Behavior of rare elements in Late Cretaceous pegmatites from the Setouchi Province, Inner Zone of Southwest Japan

Abstract

This paper reports the paragenesis and reaction textures of rare earth minerals such as monazite, zircon, allanite, thorite, xenotime, and apatite in pegmatites within the Setouchi Province, Inner Zone of Southwest Japan. One of the zircon grains described shows parallel growth with thorite, suggesting their limited solid solution. Xenotime grains are sometimes disseminated by thorite and overgrown by zircon. Monazite in most pegmatites is corroded and replaced with an aggregate of allanite, apatite, thorite, and occasional xenotime and zircon. Core-mantle domains in zircon grains in one of the pegmatites give high Th/U ratios (0.8-0.4) with lower U and Th contents, and rim domains give low Th/U ratios (0.5-0.2 or less) with higher U and Th contents. The decrease of the Th/U ratio toward the rim results from the change of co-precipitating phase from monazite to thorite. The indistinguishable monazite age (81.7 ± 6.4 Ma) and zircon age for the rim domains (82.9 ± 2.1 Ma) suggest that the paragenetic change occurred during a single evolution of pegmatite. The composition of the metasomatic fluid changed from Ca-poor to Ca-rich, causing monazite corrosion and the consequent precipitation of thorite and allanite. This, in turn, regulated the Th and U contents and the Th/U ratio in associated zircon, as well as the fluid composition.