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

摘要

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–in–zircon 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.