Chemical composition and CHIME dating of monazite from the Hikami Granites associated with the Tsubonosawa Metamorphic Rock in the South Kitakami Belt, NE Japan

Abstract

Monazite crystals from the Hikami Granites associated with the Tsubonosawa Metamorphic Rock in the South Kitakami Belt were analyzed for U, Th, Pb, and rare earth element contents by electron microprobe analysis. Back-scattered electron imaging revealed that the crystals have cores with a high Si/(Th + U) ratio, which are surrounded and partially replaced by mantles with a low Si/(Th + U) ratio. The empirical formulae of the average core and mantle compositions are (Ce_0.445La_0.234Nd_0.156Y_0.027Pr_0.046Sm_0.021Gd_0.013Dy_0.005Er_0.001Th_0.034U_0.001Ca_0.018)_Σ1.001(P_0.981Si_0.019)_Σ1.000O₄ and (Ce_0.417La_0.218Nd_0.152Y_0.044Pr_0.043Sm_0.023Gd_0.017Dy_0.008Er_0.002Th_0.036U_0.003Ca_0.034)_Σ0.997(P_0.995Si_0.007)_Σ1.002O₄, respectively. The distinct chemical trends observed in the core and mantle suggest they formed under different conditions. Analytical data for the core and mantle show linear arrays on a PbO–ThO₂* (ThO₂* = ThO₂ plus the equivalent of UO₂) diagram and yield a single isochron corresponding to a CHIME age of 441 ± 33 Ma (2σ). The core and mantle are interpreted to have formed during Silurian magmatic and hydrothermal stages, respectively.