Abstract

This study presents the K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, in central Japan. Borehole samples from the Toki granite were collected and mechanically and chemically processed to separate plagioclase from the rock and remove bulk impurities. The sample fraction of cleaned plagioclase powder was further processed to a smaller size fraction, allowing separation of the altered K-feldspar from the plagioclase host. The resulting K-feldspar represented the hydrothermal alteration product and was characterized crystallographically as microcline, and its K-Ar ages were measured. The results of the K-Ar dating and petrographic characterization indicated that in this setting, plagioclase alteration occurred through a combination of solid-state replacement and dissolution–precipitation processes. The K-feldspathization age enables constraint of the temporal conditions of the solid-state replacement process to 62.2 ± 1.4 Ma. The time-temperature (t-T) path of the sampling site is an effective tool for determining both the timing and thermal conditions of the hydrothermal microcline formation in plagioclase alteration. The combination of the t-T path and the microcline K-Ar age provides formation temperatures of about 307–325 °C. The timing and thermal conditions of solid-state replacement (62.2 ± 1.4 Ma and 325–307 °C) indicate an older age and a higher temperature than those of dissolution–precipitation (59.2 ± 1.4 Ma and 305–290 °C: Yuguchi et al., 2019A). The plagioclase alteration consists of serial processes from solid-state replacement to dissolution–precipitation. Addition of the thermal conditions and timing into petrography have implications for the sequential phenomenal variation in granite.

Highlights

  • Fluid is vital in hydrothermal reactions (Putnis et al, 2007) because fluid chemistry determines mineral assemblages in hydrothermal alteration

  • This study focuses on K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, central Japan (Figure 1)

  • Illitization age of 59.2 Æ 1.4 Ma reflects the dissolution–precipitation process determined by illite K-Ar geochronology (Yuguchi et al, 2019A)

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Summary

Introduction

Fluid is vital in hydrothermal reactions (Putnis et al, 2007) because fluid chemistry determines mineral assemblages in hydrothermal alteration. Formation ages and temperature determinations for hydrothermal minerals are significant constraints to the timing and thermal conditions of the alteration process by hydrothermal fluids. This study focuses on K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, central Japan (Figure 1). Hydrothermal alterations of granitic rocks involve ubiquitous biotite chloritization and plagioclase alteration. The Toki granite experiences serial alteration from biotite chloritization to plagioclase alteration occurring 68–51 Ma at temperatures of 180–350 C (Yuguchi et al, 2015, 2019A). Plagioclase alteration involves albitization and K-feldspathization, occurring through solid-state replacement within plagioclase, and illite, calcite, fluorite, and epidote formation, occurring via dissolution of plagioclase around micropores and precipitation into vacancies (Yuguchi et al, 2019A). This study presents companion data and follows the paper by Yuguchi et al (2019A)

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