Biotite zone in the Sambagawa metamorphic terrain east of Onishi-machi, Kanto Mountains

Sakai, Chihiro

doi:10.5575/geosoc.86.517

Cited by 5 publications

(6 citation statements)

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“…The foliation of the schists generally dips slightly north in the area. The lithology is known to be nearly parallel to the foliation (Sakai, 1980;Hashimoto et al, 1992).…”

Section: Geologic Settingmentioning

confidence: 99%

Spatial distribution of garnet indicating control of bulk rock chemistry in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan

Inui

Tanifuji

2018

Journal of Mineralogical and Petrological Sciences

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Traditional metamorphic geology admitted that the appearance of specific index mineral, such as garnet, indicated the rocks experienced higher peak metamorphic condition. It is largely true, but it is still unclear what is going on in the rocks at the very start of the garnet formation. One of the key must lie in the area where apparent isograd is outcropped. Nagatoro area is located in the Kanto Mountains, and it is where the low grade metamorphic rocks of the Sanbagawa metamorphic belt are exposed. Appearance of garnet is the index of the higher metamorphic grade in this area. Several outcrops in Nagatoro area are known to contain garnet, thus being good samples of the outcropped isograd. In this study, spatial distribution of garnet within such an outcrop several tens of meters long was investigated. The samples are pelitic schists, and the mineral assemblage was basically quartz, plagioclase, muscovite, and chlorite. 36 of the 55 samples contained garnet. Garnet grains were small, most of them with diameters less than 100 µm. Most of the garnet grains were euhedral to subhedral and were found within micaceous lamellae which form the foliations of the pelitic metamorphic rocks. The micaceous lamellae were constituted mainly by muscovite with lesser amount of chlorite. Occurrence of garnet-bearing rocks within the outcrop seemed to be restricted in certain structural layers. The structural layers are known to be nearly parallel to the lithologic boundary in this area. Mapped chemical profiles of garnet revealed that the garnet grains exhibited euhedral growth of the crystal. The core part was relatively large and homogeneous, with quite a high Mn end-member (spessartine) content (X Sps > 50%). Irregular shaped inner core was preserved. These features indicate that the texture preserves the beginning stage of garnet growth. The trend of chemical composition of garnet rim and adjacent chlorite is consistent with the bulk rock chemistry control. Spatial distribution of garnet, at the start of its growth, was probably controlled by the bulk rock chemistry.

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“…The foliation of the schists generally dips slightly north in the area. The lithology is known to be nearly parallel to the foliation (Sakai, 1980;Hashimoto et al, 1992).…”

Section: Geologic Settingmentioning

confidence: 99%

Spatial distribution of garnet indicating control of bulk rock chemistry in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan

Inui

Tanifuji

2018

Journal of Mineralogical and Petrological Sciences

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“…Carbonaceous materials from the Asemi river route were separated by Itaya (1981), whose separation-method is different from mine regarding the use of HNO3 and drying. The mineral zone boundaries by Higashino (1975) and by Sakai (1980) are duplicated on Fig. 6.…”

Section: Separationmentioning

confidence: 99%

A measurement of the graphitizing-degree by the X-ray powder diffractometer.

Tagiri

1981

J. Japan. Assoc. Min. Petr. Econ. Geol.

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“…The Nagatoro area is located to the northwest of Tokyo, within the Kanto Mountains, and forms the easternmost part of the Sanbagawa metamorphic belt. The Kanto Mountains can be divided roughly into chlorite, garnet, and biotite zones, with increasing metamorphic grade, based on the mineral assemblages and extent of graphitization in pelitic schists [19,20]. The bedding in this area is sub-parallel to the schistosity [19,20].…”

Section: Geological Settingmentioning

confidence: 99%

“…The Kanto Mountains can be divided roughly into chlorite, garnet, and biotite zones, with increasing metamorphic grade, based on the mineral assemblages and extent of graphitization in pelitic schists [19,20]. The bedding in this area is sub-parallel to the schistosity [19,20]. Pelitic schists of the chlorite zone generally contain quartz, plagioclase, muscovite, and chlorite, with accessory calcite, titanite, and pyrite.…”

Section: Geological Settingmentioning

confidence: 99%

Nucleation and Initial Growth of Garnet in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt, Kanto Mountains, Japan

2020

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The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.

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Biotite zone in the Sambagawa metamorphic terrain east of Onishi-machi, Kanto Mountains

Cited by 5 publications

References 0 publications

Spatial distribution of garnet indicating control of bulk rock chemistry in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan

Spatial distribution of garnet indicating control of bulk rock chemistry in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan

A measurement of the graphitizing-degree by the X-ray powder diffractometer.

Nucleation and Initial Growth of Garnet in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt, Kanto Mountains, Japan

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