Chemical compositions and U–Th–Pb chemical ages of monazite in the Yamanoo pegmatite from Sakuragawa, Ibaraki Prefecture, Japan

Abstract

Monazite in a granite pegmatite from Yamanoo, Sakuragawa, Ibaraki Prefecture, Japan has been examined in terms of U, Th, Pb, and rare earth element (REE) chemistry by electron microprobe analysis. This pegmatite occurs as a lenticular vein in the Kabasan granitic body. Oscillatory and sector zoning were observed by back–scattered electron images. An averaged empirical formula was (Ce0.286Nd0.180Sm0.104La0.077Y0.069Pr0.041Gd0.069Dy0.020Er0.004Th0.069U0.011Ca0.052)Σ0.982(P0.983 Si0.027)Σ1.010O4. Compositional variations suggest that the (Th + U) contents are controlled mainly by the coupled substitution (Th,U) + Ca = 2REE. Calculated U–Th–Pb chemical ages based on the assumption that the initial Pb is negligible are 61 ± 11–69 ± 10 Ma (2σ) and consistent with previously reported radiometric ages for the Kabasan granitic body.