Coexistence of jadeite and quartz in garnet of the Sanbagawa metapelite from the Asemi–gawa region, central Shikoku, Japan

Taguchi, Tomoki; Enami, Masaki

doi:10.2465/jmps.140323

Cited by 20 publications

(22 citation statements)

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“…Wallis and Aoya () proposed a nappe structure subdividing the higher‐grade areas in the Besshi region into eclogite and non‐eclogite units; the former is located at the highest structural level of the Sanbagawa belt. The distribution of the unit boundary, and thus, the areal extent of the eclogite unit in the Besshi region, was proposed by Kouketsu et al () and Taguchi and Enami (, ) based on a combination of data on (i) chemical zoning of garnet, (ii) the nature of sodic‐phase inclusions, and (iii) the residual pressure retained by quartz inclusions in garnet hosts in metapelites (Figure a). Rocks of the eclogite unit might have experienced the P–T evolution of prograde eclogite facies stage → exhumation and hydration stage → prograde epidote–amphibolite facies stage (e.g.…”

Section: Geological Settingmentioning

confidence: 98%

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

et al. 2018

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Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.

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Section: Geological Settingmentioning

confidence: 98%

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

et al. 2018

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“…In contrast, the Otaguchi area corresponds entirely to the low‐grade chlorite zone (Sakaguchi & Ishizuka, ). There is definitive evidence (jadeite + quartz inclusions in garnet) for eclogite metamorphism in the Asemigawa area (Taguchi & Enami, ), but the areal extent of the eclogite‐facies unit has not been clarified yet. In the study area, both the lithological boundary and main schistosity of the Shirataki unit are subhorizontal in low‐grade parts and dip moderately to the north in higher‐grade parts.…”

Section: Lithostratigraphic Unit Division and Structurementioning

confidence: 99%

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

Endo

Miyazaki

Danhara³

et al. 2018

Island Arc

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The main tectono‐stratigraphic unit (Shirataki unit) of the Sanbagawa metamorphic complex in central Shikoku is characterized by abundant mafic schist layers that show the mid‐ocean ridge basalt (MORB) affinity. These MORB‐derived schist layers are absent in a southern (structurally lower) domain within the unit. Instead, sporadic occurrences of small metabasite lenses that contain relict igneous minerals (Ti‐rich augite and kaersutite) indicative of alkali basalt magmatism are newly recognized in the southern domain. Compositions of relict clinopyroxene in metabasalt are useful to identify the tectonic setting and origin of the protolith basalt, and those in each unit of the Sanbagawa metamorphic complex are presented. The metamorphic grade of the Shirataki unit generally increases structurally upwards in the southern side of the highest‐grade zone, and metamorphic zonation is subparallel to lithostratigraphic succession. The protolith assemblage of the Shirataki unit shows a distinct change from the southern low‐grade domain (lower Shirataki subunit) composed of terrigenous sedimentary rocks (mudstone and sandstone) with minor alkali basalt to the northern higher‐grade domain (upper Shirataki subunit) consisting of terrigenous and pelagic sedimentary rocks with abundant MORB. The youngest detrital zircon U–Pb ages (ca 95–90 Ma) suggest that both domains have Late Cretaceous depositional ages at the trench. Progressive peeling of oceanic plate stratigraphy during subduction can account for the observed change of lithological association in the Shirataki unit.

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“…Taguchi and Enami (2014a, 2014b) recently reported quartz inclusions with ∆ω 1 >10.0 cm −1 from pelitic schists in the albite–biotite zone of the Asemi‐gawa region. In the present study, the ∆ω 1 values of the pelitic schists also show high values of >8.5 cm −1 , suggesting eclogite facies conditions (Figure a).…”

Section: Discussionmentioning

confidence: 99%

“…IR: Iratsu body; HA: Higashi‐akaishi ultramafic body; TN: Tonaru body; SB: Seba body; SG: Shiraga ultramafic body; ISTL: Itoigawa‐Shizuoka Tectonic Line; MTL: Median Tectonic Line. (b) Lithological map of the Asemi‐gawa region, which was modified after Mori and Wallis () and Taguchi and Enami (2014a, 2014b). Study sample location denoted by red stars.…”

Section: Introductionmentioning

confidence: 99%

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts

Taguchi

Enami

Kouketsu

2018

Journal Metamorphic Geology

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The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1 and <8.5 cm−1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.

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Coexistence of jadeite and quartz in garnet of the Sanbagawa metapelite from the Asemi–gawa region, central Shikoku, Japan

Cited by 20 publications

References 46 publications

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts

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