The spatial variation of initial 87Sr/86Sr ratios in the Toki granite, Central Japan: Implications for the intrusion and cooling processes of a granitic pluton

Abstract

The spatial variation in initial 87 Sr/ 86 Sr ratios (SrI) in the Toki granite, Central Japan, shows heterogeneity ranging from 0.708942 to 0.710069, which provides information on the intrusion and cooling processes of plutons. The Toki granite has three mineralogy -based rock facies: muscovite -biotite granite (MBG), hornblende -biotite granite (HBG) and biotite granite (BG). Large SrI values were found to be distributed at the western margin (west MBG) and the lithologically central region (central BG), while small SrI values were found at the northeast margin (northeast MBG). Regions with high and low Sr concentrations were also found in the Toki granite. In the Sr -rich samples, SrI (0.708942 -0.709789) increases with 100/Sr (0.7 -1.5). This geochemical trend extends towards the country sedimentary rocks of the Mino Terrane, which can be interpreted to result from assimilation and fractional crystallization (AFC) between the original granitic magma and the Mino sedimentary rocks. The SrI values in the Sr -rich regions show a correlation with the Alumina Saturation Index (ASI). In particular, the west MBG, with large SrI values, is classified as a peraluminous granitoid with large ASI, suggesting that the western margin of the pluton was strongly affected by assimilation during the intrusion process. The Srpoor samples are present both in the central BG, with large SrI values, and in the northeast MBG, with small SrI values. The Sr -poor samples have small ASI and large differentiation indices, indicating that the central BG and the northeast MBG were generated either by different AFC process with different amounts of contaminants or by the intrusion and fractionation of different source magma with different SrI values. Overall, the geochemical spatial variations found in the Toki granite can be explained by various degrees of assimilation and fractional crystallization in the magma chamber and/or multi -stage intrusions with different degrees of crystallization of plural source magmas with different SrI values. Previous petrographical and thermochronological studies revealed that the Toki granite effectively cooled from the wall at the western margin in the cooling stage at temperatures below 500 °C. Larger SrI and ASI values found in this study corresponded to the western margins where the earlier and more rapid cooling took place, indicating that the Toki granite was preferentially cooled from the peraluminous marginal regions where the assimilation of country rock was the most extensive.

Hydrothermal chloritization processes from biotite in the Toki granite, Central Japan: Temporal variations of of the compositions of hydrothermal fluids associated with chloritization

“…1; Ishihara and Chappell 2007). A series of studies by the authors have presented the processes from intrusion through emplacement to cooling of the Toki granitic pluton (Yuguchi et al 2010(Yuguchi et al , 2011b(Yuguchi et al , 2011c(Yuguchi et al , 2011d(Yuguchi et al , 2013. The Toki granite is a good candidate for such studies because several occurrences (types) of chloritization are recognized in terms of mineral assemblage (see petrography below).…”

Ecological and genomic profiling of anaerobic methane-oxidizing archaea in a deep granitic environment

“…The Mizunami URL consists of a main shaft, a ventilation shaft, access tunnels at 300 and 500 m below ground level (mbgl) and substages ( Figure 1a). Cretaceous Toki granite is overlain by the Tertiary sedimentary rocks at 100-200 mbgl around the Mizunami URL (Yuguchi et al, 2013).…”

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

“…The Toki granite is characterized by systematic spatial changes in rock facies (mode and mineral assemblage) and a corresponding change in the bulk chemical composition, and thus is recognized as a zoned pluton ( Figure 1c; Yuguchi et al, 2010). The geology and petrography of the Toki granite were described in detail by Yuguchi et al (2010), Yuguchi, Tsuruta, and Nishiyama (2011c, 2011d, and Yuguchi, Tsuruta, Hama, and Nishiyama (2013).…”