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

Yuguchi, Takashi*; Ito, Daichi*; Yokoyama, Tatsunori   ; Sakata, Shuhei*; Suzuki, Satoshi*; Ogita, Yasuhiro  ; Yagi, Koshi*; Imura, Takumi*; Motai, Satoko*; Ono, Takeshi*

We propose a new method for elucidating zircon growth in granitic plutons, based on variations in three-dimensional 3D cathodoluminescence (CL) patterns, U-Pb ages, titanium concentration, and Th/U ratios. We focused on the zircon growth processes in the Okueyama granite (OKG) in central Kyushu, Japan, to obtain interpretations of magma chamber processes that result in the formation of granitic plutons. The OKG consists of three lithofacies: biotite granite (BG), hornblende granite (HG), and hornblende granodiorite (HGD). To determine the 3D internal structure and growth pattern of a zircon crystal, we performed CL observations for multi-sections of the samples. Simultaneously, we also determined the zircon U-Pb age and titanium concentration of the center sections of the samples. The 3D distribution of the oscillatory zoning can be used to determine the crystal nucleus. The simultaneous determination of zircon U-Pb ages and Ti concentrations of the granite samples indicates the time-temperature (t-T) history of granitic magma before its solidification. The t-T histories of the BG, HG, and HGD represented similar cooling behaviors within the magma chamber: rapid cooling from the zircon crystallization temperature to the closure temperature of the biotite K-Ar system between 16 Ma and 10 Ma. The variations in the Th/U ratios against temperature also demonstrate a different trend at the boundary of approximately 670 C. Fractional crystallization in the magma chamber progressed significantly at temperatures above 670 C; below 670 C, crystallization progressed slowly, indicating only minimal changes in the magma composition. The variations in the Th/U ratio against temperature in the BG, HG, and HGD portrayed common tendencies, indicating the same behavior in the progression of fractional crystallization among the three lithofacies, which in turn, represented the same behavior within the entire magma chamber.