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Late chloritoid-staurolite assemblage in a garnet-kyanite–bearing metapelite from the ultrahigh-pressure Brossasco-Isasca unit (Dora-Maira Massif, Western Alps): New petrological constraints for a portion of the decompressional path

By
C. Groppo
C. Groppo
1
Department of Mineralogical and Petrological Sciences, via Valperga Caluso 35, I-10125 Torino, Italy
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D. Castelli
D. Castelli
2
Department of Mineralogical and Petrological Sciences, via Valperga Caluso 35, I-10125 Torino, Italy, and CNR (National Research Council), Institute of Geosciences and Georesources–Torino, c/o DSMP (Department of Mineralogical and Petrological Sciences), via Valperga Caluso 35, I-10125 Torino, Italy
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R. Compagnoni
R. Compagnoni
2
Department of Mineralogical and Petrological Sciences, via Valperga Caluso 35, I-10125 Torino, Italy, and CNR (National Research Council), Institute of Geosciences and Georesources–Torino, c/o DSMP (Department of Mineralogical and Petrological Sciences), via Valperga Caluso 35, I-10125 Torino, Italy
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Published:
January 01, 2006

We conducted a detailed study of a unique metapelite from the ultrahigh-pressure (UHP) Brossasco-Isasca unit (Dora-Maira Massif). It consists of pre-Alpine porphyro-blastic garnet (Grt1), Alpine-age idioblastic garnet (Grt2), phengite, and kyanite. The pre-Alpine garnet contains inclusions of staurolite, kyanite, paragonite, chloritoid, chlorite, quartz, ilmenite, and rutile. The metapelite also includes quartz aggregates after former coesite and fine-grained paragonite aggregates after jadeite. Late chlori-toid and staurolite idioblasts, first reported from a Brossasco-Isasca unit metapelite, are also present in the rock matrix: they grew randomly across the main Alpine UHP foliation defined by phengite. The decompression pressure-temperature (P-T) path of the unit was constructed employing a P-T pseudosection calculated for the model MnNKFMASH (MnO-Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O) system in the pressure interval 0.6–1.8 GPa, i.e., below the coesite-quartz transition and within the albite stability field. The absence of albite and biotite, together with the compositions of staurolite, chloritoid, chlorite, and phengite allow us to constrain the 580–590 °C portion of the retrograde P-T path from P = 1.12–0.96 GPa. The results are in good agreement with previously estimated P-T paths, mostly inferred from retrograde assemblages in whiteschist and eclogite of the Brossasco-Isasca unit.

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GSA Special Papers

Ultrahigh-pressure metamorphism: Deep continental subduction

Bradley R. Hacker
Bradley R. Hacker
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William C. McClelland
William C. McClelland
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J.G. Liou
J.G. Liou
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Geological Society of America
Volume
403
ISBN print:
9780813724034
Publication date:
January 01, 2006

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