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
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
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DOI：
10.1016/j.lithos.2023.107026
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发表时间：
2023-03
期刊：
影响因子：
3.5
通讯作者：
T. Yuguchi;Daichi Itoh;T. Yokoyama;S. Sakata;Satoshi Suzuki;Yasuhiro Ogita;K. Yagi;T. Imura;S. Motai;T. Ohno
中科院分区：
文献类型：
--
作者：
T. Yuguchi;Daichi Itoh;T. Yokoyama;S. Sakata;Satoshi Suzuki;Yasuhiro Ogita;K. Yagi;T. Imura;S. Motai;T. Ohno
本文提出了一种基于三维阴极发光模式、U-Pb年龄、Ti浓度和Th/U比值的变化来解释花岗岩中锆石生长的新方法。我们专注于在日本九州中部的Okueyama花岗岩（OKG）的锆石生长，以获得解释的岩浆房过程，导致形成的花岗岩岩体。OKG的三个岩相取样：黑云母花岗岩，角化花岗岩，角化花岗闪长岩。为了确定锆石的三维内部结构和生长模式，我们对多个晶体进行了CL观察，这些晶体被切成5-10 μm的深度间隔。同时测定了晶体中心部分的锆石U、Pb年龄和Ti含量。锆石晶体内部结构的3D分布包括以下五种纹理：1）振荡环带（OZ），2）多孔，3）混乱，4）局部扰动，和5）具有继承核的晶体。OZ的3D分布可用于近似锆石成核的位置。同时测定花岗岩样品的锆石U、Pb年龄和Ti含量，揭示了花岗岩岩浆凝固前的时间-温度（t-T）历史。三种岩相在锆石结晶温度上记录了相似的冷却历史。岩浆房在大约13 Ma时从900 °C冷却到650 °C，然后冷却到黑云母K单键Ar闭合温度。同时测定了花岗岩中的锆石U、Pb年龄和Ti含量，并将Th/U与视温度进行了关联。锆石Th/U随温度降低而降低，表明岩浆房在冷却过程中进行了分离结晶。锆石Th/U值随温度的降低可能与副产物独居石和锆石的共结晶有关。Th/U与温度的关系表明，770 °C以上岩浆房中的分离结晶作用比770 °C以下岩浆房中的分离结晶作用更强。三种岩相Th/U与温度的关系具有共同的趋势，表明三种岩相在冷却岩浆房中的分离结晶路径相同。
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, Usingle bondPb 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 Usingle bondPb 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 Usingle bondPb 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 Ksingle bondAr closure temperature. The simultaneous determination of zircon Usingle bondPb 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.