U-Pb zircon dating, rare earth elements (REE) analysis and Sr-Nd isotope analysis were conducted on samples from the Soeda Granodiorite in the eastern part of northern Kyushu, Southwest Japan. The Soeda Granodiorite is divided into two units, based on whole-rock Sr content: the Ochiai and Irahara Granodiorites. These granodiorites are composed mainly of medium-grained hornblende - biotite granodiorite to biotite granite. U-Pb zircon ages of 107.41 ± 0.82 Ma and 103.11 ± 0.69 Ma were obtained from the Ochiai and Irahara Granodiorites, respectively. These suggest that timing of intrusion of each of the granodiorite magmas were different. REE and Sr-Nd isotopic compositions of source magmas of the Ochiai and Irahara Granodiorites also differ.
Recent reexamination of the Hidaka Supergroup (Hokkaido, Japan) has left several stratigraphic units undated, and it is therefore unclear whether these units should be considered as protoliths of the Hidaka Metamorphic Belt. Furthermore, the Erimo Complex remains uncorrelated, precluding a thorough interpretation of the Horoizumi Shear Zone. We present results from detrital zircon U-Pb dating of the sedimentary complexes around the Hidaka Mountains to clarify the tectonic and orogenic history of the area. The turbidite sandstone of the T-Unit (Horobetsugawa Complex, Idonnappu Zone; youngest age cluster of 64.3 ± 0.9 Ma, early Paleocene) is significantly older than the Nakanogawa Group (Hidaka Belt). The turbidite sandstone of the Erimo Complex (youngest age cluster of 52.8 ± 0.3 Ma, early Eocene) is significantly younger than the T-Unit turbidite sandstone and of similar age to the Bisei Formation (Nakanogawa Group, northern unit). We conclude that the Erimo Complex is closely related to the Nakanogawa Group (Hidaka Belt). This relationship implies that the Horoizumi Shear Zone (between the Hidaka Metamorphic Belt and the Erimo Complex) is not a southern extension of the Hidaka Main Thrust but rather a right-lateral or normal fault. The acidic tuff bed, Gm-01, at the base of the turbidite facies of the Hiroo Complex (Nakanogawa Group, southern unit), is dated at 64.4 ± 1.0 Ma, constraining the upper age limit of the Hiroo Complex turbidite deposits. The acidic tuff, Ym-1 (Nakanogawa Group, northern unit), was deposited in a similar hemipelagic environment to that of the acidic tuff bed, but its depositional age (57.0 +1.3/−0.5 Ma; Nanayama et al., 2018) is younger than that of Gm-01. This age differential suggests an age polarity between the northern and southern units of the Nakanogawa Group. The youngest age cluster (48.8 ± 0.4 Ma, early Eocene) of a turbidite sandstone from the northeastern extension of the previously undated Bisei Formation represents the youngest age yet obtained for the Nakanogawa Group and the Hidaka Supergroup as a whole.
This report describes and explains the systematic procedure for preserving rock samples in the departmental museum at Yamaguchi University. The rock samples were collected during fieldwork and their preservation is important for both scientific and education purposes. The archiving process consists of the following steps: 1) sample selection and creation of a temporary record; 2) sample cleaning and cutting to size; 3) sample numbering and storing the numerical identifiers within the rock registry; 4) adding the name and locality of specimens to the rock registry; 5) sample labeling and storage; and 6) displaying the samples for viewing by both students and the general public, and establishing a database. Maintaining a sample numbering system and ensuring that sample details are entered into the registry are fundamental for the proper upkeep of a rock collection. Cutting the samples to a smaller size is important for storing them in the restricted space of a small departmental museum. Contributing to education and enabling public display of the rock samples is likely to encourage the long-term viability of the museum.
A “coarse-grained” metabasite body has recently been discovered within the Sambagawa metamorphic rocks in Kumakogen Town, Ehime Prefecture, Southwest Japan. The metabasite is exposed at only a few outcrops, although gravel-sized clasts from the metabasite are also found in the overlying conglomerate beds of the Eocene Hiwadatoge Formation. The metabasite rocks generally have a weak foliation compared with the well-developed foliation in the surrounding Sambagawa crystalline schists. The metabasite includes white to gray “porphyroclasts” (composed mainly of albite and zoisite) within a greenish-brown matrix (including actinolite and chlorite). Rock textures and modal mineral compositions of the metabasite gravels in the Hiwadatoge Formation are more varied than those found in outcrops of the metabasite unit itself. Mineral assemblages found in the metabasite and the surrounding mafic schist show that both rock units were metamorphosed under conditions corresponding to greenschist facies.
Laser-ablation ICP-MS U−Pb dating was carried out on rock samples from granitic dikes of the Hida Metamorphic Belt at Kagasawa, Toyama City, where Precambrian Rb−Sr whole-rock ages disagree with Triassic (240-230 Ma) CHIME zircon ages. Two medium-grained granodiorite samples yielded zircon 238U-206Pb weighted mean ages of young age clusters of 241.3±3.0 Ma and 229.3±1.9 Ma, and a coarse-grained granite yielded a zircon 238U-206Pb weighted mean age of 241.4±2.0 Ma. These ages are similar to the CHIME zircon ages. The medium-grained granodiorites contain several Precambrian zircons with core ages of ca. 2500 Ma, 2000 Ma, and 1600 Ma. The coarse-grained granite, however, contains no older zircon.