Chemical compositions of the Paleogene granitoids of eastern Shimane Prefecture, Sanin District, Southwest Japan

Ishihara, Shunsō; Chappell, B. W.

doi:10.9795/bullgsj.59.225

Cited by 9 publications

(11 citation statements)

References 23 publications

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“…Al 2 O 3 , CaO* + Na 2 O and K 2 O (A–CN–K) diagram for core 13Nk‐8c samples (in molar proportion). Also plotted are the compositions of Daito granodiorite, Yokota granite, Yamasa leucogranite and Seikyuzen andesite (Ishihara and Chappell, ). Dashed line parallel to the A–CN axis is the ideal weathering trend (IWT).…”

Section: Discussionmentioning

confidence: 97%

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

Ota

Kawahata

Sato

et al. 2017

J Quaternary Science

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Management of flood risk can benefit from a knowledge‐based approach to flooding history. However, the data on such events are often insufficient for an objective evaluation of flooding characteristics such as their intensity. Therefore, we analyzed a 14C‐dated sediment core recovered from near the Iinashi River mouth in Lake Nakaumi, western Japan, and reconstructed flooding records over the past 700 years from rock magnetism, grain‐size distribution and sediment geochemistry. The sediment core contained three black layers, characterized by high magnetic susceptibility, low anhysteretic remanent magnetization to saturation isothermal remanent magnetization ratio, high total organic carbon (TOC) to total nitrogen ratio, and coarse mean grain size, which we identified as possible flooding event deposits. Moreover, the chemical index of alteration was low and the TOC to total sulfur ratio was high in these layers, indicating intensified erosion associated with more freshwater inflow due to the flood events. We tentatively correlated the three event deposits with historical floods along the Iinashi River in AD 1596, 1666 and 1826. The variations in rock magnetic and geochemical properties suggest that the flooding in AD 1666 may have been the least extreme of the events detected in the core.

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Section: Discussionmentioning

confidence: 97%

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

Ota

Kawahata

Sato

et al. 2017

J Quaternary Science

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“…In addition to oxidized magmatic conditions, adakite‐like intrusions, characterized by high Sr/Y, are widely promoted as hallmarks of arc segments that are especially fertile for porphyry Cu genesis (Loucks, ). High Sr/Y – commonly ascribed to suppression of plagioclase fractionation due to the elevated magmatic water contents that are conducive to porphyry Cu formation (Richards, ) – are reported from a few Early Cretaceous intrusions throughout Japan (Kamei, ; Takahashi et al , ; Tsuchiya et al , , ; Ishihara & Chappell, ) and several Quaternary volcanoes in SW Japan (Mahony et al , ; Shibata et al , , and references therein), but apparently not from post‐Jurassic magmatic rocks in South Korea (Kim et al , ). Although most of the Early Cretaceous intrusions are barren, several in the Kitakami belt, including that alongside the Kamaishi Cu‐Fe skarn (Fig.…”

Section: Regional Factors Influencing Porphyry Cu Potentialmentioning

confidence: 99%

Why No Porphyry Copper Deposits in Japan and South Korea?

Sillitoe¹

2018

Resource Geology

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This analysis concludes that there is little realistic likelihood of discovering large, high‐grade porphyry Cu deposits in Japan and South Korea because of an unusual combination of geological factors. First, weak inter‐plate coupling, trench retreat, and extension – a tectonic regime unfavorable for porphyry Cu formation – appears to have characterized the convergent margin during much of Cretaceous through Neogene time. Second, Cretaceous through Quaternary magmatism involved widespread ash‐flow caldera development, a volcanic style that suppresses porphyry Cu formation because the all‐important Cu‐bearing magmatic fluids – if present – would be explosively discharged during silicic ignimbrite eruption and lost to the atmosphere. Third, due to the trench retreat, crustal profiles beneath Japan are dominated by accretionary complexes containing chemically reduced lithologies, which decrease the redox state of subduction‐related magmas during their ascent. The most reduced intrusions belong to the ilmenite‐series, which may lock up chalcophile elements at depth and produce magmatic fluids that are incapable of transporting the Cu required for significant porphyry deposit formation. Fourth, many of the caldera‐related magmatic rocks, including the principal intrusions, are not only derived by partial melting of reduced crust but are commonly also highly fractionated; hence, any associated mineralization tends to be Mo ± W‐rich rather than Cu dominated. Fifth, intrusive rocks with high Sr/Y, typical of hydrous magmas responsible for porphyry Cu belts worldwide, appear to be present only locally. Sixth, two important magmatic and metallogenic tracts in Japan, the Kuroko‐bearing Green Tuff belt and Kitami low‐sulfidation epithermal Au region, are characterized by bimodal magmatism, which worldwide is compositionally unsuited to porphyry Cu generation. Seventh, the Pliocene‐Quaternary andesitic–dacitic volcanoes that could be the surface expressions of porphyry Cu systems are mostly too shallowly eroded to reveal underlying deposits should they exist. Consequently, few magmatic suites in Japan and South Korea provide propitious environments for porphyry Cu exploration, with the last hope believed to lie at depth below late Miocene–Pliocene advanced argillic lithocaps, several of which contain relatively minor high‐sulfidation epithermal Au mineralization. However, even there, the reduced crust may have militated against effective Cu transport into the porphyry environment. The magmatic and metallogenic factors that downgrade the porphyry Cu potential of Japan and South Korea are believed to be equally applicable to other subduction‐related magmatic arc segments worldwide.

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“…Around the two large iron‐placer mines at Zakka and Katsura‐ga‐dani to the east, the silica contents vary from 51.3 to 59.6 % (Ishihara & Chappell, ). Two of the most mafic rocks were selected in this study from the main Zakka part.…”

Section: Petrography Of the Studied Mafic Rocksmentioning

confidence: 99%

Geneses of Two Types of Mafic Rocks to Carry Placer‐Magnetite Ores in the Sanin Granitic Belt, SW Japan

Ishihara

Hirano

2016

Resource Geology

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Two types of mafic rocks from the central Sanin district, and their mafic minerals, were studied chemically and microscopically. They are classified into pyroxene‐containing gabbroid and hornblende–biotite quartz diorite. The gabbroid had higher color index but lower magnetite content; while the quartz diorite had lower color index, but higher magnetite content. The magnetite contents are also related to the amounts of hydrous mafic silicates. The gabbroic magma having pyroxene–amphibole assemblage, originated in the upper mantle, was considered essentially anhydrous, but became partly hydrous on the way to the site of solidification in the continental crust, and crystallized some magnetites with hypersthene and amphibole. The quartz dioritic magma was formed by partial melting of possibly subducting ocean‐floor basalts, once exposed to the sea‐floor then altered; thus the magmas became hydrous and oxidized originally, and precipitated abundant magnetite and hydrous mafic silicates from the early crystallization stage onward. Their weathered parts provided the most placer magnetite ores in the history.

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Chemical compositions of the Paleogene granitoids of eastern Shimane Prefecture, Sanin District, Southwest Japan

Cited by 9 publications

References 23 publications

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

Why No Porphyry Copper Deposits in Japan and South Korea?

Geneses of Two Types of Mafic Rocks to Carry Placer‐Magnetite Ores in the Sanin Granitic Belt, SW Japan

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