Geology and petrology of the coesite-bearing terrain, Dora Maira massif, Western Alps.

Chopin, Christian; Henry, Caroline; Michard, André

doi:10.1127/ejm/3/2/0263

Cited by 285 publications

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“…Chopin, 1984;Chopin et al, 1991;Schertl et al, 1991). These assemblages include talc + kyanite + pyrope + coesite, talc + phengite (Si > 3.5) + kyanite + coesite, and talc + jadeite.…”

Section: Discussionmentioning

confidence: 99%

“…Talc-bearing assemblages in manganiferous quartzite from the Zermatt-Saas zone, Western Alps (Reinecke, 1991), and in Mg-rich quartzite from the Dora Maira Schertl et al, 1991) have been identified recently in some ultrahigh-P metamorphic (UHPM) terranes and their stabilities have been experimentally delineated (e.g. Schreyer, 1988a;Chopin et al, 1991). The classic whiteschist assemblage talc + kyanite has a very large stability field under H20-saturated conditions from 6 kbar to probably 40 kbar at 550-800~ (Schreyer, 1988a).…”

Section: Introductionmentioning

confidence: 99%

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Significance of ultrahigh-P talc-bearing eclogitic assemblages

Liou

Zhang

1995

Mineral. mag.

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Talc was identified as an equilibrium associate of omphacite, kyanite, zoisite, and garnet in some coesitebearing eclogites and as a retrograde phase in associated garnet peridotite from the Bixiling mafic-ultramafic complex in the Dabie ultrahigh-P metamorphic terrane, central China. Observed talc-and tremolite-bearing assemblages within the coexisting mafic and ultramafic rocks together with established phase relations for the simple system CaO-MgO-A12Oa-SiO2-H20 constrain the P-T path for retrograde metamorphism. This, together with reported talc occurrences from the Dora Maira massif, leads to the conclusion that UHP talc assemblages are confined to mafic and pelitic rocks of unusual chemistry, and are not stable at ultrahigh pressures in ultramafic rocks. Thus, talc cannot be an important H20-bearing phase in deep mantle ultramafic rocks.

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“…Chopin, 1984;Chopin et al, 1991;Schertl et al, 1991). These assemblages include talc + kyanite + pyrope + coesite, talc + phengite (Si > 3.5) + kyanite + coesite, and talc + jadeite.…”

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Significance of ultrahigh-P talc-bearing eclogitic assemblages

Liou

Zhang

1995

Mineral. mag.

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“…Having experienced peak metamorphism under epidote-blueschist facies conditions, the Pinerolo Unit could have not been reset for its white mica K-Ar system during the Alpine metamorphism and could have preserved large amounts of radiogenic 40 Ar formed by the 40 K decay in micas. (3) Another possible source may be the overlying Rocca Solei Unit (quartz-eclogite metamorphic overprint), which also experienced lower peak-T conditions than those experienced by the BIU (T;~550°C, P; 1.5 GPa: Chopin et al, 1991;Matsumoto and Hirajima, 2000). In either case, the argon release could have been enhanced by the pervasive deformation occurred during exhumation.…”

Section: Possible Source Of Eaw In the Dora-maira Massifmentioning

confidence: 99%

Laser step–heating <sup>40</sup>Ar/<sup>39</sup>Ar analyses of biotites from meta–granites in the UHP Brossasco–Isasca Unit of Dora–Maira Massif, Italy

Itaya¹,

Hyodo

Imayama

et al. 2018

Journal of Mineralogical and Petrological Sciences

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Two undeformed metagranite samples were collected from the UHP Brossasco-Isasca Unit (BIU) of Dora-Maira Massif, Italy to carry out the laser step-heating 40 Ar/ 39 Ar analyses of individual biotite crystals. The metagranites occur as undeformed domains (m-to ten of meters in size) within strongly deformed augen-gneiss. They still preserve their medium-to coarse-grained igneous texture and are composed mainly of K-feldspar, plagioclase pseudomorph, quartz, and biotite that preserve their original igneous shape but are either re-equilibrated or replaced by new phases.Three biotite crystals from the first sample have similar age spectra showing 400 to 300 Ma, except for the first fraction (500-1500 Ma). On the contrary, the age spectra of five biotite crystals from the second sample are significantly different; these biotites show saddle shape 40 Ar/ 39 Ar age spectra, except for one crystal. The oldest fractions have ages (800 to 1300 Ma) three to four times older than that of the granite protolith (which is late Permian). This extremely high intensity of excess argon could be due to an 'Excess-Argon Wave' (EAW) phenomenon, occurred during the quick exhumation of the BIU, combined with the extremely short ductile deformation history. The observed variation of the biotite age spectra may reflect the different trapping processes of EAW and/or localized source of EAW.

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“…As a fundamental contribution of petrology to field mapping, it has been recognized that most crystalline units exposed in the area, though deriving from the same Variscan amphibolite-facies basement intruded by Permian granitoids, were characterized by different early-Alpine high-P peak metamorphic overprints, ranging from the epidote-blueschist to the coesiteeclogite-facies conditions (e.g., Chopin et al, 1991;Compagnoni & Rolfo, 2003). The coesite-eclogite bearing BIU is ca.…”

Section: Introductionmentioning

confidence: 99%

“…After the discovery of coesite (Chopin, 1984) in the southern sector of the intermediate Polymetamorphic unit of Vialon (1966), the scientific interest for the southern DMM increased rapidly and a number of detailed petrologic studies were produced (e.g., Castelli, Rolfo, Groppo, & Compagnoni, 2007;Chopin, Henry, & Michard, 1991;Chopin & Schertl, 2000;Compagnoni & Hirajima, 2001;Compagnoni, Hirajima, & Chopin, 1995;Ferrando, Frezzotti, Petrelli, & Compagnonilli, 2009;Groppo, Castelli, & Compagnoni, 2006;Groppo, Lombardo, Castelli, & Compagnoni, 2007a, Groppo, Rolfo, & Castelli, 2007Michard, Henry, & Chopin, 1995;Nowlan, Schertl, & Schreyer, 2000;Schertl, Schreyer, & Chopin, 1991;) aimed at determining the peak metamorphic conditions. The UHP (ultra-high pressure) unit has also been mapped in great detail, and special attention has been paid to recognize the relics of both pre-Alpine and early-Alpine structures and minerals which are essential to understand the nature and relationships among lithologies and to check the internal coherency of the unit.…”

Section: Introductionmentioning

confidence: 99%

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

et al. 2012

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In the southern Dora-Maira Massif, Western Alps, slivers of continental crust with similar lithologies, but recrystallized during the Alpine orogeny at different peak-P conditions, are exposed. They include the Brossasco-Isasca Unit (BIU) where coesite was first discovered in continental crust. A new 1:20,000-scale geologic map and related cross-sections of the whole BIU and adjoining units is presented, in which the most significant features useful to infer the pre-Alpine history and the Alpine tectonic and metamorphic evolution, are summarized. Thanks to detailed petrography and petrology, the geologic map shows the precise location of ultra-high pressure (UHP) minerals (such as coesite), and the locations of the most significant mineral assemblages (such as kyanite + jadeite). This innovative approach is used to distinguish the BIU from the adjacent units. Relict pre-Alpine structures (such as igneous intrusive contacts with basement xenoliths and metagranitoids) are summarized in a sketch illustrating the geologic setting of the UHP metamorphic unit as inferred before the Alpine orogeny.

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Geology and petrology of the coesite-bearing terrain, Dora Maira massif, Western Alps.

Cited by 285 publications

References 0 publications

Significance of ultrahigh-P talc-bearing eclogitic assemblages

Significance of ultrahigh-P talc-bearing eclogitic assemblages

Laser step–heating <sup>40</sup>Ar/<sup>39</sup>Ar analyses of biotites from meta–granites in the UHP Brossasco–Isasca Unit of Dora–Maira Massif, Italy

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

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