Crystallization of REE minerals and redistribution of U, Th, and REE at contact boundary between granite and gabbro during hydrothermal alteration

In this study, redistribution behaviors of U, Th, and rare earth elements (REE) in association with igneous activity in the Unazuki region of southwest Japan were investigated. At the contact boundary between granite and gabbro, REE minerals such as monazite, xenotime, and REE oxides formed with Fe–Ti oxides in K-feldspar assimilated by gabbro. U was enriched in xenotime (UO2 > 0.72 wt.%) and REE oxides (UO2 > 0.10 wt.%), whereas Th was concentrated in REE oxides (ThO2 > 1.32 wt.%). Considering the similarity between the ionic radii of U4+ and Y3+, U was incorporated into xenotime and REE oxides together with heavy REE. Th incorporation into xenotime (eightfold coordinated site) and monazite (ninefold coordinated site) was constrained by the lower contents of Gd and Dy, respectively. Saussuritization of plagioclase in gabbro and chloritization of biotite in granite at the contact boundary suggest low-temperature hydrothermal alteration at late stages of gabbro intrusion that occurred at 253 ± 1 Ma. In contrast, zircon in the chloritized granite contained altered domains characterized by a dark response between back-scattered electron and cathodoluminescence images; this was caused by a low average atomic number due to the release of Zr and Hf; the incorporation of non-formula elements such as Ca, Fe, and Al; and low crystallinity because of radiation damage. The altered domains showed enrichments of Th (ThO2 > 1.50 wt.%) and light REE (LREE) to middle REE (MREE) as well as positive Eu anomalies; this suggests the preferential incorporation of Th and LREE–MREE (particularly Eu2+) with Ca occurring during the hydrothermal alteration. The redistribution behaviors of U, Th, and REE during low-temperature hydrothermal alteration strongly depend on their ionic radii. Thorite in zircon was probably formed by the exsolution of high-Th altered domains.