Chemically sector-zoned garnet in Sanbagawa schists; its mode of occurrence and growth timing.

Shirahata, Katsushi; Hirajima, Tsuyoshi

doi:10.2465/ganko.90.69

Cited by 15 publications

(19 citation statements)

References 12 publications

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“…Distinctly small euhedral garnet grains with diameters around 100 μm are also observed in zone A. The occurrence of the small grains is similar to that of garnet with intrasectoral chemical zoning occasionally found in the Besshi district (Shirahata and Hirajima, 1995;Inui and Toriumi, 2002). It is known that the chemical composition of such small garnet is not always similar to that of the porphyroblastic garnet, indicating that they did not grow simultaneously in equilibrium.…”

Section: Sample Descriptionmentioning

confidence: 65%

A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

Inui

2008

Journal of Mineralogical and Petrological Sciences

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The EPMA analysis of a pelitic sample from the Besshi district, central Shikoku, in the Sambagawa metamorphic belt reveals that two types of garnet porphyroblasts with different sizes and chemical compositions are present within a single thin section. One type consisting of medium -sized (d ~ < 1 mm) grains has a high Mn content at the core and shows a pyrope -rich profile. The other type is comparatively larger in size (d ~ 2 mm). The rim compositions of both types are different. The influence of the initial bulk rock chemistry on the garnet is calculated by the forward model. A comparison of the observations with the calculated results indicates that the difference between the two types of garnet grains is attributed to the initial Fe/Mg ratio of the local bulk rocks.

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Section: Sample Descriptionmentioning

confidence: 65%

A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

Inui

2008

Journal of Mineralogical and Petrological Sciences

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“…Based on the size distributions, they are divided into Higashino, 1990;Aoya et al, 2013) and locality of the studied sample (IR09). Sampling localities of previously reported Mn-caldera shaped zoned garnet (Xu et al, 1994), chemically sector-zoned garnet in the Tonaru area (Shirahata and Hirajima, 1995) and the Asemi-gawa area (Inui, 2010), and evidences of the eclogite facies metamorphism in the related area (Aoki et al, 2009;Taguchi and Enami, 2014a;b) are also shown. Broken line shows the suggested boundary of the continuous eclogite nappe (Aoya et al, 2013).…”

Section: Zoning Pattern and Mineral Inclusions Of Garnetmentioning

confidence: 80%

“…Some retrograde reactions are reported to generate Mn-increase, which is known as Mn-reverse zoning, at the rim of garnet due to resorption of garnet (e.g., Itaya, 1978;Takasu, 1986;Takasu and Kohsaka, 1987;Nomizo, 1992). Other types of compositional zoning such as chemically sector-zon-ing (Shirahata and Hirajima, 1995;Inui, 2010), and complex and discontinuous zonings (e.g., Takasu, 1986;Endo et al, 2009;Kabir and Takasu, 2011;Yoshida and Hirajima, 2012;Kouketsu et al, 2014a) are also reported, especially within eclogite-facies rocks and their proximal area of the Besshi district, central Shikoku. Furthermore, garnet overgrowth on the detrital garnet grains is reported in some metasedimentary rocks in the Sanbagawa metamorphic belt, which are characterized by discontinuous compositional profile (e.g., Higashino and Takasu, 1982).…”

Section: Introductionmentioning

confidence: 99%

3D chemical mapping of ‘Mn–caldera shaped zoning’ garnet found from the Sanbagawa metamorphic belt of the Besshi district, SW Japan

Yoshida

Hirajima

2015

Journal of Mineralogical and Petrological Sciences

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Garnet with a complex compositional zoning was found from the northern edge of the Western Iratsu body in the Sanbagawa metamorphic belt of the Besshi district, southwest Japan. The studied garnet shows incipient Mn-reverse (increase) zoning from the center part and subsequent Mn-bell shape type (decrease) zoning towards the outer part, which is almost identical to the 'Mn-caldera shaped zoning' described by Xu et al. (1994) and Banno and Nakamura (2004). In order to display the chemical characteristics sterically, three-dimensional X-ray chemical mapping was performed for one very-coarse grain of garnet. The result clearly displays that there is a high-Mn layer with faceted euhedral shape of the garnet at the intermediate part, and that its composition continuously changes from the inside to the outside. Inclusion arrays in garnet getting across the highMn layer also suggest the continuous growth of the garnet. In the same sample, garnets without Mn-caldera shaped zoning are ubiquitously observed, which are relatively small in size up to 5 mm in diameter. Inclusion mineral assemblage in garnet and Raman barometry suggest the peak P-T conditions of the studied sample not having reached the eclogite facies, but being the epidote-amphibolite facies. The formation process of the 'Mncaldera shaped zoning' can be best explained by the disequilibrium crystal growth under oversaturation of garnet in MnO-(MgO + FeO) binary system. Such oversaturation can be triggered by rapid heating. The finding of the remnant of rapid heating event would provide an important clue to discuss the exhumation history of the Sanbagawa metamorphic belt.

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“…The localities reported previously are as follows: the oligoclase -biotite zone rocks in the vicinity of the northern border of the Tonaru metagabbro mass (Kitamura et al, 1993;Shirahata and Hirajima, 1995), the albite -biotite zone rocks in the vicinity of the Sebadani eclogite mass (Takasu and Kondo, 1993;Takasu and Fujita, 1994) and in the vicinity of the Iratsu metagabbro mass (Inui and Toriumi, 2002). The formation of sector zoning in these garnets is considered to be related to the occurrence of the eclogitic rock masses because all the previously mentioned localities are close to the amphibolites (retrograded eclogites).…”

Section: Implication Of Type -B Garnetmentioning

confidence: 99%

“…The Asemigawa River route is particularly well known because rocks of various metamorphic grades are exposed along the river, including the highest -grade rocks in the Sanbagawa metamorphic belt (Higashino, 1990). In the last two decades, many scientists have been able to detect anomalous chemical zoning, including sector zoning, in the garnets obtained from the central Shikoku area (Takasu, 1994;Shirahata and Hirajima, 1995;Inui, 2004), because of the increased availability of sophisticated instruments for chemical mapping. The presence of chemical sector zoning indicates that garnets had not grown in chemical equilibrium.…”

Section: Introductionmentioning

confidence: 99%

Two types of garnet in Sanbagawa pelitic schists along Asemigawa River, central Shikoku

Inui

2010

Journal of Mineralogical and Petrological Sciences

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The mode of occurrence of garnet in pelitic schists of the Sanbagawa metamorphic belt along the Asemigara River in central Shikoku is examined. Two different types of garnets, A and B, are identified from the albite -biotite and the oligoclase -biotite zones of the Sanbagawa belt. Type -A garnets are larger than type -B garnets and show concentric normal chemical zoning. There is no evidence of disequilibrium crystal growth in type -A garnets. On the contrary, type -B garnets are small, and some of them occasionally show sector zoning, which is a strong evidence for a disequilibrium crystal growth. The chemical trend observed between the core and the rim of type -B garnets is different from that observed in the case of type -A garnets, although only some of the type -B garnets show chemical sector zoning. Type -B garnets are typically found in clusters in muscovite -rich layers or are included in albite porphyroblasts. These two types of garnets can be distinguished from each other on the basis of grain size, cluster formation, and chemical composition. Type -B grains are found only in the structurally lower part of the oligoclase -biotite zone and in the lower albite -biotite zone. These two types of garnets are thought to represent two stages of garnet growth. Further, in the case of type -A garnets (early garnet), chemical equilibrium is maintained during prograde metamorphism, and hence, these garnets are suitable for the use in thermobarometric studies.

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Chemically sector-zoned garnet in Sanbagawa schists; its mode of occurrence and growth timing.

Cited by 15 publications

References 12 publications

A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

3D chemical mapping of ‘Mn–caldera shaped zoning’ garnet found from the Sanbagawa metamorphic belt of the Besshi district, SW Japan

Two types of garnet in Sanbagawa pelitic schists along Asemigawa River, central Shikoku

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