Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Nowlan, Elke Ursula; Schertl, Hans‐Peter; Schreyer, Werner

doi:10.1016/s0024-4937(99)00091-2

Cited by 61 publications

(52 citation statements)

References 29 publications

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“…It is, however, quite remarkable that one garnet porphyroblast still preserves its compositional growth zoning; this may have been caused by accelerated rates of subduction and exhumation. One major problem concerning P±T estimates of high-grade metamorphic rocks bearing garnet with diffusion-controlled zoning is uncertainty about equilibrium compositions and the relative timing of the thermal equilibrium (recently discussed for UHP eclogites in Nowlan et al 2000). The`¯at' zonation pattern in the core of the garnet probably re¯ects a temperature peak at a pressure below the peak pressure of the metamorphic evolution.…”

Section: Discussionmentioning

confidence: 99%

High‐Si phengite, mineral chemistry and P–T evolution of ultra‐high‐pressure eclogites and calc‐silicates from the Dabie Shan, eastern China

et al. 2000

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A suite of coesite±eclogites and associated calc-silicate rocks from the ultra-high-pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high-Si phengite in the eclogites and the enclosing calc-silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are re¯ected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite-facies rocks, maximum pressure± temperature (P±T) conditions of 40±48 kbar at`750 C were estimated for the peak-metamorphic mineral assemblages. These P±T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion-controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite-and amphibolite-facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to`20 kbar at`600 C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal in¯uence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper-mantle depths.

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Section: Discussionmentioning

confidence: 99%

High‐Si phengite, mineral chemistry and P–T evolution of ultra‐high‐pressure eclogites and calc‐silicates from the Dabie Shan, eastern China

et al. 2000

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“…It is problematic to select an appropriate composition for phengite at the peak pressure condition. Generally in eclogites, phengites with higher Si contents yield maximum pressures (e.g., Nowlan et al, 2000;Tsujimori et al, 2006a). However, petrographical relations shown by the direct contact of the rim portion of coarse-grained phengite with garnet rim II do not reflect any Si-Al exchange between the two minerals (Figs.…”

Section: Peak Stage (M3)mentioning

confidence: 99%

Prograde and retrograde evolution of eclogites from the Bantimala Complex in South Sulawesi, Indonesia

Setiawan

Osanai

Nakano

et al. 2016

Journal of Mineralogical and Petrological Sciences

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This contribution reports the metamorphic evolution of the high-pressure metamorphic rocks from the Bantimala Complex, South Sulawesi, Indonesia. Barroisite-bearing and barroisite-free eclogites were examined to assess their metamorphic evolutions, which have implications regarding the tectonic conditions in this region. The eclogites mainly consist of garnet, omphacite, phengite, rutile, and epidote, with or without barroisite. The variations in mineral assemblages are interpreted to depend upon local changes in the bulk chemical composition. The barroisite-bearing eclogites contain two types of euhedral garnet: coarse-(1-1.5 mm) and finegrained (<0.5 mm). Mineral inclusions in the coarse-grained garnet core and mantle show epidote + titanite and glaucophane + epidote assemblages, that stabilized at 0.9-1.5 GPa and 350-550°C within epidote blueschist-facies conditions. Mineral chemistry and chemical-mapping analyses indicate that both fine-grained garnet and the rim of coarse-grained garnet formed at peak P-T conditions, which were estimated as 2.3-2.7 GPa at 615-680°C based on the garnet-omphacite-phengite-quartz equilibrium. Peak P-T conditions for barroisitefree eclogite were similar (2.5-2.7 GPa at 650-690°C) to those for barroisite-bearing eclogite. Actinolite rims overgrowing matrix sodic-calcic amphiboles attest to retrogression at P < 0.5 GPa and T < 350°C in a clockwise P-T path. The very low geothermal gradient experienced during the prograde path (~5°C/km) likely suggests the subduction of an old and cold oceanic crust. The low geothermal gradient on the retrograde path suggests decompressional cooling during exhumation, possibly favored by a serpentinite-dominated matrix within a subduction channel environment.

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“…This P T estimations are consistent with the occurrence of paragonite inclusion in garnet and omphacite. The combination of the reaction (1) and Grt Cpx geothermometer (GCPQ), that has been extensively employed for the P T estimation of HP rocks (e.g., Carswell et al, 1997;Cuthbert et al, 2000;Nowlan et al, 2000), gives slightly more scattered P T conditions of 2.2 2.5 GPa/665 830 °C than the GCKPQ geothermobarometer. Enami (1996) and Ota et al (2004) reported P T conditions of 1.9 2.6 GPa/700 760 °C for the kyanite quartz eclogite and 1.4 2.5 GPa/550 790 °C for eclogite assemblages from the Western and Eastern Iratsu bodies, respectively.…”

Section: Geothermobarometrymentioning

confidence: 99%

Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan

Miyamoto

Enami

Tsuboi

et al. 2007

Journal of Mineralogical and Petrological Sciences

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Kyanite bearing eclogitic assemblages occur in the highest grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca rich/Mg poor inner and Ca poor/Mg rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower pressure conditions of subsequent epidote amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3 2.4 GPa/675 740 °C. The metamorphic P T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg rich composition was in equilibrium with other eclogite facies phases. Therefore, P T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.

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Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Cited by 61 publications

References 29 publications

High‐Si phengite, mineral chemistry and P–T evolution of ultra‐high‐pressure eclogites and calc‐silicates from the Dabie Shan, eastern China

High‐Si phengite, mineral chemistry and P–T evolution of ultra‐high‐pressure eclogites and calc‐silicates from the Dabie Shan, eastern China

Prograde and retrograde evolution of eclogites from the Bantimala Complex in South Sulawesi, Indonesia

Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan

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