三重県竹原鉱山産ゼノタイムの化学組成とCHIME年代

抄録

Xenotime in a granite pegmatite from the Takehara mine, Tsu, Mie Prefecture, Japan, has been examined in terms of U, Th, Pb, and rare earth element (REE) chemistry by electron microprobe analysis. The pegmatite occurs as a vein in the Cretaceous Kimigano granodiorite. Oscillatory and sector zoning were observed in back-scattered electron images. An averaged empirical formula was (Y_0.701Dy_0.065Er_0.039Yb_0.038Gd_0.036Sm_0.016Ho_0.010Tb_0.009Nd_0.008Tm_0.005U_0.040Th_0.011Ca_0.001)_Σ0.979(P_0.958Si_0.057)_Σ1.015O₄. Compositional variations suggest that the (U + Th) contents are mainly controlled by the coupled substitution (U, Th) + Si = REE + P. The CHIME age of the xenotime grains is 62 ± 5 Ma (2σ), which is much younger than the Kimigano granodiorite dated at 94 ± 2 Ma. This result indicates that the granite pegmatite and the host granitic rock are not of the same magmatic origin.