The North Kitakami belt in northeastern Japan is a Jurassic accretionary complex with abundant greenstones. The Shimamori Formation greenstones in the Hachinohe area, Aomori Prefecture, comprise basaltic massive lava, pillow lava and hyaloclastite associated with limestone as well as dykes of dolerite and gabbro. Association of these greenstones with limestone, without any chert, suggests their formation at a depth shallower than the carbonate compensation depth. Bulk rock major and trace element chemistry of these greenstones reveals that they are within-plate tholeiitic basalt enriched in Nb, Ti and LREE. The relic mineral chemistry such as Ti-poor (TiO2<1.3 wt.%) clinopyroxene and Cr-rich (Cr#=60) chromian spinel also supports “tholeiite” identification. These chemical and mineralogical characteristics indicate that the Shimamori Formation represents a seamount fragment mainly formed by within-plate tholeiitic (ocean island tholeiitic) magmatism, and accreted to the Eurasian continental margin through subduction process. The tholeiitic magmatism, however, is contrasting to the alkali basalt magmatism, which dominates the accreted greenstones in the main part of the North Kitakami belt to the south.
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.
Analytical results of major and trace elements (V, Cr, Ni, Rb, Sr, Ba, Y, Zr, Nb) for bulk rocks by XRF using 1 : 5 dilution glass beads were evaluated. The results show relatively good accuracy and precision for major and trace elements. Analytical conditions, the accuracies and precisions are reported. If a rock contains V>40 (ppm), Cr>20, Ni>10, Rb>5, Sr>5, Ba>75, Y>4, Zr>10, Nb>5, the relative standard deviations would be below 10%. This method could be suitable to arc andesites.
Practical methods of image simulations for high-resolution transmission electron microscopy (HRTEM) of minerals are described by using a commercial software product. The parameters used for the simulation are discussed to obtain the correspondence between calculated and observed HRTEM images. The simulations of mica, apatite and mica surfaces are presented with the comparison of their HRTEM images.