ABSTRACT

The intra-grain distribution of rare earth elements (REEs) in coexisting apatite, monazite, xenotime and zircon was determined on an electron microprobe for a sample of Cretaceous Busetsu Granite, Okazaki, Japan. The REE distribution suggests that these accessory minerals did not originate as a restite component, but crystallized successively from magmatic melt despite of their possible cotectic relationships. The crystallization sequence is early zircon and euhedral apatite, followed by monazite, xenotime and anhedral apatite. Euhedral apatite shows a core to rim increase in light REEs and a decrease in middle to heavy REEs. Monazite inclusions within biotite show a core to rim decrease in La and Ce with a counterbalancing increase in intermediate REEs and Y. Some large monazite grains in the intergranular space of major minerals display multiple zonation patterns. Zircon shows a core to rim increase in Y content and Hf/(Zr + Hf) ratio, and xenotime shows a core to rim decrease in Zr. Anhedral apatite exhibits an extreme core to rim increase in light and heavy REEs. The REE distribution in these accessory minerals is closely related to the sequence of their crystallization. The successive appearance and disappearance of accessory minerals in the crystallization sequence are explained in terms of deviation from cotectic saturation levels owing to kinetic crystallization from the supersaturated melt.