西南日本領家帯 (四国)，高浜花崗岩類の冷却史，特に新第三紀貫入岩類による熱的影響

抄録

We evaluated the cooling history of the Cretaceous Ryoke Granitic Rocks (Takahama Granitic Rocks, TGR) exposed in the Takahama area (Shikoku), with special reference to the thermal effect caused by later intrusive rocks.
The Neogene andesitic intrusive rocks intrude into the TGR in the southern part of the Takahama area. These rocks together belong to the Setouchi Volcanic Rock Suite. The K-Ar whole rock age of a sample, determined in this study, is 12.2±0.6 Ma.
For the evaluation of the thermal effect by Takahama-Kuroiwa Intrusive Body (TKIB) of the Neogene intrusive rocks to the TGR, we determined K-Ar and FT mineral ages of five rock samples of the TGR from various distances (0.1, 1, 8.5, 51 and 90 m) from the contact with the TKIB. Annealing effects by the later intrusion on FT ages of zircon and apatite are observed in the samples from 0.1 to 8.5 m and from 0.1 to 51 m, respectively. FT ages of zircon at 0.1 m (13.1±0.4 Ma) and apatites from 0.1 to 8.5 m (14.4±1.4, 13.0±1.3 and 12.4±1.1 Ma) are nearly identical to the intrusive age of the TKIB. Therefore, the temperatures experienced by these minerals were higher than the upper limits of PAZs (partial annealing zones). By application of the relationships between track lengths of zircon, annealing temperatures, and annealing duration (Yamada et al., 1995), we tried to estimate the annealing temperatures of zircon. Annealing duration was estimated as being between about 10² and 10³ years by the using a one-dimensional non-steady state thermal diffusion model. The above results showed that the peak temperatures of annealing were >450°C at 0.1 m, 300-370°C at 1 m, 270-300°C at 8.5 m and >60-90°C at 51 m from the contact of the TKIB.
K-Ar ages of hornblende and biotite of a TGR sample taken from the northern part of the area, where no later intrusive rocks are found, are 87.6±4.4 Ma and 86.4±4.3 Ma, respectively. Fission Track ages of zircon and apatite from the same sample are 73.6±3.0 Ma and 54.8±3.7 Ma, respectively. K-Ar ages of hornblende and biotite, Fission Track ages of zircon and apatite of a TGR sample, which was not subjected to thermal effect by the intrusion, at 90 m distance from the TKIB are 91.1±4.6 Ma, 89.1±4.5 Ma, 76.3±4.3 Ma and 54.7±3.5 Ma, respectively.
Assuming reasonable closure temperatures, the cooling history of the TGR that we deduced is as follows: emplacement at about 90 Ma, followed by rapid cooling with a rate of about 105-175°C/m.y. (within a temperature range of about 510-300°C) and slow cooling with a rate of about 6°C/m.y. (within a temperature range of about 300-100°C). The earlier high rate resulted from the rapid cooling of the TGR after their emplacement and solidification, while the later slow cooling rate resulted from the slow exhumation of the TGR after rapid cooling.