Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.69 No.1 (2018)

Cover photograph | Table of Contents | Abstract

Cover photograph

Lithofacies of the Jurassic accretionary complex distributed in the western area of Lake Towada, Northeast Japan.

This accretionary complex, which is correlated to that in the North Kitakami Belt, rarely occurs in this area because it is broadly covered and/or intruded by the Cenozoic erathem. The accretionary complex consists of pelitic rocks with minor sandstone and chert. They are weakly metamorphosed; particularly the pelitic rocks frequently become semi-schist.

Upper left: Outcrop of the pelitic semi-schist. Upper right: Polished sample of the semi-schist showing alternating thin layers of dark gray and gray parts, and intrusions of thin quartz veins. The width is ca. 10 cm. Lower left: Thin section photomicrograph of the semi-schist (cross-polarized light) showing alternating thin layers of a very fine mineral band of quartz and felspar and a mica band of sericite and muscovite (high interference color). The width is ca. 4 mm. Lower right: Thin section photomicrograph of the coarse sandstone (felspathic wache) from which a zircon U–Pb age (174.6 Ma) was determined (cross-polarized light). The mineral in the middle with a high interference color is zircon. The width is ca. 1.6 mm.

(Photograph and Caption by Takayuki Uchino)

Table of Contents

All the pages PDF ： 69_01_full.pdf ［31MB］

Abstract

Evaluation of straightforward and rapid multi-element analyses of stream sediments for geochemical mapping in the remote islands of Japan — Seto Inland Sea region —

Atsuyuki Ohta

The straightforward and rapid determination of 53 elements in stream sediments using ICPAES (inductively coupled plasma atomic emission spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), and AAS (atomic absorption spectrometry) were evaluated for the geochemical mapping of isolated islands. Samples of 0.1 g were decomposed with HF, HNO₃, and HClO₄ at 125–145℃ for 3 h to improve the determination of elements, including refractory minerals. The concentrations of rare earth elements (REEs), Nb, and Ta increased by 5–15% on average, and those of Zr and Hf increased by 30% on average. For arsenic determination, 0.1 g samples were digested using a mixed acid solution with an oxidizing reagent (KMnO₄) at 120°C for 20 min. Decomposition without using the oxidizing reagent and/or extended decomposition times has been shown to cause a decrease in As concentrations in geochemical reference materials. However, similar As concentrations in stream sediment samples were obtained irrespective of KMnO₄ addition and decomposition time. AAS was used to Hg measurement after thermal decomposition of about 50 mg samples without pre-treatment. The estimated concentrations of 53 elements in geochemical reference materials measured using ICP-AES, ICP-MS and AAS were agreement with the recommended values. Thus, it is concluded that the precision and accuracy of the rapid and straightforward analysis for geochemical mapping were satisfactory.
Geochemical features of stream sediments in the isolated islands of the Seto Inland Sea were strongly influenced by the parent lithology distributed in their watershed. Enrichments of Na₂O, Al₂O₃, K₂O, Be, Rb, Nb, REEs, Ta, Th and U were observed in sediments from isolated islands with widely underlain granitic rocks. In contrast, MgO, TiO₂, V, Cr, MnO, Fe₂O₃, Ni, and Co were abundant in sediments from Shodoshima Island where mafic volcanic rock had erupted. Extreme enrichments of Cu, Zn, As, Mo, Cd, Sn, Sb, Hg, Pb, and Bi were found in sediments influenced by mineral deposits and anthropogenic activity.

The Toya Ash interbedded with the eruptive deposits during the pre-caldera stage of Towada volcano, Northeast Japan

Takashi Kudo

A vitric fine ash layer was found at four sites of the proximal eruptive deposits during the precaldera stage of Towada volcano. Results of mineral composition and refractive index analyses of tephra samples show that the vitric ash layer is correlated to the Toya Ash. Stratigraphic relations of the Toya Ash and the eruptive deposits from Towada volcano indicate that Towada volcano had been erupting before and after fallout of the Toya Ash and had erupted more frequently in the pre-caldera stage than previously thought.

Detrital zircon U–Pb age of the Jurassic accretionary complex in the western area of Lake Towada located between Akita and Aomori prefectures, Northeast Japan

Takayuki Uchino

Jurassic accretionary complexes are broadly distributed in the North Kitakami Belt in the Kitakami Massif, Northeast Japan. Meanwhile, the accretionary complexes rarely occur in the western area of the N–S-trending Ou Mountain Range because they are broadly covered and intruded by the Cenozoic erathem. Therefore, the accretionary complexes in the western area provide insufficient geoinformation: their geologic age and correlation to the accretionary complexes in the Kitakami Massif.
In this study, the U–Pb ages of detrital zircon from sandstone were examined to understand the geologic age of the accretionary complexes, which are sparsely distributed in the western area of Lake Towada within the Ou Mountain Range. Consequently, 174.6 ± 0.7 Ma was obtained as a weighted mean age of the youngest age component of grain-age distribution (i.e., youngest cluster). Therefore, it has been confirmed that the sandstone was deposited after Middle Jurassic.
A pattern of the relative probability of the detrital zircon ages shows the distribution of intensive peaks for the Permian–Jurassic period of the Phanerozoic and minor peaks for Paleoproterozoic. This pattern is considerably similar to that obtained for the Middle Jurassic–Lower Cretaceous shallow-marine deposits in the South Kitakami Belt. The geologic age of the Mesozoic accretionary complexes in the Tohoku area shows a younging trend from the Late Triassic to earliest Cretaceous toward the Pacific Ocean according to previous studies. Based on this younging trend, the accretionary complex in the western area of Lake Towada probably correlates to the Middle Jurassic accretionary complex.

Whole-rock geochemical compositions of igneous-origin rocks from the 1:200,000, Hiroo Quadrangle and related area

Toru Yamasaki, Futoshi Nanayama, Yutaka Takahashi and Kohei Yamashita

Whole-rock major and trace geochemical analyses of igneous rocks and igneous-origin rocks from the 1:200,000, Hiroo Quadrangle and related area have been carried out to confirm geotectonic attribution and correlation. Studied samples are; 1) Okutokachi Amphibolite, 2) Rekifune Volcanic Rocks, 3) dolerite dike in the Okawa Formation (Middle Miocene), 4) greenstone in the Toyokoro area (originated from Nikoro Group, Tokoro Belt), 5) granitic gravels in the Toyonigawa Formation (Middle–Late Miocene), 6) Nupinai Granite in the Hidaka Metamorphic Belt, 7) mafic volcanic gravels in the Nakanogawa Group, Hidaka Belt (Paleocene–Eocene), 8) greenstone from the Daimaruyama Greenstone body in the Nakanogawa Group, 9) granitic gravel in the Erimo Formation (Utaro Conglomerate: Late Oligocene), and 10) acidic tuff in the Sakashita Formation, Nakanogawa Group, Hidaka Belt. Obtained data are important as the scientific bases of the 1:200,000, Hiroo Quadrangle (2nd edition), and valuable information for future researches. Based on the analytical results, geotectonic attribution, origin and tectonics of the studied samples are preliminary discussed.