Zircon growth in a granitic pluton with specific mechanisms, crystallization temperatures and U–Pb ages: Implication to the ‘spatiotemporal’ formation process of the Toki granite, central Japan

Abstract

Zircons collected from a granitic pluton provide evidence of serial growth events with specific mechanisms, crystallization temperatures and U-Pb ages, revealing details of the sequential formation process from intrusion through emplacement to crystallization/solidification. The events have been identified by: 1) the study of the internal structure of zircon using cathodoluminescence, 2) deriving crystallization temperatures using Ti-inzircon thermometry of the internal structure and 3) U-Pb age dating of the internal structure. The magmatic zircons from the Toki granite, central Japan, show two kinds in their internal structure: a low luminescence core (LLC) and oscillatory zonation (OZ). The LLC was produced by interfacial reaction-controlled growth in the granitic magma with cooling from about 910 to 760°C. The formation of OZ occurred by diffusion-controlled growth in a cooling magma chamber from about 850 to 690°C. The U-Pb ages derived from the LLC ranges from 74.7 ± 4.2 to 70.5 ± 1.3 Ma, indicating the incipient intrusion timing of the magma into the shallow crust. The OZ ages distribute from 72.7 ± 0.6 to 70.4 ± 1.7 Ma, which mean the timing from emplacement to crystallization/solidification of the granite pluton. Thus, the serial processes from intrusion through emplacement to crystallization/solidification occurred within a few million years. The old LLC and OZ ages are recognized in the western margins of the Toki granite, implying that the magma forming the western margins was the first to intrude, emplace and crystallize/solidify. The western margins with initial intrusion may accompany the crustal assimilation in order to create sufficient magma reservoir space, which is consistent with larger SrI and ASI values found in the western margins of the granite.

Spatial distribution of the apatite fission‐track ages in the Toki granite, central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

“…These mineral ages of the Naegi–Agematsu granite are similar to those of the Toki granite (e.g. Suzuki & Adachi, ; Yuguchi et al, , ). The consistency of zircon U–Pb and monazite CHIME ages between these plutons indicates a similar emplacement age.…”

“…Figure presents the AFT inversion results, which show the t–T paths for samples DH9‐10 (acceptable‐fit paths: 3967, good‐fit paths: 575), DH13‐2 (acceptable‐fit paths: 4301, good‐fit paths: 2159) and DH6‐2 (acceptable‐fit paths: 3136, good‐fit paths: 1200). The Toki granite has zircon U–Pb ages ranging from 74.7 ±4.2 Ma to 70.4 ±1.7 Ma (Yuguchi et al, ). Such a zircon U–Pb age does not have difference with temporal conditions at about 300°C in the generated t–T path (e.g.…”

“…In the Toki granite, zircon U–Pb dates (700–900°C of closure temperature), biotite K–Ar dates (350–400°C of closure temperature) and ZFT dates (190–350°C of ZFT PAZ) often have consistent ages in the same sample; e.g. sample DH8FT5 (47.3 meters above sea level [masl])) has zircon U–Pb ages of 73.1 ±1.5 Ma to 72.6 ±1.0 Ma, biotite K‐Ar age of 72.9 ±1.8 Ma and ZFT age of 75.6 ±3.3 Ma (Yuguchi et al, , ). These chronological data indicate the rapid cooling from zircon U–Pb closure temperature to ZFT PAZ.…”

“…The granitic magma that formed the Toki granite was emplaced in the East Asian continental margin at the timing of about 70 Ma (Yuguchi et al, ). The Toki granite started cooling and exhuming after the intrusion/emplacement.…”

“…Shibata and Ishihara () revealed that the Toki granite has a whole‐rock Rb–Sr isochron age of 72.3 ±3.9 Ma, and Suzuki and Adachi () obtained a monazite chemical Th–U‐total Pb isochron (CHIME) age of 68.3 ±1.8 Ma. The Toki granite has zircon U–Pb ages of 74.7 ±4.2 Ma to 70.4 ±1.7 Ma, which signify a time period from the incipient intrusion into the shallow crust through emplacement to crystallization/ solidification of the granite pluton (Yuguchi et al, ). Yuguchi, Amano, Tsuruta, Danhara, and Nishiyama () demonstrated that the Toki granite has hornblende K–Ar ages of 74.3 ±3.7 Ma, with a closure temperature of 510 ±25 °C (e.g.…”