Outlining zircon growth in a granitic pluton using 3D cathodoluminescence patterns, UPb age, titanium concentration, and Th/U: Implications for the magma chamber process of Okueyama granite, Kyushu, Japan

In this study, we propose a new method for elucidating zircon growth in granitic plutons, based on the variations in three-dimensional (3D) cathodoluminescence (CL) patterns, UPb age, titanium concentration, and Th/U. We focused on the zircon growth in the Okueyama granite (OKG) in central Kyushu, Japan, to obtain interpretations of magma chamber processes that result in the formation of granitic plutons. Three lithofacies of the OKG were sampled: biotite granite, hornblende granite, and hornblende granodiorite. To determine the 3D internal structure and growth pattern of zircon, we performed CL observations for multiple crystals that were sectioned in depth intervals of 5–10 μm. The zircon UPb ages and Ti concentrations for the center sections of the crystals were simultaneously determined. The 3D distribution of the internal structure of zircon crystals comprises the following five textures: 1) oscillatory zoned (OZ), 2) porous, 3) chaotic, 4) locally disturbed, and 5) crystals with inherited cores. The 3D distribution of the OZ can be used to approximate the location of zircon nucleation. The simultaneous determination of zircon UPb ages and Ti concentrations of the granite samples indicates the time–temperature (t–T) history of granitic magma before its solidification. The three lithofacies record similar cooling histories among the temperatures of zircon crystallization. The magma chamber cooled from 900 °C to 650 °C at approximately 13 Ma and then cooled to the biotite KAr closure temperature. The simultaneous determination of zircon UPb age and Ti concentration in the granite also enabled us to correlate the Th/U with apparent temperature. A decrease in zircon Th/U with decreasing temperature indicates progressive fractional crystallization during the cooling of the magma chamber. This decrease in zircon Th/U with decreasing temperature may be related to the co-crystallization of accessory monazite and zircon. The relationship between Th/U and temperature indicates that fractional crystallization in the magma chamber at temperatures above 770 °C was more progressive than that at temperatures below 770 °C. The three lithofacies demonstrate common tendencies in the relationship between Th/U and temperature, which indicate the same path of fractional crystallization among the three lithofacies in the cooling magma chamber.