN INDICATOR OF THE ORIGIN OF GRANITIC ROCKS IN JAPAN: IMPORTANCE OF Sr–Nd ISOTOPIC AND CHEMICAL COMPOSITION

Mihoko Hoshino^1,, Mitsuyoshi Kimata¹, Yoji Arakawa¹, Masahiro Shimizu¹, Norimasa Nishida² and Shunichi Nakai³

¹ Earth Evolution Sciences, Life and Environmental Sciences, University of Tsukuba, Tennodai 1–1–1, Tsukuba, Ibaraki, 305–8572, Japan
² Chemical Analysis Division, Research Facility Center for Science and Technology, University of Tsukuba, Tennodai 1–1–1, Tsukuba, Ibaraki, 305–8577, Japan
³ Earthquake Research Institute, University of Tokyo, Yayoi 1–1–1, Bunkyo, Tokyo, 113–0032, Japan

E-mail address: hossy716{at}geol.tsukuba.ac.jp

The isotopic compositions of Sr and Nd were analyzed for sixteen samples of allanite-(Ce) from Mesozoic–Cenozoic granitic rocks in Japan (11 samples of an Mn-rich type, and five samples of an Mn-poor type). The Mn-rich allanite-(Ce) crystals have an initial ⁸⁷Sr/⁸⁶Sr value in the range 0.70811–0.71398 and an initial ¹⁴³Nd/¹⁴⁴Nd value in the range 0.512148–0.512471, whereas for crystals of Mn-poor allanite-(Ce), the analogous intervals are 0.70499–0.70704 and 0.512547–0.512606. There is a negative correlation between ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values. The trend of these isotopic data for Mn-rich and Mn-poor allanite-(Ce) is parallel to that for ilmenite- and magnetite-series granitic rocks from Japan, respectively. This parallelism suggests that the genesis of these two groups of allanite-(Ce) is related to that of ilmenite-series and magnetite-series granitic rocks. The characteristics of allanite-(Ce) can shed light on the petrogenesis of granitic rocks.

Keywords: allanite-(Ce), Mn content, rare-earth elements, strontium isotopes, neodymium isotopes, granitic rocks, origin, Japan.